Abstract:
PDGF-D, a new member of the PDGF/VEGF family of growth factors, is described, as well as the nucleotide sequence encoding it, methods for producing it, antibodies and other antagonists to it, transfected and transformed host cells expressing it, pharmaceutical compositions containing it, and uses thereof in medical and diagnostic applications.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/107,852, filed Nov. 10, 1998; U.S. Provisional Application No. 60/113,997, filed Dec. 28, 1998; U.S. Provisional Application No. 60/150,604, filed Aug. 26, 1999; U.S. Provisional Application No. 60/157,108, filed Oct. 4, 1999; and U.S. Provisional Application No. 60/157,756, filed Oct. 5, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to growth factors for cells expressing receptors to a novel growth factor that include endothelial cells, connective tissue cells (such as fibroblasts) myofibroblasts and glial cells, and in particular to a novel platelet-derived growth factor/vascular endothelial growth factor-like growth factor, a polynucleotide sequence encoding the factor, and to pharmaceutical and diagnostic compositions and methods utilizing or derived from the factor. 
     BACKGROUND OF THE INVENTION 
     In the developing embryo, the primary vascular network is established by in situ differentiation of mesodermal cells in a process called vasculogenesis. It is believed that all subsequent processes involving the generation of new vessels in the embryo and neovascularization in adults, are governed by the sprouting or splitting of new capillaries from the pre-existing vasculature in a process called angiogenesis (Pepper et al., Enzyme &amp; Protein, 1996 49 138-162; Breier et al., Dev. Dyn. 1995 204 228-239; Risau, Nature, 1997 386 671-674). Angiogenesis is not only involved in embryonic development and normal tissue growth, repair, and regeneration, but is also involved in the female reproductive cycle, establishment and maintenance of pregnancy, and in repair of wounds and fractures. In addition to angiogenesis which takes place in the normal individual, angiogenic events are involved in a number of pathological processes, notably tumor growth and metastasis, and other conditions in which blood vessel proliferation, especially of the microvascular system, is increased, such as diabetic retinopathy, psoriasis and arthropathies. Inhibition of angiogenesis is useful in preventing or alleviating these pathological processes. 
     On the other hand, promotion of angiogenesis is desirable in situations where vascularization is to be established or extended, for example after tissue or organ transplantation, or to stimulate establishment of collateral circulation in tissue infarction or arterial stenosis, such as in coronary heart disease and thromboangitis obliterans. 
     The angiogenic process is highly complex and involves the maintenance of the endothelial cells in the cell cycle, degradation of the extracellular matrix, migration and invasion of the surrounding tissue and finally, tube formation. The molecular mechanisms underlying the complex angiogenic processes are far from being understood. 
     Because of the crucial role of angiogenesis in so many physiological and pathological processes, factors involved in the control of angiogenesis have been intensively investigated. A number of growth factors have been shown to be involved in the regulation of angiogenesis; these include fibroblast growth factors (FGFs), platelet-derived growth factor (PDGF), transforming growth factor alpha (TGFα), and hepatocyte growth factor (HGF). See for example Folkman et al., J. Biol. Chem., 1992 267 10931-10934 for a review. 
     It has been suggested that a particular family of endothelial cell-specific growth factors, the vascular endothelial growth factors (VEGFs), and their corresponding receptors is primarily responsible for stimulation of endothelial cell growth and differentiation, and for certain functions of the differentiated cells. These factors are members of the PDGF family, and appear to act primarily via endothelial receptor tyrosine kinases (RTKs). Nine different proteins have been identified in the PDGF family, namely two PDGFs (A and B), VEGF and six members that are closely related to VEGF. The six members closely related to VEGF are: VEGF-B, described in International Patent Application PCT/US96/02957 (WO 96/26736) and in U.S. Pat. Nos. 5,840,693 and 5,607,918 by Ludwig Institute for Cancer Research and The University of Helsinki; VEGF-C, described in Joukov et al., EMBO J., 1996 15 290-298 and Lee et al., Proc. Natl. Acad. Sci. USA, 1996 93 1988-1992; VEGF-D, described in International Patent Application No. PCT/US97/14696 (WO 98/07832), and Achen et al., Proc. Natl. Acad. Sci. USA, 1998 95 548-553; the placenta growth factor (PlGF), described in Maglione et al., Proc. Natl. Acad. Sci. USA, 1991 88 9267-9271; VEGF2, described in International Patent Application No. PCT/US94/05291 (WO 95/24473) by Human Genome Sciences, Inc; and VEGF3, described in International Patent Application No. PCT/US95/07283 (WO 96/39421) by Human Genome Sciences, Inc. Each VEGF family member has between 30% and 45% amino acid sequence identity with VEGF. The VEGF family members share a VEGF homology domain which contains the six cysteine residues which form the cysteine knot motif. Functional characteristics of the VEGF family include varying degrees of mitogenicity for endothelial cells, induction of vascular permeability and angiogenic and lymphangiogenic properties. 
     Vascular endothelial growth factor (VEGF) is a homodimeric glycoprotein that has been isolated from several sources. VEGF shows highly specific mitogenic activity for endothelial cells. VEGF has important regulatory functions in the formation of new blood vessels during embryonic vasculogenesis and in angiogenesis during adult life (Carmeliet et al., Nature, 1996 380 435-439; Ferrara et al., Nature, 1996 380 439-442; reviewed in Ferrara and Davis-Smyth, Endocrine Rev., 1997 18 4-25). The significance of the role played by VEGF has been demonstrated in studies showing that inactivation of a single VEGF allele results in embryonic lethality due to failed development of the vasculature (Carmeliet et al., Nature, 1996 380 435-439; Ferrara et al., Nature, 1996 380 439-442). In addition VEGF has strong chemoattractant activity towards monocytes, can induce the plasminogen activator and the plasminogen activator inhibitor in endothelial cells, and can also induce microvascular permeability. Because of the latter activity, it is sometimes referred to as vascular permeability factor (VPF). The isolation and properties of VEGF have been reviewed; see Ferrara et al., J. Cellular Biochem., 1991 47 211-218 and Connolly, J. Cellular Biochem., 1991 47 219-223. Alterative mRNA splicing of a single VEGF gene gives rise to five isoforms of VEGF. 
     VEGF-B has similar angiogenic and other properties to those of VEGF, but is distributed and expressed in tissues differently from VEGF. In particular, VEGF-B is very strongly expressed in heart, and only weakly in lung, whereas the reverse is the case for VEGF. This suggests that VEGF and VEGF-B, despite the fact that they are co-expressed in many tissues, may have functional differences. 
     VEGF-B was isolated using a yeast co-hybrid interaction trap screening technique by screening for cellular proteins which might interact with cellular resinoid acid-binding protein type I (CRABP-I). Its isolation and characteristics are described in detail in PCT/US96/02957 and in Olofsson et al., Proc. Natl. Acad. Sci. USA, 1996 93 2576-2581. 
     VEGF-C was isolated from conditioned media of the PC-3 prostate adenocarcinoma cell line (CRL1435) by screening for ability of the medium to produce tyrosine phosphorylation of the endothelial cell-specific receptor tyrosine kinase VEGFR-3 (Flt4), using cells transfected to express VEGFR-3. VEGF-C was purified using affinity chromatography with recombinant VEGFR-3, and was cloned from a PC-3 cDNA library. Its isolation and characteristics are described in detail in Joukov et al., EMBO J., 1996 15 290-298. 
     VEGF-D was isolated from a human breast cDNA library, commercially available from Clontech, by screening with an expressed sequence tag obtained from a human cDNA library designated “Soares Breast 3NbHBst” as a hybridization probe (Achen et al., Proc. Natl. Acad. Sci. USA, 1998 95 548-553). Its isolation and characteristics are described in detail in International Patent Application No. PCT/US97/14696 (WO98/07832). 
     The VEGF-D gene is broadly expressed in the adult human, but is certainly not ubiquitously expressed. VEGF-D is strongly expressed in heart, lung and skeletal muscle. Intermediate levels of VEGF-D are expressed in spleen, ovary, small intestine and colon, and a lower expression occurs in kidney, pancreas, thymus, prostate and testis. No VEGF-D mRNA was detected in RNA from brain, placenta, liver or peripheral blood leukocytes. 
     PlGF was isolated from a term placenta cDNA library. Its isolation and characteristics are described in detail in Maglione et al., Proc. Natl. Acad. Sci. USA, 1991 88 9267-9271. Presently its biological function is not well understood. 
     VEGF2 was isolated from a highly tumorgenic, oestrogen-independent human breast cancer cell line. While this molecule is stated to have about 22% homology to PDGF and 30% homology to VEGF, the method of isolation of the gene encoding VEGF2 is unclear, and no characterization of the biological activity is disclosed. 
     VEGF3 was isolated from a cDNA library derived from colon tissue. VEGF3 is stated to have about 36% identity and 66% similarity to VEGF. The method of isolation of the gene encoding VEGF3 is unclear and no characterization of the biological activity is disclosed. 
     Similarity between two proteins is determined by comparing the amino acid sequence and conserved amino acid substitutions of one of the proteins to the sequence of the second protein, whereas identity is determined without including the conserved amino acid substitutions. 
     PDGF/VEGF family members act primarily by binding to receptor tyrosine kinases. Five endothelial cell-specific receptor tyrosine kinases have been identified, namely VEGFR-1 (Flt-1), VEGFR-2 (KDR/Flk-1), VEGFR-3 (Flt4), Tie and Tek/Tie-2. All of these have the intrinsic tyrosine kinase activity which is necessary for signal transduction. The essential, specific role in vasculogenesis and angiogenesis of VEGFR-1, VEGFR-2, VEGFR-3, Tie and Tek/Tie-2 has been demonstrated by targeted mutations inactivating these receptors in mouse embryos. 
     The only receptor tyrosine kinases known to bind VEGFs are VEGFR-1, VEGFR-2 and VEGFR-3. VEGFR-1 and VEGFR-2 bind VEGF with high affinity, and VEGFR-1 also binds VEGF-B and PlGF. VEGF-C has been shown to be the ligand for VEGFR-3, and it also activates VEGFR-2 (Joukov et al., The EMBO Journal, 1996 15 290-298). VEGF-D binds to both VEGFR-2 and VEGFR-3. A ligand for Tek/Tie-2 has been described in International Patent Application No. PCT/US95/12935 (WO 96/11269) by Regeneron Pharmaceuticals, Inc. The ligand for Tie has not yet been identified. 
     Recently, a novel 130-135 kDa VEGF isoform specific receptor has been purified and cloned (Soker et al., Cell, 1998 92 735-745). The VEGF receptor was found to specifically bind the VEGF 165  isoform via the exon 7 encoded sequence, which shows weak affinity for heparin (Soker et al., Cell, 1998 92 735-745). Surprisingly, the receptor was shown to be identical to human neuropilin-1 (NP-1), a receptor involved in early stage neuromorphogenesis. PlGF-2 also appears to interact with NP-1 (Migdal et al., J. Biol. Chem., 1998 273 22272-22278). 
     VEGFR-1, VEGFR-2 and VEGFR-3 are expressed differently by endothelial cells. Both VEGFR-1 and VEGFR-2 are expressed in blood vessel endothelia (Oelrichs et al., Oncogene, 1992 8 11-18; Kaipainen et al., J. Exp. Med., 1993 178 2077-2088; Dumont et al., Dev. Dyn., 1995 203 80-92; Fong et al., Dev. Dyn., 1996 207 1-10) and VEGFR-3 is mostly expressed in the lymphatic endothelium of adult tissues (Kaipainen et al., Proc. Natl. Acad. Sci. USA, 1995 9 3566-3570). VEGFR-3 is also expressed in the blood vasculature surrounding tumors. 
