WO2004077010A2 - Water channel assays - Google Patents

Abstract

The invention is directed to methods for identifying agents that can modulate water channel activity. Such agents can modulate the movement of water into or out of various cell types.

Description

WATER CHANNEL ASSAYS

Applicant claims priority from U.S. Provisional Application 60/444,846, which was filed February 3, 2003.

The invention described herein was made with United States Government support under Grant Number RO1 HL 48908 awarded by the National Institutes of Health. The United States Government has certain rights in this invention.

FIELD OF THE INVENTION

The invention relates to methods for using aquaporin polypeptides to identify agents that can modulate the activity of water channels. Such agents are useful for treating hypertension, congestive heart failure and clinical states relating to abnormal water balance. Such agents could also be used to alter water uptake by plant roots and to affect the hydraulic conductivity of other plant tissues.

BACKGROUND OF THE INVENTION Cell membranes regulate the passage of materials into and out of a cell, a function that makes it possible to maintain the structural and functional integrity of the cell. It has long been recognized that the basic structure of cell membranes consists of a lipid bilayer having proteins embedded throughout. Some types of cells, especially unicellular organisms, also have a cell wall that protects and strengthens the cell. Proteins associated with the cell membrane also contribute to the structural strength of the membrane, and can also act as enzymes to promote chemical reactions, act as carriers for the transport of substances through the membrane and provide breaks in the lipid bilayer so as to form pores through the membrane. Membranes of various cell types differ in biological function largely due to the different kinds of proteins embedded in the lipid bilayer.

Aquaporins are a class of water channel proteins that play a role in water transport and human physiology. Aquaporins are members of the Major Intrinsic Protein (MIP) superfarnily, found in animals, plants, insects and bacteria. Over a hundred aquaporin isoforms have been identified, eleven of which are mammalian. Modulation of aquaporin activities in these organisms may modulate the ability of the organism to withstand fluid overloading or drought conditions. Aquaporin inhibitors may also be useful for treatment of clinical states relating to abnormal water balance. For example, hypertension (high blood pressure) affects one in four Americans and is the primary or contributing cause of death for over 10 percent of the U.S. population. See American Heart Association (2001) 2002 Heart and Stroke Update. Dallas, TX, American Heart Association. Ninety percent of the U.S. population will develop hypertension by age 65. Additionally, congestive heart failure affects almost 5 million Americans, and over half a million new cases occur annually (American Heart Association, 2001). Hyponatremia (low intracellular sodium concentrations) and hypoosmolarity (low intracellular salt concentrations) correlate with the severity of cardiac disease and increased risk of sudden death. Schrier and Martin

(1998). Advances in Experimental Medicine and Biology 449: 415-426. While mercurial kidney aquaporin inhibitors have been used successfully for treating hypertension and congestive heart failure, such compounds are toxic and are no longer utilized. Therefore, new classes of agents are needed for the treatment of patients with hypertension, congestive heart failure and other conditions relating to fluid imbalance. Moreover, new agents are needed for modifying the activity of aquaporins in a variety of organisms including plants, fungi, bacteria, birds and mammals. For example, aquaporins in plant roots play a role in the recovery from acute oxygen deprivation such as occurs in winter or following irrigation. Tournaire-Roux, et al. (2003). Nature 425: 393-397.

SUMMARY OF THE INVENTION

The invention is directed to methods for identifying agents that can modulate water channel activity. Such agents can modulate the water retention or water influx of various cell types. Agents that can modulate water channel activity can be used in a variety of ways, including treatment of conditions relating to abnormal water balance, and development of crops that are tolerant to water stresses. Thus, the invention provides a method for identifying agents that can modulate water channel activity comprising: (a) obtaining an aquaporin- expressing cell in an osmotically acceptable solution; (b) making the aquaporin- expressing cell osmotically sensitive; (c) contacting the aquaporin-expressing cell that is osmotically sensitive with a test agent; (d) osmotically shocking the aquaporin-expressing cell that is osmotically sensitive in the presence of the test agent; and (e) observing whether water channel activity of the aquaporin is modulated. The method can further involve comparing the response of aquaporin-expressing, osmotically sensitive cells to the response of osmotically sensitive cells that do not express aquaporin.

In another embodiment, the invention is directed to a method for identifying an agent that can modulate water channel activity comprising: (a) obtaining a first population of aquaporin-expressing Pichia pastoris cells in an osmotically-acceptable solution comprising 1M sorbitol; (b) treating the first population with yeast lytic enzyme to generate a second population comprising osmotically-sensitive aquaporin-expressing cells; (c) contacting a first aliquot of the second population with a test agent to generate a test mixture; (d) reserving a second aliquot of the second population to generate a control; (e) separately osmotically shocking the aquaporin-expressing cells in the test mixture and the control; and (e) comparing the amount of cell lysis or cell swelling in the test mixture and the control to thereby identify an agent that can modulate the water channel activity of an aquaporin.

The aquaporin expressed by the cell can, for example, be any one of AQP0, AQP1, AQP2, AQP3, AQP4, AQP5, AQP6, AQP7, AQP8, AQP9, OR AQP10. In some embodiments, the aquaporin expressed by the cell comprises SEQ ED NO:l, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:7, or SEQ ID NO:9.

While the aquaporin-expressing cell can be any convenient cell that can be made to be osmotically sensitive, in some embodiments it is a Pichia pastoris yeast cell. Thus, the invention is also directed to a Pichia pastoris cell that can recombinantly express a transgenic aquaporin. In some embodiments, the aquaporin-expressing Pichia pastoris cells can be Pichia pastoris yeast strain KM71H. The osmotically acceptable solution can, for example, be a solution with an osmolarity that is substantially the same as the osmolarity of cytoplasm in the aquaporin-expressing cell. In some embodiments, the osmotically acceptable solution is about 0.9 molar to about 1.5 molar sorbitol. In other embodiments, the osmotically acceptable solution is about 1 molar sorbitol. Other osmotically active solutions can be used, including solutions of complex carbohydrates, polyethylene glycols, dextrans and other substances that cannot be metabolized by the aquaporin-expressing cell. The aquaporin-expressing cell can be made osmotically sensitive by digesting some, or substantially all, of its cell wall components. One way to accomplish this is by digestion with a lytic enzyme.

The aquaporin-expressing cell can be osmotically shocked by dilution of the osmotically acceptable solution containing the agent and the aquaporin- expressing cell. In some embodiments, the aquaporin-expressing cells can be osmotically shocked by dilution of the osmotically acceptable solution into a dilution solution comprising less than about 0.8 M sorbitol. Convenient dilutions that may be used include two-fold, three-fold, four-fold, five-fold, tenfold, twenty- fold and other such dilutions.

The water channel activity can be observed by observing whether the light scattering or optical absorbance of a population of aquaporin-expressing cells changes, for example, decreased light scattering or optical absorbance following hypotonic shock is one measure of water channel activity. The water channel activity can also be observed by observing whether the aquaporin- expressing cell lyses or expands, or whether an increase in cell lysis or cell expansion is observed amongst a population of aquaporin-expressing cells when compared to a population of non-aquaporin-expressing cells. Such cell lysis can be observed by observing whether the light scattering or optical absorbance of the aquaporin-expressing cells decreases following hypotonic shock.

In some embodiments, the water channel activity of the aquaporin can be observed for modulation thereof by comparing the water channel activity of the aquaporin exposed to the test agent with the water channel activity of an aquaporin that has not been exposed lo the test agent.

The agents that are identified by these methods have utility for treating conditions relating to abnormal water balance. For example, such conditions can include hepatic cirrhosis, end-stage liver disease, hepatorenal syndrome, nephritic syndrome, nephrotic syndrome, renal failure, endocrine disorders such as Cushing's syndrome and hyperaldosteronism, water retention related to use of drugs with mineralocorticoid activity (e.g. corticosteroids, estrogen, progesterone), cancers which secrete molecules with mineralocorticoid activity, venous insufficiency, preeclampsia, cerebral edema, and any condition with abnormal capillary permeability such as respiratory distress syndrome, multiorgan system failure, anaphylaxis, allergic reactions, insect and animal bites (for example, bee stings and snake bites), toxic shock syndrome, sepsis from viral, bacterial or fungal organisms, and drugs that may alter capillary permeability (for example, calcium channel blockers). In addition, aquaporin inhibitors would be useful for treating hypertension and congestive heart failure. Agents that can modulate water channel activity that are identified by the present methods also have utility for treating conditions relating to abnormal water balance in agricultural situations. Agricultural uses for agents that can modulate water channel activity (e.g. aquaporin activity) include conditions such as root anoxia as a result of flooding, irrigation, winter freezing or growth in microgravity.

DESCRIPTION OF THE FIGURES Figure 1 illustrates an in vivo water permeability assay comparing the absorbance of α-TIP expressing yeast (Q) with the control, parent yeast Pichia pastoris strain KM71H (| |) at different concentrations of sorbitol (to provide a shock gradient, ΔM sorbitol). The change in molarity from the original 1 molar sorbitol solution used to stabilize the osmotically sensitive spheroplasts is plotted on the x-axis, and the absorbance of the yeast cell suspension is plotted on the y- axis. Also shown are similar assays containing an aquaporin inhibitor (3 mM HgCl₂) added 10 minutes prior to absorbance measurement of α-TIP expressing yeast (solid circles, ) and KM71H yeast (solid squares, ^).

Figure 2A provides a bar graph showing the changes in the absorbance of spheroplasts of α-TD? expressing yeast at 0.5 M sorbitol (open squares, [J), 0.75 sorbitol (gray squares, Q)_» an ¹ 1.0 M sorbitol (solid squares, Q) at increasing intervals of time after shock. In this osmotic shock assay, absorbance is a measure of water permeability. Figure 2B provides a copy of a Western immunoblot illustrating α-TIP expression as a function of time after osmotic shock. As illustrated, the change in absorbance was directly related to the induction of α-TIP aquaporin expression.

DETAILED DESCRIPTION OF THE LWENΪION

The invention provides methods and cells for identifying agents that can modulate water transport in animals, insects, plants, fungi, enveloped viruses and bacteria. Such methods involve contacting an aquaporin-expressing cell with an agent and observing whether the water permeability of the cell is altered. Water permeability of the aquaporin-expressing cell that has been exposed to the agent can be compared to an aquaporin-expressing cell of the same genetic background that has not been exposed to the agent. In some embodiments, the water permeability of the aquaporin-expressing cell that has been exposed to the agent can also be compared to the water permeability of a control cell that does not express the aquaporin but otherwise has the same genetic background.

Definitions

The term "% homology" is used interchangeably herein with the term "% identity" herein and refers to the level of identity between two sequences, i.e. 70% homology means 70% sequence identity. Such sequence identity can be determined by using the BLASTN program when evaluating a given nucleic acid sequence relative to nucleic acid sequences in the GenBank DNA Sequences and other public databases. The BLASTX program is preferred for searching nucleic acid sequences that have been translated in all reading frames against amino acid sequences in the GenBank Protein Sequences and other public databases. Both BLASTN and BLASTX can be run using default parameters of an open gap penalty of 11.0, and an extended gap penalty of 1.0, and utilize the BLOSUM-62 matrix. See, Altschul, et al., J. Mol. Biol. (1997) 215:403-410. A sequence preferably has at least about 70%, or about 80%, or about 85%, or about 90%, or about 95% sequence identity over a length of the given sequence.

A nucleic acid sequence is considered to be "selectively hybridizable" to a reference nucleic acid sequence if the two sequences specifically hybridize to one another under moderate or high stringency hybridization and wash conditions. Exemplary conditions include hybridization conducted as described in Sambrook et al. (MOLECULAR CLONING: A LABORATORY MANUAL, Cold Spring Harbor, NY (1989); or Sambrook et al., MOLECULAR CLONING: A LABORATORY MANUAL, Cold Spring Harbor, NY (2001)), expressly incorporated by reference herein. For example, hybridization can be conducted in 1 mM EDTA, 0.25 M Na₂ HPO₄ and 7% SDS at 60° C, followed by washing in 1 mM EDTA, 40 mM NaPO₄, 5% SDS, and 1 mM EDTA, 40 mM NaPO_4s 1% SDS.

As used herein, the term "expression" refers to the process by which a polypeptide is produced based on the nucleic acid sequence of a gene. The process includes both transcription and translation.

As used herein, the term "modulate" refers to a change in biological activity of a water channel protein such as an aquaporin. Modulation may relate to an increase or a decrease in the flow of water through a water channel protein such as an aquaporin.

Osmotic shock as used herein constitutes exposure of a cell or a suspension of cells to a solution that has an osmolarity above or below the osmolarity of the cytoplasm of the cell(s). Hypotonic solutions have an osmolarity below the osmolarity of the cytoplasm of the cell(s). Hypertonic solutions have an osmolarity above the osmolarity of the cytoplasm of the cell(s). hi some embodiments, the osmotic shock involves exposure of cells to hypotonic solutions.

Screening Methods The invention provides methods for identifying agents that can modulate water channel activity of an aquaporin. The methods involve obtaining a cell that expresses an aquaporin, making the cell osmotically sensitive, contacting such an osmotically sensitive cell with a test agent, osmotically shocking the cells and observing whether the water channel activity of the aquaporin is modulated. Such methods are useful for identifying drugs to treat conditions, injuries and diseases that adversely affect the water balance in an organism. While any convenient method can be used to observe water channel activity, simple, easily reproducible and readily automated assays are preferred. One traditional assay for determining plasma membrane water channel activity in vivo involves the use of a stopped-flow light-scattering spectrophotometer. Zeidel etal. (1992) Biochemistry 31: 1 -36-1 AA; Laize et al. (1995) FEBS Letters 373: 269-21 A. Unfortunately, the complexity of this instrument and the time required per sample generally precludes its use in high-throughput applications.

Therefore, in some embodiments, the invention employs a modified osmotic fragility assay that uses a simple spectrophotometer to test the water channel activity of a recombinant aquaporin that is overexpressed in osmotically fragile cells. Any osmotically fragile host cells available to one of skill in the art can be employed, including bacterial, fungal, insect, plant or animal (e.g. mammalian or bird) cells. Osmotically fragile host cells can also be generated by enzymatic removal of cell wall components so that the cellular membrane remains largely intact but vulnerable to osmotic lysis through aquaporin water channels. The cell wall of a prokaryotic cell confers rigidity and shape. The peptidoglycan layer as one of the important components of the cell walls is composed of two sugar derivatives: N-acetylglucosamine and N-acetylmuramic acid, and a small group of amino acids. The basic structure is a thin sheet in which the glycan chains formed by the sugars are connected by peptide cross- links formed by the amino acids. The full strength of the peptidoglycan structure is obtained when these chains are joined by cross-linking peptides. The cross- linking occurs to characteristically different extents in different bacteria. In gram-positive bacteria, cross-linkage is usually by a peptide interbridge, the kinds and numbers of crosslinking amino acids varying from organism to organism.

