COMBINED USE OF GDF TRAPS AND ERYTHROPOIETIN RECEPTOR ACTIVATORS TO INCREASE RED BLOOD CELL LEVELS The present application is a divisional application of Australian Application 5 No. 2010282361, which is incorporated in its entirety herein by reference. BACKGROUND OF THE INVENTION The mature red blood cell, or erythrocyte, is responsible for oxygen transport in the circulatory systems of vertebrates. Red blood cells contain high concentrations of 10 hemoglobin, a protein that binds oxygen in the lungs at relatively high partial pressure of oxygen (pO2) and delivers oxygen to areas of the body with a relatively low PO2. Mature red blood cells are produced from pluripotent hematopoietic stem cells in a process termed erythropoiesis. Postnatal erythropoiesis occurs primarily in the bone marrow and in the red pulp of the spleen. The coordinated action of various signaling 15 pathways control the balance of cell proliferation, differentiation, survival and death. Under normal conditions, red blood cells are produced at a rate that maintains a constant red cell mass in the body, and production may increase or decrease in response to various stimuli, including increased or decreased oxygen tension or tissue demand. The process of erythropoiesis begins with the formation of lineage committed precursor cells 20 and proceeds through a series of distinct precursor cell types. The final stages of erythropoiesis occur as reticulocytes are released into the bloodstream and lose their mitochondria and ribosomes while assuming the morphology of mature red blood cell. An elevated level of reticulocytes, or an elevated reticulocyte:erythrocyte ratio, in the blood is indicative of increased red blood cell production rates. 25 Erythropoietin (EPO) is widely recognized as the most significant positive regulator of postnatal erythropoiesis in vertebrates. EPO regulates the compensatory erythropoietic response to reduced tissue oxygen tension (hypoxia) and low red blood cell levels or low hemoglobin levels. In humans, elevated EPO levels promote red blood cell formation by stimulating the generation of erythroid progenitors in the bone marrow 30 and spleen. In the mouse, EPO enhances erythropoiesis primarily in the spleen. Effects of EPO are mediated by a cell-surface receptor belonging to the cytokine receptor superfamily. The human EPO receptor gene encodes a 483 amino-acid - 1 transmembrane protein, whereas the active EPO receptor is thought to exist as a multimeric complex even in the absence of ligand (See U.S. Pat. No. 6,319,499). The cloned full-length EPO receptor expressed in mammalian cells binds EPO with an affinity similar to that of the 5 - 1a native receptor on erythroid progenitor cells. Binding of EPO to its receptor causes a conformational change resulting in receptor activation and biological effects including increased proliferation of immature erythroblasts, increased differentiation of immature erythroblasts, and decreased apoptosis in crythroid progenitor cells (Liboi ct al., 1993, Proc 5 Natl Acad Sci USA 90:1 1351-11355; Koury et al., 1990, Science 248:378-381). Various forms of recombinant EPO are used by physicians to increase red blood cell levels in a variety of clinical settings, and particularly for the treatment of anemia. Anemia is a broadly-defined condition characterized by lower than normal levels of hemoglobin or red blood cells in the blood. In some instances, anemia is caused by a primary disorder in the 10 production or survival of red blood cells. More commonly, anemia is secondary to diseases of other systems (Weatherall & Provan (2000) Lancet 355, 1169-1175). Anemia may result from a reduced rate of production or increased rate of destruction of red blood cells or by loss of red blood cells due to bleeding. Anemia may result from a variety of disorders that include, for example, chronic renal failure, chemotherapy treatment, myclodysplastic 15 syndrome, rheumatoid arthritis, and bone marrow transplantation. Treatment with EPO typically causes a rise in hemoglobins by about 1-3 g/dL in healthy humans over a period of weeks. When administered to anemic individuals, this treatment regimen often provides substantial increases in hemoglobin and red blood cell levels and leads to improvements in quality of life and prolonged survival. EPO is not 20 uniformly effective, and many individuals are refractory to even high doses (Horl et al. (2000) Nephrol Dial Transplant 15, 43-50). Over 50% of patients with cancer have an inadequate response to EPO, approximately 10% with end-stage renal disease are hyporesponsive (Glaspy et al. (1997) J Clin Oncol 15, 1218-1234; Demetri et al. (1998) J Clin Oncol 16, 3412-3425), and less than 10% with myclodysplastic syndrome respond 25 favorably (Estey (2003) Curr Opin Hematol 10, 60-67). Several factors, including inflammation, iron and vitamin deficiency, inadequate dialysis, aluminum toxicity, and hyperparathyroidism may predict a poor therapeutic response. The molecular mechanisms of resistance to EPO are as yet unclear. Recent evidence suggests that higher doses of EPO may be associated with an increased risk of cardiovascular morbidity, tumor growth, and mortality 30 in some patient populations (Krapf et al., 2009, Clin J Am Soc Nephrol 4:470-480; Glaspy, 2009, Annu Rev Med 60:181-192). It has therefore been recommended that EPO-based therapeutic compounds (erythropoietin-stimulating agents, ESAs) be administered at the 2 lowest dose sufficient to avoid the need for red blood cell transfusions (Jelkmann et al., 2008, Crit Rev Oncol. Hematol 67:39-61). Thus, it is an object of the present disclosure to provide alternative methods for increasing red blood cell levels in patients, which would permit use of reduced doses of 5 erythropoietin receptor activators. SUMMARY OF THE INVENTION In part, the disclosure demonstrates that GDF Traps may be used in combination (e.g., administered at the same time or different times, but generally in such a manner as to achieve 10 overlapping pharmacological effects) with EPO receptor activators to increase red blood cell levels (crythropoiesis) or treat anemia in patients in need thereof In pail, the disclosure demonstrates that a GDF Trap can be administered in combination with an EPO receptor activator to synergistically increase formation of red blood cells in a patient. Thus, the effect of this combined treatment can be significantly greater than the sum of the effects of the GDF 15 Trap and the EPO receptor activator when administered separately at their respective doses. In certain embodiments, this synergism may be advantageous since it enables target levels of red blood cells to be attained with lower doses of an EPO receptor activator, thereby avoiding potential adverse effects or other problems associated with higher levels of EPO receptor activation. 20 An EPO receptor activator may stimulate crythropoiesis by directly contacting and activating EPO receptor. In certain embodiments, the EPO receptor activator is one of a class of compounds based on the 165 amino-acid sequence of native EPO and generally known as erythropoiesis-stimulating agents (ESAs), examples of which are epoetin alfa, epoetin beta, epoetin delta, and cpoetin omega. In other embodiments, ESAs include synthetic EPO 25 proteins (SEPs) and EPO derivatives with nonpeptidic modifications conferring desirable pharmacokinetic properties (lengthened circulating half-life), examples of which are darbepoetin alfa and methoxy-polyethylene-glycol epoetin beta. In certain embodiments, an EPO receptor activator may be an EPO receptor agonist that does not incorporate the EPO polypeptide backbone or is not generally classified as an ESA. Such EPO receptor agonists 30 may include, but are not limited to, peptidic and nonpeptidic mimetics of EPO, agonistic 3 antibodies targeting EPO receptor, fusion proteins comprising an EPO mimetic domain, and erythropoietin receptor extended-duration limited agonists (EREDLA). In certain embodiments, an EPO receptor activator may stimulate erythropoiesis indirectly, without contacting EPO receptor itself, by enhancing production of endogenous 5 EPO. For example, hypoxia-inducible transcription factors (HIFs) are endogenous stimulators of EPO gene expression that are suppressed (destabilized) under normoxic conditions by cellular regulatory mechanisms. In part, the disclosure provides increased crythropoiesis in a patient by combined treatment with a GDF Trap and an indirect EPO receptor activator with HIF stabilizing properties, such as a prolyl hydroxylase inhibitor. 10 Variant ActRIIB polypeptides having a significantly decreased affinity for activin (e.g., activin A and/or activin B) relative to other ActRIIB ligands, such as GDFI I and/or myostatin, are referred to as GDF Traps. ActRIJB variants described herein are GDF Traps unless otherwise stated. In particular, the disclosure demonstrates that a GDF Trap which is a soluble form of ActRiIB polypeptide having an acidic residue at position 79 of SEQ ID NO: 15 1, when administered in vivo, increases red blood cell levels in the blood. Therefore, in certain embodiments, the disclosure provides methods for using GDF Traps to increase red blood cell and hemoglobin levels in patients and to treat disorders associated with low red blood cell or hemoglobin levels in patients in need thereof. As described in U.S. Patent Application No. 12/012,652, incorporated by reference herein, GDF Traps can be used to 20 increase muscle mass and decrease fat mass. In certain aspects, the present disclosure provides GDF Traps that are variant ActRIlB polypeptides, including ActR HI polypeptidcs having nmino- and cnrhoxy-termin al truncations and sequence alterations. Optionally, GDF Traps of the invention may be designed to preferentially antagonize one or more ligands of ActRIIB receptors, such as 25 GDF8 (also called myostatin), GDFl1I, Nodal, and BMP7 (also called OP-i). Examples of GDF Traps include a set of variants derived from ActRIIB that have greatly diminished affinity for activin. These variants exhibit desirable effects on red blood cells while reducing effects on other tissues. Examples of such variants include those having an acidic amino acid (e.g., aspartic acid, D, or glutamic acid, E) at the position corresponding to position 79 of 30 SEQ ID NO. 1. In certain embodiments, the GDF Trap polypeptide comprises an amino acid sequence that comprises, consists of, or consists essentially of, the amino acid sequence of 4 SEQ ID NO: 7, 26, 28, 29, 32, 37 or 38, and polypeptides that are at least 80%, 85%, 90%, 95%, 97%, 98%, or 99% identical to any of the foregoing. In certain aspects, the disclosure provides pharmaceutical preparations comprising a GDF Trap that binds to an ActRliB ligand such as GDF8, GDF 11, activin (e.g., activin B), 5 BMP7 or nodal, and a pharmaceutically acceptable carrier. Optionally, the GDF Trap binds to an ActRilB ligand with a Kd less than 10 micromolar, less than 1 micromolar, less than 100 nanomolar, less than 10 nanomolar, or less than 1 nanomolar. Optionally, the GDF Trap inhibits ActRIIB signaling, such as intracellular signal transduction events triggered by an ActRIB ligand. A GDF Trap for use in such a preparation may be any of those disclosed 10 herein, including, for example, GDF Traps having an amino acid sequence selected from SEQ ID NOs: 2, 3, 7, I1, 26, 28, 29, 32, 37, 38 or 40, or GDF Traps having an amino acid sequence that is at least 80%, 85%, 90%, 95%, 97% or 99% identical to an amino acid sequence selected from SEQ ID NOs: 2, 3, 7, 11, 26, 28, 29, 32, 37, 38 or 40, or GDF Traps having an amino acid sequence that is at least 80%, 85%, 90%, 95%, 97% or 99% identical to 15 an amino acid sequence selected from SEQ I D NOs: 2, 3, 7, 11, 26, 28, 29, 32, 37, 38 or 40 wherein the position corresponding to L79 in SEQ ID NO: I is an acidic amino acid. A preferred GDF Trap for use in such a preparation consists of, or consists essentially of, the amino acid sequence of SEQ ID NO: 26. A GDF Trap may include a functional fragment of a natural ActRIlB polypeptide, such as one comprising at least 10, 20 or 30 amino acids of a 20 sequence selected from SEQ ID NOs: 2, 3, 7, I1, 26, 28, 29, 32, 37, 38 or 40 or a sequence of SEQ ID NO: 2, lacking the C-terminal 1, 2, 3, 4, 5 or 10 to 15 amino acids and lacking 1, 2, 3, 4 or 5 amino acids at the N-terminus. A preferred polypeptide will comprise a truncation relative to SEQ ID NO: 2 or 40 of between 2 and 5 amino acids at the N-terminus and no more than 3 amino acids at the C-terminus. A GDF Trap may include one or more alterations 25 in the amino acid sequence of an ActRIIB polypeptide (e.g., in the ligand-binding domain) relative to a naturally occurring ActRIIB polypeptide. The alteration in the amino acid sequence may, for example, alter glycosylation of the polypeptide when produced in a mammalian, insect or other eukaryotic cell or alter proteolytic cleavage of the polypeptide relative to the naturally occurring ActRIIB polypeptide. 30 A GDF Trap may be a fusion protein that has, as one domain, an ActRILB polypeptide (e.g., a ligand-binding domain of an ActRIlB with one or more sequence variations) and one or more additional domains that provide a desirable property, such as improved 5 pharmacokinetics, easier purification, targeting to particular tissues, etc. For example, a domain of a fusion protein may enhance one or more of in vivo stability, in vivo half life, uptake/administration, tissue localization or distribution, formation of protein complexes, multimerization of the fusion protein, and/or purification. GDF Trap fusion proteins may 5 include an immunoglobulin Fe domain (wild-type or mutant) or a serum albumin. In certain embodiments, a GDF Trap fusion comprises a relatively unstructured linker positioned between the Fe domain and the extracellular ActRIIB domain. This unstructured linker may correspond to the roughly 15 amino acid unstructured region at the C-terminal end of the extracellular domain of ActRIIB (the "tail"), or it may be an artificial sequence of between 3 10 and 5, 15, 20, 30, 50 or more amino acids that are relatively free of secondary structure. A linker may be rich in glycine and proline residues and may, for example, contain repeating sequences of threonine/serine and glycines (e.g., TG4 (SEQ ID NO: 13) or SG (SEQ ID NO: 14) singlets or repeats) or a series of three glycines. A fusion protein may include a purification subsequence, such as an epitope tag, a FLAG tag, a polyhistidine sequence, and a 15 GST fusion. In certain embodiments, a GDF Trap fusion comprises a leader sequence. The leader sequence may be a native ActRIIB leader sequence or a heterologous leader sequence. In certain embodiments, the leader sequence is a Tissue Plasminogen Activator (TPA) leader sequence. In an embodiment, a GDF Trap fusion protein comprises an amino acid sequence as set forth in the formula A-B-C. The B portion is an N- and C-terminally truncated ActRIIB 20 polypeptide consisting of the amino acid sequence corresponding to amino acids 25-131 of SEQ ID NO: 2 or 40. The A and C portions may be independently zero, one or more than one amino acids, and both A and C portions are heterologous to B. The A and/or C portions may b; attached to te 13 portion iia a linker scqucc. Optionally, a GDF Trap includes a variant ActRIIB polypeptide having one or more 25 modified amino acid residues selected from: a glycosylated amino acid, a PEGylated amino acid, a farnesylated amino acid, an acetylated amino acid, a biotinylated amino acid, an amino acid conjugated to a lipid moiety, and an amino acid conjugated to an organic derivatizing agent. A pharmaceutical preparation may also include one or more additional compounds such as a compound that is used to treat an ActRIIB-associated disorder. 30 Preferably, a pharmaceutical preparation is substantially pyrogen free. In general, it is preferable that a GDF Trap be expressed in a mammalian cell line that mediates suitably natural glycosylation of the GDF Trap so as to diminish the likelihood of an unfavorable 6 immune response in a patient. Human and CHO cell lines have been used successfully, and it is expected that other common mammalian expression vectors will be useful. In certain aspects, the disclosure provides packaged pharmaceuticals comprising a pharmaceutical preparation described herein and labeled for use in increasing red blood cell 5 levels in a human. In certain aspects, the disclosure provides GDF Traps which are soluble ActRIIB polypeptides comprising an altered ligand-binding (e.g., GDF8-binding) domain. GDF Traps with altered ligand-binding domains may comprise, for example, one or more mutations at amino acid residues such as E37, E39, R40, K55, R56, Y60, A64, K74, W78, L79, D80, F82 10 and F101 of human ActRiIB (numbering is relative to SEQ ID NO: 1). Optionally, the altered ligand-binding domain can have increased selectivity for a ligand such as GDF8/GDF 11 relative to a wild-type ligand-binding domain of an ActR.IIB receptor. To illustrate, these mutations are demonstrated herein to increase the selectivity of the altered ligand-binding domain for GDF1 I (and therefore, presumably, GDF8) over activin: K74Y, 15 K74F, K741, L79D, L79E, and D801 . The following mutations have the reverse effect, increasing the ratio of activin binding over GDF 11: D54A, K55A, L79A and F82A. The overall (GDF1 I and activin) binding activity can be increased by inclusion of the "tail" region or, presumably, an unstructured linker region, and also by use of a K74A mutation. Other mutations that caused an overall decrease in ligand binding affinity, include: R40A, 20 E37A, R56A, W78A, D80K, D80R, D80A, D80G, D80F, D80M and D80N. Mutations may be combined to achieve desired effects. For example, many of the mutations that affect the ratio of GDFl I :Activin binding have an overall negative effect on ligand binding, and therefore, these may be combined with mutations that generally increase ligand binding to produce an improved binding protein with ligand selectivity. In an exemplary embodiment, a 25 GDF Trap is an ActRIIB polypeptide comprising an L79D or L79E mutation, optionally in combination with additional amino acid substitutions, additions or deletions. Optionally, a GDF Trap comprising an altered ligand-binding domain has a ratio of
K
4 for activin binding to K 4 for GDF8 binding that is at least 2, 5, 10, or even 100 fold greater relative to the ratio for the wild-type ligand-binding domain. Optionally, the GDF 30 Trap comprising an altered ligand-binding domain has a ratio of IC 50 for inhibiting activin to
IC
50 for inhibiting GDF8/GDF II that is at least 2, 5, 10, or even 100 fold greater relative to the wild-type ActRIlIB ligand-binding domain. Optionally, the GDF Trap comprising an 7 altered ligand-binding domain inhibits GDF8/GDF I with an ICso at least 2, 5, 10, or even 100 times less than the ICso for inhibiting activin. These GDF Traps can be fusion proteins that include an immunoglobulin Fe domain (either wild-type or mutant). In certain cases, the subject soluble GDF Traps are antagonists (inhibitors) of GDF8 and/or GDFl 1. 5 Other GDF Traps are contemplated, such as the following. A GDF Trap fusion protein comprising a portion derived from the ActRIIB sequence of SEQ ID NO: I or 39 and a second polypeptide portion, wherein the portion derived from ActRIIB corresponds to a sequence beginning at any of amino acids 21-29 of SEQ ID NO: I or 39 (optionally beginning at 22-25 of SEQ ID NO: I or 39) and ending at any of amino acids 109-134 of 10 SEQ ID NO: I or 39, and wherein the GDF Trap fusion protein inhibits signaling by activin, myostatin and/or GDF I in a cell-based assay. The GDF Trap fusion protein above, wherein the portion derived from ActRIIB corresponds to a sequence beginning at any of amino acids 20-29 of SEQ ID NO: I or 39 (optionally beginning at 22-25 of SEQ ID NO: I or 39) and ending at any of amino acids 109-133 of SEQ ID NO: I or 39. The GDF Trap fusion protein 15 above, wherein the portion derived from ActRIIB corresponds to a sequence beginning at any of amino acids 20-24 of SEQ ID NO: I or 39 (optionally beginning at 22-25 of SEQ ID NO: 1 or 39) and ending at any of amino acids 109-133 of SEQ ID NO: I or 39. The GDF Trap fusion protein above, wherein the portion derived from ActRIIB corresponds to a sequence beginning at any of amino acids 21-24 of SEQ ID NO: 1 or 39 and ending at any of amino 20 acids 109-134 of SEQ ID NO: 1 or 39. The GDF Trap fusion protein above, wherein the portion derived from ActRIIB corresponds to a sequence beginning at any of amino acids 20 24 of SEQ ID NO: 1 or 39 and ending at any of amino acids 118-133 of SEQ ID NO: I or 39. The GDF Trap fusion protein above, wherein the portion derived from ActRIlIB corresponds to a sequence beginning at any of amino acids 21-24 of SEQ ID NO: 1 or 39 25 and ending at any of amino acids 118-134 of SEQ ID NO: 1 or 39. The GDF Trap fusion protein above, wherein the portion derived from ActRIIB corresponds to a sequence beginning at any of amino acids 20-24 of SEQ ID NO: I or 39 and ending at any of amino acids 128-133 of SEQ ID NO: I or 39. The GDF Trap fusion protein above, wherein the portion derived from ActRIIB corresponds to a sequence beginning at any of amino acids 20 30 24 of SEQ ID NO: I or 39 and ending at any of amino acids 128-133 of SEQ ID NO: I or 39. The GDF 'rap fusion protein above, wherein the portion derived from ActRIlB corresponds to a sequence beginning at any of amino acids 21-29 of SEQ ID NO: 1 or 39 and ending at any of amino acids 118-134 of SEQ ID NO: I or 39. The GDF Trap fusion 8 protein above, wherein the portion derived from ActRIIB corresponds to a sequence beginning at any of amino acids 20-29 of SEQ ID NO: 1 or 39 and ending at any of amino acids 118-133 of SEQ ID NO: I or 39. The GDF Trap fusion protein above, wherein the portion derived from ActRIIB corresponds to a sequence beginning at any of amino acids 21 5 29 of SEQ ID NO: I or 39 and ending at any of amino acids 128-134 of SEQ ID NO: I or 39. The GDF Trap fusion protein above, wherein the portion derived from ActRIIB corresponds to a sequence beginning at any of amino acids 20-29 of SEQ ID NO: I and ending at any of amino acids 128-133 of SEQ ID NO: I or 39. Surprisingly, constructs beginning at 22-25 of SEQ ID NO: 1 or 39 have activity levels greater than proteins having 10 the full extracellular domain of human ActRIIB. In a preferred embodiment, the GDF Trap fusion protein comprises, consists essentially of, or consists of, an amino acid sequence beginning at amino acid position 25 of SEQ ID NO: I or 39 and ending at amino acid position 131 of SEQ ID NO: 1 or 39. In another preferred embodiments, the GDF Trap polypeptide consists of, or consists essentially of, the amino acid sequence of SEQ ID NO: 7, 15 26, 28, 29, 32, 37 or 38. Any of the above GDF Trap fusion proteins may be produced as a homodimer. Any of the above GDF Trap fusion proteins may have a heterologous portion that comprises a constant region from an IgG heavy chain, such as an Fc domain. Any of the above GDF Trap fusion proteins may comprise an acidic amino acid at the position corresponding to position 79 of SEQ ID NO: 1, optionally in combination with one or more 20 additional amino acid substitutions, deletions or insertions relative to SEQ ID NO: . Other GDF Trap proteins are contemplated, such as the following. A GDF Trap protein comprising an amino acid sequence that is at least 80% identical to the sequence of amino acids 29-109 of SEQ ID NO: I or 39, wherein the position corresponding to 64 of SEQ ID NO: I is an R or K, and wherein the GDF Trap protein inhibits signaling by activin, 25 myostatin and/or GDF1 I in a cell-based assay. The GDF Trap protein above, wherein at least one alteration with respect to the sequence of SEQ ID NO: 1 or 39 is positioned outside of the ligand binding pocket. The GDF Trap protein above, wherein at least one alteration with respect to the sequence of SEQ ID NO: 1 or 39 is a conservative alteration positioned within the ligand binding pocket. The GDF Trap protein above, wherein at least one 30 alteration with respect to the sequence of SEQ ID NO: 1 or 39 is an alteration at one or more positions selected from the group consisting of K74, R40, Q53, K55, F82 and L79. The GDF Trap protein above, wherein the protein comprises at least one N-X-S/T sequence at a 9 position other than an endogenous N-X-SIT sequence of ActRIIB, and at a position outside of the ligand binding pocket. Other GDF Traps are contemplated, such as the following. A GDF Trap protein comprising an amino acid sequence that is at least 80% identical to the sequence of amino 5 acids 29-109 of SEQ ID NO: 1 or 39, and wherein the protein comprises at least one N-X-S/T sequence at a position other than an endogenous N-X-S/T sequence of ActRIIB, and at a position outside of the ligand binding pocket. The GDF Trap above, wherein the GDF Trap protein comprises an N at the position corresponding to position 24 of SEQ ID NO: I or 39 and an S or T at the position corresponding to position 26 of SEQ ID NO: 1 or 39, and 10 wherein the GDF Trap inhibits signaling by activin, myostatin and/or GDFI I in a cell-based assay. The GDF Trap above, wherein the GDF Trap protein comprises an R or K at the position corresponding to position 64 of SEQ ID NO: 1 or 39. The GDF Trap above, wherein the ActRI1B protein comprises a D or E at the position corresponding to position 79 of SEQ ID NO: I or 39 and wherein the GDF Trap inhibits signaling by activin, myostatin and/or 15 GDF I in a cell-based assay. The GDF Trap above, wherein at least one alteration with respect to the sequence of SEQ ID NO: I or 39 is a conservative alteration positioned within the ligand binding pocket. The GDF Trap above, wherein at least one alteration with respect to the sequence of SEQ ID NO: I or 39 is an alteration at one or more positions selected from the group consisting of K74, R40, Q53, K55, F82 and L79. The GDF Trap above, wherein 20 the protein is a fusion protein further comprising a heterologous portion. Any of the above GDF Trap fusion proteins may be produced as a homodimer. Any of the above GDF Trap fusion proteins may have a heterologous portion that comprises a constant region from an IgG heavy chain, such as an Fc domain. In certain aspects, the disclosure provides nucleic acids encoding a GDF Trap 25 polypeptide. An isolated polynucleotide may comprise a coding sequence for a soluble GDF Trap polypeptide, such as described above. For example, an isolated nucleic acid may include a sequence coding for a GDF Trap comprising an extracellular domain (e.g., ligand binding domain) of an ActRIlB polypeptide having one or more sequence variations and a sequence that would code for part or all of the transmembrane domain and/or the cytoplasmic 30 domain of an ActRIlB polypeptide, but for a stop codon positioned within the transmembrane domain or the cytoplasmic domain, or positioned between the extracellular domain and the transmembrane domain or cytoplasmic domain. For example, an isolated polynucleotide 10 coding for a GDF Trap may comprise a full-length ActRIIB polynucleotide sequence such as SEQ ID NO: 4 having one or more variations, or a partially truncated version, said isolated polynucleotide further comprising a transcription termination codon at least six hundred nuelcotides before the 3'-terminus or otherwise positioned such that translation of the 5 polynuclcotide gives rise to an extracellular domain optionally fused to a truncated portion of a full-length ActRiIB. Nucleic acids disclosed herein may be operably linked to a promoter for expression, and the disclosure provides cells transformed with such recombinant polynucleotides. Preferably the cell is a mammalian cell such as a CHO cell. In certain aspects, the disclosure provides methods for making a GDF Trap 10 polypeptide. Such a method may include expressing any of the nucleic acids (e.g., SEQ ID NO: 5, 25, 27, 30 or 3 1) disclosed herein in a suitable cell, such as a Chinese hamster ovary (CHO) cell. Such a method may comprise: a) culturing a cell under conditions suitable for expression of the GDF Trap polypeptide, wherein said cell is transformed with a GDF Trap expression construct; and b) recovering the GDF Trap polypeptide so expressed. GDF Trap 15 polypeptides may be recovered as crude, partially purified or highly purified fractions using any of the well known techniques for obtaining protein from cell cultures. In certain aspects, a GDF Trap polypeptide disclosed herein may be used in a method for promoting red blood cell production or increasing red blood cell levels in a subject. In certain embodiments, the disclosure provides methods for treating a disorder associated with 20 low red blood cell counts or low hemoglobin levels (e.g., an anemia), or to promote red blood cell production, in patients in need thereof. A method may comprise administering to a subject in need thereof an effective amount of a GDF Trap polypeptide. In certain aspects, the disclosure provides uses of GDF Trap polypeptides for making a medicament for the treatment of a disorder or condition as described herein. 