     Disruption of the VEGFR genes results in aberrant development of the vasculature leading to embryonic lethality around midgestation. Analysis of embryos carrying a completely inactivated VEGFR-1 gene suggests that this receptor is required for functional organization of the endothelium (Fong et al., Nature, 1995 376 66-70). However, deletion of the intracellular tyrosine kinase domain of VEGFR-1 generates viable mice with a normal vasculature (Hiratsuka et al., Proc. Natl. Acad. Sci. USA 1998 95 9349-9354). The reasons underlying these differences remain to be explained but suggest that receptor signalling via the tyrosine kinase is not required for the proper function of VEGFR-1. Analysis of homozygous mice with inactivated alleles of VEGFR-2 suggests that this receptor is required for endothelial cell proliferation, hematopoesis and vasculogenesis (Shalaby et al., Nature, 1995 376 62-66; Shalaby et al., Cell, 1997 89 981-990). Inactivation of VEGFR-3 results in cardiovascular failure due to abnormal organization of the large vessels (Dumont et al. Science, 1998 282 946-949). 
     Although VEGFR-1 is mainly expressed in endothelial cells during development, it can also be found in hematopoetic precursor cells during early stages of embryogenesis (Fong et al., Nature, 1995 376 66-70). It is also is expressed by most, if not all, vessels in embryos (Breier et al., Dev. Dyn., 1995 204 228-239; Fong et al., Dev. Dyn., 1996 207 1-10). In adults, monocytes and macrophages also express this receptor (Barleon et al., Blood, 1996 87 3336-3343). 
     The receptor VEGFR-3 is widely expressed on endothelial cells during early embryonic development, but as embryogenesis proceeds, it becomes restricted to venous endothelium and then to the lymphatic endothelium (Kaipainen et al., Cancer Res., 1994 54 6571-6577; Kaipainen et al., Proc. Natl. Acad. Sci. USA, 1995 92 3566-3570). VEGFR-3 continues to be expressed on lymphatic endothelial cells in adults. This receptor is essential for vascular development during embryogenesis. Targeted inactivation of both copies of the VEGFR-3 gene in mice resulted in defective blood vessel formation characterized by abnormally organized large vessels with defective lumens, leading to fluid accumulation in the pericardial cavity and cardiovascular failure at post-coital day 9.5. On the basis of these findings it has been proposed that VEGFR-3 is required for the maturation of primary vascular networks into larger blood vessels. However, the role of VEGFR-3 in the development of the lymphatic vasculature could not be studied in these mice because the embryos died before the lymphatic system emerged. 
     Nevertheless it is assumed that VEGFR-3 plays a role in development of the lymphatic vasculature and lymphangiogenesis given its specific expression in lymphatic endothelial cells during embryogenesis and adult life. This is supported by the finding that ectopic expression of VEGF-C, a ligand for VEGFR-3, in the skin of transgenic mice, resulted in lymphatic endothelial cell proliferation and vessel enlargement in the dermis. Furthermore this suggests that VEGF-C may have a primary function in lymphatic endothelium, and a secondary function in angiogenesis and permeability regulation which is shared with VEGF (Joukov et al., EMBO J., 1996 15 290-298). 
     Some inhibitors of the VEGF/VEGF-receptor system have been shown to prevent tumor growth via an anti-angiogenic mechanism; see Kim et al., Nature, 1993 362 841-844 and Saleh et al., Cancer Res., 1996 56 393-401. 
     As mentioned above, the VEGF family of growth factors are members of the PDGF family. PDGF plays a important role in the growth and/or motility of connective tissue cells, fibroblasts, myofibroblasts and glial cells (Heldin et al., “Structure of platelet-derived growth factor: Implications for functional properties”, Growth Factor, 1993 8 245-252). In adults, PDGF stimulates wound healing (Robson et al., Lancet, 1992 339 23-25). Structurally, PDGF isoforms are disulfide-bonded dimers of homologous A- and B-polypeptide chains, arranged as homodimers (PDGF-AA and PDGF-BB) or a heterodimer (PDGF-AB). 
     PDGF isoforms exert their effects on target cells by binding to two structurally related receptor tyrosine kinases (RTKs). The alpha-receptor binds both the A- and B-chains of PDGF, whereas the beta-receptor binds only the B-chain. These two receptors are expressed by many in vitro grown cell lines, and are mainly expressed by mesenchymal cells in vivo. The PDGFs regulate cell proliferation, cell survival and chemotaxis of many cell types in vitro (reviewed in Heldin et al., Biochim Biophys Acta., 1998 1378 F79-113). In vivo, they exert their effects in a paracrine mode since they often are expressed in epithelial (PDGF-A) or endothelial cells (PDGF-B) in close apposition to the PDGFR expressing mesenchyme. In tumor cells and in cell lines grown in vitro, coexpression of the PDGFs and the receptors generate autocrine loops which are important for cellular transformation (Betsholtz et al., Cell, 1984 39 447-57; Keating et al., J. R. Coll Surg Edinb., 1990 35 172-4). Overexpression of the PDGFs have been observed in several pathological conditions, including maligancies, arteriosclerosis, and fibroproliferative diseases (reviewed in Heldin et al., The Molecular and Cellular Biology of Wound Repair, New York: Plenum Press, 1996, 249-273). 
     The importance of the PDGFs as regulators of cell proliferation and survival are well illustrated by recent gene targeting studies in mice that have shown distinct physiological roles for the PDGFs and their receptors despite the overlapping ligand specificities of the PDGFRs. Homozygous null mutations for either of the two PDGF ligands or the receptors are lethal. Approximately 50% of the homozygous PDGF-A deficient mice have an early lethal phenotype, while the surviving animals have a complex postnatal phenotype with lung emphysem a due to improper alveolar septum formation because of a lack of alveolar myofibroblasts (Boström et al., Cell, 1996 85 863-873). The PDGF-A deficient mice also have a dermal phenotype characterized by thin dermis, misshapen hair follicles and thin hair (Karlsson et al., Development, 1999 126 2611-2). PDGF-A is also required for normal development of oligodendrocytes and subsequent myelination of the central nervous system (Fruttiger et al., Development, 1999 126 457-67). The phenotype of PDGFR-alpha deficient mice is more severe with early embryonic death at E10, incomplete cephalic closure, impaired neural crest development, cardiovascular defects, skeletal defects, and odemas (Soriano et al., Development, 1997 124 2691-70). The PDGF-B and PDGFR-beta deficient mice develop similar phenotypes that are characterized by renal, hematological and cardiovascular abnormalities (Leveen et al., Genes Dev., 1994 8 1875-1887; Soriano et al., Genes Dev., 1994 8 1888-96; Lindahl et al., Science, 1997 277 242-5; Lindahl, Development, 1998 125 3313-2), where the renal and cardiovascular defects, at least in part, are due to the lack of proper recruitment of mural cells (vascular smooth muscle cells, pericytes or mesangial cells) to blood vessels (Leveen et al., Genes Dev., 1994 8 1875-1887; Lindahl et al., Science, 1997 277 242-5; Lindahl et al., Development, 1998 125 3313-2). 
     SUMMARY OF THE INVENTION 
     The invention generally provides an isolated novel growth factor which has the ability to stimulate and/or enhance proliferation or differentiation and/or growth and/or motility of cells expressing a PDGF-D receptor including, but not limited to, endothelial cells, connective tissue cells, myofibroblasts and glial cells, an isolated polynucleotide sequence encoding the novel growth factor, and compositions useful for diagnostic and/or therapeutic applications. 
     According to one aspect, the invention provides an isolated and purified nucleic acid molecule which comprises a polynucleotide sequence having at least 85% identity, more preferably at least 90%, and most preferably at least 95% identity to at least nucleotides 1 to 600 of the sequence set out in FIG. 3 (SEQ ID NO:3), at least nucleotides 1 to 966 of the sequence set out in FIG. 5 (SEQ ID NO:5), at least nucleotides 176 to 1285 of the sequence set out in FIG. 7 (SEQ ID NO:7) or at least nucleotides 935 to 1285 set out in FIG. 7 (SEQ ID NO:7). The sequence of at least nucleotides 1 to 600 of the sequence set out in FIG. 3 or at least nucleotides 1 to 966 of the sequence set out in FIG. 5 encodes a 5′-truncated polypeptide, designated PDGF-D (formally designated “VEGF-G”), while at least nucleotides 176 to 1285 of the sequence set out in FIG. 7 (SEQ ID NO:7) encodes a full-length PDGF-D. PDGF-D is structurally homologous to PDGF-A, PDGF-B, VEGF, VEGF-B, VEGF-C and VEGF-D. The sequence of at least nucleotides 935 to 1285 set out in FIG. 7 (SEQ ID NO:7) encodes a portion of the PDGF/VEGF homology domain, which is the bioactive fragment of PDGF-D. This bioactive fragment would also be encoded by the sequence of at least nucleotides 1 to 600 of the sequence set out in FIG. 3 or at least nucleotides 1 to 966 of the sequence set out in FIG.  5 . In a preferred embodiment, the nucleic acid molecule is a cDNA which comprises at least nucleotides 1 to 600 of the sequence set out in FIG. 3 (SEQ ID NO:3), at least nucleotides 1 to 966 of the sequence set out in FIG. 5 (SEQ ID NO:5), at least nucleotides 176 to 1285 of the sequence set out in FIG. 7 (SEQ ID NO:7) or at least nucleotides 935 to 1285 set out in FIG. 7 (SEQ ID NO:7). This aspect of the invention also encompasses DNA molecules having a sequence such that they hybridize under stringent conditions with at least nucleotides 1 to 600 of the sequence set out in FIG. 3 (SEQ ID NO:3), at least nucleotides 1 to 966 of the sequence set out in FIG. 5 (SEQ ID NO:5), at least nucleotides 176 to 1285 of the sequence set out in FIG. 7 (SEQ ID NO:7) or at least nucleotides 935 to 1285 set out in FIG. 7 (SEQ ID NO:7) or fragments thereof. 
     According to a second aspect, the polypeptide of the invention has the ability to stimulate and/or enhance proliferation and/or differentiation and/or growth and/or motility of cells expressing a PDGF-C receptor including, but not limited to, endothelial cells, connective tissue cells, myofibroblasts and glial cells and comprises a sequence of amino acids corresponding to the amino acid sequence set out in FIG. 4 (SEQ ID NO:4) or FIG. 6 (SEQ ID NO:6), or FIG. 8 (SEQ ID NO:8), or a fragment or analog thereof which has the ability to stimulate endothelial cell proliferation, differentiation, migration and/or survival of and/or growth and/or motility of connective tissue cells (such as fibroblasts), myofibroblasts and glial cells. Preferably the polypeptides have at least 85% identity, more preferably at least 90%, and most preferably at least 95% identity to the amino acid sequence of in FIG. 4 (SEQ ID NO:4) or FIG. 6 (SEQ ID NO:6) or FIG. 8 (SEQ ID NO:8), or a fragment or analog thereof having the biological activity of PDGF-D. A preferred fragment is a truncated form of PDGF-D comprising a portion of the PDGF/VEGF homology domain (PVHD) of PDGF-D. The portion of the PVHD is from residues 254-370 of FIG. 8 where the putative proteolytic processing site RKSK starts at amino acid residue 254 (SEQ ID NO:8). However, the PVHD extends toward the N terminus up to residue 234 of FIG. 8 (SEQ ID NO:8). Herein the PVHD is defined as truncated PDGF-D. The truncated PDGF-D is the putative activated form of PDGF-D. 
     As used in this application, percent sequence identity is determined by using the alignment tool of “MEGALIGN” from the Lasergene package (DNASTAR, Ltd. Abacus House, Manor Road, West Ealing, London W130AS United Kingdom). The MEGALIGN is based on the J. Hein method (Methods in Enzymology, 1990 183 626-645). The PAM 250 residue weight table is used with a gap penalty of eleven and a gap length penalty of three and a K-tuple value of two in the pairwise alignments. The alignment is then refined manually, and the number of identities are estimated in the regions available for a comparison. 
     Preferably the polypeptide or the encoded polypeptide from a polynucleotide has the ability to stimulate one or more of proliferation, differentiation, motility, survival or vascular permeability of cells expressing a PDGF-D receptor including, but not limited to, vascular endothelial cells, lymphatic endothelial cells, connective tissue cells (such as fibroblasts), myofibroblasts and glial cells. Preferably the polypeptide or the encoded polypeptide from a polynucleotide has the ability to stimulate wound healing. PDGF-D can also have antagonistic effects on cells, but are included in the biological activities of PDGF-D. These abilities are referred to hereinafter as “biological activities of PDGF-D” and can be readily tested by methods known in the art. 
