Cellular volume regulation by anoctamin 6: Ca²⁺, phospholipase A2 and osmosensing

Functional Interdependence of Anoctamins May Influence Conclusions from Overexpression Studies

Anoctamin 6 (ANO6, TMEM16F) is a phospholipid (PL) scramblase that moves PLs between both plasma membrane (PM) leaflets and operates as an ion channel. It plays a role in development and is essential for hemostasis, bone mineralization and immune defense. However, ANO6 has also been shown to regulate cellular Ca2+ signaling and PM compartments, thereby controlling the expression of ion channels such as CFTR. Given these pleiotropic effects, we investigated the functional interdependence of the ubiquitous ANO6 with other commonly co-expressed anoctamins. As most expression studies on anoctamins use HEK293 human embryonic kidney cells, we compared ion currents, PL scrambling and Ca2+ signals induced by the overexpression of anoctamins in HEK293 wild-type parental and ANO6-knockout cells. The data suggest that the endogenous expression of ANO6 significantly affects the results obtained from overexpressed anoctamins, particularly after increasing intracellular Ca2+. Thus, a significant interdependence of anoctamins may influence the interpretation of data obtained from the functional analysis of overexpressed anoctamins.

The ion channel Anoctamin 10/TMEM16K coordinates organ morphogenesis across scales in the urochordate notochord

During embryonic development, tissues and organs are gradually shaped into their functional morphologies through a series of spatiotemporally tightly orchestrated cell behaviors. A highly conserved organ shape across metazoans is the epithelial tube. Tube morphogenesis is a complex multistep process of carefully choreographed cell behaviors such as convergent extension, cell elongation, and lumen formation. The identity of the signaling molecules that coordinate these intricate morphogenetic steps remains elusive. The notochord is an essential tubular organ present in the embryonic midline region of all members of the chordate phylum. Here, using genome editing, pharmacology and quantitative imaging in the early chordate Ciona intestinalis we show that Ano10/Tmem16k, a member of the evolutionarily ancient family of transmembrane proteins called Anoctamin/TMEM16 is essential for convergent extension, lumen expansion, and connection during notochord morphogenesis. We find that Ano10/Tmem16k works in concert with the plasma membrane (PM) localized Na+/Ca2+ exchanger (NCX) and the endoplasmic reticulum (ER) residing SERCA, RyR, and IP3R proteins to establish developmental stage specific Ca2+ signaling molecular modules that regulate notochord morphogenesis and Ca2+ dynamics. In addition, we find that the highly conserved Ca2+ sensors calmodulin (CaM) and Ca2+/calmodulin-dependent protein kinase (CaMK) show an Ano10/Tmem16k-dependent subcellular localization. Their pharmacological inhibition leads to convergent extension, tubulogenesis defects, and deranged Ca2+ dynamics, suggesting that Ano10/Tmem16k is involved in both the "encoding" and "decoding" of developmental Ca2+ signals. Furthermore, Ano10/Tmem16k mediates cytoskeletal reorganization during notochord morphogenesis, likely by altering the localization of 2 important cytoskeletal regulators, the small GTPase Ras homolog family member A (RhoA) and the actin binding protein Cofilin. Finally, we use electrophysiological recordings and a scramblase assay in tissue culture to demonstrate that Ano10/Tmem16k likely acts as an ion channel but not as a phospholipid scramblase. Our results establish Ano10/Tmem16k as a novel player in the prevertebrate molecular toolkit that controls organ morphogenesis across scales.

