Modulation of von Willebrand Factor A1 Domain-Mediated Platelet Aggregation/Agglutination by α-Thrombin.

Evaluation of the crystal structures of the amino terminal domain of platelet glycoprotein (GP) Ibα bound to the von Willebrand factor A1 domain (VWFA1) or to α-thrombin indicate the absence of significant steric hindrance in a putative triple complex of the two ligands interacting with the same receptor molecule. Superposition of the models reveals that intermolecular contacts may be established between VWFA1 and α-thrombin concurrently bound to GP Ibα, and suggests that these additional interactions could stabilize the intrinsically low affinity binding of the VWF A1 domain. To verify the predictions of the model, we used gel electrophoresis under native conditions and purified components in solution to demonstrate directly the formation of a triple complex. We then sought to evaluate whether α-thrombin could influence the functional effects of the VWF-GP Ibα interaction. For this purpose, we established a model of platelet agglutination/aggregation dependent on the interaction between recombinant dimeric VWFA1 domain, purified from the culture medium of stably transfected D. melanogaster cell lines, and GP Ibα. In this assay, platelet rich plasma prepared from individual donor blood collected with the thrombin inhibitor D-phenyl alanyl-L-prolyl-L-arginine chloromethyl ketone dihydrochloride (PPACK) as an anticoagulant (80 μM) was mixed with varying concentrations of dimeric VWFA1 (0.5-10 μg/ml) and exposed to variable shear rate levels in a cone-and-plate viscometer. Platelet aggregation was observed at shear rates between 6 and 108 dyn/cm². The response in different normal controls was reproducible but variable in extent, and individuals could be assigned to one of two categories, low responder and high responder. An agglutination response was observed after platelets were treated with 10 μM prostaglandin E1 to block activation, and the distinction between low and high responders remained true under these conditions. For simplicity, agglutinated platelets were still defined as “aggregates”. With activation blocked platelets, aggregates were stable up to a shear rate of 30 dyn/cm², but began to dissipate at higher levels. The addition of α-thrombin with the active site irreversibly blocked by PPACK at concentrations between 5 and 10 μg/ml substantially increased the extent of the platelet response. This was demonstrated by a faster rate of platelet agglutination/aggregation, a greater stability of aggregates at higher shear rates, and an overall increase in the size of aggregates formed. To demonstrate the latter, samples were exposed to shear stress under selected conditions and immediately fixed with 1% glutaraldehyde for quantitative image analysis. Maximum aggregate size was increased several fold in the presence of α-thrombin, and the difference was particularly evident in low responder individuals in whom dimeric VWFA1 alone caused the formation of small and unstable aggregates. PPACK-blocked thrombin by itself had no effect on platelet aggregate formation at any shear rate tested. Our findings delineate a mechanism through which α-thrombin may stabilize platelet-platelet contacts by mediating a tighter association between VWF A1 domain and GP Ibα receptor. Such a function, independent of proteolytic activity, may enhance platelet deposition at sites of vascular injury.