LAMININ 8 AND METHODS FOR ITS USE

Abstract: The present invention provides substantially purified laminin 8, methods for making recombinant laminin 8, cells that express recombinant laminin 8, and methods for using the recombinant laminin 8 to accelerate the healing of injuries to vascular tissue and tissue of mesenchymal origin, and to promote cell attachment and migration.

Description: LAMININ 8 AND METHODS FOR ITS USE Cross Reference This application claims priority to U.S. Provisional Patent Application Serial Nos. 60/131,720 filed April 30, 1999; 60/149,738 filed August 19, 1999; 60/155,945 filed September 24, 1999; and 60/182,012 filed February 11, 2000; all of which are incoφorated herein by reference in their entirety. Field of the Invention This application relates to purified laminin 8 and methods for its use. Background of the Invention Basal laminae (basement membranes) are sheet-like, cell-associated extracellular matrices that play a central role in cell growth, tissue development, and tissue maintenance. They are present in virtually all tissues, and appear in the earliest stages of embryonic development. Basal laminae are central to a variety of architectural and cell-interactive functions: (See for example, Malinda and Kleinman, Int. J. Biochem. Cell Biol. 28:957- 959 (1996); Aumailley and Krieg, J. Invest. Dermatology 106:209-214 (1996)) 1. They serve as architectural supports for tissues, providing adhesive substrata for cells. 2. They create perm-selective barriers between tissue compartments that impede the migration of cells and passively regulate the exchange of macromolecules. These properties are illustrated by the kidney glomerular basement membrane, which functions as an important filtration structure, creating an effective blood- tissue barrier that is not permeable to most proteins and cells. 3. Basal laminae create highly interactive surfaces that can promote cell migration and cell elongation during embryogenesis and wound repair. Following an injury, they provide a surface upon which cells regenerate to restore normal tissue function. 4. Basal laminae present information encoded in their structure to contacting cells that is important for differentiation and tissue maintenance. This information is communicated to the cells through various receptors that include the integrins, dystroglycan, and cell surface proteoglycans. Signaling is dependent not only on the presence of matrix ligands and corresponding receptors that interact with sufficient affinities, but also on such topographical factors as ligand density in a three-dimensional matrix "landscape", and on the ability of basal lamina components to cluster receptors. Because these matrix proteins can be long- lived, basal laminae create a "surface memory" in the basal lamina for resident and transient cells. The basal lamina is largely composed of laminin and type IV collagen heterotrimers that in turn become organized into complex polymeric structures. To date, six type IV collagen chains and at least twelve laminin subunits have been identified. These chains possess shared and unique functions and are expressed with specific temporal (developmental) and spatial (tissue-site specific) patterns. Laminins are a family of heterotrimeric glycoproteins that reside primarily in the basal lamina. They function via binding interactions with neighboring cell receptors, and by forming laminin networks, and they are important signaling molecules that can strongly influence cellular function. Laminins are important in both maintaining cell/tissue phenotype as well as promoting cell growth and differentiation in tissue repair and development. Laminins are large, multi-domain proteins, with a common structural organization. The laminin molecule integrates various matrix and cell interactive functions into one molecule. The laminin molecule is comprised of an α-, β-, and γ-chain subunit joined together through a coiled-coil domain. Within this structure are identifiable domains that possess binding activity towards other laminin and basal lamina molecules, and membrane-bound receptors. Domains VI, IVb, and IVa form globular structures, and domains V, Illb, and Ilia (which contain cysteine-rich EGF-like elements) form rodlike structures. (Kamiguchi et al., Ann. Rev. Neurosci. 21 :97-125 (1998)) Domains I and II of the three chains participate in the formation of a triple-stranded coiled-coil structure (the long arm). WO 00/66732 PCT/USOO/l 1543 Table 1 shows the individual chains that each laminin type is composed of: TABLE 1. Known laminin family members Four structurally-defined family groups of laminins have been identified. The first group of five identified laminin molecules all share the βl and γl chains, and vary by their α-chain composition (αl to α5 chain). The second group of five identified laminin molecules all share the β2 and γl chain, and again vary by their α- chain composition. The third group of identified laminin molecules has one identified member, laminin 5, with a chain composition of α3β3γ2. The fourth group of identified laminin molecules has one identified member, laminin 12, with the newly identified γ3 chain (α2βlγ3) Some progress has been made in elucidating the relationship between domain structure and function. (See, for example, Wewer and Engvall, Neuromusc. Disord. 6:409-418 (1996).) The overall sequence similarity among the homologous domains in different chains varies, but it is highest in domain VI (thought to play a key role in laminin polymerization), followed by domains V (possibly involved in protein- protein interactions) and III (entactin/nidogen binding; possible cell adhesion sites), and is lowest in domains I, II (both thought to be involved in intermolecular assembly, and containing possible cell adhesion sites), and G. Not all domains are present in all 3 types of chains. The globular G domain (thought to be involved in cell receptor binding) is present only in the α chains. Other domains may not be present in all chains within a certain chain type. For example, domain VI is absent from α.3, α4, and γ2 chains. (Wewer and Engvall, 1996) As a result of their large size (>600 kD) and unique structure, the laminin molecules can be resolved in the electron microscope. (Wewer and Engvall, 1996) Typically, laminins appear as cross-shaped molecules in an EM. The three short arms of the cross represent the amino terminal portions of each of the three separate laminin chains (one short arm per chain). The long arm of the cross is composed of the C-terminal parts of the three chains, which together form a coiled coil structure. (Wewer and Engvall, 1996) The long arm ends with the globular G domain. The coiled-coil domain of the long arm is crucial for assembly of the three chains of laminin. (Yurchenco et al., Proc. Natl. Acad. Sci. 94:10189-10194 (1997)). Disulfide bonds bridge and stabilize all three chains in the most proximal region of the long arm and join the β and γ chains in the most distal region of the long arm. A model of laminin receptor-facilitated self-assembly, based on studies conducted with cultured skeletal myotubes and Schwann cells, predicts that laminins bind to their receptors, which freely diffuse in a fluidic membrane, when ligand-free. Receptor engagement forces the laminins into a high local two-dimensional concentration, facilitating their mass-action driven assembly into ordered surface polymers. In this process, the engaged receptors are also reorganized, accompanied by cytoskeletal rearrangements. (Colognato, J. Cell Biol. 145:619-631 (1999)) This reorganization activates the receptors, causing signal transduction with the alteration of cell expression, shape and/or behavior. The evidence is that laminins must possess both cell-interacting and architecture-forming sites, which are located in different protein domains and on different subunits. One class of laminin receptors are the integrins, which are cell surface receptors that mediate many cell-matrix and cell-cell interactions. Integrins are heterodimers, consisting of an α and a β subunit. 16 α- and 8 β-subunits are known, and at least 22 combinations of α and β subunits have been identified to date. Some integrins have only one or a few known ligands, whereas others appear to be very promiscuous. Binding to integrins is generally of low affinity, and is dependent on divalent cations. Integrins, activated through binding to their ligands, transduce signals via kinase activation cascades, such as focal adhesion and mitogen-activated kinases. Several different integrins bind different laminin isoforms more or less specifically. (Aumailley et al., In The Laminins, Timpl and Ekblom, eds., Harwood Academic Publishers, Amsterdam, pp. 127-158 (1996)) Laminin 8, a recently identified laminin, is composed of α4, βl, and γl laminin chains. The laminin α4 chain is widely distributed both in adults and during development. (Iivanainen et al., J. Biol. Chem. 272:27862-27868 (1997)) In adults it is found in the basement membrane surrounding cardiac, skeletal, and smooth muscle fibers, and in lung alveolar septa. Furthermore, it is found in the endothelial basement membrane both in capillaries and larger vessels, and in the perineurial basement membrane of peripheral nerves, as well as in intersinusoidal spaces, large arteries, and smaller arterioles of bone marrow. (Frieser et al., Eur. J. Biochemistry 246:727-735 (1997); Miner et al., J. Cell Biol. 137:685-701 (1997); Geberhiwot et al., Exptl. Cell Res. 253:723-732 (1999); Gu et al., Blood 93:2533-2542 (1999); Iivanainen et al., J. Biol. Chem. 272:27862-27868 (1997)) Laminin 8 is a major laminin isoform in the vascular endothelium (Iivanainen et al., J. Biol. Chem. 272:27862-27868 (1997); Frieser et al, 1997), is expressed and adhered to by platelets (Geberhiwot et al., Exptl. Cell Res. 253:723-732 (1999)), and is the only laminin isoform synthesized in 3T3-L1 adipocytes, with its level of synthesis shown to increase upon adipose conversion of the cells. (Niimi et al., Matrix Biology 16:223-230 (1997)) Laminin 8 was further speculated to be the laminin isoform generally expressed in mesenchymal cell lineages to induce microvessels in connective tissues. (Niimi et al., 1997). Laminin 8 has also been identified in mouse bone marrow primary cell cultures, arteriolar walls, and intersinusoidal spaces where data indicated that it is the major laminin isoform in the developing bone marrow. (Gu et al., Blood 93:2533- 2542 (1999). The investigators concluded that, due to its localization in adult bone marrow adjacent to hematopoietic cells, laminin isoforms containing the α4 chain are the most likely to have biologically relevant interactions with developing hematopoietic cells. (Gu et al., 1999) Despite the broad tissue distribution of the laminin α4 chain and laminin 8, there is not a means to prepare substantially purified laminin 8 from cell or tissue sources for research and therapeutic purposes, nor has a means for recombinant expression of laminin 8 been developed. Such research and therapeutic purposes include, but are not limited to, methods for treating injuries to tissue of mesenchymal origin, such as bone, cartilage, tendon, and ligament, treating injuries to vascular tissue, promoting cell attachment and migration, promoting therapeutic angiogenesis and neural regeneration, ex vivo cell therapy, improving the biocompatibility of medical devices, and preparing improved cell culture devices and media. Thus, there is a need in the art for adequate amounts of substantially purified laminin-8 for research and therapeutic purposes, and methods for making laminin 8. Such laminin 8 could be prepared either from cell lines in culture, or via recombinant DNA technology. A preferred method of production is the use of recombinant DNA technology to engineer a human cell line of choice to produce recombinant laminin-8 ("r-laminin 8"). A recombinant-based method of laminin-8 production has several advantages over purification from human tissue or isolation from human cell lines in culture: 1. The recombinant produced protein is free of human pathogens. While this is also true for endogenous cell culture produced protein, protein derived from human tissue carries a risk for contamination by HIV, hepatitis, and other infectious agents. 2. Expression levels of the protein, and hence yields, can be improved through the use of genetically engineered genes/vectors that enhance the production of the encoded protein. 3. It is possible to engineer additional peptide sequences to the protein chain that provides a binding site for a commercially viable affinity purification procedure. 