TMEM16F Plays a Vital Role In Platelet Microparticle Generation and Thrombosis

Transmembrane (TMEM)16f is member of the TMEM16 family of ion channels, recently shown to be essential for optimal Ca2+-dependent phospholipid scrambling in platelets, platelet-dependent pro-coagulant activity and thrombosis. However, relatively little is known about the effect of TMEM16f on platelet signaling, functional activity, and microparticle formation. TMEM16f-/- mice were obtained from Andrea Vortkamp and the platelets of these mice were isolated, stimulated with either dual agonist (thrombin + convulxin) or calcium ionophore (A23187), and platelet surface exposure of phosphatidylserine (PS) and microparticle generation were assessed by both flow cytometry and high resolution immunofluorescent confocal microscopy. To measure PS exposure, annexin V binding to platelet surfaces was quantified: In response to thrombin and convulxin, only 10.65% ± 1.35% of TMEM16f-/- platelets exposed PS after dual agonist exposure, compared to 16.1% ± 2.3%PS+ WT platelets when analyzed by high resolution microscopy. When analyzed by flow cytometry, dual agonist exposure of WT platelets yielded 52.4% ± 7.0% of maximal annexin V binding achieved by 10μM A23187; in contrast, there was no significant increase in annexin V binding detected in TMEM16f-/- platelets treated with dual agonist. Responses to calcium ionophore were also reduced in TMEM16f-/- platelets relative to WT (17.3% ± 8.3% PS+ TMEM16f-/- platelets in response to 1μM A23187 compared to 40% ± 1.4% PS+ WT platelets, by high resolution microscopy). Microparticle generation from TMEM16f-/- platelets compared to WT platelets was also evaluated by high resolution immunofluorescent microscopy and flow cytometry. On average, approximately 5.7 ± 0.33 microparticles were generated by untreated WT platelets however thrombin and convulxin treated platelets yielded 10.66 ± 0.92 microparticles per platelet whereas no significant increase in microparticle generation was observed in TMEM16f-/- platelets after dual agonist treatment. These data suggest that TMEM16f is required not only for PS exposure on platelet membranes, but also for the shedding of PS+ microparticles. TMEM16f-/- mice also have a significant defect in stable occlusive thrombus formation following 10% ferric chloride injury of the carotid artery (0/6 TMEM16f-/- mice form stable thrombi compared to 5/6 WT mice forming stable thrombi under the same conditions). TMEM16f-/- platelets had a slight (15%) reduction in total fibrinogen binding stimulated by PAR4 peptide agonist compared to WT, but showed no significant differences in aggregation to PAR4 agonist peptide or ADP compared to WT control platelets, suggesting that the defect in thrombus formation is likely due to PS- or microparticle-dependent procoagulant activity. This hypothesis is supported by preliminary results demonstrating that when microparticles isolated from ionophore-stimulated WT platelets were injected into TMEM16f knockout mice, 3/3 TMEM16f-/- mice formed stable thrombi, compared to 5/6 WT mice injected with vehicle control. In conclusion, TMEM16f-/- mice are deficient in platelet PS exposure, platelet-derived microparticle formation, and injury-induced thrombus formation, and platelet-derived microparticles appear to contribute to the defect in thrombus formation.