Similarly, the cell wall of unicellular fungi like yeast provides a protective layer conferring rigidity and shape. The yeast cell wall contains yeast glucan along with smaller amounts of lipid and protein. Yeast glucan is mainly composed of a backbone chain of β(l-3) linked glucose units with a low degree of inter- and intra-molecular branching through β(l-6) linkages. A minor component of yeast glucan consists mainly of a highly branched β(l-6) linked glucan.

Enzymatic removal of cell wall components can be accomplished using enzymes that digest certain cell wall components, for example, the peptidoglycan or polysaccharide components of cell walls. Examples of such enzymes include yeast lytic enzyme, lysozyme, chitinase, glucanase or a mixture of these enzymes.

Examples of glucanases that can be utilized include β- 1,3 -glucanase and β -1,6-glucanase. The enzyme(s) can be used in substantially pure form or as crude preparations as long as impurities in the enzyme preparation do not interfere with the methods of the invention. Crude preparations from natural origins containing the enzymes are commercially available and can be used instead of purified enzymes. One such enzyme preparation is marketed as NovoZyme™ 234, ex NOVO, Denmark, which is a mixture of lytic enzymes containing inter alia chitinase as well as β -1,3-glucanase and small amounts of β- 1,6-glucanase. Such a preparation is produced by fermentation of the fungus Trichoderma harzianum according to U.S. Pat. No. 4,353,891; further details on its use are described in WO 90/03732. Other natural mixtures of chitinase and glucanases can be obtained from vegetable sources, particularly from plants that are able to produce glucanase and chitinase as described, for example, in Plant Physiology 101: 857-863 (1993).

Chitinase as well as glucanase can be used at a concentration of about 0.001 to 2 wt. % calculated on the composition. The amount of enzyme employed can also be about 0.00001-0.1 wt. %, or about 0.0001-0.02 wt. % of the composition. An acceptable ratio of chitinase to β- 1,3 -glucanase is about 1:9 to about 9:1. Concentrations and ratios can be optimized by one of skill in the art and can vary depending upon the cell type and upon the other ingredients in the enzymatic reaction mixture. Lysozymes (Muramidase; mucopeptide N-acetylmucamoylhydrolase; l,4-.beta-N acetylhexosaminodase, E.G. 3.2.1.17) are mucolytic enzymes which have been isolated from various sources and are well characterized proteins. The antimicrobial activity of lysozymes against gram positive bacteria is well documented, for example by V. N. Procter et al in CRC Crit. Reviews in Food Science and Nutrition, 1988, 26(4):359-395. The molecular weight of egg white lysozyme is approximately 14,300 to 14,600, the isoelectric point is pH 10.5- 10.7. It is composed of 129 amino acids, which are interconnected by four disulfide bridges. Similar enzymes have been isolated and characterized from other sources including such diverse producers as Escherichia coli bacteriophage T4 and human tears. Despite slight differences (for example, the human lysozyme has 130 amino acids) the capacity for hydrolysis of acetylhexosamine polymers remains essentially the same. Accordingly, for purposes of this invention, the term lysozyme is intended to include those cell wall or peptidoglycan degrading enzymes that have the ability to hydrolyze acetylhexosamine and related polymers.

The optimal conditions under which cell walls are enzymatically digested are generally available and largely correspond to the pH, temperature and cofactor requirements of the selected enzyme(s). Some adjustment of those conditions may be made by one of skill in the art to preserve the integrity of the resulting osmotically sensitive cells. For example, the lytic reaction can be conducted under conditions where the osmolarity of the reaction mixture is substantially equivalent to the cytoplasm of the cells, the temperature and pH can be adjusted to a temperature and pH that is readily tolerated by the cells, and smaller amounts of lytic enzyme may be used to generate osmotically sensitive cells rather than to promote complete lysis.

One of skill in the art can readily define temperature, pH and osmolarity conditions useful for preparing osmotically sensitive cells. For example, in some experiments the water channel activities of yeast spheroplasts derived from Pichia pastoris expressing recombinant aquaporin are tested. To generate such spheroplasts, Pichia pastoris yeast cells are harvested by centrifugation 24 to 36 hours following the start of aquaporin expression. The cell pellet can be resuspended in BMMY, a buffered yeast medium containing methanol, that is supplemented with 1.0M sorbitol. The cell suspension can then be incubated at 30°C with moderate agitation for about one hour. A preparation of yeast lytic enzyme is then added (e.g., at about 60 units/OD₆o_0nm) and incubation is continued for about one hour at 30°C with gentle agitation. The resulting material constitutes a suspension of osmotically sensitive yeast spheroplasts. Osmotically sensitive cells that express an aquaporin are osmotically shocked by significantly changing the osmolarity of the cell culture medium, for example, with a hypotonic solution. This can be done, for example, by rapid dilution of the culture medium. Under these conditions aquaporin-mediated water flow will lead to cell lysis or expansion. However, such dilution should not lyse osmotically sensitive cells that are not expressing aquaporin. One of skill in the art can readily ascertain what osmotic shock conditions are useful by observing whether lysis or cell expansion occurs in aquaporin-expressing and non-aquaporin-expressing that have been made osmotically sensitive. Cellular lysis or cell expansion can be detected by standard procedures, for example, by spectrophotometric absorbance at a convenient wavelength for detecting light scattering (e.g., about 450 nm to about 650 nm). For cells that express, or overexpress, an aquaporin, such a hypotonic shock results in more cell lysis than for control cells that do not express substantial aquaporin. The increased cell lysis in the aquaporin-expressing cells will result in a lower optical absorbance or reduced light scattering compared with the control cells.

For example, Pichia pastoris yeast spheroplasts prepared as described above are osmotically shocked by a ten-fold dilution into various concentrations of sorbitol solutions that are significantly lower than about 1M. The absorbance of a Pichia pastoris yeast spheroplast suspension at λ=600nm can then be measured following the osmotic shock using a standard spectrophotometer (e.g. Pharmacia Ultrospec 2000). Typical results are shown in Figure 1.

Mercury is an inhibitor of aquaporins. As a test of the assay conditions, mercury chloride can be added to an aliquot of the aquaporin-expressing osmotically sensitive cells. If the cells become less sensitive to osmotic shock, the aquaporin channels are being blocked by mercury chloride. Hence, lysis of cells in response to osmotic shock is due to the water channel activity of the aquaporin and not to some other toxic effect by the agent or some other component in the assay mixture. Figure 1 shows that the osmotic shock response of cells that do not express substantial aquaporin is the same in the presence and absence of mercury chloride. For some experiments the aquaporin inhibitor mercury chloride was added, for example, at a concentration of about 2-3 mM, at approximately ten minutes before the end of incubation.

Cells that do not express substantial aquaporin can be used as control cells against which the water channel activity of an aquaporin-expressing cell can be assessed. The term "do not express substantial aquaporin" means that the cells were not intentionally manipulated to express or overexpress an aquaporin. Some small or insubstantial levels of aquaporin maybe present in the cells, either because aquaporin is expressed endogenously at low levels or because the cells have absorbed small amounts of aquaporin. Other types of control cells would include but would not be limited to cells expressing nonfunctional aquaporin or a homologous protein in the MD? family that does not function as an aquaporin. Control cells, including those that do not express substantial aquapo,rin can be made to be osmotically sensitive as described herein.

In some embodiments, the methods of the invention are used for high throughput screening of agents that can modulate the water permeability of cells. High throughput drug screening (HTS) involves the rapid assay of a vast number of small molecules to identify potential therapeutic drugs. Cox et al. (2000) Progress in Medicinal Chemistry 37: 83-133.

To perform such high throughput screening assays, aliquots of osmotically sensitive cells (e.g. yeast spheroplasts) can be distributed over a multiwell plate, and test compounds can be added to each well, followed by addition of an aliquot of hypotonic buffer. If the water channels are active, the osmotic shock will result in swelling and lysis of the osmotically sensitive cells. The associated effects on optical density can be measured on a multiwell plate reader. The results are therefore expressed as a change in optical density relative to control samples. Compounds that inhibit the water channel activity of aquaporins cause aquaporin-expressing cells to behave as, or more like, control cells that do not express aquaporin. For example such compounds may prevent cell lysis or cell expansion of aquaporin-expressing cells.

High throughput screening assays can employ miniaturization, and fully automated robotic driven systems. Persidis (1998) Nature Biotechnology 16: 488-489; Cox et al. (2000) Progress in Medicinal Chemistry 57: 83-133. Such miniaturization and automation can enable high throughput programs that screen more than 100,000 compounds a day. Early high throughput screening systems used a 96-well microplate format. Higher density microtiter plates, including 384- well, 1,536-well and even 9,600- well procedures can also be employed (Persidis, 1998). The cell-based, light absorbance assay for aquaporin activity described herein is more biologically relevant than a solution-based in vitro test because the activity of a lead compound to its target is measured in situ. Cox ei al. , (2000); Silverman et al. (1998) Current Opinion in Chemical Biology 2: 397- 403; Sundberg (2000) Current Opinion in Biotechnology 11: 47-53. The simplicity of measuring absorbance for drug activity should readily allow the screening of thousands of compounds a day.

Agents that Can Modulate Water Permeability Agents that can be tested for the ability to modulate water channel activity include any agent suspected of being able to do so by one of skill in the art. Such agents can be any small molecular weight compound, any peptide, any polypeptide or protein or any antibody. Libraries of compounds, peptides, proteins, inhibitors, antibodies and the like can be screened using the methods of the invention. Hence, one of skill in the art can readily test rather than attempt to predict which types of molecules can modulate water channel activity.

Nevertheless, one of skill in the art may choose to select agents of a particular size, shape or charge for screening as agents that can modulate water channel activity. In this regard the structure and properties of aquaporin pores may be of use. Recent high resolution structures derived by electron and X-ray crystallography of aquaporin molecules suggest that selectivity for water is accomplished by a filter that excludes larger molecules and a hydrophobic entrance to the pore that blocks the passage of hydrated ions. Electrostatic interactions between highly conserved asparagine residues and water molecules in the pore may disrupt the hydrogen bonding pattern that would be formed by the chain of water molecules, thereby preventing the conduction of protons. Murata et al., (2000) Nature 407: 599-605. The unusual combination of a hydrophobic pore and a small number of solute binding sites was proposed to facilitate water transport by reducing interactions between water molecules and the pore. Sui et al. (2001) Nature 414: 872-877.

Cysteine residues that reside at the mouth of the pore can be modified by sulfhydryl-reactive compounds, thereby blocking water transport. Such reagents include mercury, lead, and silver sulfhydryl reagents, including but not limited to the compounds mercury chloride, lead chloride, and silver sulfadiazine, which are the only known inhibitors of aquaporin activity. The mechanism of inhibition is thought to involve steric or ionic blockage of the water pore. It is noteworthy that mercurial diuretics were used for over half a century to treat congestive heart failure, and it is thought that their mechanism of action was by inhibition of aquaporins in the proximal tubule of the kidney. Unfortunately, mercury toxicity causes renal failure, and such diuretics are no longer used. Nevertheless, it is clear that kidney aquaporins would be an important target for a novel class of drugs to treat hypertension, congestive heart failure and other fluid overload states.

Aquaporin Binding and Specificity to Promising Agents

There are several methods available to determine the binding specificity of lead compounds and promising agents for the target aquaporin. For example, in one method, a labeled (e.g. tritiated) lead compound or agent is incubated with membranes bearing the target aquaporin. Membranes are then isolated by centrifugation and the label (e.g. radioactivity) in the supernatant is measured (e.g. using a scintillation counter). Non-specific binding can be determined by co-incubation with unlabeled lead compound or agent. The association rate constant corresponds to the time-dependent adsorption of labeled lead compound to the membrane-intrinsic aquaporin target. The time-dependent displacement of bound labeled compound by free unlabeled compound can be used to determine the dissociation rate constant. An example of this procedure is described in Tiffany et al. (2001). This assay can be performed in a multiwell plate format and is thus readily automated. In another method, an immobilized protein affinity selection procedure followed by electrospray ionization mass spectrometry (ESI-MS) can be used to identify aquaporin-binding compounds. Developed by Cancilla and colleagues (2000), this method screens for binding ligands by identification of compounds that show a decrease in ion abundance following incubation with immobilized protein. Aquaporins can be bound to gel beads and incubated with a solution of test compound(s). Following centrifugation of this suspension, an aliquot of the supernatant is subjected to ESI-MS. The mass spectrum of the supernatant is then compared with the pre-incubation mass spectrum of the test compound solution. Ion mass peak intensities that decrease indicate that the compound or agent is an aquaporin-binding compound or agent. Compounds/Agents with binding constants of lOOμM and less can be identified using this method (Cancilla et al, 2000). Individual aquaporins (i.e. AQP0 to AQP9) can be bound to gel beads and then incubated with solutions of lead compound(s). Binding specificity is indicated by a mass peak of a lead compound/agent that decreases only upon incubation with the target aquaporin. This method can be employed using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICRMS). The high mass accuracy of ESI FT-ICRMS (about 1.5 ppm) allows for the identification of individual ligands within a complex mixture of test compounds, so this procedure can also be used for the preliminary high-throughput screening of aquaporin inhibitor compounds.

A third method utilizes frontal affinity chromatography directly coupled to ESI-MS (FAC/ESI-MS). Samples containing test compounds or test agents are loaded onto a column containing an immobilized target aquaporin protein. The elution time of each individual inhibitor corresponds to its binding affinity. Compounds with low affinity eluting earlier than high affinity compounds. Eluted ligands are identified by ESI-MS; additionally, their dissociation rate constant can be determined from the ligand elution time. An example of this method is described by Zhang and colleagues (2001).

Tissue Specificity Assay of Lead Compounds

Cell homogenates from major tissues and organs can be prepared in order to determine the cellular specificity of lead compounds that bind thereto. Samples of brain, heart, lung, liver, spleen, pancreas, kidney, skeletal muscle, and reproductive tissue can be homogenized and impurities can be removed by low speed centrifugation. High speed centrifugation is then used to obtain an enriched membrane pellet that can be resuspended and used for binding studies.

