ABSTRACT: Receptor-mediated platelet activation requires phospholipase C (PLC) activity to elevate intracellular calcium and induce actin cytoskeleton reorganization. PLCs are classified into structurally distinct ?, ?, ?, ?, ?, and ? isoforms. There are two PLC? isoforms (PLC?1, PLC?2), which are critical for activation by tyrosine kinase-dependent receptors. Platelets express both PLC?1 and PLC?2. Although PLC?2 has been shown to play a dominant role in platelet activation, the extent to which PLC?1 contributes has not been evaluated. To ascertain the relative contributions of PLC?1 and PLC?2 to platelet activation, we generated conditionally PLC?1-deficient, wild-type (WT), PLC?2-deficient, and PLC?1/PLC?2 double-deficient mice and measured the ability of platelets to respond to different agonists. We found that PLC?2 deficiency abrogated ?IIb?3-dependent platelet spreading, GPVI-dependent platelet aggregation, and thrombus formation on collagen-coated surfaces under shear conditions, which is dependent on both GPVI and ?IIb?3. Addition of exogenous ADP overcame defective spreading of PLC?2-deficient platelets on immobilized fibrinogen, suggesting that PLC?2 is required for granule secretion in response to ?IIb?3 ligation. Consistently, ?IIb?3-mediated release of granule contents was impaired in the absence of PLC?2. In contrast, PLC?1-deficient platelets spread and released granule contents normally on fibrinogen, exhibited normal levels of GPVI-dependent aggregation, and formed thrombi normally on collagen-coated surfaces. Interestingly, enforced expression of PLC?1 fully restored GPVI-dependent aggregation and ?IIb?3-dependent spreading of PLC?2-deficient platelets. We conclude that platelet activation through GPVI and ?IIb?3 utilizes PLC?2 because PLC?1 levels are insufficient to support responsiveness, but that PLC?1 can restore responsiveness if expressed at levels normally achieved by PLC?2.
