The metalloprotease ADAMTS13 cleaves highly adhesive ultra large von Willebrand factor (UL-VWF) multimers after their release from the endothelium. UL-VWF is stored in Weibel-Palade bodies of endothelial cells. ADAMTS13 deficiency is linked to a life threatening disorder, thrombotic thrombocytopenic purpura (TTP), which is characterized by VWF and platelet-rich thrombi in the microvasculature, and may lead to organ ischemia, neurological dysfunction and ultimately death. We previously confirmed in vivo that in Adamts13−/− mice and not in wild-type mice “strings of platelets” formed after endothelial activation. The VWF strings remain attached at one end to the endothelium and “waved” the other end in the blood stream. In addition, we showed prolonged adhesion of platelets to secreted VWF on endothelium of Adamts13−/− mice. The possible role of ADAMTS13 in platelet plug formation has not been addressed previously. We investigated thrombosis in venules and arterioles of Adamts13−/− mice by intravital microscopy. Mesenteric microvenules were stimulated with calcium ionophore A23187, a secretagouge of Weibel-Palade bodies. In the Adamts13−/− mice we observed spontaneous thrombus formation in the activated microvenules. Inhibitory antibody to plasma ADAMTS13 induced thrombi formation in the activated microvenules and prolonged adhesion of platelets to secreted VWF on endothelium of wild-type mice. In the ferric chloride arterial injury model we found that the number of fluorescent platelets deposited within 2–3 min on the sub-endothelium after injury was unexpectedly higher in Adamts13−/− mice as compared to Adamts13+/+ mice (P<0.05). In the Adamts13−/− mice, platelet plug grew faster as thrombi >30 um were seen at 6.6±0.9 min compared to 10.8±0.8 min in the Adamts13+/+ mice (P<0.005). The platelet plug reached occlusive size in 10.6±0.7 min in Adamts13−/− mice, whereas in the Adamts13+/+ mice all the vessels were still open at this time. In the Adamts13+/+, the mean vessel occlusion time was 16.7±1.2 min after injury (P<0.0005). Thus ADAMTS13, by cleaving VWF multimers, down regulates both platelet adhesion to exposed subendothelium and thrombus growth. Furthermore, we observed that recombinant human ADAMTS13 inhibited formation of VWF platelet strings in histamine-activated venules and promoted thrombi dissolution in the Adamts13−/− mice. Our findings reveal that ADAMTS13 has natural anti-thrombotic activity and recombinant ADAMTS13 could be used as an anti-thrombotic agent.

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