Regulated Diversity of Heparan Sulfate

Abstract

heparan sulfate N-acetylated N-sulfated fibroblast growth factor platelet-derived growth factor N-deacetylase/N-sulfotransferase GlcN 6-O-sulfotransferase GlcN 3-O-sulfotransferase IdoA 2-O-sulfotransferase 3′-phosphoadenosine 5′-phosphosulfate. Large numbers of proteins in animal tissues occur immobilized in the extracellular space, at cell surfaces or in the extracellular matrix. Some are anchored through interactions with other proteins. However, current research increasingly implicates proteoglycans as scaffold structures, designed to accommodate proteins through noncovalent binding to their glycosaminoglycan side chains (1Kjellén L. Lindahl U. Annu. Rev. Biochem. 1991; 60: 443-475Crossref PubMed Scopus (1660) Google Scholar). In particular, heparan sulfate (HS)1 proteoglycans are recognized as ubiquitous protein ligands. Binding of proteins to HS chains may serve a variety of functional purposes, from simple immobilization or protection against proteolytic degradation to distinct modulation of biological activity. Because of such interactions HS proteoglycans are critically involved in a variety of biological phenomena at various levels of complexity, including organogenesis in embryonic development, angiogenesis, regulation of blood coagulation and growth factor/cytokine action, cell adhesion, lipid metabolism, etc. (2Lindahl U. Lidholt K. Spillmann D. Kjellén L. Thromb. Res. 1994; 75: 1-32Abstract Full Text PDF PubMed Scopus (341) Google Scholar, 3Salmivirta M. Lidholt K. Lindahl U. FASEB J. 1996; 10: 1270-1279Crossref PubMed Scopus (392) Google Scholar, 4Rosenberg R.D. Shworak N.W. Liu J. Schwartz J.J. Zhang L. J. Clin. Invest. 1997; 99: 2062-2070Crossref PubMed Scopus (253) Google Scholar). In this review we outline the evidence for regulated expression of specific domains in HS chains and its relation to selective protein binding. Further, we consider the current information on HS biosynthesis, with the aim of understanding the mechanisms in control of generating specific saccharide sequences. Contrary to heparin, which is synthesized exclusively as serglycin proteoglycan in connective tissue-type mast cells (1Kjellén L. Lindahl U. Annu. Rev. Biochem. 1991; 60: 443-475Crossref PubMed Scopus (1660) Google Scholar), the structurally related HS is produced by most mammalian (and many other) cell types. A variety of core proteins have been shown to carry HS chains, including agrin and perlecan in the extracellular matrix (5Iozzo R.V. Matrix Biol. 1994; 14: 203-208Crossref PubMed Scopus (126) Google Scholar) and the syndecan- and glypican-type species at the cell surface (6David G. FASEB J. 1993; 7: 1023-1030Crossref PubMed Scopus (373) Google Scholar). There is so far no evidence of any correlation between the structures of core proteins and those of the attached HS chains, whereas, conversely, the fine structure of the HS chains on a given proteoglycan species may differ between cell types (7Kato M. Wang H. Bernfield M. Gallagher J.T. Turnbull J.E. J. Biol. Chem. 1994; 269: 18881-18890Abstract Full Text PDF PubMed Google Scholar). The carbohydrate backbone in heparin and HS consists of alternating hexuronic acid (d-glucuronic acid (GlcA) or l-iduronic acid (IdoA)) and d-glucosamine (GlcN) units. The GlcN residues in heparin are predominantly N-sulfated, whereas those in HS show a more varied N-substitution pattern with appreciable proportions of both N-sulfated and N-acetylated and a smaller amount of N-unsubstituted GlcN units (3Salmivirta M. Lidholt K. Lindahl U. FASEB J. 1996; 10: 1270-1279Crossref PubMed Scopus (392) Google Scholar, 8Gallagher J.T. Walker A. Biochem. J. 1985; 230: 665-674Crossref PubMed Scopus (190) Google Scholar,9van den Born J. Gunnarsson K. Bakker M.A.H. Kjellén L. Kusche-Gullberg M. Maccarana M. Berden J.H.M. Lindahl U. J. Biol. Chem. 1995; 270: 31303-31309Abstract Full Text Full Text PDF PubMed Scopus (133) Google Scholar). These structures are generated through the formation of a [GlcA-GlcNAc]n polymer that is subsequently modified by partial N-deacetylation/N-sulfation of GlcNAc units, C-5 epimerization of GlcA to IdoA residues, and, finally, incorporation of O-sulfate groups at various positions (Fig. 1). Compositional analysis of several HS preparations derived from different bovine organs revealed certain domain characteristics of unexpected regularity (10Maccarana M. Sakura Y. Tawada A. Yoshida K. Lindahl U. J. Biol. Chem. 1996; 271: 17804-17810Abstract Full Text Full Text PDF PubMed Scopus (240) Google Scholar). Thus 20–30% of the disaccharide units in all preparations were consistently arranged in alternatingN-acetylated and N-sulfated sequence (NA/NS domain in Fig. 1). Moreover, although the proportions of consecutiveN-sulfated disaccharide units (NS domain in Fig. 1) varied between preparations, those of IdoA residues within the NS domains and the extent of 2-O-sulfation of these units were remarkably constant. Very few 2-O-sulfate groups were located outside