Plasmin-mediated cleavage of GPIbα contributes to breakdown of platelet-von Willebrand factor complexes

Abstract Von Willebrand factor (VWF) is an essential contributor to hemostasis through its interaction with the platelet glycoprotein (GP) Ibα receptor. VWF is cleaved by ADAMTS13 to limit its prothrombotic properties. Failure to do so can result in platelet-VWF complexes that occlude the microcirculation, as seen in thrombotic thrombocytopenic purpura (TTP). In this setting, plasmin becomes active to cleave VWF, forming a distinct plasmin-generated cleavage product of VWF (cVWF) that is detectable during acute attacks in patients with TTP and following therapeutic plasminogen activation in a mouse model of TTP. However, it remains unclear whether plasmin-mediated proteolysis of VWF alone accounts for the breakdown of platelet-VWF complexes. Using ristocetin-induced platelet agglutinations, we show that plasmin cleavage of VWF does not impair its platelet-binding capacity, whereas plasmin-mediated cleavage of GPIbα reduces the ability of platelets released from agglutinates to bind VWF. Furthermore, platelets in suspension are relatively resistant to plasmin cleavage. We therefore propose that VWF binding may enhance GPIbα cleavage by recruiting plasmin(ogen) to the platelet surface. In a TTP mouse model, plasminogen activation led to a VWF-dependent reduction in GPIbα detectability, although to a lesser extent than observed in vitro. In patients with acute TTP, soluble GPIbα levels were elevated, indicating increased GPIbα shedding during attacks of thrombotic microangiopathy, although the extent to which this is plasmin-mediated remains unclear. Together, our findings demonstrate that plasmin cleavage of GPIbα drives the disruption of ristocetin-induced agglutinates, while its contribution to the breakdown of platelet-VWF complexes in vivo appears limited.