US20070212707A1 - Cell cycle markers - Google Patents

Cell cycle markers Download PDF

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US20070212707A1
US20070212707A1 US11/572,508 US57250805A US2007212707A1 US 20070212707 A1 US20070212707 A1 US 20070212707A1 US 57250805 A US57250805 A US 57250805A US 2007212707 A1 US2007212707 A1 US 2007212707A1
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cell
hdhb
cell cycle
test agent
construct according
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Suzanne Hancock
Simon Stubbs
Nicholas Thomas
Ellen Fanning
Jinming Gu
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GE Healthcare UK Ltd
Vanderbilt University
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GE Healthcare UK Ltd
Vanderbilt University
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    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07K—PEPTIDES
    • C07K14/00—Peptides having more than 20 amino acids; Gastrins; Somatostatins; Melanotropins; Derivatives thereof
    • C07K14/435—Peptides having more than 20 amino acids; Gastrins; Somatostatins; Melanotropins; Derivatives thereof from animals; from humans
    • C07K14/46—Peptides having more than 20 amino acids; Gastrins; Somatostatins; Melanotropins; Derivatives thereof from animals; from humans from vertebrates
    • C07K14/47—Peptides having more than 20 amino acids; Gastrins; Somatostatins; Melanotropins; Derivatives thereof from animals; from humans from vertebrates from mammals
    • C07K14/4701—Peptides having more than 20 amino acids; Gastrins; Somatostatins; Melanotropins; Derivatives thereof from animals; from humans from vertebrates from mammals not used
    • C07K14/4738—Cell cycle regulated proteins, e.g. cyclin, CDC, INK-CCR
    • C—CHEMISTRY; METALLURGY
    • C12—BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
    • C12N—MICROORGANISMS OR ENZYMES; COMPOSITIONS THEREOF; PROPAGATING, PRESERVING, OR MAINTAINING MICROORGANISMS; MUTATION OR GENETIC ENGINEERING; CULTURE MEDIA
    • C12N15/00—Mutation or genetic engineering; DNA or RNA concerning genetic engineering, vectors, e.g. plasmids, or their isolation, preparation or purification; Use of hosts therefor
    • C12N15/09—Recombinant DNA-technology
    • C12N15/10—Processes for the isolation, preparation or purification of DNA or RNA
    • C12N15/1034—Isolating an individual clone by screening libraries
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07K—PEPTIDES
    • C07K2319/00—Fusion polypeptide
    • C07K2319/60—Fusion polypeptide containing spectroscopic/fluorescent detection, e.g. green fluorescent protein [GFP]

Definitions

  • the present invention relates to G1/S cell cycle specific markers and methods for determining the transition between G1-phase and S-phase of the cell cycle in mammalian cells.
  • Eukaryotic cell division proceeds through a highly regulated cell cycle comprising consecutive phases termed G1, S, G2 and M.
  • Disruption of the cell cycle or cell cycle control can result in cellular abnormalities or disease states such as cancer which arise from multiple genetic changes that transform growth-limited cells into highly invasive cells that are unresponsive to normal control of growth. Transition of normal cells into cancer cells can arise though loss of correct function in DNA replication and DNA repair mechanisms. All dividing cells are subject to a number of control mechanisms, known as cell-cycle checkpoints, which maintain genomic integrity by arresting or inducing destruction of aberrant cells. Investigation of cell cycle progression and control is consequently of significant interest in designing anticancer drugs (Flatt, P. M. and Pietenpol, J. A. Drug Metab. Rev., (2000), 32(3-4), 283-305; Buolamwini, J. K. Current Pharmaceutical Design, (2000), 6, 379-392).
  • cell cycle status for cell populations has been determined by flow cytometry using fluorescent dyes which stain the DNA content of cell nuclei (Barlogie, B. et al, Cancer Res., (1983), 43(9), 3982-97).
  • Flow cytometry yields quantitative information on the DNA content of cells and hence allows determination of the relative numbers of cells in the G1, S and G2+M phases of the cell cycle.
  • this analysis is a destructive non-dynamic process and requires serial sampling of a population to determine cell cycle status with time.
  • Cell cycle progression is tightly regulated by defined temporal and spatial expression, localisation and destruction of a number of cell cycle regulators which exhibit highly dynamic behaviour during the cell cycle (Pines, J., Nature Cell Biology, (1999), 1, E73-E79). For example, at specific cell cycle stages some proteins translocate from the nucleus to the cytoplasm, or vice versa, and some are rapidly degraded. For details of known cell cycle control components and interactions, see Kohn, Molecular Biology of the Cell (1999), 10, 2703-2734.
  • U.S. Pat. No. 6,048,693 describes a method for screening for compounds affecting cell cycle regulatory proteins, wherein expression of a reporter gene is linked to control elements which are acted on by cyclins or other cell cycle control proteins.
  • temporal expression of a reporter gene product is driven in a cell cycle specific fashion and compounds acting on one or more cell cycle control components may increase or decrease expression levels.
  • U.S. Pat. No. 6,159,691 relates to a method for assaying for putative regulators of cell cycle progression.
  • nuclear localisation signals derived from the cell cycle phase specific transcription factors DP-3 and E2F-1 are used to assay the activity of compounds which act as agonists or antagonists to increase or decrease nuclear localisation of an NLS fused to a detectable marker.
  • WO 03/031612 describes DNA reporter constructs and methods for determining the cell cycle position of living mammalian cells by means of cell cycle phase-specific expression control elements and destruction control elements.
  • the present invention describes a method which utilises key components of the cell cycle regulatory machinery in defined combinations to provide novel means of determining cell cycle status for individual living cells in a non-destructive process providing dynamic read out.
  • the present invention further provides DNA constructs, and stable cell lines containing such constructs, that exhibit translocation of a detectable reporter molecule in a cell cycle phase specific manner, by direct linkage of reporter signal switching to a G1/S cell dependent location control sequence.
  • nucleic acid reporter construct comprising a nucleic acid sequence encoding a detectable live-cell reporter molecule operably linked to and under the control of:
  • a G1/S cell cycle phase-dependent location control element wherein the translocation of said reporter construct within a mammalian cell is indicative of the cell cycle position.