25 In certain aspects, the disclosure provides methods for administering a GDF Trap polypeptide to a patient. In part, the disclosure demonstrates that GDF Trap polypeptides can be used to increase red blood cell and hemoglobin levels. GDF Trap polypeptides may also be used for treating or preventing other therapeutic uses such as promoting muscle growth. In certain instances, when administering a GDF Trap polypeptide for promoting muscle 30 growth, it may be desirable to monitor the effects on red blood cells during administration of the GDF Trap polypeptide, or to determine or adjust the dosing of the GDF Trap polypeptide, 11 in order to reduce undesired effects on red blood cells. For example, increases in red blood cell levels, hemoglobin levels, or hematocrit levels may cause increases in blood pressure. BRIEF DESCRIPTION OF THE DRAWINGS 5 The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. Figure 1 shows an alignment of the extracellular domains of human ActRIIA (SEQ ID NO: 15) and human ActRIIB (SEQ ID NO: 2) with the residues that are deduced herein, 10 based on composite analysis of multiple ActRIlIB and ActRlIA crystal structures to directly contact ligand (the ligand binding pocket) indicated with boxes. Figure 2 shows a multiple sequence alignment of various vertebrate ActRIIB proteins and human ActRIIA (SEQ ID NOs: 16-23). Figure 3 shows the full amino acid sequence for the GDF Trap ActRIIB(L79D 20 15 134)-hFe (SEQ ID NO: I1), including the TPA leader sequence (double underlined), ActRIIB extracellular domain (residues 20-134 in SEQ ID NO: I- underlined), and hFc domain. The aspartate substituted at position 79 in the native sequence is double underlined and highlighted, as is the glycine revealed by sequencing to be the N-terminal residue in the mature fusion protein. 20 Figure 4 shows a nucleotide sequence encoding ActRIlB(L79D 20-134)-liFe. SEQ ID NO: 25 conespoLnd to the Csne -,trand, and SEQ ID NO. 33 cuncspunds to thc antiscnse strand. The TPA leader (nucleotides 1-66) is double underlined, and the ActRIlB extracellular domain (nucleotides 76-420) is underlined. Figure 5 shows the full amino acid sequence for the truncated GDF Trap 25 ActRIlIB(L79D 25-131)-hFc (SEQ ID NO: 26), including the TPA leader (double underlined), truncated ActRIIB extracellular domain (residues 25-131 in SEQ ID NO: I; underlined), and hFc domain. The aspartate substituted at position 79 in the native sequence is double underlined and highlighted, as is the glutamate revealed by sequencing to be the N terminal residue in the mature fusion protein. 12 Figure 6 shows a nucleotide sequence encoding ActRIIB(L79D 25-131)-hFc. SEQ ID NO: 27 corresponds to the sense strand, and SEQ ID NO: 34 corresponds to the antisense strand. The TPA leader (nucleotides 1-66) is double underlined, and the truncated ActRIIB extracellular domain (nucleotides 76-396) is underlined. The amino acid sequence for the 5 ActRIIB extracellular domain (residues 25-131 in SEQ ID NO: 1) is also shown. Figure 7 shows the amino acid sequence for the truncated GDF Trap ActRlIB(L79D 25-131)-hFc without a leader (SEQ ID NO: 28). The truncated ActRIlB extracellular domain (residues 25-131 in SEQ ID NO: 1) is underlined. The aspartate substituted at position 79 in the native sequence is double underlined and highlighted, as is the glutamate revealed by 10 sequencing to be the N-terminal residue in the mature fusion protein. Figure 8 shows the amino acid sequence for the truncated GDF Trap ActRIIB(L79D 25-131) without the leader, hFc domain, and linker (SEQ ID NO: 29). The aspartate substituted at position 79 in the native sequence is underlined and highlighted, as is the glutamate revealed by sequencing to be the N-terminal residue in the mature fusion protein. 15 Figure 9 shows an alternative nucleotide sequence encoding ActRIIB(L79D 25-13 1) hFc. SEQ ID NO: 30 corresponds to the sense strand, and SEQ ID NO: 35 corresponds to the antisense strand. The TPA leader (nucleotides 1-66) is double underlined, the truncated ActRIIB extracellular domain (nucleotides 76-396) is underlined, and substitutions in the wildtype nucleotide sequence of the extracellular domain are double underlined and 20 highlighted (compare with SEQ ID NO: 27, Figure 6). The amino acid sequence for the ActRIIB extracellular domain (residues 25-131 in SEQ ID NO: 1) is also shown. Figure 10 shows iucleotides 76-39 (SEQ ID NO: 3 1) of the alternative nudlcotide sequence shown in Figure 9 (SEQ ID NO: 30). The same nucleotide substitutions indicated in Figure 9 are also underlined and highlighted here. SEQ ID NO: 31 encodes only the 25 truncated ActRIIB extracellular domain (corresponding to residues 25-131 in SEQ ID NO: 1) with a L79D substitution, e.g., ActRIIB(L79D 25-13 1). Figure 11 shows the effect of ActRIIB(L79D 25-13 1)-hFc on hemoglobin concentration in a mouse model of chemotherapy-induced anemia. Data are means ± SEM. **, P < 0.01 vs. paclitaxel at the same time point. This GDF Trap offset the anemia induced 30 by paclitaxel treatment. 13 Figure 12 shows the effect of ActRIIB(L79D 25-13 1)-hFc on red blood cell (RBC) levels in a unilaterally nephrectomized (NEPHX) mouse model of chronic kidney disease. Data are means ± SEM. ***, P < 0.001 vs. baseline. This GDF Trap reversed the nephrectomy-induced anemia observed in control mice. 5 Figure 13 shows the effect of ActRIIB(L79D 25-131 )-hFc on red blood cell (RBC), hemoglobin (HGB), and hematocrit (HCT) levels in a unilaterally nephrectomized (NEPHX) mouse model of chronic kidney disease. Data are mean changes from baseline over 4 weeks (± SEM). *, P < 0.05; **, P < 0.0 1; ***, P < 0.001 vs. NEPHX controls. This GDF Trap prevented the nephrectomy-associated decline in these erythrocytic parameters, increasing 10 each by a magnitude similar to that in kidney-intact (sham) mice . Figure 14 shows the effect of ActRIlIB(L79D 25-13 1)-hFc on red blood cell (RBC) levels in a rat model of anemia induced by acute blood loss. Blood removal occurred on Day -1, with dosing on Days 0 and 3. Data are means ± SEM. **, P < 0.01; ***, P < 0.00 1 vs. vehicle at same time point. This GDF Trap improved the rate and extent of recovery from 15 blood-loss-induced anemia. Figure 15 shows the effect of treatment with ActRIlB(L79D 20-134)-hFc (gray) or ActRIIB(L79D 25-131)-hFc (black) on the absolute change in red blood cell concentration from baseline in cynomolgus monkey. VEH = vehicle. Data are means + SEM. n = 4-8 per group. 20 Figure 16 shows the effect of treatment with ActRIlIB(L79D 20-134)-hFc (gray) or ActRilB(L79D 25-131)-hFc (black) on the absolute change in hematocrit from baseline in cynoniogus monkey. VEH - vehicle. Data are means T SEM. n - 4-8 per group. Figure 17 shows the effect of treatment with ActRIIB(L79D 20-134)-hFc (gray) or ActRIIB(L79D 25-131)-hFc (black) on the absolute change in hemoglobin concentration 25 from baseline in cynomolgus monkey. VEH = vehicle. Data are means + SEM. n = 4-8 per group. Figure 18 shows the effect of treatment with ActRIIB(L79D 20-134)-hFc (gray) or ActRIB(L79D 25-131)-hFc (black) on the absolute change in circulating reticulocyte concentration from baseline in cynomolgus monkey. VEH = vehicle. Data are means + 30 SEM. n = 4-8 per group. 14 Figure 19 shows the effect of combined treatment with erythropoietin (EPO) and ActRIIB(L79D 25-131)-hFc for 72 hours on hematocrit in mice. Data are means ± SEM (n 4 per group), and means that are significantly different from each other (p < 0.05, unpaired t test) are designated by different letters. Combined treatment increased hematocrit by 23% 5 compared to vehicle, a synergistic increase greater than the sum of the separate effects of EPO and ActRIIB(L79D 25-131)-hFc. Figure 20 shows the effect of combined treatment with EPO and ActRIIB(L79D 25 13 l)-hFc for 72 hours on hemoglobin concentrations in mice. Data are means ± SEM (n = 4 per group), and means that are significantly different from each other (p < 0.05) are 10 designated by different letters. Combined treatment increased hemoglobin concentrations by 23% compared to vehicle, which was also a synergistic effect. Figure 21 shows the effect of combined treatment with EPO and ActRIIB(L79D 25 13 i)-hFc for 72 hours on red blood cell concentrations in mice. Data are means ± SEM (n = 4 per group), and means that are significantly different from each other (p < 0.05) are 15 designated by different letters. Combined treatment increased red blood cell concentrations by 20% compared to vehicle, which was also a synergistic effect. Figure 22 shows the effect of combined treatment with EPO and ActRIIB(L79D 25 131 )-hFc for 72 hours on numbers of erythropoietic precursor cells in mouse spleen. Data are means ± SEM (n = 4 per group), and means that are significantly different from each other (p 20 < 0.01) are designated by different letters. Whereas EPO alone increased the number of basophilic crythroblasts (Baso.E) dramatically at the expense of late-stage precursor maturation. combined treatment increased BasoF numbers to a lesser but still significant extent while supporting undiminished maturation of late-stage precursors. 25 DETAILED DESCRIPTION OF THE INVENTION 1L Overview EPO is a glycoprotein hormone involved in the growth and maturation of crythroid progenitor cells into erythrocytes. EPO is produced by the liver during fetal life and by the kidney in adults. Decreased production of EPO, which commonly occurs in adults as a 30 consequence of renal failure, leads to anemia. EPO has been produced by genetic engineering techniques based on expression and secretion of the protein from a host cell 15 transfected with the EPO gene. Administration of such recombinant EPO has been effective in the treatment of anemia. For example, Eschbach et al. (1987, N Engl J Med 316:73) describe the use of EPO to correct anemia caused by chronic renal failure. Effects of EPO are mediated through its binding to, and activation of, a cell surface 5 receptor belonging to the cytokine receptor superfamily and designated the EPO receptor. The human and murine EPO receptors have been cloned and expressed (D'Andrea et al., 1989, Cell 57:277; Jones et a., 1990, Blood 76:3 1; Winkelman et al., 1990, Blood 76:24; WO 90/08822/U.S. Pat. No. 5,278,065). The human EPO receptor gene encodes a 483 amino acid transmembrane protein comprising an extracellular domain of approximately 224 amino 10 acids and exhibits approximately 82% amino acid sequence identity with the murine EPO receptor (See U.S. Pat. No. 6,319,499). The cloned, full-length EPO receptor expressed in mammalian cells (66-72 kDa) binds EPO with an affinity (KI) = 100-300 nM) similar to that of the native receptor on erythroid progenitor cells. Thus, this form is thought to contain the main EPO binding determinant and is referred to as the EPO receptor. By analogy with other 15 closely related cytokine receptors, the EPO receptor is thought to dimerize upon agonist binding. Nevertheless, the detailed structure of the EPO receptor, which may be a multimeric complex, and its specific mechanism of activation are not completely understood (U.S. Pat. No. 6,319,499). Activation of the EPO receptor results in several biological effects. These include 0 increased proliferation of immature crythroblasts, increased differentiation of immature crythroblasts, and decreased apoptosis in erythroid progenitor cells (Liboi et al., 1993, Proc Nat] Acad Sci USA 90:11351-11355; Koury et al., 1990, Science 248:378-381). The EPO receptor signal transduction pathways mediating proliferation and differentiation appear to be distinct (Noguchi et al., 1988, Mol Cell Biol 8:2604; Patel et al., 1992, J Biol Chem 1992, 25 267:21300; Liboi et al., ibid). Some results suggest that an accessory protein may be required for mediation of the differentiation signal (Chiba et a., 1993, Nature 362:646; Chiba et al., 1993, Proc Natl Acad Sci USA 90:11593); however, there is controversy regarding the role of accessory proteins in differentiation since a constituitively activated form of the receptor can stimulate both proliferation and differentiation (Pharr ct al., 1993, Proc Natl Acad Sci USA 30 90:938). EPO receptor activators include small-molecule crythropoicsis-stimulating agents (ESAs) as well as EPO-based compounds. An example of the former is a dimeric peptide 16 based agonist covalently linked to polyethylene glycol (proprietary name Hematide), which has shown erythropoiesis-stimulating properties in healthy volunteers and in patients with both chronic kidney disease and endogenous anti-EPO antibodies (Stead et al., 2006, Blood 108:1830-1834; Macdougall et al, 2009, N Engl J Med 361:1848-1855). Other examples 5 include nonpeptide-based ESAs (Qureshi et al., 1999, Proc Nat[ Acad Sci USA 96:12156 12161). EPO receptor activators also include compounds that stimulate erythropoicsis indirectly, without contacting EPO receptor itself, by enhancing production of endogenous EPO. For example, hypoxia-inducible transcription factors (HIFs) are endogenous 10 stimulators of EPO gene expression that are suppressed (destabilized) under normoxic conditions by cellular regulatory mechanisms. Therefore, inhibitors of HIF prolyl hydroxylase enzymes are being investigated for EPO-inducing activity in vivo. Other indirect activators of EPO receptor include inhibitors of GATA-2 transcription factor (Nakano et al., 2004, Blood 104:4300-4307), which tonically inhibits EPO gene expression, 15 and inhibitors of hemopoictic cell phosphatase (HCP or SHP-I), which functions as a negative regulator of EPO receptor signal transduction (Klingmuller et al., 1995, Cell 80:729 738). The transforming growth factor-beta (TGF-beta) superfamily contains a variety of growth factors that share common sequence elements and structural motifs. These proteins 20 are known to exert biological effects on a large variety of cell types in both vertebrates and invertebrates. Members of the superfamily perform important functions during embryonic development in pattern formation and tissue specification and can influence a variety of differentiation processes, including adipogenesis, myogenesis, chondrogenesis, cardiogenesis, hematopoiesis, neurogenesis, and epithelial cell differentiation. The family is 25 divided into two general branches: the BMP/GDF and the TGF-beta/ActivinI/BMP0 branches, whose members have diverse, often complementary effects. By manipulating the activity of a member of the TGF-beta family, it is often possible to cause significant physiological changes in an organism. For example, the Piedmontese and Belgian Blue cattle breeds carry a loss-of-function mutation in the GDF8 (also called myostatin) gene that causes 30 a marked increase in muscle mass. Grobet et al., Nat Genet. 1997, 17(l):71-4. Furthermore, in humans, inactive alleles of GDF8 are associated with increased muscle mass and, reportedly, exceptional strength. Schuelke et al., N Engl J Med 2004, 350:2682-8. 17 TGF- signals are mediated by heteromeric complexes of type I and type II serine/threonine kinase receptors, which phosphorylate and activate downstream Smad proteins upon ligand stimulation (Massagu&, 2000, Nat. Rev. Mol. Cell Biol. 1:169-178). These type I and type II receptors are transmembrane proteins, composed of a ligand-binding 5 extracellular domain with cysteinc-rich region, a transmembrane domain, and a cytoplasmic domain with predicted serine/threonine specificity. Type I receptors are essential for signaling. Type 11 receptors are required for binding ligands and for expression of Type I receptors. Type I and 11 activin receptors form a stable complex after ligand binding, resulting in phosphorylation of Type I receptors by Type IT receptors. 10 Two related Type II receptors (ActRil), ActRIIA and ActRIIB, have been identified as the Type 11 receptors for activins (Mathews and Vale, 1991, Cell 65:973-982; Attisano et al., 1992, Cell 68: 97-108). Besides activins, ActRIIA and ActRIIB can biochemically interact with several other TGF- family proteins, including BMP7, Nodal, GDF8, and GDF I I (Yamashita et al., 1995, J. Ccl] Biol. 130:217-226; Lee and McPherron, 2001, Proc. 15 Nati. Acad. Sci. 98:9306-9311; Yeo and Whitman, 2001, Mol. Cell 7: 949-957; Oh et al., 2002, Genes Dev. 16:2749-54). ALK4 is the primary type I receptor for activins, particularly for activin A, and ALK-7 may serve as a receptor for activins as well, particularly for activin B. In certain embodiments, the present invention relates to antagonizing a ligand of ActRiIB receptors (also referred to as an ActRilB ligand) with a subject GDF Trap polypeptide. 20 Exemplary ligands of ActRIlB receptors include some TGF-p family members, such as activin, Nodal, GDF8, GDF 11, and BMP7. Activins are dimeric polypeptide growth factors that belong to the TGF-beta superfamily. There are three principal activin forms (A, B, and AB) that are homo/heterodimers of two closely related p subunits (pAPA, 1 pp, and PAPB, respectively). 25 The human genome also encodes an activin C and an activin E, which are primarily expressed in the liver, and heterodimeric forms containing Pc or pE.are also known. In the TGF-beta superfamily, activins are unique and multifunctional factors that can stimulate hormone production in ovarian and placental cells, support neuronal cell survival, influence cell-cycle progress positively or negatively depending on cell type, and induce mesodermal 30 differentiation at least in amphibian embryos (DePaolo et aL, 1991, Proc Soc Ep Biol Med. 198:500-512; Dyson et al., 1997, CUrT Biol. 7:81-84; Woodruff, 1998, Biochem Pharmacol. 55:953-963). Moreover, erythroid differentiation factor (EDF) isolated from the stimulated 18 human monocytic leukemic cells was found to be identical to activin A (Murata et aL., 1988, PNAS, 85:2434). It has been suggested that activin A promotes erythropoiesis in the bone marrow. In several tissues, activin signaling is antagonized by its related heterodimer, inhibin. For example, during the release of follicle-stimulating hormone (FSH) from the 5 pituitary, activin promotes FSH secretion and synthesis, while inhibin prevents FSH secretion and synthesis. Other proteins that may regulate activin bioactivity and/or bind to activin include follistatin (FS), follistatin-related protein (FSRP) and U 2 -macroglobulin. Nodal proteins have functions in mesoderm and endoderm induction and formation, as well as subsequent organization of axial structures such as heart and stomach in early 10 embryogenesis. It has been demonstrated that dorsal tissue in a developing vertebrate embryo contributes predominantly to the axial structures of the notochord and pre-chordal plate while it recruits surrounding cells to form non-axial embryonic structures. Nodal appears to signal through both type I and type 11 receptors and intracellular effectors known as Smad proteins. Recent studies support the idea that ActRIIA and ActRIIB serve as type 11 15 receptors for Nodal (Sakuma et al., Genes Cells. 2002, 7:401-12). It is suggested that Nodal ligands interact with their co-factors (e.g., cripto) to activate activin type I and type II receptors, which phosphorylate Smad2. Nodal proteins are implicated in many events critical to the early vertebrate embryo, including mesoderm formation, anterior patterning, and left right axis specification. Experimental evidence has demonstrated that Nodal signaling 20 activates pAR3-Lux, a luciferase reporter previously shown to respond specifically to activin and TGF-beta. However, Nodal is unable to induce pTlx2-Lux, a reporter specifically responsive to bone morphogenetic proteins. Recent results provide direct biochemical evidence that Nodal signaling is mediated by both activin-TGF-beta pathway Smads, Smad2 and Smad3. Further evidence has shown that the extracellular cripto protein is required for 25 Nodal signaling, making it distinct from activin or TGF-beta signaling. Growth and Differentiation Factor-8 (GDF8) is also known as myostatin. GDF8 is a negative regulator of skeletal muscle mass. GDF8 is highly expressed in the developing and adult skeletal muscle. The GDF8 null mutation in transgenic mice is characterized by a marked hypertrophy and hyperplasia of the skeletal muscle (McPherron et al., Nature, 1997, 30 387:83-90). Similar increases in skeletal muscle mass are evident in naturally occurring mutations of GDF8 in cattle (Ashmore et al., 1974, Growth, 38:501-507; Swatland and Kieffer, J. Anim. Sci., 1994, 38:752-757; McPherron and Lee, Proc. NatI. Acad. Sci. USA, 19 1997, 94:12457-12461; and Kambadur et al., Genome Res., 1997, 7:910-915) and, strikingly, in humans (Schuelke ct al., N Engl J Med 2004;350:2682-8). Studies have also shown that muscle wasting associated with HIV-infection in humans is accompanied by increases in GDF8 protein expression (Gonzalez-Cadavid ct al., PNAS, 1998, 95:1493843). In addition, 5 GDF8 can modulate the production of muscle-specific enzymes (e.g., creatine kinase) and modulate myoblast cell proliferation (WO 00/43781). The GDF8 propeptide can noncovalently bind to the mature GDF8 domain dimer, inactivating its biological activity (Miyazono et al. (1988) J. Biol. Chem., 263: 6407-6415; Wakefield ct al. (1988) J. Biol. Chem., 263; 7646-7654; and Brown et al. (1990) Growth Factors, 3: 35-43). Other proteins 10 which bind to GDF8 or structurally related proteins and inhibit their biological activity include follistatin, and potentially, follistatin-related proteins (Gamer et al. (1999) Dev. Biol., 208: 222-232). Growth and Differentiation Factor-I I (GDFl 1), also known as BMPl 1, is a secreted protein (McPherron et al., 1999, Nat. Genet. 22: 260-264). GDF I I is expressed in the tail 15 bud, limb bud, maxillary and mandibular arches, and dorsal root ganglia during mouse development (Nakashima et al,, 1999, Mech. Dev. 80: 185-189). GDFI I plays a unique role in patterning both mesodermal and neural tissues (Gainer et al., 1999, Dev Biol., 208:222 32). GDFI I was shown to be a negative regulator of chondrogenesis and myogenesis in developing chick limb (Gamer ct al., 2001, Dev Biol. 229:407-20). The expression of 20 GDF I I in muscle also suggests its role in regulating muscle growth in a similar way to GDF8. In addition, the expression of GDFI 1 in brain suggests that GDFI 1 may also possess activities that relate to the function of the nervous system. Interestingly, GDF1 1 was found to inhibit neurogenesis in the olfactory epithelium (Wu et al., 2003, Neuron. 