Broadening the clinical spectrum: molecular mechanisms and new phenotypes of ANO3-dystonia

Anoctamin 3 (ANO3) belongs to a family of transmembrane proteins that form phospholipid scramblases and ion channels. A large number of ANO3 variants were identified as the cause of craniocervical dystonia, but the underlying pathogenic mechanisms remain obscure. It was suggested that ANO3 variants may dysregulate intracellular Ca2+ signalling, as variants in other Ca2+ regulating proteins like hippocalcin were also identified as a cause of dystonia. In this study, we conducted a comprehensive evaluation of the clinical, radiological and molecular characteristics of four individuals from four families who carried heterozygous variants in ANO3. The median age at follow-up was 6.6 years (ranging from 3.8 to 8.7 years). Three individuals presented with hypotonia and motor developmental delay. Two patients exhibited generalized progressive dystonia, while one patient presented with paroxysmal dystonia. Additionally, another patient exhibited early dyskinetic encephalopathy. One patient underwent bipallidal deep brain stimulation (DBS) and showed a mild but noteworthy response, while another patient is currently being considered for DBS treatment. Neuroimaging analysis of brain MRI studies did not reveal any specific abnormalities. The molecular spectrum included two novel ANO3 variants (V561L and S116L) and two previously reported ANO3 variants (A599D and S651N). As anoctamins are suggested to affect intracellular Ca2+ signals, we compared Ca2+ signalling and activation of ion channels in cells expressing wild-type ANO3 and cells expressing anoctamin variants. Novel V561L and S116L variants were compared with previously reported A599D and S651N variants and with wild-type ANO3 expressed in fibroblasts isolated from patients or when overexpressed in HEK293 cells. We identified ANO3 as a Ca2+-activated phospholipid scramblase that also conducts ions. Impaired Ca2+ signalling and compromised activation of Ca2+-dependent K+ channels were detected in cells expressing ANO3 variants. In the brain striatal cells of affected patients, impaired activation of KCa3.1 channels due to compromised Ca2+ signals may lead to depolarized membrane voltage and neuronal hyperexcitability and may also lead to reduced cellular viability, as shown in the present study. In conclusion, our study reveals the association between ANO3 variants and paroxysmal dystonia, representing the first reported link between these variants and this specific dystonic phenotype. We demonstrate that ANO3 functions as a Ca2+-activated phospholipid scramblase and ion channel; cells expressing ANO3 variants exhibit impaired Ca2+ signalling and compromised activation of Ca2+-dependent K+ channels. These findings provide a mechanism for the observed clinical manifestations and highlight the importance of ANO3 for neuronal excitability and cellular viability.

The anoctamins: Structure and function

When activated by increase in intracellular Ca2+, anoctamins (TMEM16 proteins) operate as phospholipid scramblases and as ion channels. Anoctamin 1 (ANO1) is the Ca2+-activated epithelial anion-selective channel that is coexpressed together with the abundant scramblase ANO6 and additional intracellular anoctamins. In salivary and pancreatic glands, ANO1 is tightly packed in the apical membrane and secretes Cl-. Epithelia of airways and gut use cystic fibrosis transmembrane conductance regulator (CFTR) as an apical Cl- exit pathway while ANO1 supports Cl- secretion mainly by facilitating activation of luminal CFTR and basolateral K+ channels. Under healthy conditions ANO1 modulates intracellular Ca2+ signals by tethering the endoplasmic reticulum, and except of glands its direct secretory contribution as Cl- channel might be small, compared to CFTR. In the kidneys ANO1 supports proximal tubular acid secretion and protein reabsorption and probably helps to excrete HCO3-in the collecting duct epithelium. However, under pathological conditions as in polycystic kidney disease, ANO1 is strongly upregulated and may cause enhanced proliferation and cyst growth. Under pathological condition, ANO1 and ANO6 are upregulated and operate as secretory channel/phospholipid scramblases, partly by supporting Ca2+-dependent processes. Much less is known about the role of other epithelial anoctamins whose potential functions are discussed in this review.