  • translocation is defined as the detectable movement of the reporter from one sub-cellular location to another, typically from the nucleus to the cytoplasm or vice versa.
  • live cell as it relates to a reporter molecule, defines a reporter molecule which produces a detectable signal in living cells and is thus suitable for use in live-cell imaging systems, such as the IN Cell Analyzer (GE Healthcare).
  • operably linked indicates that the elements are arranged so that they function in concert for their intended purposes, e.g. transcription initiates in a promoter and proceeds through the DNA sequence coding for the reporter molecule of the invention.
  • the expression control element controls transcription over an extended time period.
  • the expression control element is the ubiquitin C promoter which provides transcription over an extended period which is required for the production of stable cell lines.
  • the cell cycle phase-specific dependent location control element is selected from the group consisting of Rag2, Chaf1B, Fen1, PPP1R2, helicase B, sgk, CDC6 or motifs therein such as the phosphorylation-dependent subcellular localization domain of the C-terminal special control region of helicase B (PSLD).
  • the phase-specific dependent location element is the phosphrylation-dependent subcellular localization domain of the C-terminal special control region of helicase B (PSLD).
  • a human helicase B homolog has been reported and characterised (Taneja et al J. Biol. Chem., (2002), 277, 40853-40861); the nucleic acid sequence for MGC clone NM 033647 and the corresponding protein are given in SEQ ID No. 1 and SEQ ID No. 2, respectively.
  • the report demonstrates that helicase activity is needed during G1 to promote the G1/S transition.
  • Gu et al Mol. Biol. Cell., (2004), 15, 3320-3332
  • a small C-terminal region of the helicase B gene termed the phosphorylation-dependent subcellular localization domain (PSLD) is phosphorylated by Cdk2/cyclin E and contains NLS and NES sequences.
  • PSLD phosphorylation-dependent subcellular localization domain
  • the live-cell reporter molecule is selected from the group consisting of fluorescent protein, enzyme reporter and antigenic tag.
  • the fluorescent protein is selected from Green Fluorescent Protein (GFP) and a functional GFP analogue in which the amino acid sequence of wild type GFP has been altered by amino acid deletion, addition, or substitution.
  • GFP Green Fluorescent Protein
  • the GFP is Enhanced Green Fluorescent Protein (EGFP), Emerald or J-Red.
  • the enzyme reporter is halo-tag (Promega Corporation, USA).
  • the cell cycle phase-dependent location control element is PSLD.
  • the reporter molecule is a GFP and the cell cycle phase-dependent location control element is PSLD. More preferably, the reporter molecule is EGFP and the cell cycle phase-dependent location control element is PSLD.
  • the reporter construct comprises a CMV promoter, a PSLD and EGFP.
  • the reporter construct comprises a human ubiquitin C promoter, a PSLD and a green fluorescent protein.
  • the construct additionally comprises an inert group to increase the size of the expressed protein.
  • the purpose of such a group is to allow the translated protein to be comparable in size to the ‘parent’ protein if, for example, only a portion of the protein has been used as the cell cycle phase-dependent location control element (e.g. only the PSLD domain of the complete helicase B protein).
  • the inert group is ⁇ Gal.
  • nucleic acid reporter construct comprising an expression vector comprising:
  • a vector backbone comprising:
  • the construct additionally contains a eukaryotic drug resistance gene, preferably a mammalian drug resistance gene.
  • Expression vectors may also contain other nucleic acid sequences, such as polyadenylation signals, splice donor/splice acceptor signals, intervening sequences, transcriptional enhancer sequences, translational enhancer sequences and the like.
  • the drug resistance gene and reporter gene may be operably linked by an internal ribosome entry site (IRES), which is cell cycle independent (Jang et al., J. Virology, (1988), 62, 2636-2643) rather than the two genes being driven by separate promoters.
  • IRES internal ribosome entry site
  • the pIRES-neo and pIRES vectors commercially available from Clontech may be used.
  • Suitable vector backbones which include bacterial and mammalian drug resistance genes and a bacterial origin of replication include, but are not limited to: pCI-neo (Promega), pcDNA (Invitrogen) and pTriEx1 (Novagen).
  • Suitable bacterial drug resistance genes include genes encoding for proteins that confer resistance to antibiotics including, but not restricted to: ampicillin, kanamycin, tetracyclin and chloramphenicol.
  • Eurkaryotic drug selection markers include agents such as: neomycin, hygromycin, puromycin, zeocin, mycophenolic acid, histidinol, gentamycin and methotrexate.
  • the DNA construct may be prepared by the standard recombinant molecular biology techniques of restriction digestion, ligation, transformation and plasmid purification by methods familiar to those skilled in the art and are as described in Sambrook, J. et al (1989), Molecular Cloning—A Laboratory Manual, Cold Spring Harbor Laboratory Press.
  • the construct can be prepared synthetically by established methods, eg. the phosphoramidite method described by Beaucage and Caruthers, (Tetrahedron Letters, (1981), 22, 1859-1869) or the method described by Matthes et al (EMBO J., (1984), 3, 801-805).
  • oligonucleotides are synthesised, eg.
  • DNA construct may also be prepared by polymerase chain reaction (PCR) using specific primers, for instance, as described in U.S. Pat. No. 4,683,202 or by Saiki et al (Science, (1988), 239, 487-491).
  • PCR polymerase chain reaction
  • the gene sequence encoding the reporter must be joined in frame with the G1/S cell cycle phase-dependent location control element.
  • the resultant DNA construct should then be placed under the control of one or more suitable cell cycle phase independent expression control elements.
  • a polypeptide encoded by a nucleic acid construct as hereinbefore described is provided.
  • a polypeptide as hereinbefore described in the third aspect for determining the cell cycle position of a mammalian cell.
  • a host cell transfected with a nucleic acid construct as hereinbefore described The host cell into which the construct or the expression vector containing such a construct is introduced may be any mammalian cell which is capable of expressing the construct.
  • the prepared DNA reporter construct may be transfected into a host cell using techniques well known to the skilled person.
  • One approach is to temporarily permeabilise the cells using either chemical or physical procedures. These techniques may include: electroporation (Tur-Kaspa et al, Mol. Cell Biol. (1986), 6, 716-718; Potter et al, Proc. Nat. Acad. Sci. USA, (1984), 81, 7161-7165), a calcium phosphate based method (eg. Graham and Van der Eb, Virology, (1973), 52, 456-467 and Rippe et al, Mol. Cell Biol., (1990), 10, 689-695) or direct microinjection.
