Platelets Lacking Pip5ki Gamma Have Normal Integrin Activation But Impaired Cytoskeletal-Membrane Integrity And Adhesion

Three isoforms of phosphatidylinositol-4-phosphate 5-kinase (PIP5KI alpha, PIP5KI beta, and PIP5KI gamma) can each catalyze the final step in the synthesis of phosphatidylinositol-4,5-bisphosphate (PIP2), which in turn can be either converted to second messengers or bind directly to and thereby regulate proteins such as talin. A widely quoted model speculates that only p90, a longer splice form of platelet-specific PIP5KI gamma, but not the shorter p87 PIP5KI gamma, regulates the ligand-binding activity of integrins via talin. However, when we used mice genetically engineered to lack only p90 PIP5KI gamma, we found that p90 PIP5KI gamma is not critical for integrin activation or platelet adhesion on collagen. However, p90 PIP5KI gamma-null platelets do have impaired anchoring of their integrins to the underlying cytoskeleton. Platelets lacking both the p90 and p87 PIP5KI gamma isoforms had normal integrin activation and actin dynamics, but impaired anchoring of their integrins to the cytoskeleton. Most importantly, they formed weak shear-resistant adhesions ex vivo and unstable vascular occlusions in vivo. Together, our studies demonstrate that, although PIP5KI gamma is essential for normal platelet function, individual isoforms of PIP5KI gamma fulfill unique roles for the integrin-dependent integrity of the membrane cytoskeleton and for the stabilization of platelet adhesion. (Blood. 2013;121(14):2743-2752)