     As used herein, the term “PDGF-D” collectively refers to the polypeptides of FIG. 4 (SEQ ID NO:4), FIG. 6 (SEQ ID NO:6) or FIG. 8 (SEQ ID NO:8), and fragments or analogs thereof which have the biological activity of PDGF-D as defined above, and to a polynucleotide which can code for PDGF-D, or a fragment or analog thereof having the biological activity of PDGF-D. The polynucleotide can be naked and/or in a vector or liposome. 
     In another preferred aspect, the invention provides a polypeptide possessing an amino acid sequence: 
     PXCLLVXRCGGNCXC (SEQ ID NO:25) 
     which is unique to PDGF-D and differs from the other members of the PDGF/VEGF family of growth factors because of the insertion of the three amino acid residues (NCG) between the third and fourth cysteines (see FIG.  9 —SEQ ID NOs:10-18). 
     Polypeptides comprising conservative substitutions, insertions, or deletions, but which still retain the biological activity of PDGF-D are clearly to be understood to be within the scope of the invention. Persons skilled in the art will be well aware of methods which can readily be used to generate such polypeptides, for example the use of site-directed mutagenesis, or specific enzymatic cleavage and ligation. The skilled person will also be aware that peptidomimetic compounds or compounds in which one or more amino acid residues are replaced by a non-naturally occurring amino acid or an amino acid analog may retain the required aspects of the biological activity of PDGF-D. Such compounds can readily be made and tested for their ability to show the biological activity of PDGF-D by routine activity assay procedures such as the fibroblast proliferation assay and are also within the scope of the invention. 
     In addition, possible variant forms of the PDGF-D a polypeptide which may result from alternative splicing, as are known to occur with VEGF and VEGF-B, and naturally-occurring allelic variants of the nucleic acid sequence encoding PDGF-D are encompassed within the scope of the invention. Allelic variants are well known in the art, and represent alternative forms or a nucleic acid sequence which comprise substitution, deletion or addition of one or more nucleotides, but which do not result in any substantial functional alteration of the encoded polypeptide. 
     Such variant forms of PDGF-D can be prepared by targeting non-essential regions of the PDGF-D polypeptide for modification. These non-essential regions are expected to fall outside the strongly-conserved regions indicated in FIG. 9 (SEQ ID NOs:10-18). In particular, the growth factors of the PDGF family, including PDGF-D, are dimeric. PDGF-D differs slightly from VEGF, VEGF-B, VEGF-C, VEGF-D, PlGF, PDGF-A and PDGF-B because it shows complete conservation of only seven of the eight cysteine residues in the PVHD (Olofsson et al., Proc. Natl. Acad. Sci. USA, 1996 93 2576-2581; Joukov et al., EMBO J., 1996 15 290-298). These cysteines are thought to be involved in intra- and inter-molecular disulfide bonding. Loops 1, 2 and 3 of each subunit, which are formed by intra-molecular disulfide bonding, are involved in binding to the receptors for the PDGF/VEGF family of growth factors (Andersson et al., Growth Factors, 1995 12 159-164). 
     Persons skilled in the art thus are well aware that these cysteine residues should be preserved in any proposed variant form, and that the active sites present in loops 1, 2 and 3 also should be preserved. However, other regions of the molecule can be expected to be of lesser importance for biological function, and therefore offer suitable targets for modification. Modified polypeptides can readily be tested for their ability to show the biological activity of PDGF-D by routine activity assay procedures such as the fibroblast proliferation assay. 
     It is contemplated that some modified PDGF-D polypeptides will have the ability to bind to PDGF-D receptors on cells including, but not limited to, endothelial cells, connective tissue cells, myofibroblasts and/or glial cells, but will be unable to stimulate cell proliferation, differentiation, migration, motility or survival or to induce vascular proliferation, connective tissue development or wound healing. These modified polypeptides are expected to be able to act as competitive or non-competitive inhibitors of the PDGF-D polypeptides and growth factors of the PDGF/VEGF family, and to be useful in situations where prevention or reduction of the PDGF-D polypeptide or PDGF/VEGF family growth factor action is desirable. Thus such receptor-binding but non-mitogenic, non-differentiation inducing, non-migration inducing, non-motility inducing, non-survival promoting, non-connective tissue promoting, non-wound healing or non-vascular proliferation inducing variants of the PDGF-D polypeptide are also within the scope of the invention, and are referred to herein as “receptor-binding but otherwise inactive variants”. Because PDGF-D forms a dimer in order to activate its only known receptor, it is contemplated that one monomer comprises the receptor-binding but otherwise inactive variant modified PDGF-D polypeptide and a second monomer comprises a wild-type PDGF-D or a wild-type growth factor of the PDGF/VEGF family. These dimers can bind to its corresponding receptor but cannot induce downstream signaling. 
     It is also contemplated that there are other modified PDGF-D polypeptides that can prevent binding of a wild-type PDGF-D or a wild-type growth factor of the PDGF/VEGF family to its corresponding receptor on cells including, but not limited to, endothelial cells, connective tissue cells (such as fibroblasts), myofibroblasts and/or glial cells. Thus these dimers will be unable to stimulate endothelial cell proliferation, differentiation, migration, survival, or induce vascular permeability, and/or stimulate proliferation and/or differentiation and/or motility of connective tissue cells, myofibroblasts or glial cells. These modified polypeptides are expected to be able to act as competitive or non-competitive inhibitors of the PDGF-D growth factor or a growth factor of the PDGF/VEGF family, and to be useful in situations where prevention or reduction of the PDGF-D growth factor or PDGF/VEGF family growth factor action is desirable. Such situations include the tissue remodeling that takes place during invasion of tumor cells into a normal cell population by primary or metastatic tumor formation. Thus such the PDGF-D or PDGF/VEGF family growth factor-binding but non-mitogenic, non-differentiation inducing, non-migration inducing, non-motility inducing, non-survival promoting, non-connective tissue promoting, non-wound healing or non-vascular proliferation inducing variants of the PDGF-D growth factor are also within the scope of the invention, and are referred to herein as “the PDGF-D growth factor-dimer forming but otherwise inactive or interfering variants”. 
     An example of a PDGF-D growth factor-dimer forming but otherwise inactive or interfering variant is where the PDGF-D has a mutation which prevents cleavage of CUB domain from the protein. It is further contemplated that a PDGF-D growth factor-dimer forming but otherwise inactive or interfering variant could be made to comprise a monomer, preferably an activated monomer, of VEGF, VEGF-B, VEGF-C, VEGF-D, PDGF-C, PDGF-A, PDGF-B, PDGF-C, PDGF-D or PlGF linked to a CUB domain that has a mutation which prevents cleavage of CUB domain from the protein. Dimers formed with the above mentioned PDGF-D growth factor-dimer forming but otherwise inactive or interfering variants and the monomers linked to the mutant CUB domain would be unable to bind to their corresponding receptors. 
     A variation on this contemplation would be to insert a proteolytic site between an activated monomer of VEGF, VEGF-B, VEGF-C, VEGF-D, PDGF-C, PDGF-A, PDGF-B, PDGF-C, PDGF-D or PlGF and the mutant CUB domain linkage which is dimerized to an activated monomer of VEGF, VEGF-B, VEGF-C, VEGF-D, PDGF-A, PDGF-B, PDGF-C, PDGF-D or PlGF. An addition of the specific protease(s) for this proteolytic site would cleave the CUB domain and thereby release an activated dimer that can then bind to its corresponding receptor. In this way, a controlled release of an activated dimer is made possible. 
     According to a third aspect, the invention provides a purified and isolated nucleic acid encoding a polypeptide or polypeptide fragment of the invention as defined above. The nucleic acid may be DNA, genomic DNA, cDNA or RNA, and may be single-stranded or double stranded. The nucleic acid may be isolated from a cell or tissue source, or of recombinant or synthetic origin. Because of the degeneracy of the genetic code, the person skilled in the art will appreciate that many such coding sequences are possible, where each sequence encodes the amino acid sequence shown in FIG. 4 (SEQ ID NO:4), FIG. 6 (SEQ ID NO:6) or FIG. 8 (SEQ ID NO:8), a bioactive fragment or analog thereof, a receptor-binding but otherwise inactive or partially inactive variant thereof or a PDGF-D dimer-forming but otherwise inactive or interfering variants thereof. 
     A fourth aspect of the invention provides vectors comprising the cDNA of the invention or a nucleic acid molecule according to the third aspect of the invention, and host cells transformed or transfected with nucleic acids molecules or vectors of the invention. These may be eukaryotic or prokaryotic in origin. These cells are particularly suitable for expression of the polypeptide of the invention, and include insect cells such as Sf9 cells, obtainable from the American Type Culture Collection (ATCC SRL-171), transformed with a baculovirus vector, and the human embryo kidney cell line 293—EBNA transfected by a suitable expression plasmid. Preferred vectors of the invention are expression vectors in which a nucleic acid according to the invention is operatively connected to one or more appropriate promoters and/or other control sequences, such that appropriate host cells transformed or transfected with the vectors are capable of expressing the polypeptide of the invention. Other preferred vectors are those suitable for transfection of mammalian cells, or for gene therapy, such as adenoviral-, vaccinia- or retroviral-based vectors or liposomes. A variety of such vectors is known in the art. 
     The invention also provides a method of making a vector capable of expressing a polypeptide encoded by a nucleic acid molecule according to the invention, comprising the steps of operatively connecting the nucleic acid molecule to one or more appropriate promoters and/or other control sequences, as described above. 
     The invention further provides a method of making a polypeptide according to the invention, comprising the steps of expressing a nucleic acid or vector of the invention in a host cell, and isolating the polypeptide from the host cell or from the host cell&#39;s growth medium. 
     In yet a further aspect, the invention provides an antibody specifically reactive with a polypeptide of the invention or a fragment of the polypeptide. This aspect of the invention includes antibodies specific for the variant forms, immunoreactive fragments, analogs and recombinants of PDGF-D. Such antibodies are useful as inhibitors or agonists of PDGF-D and as diagnostic agents for detecting and quantifying PDGF-D. Polyclonal or monoclonal antibodies may be used. Monoclonal and polyclonal antibodies can be raised against polypeptides of the invention or fragment or analog thereof using standard methods in the art. In addition the polypeptide can be linked to an epitope tag, such as the FLAG® octapeptide (Sigma, St. Louis, Mo.), to assist in affinity purification. For some purposes, for example where a monoclonal antibody is to be used to inhibit effects of PDGF-D in a clinical situation, it may be desirable to use humanized or chimeric monoclonal antibodies. Such antibodies may be further modified by addition of cytotoxic or cytostatic drug(s). Methods for producing these, including recombinant DNA methods, are also well known in the art. 
     This aspect of the invention also includes an antibody which recognizes PDGF-D and is suitably labeled. 
     Polypeptides or antibodies according to the invention may be labeled with a detectable label, and utilized for diagnostic purposes. Similarly, the thus-labeled polypeptide of the invention may be used to identify its corresponding receptor in situ. The polypeptide or antibody may be covalently or non-covalently coupled to a suitable supermagnetic, paramagnetic, electron dense, ecogenic or radioactive agent for imaging. For use in diagnostic assays, radioactive or non-radioactive labels may be used. Examples of radioactive labels include a radioactive atom or group, such as  125 I or  32 P. Examples of non-radioactive labels include enzymatic labels, such as horseradish peroxidase or fluorimetric labels, such as fluorescein-5-isothiocyanate (FITC). Labeling may be direct or indirect, covalent or non-covalent. 