  • cationic lipid based methods may be used to introduce DNA into cells (Stewart et al, Human Gene Therapy, (1992), 3, 267; Torchilin et al, FASEB J, (1992), 6, 2716; Zhu et al, Science, (1993), 261, 209-211; Ledley et al, J. Pediatrics, (1987), 110, 1; Nicolau et al, Proc. Nat. Acad. Sci., USA, (1983), 80, 1068; Nicolau and Sene, Biochem. Biophys. Acta, (1982), 721, 185-190).
  • Jiao et al, Biotechnology, (1993), 11, 497-502 describe the use of bombardment mediated gene transfer protocols for transferring and expressing genes in brain tissues which may also be used to transfer the DNA into host cells.
  • a further alternative method for transfecting the DNA construct into cells utilises the natural ability of viruses to enter cells.
  • Such methods include vectors and transfection protocols based on, for example, Herpes simplex virus (U.S. Pat. No. 5,288,641), cytomegalovirus (Miller, Curr. Top. Microbiol. Immunol., (1992), 158, 1), vaccinia virus (Baichwal and Sugden, 1986, in Gene Transfer, ed. R. Kucherlapati, New York, Plenum Press, p 117-148), and adenovirus and adeno-associated virus (Muzyczka, Curr. Top. Microbiol. Immunol., (1992), 158, 97-129).
  • suitable recombinant host cells include HeLa cells, Vero cells, Chinese Hamster ovary (CHO), U2OS, COS, BHK, HepG2, NIH 3T3 MDCK, RIN, HEK293 and other mammalian cell lines that are grown in vitro.
  • the host cell is a human cell.
  • Such cell lines are available from the American Tissue Culture Collection (ATCC), Bethesda, Md., U.S.A. Cells from primary cell lines that have been established after removing cells from a mammal followed by culturing the cells for a limited period of time are also intended to be included in the present invention.
  • the cell line is a stable cell line comprising a plurality of host cells according to the third aspect.
  • Cell lines which exhibit stable expression of a cell cycle position reporter may also be used in establishing xenografts of engineered cells in host animals using standard methods. (Krasagakis, K. J et al, Cell Physiol., (2001), 187(3), 386-91; Paris, S. et al, Clin. Exp. Metastasis, (1999), 17(10), 817-22). Xenografts of tumour cell lines engineered to express cell cycle position reporters will enable establishment of model systems to study tumour cell division, stasis and metastasis and to screen new anticancer drugs.
  • cells transfected with the DNA reporter construct may be cultured under conditions and for a period of time sufficient to allow expression of the reporter molecule at a specific stage of the cell cycle.
  • expression of the reporter molecule will occur between 16 and 72 hours post transfection, but may vary depending on the culture conditions.
  • the reporter molecule is based on a green fluorescent protein sequence the reporter may take a defined time to fold into a conformation that is fluorescent. This time is dependent upon the primary sequence of the green fluorescent protein derivative being used.
  • the fluorescent reporter protein may also change colour with time (see for example, Terskikh, Science, (2000), 290, 1585-8) in which case imaging is required at specified time intervals following transfection.
  • a method for determining the cell cycle position of a mammalian cell by monitoring the expression of the reporter molecule and detecting signals emitted by the reporter using an appropriate detection device. If the reporter molecule produces a fluorescent signal, then, either a conventional fluorescence microscope, or a confocal based fluorescence microscope may be used. If the reporter molecule produces luminescent light, then a suitable device such as a luminometer may be used. Using these techniques, the proportion of cells expressing the reporter molecule may be determined. If the DNA construct contains translocation control elements and the cells are examined using a microscope, the location of the reporter may also be determined.
  • the fluorescence of cells transformed or transfected with the DNA construct may suitably be measured by optical means in for example; a spectrophotometer, a fluorimeter, a fluorescence microscope, a cooled charge-coupled device (CCD) imager (such as a scanning imager or an area imager), a fluorescence activated cell sorter, a confocal microscope or a scanning confocal device, where the spectral properties of the cells in culture may be determined as scans of light excitation and emission.
  • a spectrophotometer a fluorimeter, a fluorescence microscope, a cooled charge-coupled device (CCD) imager (such as a scanning imager or an area imager), a fluorescence activated cell sorter, a confocal microscope or a scanning confocal device, where the spectral properties of the cells in culture may be determined as scans of light excitation and emission.
  • CCD charge-coupled device
  • the nucleic acid reporter construct comprises a drug resistance gene
  • cells expressing the modified reporter gene may be selected by growing the cells in the presence of an antibiotic for which transfected cells are resistant due, to the presence of a selectable marker gene.
  • the purpose of adding the antibiotic is to select for cells that express the reporter gene and that have, in some cases, integrated the reporter gene, with its associated promoter, into the genome of the cell line.
  • a clonal cell line expressing the construct can be isolated using standard techniques. The clonal cell line may then be grown under standard conditions and will express reporter molecule and produce a detectable signal at a specific point in the cell cycle.
  • Cells transfected with the nucleic acid reporter construct according to the present invention may be grown in the absence and/or the presence of a test agent to be studied and whose effect on the cell cycle of a cell is to be determined.
  • a test agent to be studied and whose effect on the cell cycle of a cell is to be determined.
  • a test agent on the cell cycle position of a mammalian cell, the method comprising:
  • a test agent on the cell cycle position of a mammalian cell, the method comprising:
  • a test agent on the cell cycle position of a mammalian cell, the method comprising:
  • a method of determining the effect of the mammalian cell cycle on the expression, translocation or sub-cellular distribution of a first detectable reporter which is known to vary in response to a test agent comprising:
  • the test agent is a form of electromagnetic radiation or is a chemical entity.
  • the test agent is a chemical entity selected from the group consisting of drug, nucleic acid, hormone, protein and peptide.
  • the test agent is selected from a peptide or protein that is expressed in the cell under study.
  • FIG. 1 is the localisation of HDHB in the nucleus or cytoplasm.
