Platelet hyperreactivity and a prothrombotic phenotype in mice with a gain‐of‐function mutation in phospholipase Cγ2

Summary. Background: Agonist‐induced platelet activation involves different signaling pathways leading to the activation of phospholipase C (PLC) β or PLCγ2. Activated PLC produces inositol 1,4,5‐trisphosphate and diacylglycerol, which trigger Ca²⁺ mobilization and the activation of protein kinase C, respectively. PLCβ is activated downstream of Gq‐coupled receptors for soluble agonists with only short interaction times in flowing blood. In contrast, PLCγ2 becomes activated downstream of receptors that interact with immobilized ligands such as the collagen receptor glycoprotein (GP) VI or activated integrins. Objective and methods: We speculated that PLCγ2 activity might be optimized for sustained but submaximal signaling to control relatively slow platelet responses. To test this hypothesis, we analyzed platelets from mice heterozygous for a gain‐of‐function mutation in the Plcg2 gene (Plcg2^Ali5/+). Results: Plcg2^Ali5/+ platelets showed enhanced Ca²⁺ mobilization, integrin activation, granule secretion and phosphatidylserine exposure upon GPVI or C‐type lectin‐like receptor‐2 stimulation. Furthermore, integrin α_IIbβ₃ outside‐in signaling was markedly enhanced in the mutant platelets, as shown by accelerated spreading on different matrices and faster clot retraction. These defects translated into virtually unlimited thrombus formation on collagen under flow in vitro and a prothrombotic phenotype in vivo. Conclusions: These results demonstrate that the enzymatic activity of PLCγ2 is tightly regulated to ensure efficient but limited platelet activation at sites of vascular injury.