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 beta(3) (-/-) platelets or by integrin antagonists. The impaired MV release and PS exposure in beta(3) (-/-) platelets were rescued by expression of wild-type b3 but not a Ga13 binding-deficient beta(3) mutant ((EEE)-E-733 to AAA), which blocks outside-in signaling but not ligand binding. Inhibition of G alpha(13) 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 G alpha(13)-integrin interaction, suggesting that G alpha(13)-dependent transmission of mechanical signals by integrins induces PS exposure. Inhibition of G alpha(13) delayed coagulation in vitro. Furthermore, inhibition or platelet-specific knockout of G alpha(13) diminished laser-induced intravascular fibrin formation in arterioles in vivo. Thus, beta(3) integrins serve as a shear sensor activating the G alpha(13)-dependent outside-in signaling pathway to facilitate platelet procoagulant function. Pharmacological targeting of G alpha(13)-integrin interaction prevents occlusive thrombosis in vivo by inhibiting both coagulation and platelet thrombus formation.