  • (F) Walker B mutants (MutB) of GFP-HDHB transiently expressed in microinjected U2OS cells were visualized by fluorescence microscopy. Nuclei were stained with Hoechst dye. Bar, 10 ⁇ m.
  • (G) U2OS cells transiently expressing GFP-HDHB wt, MutA, and MutB were extracted with 0.5% Triton X-100 before fixation and fluoresence microscopy. Nuclei were stained with Hoechst dye. Bar, 10 ⁇ m.
  • FIG. 2 is the GFP-HDHB nuclear focus formation increases upon DNA damage.
  • U2OS cells transiently expressing GFP-HDHB were treated with DMSO (control), 20 ⁇ M etoposide, 10 ⁇ M camptothecin, or 1 ⁇ M mitomycin C as indicated.
  • FIG. 3 is the subcellular localization of GFP-HDHB is cell cycle-dependent.
  • Cytoplasmic and nuclear extracts of synchronized U2OS cells (G1 and S phase) were analyzed by denaturing gel electrophoresis and western blotting with antibody against recombinant HDHB, ⁇ -tubulin, and PCNA. Immunoreactive proteins were detected by chemiluminescence.
  • FIG. 4 is the identification of a domain required for nuclear localization of HDHB.
  • GFP-HDHB- ⁇ SLD was transiently expressed in U2OS cells in G1 or S phase and visualized by fluorescence microscopy. Nuclei were stained with Hoechst dye. Bar, 10 ⁇ m.
  • FIG. 5 is the GFP- ⁇ Gal-PSLD subcellular localization pattern varies with the cell cycle.
  • GFP- ⁇ Gal, GFP- ⁇ Gal-SLD, and GFP- ⁇ Gal-PSLD were transiently expressed in U2OS cells in G1 (left) and S phase (right) of the cell cycle. Nuclei were stained with Hoechst dye. Bar, 10 ⁇ m.
  • FIG. 6 is the identification of a functional rev-type nuclear export signal (NES) in SLD of HDHB.
  • GFP-HDHB and FLAG-HDHB in asynchronous, G1, and S phase cells was quantified and expressed as a percentage of the total number of GFP-positive cells in that sample.
  • C GFP-HDHB and GFP- ⁇ Gal-PSLD carrying the Mut1 or Mut2 mutations, or the corresponding proteins without the mutations, were transiently expressed in asynchronous U2OS cells and visualized by fluorescence microscopy. Cells showing the most frequently observed fluorescence pattern are shown. Nuclei were stained with Hoechst dye. Bar, 10 ⁇ m.
  • D The subcellular localization of wild type and mutant GFP-HDHB and GFP- ⁇ Gal-PSLD in asynchronous U2OS cells was quantified and expressed as a percentage of the total number of GFP-positive cells in that sample.
  • FIG. 7 is the cell cycle-dependent phosphorylation of FLAG-HDHB in vivo.
  • U2OS cells transiently expressing FLAG-HDHB (lane 1) and its truncation mutants 1-1039 (lane 2) and 1-874 (lane 3) were labeled with [32P] ortho-phosphate.
  • Cell extracts were immunoprecipitated with anti-FLAG resin.
  • the precipitated proteins were separated by 7.5% SDS-PAGE, transferred to a PVDF membrane, and detected by autoradiography (top) or western blotting (bottom). The positions of marker proteins of known molecular mass are indicated at the left.
  • FLAG-HDHB expressed in U2OS cells was immunoprecipitated with anti-FLAG resin, incubated with (+) or without ( ⁇ ) ⁇ -phosphatase ( ⁇ -PPase) in the presence (+) or absence ( ⁇ ) of phosphatase inhibitors, as indicated, and analyzed by SDS-PAGE and immunoblotting with anti-HDHB antibody.
  • FIG. 8 is the identification of a major in vivo phosphorylation site in HDHB.
  • FIG. 9 is the identification of cyclin E/CDK2 as the potential G1/S kinase of HDHB S967.
  • FIG. 10 is the subcellular localization of HDHB is regulated by phosphorylation of S967.
  • FIG. 11 is the vector map of pCORON1002-EGFP-C1-PSLD.
  • FIG. 12 is the vector map of pCORON1002-EGFP-C1- ⁇ Gal-PSLD.
  • FIG. 13 is the EGFP-C1-PSLD transiently expressed in U2-OS cells. Image obtained on In Cell Analyzer 1000, 10 ⁇ objective (Hoechst nuclear stain also shown to highlight a number of untransfected/low expressing cells).
  • FIG. 14 is the EGFP-C1-PSLD stably expressed in U2-OS cells. IN Cell Analyzer 1000, 10 ⁇ objective.
  • FIG. 15 is the flow cytometry data comparing brightness and homogeneity of signal for representative stable cell lines developed with pCORON1002-EGFP-C1-PSLD, pCORON1002-EGFP-C1-Gal-PSLD and the parental U2OS cell line.
  • pGFP-HDHB and mutant derivatives of it were created by inserting the full-length HDHB cDNA as a Bglll/NotI fragment (Taneja et al., J. Biol. Chem., (2002) 277, 40853-40861) into the NotI site of the pEGFP-C1 vector (Clontech, Palo Alto, Calif.).
  • pFLAG-HDHB was constructed by inserting a HindIII/NotI fragment containing full-length HDHB cDNA into the NotI site of pFlag-CMV2 vector (Eastman Kodak Co., Rochester, N.Y.).
  • Tagged HDHB-.SLD (1-1039) was constructed by cleaving the tagged HDHB plasmid with Nrul following the coding sequence for residue 1034 and with NotI in the polylinker and replacing the small fragment by a duplex adaptor oligonucleotide with a blunt end encoding residues 1035 to 1039, a stop codon, and an overhanging NotI-compatible 5′ end.
  • pFLAG-HDHB (1-874
  • Stul-digested pFLAG-HDHB DNA was treated with Klenow polymerase to generate blunt ends and ligated into the pFLAG-CMV2 vector.
  • pEGFP- ⁇ Gal a DNA fragment encoding E.
  • coli ⁇ -galactosidase ( ⁇ Gal) was amplified by PCR from the p ⁇ Gal-control vector (Clontech) and inserted in frame at the 3′ end of the GFP coding sequence in pEGFP-C1, using the HindIII restriction site.
