This Reactome event shows Ca²⁺-dependent, anoctamin-6 (ANO6)-mediated translocation of phosphatidylserine (PS) and phosphatidylethanolamine (PE) to the external side of the platelet cell surface. Externalized anionic phospholipids serve as a platform for assembly of coagulation complexes.
Sustained increase of intracellular Ca²⁺ disrupts asymmetrical distribution of phospholipids in the cell membranes by inhibiting flippase activity and inducing phospholipid scrambling activity (reviewed by Sakuragi T & Nagata S 2023). ANO6 (also known as TMEM16F) functions as a Ca²⁺-induced phospholipid scramblase (Suzuki J et al., 2010; Clark SR et al., 2013; reviewed by Kunzelmann K et al., 2014). ANO6 translocates negatively charged PS and PE from the inner leaflet to the outer leaflet of the plasma membrane.
In procoagulant platelets, exposed PS (and PE) creates a negatively charged membrane surface, promoting recruitment of blood clotting factors to the platelet cell surface, facilitating the formation of the tenase FIXa:FVIIIa and prothrombinase FXa:FVa complexes on the PS-exposing platelets (reviewed by Lentz BR 2003; Majumder R 2022). FXa:FVa converts prothrombin (FII) to thrombin (FIIa), which in turn activates more FV, FVIII and FXI in positive feedback loops. Furthermore, thrombin cleaves platelet-bound fibrinogen to generate fibrin monomers that upon polymerization form a platelets-fibrin clot.
The significance of ANO6 function in blood coagulation is evident in patients with Scott syndrome, a bleeding disorder, where loss-of-function mutations in ANO6 lead to diminished PS exposure on the platelet surface (Suzuki J et al., 2010; Montague SJ et al., 2024). The defective scramblase activity and bleeding phenotype was also observed in ANO6 (TMEM16F) knockout mice, which serve as a model for Scott syndrome (Yang H et al., 2012; Fujii T et al., 2015; Mattheij NJA et al., 2016).
In addition to its (Ca²⁺)-dependent phospholipid scramblase activity, ANO6 can function as an ion channel (Yang H et al., 2012). Structural and biochemical studies using mouse Ano6 reveal that Ca²⁺ binds Ano6 directly to initiate both ion currents and lipid translocation (Alvadia C et al., 2019; Le T et al., 2019; Feng S et al., 2019). However, ANO6 undergoes two alternate conformational rearrangements to regulate these two functions (Alvadia C et al., 2019; Le T et al., 2019; Feng S et al., 2019, 2023; Jia Z et al., 2022). Scramblase activity of mammalian ANO6 may also contribute to extracellular vesiculation in various cell types including platelets, neutrophils and T cells (Fujii T et al., 2015; Headland SE et al., 2015; Hu Y et al., 2016; Bricogne C et al., 2019; Han TW et al., 2019).
