1. Introduction

and adhesion. Such cellular actions of HSPGs regulate many biological events including angiogenesis, inflammation, hemostasis, lipoprotein metabolism, axonal guidance, and developmental inductions.

Although HSPGs control a myriad of functions, this chapter selectively focuses on the role of the HS chain in regulating endothelial cell (EC) signaling by “heparin-binding growth factors.” For ease of discussion, this term is defined as any signaling ligand that exhibits high affinity to heparin, a particular flavor of highly sulfated HS. This definition encompasses classic growth factors [such as vascular endothelial growth factors (VEGFs), fibroblast growth factors (FGFs), heparin-binding epidermal growth factor, and Wnts], cytokines (such as GM-CSF, interleukin-3, and interferon-γ), most chemokines and even growth inhibitors such as endostatin. Consequently, this term includes the majority of HS-binding ligands (Table 1). “Heparin-binding” is a historical term, which derives from many of these factors being originally purified by heparin chromatography. However, heparin only occurs within mast cell granules, so it is unlikely to contribute to the in vivo roles of most of these ligands. The physiological activities of heparin-binding growth factors instead predominantly involve their interaction with the HS chains of extracellular matrix and cell surface HSPGs, which are produced by almost all cell types.

2. Historical Perspective

Although heparin is only found in mast cells, it was the first form of HS discovered and consequently dominated the early history of the field. In 1916, Jay McLean, a second-year medical student, found that an extract from dog liver exhibited strong anticoagulant activity.1 This activity was designated as heparin to denote its isolation from liver. By 1935, clinical trials were started, which heralded the long-standing use of heparin as an efficacious anticoagulant (reviewed in Ref. 2). The acceptance of heparin as a therapeutic agent motivated studies to discover its structure and mechanism of action.

The paradigm for how HS motifs convey biological activity derives from the original elucidation of heparin’s mechanism of action. McLean’s mentor, William Henry Howell, determined in 1925 that

122 N. W. Shworak

heparin was a form of polysaccharide and proposed that its anticoagulant activity required a plasma cofactor.3 Potentially, this cofactor might be “antithrombin” — an activity in defibrinated plasma, discovered at the end of the 19th century, known to slowly neutralize thrombin.4 However, almost six decades of research were required to unravel how heparin’s intricate structure conveys its mechanism of action (reviewed in Ref. 2). In 1968, Abildgaard ultimately determined that Howell’s hypothetical plasma cofactor was indeed antithrombin.5 Shortly after, Rosenberg and Damus found that the binding of heparin to antithrombin dramatically catalyzed antithrombin’s ability to neutralize coagulation proteases.6 Initially, such binding was largely considered to occur by non-specific ionic interactions. This notion was discounted in 1976, when the three independent groups of Lindahl, Rosenberg and Sims showed that only one-third of heparin molecules could bind antithrombin and only this population of molecules exhibited anticoagulant activity.7−9 This landmark observation suggested structural specificity must exist. In the early 1980s, the groups of Choay, Lindahl and Rosenberg demonstrated that the active component of heparin was a pentasaccharide motif with a specific arrangement of sulfate groups (reviewed in Ref. 2). It is now appreciated that many of the distinct activities of HS are conveyed by specific motifs, with a given motif bound by a distinct effector protein.

Although the initial HS landscape was dominated by heparin, it was appreciated as early as 1937 that heparin must play only a limited role in the body, as it is only present in the basophilic granules of mast cells.10 The rich heparin content of liver derives from high levels of mast cells in the liver capsule. The ubiquitous nature of HS was eventually realized in the early 1970s, when it was determined that virtually all cell types produce HSPGs (reviewed in Ref. 2). In the 1980s, the various core proteins were identified, and the 1990s heralded the cloning of the HS biosynthetic enzymes. The same family of enzymes were found to generate both the ubiquitous HS and heparin, which led to the appreciation that heparin is simply one type of HS (reviewed in Refs. 11 to 13). Indeed, certain cell types can produce an HS subpopulation that exhibits a high sulfate content indistinguishable from that of mast cell heparin.