  • the HDHB coding sequence for amino acids 1040-1087(SLD) and 957-1087(PSLD) were PCR amplified and inserted in frame at the 3′ end of the ⁇ Gal cDNA in pEGFP- ⁇ Gal to create pGFP- ⁇ Gal-SLD and pGFP- ⁇ Gal-PSLD respectively.
  • the HDHB Walker A and Walker B mutants, MutA and MutB, were described previously (Taneja et al., J. Biol. Chem., (2002) 277, 40853-40861).
  • the NES mutants and phosphorylation site mutants were created in the HDHB cDNA by site-directed mutagenesis (QuikChange, Stratagene, La Jolla, Calif.) according to the manufacturer's protocol using Pfu Turbo polymerase (Stratagene) and oligonucleotides containing the desired DNA sequence changes as primers in the PCR reactions.
  • pCORON1002-EGFP-C1-PSLD was constructed by PCR amplification of the 390 bp PSLD region from the DNA construct pEGFP-Cl-Gal-PSLD (see above). Introduction of a NheI restriction enzyme site at the 5′ end and a SalI restriction enzyme site at the 3′ end of the PSLD fragment allowed for sub-cloning into the in house vector pCORON1002-EGFP-C1.
  • the resulting 6704 bp DNA construct pCORON1002-EGFP-C1-PSLD contains an ubiquitin C promoter, a bacterial ampicillin resistance gene and a mammalian neomycin resistance gene ( FIG. 11 ).
  • the nucleic acid sequence of the vector is shown in SEQ ID No. 3.
  • pCORON1002-EGFP-C1-Gal-PSLD was constructed by NheI and XmaI restriction enzyme digest of the DNA construct pEGFP-Cl-Gal-PSLD (see above).
  • the 4242 bp EGFP-C1-Gal-PSLD fragment was then ligated into the NheI and XmaI restriction enzyme digested pCORON1002 vector.
  • the resulting 9937 bp DNA construct pCORON1002-EGFP-C1-Gal-PSLD contains an ubiquitin C promoter, a bacterial ampicillin resistance gene and a mammalian neomycin resistance gene ( FIG. 12 ).
  • the nucleic acid sequence of the vector is shown in SEQ ID No. 4.
  • Anti-HDHB antibody was generated against purified recombinant HDHB (Bethyl Laboratories, Montgomery, Tex.) and affinity-purified on immobilized HDHB (Harlow & Lane, Antibodies: A laboratory manual. Cold Spring Harbor Laboratory). Initial characterization of these antibodies revealed that they were not ideal for indirect immunofluorescence or immunoprecipitation, but detected purified recombinant HDHB and endogenous HDHB in human cell extracts by western blotting.
  • U2OS cells were cultured as exponentially growing monolayers in Dulbecco-modified Eagle medium (DMEM) (Gibco BRL Lifetechnologies, Carlsbad, Calif.) supplemented with 10% fetal bovine serum (FBS) (Atlanta Biologicals, Norcross, Ga.) at 37° C. Exponentially growing U2OS cells were arrested at G1/S in incubation in DMEM containing 5 mM thymidine (Sigma-Aldrich, St. Louis, Mo.), for 24 h. To release the cells into S phase, the medium was aspirated and the cells washed three times with warm DMEM plus 10% FBS, and incubated in fresh DMEM plus 10% FBS.
  • DMEM Dulbecco-modified Eagle medium
  • FBS fetal bovine serum
  • U2OS cells Exponentially growing U2OS cells were arrested in G2/M for 16 h in DMEM containing 30 ng/ml nocodazole (Sigma-Aldrich). To release cells into G1, mitotic cells were collected by gently shaking them off, washed three times with DMEM plus 10% FBS, and then plated on glass coverslips for microinjection, or in culture dishes for further manipulation.
  • the cells were incubated with Texas Red-conjugated goat anti-mouse secondary antibody (Jackson ImmunoResearch Laboratories, West Grove, Pa.) at a dilution of 1:100 in PBS plus 10% FBS for 1 h at room temperature. After three washes, the cells were incubated for 10 min with Hoechst 33258 at a concentration of 2 ⁇ M in PBS. Coverslips were mounted in ProLong Antifade (Molecular Probes, Eugene, Oreg.). Images were obtained with a Hamamatsu digital camera using the Openlab 3.0 software (Improvision, Lexington, Mass.) on the Zeiss Axioplan 2 Imaging system (Carl Zeiss Inc.). The number of cells that exhibited each pattern of subcellular localization was counted and expressed as a percentage of the total number of cells scored (100 to 150 cells in each experiment). The subcellular distribution of each protein was quantitatively evaluated in at least two independent experiments.
  • Triton X-100 extraction cells were washed twice with cold cytoskeleton buffer (CSK, 10 mM HEPES [pH 7.4], 300 mM sucrose, 100 mM NaCl, 3 mM MgCl2), and extracted for 5 min on ice with 0.5% Triton X-100 in CSK buffer (supplemented with 1 ⁇ protease inhibitors) and then fixed as described above.
  • CSK cold cytoskeleton buffer
  • HEPES 10 mM HEPES [pH 7.4]
  • sucrose 100 mM NaCl
  • 3 mM MgCl2 3 mM MgCl2
  • Flourescence microscopy was conducted using a confocal imaging system (In Cell Analyzer 1000, GE Healthcare, Amersham, UK) on cells transfected with the pCORON1002-EGFP-C1-PSLD or the pCORON1002-EGFP-C1-Gal-PSLD vectors.
  • U2OS cells (80-90% confluent) were transfected with pGFP-HDHB according to the manufacturer's protocol (Lipofectamine 2000, Invitrogen). At 4 h after transfection, cells were treated with DMSO (control), 20 ⁇ M etoposide, 10 ⁇ M camptothecin, or 1 ⁇ M mitomycin C. After 20 h, cells were extracted with Triton X-100 buffer and then fixed for immunofluorescence as described above. Distinctive GFP-HDHB nuclear foci were counted in more than 100 cells in each independent assay.
