Irreversible Platelet Activation Requires PAR1 Regulation of Phosphatidylinositol Phosphates (PIP_ns) Activation of Rap1.

Thrombin is the most potent activator of platelets, through reversible and then irreversible aggregation and mediates a diverse range of vascular effects which may result in acute coronary syndrome and stroke. We demonstrate that PAR1 signals human platelet activation through generation of a phosphatidylinositol phosphate signaling pathway; inhibition of this pathway blocked the formation of a stable platelet aggregate. Early Rap1 activation and platelet aggregation are insensitive to perturbations along the PI-kinase pathway, while stable aggregate formation requires this pathway. Computational lipidomics identified a single phosphatidic acid species required for PAR1-, but not PAR4-mediated Rap1 activation and platelet aggregation. Both PI5K and PI3Kg were required for stable PAR1-mediated platelet aggregation, and we show that these enzymes are present in a complex with activated Rap1. This novel lipid signaling pathway is a potential target for anti-platelet therapy.