The relative role of PLC and PI3K in platelet activation

Stimulation of platelet G protein–coupled receptors results in the cleavage of phosphatidylinositol 4,5-trisphosphate (PIP2) into inositol 1,4,5-trisphosphate and 1,2diacylglycerol by phospholipase C (PLC ). It also results in the phosphorylation of PIP2 by the isoform of phosphatidylinositol 3-kinase (PI3K ) to synthesize phosphatidylinositol 3,4,5trisphosphate. To understand the role of PIP2 in platelet signaling, we evaluated knock-out mice lacking 2 isoforms of PLC (PLC 2 and PLC 3) or lacking the G -activated isoform of PI3K (PI3K ). Both knock-out mice were unable to form stable thrombi in a carotid injury model. To provide a functional explanation, knock-out platelets were studied ex vivo. PLC 2/ 3 / platelets failed to assemble filamentous actin, had defects in both secretion and mobilization of intracellular calcium, and were unable to form stable aggregates following low doses of agonists. Platelets lacking PI3K disaggregated following low-dose adenosine diphosphate (ADP) and had a mildly impaired ability to mobilize intracellular calcium. Yet, they exhibited essentially normal actin assembly and secretion. Remarkably, both PLC 2/ 3 / and PI3K / platelets spread more slowly upon fibrinogen. These results suggest substantial redundancy in platelet signaling pathways. Nonetheless, the diminished ability of knock-out platelets to normally spread after adhesion and to form stable thrombi in vivo suggests that both PLC 2/ 3 and PI3K play vital roles in platelet cytoskeletal dynamics. (Blood. 2005;106:110-117)