  • Asynchronously growing U2OS cells (5 ⁇ 10 6 ) were trypsinized, collected by centrifugation, and resuspended in 800 ⁇ l of 20 mM HEPES (pH 7.4), 0.7 mM Na2HPO4/NaH2PO4, 137 mM NaCl, 5 mM KCl, 6 mM glucose at a final pH of 7.4.
  • Ten ⁇ g of DNA was added and the mixture was transferred to a 0.4 cm electroporation cuvette (BioRad, Hercules, Calif.). Electroporation was performed using a Gene Pulser II apparatus and Gene Pulser II RF module (BioRad) at 300 V, 600 ⁇ F. Cells were then plated in tissue culture dishes, and 1 h later, washed with fresh medium and cultured for another 23 h.
  • U-2OS cells were transiently transfected with either the pCORON1002-EGFP-C1-PSLD vector ( FIG. 11 ) or the pCORON1002-EGFP-C1-Gal-PSLD vector ( FIG. 12 ) using FuGENE 6 (Roche Biochemicals).
  • FuGENE 6 FuGENE 6: DNA ratios were investigated. Working with transiently transfected cells proved difficult due to low transfection efficiency and heterogeneity of expression.
  • U-2OS cells were transfected with either the pCORON1002-EGFP-C1-PSLD vector or the pCORON1002-EGFP-C1-Gal-PSLD vector using FuGENE 6.
  • Stable clones expressing the recombinant fusion protein were selected using 1 mg/ml Geneticin G418 (Sigma). Isolated clones were chosen, FACS analysed, grown in culture and stocks frozen.
  • U2OS cells (2.5 ⁇ 10 6 ) were transiently transfected with wild type or mutant FLAGHDHB by electroporation. After 24 h, cells were incubated in phosphate-depleted DMEM (Gibco BRL Lifetechnologies) for 15 min and then radiolabeled with 32P-H3PO4 (0.35 mCi per ml of medium; ICN Pharmaceuticals Inc., Costa Mesa, Calif.) for 4 h. Phosphate-labeled FLAG-HDHB was immunoprecipitated from extracts, separated by 7.5% SDS/PAGE, and then transferred to a polyvinylidene difluoride (PVDF) membrane as described below.
  • PVDF polyvinylidene difluoride
  • FLAG-HDHB-transfected cultures to be analyzed by immunoprecipitation and immunoblotting were lysed in lysis buffer (50 mM Tris-HCl pH 7.5, 10% glycerol, 0.1% NP-40, 1 mM DTT, 25 mM NaF, 100 ⁇ g/ml PMSF, 1 ⁇ g/ml aprotinin, 1 ⁇ g/ml leupeptin) (0.5 ml per 35 mm or 1 ml per 60 mm dish or 75 cm flask).
  • the extract was scraped off the dish, incubated for 5 min on ice, and centrifuged for 10 min at 14 000 g.
  • U2OS cells For selective nuclear and cytoplasmic protein extraction, 80-90% confluent U2OS cells were harvested by trypsinization and washed with PBS. They were resuspended and lysed in 10 mM Tris-HCl [pH 7.5], 10 mM KCl, 1.5 mM MgCl2, 0.25 M sucrose, 10% glycerol, 75 ⁇ g/ml digitonin, 1 mM DTT, 10 mM NaF, 1 mM Na3VO4, 100 ⁇ g/ml PMSF, 1 ⁇ g/ml aprotinin, and 1 ⁇ g/ml leupeptin for 10 min on ice, and centrifuged at 1000 ⁇ g for 5 min.
  • 10 mM Tris-HCl [pH 7.5] 10 mM KCl, 1.5 mM MgCl2, 0.25 M sucrose, 10% glycerol, 75 ⁇ g/ml digitonin, 1 mM DTT
  • the supernatant fraction was collected as the cytosolic extract.
  • the pellet washed, resuspended in high salt buffer (10 mM Tris-HCl [pH 7.5], 400 mM NaCl 1 mM EDTA, 1 mM EGTA, 1 mM DTT, 1% NP-40, 100 ⁇ g/ml PMSF, 1 ⁇ g/ml aprotinin, and 1 ⁇ g/ml leupeptin), and rocked for 10 min at 4° C. After sonication, the suspended material, containing both soluble and chromatin-bound protein, was analyzed as nuclear extract. Proteins in the nuclear and cytoplasmic extracts were analyzed by 8.5% SDS-PAGE, followed by western blotting with antibodies against ⁇ -tubulin, PCNA (both Santa Cruz Biotechnology), and recombinant HDHB.
  • FLAG-HDHB bound to anti-FLAG beads was incubated with 100 U of ⁇ -phosphatase (New England Biolabs, Beverly, Mass.) in phosphatase buffer (50 mM Tris-HCl [pH 7.5], 0.1 mM EDTA, 0.01% NP-40) for 1 h at 30° C. The reaction was carried out in the presence or absence of phosphatase inhibitors (5 mM Na3VO4, 50 mM NaF). The proteins were separated by 7.5% SDSPAGE (acrylamide-bisacrylamide ratio, 30:0.36) and HDHB was detected by western blotting with anti-HDHB-peptide serum and chemiluminescence.
  • phosphatase buffer 50 mM Tris-HCl [pH 7.5], 0.1 mM EDTA, 0.01% NP-40
  • radiolabeled FLAG-HDHB-transfected cultures to be used for immunoprecipitation and phosphoamino acid or phosphopeptide mapping were processed as above, except that lysis buffer was substituted by RIPA buffer (50 mM Tris-HCl [pH7.5], 150 mM NaCl, 1% NP-40, 0.5% deoxycholic acid, 1% SDS, 50 mM NaF, 1 mM EDTA, 5 mM Na3VO4, 100 ⁇ g/ml PMSF, 1 ⁇ g/ml aprotinin, and 1 ⁇ g/ml leupeptin). Immunoprecipitated proteins were separated by 7.5% SDS-PAGE and transferred to PVDF membranes.
  • RIPA buffer 50 mM Tris-HCl [pH7.5], 150 mM NaCl, 1% NP-40, 0.5% deoxycholic acid, 1% SDS, 50 mM NaF, 1 mM EDTA, 5 mM Na3VO4, 100 ⁇ g/
  • the membranes containing radiolabeled HDHB were rinsed well with deionized H2O twice before visualization of phosphoproteins by autoradiography.
