Apoptosis, or programmed cell death, is appreciated as the main physiologic mechanism that regulates cell life-span and serves for controlled deletion of unwanted cells. Since its discovery in 1972, apoptosis was long attributed exclusively to nucleate cells. It took more than 20 years to recognize apoptosis in enucleated cells cytoplasts and anucleate platelets. During the following years, apoptosis has been demonstrated in platelets treated with natural and artificial agonists, in platelet concentrates aged during storage under standard blood banking conditions, and in animal models of suppressed thrombopoiesis and thrombocytopenia. Other studies documented that mechanical forces (shear stresses) stimulate platelet activation and signaling in the absence of exogenous chemical stimuli. We analysed whether shear stresses can trigger platelet apoptosis, a question that has not yet been studied. Using a cone-and-plate viscometer (CAP-2000, Brookfield Engineering Labs, Inc., Middleboro, MA), we exposed human platelet-rich plasma to different shear stresses, ranging from physiologic arterial and arterioles levels (10–44 dynes/cm2) to pathologic high levels (117–388 dynes/cm2) occurring in stenosed coronary, peripheral or cerebral arteries. We found that pathologic shear stresses induce not only platelet activation (P-selectin upregulation and GPIb-alpha downregulation) but also trigger apoptosis events, including mitochondrial transmembrane potential depolarization, caspase 3 activation, phosphatidylserine exposure, and platelet shrinkage and fragmentation into microparticles, whereas physiologic shear stresses are not effective. Platelets subjected to pathologic shear stresses are characterized by impaired platelet function as shown by the absence of ADP-induced platelet aggregation. Apoptosis changes were also induced by the treatment of platelets with calcium ionophore A23187 (10 μM) and thrombin (1 U/mL). Thus, in the present work, we have demonstrated that platelet apoptosis can be induced by chemical stimuli and by mechanical rheological forces (pathologic high shear stresses). Most of shear-induced activation and apoptosis events occur inside of the platelet, including translocation of CD62 from alpha-granules to the platelet surface, depolarization of mitochondrial inner membrane potential, activation of cytosolic enzyme caspase 3, and translocation of phosphatidylserine from the inner to the outer plasma membrane leaflet. These data suggest that the effects of shear stress on platelet activation and apoptosis are mediated by mechanoreceptor(s) that transmit activation and apoptosis signals to the cell interior. The platelet paradigm of apoptosis induced by chemical agonists and shear stresses suggests that apoptotic cytoplasmic machinery may function without nuclear participation.

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