VSI: The anoctamins: Structure and function: "Intracellular" anoctamins

Plasma membrane localized anoctamin 1, 2 and 6 (TMEM16A, B, F) have been examined in great detail with respect to structure and function, but much less is known about the other seven intracellular members of this exciting family of proteins. This is probably due to their limited accessibility in intracellular membranous compartments, such as the endoplasmic reticulum (ER) or endosomes. However, these so-called intracellular anoctamins are also found in the plasma membrane (PM) which adds to the confusion regarding their cellular role. Probably all intracellular anoctamins except of ANO8 operate as intracellular phospholipid (PL) scramblases, allowing for Ca2+-activated, passive transport of phospholipids like phosphatidylserine between both membrane leaflets. Probably all of them also conduct ions, which is probably part of their physiological function. In this brief overview, we summarize key findings on the biological functions of ANO3, 4, 5, 7, 8, 9 and 10 (TMEM16C, D, E, G, H, J, K) that are gradually coming to light. Compartmentalized regulation of intracellular Ca2+ signals, tethering of the ER to specific PM contact sites, and control of intracellular vesicular trafficking appear to be some of the functions of intracellular anoctamins, while loss of function and abnormal expression are the cause for various diseases.

Calcium chelation independent effects of BAPTA on endogenous ANO6 channels in HEK293T cells

Selective calcium chelator 1,2-Bis(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA) is a common tool to investigate calcium signaling. However, BAPTA expresses various effects on intracellular calcium signaling, which are not related to its ability to bind Ca2+. In patch clamp experiments, we investigated calcium chelation independent effects of BAPTA on endogenous calcium-activated chloride channels ANO6 (TMEM16F) in HEK293T cells. We have found that application of BAPTA to intracellular solution led to two distinct effects on channels properties. On the one hand, application of BAPTA acutely reduced amplitude of endogenous ANO6 channels induced by 10 μM Ca2+ in single channel recordings. On the other hand, BAPTA application by itself induced ANO6 channel activity in the absence of the intracellular calcium elevation. Open channel probability was enhanced by increasing the intracellular BAPTA concentration from 0.1 to 1 and 10 mM. Another calcium chelator EGTA did not demonstrate chelation independent effects on the ANO6 activity in the same conditions. Due to off-target effects BAPTA should be used with caution when studying calcium-activated ANO6 channels.

Hippo pathway in intestinal diseases: focusing on ferroptosis

The incidence of intestinal diseases, such as inflammatory bowel disease, gastric cancer, and colorectal cancer, has steadily increased over the past decades. The Hippo pathway is involved in cell proliferation, tissue and organ damage, energy metabolism, tumor formation, and other physiologic processes. Ferroptosis is a form of programmed cell death characterized by the accumulation of iron and lipid peroxides. The Hippo pathway and ferroptosis are associated with various intestinal diseases; however, the crosstalk between them is unclear. This review elaborates on the current research on the Hippo pathway and ferroptosis in the context of intestinal diseases. We summarized the connection between the Hippo pathway and ferroptosis to elucidate the underlying mechanism by which these pathways influence intestinal diseases. We speculate that a mutual regulatory mechanism exists between the Hippo pathway and ferroptosis and these two pathways interact in several ways to regulate intestinal diseases.

Chloride channels in the lung: Challenges and perspectives for viral infections, pulmonary arterial hypertension, and cystic fibrosis

Fine control over chloride homeostasis in the lung is required to maintain membrane excitability, transepithelial transport as well as intra- and extracellular ion and water homeostasis. Over the last decades, a growing number of chloride channels and transporters have been identified in the cells of the pulmonary vasculature and the respiratory tract. The importance of these proteins is underpinned by the fact that impairment of their physiological function is associated with functional dysregulation, structural remodeling, or hereditary diseases of the lung. This paper reviews the field of chloride channels and transporters in the lung and discusses chloride channels in disease processes such as viral infections including SARS-CoV- 2, pulmonary arterial hypertension, cystic fibrosis and asthma. Although chloride channels have become a hot research topic in recent years, remarkably few of them have been targeted by pharmacological agents. As such, we complement the putative pathophysiological role of chloride channels here with a summary of their therapeutic potential.