4. The method can provide for the modification of protein structure/function through the addition, substitution, elimination, and/or other modifications of protein domain structures. For example, it may be desirable to introduce an integrin binding site (e.g. RGD), switch integrin recognition sites, or engineer in a stable binding site to a synthetic substrate. Thus, the creation of expression vectors that express laminin chains generates enormous flexibility for future uses and creates a basis for creating second generation "designer" laminins. Summary of the Invention The present invention fulfills the need in the art for a source of substantially purified laminin 8 protein, methods for making substantially purified recombinant laminin 8 (hereinafter referred to as r-laminin 8), pharmaceutical compositions comprising laminin 8, and methods of using laminin 8 for treating injuries to tissue of mesenchymal origin, such as bone, cartilage, tendon, and ligament, treating injuries to vascular tissue, promoting cell attachment and migration, ex vivo cell therapy, improving the biocompatibility of medical devices, and preparing improved cell culture devices and media. In one aspect, the present invention provides recombinant host cells that express laminin 8 chains and secrete r-laminin 8. In another aspect, the present invention provides substantially purified laminin 8, and methods for producing substantially purified r-laminin 8. In a further aspect, the present invention provides pharmaceutical compositions, comprising laminin 8 together with a pharmaceutically acceptable carrier. Such pharmaceutical compositions can optionally be provided with other extracellular matrix components. In further aspect, the present invention provides methods and kits for accelerating the healing of injuries to tissue of mesenchymal origin, such as bone, cartilage, tendon, and ligament, treating injuries to vascular tissue, and for improving the biocompatibility of grafts used for treating such injuries. In specific examples, laminin 8 or pharmaceutical compositions thereof are used to: a. promote re-endothelialization at the site of vascular injuries; b. improve the "take" of grafts; c. improve the biocompatibility of medical devices; d. treat neural injuries (neural regeneration); e. regulate angiogenesis; and d. promote cell attachment and migration by providing an amount effective of r-laminin 8 for the various methods. In preferred embodiments of all of these methods, recombinant laminin 8 is used. The kits comprise an amount of laminin 8 effective for the desired effect, and instructions for the use thereof. In a further aspect, the present invention provides improved medical devices and grafts, and methods for preparing improved medical devices and grafts, wherein the improvement comprises applying an amount effective of laminin 8 or the pharmaceutical compositions of the invention to the device or graft for the desired application. In a further aspect, the invention provides improved cell culture devices, and methods for preparing improved cell culture devices, for the growth and maintenance of cells in culture, by providing an amount effective of laminin 8 for the attachment of cells to a cell culture device for the subsequent proliferation/differentiation/stasis of the cells. In another aspect, the invention provides a cell culture growth supplement, comprising laminin 8. In another aspect, the invention provides an improved cell culture growth media, wherein the improvement comprises the addition of r-laminin 8. Brief Description of the Figures Figure 1 is a photograph of a 3-12% gradient SDS-PAGE gel. LN-1 is laminin 1/nidogen (ndg) complex with component chain identities indicated on the left; LN-8 is recombinant laminin 8. Inteφretation of r-laminin 8 protein band identities are indicated based on western blotting data: α4 = reactivity with anti-human laminin α4 and also anti-FLAG monoclonal antibody (mAb); βl = reactivity with polyclonal anti- murine laminin αl/βl/γl; γl = reactivity with anti-human laminin γl mAb; * = reactivity with both anti-laminin γl mAb and anti-murine αl/βl/γl. Both samples were run on the same gel which was subsequently silver stained. Figure 2 is a rotary shadowed electron micrograph or r-laminin 8. Top: low magnification field showing several r-laminin 8 molecules. Bottom: Individual molecules. Each molecule has two short arms and one long arm. In some molecules, a very short (5-10 nm) rod-like stub can be seen at the junction of the arms. Arrow: G- domain can be seen as consisting of two moieties in some molecules. (Bar = 50 nm) Figure 3 is a graph depicting a titration of cell adhesion to r-laminin 8 or laminin 1. Figure 4 is a graph depicting HT-1080 cell adhesion to r-laminin 8 or laminin 1 coated at 10 μg/ml on 96 well plates in the presence and absence of various function-blocking integrin antibodies and other compounds. Figure 5 is a graph depicting bovine capillary endothelial (BCE) cell adhesion to r- laminin 8 or laminin 1 coated at 10 μg/ml on 96 well plates in the presence and absence of various function-blocking integrin antibodies and other compounds. Figure 6 is a graph depicting immortomouse brain endothelial (IBE) cell adhesion to r- laminin 8 or laminin 1 coated at 10 μg/ml on 96 well plates in the presence and absence of various function-blocking integrin antibodies and other compounds. Figure 7 is a graph depicting integrin α6β4-transfected K562 cell adhesion to r-laminin 8 or laminin 1 coated at 10 μg/ml on 96 well plates in the presence and absence of various function-blocking integrin antibodies and other compounds. Figure 8 is a graph depicting integrin α6-transfected K562 cell adhesion to r-laminin 8 or laminin 1 coated at 10 μg/ml on 96 well plates in the presence and absence of various function-blocking integrin antibodies and other compounds. Detailed Description of the Preferred Embodiments All references, patents and patent applications are hereby incoφorated by reference in their entirety. Within this application, unless otherwise stated, the techniques utilized may be found in any of several well-known references such as: Molecular Cloning: A Laboratory Manual (Sambrook, et al., 1989, Cold Spring Harbor Laboratory Press), Gene Expression Technology (Methods in Enzymology, Vol. 185, edited by D. Goeddel, 1991. Academic Press, San Diego, CA), "Guide to Protein Purification" in Methods in Enzymology (M.P. Deutshcer, ed., (1990) Academic Press, Inc.); PCR Protocols: A Guide to Methods and Applications (Innis, et al. 1990. Academic Press, San Diego, CA), Culture of Animal Cells: A Manual of Basic Technique, 2" Ed. (R.I. Freshney. 1987. Liss, Inc. New York, NY), Gene Transfer and Expression Protocols, pp. 109-128, ed. E.J. Murray, The Humana Press Inc., Clifton, N.J.), and the Ambion 1998 Catalog (Ambion, Austin, TX). As used herein "laminin 8" encompasses both r-laminin 8 and heterotrimeric laminin 8 from naturally occurring sources. As used herein, the term "r-laminin 8" refers to recombinant heterotrimeric laminin 8, expressed by a cell that has been transfected with one or more expression vectors comprising at least one nucleic acid sequence encoding a laminin 8 chain selected from the α4, βl and γl chains, or a portion of the chains that are capable of forming a heterotrimeric laminin 8 and maintaining laminin 8 activity, or processed forms thereof. Such r-laminin 8 can thus comprise α4, βl, and γl sequences from a single organism, or from different organisms. Various laminin 8 chain DNA sequences are known in the art, and the use of each to prepare the r-laminin 8 of the invention is contemplated. (See, for example, Iivanainen et al., FEBS Letters 365:183- 188 (1995); Frieser et al., Eur. J. Biochem. 246:727-735 (1997); Richards et al., Eur. J. Biochem. 238:813-821 (1996); Liu and Mayne, 15:433-437 (1996); Vuolteenaho et al, J. Biol. Chem. 265:15611-15616 (1990); Kallunki et al., J. Biol. Chem. 266:221-228 (1991); Sasaki et al., J. Biol. Chem. 263:16536-16544 (1988); Sasaki and Yamada, J. Biol. Chem. 262:17111-17117 (1987); Sasaki et al, Proc. Natl. Acad. Sci. 84:935-939 (1987); Pikkarainen et al., J. Biol. Chem. 262:10454-10462 (1987); all references incoφorated by reference herein in their entirety). The invention encompasses those laminin molecules wherein one or two of the chains that make up the recombinant heterotrimeric laminin 8 are encoded by endogenous laminin 8 chains. In a preferred embodiment, cells are transfected with one or more expression vectors comprising nucleic acid sequences encoding each of the α4, βl and γl chains, or a portion of each of the chains that are capable of forming a heterotrimeric laminin 8 and maintaining laminin 8 activity. In the present invention, laminin 8 is a secreted protein, which is capable of being directed to the ER, secretory vesicles, and the extracellular space as a result of a signal sequence, as well as those proteins released into the extracellular space without necessarily containing a signal sequence. If the secreted protein is released into the extracellular space, the secreted protein can undergo extracellular processing to produce a "mature" protein. Such processing event can be variable, and thus may yield different versions of the final "mature protein". The substantially purified laminin 8 of the present invention includes heterotrimers comprising both the full length and any such processed laminin 8 chains. As used herein, the term "substantially purified" means that the laminin 8 so designated has been separated from its in vivo cellular environment. As used herein, a laminin 8 polypeptide chain refers to a polypeptide chain according to one or more of the following: (a) comprises a polypeptide structure selected from the group consisting of: 1. R1-R2-R3 2. Rl-R2-R3(e) 3. R3 4. R3(e) 5. R1-R3 6. Rl-R3(e) 7. R2-R3 8. R2-R3(e) wherein Rl is an amino terminal methionine; R2 is a signal sequence that is capable of directing secretion of the polypeptide, wherein the signal sequence may be the natural signal sequence for the particular laminin chain, that of another secreted protein, or an artificial sequence; R3 is a secreted laminin chain selected from the α4, βl, and γl chains; and R3(e) is a secreted laminin chain selected from the α4, βl, and γl chains that further comprises an epitope tag (such as those described below), which can be placed at any position within the laminin chain amino acid sequence; and/or (b) is encoded by a polynucleotide that is substantially similar to one or more of the disclosed laminin chain polynucleotide sequences or portions thereof (SEQ ID NOS.: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21,23, 25, or 27); and/or (c) is encoded by a polynucleotide that hybridizes under high or low stringency conditions to the coding regions, or portions thereof, of one or more of the recombinant laminin 8 chain DNA sequences disclosed herein (SEQ ID NOS.: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27), or complementary sequences thereof; and/or (d) has at least 70% identity to one or more of the disclosed laminin 8 polypeptide chain amino acid sequences (SEQ ID NOS.: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, or 28), preferably at least 80% identity, and most preferably at least about 90%) identity. WO 00/66732 PCT/USOO/l 1543 The phrase "substantially similar" is used herein in reference to polynucleotide or polypeptide sequences having one or more conservative variations from the laminin 8 sequences disclosed herein, including but not limited to deletions, insertions, inversions, repeats, and substitutions, wherein the resulting laminin chain is functionally equivalent to those disclosed herein. For example, conservative polynucleotide variants may contain alterations in coding regions, non-coding regions, or both. Especially preferred are polynucleotide variants containing alterations which produce silent substitutions, additions, or deletions, but do not alter the properties or activities of the encoded polypeptide. Nucleotide variants produced by silent substitutions due to the degeneracy of the genetic code are preferred. Moreover, variants in which 5-10, 1-5, or 1-2 amino acids are substituted, deleted, or added in any combination are also preferred. Polynucleotide variants can be produced for a variety of reasons, including but not limited to optimizing codon expression for a particular host (change codons in the human mRNA to those preferred by a bacterial host such as E. coli). Naturally occurring conservative variants are called "allelic variants," and refer to one of several alternate forms of a gene occupying a given locus on a chromosome of an organism. (Genes II, Lewin, B., ed., John Wiley & Sons, New York (1985).) These allelic variants can vary at either the polynucleotide and/or polypeptide level. Alternatively, non-naturally occurring conservative variants may be produced by mutagenesis techniques or by direct synthesis. Using known methods of protein engineering and recombinant DNA technology, conservative polynucleotide variants may be generated to improve or alter the characteristics of the expressed laminin chain polypeptides. For instance, one or more amino acids can be deleted from the N-terminus or C-terminus of the secreted protein. (See, for example, Ron et al, J. Biol. Chem. 268: 2984-2988 (1993); Dobeli et al., J. Biotechnology 7:199-216 (1988)) Ample evidence demonstrates that variants often retain a biological activity similar to that of the naturally occurring protein. (See, for example, Gayleet al., J. Biol. Chem 268:22105-22111 (1993)) Furthermore, even if deleting one or more amino acids from the N-terminus or C-terminus of a polypeptide results in modification or loss of one or more biological functions, other biological activities may still be retained. Guidance concerning how to make phenotypically silent amino acid substitutions is provided in Bowie, J. U. et al., Science 247:1306-1310 (1990), wherein WO 00/66732 PCT/USOO/l 1543 the authors indicate that there are two main strategies for studying the tolerance of an amino acid sequence to change. The first strategy exploits the tolerance of amino acid substitutions by natural selection during the process of evolution. By comparing amino acid sequences in different species, conserved amino acids can be identified. These conserved amino acids are likely important for protein function. In contrast, the amino acid positions where substitutions have been tolerated by natural selection indicates that these positions are not critical for protein function. Thus, positions tolerating amino acid substitution could be modified while still maintaining biological activity of the protein. The second strategy uses genetic engineering to introduce amino acid changes at specific positions of a cloned gene to identify regions critical for protein function. For example, site directed mutagenesis or alanine-scanning mutagenesis (introduction of single alanine mutations at every residue in the molecule) can be used. (Cunningham and Wells, Science 244:1081-1085 (1989).) The resulting mutant molecules can then be tested for biological activity. As the authors state, these two strategies have revealed that proteins are suφrisingly tolerant of amino acid substitutions. The authors further indicate which amino acid changes are likely to be permissive at certain amino acid positions in the protein. For example, most buried (within the tertiary structure of the protein) amino acid residues require nonpolar side chains, whereas few features of surface side chains are generally conserved. Moreover, tolerated conservative amino acid substitutions involve replacement of the aliphatic or hydrophobic amino acids Ala, Vai, Leu and He; replacement of the hydroxyl residues Ser and Thr; replacement of the acidic residues Asp and Glu; replacement of the amide residues Asn and Gin, replacement of the basic residues Lys, Arg, and His; replacement of the aromatic residues Phe, Tyr, and Tφ, and replacement of the small-sized amino acids Ala, Ser, Thr, Met, and Gly. The "substantially similar" polypeptides of the present invention also include (i) substitutions with one or more of the non-conserved amino acid residues, where the substituted amino acid residues may or may not be one encoded by the genetic code, or (ii) substitution with one or more amino acid residues having substituents groups, or (iii) fusion of the mature polypeptide with another compound, such as a compound to increase the stability and/or solubility of the polypeptide (for example, polyethylene glycol), or (iv) fusion of the polypeptide with additional amino acids, such as an IgG Fc fusion region peptide, or leader or secretory sequence, or a sequence facilitating purification. Such variant polypeptides are deemed to be within the scope of those skilled in the art from the teachings herein. For example, polypeptide variants containing amino acid substitutions of charged amino acids with other charged or neutral amino acids may produce proteins with improved characteristics, such as less aggregation. Aggregation of pharmaceutical formulations both reduces activity and increases clearance due to the aggregate's immunogenic activity. (Pinckard et al., Clin. Exp. Immunol. 