Gating and Regulatory Mechanisms of TMEM16 Ion Channels and Scramblases

Abstract

The transmembrane protein 16 (TMEM16) family consists of Ca2+-activated ion channels and Ca2+-activated phospholipid scramblases (CaPLSases) that passively flip-flop phospholipids between the two leaflets of the membrane bilayer. Owing to their diverse functions, TMEM16 proteins have been implicated in various human diseases, including asthma, cancer, bleeding disorders, muscular dystrophy, arthritis, epilepsy, dystonia, ataxia, and viral infection. To understand TMEM16 proteins in health and disease, it is critical to decipher their molecular mechanisms of activation gating and regulation. Structural, biophysical, and computational characterizations over the past decade have greatly advanced the molecular understanding of TMEM16 proteins. In this review, we summarize major structural features of the TMEM16 proteins with a focus on regulatory mechanisms and gating.

Functional coupling between TRPV4 channel and TMEM16F modulates human trophoblast fusion

“…TMEM16F KO BeWo cells lack Ca 2+ -, voltage- and time-dependent TMEM16F conductance ( Figure 3—figure supplement 1 ); and GSK101 only triggers small, outward-rectifying TRPV4 conductance ( Figure 3 ). It is worth noting that, for reasons unclear, there is always a long delay to activate TMEM16F CaPLSase and channel activities in the whole-cell configuration ( Le et al, 2021 ; Yu et al, 2015 ; Grubb et al, 2013 ; Shimizu et al, 2013 ; Lin et al, 2018 ). Using whole-cell patch clamp-lipid scrambling fluorometry (PCLSF) to monitor both ionic current and PS exposure ( Yu et al, 2015 ; Liang and Yang, 2021 ), we confirm that TMEM16F-mediated ion and phospholipid permeation happen almost simultaneously 7–10 min after GSK101 application ( Figure 3—figure supplement 2 ).…”

“…However, the apparent Ca 2+ sensitivity of TMEM16F is lower than that of TMEM16A and TMEM16B. Differing from CaCCs that are readily activated by elevated cytosolic Ca 2+ from either cell surface Ca 2+ channels or from internal stores ( Yang et al, 2012 ; Cabrita et al, 2017 ; Genovese et al, 2019 ; Jin et al, 2013 ; Shah et al, 2020 ; Zhang et al, 2017 ; Takayama et al, 2014 ), TMEM16F activation requires more robust and sustained intracellular Ca 2+ elevation ( Suzuki et al, 2010 ; Le et al, 2021 ; Yu et al, 2015 ; Yang et al, 2012 ; Grubb et al, 2013 ; Liang and Yang, 2021 ; Shimizu et al, 2013 ; Ye et al, 2018 ; Ye et al, 2019 ). Ca 2+ ionophores are widely used to artificially trigger TMEM16F activation and these pharmacological tools allow us to study the biophysical properties or functional expression of TMEM16F in native cells ( Suzuki et al, 2010 ; Le et al, 2019 ; Yu et al, 2015 ; Zhang et al, 2020 ).…”

“…Phospholipids are essential building blocks of mammalian cell membranes and are asymmetrically distributed between the inner and outer leaflets ( Bevers and Williamson, 2016 ; Nagata et al, 2016 ). TMEM16F is a Ca 2+ -activated phospholipid scramblase (CaPLSase) that catalyzes trans-bilayer movement of the phospholipids down their concentration gradients ( Suzuki et al, 2010 ; Le et al, 2021 ). In response to increased intracellular Ca 2+ , phosphatidylserine (PS), an anionic phospholipid that is usually sequestered in the inner membrane leaflet, can rapidly permeate through a hydrophilic pathway of TMEM16F and become exposed on the cell surface ( Le et al, 2019 ; Feng et al, 2019 ; Alvadia et al, 2019 ; Yu et al, 2015 ).…”

“…Ca 2+ is required for TMEM16 activation ( Le et al, 2021 ; Tien et al, 2014 ; Yu et al, 2012 ; Brunner et al, 2014 ; Pedemonte and Galietta, 2014 ). Ca 2+ binds to the highly conserved Ca 2+ binding sites to activate TMEM16 proteins including TMEM16F CaPLSase and TMEM16A and TMEM16B Ca 2+ -activated Cl − channels (CaCCs) ( Le et al, 2021 ; Yang et al, 2012 ; Pedemonte and Galietta, 2014 ). However, the apparent Ca 2+ sensitivity of TMEM16F is lower than that of TMEM16A and TMEM16B.…”

PIEZO channels and newcomers in the mammalian mechanosensitive ion channel family

“…At variance, however, OSCA1.1 and OSCA1.2 have higher mechanical activation threshold and detectable permeability for chloride ions. High-resolution cryo-EM structures of OSCA1.2 reveals a symmetrical dimeric architecture (Figure 1C) and topological similarities to TMEM16 proteins (TMEM16A/ANO1, TMEM16B/ANO2), a family of ion channels that includes Ca 2+ -activated Cl À channels and Ca 2+activated phospholipid scramblases (Jojoa-Cruz et al, 2018;Le et al, 2021;Zhang et al, 2018b). The ion permeation pathway of OSCA1.2 lies within each subunit, and current model for activation suggests expansion of its cross-sectional area in response to membrane tension, a principle common for many MS ion channels (Guo and MacKinnon, 2017).…”