Activation of G(12)/G(13) Results in Shape Change and Rho/Rho-Kinase–mediated Myosin Light Chain Phosphorylation in Mouse Platelets

Platelets respond to various stimuli with rapid changes in shape followed by aggregation and secretion of their granule contents. Platelets lacking the α-subunit of the heterotrimeric G protein G(q) do not aggregate and degranulate but still undergo shape change after activation through thromboxane-A(2 )(TXA(2)) or thrombin receptors. In contrast to thrombin, the TXA(2) mimetic U46619 led to the selective activation of G(12) and G(13) in Gα(q)-deficient platelets indicating that these G proteins mediate TXA(2) receptor-induced shape change. TXA(2) receptor-mediated activation of G(12)/G(13) resulted in tyrosine phosphorylation of pp72(syk) and stimulation of pp60(c-src) as well as in phosphorylation of myosin light chain (MLC) in Gα(q)-deficient platelets. Both MLC phosphorylation and shape change induced through G(12)/G(13) in the absence of Gα(q) were inhibited by the C3 exoenzyme from Clostridium botulinum, by the Rho-kinase inhibitor Y-27632 and by cAMP-analogue Sp-5,6-DCl-cBIMPS. These data indicate that G(12)/G(13) couple receptors to tyrosine kinases as well as to the Rho/Rho-kinase–mediated regulation of MLC phosphorylation. We provide evidence that G(12)/G(13)-mediated Rho/Rho-kinase–dependent regulation of MLC phosphorylation participates in receptor-induced platelet shape change.