Cell Surface Heparan Sulfate Proteoglycans: Selective Regulators of Ligand-Receptor Encounters

heparan sulfate proteoglycan heparan sulfate N-sulfated N-acetyl fibroblast growth factor fibroblast growth factor receptor N-deacetylase/N-sulfotransferase mitogen-activated protein tissue inhibitor of metalloproteinase Cell surface heparan sulfate proteoglycans (HSPGs),1 substantially more abundant than most receptors, modulate encounters of extracellular protein ligands with their receptors by forming HS-protein complexes. Two gene families account for most cell surface HSPGs. Both consist of discrete core proteins covalently attached to two or three chains of HS, an N- and O-sulfated linear polysaccharide of repeating disaccharides containing N-acetylglucosamine (GlcNAc) and uronic acid (glucuronic acid (GlcA) or iduronic acid (IdoUA)). The syndecan family was the first discovered, which in mammals contains four gene products with distinctive extracellular domains (ectodomains) and highly conserved short cytoplasmic domains. These apparently extended proteins place the HS chains distal from the plasma membrane (1Bernfield M. Götte M. Park P.W. Reizes O. Fitzgerald M.L. Lincecum J. Zako M. Annu. Rev. Biochem. 1999; 68: 729-777Crossref PubMed Scopus (2323) Google Scholar, 2Bernfield M. Kokenyesi R. Kato M. Hinkes M.T. Spring J. Gallo R.L. Lose E.J. Annu. Rev. Cell Biol. 1992; 8: 365-393Crossref PubMed Scopus (968) Google Scholar). The syndecan family contrasts with the glypican family, which in mammals contains six gene products that are covalently linked to plasma membrane lipid by glycosylphosphatidylinositol anchor (1Bernfield M. Götte M. Park P.W. Reizes O. Fitzgerald M.L. Lincecum J. Zako M. Annu. Rev. Biochem. 1999; 68: 729-777Crossref PubMed Scopus (2323) Google Scholar, 3David G. FASEB J. 1993; 7: 1023-1030Crossref PubMed Scopus (374) Google Scholar). The glypican core proteins contain six invariant disulfide bonds, are likely to be globular, and place HS chains adjacent to the plasma membrane. Expression of both the syndecans and glypicans is extensively regulated during mouse embryogenesis and results in discrete adult expression patterns for each HSPG such that every adherent cell exhibits a distinct repertoire of cell surface HSPGs. Binding to HS chains is remarkably widespread among extracellular proteins, especially matrix proteins, proteases and their inhibitors, lipases, lipoproteins, growth factors and their binding proteins, cytokines, chemokines, collectins, and antimicrobial peptides. These proteins are involved in morphogenesis, tissue repair, energy balance, and host defense (Fig. 1). Additionally, numerous pathogens (e.g. herpes simplex virus,Neisseria, Plasmodium) bind to the cell surface via HS (4Rostand K.S. Esko J.D. Infect. Immun. 1997; 65: 1-8Crossref PubMed Google Scholar). Importantly, many of these ligand-HS interactions are essentially identical from Drosophila to the mouse, including those involved in generation of the basic metazoan body plan,e.g. dpp (bone morphogenetic proteins 2–4), wg (Wnt-1), and sog (chordin). Formation of the complexes can enhance or reduce receptor activation, often depending on the concentrations of ligand, receptor, and HSPG. The HS chains catalyze encounters between ligand and signaling receptor by bringing them together. Because binding to the HS chain reduces the dimensionality of this interaction from three (when the ligand is soluble) to two (when the ligand is bound to the HS chain), interaction could result from a localized increase in ligand concentration at optimal HS concentrations (5Schlessinger J. Lax I. Lemmon M. Cell. 1995; 83: 357-360Abstract Full Text PDF PubMed Scopus (451) Google Scholar). However, at HS levels lower or higher than optimal, the effective ligand concentration for engaging the receptor will fall, potentially accounting for the bell-shaped activity curve typically seen experimentally when HSPG (or heparin) concentrations are varied. The curve may be concave or convex depending on whether ligand binding to the HSPG is inhibitory or stimulatory (6Lander A.D. Matrix Biol. 1998; 17: 465-472Crossref PubMed Scopus (103) Google Scholar). Furthermore, the cytoplasmic domains of the syndecans also form complexes with cytoplasmic enzymes and scaffolding proteins, adding to the modulating influence of these proteoglycans (7Couchman J.R. Woods A. J. Cell. Biochem. 