Modulation of the von Willebrand factor-dependent platelet adhesion through alternative proteolytic pathways

INTRODUCTION: Platelet adhesion to collagen under high shear rates depends on the optimal size of the von Willebrand factor (VWF) multimers, which is determined by their limited proteolysis. The present study attempts to identify the role of hemostatic-fibrinolytic enzymes (thrombin, plasmin) and leukocyte-derived proteases (matrix metalloproteinase (MMP)-8, MMP-9, neutrophil elastase) in the cleavage of VWF and to characterize the effect of flow and platelets on this proteolysis and its functional consequences on platelet adhesion. Methods and results According to VWF immunoblots, plasmin, neutrophil elastase and thrombin at concentrations of in vivo relevance resulted in extensive degradation of VWF within several minutes. Platelets protected VWF against this proteolysis under static conditions, whereas perfusion of the proteases at 3350 s(-1) shear rate over VWF immobilized on artery cross sections enhanced its degradation and blocked the protective effect of platelets. In parallel with VWF digestion, the examined proteases impaired the VWF-dependent platelet adhesion as reflected in the decreased surface-bound GpIIb/IIIa immunoreactivity following perfusion of collagen-coated surfaces or artery sections with blood and plasmin, neutrophil elastase or thrombin. Within the time frame of minutes no VWF cleavage could be detected under static or flow conditions after exposure to MMP-8 and MMP-9 at concentrations relevant to physiological neutrophil counts. CONCLUSION: Our results indicate a shear- and platelet-dependent role for several proteases in the local modulation of the VWF function.