Role of Thrombospondin-1 in Control of von Willebrand Factor Multimer Size in Mice

Abstract

Plasma von Willebrand factor (VWF) is a multimeric glycoprotein from endothelial cells and platelets that mediates adhesion of platelets to sites of vascular injury. In the shear force of flowing blood, however, only the very large VWF multimers are effective in capturing platelets. The multimeric size of VWF can be controlled by proteolysis at the Tyr 842 -Met 843 peptide bond by ADAMTS13 or cleavage of the disulfide bonds that hold VWF multimers together by thrombospondin-1 (TSP-1). The average multimer size of plasma VWF in TSP-1 null mice was significantly smaller than in wild type mice. In addition, the multimer size of VWF released from endothelium in vivo was reduced more rapidly in TSP-1 null mice than in wild type mice. TSP-1, like ADAMTS13, bound to the VWF A3 domain. TSP-1 in the wild type mice, therefore, may compete with ADAMTS13 for interaction with the A3 domain and slow the rate of VWF proteolysis. TSP-1 is stored in platelet ␣-granules and is released upon platelet activation. Significantly, platelet VWF multimer size was reduced upon lysis or activation of wild type murine platelets but not TSP-1 null platelets. This difference had functional consequences in that there was an increase in collagen-and VWF-mediated aggregation of the TSP-1 null platelets under both static and shear conditions. These findings indicate that TSP-1 influences plasma and platelet VWF multimeric size differently and may be more relevant for control of the VWF released from platelets.

Thrombospondin 1 requires von Willebrand factor to modulate arterial thrombosis in mice

“…22 The K d for soluble TSP1 binding to VWF A2 domain is not known, but we speculate that it could be similar (;3 mM) to A3 domain because in the residual collagen binding assay it was shown that TSP1 (100 nM) competes with both VWF A2 and A3 domain to a similar extent. 21 On the other hand, the levels of ADAMTS13 in plasma is ;1000 ng/mL (;7 nM), 44 and has much higher affinity for VWF (K d ; 15 nM).…”

“…21,22 On the other hand, TSP1 has also been suggested to act as a reductase that controls VWF multimer size, and thereby may negatively regulate thrombus growth. 22,23 Second, TSP1 could directly interact with its counter receptors on platelets, particularly CD36, 20 to mediate platelet adhesion under arterial shear and thereby promote thrombosis. Although these studies suggest VWF dependent and independent roles for TSP1 in arterial thrombosis, definitive in vivo evidence for either was still lacking.…”

“…22,23,[30][31][32] However, in vivo evidence to support this mechanism remains unclear and questionable. This prompted us to investigate the role of TSP1 in arterial thrombosis in the context of VWF deficiency.…”

“…21 TSP1 has also been shown to regulate multimer size of circulating VWF, or VWF released from activated platelets. 22,23 Together, these studies suggest that TSP1 may modulate arterial thrombosis via VWF; however, definitive in vivo evidence to support this mechanism remains to be elucidated.…”

Thrombospondins: from structure to therapeutics

“…Under some conditions, TSP1 promotes platelet aggregation [92], but recombinant domains and peptides derived from TSP1 can either increase or delay platelet aggregation [93][94][95][96]. TSP1 also regulates the processing of the platelet adhesion factor von Willebrand factor (vWF) by ADAMTS13 [97], which is associated with increased collagen-and vWF-mediated aggregation of TSP-1 null platelets under static and shear conditions [98]. However, a general physiological role for TSP1 in hemostasis was put in doubt by the failure to find clotting defects in mice with a disrupted Thbs1 gene, whose platelets lacked any detectable thrombospondins [97,99].…”