37:197-207). Hence, GDFI I may have in vitro and in vivo applications in the treatment of diseases such as 25 muscle diseases and neurodegenerative diseases (e.g., amyotrophic lateral sclerosis). Bone morphogenetic protein (BMP7), also called osteogenic protein-I (OP-I), is well known to induce cartilage and bone formation. In addition, BMP7 regulates a wide array of physiological processes. For example, BMP7 may be the ostcoinductive factor responsible for the phenomenon of epithelial osteogenesis. It is also found that BMP7 plays a role in 30 calcium regulation and bone homeostasis. Like activin, BMP7 binds to Type I receptors, ActRilA and ActRlB. However, BMP7 and activin recruit distinct Type I receptors into heteromeric receptor complexes. The major BMP7 Type I receptor observed was ALK2, 20 while activin bound exclusively to ALK4 (ActRIIB). BMP7 and activin elicited distinct biological responses and activated different Smad pathways (Macias-Silva et al., 1998, J Biol Chem. 273:25628-36). As demonstrated herein, a GDF Trap polypeptide, which is a variant ActRIIB 5 polypeptide (ActRIIB), is more effective at increasing red blood cell levels in vivo as compared to a wild-type soluble ActRIIB polypeptide and has beneficial effects in a variety of models for anemias. Additionally, it is shown that the use of a GDF Trap polypeptide in combination with an EPO receptor activator causes a substantial increase in red blood cell formation. It should be noted that hematopoiesis is a complex process, regulated by a variety 10 of factors, including crythropoietin, G-CSF and iron homeostasis. The terms "increase red blood cell levels" and "promote red blood cell formation" refer to clinically observable metrics, such as hematocrit, red blood cell counts and hemoglobin measurements, and are intended to be neutral as to the mechanism by which such changes occur. In addition to stimulating red blood cell levels, GDF Trap polypeptides are useful for 15 a variety of therapeutic applications, including, for example, promoting muscle growth (see PCT Publication Nos. WO 2006/012627 and WO 2008/097541, which are hereby incorporated by reference in their entirety). In certain instances, when administering a GDF Trap polypeptide for the purpose of increasing muscle, it may be desirable to reduce or minimize effects on red blood cells. By monitoring various hematologic parameters in 20 patients being treated with, or who are candidates for treatment with, a GDF Trap polypeptide, appropriate dosing (including amounts and frequency of administration) may be determined based on an individual patient's needs, baseline hematologic parameters. and purpose for treatment. Furthermore, therapeutic progress and effects on one or more hematologic parameters over time may be useful in managing patients being dosed with a 25 GDF Trap polypeptide by facilitating patient care, determining appropriate maintenance dosing (both amounts and frequency), etc. The terms used in this specification generally have their ordinary meanings in the art, within the context of this invention and in the specific context where each term is used. Certain terms are discussed below or elsewhere in the specification, to provide additional 30 guidance to the practitioner in describing the compositions and methods of the invention and how to make and use them. The scope or meaning of any use of a term will be apparent from the specific context in which the term is used. 21 "About" and "approximately" shall generally mean an acceptable degree of error for the quantity measured given the nature or precision of the measurements. Typically, exemplary degrees of error are within 20 percent (%), preferably within 10%, and more preferably within 5% of a given value or range of values. 5 Alternatively, and particularly in biological systems, the terms "about" and "approximately" may mean values that are within an order of magnitude, preferably within 5 fold and more preferably within 2-fold of a given value. Numerical quantities given herein are approximate unless stated otherwise, meaning that the terrn "about" or "approximately" can be inferred when not expressly stated. 10 The methods of the invention may include steps of comparing sequences to each other, including wild-type sequence to one or more mutants (sequence variants). Such comparisons typically comprise alignments of polymer sequences, e.g., using sequence alignment programs and/or algorithms that are well known in the art (for example, BLAST, FASTA and MEGALIGN, to name a few). The skilled artisan can readily appreciate that, in 15 such alignments, where a mutation contains a residue insertion or deletion, the sequence alignment will introduce a "gap" (typically represented by a dash, or "A") in the polymer sequence not containing the inserted or deleted residue. "Homologous," in all its grammatical forms and spelling variations, refers to the relationship between two proteins that possess a "common evolutionary origin," including 20 proteins from superfamilies in the same species of organism, as well as homologous proteins from different species of organism. Such proteins (and their encoding nucleic acids) have - sequenCe homology, Ps reflected by their sequence similarity. whether in terms of percent identity or by the presence of specific residues or motifs and conserved positions. The term "sequence similarity," in all its grammatical forms, refers to the degree of 25 identity or correspondence between nucleic acid or amino acid sequences that may or may not share a common evolutionary origin. However, in common usage and in the instant application, the term "homologous," when modified with an adverb such as "highly," may refer to sequence similarity and may or may not relate to a common evolutionary origin. 30 22 2. GDF Trap Polypeptides In certain aspects, the invention relates to GDF Trap polypeptides, e.g., soluble variant ActRIlB polypeptides, including, for example, fragments, functional variants, and modified forms of ActRIIB polypeptides. In certain embodiments, the GDF Trap 5 polypeptides have at least one similar or same biological activity as a corresponding wild type ActRllB polypeptide. For example, a GDF Trap polypeptide of the invention may bind to and inhibit the function of an ActRIIB ligand (e.g., activin A, activin AB, activin B, Nodal, GDF8, GDF1I1 or BMP7). Optionally, a GDF Trap polypeptide increases red blood cell levels. Examples of GDF Trap polypeptides include human ActRIlB precursor polypeptides 10 (SEQ ID NO: 1 or 39) having one or more sequence variations, and soluble human ActRIIB polypeptides (e.g., SEQ ID NOs: 2, 3, 7, 11, 26, 28, 29, 32, 37, 38,40 and 41) having one or more sequence variations. A GDF Trap refers to an ActRIIB polypeptide having a decreased affinity for activin relative to other ActRIIB ligands, including for example GDF1 1 and/or myostatin. 15 As used herein, the term "ActRIIB" refers to a family of activin receptor type l~b (ActRIIB) proteins from any species and variants derived from such ActRIIB proteins by mutagenesis or other modification. Reference to ActRIIB herein is understood to be a reference to any one of the currently identified forms. Members of the ActRIIB family are generally transmembrane proteins, composed of a ligand-binding extracellular domain with a 20 cysteine-rich region, a transmembrane domain, and a cytoplasmic domain with predicted serine/threonine kinase activity. Amino acid sequences of human ActRIIA soluble extracellular domain (provided for comparison) and ActRIIB soluble extracellular domain are illustrated in Figure 1. The term "ActRiIB polypeptide" includes polypeptides comprising any naturally 25 occurring polypeptide of an ActRIIB family member as well as any variants thereof (including mutants, fragments, fusions, and peptidomimetic forms) that retain a useful activity. See, for example, WO 2006/012627. For example, ActRIlIB polypeptides include polypeptides derived from the sequence of any known ActRIIB having a sequence at least about 80% identical to the sequence of an ActRIIB polypeptide, and optionally at least 85%, 30 90%, 95%, 97%, 99% or greater identity. For example, an ActRI1B polypeptide may bind to and inhibit the function of an ActRIIB protein and/or activin. An ActRIIB polypeptide which is a GDF Trap may be selected for activity in promoting red blood cell formation in vivo. 23 Examples of ActRIIB polypeptides include human ActRIIB precursor polypeptide (SEQ ID NO: I and 39) and soluble human ActRIIB polypeptides (e.g., SEQ ID NO: 2, 3, 7, 11, 26, 28, 29, 32, 37, 38, 40 and 41). Numbering of amino acids for all ActRIIB-related polypeptides described herein is based on the numbering for SEQ ID NO: 1, unless 5 specifically designated otherwise. The human ActRIIB precursor protein sequence is as follows: MTAPWVALALLWGSLWPGSGRGEAETRECIYYNANWELERT-QSGLERC EGEQDKRLHCYASWRQSSGTIELVKKGCWLDDFNCYDRQECVATEENPQ VYFCCCEGNFCNERFTHLPEAGGPEVTYEPPPTAPTLLTVLAYSLLPIG 10 GLSLIVLLAFWMYRHRKPPYGHVDIHEDPGPPPPSPLVGLKPLQLLEIK ARGRFGCVWKAQLMNDFVAVKIFPLQDKQSWQSEREIFSTPGMKHENLL QFIAAEKRGSNLEVELWLITAFHDKGSLTDYLKGN IITWNELCHVAETM SRGLSYLHEDVPWCRGEGHKPSIAHRDFKSKNVLLKSDLTAVLADFGLA VRFE PGKPPGDTHGQVGTRRYMAPEVLEGAINFQRDAFLRIDMYAMGLV 15 LWELVSRCKAADGPVDEYMLPFEEEIGQHPSLEELQEVVVHKKMRPTIK DHWLKHPGLAQLCVTIEECWDHDAEARLSAGCVEERVSLIRRSVNGTTS DCLVSLVTSVTNVDLPPKESSI (SEQ ID NO: 1) The signal peptide is single underlined; the extracellular domain is in bold and the potential N-linked glycosylation sites are in boxes. 20 A form with an alanine at position 64 is also reported in the literature, as follows: MTA PWVALALLWGSLWPGSGRGEAETRECIYYNANWELERTEQSGLERC EGEQDKRLHCYASWAjSSGTIELVKKGCWLDDFNCYDRQECVATEENPQ VYFCCCEGNFCNERFTRLPEAGGPEVTYEPPPTAPTDL ± V Lutu± QLit i -j GLSLIVLLAFWMYRHRKPPYGHVDIHEDPGPPPPSPLVGLKPLQLLEIK 25 ARGRFGCVWKAQLMNDFVAVKIFPLQDKQSWQSEREIFSTPGMKHENLL QFIAAEKRGSNLEVELWLITAFHDKGSLTDYLKGNIITWNELCHVAETM SRGLSYLHEDVPWCRGEGHKPSIAHRDFKSKNVLLKSDLTAVLADFGLA VRFEPGKPPGDTHGQVGTRRYMAPEVLEGAINFQRDAFLRIDMYAMAGLV LWELVSRCKAADGPVDEYMLPFEEEIGQHPSLEELQEVVVHKKMRPTIK 30 DHWLKHPGLAQLCVTIEECWDHDAEARLSAGCVEERVSLIRRSVNGTTS DCLVSLVTSVTNVDLPPKESSI (SEQ ID NO: 39) 24 The human ActRIIB soluble (extracellular), processed polypeptide sequence is as follows: GRGEAETRECIYYNANWELERTNQSGLERCEGEQDKRLHCYASWRNSSG TIELVKKGCWLDDFNCYDRQECVATEENPQVYFCCCEGNFCNERFTHLP 5 EAGGPEVTYEPPPTAPT (SEQ ID NO: 2) The alternative form with an A64 is as follows: GRGEAETREC IYYNANWELERTNQSGLERCEGEQDKRLHCYASWANSSG TIELVKKGCWLDDFNCYDRQECVATEENPQVYFCCCEGNFCNERFTHLP EAGGPEVTYEPPPTAPT (SEQ ID NO: 40) 10 In some conditions, the protein may be produced with an "SGR.." sequence at the N terminus. The C-terminal "tail" of the extracellular domain is underlined. The sequence with the "tail" deleted (a A] 5 sequence) is as follows: GRGEAETREC IYYNANWELERTNQSGLERCEGEQDKRLHCYASWRNSSG TIELVKKGCWLDDFNCYDRQECVATEENPQVYFCCCEGNFCNERFTHLP 15 EA (SEQ ID NO: 3) The alternative form with an A64 is as follows: GRGEAETREC IYYNANWELERTNQSGLERCEGEQDKRLHCYASWANSSG TIELVKKGCWLDDFNCYDRQECVATEENPQVYFCCCEGNFCNERFTHLP EA (SEQ ID NO: 41) 20 In some conditions, the protein may be produced with an "SGR.." sequence at the N terminus. The nucleic acid sequence encoding a human ActRIIB precursor protein is as follows. (nucleotLides 5-1543 of Genbank einry NI_00 l106)(ie sequence as shown prox ides an alanine at position 64, and may be modified to provide an arginine instead) 25 ATGACGGCGCCCTGGGTGGCCCTCGCCCTCCTCTGGGGATCGCTGTGGC CCGGCTCTGGGCGTGGGGAGGCTGAGACACGGGAGTGCATCTACTACAA CGCCAACTGGGAGCTGGAGCGCACCAACCAGAGCGGCCTGGAGCGCTGC GAAGGCGAGCAGGACAAGCGGCTGCACTGCTACGCCTCCTGGGCCAACA GCTCTGGCACCATCGAGCTCGTGAAGAAGGGCTGCTGGCTAGATGACTT 30 CAACTGCTACGATAGGCAGGAGTGTGTGGCCACTGAGGAGAACCCCCAG GTGTACTTCTGCTGCTGTGAAGGCAACTTCTGCAACGAGCGCTTCACTC ATTTGCCAGAGGCTGGGGGCCCGGAAGTCACGTACGAGCCACCCCCGAC AGCCCCCACCCTGCTCACGGTGCTGGCCTACTCACTGCTGCCCATCGGG 25 GGCCTTTCCCTCATCGTCCTGCTGGCCTTTTGGATGTACCGGCATCGCA AGCCCCCCTACGGTCATGTGGACATCCATGAGGACCCTGGGCCTCCACC ACCATCCCCTCTGGTGGGCCTGAAGCCACTGCAGCTGCTGGAGATCAAG GCTCGGGGGCGCTTTGGCTGTGTCTGGAAGGCCCAGCTCATGAATGACT 5 TTGTAGCTGTCAAGATCTTCCCACTCCAGGACAAGCAGTCGTGGCAGAG TGAACGGGAGATCTTCAGCACACCTGGCATGAAGCACGAGAACCTGCTA CAGTTCATTGCTGCCGAGAAGCGAGGCTCCAACCTCGAAGTAGAGCTGT GGCTCATCACGGCCTTCCATGACAAGGGCTCCCTCACGGATTACCTCAA GGGGAACATCATCACATGGAACGAACTGTGTCATGTAGCAGAGACGATG 10 TCACGAGGCCTCTCATACCTGCATGAGGATGTGCCCTGGTGCCGTGGCG AGGGCCACAAGCCGTCTATTGCCCACAGGGACTTTAAAAGTAAGAATGT ATTGCTGAAGAGCGACCTCACAGCCGTGCTGGCTGACT TTGGCTTGGCT GTTCGATTTGAGCCAGGGAAACCTCCAGGGGACACCCACGGACAGGTAG GCACGAGACGGTACATGGCTCCTGAGGTGCTCGAGGGAGCCATCAACTT 15 CCAGAGAGATGCCTTCCTGCGCATTGACATGTATGCCATGGGGTTGGTG CTGTGGGAGCTTGTGTCTCGCTGCAAGGCTGCAGACGGACCCGTGGATG AGTACATGCTGCCCTTTGAGGAAGAGATTGGCCAGCACCCTTCGTTGGA GGAGCTGCAGGAGGTGGTGGTGCACAAGAAGATGAGGCCCACCATTAAA GATCACTGGTTGAAACACCCGGGCCTGGCCCAGCTTTGTGTGACCATCG 20 AGGAGTGCTGGGACCATGATGCAGAGGCTCGCTTGTCCGCGGGCTGTGT GGAGGAGCGGGTGTCCCTGATTCGGAGGTCGGTCAACGGCACTACCTCG GACTGTCTCGTTTCCCTGGTGACCTCTGTCACCAATGTGGACCTGCCCC CTAAAGAGTCAAGCATCTAA (SEQ ID NO: 4) 25 The nucleic acid sequence encoding a human ActRIIB soluble (extracellular) polypeptide is as follows (the sequence as shown provides an alanine at position 64, and may be modified to provide an arginine instead): GGGCGTGGGGAGGCTGAGACACGGGAGTGCATCTACTACAACGCCAACT GGGAGCTGGAGCGCACCAACCAGAGCGGCCTGGAGCGCTGCGAAGGCGA 30 GCAGGACAAGCGGCTGCACTGCTACGCCTCCTGGGCCAACAGCTCTGGC ACCATCGAGCTCGTGAAGAAGGGCTGCTGGCTAGATGACTTCAACTGCT ACGATAGGCAGGAGTGTGTGGCCACTGAGGAGAACCCCCAGGTGTACTT CTGCTGCTGTGAAGGCAACTTCTGCAACGAGCGC TTCACTCATTTGCCA 26 GAGGCTGGGGGCCCGGAAGTCACGTACGAGCCACCCCCGACAGCCCCCA CC (SEQ ID NO: 5) In a specific embodiment, the invention relates to GDF Trap polypeptides which are 5 variant forms of soluble ActRIIB polypeptides. As described herein, the term "soluble ActRIIB polypeptide" generally refers to polypeptides comprising an extracellular domain of an ActRIIB protein. The term "soluble ActRIIB polypeptide," as used herein, includes any naturally occurring extracellular domain of an ActRilB protein as well as any variants thereof (including mutants, fragments and peptidomimetic forms) that retain a useful activity. For 10 example, the extracellular domain of an ActR1IB protein binds to a ligand and is generally soluble. Examples of soluble ActRIIB polypeptides include ActRilB soluble polypeptides (e.g., SEQ ID NOs: 22, 3, 7, 11, 26, 28, 29, 32, 37, 38, 40 and 41). Other examples of soluble ActRIIB polypeptides comprise a signal sequence in addition to the extracellular domain of an ActRIIB protein, see Example 1. The signal sequence can be a native signal 15 sequence of an ActRIIB, or a signal sequence from another protein, such as a tissue plasminogen activator (TPA) signal sequence or a honey bee melittin (HBM) signal sequence. The disclosure identifies functionally active portions and variants of ActRIIB. Applicants have ascertained that an Fc fusion protein having the sequence disclosed by 20 Hilden et aL (Blood. 1994 Apr 15;83(8):2163-70), which has an Alanine at the position corresponding to amino acid 64 of SEQ ID NO: 1 (A64), has a relatively low affinity for activin and GDF-l 1. By contrast, the same Fe fusion protein with an Arginine at position 64 (R64) has an affinity for activin and GDF- 1 in the low nanomolar to high picomolar range. Therefore, a sequence with an R64 is used as the wild-type reference sequence for human 25 ActR11B in this disclosure. Attisano et al. (Cell. 1992 Jan 10;68(1):97-108) showed that a deletion of the proline knot at the C-terminus of the extracellular domain of ActRIIB reduced the affinity of the receptor for activin, An ActRIlB-Fc fusion protein containing amino acids 20-119 of SEQ ID NO: 1, "ActRIIB(20- 19)-Fc", has reduced binding to GDF- 1 and activin relative to an 30 ActRIIB(20-134)-Fc, which includes the proline knot region and the complete juxtamembrane domain. However, an ActRIIB(20-129)-Fc protein retains similar but somewhat reduced activity relative to the wild type, even though the proline knot region is 27 disrupted. Thus, ActRIB extracellular domains that stop at amino acid 134, 133, 132, 131, 130 and 129 are all expected to be active, but constructs stopping at 134 or 133 may be most active. Similarly, mutations at any of residues 129-134 are not expected to alter ligand binding affinity by large margins. In support of this, mutations of P129 and P130 do not 5 substantially decrease ligand binding. Therefore, a GDF Trap polypeptide which is an ActRI1B-Fc fusion protein may end as early as amino acid 109 (the final cysteine), however, forms ending at or between 109 and 119 are expected to have reduced ligand binding. Amino acid 119 is poorly conserved and so is readily altered or truncated. Forms ending at 128 or later retain ligand binding activity. Forms ending at or between 119 and 127 will have an 10 intermediate binding ability. Any of these forms may be desirable to use, depending on the clinical or experimental setting. At the N-terminus of ActRIIB, it is expected that a protein beginning at amino acid 29 or before will retain ligand binding activity. Amino acid 29 represents the initial cysteine. An alanine to asparagine mutation at position 24 introduces an N-linked glycosylation 15 sequence without substantially affecting ligand binding. This confirms that mutations in the region between the signal cleavage peptide and the cysteine cross-linked region, corresponding to amino acids 20-29 are well tolerated. In particular, constructs beginning at position 20, 21, 22, 23 and 24 will retain activity, and constructs beginning at positions 25, 26, 27, 28 and 29 are also expected to retain activity. Data shown in the Examples 20 demonstrates that, surprisingly, a construct beginning at 22, 23, 24 or 25 will have the most activity. Taken together,. an active portion of ActRIlIB comprises amino acids 29-109 of SEQ ID NO: 1, and GDF Trap constructs may, for example, comprise a portion of ActRIIB beginning at a residue corresponding to amino acids 20-29 of SEQ ID NO: 1 or 39 and 25 ending at a position corresponding to amino acids 109-134 of SEQ ID NO: I or 39. Other examples include constructs that begin at a position from 20-29 or 21-29 and end at a position from 119-134, 119-133, 129-134, or 129-133 of SEQ ID NO: I or 39. Other examples include constructs that begin at a position from 20-24 (or 21-24, or 22-25) and end at a position from 109-134 (or 109-133), 119-134 (or 119-133) or 129-134 (or 129-133) of SEQ 30 ID NO: 1 or 39. Variants within these ranges are also contemplated, particularly those having at least 80%, 85%, 90%, 95% or 99% identity to the corresponding portion of SEQ ID NO: 1 or 39. In certain embodiments, the GDF Trap polypeptide comprises, consists 28 essentially of, or consists of, a polypeptide having an amino acid sequence that is at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or 100% identical to amino acid residues 25 131 of SEQ ID NO: I or 39. In certain embodiments, the GDF Trap polypeptide comprises, consists essentially of, or consists of, a polypeptide having an amino acid sequence that is at 5 least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or 100% identical to SEQ ID NOs: 7, 26, 28, 29, 32, 37 or 38. In preferred embodiments, the GDF Trap polypeptide consists of, or consists essentially of, the amino acid sequence of SEQ ID NO: 7, 26, 28, 29, 32, 37 or 38. The disclosure includes the results of an analysis of composite ActRIlB structures, shown in Figure 1, demonstrating that the ligand binding pocket is defined by residues Y3 1, 10 N33, N35, L38 through T41, E47, E50, Q53 through K55, L57, H58, Y60, S62, K74, W78 through N83, Y85, R87, A92, and E94 through F101. At these positions, it is expected that conservative mutations will be tolerated, although a K74A mutation is well-tolerated, as arc R40A, K55A, F82A and mutations at position L79. R40 is a K in Xenopus, indicating that basic amino acids at this position will be tolerated. Q53 is R in bovine ActRIlB and K in 15 Xenopus ActRIIB, and therefore amino acids including R, K, Q, N and H will be tolerated at this position. Thus, a general formula for a GDF Trap protein is one that comprises amino acids 29-109 of SEQ ID NO: 1 or 39, but optionally beginning at a position ranging from 20 24 or 22-25 and ending at a position ranging from 129-134, and comprising no more than 1, 2, 5, 10 or 15 conservative amino acid changes in the ligand binding pocket, and zero, one or 20 more non-conservative alterations at positions 40, 53, 55, 74, 79 and/or 82 in the ligand binding pocket. Such a protein may retain greater than 80%, 90%, 95% or 99% sequence identity to the sequence of amino acids 29-109 of SEQ ID NO: 1 or 39. Sites outside the binding pocket, at which variability may be particularly well tolerated, include the amino and carboxy termini of the extracellular domain (as noted above), and positions 42-46 and 65-73. 25 An asparagine to alanine alteration at position 65 (N65A) actually improves ligand binding in the A64 background, and is thus expected to have no detrimental effect on ligand binding in the R64 background. This change probably eliminates glycosylation at N65 in the A64 background, thus demonstrating that a significant change in this region is likely to be tolerated. While an R64A change is poorly tolerated, R64K is well-tolerated, and thus 30 another basic residue, such as H may be tolerated at position 64. ActRIlB is well-conserved across nearly all vertebrates, with large stretches of the extracellular domain conserved completely. Many of the ligands that bind to ActRIIB are 29 also highly conserved. Accordingly, comparisons of ActRllB sequences from various vertebrate organisms provide insights into residues that may be altered. Therefore, an active, human ActRIIB variant polypeptide useful as a GDF Trap may include one or more amino acids at corresponding positions from the sequence of another vertebrate ActRIJB, or may 5 include a residue that is similar to that in the human or other vertebrate sequence. The following examples illustrate this approach to defining an active ActRiIB variant. L46 is a valine in Xenopus ActRIIB, and so this position may be altered, and optionally may be altered to another hydrophobic residue, such as V, I or F, or a non-polar residue such as A. E52 is a K in Xenopus, indicating that this site may be tolerant of a wide variety of changes, 10 including polar residues, such as E, D, K, R, H, S, T, P, G, Y and probably A. T93 is a K in Xenopus, indicating that a wide structural variation is tolerated at this position, with polar residues favored, such as S, K, R, E, D, H, G, P, G and Y. F108 is a Y in Xenopus, and therefore Y or other hydrophobic group, such as I, V or L should be tolerated. El I is K in Xenopus, indicating that charged residues will be tolerated at this position, including D, R, K 15 and H, as well as Q and N. RI 12 is K in Xenopus, indicating that basic residues are tolerated at this position, including R and H. A at position 119 is relatively poorly conserved, and appears as P in rodents and V in Xenopus, thus essentially any amino acid should be tolerated at this position. The disclosure demonstrates that the addition of a further N-linked glycosylation site 20 (N-X-S/T) increases the serum half-life of an ActRIlB-Fc fusion protein, relative to the ActRIIB(R64)-Fc form. By introducing an asparagine at position 24 (A24N construct), an NXT sequence is created that confers a longer half-life. Other NX(T/S) sequences are found at 42-44 (NQS) and 65-67 (NSS), although the latter may not be efficiently glycosylated with the R at position 64. N-X-S/T sequences may be generally introduced at positions outside the 25 ligand binding pocket defined in Figure 1. Particularly suitable sites for the introduction of non-endogenous N-X-S/T sequences include amino acids 20-29, 20-24, 22-25, 109-134, 120 134 or 129-134. N-X-S/T sequences may also be introduced into the linker between the ActRIIB sequence and the Fc or other fusion component. Such a site may be introduced with minimal effort by introducing an N in the correct position with respect to a pre-existing S or 30 T, or by introducing an S or T at a position corresponding to a pre-existing N. Thus, desirable alterations that would create an N-linked glycosylation site are: A24N, R64N, S67N (possibly combined with an N65A alteration), E06N, R I 12N, G120N, E123N, PI29N, A I32N, RI 12S and R I 12T. Any S that is predicted to be glycosylated may be altered to a T 30 without creating an immunogenic site, because of the protection afforded by the glycosylation. Likewise, any T that is predicted to be glycosylated may be altered to an S. Thus the alterations S67T and S44T are contemplated. Likewise, in an A24N variant, an S26T alteration may be used. Accordingly, a GDF Trap may be an ActRIB variant having 5 one or more additional, non-endogenous N-linked glycosylation consensus sequences. Position L79 of ActRI1B may be altered to confer altered activin - myostatin (GDF 11) binding properties. L79A or L79P reduces GDF-1 1 binding to a greater extent than activin binding. L79E or L79D retains GDF-I I binding. Remarkably, the L79E and L79D variants have greatly reduced activin binding. In vivo experiments indicate that these non 10 activin receptors retain significant ability to increase red blood cells but show decreased effects on other tissues. These data demonstrate the desirability and feasibility for obtaining polypeptides with reduced effects on activin. In exemplary embodiments, the methods described herein utilize a GDF Trap polypeptide which is a variant ActRIB polypeptide comprising an acidic amino acid (e.g., D or E) at the position corresponding to position 79 of 15 SEQ ID NO: 1 or 39, optionally in combination with one or more additional amino acid substitutions, additions, or deletions. The variations described may be combined in various ways. Additionally, the results of the mutagenesis program described herein indicate that there are amino acid positions in ActRIIB that are often beneficial to conserve. These include position 64 (basic amino acid), 20 position 80 (acidic or hydrophobic amino acid), position 78 (hydrophobic, and particularly tryptophan), position 37 (acidic, and particularly aspartic or glutamic acid), position 56 (basic amino acid), position 60 (hydrophobic amino acid, particularly phenylalanine or tyrosine). Thus, in each of the variants disclosed herein, the disclosure provides a framework of amino acids that may be conserved. Other positions that may be desirable to conserve are as 25 follows: position 52 (acidic amino acid), position 55 (basic amino acid), position 81 (acidic), 98 (polar or charged, particularly E, D, R or K). In certain embodiments, isolated fragments of ActRIIB polypeptides can be obtained by screening polypeptides recombinantly produced from the corresponding fragment of the nucleic acid encoding an ActRIB polypeptide (e.g., SEQ ID NOs: 4 and 5). In addition, 30 fragments can be chemically synthesized using techniques known in the art such as conventional Merrifield solid phase f-Moe or t-Boc chemistry. The fragments can be produced (recombinantly or by chemical synthesis) and tested to identify those peptidyl 31 fragments that can function, for example, as antagonists (inhibitors) or agonists (activators) of an ActRIIB protein or an ActRJIB ligand. In certain embodiments, GDF Trap polypeptide is a variant ActRllB polypeptide having an amino acid sequence that is at least 75% identical to an amino acid sequence 5 selected from SEQ ID NOs: 2, 3, 7, 11, 26, 28, 29, 32, 37, 38, 40 or 41. In certain cases, the GDF Trap has an amino acid sequence at least 80%, 85%, 90%, 95%, 97%, 98%, 99% or 100% identical to an amino acid sequence selected from SEQ ID NOs: 2, 3, 7, 11, 26, 28, 29, 32, 37, 38, 40 or 41. In certain emobdiments, the GDF Trap comprises, consists essentially of, or consists of, an amino acid sequence at least 80%, 85%, 90%, 95%, 97%, 98%, 99% or 10 100% identical to an amino acid sequence selected from SEQ ID NOs: 2, 3, 7, 1I, 26, 28, 29, 32, 37, 38, 40 or 41, wherein the position corresponding to L79 of SEQ ID NO: 1 is an acidic amino acid (e.g., a D or E amino acid residue). In certain embodiments, the present invention contemplates making functional variants by modifying the structure of a GDF Trap polypeptide for such purposes as 15 enhancing therapeutic efficacy, or stability (e.g., cx vivo shelf life and resistance to proteolytic degradation in vivo). GDF Trap polypeptides can also be produced by amino acid substitution, deletion, or addition. For instance, it is reasonable to expect that an isolated replacement of a leucine with an isoleucine or valine, an aspartate with a glutamate, a threonine with a serine, or a similar replacement of an amino acid with a structurally related 20 amino acid (e.g., conservative mutations) will not have a major effect on the biological activity of the resulting molecule. Conservative replacements are those that take place within a family of amino acids that are related in their side chains. Whether a change in the amino acid sequence of a GDF Trap polypeptide results in a functional variant can be readily determined by assessing the ability of the GDF Trap polypeptide to produce a response in 25 cells relative to the unmodified GDF Trap polypeptide or a wild-type ActRIlB polypeptide, or to bind to one or more ligands, such as activin, GDF-I l or myostatin as compared to the unmodified GDF Trap polypeptide or a wild-type ActRIIB polypeptide. In certain specific embodiments, the present invention contemplates making mutations in the extracellular domain (also referred to as ligand-binding domain) of an 30 ActRIlB polypeptide such that the ActRIlIB polypeptide has altered ligand-binding activities (e.g., binding affinity or binding specificity). In certain cases, such GDF Trap polypeptides 32 have altered (elevated or reduced) binding affinity for a specific ligand. In other cases, the GDF Trap polypeptides have altered binding specificity for ActRIlB ligands. For example, the disclosure provides GDF Trap polypeptides that preferentially bind to GDF8/GDFI I relative to activins. The disclosure further establishes the desirability of 5 such polypeptides for reducing off-target effects, although such selective variants may be less desirable for the treatment of severe diseases where very large gains in red blood cell levels may be needed for therapeutic effect and where some level of off-target effect is acceptable. For example, amino acid residues of the ActRIIB protein, such as E39, K55, Y60, K74, W78, D80, and F101, are in the ligand-binding pocket and mediate binding to its ligands such as 10 activin and GDF8. Thus, the present invention provides a GDF Trap comprising an altered ligand-binding domain (e.g., GDF8-binding domain) of an ActRIIB receptor, which comprises one or more mutations at those amino acid residues. Optionally, the altered ligand-binding domain can have increased selectivity for a ligand such as GDF8 relative to a wild-type ligand-binding domain of an ActRIIB receptor. To illustrate, these mutations 15 increase the selectivity of the altered ligand-binding domain for GDF8 over activin. Optionally, the altered ligand-binding domain has a ratio of Kd for activin binding to Kd for GDF8 binding that is at least 2, 5, 10, or even 100 fold greater relative to the ratio for the wild-type ligand-binding domain. Optionally, the altered ligand-binding domain has a ratio of IC 5 o for inhibiting activin to IC 5 for inhibiting GDF8 that is at least 2, 5, 10, or even 100 20 fold greater relative to the wild-type ligand-binding domain. Optionally, the altered ligand binding domain inhibits GDF8 with an IC 5 at least 2, 5, 10, or even 100 times less than the
IC
5 o for inhibiting activin. As a specific example, the positively-charged amino acid residue Asp (D80) of the ligand-binding domain of ActRiIB can be mutated to a different amino acid residue to 25 produce a GDF Trap polypeptide that preferentially binds to GDF8, but not activin. Preferably, the D80 residue is changed to an amino acid residue selected from the group consisting of: an uncharged amino acid residue, a negative amino acid residue, and a hydrophobic amino acid residue. As a further specific example, the hydrophobic residue, L79, can be altered to the acidic amino acids aspartic acid or glutamic acid to greatly reduce 30 activin binding while retaining GDFI I binding. As will be recognized by one of skill in the art, most of the described mutations, variants or modifications may be made at the nucleic 33 acid level or, in some cases, by post translational modification or chemical synthesis. Such techniques are well known in the art. In certain embodiments, the present invention contemplates GDF Trap polypeptides having specific mutations in ActRIIB so as to alter the glycosylation of the ActRIIB 5 polypeptide. Exemplary glycosylation sites in GDF Trap polypeptides are illustrated in Figure 1 (e.g., the underlined NX(S/T) sites). Such mutations may be selected so as to introduce or eliminate one or more glycosylation sites, such as 0-linked or N-linked glycosylation sites. Asparagine-linked glycosylation recognition sites generally comprise a tripeptide sequence, asparagine-X-thrconine (where "X" is any amino acid) which is 10 specifically recognized by appropriate cellular glycosylation enzymes. The alteration may also be made by the addition of, or substitution by, one or more shrine or threonine residues to the sequence of the wild-type ActRUB polypeptide (for 0-linked glycosylation sites). A variety of amino acid substitutions or deletions at one or both of the first or third amino acid positions of a glycosylation recognition site (and/or amino acid deletion at the second 15 position) results in non-glycosylation at the modified tripeptide sequence. Another means of increasing the number of carbohydrate moieties on a GDF Trap polypeptide is by chemical or enzymatic coupling of glycosides to the GDF Trap polypeptide. Depending on the coupling mode used, the sugar(s) may be attached to (a) arginine and histidine; (b) free carboxyl groups; (c) free sulfhydryl groups such as those of cysteine; (d) free hydroxyl groups such as 20 those of shrine, threonine, or hydroxyproline; (e) aromatic residues such as those of phenylalanine, tyrosine, or tryptophan; or (f) the amide group of glutamine. These methods are described in WO 87/05330 and in Aplin and Wriston (1981) CRC Crit. Rev. Biochem., pp. 259-306, incorporated by reference herein. Removal of one or more carbohydrate moieties present on a GDF Trap polypeptide may be accomplished chemically and/or 25 enzymatically. Chemical deglycosylation may involve, for example, exposure of the GDF Trap polypeptide to the compound trifluoromethanesulfonic acid, or an equivalent compound. This treatment results in the cleavage of most or all sugars except the linking sugar (N acetylglucosamine or N-acetylgalactosamine), while leaving the amino acid sequence intact. Chemical deglycosylation is further described by Hakimuddin et al. (1987) Arch. Biochem. 30 Biophys. 259:52 and by Edge et al. (1981) Anal. Biochem. 118:131. Enzymatic cleavage of carbohydrate moieties on GDF Trap polypeptides can be achieved by the use of a variety of endo- and exo-glycosidases as described by Thotakura et al. (1987) Meth. Enzymol, 138:350. The sequence of a GDF Trap polypeptide may be adjusted, as appropriate, depending on the 34 type of expression system used, as mammalian, yeast, insect and plant cells may all introduce differing glycosylation patterns that can be affected by the amino acid sequence of the peptide. In general, GDF Trap polypeptides for use in humans will be expressed in a mammalian cell line that provides proper glycosylation, such as HEK293 or CHO cell lines, 5 although other mammalian expression cell lines are expected to be useful as well. This disclosure further contemplates a method of generating variants, particularly sets of combinatorial variants of a GDF Trap polypeptide, including, optionally, truncation variants; pools of combinatorial mutants are especially useful for identifying GDF Trap sequences. The purpose of screening such combinatorial libraries may be to generate, for 10 example, GDF Trap polypeptide variants which have altered properties, such as altered pharmacokinetics, or altered ligand binding. A variety of screening assays are provided below, and such assays may be used to evaluate variants. For example, a GDF Trap polypeptide variant may be screened for the ability to bind to an ActR1IB polypeptide, to prevent binding of an ActRIlB ligand to an ActRIIB polypeptide or to interfere with signaling 15 caused by an ActRIIB ligand, The activity of a GDF Trap polypeptide or its variants may also be tested in a cell based or in vivo assay. For example, the effect of a GDF Trap polypeptide variant on the expression of genes involved in hematopoiesis may be assessed. This may, as needed, be performed in the presence of one or more recombinant ActRIIB ligand proteins (e.g., activin), 20 and cells may be transfected so as to produce a GDF Trap polypeptide and/or variants thereof, and optionally, an ActRIIB ligand. Likewise, a GDF Trap polypeptide may be administered to a mouse or other animal, and one or more blood measurements, such as an RBC count, hemoglobin levels, hematocrit levels, iron stores, or reticulocyte count may be assessed using art recognized methods. 25 Combinatorially-derived variants can be generated which have a selective potency relative to a reference GDF Trap polypeptide. Such variant proteins, when expressed from recombinant DNA constructs, can be used in gene therapy protocols. Likewise, mutagenesis can give rise to variants which have intracellular half-lives dramatically different than the corresponding unmodified GDF Trap polypeptide. For example, the altered protein can be 30 rendered either more stable or less stable to proteolytic degradation or other processes which result in destruction of, or otherwise inactivation of an unmodified GDF Trap polypeptide. Such variants, and the genes which encode them, can be utilized to alter GDF Trap 35 polypeptide levels by modulating the half-life of the GDF Trap polypeptides. For instance, a short half-life can give rise to more transient biological effects and, when part of an inducible expression system, can allow tighter control of recombinant GDF Trap polypeptide levels within the cell. In an Fc fusion protein, mutations may be made in the linker (if any) and/or 5 the Fc portion to alter the half-life of the protein. In certain embodiments, the GDF Trap polypeptides of the invention may further comprise post-translational modifications in addition to any that are naturally present in the ActRIIB polypeptides. Such modifications include, but are not limited to, acetylation, carboxylation, glycosylation, phosphorylation, lipidation, and acylation. As a result, GDF 10 Trap polypeptides may contain non-amino acid elements, such as polyethylene glycols, lipids, poly- or mono-saccharide, and phosphates. Effects of such non-amino acid elements on the functionality of a GDF Trap polypeptide may be tested as described herein for other GDF Trap polypeptide variants. When a GDF Trap polypeptide is produced in cells by cleaving a nascent form of the GDF Trap polypeptide, post-translational processing may also 15 be important for correct folding and/or function of the protein. Different cells (such as CHO, HcLa, MDCK, 293, W138, NIH-3T3 or HEK293) have specific cellular machinery and characteristic mechanisms for such post-translational activities and may be chosen to ensure the correct modification and processing of the GDF Trap polypeptides. In certain aspects, GDF Trap polypeptides include fusion proteins having at least a 20 portion of an ActRIIB polypeptide and one or more fusion domains. Well known examples of such fusion domains include, but are not limited to, polyhistidine, Glu-Glu, glutathione S transferase (GST), thioredoxin. protein A, protein G, an immunoglobulin heavy chain constant region (e.g., an Fc), maltose binding protein (MBP), or human serum albumin. A fusion domain may be selected so as to confer a desired property. For example, some fusion 25 domains are particularly useful for isolation of the fusion proteins by affinity chromatography. For the purpose of affinity purification, relevant matrices for affinity chromatography, such as glutathione-, amylase-, and nickel- or cobalt- conjugated resins are used. Many of such matrices are available in "kit" form, such as the Pharmacia GST purification system and the QlAcxpress"' 1 system (Qiagen) useful with (HIS 6 ) fusion 30 partners. As another example, a fusion domain may be selected so as to facilitate detection of the GDF Trap polypeptides. Examples of such detection domains include the various fluorescent proteins (e.g., GFP) as well as "epitope tags," which are usually short peptide 36 sequences for which a specific antibody is available. Well known epitope tags for which specific monoclonal antibodies are readily available include FLAG, influenza virus haemagglutinin (HA), and c-myc tags. In some cases, the fusion domains have a protease cleavage site, such as for Factor Xa or Thrombin, which allows the relevant protease to 5 partially digest the fusion proteins and thereby liberate the recombinant proteins therefrom. The liberated proteins can then be isolated from the fusion domain by subsequent chromatographic separation. In certain preferred embodiments, a GDF Trap polypeptide is fused with a domain that stabilizes the GDF Trap polypeptide in vivo (a "stabilizer" domain). By "stabilizing" is meant anything that increases serum half life, regardless of whether this is 10 because of decreased destruction, decreased clearance by the kidney, or other pharmacokinetic effect. Fusions with the Fc portion of an immunoglobulin are known to confer desirable pharmacokinetic properties on a wide range of proteins. Likewise, fusions to human serum albumin can confer desirable properties. Other types of fusion domains that may be selected include multimerizing (e.g., dimerizing, tetramerizing) domains and 15 functional domains (that confer an additional biological function, such as further increasing red blood cell levels). As a specific example, the present invention provides GDF Trap that is an ActRIIB Fe fusion protein which comprises an extracellular (e.g., ligand-binding) domain of ActRIIB polypeptide fused to an Fe domain. The sequence of an exemplary Fe domain is shown 20 below (SEQ ID NO: 6). THTCPPCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCVVVD(A)VSHEDPEVKFNWYVDG VEVHNAKTKPREEQYNSTYRVVSVLTVLHQDWLNGKEYKCK (A) VSNKALPVPIEKTISKAK GQPREPQVYTL PPSREEMTKNQVSLTCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDG PFFLYSKLTVDKSRWQQGNVFSCSVMHEALHN(A)HYTQKSLSLSPGK* 25 Optionally, the Fc domain has one or more mutations at residues such as Asp-265, lysine 322, and Asn-434. In certain cases, the mutant Fe domain having one or more of these mutations (e.g., Asp-265 mutation) has reduced ability of binding to the Fey receptor relative to a wildtype Fc domain. In other cases, the mutant Fe domain having one or more of these mutations (e.g., Asn-434 mutation) has increased ability of binding to the MHC class I 30 related Fc-receptor (FcRN) relative to a wildtype Fe domain. It is understood that different elements of the fusion proteins may be arranged in any manner that is consistent with the desired functionality. For example, a GDF Trap 37 polypeptide may be placed C-terminal to a heterologous domain, or, alternatively, a heterologous domain may be placed C-terminal to a GDF Trap polypeptide. The GDF Trap polypeptide domain and the heterologous domain need not be adjacent in a fusion protein, and additional domains or amino acid sequences may be included C- or N-terminal to either 5 domain or between the domains. In certain embodiments, a GDF Trap fusion protein comprises an amino acid sequence as set forth in the formula A-B-C. The B portion is an N- and C-terminally truncated ActRIIB polypeptide consisting of the amino acid sequence corresponding to amino acids 26-132 of SEQ ID NO: 26. The A and C portions may be independently zero, one or 10 more than one amino acids, and both the A and C portions when present are heterologous to B. The A and/or C portions may be attached to the B portion via a linker sequence. Exemplary linkers are include short polypeptide linkers such as 2-10, 2-5, 2-4, 2-3 Glycine residues, such as, for example, a Gly-Gly-Gly linker. Other suitable linkers are described herein above. In certain embodiments, a GDF Trap fusion protein comprises an amino acid 15 sequence as set forth in the formula A-B-C, wherein A is a leader sequence, B consists of amino acids 26-132 of SEQ ID NO: 26, and C is a polypeptide portion that enhances one or more of in vivo stability, in vivo half life, uptake/administration, tissue localization or distribution, formation of protein complexes, and/or purification. In certain embodiments, a GDF Trap fusion protein comprises an amino acid sequence as set forth in the formula A-B 20 C, wherein A is a TPA leader sequence, B consists of amino acids 26-132 of SEQ ID NO: 26, and C is an immunoglobulin Fe domain. A preferred GDF Trap fusion protein comprises the amino acid sequence set forth in SEQ ID NO: 26. In certain embodiments, the GDF Trap polypeptides of the present invention contain one or more modifications that are capable of stabilizing the GDF Trap polypeptides. For 25 example, such modifications enhance the in vitro half life of the GDF Trap polypeptides, enhance circulatory half life of the GDF Trap polypeptides or reducing proteolytic degradation of the GDF Trap polypeptides. Such stabilizing modifications include, but are not limited to, fusion proteins (including, for example, fusion proteins comprising an GDF Trap polypeptide and a stabilizer domain), modifications of a glycosylation site (including, 30 for example, addition of a glycosylation site to a GDF Trap polypeptide), and modifications of carbohydrate moiety (including, for example, removal of carbohydrate moieties from a GDF Trap polypeptide). In the case of fusion proteins, a GDF Trap polypeptide is fused to a 38 stabilizer domain such as an IgG molecule (e.g., an Fe domain). As used herein, the term "stabilizer domain" not only refers to a fusion domain (e.g., Fc) as in the case of fusion proteins, but also includes nonproteinaceous modifications such as a carbohydrate moiety, or nonproteinaceous polymer, such as polyethylene glycol. 5 In certain embodiments, the present invention makes available isolated and/or purified forms of the GDF Trap polypeptides, which are isolated from, or otherwise substantially free of, other proteins. In certain embodiments, GDF Trap polypeptides (unmodified or modified) of the invention can be produced by a variety of art-known techniques. For example, such GDF 10 Trap polypeptides can be synthesized using standard protein chemistry techniques such as those described in Bodansky, M. Principles of Peptide Synthesis, Springer Verlag, Berlin (1993) and Grant G. A. (ed.), Synthetic Peptides: A User's Guide, W. H. Freeman and Company, New York (1992). In addition, automated peptide synthesizers are commercially available (e.g., Advanced ChemTech Model 396; Milligen/Biosearch 9600). Alternatively, 15 the GDF Trap polypeptides, fragments or variants thereof may be recombinantly produced using various expression systems (e.g., E. coli, Chinese Hamster Ovary (CHO) cells, COS cells, baculovirus) as is well known in the art. In a further embodiment, the modified or unmodified GDF Trap polypeptides may be produced by digestion of recombinantly produced full-length GDF Trap polypeptides by using, for example, a protease, e.g., trypsin, 20 thermolysin, chymotrypsin, pepsin, or paired basic amino acid converting enzyme (PACE). Computer analysis (using a commercially available software, e.g., MacVector, Omega, PCGene, Molecular Simulation, Inc.) can be used to identify proteolytic cleavage sites. Alternatively, such GDF Trap polypeptides may be produced from recombinantly produced full-length GDF Trap polypeptides such as standard techniques known in the art, such as by 25 chemical cleavage (e.g., cyanogen bromide, hydroxylamine). 3. Nucleic Acids Encoding GDF Trap Polypeptides In certain aspects, the invention provides isolated and/or recombinant nucleic acids encoding any of the GDF Trap polypeptides disclosed herein. SEQ ID NO: 4 encodes a 30 naturally occurring ActRIIB precursor polypeptide, while SEQ ID NO: 5 encodes a soluble ActRIIB polypeptide, and SEQ ID NOs: 25, 27, 30 and 31 encode soluble GDF Traps. The 39 subject nucleic acids may be single-stranded or double stranded. Such nucleic acids may be DNA or RNA molecules. These nucleic acids may be used, for example, in methods for making GDF Trap polypeptides or as direct therapeutic agents (e.g., in a gene therapy approach). 5 In certain aspects, the subject nucleic acids encoding GDF Trap polypeptides are further understood to include nucleic acids that are variants of SEQ ID NOs: 5, 25, 27, 30 and 31. Variant nucleotide sequences include sequences that differ by one or more nucleotide substitutions, additions or deletions, such as allelic variants; and will, therefore, include coding sequences that differ from the nucleotide sequence of the coding sequence designated 10 in SEQ ID NOs: 5, 25, 27, 30 and 31. In certain embodiments, the invention provides isolated or recombinant nucleic acid sequences that are at least 80%, 85%, 90%, 95%, 97%, 98%, 99% or 100% identical to SEQ ID NO: 5, 25, 27, 30 or 31. One of ordinary skill in the art will appreciate that nucleic acid sequences complementary to SEQ ID NO: 5, 25, 27, 30 or 31, and variants of SEQ ID NO: 5, 15 25, 27, 30 or 31, are also within the scope of this invention. In further embodiments, the nucleic acid sequences of the invention can be isolated, recombinant, and/or fused with a heterologous nucleotide sequence, or in a DNA library. In other embodiments, nucleic acids of the invention also include nucleotide sequences that hybridize tinder highly stringent conditions to the nucleotide sequence 20 designated in SEQ ID NO: 5, 25, 27, 30 or 31, complement sequence of SEQ ID NO: 5, 25, 27, 30 or 31, or fragments thereof. As discussed above, one of ordinary skill in the art will undertamd readily that appropriate stringency conditions which promote DN\ hybridi7ation can be varied, For example, one could perform the hybridization at 6.0 x sodium chloride/sodium citrate (SSC) at about 45 *C, followed by a wash of 2.0 x SSC at 50 *C. For 25 example, the salt concentration in the wash step can be selected from a low stringency of about 2.0 x SSC at 50 *C to a high stringency of about 0.2 x SSC at 50 *C. In addition, the temperature in the wash step can be increased from low stringency conditions at room temperature, about 22 'C, to high stringency conditions at about 65 *C. Both temperature and salt may be varied, or temperature or salt concentration may be held constant while the 30 other variable is changed. In one embodiment, the invention provides nucleic acids which hybridize under low stringency conditions of 6 x SSC at room temperature followed by a wash at 2 x SSC at room temperature. 