Von Willebrand factor and platelet adhesion to the vessel wall

Platelet adhesion to damaged arterial blood vessel walls relies on the ability of multimeric von Willebrand factor (vWF) to interact with its platelet receptor GPIb in high shear conditions. The vWF-GPIb interaction results in rolling of the platelets over the damaged area, and by this slowing down, another receptor, the alphaIIbbeta3 integrin, is involved to recruit circulating platelets from the blood flow, forming a platelet aggregate. The mechanism of thrombus growth in high shear conditions thus requires two platelet receptors, which bind to two distinct domains of vWF, the A I domain binds to GPIb while the RGDS sequence binds to alphaIIbbeta3. We have focused our studies on the high shear-dependent mechanism of platelet aggregate formation. Using mutated recombinant vWF in the A1 domain, expressing a gain-of-function mutation, we have studied shear-induced platelet aggregation (SIPA) in a coaxial cylinder shearing device, which allows shear rates between 200 and 4,000 sec(-1). We found a full aggregation in the presence of this gain-of-function mutated rvWF. Surprisingly, we could demonstrate that SIPA was completely dependent on the vWFGPIb interaction, while alphaIIbbeta3 was not involved in thrombus forming. We have then attempted to analyse the tyrosine phosphorylation events resulting from the vWF-GPIb-high shear interaction. Following exposure to shear, platelet lysates were separated on SDS-PAGE and blotted with antibodies against phosphorylated proteins or phosphorylated p125FAK. Our data suggest that a novel signalling pathway is involved which lead to increased phosphorylation of p125-FAK in the process of platelet aggregation without any involvement of the alphaIIbbeta3 integrin.