Activation of G(12)/G(13) results in shape change and Rho/Rho-kinase-mediated myosin light chain phosphorylation in mouse platelets

Abstract

Platelets respond to various stimuli with rapid changes in shape followed by aggregation and secretion of their granule contents. Platelets lacking the alpha-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 G alpha(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 induced through G(12)/G(13) in the absence of G alpha(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.