Abstract 18994: Shear-Dependent Membrane Scrambling and Microvesiculation and an Important Role for Rac1

Abstract

Surface exposure of phosphatidylserine (PS) and the release of membrane microvesicles (microparticles) are critically important for blood platelets and leukocytes to promote coagulation and inflammation during vascular injury. The caspase-dependent apoptotic signaling pathway and high intracellular calcium-dependent mechanisms have been implicated in the induction of these events under static conditions. However, it is known that when stimulated with physiological agonists under static conditions only a small percentage of platelets actually respond in a procoagulant manner. Thus, these mechanisms do not fully explain the potent procoagulant response of platelets to physiological stimuli. Here we demonstrate a novel shear stress-dependent mechanism of PS exposure and microvesiculation in platelets stimulated by physiological agonists. We show that exposing agonist-stimulated mouse and human platelets to increasing levels of shear led to greatly increased levels of surfaced exposed PS, shedding of membrane microvesicles, and in particular an increased sensitivity of platelets to display procoagulant activity at low concentrations of physiological agonists. Thus, shear stress is critical for inducing the surface exposure of PS and the shedding of PS-exposed microvesicles in platelets. Moreover, using the Rac1 inhibitor NSC23766 and platelet-specific Rac1 -/- platelets we identified Rac1, a member of the Rho family small GTPases, as being a critical mediator of the platelet procoagulant response to shear. We also show that Rac1 regulates this shear-induced response independent of the known apoptosis signaling pathway, as Rac1 -/- platelets showed no difference in the caspase-dependent pathway of PS exposure induced by ABT737. Importantly, the role of Rac1 in mediating shear-dependent PS exposure and microvesicle release appears to be independent of its known role in stimulating platelet secretion, and represents a common pathway of PS exposure and microvesicle release induced via several different agonists. This novel PS exposure and microvesculation mechanism is important for the function of platelets to promote coagulation.