The function of ultra-large von Willebrand factor multimers in high shear flow controlled by ADAMTS13.

The paradigm that platelet aggregation, which contributes to bleeding arrest and also to thrombovascular disorders, initiates after signaling-induced platelet activation has been refuted in past recent years. Platelets can form aggregates independently of activation when soluble von Willebrand factor (VWF) is present and the shear rate exceeds a certain threshold where active A1 domains become exposed in soluble VWF multimers and can bind to platelet glycoprotein Ib. Subsequently - fostering each other - VWF can self-assemble into large nets combining with platelets into large conglomerates, which are entirely reversible when they enter a flow region with shear rates below the threshold. In addition the threshold changes from approximately 20 000 s⁻¹ in wall parallel flow to approximately 10 000 s⁻¹ in stagnation point flow. VWF containing ultra-large multimers - as when just released from endothelial storage sites - has been shown to have the highest binding potential to platelets and to each other, thus facilitating rapid platelet accrual to sites of vessel injury and exposed subendothelial structures, i.e. collagen. The VWF nets as well as the platelet-VWF conglomerates are controlled by the cleaving protease ADAMTS13 within minutes under high shear flow. Therewith the hemostatic potential is delivered where needed and the thrombogenic potential is highly controlled twofold: by flow and enzymatic proteolytic cleavage.