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Like all eukaryotic cells, platelets maintain plasma membrane phospholipid asymmetry in normal blood circulation via lipid transporters, which control transbilayer movement. Upon platelet activation, the asymmetric orientation of membrane phospholipids is rapidly disrupted, resulting in a calcium‐dependent exposure of the anionic phospholipid, phosphatidylserine (PS), at the outer platelet surface. This newly‐exposed PS surface is a major component of normal hemostasis because it supports platelet procoagulant function. Binding of blood clotting enzyme complexes to this negatively‐charged membrane surface allows a dramatic increase in the rate of conversion of zymogens to active serine proteases, which in turn produce a burst of thrombin leading to the formation of a fibrin clot and further platelet activation. Cells have the capacity to catalyze transbilayer phospholipid exchange via ATP‐requiring translocase enzymes (flippases and floppases), which control unidirectional phospholipid transport against a concentration gradient. They also use an energy‐independent, calcium‐dependent scramblase activity to govern the bidirectional exchange of phospholipids between the two leaflets of the bilayer; this activity is essential for PS exposure during platelet activation. Scramblase activity, biochemically characterized in the 1980s, is deficient in patients with Scott syndrome, a rare inherited bleeding disorder with defective platelet procoagulant activity. Despite considerable efforts, the platelet scramblase protein remained elusive for years but a significant advance has recently been made with the identification of TMEM16F, a membrane protein essential for calcium‐dependent PS exposure whose loss of function mutations are found in Scott syndrome. This review recalls historical aspects of platelet membrane asymmetry characterization, summarizes the mechanisms and roles of PS exposure following platelet activation and discusses the recent identification of TMEM16F and its significance in the scrambling process.

Background: Heparin‐induced thrombocytopenia (HIT) is a rare but severe complication of heparin therapy in which immunoglobulin G (IgG) antibodies against the platelet factor 4–heparin complex activate platelets through the FcγRIIA receptor. Clustering of FcγRIIA initiates signaling cascades involving tyrosine kinases including the spleen tyrosine kinase (Syk). Moreover, besides the critical role of platelets, the expression of tissue factor (TF) by human monocytes triggered by HIT antibodies has been shown to contribute to the hypercoagulability and the thrombotic complications in HIT patients. Objectives: We investigated the effect of R406, a small molecule inhibitor of Syk developed as a potential treatment of autoimmune diseases, allergic disorders and B‐cell related hematological malignancies, on FcγRIIA‐mediated platelet activation. To further assess the potential activity of Syk inhibitors in HIT treatment, the effect of R406 was also evaluated on HIT antibodies‐induced expression of TF and procoagulant activity of monocytic cells. Results: We show that R406 is a potent inhibitor of platelet signaling and functions initiated by FcγRIIA cross‐linking by specific antibodies or by sera from HIT patients. Syk inhibition efficiently prevents FcγRIIA‐induced LAT phosphorylation and activation of phosphoinositide 3‐kinase, Akt, phospholipase Cγ2 and p38 MAP‐kinase. As a consequence, FcγRIIA‐induced platelet aggregation, granule secretion and microparticles production are strongly inhibited by R406. Moreover, the Syk inhibitor efficiently impairs the expression of TF and the procoagulant activity of human monocytes triggered by HIT antibodies. Conclusion: Syk inhibitors may be of therapeutic interest in the treatment of HIT by reducing HIT antibodies‐mediated platelet activation and monocyte procoagulant activity.