Binding experiments can be performed by incubating labeled and unlabeled lead compound(s) with and without membrane samples. Four experiments can be performed in parallel for each lead compound: 1) labeled compound incubated with membranes, 2) labeled compound plus unlabeled compound in molar excess (>1000-fold) incubated with membranes, 3) labeled compound incubated without membranes, and 4) labeled compound plus unlabeled compound in molar excess (>1000-fold) incubated without membranes. Following incubation, the samples can be vacuum filtered through paper disks and the label (e.g. radioactivity) bound to the disks measured (e.g. using a scintillation counter). Filter-bound label corresponds to the lead compound that is bound to the aquaporin. Comparing the amount of filter-bound label between the first two experiments indicates whether the binding of the lead compound is specific or non-specific. Comparing the filter-bound radioactivity between the last two experiments is a control for binding of lead compound to the filter disk.

Aquaporin-specific binding in tissue samples can be determined by incubating labeled lead compound with membrane samples, washing unbound label from the membrane, elec rophoretically separating intrinsic membrane proteins and observing which bands on the gel are labeled. For example, if the lead compound is radioactively labeled, the bands on the gel can be detected by autoradiography of the electrophoretically separated membrane proteins. Aquaporin-specific binding is indicated by a single labeled protein of about 28kDa molecular weight.

Animal Models to Determine Efficacy of Lead Compounds

There are several animal models available for testing the efficacy of lead compounds in the treatment of hypertension and congestive heart failure. The models involve specific genetic strains or gene knock-outs. The choice of model depends on the disease to be treated.

For example, a common hypertension model is the Dahl salt-sensitive rat. From the time of weaning, control and Dahl salt-sensitive rats are fed laboratory chow supplemented with 0.3% sodium chloride. At seven weeks of age, the salt content of the diet is increased to 8% sodium chloride, which is the standard treatment in this model. The rats are randomized to sham treatment (negative control), treatment with a known anti-hypertensive agent such as a calcium channel blocker (positive control group), or treatment with aquaporin-active lead compounds (experimental group). Blood pressure and heart rate are measured periodically. These physiological responses to the various treatment regimes, as well as effects on cardiac hypertrophy, provide a measure of the efficacy of a selected compound or agent.

Many organisms have been shown to possess a class of protein channels, termed "aquaporins", which are specialized to facilitate the transcellular movement of water. Aquaporins are members of the Major Intrinsic Protein (MIP) superfamily, found in animals, plants, insects and bacteria (Pao et al., 1991; Chepelinsky, 1994; Caherec et al., 1996; Park and Saier, 1996). Generally, aquaporins facilitate the movement of water across cell membranes in response to osmotic gradients, functioning in cellular and organismal osmoregulation and solute transport (Agre et al., 1993; Chrispeels and Agre, 1994; Maurel, 1997).

In some embodiments, the aquaporin employed is a transgenic aquaporin. Such a transgenic aquaporin is present within a host cell but the transgenic aquaporin has a nucleotide sequence or amino acid sequence that is not naturally found in the cell. Nucleic acids encoding the aquaporin can be recombinantly introduced into the host cell so that the host cell can express the transgenic aquaporin. As described below, the host cell can typically be any convenient host cell. However, in some embodiments, the host is a Pichia pastoris cell.

Any aquaporin available to one of skill in the art can be used in the cells and methods of the invention. To date, over a hundred isoforms have been identified. The invention contemplates using all such isoforms, as well as variants and derivatives thereof. At least eleven mammalian aquaporins exist, designated AQP0 to AQP10. At least seven aquaporin isoforms are present in the kidney, located at distinct sites along the nephron and collecting duct, where they act to facilitate osmotically driven water reabsorption (Nielsen et al., 2002). Knockout mice in which these aquaporin genes are deleted show significant urine concentrating defects. The four major kidney aquaporins, AQPl to AQP4, are therefore potential targets for agents that can act as diuretics.

AQPl is the main water transport channel of the renal proximal tubule and descending vasa recta. In humans the Colton blood group antigen corresponds to the AQPl protein. Colton null individuals appear phenotypically normal (Preston et al, 1994). However, a patient with mutant AQPl exhibits chronic renal insufficiency of a clinically unknown origin (Leo et al, 1997). Mice in which the AQPl gene is deleted are generally normal, but when deprived of water they are unable to concentrate urine and become severely hyponatremic (Ma and Verkman, 2000).

A sequence for the human AQPl gene product is provided by the NCBI database (accession number NP 000376; gi: 4502177), and recited below as SEQ ID NO:l:

1 MASEFKKKLF WRAWAEFLA TTLFVFISIG SALGFKYPVG 41 NNQTAVQDNV KVSLAFGLSI ATLAQSVGHI SGAHLNPAVT

81 LG LLSCQIS IFRALMYIIA QCVGAIVATA ILSGITSSLT

121 GNSLGRNDLA DGVNSGQGLG IEIIGTLQLV LCVLATTDRR

161 RRDLGGSAPL AIGLSVALGH LLAIDYTGCG INPARSFGSA 201 VITHNFSNHW IFWVGPFIGG ALAVLIYDFI LAPRSSDLTD

241 RVKVWTSGQV EEYDLDADDI NSRVEMKPK

A nucleotide sequence for the human AQPl codingregion is providedby the NCBI database (accessionnumberNM 000385; gi:4755121), and is recited below as SEQ ID NO:2:

1 GCACCCGGCA GCGGTCTCAG GCCAAGCCCC CTGCCAGCAT

41 GGCCAGCGAG TTCAAGAAGA AGCTCTTCTG GAGGGCAGTG

81 GTGGCCGAGT TCCTGGCCAC GACCCTCTTT GTCTTCATCA

121 GCATCGGTTC TGCCCTGGGC TTCAAATACC CGGTGGGGAA 161 CAACCAGACG GCGGTCCAGG ACAACGTGAA GGTGTCGCTG

201 GCCTTCGGGC TGAGCATCGC CACGCTGGCG CAGAGTGTGG

241 GCCACATCAG CGGCGCCCAC CTCAACCCGG CTGTCACACT

281 GGGGCTGCTG CTCAGCTGCC AGATCAGCAT CTTCCGTGCC

321 CTCATGTACA TCATCGCCCA GTGCGTGGGG GCCATCGTCG 361 CCACCGCCAT CCTCTCAGGC ATCACCTCCT CCCTGACTGG

401 GAACTCGCTT GGCCGCAATG ACCTGGCTGA TGGTGTGAAC

441 TCGGGCCAGG GCCTGGGCAT CGAGATCATC GGGACCCTCC

481 AGCTGGTGCT ATGCGTGCTG GCTACTACCG ACCGGAGGCG

521 CCGTGACCTT GGTGGCTCAG CCCCCCTTGC CATCGGCCTC 561 TCTGTAGCCC TTGGACACCT CCTGGCTATT GACTACACTG

601 GCTGTGGGAT TAACCCTGCT CGGTCCTTTG GCTCCGCGGT

641 GATCACACAC AACTTCAGCA AC CTGGAT TTTCTGGGTG

681 GGGCCATTCA TCGGGGGAGC CCTGGCTGTA CTCATCTACG

721 ACTTCATCCT GGCCCCACGC AGCAGTGACC TCACAGACCG 761 CGTGAAGGTG TGGACCAGCG GCCAGGTGGA GGAGTATGAC

801 CTGGATGCCG ACGACATCAA CTCCAGGGTG GAGATGAAGC

841 CCAAATAGAA GGGGTCTGGC CCGGGCATCC ACGTAGGGGG

881 CAGGGGCAGG GGCGGGCGGA GGGAGGGGAG GGGTGAAATC

921 CATACTGTAG ACACTCTGAC AAGCTGGCCA AAGTCACTTC 961 CCCAAGATCT GCCAGACCTG CATGGTCAAG CCTCTTATGG

1001 GGGTGTTTCT ATCTCTTTCT TTCTCTTTCT GTTTCCTGGC

1041 CTCAGAGCTT CCTGGGGACC AAGATTTACC AATTCACCCA

1081 CTCCCTTGAA GTTGTGGAGG AGGTGAAAGA AAGGGACCCA

1121 CCTGCTAGTC GCCCCTCAGA GCATGATGGG AGGTGTGCCA 1161 GAAAGTCCCC CCTCGCCCCA AAGTTGCTCA CCGACTCACC

1201 TGCGCAAGTG CCTGGGATTC TACCGTAATT GCTTTGTGCC

1241 TTTGGGCAGG CCCTCCTTCT TTTCCTAACA TGCACCTTGC

1281 TCCCAATGGT GCTTGGAGGG GGAAGAGATC CCAGGAGGTG

1321 CAGTGGAGGG GGCAAGCTTT GCTCCTTCAG TTCTGCTTGC 1361 TCCCAAGCCC CTGACCCGCT CGGACTTACT GCCTGACCTT

1401 GGAATCGTCC CTATATCAGG GCCTGAGTGA CCTCCTTCTG

1441 CAAAGTGGCA GGGACCGGCA GAGCTCTACA GGCCTGCAGC

1481 CCCTAAGTGC AAACACAGCA TGGGTCCAGA AGACGTGGTC

1521 TAGACCAGGG CTGCTCTTTC CACTTGCCCT GTGTTCTTTC 1561 CCCAGGGGCA TGACTGTCGC CACACGCCTC TGCATATATG 1601 TCTCTTTGGA GTTGGAATTT CATTATATGT TAAGAAAATA 1641 AAGGAAAATG ACTTGTAAGG TC

Additional aquaporin 1 sequences can be found in the NCBI database.

See website at www.ncbi.nlm.nih.gov.

AQP2 is a vasopressin-regulated aquaporin found on the apical membrane of collecting duct cells. Patients with congestive heart failure often develop hyponatremia associated with an increase in arginine vasopressin (AVP) concentration. Syndromes that lead to water retention as a result of non-osmotic

AVP release, including congestive heart failure, hepatic cirrhosis, and adrenal insufficiency, show elevated expression and urinary excretion of AQP2 (Schrier and Martin, 1998; Laski and Pressley, 1999).

A sequence for the human AQP2 gene product is provided by the NCBI database (accession number AAD38692; gi:5052748), and is recited below as

SEQ ID NO:3:

1 MWELRSIAFS RAVFAEFLAT LLFVFFGLGS ALNWPQALPS

41 VLQIAMAFGL GIGTLVQALG HISGAHINPA VTVACLVGCH

81 VSVLRAAFYV AAQLLGAVAG AALLHEITPA DIRGDLAVNA 121 LSNSTTAGQA VTVELFLTLQ LVLCIFASTD ERRGENPGTP

161 ALSIGFSVAL GHLLGIHYTG CSMNPARSLA PAVVTGKFDD

201 HWVFWIGPLV GAILGSLLYN YVLFPPAKSL SERLAVLKGL

241 EPDTDWEERE VRRRQSVELH SPQSLPRGTK A

A nucleotide sequence for the human AQP2 coding region is provided by the NCBI database (accession number AH007817; gi:14190630), and is recited below as SEQ ID NO:4:

1 CCCCAATCTA ATGGGCTACA GAGTCAGTTT TGCTACCTCT

41 GGCGGGGGGA CCATGGGCAT CCTGGGGGAT CAGGGGCTGC

81 CTTTGGGCCA GGGCCCAGGA AGAAGGGATC AGTCGTTGCA 121 GCTAAGGGCG TCTGGCAAGC CCAGGTGTTC CGGCTCCCAG

161 CCCAGAGGCC CCCTGGTGCC TCGACTGCAG GTGGACAGGA

201 AGATGGAGCC AGAGAGGAAA GTGGGCTCAG TGTTCCCCTA

241 CCCGCCTCTT CTCTGTCCCC AGCTCAGCAA CAGCACGACG

281 GCTGGCCAGG CGGTGACTGT GGAGCTCTTC CTGACACTGC 321 AGCTGGTGCT CTGCATCTTC GCCTCCACCG ATGAGCGCCG

361 CGGAGAGAAC CCGGGCACCC CTGCTCTCTC CATAGGCTTC

401 TCCGTGGCCC TGGGCCACCT CCTTGGGGTA GGTCATGGCC 441 ATGGGTTCCA GCCTCCCTGG AGAAACAGAC ACACAGACCA 481 CTCCAGAGAC AGACACAGAG ACCCCAAGAG GGACACATAC 521 ACAGAACTCT CAAGAGGAAC AGACACCCCA GAGGTTTGAC 561 TCCTAGATAC CCAGAGGACA GATATCACTC CAGCCCATCT 601 GTAAATAAAA CGTGATGTTA ATTGTCCATC ACGTGGGTTC 641 CCTTTAGGCT GAGGTCAAGC ACTGCAGGTG CGGGACAAGG

681 ACTTCCTGCC CTGTCCTCAC CTCCCTTCTC TCTTTGATGC

721 CCTCCTCCCA CTGCAGATCC ATTACACCGG CTGCTCTATG

761 AATCCTGCCC GCTCCCTGGC TCCAGCTGTC GTCACTGGCA 801 AATTTGATGA CCACTGGGTA ATGGCTGAAA CCCCCTGCCC

841 TCCCCTTCTC TAGAAACCCA TTTTAGAGGG AGAACAAGAG

881 CTGGAATAGC ATGGGATGGG GGCTCAGCAG CGGTACCCCA

921 AACCCTCCAC ACTCCTCCTG GTCCTGGGGA GCCTTGGGTT

961 CCACCCCTCA GATCTGATGC CAAAGACTCA GTTTCCATGT 1001 CTGTGAATGA GGATGACAAC AGCTTACCTC ACTGGCTTCT

1041 TGGGAACAGT AAGTGAGGTT ACCGGTGTAA CCCAGATAAT

1081 GCAGTGTCTT GGCACTTAGA ACTCTATAAG TGTGATATTG

1121 ACCTATCTGG TGCCTTAGTA TGTGGTGAGC CTGTTCTGAG

1161 TGCTTTGCAA CATTAACTCA GTTTTCACAA CCACCCAGGA 1201 GGTAGACATT CTTTAGAGTG AAAGGCACAG AGAGGGTAAG

1241 TAACTTGTCG AAGTGCACAC AGCACTTAAG TGGTGGAATC

1281 AGGACACACA CAGGCAGTGG CTTCAGAATT CGCACCCTTA

1321 ACCCCGCACT GACAAGGCTT CCCCAGCAGC TGGCGTTGTC

1361 GTTGTAATTA CATAAATAAG CATTTTACTA GATTAATGTC 1401 GGGGAGGAGG GGTGCGGCCG CAGAGTGTGC CGCCGGGGCC

1441 TGAGGGCTCC GCGTGCCGGT GCGGGCGCGG GTGCCAAGCC

1481 GCCCTCTCCG CTCGCCCCCA GGTCTTCTGG ATCGGACCCC

1521 TGGTGGGCGC CATCCTGGGC TCCCTCCTCT ACAACTACGT

1561 GCTGTTTCCG CCAGCCAAGA GCCTGTCGGA GCGCCTGGCA 1601 GTGCTGAAGG GCCTGGAGCC GGACACCGAT TGGGAGGAGC

1641 GCGAGGTGCG ACGGCGGCAG TCGGTGGAGC TGCACTCGCC

1681 GCAGAGCCTG CCACGGGGTA CCAAGGCCTG AGGGCCGCCA

1721 GCGGCCTCTA AGGCCCCGAC GGACGCTTGT GAGGCCCGAG

1761 GCAGAAGGGC CCACCCCGTC CCTCCTCTCC CGCAGGTCTG 1801 AAGTTGGCCC CCCAGCGCAG AGTAGCTGCT TCCTGGACGT

1841 GCGCGCCCAG GCCAGTGCTG TGAGCAGGCG GGGAGGAGGC

1881 TGCCGGAGGG AGCCCTGAGC CTGGCAGGTC CCCTGCCCTG

1921 AGGCTGTGAG CAGCTAGTGG TGGCTTCTCC AGCCTTTTTC

1961 AGGGAACTGG GAACTTAGGG GACTGAGCTG GGGAGGGAGG 2001 CA

Additional aquaporin 2 sequences can be found in the NCBI database. See website at www.ncbi.nlm.nih.gov.