Polymodal Control of TMEM16x Channels and Scramblases

The TMEM16A/anoctamin-1 calcium-activated chloride channel (CaCC) contributes to a range of vital functions, such as the control of vascular tone and epithelial ion transport. The channel is a founding member of a family of 10 proteins (TMEM16x) with varied functions; some members (i.e., TMEM16A and TMEM16B) serve as CaCCs, while others are lipid scramblases, combine channel and scramblase function, or perform additional cellular roles. TMEM16x proteins are typically activated by agonist-induced Ca2+ release evoked by Gq-protein-coupled receptor (GqPCR) activation; thus, TMEM16x proteins link Ca2+-signalling with cell electrical activity and/or lipid transport. Recent studies demonstrate that a range of other cellular factors—including plasmalemmal lipids, pH, hypoxia, ATP and auxiliary proteins—also control the activity of the TMEM16A channel and its paralogues, suggesting that the TMEM16x proteins are effectively polymodal sensors of cellular homeostasis. Here, we review the molecular pathophysiology, structural biology, and mechanisms of regulation of TMEM16x proteins by multiple cellular factors.

Calmodulin-Dependent Regulation of Overexpressed but Not Endogenous TMEM16A Expressed in Airway Epithelial Cells

Regulation of the Ca²⁺-activated Cl⁻ channel TMEM16A by Ca²⁺/calmodulin (CAM) is discussed controversially. In the present study, we compared regulation of TMEM16A by Ca²⁺/calmodulin (holo-CAM), CAM-dependent kinase (CAMKII), and CAM-dependent phosphatase calcineurin in TMEM16A-overexpressing HEK293 cells and TMEM16A expressed endogenously in airway and colonic epithelial cells. The activator of the Ca²⁺/CAM-regulated K⁺ channel KCNN4, 1-EBIO, activated TMEM16A in overexpressing cells, but not in cells with endogenous expression of TMEM16A. Evidence is provided that CAM-interaction with TMEM16A modulates the Ca²⁺ sensitivity of the Cl⁻ channel. Enhanced Ca²⁺ sensitivity of overexpressed TMEM16A explains its activity at basal (non-elevated) intracellular Ca²⁺ levels. The present results correspond well to a recent report that demonstrates a Ca²⁺-unbound form of CAM (apo-CAM) that is pre-associated with TMEM16A and mediates a Ca²⁺-dependent sensitization of activation (and inactivation). However, when using activators or inhibitors for holo-CAM, CAMKII, or calcineurin, we were unable to detect a significant impact of CAM, and limit evidence for regulation by CAM-dependent regulatory proteins on receptor-mediated activation of endogenous TMEM16A in airway or colonic epithelial cells. We propose that regulatory properties of TMEM16A and and other members of the TMEM16 family as detected in overexpression studies, should be validated for endogenous TMEM16A and physiological stimuli such as activation of phospholipase C (PLC)-coupled receptors.

Ca2+ Dependence of Volume-Regulated VRAC/LRRC8 and TMEM16A Cl- Channels

All vertebrate cells activate Cl- currents (ICl ,swell) when swollen by hypotonic bath solution. The volume-regulated anion channel VRAC has now been identified as LRRC8/SWELL1. However, apart from VRAC, the Ca2+-activated Cl- channel (CaCC) TMEM16A and the phospholipid scramblase and ion channel TMEM16F were suggested to contribute to cell swelling-activated whole-cell currents. Cell swelling was shown to induce Ca2+ release from the endoplasmic reticulum and to cause subsequent Ca2+ influx. It is suggested that TMEM16A/F support intracellular Ca2+ signaling and thus Ca2+-dependent activation of VRAC. In the present study, we tried to clarify the contribution of TMEM16A to ICl ,swell. In HEK293 cells coexpressing LRRC8A and LRRC8C, we found that activation of ICl ,swell by hypotonic bath solution (Hypo; 200 mosm/l) was Ca2+ dependent. TMEM16A augmented the activation of LRRC8A/C by enhancing swelling-induced local intracellular Ca2+ concentrations. In HT29 cells, knockdown of endogenous TMEM16A attenuated ICl ,swell and changed time-independent swelling-activated currents to VRAC-typical time-dependent currents. Activation of ICl ,swell by Hypo was attenuated by blocking receptors for inositol trisphosphate and ryanodine (IP3R; RyR), as well as by inhibiting Ca2+ influx. The data suggest that TMEM16A contributes directly to ICl ,swell as it is activated through swelling-induced Ca2+ increase. As activation of VRAC is shown to be Ca2+-dependent, TMEM16A augments VRAC currents by facilitating Hypo-induced Ca2+ increase in submembraneous signaling compartments by means of ER tethering.