1996; 61: 578-584Crossref PubMed Scopus (94) Google Scholar). The HS chains are structurally diverse by virtue of their biosynthesis. A non-sulfated repeating disaccharide precursor is generated while attached to a core protein and is then sequentially modified by a variety of enzymes in reactions that do not go to completion. The details of this biosynthetic scheme have been recently reviewed in this series (8Lindahl U. Kusche-Gullberg M. Kjellen L. J. Biol. Chem. 1998; 273: 24979-24982Abstract Full Text Full Text PDF PubMed Scopus (574) Google Scholar). Three major characteristics of this scheme produce HS chains with selectivity for protein binding. First, the process yields an extraordinary variety of saccharide sequences. Second, clustering of the modifications along the HS chain yields highly N-sulfated domains (NS domains) of approximately 12–20 residues that alternate with typically larger sized, relatively unmodified N-acetyl-rich domains (NA domains). The NS domains are rich in IdoUA, which can assume several different conformations and thus influence the orientation of the sulfate residues in space. This domain organization places relatively flexible NA domains adjacent to relatively rigid NS domains, thus facilitating protein interactions with the sulfate residues. Finally, this microsequence diversity and macro-organization are cell type-specific and do not appear to be core protein-specific (e.g. HS chains of syndecan-1 and -4 from mammary epithelia are indistinguishable), presumably the result of cell type-specific repertoires of the HS chain-modifying enzymes. Distinct oligosaccharide sequences in HS chains are recognized by the various proteins whose function depends on this interaction. The best characterized of these interactions is the recognition of a specific pentasaccharide sequence by antithrombin III (9Lindahl U. Thunberg L. Backstrom G. Riesenfeld J. Nordling K. Bjork I. J. Biol. Chem. 1984; 259: 12368-12376Abstract Full Text PDF PubMed Google Scholar). FGF-2 binds most tightly to a specific hexasaccharide sequence, but an additional 4–6 sugar residues are required to activate the receptor (10Rapraeger A.C. Chem. Biol. 1995; 2: 645-649Abstract Full Text PDF PubMed Scopus (78) Google Scholar). Specific oligosaccharide binding sequences are known for multiple ligands (11Parthasarathy N. Goldberg I.J. Sivaram P. Mulloy B. Flory D.M. Wagner W.D. J. Biol. Chem. 1994; 269: 22391-22396Abstract Full Text PDF PubMed Google Scholar, 12Stringer S.E. Gallagher J.T. J. Biol. Chem. 1997; 272: 20508-20514Abstract Full Text Full Text PDF PubMed Scopus (169) Google Scholar, 13Feyzi E. Lustig F. Fager G. Spillmann D. Lindahl U. Salmivirta M. J. Biol. Chem. 1997; 272: 5518-5524Abstract Full Text Full Text PDF PubMed Scopus (119) Google Scholar, 14Conrad H.E. Heparin-binding Proteins. Academic Press, San Diego1998Google Scholar); however, there is no universal consensus amino acid sequence for protein binding to HS chains. Most studies suggest that multiple arginine and/or lysine residues aligned on the protein surface accommodate a distinctive array of anionic sites on the HS chain (14Conrad H.E. Heparin-binding Proteins. Academic Press, San Diego1998Google Scholar). Cell surface HSPGs bind to a large number of proteins; thus, some of their molecular interactions have been considered nonspecific. This perception is challenged by both in vivo evidence showing that cell surface HSPGs are required for specific morphogenetic events and by a growing body of data demonstrating that HS interactions with ligands depend on specific HS sequences. These interactions are mostly modulating or regulatory and do not typically result in intracellular signaling from the HSPG. The studies reviewed below suggest that when HSPGs accelerate ligand-receptor encounters, the subsequent receptor action depends on whether the ligand is soluble or insoluble. Extensive research has focused on HSPGs as coreceptors for a large variety of soluble ligands, including FGFs, transforming growth factors-β1 and -β2, vascular endothelial growth factor (VEGF165,189), CC and C XC chemokines, and various cytokines (1Bernfield M. Götte M. Park P.W. Reizes O. Fitzgerald M.L. Lincecum J. Zako M. Annu. Rev. Biochem. 