Structural basis for anion conduction in the calcium-activated chloride channel TMEM16A

Regulation of TMEM16A/ANO1 and TMEM16F/ANO6 ion currents and phospholipid scrambling by Ca2+ and plasma membrane lipid

KEY POINTS

TMEM16 proteins can operate as Ca2+ -activated Cl- channels or scramble membrane phospholipids, which are both highly relevant mechanisms during disease. Overexpression of TMEM16A and TMEM16F were found to be partially active at 37°C and at resting intracellular Ca2+ concentrations. We show that TMEM16 Cl- currents and phospholipid scrambling can be activated by modification of plasma membrane phospholipids, through reactive oxygen species and phospholipase A2. While phospholipids and Cl- ions are likely to use the same pore within TMEM16F, TMEM16A only conducts Cl- ions. Lipid regulation of TMEM16 proteins is highly relevant during inflammation and regulated cell death such as apoptosis and ferroptosis.

ABSTRACT

TMEM16/anoctamin (ANO) proteins form Ca2+ -activated ion channels or phospholipid scramblases. We found that both TMEM16A/ANO1 and TMEM16F/ANO6 produced Cl- currents when activated by intracellular Ca2+ , but only TMEM16F was able to expose phosphatidylserine to the outer leaflet of the plasma membrane. Mutations within TMEM16F or TMEM16A/F chimeras similarly changed Cl- currents and phospholipid scrambling, suggesting the same intramolecular pathway for Cl- and phospholipids. When overexpressed, TMEM16A and TMEM16F produced spontaneous Cl- currents at 37°C even at resting intracellular Ca2+ levels, which was abolished by inhibition of phospholipase A2 (PLA2 ). Connversely, activation of PLA2 or application of active PLA2 , as well as lipid peroxidation induced by reactive oxygen species (ROS) using staurosporine or tert-butyl hydroperoxide, enhanced ion currents by TMEM16A/F and in addition activated phospholipid scrambling by TMEM16F. Thus, TMEM16 proteins are activated by an increase in intracellular Ca2+ , or independent of intracellular Ca2+ , by modifications occurring in plasma and intracellular membrane phospholipids. These results may help to explain why regions distant to the TMEM16 pore and the Ca2+ binding sites control Cl- currents and phospholipid scrambling. Regulation of TMEM16 proteins through modification of membrane phospholipids occurs during regulated cell death such as apoptosis and ferroptosis. It contributes to inflammatory and nerve injury-induced hypersensitivity and generation of pain and therefore provides a regulatory mechanism that is particularly relevant during disease.

Phosphatidylinositol 4,5-bisphosphate, cholesterol, and fatty acids modulate the calcium-activated chloride channel TMEM16A (ANO1)

The TMEM16A-mediated Ca²⁺-activated Cl^- current drives several important physiological functions. Membrane lipids regulate ion channels and transporters but their influence on members of the TMEM16 family is poorly understood. Here we have studied the regulation of TMEM16A by phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2), cholesterol, and fatty acids using patch clamp, biochemistry and fluorescence microscopy. We found that depletion of membrane PI(4,5)P2 causes a decline in TMEM16A current that is independent of cytoskeleton, but is partially prevented by removing intracellular Ca²⁺. On the other hand, supplying PI(4,5)P2 to inside-out patches attenuated channel rundown and/or partially rescued activity after channel rundown. Also, depletion (with methyl-β-cyclodextrin M-βCD) or restoration (with M-βCD+cholesterol) of membrane cholesterol slows down the current decay observed after reduction of PI(4,5)P2. Neither depletion nor restoration of cholesterol change PI(4,5)P2 content. However, M-βCD alone transiently increases TMEM16A activity and dampens rundown whereas M-βCD+cholesterol increases channel rundown. Thus, PI(4,5)P2 is required for TMEM16A function while cholesterol directly and indirectly via a PI(4,5)P2-independent mechanism regulate channel function. Stearic, arachidonic, oleic, docosahexaenoic, and eicosapentaenoic fatty acids as well as methyl stearate inhibit TMEM16A in a dose- and voltage-dependent manner. Phosphatidylserine, a phospholipid whose hydrocarbon tails contain stearic and oleic acids also inhibits TMEM16A. Finally, we show that TMEM16A remains in the plasma membrane after treatment with M-βCD, M-βCD+cholesterol, oleic, or docosahexaenoic acids. Thus, we propose that lipids and fatty acids regulate TMEM16A channels through a membrane-delimited protein-lipid interaction.

Activation mechanism of the calcium-activated chloride channel TMEM16A revealed by cryo-EM

The calcium-activated chloride channel TMEM16A is a ligand-gated anion channel that opens in response to an increase in intracellular Ca²⁺ concentration. The protein is broadly expressed and contributes to diverse physiological processes, including transepithelial chloride transport and the control of electrical signalling in smooth muscles and certain neurons. As a member of the TMEM16 (or anoctamin) family of membrane proteins, TMEM16A is closely related to paralogues that function as scramblases, which facilitate the bidirectional movement of lipids across membranes. The unusual functional diversity of the TMEM16 family and the relationship between two seemingly incompatible transport mechanisms has been the focus of recent investigations. Previous breakthroughs were obtained from the X-ray structure of the lipid scramblase of the fungus Nectria haematococca (nhTMEM16), and from the cryo-electron microscopy structure of mouse TMEM16A at 6.6 Å (ref. 14). Although the latter structure disclosed the architectural differences that distinguish ion channels from lipid scramblases, its low resolution did not permit a detailed molecular description of the protein or provide any insight into its activation by Ca²⁺. Here we describe the structures of mouse TMEM16A at high resolution in the presence and absence of Ca²⁺. These structures reveal the differences between ligand-bound and ligand-free states of a calcium-activated chloride channel, and when combined with functional experiments suggest a mechanism for gating. During activation, the binding of Ca²⁺ to a site located within the transmembrane domain, in the vicinity of the pore, alters the electrostatic properties of the ion conduction path and triggers a conformational rearrangement of an α-helix that comes into physical contact with the bound ligand, and thereby directly couples ligand binding and pore opening. Our study describes a process that is unique among channel proteins, but one that is presumably general for both functional branches of the TMEM16 family.

Lipids and ions traverse the membrane by the same physical pathway in the nhTMEM16 scramblase

From bacteria to mammals, different phospholipid species are segregated between the inner and outer leaflets of the plasma membrane by ATP-dependent lipid transporters. Disruption of this asymmetry by ATP-independent phospholipid scrambling is important in cellular signaling, but its mechanism remains incompletely understood. Using MD simulations coupled with experimental assays, we show that the surface hydrophilic transmembrane cavity exposed to the lipid bilayer on the fungal scramblase nhTMEM16 serves as the pathway for both lipid translocation and ion conduction across the membrane. Ca²⁺ binding stimulates its open conformation by altering the structure of transmembrane helices that line the cavity. We have identified key amino acids necessary for phospholipid scrambling and validated the idea that ions permeate TMEM16 Cl^- channels via a structurally homologous pathway by showing that mutation of two residues in the pore region of the TMEM16A Ca²⁺-activated Cl^- channel convert it into a robust scramblase.

Poring over furrows

Cryo-electron microscopy reveals the structure of a chloride channel that is closely related to a protein that transports lipids.