A novel pathway of phospholipase Cg2 activation downstream of the platelet integrin aIIbb3

Abstract number: P0226

Wonerow P., Pearce A., Watson S. P., Vaux D. J.

University of Oxford, UK

Adhesion of platelets to fibrinogen leads to reorganization of the platelet cytoskeleton via the integrin aIIbb3. Many of the proteins that participate in this outside-in signaling cascade are regulated by the collagen receptor GPVI, including FcR g-chain, Syk, SLP-76, Btk, Vav and PLCg2. Important differences, however, exist between the two signaling cascades including a dramatic tyrosine phosphorylation of FAK but absence of phosphorylation of the raft-associated adapter LAT downstream of aIIbb3. In further contrast to signaling by GPVI, tyrosine phosphorylation of PLCg2 by aIIbb3 is unaltered in murine platelets which lack the FcR g-chain or LAT but is abolished in the presence of cytochalasin D. aIIbb3 outside-in signaling activates PLCg2, as demonstrated by measurement of phosphatidic acid in PLCg2^-/- mice. aIIbb3 also elevates cytosolic Ca²⁺ through a pathway that is inhibited by cytochalasin D, the tyrosine kinase inhibitor PP2 and the PLC inhibitor U71322. Spreading of human platelets on fibrinogen is dependent on Ca²⁺ elevation and activation of PLC. These results describe a novel pathway of Ca²⁺ elevation downstream of the platelet integrin aIIbb3 which supports platelet responses on fibrinogen-coated surfaces. aIIbb3 activation of PLCg2 is ITAM- and LAT-independent but requires cytoskeletal reorganizations.