Lysophosphatidic acid-induced pro-thrombotic phosphatidylserine exposure and ionophore-induced microvesiculation is mediated by the scramblase TMEM16F in erythrocytes

Recent studies indicate that erythrocytes actively modulate blood clotting and thrombus formation. The lipid mediator lysophosphatidic acid (LPA) is produced by activated platelets, and triggers a signaling process in erythrocytes. This results in cellular calcium uptake and exposure of phosphatidylserine (PS) at the cell surface, thereby generating activated membrane binding sites for factors of the clotting cascade. Moreover, erythrocytes of patients with a bleeding disorder and mutations in the scramblase TMEM16F show impaired PS exposure and microvesiculation upon treatment with calcium ionophore. We report that TMEM16F inhibitors tannic acid (TA) and epigallocatechin-3-gallate (EGCG) inhibit LPA-induced PS exposure and calcium uptake at low micromolar concentrations; fluoxetine, an antidepressant and a known activator of TMEM16F, enhances these processes. These effectors likewise modulate erythrocyte PS exposure and microvesicle shedding induced by calcium ionophore treatment. Further, LPA-treated erythrocytes triggered thrombin generation in platelet-free plasma which was partially impaired in the presence of TA and EGCG. Thus, this study suggests that LPA activates the scramblase TMEM16F in erythrocytes, thereby possibly mediating a pro-thrombotic function in these cells. EGCG as well as fluoxetine, substances with potentially high plasma concentrations due to alimentation or medical treatment, should be considered as potential effectors of systemic hemostatic regulation.

Pharmacological Inhibition and Activation of the Ca2+ Activated Cl- Channel TMEM16A

TMEM16A is a Ca²⁺ activated Cl⁻ channel with important functions in airways, intestine, and other epithelial organs. Activation of TMEM16A is proposed as a therapy in cystic fibrosis (CF) to reinstall airway Cl⁻ secretion and to enhance airway surface liquid (ASL). This CFTR-agnostic approach is thought to improve mucociliary clearance and lung function in CF. This could indeed improve ASL, however, mucus release and airway contraction may also be induced by activators of TMEM16A, particularly in inflamed airways of patients with asthma, COPD, or CF. Currently, both activators and inhibitors of TMEM16A are developed and examined in different types of tissues. Here we compare activation and inhibition of endogenous and overexpressed TMEM16A and analyze potential off-target effects. The three well-known blockers benzbromarone, niclosamide, and Ani9 inhibited both TMEM16A and ATP-induced Ca²⁺ increase by variable degrees, depending on the cell type. Niclosamide, while blocking Ca²⁺ activated TMEM16A, also induced a subtle but significant Ca²⁺ store release and inhibited store-operated Ca²⁺ influx. Niclosamide, benzbromarone and Ani9 also affected TMEM16F whole cell currents, indicating limited specificity for these inhibitors. The compounds Eact, cinnamaldehyde, and melittin, as well as the phosphatidylinositol diC8-PIP₂ are the reported activators of TMEM16A. However, the compounds were unable to activate endogenous TMEM16A in HT₂₉ colonic epithelial cells. In contrast, TMEM16A overexpressed in HEK293 cells was potently stimulated by these activators. We speculate that overexpressed TMEM16A might have a better accessibility to intracellular Ca²⁺, which causes spontaneous activity even at basal intracellular Ca²⁺ concentrations. Small molecules may therefore potentiate pre-stimulated TMEM16A currents, but may otherwise fail to activate silent endogenous TMEM16A.