     Clinical applications of the invention include diagnostic applications, acceleration of angiogenesis in tissue or organ transplantation, or stimulation of wound healing, or connective tissue development, or to establish collateral circulation in tissue infarction or arterial stenosis, such as coronary artery disease, and inhibition of angiogenesis in the treatment of cancer or of diabetic retinopathy and inhibition of tissue remodeling that takes place during invasion of tumor cells into a normal cell population by primary or metastatic tumor formation. Quantitation of PDGF-D in cancer biopsy specimens may be useful as an indicator of future metastatic risk. 
     PDGF-D may also be relevant to a variety of lung conditions. PDGF-D assays could be used in the diagnosis of various lung disorders. PDGF-D could also be used in the treatment of lung disorders to improve blood circulation in the lung and/or gaseous exchange between the lungs and the blood stream. Similarly, PDGF-D could be used to improve blood circulation to the heart and O 2  gas permeability in cases of cardiac insufficiency. In a like manner, PDGF-D could be used to improve blood flow and gaseous exchange in chronic obstructive airway diseases. 
     Thus the invention provides a method of stimulation of angiogenesis, lymphangiogenesis, neovascularization, connective tissue development and/or wound healing in a mammal in need of such treatment, comprising the step of administering an effective dose of PDGF-D, or a fragment or an analog thereof which has the biological activity of PDGF-D to the mammal. Optionally the PDGF-D, or fragment or analog thereof may be administered together with, or in conjunction with, one or more of VEGF, VEGF-B, VEGF-C, VEGF-D, PlGF, PDGF-A, PDGF-B, PDGF-C, FGF and/or heparin. 
     Conversely, PDGF-D antagonists (e.g. antibodies and/or competitive or noncompetitive inhibitors of binding of PDGF-D in both dimer formation and receptor binding) could be used to treat conditions, such as congestive heart failure, involving accumulation of fluid in, for example, the lung resulting from increases in vascular permeability, by exerting an offsetting effect on vascular permeability in order to counteract the fluid accumulation. Administrations of PDGF-D could be used to treat malabsorptive syndromes in the intestinal tract, liver or kidneys as a result of its blood circulation increasing and vascular permeability increasing activities. 
     Thus, the invention provides a method of inhibiting angiogenesis, lymphangiogenesis, neovascularization, connective tissue development and/or wound healing in a mammal in need of such treatment, comprising the step of administering an effective amount of an antagonist of PDGF-D to the mammal. The antagonist may be any agent that prevents the action of PDGF-D, either by preventing the binding of PDGF-D to its corresponding receptor on the target cell, or by preventing activation of the receptor, such as using receptor-binding PDGF-D variants. Suitable antagonists include, but are not limited to, antibodies directed against PDGF-D; competitive or non-competitive inhibitors of binding of PDGF-D to the PDGF-D receptor(s), such as the receptor-binding or PDGF-D dimer-forming but non-mitogenic PDGF-D variants referred to above; and anti-sense nucleotide sequences as described below. 
     A method is provided for determining agents that bind to an activated truncated form of PDGF-D. The method comprises contacting an activated truncated form of PDGF-D with a test agent and monitoring binding by any suitable means. Agents can include both compounds and other proteins. 
     The invention provides a screening system for discovering agents that bind an activated truncated form of PDGF-D. The screening system comprises preparing an activated truncated form of PDGF-D, exposing the activated truncated form of PDGF-D to a test agent, and quantifying the binding of said agent to the activated truncated form of PDGF-D by any suitable means. This screening system can also be used to identify agents which inhibit the proteolytic cleavage of the full length PDGF-D protein and thereby prevent the release of the activated truncated form of PDGF-D. For this use, the full length PDGF-D must be prepared. 
     Use of this screen system provides a means to determine compounds that may alter the biological function of PDGF-D. This screening method may be adapted to large-scale, automated procedures such as a PANDEX® (Baxter-Dade Diagnostics) system, allowing for efficient high-volume screening of potential therapeutic agents. 
     For this screening system, an activated truncated form of PDGF-D or full length PDGF-D is prepared as described herein, preferably using recombinant DNA technology. A test agent, e.g. a compound or protein, is introduced into a reaction vessel containing the activated truncated form of or full length PDGF-D. Binding of the test agent to the activated truncated form of or full length PDGF-D is determined by any suitable means which include, but is not limited to, radioactively- or chemically-labeling the test agent. Binding of the activated truncated form of or full length PDGF-D may also be carried out by a method disclosed in U.S. Pat. No. 5,585,277, which is incorporated by reference. In this method, binding of the test agent to the activated truncated form of or full length PDGF-D is assessed by monitoring the ratio of folded protein to unfolded protein. Examples of this monitoring can include, but are not limited to, monitoring the sensitivity of the activated truncated form of or full length PDGF-D to a protease, or amenability to binding of the protein by a specific antibody against the folded state of the protein. 
     Those of skill in the art will recognize that IC 50  values are dependent on the selectivity of the agent tested. For example, an agent with an IC 50  which is less than 10 nM is generally considered an excellent candidate for drug therapy. However, an agent which has a lower affinity, but is selective for a particular target, may be an even better candidate. Those skilled in the art will recognize that any information regarding the binding potential, inhibitory activity or selectivity of a particular agent is useful toward the development of pharmaceutical products. 
     Where PDGF-D or a PDGF-D antagonist is to be used for therapeutic purposes, the dose(s) and route of administration will depend upon the nature of the patient and condition to be treated, and will be at the discretion of the attending physician or veterinarian. Suitable routes include oral, subcutaneous, intramuscular, intraperitoneal or intravenous injection, parenteral, topical application, implants etc. Topical application of PDGF-D may be used in a manner analogous to VEGF. Where used for wound healing or other use in which enhanced angiogenesis is advantageous, an effective amount of the truncated active form of PDGF-D is administered to an organism in need thereof in a dose between about 0.1 and 1000 μg/kg body weight. 
     The PDGF-D or a PDGF-D antagonist may be employed in combination with a suitable pharmaceutical carrier. The resulting compositions comprise a therapeutically effective amount of PDGF-D or a PDGF-D antagonist, and a pharmaceutically acceptable non-toxic salt thereof, and a pharmaceutically acceptable solid or liquid carrier or adjuvant. Examples of such a carrier or adjuvant include, but are not limited to, saline, buffered saline, Ringer&#39;s solution, mineral oil, talc, corn starch, gelatin, lactose, sucrose, microcrystalline cellulose, kaolin, mannitol, dicalcium phosphate, sodium chloride, alginic acid, dextrose, water, glycerol, ethanol, thickeners, stabilizers, suspending agents and combinations thereof. Such compositions may be in the form of solutions, suspensions, tablets, capsules, creams, salves, elixirs, syrups, wafers, ointments or other conventional forms. The formulation to suit the mode of administration. Compositions which comprise PDGF-D may optionally further comprise one or more of PDGF-A, PDGF-B, PDGF-C, VEGF, VEGF-B, VEGF-C, VEGF-D, PlGF and/or heparin. Compositions comprising PDGF-D will contain from about 0.1% to 90% by weight of the active compound(s), and most generally from about 10% to 30%. 
     For intramuscular preparations, a sterile formulation, preferably a suitable soluble salt form of the truncated active form of PDGF-D, such as hydrochloride salt, can be dissolved and administered in a pharmaceutical diluent such as pyrogen-free water (distilled), physiological saline or 5% glucose solution. A suitable insoluble form of the compound may be prepared and administered as a suspension in an aqueous base or a pharmaceutically acceptable oil base, e.g. an ester of a long chain fatty acid such as ethyl oleate. 
     According to yet a further aspect, the invention provides diagnostic/prognostic devices typically in the form of test kits. For example, in one embodiment of the invention there is provided a diagnostic/prognostic test kit comprising an antibody to PDGF-D and a means for detecting, and more preferably evaluating, binding between the antibody and PDGF-D. In one preferred embodiment of the diagnostic/prognostic device according to the invention, a second antibody (the secondary antibody) directed against antibodies of the same isotype and animal source of the antibody directed against PDGF-D (the primary antibody) is provided. The secondary antibody is coupled directly or indirectly to a detectable label, and then either an unlabeled primary antibody or PDGF-D is substrate-bound so that the PDGF-D/primary antibody interaction can be established by determining the amount of label bound to the substrate following binding between the primary antibody and PDGF-D and the subsequent binding of the labeled secondary antibody to the primary antibody. In a particularly preferred embodiment of the invention, the diagnostic/prognostic device may be provided as a conventional enzyme-linked immunosorbent assay (ELISA) kit. 
     In another alternative embodiment, a diagnostic/prognostic device may comprise polymerase chain reaction means for establishing sequence differences of a PDGF-D of a test individual and comparing this sequence structure with that disclosed in this application in order to detect any abnormalities, with a view to establishing whether any aberrations in PDGF-D expression are related to a given disease condition. 
     In addition, a diagnostic/prognostic device may comprise a restriction length polymorphism (RFLP) generating means utilizing restriction enzymes and genomic DNA from a test individual to generate a pattern of DNA bands on a gel and comparing this pattern with that disclosed in this application in order to detect any abnormalities, with a view to establishing whether any aberrations in PDGF-D expression are related to a given disease condition. 
     In accordance with a further aspect, the invention relates to a method of detecting aberrations in PDGF-D gene structure in a test subject which may be associated with a disease condition in the test subject. This method comprises providing a DNA sample from said test subject; contacting the DNA sample with a set of primers specific to PDGF-D DNA operatively coupled to a polymerase and selectively amplifying PDGF-D DNA from the sample by polymerase chain reaction, and comparing the nucleotide sequence of the amplified PDGF-D DNA from the sample with the nucleotide sequences shown in FIG. 3 (SEQ ID NO:3), FIG. 5 (SEQ ID NO:5) or FIG. 7 (SEQ ID NO:7). The invention also includes the provision of a test kit comprising a pair of primers specific to PDGF-D DNA operatively coupled to a polymerase, whereby said polymerase is enabled to selectively amplify PDGF-D DNA from a DNA sample. 
     The invention also provides a method of detecting PDGF-D in a biological sample, comprising the step of contacting the sample with a reagent capable of binding PDGF-D, and detecting the binding. Preferably the reagent capable of binding PDGF-D is an antibody directed against PDGF-D, particularly a monoclonal antibody. In a preferred embodiment the binding and/or extent of binding is detected by means of a detectable label; suitable labels are discussed above. 