2:331-340 (1967); Robbins et al., Diabetes 36: 838-845 (1987); Cleland et al., Crit. Rev. Therapeutic Drug Carrier Systems 10:307-377 (1993).) "Stringency of hybridization" is used herein to refer to conditions under which nucleic acid hybrids are stable. The invention also includes nucleic acids that hybridize under high stringency conditions (as defined herein) to all or a portion of the coding sequences of the laminin chain polynucleotides disclosed herein, or their complements. The hybridizing portion of the hybridizing nucleic acids is typically at least 50 nucleotides in length. As known to those of skill in the art, the stability of hybrids is reflected in the melting temperature (T M ) of the hybrids. T M decreases approximately 1-1.5°C with every 1% decrease in sequence homology. In general, the stability of a hybrid is a function of sodium ion concentration and temperature. Typically, the hybridization reaction is performed under conditions of lower stringency, followed by washes of varying, but higher, stringency. Reference to hybridization stringency relates to such washing conditions. Thus, as used herein, high stringency refers to an overnight incubation at 42° C in a solution comprising 50%o formamide, 5x SSC (750 mM NaCl, 75 mM sodium citrate), 50 mM sodium phosphate (pH 7.6), 5x Denhardt's solution, 10%) dextran sulfate, and 20 μg/ml denatured, sheared salmon sperm DNA, followed by washing the filters in 0. lx SSC at about 65°C. Also contemplated are laminin 8-encoding nucleic acid sequences that hybridize to the polynucleotides of the present invention at lower stringency hybridization conditions. Changes in the stringency of hybridization and signal detection are primarily accomplished through the manipulation of formamide concentration (lower percentages of formamide result in lowered stringency); salt conditions, or temperature. For example, lower stringency conditions include an overnight incubation at 37°C in a solution comprising 6X SSPE (20X SSPE = 3M NaCl; 0.2M NaH 2 PO 4 ; 0.02M EDTA, pH 7.4), 0.5% SDS, 30% formamide, 100 ug/ml salmon sperm blocking DNA; followed by washes at 50°C with 1XSSPE, 0.1% SDS. In addition, to achieve even lower stringency, washes performed following stringent hybridization can be done at higher salt concentrations (e.g. 5X SSC). Note that variations in the above conditions may be accomplished through the inclusion and/or substitution of alternate blocking reagents used to suppress background in hybridization experiments. Typical blocking reagents include Denhardt's reagent, BLOTTO, heparin, denatured salmon sperm DNA, and commercially available proprietary formulations. The inclusion of specific blocking reagents may require modification of the hybridization conditions described above, due to problems with compatibility. As used herein, "percent identity" of two amino acids or of two nucleic acids is determined using the algorithm of Karlin and Altschul (Proc. Natl. Acad. Sci. USA 87:2264-2268, 1990), modified as in Karlin and Altschul (Proc. Natl. Acad. Sci. USA 90:5873-5877, 1993). Such an algorithm is incoφorated into the NBLAST and XBLAST programs of Altschul et al. (J. Mol. Biol. 215:403-410, 1990). BLAST nucleotide searches are performed with the NBLAST program, score = 100, wordlength = 12, to obtain nucleotide sequences homologous to the nucleic acid molecules of the invention. BLAST protein searches are performed with the XBLAST program, score = 50, wordlength = 3, to obtain an amino acid sequence homologus to a polypeptide of the invention. To obtain gapped alignments for comparison puφoses, Gapped BLAST is utilized as described in Altschul et al. (Nucleic Acids. Res. 25:3389-3402, 1997). When utilizing BLAST and Gapped BLAST programs, the default parameters of the respective programs (e.g., XBLAST and NBLAST) are used. See http://www.ncbi.nlm.nih.gov. Further embodiments of the present invention include polynucleotides encoding laminin 8 chain polypeptides having at least 70% identity, preferably at least 80% identity, and most preferably at least 90% identity to one or more of the polypeptide sequences contained in SEQ ID NO:2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, or fragments thereof. As used herein, "α4 polynucleotide" refers to polynucleotides encoding an α4 laminin chain of the same name. Such polynucleotides can be characterized by one or more of the following: (a) the nucleotides of said polynucleotide may encode an amino acid sequence substantially similar to the sequence set forth in SEQ ID NO: 2, 4, 6, 8, 10, 12 or fragments thereof; (b) polynucleotides that encode polypeptides which share at least 70% identity, preferably 80% identity, and most preferably at least 90% identity with the sequence set forth in SEQ ID NO: 2, 4, 6, 8, 10, 12, or fragments thereof; (c) the α.4 polynucleotides hybridize under low or high stringency conditions to the coding sequence set forth in one or more of SEQ ID NO: 1, 3, 5, 7, 9, 11, or fragments thereof, or complementary sequences thereof; or (d) the α.4 polynucleotides encode a polypeptide with a general structure selected from (1) R1-R2-R3; (2) Rl-R2-R3(e); (3) R3; (4) R3(e); (5) R1-R3; (6) Rl-R3(e); (7) R2-R3; and (8) R2-R3(e); wherein Rl and R2 are as described above, and R3 and R3(e) are as described above but comprise secreted α4 chain polypeptides. As used herein, "βl polynucleotides" refers to polynucleotides encoding a βl laminin chain of the same name. Such polynucleotides can be characterized by one or more of the following: (a) the nucleotides of said polynucleotides may encode an amino acid sequence substantially similar to the sequence set forth in SEQ ID NO: 14, 16, 18, 20 or fragments thereof; (b) polynucleotides that encode polypeptides which share at least 70% identity, preferably at least 80%., and most preferably at least 90%> identity with one or more of the sequences set forth in SEQ ID NO: 14, 16, 18, 20 or fragments thereof; (c) the βl polynucleotides hybridize under low or high stringency conditions to the coding sequence set forth in one or more of SEQ ID NO: 13, 15, 17, 19, fragments thereof, or complementary sequences thereof; or (d) the βl polynucleotides encode a polypeptide with a general structure selected from (1) R1-R2-R3; (2) Rl-R2-R3(e); (3) R3; (4) R3(e); (5) R1-R3; (6) Rl-R3(e); (7) R2-R3; and (8) R2-R3(e); wherein Rl and R2 are as described above, and R3 and R3(e) are as described above but comprise secreted βl chain polypeptides. As used herein, "γl polynucleotides" refers to polynucleotides encoding a γl laminin chain of the same name. Such polynucleotides can be characterized by one or more of the following: (a) the nucleotides of said polynucleotides may encode an amino acid that is substantially similar to one or more of the sequences set forth in SEQ ID NO: 22, 24, 26, 28 or fragments thereof; (b) polynucleotides that encode polypeptides which share at least 70% identity, preferably at least 80%, and most preferably at least 90% identity with at one or more of the sequences set forth in SEQ ID NO: 22, 24, 26, 28 or fragments thereof; (c) the γl polynucleotides hybridize under low or high stringency conditions to the coding sequence set forth in one or more of SEQ ID NO: 21, 23, 25, 27 or complementary sequences thereof; or (d) the γl polynucleotides encode a polypeptide with a general structure selected from (1) R1-R2-R3; (2) R1-R2- R3(e); (3) R3; (4) R3(e); (5) R1-R3; (6) Rl-R3(e); (7) R2-R3; and (8) R2-R3(e); wherein Rl and R2 are as described above, and R3 and R3(e) are as described above but comprise secreted γl chain polypeptides. As used herein, the term "epitope tag" refers to a polypeptide sequence that is expressed as part of a chimeric protein, where the epitope tag serves as a recognition site for binding of antibodies generated against the epitope tag, or for binding of other molecules that can be used for affinity purification of sequences containing the tag. As used herein, the term "increased biocompatibility" refers to reduced induction of acute or chronic inflammatory response, and reduced disruption of the proper differentiation of implant-surrounding tissues for laminin 8-coated biomaterials relative to an analogous, non-coated biomaterial. As used herein the term "graft" refers to both natural and prosthetic grafts and implants. In one aspect, the present invention provides r-laminin 8 expressing-cells that have been transfected with an expression vector containing promoter sequences that are operatively linked to nucleic acid sequences encoding at least one polypeptide sequence comprising the α4, βl and γl chains of laminin 8, or fragments thereof, wherein the transfected cells secrete heterotrimeric laminin 8 containing the recombinant laminin chain. In a preferred embodiment, the cells are systematically transfected with recombinant expression vectors containing promoter sequences that are operatively linked to nucleic acid sequences encoding polypeptide sequences comprising the α4, βl and γl chains of laminin 8 After the multiple transfections, the cells express each of the recombinant laminin 8 chains, which form the heterotrimer, before r-laminin 8 secretion into the media. In a preferred embodiment, cDNAs encoding the α4, βl and γl chains, or fragments thereof, are subcloned into an expression vector. Alternatively, laminin 8 α4, βl and/or γl gene sequences, including one or more introns, can be used. Any cell capable of expressing and secreting the r-laminin 8 can be used. Preferably, eukaryotic cells are used, and most preferably mammalian cells are used, including but not limited to kidney and epithelial cell lines. In a most preferred embodiment, the mammalian cells do not express all of the laminin 8 chains endogenously. Carbohydrate and disulfide post-translational modifications are believed to be required for laminin 8 protein folding and function. This makes the use of eukaryotic cells preferable for producing functional r-laminin 8, although other systems are useful for obtaining, for example, antigens for antibody production. "Recombinant expression vector" includes vectors that operatively link a nucleic acid coding region or gene to any promoter capable of effecting expression of the gene product. The promoter sequence used to drive expression of the individual chains or r-laminin 8 may be constitutive (driven by any of a variety of promoters, including but not limited to, CMV, SV40, RSV, actin, EF) or inducible (driven by any of a number of inducible promoters including, but not limited to, tetracycline, ecdysone, steroid-responsive). The expression vector must be rephcable in the host organisms either as an episome or by integration into host chromosomal DNA. In a preferred embodiment, the expression vector comprises a plasmid. However, the invention is intended to include other expression vectors that serve equivalent functions, such as viruses. In one embodiment, at least one of the laminin chain polypeptide sequences, or fragments thereof, is operatively linked to a nucleic acid sequence encoding an "epitope tag", so that at least one of the chains is expressed as a fusion protein with an expressed epitope tag. The epitope tag may be expressed as the amino terminus, the carboxy terminus, or internal to any of the polypeptide chains comprising r-laminin 8, so long as the resulting r-laminin 8 remains functional. Any epitope tag may be utilized, so long as it can be used as the basis for affinity purification of the resulting r-laminin 8. Examples of such epitope tags include, but are not limited to FLAG (Sigma Chemical, St. Louis, MO), myc (9E10) (Invitrogen, Carlsbad, CA), 6-His (Invitrogen; Novagen, Madison, WI), and HA (Boehringer Manheim Biochemicals). In another embodiment, one of the r-laminin 8 chains is expressed as a fusion protein with a first epitope tag, and at least one