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 b y aggregation and secretion of their granule contents. Platelets lacking th e alpha-subunit of the heterotrimeric G protein G(q) do not aggregate and d egranulate but still undergo shape change after activation through thrombox ane-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 G al pha(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 G alpha(q)-deficient platelets. Both MLC phosphorylation and shape change in duced through G(12)/G(13) in the absence of G alpha(q) were inhibited by th e C3 exoenzyme from Clostridium botulinum, by the Rho-kinase inhibitor Y-27 632 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.