AQP3 and AQP4 are found on the basolateral membrane of collecting duct cells, presumably forming an exit pathway for water entering cells via AQP2 (Nielsen et al, 2002). Expression of AQP3 is vasopressin-regulated in parallel with AQP2. AQP4 differs from most aquaporins in that it also conducts urea and glycerol. Knockout mice lacking AQP3 or AQP4 show defects in urine concentration. Notably, AQP4 is found predominantly in the brain and spinal cord, suggesting that it functions in cerebral water transport and osmoregulation (Hasegawa et al, 199A; Jung et al, 1994; Rash et al, 1998; Manley et al, 2000). AQP4 inhibitors could therefore be used to treat cerebral edema associated with conditions such as stroke, head trauma and hydrocephalus

(Manleyet α/., 2000).

A sequence for the human AQP3 gene product is provided by the NCBI database (accession number BAA19237; gi: 1854374), and is recited below as

SEQ ID NO:5:

1 MGRQKELVSR CGEMLHIRYR LLRQALAECL GTLILVMFGC

41 GSVAQWLSR GTHGGFLTIN LAFGFAVTLG ILIAGQVSGA

81 HLNPAVTFAM CFLAREPWIK LPIYTLAQTL GAF GAGIVF 121 GLYYDAIWHF ADNQLFVSGP NGTAGIFATY PSGHLDMING

161 FFDQFIGTAS LIVCVLAIVD PYNNPVPRGL EAFTVGLWL

201 VIGTSMGFNS GYAVNPARDF GPRLFTALAG WGSAVFTTGO

241 HWW VPIVSP LLGSIAGVFV YQLMIGCHLE QPPPSNEEEN

281 VKLAHVKHKE QI A nucleotide sequence of the coding region for human AQP3 is provided by the NCBI database (accession number AB001325; gi: 1854373), and is recited below as SEQ ID NO:6:

1 CCGGGGATCC ACGCGCGCCG CCACCCCTGC CCGCCCGACA 41 GCGCCGCCGC CTGCCCCGCC ATGGGTCGAC AGAAGGAGCT 81 GGTGTCCCGC TGCGGGGAGA TGCTCCACAT CCGCTACCGG 121 TTGCTCCGAC AGGCGCTGGC CGAGTGCCTG GGGACCCTCA 161 TCCTCGTGAT GTTTGGCTGT GGCTCCGTGG CCCAGGTTGT 201 GCTCAGCCGG GGCACCCACG GTGGTTTCCT CACCATCAAC 241 CTGGCCTTTG GCTTTGCTGT CACTCTGGGC ATCCTCATCG 281 CTGGCCAGGT CTCTGGGGCC CACCTGAACC CTGCCGTGAC 321 CTTTGCCATG TGCTTCCTGG CTCGTGAGCC CTGGATCAAG 361 CTGCCCATCT ACACCCTGGC ACAGACGCTG GGAGCCTTCT 401 TGGGTGCTGG AATAGTTTTT GGGCTGTATT ATGATGCAAT 441 CTGGCACTTT GCCGACAACC AGCTTTTTGT TTCGGGCCCC 481 AATGGCACAG CCGGCATCTT TGCTACCTAC CCCTCTGGAC 521 ACTTGGATAT GATCAATGGC TTCTTTGACC AGTTCATAGG 561 CACAGCCTCC CTTATCGTGT GTGTGCTGGC CATTGTTGAC 601 CCTTACAACA ACCCCGTCCC CCGAGGCCTG GAGGCCTTCA 641 CCGTGGGCCT GGTGGTCCTG GTCATTGGCA CCTCCATGGG 681 CTTCAACTCC GGCTATGCCG TCAACCCTGC CCGGGACTTT 721 GGCCCCCGCC TTTTTACAGC CCTTGCGGGC TGGGGCTCTG 761 CAGTCTTCAC GACCGGCCAG CATTGGTGGT GGGTGCCCAT 801 CGTGTCCCCA CTCCTGGGCT CCATTGCGGG TGTCTTCGTG 841 TACCAGCTGA TGATCGGCTG CCACCTGGAG CAGCCCCCAC 881 CCTCCAACGA GGAAGAGAAT GTGAAGCTGG CCCATGTGAA

921 GCACAAGGAG CAGATCTGAG TGGGAAGGGC CATCTCCCAC

961 TCCGCTGCCC TGGCCTTGAG CATCCACTGA CTGTCCAAGG

1001 CCACTCCCAA GAAGCCCCCC TTCACGATCC ACCCTTTCAG

1041 GCTAAGGAGC TCCCTATCTA CCCTCACCCC ACGAAGACAG 1081 CCCCTTCAGG ATTTCCACTG GACCTTGCCC AAATAGCACC 1121 TTAGGCCACT GCCCCTAAGC TGGGGTGGAA CCGGAATTTG 1161 GGTCAATACA TCCTTTTGTC TCCCAAGGGA AGAGAATGGG

1201 CAGCAGGTAT GTGTGTGTGT GTGCATGTGT GCATGTGTGT

1241 GCATGTGTGT GCAGGGGTGT GTGTGTGGGG GGGGTTCCCA

1281 GATATTCAGG GCAAGACCAG TCGGAAGGAT CTGCTATTGG 1321 GGACCCAGAG ACAGGGAGGC AGCCTGTCCA TCTGTGCATA

1361 AGGAGAGGAA AGTTCCAGGG TGTGTATGTT TTCAGGGGCC

1401 TTCACATGGA GGAGCTGCAG ATAGATATGT GTTTCTCCGG

1441 AA

Additional aquaporin 3 sequences can be found in the NCBI database.

See website at www.ncbi.nlm.nih.gov.

A sequence for the human AQP4 gene product is provided by the NCBI database (accession number AAH22286; gi: 18490380), and is recited below as

SEQ ID NO:7: 1 MSDRPTARRW GKCGPLCTRE NIMVAFKGVW TQAFWKAVTA

41 EFLAMLIFVL LSLGSTIN G GTEKPLPVDM VLISLCFGLS

81 IATMVQCFGH ISGGHINPAV TVAMVCTRKI SIAKSVFYIA

121 AQCLGAIIGA GILYLVTPPS VVGGLGVTMV HGNLTAGHGL

161 LVELIITFQL VFTIFASCDS KRTDVTGSIA LAIGFSVAIG 201 HLFAINYTGA SMNPARSFGP AVIMGNWENH WIY VGPIIG

241 AVLAGGLYEY VFCPDVEFKR RFKEAFSKAA QQTKGSYMEV

281 EDNRSQVETD DLILKPGVVH VIDVDRGEEK KGKDQSGEVL

321 SSV A nucleotide sequence of the coding region for human AQP4 is provided by the NCBI database (accession number BC022286; gi: 18490379), and is recited below as SEQ ID NO:8:

1 GCACTCTGGC TGGGGAAGGC ATGAGTGACA GACCCACAGC 41 AAGGCGGTGG GGTAAGTGTG GACCTTTGTG TACCAGAGAG 81 AACATCATGG TGGCTTTCAA AGGGGTCTGG ACTCAAGCTT 121 TCTGGAAAGC AGTCACAGCG GAATTTCTGG CCATGCTTAT 161 TTTTGTTCTC CTCAGCCTGG GATCCACCAT CAACTGGGGT 201 GGAACAGAAA AGCCTTTACC GGTCGACATG GTTCTCATCT 241 CCCTTTGCTT TGGACTCAGC ATTGCAACCA TGGTGCAGTG 281 CTTTGGCCAT ATCAGCGGTG GCCACATCAA CCCTGCAGTG 321 ACTGTGGCCA TGGTGTGCAC CAGGAAGATC AGCATCGCCA 361 AGTCTGTCTT CTACATCGCA GCCCAGTGCC TGGGGGCCAT 401 CATTGGAGCA GGAATCCTCT ATCTGGTCAC ACCTCCCAGT 441 GTGGTGGGAG GCCTGGGAGT CACCATGGTT CATGGAAATC 481 TTACCGCTGG TCATGGTCTC CTGGTTGAGT TGATAATCAC 521 ATTTCAATTG GTGTTTACTA TCTTTGCCAG CTGTGATTCC 561 AAACGGACTG ATGTCACTGG CTCAATAGCT TTAGCAATTG 601 GATTTTCTGT TGCAATTGGA CATTTATTTG CAATCAATTA 641 TACTGGTGCC AGCATGAATC CCGCCCGATC CTTTGGACCT 681 GCAGTTATCA TGGGAAATTG GGAAAACCAT TGGATATATT 721 GGGTTGGGCC CATCATAGGA GCTGTCCTCG CTGGTGGCCT 761 TTATGAGTAT GTCTTCTGTC CAGATGTTGA ATTCAAACGT 801 CGTTTTAAAG AAGCCTTCAG CAAAGCTGCC CAGCAAACAA

841 AAGGAAGCTA CATGGAGGTG GAGGACAACA GGAGTCAGGT

881 AGAGACGGAT GACCTGATTC TAAAACCTGG AGTGGTGCAT

921 GTGATTGACG TTGACCGGGG AGAGGAGAAG AAGGGGAAAG 961 ACCAATCTGG AGAGGTATTG TCTTCAGTAT GACTAGAAGA

1001 TCGCACTGAA AGCAGACAAG ACTCCTTAGA ACTGTCCTCA

1041 GATTTCCTTC CACCCATTAA GGAAACAGAT TTGTTATAAA

1081 TTAGAAATGT GCAGGTTTGT TGTTTCATGT CATATTACTC

1121 AGTCTAAACA ATAAATATTT CATAATTTAC AAAGTAAAAA 1161 AAAAAAAAAA AAAAAAAAAA AAA

Additional aquaporin 4 sequences can be found in the NCBI database. See website at www.ncbi.nlm.nih.gov. hi other embodiments, a plant aquaporin is utilized. Any plant aquaporin available to one of skill in the art can be employed in the assays of the invention.

For example, the α-TIP aquaporin from Phaseolus vulgaris (Daniels et al, 1999) has been employed in the assays of the invention. α-TIP aquaporins are found abundantly in vacuolar membranes of cotyledons (seed storage organs) and are synthesized during seed maturation. The α-TD? aquaporins may function in seed desiccation, cytoplasmic osmoregulation, and/or seed rehydration. A sequence for the Phaseolus vulgaris α-TIP aquaporin is provided by the NCBI database (accession number P23958; gi: 135859), and is recited below as SEQ ID

NO:9:

1 MATRRYSFGR TDEATHPDSM RASLAEFAST FIFVFAGEGS 41 GLALVKIYQD SAFSAGELLA LALAHAFALF AAVSAS HVS

81 GGHVNPAVSF GALIGGRISV IRAVYYWIAQ LLGSIVAALV

121 LRLVTNN RP SGFHVSPGVG VGHMFILEW MTFGLMYTVY

161 GTAIDPKRGA VSYIAPLAIG LIVGANILVG GPFDGACMNP

201 ALAFGPSLVG WQ HQHWIF VGPLLGAALA ALVYEYAVIP 241 IEPPPHHHQP LATEDY

A nucleotide sequence of the coding region for the α-TIP aquaporin from

Phaseolus vulgaris is provided by the NCBI database (accession number

X62873; gi:21054), and is recited below as SEQ ID NO: 10: 1 GGCACTCTTT CAGTTCCACC ATAGTTTCTC ACCTTTCAAA

41 TCTCCATTCT GTAAAGAGAA AACCCTTTTT TCTGAAGAAA

81 GAATAATATA GTCTGAGCCG TTCAAGGATC TTTGATCTTG

121 AAAGGCTAGT TGTGTGTGAC AACCTTAGTT GTTAGTCCTA

161 AGGTCTTGTA AGAGAGTGAG CTCATGGCAA CCCGAAGATA 201 TTCTTTTGGA AGGACTGATG AGGCCACTCA TCCAGACTCC

241 ATGAGGGCCT CTTTGGCTGA ATTTGCTTCT ACTTTCATCT

281 TTGTCTTTGC TGGAGAAGGC TCTGGCCTTG CTTTGGTTAA 321 GATTTACCAG GATTCAGCTT TCTCAGCTGG TGAACTGTTA

361 GCACTTGCAC TTGCCCATGC ATTTGCTCTA TTTGCTGCTG

401 TTTCTGCTAG CATGCATGTA TCAGGTGGTC ATGTCAACCC

441 AGCTGTGTCA TTCGGTGCTC TCATTGGGGG GAGGATCTCT 481 GTGATCCGTG CAGTATACTA CTGGATTGCT CAACTTCTGG

521 GTTCTATAGT GGCTGCCCTT GTGCTCAGGC TTGTCACTAA

561 TAACATGAGG CCATCAGGGT TCCACGTGTC ACCAGGTGTT

601 GGAGTTGGAC ACATGTTTAT ACTTGAGGTT GTGATGACAT

641 TTGGGCTGAT GTATACTGTA TATGGTACTG CAATAGATCC 681 GAAAAGGGGT GCTGTTAGCT ATATTGCACC CTTGGCAATT

721 GGGCTCATTG TTGGTGCAAA CATCCTTGTT GGTGGGCCAT

761 TTGATGGAGC ATGCATGAAC CCTGCTCTGG CTTTTGGACC

801 TTCCTTGGTG GGCTGGCAAT GGCACCAGCA CTGGATCTTC

841 TGGGTGGGTC CACTACTTGG GGCTGCACTG GCAGCACTGG 881 TGTATGAATA TGCTGTGATC CCAATTGAAC CACCCCCACA

921 CCACCACCAA CCTTTGGCAA CTGAAGATTA CTAGTTGCTT

961 CTACCAGTTG TTGCATTTGT GCATCAAAAT ATGTTACTAC

1001 TTGCTTTTGG GTATCTAAGT AGAATTGTAT GCTTGTGTTT

1041 TCCTGTATTT TCTCAACCAA CCTTCTCCTT GAGTGGTTAC 1081 TAGGCTTCTG CATCTGCATT TTTATTTTCT TGTTTTGGGG

1121 TCACAACTTC CAAAACCTAG TTAAGTGTGT AGCTGTTGTA

1161 TTTTATCTTT TACACATTGA ATAAAATGTG TGGAACCCTT

1201 TGTTGTGT

Other plant aquaporin sequences can be found in the NCBI database. See website at www.ncbi.nlm.nih.gov. For example, sequences for the following types of aquaporins are readily available: wheat (accession number AAM00368.1, gi:19880505; or AAM00369.1, gi:19880507), corn (Zea mays): accession number AF037061.1, gi:3004949; rice (Oryza sativa) (accession number AF062393.1, gi:3135542); soybean (Glycine max) (accession number L12258.1, gi:310577) and Arabidopsis thaliana (accession number P25818 gi: 135860). These sequences are merely exemplary; additional sequences are available to one of skill in the art.