other r-laminin chain is expressed as a fusion protein with a second epitope tag. This permits multiple rounds of purification to be carried out. Alternatively, the same epitope tag can be used to create fusion proteins with more than one of the r-laminin chains. In a further embodiment, the epitope tag can be engineered to be cleavable from the r-laminin 8 chain(s). Alternatively, no epitope tag is fused to any of the r- laminin 8 chains, and the r-laminin 8 is purified by standard techniques, including but not limited to affinity chromatography using laminin 8 specific antibodies or other laminin 8 binding molecules. Transfection of the expression vectors into eukaryotic cells can be accomplished via any technique known in the art, including but not limited to calcium phosphate co- precipitation, electroporation, or liposome mediated-, DEAE dextran mediated-, polycationic mediated-, or viral mediated transfection. Transfection of bacterial cells can be done by standard methods. In a preferred embodiment, the cells are stably transfected. Methods for stable transfection and selection of appropriate transfected cells are known in the art. In a most preferred embodiment, a CMV promoter driven expression vector is used in a human kidney embryonic 293 cell line. Media from cells transfected with a single laminin chain are initially analyzed on Western blots using laminin chain-specific antibodies. The expression of single laminin chains following transfection is generally intracellular. Clones showing reactivity against individual transfected chain(s) are verified by any appropriate method, such as PCR, reverse transcription-PCR, or nucleic acid hybridization, to confirm incoφoration of the transfected gene. Preferably, analysis of genomic DNA preparations from such clones is done by PCR using laminin chain-specific primer pairs. Media from transfected clones producing all three chains are further analyzed for r-laminin 8 secretion and/or activity, by any appropriate method, including Western blot analysis and cell binding assays. Activity of the r-laminin 8 is preferably analyzed in a cell adhesion assay. In another aspect, the present invention provides substantially purified laminin 8, preferably r-laminin 8. In one embodiment, the substantially purified laminin 8 comprises a first chain comprising an α4 chain polypeptide; a second chain comprising a βl chain polypeptide; and a third chain comprising a γl chain polypeptide. Alternatively, the r-laminin 8 comprises a first chain that is substantially similar to at least one of the sequences shown in SEQ ID NO: 2, 4, 6, 8, 10, 12 or fragments thereof; a second chain that is substantially similar to at least one of the sequence shown in SEQ ID NO: 14, 16, 18, 20 or fragments thereof; and a third chain that is substantially similar to the sequence shown in SEQ ID NO: 22, 24, 26, 28 or fragments thereof. In another embodiment, the substantially purified r-laminin 8 comprises a first chain comprising a polypeptide that is at least about 70% identical to at least one of the sequences shown in SEQ ID NO: 2, 4, 6, 8, 10, 12 or fragments thereof; a second chain comprising a polypeptide that is at least 70% identical to at least one of the sequences shown in SEQ ID NO: 14, 16, 18, 20 or fragments thereof; and a third chain comprising a polypeptide that is at least 70% identical to at least one of the sequences shown in SEQ ID NO: 22, 24, 26, 28 or fragments thereof, wherein the first, second, and third polypeptides are produced recombinantly, and wherein the first, second, and third chains assemble into a recombinant heterotrimeric laminin 8. In a preferred embodiment, at least one of the first, second, or third chains of the substantially purified human r-laminin 8 is expressed as a fusion protein with an epitope tag. Alternatively, the r-laminin 8 comprises a heterotrimeric polypeptide structure, wherein each individual chain comprises a general structure selected from the group consisting of: (1) R1-R2-R3; (2) Rl-R2-R3(e); (3) R3; (4) R3(e); (5) R1-R3; (6) Rl- R3(e); (7) R2-R3; and (8) R2-R3(e) wherein Rl is a amino terminal methionine; R2 is a signal sequence that is capable of directing secretion of the polypeptide, wherein the signal sequence may be the natural signal sequence for the particular laminin chain, that of another secreted protein, or an artificial sequence; R3 is a secreted α4, βl, or γl laminin chain; and R3(e) is a secreted laminin α4, βl, and γl chain that further comprises an epitope tag (such as those described above), which can be placed at any position within the laminin chain amino acid sequence. In a preferred embodiment, purification of r-laminin 8 is accomplished by passing media from the transfected cells through an antibody affinity column. In one embodiment, antibodies against a peptide epitope expressed on at least one of the recombinant chains are attached to an affinity column, and bind the r-laminin 8 that has been secreted into the media. The r-laminin 8 is removed from the column by passing excess peptide over the column. Eluted fractions are analyzed by any appropriate method, including gel electrophoresis and Western blot analysis. In a further embodiment, the peptide epitope can be cleaved after purification. In other embodiments, two or three separate r-laminin chains are expressed as fusion proteins, each with a different epitope tag, permitting two or three rounds of purification and a doubly or triply purified r-laminin 8. The epitope tag can be engineered so as to be cleavable from the r-laminin 8 chain(s) after purification. Alternatively, no epitope tag is fused to any of the r-laminin 8 chains, and the r-laminin 8 is purified by standard techniques, including but not limited to affinity chromatography using laminin 8 specific antibodies or other laminin 8 binding molecules. The present invention further provides pharmaceutical compositions comprising substantially purified laminin 8 and a pharmaceutically acceptable carrier. In a preferred embodiment, the pharmaceutical composition comprises substantially purified r-laminin 8. According to this aspect of the invention, other agents can be included in the pharmaceutical compositions, depending on the condition being treated. The pharmaceutical composition may further comprise one or more other compounds, including but not limited to any of the collagens, other laminin types, fibronectin, vitronectin, cadherins, integrins, α-dystroglycan, entactin nidogen, α-dystroglycan, glycoproteins, proteoglycans, heparan sulfate proteoglycan, glycosaminoglycans, epidermal growth factor, vascular endothelial growth factor, fibroblast growth factor, or nerve growth factors, and peptide fragments thereof. Pharmaceutical preparations comprising substantially purified laminin 8 can be prepared in any suitable form, and generally comprise the laminin 8 in combination with any of the well known pharmaceutically acceptable carriers. The carriers can be injectable carriers, topical carriers, transdermal carriers, and the like. The preparation may advantageously be in a form for topical administration, such as an ointment, gel, cream, spray, dispersion, suspension or paste. The preparations may further advantageously include preservatives, antibacterials, antifungals, antioxidants, osmotic agents, and similar materials in composition and quantity as is conventional. Suitable solutions for use in accordance with the invention are sterile, are not harmful for the proposed application, and may be subjected to conventional pharmaceutical operations such as sterilization and/or may contain conventional adjuvants, such as preservatives, stabilizers, wetting agents, emulsifiers, buffers etc. For assistance in formulating the compositions of the present invention, one may refer to Remington's Pharmaceutical Sciences, 15th Ed., Mack Publishing Co., Easton, Pa. (1975). In further aspect, the present invention provides methods and kits comprising laminin 8, or pharmaceutical compositions thereof (and instructions for using the laminin 8 in the kits) for accelerating the healing of injuries to tissue of mesenchymal origin, such as bone, cartilage, tendon, and ligament, treating injuries to vascular and neural tissue, and for improving the biocompatibility of grafts used for treating such injuries. In a preferred embodiment of each of the methods disclosed below, r-laminin 8 is used. In specific examples, substantially purified laminin 8, r-laminin 8, or pharmaceutical compositions thereof are used to: a. promote re-endothelialization at the site of vascular injuries; b. improve the "take" of grafts; c. improve the biocompatibility of medical devices; d. treat neural injuries (neural regeneration); e. regulate angiogenesis; and d. promote cell attachment and migration by providing an amount effective of laminin 8 or pharmaceutical compositions thereof for the various methods. In one embodiment, laminin 8 is used to promote re-endothelialization, and to thus inhibit abnormal smooth muscle cell proliferation, at the site of a vascular injury. The α4 chain is associated with mesenchymally derived cell populations, including but not limited to endothelium and smooth muscle cells, and laminin 8 has been shown to be a primary laminin of the vascular endothelium. The value of angioplasty in clearing occluded coronary arteries is limited by a restenosis/reocclusion rate of 50-70%. Several studies have indicated that the insertion of a vascular stent following angioplasty appears to decrease the occurrence of restenosis, but the problem still limits the effectiveness of this treatment. Restenosis appears to arise in part from the proliferation of vascular smooth muscle cells in response to the angioplasty treatment. It is likely that the scraping action of angioplasty removes not only the problematic occlusion, but also sections of the vascular basal lamina. The discontinuous basal lamina that results could contribute to what appears to be abnormal growth of the vascular smooth muscle cells that leads to restenosis. The attachment of laminin 8 to vascular stents can be used to limit restenosis, by promoting re-endothelialization. The interaction of vascular endothelial cells with the laminin 8 coated stents promotes their adhesion and attachment, thereby leading to homeostasis and a normal cell growth response, instead of the injury/activation endothelial cell response seen with restenosis. While activated platelets adhere to laminin 8, non-activated platelets do not. Furthermore, is has been shown that soluble laminin 8 does not cause platelet activation, but has an inhibitory effect on platelet activation by classical activators such as thrombin, collagen I, and ADP (unpublished observations). A more normal and controlled rate of re-endothelialization will decrease the incidence of re-occlusions, and improve the outcome of the angioplasty procedure. Similarly, synthetic vascular grafts can induce blood clotting and thrombosis through interactions of blood clotting factors with the synthetic graft material. Coating vascular grafts with laminin 8 promotes endothehahzation of the synthetic vessel, thereby providing for a non-thrombogenic surface. Vascular endothelial cells, like other cells that sit upon a basement membrane, prefer to adhere to an appropriate basement membrane substrate. Laminin-8 has been identified as a component of the vascular basal lamina, and is suspected to be involved in the attachment of vascular endothelial cells to the supporting basal lamina. Providing this substrate in a graft material creates a non-thrombogenic surface, promotes endothelialization, and inhibits intravascular thrombosis and vascular obstruction. Administration to the injured blood vessel can be accomplished in some cases by simply coating laminin 8 or pharmaceutical compositions thereof into an injured area. In other embodiments, delivery can be accomplished by: 1. Coating a stent; 2. Coating a biodegradable sleeve over the stent; or 3. Forcing a liquid preparation of the laminin 8 or pharmaceutical compositions thereof through a porous catheter to the injured site. In another embodiment, the present invention provides methods to promote bone and connective tissue repair in a subject. The incoφoration of laminin 8 or pharmaceutical compositions thereof into wound repair dressings and matrices as well as tissue grafts to accelerate the healing of bone and connective tissue repair provides a natural ligand interactive surface to promote normal cell adherence, cell growth and tissue development. Many grafts are used to replace connective tissue that has a cell layer adherent to a basal lamina. When an inappropriate surface is provided to these cells following grafting, the graft is at risk for failure of restoration of the normal cell layer. The advantage of coating these grafts with laminin 8 is to create a surface that sufficiently recapitulates a normal basal lamina surface to promote cell re-population. As used herein the term "graft" refers to both natural and prosthetic grafts. The methods of the present invention have application in the healing of tendon, cartilage, or ligament tears, deformities and defects, bone fractures, defects, as well as use in the improved fixation of tendon, cartilage, or ligament to bone or other tissues. In addition, bony in-growth into various prosthetic devices can be greatly enhanced so that such artificial parts are firmly and permanently anchored into the surrounding skeletal tissue through a natural osseous bridge. In a further aspect, the present invention comprises medical devices with improved biocompatibility, wherein the devices are coated with laminin 8 or pharmaceutical compositions thereof, alone or in combination with other proteins or agents that serve to increase the biocompatibility of the device surface. The coated device stimulates cell attachment and provides for diminished inflammation and/or infection