Platelet von Willebrand factor--structure, function and biological importance.

Besides circulating in normal plasma, von Willebrand factor (VWF) is also stored at relatively high concentration within the alpha-granules of platelets. This pool of platelet VWF exists distinct from plasma VWF, and is enriched in haemostatically-active high molecular weight multimers. Interestingly, the glycosylation profile of platelet VWF differs significantly from that of plasma VWF. Total sialic acid and galactose expression are reduced twofold on platelet VWF, and ABO blood group carbohydrate determinants are not present on the N-linked glycans of platelet VWF. Consequently, in view of the critical role played by VWF glycans in modulating its activity, it is not surprising that the functional properties of platelet VWF differ markedly compared to those of plasma VWF. Nevertheless, animal model studies suggest that both plasma and platelet VWF play important roles in securing primary haemostasis. In addition, platelet VWF antigen and activity levels vary markedly between patients with different types of von Willebrand disease (VWD). Future studies to define the biochemical mechanisms responsible for these differences between plasma and platelet VWF are thus not only of basic scientific interest, but also of direct translational importance.