The methods of the invention can also be performed using variant nucleic acids that hybridize under moderate or, preferably, high stringency conditions to a reference nucleic acid encoding an aquaporin, so long as the variant nucleic acid encodes an aquaporin that can function as a water channel. For example, such a reference nucleic acid can be any one of SEQ ID NO:2, 4, 6, 8 or 10. Moderate and stringent hybridization conditions are well known to the art, see, for example sections 0.47-9.51 of Sambrook et al. (MOLECULAR

CLONING: A LABORATORY MANUAL, Cold Spring Harbor, NY (1989); see also Sambrook et al., MOLECULAR CLONING: A LABORATORY MANUAL, Cold Spring Harbor, NY (2001)). For example, stringent conditions are those that (1) employ low ionic strength and high temperature for washing, for example, 0.015 M NaCl/0.0015 M sodium citrate (SSC); 0.1% sodium lauryl sulfate (SDS) at 50 °C, or (2) employ a denaturing agent such as fonnamide during hybridization, e.g., 50% formamide with 0.1% bovine serum albumin/0.1% Ficoll/0.1% polyvinylpyrrolidone/50 mM sodium phosphate buffer at pH 6.5 with 750 mM NaCl, 75 mM sodium citrate at 42 °C. Another example is the use of 50% formamide, 5 x SSC (0.75 M NaCl, 0.075 M sodium citrate), 50 mM sodium phosphate (pH 6.8), 0.1% sodium pyrophosphate, 5 x Denhardt's solution, sonicated salmon sperm DNA (50 μg/ml), 0.1% sodium dodecylsulfate (SDS), and 10% dextran sulfate at 42 °C, with washes at 42 °C in 0.2 x SSC and 0.1% SDS.

I Aquaporin Expression Systems

The invention is also directed to expression cassettes, expression vectors and host cells that are useful for performing the methods of the invention. Expression cassettes and vectors that are capable of directing the expression of an aquaporin gene product can be prepared by standard molecular biological techniques. See generally, Sambrook et al., 1989, Molecular Cloning, A

Laboratory Manual, 2d ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.; Sambrook and Russell, Molecular Cloning: A Laboratory Manual, 3rd edition (January 15, 2001) Cold Spring Harbor Laboratory Press, ISBN: 0879695765; Ausubel et al., Current Protocols in Molecular Biology, Green Publishing Associates and Wiley fnterscience, NY (1989)).

For example, an expression cassette or expression vector capable of expressing an aquaporin protein can be generated by inserting a nucleic acid encoding an aquaporin protein into a selected cassette or vector. To obtain expression of the aquaporin protein, the expression cassette or vector (with the aquaporin insert) can then be introduced into a selected host cell, for example, a bacterial cell type, a yeast cell type, an insect cell type or a mammalian cell type. The cells can then be plated and screened for expression of an aquaporin polypeptide. Any vector that can replicate in a selected host cell can be utilized for expression of the aquaporin protein. In general, the vector is an expression vector that provides the nucleic acid segments needed for expression of the aquaporin. Various vectors are publicly available. The vector may, for example, be in the form of a plasmid, cosmid, viral particle, or phage. Similarly, any expression cassette that is available lo one of skill in the art can be employed in the methods of the invention.

Expression cassettes and expression vectors generally provide components that facilitate expression of an operably linked nucleic acid that encodes a polypeptide of interest (e.g. an aquaporin). Such components include, but are not limited to, signal sequences, origins of replication, one or more marker genes, enhancer elements, promoters, and transcription termination sequences.

The aquaporin nucleic acids may be inserted into the vector by a variety of procedures. In general, DNA is inserted into an appropriate restriction endonuclease site(s) using techniques known in the art. See generally, Sambrook et al., 1989, Molecular Cloning, A Laboratory Manual, 2d ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.; Sambrook and Russell, Molecular Cloning: A Laboratory Manual, 3rd edition (January 15, 2001) Cold Spring Harbor Laboratory Press, ISBN: 0879695765; Ausubel et al., Current Protocols in Molecular Biology, Green Publishing Associates and Wiley Interscience, NY (1989)). Construction of suitable expression vectors encoding an aquaporin employs standard ligation techniques that are known to the skilled artisan. The invention therefore provides an expression cassette or vector capable of directing the expression of a binding agent polypeptide. The expression cassette can be generated and then placed within a vector to generate an expression vector.

The expression cassettes and vectors include a promoter. Any promoter able to direct transcription of an operably linked coding sequence may be used. A promoter is a nucleotide sequence that controls expression of an operably linked nucleic acid sequence by providing a recognition site for RNA polymerase, and possibly other factors, required for proper transcription. A promoter includes a minimal promoter, consisting only of all basal elements needed for transcription initiation, such as a TATA-box and/or other sequences that serve to specify the site of transcription initiation.

Many different promoter types may be included within the expression systems of the invention. Some useful promoters include constitutive promoters, inducible promoters, regulated promoters, cell specific promoters, viral promoters, and synthetic promoters. A promoter may be obtained from a variety of different sources. For example, a promoter may be derived entirely from a native gene, be composed of different elements derived from different promoters found in nature, or be composed of nucleic acid sequences that are entirely synthetic. A promoter may be derived from many different types of organisms and tailored for use within a given cell.

For expression of a polypeptide in a bacterium, an expression system having a bacterial promoter is used. A bacterial promoter is any DNA sequence capable of binding bacterial RNA polymerase and initiating the downstream (3') transcription of a coding sequence into mRNA. A promoter will have a transcription initiation region that is usually placed proximal to the 5' end of the coding sequence. This transcription initiation region usually includes an RNA polymerase binding site and a transcription initiation site. A second domain called an operator may be present and overlap an adjacent RNA polymerase binding site at which RNA synthesis begins. The operator permits negatively regulated (inducible) transcription, as a gene repressor protein may bind the operator and thereby inhibit transcription of a specific gene. Constitutive expression may occur in the absence of negative regulatory elements, such as the operator. In addition, positive regulation may be achieved by a gene activator protein binding sequence, which, if present, is usually proximal (5') to the RNA polymerase binding sequence. An example of a gene activator protein is the catabolite activator protein (CAP), which helps initiate transcription of the lac operon inE. coli (Raibaud et al., Ann. Rev. Genet., 18:173 (1984)). Regulated expression may therefore be positive or negative, thereby either enhancing or reducing transcription.

Sequences encoding metabolic pathway enzymes provide particularly useful promoter sequences. Examples include promoter sequences derived from sugar metabolizing enzymes, such as galactose, lactose (lac) (Chang et al., Nature, 198:1056 (1977)), and maltose. Additional examples include promoter sequences derived from biosynthetic enzymes such as tryptophan (trp) (Goeddel et al., N.A.R., 8: 4057 (1980); Yelverton et al., N.A.R., 9: 731 (1981); U.S. Pat. No. 4,738,921; and EPO Publ. Nos. 036776 and 121 775). The β-lactamase (bla) promoter system (Weissmann, "The cloning of interferon and other mistakes", in: Interferon 3 (ed. I. Gresser), 1981), and bacteriophage lambda P (Shimatake et al., Nature, 292:128 (1981)) and T5 (U.S. Pat. No. 4,689,406) promoter systems also provide useful promoter sequences. Another promoter is the Chlorella virus promoter (U.S. Patent No. 6,316,224).

Synthetic promoters that do not occur in nature also function as bacterial promoters. For example, transcription activation sequences of one bacterial or bacteriophage promoter may be joined with the operon sequences of another bacterial or bacteriophage promoter, creating a synthetic hybrid promoter (U.S. Pat. No. 4,551,433). For example, the tac promoter is a hybrid trp-lac promoter that is regulated by the lac repressor and that is comprised of both the trp promoter and the lac operon sequences (Amann et al., Gene, 25: 167 (1983); de Boer et al., Proc. Natl. Acad. Sci. USA. 80: 21 (1983)). Furthermore, a bacterial promoter can include naturally occurring promoters of non-bacterial origin that have the ability to bind bacterial RNA polymerase and initiate transcription. A naturally occurring promoter of non-bacterial origin can also be coupled with a compatible RNA polymerase to produce high levels of expression of some genes in prokaryotes. The bacteriophage T7 RNA polymerase/promoter system is an example of a coupled promoter system (Studier et al., J. Mol. Biol., 189: 113 (1986); Tabor et al., Proc. Natl. Acad. Sci. USA. 82:1074 (1985)). In addition, a hybrid promoter can also be comprised of a bacteriophage promoter and an E. coli operator region (EPO Publ. No. 267 851).

An expression cassette having an insect promoter such as a baculovirus promoter can be used for expression of a polypeptide in an insect cell. A baculovirus promoter is any DNA sequence capable of binding a baculovirus RNA polymerase and initiating transcription of a coding sequence into mRNA. A promoter will have a transcription initiation region that is usually placed proximal to the 5' end of the coding sequence. This transcription initiation region usually includes an RNA polymerase binding site and a transcription initiation site. A second domain called an enhancer may be present and is usually distal to the structural gene. A baculovirus promoter may be a regulated promoter or a constitutive promoter. Useful promoter sequences may be obtained from structural genes that are transcribed at times late in a viral infection cycle. Examples include sequences derived from the gene encoding the baculovirus polyhedron protein (Friesen et al., "The Regulation of Baculovirus Gene Expression", in: The Molecular Biology of Baculo viruses (ed. Walter Doerfler), 1986; and EPO Publ. Nos. 127 839 and 155 476) and the gene encoding the baculoviral plO protein (Vlak et al., J. Gen. Virol.. 69: 165 (1988)).

Promoters that are functional in yeast are known to those of ordinary skill in the art. In addition to an RNA polymerase binding site and a transcription initiation site, a yeast promoter may also have a second region called an upstream activator sequence. The upstream activator sequence permits regulated expression that may be induced. Constitutive expression occurs in the absence of an upstream activator sequence. Regulated expression can be positive or negative, thereby either enhancing or reducing transcription. Promoters for use in yeast may be obtained from yeast genes that encode enzymes active in metabolic pathways. Examples of such genes include alcohol oxidase (AOX1), alcohol dehydrogenase (ADH) (EPO Publ. No. 284 044), enolase, glucokinase, glucose-6-phosphate isomerase, glyceraldehyde-3- phosphatedehydrogenase (GAP or GAPDH), hexokinase, phosphofructokinase, 3-phosphoglyceratemutase, and pyruvate kinase (PyK). (EPO Publ. No. 329 203). The yeast PHO5 gene, encoding acid phosphatase, also provides useful promoter sequences. (Myanohara et al., Proc. Natl. Acad. Sci. USA, .80: 1 (1983)).

Synthetic promoters that do not occur in nature may also be used for expression in yeast. For example, upstream activator sequences from one yeast promoter may be joined with the transcription activation region of another yeast promoter, creating a synthetic hybrid promoter. Examples of such hybrid promoters include the ADH regulatory sequence linked to the GAP transcription activation region (U.S. Pat. Nos. 4,876,197 and 4,880,734). Other examples of hybrid promoters include promoters that consist of the regulatory sequences of either the ADH2, GAL4, GAL 10, or PHO5 genes, combined with the transcriptional activation region of a glycolytic enzyme gene such as GAP or PyK (EPO Publ. No. 164 556). Furthermore, a yeast promoter can include naturally occurring promoters of non-yeast origin that have the ability to bind yeast RNA polymerase and initiate transcription. Examples of such promoters are known in the art. (Cohen et al., Proc. Natl. Acad. Sci. USA. 77: 1078 (1980); Henikoff et al., Nature. 283:835 (1981); Hollenberg et al., Curr. Topics Microbiol. Immunol., 96: 119 (1981); Hollenberg et al., "The Expression of Bacterial Antibiotic Resistance Genes in the Yeast Saccharomyces cerevisiae", in: Plasmids of Medical, Environmental and Commercial Importance (eds. K. N. Tirnmis and A. Puhler), 1979; Mercerau-Puigalon et al, Gene, 11: 163 (1980); Panthier et al, Curr. Genet., 2:109 (1980)).

Many mammalian promoters are known in the art that may be used in conjunction with the expression cassette of the invention. Mammalian promoters often have a transcription initiating region, which is usually placed proximal to the 5' end of the coding sequence, and a TATA box, usually located 25-30 base pairs (bp) upstream of the transcription initiation site. The TATA box is thought to direct RNA polymerase II to begin RNA synthesis at the correct site. A mammalian promoter may also contain an upstream promoter element, usually located within 100 to 200 bp upstream of the TATA box. An upstream promoter element determines the rate at which transcription is initiated and can act in either orientation (Sambrook et al., "Expression of Cloned Genes in Mammalian Cells", in: Molecular Cloning: A Laboratory Manual, 2nd ed., 1989).

Mammalian viral genes are often highly expressed and have a broad host range; therefore, sequences encoding mammalian viral genes often provide useful promoter sequences. Examples include the S V40 early promoter, mouse mammary tumor virus LTR promoter, adenovirus major late promoter (Ad MLP), and herpes simplex virus promoter, hi addition, sequences derived from non- viral genes, such as the murine metallothionein gene, also provide useful promoter sequences. Expression may be either constitutive or regulated.

