Abstract 74: Thrombospondin 1 Modulates Arterial Thrombosis via von Willebrand Factor in Mice

Abstract

Background and Hypothesis: Thrombospondin 1 (TSP1) is a large adhesive glycoprotein that is stored in platelet α-granules. Upon platelet activation, TSP1 is released in large amount (up to 30 μg/mL) in circulation. Previous studies have shown smaller plasma von Willebrand factor (VWF) multimer size in Tsp1 -/- mice compared with wild-type (WT), suggesting that TSP1 protects VWF from excessive proteolysis by ADAMTS13. It remains unclear whether TSP1/VWF axis determines the rate of thrombus growth in injured vessels. We tested the hypothesis that TSP1 modulates arterial thrombosis via VWF. Model and Methods: We generated Vwf -/- / Tsp1 -/- mice and compared with Tsp1 -/- , Vwf -/- and WT mice. Platelet adhesion and susceptibility to thrombosis was assessed in a ferric-chloride injury-induced mesenteric artery thrombosis model. Using high-resolution intravital microscopy, we measured the time to form 1 st thrombus (>20 μm), thrombus growth kinetics and occlusion time in the injured arterioles. Results: We found that the number of fluorescently labeled platelets that adhered transiently to 250μm vessel wall segment within 3-4 min following ferric-chloride injury was similar in Tsp1 -/- mice compared with WT mice. The mean time to form first thrombus was significantly increased in Tsp1 -/- mice compared with WT (9.98±0.52 vs. 7.12±0.55min, P<0.05). The rate of thrombus growth was decreased in Tsp1 -/- mice ( P <0.05 vs. WT). The time to stable occlusion of the mesenteric artery was modestly but significantly prolonged in Tsp1 -/- mice compared with WT mice (15.8±0.59 vs. 13.3±0.63min, P<0.05). Because TSP1 binds to A3 domain of VWF, we investigated TSP1 modulates thrombosis via binding to VWF. Compared to Vwf -/- mice, Vwf -/- / Tsp1 -/ - littermates exhibited similar platelet adhesion, time to form first thrombus, and non-occlusive thrombus growth kinetics, suggesting that TSP1 modulates thrombus growth via VWF. Conclusion: We provide evidence for the first time that TSP1 does not mediate initial platelet adhesion to injured vessel wall but modulates arterial thrombus growth via a mechanism that requires VWF. These results suggest that TSP1 may participate in amplifying platelet aggregation following injury by protecting VWF from excessive cleavage by ADAMTS13.