The initial event in platelet activation is the reorganization of the cytoskeleton causing the platelets to change from a discoid to a spiculated spheroid shape. Platelet shape change is primarily regulated by the phosphorylation of myosin light chain kinase. We have shown that this process is mediated through both calcium-dependent and calcium-independent Rho kinase pathways. CPI-17, a Protein kinase C (PKC) phosphorylated inhibitory protein of myosin light chain phosphatase, has been shown to have a role in platelet shape change downstream of thrombin-induced platelet activation. CPI-17 is a 17 kDa protein expressed in human platelets shown to inhibit myosin light chain phosphotase activity via PKCs. In this study we examined the role of CPI-17 in ADP-induced shape change and phosphorylation of CPI-17, downstream of the Gq coupled, P2Y1, and the Gi coupled, P2Y12 receptors. CPI-17 phosphorylation occurred upon activation of platelets with 2MeSADP. This phosphorylation was abolished in the presence of the P2Y1 receptor antagonist, MRS-2179. These results indicated that Gq signaling is important for platelet shape change and phosphorylation of CPI-17. In the presence of the calcium chelator, BAPTA, platelets changed shape in response to 2MeSADP; CPI-17 phosphorylation, however, was unaffected by BAPTA treatment under these conditions. However, CPI-17 phosphorylation was inhibited in the presence of the pan PKC inhibitors. These results indicate that CPI-17 phosphorylation occurs downstream of PKC activation. In the presence of BAPTA, treatment with PKC inhibitors decreased platelet shape change possibly due to reduced CPI-17 phosphorylation. The shape change caused by p160ROCK downstream of G12/13 pathways was unaffected by pan PKC inhibitors, but abolished by p160ROCK inhibitors H1152 or Y27632. Platelets incubated with BAPTA, pan PKC inhibitors, and p160ROCK inhibitor H1152, abolished ADP-induced platelet shape change and CPI-17 phosphorylation. In conclusion, ADP-induced platelet shape change occurs through a Gq-mediated, calcium-independent signaling pathway regulated by CPI-17 phosphorylation via PKC activation.

Author notes

Disclosure: No relevant conflicts of interest to declare.