RGS/Gi2α interactions modulate platelet accumulation and thrombus formation at sites of vascular injury

Although much is known about extrinsic regulators of platelet function such as nitric oxide and prostaglandin I-2 (PGI(2)), considerably less is known about intrinsic mechanisms that prevent overly robust platelet activation after vascular injury. Here we provide the first evidence that regulators of G-protein signaling (RGS) proteins serve this role in platelets, using mice with a G184S substitution in G(i2 alpha) that blocks RGS/G(i2) interactions to examine the consequences of lifting constraints on G(i2)-dependent signaling with-out altering receptor: effector coupling. The results show that the G(i2 alpha)(G184S) allele enhances platelet aggregation in vitro and increases platelet accumulation after vascular injury when expressed either as a global knock-in or limited to hematopoietic cells. Biochemical studies show that these changes occur in concert with an attenuated rise in cyclic adenosine monophosphate levels in response to prostacyclin and a substantial increase in basal Akt activation. In contrast, basal cyclic adenosine monophosphate (cAMP) levels, agonist-stimulated increases in [Ca++](i), Rap1 activation, and alpha-granule secretion were unaffected. Collectively, these observations (1) demonstrate an active role for RGS proteins in regulating platelet responsiveness, (2) show that this occurs in a pathway-selective manner, and (3) suggest that RGS proteins help to prevent unwarranted platelet activation as well as limiting the magnitude of the normal hemostatic response. (Blood. 2010; 116(26):6092-6100)