Glycoprotein Ibα Forms Catch Bonds with von Willebrand Factor A1 Domain but Not with Mutant A1 Domains Exhibiting Properties of Type 2B von Willebrand Disease.

Interactions between glycoprotein Ibα (GPIbα) and von Willebrand factor (VWF) initiate platelet adhesion to injured vascular surfaces, which is enhanced by arterial blood flow. The flow requirement for adhesion is reduced (i.e., gain-of-function) by single-residue substitutions of the VWF-A1 domain, e.g., R543Q that occurs in some patients with type 2B von Willebrand disease (VWD) and R687E that was designed to exhibit type 2B VWD properties. Yet the mechanisms for flow-enhanced adhesion through GPIbα-VWF interactions are not understood. By probing single bonds with atomic force microscopy, we showed that lifetimes of GPIbα/VWF-A1 bonds first increased (catch) and then decreased (slip) with increasing force applied to the bond. Remarkably, the catch bond aspect of the GPIbα/VWF-A1 bonds was eliminated by the R543Q and R687E mutations, which exhibited slip bonds only with prolonged lifetimes at low forces. Flow chamber experiments showed that catch-slip transitional bonds governed flow-enhanced rolling of platelets and GPIbα-coated microspheres on wild-type A1, such that rolling velocities first decreased and then increased with increasing flow. By contrast, slip bonds governed rolling velocities on R543Q and R687E A1 mutants, which increased monotonically with increasing flow. We changed fluid viscosity by adding Ficoll to the medium, tether force by using microspheres of different radii, and platelet deformability by using a chemical fixative. The rolling velocity vs. flow curves aligned with tether force but not with transport parameters and were minimally affected by fixation, which respectively rule out transport-enhanced GPIbα/VWF-A1 bond formation and force-induced enlargement of platelet-surface contact area as the causes for flow-enhanced rolling. Flowing platelets agglutinated with microspheres bearing R543Q and R687E A1 mutants but not with those bearing wild-type A1, suggesting that GPIbα/VWF-A1 catch bonds prevent agglutination of circulating platelets via binding to VWF multimers and that platelet agglutination in patients with type 2B VWD may be explained by the prolonged lifetimes at low forces resulting from elimination of catch bonds in the interactions of GPIbα with VWF-A1 mutants.