40 Isolated nucleic acids which differ from the nucleic acids as set forth in SEQ ID NO: 5, 25, 27, 30 or 31 due to degeneracy in the genetic code are also within the scope of the invention, For example, a number of amino acids are designated by more than one triplet. Codons that specify the same amino acid, or synonyms (for example, CAU and CAC are 5 synonyms for histidine) may result in "silent" mutations which do not affect the amino acid sequence of the protein. In certain embodiments, the GDF Trap polypeptide will be encoded by an alternative nucleotide sequence. Alternative nucleotide sequences are degenerate with respect to the native GDF Trap nucleic acid sequence but still encode for the same fusion protein. In certain embodiments, the GDF Trap having SEQ ID NO: 26 is encoded by an 10 alternative nucleic acid sequence comprising SEQ ID NO: 30. However, it is expected that DNA sequence polymorphisms that do lead to changes in the amino acid sequences of the subject proteins will exist among mammalian cells. One skilled in the art will appreciate that these variations in one or more nucleotides (up to about 3-5% of the nucleotides) of the nucleic acids encoding a particular protein may exist among individuals of a given species 15 due to natural allelic variation. Any and all such nucleotide variations and resulting amino acid polymorphisms are within the scope of this invention. In certain embodiments, the recombinant nucleic acids of the invention may be operably linked to one or more regulatory nucleotide sequences in an expression construct. Regulatory nucleotide sequences will generally be appropriate to the host cell used for 20 expression. Numerous types of appropriate expression vectors and suitable regulatory sequences are known in the art for a variety of host cells. Typically, said one or more regulatory nucleotide sequences may include, but are not limited to, promoter sequences, leader or signal sequences, ribosomal binding sites, transcriptional start and termination sequences, translational start and termination sequences, and enhancer or activator sequences. 25 Constitutive or inducible promoters as known in the art are contemplated by the invention. The promoters may be either naturally occurring promoters, or hybrid promoters that combine elements of more than one promoter. An expression construct may be present in a cell on an episome, such as a plasmid, or the expression construct may be inserted in a chromosome. In a preferred embodiment, the expression vector contains a selectable marker 30 gene to allow the selection of transformed host cells. Selectable marker genes are well known in the art and will vary with the host cell used. 41 In certain aspects of the invention, the subject nucleic acid is provided in an expression vector comprising a nucleotide sequence encoding a GDF Trap polypeptide and operably linked to at least one regulatory sequence. Regulatory sequences are art-recognized and are selected to direct expression of the GDF Trap polypeptide. Accordingly, the term 5 regulatory sequence includes promoters, enhancers, and other expression control elements. Exemplary regulatory sequences are described in Goeddel; Gene Expression Technology: Methods in Enzymology, Academic Press, San Diego, CA (1990). For instance, any of a wide variety of expression control sequences that control the expression of a DNA sequence when operatively linked to it may be used in these vectors to express DNA sequences encoding a 10 GDF Trap polypeptide. Such useful expression control sequences, include, for example, the early and late promoters of SV40, tet promoter, adenovirus or cytomegalovirus immediate early promoter, RSV promoters, the lac system, the trp system, the TAC or TRC system, T7 promoter whose expression is directed by T7 RNA polymerase, the major operator and promoter regions of phage lambda, the control regions for fd coat protein, the promoter for 3 15 phosphoglycerate kinase or other glycolytic enzymes, the promoters of acid phosphatase, e.g., Pho5, the promoters of the yeast a-mating factors, the polyhedron promoter of the baculovirus system and other sequences known to control the expression of genes of prokaryotic or eukaryotic cells or their viruses, and various combinations thereof, It should be understood that the design of the expression vector may depend on such factors as the 20 choice of the host cell to be transformed and/or the type of protein desired to be expressed. Moreover, the vector's copy number, the ability to control that copy number and the expression of any other protein encoded by the vector, such as antibiotic markers, should also be con ,dred.- A recombinant nucleic acid of the invention can be produced by ligating the cloned 25 gene, or a portion thereof, into a vector suitable for expression in either prokaryotic cells, eukaryotic cells (yeast, avian, insect or mammalian), or both. Expression vehicles for production of a recombinant GDF Trap polypeptide include plasmids and other vectors. For instance, suitable vectors include plasmids of the types: pBR322-derived plasmids, pEMBL derived plasmids, pEX-derived plasmids, pBTac-derived plasmids and pUC-derived plasmids 30 for expression in prokaryotic cells, such as E. coli. Some mammalian expression vectors contain both prokaryotic sequences to facilitate the propagation of the vector in bacteria, and one or more eukaryotic transcription units that 42 are expressed in eukaryotic cells. The pcDNAI/amp, pcDNAI/nco, pRc/CMV, pSV2gpt, pSV2neo, pSV2-dhfr, pTk2, pRSVneo, pMSG, pSVT7, pko-neo and pHyg derived vectors are examples of mammalian expression vectors suitable for transfection of eukaryotic cells. Some of these vectors are modified with sequences from bacterial plasmids, such as pBR322, 5 to facilitate replication and drug resistance selection in both prokaryotic and eukaryotic cells. Alternatively, derivatives of viruses such as the bovine papilloma virus (BPV-l), or Epstein Barr virus (pH EBo, pREP-derived and p205) can be used for transient expression of proteins in eukaryotic cells. Examples of other viral (including retroviral) expression systems can be found below in the description of gene therapy delivery systems. The various methods 10 employed in the preparation of the plasmids and in transformation of host organisms are well known in the art. For other suitable expression systems for both prokaryotic and eukaryotic cells, as well as general recombinant procedures, see Molecular Cloning A Laboratory Manual, 2nd Ed., ed. by Sambrook, Fritsch and Maniatis (Cold Spring Harbor Laboratory Press, 1989) Chapters 16 and 17. In some instances, it may be desirable to 15 express the recombinant polypeptides by the use of a baculovirus expression system. Examples of such baculovirus expression systems include pVL-derived vectors (such as pVL 1392, pVL1393 and pVL94 1), pAcUW-dcrived vectors (such as pAcUW I), and pBlucBac-derived vectors (such as the B-gal containing pBlueBac 111). In a preferred embodiment, a vector will be designed for production of the subject 20 GDF Trap polypeptides in CHO cells, such as a Pcmv-Script vector (Stratagene, La Jolla, Calif.), pcDNA4 vectors (Invitrogen, Carlsbad, Calif.) and pCl-neo vectors (Promega, Madison, Wisc.). As will be apparent, the subject gene constructs can be used to cause expression of the subject GDF Trap polypeptides in cells propagated in culture, e.g., to produce proteins, including fusion proteins or variant proteins, for purification. 25 This invention also pertains to a host cell transfected with a recombinant gene including a coding sequence (e.g., SEQ ID NO: 4, 5, 25, 27, 30 or 31) for one or more of the subject GDF Trap polypeptides. The host cell may be any prokaryotic or eukaryotic cell. For example, a GDF Trap polypeptide of the invention may be expressed in bacterial cells such as E. co/i, insect cells (e.g., using a baculovirus expression system), yeast, or mammalian cells. 30 Other suitable host cells are known to those skilled in the art. Accordingly, the present invention further pertains to methods of producing the subject GDF Trap polypeptides. For example, a host cell transfected with an expression 43 vector encoding a GDF Trap polypeptide can be cultured under appropriate conditions to allow expression of the GDF Trap polypeptide to occur. The GDF Trap polypeptide may be secreted and isolated from a mixture of cells and medium containing the GDF Trap polypeptide. Alternatively, the GDF Trap polypeptide may be retained cytoplasmically or in 5 a membrane fraction and the cells harvested, lysed and the protein isolated. A cell culture includes host cells, media and other byproducts. Suitable media for cell culture are well known in the art. The subject GDF Trap polypeptides can be isolated from cell culture medium, host cells, or both, using techniques known in the art for purifying proteins, including ion-exchange chromatography, gel filtration chromatography, ultrafiltration, 10 electrophoresis, and immunoaffinity purification with antibodies specific for particular epitopes of the GDF Trap polypeptides. In a preferred embodiment, the GDF Trap polypeptide is a fusion protein containing a domain which facilitates its purification. In another embodiment, a fusion gene coding for a purification leader sequence, such as a poly-(His)/enterokinase cleavage site sequence at the N-terminus of the desired portion 15 of the recombinant GDF Trap polypeptide, can allow purification of the expressed fusion protein by affinity chromatography using a Ni 2 < metal resin. The purification leader sequence can then be subsequently removed by treatment with enterokinase to provide the purified GDF Trap polypeptide (e.g., see Hochuli et al., (1987) J. Chromatography 411:177; and Janknecht et al., PNAS USA 88:8972). 20 Techniques for making fusion genes are well known. Essentially, the joining of various DNA fragments coding for different polypeptide sequences is performed in accordance with conventional techniques, employing blunt-ended or stagger-ended termini for ligation, restriction enzyme digestion to provide for appropriate termini, filling-in of cohesive ends as appropriate, alkaline phosphatase treatment to avoid undesirable joining, 25 and enzymatic ligation. In another embodiment, the fusion gene can be synthesized by conventional techniques including automated DNA synthesizers. Alternatively, PCR amplification of gene fragments can be carried out using anchor primers which give rise to complementary overhangs between two consecutive gene fragments which can subsequently be annealed to generate a chimeric gene sequence (see, for example, Current Protocols in 30 Molecular Biology, eds. Ausubel et al., John Wiley & Sons: 1992). 44 4. Screening Assays In certain aspects, the present invention relates to the use of the subject GDF Trap polypeptides (e.g., soluble variant ActRIIB polypeptides) to identify compounds (agents) which are agonist or antagonists of ActRIIB polypeptides. Compounds identified through 5 this screening can be tested to assess their ability to modulate red blood cell, hemoglobin and/or reticulocyte levels in vivo or in vitro. These compounds can be tested, for example, in animal models. There are numerous approaches to screening for therapeutic agents for increasing red blood cell or hemoglobin levels by targeting ActRIIB signaling. In certain embodiments, 10 high-throughput screening of compounds can be carried out to identify agents that perturb ActRIIB-mcdiated effects on a selected cell line. In certain embodiments, the assay is carried out to screen and identify compounds that specifically inhibit or reduce binding of an ActRIIB polypeptide to its binding partner, such as an ActRIIB ligand (e.g., activin, Nodal, GDF8, GDFI I or BMP7). Alternatively, the assay can be used to identify compounds that 15 enhance binding of an ActRIIB polypeptide to its binding partner such as an ActRIIB ligand. In a further embodiment, the compounds can be identified by their ability to interact with an ActRIiB polypeptide. A variety of assay formats will suffice and, in light of the present disclosure, those not expressly described herein will nevertheless be comprehended by one of ordinary skill in the 20 art. As described herein, the test compounds (agents) of the invention may be created by any combinatorial chemical method. Alternatively, the subject compounds may be naturally occurring biomolecules synthesized in riro or in vitro. Compounds (agents) to be tested for their ability to act as modulators of tissue growth can be produced, for example, by bacteria, yeast, plants or other organisms (e.g., natural products), produced chemically (e.g., small 25 molecules, including peptidomimeties), or produced recombinantly. Test compounds contemplated by the present invention include non-peptidyl organic molecules, peptides, polypeptides, peptidomimetics, sugars, hormones, and nucleic acid molecules. In a specific embodiment, the test agent is a small organic molecule having a molecular weight of less than about 2,000 Daltons. 30 The test compounds of the invention can be provided as single, discrete entities, or provided in libraries of greater complexity, such as made by combinatorial chemistry. These libraries can comprise, for example, alcohols, alkyl halides, amines, amides, esters, 45 aldehydes, ethers and other classes of organic compounds. Presentation of test compounds to the test system can be in either an isolated form or as mixtures of compounds, especially in initial screening steps. Optionally, the compounds may be optionally derivatized with other compounds and have derivatizing groups that facilitate isolation of the compounds. Non 5 limiting examples of derivatizing groups include biotin, fluorescein, digoxygenin, green fluorescent protein, isotopes, polyhistidine, magnetic beads, glutathione S transferase (GST), photoactivatible crosslinkers or any combinations thereof. In many drug screening programs which test libraries of compounds and natural extracts, high throughput assays are desirable in order to maximize the number of compounds 10 surveyed in a given period of time. Assays which are performed in cell-free systems, such as may be derived with purified or semi-purified proteins, are often preferred as "primary" screens in that they can be generated to permit rapid development and relatively easy detection of an alteration in a molecular target which is mediated by a test compound. Moreover, the effects of cellular toxicity or bioavailability of the test compound can be 15 generally ignored in the in vitro system, the assay instead being focused primarily on the effect of the drug on the molecular target as may be manifest in an alteration of binding affinity between an ActRIIB polypeptide and its binding partner (e.g., an ActRJIB ligand). Merely to illustrate, in an exemplary screening assay of the present invention, the compound of interest is contacted with an isolated and purified ActRIIB polypeptide which is 20 ordinarily capable of binding to an ActRIlIB ligand, as appropriate for the intention of the assay. To the mixture of the compound and ActRIIB polypeptide is then added to a composition containing an ActRIIB ligand. Detection and quantification of ActRiIB/ActRIIB ligand complexes provides a means for determining the compound's efficacy at inhibiting (or potentiating) complex formation between the ActRIlB polypeptide 25 and its binding protein. The efficacy of the compound can be assessed by generating dose response curves from data obtained using various concentrations of the test compound. Moreover, a control assay can also be performed to provide a baseline for comparison. For example, in a control assay, isolated and purified ActRIB ligand is added to a composition containing the A.ctRIIB polypeptide, and the formation of ActRIlIB/ActRIlIB ligand complex 30 is quantitated in the absence of the test compound. It will be understood that, in general, the order in which the reactants may be admixed can be varied, and can be admixed 46 simultaneously. Moreover, in place of purified proteins, cellular extracts and lysates may be used to render a suitable cell-free assay system. Complex formation between the ActRIB polypeptide and its binding protein may be detected by a variety of techniques. For instance, modulation of the formation of complexes 5 can be quantitated using, for example, detectably labeled proteins such as radiolabeled (e.g., 32 P, 35S, ' 4 C or 3 H), fluorescently labeled (e.g., FITC), or enzymatically labeled ActR11B polypeptide or its binding protein, by immunoassay, or by chromatographic detection. In certain embodiments, the present invention contemplates the use of fluorescence polarization assays and fluorescence resonance energy transfer (FRET) assays in measuring, 10 either directly or indirectly, the degree of interaction between an ActRlIB polypeptide and its binding protein. Further, other modes of detection, such as those based on optical waveguides (PCT Publication WO 96/26432 and U.S. Pat. No. 5,677,196), surface plasmon resonance (SPR), surface charge sensors, and surface force sensors, are compatible with many embodiments of the invention. 15 Moreover, the present invention contemplates the use of an interaction trap assay, also known as the "two hybrid assay," for identifying agents that disrupt or potentiate interaction between an ActRlIB polypeptide and its binding partner. See for example, U.S. Pat. No. 5,283,317; Zervos et al. (1993) Cell 72:223-232; Madura et al. (1993) J Biol Chem 268:12046-12054; Bartel et al. (1993) Biotechniques 14:920-924; and Iwabuchi et al. (1993) 20 Oncogene 8:1693-1696). In a specific embodiment, the present invention contemplates the use of reverse two hybrid systems to identify compounds (e.g., small molecules or peptides) that dissociate interactions.between an \etRIIB polypeptide and its binding protein. See for example, Vidal and Legrain, (1999) Nucleic Acids Res 27:919-29; Vidal and Legrain, (1999) Trends Biotechnol 17:374-81; and U.S. Pat. Nos. 5,525,490; 5,955,280; and 5,965,368. 25 In certain embodiments, the subject compounds are identified by their ability to interact with an ActR.lIB polypeptide. The interaction between the compound and the ActRIiB polypeptide may be covalent or non-covalent. For example, such interaction can be identified at the protein level using in vitro biochemical methods, including photo crosslinking, radiolabeled ligand binding, and affinity chromatography (Jakoby WB et al., 30 1974, Methods in Enzymology 46: 1). In certain cases, the compounds may be screened in a mechanism based assay, such as an assay to detect compounds which bind to an ActRIlB polypeptide. This may include a solid phase or fluid phase binding event. Alternatively, the 47 gene encoding an ActRIlB polypeptide can be transfected with a reporter system (e.g., p galactosidase, luciferase, or green fluorescent protein) into a cell and screened against the library preferably by a high throughput screening or with individual members of the library. Other mechanism based binding assays may be used, for example, binding assays which 5 detect changes in free energy. Binding assays can be performed with the target fixed to a well, bead or chip or captured by an immobilized antibody or resolved by capillary electrophoresis. The bound compounds may be detected usually using colorimetric or fluorescence or surface plasmon resonance. 10 5. Exemplary Therapeutic Uses In certain embodiments, the GDF Trap polypeptides of the present invention can be used to increase red blood cell levels in mammals such as rodents and primates, and particularly human patients. Additionally, as shown herein, GDF Trap polypeptides may be used in combination with EPO receptor activators to achieve an increase in red blood cells at 15 lower dose ranges. This may be beneficial in reducing the known off-target effects and risks associated with high doses of EPO receptor activators. In certain embodiments, the present invention provides methods of treating or preventing anemia in an individual in need thereof by administering to the individual a therapeutically effective amount of a GDF Trap polypeptide or a combination (or concomitant therapy) of a GDF Trap polypeptide and a EPO 20 receptor activator. These methods may be used for therapeutic and prophylactic treatments of mammals, and particularly humans. I he GUDF i rap polypepides mdy be used in cbmbniation waih L-PU receptor activators to reduce the required dose of these activators in patients that are susceptible to adverse effects of EPO. The primary adverse effects of EPO are an excessive increase in the 25 hematocrit or hemoglobin levels and polycythemia. Elevated hematocrit levels can lead to hypertension (more particularly aggravation of hypertension) and vascular thrombosis. Other adverse effects of EPO which have been reported, some of which related to hypertension, are headaches, influenza-like syndrome, obstruction of shunts, myocardial infarctions and cerebral convulsions due to thrombosis, hypertensive encephalopathy, and red cell blood cell 30 applasia (Singibarti, (1994) J. Clin Investig 72(suppl 6), S36-S43; Horl et al. (2000) Nephrol Dial Transplant 15(suppl 4), 51-56; Delanty et al. (1997) Neurology 49, 686-689; Bunn (2002) N Engl J Med 346(7), 522-523). 48 The rapid effect on red blood cell levels of the GDF Trap polypeptides disclosed herein indicate that these agents act by a different mechanism than EPO. Accordingly, these antagonists may be useful for increasing red blood cell and hemoglobin levels in patients that do not respond well to EPO. For example, a GDF Trap polypeptide may be beneficial for a 5 patient in which administration of a normal to increased (>300 IU/kg/week) dose of EPO does not result in the increase of hemoglobin level up to the target level. Patients with an inadequate EPO response are found for all types of anemia, but higher numbers of non responders have been observed particularly frequently in patients with cancers and patients with end-stage renal disease. An inadequate response to EPO can be either constitutive (i.e. 10 observed upon the first treatment with EPO) or acquired (e.g. observed upon repeated treatment with EPO). As used herein, a therapeutic that "prevents" a disorder or condition refers to a compound that, in a statistical sample, reduces the occurrence of the disorder or condition in 15 the treated sample relative to an untreated control sample, or delays the onset or reduces the severity of one or more symptoms of the disorder or condition relative to the untreated control sample. The term "treating" as used herein includes prophylaxis of the named condition or amelioration or elimination of the condition once it has been established. In either case, prevention or treatment may be discerned in the diagnosis provided by a 20 physician or other health care provider and the intended result of administration of the therapeutic agent. As shown herein. GDF Trap polypeptides, optionally combined with an FPO receptor activator, may be used to increase red blood cell, hemoglobin or reticulocyte levels in healthy individuals, and such GDF Trap polypeptides may be used in selected patient populations. 25 Examples of appropriate patient populations include those with undesirably low red blood cell or hemoglobin levels, such as patients having an anemia, and those that are at risk for developing undesirably low red blood cell or hemoglobin levels, such as those patients that are about to undergo major surgery or other procedures that may result in substantial blood loss. In one embodiment, a patient with adequate red blood cell levels is treated with a GDF 30 Trap polypeptide to increase red blood cell levels, and then blood is drawn and stored for later use in transfusions. 