A mammalian promoter may also be associated with an enhancer. The presence of an enhancer will usually increase transcription from an associated promoter. An enhancer is a regulatory DNA sequence that can stimulate transcription up to 1000-fold when linked to homologous or heterologous promoters, with synthesis beginning at the normal RNA start site. Enhancers are active when they are placed upstream or downstream from the transcription initiation site, in either normal or flipped orientation, or at a distance of more than 1000 nucleotides from the promoter. (Maniatis et al., Science, 236:1237 (1987); Alberts et al., Molecular Biology of the Cell, 2nd ed., 1989)). Enhancer elements derived from viruses are often times useful, because they usually have a broad host range. Examples include the SV40 early gene enhancer (Dijkema et al, EMBO J., 4:761 (1985) and the enhancer/promoters derived from the long terminal repeat (LTR) of the Rous Sarcoma Virus (Gorman et al., Proc. Natl. Acad. Sci. USA. 79:6777 (1982b)) and from human cytomegalovirus (Boshart et al., Cell, 41: 521 (1985)). Additionally, some enhancers are regulatable and become active only in the presence of an inducer, such as a hormone or metal ion (Sassone-Corsi and Borelli, Trends Genet., 2:215 (1986); Maniatis et al., Science. 236:1237 (1987)).

It is understood that many promoters and associated regulatory elements may be used within the expression system of the invention to transcribe an encoded polypeptide. The promoters described above are provided merely as examples and are not to be considered as a complete list of promoters that are included within the scope of the invention.

The expression system of the invention may contain a nucleic acid sequence for increasing the translation efficiency of an mRNA encoding a binding agent of the invention. Such increased translation serves to increase production of the binding agent. The presence of an efficient ribosome binding site is useful for gene expression in prokaryotes. In bacterial mRNA a conserved stretch of six nucleotides, the Shine-Dalgarno sequence, is usually found upstream of the initiating AUG codon. (Shine et al., Nature, 254: 34 (1975)). This sequence is thought to promote ribosome binding to the mRNA by base pairing between the ribosome binding site and the 3' end of Escherichia coli 16S rRNA. (Steitz et al., "Genetic signals and nucleotide sequences in messenger RNA", in: Biological Regulation and Development: Gene Expression (ed. R. F. Goldberger), 1979)). Such a ribosome binding site, or an operable derivative thereof, is included within the expression cassette of the invention. A translation initiation sequence can be derived from any expressed

Escherichia coli gene and can be used within an expression cassette of the invention. Preferably the gene is a highly expressed gene. A translation initiation sequence can be obtained via standard recombinant methods, synthetic techniques, purification techniques, or combinations thereof, which are all well known. (Ausubel et al., Current Protocols in Molecular Biology, Green . Publishing Associates and Wiley Interscience, NY. (1989); Beaucage and Caruthers, Tetra. Letts., 22:1859 (1981); VanDevanter et al., Nucleic Acids Res., 12:6159 (1984). Alternatively, translational start sequences can be obtained from numerous commercial vendors. (Operon Technologies; Life Technologies Inc, Gaithersburg, MD). In a preferred embodiment, the T7 leader sequence is used. The T7 tag leader sequence is derived from the highly expressed T7 Gene 10 cistron. Other examples of translation initiation sequences include, but are not limited to, the maltose-binding protein (Mai E gene) start sequence (Guan et al., Gene, 67:21 (1997)) present in the pMalc2 expression vector (New England Biolabs, Beverly, MA) and the translation initiation sequence for the following genes: thioredoxin gene (Novagen, Madison, WI), Glutathione-S-transferase gene (Pharmacia, Piscataway, NJ), β-galactosidase gene, chloramphenicol acetyltransferase gene and E. coli Trp E gene (Ausubel et al, 1989, Current Protocols in Molecular Biology. Chapter 16, Green Publishing Associates and Wiley Interscience, NY).

Eukaryotic mRNA does not contain a Shine-Dalgarno sequence. Instead, the selection of the translational start codon is- usually determined by its proximity to the cap at the 5' end of an mRNA. The nucleotides immediately surrounding the start codon in eukaryotic mRNA influence the efficiency of translation. Accordingly, one skilled in the art can determine what nucleic acid sequences will increase translation of a polypeptide encoded by the expression system of the invention.

Termination sequences can also be included in the cassettes and vectors of the invention. Usually, transcription termination sequences recognized by bacteria are regulatory regions located 3' to the translation stop codon, and thus together with the promoter flank the coding sequence. These sequences direct the transcription of an mRNA that can be translated into the polypeptide encoded by the DNA. Transcription termination sequences frequently include DNA sequences of about 50 nucleotides capable of forming stem loop structures that aid in terminating transcription. Examples include transcription termination sequences derived from genes with strong promoters, such as the trp gene in E. coli as well as other biosynthetic genes. Usually, transcription termination and polyadenylation sequences recognized by mammalian cells are regulatory regions located 3' to the translation stop codon and thus, together with the promoter elements, flank the coding sequence. The 3' terminus of the mature mRNA is formed by site-specific post-transcriptional cleavage and polyadenylation (Birnstiel et al., Cell. 41:349 (1985); Proudfoot and Whitelaw, "Termination and 3' end processing of eukaryotic RNA", in: Transcription and Splicing (eds. B. D. Hames and D. M. Glover) 1988; Proudfoot, Trends Biochem. Sci.. 14:105 (1989)). These sequences direct the transcription of an mRNA that can be translated into the polypeptide encoded by the DNA. Examples of transcription terminator/ polyadenylation signals include those derived from SV40 (Sambrook et al ., "Expression of cloned genes in cultured mammalian cells", in: Molecular Cloning: A Laboratory Manual, 1989).

Transcription termination sequences recognized by yeast are regulatory regions that are usually located 3' to the translation stop codon. Examples of transcription terminator sequences that may be used as termination sequences in yeast and insect expression systems are well known. (Lopez-Ferber et al., Methods Mol. Biol.. 39:25 (1995); King and Possee, The baculovirus expression system. A laboratory guide. Chapman and Hall, London, England (1992); Gregor and Proudfoot, EMBO J..17:4771 (1998); O'Reilly et al, Baculovirus expression vectors: a laboratory manual. W.H. Freeman & Company, New York, NY (1992); Richardson, Crit. Rev. Biochem. Mol. Biol..28:1 (1993); Zhao et al., Microbiol. Mol. Biol. Rev.. 63:405 (1999)).

As indicated above, any expression cassette or expression vector can be utilized in the methods of the invention. Vectors that may be used include, but are not limited to, those able to be replicated in prokaryotes and eukaryotes. For example, vectors may be used that are replicated in bacteria, yeast, insect cells, plant cells and mammalian cells. Examples of vectors include plasmids, phagemids, bacteriophages, viruses, cosmids, and F-factors. The invention includes any vector into which the aquaporin nucleic acids may be inserted and replicated in vitro or in vivo. Specific vectors may be used for specific cell types. Additionally, shuttle vectors may be used for cloning and replication in more than one cell type. Such shuttle vectors are known in the art. The aquaporin nucleic acids maybe carried extrachromosomally within a host cell or may be integrated into a host cell chromosome. Numerous examples of vectors are known in the art and are commercially available. (Sambrook and Russell, Molecular Cloning: A Laboratory Manual, 3rd edition (January 15, 2001) Cold Spring Harbor Laboratory Press, ISBN: 0879695765; New England Biolab, Beverly, MA; Stratagene, La Jolla, CA; Promega, Madison, WI; ATCC, Rockville, MD; CLONTECH, Palo Alto, CA; Invitrogen, Carlabad, CA; Origene, Rockville, MD; Sigma, St. Louis, MO; Pharmacia, Peapack, NJ; USB, Cleveland, OH). These vectors also provide many promoters and other regulatory elements that those of skill in the art may select to be inserted through use of known recombinant techniques.

A vector for use in a prokaryotic host, such as a bacterial cell, includes a replication system allowing it to be maintained in the host for expression or for cloning and amplification. In addition, a vector may be present in the cell in either high or low copy number. Generally, about 5 to about 200, and usually about 10 to about 150 copies of a high copy number vector are present within a host cell. A host cell containing a high copy number vector will preferably contain at least about 10, and more preferably at least about 20 plasmid vectors. Generally, about 1 to 10, and usually about 1 to 4 copies of a low copy number vector will be present in a host cell. The copy number of a vector may be controlled by selection of different origins of replication according to methods known in the art. Sambrook and Russell, Molecular Cloning: A Laboratory Manual, 3rd edition (January 15, 2001) Cold Spring Harbor Laboratory Press, ISBN: 0879695765.

A nucleic acid construct containing an expression cassette can be integrated into the genome of a bacterial host cell through use of an integrating vector. Integrating vectors usually contain at least one sequence that is homologous to the bacterial chromosome that allows the vector to integrate. Integrations are thought to result from recombination events between homologous DNA in the vector and the bacterial chromosome. For example, integrating vectors constructed with DNA from various Bacillus strains integrate into the Bacillus chromosome (EPO Publ. No. 127 328). Integrating vectors may also contain bacteriophage or transposon sequences.

Extrachromosomal and integrating vectors may contain selectable markers to allow for the selection of bacterial strains that have been transformed. Selectable markers can be expressed in the bacterial host and may include genes that render bacteria resistant to drugs such as ampicillin, chloramphenicol, erythromycin, kanamycin (neomycin), and tetracycline (Davies et al., Ann. Rev. Microbiol.. 32: 469 (1978)). Selectable markers may also include biosynthetic genes, such as those in the hislidine, tryptophan, and leucine biosjnthelic pathways.

Numerous vectors, either extra-chromosomal or integrating vectors, have been developed for transformation into many bacteria. For example, vectors have been developed for the following bacteria: B. subiilis (Palva et al., Proc. Natl. Acad. Sci. USA. 79: 5582 (1982); EPO Publ. Nos. 036 259 and 063 953; PCT Publ. No. WO 84/04541), E. coli (Shimatake et al, Nature. 292:128 (1981); Amann et al., Gene. 40:183 (1985); Studier et al., J. Mol. Biol.. 189:113 (1986); EPO Publ. Nos. 036 776, 136 829 and 136 907)), Streptococcus cremoris (Powell et al., Appl. Environ. Microbiol.. 54: 655 (1988)); Streptococcus lividans (Powell et al., Appl. Environ. Microbiol.. 54:655 (1988)), and

Streptomyces lividans (U.S. Pat. No. 4,745,056). Numerous vectors are also commercially available (New England Biolabs, Beverly, MA; Stratagene, La Jolla, CA).

Many vectors may be used for the expression vectors or libraries of the invention that provide for the selection and expression of binding agents in yeast. Such vectors include, but are not limited to, plasmids and yeast artificial chromosomes. Preferably the vector has two replication systems, thus allowing it to be maintained, for example, in yeast for expression and in a prokaryotic host for cloning and amplification. Examples of such yeast-bacteria shuttle vectors include YEp24 (Botstein, et al., Gene, 8: 17 (1979)), pCl/1 (Brake et al., Proc. Natl. Acad. Sci. USA. 81:4642 (1984)), and YRpl7 (Stinchcomb et al., J. Mol. BjoL, 158:157 (1982)).

An expression vector may also be integrated into the yeast genome with an integrating vector. Integrating vectors usually contain at least one sequence homologous to a yeast chromosome that allows the vector to integrate, and preferably contain two homologous sequences flanking an expression cassette of the invention. Integrations appear lo result from recombination events between homologous DNA in the vector and the yeast chromosome. (Orr- Weaver et al., Methods in Enzymol., 1_01 :228 (1983)). An integrating vector may be directed to a specific locus in yeast by selecting the appropriate homologous sequence for inclusion in the vector. One or more expression cassettes may integrate, which may affect the level of recombinant protein produced. (Rine et al., Proc. Natl. Acad. Sci. USA. 80:6750 (1983)). The chromosomal sequences included in the vector can occur either as a single segment in the vector, which results in the integration of the entire vector, or two segments homologous to adjacent segments in the chromosome and flanking an expression cassette included in the vector, which can result in the stable integration of only the expression cassette. Extrachromosomal and integrating expression vectors may contain selectable markers that allow for selection of yeast strains that have been transformed. Selectable markers may include, but are not limited to, biosynthetic genes that can be expressed in the yeast host, such as ADE2, HIS4, LEU2, TRPl, and ALG7, and the G418 resistance gene, which confer resistance in yeast cells to tunicamycin and G418, respectively. In addition, a selectable marker may also provide yeast with the ability to grow in the presence of toxic compounds, such as metal. For example, the presence of CUP 1 allows yeast to grow in the presence of copper ions. (Butt et al., Microbiol. Rev., 51:351 (1987)). Alternatively, a selectable marker may provide yeast with the ability to grow in the presence of antibiotics. For example, the presence of the sh ble gene and its associated gene product allows yeast to grow in the presence of the antibiotic zeocin (Invitrogen, 2000).

Many vectors have been developed for transformation into many yeast strains. For example, vectors have been developed for the following yeasts: Candida albicans (Kurtz et al., Mol. Cell. Biol., 6:142 (1986)), Candida maltose (Kunze et al., J. Basic Microbiol., 25:141 (1985)), Hansenula polymorpha

(Gleeson et al., J. Gen. Microbiol., 132:3459 (1986); Roggenkamp et al., Mol. Gen. Genet., 202:302 (1986), Kluyveromyces fragilis (Das et al., J. Bacteriol.. 158: 1165 (1984)), Kluyveromyces lactis (De Louvencourt et al., J. Bacteriol., 154:737 (1983); van den Berg et al, Bio/Technology. 8:135 (1990)), Pichia guillerimondii (Kunze et al., J. Basic Microbiol.. 25: 141 (1985)), Pichia pastoris (Cregg et al, Mol. Cell. Biol. 5: 3376, 1985; U.S. Pat. Nos. 4,837,148 and 4,929,555), Saccha omyces cerevisiae (Hinnen et al., Proc. Natl. Acad. Sci. USA. 75:1929 (1978); Ito et al., J. Bacteriol.. 153:163 (1983)), Schizosaccharomyces pombe (Beach and Nurse, Nature, 300:706 (1981)), and Yarrowia lipolytica (Davidow et al., Curr. Genet., 10:39 (1985); Gaillardin et al., Curr. Genet.. 10:49 (1985)).

Baculovirus vectors have been developed for infection into several insect cells and may be used to produce nucleic acid constructs that encode a binding agent polypeptide of the invention. For example, recombinant baculo viruses have been developed for Aedes aegypti, Aulographa californica, Bombyx mori, Drosophila melanogaster, Spodopterafi'ugiperda, and Trichoplusia ni (PCT Pub. No. WO 89/046699; Carbonell et al., J. Virol.. 56:153 (1985); Wright, Nature, 321: 718 (1986); Smith et al, Mol. Cell. Biol, 3: 2156 (1983); and see generally, Fraser et al.. In Vitro Cell. Dev. Biol., 25:225 (1989)). Such a baculovirus vector may be used to introduce an expression cassette into an insect and provide for the expression of a binding agent polypeptide within the insect cell.

