Mechanism of platelet adhesion to von Willebrand factor and microparticle formation under high shear stress

We describe here the mechanism of platelet adhesion to immobilized von Willebrand factor (VWF) and subsequent formation of platelet-derived microparticles mediated by glycoprotein lb alpha (GPlb alpha) under high shear stress. As visualized in whole blood perfused in a flow chamber, platelet attachment to VWF involved one or few membrane areas of 0.05 to 0.1 mu M-2 that formed discrete adhesion points (DAPs) capable of resisting force in excess of 160 pN. Under the influence of hydrodynamic drag, membrane tethers developed between the moving platelet body and DAPs firmly adherent to immobilized VWF. Continued stretching eventually caused the separation of many such tethers, leaving on the surface tube-shaped or spherical microparticles with a diameter as low as 50 to 100 nm. Adhesion receptors (GPIb alpha, aIIb beta 3) and phosphatidylserine were expressed on the surface of these microparticles, which were procoagulant. Shearing platelet-rich plasma at the rate of 10 000 s(-1) in a cone-and-plate viscosimeter increased microparticle counts up to 55-fold above baseline. Blocking the GPlb-VWF interaction abolished microparticle generation in both experimental conditions. Thus, a biomechanical process mediated by GPlb alpha-VWF bonds in rapidly flowing blood may not only initiate platelet arrest onto reactive vascular surfaces but also generate procoagulant microparticles that further enhance thrombus formation.