49 GDF Trap polypeptides, optionally combined with an EPO receptor activator, disclosed herein may be used to increase red blood cell levels in patients having an anemia. When observing hemoglobin levels in humans, a level of less than normal for the appropriate age and gender category may be indicative of anemia, although individual variations are 5 taken into account. For example, a hemoglobin level of 12 g/dl is generally considered the lower limit of normal in the general adult population. Potential causes include blood-loss, nutritional deficits, medication reaction, various problems with the bone marrow and many diseases. More particularly, anemia has been associated with a variety of disorders that include, for example, chronic renal failure, myclodysplastic syndrome, rheumatoid arthritis, 10 bone marrow transplantation. Anemia may also be associated with the following conditions: solid tumors (e.g. breast cancer, lung cancer, colon cancer); tumors of the lymphatic system (e.g. chronic lymphocyte leukemia, non-Hodgkins and Hodgkins lymphomas); tumors of the hematopoictic system (e.g. leukemia, myclodysplastic syndrome, multiple myeloma); radiation therapy; chemotherapy (e.g. platinum containing regimens); inflammatory and 15 autoimmune diseases, including, but not limited to, rheumatoid arthritis, other inflammatory arthritides, systemic lupus crythematosis (SLE), acute or chronic skin diseases (e.g. psoriasis), inflammatory bowel disease (e.g. Crohn's disease and ulcerative colitis); acute or chronic renal disease or failure including idiopathic or congenital conditions; acute or chronic liver disease; acute or chronic bleeding; situations where transfusion of red blood cells is not 20 possible due to patient allo- or auto-antibodies and/or for religious reasons (e.g. some Jehovah's Witnesses); infections (e.g. malaria, osteomyelitis); hemoglobinopathies, including, for example, sickle cell disease, thalassemias; drug use or abuse, e.g. alcohol misuse; pediatric patient \ ith anemia from any cause to i old transfusion; and elderly patients or patients with underlying cardiopulmonary disease with anemia who cannot receive 25 transfusions due to concerns about circulatory overload. GDF Trap polypeptides, optionally combined with an EPO receptor activator, would be appropriate for treating anemias of hypoproliferative bone marrow, which are typically associated with little change in red blood cell (RBC) morphology. Hypoproliferative anemias include: 1) anemia of chronic disease, 2) anemia of kidney disease, and 3) anemia associated 30 with hypometabolic states. In each of these types, endogenous erythropoictin levels are inappropriately low for the degree of anemia observed. Other hypoproliferative anemias include: 4) early-stage iron-deficient anemia, and 5) anemia caused by damage to the bone 50 marrow. In these types, endogenous erythropoietin levels are appropriately elevated for the degree of anemia observed. The most common type is anemia of chronic disease, which encompasses inflammation, infection, tissue injury, and conditions such as cancer, and is distinguished by 5 both low erythropoietin levels and an inadequate response to erythropoietin in the bone marrow (Adamson, 2008, Harrison's Principles of Internal Medicine, 17th ed.; McGraw Hill, New York, pp 628-634). Many factors can contribute to cancer-related anemia. Some are associated with the disease process itself and the generation of inflamatory cytokines such as interleukin-1, interferon-gamma, and tumor necrosis factor (Bron et al, 2001, Semin Oncol 10 28(Suppl 8):1-6). Among its effects, inflammation induces the key iron-regulatory peptide hepcidin, thereby inhibiting iron export from macrophages and generally limiting iron availability for crythropoiesis (Ganz, 2007, J Am Soc Nephrol 18:394-400). Blood loss through various routes can also contribute to cancer-related anemia. The prevalence of anemia due to cancer progression varies with cancer type, ranging from 5% in prostate cancer 15 up to 90% in multiple myeloma. Cancer-related anemia has profound consequences for patients, including fatigue and reduced quality of life, reduced treatment efficacy, and increased mortality. Chronic kidney disease is associated with hypoproliferative anemia that varies in severity with the degree of renal impairment. Such anemia is primarily due to inadequate 20 production of erythropoietin and reduced survival of red blood cells. Chronic kidney disease usually proceeds gradually over a period of years or decades to end-stage (Stage-5) disease, at which point dialysis or kidney transplantation is required for patient survival. Anemia often develops early in this process and worsens as disease progresses. The clinical consequences of anemia of kidney disease are well-documented and include development of 25 left ventricular hypertrophy, impaired cognitive function, reduced quality of life, and altered immune function (Levin et al., 1999, Am J Kidney Dis 27:347-354; Nissenson, 1992, Am J Kidney Dis 20(Suppl 1):21-24; Revicki et al., 1995, Am J Kidney Dis 25:548-554; Gafter et al., 1994, Kidney lit 45:224-23 1). As demonstrated by the Applicants in a mouse model of chronic kidney disease (see Example below), a GDF Trap polypeptide, optionally combined 30 with an EPO receptor activator, can be used to treat anemia of kidney disease. Many conditions resulting in a hypometabolic rate can produce a mild-to-moderate hypoproliferative anemia. Among such conditions are endocrine deficiency states. For 51 example, anemia can occur in Addison's disease, hypothyroidism, hyperparathyroidism, or males who are castrated or treated with estrogen. Mild-to-moderate anemia can also occur with reduced dietary intake of protein, a condition particularly prevalent in the elderly. Finally, anemia can develop in patients with chronic liver disease arising from nearly any 5 cause (Adamson, 2008, Harrison's Principles of Internal Medicine, 17th ed.; McGraw Hill, New York, pp 628-634). Anemia resulting from acute blood loss of sufficient volume, such as fiom trauma or postpartum hemorrhage, is known as acute post-hemorrhagic anemia. Acute blood loss initially causes hypovolemia without anemia since there is proportional depletion of RBCs 10 along with other blood constituents. However, hypovolemia will rapidly trigger physiologic mechanisms that shift fluid from the extravascular to the vascular compartment, which results in hemodilution and anemia. If chronic, blood loss gradually depletes body iron stores and eventually leads to iron deficiency. As demonstrated by the Applicants in a mouse model (see Example below), a GDF Trap polypeptide, optionally combined with an EPO receptor 15 activator, can be used to speed recovery from anemia of acute blood loss. Iron-deficiency anemia is the final stage in a graded progression of increasing iron deficiency which includes negative iron balance and iron-deficient crythropoiesis as intermediate stages. Iron deficiency can result from increased iron demand, decreased iron intake, or increased iron loss, as exemplified in conditions such as pregnancy, inadequate 20 diet, intestinal malabsorption, acute or chronic inflammation, and acute or chronic blood loss. With mild-to-moderate anemia of this type, the bone marrow remains hypoproliferative, and RBC morphology is largely normal; however, even mild anemia can result in some microcytic hypochromic RBCs, and the transition to severe iron-deficient anemia is accompanied by Iyperproliferation of the bone marrow and increasingly prevalent microcytic 25 and hypochromic RBCs (Adamson, 2008, Harrison's Principles of Internal Medicine, 17th ed.; McGraw Hill, New York, pp 628-634). Appropriate therapy for iron-deficiency anemia depends on its cause and severity, with oral iron preparations, parenteral iron formulations, and RBC transfusion as major conventional options. A GDF Trap polypeptide, optionally combined with an EPO receptor activator, could be used to treat chronic iron-deficiency 30 anemias alone or in combination with conventional therapeutic approaches, particularly to treat anemias of multifactorial origin. 52 Hypoproliferative anemias can result from primary dysfunction or failure of the bone marrow, instead of dysfunction secondary to inflammation, infection, or cancer progression. Prominent examples would be myclosuppression caused by cancer chemotherapeutic drugs or cancer radiation therapy. A broad review of clinical trials found that mild anemia can occur 5 in 100% of patients after chemotherapy, while more severe anemia can occur in up to 80% of such patients (Groopman et al., 1999, J Natl Cancer Inst 91:1616-1634). Myclosuppressive drugs include: 1) alkylating agents such as nitrogen mustards (e.g., melphalan) and nitrosourcas (e.g., streptozocin); 2) antimetabolites such as folic acid antagonists (e.g., methotrexate), purine analogs (e.g., thioguanine), and pyrimidine analogs (e.g., gemcitabine); 10 3) cytotoxic antibotics such as anthracyclines (e.g., doxorubicin); 4) kinase inhibitors (e.g., gefitinib); 5) mitotic inhibitors such as taxanes (e.g., paclitaxel) and vinca alkaloids (e.g., vinorelbine); 6) monoclonal antibodies (e.g., rituximab); and 7) topoisomerase inhibitors (e.g., topotecan and etoposide). As demonstrated in a mouse model of chemotherapy-induced anemia (see Example below), a GDF Trap polypeptide, optionally combined with an EPO 15 receptor activator, can be used to treat anemia caused by chemotherapeutic agents and/or radiation therapy. GDF Trap polypeptides, optionally combined with an EPO receptor activator, would also be appropriate for treating anemias of disordered RBC maturation, which are characterized in part by undersized (microcytic), oversized (macrocytic), misshapen, or 20 abnormally colored (hypochromic) RBCs. Patients may be treated with a dosing regimen intended to restore the patient to a target hemoglobin level, usually between about 10 g/dl and about 12.5 g/dl, and typically about 11.0 g/dl (see also Jacobs et al. (2000) Nephrol Dial Transplant 15, 15-19), although lower target levels may cause fewer cardiovascular side effects. Alternatively, hematocrit 25 levels (percentage of the volume of a blood sample occupied by the cells) can be used as a measure for the condition of red blood cells. Hematocrit levels for healthy individuals range from 41 to 5 1% for adult males and from 35 to 45% for adult females. Target hematocrit levels are usually around 30-33%. Moreover, hemoglobin/hematocrit levels vary from person to person. Thus, optimally, the target hemoglobin/hematocrit level can be 30 individualized for each patient. In certain embodiments, the present invention provides methods for managing a patient that has been treated with, or is a candidate to be treated with, a GDF Trap 53 polypeptide by measuring one or more hematologic parameters in the patient. The hematologic parameters may be used to evaluate appropriate dosing for a patient who is a candidate to be treated with a GDF Trap polypeptide, to monitor the hematologic parameters during treatment with a GDF Trap polypeptide, to evaluate whether to adjust the dosage 5 during treatment with a GDF Trap polypeptide, and/or to evaluate an appropriate maintenance dose of a GDF Trap polypeptide. If one or more of the hematologic parameters are outside the normal level, dosing with a GDF Trap polypeptide may be reduced, delayed or terminated. Hematologic parameters that may be measured in accordance with the methods 10 provided herein include, for example, red blood cell levels, blood pressure, iron stores, and other agents found in bodily fluids that correlate with increased red blood cell levels, using art recognized methods. Such parameters may be determined using a blood sample from a patient. Increases in red blood cell levels, hemoglobin levels, and/or hematocrit levels may cause increases in blood pressure. 15 In one embodiment, if one or more hematologic parameters are outside the normal range, or on the high side of normal, in a patient who is a candidate to be treated with a GDF Trap polypeptide then onset of administration of the GDF Trap polypeptide may be delayed until the hematologic parameters have returned to a normal or acceptable level either naturally or via therapeutic intervention. For example, if a candidate patient is hypertensive 20 or prehypertensive, then the patient may be treated with a blood pressure lowering agent in order to reduce the patient's blood pressure. Any blood pressure lowering agent appropriate for the individual patient's condition may be used including, for example, diuretics, adrenergic inhibitors (including alpha blockers and beta blockers), vasodilators, calcium channel blockers, angiotensin-converting enzyme (ACE) inhibitors, or angiotensin 11 receptor 25 blockers. Blood pressure may alternatively be treated using a diet and exercise regimen. Similarly, if a candidate patient has iron stores that are lower than normal, or on the low side of normal, then the patient may be treated with an appropriate regimen of diet and/or iron supplements until the patient's iron stores have returned to a normal or acceptable level. For patients having higher than normal red blood cell levels and/or hemoglobin levels, then 30 administration of the GDF Trap polypeptide may be delayed until the levels have returned to a normal or acceptable level. 54 In certain embodiments, if one or more hematologic parameters are outside the normal range, or on the high side of normal, in a patient who is a candidate to be treated with a GDF Trap polypeptide then the onset of administration may be not be delayed. However, the dosage amount or frequency of dosing of the GDF Trap polypeptide may be set at an 5 amount that would reduce the risk of an unacceptable increase in the hematologic parameters arising upon administration of the GDF Trap polypeptide. Alternatively, a therapeutic regimen may be developed for the patient that combines a GDF Trap polypeptide with a therapeutic agent that addresses the undesirable level of the hematologic parameter. For example, if the patient has elevated blood pressure, then a therapeutic regimen involving 10 administration of a GDF Trap polypeptide and a blood pressure lowering agent may be designed. For a patient having lower than desired iron stores, a therapeutic regimen of a GDF Trap polypeptide and iron supplementation may be developed. In one embodiment, baseline parameter(s) for one or more hematologic parameters may be established for a patient who is a candidate to be treated with a GDF Trap 15 polypeptide and an appropriate dosing regimen establish for that patient based on the baseline value(s). Alternatively, established baseline parameters based on a patient's medical history could be used to inform an appropriate GDF Trap polypeptide dosing regimen for a patient. For example, if a healthy patient has an established baseline blood pressure reading that is above the defined normal range it may not be necessary to bring the patient's blood pressure 20 into the range that is considered normal for the general population prior to treatment with the GDF Trap polypeptide. A patient's baseline values for one or more hematologic parameters prior to treatment with a GDF Trap polypeptide may also be used as the relevant comparative values for monitoring any changes to the hematologic parameters during treatment with the GDF Trap polypeptide. 25 In certain embodiments, one or more hematologic parameters are measured in patients who are being treated with a GDF Trap polypeptide. The hematologic parameters may be used to monitor the patient during treatment and permit adjustment or termination of the dosing with the GDF Trap polypeptide or additional dosing with another therapeutic agent. For example, if administration of a GDF Trap polypeptide results in an increase in blood 30 pressure, red blood cell level, or hemoglobin level, or a reduction in iron stores, then the dose of the GDF Trap polypeptide may be reduced in amount or frequency in order to decrease the effects of the GDF Trap polypeptide on the one or more hematologic parameters. If 55 administration or a GDF Trap polypeptide results in a change in one or more hematologic parameters that is adverse to the patient, then the dosing of the GDF Trap polypeptide may be terminated either temporarily, until the hematologic parameter(s) return to an acceptable level, or permanently. Similarly, if one or more hematologic parameters are not brought 5 within an acceptable range after reducing the dose or frequency of administration of the GDF Trap polypeptide then the dosing may be terminated. As an alternative, or in addition to, reducing or terminating the dosing with the GDF Trap polypeptide, the patient may be dosed with an additional therapeutic agent that addresses the undesirable level in the hematologic parameter(s), such as, for example, a blood pressure lowering agent or an iron supplement. 10 For example, if a patient being treated with a GDF Trap polypeptide has elevated blood pressure, then dosing with the GDF Trap polypeptide may continue at the same level and a blood pressure lowering agent is added to the treatment regimen, dosing with the GDF Trap polypeptide may be reduce (e.g., in amount and/or frequency) and a blood pressure lowering agent is added to the treatment regimen, or dosing with the GDF Trap polypeptide may be 15 terminated and the patient may be treated with a blood pressure lowering agent. In certain embodiments, patients being treated with a GDF Trap polypeptide, or candidate patients to be treated with a GDF Trap polypeptide, are patients in need of muscle growth, such as patients suffering from, or at risk of developing, a neuromuscular disorder or musculogenerative disorder. For example, patients or candidate patients may be suffering 20 from, or at risk for developing, Lou Gehrig's disease (ALS), cancer anorexia-cachexia syndrome, muscular dystrophy, muscle atrophy, congestive obstructive pulmonary disease (and muscle wasting associated with COPD), muscle wasting syndrome, sarcopenia, or C1CheXI. Muscular dystrophy feis to a group of dcgeneratic eiusclc dicases characterized by gradual weakening and deterioration of skeletal muscles and sometimes the 25 heart and respiratory muscles. Exemplary muscular dystrophies that can be treated with a regimen including the subject GDF Trap polypeptides include: Duchenne Muscular Dystrophy (DMD), Becker Muscular Dystrophy (BMD), Emery-Dreifuss Muscular Dystrophy (EDMD), Limb-Girdle Muscular Dystrophy (LGMD), Facioscapulohumeral Muscular Dystrophy (FSH or FSHD) (also known as Landouzy-Dejerine), Myotonic 30 Dystrophy (MMD) (also known as Steinert's Disease), Oculopharyngeal Muscular Dystrophy (OPMD), Distal Muscular Dystrophy (DD), Congenital Muscular Dystrophy (CMD). 56 6. Pharmaceutical Compositions In certain embodiments, compounds (e.g., GDF Trap polypeptides) of the present invention are formulated with a pharmaceutically acceptable carrier. For example, a GDF Trap polypeptide can be administered alone or as a component of a pharmaceutical 5 formulation (therapeutic composition). The subject compounds may be formulated for administration in any convenient way for use in human or veterinary medicine. In certain embodiments, the therapeutic method of the invention includes administering the composition systemically, or locally as an implant or device. When administered, the therapeutic composition for use in this invention is, of course, in a pyrogen 10 free, physiologically acceptable fonm. Therapeutically useful agents other than the GDF Trap polypeptides which may also optionally be included in the composition as described above, may be administered simultaneously or sequentially with the subject compounds (e.g., GDF Trap polypeptides) in the methods of the invention. Typically, compounds will be administered parenterally. Pharmaceutical 15 compositions suitable for parenteral administration may comprise one or more GDF Trap polypeptides in combination with one or more pharmaceutically acceptable sterile isotonic aqueous or nonaqueous solutions, dispersions, suspensions or emulsions, or sterile powders which may be reconstituted into sterile injectable solutions or dispersions just prior to use, which may contain antioxidants, buffers, bacteriostats, solutes which render the formulation 20 isotonic with the blood of the intended recipient or suspending or thickening agents. Examples of suitable aqueous and nonaqucous carriers which may be employed in the phrmuaceutica! compositions of the invention include water. ethanol. polyols (such as glycerol, propylene glycol, polyethylene glycol, and the like), and suitable mixtures thereof, vegetable oils, such as olive oil, and injectable organic esters, such as ethyl oleate. Proper 25 fluidity can be maintained, for example, by the use of coating materials, such as lecithin, by the maintenance of the required particle size in the case of dispersions, and by the use of surfactants. Further, the composition may be encapsulated or injected in a form for delivery to a target tissue site (e.g., bone marrow). In certain embodiments, compositions of the present 30 invention may include a matrix capable of delivering one or more therapeutic compounds (e.g., GDF Trap polypeptides) to a target tissue site (e.g., bone marrow), providing a structure for the developing tissue and optimally capable of being resorbed into the body. For 57 example, the matrix may provide slow release of the GDF Trap polypeptides. Such matrices may be formed of materials presently in use for other implanted medical applications. The choice of matrix material is based on biocompatibility, biodegradability, mechanical properties, cosmetic appearance and interface properties. The particular 5 application of the subject compositions will define the appropriate formulation. Potential matrices for the compositions may be biodegradable and chemically defined calcium sulfate, tricalciumphosphate, hydroxyapatite, polylactic acid and polyanhydrides. Other potential materials are biodegradable and biologically well defined, such as bone or dermal collagen. Further matrices are comprised of pure proteins or extracellular matrix components. Other 10 potential matrices are non-biodegradable and chemically defined, such as sintered hydroxyapatite, bioglass, aluminates, or other ceramics. Matrices may be comprised of combinations of any of the above mentioned types of material, such as polylactic acid and hydroxyapatite or collagen and tricalciumphosphate. The bioceramics may be altered in composition, such as in calcium-aluminate-phosphate and processing to alter pore size, 15 particle size, particle shape, and biodegradability. In certain embodiments, methods of the invention can be administered for orally, e.g., in the form of capsules, cachets, pills, tablets, lozenges (using a flavored basis, usually sucrose and acacia or tragacanth), powders, granules, or as a solution or a suspension in an aqueous or non-aqueous liquid, or as an oil-in-water or water-in-oil liquid emulsion, or as an 20 elixir or syrup, or as pastilles (using an inert base, such as gelatin and glycerin, or sucrose and acacia) and/or as mouth washes and the like, each containing a predetermined amount of an agent as an active ingredient. An agent may also be administered as a bolus, electuary or paste. In solid dosage forms for oral administration (capsules, tablets, pills, dragees, 25 powders, granules, and the like), one or more therapeutic compounds of the present invention may be mixed with one or more pharmaceutically acceptable carriers, such as sodium citrate or dicalcium phosphate, and/or any of the following: (1) fillers or extenders, such as starches, lactose, sucrose, glucose, mannitol, and/or silicic acid; (2) binders, such as, for example, carboxymethylcellulose, alginates, gelatin, polyvinyl pyrrolidone, sucrose, and/or acacia; (3) 30 humectants, such as glycerol; (4) disintegrating agents, such as agar-agar, calcium carbonate, potato or tapioca starch, alginic acid, certain silicates, and sodium carbonate; (5) solution retarding agents, such as paraffin; (6) absorption accelerators, such as quaternary ammonium 58 compounds; (7) wetting agents, such as, for example, cetyl alcohol and glycerol monostearate; (8) absorbents, such as kaolin and bentonite clay; (9) lubricants, such a talc, calcium stearate, magnesium stearate, solid polyethylene glycols, sodium lauryl sulfate, and mixtures thereof; and (10) coloring agents. In the case of capsules, tablets and pills, the 5 pharmaceutical compositions may also comprise buffering agents. Solid compositions of a similar type may also be employed as fillers in soft and hard-filled gelatin capsules using such excipients as lactose or milk sugars, as well as high molecular weight polyethylene glycols and the like. Liquid dosage forms for oral administration include pharmaceutically acceptable 10 emulsions, microemulsions, solutions, suspensions, syrups, and elixirs. In addition to the active ingredient, the liquid dosage forms may contain inert diluents commonly used in the art, such as water or other solvents, solubilizing agents and emulsifiers, such as ethyl alcohol, isopropyl alcohol, ethyl carbonate, ethyl acetate, benzyl alcohol, benzyl benzoate, propylene glycol, 1,3-butylene glycol, oils (in particular, cottonseed, groundnut, corn, germ, olive, 15 castor, and sesame oils), glycerol, tetrahydrofuryl alcohol, polyethylene glycols and fatty acid esters of sorbitan, and mixtures thereof. Besides inert diluents, the oral compositions can also include adjuvants such as wetting agents, emulsifying and suspending agents, sweetening, flavoring, coloring, perfuming, and preservative agents. Suspensions, in addition to the active compounds, may contain suspending agents 20 such as ethoxylated isostearyl alcohols, polyoxyethylene sorbitol, and sorbitan esters, microcrystalline cellulose, aluminum metahydroxide, bentonite, agar-agar and tragacanth, and mixtures thereof The compositions of the invention may also contain adjuvants, such as preservatives, wetting agents, emulsifying agents and dispersing agents. Prevention of the action of 25 microorganisms may be ensured by the inclusion of various antibacterial and antiftngal agents, for example, paraben, chlorobutanol, phenol sorbic acid, and the like. It may also be desirable to include isotonic agents, such as sugars, sodium chloride, and the like into the compositions. In addition, prolonged absorption of the injectable pharmaceutical form may be brought about by the inclusion of agents which delay absorption, such as aluminum 30 monostearate and gelatin. It is understood that the dosage regimen will be determined by the attending physician considering various factors which modify the action of the subject compounds of the 59 invention (e.g., GDF Trap polypeptides). The various factors include, but are not limited to, the patient's red blood cell count, hemoglobin level or other diagnostic assessments, the desired target red blood cell count, the patient's age, sex, and diet, the severity of any disease that may be contributing to a depressed red blood cell level, time of administration, and other 5 clinical factors. The addition of other known growth factors to the final composition may also affect the dosage. Progress can be monitored by periodic assessment of red blood cell and hemoglobin levels, as well as assessments of reticulocyte levels and other indicators of the hematopoictic process. In certain embodiments, the present invention also provides gene therapy for the in 10 vivo production of GDF Trap polypeptides. Such therapy would achieve its therapeutic effect by introduction of the GDF Trap polynucleotide sequences into cells or tissues having the disorders as listed above. Delivery of GDF Trap polynucleotide sequences can be achieved using a recombinant expression vector such as a chimeric virus or a colloidal dispersion system. Preferred for therapeutic delivery of GDF Trap polynucleotide sequences is the use 15 of targeted liposomes. Various viral vectors which can be utilized for gene therapy as taught herein include adenovirus, herpes virus, vaccinia, or an RNA virus such as a retrovirus. The retroviral vector may be a derivative of a murine or avian retrovirus. Examples of retroviral vectors in which a single foreign gene can be inserted include, but are not limited to: Moloney murine 20 leukemia virus (MoMuLV), Harvey murine sarcoma virus (HaMuSV), murine mammary tumor virus (MuMTV), and Rous Sarcoma Virus (RSV). A number of additional retroviral vectors can incorporate multiple genes. All of these vectors can transfer or incorporate a gene for a selectable marker so that transduced cells can be identified and generated. Retroviral vectors can be made target-specific by attaching, for example, a sugar, a 25 glycolipid, or a protein. Preferred targeting is accomplished by using an antibody. Those of skill in the ail will recognize that specific polynucleotide sequences can be inserted into the retroviral genome or attached to a viral envelope to allow target specific delivery of the retroviral vector containing the GDF Trap polynucleotide. Alternatively, tissue culture cells can be directly transfected with plasmids encoding 30 the retroviral structural genes gag, pol and env, by conventional calcium phosphate transfection. These cells are then transfected with the vector plasmid containing the genes of interest. The resulting cells release the retroviral vector into the culture medium. 