Interactions of signaling proteins, growth factors and other proteins with heparan sulfate: mechanisms and mysteries

Abstract

Heparan sulfate (HS) is a component of cell surface and matrix-associated proteoglycans (HSPGs) that collectively, play crucial roles in many physiologic processes including cell differentiation, organ morphogenesis and cancer. A key function of HS is to bind and interact with signaling proteins, growth factors, plasma proteins, immune-modulators and other factors. In so doing, the HS chains and HSPGs are able to regulate protein distribution, bio-availability and action on target cells and can also serve as cell surface co-receptors, facilitating ligand-receptor interactions. These proteins contain an HS/heparin-binding domain (HBD) that mediates their association and contacts with HS. HBDs are highly diverse in sequence and predicted structure, contain clusters of basic amino acids (Lys, Arg) and possess an overall net positive charge, most often within a consensus Cardin-Weintraub (CW) motif. Interestingly, other domains and residues are now known to influence protein-HS interactions, as well as interactions with other glycosaminoglycans, such as chondroitin sulfate. In this review we provide a description and analysis of HBDs in proteins including amphiregulin, fibroblast growth factor family members, heparanase, sclerostin and hedgehog protein family members. We discuss HBD structural and functional features and important roles carried out by other protein domains, and also provide novel conformational insights into the diversity of CW motifs present in Sonic, Indian and Desert hedgehogs. Finally, we review progress in understanding the pathogenesis of a rare pediatric skeletal disorder, Hereditary Multiple Exostoses (HME), characterized by HS deficiency and cartilage tumor formation. Advances in understanding protein-HS interactions will have broad implications for basic biology and translational medicine as well as for the development of HS-based therapeutics.

Domains with highest heparan sulfate–binding affinity reside at opposite ends in BMP2/4 versus BMP5/6/7: Implications for function

“…Comparative analyses on vitronectin, platelet factor-4, apoliprotein A and apoliprotein B led to the identification of two HS-binding motifs -XBBXBX and XBBBXXBX-where B represents a basic residue (Arg or Lys) and X represents a noncharged residue. Subsequent studies on many other proteins have verified and greatly extended those original findings (25)(26)(27). The HS-binding domains in BMP2 and BMP4 have been characterized in previous studies (18,28).…”

“…Deciphering the exact 3D organization and features of BMP dimers will require further analysis, and the contribution of single or groups of basic residues -distant from the CW motifs-will need to be taken into consideration to precisely delineate the overall structural basis of BMP-HS interactions (25,46,47). It will be necessary also to clarify the spatial orientation of different BMPs with respect to the HS chain backbone given that their electropositive surfaces differ in location and structure as suggested above.…”

“…Such multiple serial biosynthetic steps result in chains with highly diverse sulfation and sugar modification patterns within 6-12 sugar residue-long segments flanked by largely unmodified and un-sulfated segments (19,23). These features are thought to represent the basis for the ability of HS chains to strongly and specifically interact with proteins (23,25,48). Some proteins such as fibroblast growth factor 2 (FGF2) and antithrombin require specific modifications of HS for optimal binding including 3-O sulfation (49)(50)(51), whereas other proteins such as IL-8 and thrombin mainly rely on HS domain structure or charge density, respectively (52,53).…”

“…These distinct sequences are highly conserved, and experimental mutations of their basic residues were shown to alter HS binding and biological function (18,30). Notably, these sequences reiterate a fundamental and yet largely unexplained feature of HS-binding domains characterized in these and many other proteins -namely, that the amino acid sequence and organization of each domain vary greatly from protein to protein (25).…”

Heparan sulfate disaccharide measurement from biological samples using pre-column derivatization, UPLC-MS and single ion monitoring

“…Although the molecular mechanism of the action of GAGs is not well understood, accumulating evidence suggests that the effects of heparan sulfate glycosaminoglycans may be attributable at least in part to their binding of the growth factors and regulating of their signaling [6,7].…”