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

Cell surface heparan sulfate proteoglycans (HSPGs), substantially more abundant than most receptors, modulate encounters of extracellular protein ligands with their receptors by forming HSprotein 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 Nand 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 (1, 2). 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 (1, 3). 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 (4). 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 (5). 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 (6). Furthermore, the cytoplasmic domains of the syndecans also form complexes with cytoplasmic enzymes and scaffolding proteins, adding to the modulating influence of these proteoglycans (7). 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 (8). 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 typespecific 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 (9). FGF-2 binds most tightly to a specific hexasaccharide sequence, but an additional 4–6 sugar residues are required to activate the receptor (10). Specific oligosaccharide binding sequences are known for multiple ligands (11–14); 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 (14).