60 Another targeted delivery system for GDF Trap polynucleotides is a colloidal dispersion system. Colloidal dispersion systems include macromolecule complexes, nanocapsules, microspheres, beads, and lipid-based systems including oil-in-water emulsions, micelles, mixed micelles, and liposomes. The preferred colloidal system of this invention is a 5 liposomes. Liposomes are artificial membrane vesicles which are useful as delivery vehicles in vitro and in vivo. RNA, DNA and intact visions can be encapsulated within the aqueous interior and be delivered to cells in a biologically active form (see e.g., Fraley, et al., Trends Biochem. Sci., 6:77, 1981). Methods for efficient gene transfer using a liposome vehicle, are known in the art, see e.g., Mannino, et al., Biotechniques, 6:682, 1988. The composition of 10 the liposome is usually a combination of phospholipids, usually in combination with steroids, especially cholesterol. Other phospholipids or other lipids may also be used. The physical characteristics of liposomes depend on pH, ionic strength, and the presence of divalent cations. Examples of lipids useful in liposome production include phosphatidyl compounds, 15 such as phosphatidylglycerol, phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, sphingolipids, cerebrosides, and gangliosides. Illustrative phospholipids include egg phosphatidylcho line, dipalmitoylphosphatidylcholine, and distearoylphosphatidylcholine. The targeting of liposomes is also possible based on, for example, organ-specificity, cell-specificity, and organelle-specificity and is known in the art. 20 EXEMPLIFICATION fhe invention iow beimg generally described, it will be m6re readily understood by reference to the following examples, which are included merely for purposes of illustration of certain embodiments and embodiments of the present invention, and are not intended to limit 25 the invention. Example 1. Generation of a GDF Trap. Applicants constructed a GDF Trap as follows. A polypeptide having a modified extracellular domain of ActRIIB with greatly reduced activin A binding relative to GDF II and/or myostatin (as a consequence of a leucine-to-aspartate substitution at position 79 in 30 SEQ ID NO: 1) was fused to a human or mouse Fe domain with a minimal linker (three glycine amino acids) in between. The constructs are referred to as ActRIlIB(L79D 20-134) 61 hFc and ActRllB(L79D 20-134)-mFe, respectively. Alternative forms with a glutamate rather than an aspartate at position 79 performed similarly (L79E). Alternative forms with an alanine rather than a valine at position 226 with respect to SEQ ID NO: 7, below were also generated and performed equivalently in all respects tested. The aspartate at position 79 5 (relative to SEQ ID NO: 1, or position 60 relative to SEQ ID NO: 7) is highlighted in gray below. The valine at position 226 relative to SEQ ID NO: 7 is also highlighted in gray below. The GDF Trap ActRIlIB(L79D 20-134)-hFc is shown below as purified from CHO cell lines (SEQ ID NO: 7). 10 GRGEAETRECIYYNANWELERTNQSGLERCEGEQDKRL ICYASWRNSSGTIELVKK GCWDDDFNCYDRQECVATEENPQVYFCCCEGNFCNERFTHLPEAGGPEVTYEPPPT APTGGGTHTCPPCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVK FNWYVDGVEVHNAKTKPREEQYNSTYRVVSVLTVLHQDWLNGKEYKCKVSNKAL PVPIEKTISKAKGOPREPOVYTLPPSREEMTKNOVSLTCLVKGFYPSDIAVEWESNGQ 15 PENNYKTTPPVLDSDGSFFLYSKLTVDKSRWQOGNVFSCSVMHEALHNHYTOKSLS LSPGK The ActRIIB-derived portion of the GDF Trap has an amino acid sequence set forth below (SEQ ID NO: 32), and that portion could be used as a monomer or as a non-Fc fusion protein as a monomer, dimer or greater order complex. 20 GRGEAETRECIYYNANWELERTNQSGLERCEGEQDKRLHCYASWRNSSGTIE LVKKGCWDDDFNCYDRQECVATEENPQVYFCCCEGNFCNERFTH LPEAGGPEVTYE PPPi API (SEQ ID NU: 32) The GDF Trap protein was expressed in CHO cell lines. Three different leader sequences were considered: 25 (i) Honey bee melittin (HBML): MKFLVNVALVFMVVYISYIYA (SEQ ID NO: 8) (ii) Tissue Plasminogen Activator (TPA): MDAMKRGLCCVLLLCGAVFVSP (SEQ ID NO: 9) (iii) Native: MTAPWVALALLWGSLCAGS (SEQ ID NO: 10). The selected forrn employs the TPA leader and has the following unprocessed amino 30 acid sequence: 62 MDAMKRGLCCVLLLCGAVFVSPGASGRGEAETRECIYYNANWELERTNQSGLERCE GEQDKRLHCYASWRNSSGTIELVKKGCWDDDFNCYDRQECVATEENPQVYFCCCE GNFCNERFTHLPEAGGPEVTYEPPPTAPTGGGTHTCPPCPAPELLGGPSVFLFPPKPKD TLMISRTPEVTCVVVDVSHEDPEVKFNWYVDGVEVHNAKTKPREEQYNSTYRVVSV 5 LTVLHQDWLNGKEYKCKVSNKALPVPIEKTISKAKGOPREPOVYTLPPSREEMTKNQ VSLTCLVKGFYPSDIAVEWESNGOPENNYKTTPPVLDSDGSFFLYSKLTVDKSRWQQ GNVFSCSVMHEALHNHYTOKSLSLSPGK (SEQ ID NO: 11) This polypeptide is encoded by the following nucleic acid sequence (SEQ ID NO: 12): A TGGATGCAAT GAAGAGAGGG CTCTGCTGTG TGCTGCTGCT GTGTGGAGCA GTCTTCGTTT 10 CGCCCGGCGC CTCTGGGCGT GGGGAGGCTG AGACACGGGA GTGCATCTAC TACAACGCCA ACTGGGAGCT GGAGCGCACC AACCAGAGCG GCCTGGAGCG CTGCGAAGGC GAGCAGGACA AGCGGCTGCA CTGCTACGCC TCCTGGCGCA ACAGCTCTGG CACCATCGAG CTCGTGAAGA AGGGCTGCTG GGACGATGAC TTCAACTGCT ACGATAGGCA GGAGTGTGTG GCCACTGAGG AGAACCCCCA GGTGTACTTC TGCTGCTGTG AAGGCAACTT CTGCAACGAG CGCTTCACTC 15 ATTTGCCAGA GGCTGGGGGC CCGGAAGTCA CGTACGAGCC ACCCCCGACA GCCCCCACCG GTGGTGGAAC TCACACATGC CCACCGTGCC CAGCACCTGA ACTCCTGGGG GGACCGTCAG TCTTCCTCTT CCCCCCAAAA CCCAAGGACA CCCTCATGAT CTCCCGGACC CCTGAGGTCA CATGCGTGGT GGTGGACGTG AGCCACGAAG ACCCTGAGGT CAAGTTCAAC TGGTACGTGG ACGGCGTGGA GGTGCATAAT GCCAAGACAA AGCCGCGGGA GGAGGTAC AACAGCACGT 20 ACCGTGTGGT CAGCGTCCTC ACCGTCCTGC ACCAGGACTG GCTGAATGGC AAGGAGTACA AGTGCAAGGT CTCCAACAAA GCCCTCCCAG TCCCCATCGA GAAAACCATC TCCAAAGCCA AAGGGCAGCC CCGAGAACCA CAGGTGTACA CCCTGCCCCC ATCCCGGGAG GAGATGACCA AGAACCAGGT CAGCCTGACC TGCCTGGTCA AAGGCTTCTA TCCCAGCGAC ATCGCCGTGG AGTGGGAGAG CAATGGGCAG CCGGAGAACA ACTACAAGAC CACGCCTCCC GTGCTGGACT 25 CCGACGGCTC CTTCTTCCTC TATAGCAAGC TCACCGTGGA CAAGAGCAGG TGGCAGCAGG GGAACGTCTT CTCATGCTCC GTGATGCATG AGGCTCTGCA CAACCACTAC ACGCAGAAGA GCCTCTCCCT GTCTCCGGGT AAATGA Purification could be achieved by a series of column chromatography steps, including, 30 for example, three or more of the following, in any order: protein A chromatography, Q sepharose chromatography, phenylsepharose chromatography, size exclusion chromatography, and cation exchange chromatography. The purification could be completed with viral filtration and buffer exchange. In an example of a purification scheme, the cell culture medium is passed over a protein A column, washed in 150 mM Tris/NaCl (pH 8.0), 35 then washed in 50 mM Tris/NaCl (pH 8.0) and eluted with 0.1 M glycine, pH 3.0. The low pH cluate is kept at room temperature for 30 minutes as a viral clearance step. The eluate is then neutralized and passed over a Q sepharose ion exchange column and washed in 50 mM Tris pH 8,0, 50 mM NaCl, and cluted in 50 mM Tris pH 8.0, with an NaCl concentration between 150 mM and 300 mM. The cluate is then changed into 50 mM Tris pH 8.0, 1.1 M 40 ammonium sulfate and passed over a phenyl sepharose column, washed, and eluted in 50 mM 63 Tris pH 8.0 with ammonium sulfate between 150 and 300 mM. The cluate is dialyzed and filtered for use. Additional GDF Traps (ActRIIB-Fc fusion proteins modified so as to reduce the ratio of activin A binding relative to myostatin or GDF 1) are described in 5 PCT/US2008/001506 and WO 2006/012627, incorporated by reference herein. Example 2. Bioassay for GDF-1I1 and Activin-mediated signaling. An A-204 Reporter Gene Assay was used to evaluate the effects of ActRIIB-Fc proteins and GDF Traps on signaling by GDF-i 1 and Activin A. Cell line: Human 10 Rhabdomyosarcoma (derived from muscle). Reporter vector: pGL3(CAGA) 12 (Described in Dennler et al, 1998, EMBO 17: 3091-3 100). The CAGA12 motif is present in TGF-Beta responsive genes ( PA-i gene) , so this vector is of general use for factors signaling through Smad2 and 3. Day 1: Split A-204 cells into 48-well plate. 15 Day 2: A-204 cells transfected with 10 ug pGL3(CAGA)12 or pGL3(CAGA)12(10 ug)+ pRLCMV (1 ug) and Fugene. Day 3: Add factors (diluted into medium+ 0.1 % BSA). Inhibitors need to be preincubated with Factors for 1 hr before adding to cells. 6 hrs later, cells rinsed with PBS, and lyse cells. 20 This is followed by a Luciferase assay. In the absence of any inhibitors, Activin A showed 10 fold stimulation of reporter gene expression and an ED50 - 2 ng/ml. GDF-I 1: 16 fold stimulation, ED50: - 1.5 ng/ml. ActRIIB(20-134) is a potent inhibitor of activin, GDF-8 and GDF-I I activity in this assay. Variants were tested in this assay as well. 25 Example 3. GDF-1 I Inhibition by N-terminal and C-terminal Truncations Variants of ActRIIB(20-i34)-hFc with truncations at the N-terminus and/or C terminus were generated and tested for activity as inhibitors of GDF- 1 and activin. The activities are shown below (as measured in conditioned media): 64 C-terminal ActRIIB-hFc Truncations: IC50 (ng/mL) GDF- 1 Activin ActRIIB(20-134)-hFc 45 22 ActRIIB(20-132)-hFc 87 32 ActR1lB(20-131)-hFc 120 44 ActRIlIB(20-128)-hFc 130 158 As can be seen, truncations of three (ending with ... PPT), six (ending with ... YEP) or more amino acids at the C-terminus causes a threefold or greater decrease in the activity of 5 the molecule. The truncation of the final 15 amino acids of the ActRIIB portion causes a greater loss of activity (see W02006/0 12627). Amino terminal truncations were made in the background of an ActRIIB(20-131)-hFc protein. The activities are shown below (as measured in conditioned media): N-terminal ActRIIB-hFe Truncations: IC50 (ng/rnL) GDF-1 Activin ActR 13B(20-1 )-hFc 183 201 RG..) ActR1I B(21-13 1)-hFe 121 325 (RGE...) ActRIlB(22-131)-hFc 71 100 (GEA...) d ActRIB(23-13 1)-hFc 0 43 (EAE...) ActRIB(24-13 1)-hFc 9 105 (AET...) 10 Accordingly, truncations of two, three or four amino acids from the N-terminus lead to the production of a more active protein than the versions with a full-length extracellular domain. Additional experiments show that a truncation of five amino acids, ActRIIB(25 13 1)-hFc has activity equivalent to the untruncated form, and additional deletions at the N 65 terminus continue to degrade the activity of the protein. Therefore, optimal constructs will have a C-terminus ending between amino acid 133-134 of SEQ ID NO: I and an N-terminus beginning at amino acids 22-24 of SEQ ID NO: 1. An N-terminus corresponding to amino acids 21 or 25 will give activity that is similar to the ActRIIB(20-134)-hFc construct. These 5 truncations may also be used in the context of GDF Traps, such as an L79D or L79E variant. Example 4. ActRIIB-Fc Variants, Cell-based Activity. Activity of ActRiIB-Fc proteins and GDF Traps was tested in a cell based assay, as described above. Results are summarized in the table below. Some variants were tested in 10 different C-terminal truncation constructs. As discussed above, truncations of five or fifteen amino acids caused reduction in activity. The GDF Traps (L79D and L79E variants) showed substantial loss of activin binding while retaining almost wild-type inhibition of GDF-11. 66 Soluble ActRIIB-Fc binding to GDF11 and Activin A: ActRIIB-Fc Portion of ActRIIB GDFI 1 Inhibition Activin Inhibition Variations (corresponds to amino Activity Activity acids of SEQ ID NO: ) R64 20-134 +++ (approx. 10 M KI) (approx. 10' M K 1 ) A64 20-134 + + (approx. 10- M K 1 ) (approx. 106 M K 1 ) R64 20-129 +++ +++ R64 K74A 20-134 ++++ ++++ R64 A24N 20-134 +++ +++ R64 A24N 20-119 ++ ++ R64 A24N K74A 20-119 + + R64 L79P 20-134 + + R64 L79P K74A 20-134 + + R64 L79D 20-134 +++ + R64 L79E 20-134 +++ + R64K 20-134 ++++++ R64K 20-129 +++++ R64 P129S P130A 20-134 +++ ++ R64N 20-134 + + 5 + Poor activity (roughly 1x10- 6 KI) ++ Moderate activity (roughly 1x10-7 K) ++ Good (wild-type) activity (roughly Ix 1 0- K 1 ) ++++ Greater than wild-type activity 10 Several variants have been assessed for serum half-life in rats. ActRIB(20-1 34)-Fe has a serum half-life of approximately 70 hours. ActRIIB(A24N 20-134)-Fc has a serum half-life of approximately 100-150 hours. The A24N variant has activity in the cell-based assay (above) and in vivo assays (below) that are equivalent to the wild-type molecule. Coupled with the longer half-life, this means that over time an A24N variant will give greater effect 15 per unit of protein than the wild-type molecule. The A24N variant, and any of the other variants tested above, may be combined with the GDF Trap molecules, such as the L79D or L79E variants. 67 Example 5. GDF-1 1 and Activin A Binding. Binding of certain ActR1IB-Fc proteins and GDF Traps to ligands was tested in a BiaCoreFM assay. The ActRIIB-Fc variants or wild-type protein were captured onto the system using an 5 anti-hFc antibody. Ligands were injected and flowed over the captured receptor proteins. Results are summarized in the tables, below. Ligand binding specificity IIB variants. GDF11 Protein Kon (1/Ms) Koff (1/s) KD (M) ActRIIB(20-134)-hFc 1.34e-6 1.13c-4 8.42e-1 1 ActRlIB(A24N 20-134)-hFc 1.21 e-6 6.35c-5 5.19e-11 ActRIlB(L79D 20-134)-hFc 6.7c-5 4.39c-4 6.55c- 10 ActRIlB(L79E 20-134)-hFc 3.8-5 2.74e-4 7.1 6e-10 ActRIIB(R64K 20-134)-hFc 6.77c-5 2.41c-5 3.56c-11 GDF8 Protein Kon (1/Ms) |Koff (1/s) KD (M) ActRIIB(20-134)-hFe 3.69e-5 3.45c-5 9.35e- 11 ActRIIB(A24N 20-134)-hiFe ActRIB(L79D 20-134)-hFc 3.85e-5 8.3e-4 2.15e-9 ActRIIB(L79E 20-134)-hFc 3.74e-5 9C-4 2.41 c-9 ActRIIB(R64K 20-134)-hFc 2.25e-5 4.71 c-5 2.le-10 ActRIB(R64K 20-129)-hFc 9.74e-4 2.09e-4 2.15c--9 ActRIIB(P ]29S, PI30R 20- 1.08e-5 1.8c-4 1.67c-9 134)-hFc ActRIIB(K74A 20-134)-hFc 2.8c-5 2.03e-5 7.18C-11 ________________________________A clivinAk Protein Kon (1/Ms) Koff (1/s) KD (M) ActRlB(20-134)-hFc 5.94e6 1.59e-4 2.68c- 11 ActRIIB(A24N 20-134)-hFe 3.34e6 3.46e-4 1.04e-10 ActRIIB(L79D 20-134)-hFc Low binding ActRIIB(L79E 20-134)-hFc Low binding ActRIIB(R64K 20-134)-hFe 6.82e6 3.25e-4 4.76e-1 I ActRIIB(R64K 20-129)-hFc 7.46e6 6.28e-4 8.41e-I I ActRIIB(P129S, Pl30R 20- 5.02c6 4.17e-4 8.31e-1 I 134)-iFe 10 These data confirm the cell based assay data, demonstrating that the A24N variant retains ligand-binding activity that is similar to that of the ActRIB(20-134)-hFc molecule, 68 and that the L79D or L79E Molecule retains myostatin and GDFI I binding but shows markedly decreased (non-quantifiable) binding to Activin A. Other variants have been generated and tested, as reported in W02006/012627 (incorporated herein by reference in its entirety) see e.g., pp. 59-60, using ligands coupled to 5 the device and flowing receptor over the coupled ligands. Notably, K74Y, K74F, K741 (and presumably other hydrophobic substitutions at K74, such as K74L), and D801, cause a decrease in the ratio of Activin A binding to GDFI 1 binding, relative to the wild-type K74 molecule. A table of data with respect to these variants is reproduced below: Soluble ActRIIB-Fc variants binding to GDF1 1 and Activin A (BiaCore assay) 10 ActRIIB ActA GDF1 1 WT (64A) KD=1.8c-7M KD= 2.6e-7M (+) (+) WT (64R) na KD= 8.6e-8M +I5tail KD -2.6 c-8M KD= 1.9c-8M E37A * * R40A D54A * K55A ++ * R56A * * K74A KD=4.35c-9 M KD=5.3c-9M K74Y * K74F * K741 * W78A * * L79A + * D80K * * D80R * * D80A * * D80F * * D80G * * D80M * * D80N * * D80 * F82A ++ * No observed binding -- < 1/5 WTbinding 15 - 1/2 WT binding + WT 69 ++ < 2x increased binding +++ ~5x increased binding ++++ -Ox increased binding +++++~ 40x increased binding 5 Example 6. ActRIIB-hFc Stimulates Erythropoiesis in Non-Human Primates ActRIIB(20-134)-hFc (IgG1) was administered once a week for I month to male and female cynomolgus monkeys by subcutaneous injection. Forty-eight cynomolgus monkeys (24/sex) were assigned to one of four treatment groups (6 animals/sex/group) and were 10 administered subcutaneous injections of either vehicle or ActRIIB-hFe at 3, 10, or 30 mg/kg once weekly for 4 weeks (total of 5 doses). Parameters evaluated included general clinical pathology (hematology, clinical chemistry, coagulation, and urinalysis). ActRIIB-hFc caused statistically significant elevated mean absolute reticulocyte values by day 15 in treated animals. By day 36, ActRIIB-hFc caused several hematological changes, including elevated 15 mean absolute reticulocyte and red blood cell distribution width values and lower mean corpuscular hemoglobin concentration. All treated groups and both sexes were affected. These effects are consistent with a positive effect of ActRIIB-hFc on the release of immature reticulocytes from the bone marrow. This effect was reversed after drug was washed out of the treated animals (by study day 56). Accordingly, we conclude that ActRIIB-hFc 20 stimulates erythropoicsis. Example 7. ActRIIB-mFc Promotes Aspects of Erythropoiesis in Mice by Stimulation of Splenic Erythropoietic Activities In this study the effects of the in vivo administration of ActRilB(20-134)-mFc on the 25 frequency of hematopoictic progenitors in bone marrow and spleen was analyzed. One group of C57BL/6 mice was injected with PBS as a control and a second group of mice administered two doses of ActRiIB-mFc at 10 mg/kg and both groups sacrificed after 8 days. Peripheral blood was used to perform complete blood counts and femurs and spleens were used to perform in vitro clonogenic assays to assess the lymphoid, erythroid and mycloid 30 progenitor cell content in each organ. In the brief time frame of this study, no significant changes were seen in red blood cell, hemoglobin or white blood cell levels in treated mice. In the femurs there was no difference in the nucleated cell numbers or progenitor content between the control and treated groups. In the spleens, the compound treated group 70 experienced a statistically significant increase in the mature erythroid progenitor (CFU-E) colony number per dish, frequency and total progenitor number per spleen. In addition, and increase was seen in the number of mycloid (CFU-GM), immature erythroid (BFU-E) and total progenitor number per spleen. 5 Animals: Sixteen C57BL/6 female mice 6-8 weeks of age were used in the study. Eight mice were injected subcutaneously with test compound ActRIIB-mFc at days I and 3 at a dose of 10 mg/kg and eight mice were injected subcutaneously with vehicle control, phosphate buffered saline (PBS), at a volume of 100 ttL per mouse. All mice were sacrificed 8 days 10 after First injection in accordance with the relevant Animal Care Guidelines. Peripheral blood (PB) samples from individual animals were collected by cardiac puncture and used for complete blood counts and differential (CBC/Diff). Femurs and spleens were harvested from each mouse. Tests performed: 15 CBC/Diff Counts PB from each mouse was collected via cardiac puncture and placed into the appropriate microtainer tubes. Samples were sent to CLV for analysis on a CellDyn 3500 counter. Clonogenic Assays 20 Clonogenic progenitors of the mycloid, erythroid and lymphoid lineages were asCsssed using the in vitro methyicellulose-based media system descubed below. Mature Erythroid Progenitors: Clonogenic progenitors of the mature crythroid (CFU-E) lineages were cultured in MethoCultTM 3334, a methylcellulose-based medium containing recombinant human (rh) 25 Erythropoietin (3 U/mL). 71 Lymphoid Progenitors: Clonogenic progenitors of the lymphoid (CFU-pre-B) lineage were cultured in MethoCult@ 3630, a methylcellulose-based medium containing rh Interleukin 7 (10 ng/mL). Myeloid and Immature Erythroid Progenitors: 5 Clonogenic progenitors of the granulocyte-monocyte (CFU-GM), erythroid (BFU-E) and multipotential (CFU-GEMM) lineages were cultured in MethoCultTM 3434, a methylcellulose-based medium containing recombinant murine (rm) Stem Cell Factor (50 ng/mL), rh Interleukin 6 (10 ng/mL), rm Interleukin 3 (10 ng/mL) and rh Erythropoietin (3 U/mL). 10 Methods: Mouse femurs and spleens were processed by standard protocols. Briefly, bone marrow was obtained by flushing the femoral cavity with Iscove's Modified Dulbecco's Media containing 2% fetal bovine serum (IMDM 2% FBS) using a 21 gauge needle and 1 cc syringe. Spleen cells were obtained by crushing spleens through a 70 jiM filter and rinsing 15 the filter with IMDM 2% FBS, Nucleated cell counts in 3% glacial acetic acid were then performed on the single cells suspensions using a Neubauer counting chamber so that the total cells per organ could be calculated. To remove contaminating red blood cells, total spleen cells were then diluted with 3 times the volume of ammonium chloride lysis buffer and incubated on ice 10 minutes. The cells were then washed and resuspended in IMDM 2% 20 FBS and a second cell count were performed to determine the cell concentration of cells after lysis. Cell stocks were made and added to each methylcellulose-based media formulation to obtain the optimal plating concentrations for each tissue in each media formulation. Bone marrow cells were plated at lx105 cells per dish in MethoCultTM 3334 to assess mature 25 erythroid progenitors, 2x 105 cells per dish in MethoCultTM 3630 to assess lymphoid progenitors and 3x 104 cells per dish in MethoCultTM 3434 to assess immature crythroid and mycloid progenitors. Spleen cells were plated at 4x10 5 cells per dish in MethoCultTM 3334 to assess mature crythroid progenitors, 4x10 5 cells per dish in MethoCultTM 3630 to assess lymphoid progenitors and 2x 10- cells per dish in MethoCultTM 3434 to assess immature 30 erythroid and myeloid progenitors. Cultures plated in triplicate dishes were incubated at 37 0 C, 5% C02 until colony enumeration and evaluation was performed by trained personnel. 72 Mature erythroid progenitors were cultured for 2 days, lymphoid progenitors were cultured for 7 days and mature erythroid and myeloid progenitors were cultured for 12 days. Analysis: The mean +/- 1 standard deviation was calculated for the triplicate cultures of the 5 clonogenic assays and for the control and treatment groups for all data sets. Frequency of colony forming cells (CFC) in each tissue was calculated as follows: Cells plated per dish Mean CFC scored per dish Total CFC per femur or spleen was calculated as follows: 10 Total CFC scored x nucleated cell count per femur or spleen (following RBC lysis) Number of nucleated cells cultured Standard t-tests were performed to assess if there was a differences in the mean number of cells or hematopoictic progenitors between the PBS control mice and compound treated mice. Due to the potential subjectivity of colony enumeration, a p value of less than 15 0.01 is deemed significant. Mean values (+/- SD) for each group are shown in the tables below. Table: Hematologic Parameters Treatment White Blood ] Red Blood Cells Hemoglobin Hematocrit Group Celis (x104'L) (x1 I0/L) (gL) (L/L) PBS 9.53 +/- 1.44 10.5 +/- 1.1 160.9+/ 13.3 0.552 +/- 0.057 (n=8) ActRIIB-mFc 9.77+ 1.19 10.8+/-0.3 162.1 +4.1 0.567+/-0.019 (n=8) 20 73 Table: CFC From Femur and Spleen Treatment Total CFC per Total CFC per Total CFU-E per Total CFU-E per Group Femur Spleen Femur Spleen PBS 88+/-10 54+/-14 156+/-27 131+/-71 (n=8) ActRIB-mFc 85 +/-9 79 +/- 6* 164+/- 23 436 +/- 86* (n=8) 1 1 1 1 1 * preliminary analysis indicates p<0.05 Treatment of mice with ActRIIB(20-134)-mFc, in the brief time frame of this study, did not result in significant increases in red blood cell or hemoglobin content. However, the 5 effect on progenitor cell content was notable. In the femurs there was no difference in the nucleated cell numbers or progenitor content between the control and treated groups. In the spleens, the compound treated group experienced a statistically significant increase in the nucleated cell number before red blood cell lysis and in the mature crythroid progenitor (CFU-E) colony number per dish, frequency and total progenitor number per spleen. In 10 addition, an increase was seen in the number of mycloid (CFU-GM), immature crythroid (BFU-E) and total progenitor number per spleen. Accordingly, it is expected that over a longer time course, ActRIB(20-134)-mFc treatment may result in elevated red blood cell and hemoglobin content. 15 Example 8: A GDF Trap Increases Red Blood Cell Levels in vivo Twelve-week-old male C57BL/6NTac mice were assigned to one of two treatment groups (N=10). Mice were dosed with either vehicle or with a variant ActR11B polypeptide ("GDF Trap") [ActRIlB(L79D 20-134)-hFe] by subcutaneous injection (SC) at 16 mg/kg twice per week for 4 weeks. At study termination, whole blood was collected by cardiac 20 puncture into EDTA containing tubes and analyzed for cell distribution using an HM2 hematology analyzer (Abaxis, Inc). Grou Designation Group N Mice Injection Dose Route Frequency I_ _ (ng/kg) 1 10 C57BL/6 PBS 0 SC Twice/week GDF Trap 2 10 C57BL/6 [ActRIIB(L79D 10 SC Twice/week 20-134)-hFc) 74 Treatment with the GDF Trap did not have a statistically significant effect on the number of white blood cells (WBC) compared to the vehicle controls. Red blood cell (RBC) numbers were increased in the treated group relative to the controls (see table below). Both the hemoglobin content (HGB) and hematocrit (HCT) were also increased due to the 5 additional red blood cells. The average width of the red blood cells (RDWc) was higher in the treated animals, indicating an increase in the pool of immature red blood cells. Therefore, treatment with the GDF Trap leads to increases in red blood cells, with no distinguishable effects on white blood cell populations. 10 Hematology Results RBC HGB HCT RDWc 14O/L (g/dL) (%) (%) PBS 10.7 0.1 14.8 0.6 44.8 0.4 17.0 + 0.1 GDF Trap 12.4+ 17.0 48.8+ 1.8* 18.4 0.4** 0.7* 0.2** *=peZO.