Trpv4 Mediates Hypotonic Inhibition of Central Osmosensory Neurons via Taurine Gliotransmission

The maintenance of hydromineral homeostasis requires bidirectional detection of changes in extracellular fluid osmolality by primary osmosensory neurons (ONs) in the organum vasculosum laminae terminalis (OVLT). Hypertonicity excites ONs in part through the mechanical activation of a variant transient receptor potential vanilloid-1 channel (dn-Trpv1). However, the mechanism by which local hypotonicity inhibits ONs in the OVLT remains unknown. Here, we show that hypotonicity can reduce the basal activity of dn-Trpv1 channels and hyperpolarize acutely isolated ONs. Surprisingly, we found that mice lacking dn-Trpv1 maintain normal inhibitory responses to hypotonicity when tested in situ. In the intact setting, hypotonicity inhibits ONs through a non-cell-autonomous mechanism that involves glial release of the glycine receptor agonist taurine through hypotonicity activated anion channels (HAAC) that are activated subsequent to Ca²⁺ influx through Trpv4 channels. Our study clarifies how Trpv4 channels contribute to the inhibition of OVLT ONs during hypotonicity in situ.

TMEM16A chloride channel does not drive mucus production

Despite being essential for airway hydration, TMEM16A is not required for mucus (MUC5AC) production. Cell proliferation is the main driver for TMEM16A up-regulation during inflammation.

Airway mucus obstruction is the main cause of morbidity in cystic fibrosis, a disease caused by mutations in the CFTR Cl⁻ channel. Activation of non-CFTR Cl⁻ channels such as TMEM16A can likely compensate for defective CFTR. However, TMEM16A was recently described as a key driver in mucus production/secretion. Here, we have examined whether indeed there is a causal relationship between TMEM16A and MUC5AC production, the main component of respiratory mucus. Our data show that TMEM16A and MUC5AC are inversely correlated during differentiation of human airway cells. Furthermore, we show for the first time that the IL-4–induced TMEM16A up-regulation is proliferation-dependent, which is supported by the correlation found between TMEM16A and Ki-67 proliferation marker during wound healing. Consistently, the notch signaling activator DLL4 increases MUC5AC levels without inducing changes neither in TMEM16A nor in Ki-67 expression. Moreover, TMEM16A inhibition decreased airway surface liquid height. Altogether, our findings demonstrate that up-regulation of TMEM16A and MUC5AC is only circumstantial under cell proliferation, but with no causal relationship between them. Thus, although essential for airway hydration, TMEM16A is not required for MUC5AC production.

Hypotonic stress response of human keratinocytes involves LRRC8A as component of volume-regulated anion channels

The barrier function of the human epidermis is constantly challenged by environmental osmotic fluctuations. Hypotonic stress triggers cell swelling, which is counteracted by a compensatory mechanism called regulatory volume decrease (RVD) involving volume-regulated anion channels (VRACs). Recently, it was discovered that VRACs are composed of LRRC8 heteromers and that LRRC8A functions as the essential VRAC subunit in various mammalian cell types; however, the molecular identity of VRACs in the human epidermis remains to be determined. Here, we investigated the expression of LRRC8A and its role in hypotonic stress response of human keratinocytes. Immunohistological staining showed that LRRC8A is preferentially localized in basal and suprabasal epidermal layers. RNA sequencing revealed that LRRC8A is the most abundant subunit within the LRRC8 gene family in HaCaT cells as well as in primary normal human epidermal keratinocytes (NHEKs). To determine the contribution of LRRC8A to hypotonic stress response, we generated HaCaT- and NHEK-LRRC8A knockout cells by using CRISPR-Cas9. I^- influx assays using halide-sensitive YFP showed that LRRC8A is crucially important for mediating VRAC activity in HaCaTs and NHEKs. Moreover, cell volume measurements using calcein-AM dye further revealed that LRRC8A also substantially contributes to RVD. In summary, our study provides new insights into hypotonic stress response and suggests an important role of LRRC8A as VRAC component in human keratinocytes.