     In another aspect, the invention relates to a protein dimer comprising the PDGF-D polypeptide, particularly a disulfide-linked dimer. The protein dimers of the invention include both homodimers of PDGF-D polypeptide and heterodimers of PDGF-D and VEGF, VEGF-B, VEGF-C, VEGF-D, PlGF, PDGF-A, PDGF-B or PDGF-C. 
     According to a yet further aspect of the invention there is provided a method for isolation of PDGF-D comprising the step of exposing a cell which expresses PDGF-D to heparin to facilitate release of PDGF-D from the cell, and purifying the thus-released PDGF-D. 
     Another aspect of the invention involves providing a vector comprising an anti-sense nucleotide sequence which is complementary to at least a part of a DNA sequence which encodes PDGF-D or a fragment or analog thereof that has the biological activity of PDGF-D. In addition the anti-sense nucleotide sequence can be to the promoter region of the PDGF-D gene or other non-coding region of the gene which may be used to inhibit, or at least mitigate, PDGF-D expression. 
     According to a yet further aspect of the invention such a vector comprising an anti-sense sequence may be used to inhibit, or at least mitigate, PDGF-D expression. The use of a vector of this type to inhibit PDGF-D expression is favored in instances where PDGF-D expression is associated with a disease, for example where tumors produce PDGF-D in order to provide for angiogenesis, or tissue remodeling that takes place during invasion of tumor cells into a normal cell population by primary or metastatic tumor formation. Transformation of such tumor cells with a vector containing an anti-sense nucleotide sequence would suppress or retard angiogenesis, and so would inhibit or retard growth of the tumor or tissue remodeling. 
     Another aspect of the invention relates to the discovery that the full length PDGF-D protein is likely to be a latent growth factor that needs to be activated by proteolytic processing to release an active PDGF/VEGF homology domain. A putative proteolytic site is found in residues 255-258 in the full length protein, residues -RKSK-(SEQ ID NO:9). This is a dibasic motif. The -RKSK-(SEQ ID NO:9) putative proteolytic site is also found in PDGF-A, PDGF-B, VEGF-C and VEGF-D. In these four proteins, the putative proteolytic site is also found just before the minimal domain for the PDGF/VEGF homology domain. Together these facts indicate that this is the proteolytic site. 
     Preferred proteases include, but are not limited, to plasmin, Factor X and enterokinase. The N-terminal CUB domain may function as an inhibitory domain which might be used to keep PDGF-D in a latent form in some extracellular compartment and which is removed by limited proteolysis when PDGF-D is needed. 
     According to this aspect of the invention, a method is provided for producing an activated truncated form of PDGF-D or for regulating receptor-binding specificity of PDGF-D. These methods comprise the steps of expressing an expression vector comprising a polynucleotide encoding a polypeptide having the biological activity of PDGF-D and supplying a proteolytic amount of at least one enzyme for processing the expressed polypeptide to generate the activated truncated form of PDGF-D. 
     This aspect also includes a method for selectively activating a polypeptide having a growth factor activity. This method comprises the step expressing an expression vector comprising a polynucleotide encoding a polypeptide having a growth factor activity, a CUB domain and a proteolytic site between the polypeptide and the CUB domain, and supplying a proteolytic amount of at least one enzyme for processing the expressed polypeptide to generate the activated polypeptide having a growth factor activity. 
     In addition, this aspect includes the isolation of a nucleic acid molecule which codes for a polypeptide having the biological activity of PDGF-D and a polypeptide thereof which comprises a proteolytic site having the amino acid sequence RKSR (SEQ ID NO:9) or a structurally conserved amino acid sequence thereof. 
     Also this aspect includes an isolated dimer comprising an activated monomer of PDGF-D and an activated monomer of VEGF, VEGF-B, VEGF-C, VEGF-D, PDGF-D, PDGF-A, PDGF-B, PDGF-C or PlGF linked to a CUB domain, or alternatively, an activated monomer of VEGF, VEGF-B, VEGF-C, VEGF-D, PDGF-D, PDGF-A, PDGF-B or PlGF and an activated monomer of PDGF-D linked to a CUB domain. The isolated dimer may or may not include a proteolytic site between the activator monomer and the CUB domain linkage. 
     Polynucleotides of the invention such as those described above, fragments of those polynucleotides, and variants of those polynucleotides with sufficient similarity to the non-coding strand of those polynucleotides to hybridize thereto under stringent conditions all are useful for identifying, purifying, and isolating polynucleotides encoding other, non-human, mammalian forms of PDGF-D. Thus, such polynucleotide fragments and variants are intended as aspects of the invention. Exemplary stringent hybridization conditions are as follows: hybridization at 42° C. in 5×SSC, 20 mM NaPO 4 , pH 6.8, 50% formamide; and washing at 42° C. in 0.2×SSC. Those skilled in the art understand that it is desirable to vary these conditions empirically based on the length and the GC nucleotide base content of the sequences to be hybridized, and that formulas for determining such variation exist. See for example Sambrook et al, “Molecular Cloning: A Laboratory Manual”, Second Edition, pages 9.47-9.51, Cold Spring Harbor, N.Y.: Cold Spring Harbor Laboratory (1989). 
     Moreover, purified and isolated polynucleotides encoding other, non-human, mammalian PDGF-D forms also are aspects of the invention, as are the polypeptides encoded thereby and 30 antibodies that are specifically immunoreactive with the non-human PDGF-D variants. Thus, the invention includes a purified and isolated mammalian PDGF-D polypeptide and also a purified and isolated polynucleotide encoding such a polypeptide. 
     It will be clearly understood that nucleic acids and polypeptides of the invention may be prepared by synthetic means or by recombinant means, or may be purified from natural sources. 
     It will be clearly understood that for the purposes of this specification the word “comprising” means “included but not limited to”. The corresponding meaning applies to the word “comprises”. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 (SEQ ID NO:1) shows a nucleotide sequence that includes a cDNA sequence encoding the C-terminal part of human PDGF-D (hPDGF-D). The nucleotides which encode for the partial fragment of hPDGF-D are 1 to 198. The deduced partial amino acid sequence of hPDGF-D (66 amino acid residues-SEQ ID NO:2) derived from nucleotides 1 to 198 of FIG. 1 is shown in FIG. 2; 
     FIG. 3 (SEQ ID NO:3) shows an extended sequence of a partial human cDNA encoding for the hPDGF-D. The translated cDNA sequence is from nucleotide 1 to 600. The deduced partial amino acid sequence of hPDGF-D (200 residues-SEQ ID NO:4) derived from nucleotides 1 to 600 of FIG. 3 is shown in FIG. 4; 
     FIG. 5 shows a still further extended nucleotide sequence of a partial human cDNA. The nucleotides which encode for the 5′-truncated full-length hPDGF-D are 1 to 966 (SEQ ID NO:5). The deduced partial amino acid sequence of hPDGF-D (322 residues-SEQ ID NO:6) derived from nucleotides 1 to 966 of FIG. 5 is shown in FIG. 6; 
     FIG. 7 (SEQ ID NO:7) shows the complete nucleotide sequence of cDNA encoding a hPDGF-D(1116 bp) and the deduced amino acid sequence of full-length hPDGF-D encoded thereby which consists of 370 amino acid residues (FIG.  8 -SEQ ID NO:8); 
     FIG. 9 shows an amino acid sequence alignment of the PDGF/VEGF-homology domain in hPDGF-D with several growth factors belonging to the VEGF/PDGF family (SEQ ID NOs:10-18, respectively); 
     FIG. 10 shows a phylogenetic tree of several growth factors belonging to the VEGF/PDGF family; 
     FIG. 11 provides the amino acid sequence alignment of the CUB domain present in hPDGF-D (SEQ ID NO:19) and other CUB domains present in human bone morphogenic protein-1 (hBMP-1, 3 CUB domains CUB1-3) (SEQ ID NOs:20-22, respectively) and in human neuropilin-1 (2 CUB domains) (SEQ ID NOs:23-24, respectively); 
     FIG. 12 shows that conditioned medium (CM) containing truncated PDGF-D stimulates tyrosine phosphorylation of PDGF beta-receptors in PAE-1 cells; and 
     FIG. 13 provides a graphical representation of results which show that conditioned medium (CM) containing truncated PDGF-D competes for binding with PDGF-BB homodimers for the PDGF beta-receptors, but not with PDGF-AA homodimers for the PDGF alpha-receptors. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a nucleotide sequence of human cDNA which encodes a C-terminal portion of a novel growth factor, referred to herein as PDGF-D (formerly VEGF-G). PDGF-D is a new member of the VEGF/PDGF family. The nucleotide sequence of FIG. 1 (SEQ ID NO:1) was derived from a human EST sequence (id. AI488780) in the dbEST database at the NCBI in Washington, DC. The nucleotides 1 to 198 of the cDNA of FIG. 1 (SEQ ID NO:1) encodes a 66 amino acid polypeptide (FIG.  2 —SEQ ID NO:2) which shows some sequence similarity to the known members of the VEGF/PDGF family. 
     The amino acid sequence of the polypeptide encoded by the nucleotides 1 to 198 of the polynucleotide of FIG. 1 (SEQ ID NO:1) is shown in FIG. 2 (SEQ ID NO:2). 
     To generate more sequence information on human PDGF-D, a human fetal lung λgt10 cDNA library was screened using a 327 bp polymerase chain reaction (PCR)-generated probe, based on the originally identified EST sequence. The probe was generated from DNA from a commercially available human fetal lung cDNA library (Clontech) which was amplified by PCR using two primers derived from the identified EST (AI488780). The primers were: 
     5′-GTCGTGGAACTGTCAACTGG (forward) (SEQ ID NO:26) and 
     5′-CTCAGCAACCACTTGTGTTC (reverse) (SEQ ID NO:27). 
     The amplified 327 bp fragment was cloned into the pCR2.1 vector (Invitrogen). Nucleotide sequencing verified that the insert corresponded to the EST. The screen identified several positive clones. The inserts from two of these clones, clones 5 and 8 were subcloned into pBluescript and subjected to nucleotide sequencing using internal or vector-specific primers. The nucleotide sequences determined were identical in both clones and are shown in FIG. 3 (SEQ ID NO:3). The coding region of the 690 bp polynucleotide is nucleotides 1-600 (SEQ ID NO:3) that encodes for a large portion of hPDGF-D with the exception of the 5′-end. This portion of hPDGF-D includes the bioactive fragment of hPDGF-D. The deduced partial amino acid sequence of hPDGF-D (200 residues-SEQ. ID NO:4) derived from nucleotides 1 to 600 of FIG. 3 (SEQ ID NO:3) is shown in FIG. 4 (SEQ ID NO:4). 
     Extended nucleotide sequencing of the isolated human PDGF-D cDNA clones from this human fetal lung cDNA library has provided additional sequence. FIG. 5 (SEQ ID NO:5) shows a nucleotide sequence of a partial human cDNA (1934 bp) that encodes hPDGF-D. The coding region of the 1934 bp polynucleotide is nucleotides 1 to 966 that encodes for hPDGF-D except for the most 5′-end of the polypeptide. The deduced partial amino acid sequence of hPDGF-D (322 residues-SEQ ID NO:6) derived from nucleotides 1 to 966 of FIG. 5 (SEQ ID NO:5) is shown in FIG. 6 (SEQ ID NO:6). 
     FIG. 7 (SEQ ID NO:7) shows a polynucleotide sequence of cDNA encoding a full-length hPDGF-D. The region encoding PDGF-D is 1116 bp. The deduced amino acid sequence of full-length hPDGF-D is 370 amino acid residues (FIG.  8 -SEQ ID NO:8). 
     The sequence for the 5′ end of full-length PDGF-D was obtained using Rapid Amplification of cDNA Ends (RACE) PCR, and clones containing cDNA from the human heart (Marathon-Ready™ cDNA, Clontech, Cat# 7404-1). These cDNA clones have an adaptor sequence attached to the 5′ end of each clone, including a site for primer called Adaptor Primer 1 (Clontech: 5′9 CCATCCTAATACGACTCACTATAGGGC 3′9) (SEQ ID NO:28). This primer and a second primer 5′AGTGGGATCCGTTACTGA TGGAGAGTTAT 3′ (SEQ ID NO:29) were used to amplify the sequence found at the 5′ end of PDGF-D. In the PCR reaction a special polymerase mix was used (Advantage&lt;&lt;-GC cDNA PCR Kit, Clontech, Cat# K1907-1). The reaction mix included (in microliters): 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Adaptor Primer 1 
                 1 
               
