Impaired activation of murine platelets lacking G alpha(i2)

The intracellular signaling pathways by which G protein-coupled receptors on the platelet surface initiate aggregation, a critical process for hemostasis and thrombosis, are not well understood. In particular, the contribution of the G(i) pathway has not been directly addressed. We have investigated the activation of platelets from mice in which the gene for the predominant platelet G alpha(i) subtype, G alpha(i2), has been disrupted. In intact platelets from G alpha(i2)-deficient mice, the inhibition of adenylyl cyclase by ADP was found to be partially impaired compared with wild-type platelets. Moreover, both ADP-dependent platelet aggregation and the activation of the integrin alpha IIb beta 3 (GPIIb-IIIa) were strongly reduced in platelets from G alpha(i2)-deficient mice. In addition, G alpha(i2)-deficient platelets displayed impaired activation at low thrombin concentrations. This defect was mimicked by blocking the adenylyl cyclase--coupled platelet ADP receptor (P2Y(12)) on wild-type platelets with a selective antagonist. These observations suggest that G alpha(i2) is involved in the inhibition of platelet adenylyl cyclase in vivo and is a critical component of the signaling pathway for integrin activation by ADP, resulting in platelet aggregation. In addition, thrombin-dependent activation of mouse platelets is mediated, at least in part, by secreted ADP acting on the G alpha(i2)-linked ADP receptor.