at the site of entry of the appliance. Such medical devices can be of any material used for implantation into the body, and preferably are made of or coated with a biocompatible metal that may be either stainless steel or titanium. Alternatively, the device is made of or coated with a ceramic material, or a polymer including but not limited to polyester, polyglycolic acid or a polygalactose-polyglycolic acid copolymer. One particular use of the present invention is to increase cell adhesion to target surfaces, including but not limited to endothelial, skeletal muscle, smooth muscle, and other mesenchymally-derived cells. For example, vascular grafts and stents may be coated with laminin 8 or pharmaceutical compositions thereof to stimulate endothelial cell attachment. Alternatively, bone or connective tissue grafts or prostheses may be coated with laminin 8 or pharmaceutical compositions thereof to stimulate adhesion of the appropriate cell type and improved grafting efficiency. If the device is made of a natural or synthetic biodegradable material in the form of a mesh, sheet or fabric, laminin 8 or pharmaceutical compositions thereof may be applied directly to the surface thereof. Appropriate cells may then be cultured on the matrix to form transplantable or implantable devices, including dental abutment pieces, needles, metal pins or rods, indwelling catheters, colostomy tubes, surgical meshes and any other appliance for which coating with laminin 8 is desirable. Alternatively, the devices may be implanted and cells may be permitted to attach in vivo. Coupling of the substantially purified laminin 8 may be non-covalent (such as by adsoφtion), or by covalent means. The device may be immersed in, incubated in, or sprayed with the laminin 8 or pharmaceutical compositions thereof. The dosage regimen for various treatments using the laminin 8 of the present invention is based on a variety of factors, including the type of injury or condition, the age, weight, sex, medical condition of the individual, the severity of the condition, and the route of administration. Thus, the dosage regimen may vary widely, but can be determined routinely by a physician using standard methods. Laminins are extremely potent molecules, and one or a few molecules per cell could produce an effect. Thus, effective doses in the pico-gram per milliliter range are possible if the delivery is optimized. Laminins are sometimes present in an insoluble form in the basement membrane and have the capability of polymerizing at concentrations as low as about 50 μg/ml, depending on the laminin isoform and the conditions. Laminins can also polymerize into a gel at a concentration of about 2-3 mg/ml. Dosage levels of the order of between 1 ng/ml and 10 mg/ml are thus useful for all methods disclosed herein, preferably between about 1 μg/ml and about 3 mg/ml. The present invention also provides a method for inducing cell attachment to the device (as disclosed above), comprising coating the appliance with laminin 8 or pharmaceutical compositions thereof prior to incubation with cells appropriate for the desired application. Laminin preparations are known to induce the growth and differentiation of neurons (U.S. Patent No. 5,229,365), and have been used in combination with Type I collagen to coat a hollow conduit and promote nerve regeneration across a gap of severed nerve. (U.S. Patent No. 5,019,087) Thus, in another embodiment, a method is provided for nerve regeneration, comprising administering to a subject in need thereof an amount effective of laminin 8 or pharmaceutical compositions thereof to promote nerve regeneration. The graft can comprise a nerve graft, or a prosthetic graft. Both bioresorbable and non-resorbable materials have been used in tubes for bridging nerve gaps. (See for example, Nyilas, et al., (Trans. Soc. Biomater., 6, 85, 1983), Molander, et al. (Biomaterials, Vol. 4, pp. 276-280, October, 1983), Colin, et al, (Journal of Dental Research July, 1984, pp. 987- 993). The method can be used to treat diseases and injuries characterized by the loss of function and or/degeneration of neurons and nerves. Laminins, or cell extracts containing laminins have been shown to regulate angiogenesis in a biphasic manner. (See, for example, Nicosia et al., Dev. Biol. 164:197-206 (1994); Bonfil et al., Int. J. Cancer 58:233-239 (1994)). At lower concentrations (30-300 μg/ml), a laminin-entactin complex stimulated angiogenesis in a three-dimensional culture, while at 3000 μg/ml the same complex was inhibitory to angiogenesis. Thus, in another aspect, the present invention provides methods for regulating angiogenesis, comprising contacting a tissue or culture substrate with an amount effective of laminin 8 or pharmaceutical compositions thereof to regulate angiogenesis. In one embodiments, the laminin 8 is used to promote angiogenesis by contacting a tissue or culture substrate with an amount effective of laminin 8 to promote angiogenesis. In another embodiment, the laminin 8 is used to inhibit angiogenesis, by contacting the tissue or culture substrate with an amount effective of laminin 8 to inhibit angiogenesis. An example of culture substrates to be contacted with laminin 8 to regulate angiogenesis are those used for tissue engineering puφoses. In another aspect of the present invention, laminin 8 is used for the culture of cells, including but not limited to endothelial cells, nerve cells, cells of hematopoietic lineage, and mesenchymally-derived cells including but not limited to cells derived from bone, connective tissue, and adipose tissue, skeletal muscle cells, and smooth muscle cells, by contacting the cells with an amount effective of laminin 8 to stimulate attachment and proliferation/differentiation/stasis of cells. The laminin 8 can either be provided in the cell culture medium, or as a cell culture medium supplement, or may be coated on the surface of a cell growth substrate. In a preferred embodiment, the method further includes contacting the cells with other compounds, including but not limited to any of the collagens, other laminin types, fibronectin, α-dystroglycan, cadherins, integrins, entactin/nidogen, α-dystroglycan, glycoproteins, proteoglycans, heparan sulfate proteoglycan, glycosaminoglycans, epidermal growth factor or nerve growth factors, vascular endothelial growth factor, fibroblast growth factor, and peptide fragments thereof. The cells may comprise primary cells or cell culture cell lines. The methods of this aspect of the invention can be used in vivo, ex vivo, or in vitro. In a preferred embodiment, laminin 8 is used to coat the surface of a substrate, to promote cell adhesion to the substrate, and to stimulate cell proliferation/differentiation/stasis. The substrate used herein may be any desired substrate. For laboratory use, the substrate may be as simple as glass or plastic. For use in vivo, the substrate may be any biologically compatible material capable of supporting cell adhesion. Suitable substrate materials include shaped articles made of or coated with such materials as collagen, regenerated collagen, polyglycolic acid, polygalactose, polylactic acid or derivatives thereof; biocompatible metals such as titanium and stainless steel; ceramic materials including prosthetic material such as hydroxylapatite; synthetic polymers including polyesters and nylons; polystyrene; polyacrylates; polytetrafluoroethylene and virtually any other material to which biological molecules can readily adhere. The determination of the ability of a particular material to support adhesion of the r-laminin 8 of the invention requires only routine experimentation by the skilled artisan. In a further aspect, the present invention provides cell growth substrates for adhesion and culturing of cells, by providing an amount effective of laminin 8 for the attachment of cells to a cell culture device for the attachment and subsequent proliferation/differentiation/stasis of the cells. The substrates may comprise any of the substrates discussed above. In another aspect of the present invention, an improved cell culture medium is provided, wherein the improvement comprises addition to the cell culture medium of an effective amount of laminin 8 to the cell culture medium to promote the adherence, proliferation, and/or maintenance of cells. Any cell culture media that can support the growth of cells can be used with the present invention. Such cell culture media include, but are not limited to Basal Media Eagle, Dulbecco's Modified Eagle Medium, Iscove's Modified Dulbecco's Medium, McCoy's Medium, Minimum Essential Medium, F-10 Nutrient Mixtures, Opti-MEM® Reduced-Serum Medium, RPMI Medium, and Macrophage-SFM Medium or combinations thereof. The improved cell culture medium can be supplied in either a concentrated (ie: 1 OX) or non-concentrated form, and may be supplied as either a liquid, a powder, or a lyophilizate. The cell culture may be either chemically defined, or may contain a serum supplement. Culture media is commercially available from many sources, such as GIBCO BRL (Gaithersburg, MD) and Sigma (St. Louis, MO).In an alternative embodiment, the laminin 8 is used as a cell culture supplement. The laminin 8 or pharmaceutical compositions thereof of the present invention can be used for the treatment of a variety of conditions and diseases as described herein, including but not limited to various vascular, neural, and mesenchymal tissue injuries, including but not limited to angioplasty restenosis, tissue ischemia, neural damage, vascular surgical procedures, atherosclerosis, bone fractures, defects, and disorders which result in weakened bones such as osteoporosis, osteoarthritis, and periodontal disease; bone loss resulting from cancer or side effects of other medical treatment; age-related loss of bone mass; articular cartilage tears, deformities and other cartilage defects such as arthritis and cartilaginous tissue damage, tendon or ligament tears, deformities and other tendon or ligament defects such as tendinitis and caφal tunnel syndrome, periodontal ligament injury, and tendon-to-bone detachment. The amount of laminin 8 or pharmaceutical compositions thereof used in such treatments will, of course, depend upon the type and severity of the condition or disease being treated, the route of administration chosen, and will be determined by the attending physician or veterinarian. The term "therapeutically effective amount" of laminin 8 or pharmaceutical compositions thereof refers to the amount of laminin 8 or pharmaceutical compositions thereof, in the absence of other exogenously applied factors, determined to produce a therapeutic response in a mammal. Such therapeutically effective amounts are readily ascertained by one of ordinary skill in the art. The present invention may be better understood with reference to the accompanying examples that are intended for puφoses of illustration only and should not be construed to limit the scope of the invention, as defined by the claims appended hereto. EXAMPLES Expression Constructs For expression of the human laminin α4 chain containing a C-terminal FLAG epitope, the full length cDNA was constructed and modified as follows. Complementary DNA lambda clones subcloned into pBluescript™ or pCRscript™ (Stratagene) plasmid vectors from an earlier study (Iivanainen et al., 1995) were used as cDNA source, except for clone FL136. The EcoRI insert from FL136 lambda DNA was cloned into the pBluescript™ EcoRI site to make FL136E. The 0.78 kb Sacl- BamHI fragment from clone FL76 was ligated into SacI-BamHI digested pSL1180 (Pharmacia) to make FL76SB. A sequence corresponding to nucleotides 2378-4274 of human laminin α4 cDNA was PCR-amplified using cDNA library as a template, digested with Sad and cloned into the FL76SB Sad site and its orientation confirmed to make HL4-SB. The FL64 BamHI-Sall fragment was cloned into HL4-SB BamHI- SalI to make HL4-3'. The Eco72I-XhoI fragment from clone FL117 was ligated into the Eco72I-XhoI sites of FL136E to make HL4-5'. Both mouse and human laminin α4 cDNAs have poorly conserved Kozak-sequences at the translation initiation site, as well as several extra 5' untranslated region (UTR) ATG sequences. To ensure efficient and correct translation initiation, the Kozak sequence was edited to match the consensus and the rest of the 5' UTR was deleted using standard molecular biology techniques. The resulting product was EcoRI-Eagl-digested and cloned to the EcoRI-Eagl-digested HL4-5' to make HL4Mut-5'. The Spel-Xhol fragment from HL4Mut-5' was cloned into HL4-3' to make clone HL4-Full with full length cDNA. The EcoRI insert from HL4-Full was cloned into pcDNA3.1/Zeo(-) expression vector (Invitrogen) to make HL4-Full.pcDNA. (SEQ ID NOT) The sequence encoding the FLAG epitope (SEQ ID NO:3) was inserted as follows. The FL64 BamHI-Hindlll fragment was cloned into pUC19 to make FL64BH. PCR was performed using primers to introduce the FLAG epitope, using HL4-3 ' as template. The product was digested with Xbal and Hindlll and cloned into Xbal-Hindlll digested FL64BH to make HL4FLAG-3'. This also resulted in deletion of the original 3' UTR. The BamHI-Hindlll fragment from HL4FLAG-3' was cloned into BamHI-Hindlll-digested HL4-Full.pcDNA vector, replacing the original BamHI- Hindlll fragment to make HL4FLAG-B, which lacked the BamHI-BamHI fragment. The final expression construct named HL4FLAG-Full was made by inserting the missing BamHl fragment in the correct orientation. All PCR-derived parts of the cDNA sequence were sequenced to ensure that no mutations had occurred during amplification. The construct used for expression of the mouse laminin βl chain (SEQ ID NO: 15) has been previously described (Yurchenco et al., Proc. Natl. Acad. Sci. U. S. A. 94(19), 10189-94 (1997)). To make the construct named HG1 for expression of the human laminin γl chain, full length cDNA (SEQ ID NO: 19) encoding the human laminin