Methods to form an expression cassette of the invention inserted into a baculovirus vector are available in the art. Briefly, an expression cassette of the invention is inserted into a transfer vector, usually a bacterial plasmid that contains a fragment of the baculovirus genome, through use of common recombinant methods. The plasmid may also contain a polyhedrin polyadenylation signal (Miller et al., Ann. Rev. Microbiol.. 42:177 (1988)) and a prokaryotic selection marker, such as ampicillin resistance, and an origin of replication for selection and propagation in Escherichia coli. A convenient transfer vector for introducing foreign genes into AcNPV is pAc373. Many other vectors, known to those of skill in the art, have been designed. Such a vector is pVL985 (Luckow and Summers, Virology, 17:31 (1989)). A wild-type baculoviral genome and the transfer vector having a nucleic acid construct of the invention are transfected into an insect host cell where the vector and the wild-type viral genome recombine. Methods for introducing a nucleic acid construct into a desired site in a baculovirus virus are available in the art. (Summers and Smith, Texas Agricultural Experiment Station Bulletin No. 1555, 1987. Smith et al., Mol. Cell. Biol., 3:2156 (1983); and Luckow and Summers, Virology. 17:31 (1989)). For example, the insertion can be into a gene such as the polyhedrin gene, by homologous double crossover recombination; insertion can also be into a restriction enzyme site engineered into the desired baculovirus gene (Miller et al., Bioassays, 4:91 (1989)). The packaged recombinant virus is expressed and recombinant plaques are identified and purified. Materials and methods for baculovirus and insect cell expression systems are commercially available in kit form. (Invitrogen, San Diego, Calif., USA ("MaxBac" kit)). These techniques are generally known to those skilled in the art and fully described in Summers and Smith, Texas Agricultural Experiment Station Bulletin No. 1555, 1987.

Plasmid-based expression systems have also been developed that may be used to introduce a nucleic acid construct of the invention into an insect cell and produce a binding agent polypeptide. (McCarroll and King, Curr. Opin. Biotechnol.. 8:590 (1997)). These plasmids offer an alternative to the production of a recombinant virus for the production of binding agent polypeptides.

A nucleic acid construct, an expression vector or a library of the invention may be inserted into any mammalian vectors that are known in the art or that are commercially available. (CLONTECH, Carlsbad, CA; Promega, Madision, WI; Invitrogen, Carlsbad, CA). Such vectors may contain additional elements such as enhancers and introns having functional splice donor and acceptor sites. Nucleic acid constructs may be maintained extrachromosomally or may integrate in the chromosomal DNA of a host cell. Mammalian vectors include those derived from animal viruses, which require trans-acting factors to replicate. For example, vectors containing the replication systems of papovaviruses, such as SV40 (Gluzman, Cell, 23:175 (1981)) or polyomaviruses, replicate to extremely high copy number in the presence of the appropriate viral T antigen. Additional examples of mammalian vectors include those derived from bovine papillomavirus and Epstein-Barr virus. Additionally, the vector may have two replication systems, thus allowing it to be maintained, for example, in mammalian cells for expression and in a prokaryotic host for cloning and amplification. Examples of such mammalian-bacteria shuttle vectors include ρMT2 (Kaufman et al., Mol. Cell. Biol. 9:946 (1989)) and pHEBO (Shimizu et al, Mol. Cell. Biol, 6: 1074 (1986)).

The invention is directed to host cells that can express aquaporin polypeptides. These cells may be prokaryotic or eukaryotic cells derived, for example, from animals, plants, fungi or insects. In some embodiments, bacteria are used as host cells. Examples of bacteria include, but are not limited to, Gram-negative and Gram-positive organisms. Escherichia coli is a desirable organism for screening libraries, expressing binding agent polypeptides and amplifying nucleic acid constructs. Many publicly available E. coli strains include K-strains such as MM294 (ATCC 31, 466); X1776 (ATCC 31, 537); KS 772 (ATCC 53, 635); JM109; MCI 061; HMS174; and the B-strain BL21. Recombination minus strains may be used for nucleic acid construct amplification to avoid recombination events. Such recombination events may remove concatamers of open reading frames as well as cause inactivation of a nucleic acid construct. Furthermore, bacterial strains that do not express a specific protease may also be useful for expression of binding agent polypeptides to reduce proteolytic processing of expressed polypeptides. One example of such a strain is Y1090hsdR that is deficient in the Ion protease. Eukaryotic cells are frequently used in the methods of the invention.

Eukaryotic cells are useful because they may more accurately mimic the water transport properties of mammalian cells. Moreover, an aquaporin polypeptide maybe expressed in a eukaryotic cell when glycosylation of the polypeptide is desired. Examples of eukaryotic cell lines that may be used include, but are not limited to: AS52, HI 87, mouse L cells, NIH-3T3, HeLa, Jurkat, CHO-Kl , COS- 7, BHK-21, A-431, HEK293, L6, CV-1, HepG2, HC11, MDCK, silkworm cells, mosquito cells, and yeast.

Yeast cells are desirable cells for expressing aquaporins and for use in the methods of the invention. Many different strains of yeast are available and all strains available to one of skill in the art can be used in the methods of the invention, particularly those strains where the cell wall can be digested with a convenient enzyme so that osmotically sensitive cells can be generated. Examples of yeast strains that may be used include, but are not limited to the following: Candida albicans (Kurtz et al., Mol. Cell. Biol., 6:142 (1986)), Candida maltose (Kunze et al., J. Basic Microbiol., 25:141 (1985)), Hansenula polymorpha (Gleeson et al., J. Gen. Microbiol., 132:3459 (1986); Roggenkamp et al., Mol. Gen. Genet., 202:302 (1986), Kluyveromyces fragilis (Das et al., J. Bacteriol, 158: 1165 (1984)), Kluyveromyces lactis (De Louvencourt et al., J. Bacteriol., 154:737 (1983); van den Berg et al., Bio/Technology, 8:135 (1990)), Pichia guillerimondii (Kunze et al., J. Basic Microbiol., 25:141 (1985)), Pichia pastoris (Cregg et al., Mol. Cell. Biol, 5: 3376, 1985; U.S. Pat. Nos. 4,837,148 and 4,929,555), Saccharomyces cerevisiae (Hinnen et al., Proc. Natl. Acad. Sci. USA, 75:1929 (1978); Ito et al., J. Bacteriol., 153:163 (1983)), Schizosaccharomyces pombe (Beach and Nurse, Nature, 300:706 (1981)), and Yarrowia lipolytica (Davidow et al, Curr. Genet., 10:39 (1985); Gaillardin et al., Curr. Genet, 10:49 (1985). Many strains of yeast and other cell lines are available from the American Type Culture Collection. For example, Candida albicans (ATCC Deposit No. 10231), Kluyveromyces fragilis (ATCC Deposit No. 10022)

Kluyveromyces lactis (ATCC Deposit No. 34440), Saccharomyces cerevisiae (ATCC Deposit No. 10274), Schizosaccharomyces pombe (ATCC Deposit No. 14548) and Yarrowia lipolytica (ATCC Deposit No. 16617) are available from the American Type Culture Collection. In some embodiments, Pichia pastoris host cells are utilized. In other embodiments, Pichia pastoris strain KM71H is used, which is derived from strain KM71 (ATCC number 201178). Pichia pastoris strain KM71H is available from Invitrogen (Carlsbad, CA). Pichia methanolica is a related yeast strain that can also be used and that is available from the American Type Culture Collection (ATCC number 46071). See also, Raymond CK, Bukowski T, Holderman SD, Ching AF, Vanaja E, Stamm MR. Development of the methylotrophic yeast Pichia methanolica for the expression of the 65 kilodalton isoform of human glutamate decarboxylase. Yeast, 14: 11-23 (1998). A protein expression system for this yeast is available in kit form from Invitrogen (Carlsbad, CA).

Methods for introducing exogenous DNA into bacteria are available in the art, and usually include either the transformation of bacteria treated with CaCl₂ or other agents, such as divalent cations and DMSO. DNA can also be introduced into bacterial cells by electroporation, use of a bacteriophage, or ballistic transformation. Transformation procedures usually vary with the bacterial species to be transformed (Masson et al., FEMS Microbiol. Lett., 60:273 (1989); Palva et al, Proc. Natl. Acad. Sci. USA, 79:5582 (1982); EPO Publ. Nos. 036 259 and 063 953; PCT Publ. No. WO 84/04541 [Bacillus], Miller et al., Proc. Natl. Acad. Sci. USA. 8:856 (1988); Wang et al., J. Bacteriol.. 172:949 (1990) [Campylobacter], Cohen et al., Proc. Natl. Acad. Sci. USA. 69:2110 (1973); Dower et al., Nuc. Acids Res.. 16:6127 (1988); Kushner, "An improved method for transformation of Escherichia coli with ColEl-derived plasmids", in: Genetic Engineering: Proceedings of the International Symposium on Genetic Engineering (eds. H. W. Boyer and S. Nicosia), 1978; Mandel et al., J. Mol. Biol. 53:159 (1970); Taketo, Biochim. Biophvs. Acta, 949:318 (1988) [Escherichia], Chassy et al, FEMS Microbiol. Lett.. 44:173 (1987) [Lactobacillus], Fiedler et al, Anal. Biochem, 170:38 (1988) [Pseudomonas], Augustin et al, FEMS Microbiol. Lett., 66:203 (1990) [Staphylococcus], Barany et al, J. Bacteriol, 144:698 (1980); Harlander, "Transformation of

Streptococcus lactis by electroporation", in: Streptococcal Genetics (ed. J. Ferretti and R. Curtiss III), 1987; Perry et al., Infec. Imrnun.. 32:1295 (1981); Powell et al, Appl. Environ. Microbiol, 54:655 (1988); Somkuti et al, Proc. 4th Eur. Cong. Biotechnology. 412 (1987) [Streptococcus]. Methods for introducing exogenous DNA into yeast hosts are also available in the art, and usually include either the transformation of spheroplasts or of intact yeast cells treated with alkali cations. Transformation procedures usually vary with the yeast species to be transformed (Kurtz et al, Mol. Cell. Biol, 6:142 (1986); Kunze et al, J. Basic Microbiol. 25:141 (1985) [Candida], Gleeson et al, J. Gen. Microbiol. 132:3459 (1986); Roggenkamp et al, Mol. Gen. Genet.. 202:302 (1986) [Hansenula], Das et al, J. Bacteriol. 158:1165 (1984); De Louvencourt et al, J. Bacteriol. 754:737 (1983); Van den Berg et al, Bio/Technology, 8:135 (1990) [Kluyveromyces], Cregg et al, Mol. Cell Biol, 5:3376 (1985); Kunze et al, J. Basic Microbiol. 25:141 (1985); U.S. Pat. Nos. 4,837,148 and 4,929,555 [Pichia], Hinnen et al, Proc. Natl Acad. Sci. USA.

75:1929 (1978); Ito et al, J. Bacteriol. 153:163 (1983) [Saccharomyces], Beach and Nurse, Nature, 300:706 (1981) [Schizosaccharomyces], and Davidow et al, Curr. Genet., 10:39 (1985); Gaillardin et al, Curr. Genet.. 10:49 (1985) [Yarrowia]). Exogenous DNA is conveniently introduced into insect cells through use of recombinant viruses, such as the baculoviruses described herein.

Methods for introduction of heterologous polynucleotides into mammalian cells are known in the art and include lipid-mediated transfection, dextran-mediated transfection, calcium phosphate precipitation, polybrene- mediated transfection, protoplast fusion, electroporation, encapsulation of the ρolynucleotide(s) in liposomes, biollistics, and direct microinjection of the DNA into nuclei. The choice of method depends on the cell being transformed as certain transformation methods are more efficient with one type of cell than another. (Feigner et al, Proc. Natl. Acad. Sci.. 84:7413 (1987); Feigner et al, Biol Chem.. 269:2550 (1994); Graham and van der Eb, Virology. 52:456 (1973); Vaheri and Pagano, Virology, 27:434 (1965); Neuman et al, EMBO J„ 1:841 (1982); Zimmerman, Biochem. Biophys. Acta.. 694:227 (1982); Sanford et al, Methods Enzymol, 217:483 (1993); Kawai and Nishizawa , Mol. Cell. Biol. 4:1172 (1984); Chaney et al, Somat. Cell Mol. Genet..12:237 (1986);

Aubin et al, Methods Mol. Biol, 62:319 (1997)). In addition, many commercial kits and reagents for transfection of eukaryotic cells are available.

Following transformation or transfection of a nucleic acid into a cell, the cell maybe selected for the presence of the nucleic acid through use of a selectable marker. A selectable marker is generally encoded on the nucleic acid being introduced into the recipient cell However, co-transfection of a selectable marker can also be used during introduction of nucleic acid into a host cell. Selectable markers that can be expressed in the recipient host cell may include, but are not limited to, genes that render the recipient host cell resistant to drugs such as actinomycin C, actinomycin D, amphotericin, ampicillin, bleomycin, carbenicillin, chloramphenicol, erythromycin, geneticin, gentamycin, hygromycin B, kanamycin monosulfate, methotrexate, mitomycin C, neomycin B sulfate, novobiocin sodium salt, penicillin G sodium salt, puromycin dihydrochloride, rifampicin, streptomycin sulfate, tetracycline hydrochloride, and zeocin. (Davies et al, Ann. Rev. Microbiol, 32: 469 (1978)). Selectable markers may also include biosynthetic genes, such as those in the histidine, tryptophan, and leucine biosynthetic pathways. Upon transfection or transformation of a host cell, the cell is placed into contact with an appropriate selection agent. For example, if a bacterium is transformed with an expression vector that encodes resistance to the antibiotic zeocin, the transformed bacterium may be placed on an agar plate containing zeocin. Thereafter, cells into which the expression vector was not introduced would be prohibited from growing to produce a colony while colonies would be formed by those bacteria that were successfully transformed. Thus, if one of skill in the art wishes to place the expression vector or expression cassette into a yeast cell, resistance to zeocin can also be used for selection of yeast transformants. Similar selection agents and methods may be used to select for other types of cells, including both prokaryotic and eukaryotic cells.