               
                   
                 Gene specific primer 
                 1 
               
               
                   
                 Template (Human Heart cDNA) 
                 5 
               
               
                   
                 GC-Melt (from the K1907-1 Kit) 
                 5 
               
               
                   
                 5xGC cDNA PCR Reaction Buffer 
                 10 
               
               
                   
                 50x dNTP mix 
                 1 
               
               
                   
                 Sterile H2O 
                 27 
               
               
                   
                 Total 
                 50 
               
               
                   
                   
               
             
          
         
       
     
     The 5′ end of PDGF-D was amplified for 31 cycles, five cycles consisted of 45 seconds denaturation at 94° C. and four minutes extension at 72° C., five cycles consisted of 45 seconds denaturation at 94° C. and four minutes extension at 70° C., and five cycles consisted of 45 seconds denaturation at 94° C. and four minutes extension at 68° C. and an initial denaturation step at 94° C. for two minutes. From this PCR, an approximately 790 bp long product was obtained. This product was run on a 1% agarose gel, purified (QIAquick gel extraction Kit, Qiagen, Cat # 28706) from the gel, cloned into a vector (TOPO TA Cloning Kit, Invitrogen) and transformed into bacteria ( E.Coli ). Transformed bacteria were plated, and incubated at 37° C. overnight. Single colonies were picked and grown in fresh media overnight. Plasmids were prepared (QIAprep Spin Miniprep Kit, Qiagen, Cat# 27106) and sequenced with the plasmid primers, T7 and M13R. The result of this sequencing was that 312 bp of previously unknown PDGF-D sequence was obtained. The rest of the sequence (478 bp) was identical with previously obtained sequence from other PDGF-D cDNA clones. 
     FIG. 9 shows the amino acid sequence alignment of the PDGF/VEGF-homology domain of PDGF-D (found in the C-terminal region of the polypeptide) with the PDGF/VEGF-homology domains of PDGF/VEGF family members, VEGF 165 , PlGF-2, VEGF-B 167 , Pox Orf VEGF, VEGF-C, VEGF-D, PDGF-A and PDGF-B (SEQ ID NOs:10-18, respectively). Some of the amino acid sequences in the N- and C-terminal regions in VEGF-C and VEGF-D have been deleted in this figure. Gaps were introduced to optimize the alignment. This alignment was generated using the MEGALIGN alignment tool based on the method of J. Hein, (Methods Enzymol. 1990 183 626-45) The PAM 250 residue weight table is used with a gap penalty of eleven and a gap length penalty of three and a K-tuple value of two in the pairwise alignments. The alignment is then refined manually, and the number of identities are estimated in the regions available for a comparison. The boxed residues indicate amino acids which match the VEGF-D within two distance units. 
     The alignment shows that PDGF-D has the expected pattern of invariant cysteine residues, a hallmark of members of this family, with two exceptions. The first exception occurs between cysteine 3 and 4. Normally these two cysteines are spaced by 2 residues and with PDGF-D there is an insertion of three extra amino acids (NCA). This feature of the sequence in PDGF-D was highly unexpected. The second is that the invariant fifth cysteine found in the other members of the PDGF/VEGF family is not conserved in PDGF-D. This feature is unique to PDGF-D. 
     Based on the amino acid sequence alignments in FIG. 9, a phylogenetic tree was constructed and is shown in FIG.  10 . The data show that the PVHD of PDGF-D is closely related to the PVHDs of VEGF-C and VEGF-D. 
     CUB Domain 
     The N-terminal region of the partial PDGF-D amino acid sequence of FIG. 11 (residues 53-170) (SEQ ID NO:8) has second distinct protein domain which is referred to as a CUB domain (Bork and Beckmann, J. Mol. Biol., 1993 231, 539-545). This domain of about 115 amino acids was originally identified in complement factors C1r/C1s, but has recently been identified in several other extracellular proteins including signaling molecules such as bone morphogenic protein 1 (BMP-1) (Wozney et al.,Science, 1988 242, 1528-1534) as well as in several receptor molecules such as neuropilin-1 (NP-1) (Soker et al., Cell, 1998 92 735-745). The functional roles of CUB domains are not clear but they may participate in protein-protein interactions or in interactions with carbohydrates including heparin sulfate proteoglycans. These interactions may play a role the proteolytic activation of PDGF-D. 
     As shown in FIG. 11, the amino acid sequences from several CUB-containing proteins were aligned. The results show that the single CUB domain in human PDGF-D (SEQ ID NO:19) displays a significant identify with the most closely related CUB domains. Sequences from human BMP-1, with 3 CUB domains (CUBs1-3) (SEQ ID NOs:20-22, respectively) and human neuropilin-1 with 2 CUB domains (CUBs1-2) (SEQ ID NOs:23-24, respectively) are shown. This alignment was generated as described above. 
     EXAMPLE 1 
     Expression of PDGF-D Transcripts 
     To investigate the tissue expression of PDGF-D in several human tissues, a Northern blot was done using a commercial Multiple Tissue Northern blot (MTN, Clontech). The blots were hybridized at according to the instructions from the supplier using ExpressHyb solution at 68° C. for one hour (high stringency conditions), and probed with the 327 bp PCR-generated probe from the human fetal lung cDNA library (see description above). The blots were subsequently washed at 50° C. in 2×SSC with 0.05% SDS for 30 minutes and at 50° C. in 0.1×SSC with 0.1% SDS for an additional 40 minutes. The blots were then put on film and exposed at −70° C. The results, summarized in Table 1, showed that expression of PDGF-D transcripts were most abundant in heart, pancreas and ovary while lower expression levels were seen in placenta, liver, kidney, prostate and testis. The human PDGF-D transcript was around 4 kb in length. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Relative expression levels of PDGF-D transcripts in 
               