γl chain was released with BamHl from a baculovirus expression vector pVL941 (unpublished) and cloned into the BamHl site of a pcDNA3.1/Hygro(-) mammalian expression vector (Invitrogen). Antibodies, control proteins, and cell lines Affinity purified polyclonal anti-laminin α4 antibody (Ab) S8 was prepared as described previously. (Iivanainen et al., 1997, J. Biol. Chem. 272(44), 27862-8) Polyclonal anti-EHS-laminin Ab, anti-FLAG M2 monoclonal Ab (mAb), purified control mouse IgG, RGDS-peptide and heparin (grade I-A) were purchased from Sigma Chemical Company (St. Louis, MO). Anti-laminin γl (clone 22) mAb was from Transduction Laboratories (Lexington, KY). Mouse function blocking mAbs against integrin αl (clone FB12), integrin α2 (clone P1E6), and integrin α3 (clone P1B5) were obtained from Chemicon (Temecula, CA). Rat function blocking mAbs anti-integrin α6 (clone GoH3) and control rat IgG 2a were also from Chemicon. Rat function blocking mAbs against integrin α5 (clone BIIG2) and integrin βl (clone AIIB2) were provided by Dr. C. Damsky (Univ. of California, San Francisco) as hybridoma supernatants. Immunoglobulins were purified from the supernatants using GAMMABIND PLUS™ Sepharose (Pharmacia; Stockholm, Sweden) according to the manufacturer's instructions. Secondary Ab conjugates anti-rabbit IgG-HRP and anti- mouse IgG-HRP were from Dakopatts (Denmark). Laminin 1 from EHS-tumor, collagen type IV from EHS-tumor, and human placental laminin were obtained from Sigma. Fibronectin and some of the laminin 1 from EHS-tumor were purchased from Gibco BRL (Rockville, MD) EHS-derived laminin 1/nidogen complex was kindly provided by Dr. J. Engel (Univ. of Basel, Switzerland). Human fibrosarcoma HT-1080 (CCL-121) cells were from the American Type Tissue Collection. (Manassas, VA) IMMORTOMOUSE™ brain capillary endothelial (IBE, Kanda et al., 1999, Exp. Cell Res. 248(1), 203-13) and bovine adrenal microvascular (BCE, Folkman et al., 1979) cells were kindly provided by Dr. L. Claesson- Welsh (Medical Biochemistry and Microbiology, Univ. of Uppsala) and K. Olausson (Medical Cell Biology, Univ. of Uppsala). Three human erythroleukemic K562 cell lines transfected to express integrins α3 (Delwel et al., 1994, Mol. Biol. Cell 5(2),203-15), α6 (Delwel et al., 1993, J. Biol. Chem. 268(34), 25865-75), or both α6 and α4 (Niessen et al., 1994, Exp. Cell Res. 211(2), 360-7 Mol. Biol. Cell) were provided by Dr. A. Sonnenberg (Netherlands Cancer Institute, Amsterdam, Netherlands). Production and purification of recombinant laminin 8 Recombinant laminin 8 ("r-laminin 8") was produced in human embryonic kidney cells (HEK-293, ATCC CRL-1573) cultured in DME/pyravate/10% fetal calf serum (FCS) at 37°C in a humidified 5% CO 2 atmosphere. Wild-type cells were stably transfected with the laminin βl expression construct as previously described (Yurchenco et al., 1997) and selected using 500 μg/ml G418. All further cell culture and clonal expansion was carried out in the continuous presence of relevant selection antibiotics. A highly expressing clone was then transfected with the HL4FLAG-Full construct using standard calcium-phosphate transfection methods, and stable colonies were selected using 300 μg/ml Zeocin. Clones were isolated using cloning rings, expanded, and analyzed for laminin α4 secretion by Western blotting of medium using the anti-laminin α4 Ab S8. The clone with the highest expression was transfected with the HG1 construct, and stable clones were selected using 100 μg/ml hygromycin. These clones were then screened via Western blotting using a mAb against laminin γl, and clones showing the highest secretion were expanded further. For production of r-laminin 8, cells were grown in the culture medium for up to four days, after which the medium was collected and centrifuged to remove cell debris. After collection, Tris-Cl pH 7.5 was added to 50 mM and EDTA was added to a concentration of 10 mM. If not used immediately, the medium was stored at -70°C. For protein production into serum-free medium, confluent cultures were washed twice with PBS and the medium was changed to DME supplemented with pyruvate, insulin- transferrin-selen supplement (Sigma) and 1 μg/ml aprotinin (Sigma). r-laminin 8 was affinity purified using an anti-FLAG M2 matrix (Sigma). Before use, the matrix was washed with 0.1M glycine (pH 3.5) and TBS (50 mM Tris- HCl pH 7.5/150 mM NaCl) according to the manufacturer's instructions. Brij-20 (Fluka, Milwaukee, Wisconsin) was added to the medium to a final concentration of 0.05% (v/v), and the medium was incubated in batch mode with the matrix (25 μl matrix/ml) overnight at 4°C with agitation. The matrix was collected by passing the medium through a sintered column, and washed extensively in the column first with TBS/1 mM EDTA and then PBS/1 mM EDTA. Bound r-laminin 8 was competitively eluted with 100 μg/ml FLAG peptide (Sigma) in PBS/lmM EDTA at room temperature. The matrix was then regenerated as recommended by the manufacturer. The eluate was diluted 1 :1 with 20 mM NaPO 4 /l mM EDTA (pH 7.5), and injected into a UNO-Q ion-exchange column (Bio-Rad, Hercules, CA). At this salt concentration, the FLAG peptide passes through, but r-laminin 8 is bound. The column was then washed with 20 mM phosphate/1 mM EDTA, and the r-laminin 8 was eluted with 20 mM NaPO 4 /l .5M NaCl/1 mM EDTA. The eluate was diluted 1 :10 with 20 mM NaPO 4 (pH 7.5)/l mM EDTA to a final salt concentration of 150 mM and concentrated using 100 kD cut-off ultrafiltration (Gelman; Ann Arbor, Michigan) to approximately 0.5 mg/ml. Characterization of r-laminin 8 Secreted r-laminin 8 in cell medium and after purification was characterized using linear 5% or 6% SDS-PAGE and 3-12% gradient SDS-PAGE under reducing and non-reducing conditions. Proteins were visualized using silver staining or blotted to PVDF membranes using a semi-dry blotting system (Bio-Rad). For immunodetection, Renaissance ECL-System (Dupont, Waltham, MA) was used in conjunction with the Abs described above. Protein quantitation was done by measuring absorbance at 280 nm or using the Bradford method (Bio-Rad protein assay kit). Rotary shadowing electron micrography (EM) was performed as described previously. (Yurchenco and Chen, 1993) When purifying for rotary shadowing, the matrix was equilibrated with 0.15 M NH HCO 3 -acetate buffer (pH 7.4), and the r- laminin 8 was then eluted with FLAG-peptide in the same buffer. Adhesion assays and cell culture For adhesion assays, flat-bottom 96 well plates (Maxi-Soφ, Nunc; Rochester, NY) were coated by incubating with proteins diluted in PBS overnight at 4°C (50 μl/well). The remaining protein-binding capacity was saturated by addition of 2% heat- inactivated BSA in PBS (50 μl/well) and further incubation for at least 4 hours. Prior to assaying, the coating/blocking solution was aspirated, and the wells were washed with the binding medium (100 μl/well). Drying of coated protein was avoided, since this was found to be detrimental to adhesion in some cases. All cells were cultured in humidified 5%> CO 2 atmosphere. HT-1080 and BCE cells were cultured in DME/10% FCS/pyruvate at 37°C, BCE on gelatin-coated plastic. IBE cells were cultured in F12/10% FCS/2 U/ml γ-interferon on gelatin-coated plastic at 33°C. Transfected K562 cells were grown in suspension in RPMI/10%>FCS supplemented with 1 mg/ml G418 at 37°C. For K562 cells transfected with both α4 and β4 integrins, 0.7 mg/ml hygromycin was included in the medium. Prior to dissociation, the cells were washed twice with PBS. HT-1080 cells were disassociated using 5 mM EDTA in PBS, while the others were disassociated using trypsin-EDTA (Gibco-BRL). To remove trypsin, cells were pelleted and resuspended twice in serum- free medium. Cells were counted and suspended in buffered serum-free medium at 2-3 x 10 5 cells/ml. K562 cells were washed twice with serum-free medium and resuspended at 10 6 cells/ml. DME/25 mM HEPES/pyruvate was used for HT-1080 cells; F12/25 mM HEPES/0.25% BSA was used for other cell types. K562 cell stimulation was done using 5 ng/ml PMA (Sigma). Antibodies or other test compounds were added to the cell suspension and the cells were allowed to recover at 37°C for 30 minutes. The cells were then added to the protein-coated 96- well plates (100 μl/well) and allowed to adhere for 30 (K562) or 60 (other cells) minutes at 37°C. To remove unbound cells, wells were washed by two or three cycles of careful addition of 100 μl of binding medium followed by aspiration. The remaining cells were fixed with 1% glutaraldehyde in PBS for 10 minutes at room temperature. Cells were stained with 0.1 % crystal violet (Sigma) for 30 minutes and unbound stain was removed by four washes with water. Bound stain was solubilized in 2% SDS (100 μl/well) and quantitated by measuring the absorbance at 595 nm using a microplate reader. None of the cell lines bound appreciably to BSA. When the quantitative results were calculated, binding to BSA was given a value of zero, while the relevant control was given the value of 100. The mean and SEM were calculated from results obtained from parallel wells. RESULTS Production and characterization of r-laminin 8 Unconcentrated medium from wild-type HEK-293 cells did not react in Western blots with the anti-laminin α4, anti-laminin γl, anti-EHS-laminin, or anti-FLAG antibodies, indicating that these cells express endogenous laminins at very low amounts if at all. The transfected α4 chain could be secreted to some extent even when expressed alone, but secretion of the other chains required simultaneous expression of all three chains. Cells transfected with laminin α4, βl, and γl chain expression constructs secreted large amounts of all three chains to the medium. The best cell clones ("Gl-2" and "Gl-3") were estimated to produce 3-5 milligrams or r-laminin 8 per liter of medium. The r-laminin 8 bound to anti-FLAG M2 matrix with high specificity. When eluted competitively with the FLAG peptide, only laminin α4, βl, and γl bands were seen in silver-stained 3-12%) gradient SDS-PAGE gels. (Figure 1) Under non-reducing conditions, the purified protein hardly entered the gel, which was to be expected as the predicted molecular weight for the mature trimer is at least 570 kD. A minor fraction of the purified trimer appeared as non-covalently associated (see discussion). In this fraction, the βl and γl chains appeared as covalently associated dimers, whereas the α4 chain was non-covalently associated. Under reducing conditions, the protein appeared WO 00/66732 PCT/USOO/l 1543 as a broad band at around 200 kD, which reacted on Western blots with α4, EHS, γl, and anti-FLAG antibodies. The predicted molecular weights for mature α4, βl, and γl polypeptides are 200, 195, and 174 kD respectively. Laminins are heavily glycosylated, which may account for the slight discrepancy in molecular weight observed in SDS-PAGE. The βl and γl chains of laminin 1 purified from EHS-tumor showed similar or slightly slower mobility than those of r-laminin 8. Rotary shadowing EM revealed r-laminin 8 to be a Y-shaped molecule with two short and one long arm in accordance with the predicted structure. (Figure 2) In many cases, a very short (5-10 nm) rod-like stub could be seen at the junction of the arms. The G-domains could sometimes be seen as consisting of two moieties. Cell binding to r-laminin 8 and receptor identification We assayed the binding of human fibrosarcoma (HT-1080) and transfected K562 cells to r-laminin 8 in the presence of different blocking anti-integrin antibodies to identify integrin receptors binding to r-laminin 8. Immortomouse brain capillary endothelial (IBE) and bovine adrenal microvascular endothelial (BCE) cells were also used to study the adhesion of endothelial cells to r-laminin 8. The BCE cells express at least integrins α6βl, α6β4, αlβl, α2βl, α3βl, α5βl, αvβl, αvβ3, and α5β5 (Klein et al., 1993, Mol. Biol. Cell 4(10), 973-82), whereas IBE cells have been reported to express integrins α3, α5 and βl, but not αl, α2, or α6 (Kanda et al., 1999) When compared to laminin 1, the adhesiveness of r-laminin 8 was quantitatively similar or slightly weaker for the cell lines studied, as approximately the same number of cells bound to both substrates after washing. (Figure 3) Similar results were obtained with IBE and BCE cells (data not shown). In further cell adhesion assays (Figures 4-8), cells were allowed to bind to either-laminin 8 or laminin 1 coated at 10 μg/ml on 96 well plates. Prior to the assay, different components were added to the cell medium. Values indicated are relative to that of control antibody (normal mouse IgG for mouse antibodies and rat IgG 2a for rat monoclonal antibodies), which was designated as 100. For other substances, the same volume of buffer was added. Adhesion to bovine serum albumin (BSA) was designated zero. The text under the columns indicate the integrin subunit blocked or the added substance. Enor bars indicate SEM. Integrin monoclonal antibodies were used at 10 μg/ml, heparin at 2 mg/ml, and EDTA at 5 mM. Monoclonal antibodies against integrins αl and α2 were tested only in the HT- 1080 cell line, where they had no or only small effects on cell binding, indicating that these integrins were not major mediators of adhesion to r-laminin 8 (Figure 4). Adhesion to Type IV collagen was reduced to about 50% by the anti-integrin α2 mAb, demonstrating the presence of active α2 integrin (data not shown). Integrin αl mAb had only a slight effect on adhesion to collagen IV when used alone, but it had a synergistic effect when used in combination with the α2 mAb (not shown). The Ab against integrin α3 had only minor effects on adhesion of HT-1080 when used alone, but it had a synergistic effect when used in combination with the α2 mAb (data not shown). The monoclonal antibody to α2 integrin had only minor effects on adhesion of HT 1080 (Figure 4) cells to fibronectin or laminin, even though the