von Willebrand factor self-association on platelet GpIbα under hydrodynamic shear: effect on shear-induced platelet activation

The function of the mechanosensitive, multimeric blood protein von Willebrand factor (VWF) is dependent on its size. We tested the hypothesis that VWF may self-associate on the platelet glycoprotein Ib alpha (GpIb alpha) receptor under hydrodynamic shear. Consistent with this proposition, whereas Alexa-488- conjugated VWF (VWF-488) bound platelets at modest levels, addition of unlabeled VWF enhanced the extent of VWF-488 binding. Recombinant VWF lacking the A1-domain was conjugated with Alexa-488 to produce Delta A1-488. Although Delta A1-488 alone did not bind platelets under shear, this protein bound GpIb alpha on addition of either purified plasma VWF or recombinant full-length VWF. The extent of self-association increased with applied shear stress more than similar to 60 to 70 dyne/cm(2). Delta A1-488 bound platelets in the milieu of plasma. On application of fluid shear to whole blood, half of the activated platelets had Delta A1-488 bound, suggesting that VWF self-association may be necessary for cell activation. Shearing plate-lets with 6-mu m beads bearing either immobilized VWF or anti-GpIb alpha mAbresulted in cell activation at shear stress down to 2 to 5 dyne/cm(2). Taken together, the data suggest that fluid shear in circulation can increase the effective size of VWF bound to platelet GpIb alpha via protein self-association. This can trigger mechanotransduction and cell activation by enhancing the drag force applied on the cell-surface receptor. (Blood. 2010;116(19):3990-3998)