               
                 several human tissues as determined by Northern blot 
               
               
                 analysis 
               
             
          
           
               
                   
                 Tissue 
                   
               
               
                   
                 level* 
                 Expression 
               
               
                   
                   
               
               
                   
                 Heart 
                 +++++ 
               
               
                   
                 Brain 
                 n.d. 
               
               
                   
                 Placenta 
                 ++ 
               
               
                   
                 Lung 
                 + 
               
               
                   
                 Liver 
                 ++ 
               
               
                   
                 Skeletal muscle 
                 n.d. 
               
               
                   
                 Kidney 
                 ++ 
               
               
                   
                 Pancreas 
                 ++++ 
               
               
                   
                 Spleen 
                 + 
               
               
                   
                 Thymus 
                 + 
               
               
                   
                 Prostate 
                 ++ 
               
               
                   
                 Testis 
                 +++ 
               
               
                   
                 Ovary 
                 +++++ 
               
               
                   
                 Small intestine 
                 ++ 
               
               
                   
                 Colon 
                 + 
               
               
                   
                 Peripheral blood 
                 + 
               
               
                   
                 leucocytes 
               
               
                   
                   
               
               
                   
                 *The relative intensities of the bands were visually determined (+++++) highest expression and (+) lowest expression; n.d. not detected  
               
             
          
         
       
