Thrombospondin in Clinical Disease States

The high molecular weight glycoprotein, thrombospondin (TSP), (also termed glycoprotein G, thrombin-sensitive protein, and glycoprotein Ig), has in recent years been the subject of con­ siderable investigation, both in relation to its puta­ tive role in platelet aggregation and for its poten­ tial as a marker in some clinical disease states. It is a major constituent of the platelet α-granule representing 20 to 30% of thrombin-releasable pro­ tein (Fig. 1). In the presence of calcium ions, re­ leased TSP binds to the activated platelet surface. This functional property is shared by several other α-granule proteins, namely, fibrin(ogen), von Willebrand factor, and platelet factor 4 (PF4). Physicochemical characterization has revealed that the native 450 kd molecule consists of three ap­ parently identical 185 kd subunits. Each subunit comprises a 23 to 30 kd, N-terminal globular seg­ ment that contains a heparin-binding domain, linked to a C-terminal globular region by a thread­ like fibrous segment. The heparin-binding peptide is readily cleaved from TSP by a variety of proteo­ lytic enzymes, such as thrombin, trypsin, plasmin, and thermolysin. The more proteolytically resis­ tant 160 kd core contains the interpeptide disulfide bridges and a fibrinogen-binding domain. Susceptibility to cleavage is modulated by the stabil­ izing effect of a number of calcium-binding sites on the molecule. Antibodies directed against the 23 kd heparin-binding segment of TSP have been found to inhibit platelet aggregation and TSP-associated hemagglutinin activity, suggesting a role for TSP in platelet aggregation. Although this role remains to be fully elucidated, the characterization of TSP as the platelet endogenous lectin together with the identification of fibrinogen as the lectin receptor support a model for platelet aggregation in which cohesion is mediated by interplatelet lectinreceptor bridges. It would appear that the hemagglutinin activity of purified TSP is an artifact due to disulfidedependent aggregate formation during purifica­ tion and that the native 450 kd protein binds to erythrocytes monovalently. This view is supported by rotary shadowing electron microscopy in which the N-terminal, erythrocyte-binding domain or domains are seen to be closely aligned. Two syndromes have been described in which a quantitative or qualitative defect in platelet TSP is evident. The gray platelet syndrome (GPS) is an in­ herited disease that is characterized by a lack of platelet α-granules and, hence, a marked deficiency of all α-granule storage proteins, including TSP. Essential thrombocythemia (ET) is one of the myeloproliferative syndromes (MPS) characterized primarily by a persistent and marked elevation of platelet-megakaryocyte elements in blood. Studies on platelets of patients with ET have revealed a multiplicity of molecular defects that may contrib­ ute to the impairment of platelet function common­ ly found in this disease. One such finding is the identification of an altered form of TSP in platelets of patients with ET. ,29 These two syndromes pro­ vide a useful means of investigating TSP, both in relation to the disease condition and to platelet function. A third area of more clinical interest is the investigation of TSP as a potential marker of platelet activation in a number of clinical disease states. ' This review examines the relationship of TSP to platelet function with reference to these syndromes, and the usefulness of TSP as an in vivo marker of platelet activation in clinical disease. From the Department of Medicine, University of Sydney, Westmead Hospital, West mead, Australia. Reprint requests: Dr. Berndt, Department of Medicine, Uni­ versity of Sydney, Westmead Hospital, Westmead N.S. W. 2145, Australia.