Hydrodynamic forces regulate platelet activation and self-association of vWF in suspension

Shear-induced platelet aggregation (SIPA) occurs under conditions of abnormally high shear rates such as those encountered in stenosed arteries. This has been associated with a variety of pathological conditions such as cerebral ischemia and acute myocardial infarction. SIPA is initiated by the flow induced binding of a plasma protein von Willebrand factor (vWF) to the platelet receptor GpIb. We quantitatively examined the aspects of fluid flow that regulate this interaction by following the time-course of platelet activation. Platelet rich plasma (PRP) was subjected to a range of shear conditions in a cone-plate viscometer and platelet activation levels were determined by measuring Annexin V binding to the platelet surface. These experiments revealed that i) Fluid shear stress rather than shear rate controls platelet activation, and a threshold shear stress of ∼60 dyn/cm² is necessary to induce significant activation. Under these conditions, forces of the order of 1 pN are applied on soluble vWF molecules. ii) Nonlinear and time-variant shear flow do not significantly enhance the rate of cell activation. iii) Fluid forces may regulate platelet activation levels by enhancing vWF proteolysis. In other experiments, purified vWF was subjected to shear in the viscometer and vWF morphology was assessed using light scattering spectroscopy. These studies demonstrate, for the first time, the ability of hydrodynamic forces to induce vWF self-association in suspension. This result raises the possibility that vWF self-association may be an additional feature involved in controlling thrombus formation rates in circulation.