Regular Article PLATELETS AND THROMBOPOIESIS Both TMEM16F-dependent and TMEM16F-independent pathways contribute to phosphatidylserine exposure in platelet apoptosis and platelet activation

Transbilayer lipid asymmetry in platelets and other cells is maintained by an aminophospholipid translocase, an active transporter pumping aminophospholipids phosphatidylserine (PS) and phosphatidylethanolamine from the outer to the inner leaflet of the plasma membrane. Under particular conditions, phospholipid asymmetry is disrupted by a “scrambling” process, in which the phospholipids become increasingly randomized over both plasma membrane leaflets. As a consequence, PS, which in resting cells is exclusively located in the inner leaflet, becomes exposed at the membrane outer leaflet and can be probed with the PS-binding proteins annexin A5 or lactadherin. For apoptotic cells, externalized PS provides a signal for removal by the mononuclear phagocytic system. In blood cells, PS-exposing membranes strongly promote the process of thrombin generation and fibrin clot formation. In platelets, phospholipid scrambling and PS externalization are induced by activation with strong agonists like convulxin/thrombin and collagen/thrombin, stimulating glycoprotein VI, and the platelet thrombin receptors. Known requirements for agonist-induced phospholipid scrambling are a sustained high intracellular Ca concentration and loss of the mitochondrial membrane potential. Hence, platelets lacking cyclophilin D, a positive modulator of the mitochondrial permeability transition pore (MPTP), are protected against PS externalization upon convulxin/thrombin stimulation. Scott syndrome is a rare bleeding disorder in which the Ca-dependent phospholipid scrambling of platelets and other blood cells is impaired. Although agonist-induced Ca responses were found to be unchanged, Scott syndrome platelets show diminished PS exposure upon stimulation with convulxin/thrombin, collagen/thrombin, or Ca-ionophore ionomycin compared with healthy subjects. Recently, TMEM16F was recognized as a protein critically involved in Ca-induced phospholipid scrambling. A homozygous mutation in TMEM16F (IVS12–1G→T, causing premature termination of translation) was found in the genome of the propositus Scott patient. Another Scott syndrome patient appeared to be compound heterozygous in TMEM16F, suggesting that a defective TMEM16F is the cause of the Scott syndrome phenotype. Besides the agonist-induced pathway, platelets possess a distinct pathway to PS exposure, which was shown to be elicited with ABT737 or ABT263 (Navitoclax), both of which are BH-3 mimetics that activate caspase-dependent apoptosis in a Bax/Bakdependent manner. This apoptotic pathway may play a role in the clearance of aging platelets. As Bcl-xL is gradually degraded