1999; 68: 729-777Crossref PubMed Scopus (2323) Google Scholar). The diverse cognate receptors for these ligands include receptor tyrosine kinases and seven pass G-protein-coupled receptors. In this role, the HSPGs can modulate ligand-receptor encounters by altering ligand concentrations, stability, or conformation and by ligand or receptor oligomerization. Indeed, there seem to be several distinct mechanisms by which the HSPGs act as coreceptors. One of the best studied examples is that of the fibroblast growth factor receptor-1 (FGFR-1), a receptor tyrosine kinase, and its ligands FGF-1 and -2. Rapraeger et al. (15Rapraeger A.C. Krufka A. Olwin B.B. Science. 1991; 252: 1705-1708Crossref PubMed Scopus (1291) Google Scholar) first demonstrated that in the absence of cell surface HS, FGF-2 interacts poorly with its high affinity receptor, FGFR-1 and does not activate downstream intracellular signaling. Subsequent studies showed that HS, FGF-2, and FGFR-1 form a ternary complex at the cell surface, where HS can be provided by either syndecans or glypicans (16Steinfeld R. Van Den Berghe H. David G. J. Cell Biol. 1996; 133: 405-416Crossref PubMed Scopus (230) Google Scholar). The HS in the complex solely accelerates the ligand-receptor interactions, as is evident from the finding that its absence can be overcome by increasing either reactant concentration. Because FGFR-1 must dimerize to signal, the cell surface HS is thought to bind monomeric FGF-2 and form oligomers that, in turn, dimerize the receptor (5Schlessinger J. Lax I. Lemmon M. Cell. 1995; 83: 357-360Abstract Full Text PDF PubMed Scopus (451) Google Scholar, 6Lander A.D. Matrix Biol. 1998; 17: 465-472Crossref PubMed Scopus (103) Google Scholar). Despite its simplicity, this model is controversial. For example, it was recently shown that the minimal subunit for FGFR-1 activation is a monomer, suggesting that the HS provides a template for the FGF-2 and FGFR-1 interaction and does not act by oligomerizing either ligand or receptor (17Pye D.A. Gallagher J.T. J. Biol. Chem. 1999; 274: 13456-13461Abstract Full Text Full Text PDF PubMed Scopus (50) Google Scholar). Similarly, for the FGF-7 receptor, heparin or HSPG-induced oligomerization of either FGF-1 or -7 does not correlate with biological activity, supporting the model that HSPG-induced ligand oligomerization is not critical in formation of the receptor-ligand complex (18Berman B. Ostrovsky O. Shlissel M. Lang T. Regan D. Vlodavsky I. Ishai-Michaeli R. Ron D. J. Biol. Chem. 1999; 274: 36132-36138Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar). Binding to insoluble ligands immobilizes the HSPG in the plane of the membrane, which causes the syndecan cytoplasmic domain to interact with the actin cytoskeleton and form more stable adhesions. Because glypicans are linked only to the outer membrane leaflet, their direct interactions with cytoplasmic elements are limited. Individual syndecan cytoplasmic domains do not produce soluble cytoplasmic signals, but they can oligomerize, be phosphorylated, and interact with scaffolding and signaling molecules (19Kinnunen T. Kaksonen M. Saarinen J. Kalkkinen N. Peng H.B. Rauvala H. J. Biol. Chem. 1998; 273: 10702-10708Abstract Full Text Full Text PDF PubMed Scopus (197) Google Scholar, 20Oh E.S. Couchman J.R. Woods A. Arch. Biochem. Biophys. 1997; 344: 67-74Crossref PubMed Scopus (47) Google Scholar, 21Ott V.L. Rapraeger A.C. J. Biol. Chem. 1998; 273: 35291-35298Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar). The short syndecan cytoplasmic domains (28–34 amino acids) contain three functional subdomains, C1, V, and C2, at the C terminus. The highly conserved C1 subdomain contains binding sites for Src protein tyrosine kinases (21Ott V.L. Rapraeger A.C. J. Biol. Chem. 1998; 273: 35291-35298Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar). The variable V subdomain of syndecan-4 can interact with phosphatidylinositol bisphosphate and the catalytic domain of protein kinase Cα, interactions thought to result in oligomerization of the cytoplasmic domain and localization of syndecan-4 to focal adhesion complexes (22Woods A. Couchman J.R. J. Biol. Chem. 2000; 275: 24233-24236Abstract Full Text Full Text PDF PubMed Scopus (70) Google Scholar). A recently identified intracellular protein, syndesmos, also binds specifically to the cytoplasmic domain of syndecan-4 via the C1 and V subdomains (23Baciu P.C. Saoncella S. Lee S.H. Denhez F. Leuthardt D. Goetinck P.F. J. Cell Sci. 2000; 113: 315-324Crossref PubMed Google Scholar). This interaction can regulate cell spreading and actin stress fiber organization. The C2 subdomain contains a conserved amino acid sequence, FYA, which is recognized by type II domain binding and are domain proteins that form bind syndecan cytoplasmic domains in with syndecans at the plasma membrane in and appear to the syndecans and the is a protein an of function by two domains, in a molecular P. G. A. G. J. David G. Sci. U. S. A. 1997; PubMed Scopus Google Scholar, G. H. David G. J. Biol. Chem. 2000; 275: Full Text Full Text PDF PubMed Scopus Google Scholar). binds to the C2 subdomain in the syndecans with its domain S. M. J. 