Decreased expression of phospholipase C-β2 isozyme in human platelets with impaired function

Platelets from a patient with a mild inherited bleeding disorder and abnormal platelet aggregation and secretion show reduced generation of inositol 1,4,5-trisphosphate, mobilization of intracellular Ca²⁺, and phosphorylation of pleckstrin in response to several G protein mediated agonists, suggesting a possible defect at the level of phospholipase C (PLC) activation (see accompanying report). A procedure was developed that allows quantitation of platelet PLC isozymes. After fractionation of platelet extracts high-performance liquid chromatography, 7 out of 10 known PLC isoforms were detected immunoblot analysis. The amount of these isoforms in normal platelets decreased in the order PLC-γ2 > PLC-β2 > PLC-β3 > PLC- β1 > PLC-γ1 > PLC-δ1 > PLC-β4. Compared with normal platelets, platelets from the patient contained approximately one-third the amount of PLC-β2, whereas PLC-β4 was increased threefold. These results suggest that the impaired platelet function in the patient in response to multiple G protein mediated agonists is attributable to a deficiency of PLC-β2. They document for the first time a specific PLC isozyme deficiency in human platelets and provide an unique opportunity to understand the role of different PLC isozymes in normal platelet function.