Shear-induced integrin signaling in platelet phosphatidylserine exposure, microvesicle release, and coagulation.

It is currently unclear why agonist-stimulated platelets require shear force to efficiently externalize the procoagulant phospholipid phosphatidylserine (PS) and release PS-exposed microvesicles (MVs). We reveal that integrin outside-in signaling is an important mechanism for this requirement. PS exposure and MV release were inhibited in β platelets or by integrin antagonists. The impaired MV release and PS exposure in β platelets were rescued by expression of wild-type β but not a Gα binding-deficient β mutant (EEE to AAA), which blocks outside-in signaling but not ligand binding. Inhibition of Gα or Src also diminished agonist/shear-dependent PS exposure and MV release, further indicating a role for integrin outside-in signaling. PS exposure in activated platelets was induced by application of pulling force via an integrin ligand, which was abolished by inhibiting Gα-integrin interaction, suggesting that Gα-dependent transmission of mechanical signals by integrins induces PS exposure. Inhibition of Gα delayed coagulation in vitro. Furthermore, inhibition or platelet-specific knockout of Gα diminished laser-induced intravascular fibrin formation in arterioles in vivo. Thus, β integrins serve as a shear sensor activating the Gα-dependent outside-in signaling pathway to facilitate platelet procoagulant function. Pharmacological targeting of Gα-integrin interaction prevents occlusive thrombosis in vivo by inhibiting both coagulation and platelet thrombus formation.