0 5 , * p<O.OI Example 9: A GDF Trap is Superior to ActRIIB-Fc for Increasing Red Blood Cell Levels In vivo. 15 Nineteen-week-old male C57BL/6NTac mice were randomly assigned to one of three groups. Mice were dosed with vehicle (10 mM Tris Buffered Saline, TBS), wild-type ActRIlB(20-134)-mFc, or GDF trap ActRIIB(L79D 20-134)-hFc by subcutaneous injection twice per week for three weeks. Blood was collected cheek bleed at baseline and after three weeks of dosing and analyzed for cell distribution using a hematology analyzer (HM2, 20 Abaxis, Inc.) Treatment with ActRIIB-Fc or the GDF trap did not have a significant effect on white blood cell (WBC) numbers compared to vehicle controls. The red blood cell count (RBC), hematocrit (HCT), and hemoglobin levels were all elevated in GDF Trap treated mice compared to either the controls or the wild-type construct (see table below). Therefore, in a 25 direct comparison, the GDF trap promotes increases in red blood cells to a significantly greater extent than a wild-type ActRIIB-Fc protein. In fact, in this experiment, the wild-type ActRIIB-Fc protein did not cause a statistically significant increase in red blood cells, suggesting that longer or higher dosing would be needed to reveal this effect. 75 Hematology Results after three weeks of dosing RBC HCT H GB (10 2 /ml) % ........... g/dL TBS 11.06 ± 0.46 46.78+ 1.9 15.7 ± 0.7 ActRHIB-mFe 11.64 ± 0.09 j 49.03 ±0.3 16.5 ± 1.5 GDF Trap 13.19 ±0.2** j 53.04±0.8** 18.4 ±0.3** **=p<0.01 Example 10. Generation of a GDF Trap with Truncated ActRIUB Extracellular Domain 5 As described in Example 1, a GDF Trap referred to as ActRIlB(L79D 20-134)-hFe was generated by N-terminal fusion of TPA leader to the ActRIIB extracellular domain (residues 20-134 in SEQ ID NO: 1) containing a leucine-to-aspartate substitution (at residue 79 in SEQ ID NO: 1) and C-terminal fusion of human FEc domain with minimal linker (three glycine residues) (Figure 3). A nucleotide sequence corresponding to this fusion protein is 10 shown in Figure 4. A GDF Trap with truncated ActRIIB extracellular domain, referred to as ActRIB(L79D 25-13 1)-hFc, was generated by N-terminal fusion of TPA leader to truncated extracellular domain (residues 25-131 in SEQ ID NO: 1) containing a leucine-to-aspartate 15 substitution (at residue 79 in SEQ ID NO: 1) and C-terminal fusion of human Fe domain with minimal linker (three glycine residues) (Figure 5). A nucleotide sequence corresponding to this fusion protein is shown in Figure 6. Example 11. Selective Ligand Binding by GDF Trap with Double-Truncated ActRlIB 20 Extracelluar Domain The affinity of GDF Traps and other ActRIlB-hFc proteins for several ligands was evaluated in vitro with a BiacoreTm instrument. Results are summarized in the table below. Kd values were obtained by steady-state affinity fit due to the very rapid association and dissociation of the complex, which prevented accurate determination of ko 0 and korr. 76 Ligand Selectivity of ActRIIB-hFc Variants: Fusion Construct Activin A Activin B GDF11 (Kd e-11) (Kd e- 11) (Kd e-11) ActRIB(L79 20-134)-hFc 1.6 1.2 3.6 ActRIIB(L79D 20-134)-hFc 1350.0 78.8 12.3 ActRIIB(L79 25-13.1)-hFc 1.8 1.2 3.1 ActRIB(L79D 25-131)-hFc 2290.0 62.1 7.4 5 The GDF Trap with a truncated extracellular domain, ActRIIB(L79D 25-131)-hFec, equaled or surpassed the ligand selectivity displayed by the longer variant, ActRIIB(L79D 20-134)-hFc, with pronounced loss of activin A and activin B binding and nearly full retention of GDFI 1 binding compared to ActRIIB-hFc counterparts lacking the L79D substitution. Note that truncation alone (without L79D substitution) did not alter selectivity 10 among the ligands displayed here [compare ActRIIB(L79 25-13 1)-hFe with ActRIIB(L79 20-134)-hFc]. Example 12. Generation of ActRIIB(L79D 25-131)-hFe with Alternative Nucleotide Sequences 15 To generate ActRIIB(L79D 25-13 l)-hFc, the human ActRIlB extracellular domain with an aspartate substitution at native position 79 (SEQ ID NO: 1) and with N-terminal and C-terminal truncations (residues 25-131 in SEQ ID NO: 1) was fused N-terminally with a TPA leader sequence instead of the native ActRIIB leader and C-terminally with a human Fc domain via a minimal linker (three glycine residues) (Figure 5). One nucleotide sequence 20 encoding thi fusion protein is shown ii Figu ire 6 (SFQ ID NO: 27). and an alternative . nucleotide sequence encoding exactly the same fusion protein is shown in Figure 9 (SEQ ID NO: 30). This protein was expressed and purified using the methodology described in Example 1. 25 Example 13. GDF Trap with a Truncated ActRIIB Extracellular Domain Increases Proliferation of Erythroid Progenitors in Mice ActRIIB(L79D 25-131)-hFc was evaluated to determine its effect on proliferation of erythroid progenitors. Male C57BL/6 mice (8 weeks old) were treated with ActRIIB(L79D 25-13 1)-hFc (10 mg/kg, s.c.; n = 6) or vehicle (TBS; n = 6) on Days 1 and 4, then euthanized 30 on Day 8 for collection of spleens, tibias, femurs, and blood. Cells of the spleen and bone 77 marrow were isolated, diluted in Iscove's modified Dulbecco's medium containing 5% fetal bovine serum, suspended in specialized methylcellulose-based medium, and cultured for either 2 or 12 days to assess levels of clonogenic progenitors at the colony-forming unit erythroid (CFU-E) and burst forming unit-erythroid (BFU-E) stages, respectively. 5 Methylcellulose-based medium for BFU-E determination (MethoCult M3434, Stem Cell Technologies) included recombinant marine stern cell factor, interleukin-3, and interleukin-6, which were not present in methylcellulose medium for CFU-E determination (MethoCult M3334, Stem Cell Technologies), while both media contained crythropoictin, among other constituents. For both BFU-E and CFU-E, the number of colonies were determined in 10 duplicate culture plates derived from each tissue sample, and statistical analysis of the results was based on the number of mice per treatment group. Spleen-derived cultures from mice treated with ActRIIB(L79D 25-131)-hFe had twice the number of CFU-E colonies as did corresponding cultures from control mice (P < 0.05), whereas the number of BFU-E colonies did not differ significantly with treatment in vivo. 15 The number of CFU-E or BFU-E colonies from bone marrow cultures also did not differ significantly with treatment. As expected, increased numbers of CFU-E colonies in spleen derived cultures were accompanied by highly significant (P < 0.001) changes in red blood cell level (11.6% increase), hemoglobin concentration (12% increase), and hematocrit level (11.6% increase) at euthanasia in mice treated with ActRIIB(L79D 25-131)-hFc compared to 20 controls. These results indicate that in vivo administration of a GDF Trap with truncated ActRIB extracellular domain can stimulate proliferation of crythroid progenitors as part of its overall effect to increase red blood cell levels. Example 14. GDF Trap with a Truncated ActRIIB Extracellular Domain Offsets 25 Chemotherapy-Induced Anemia in Mice Applicants investigated the effect of ActRIIB(L79D 25-131)-hFc on crythropoictic parameters in a mouse model of chemotherapy-induced anemia based on paclitaxel, which inhibits cell division by blocking microtubule polymerization. Male C57BL/6 mice (8 weeks old) were assigned to one of four treatments: 30 1) paclitaxel (25 mg/kg, i.p.) 2) ActRIIB(L79D 25-13l)-hFc (10 mg/kg, i.p.) 3) paclitaxel + ActRIIB(L79D 25-131)-hFe 4) vehicle (TBS). 78 Paclitaxel was administered on Day 0, while ActRIIB(L79D 25-131)-hFe or vehicle were administered on Days 0 and 3. Blood samples were collected for CBC analysis from separate cohorts on Days 1, 3, and 5, and results for treatment groups 1-3 (above) were expressed as percent difference from vehicle at a given time point. Attrition due to paclitaxel toxicity was 5 an issue in the paclitaxel-only cohort on Day 3 (where n = 1); otherwise, n = 3-5 per treatment per time point. Compared to vehicle, paclitaxel alone decreased hemoglobin concentration by nearly 13% at Day 5, whereas addition of ActRIIB(L79D 25-131)-hFc prevented this paclitaxcl-induced decline (Figure 11). Similar effects were observed for hematocrit and RBC levels. In the absence of paclitaxel, ActRIIB(L79D 25-13 1)-hFc 10 increased hemoglobin concentration by 10% compared to vehicle on Days 3 and 5 (Figure 11). Thus, a GDF Trap with truncated ActRIIB extracellular domain can increase levels of red blood cells sufficiently to offset chemotherapy-induced anemia. Example 15. GDF Trap with a Truncated ActRIIB Extracellular Domain Reverses 15 Nephrectomy-Induced Anemia in Mice Applicants investigated the effect of ActRIIB(L79D 25-13 1)-hFc on anemia in a nephrectomized mouse model of chronic kidney disease. Male C57BL/6 mice (11 weeks old) underwent either a sham operation or a unilateral nephrectomy to reduce the capacity for crythropoietin production. Mice were allowed a week for postsurgical recovery and then 20 treated twice-weekly with ActRilB(L79D 25-13 1)-hFe (10 mg/kg, i.p.; n = 15 per condition) or vehicle (TBS; n = 15 per condition) for a total of 4 weeks. Blood samples were collected before the onset of dosing and after 4 weeks of treatment. Whereas vehicle-treated nepliretuized mice dizspluycd a sigificanvdeelinc in red bloud cell number o\ er the 4 week treatment period, treatment with ActRIIB(L79D 25-13 1)-liFc not only prevented the 25 decline but increased red blood cell levels 17% (P < 0.001) above baseline (Figure 12), despite reduced renal capacity for erythropoietin production. In nephrectomized mice, ActRIIB(L79D 25-13 l)-hFc also generated significant increases from baseline in hemoglobin concentration and hematocrit level and, notably, stimulated each of these crythropoietic parameters to approximately the same extent under nephrectomized conditions as under 30 sham-operated conditions (Figure 13). Thus, a GDF Trap with truncated ActRIIB extracellular domain can increase red blood cell levels sufficiently to reverse anemia in a model of chronic kidney disease. 79 Example 16. GDF Trap with a Truncated ActRIIB Extracellular Domain Improves Recovery from Anemia Induced by Blood Loss in Rats Applicants investigated the effect of ActRIIB(L79D 25-131 )-hFc on crythropoietic parameters in a rat model of anemia induced by acute blood loss (acute post-hemorrhagic 5 anemia). Male Sprague-Dawley rats (approximately 300 g) received a chronic jugular catheter at the vendor (Harlan). On Day -1, 20% of total blood volume was withdrawn from each rat over a 5-minute period via the catheter under isoflurane anesthesia. The volume of blood removed was based on a value for total blood volume calculated according to the following relationship derived by Lee and co-workers (J Nucl Med 25:72-76, 1985) for rats 10 with body weight greater than 120 g: Total blood volume (ml)= 0.062 x body weight (g) + 0.0012 An equal volume of phosphate-buffered saline was replaced via the catheter at the time of blood removal. Rats were treated with ActRIIB(L79D 25-131)-hFc (10 mg/kg, s.c.; n = 5) or vehicle (TBS; n = 5) on Days 0 and 3. Blood samples for CBC analysis were removed via 15 the catheter on Days -I (baseline), 0, 2, 4, and 6. Control rats responded to 20% blood loss with a drop of nearly 15% in red-blood-cell levels by Day 0. These levels remained significantly lower than baseline on Days 2 and 4, and had not recovered fully by Day 6 (Figure 14). Although rats treated with ActRIIB(L79D 25-131)-hFc showed a nearly identical drop in red-blood-cell levels after 20% blood loss, 20 these rats then displayed a complete recovery in such levels by Day 2, followed by further elevation on Days 4 and 6, which represents a highly significant improvement over control levels at the corresponding time points (Figure 14). Similar results were obtained for hemoglobin concentration. These findings demonstrate that a GDF Trap with truncated ActRlUB extracellular domain can produce a faster recovery of red blood cell levels from 25 anemia caused by acute hemorrhage. Example 17. GDF Trap with Truncated ActRIIB Extracelluar Domain Increases Levels of Red Blood Cells in Non-Human Primates Two GDF Traps, ActRIIB(L79D 20-1 34)-hFc and ActRIIB(L79D 25-13 I)-hFe, were 30 evaluated for their ability to stimulate red blood cell production in cynomolgus monkey. Monkeys were treated subcutaneously with GDF Trap (10 mg/kg; n = 4 males/4 females), or vehicle (n = 2 males/2 females) on Days I and 8. Blood samples were collected on Days 1 80 (pretreatment baseline), 3, 8, 15, 29, and 44, and were analyzed for red blood cell levels (Figure 15), hematocrit (Figure 16), hemoglobin levels (Figure 17), and reticulocyte levels (Figure 18). Vehicle-treated monkeys exhibited decreased levels of red blood cells, hematocrit, and hemoglobin at all post-treatment time points, an expected effect of repeated 5 blood sampling. In contrast, treatment with ActRIIB(L79D 20-134)-hFc or ActRiIB(L79D 25-131)-hFe increased these parameters by the first post-treatment time point (Day 3) and maintained them at substantially elevated levels for the duration of the study (Figures 15-17). Importantly, reticulocyte levels in monkeys treated with ActRIIB(L79D 20-134)-hFe or ActRIIB(L79D 25-13 1)-hFe were substantially increased at Days 8, 15, and 29 compared to 10 vehicle (Figure 18). This result demonstrates that GDF Trap treatment increased production of red blood cell precursors, resulting in elevated red blood cell levels. Taken together, these data demonstrate that truncated GDF Traps, as well as a fill length variants, can be used as selective antagonists of GDFI I and potentially related ligands to increase red blood cell formation in vivo. 15 Example 18. GDF Trap Derived from ActRIIB5 Others have reported an alternate, soluble form of ActRIIB (designated ActRIIB5), in which exon 4, including the ActRIlB transmembrane domain, has been replaced by a different C-terminal sequence (W02007/053775). 20 The sequence of native human ActRIIB5 without its leader is as follows: GRGEAETRECI YYNANWELERTNQSGLERCEGEQDKRLHCYASWRNS SGT IELVK TIPSGGPEATAAAGDQGSGALWLCLEGPAHE (SEQ ID NO: 36) 25 An leucine-to-aspartate substitution, or other acidic substitutions, may be performed at native position 79 (underlined and highlighted in grey) as described to construct the variant ActRIIB5(L79D), which has the following sequence: GRGEAETRECIYYNANWELERTNQSGLERCEGEQDKRLHCYASWRNSSGTIELVK KGCWDDDFNCYDRQECVATEENPQVYFCCCEGNFCNERFTHL PEAGGPEGPWAST 30 TIPSGGPEATAAAGDQGSGALWLCLEGPAHE (SEQ ID NO: 37) 81 This variant may be connected to human Fc with a TGGG linker to generate a human ActRIIB5(L79D)-hFc fusion protein with the following sequence: GRGEAETRECIYYNANWELERTNQSGLERCEGEQDKRLHCYASWRNSSGTIELVK KGCWDDDFNCY DRQECVATEENPQVYFCCCEGNFCNERFTHLPEAGGPEGPWAST 5 TI PSGGPEATAAAGDQGSGALWLCLEGPAHETGCGTHTCPPCPAPELLGGPSVFL FPPKPKDTL4ISRTPEVTCVVVDVSHEDPEVKFNWYVDGVEVHNAKTKPREEQYN STYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQVYTLP PSREEMTKNQVSLTCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLY SKLTVDKSRWQQGNVFSCSVMHEALHNHYTQKSLSLSPGK (SEQ ID NO: 38) 10 This construct may be expressed in CHO cells. Example 19. Effects in Mice of Combined Treatment with EPO and a GDF Trap with a Truncated ActRIIB Extracellular Domain EPO induces formation of red blood cells by increasing the proliferation of erythroid 15 precursors, whereas GDF Traps could potentially affect formation of red blood cells in ways that complement or enhance EPO's effects. Therefore, Applicants investigated the effect of combined treatment with EPO and ActRIB(L79D 25-13 l)-hFc on erythropoictic parameters. Male C57BL/6 mice (9 weeks old) were given a single i.p. injection of recombinant human EPO alone (epoetin alfa, 1800 units/kg), ActRIIB(L79D 25-13 1)-hFc alone (10 mg/kg), both 20 EPO and ActRIIB(L79D 25-13 1)-hFc, or vehicle (Tris-buffered saline). Mice were euthanized 72 h after dosing for collection of blood, spleens, and femurs. Spleens and femurs were processed to obtain erythroid precursor cells for flow cytometric analysis. After removal, the spleen was minced in Iscove's modified Dulbecco's medium containing 5% fetal bovine serum and mechanically dissociated by pushing through 25 a 70-am cell strainer with the plunger from a sterile l-mL syringe. Femurs were cleaned of any residual muscle or connective tissue and ends were trimmed to permit collection of marrow by flushing the remaining shaft with Iscove's modified Dulbecco's medium containing 5% fetal bovine serum through a 21-gauge needle connected to a 3-mL syringe. Cell suspensions were centrifuged (2000 rpm for 10 min) and the cell pellets resuspended in 30 PBS containing 5% fetal bovine serum. Cells (106) from each tissue were incubated with anti-mouse IgG to block nonspecific binding, then incubated with fluorescently labeled 82 antibodies against mouse cell-surface markers CD71 (transferrin receptor) and Ter 19 (an antigen associated with cell-surface glycophorin A), washed, and analyzed by flow cytrometry. Dead cells in the samples were excluded from analysis by counterstaining with propidium iodide. Erythroid differentiation in spleen or bone marrow was assessed by the 5 degree of CD71 labeling, which decreases over the course of differentiation, and Ter 19 labeling, which is increased during terminal crythroid differentiation beginning with the proerythroblast stage (Socolovsky et al., 2001, Blood 98:3261-3273; Ying et al., 2006, Blood 108:123-133). Thus, flow cytometry was used to determine the number of procrythroblasts (CD7 I ""'"Ter 191"'), basophilic erythroblasts (CD7 I3h"Terl 19 h''1), polychromatophilic + 10 orthochromatophilic erythroblasts (CD71md Terl 19 h), and late orthochromatophilic erythroblasts + reticulocytes (CD7 I*"Terl 191""), as described. Combined treatment with EPO and ActRilB(L79D 25-13 1)-hFc led to a surprisingly vigorous increase in red blood cells. In the 72-h time frame of this experiment, neither EPO nor ActRIIB(L79D 25-13 1)-hFc alone increased hematocrit significantly compared to 15 vehicle, whereas combined treatment with the two agents led to a nearly 25% increase in hematocrit that was unexpectedly synergistic, i.e., greater than the sum of their separate effects (Figure 19). Synergy of this type is generally considered evidence that individual agents are acting through different cellular mechanisms. Similar results were also observed for hemoglobin concentrations (Figure 20) and red blood cell concentrations (Figure 21), 20 each of which was also increased synergistically by combined treatment. Analysis of erythroid precursor levels revealed a more complex pattern. In the mouse, the spleen is considered the primary organ responsible for inducible ("stress") erythropoiesis. Flow cytometric analysis of splenic tissue at 72 h revealed that EPO markedly altered the erythropoietic precursor profile compared to vehicle, increasing the 25 number of basophilic erythroblasts by more than 170% at the expense of late precursors (late orthochromatophilic erythroblasts + reticulocytes), which decreased by more than one third (Figure 22). Importantly, combined treatment increased basophilic crythroblasts significantly compared to vehicle, but to a lesser extent than EPO alone, while supporting undiminished maturation of late-stage precursors (Figure 22). Thus, combined treatment 30 with EPO and AetRIIB(L79D 25-13 1)-hFc increased erythropoiesis through a balanced enhancement of precursor proliferation and maturation. In contrast to spleen, the precursor cell profile in bone marrow after combined treatment did not differ appreciably from that 83 after EPO alone. Applicants predict from the splenic precursor profile that combined treatment would lead to increased reticulocyte levels and would be accompanied by sustained elevation of mature red blood cell levels, if the experiment were extended beyond 72 h. Taken together, these findings demonstrate that a GDF Trap with a truncated ActRiIB 5 extracellular domain can be administered in combination with EPO to synergistically increase red blood cell formation in vivo. Acting through a complementary but undefined mechanism, a GDF trap can moderate the strong proliferative effect of an EPO receptor activator alone and still permit target levels of red blood cells to be attained with lower doses of an EPO receptor activator, thereby avoiding potential adverse effects or other problems associated 10 with higher levels of EPO receptor activation. INCORPORATION BY REFERENCE All publications and patents mentioned herein are hereby incorporated by reference in their entirety as if each individual publication or patent was specifically and individually 15 indicated to be incorporated by reference. While specific embodiments of the subject matter have been discussed, the above specification is illustrative and not restrictive. Many variations will become apparent to those skilled in the art upon review of this specification and the claims below. The full scope of the invention should be determined by reference to the claims, along with their full scope of 20 equivalents, and the specification, along with such variations. 84