Intracellular platelet signalling as a target for drug development.

Platelets are endowed with a repertoire of surface receptors that enable them to adhere, activate and aggregate upon vascular injury. Platelet adhesion is governed by the interaction between vascular collagen and GPIb-IX-V and GPVI-FcRγ complexes. Platelet kinases downstream 14-3-3ζ-bound GPIb and the FcRγ ITAM domain enable the activation of PLC-γ2 whereas the engagement of soluble agonists (predominantly ADP, TXA2 and thrombin) with Gq-protein coupled receptor trigger PLC-β activation. Once activated, PLC-γ2/β induces the generation of second messengers IP3 and DAG. IP3 is involved in Ca2+ cytosolic release from the dense tubular system whereas DAG induces PKC activation. CalDAG-GEFI sensors Ca2+ mobilization and, through activation of the small GTPase Rap1, induces cytoskeleton re-arrangements, extrusion of platelet granules and conversion of integrin αIIbβ3 into a high-affinity state ("inside-out" signalling). These events are found to be reinforced by PKC, MAPK, and ROS-dependent GPVI pathways. Finally, ligand-interaction with αIIbβ3 bridges platelets together and triggers "outside-in" signalling that orchestrates cytoskeletal rearrangements for platelet spreading and clot stabilization through the PI3K/PDK1/Akt/GSK3 axis. Understanding the platelet signalling machinery involved in thrombus formation is necessary to identify potential targets for the development of new antiplatelet agents.