For example, recombinant α-TIP aquaporin (e.g. SEQ ID NO:9 encoded by SEQ ID NO: 10) was expressed in the methylotrophic yeast Pichia pastoris (Eckart and Bussineau, 1996; Abdulaev et al, 1997; Doring el al, 1997; Hollenberg and Gellissen, 1997; Cereghino and Cregg, 2000). The α-TIP gene was subcloned into the pPICZ Pichia expression vector (Invitrogen, 2000), which includes a carboxy terminal hexahistidine (His₆) tag to facilitate purification by immobilized metal affinity chromatography (IMAC). The final gene construct was linearized and integrated into the yeast genome by homologous recombination, and recombinant yeast were selected by antibiotic screening using zeocin. Expression of this gene construct was under the control of the alcohol oxidase promoter. Consequently, the use of methanol as the sole carbon source in the growth media induced protein production. This method can be used to transform Pichia pastoris with any other aquaporn.

Accordingly, the invention is directed to expression systems that are useful for practicing the methods of the invention.

The following examples further illustrate the invention and are not intended to limit it in any way.

EXAMPLE

Construction and overexpression of α-TIP-G₃-Hg \Ά Pichia pastoris.

An a-TIP aquaporin cDNA from Phaseolus vulgaris was cloned by Polymerase Chain Reaction (PCR). This cDNA is described in Johnson, K., Hδfte, H. & Chrispeels, M. An intrinsic tonoplast protein of protein storage vacuoles in seeds is structurally related to a bacterial solute transporter (GlpF). Plant Cell 2, 525-532 (1990). Nucleic acid sequences flanking the Phaseolus vulgaris α-TIP aquaporin cDNA were modified during PCR amplification to facilitate cloning and protein purification. Two oligonucleotide primers were constructed for such PCR procedures. The forward strand primer was as follows:

5'-AGAATTCAAAATGGCTACCCGAAGATATTCTTTTG-3' (SEQ ID NO:l l). The SEQ ID NO: 11 oligonucleotide incorporates an EcoRI restriction site before the oi-TIP start codon. This sequence also introduced a conservative base pair change in the second codon (GCA to GCT) that created a more yeast-like translation initiation site, in this case resembling that of a yeast enolase gene. See Cigan, A.M. & Donahue, T.F. Gene 59, 1-18 (1987); Romanos et al, Yeast 8, 423-488 (1992).

The reverse strand primer was as follows: 5'- TTTCTAGATCAGTGATGGTGATGGTGATGCCCACCACCGTAAT CTTCAGTTGCCAAAG-3' (SEQ ID NO: 12) The SEQ ID NO: 12 oligonucleotide added an ammo-terminal G₃-H₆ tag before the α-TIP stop codon and introduced a Xbal restriction site.

PCR was performed using Deep Vent DNA polymerase (New England Biolabs). The resulting PCR fragment was cut with EcoRI and Xbal, purified by electrophoresis in an agarose gel, and extracted from the agarose using a gel extraction kit (Qiagen).

This α-TIP gene fragment was subcloned into the Pichia pastoris expression vector PICZ-B (Invitrogen). Prior to insertion, the PICZ-B vector was cut with EcoRI and Xbal, purified by electrophoresis in an agarose gel, and extracted from the agarose using a gel extraction kit (Qiagen). The α-TIP fragment and the linearized PICZ-B vector were ligated together using T4 DNA ligase (New England Biolabs).

This

construct was then transformed into Escherichia coli strain XLl-Blue (Stratagene). Transformants were selected by plating on low-salt LB agar containing the antibiotic zeocin at a concentration of 25 μg/mL. Plasmid DNA was isolated using a plasmid purification kit (Qiagen), and the fidelity of a-TIP-Gs-Hg was verified by DNA sequencing. Prior to transformation into Pichia pastoris by homologous recombination, the a-TIP- G₃-H₆/pPICZ plasmid was linearized with BstXI. Transformation of Pichia pastoris yeast strain KM71H (available from Invitrogen (Carlsbad, California)) with the linearized plasmid was carried out using the Pichia EasyComp kit (Invitrogen). Transformants were selected by plating the treated yeast on YPDS agar containing the antibiotic zeocin at a concentration of 100 μg/mL. Preparation of α-TIP over-expressing Pichia was started by culturing an isolated yeast colony in 10 mL BMGY medium overnight at 30°C with shaking at 275 rpm. A larger volume of BMGY medium (100 mL to 1 L) was inoculated with a 1/100 volume of the starting culture. The second culture was incubated approximately 12 hours at 30°C with shaking at 275 rpm. Cells were harvested at an OD₆oo_nm between 1 and 4 and pelleted by centrifugation for five minutes at

Induction of protein expression was initiated by resuspending the yeast cells in BMMY medium to an OD₆o_0nm of 0.5 to 2. BMMY is a buffered yeast growth medium containing 1 to 2% methanol (v/v). The composition of the BMMY medium is: 1% yeast extract (w/v); 2% peptone (w/v); lOOmM potassium phosphate, pH 6.0; 1.34% yeast nitrogen base, with ammonium sulfate, without dextrose or amino acids (w/v); purchased from QBioGene (Carlsbad, CA); 4 x 10^"5% biotin (w/v); 0.5% (v/v) methanol. Incubation was continued at 30°C with shaking at 250 rpm for baffled culture flasks or 300rpm for unbaffled culture flasks. A supplemental volume of methanol equal to 1/100 of the culture volume was added every 16 hours.

Following 24 to 40 hours of induction, the culture was chilled on ice and stored at 4°C.

Pichia osmotic shock assay. For the osmotic shock assay, 50 OD₆oo_nm units of induced cells were prepared by centrifugation for five minutes at \500xg and 4°C. The cell pellet was resuspended in 10 mL BMMY media supplemented with 1.0 M sorbitol. This suspension was then incubated at 30°C for one hour with vigorous shaking. Yeast spheroplasts were generated by adding one milliliter of yeast lytic enzyme (3000 units lytic activity; ICN Biomedicals) to the cell suspension, followed by incubation at 30°C for one hour with gentle mixing. A 100 μL aliquot of spheroplasts was transferred to a spectrophotometer cuvette (1.0 cm path length). Cells were osmotically shocked by a tenfold dilution with sorbitol at 1.8M, 1.4 M, 1.0 M, 0.50 M, or 0.25 M in water. Optical absorbance (λ= 600nm) was then digitally recorded (Ultrospec 2000, Pharmacia) for two minutes, beginning five to ten seconds following the addition of the sorbitol solution. For some experiments, 8 mg mercury chloride was added to the spheroplast preparation, ten minutes prior to the osmotic shock (producing a 3 mM solution of Hg²⁺). Osmotic shock responses were measured for wild type Pichia strain KM71H and α-TIP transformed Pichia strain KM71H.

Results

Wild-type yeast exhibited a linear relationship between absorbance and osmotic shock level (Figure 1). However, α-TIP-G -H₆ expressing Pichia, exhibited a break in this linear relationship when exposed to hypotonic shock, indicating that the water channel was present in the plasma membrane and active. The onset of osmotic shock sensitivity was concomitant with cellular accumulation of α-TIP (Figure 2). Significantly, addition of the aquaporin inhibitor mercury chloride did not affect the osmotic sensitivity of wild-type Pichia but restored the linear relationship for α-TIP expressing yeast. hnmunoblot analysis showed that α-TIP accumulated over time (Figure 2) and that a small but significant amount (i.e. faint but visible amount by a colorimetric method) of aquaporin was present upon the start of a measurable osmotic shock response. However, it is apparent that this response was not proportional to the amount of aquaporin present. This observation can be explained by the fact that the water channel activity of α-TIP is regulated by phosphorylation, not protein abundance. Consequently, the Pichia water permeability assay will not show a linear correlation between abundance and activity. Rather, α-TIP aquaporin activity will fluctuate according to the level of yeast kinase activity, which is highly dependent on cellular metabolic state and which will vary during the course of incubation. See Thevelein, J.M. Signal transduction in yeast. Yeast 10, 1753-1790 (1994). References

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All patents and publications referenced or mentioned herein are indicative of the levels of skill of those skilled in the art to which the invention pertains, and each such referenced patent or publication is hereby incorporated by reference to the same extent as if it had been incorporated by reference in its entirety individually or set forth herein in its entirety. Applicants reserve the right to physically incorporate into this specification any and all materials and information from any such cited patents or publications. The specific methods and compositions described herein are representative of preferred embodiments and are exemplary and not intended as limitations on the scope of the invention. Other objects, aspects, and embodiments will occur to those skilled in the art upon consideration of this specification, and are encompassed within the spirit of the invention as defined by the scope of the claims. It will be readily apparent to one skilled in the art that varying substitutions and modifications may be made to the invention disclosed herein without departing from the scope and spirit of the invention. The invention illustratively described herein suitably may be practiced in the absence of any element or elements, or limitation or limitations, which is not specifically disclosed herein as essential. The methods and processes illustratively described herein suitably may be practiced in differing orders of steps, and that they are not necessarily restricted to the orders of steps indicated herein or in the claims. As used herein and in the appended claims, the singular forms "a," "an," and "the" include plural reference unless the context clearly dictates otherwise. Thus, for example, a reference to "an antibody" includes a plurality (for example, a solution of antibodies or a series of antibody preparations) of such antibodies, and so forth. Under no circumstances may the patent be interpreted to be limited to the specific examples or embodiments or methods specifically disclosed herein. Under no circumstances may the patent be interpreted to be limited by any statement made by any Examiner or any other official or employee of the Patent and Trademark Office unless such statement is specifically and without qualification or reservation expressly adopted in a responsive writing by Applicants. The terms and expressions that have been employed are used as terms of description and not of limitation, and there is no intent in the use of such terms and expressions to exclude any equivalent of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention as claimed. Thus, it will be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims. The invention has been described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

The invention has been described in U.S. Provisional Application 60/444,846, which is incorporated by reference herein.

Claims

WHAT IS CLAIMED:

1. A method for identifying agents that can modulate water channel activity comprising:

(a) obtaining an aquaporin-expressing cell that is osmotically sensitive in an osmotically acceptable solution;

(b) contacting the aquaporin-expressing cell that is osmotically sensitive with a test agent;

(c) osmotically shocking the aquaporin-expressing cell that is osmotically sensitive in the presence of the test agent; and

(d) observing whether water channel activity of the aquaporin is modulated.

2. The method of claim 1 , wherein the aquaporin expressed by the cell is any one of AQP0, AQPl, AQP2, AQP3, AQP4, AQP5, AQP6, AQP7, AQP8, AQP9, OR AQP10.

3. The method of claim 1 , wherein the aquaporin expressed by the cell comprises SEQ ID NO:l, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:7, or SEQ ID NO:9.

4. The method of claim 1 , wherein the aquaporin expressed by the cell is a plant aquaporin.

5. The method of claim 1, wherein the aquaporin-expressing cell is a yeast cell that can made to be osmotically sensitive.

6. The method of claim 1, wherein the aquaporin-expressing cell is a yeast cell whose cell wall can be digested by yeast lytic enzyme.

7. The method of claim 1 , wherein the aquaporin-expressing cell is Pichia pastoris.

8. The method of claim 1 , wherein the osmotically acceptable solution comprises a solution with an osmolarity that is substantially identical to the osmolarity of the aquaporin-expressing cell's cytoplasm.

9. The method of claim 1, wherein the osmotically acceptable solution is sorbitol at a concentration of about 0.8 molar to about 1.4 molar.

10. The method of claim 1 , wherein the aquaporin-expressing cell that is osmotically sensitive has been made osmotically sensitive by digesting cell wall components of an aquaporin-expressing cell.

11. The method of claim 1 , wherein the aquaporin-expressing cell that is osmotically sensitive has been made osmotically sensitive by digestion with a lytic enzyme.

12. The method of claim 1, wherein the aquaporin-expressing cell that is osmotically sensitive is osmotically shocked by dilution of the osmotically acceptable solution containing the test agent and the aquaporin-expressing cell with a hypotonic solution.

13. The method of claim 1 , wherein the water channel activity is observed by observing whether there is a decrease in light absorbance of a population of the aquaporin-expressing cells that are osmotically sensitive.

14. The method of claim 13, where the decrease is light absorbance of the population of the aquaporin-expressing cells that are osmotically sensitive is measured relative to a population of osmotically sensitive cells that do not express substantial aquaporin.

15. The method of claim 1 , wherein the water channel activity is observed by observing whether the aquaporin-expressing cell lyses or expands more readily than cells that do not express substantial aquaporin.

16. The method of claim 1 , wherein observing whether water channel activity of the aquaporin is modulated comprises comparing the water channel activity of the aquaporin exposed to the test agent with a water channel activity of an aquaporin that has not been exposed to the test agent.

17. A method for identifying an agent that can modulate water channel activity comprising:

(a) obtaining a first population of aquaporin-expressing Pichia pastoris cells in an osmotically-acceptable solution comprising about 0.8 molar to about 1.4 molar sorbitol;

(b) treating the first population with yeast lytic enzyme to generate a second population comprising osmotically-sensitive aquaporin- expressing cells;

(c) contacting a first aliquot of the second population with a test agent to generate a test mixture;

(d) reserving a second aliquot of the second population to generate a control;

(e) separately osmotically shocking the aquaporin-expressing cells in the test mixture and the control; and

(f) comparing the amount of cell lysis or cell swelling in the test mixture and the control to thereby identify an agent that can modulate the water channel activity of an aquaporin.

18. The method of claim 17, wherein the aquaporin-expressing cells express any one of AQP0, AQPl, AQP2, AQP3, AQP4, AQP5, AQP6, AQP7, AQP8, AQP9, OR AQP10.

19. The method of claim 17, wherein the aquaporin-expressing cells express an aquaporin comprising SEQ ID NO:l, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:7, or SEQ ID NO:9.

20. The method of claim 17, wherein the aquaporin-expressing Pichia pastoris cells are Pichia pastoris yeast strain KM71H.

21. The method of claim 17, wherein the aquaporin-expressing cells are osmotically shocked by dilution of the osmotically acceptable solution into a dilution solution comprising less than about 0.8 M sorbitol.

22. The method of claim 17, wherein the cell lysis or cell swelling is observed by observing whether there is a decrease in light absorbance or reduction in light scattering of the test mixture relative to the control.

23. A Pichia pastoris cell that can recombinantly express a transgenic aquaporin.

24. The Pichia pastoris cell of claim 23, wherein the aquaporin is any one of AQP0, AQPl, AQP2, AQP3, AQP4, AQP5, AQP6, AQP7, AQP8, AQP9, OR AQP10.

25. The Pichia pastoris cell of claim 23, wherein the aquaporin is an aquaporin comprising SEQ ID NO:l, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:7, or SEQ ID NO:9.

26. The Pichia pastoris cell of claim 23 , wherein the cell is Pichia pastoris yeast strain KM71H.