     
     EXAMPLE 2 
     Receptor Binding Properties of a Truncated PDGF-D 
     To assess the interactions between a truncated PDGF-D and the VEGF receptors, truncated PDGF-D was tested for its capacity to bind to soluble Ig-fusion proteins containing the extracellular domains of human VEGFR-1, VEGFR-2 and VEGFR-3 (Olofsson et al., Proc. Natl. Acad. Sci. USA, 1998 95 11709-11714). An expression vector encoding the PDGF/VEGF homology domain of PDGF-D was generated in the vector pSecTag (Invitrogen). The primers 5′-CCCAAGCTTGAAGATCTTGAGAATAT 3′ (forward) (SEQ ID NO:30) and 5′-TGCTCTAGATCGAGGTGGTCTT 3′ (reverse) (SEQ ID NO:31) were used to amplify a 429 bp fragment (nucleotides 556 to 966 in FIG. 5) (SEQ ID NO:5) encoding amino acid residues 186 to 322 of FIG. 6 (SEQ ID NO:6) or 234 to 370 of FIG. 8 (SEQ ID NO:8), which are identical. The fragment was subsequently cloned into a HindIII and XbaI digested expression vector. COS cells were transfected with the expression vector encoding truncated PDGF-D or a control vector using calcium phosphate precipitation. The expressed polypeptide included a C-terminal c-myc tag and a 6X His tag (both derived from the pSecTag vector. 
     The Ig-fusion proteins, designated VEGFR-1-Ig, VEGFR-2-Ig and VEGFR-3-Ig, were transiently expressed in human 293 EBNA cells. All Ig-fusion proteins were human VEGFRs. Cells were incubated for 24 hours after transfection, washed with Dulbecco&#39;s Modified Eagle Medium (DMEM) containing 0.2% bovine serum albumin (BSA) and starved for 24 hours. The fusion proteins were then precipitated from the clarified conditioned medium using protein A-Sepharose beads (Pharmacia). The beads were combined with 100 microliters of lOX binding buffer (5% BSA, 0.2% Tween 20 and 10 μg/ml heparin) and 900 microliter of conditioned medium prepared from the COS cells transfected with the expression vector for truncated PDGF-D or the control vector. The cells were then metabolically labeled with  35 S-cysteine and methionine (Promix, Amersham) for 4 to 6 hours. After 2.5 hours, at room temperature, the Sepharose beads were washed 3 times with binding buffer at 4° C., once with phosphate buffered saline (PBS) and boiled in SDS-PAGE buffer. Labeled proteins that were bound to the Ig-fusion proteins were analyzed by SDS-PAGE under reducing conditions. Radiolabeled proteins were detected using a phosphorimager analyzer and/or on film. In all these analyses, radiolabeled PDGF-D failed to show any interaction with any of the VEGF receptors. These results indicate that secreted truncated PDGF-D does not bind to VEGF receptors R1, R2 and R3. 
     EXAMPLE 3 
     PDGF Beta-receptor Phosphorylation 
     To test if PDGF-D causes increased phosphorylation of the PDGF beta-receptor, truncated PDGF-D was tested for its capacity to bind to the PDGF beta-receptor and stimulate increased phosphorylation. Serum-starved porcine aortic endothelial-1 (PAE-1) cells stably expressing the human PDGF beta-receptor (Eriksson et al., EMBO J, 1992, 11, 543-550) were incubated on ice for 90 minutes with a solution of conditioned media mixed with an equal volume of PBS supplemented with 1 mg/ml BSA. The conditioned media was prepared from COS cells transfected with expression vectors for PDGF-A or truncated PDGF-D (as constructed in Example 1), or a mock control vector. Twenty-four hours after transfection, the medium was replaced by serum-free medium containing 1 mg/ml serum albumin. Conditioned medium was harvested after an additional 48 hours of incubation. Sixty minutes after the addition of the conditioned media, the cells were lysed in lysis buffer (20 mM tris-HCl, pH 7.5, 0.5% Triton X-100, 0.5% deoxycholic acid, 10 mM EDTA, 1 mM orthovanadate, 1 mM PMSF 1% Trasylol). The PDGF beta-receptors were immunoprecipitated from cleared lysates with rabbit antisera against the human PDGF beta-receptor (Eriksson et al., EMBO J, 1992 11 543-550). The precipitated receptors were applied to a SDS-PAGE gel. After SDS gel electrophoresis, the precipitated receptors were transferred to nitrocellulose filters, and the filters were probed with anti-phosphotyrosine antibody PY-20, (Transduction Laboratories). The filters were then incubated with horseradish peroxidase-conjugated anti-mouse antibodies. Bound antibodies were detected using enhanced chemiluminescence (ECL, Amersham Inc). The filters were then stripped and reprobed with the PDGF beta-receptor rabbit antisera, and the amount of receptors was determined by incubation with horseradish peroxidase-conjugated anti-rabbit antibodies. Bound antibodies were detected using enhanced chemiluminescence (ECL, Amersham Inc). The probing of the filters with PDGF beta receptor antibodies confirmed that equal amounts of the receptor were present in all lanes. Human recombinant PDGF-BB (100 ng/ml) and untreated cells were included in the experiment as a control. FIG. 11 shows that truncated PDGF-D containing conditioned medium stimulated PDGF beta-receptor tyrosine phosphorylation. This indicates that truncated PDGF-D is a PDGF beta-receptor ligand/agonist. 
     EXAMPLE 4 
     Competitive Binding Assay 
     Next, truncated PDGF-D was tested for its capacity to bind to the human PDGF beta-receptor by analyzing its ability to compete with PDGF-BB for binding to the PDGF beta-receptor. The binding experiments were performed on porcine aortic endothelial-1 (PAE-1) cells stably expressing the human PDGF alpha- and beta-receptors, respectively (Eriksson et al., EMBO J, 1992, 11, 543-550). Binding experiments were performed essentially as in Heldin et al. (EMBO J, 1988, 7 1387-1393). Conditioned media from COS cells expressing PDGF-A, truncated PDGF-D, or mock control, respectively, was diluted with an equal volume of BSA/PBS and mixed with 100 ng/ml of  125 I-PDGF-BB (beta-receptor ligand) or of  125 I-PDGF-AA (alpha-receptor ligand) in binding buffer (PBS containing 1 mg/ml of BSA) Two separate sets of conditioned media from these COS cells were analyzed. Aliquots were incubated with the receptor expressing PAE-1 cells plated in 24-well culture dishes on ice for 90 minutes. After three washes with binding buffer, cell-bound  125 I-PDGF-BB or  125 I-PDGF-AA was extracted by lysis of cells in 20 mM Tris-HCl, pH 7.5, 10% glycerol, 1% Triton X-100. The amount of cell bound radioactivity was determined in a gamma-counter. FIG. 12 provides a graphical representation of results which show that conditioned medium containing truncated PDGF-D competes for binding with PDGF-BB homodimers for the PDGF beta-receptors, but not with PDGF-AA homodimers for the PDGF alpha-receptors. 
     PDGF-D does not bind to any of the known VEGF receptors. PDGF-D is the only VEGF family member, thus far, which can bind to and increase phosphorylation of the PDGF beta-receptor. These characteristics indicate that the truncated form of PDGF-D may not be a VEGF family member, but instead a novel PDGF. Furthermore, the full length protein is likely to be a latent growth factor that needs to be activated by proteolytic processing to release the active PDGF/VEGF homology domain. The N-terminal CUB domain may be expressed as an inhibitory domain which might be used to localize this latent growth factor in some extracellular compartment (for example the extracellular matrix) and which is removed by limited proteolysis when need, for example during embryonic development, tissue regeneration, tissue remodelling including bone remodelling, active angiogenesis, tumor progression, tumor invasion, metastasis formation and/or wound healing. 
     BIOASSAYS TO DETERMINE THE FUNCTION OF PDGF-D 
     Assays are conducted to evaluate whether PDGF-D has similar activities to PDGF-A, PDGF-B, VEGF, VEGF-B, VEGF-C and/or VEGF-D in relation to growth and/or motility of connective tissue cells, fibroblasts, myofibroblasts and glial cells; to endothelial cell function; to angiogenesis; and to wound healing. Further assays may also be performed, depending on the results of receptor binding distribution studies. 
     I. Mitogenicity of PDGF-D for Endothelial Cells 
     To test the mitogenic capacity of PDGF-D for endothelial cells, the PDGF-D polypeptide is introduced into cell culture medium containing 5% serum and applied to bovine aortic endothelial cells (BAEs) propagated in medium containing 10% serum. The BAEs are previously seeded in 24-well dishes at a density of 10,000 cells per well the day before addition of the PDGF-D. Three days after addition of this polypeptide the cells were dissociated with trypsin and counted. Purified VEGF is included in the experiment as positive control. 
     II. Mitogenicity of PDGF-D for Fibroblasts 
     To test the mitogenic capacity of PDGF-D for fibroblasts, different concentrations of truncated homodimers of PDGF-DD or PDGF-AA (as control) are added to serum starved human foreskin fibroblasts in the presence of 0.2 μmCi [3H]thymidine. The fibroblasts are then incubated for 24 hours with 1 ml of serum-free medium supplemented with 1 mg/ml BSA. After trichloroacetic acid (TCA) precipitation, the incorporation of [3H]thymidine into DNA is determined using a beta-counter. The assay is performed essentially as described in Mori et al., J. Biol. Chem., 1991 266 21158-21164. 
     III. Assays of Endothelial Cell Function 
     a) Endothelial Cell Proliferation 
     Endothelial cell growth assays are performed by methods well known in the art, e.g. those of Ferrara &amp; Henzel, Nature, 1989 380 439-443, Gospodarowicz et al., Proc. Natl. Acad. Sci. USA, 1989 86 7311-7315, and/or Claffey et al., Biochem. Biophys. Acta, 1995 1246 1-9. 
     b) Cell Adhesion Assay 
     The effect of PDGF-D on adhesion of polymorphonuclear granulocytes to endothelial cells is tested. 
     c) Chemotaxis 
     The standard Boyden chamber chemotaxis assay is used to test the effect of PDGF-D on chemotaxis. 
     d) Plasminogen Activator Assay 
     Endothelial cells are tested for the effect of PDGF-D on plasminogen activator and plasminogen activator inhibitor production, using the method of Pepper et al., Biochem. Biophys. Res. Commun., 1991 181 902-906. 
     e) Endothelial Cell Migration Assay 
     The ability of PDGF-D to stimulate endothelial cells to migrate and form tubes is assayed as described in Montesano et al., Proc. Natl. Acad. Sci. USA, 1986 83 7297-7301. Alternatively, the three-dimensional collagen gel assay described in Joukov et al., EMBO J., 1996 15 290-298 or a gelatinized membrane in a modified Boyden chamber (Glaser et al., Nature, 1980 288 483-484) may be used. 
     IV. Angiogenesis Assay 
     The ability of PDGF-D to induce an angiogenic response in chick chorioallantoic membrane is tested as described in Leung et al., Science, 1989 246 1306-1309. Alternatively the rat cornea assay of Rastinejad et al., Cell, 1989 56 345-355 may be used; this is an accepted method for assay of in vivo angiogenesis, and the results are readily transferrable to other in vivo systems. 
     V. Wound Healing 
     The ability of PDGF-D to stimulate wound healing is tested in the most clinically relevant model available, as described in Schilling et al., Surgery, 1959 46 702-710 and utilized by Hunt et al., Surgery, 1967 114 302-307. 
     VI. The Haemopoietic System 
     A variety of in vitro and in vivo assays using specific cell populations of the haemopoietic system are known in the art, and are outlined below. In particular a variety of in vitro murine stem cell assays using fluorescence-activated cell sorter to purified cells are particularly convenient: 
     a) Repopulating Stem Cells 
     These are cells capable of repopulating the bone marrow of lethally irradiated mice, and have the Lin − , Rh h1 , Ly-6A/E + , c-kit +  phenotype. PDGF-D is tested on these cells either alone, or by co-incubation with other factors, followed by measurement of cellular proliferation by  3 H-thymidine incorporation. 
     b) Late Stage Stem Cells 
     These are cells that have comparatively little bone marrow repopulating ability, but can generate D13 CFU-S. These cells have the Lin − , Rh h1 , Ly-6A/E + , c-kit +  phenotype. PDGF-D is incubated with these cells for a period of time, injected into lethally irradiated recipients, and the number of D13 spleen colonies enumerated. 
     c) Progenitor-Enriched Cells 
     These are cells that respond in vitro to single growth factors and have the Lin − , Rh h1 , Ly-6A/E + , c-kit +  phenotype. This assay will show if PDGF-D can act directly on haemopoietic progenitor cells. PDGF-D is incubated with these cells in agar cultures, and the number of colonies present after 7-14 days is counted. 
     VII. Atherosclerosis 
     Smooth muscle cells play a crucial role in the development or initiation of atherosclerosis, requiring a change of their phenotype from a contractile to a synthetic state. Macrophages, endothelial cells, T lymphocytes and platelets all play a role in the development of atherosclerotic plaques by influencing the growth and phenotypic modulations of smooth muscle cell. An in vitro assay using a modified Rose chamber in which different cell types are seeded on to opposite cover slips measures the proliferative rate and phenotypic modulations of smooth muscle cells in a multicellular environment, and is used to assess the effect of PDGF-D on smooth muscle cells. 
     VIII. Metastasis 
     The ability of PDGF-D to inhibit metastasis is assayed using the Lewis lung carcinoma model, for example using the method of Cao et al., J. Exp. Med., 1995 182 2069-2077. 
     IX. Migration of Smooth Muscle Cells 
     The effects of the PDGF-D on the migration of smooth muscle cells and other cells types can be assayed using the method of Koyama et al., J. Biol. Chem., 1992 267 22806-22812. 
     X. Chemotaxis 
     The effects of the PDGF-D on chemotaxis of fibroblast, monocytes, granulocytes and other cells can be assayed using the method of Siegbahn et al., J. Clin. Invest., 1990 85 916-920. 
     XI. PDGF-D in Other Cell Types 
     The effects of PDGF-D on proliferation, differentiation and function of other cell types, such as liver cells, cardiac muscle and other cells, endocrine cells and osteoblasts can readily be assayed by methods known in the art, such as  3 H-thymidine uptake by in vitro cultures. 
     XII. Construction of PDGF-D Variants and Analogues 
     PDGF-D is a member of the PDGF family of growth factors which exhibits a high degree of homology to the other members of the PDGF family. PDGF-D contains seven conserved cysteine residues which are characteristic of this family of growth factors. These conserved cysteine residues form intra-chain disulfide bonds which produce the cysteine knot structure, and inter-chain disulfide bonds that form the protein dimers which are characteristic of members of the PDGF family of growth factors. PDGF-D interacts with a protein tyrosine kinase growth factor receptor. 
     In contrast to proteins where little or nothing is known about the protein structure and active sites needed for receptor binding and consequent activity, the design of active mutants of PDGF-D is greatly facilitated by the fact that a great deal is known about the active sites and important amino acids of the members of the PDGF family of growth factors. 
     Published articles elucidating the structure/activity relationships of members of the PDGF family of growth factors include for PDGF: Oestman et al., J. Biol. Chem., 1991 266 10073-10077; Andersson et al., J. Biol. Chem., 1992 267 11260-1266; Oefner et al., EMBO J., 1992 11 3921-3926; Flemming et al., Molecular and Cell Biol., 1993 13 4066-4076 and Andersson et al., Growth Factors, 1995 12 159-164; and for VEGF: Kim et al., Growth Factors, 1992 7 53-64; Pötgens et al., J. Biol. Chem., 1994 269 32879-32885 and Claffey et al., Biochem. Biophys. Acta, 1995 1246 1-9. From these publications it is apparent that because of the eight conserved cysteine residues, the members of the PDGF family of growth factors exhibit a characteristic knotted folding structure and dimerization, which result in formation of three exposed loop regions at each end of the dimerized molecule, at which the active receptor binding sites can be expected to be located. 
     Based on this information, a person skilled in the biotechnology arts can design PDGF-D mutants with a very high probability of retaining PDGF-D activity by conserving the eight cysteine residues responsible for the knotted folding arrangement and for dimerization, and also by conserving, or making only conservative amino acid substitutions in the likely receptor sequences in the loop 1, loop 2 and loop 3 region of the protein structure. 
     The formation of desired mutations at specifically targeted sites in a protein structure is considered to be a standard technique in the arsenal of the protein chemist (Kunkel et al., Methods in Enzymol., 1987 154 367-382). Examples of such site-directed mutagenesis with VEGF can be found in Pötgens et al., J. Biol. Chem., 1994 269 32879-32885 and Claffey et al., Biochem. Biophys. Acta, 1995 1246 1-9. Indeed, site-directed mutagenesis is so common that kits are commercially available to facilitate such procedures (e.g. Promega 1994-1995 Catalog., Pages 142-145). 
     The connective tissue cell, fibroblast, myofibroblast and glial cell growth and/or motility activity, the endothelial cell proliferation activity, the angiogenesis activity and/or the wound healing activity of PDGF-D mutants can be readily confirmed by well established screening procedures. For example, a procedure analogous to the endothelial cell mitotic assay described by Claffey et al., (Biochem. Biophys. Acta., 1995 1246 1-9) can be used. Similarly the effects of PDGF-D on proliferation of other cell types, on cellular differentiation and on human metastasis can be tested using methods which are well known in the art. 
     The foregoing description and examples have been set forth merely to illustrate the invention and are not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed broadly to include all variations falling within the scope of the appended claims and equivalents thereof.