The relative role of PLCβ and PI3Kγ in platelet activation

Stimulation of platelet G protein-coupled receptors results in the cleavage of phosphatidylinositol 4,5-tris phosphate (PIP(2)) into inositol 1,4,5-trisphosphate and 1,2-diacylglycerol by phospholipase C (PLC beta). It also results in the phosphorylation Of PIP2 by the gamma isoform of phosphatidylinositol 3-kinase (PI3K gamma) to synthesize phosphatidylinositol 3,4,5-trisphosphate. To understand the role of PIP2 in platelet signaling, we evaluated knock-out mice lacking 2 isoforms of PLC beta (PLC beta 2 and PLC beta 3) or lacking the G(beta gamma)-activated isoform of PI3K (PI3K gamma). 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 beta 2/beta 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 gamma 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 beta 2/beta 3(-/-) and PI3K gamma(-/-) 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 beta 2/beta 3 and PI3K gamma play vital roles in platelet cytoskeletal dynamics.