  • the phosphoproteins were then excised, and the membrane pieces were re-wet with methanol followed by water.
  • the membranes were blocked with 50 mM NH4HCO3 containing 0.1% Tween 20 (Sigma-Aldrich) for 30 min at room temperature and washed three times with 50 mM NH4HCO3 before enzymatic cleavage of phosphoproteins from the PVDF with L-(tosylamido-2-phenyl)ethyl chloromethyl ketonetreated bovine pancreatic trypsin (Worthington, Lakewood, N.J.).
  • the peptides were then subjected to two-dimensional phosphopeptide mapping or phosphoamino acid analysis as described in detail elsewhere (Boyle et al., Meth. Enzymology, (1991), 201, 110-149
  • FIG. 1A To determine the subcellular localization of endogenous HDHB, nuclear and cytoplasmic proteins were selectively extracted from human U2OS cells, separated by denaturing gel electrophoresis, and analyzed by western blotting ( FIG. 1A ). The presence of PCNA and ⁇ -tubulin in each extract was first monitored to assess the extraction procedure. PCNA was enriched in the nuclear extract and not in the cytoplasmic fraction, while ⁇ -tubulin was found primarily in the cytoplasmic fraction, validating the fractionation. HDHB was detected in both the nuclear and cytoplasmic fractions ( FIG. 1A ). The cytoplasmic HDHB migrated more slowly than the nuclear fraction ( FIG. 1A ), suggesting the possibility of post-translational modification.
  • HDHB was distributed throughout the cell, or that a mixed population of cells contained HDHB in either the nucleus or the cytoplasm.
  • HDHB was localized in situ in single cells. Since endogenous HDHB was not detectable by indirect immunofluorescence with antisera (data not shown), GFP- and FLAG-tagged HDHB were expressed in human U2OS cells by transient transfection. Transiently over-expressed tagged HDHB accumulated in greater amounts than the endogenous HDHB within 24 h ( FIG. 1B ). Since prolonged over-expression of tagged or untagged HDHB was cytotoxic, all experiments were conducted in the shortest time period possible (usually 24 h). Tagged HDHB localization was analyzed in individual cells by fluorescence microscopy.
  • GFP-HDHB and FLAG-HDHB displayed two major patterns of localization, either in the nucleus in discrete foci or in the cytoplasm ( FIG. 1C , D).
  • the localization patterns of HDHB tagged with the GFP protein were not detectably different than those of HDHB tagged with the FLAG peptide.
  • GFP-HDHB transiently expressed in primary human fibroblasts was also observed in either the nucleus or the cytoplasm (data not shown).
  • HDHB localization in nuclear foci depends on its biochemical activity, it is likely that HDHB executes its function in those nuclear foci.
  • the specificity of HDHB for DNA as a substrate and its sequence homology with bacterial RecD and T4 dda proteins suggests that HDHB might be involved in DNA damage signaling, processing, or repair.
  • DNA damage induces nuclear foci that are thought to contain damage-sensing and -processing proteins (Nelms et al., Science, (1998) 280, 590-592; van den Bosch et al. EMBO Rep., (2003), 4, 844-849).
  • U2OS cells transiently expressing GFP-HDHB were treated with the DNA damaging agents etoposide, camptothecin, and mitomycin C, or with DMSO as a control ( FIG. 2 ).
  • the protein carries its own nuclear location signal (NLS) and/or nuclear export signal (NES), motifs that are recognized by nuclear import or export machinery (Gorlich & Kutay, Rev. Cell Dev. Biol., (1999), 15, 607-660; Hood & Silver, Biochim. Biophys. Acta., (2000), 1471, M31-M41; Weis, Cell, (2003), 112, 441-451; Fabbro & Henderson, Exp. Cell. Res., (2003) 282, 59-69).
  • NLS nuclear location signal
  • NES nuclear export signal
  • HDHB may carry a NLS that is impaired or abolished by the C-terminal deletion in GFP-HDHB- ⁇ SLD.
  • a bacterial ⁇ -galactosidase ( ⁇ Gal) was used as a reporter protein because it has a molecular mass (112 kDa) close to that of HDHB and does not contain subcellular localization signals (Kalderon et al., Cell, (1984), 39, 499-509).
  • ⁇ Gal bacterial ⁇ -galactosidase
  • FIG. 4B GFP- ⁇ Gal expression vector
  • GFP- ⁇ Gal-SLD was found in both the nucleus and cytoplasm in asynchronous or synchronized U2OS cells ( FIG. 5 ), suggesting that SLD contains a NLS, but was not sufficient for nuclear localization of the reporter protein.
  • FIG. 4A a GFP- ⁇ Gal-PSLD was constructed, in which the C-terminal 131 residues of HDHB, containing the putative SLD and the cluster of potential CDK phosphorylation sites, were appended to the C-terminus of GFP- ⁇ Gal ( FIG. 4B ).
  • GFP- ⁇ Gal-PSLD plasmid DNA was transiently expressed in asynchronous and synchronized U2OS cells, GFP- ⁇ Gal-PSLD was found in the nucleus in over 90% of G1 phase cells, and in the cytoplasm in more than 70% of S phase cells ( FIG. 5 ).
  • GFP- ⁇ Gal-PSLD protein was distributed evenly throughout the nucleus in G1, sparing only the nucleoli ( FIG. 5 ).
  • HDHB contains a rev-type NES that functions through CRM1.
  • HDHB may not be a direct cargo of CRM1 and that its export may be indirectly mediated through some other protein(s).
  • FIG. 6A To assess whether the putative NES in HDHB was functional, we mutated Val/Leu and Leu/Leu of the NES motif to alanine to create NES mutants 1 and 2 ( FIG. 6A ). GFP-HDHB and GFP- ⁇ Gal-PSLD harboring these NES mutations were transiently expressed in either asynchronous or synchronized U2OS cells.
  • FLAG-HDHB is Phosphorylated in a Cell Cycle-Dependent Manner In Vivo.
  • the cluster of potential CDK phosphorylation sites in the PSLD domain of HDHB ( FIG. 4A ) suggested that phosphorylation of HDHB might regulate its subcellular localization in the cell cycle. If so, one would expect the PSLD region of HDHB to be phosphorylated in a cell cycle-dependent manner.