cells have been shown to express high levels of α3βl (Wayner et al., 1993, J. Cell Biol. 121(5), 1141-52). The blocking of integrin α5 had a slight stimulating effect on HT-1080 adhesion to both laminin 1 and laminin 8 (Figure 4), whereas adhesion of BCE cells to r-laminin 8 was slightly reduced (Figure 5). The mAb did block adhesion of HT-1080 cells to fibronectin almost completely, indicating the presence of active α5 integrin in these cells (not shown). α-6 subunit containing integrin(s) were identified as the major mediators of adhesion to r-laminin 8. The integrin α6 subunit is known to associate with either βl or β4 (Sonnenberg et al, 1990, J Cell Sci. 96(Pt 2), 207-17). By using a mAb (GoH3) that blocks α6βl- and α6β4-mediated binding, we could completely abolish binding of HT-1080 and BCE cells to r-laminin 8. An anti-βl integrin mAb (AIIB2) also completely blocked the binding of HT-1080 cells to r-laminin 8 indicating that integrin α6βl is crucial for adhesion of these cells to r-laminin 8 (Figure 4). In contrast, binding of BCE cells was blocked only partially (about 70%) by the anti-βl mAb, suggesting that these endothelial cells use both α6βl and α6β4 to adhere to r-laminin 8 (Figure 5). In another endothelial cell line, the mouse IBE cells, the anti-α6 subunit mAb blocked the binding to r-laminin 8 only partially (about 60%), suggesting that the cells are using, in addition to α6-subunit containing integrins, also other receptors (Figure 6). Interestingly, when adhesion to r-laminin 8 was compared to that of laminin 1, it was observed that the adhesion was quite differently affected by the blocking anti-α6 and anti-αl integrin mAbs. HT-1080 cells interacted with laminin 1 not only via α6βl integrin, but also via other βl-subunit-containing integrin(s), since the blocking was only partial with anti-α6, but complete with anti-βl. (Figure 4) Furthermore, the adhesion of BCE cells to laminin 1 was mediated by βl integrin(s) other than α6βl, since the adhesion was completely blocked by anti-βl, but was only minimally affected by anti-α6. (Figure 5) Similarly, in IBE cells, the adhesion to laminin 1 was mediated by receptors other than α6 integrin(s), since it was not affected by anti-α6. (Figure 6) To verify the role of α6βl and α6β4 integrins as r-laminin 8 receptors, transfected K562 cells were used. Parental K562 cells endogenously express only integrin α5βl, which is in an inactive state. The cells normally grow in suspension but can be made adherent with an activating anti-βl Ab or stimulation with PMA. K562 cells transfected with the α6 subunit express α6βl on the cell surface (Delwel et al., 1993). Interestingly, while these cells bound laminin 1 efficiently only after stimulation with PMA, they bound r-laminin 8 strongly even without stimulation (Figure 8). This finding demonstrates that the adhesive properties of r-laminin 8 are different from those of laminin 1. The cell adhesion to both laminin isoforms could be blocked with either anti-integrin α6 or βl mAbs, which agrees with results obtained with other cell lines (Figure 4-5). In addition to inactive α6βl, K562 cells transfected with α6 and β4 subunits express constitutively active α6β4 complex, and can bind laminin 1 even without stimulation (Niessen et al., 1994). We found that these cells bound to both laminin 1 and laminin 8 without stimulation, although activation of the βl integrins with PMA resulted in increased adhesion. The adhesion of non-stimulated cells could be completely inhibited with anti-integrin α6, but only partially with anti- βl, again indicating that α6β4 is able to mediate adhesion to r-laminin 8 (Figure 7). In contrast, K562 cells expressing α3βl adhered poorly to both laminin isoforms (not shown). This agrees with an earlier study where α3-transfected K562 cells were found to bind efficiently to laminin 8, but poorly to laminin 1 (Delwel et al., 1994). Cell adhesion to both laminin 1 and r-laminin 8 was found to be dependent on divalent cations, since it could be abolished by 5 mM EDTA in all cell lines tested (Figures 4-8). Heparin, when used at 2 mg/ml, had no effect on the adhesion of HT- 1080, BCE, and IBE to r-laminin 8 (Figures 4-6). On laminin 1, however, there was a slight decrease in adhesion of BCE cells (Figure 5), while the other cell lines were unaffected (Figures 4,6). The RGDS-peptide that is reported to block the function of various integrins (Pierschbacher and Ruoslahti, 1984, Nature 309(5963), 30-3) had no effect at 1 mM concentration on adhesion of HT-1080, BCE, or IBE cells to the laminins (data not shown). It was further observed that the cell-binding activity of r-laminin 8 was sensitive to air-drying. When the coated protein was allowed to air dry for 15 minutes at room temperature before adding the cells, the cell-binding activity of r-laminin 8 was completely lost (Figure 4). Even shorter than a 15 minute exposure could abolish the cell-binding activity (not shown). A drop of buffer was allowed to sit on the plastic, while the rest of the well was briefly exposed to air drying. On the dried area, the BCE cells were rounded, and only a few of them showed any signs of spreading. On the area kept wet, practically all cells were well spread and tightly adhered to the surface. Accordingly, all cells on the dried area were lost during washing. Laminin 1 was not as sensitive to this effect, but drying still reduced the cell binding activity by half (Figure 4). DISCUSSION The present work provides significant advances concerning the recently described laminin 8 isoform and its α4 chain. Large quantities or r-laminin 8 could be produced as native trimeric protein in cultured human cells, and the r-laminin 8 was shown to be biologically active and to have cell adhesive properties. Furthermore, r- laminin 8 was shown to have a preference for binding to the α6 integrins. The r-laminin 8 produced in this study is a species hybrid of two human (α4 and γl) and one mouse (βl) chains, and it contained a FLAG epitope tag attached to the C- terminus of the α4 chain. Despite these modifications, r-laminin 8 assembled into trimers in a manner expected from a native laminin protein, as demonstrated by rotary shadowing EM. The amount of r-laminin 8 produced by the HEK-293 cells in monolayer cultures was quite high considering the size and complexity of the protein. An amount of 3-5 mg/L of culture medium is similar to what is frequently obtained in eukaryotic systems, such as the baculovirus insect cell system. Similarly to other laminin isoforms characterized to date, all the chains of the r- laminin 8 trimer were disulfide linked to each other. Only a minor fraction consisted of disulfide-linked βl/γl containing dimers and non-crosslinked α4. These chains were also associated into trimers, since the dimers followed the FLAG-tagged α4 chain in immunoprecipitations using the anti-FLAG mAb. The presence of the α4 chain in r- laminin 8 trimers was also demonstrated by showing that all of the α4 could be immunoprecipitated after several rounds of immunoprecipitation with the anti-laminin 1 Ab that recognizes the αl, βl, and γl chains (data not shown). The reason for the two minor r-laminin 8 bands of different size reacting with EHS and γl antibodies is unclear. The larger one agrees with the size for a dimer, but the smaller one could not be accounted for. The size difference could be as large as 100 kD. It is possible that these dimers and non-covalent trimers are the products of incomplete or incorrect post-translational processing due to overexpression. The purified r-laminin 8 was shown to have biological activity, as all cell lines tested in this study adhered to and spread equally well on r-laminin 8 as on laminin 1. This activity could be abolished by drying the protein, suggesting that native conformation was important for full cell binding activity. The cell binding in all cases be abolished by EDTA, indicating dependence on divalent cations. A large variety of integrins have been implicated as receptors for different laminin isoforms. In this study, we demonstrated that integrins αόβl and α6β4 were major mediators of cell adhesion to r-laminin 8. The adhesion of HT-1080 and BCE cells was completely blocked by anti-integrin α6 mAb, despite the fact that both cell lines express a wide spectrum of βl and αv integrins, including several of those shown to bind to other laminin isoforms. (Conforti et. al., 1994, Cell Adhes. Commun. 1(4), 279-93) HT-1080 cell adhesion to r-laminin 8 is mediated solely by integrin α6βl, since the adhesion could be blocked not only by anti-α6 mAb, but also by the βl antibody. In contrast, the βl mAb only partially blocked adhesion to BCE cells, suggesting that α6β4 contributed to the binding of BCE cells to r-laminin 8. The role of α6β4 as a r-laminin 8 receptor was confirmed by assaying the binding of α6 and β4 transfected K562 cells that express both α6βl and α6β4 on the cell surface. Indeed, adhesion was completely blocked with α6 mAb, but only partially with βl mAb, indicating that the α6β4 complex also binds to r-laminin 8. K562 cells expressing α6βl bound r-laminin 8 while α3βl expressing cells did not, thus confirming that integrin α6βl binds r-laminin 8. Our results somewhat contradict the reported lack of integrin α6 subunit in IBE cells (Kanda et al, 1999), since the adhesion to r-laminin 8 was severely perturbed by the anti-integrin α6 mAb. The result suggests that these cells use yet another receptor(s) in addition to α6 integrins for binding to r-laminin 8. However, in certain cases GoH3 is not able to completely block integrin α6 in α6β4 complexes (Sonnenberg et al., 1993, J. Cell Sci. 106(Pt 4), 1083-102). Thus, the remaining adhesion could be due to incompletely blocked α6β4 complexes. Interestingly, adhesion of the cell lines tested to r-laminin 8 was found to be more dependent on integrin α6 than adhesion of the cell lines to laminin 1. Another indication of the different adhesive properties of r-laminin 8 and laminin 1 was the finding that α6βl -expressing K562 cells did bind to r-laminin 8 without stimulation, but, as also previously reported (Delwel et al., 1993), needed to be stimulated by PMA to efficiently bind to laminin 1 coated surfaces. Thus, r-laminin 8 appears to have a higher avidity or affinity than laminin 1 to αόβl. The α6βl integrin might bind r- laminin 8 even in the conformation that makes it unable to bind to laminin 1, or the cells could be stimulated by the presence of r-laminin 8 via an unknown mechanism. It could be that the avidity/affinity difference is of biological significance, and may well be one reason for the existence of large numbers of laminin isoforms. In addition to integrins, several other cell surface proteins have been reported to function as laminin receptors. Alpha-dystroglycan is a component of the dystrophin- dystroglycan complex in the skeletal muscle thought to connect the contractile cytoskeleton to the extracellular matrix. Dystroglycan has also been shown to bind laminin 2 and dystrophin, forming a link between the two. (Ervasti and Campbell, 1993, J. Cell Biol. 122(4), 809-23) Indirect evidence suggests that laminin 8 might bind to α-dystroglycan; it has been shown that laminin from laminin αd-deficient dystrophic muscle bound dystroglycan, but, in contrast to laminin from normal muscle, in a manner that was sensitive to inhibition by heparin. (McDearmon et al., 1998, J. Biol. Chem. 273(37), 24139-44) Since upregulation of laminin α4 has been observed in laminin α2 deficient muscular dystrophy (Patton et al, 1997; Ringelmann et al., 1999), it can be assumed that the laminin α4 chain is involved in the observed interaction. Alpha-dystroglycan is not restricted to skeletal muscle. (Durbeej et al. 1998, J. Histochem. Cytochem. 46(4), 449-57) It was recently shown to be a receptor for laminin 1 in bovine aorta endothelial cells, binding in a manner sensitive to heparin, dextran sulfate, and fucoidan. (Shimizu et al., 1999, J. Biol. Chem. 274(17), 11995-2000) Heparin-sensitive interactions were not detected in this study, but this does not rule out the possibility of such interactions in other cell types or in vivo. We did observe that the r-laminin 8 binds heparin-sepharose at physiological salt concentration (data not shown). In this study, integrins α6βl and α6β4 were identified as receptors for r-laminin 8 in cultured cells, and thus it is likely that these integrins mediate binding of laminin 8 in vivo, such as to endothelial and muscle cells. Endothelial cells express a wide variety of integrins depending on developmental stage, activation state, and location. At least integrins α2βl, α5βl, α6βl, α6β4, and αvβ3 have been found in endothelial cells in vivo (Sonnenberg 1990; Conforti 1992), whereas the main laminin isoforms in endothelial basement membranes (BM) are laminins 8 and 10. Other cells besides endothelial cells are likely to interact with the laminin 8 in endothelial BM; platelets contain and secrete laminin 8 when stimulated and adhere to it using the α6βl intregrin. Laminins 8 and 9 are also found in developing muscle and in the peripheral nervous system, overlapping in expression with integrin α6. In laminin α2-deficient muscle, both the laminin α4 and integrin α6 are upregulated. (Vachon et al, 1997, J. Clin. Invest. 100(7), 1870-81) Interestingly, integrin α6 and integrin β4 knock-outs result in epidemolysis bullosa (Georges-Labouesse et al, 1996, Nat. Genet. 13(3), 370-3; van der Neut et al., 1996, Nat. Genet. 13(3), 366-9), but no muscular or vascular phenotype was reported. The present invention is not limited by the aforementioned particular prefened embodiments. It will occur to those ordinarily skilled in the art that various modifications may be made to the disclosed preferred embodiments without diverting from the concept of the invention. All such modifications are intended to be within the scope of the present invention.