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J. J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar, Biochem. J. 1997; PubMed Scopus Google Scholar). is an of activation and for approximately of cell surface proteins, including growth cytokines, cell adhesion and among Because the of these molecules critical in various events such as cell and host the process of has been for and and the has been Biochem. J. 1997; PubMed Scopus Google Scholar). of the four syndecans is from the cell surface by of the core protein (1Bernfield M. Götte M. Park P.W. Reizes O. Fitzgerald M.L. Lincecum J. Zako M. Annu. Rev. Biochem. 1999; 68: 729-777Crossref PubMed Scopus (2323) Google Scholar). The of for syndecan-1 has been localized to amino adjacent to the extracellular of the plasma membrane M.L. Park P.W. G. M. J. Cell Biol. 2000; PubMed Scopus Google Scholar). the of the syndecan is its activity is cell and can be by and tissue inhibitor of but not by or or by of and M.L. Park P.W. G. M. J. 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Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar). syndecan-1 can also function as of FGF-2 via the NA domains of their HS chains M. H. Fitzgerald S. D.M. M. 1998; PubMed Scopus Google Scholar). However, when the syndecan-1 are with as during tissue the the NA domains. This the NS domains that activate FGF-2 M. H. Fitzgerald S. D.M. M. 1998; PubMed Scopus Google Scholar). Indeed, concentrations of the NS domains are in the macro-organization of HS chains has a functional of cell by syndecan-1 has also been with cell M. H. M. J. Biol. Chem. 1994; 269: Full Text PDF PubMed Google Scholar). can also potentially regulate receptor signaling events by the of cell surface HSPGs. The biological of this is evident from data showing in to FGF-2 in with either HS enzymes or that with HS (e.g. is by tissue such as growth and (1Bernfield M. Götte M. Park P.W. Reizes O. Fitzgerald M.L. Lincecum J. Zako M. Annu. Rev. Biochem. 1999; 68: 729-777Crossref PubMed Scopus (2323) Google and syndecan are in tissue such as in of and in Fitzgerald M.L. M. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar, M. H. Fitzgerald S. D.M. M. 1998; PubMed Scopus Google Scholar). Importantly, the can bind and the activity of some that syndecan are soluble of tissue The syndecans apparently during the major metazoan of the their core protein are and in conserved in and in large of their to epithelia that an from the are required to such an extracellular was which with and the of HS in the of extracellular signaling and matrix proteins was HS-protein interactions seem to be on an conserved extracellular This is with the of such proteins that depend for their function on binding HS, the many distinct protein for this the of most of these proteins in that are to an and the of major in HS-protein the this HS is and its is on interactions with proteins and its is in However, the this is not on a direct template is to and to depend on Despite the binding of HS as more most interactions to be For example, the HS chain-modifying is the most involved in HS for this of type II a M. J. K. T. F. I. A. L. E. J. Biol. Chem. 2000; 275: Full Text Full Text PDF PubMed Scopus Google G. L. L. B. G. 2000; PubMed Scopus Google Scholar). is the absence of in with a gene 1998; PubMed Scopus Google Scholar). are also specific for HSPGs. For example, the formation of multiple is a of in HS T. F. Lindahl U. K. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google and on and growth are by G. A. P. R. G. A. A. D. 1996; PubMed Scopus Google Scholar). recently for the of HS chains G. R. R. Science. 1999; PubMed Scopus Google Scholar) and for the of specific HS sequences 1999; PubMed Scopus Google Scholar) will be in the to the molecular mechanisms such distinct and for in this