  • U2OS cells were transiently transfected with expression plasmids for wild type and C-terminally truncated forms of FLAG-HDHB, radiolabeled with phosphate, and then FLAG-HDHB was immunoprecipitated from cell extracts. Immunoprecipitated proteins were analyzed by denaturing gel electrophoresis, immunoblotting, and autoradiography ( FIG.
  • U2OS cells transiently expressing FLAG-HDHB were arrested in G1/S by adding thymidine to the medium or in G2/M by adding nocodazole to the medium. The cells were released from the blocks for different time periods, and FLAG-HDHB was immunoprecipitated from cell extracts.
  • the immunoprecipitated material was incubated with or without ⁇ -PPase and then analyzed by denaturing gel electrophoresis and western blotting ( FIG. 7C ).
  • the mobility of FLAG-HDHB from cells arrested at G1/S was increased by ⁇ -PPase treatment, suggesting that the protein was phosphorylated at G1/S ( FIG. 7C , upper panel).
  • a similar mobility shift was detected after phosphatase treatment of FLAG-HDHB for at least nine hours after release from the G1/S block (upper panel), as well as in cells arrested at G2/M ( FIG. 7C , lower panel).
  • Serine 967 is the Major Phosphorylation Site of Ectopically Expressed HDHB.
  • Wild type GFP-HDHB accumulated in nuclear foci of cells in G1, but in the cytoplasm of cells in S phase as expected (not shown). However, regardless of cell cycle timing, GFP-HDHB-S967A localized in nuclear foci in about 70% of the fluorescent cells ( FIG. 9A , B). The other three substitution mutants localized in either the nucleus or the cytoplasm like wild type GFP-HDHB (data not shown). In an attempt to mimic the phosphorylation of S967, serine 967 was mutated to aspartic acid, GFP-HDHB-S967D was expressed in asynchronous and synchronized U2OS cells, and the subcellular distribution of the mutant fusion protein was examined.
  • wild type GFP-HDHB resides in prominent nuclear foci that are associated with detergent-insoluble nuclear structures ( FIG. 1, 2 , 3 ).
  • the topoisomerase inhibitors etoposide and camptothecin induced many more HDHB nuclear foci, suggesting that the helicase activity may be recruited to process sites of DNA damage ( FIG. 2 ).
  • Many proteins involved in DNA double strand break repair and recombination show a similar focal relocalization in response to DNA damage at different times in the cell cycle, including Rad52, RPA, Mre11, Ddc1 Ddc2, Rad9, Rad24, Rad51, Rad53, Rad54, and Rad55 (e.g. Lisby et al., 2003).
  • HDHB helicase-defective HDHB mutants to block G1/S progression when the mutant protein is injected into cells in early G1 (Taneja et al., J. Biol. Chem., (2002), 277, 40853-40861).
  • the MutB form of HDHB would associate with the damage, but could not process it properly, interfering with the activity of the endogenous helicase and leading to cell cycle arrest in late G1.
  • HDHB DNA replication
  • the predominantly cytoplasmic localization of HDHB during S and G2/M argues that HDHB is probably not directly involved in genomic DNA replication ( FIG. 3 ).
  • the helicase-defective HDHB did not affect DNA synthesis when injected into the nucleus of cells in late G1 or S phase (Taneja et al., 2002), probably because the injected protein was phosphorylated by CDK2 associated with cyclins E or A and rapidly targeted to the cytoplasm.
  • export to the cytoplasm may serve as a means to sequester HDHB when its function is not needed or might be detrimental in the nucleus.
  • HDHB activity is not affected by CDK phosphorylation in vitro (Taneja and Fanning, unpublished data), and prolonged over-expression of helicase-proficient HDHB mutants that cannot be exported prevents the G1/S transition and eventually results in apoptosis (Gu and Fanning, unpublished data).
  • a 131-residue domain, PSLD is sufficient to target HDHB or a ⁇ Gal reporter to either the nucleus or the cytoplasm in a cell cycle-dependent manner ( FIG. 5 ).
  • a rev-type NES resides in this domain ( FIG. 6 ), but its activity or accessibility to the nuclear export machinery depends on phosphorylation of PSLD, primarily on serine 967, at the G1/S transition ( FIG. 7-10 ).
  • S967 is a perfect match to the consensus CDK substrate recognition motif (S/T)PX(K/R).
  • cyclin E/CDK2 and cyclin A/CDK2 can modify HDHB in vitro, but the ability of cyclin E/CDK2 to complex with HDHB in cell extracts suggests that it may be the initial kinase that modifies HDHB at the G1/S transition ( FIG. 9 ). Phosphorylation of HDHB in PSLD appears to persist through the latter part of the cell cycle, correlating well with the predominantly cytoplasmic localization of HDHB. However, it is not possible yet to distinguish whether HDHB undergoes dephosphorylation at the M/G1 transition ( FIG. 7C ) or is perhaps targeted for proteolysis and rapidly re-synthesized in early G1, when it would enter the nucleus.
  • U-2OS cells were transiently transfected with pCORON1002-EGFP-C1-PSLD ( FIG. 11 ), pCORON1002-EGFP-C1-Gal-PSLD ( FIG. 12 ) or J-Red derivatives of the above vectors.
  • Cells transiently expressing EGFP-C1-PSLD were obtained ( FIG. 13 ) but proved difficult to work with due to the heterogeneity of expression and variable signal.
  • Stable clones expressing the recombinant fusion proteins were selected using 1 mg/ml G418 (Sigma) or hygromycin, where appropriate. Isolated primary clones ( ⁇ 60 per construct) were analysed by flow cytometry to confirm the level and homogeneity of expression of the sensor and where appropriate secondary clones were developed using methods above.
  • US-OS cells stably expressing EGFP-C1-PSLD (Clone 22 ; FIG. 14 ) and EGFP-C1-Gal-PSLD—were selected which were homogeneous in nature and provided a bright, uniform signal. These cells are much more useful for providing sensitive, stable and uniform assays for investigating the cell cycle and the effect of agents upon the cell cycle.
  • the fluorescent signals generated by the stable cell lines developed with pCORON1002-EGFP-C1-Gal-PSLD and pCORON1002-EGFP-C1-PSLD are shown in FIG. 15 .

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