Claims: We claim 1. Substantially purified laminin 8. 2. The substantially purified laminin 8 of claim 1, comprising recombinant laminin 8. 3. The substantially purified recombinant laminin 8 of claim 2 comprising: a first chain comprising a polypeptide that is substantially similar to an α4 laminin chain; a second chain comprising a polypeptide that is substantially similar to a βl laminin chain; and a third chain comprising a polypeptide that is substantially similar to a γl laminin chain; wherein the first, second, and third chains are assembled into recombinant heterotrimeric laminin 8. 4. The substantially purified recombinant laminin 8 of claim 2 comprising: a first chain encoded by a polynucleotide that hybridizes under high stringency conditions to a coding region of one or more of SEQ ID NOT, 3, 5, 7, 9, or fragments thereof; a second chain encoded by a polynucleotide that hybridizes under high stringency conditions to a coding region of one or more of SEQ ID NOT 1, 13, 15, 17, or fragments thereof; and a third chain encoded by a polynucleotide that hybridizes under high stringency conditions to a coding region of one or more of SEQ ID NO: 19, 21, 23, 25, or fragments thereof; wherein the first, second, and third chains are assembled into recombinant heterotrimeric laminin 8. 5. The substantially purified recombinant laminin 8 of claim 2 comprising: a first chain comprising a polypeptide at least 70% identical to one or more of SEQ ID NO:2, 4, 6, 8, 10 or fragments thereof; a second chain comprising a polypeptide at least 10% identical to one or more of SEQ ID NO: 12, 14, 16, 18 or fragments thereof; and a third chain comprising a polypeptide at least 70% identical to one or more of SEQ ID NO:20, 22, 24, 26, or fragments thereof; wherein the first, second, and third chains are assembled into recombinant heterotrimeric laminin 8. 6. The substantially purified recombinant laminin 8 of claim 2 comprising a first, second, and third polypeptide chain, wherein the first, second, and third polypeptide chains each comprise a general structure selected from the group consisting of: (1) Rl- R2-R3; (2) Rl-R2-R3(e); (3) R3; (4) R3(e); (5) R1-R3; (6) Rl-R3(e); (7) R2-R3; and (8) R2-R3(e) wherein Rl is a amino terminal methionine; R2 is a signal sequence that is capable of directing secretion of the polypeptide, wherein the signal sequence may be the natural signal sequence for the particular laminin chain, that of another secreted protein, or it may be an artificial sequence; R3 is a secreted α4 laminin chain for the first polypeptide chain, a secreted βl laminin chain for the second polypeptide chain, and γl laminin chain for the third polypeptide chain; and R3(e) is identical to R3, but further comprises an epitope tag. 7. Recombinant laminin 8-expressing host cells. 8. The recombinant laminin 8-expressing host cells of claim 7, wherein the cells express recombinant laminin 8 comprising: a first chain comprising a recombinant polypeptide that is substantially similar to an α4 laminin polypeptide; a second chain comprising a recombinant polypeptide that is substantially similar to a βl laminin polypeptide sequence; and a third chain comprising a recombinant polypeptide that is substantially similar to a γl laminin polypeptide sequence; wherein the cell expresses the first, second, and third chains, and wherein the first, second, and third chains assemble into recombinant laminin 8 that is secreted into the media by the cultured cell. 9. The recombinant laminin 8-expressing host cells of claim 7, wherein the cells express recombinant laminin 8 comprising: a first chain encoded by a polypeptide that hybridizes under high stringency conditions to a coding region of one or more of SEQ ID NOT, 3, 5, 7, or 9, or fragments thereof; a second chain encoded by a polypeptide that hybridizes under high stringency conditions to a coding region of one or more of SEQ ID NOT 1, 13, or fragments thereof; and a third chain encoded by a polypeptide that hybridizes under high stringency conditions to a coding region of one or more of SEQ ID NO: 15, 17, or fragments thereof; wherein the cell expresses the first, second, and third chains, and wherein the first, second, and third chains assemble into recombinant laminin 8 that is secreted into the media by the cultured cell. 10. The recombinant laminin 8-expressing host cells of claim 7, wherein the cells express recombinant laminin 8 comprising: a first chain comprising a polypeptide at least 70% identical to one or more of SEQ ID NO:2, 4, 6, 8, 10 or fragments thereof; a second chain comprising a polypeptide at least 70% identical to one or more of SEQ ID NO: 12, 14, 16, 18 or fragments thereof; and a third chain comprising a recombinant polypeptide at least 70% identical to one or more of SEQ ID NO:20, 22, 24, 26, or fragments thereof; wherein the cell expresses the first, second, and third chains, and wherein the first, second, and third chains assemble into recombinant laminin 8 that is secreted into the media by the cultured cell. 11. The recombinant laminin 8-expressing host cells of claim 7, wherein the cells express recombinant laminin 8 comprising a first, second, and third polypeptide chain, wherein the first, second, and third polypeptide chains each comprise a general structure selected from the group consisting of: (1) R1-R2-R3; (2) Rl-R2-R3(e); (3) R3; (4) R3(e); (5) R1-R3; (6) Rl-R3(e); (7) R2-R3; and (8) R2-R3(e) wherein Rl is a amino terminal methionine; R2 is a signal sequence that is capable of directing secretion of the polypeptide, wherein the signal sequence may be the natural signal sequence for the particular laminin chain, that of another secreted protein, or it may be an artificial sequence; R3 is a secreted α4 laminin chain for the first polypeptide chain, a secreted βl laminin chain for the second polypeptide chain, and γl laminin chain for the third polypeptide chain; and R3(e) is identical to R3, but further comprises an epitope tag . 12. The host cells of any of claims 7-11, wherein the host cell is a mammalian cell. 13. The host cells of claim 12, wherein at least one of the first, second, or third chains is expressed as a fusion protein with an epitope tag. 14. A method of purifying recombinant laminin 8, comprising: a. providing the host cells of claim 12; b. growing the cells in cell culture medium under conditions to stimulate expression of the recombinant laminin 8 chains; c. passing the cell culture medium through an affinity chromatography column, wherein the column contains a compound that binds to the recombinant laminin 8; d. washing the affinity column to remove unbound materials; and e. eluting the bound recombinant laminin 8 from the column. 15. Substantially purified recombinant laminin 8 isolated according to the method of claim 14. 16. A pharmaceutical composition comprising: a. laminin 8; and b. a pharmaceutically acceptable carrier. 17. The pharmaceutical composition of claim 16, wherein the laminin 8 comprises recombinant laminin 8. 18. A method to accelerate healing of a vascular tissue injury in a subject, comprising contacting the site of the vascular tissue injury of the subject with an amount effective of laminin 8 to promote re-endothelialization at the vascular tissue injury site. 19. The method of claim 18, wherein the vascular injury is selected from the group consisting of angioplasty restenosis, vascular surgical procedures, aneurysm, and atherosclerosis. 20. A method to accelerate healing of a bone or connective tissue injury in a subject comprising contacting the site of the bone or connective tissue injury in the subject with an amount effective of laminin 8 to accelerate healing of the bone or connective tissue injury. 21. The method of claim 20 wherein healing is accomplished by incoφoration of recombinant laminin 8 into wound repair dressings, matrices, or tissue grafts. 22. The method of claim 20 wherein the bone or connective tissue injury is selected from the group consisting of fractures, tears, deformities, or defects of bone, tendon, cartilage, and ligament. 23. A method to improve the biocompatibility of a medical device or graft, comprising contacting the medical device or graft with an amount effective of laminin 8 to improve the biocompatibility of the medical device or graft. 24. An improved medical device or graft, wherein the improvement consists of providing a medical device or graft with an amount effective of laminin 8 to improve the biocompatibility of the medical device or graft. 25. A method to regulate angiogenesis in a subject, comprising contacting a site in need of angiogenesis in the subject with an amount effective of laminin 8 to regulate angiogenesis. 26. A method to promote neural regeneration in a subject, comprising contacting a site in need of neural regeneration in the subject with an amount effective of laminin 8 to promote neural regeneration. 27. A method to promote cell adhesion to a surface, comprising contacting cells with an amount effective of the laminin 8 to promote cell adhesion to the surface. 28. An improved cell growth substrate, wherein the improvement consists of providing a cell growth substrate that has been coated with an amount effective of laminin 8 to promote cell attachment to the cell growth substrate. 29. The method of any of claims 18-28, wherein the laminin 8 comprises recombinant laminin 8. 30. A method to accelerate healing of a vascular tissue injury in a subject, comprising contacting the site of the vascular tissue injury of the subject with an amount effective of the pharmaceutical composition of claim 16 or 17 to promote re- endothelialization at the vascular tissue injury site. 31. The method of claim 30, wherein the vascular injury is selected from the group consisting of angioplasty restenosis, vascular surgical procedures, aneurysm, and atherosclerosis 32. A method to accelerate healing of a bone or connective tissue injury in a subject comprising contacting the site of the bone or connective tissue injury in the subject with an amount effective of the pharmaceutical composition of claim 16 or 17 to accelerate healing of the bone or connective tissue injury. 33. The method of claim 32 wherein healing is accomplished by incoφoration of recombinant laminin 8 into wound repair dressings, matrices, or tissue grafts. 34. The method of claim 32 wherein the bone or connective tissue injury is selected from the group consisting of fractures, tears, deformities, or defects of bone, tendon, cartilage, and ligament. 35. A method to improve the biocompatibility of a medical device or graft, comprising contacting the medical device or graft with an amount effective of the pharmaceutical composition of claim 16 or 17 to improve the biocompatibility of the medical device or graft. 36. An improved medical device or graft, wherein the improvement consists of providing a medical device or graft with an amount effective of the pharmaceutical composition of claim 16 or 17 to improve the biocompatibility of the medical device or graft. 37. A method to promote angiogenesis in a subject, comprising contacting a site in need of angiogenesis in the subject with an amount effective of the pharmaceutical composition of claim 16 or 17 to promote angiogenesis. 38. A method to promote neural regeneration in a subject, comprising contacting a site in need of neural regeneration in the subject with an amount effective of the pharmaceutical composition of claim 16 or 17 to promote neural regeneration. 39. A method to promote cell adhesion to a surface, comprising contacting cells with an amount effective of the pharmaceutical composition of claim 16 or 17 to promote cell adhesion to the surface. 40. An improved cell growth substrate, wherein the improvement consists of providing a cell growth substrate that has been coated with an amount effective of the pharmaceutical composition of claim 16 or 17 to promote cell attachment to the cell growth substrate. 41. A kit for carrying out the method of any of claims 18-28, comprising: (a) an amount effective of laminin 8 for carrying out the method; and (b) instructions for using the laminin 8 for carrying out the method. 42. A method to inhibit cell adhesion to laminin 8, comprising contacting the cell with an amount effective of an antagonist of at least one of integrin α6βl and α6β4 to inhibit cell adhesion to laminin 8.

Inventors: KORTESMAA JARRKO, TRYGGVASON KARL, KORTESMAA, JARRKO, TRYGGVASON, KARL
Applicants: BIOSTRATUM INC, KORTESMAA JARRKO, TRYGGVASON KARL, BIOSTRATUM, INC., KORTESMAA, JARRKO, TRYGGVASON, KARL
Application year: 2000
Main classification: C12N15
IPC codes: A61K38, A61L27, A61P7, A61P9, A61P19, A61P25, C07K14, C12N1, C12N5, C12N15, C12P21, C12R1, G01N31