Niclosamide repurposed for the treatment of inflammatory airway disease

Inflammatory airway diseases, such as asthma, cystic fibrosis (CF), and chronic obstructive pulmonary disease (COPD), are characterized by mucus hypersecretion and airway plugging. In both CF and asthma, enhanced expression of the Ca2+-activated Cl- channel TMEM16A is detected in mucus-producing club/goblet cells and airway smooth muscle. TMEM16A contributes to mucus hypersecretion and bronchoconstriction, which are both inhibited by blockers of TMEM16A, such as niflumic acid. Here we demonstrate that the FDA-approved drug niclosamide, a potent inhibitor of TMEM16A identified by high-throughput screening, is an inhibitor of both TMEM16A and TMEM16F. In asthmatic mice, niclosamide reduced mucus production and secretion, as well as bronchoconstriction, and showed additional antiinflammatory effects. Using transgenic asthmatic mice, we found evidence that TMEM16A and TMEM16F are required for normal mucus production/secretion, which may be due to their effects on intracellular Ca2+ signaling. TMEM16A and TMEM16F support exocytic release of mucus and inflammatory mediators, both of which are blocked by niclosamide. Thus, inhibition of mucus and cytokine release, bronchorelaxation, and reported antibacterial effects make niclosamide a potentially suitable drug for the treatment of inflammatory airway diseases, such as CF, asthma, and COPD.

TMEM16F/Anoctamin 6 in Ferroptotic Cell Death

Ca2+ activated Cl- channels (TMEM16A; ANO1) support cell proliferation and cancer growth. Expression of TMEM16A is strongly enhanced in different types of malignomas. In contrast, TMEM16F (ANO6) operates as a Ca2+ activated chloride/nonselective ion channel and scrambles membrane phospholipids to expose phosphatidylserine at the cell surface. Both phospholipid scrambling and cell swelling induced through activation of nonselective ion currents appear to destabilize the plasma membrane thereby causing cell death. There is growing evidence that activation of TMEM16F contributes to various forms of regulated cell death. In the present study, we demonstrate that ferroptotic cell death, occurring during peroxidation of plasma membrane phospholipids activates TMEM16F. Ferroptosis was induced by erastin, an inhibitor of the cystine-glutamate antiporter and RSL3, an inhibitor of glutathione peroxidase 4 (GPX4). Cell death was largely reduced in the intestinal epithelium, and in peritoneal macrophages isolated from mice with tissue-specific knockout of TMEM16F. We show that TMEM16F is activated during erastin and RSL3-induced ferroptosis. In contrast, inhibition of ferroptosis by ferrostatin-1 and by inhibitors of TMEM16F block TMEM16F currents and inhibit cell death. We conclude that activation of TMEM16F is a crucial component during ferroptotic cell death, a finding that may be useful to induce cell death in cancer cells.

Temperature-dependent increase in the calcium sensitivity and acceleration of activation of ANO6 chloride channel variants

Anoctamin-6 (ANO6) belongs to a family of calcium (Ca2+)-activated chloride channels (CaCCs), with three splicing variants (V1, V2, and V5) showing plasma membrane expression. Unlike other CaCCs, ANO6 requires a non-physiological intracellular free calcium concentration ([Ca2+]i > 1 μM) and several minutes for full activation under a whole-cell patch clamp. Therefore, its physiological role as an ion channel is uncertain and it is more commonly considered a Ca2+-dependent phospholipid scramblase. Here, we demonstrate that physiological temperature (37 °C) increases ANO6 Ca2+ sensitivity under a whole-cell patch clamp; V1 was activated by 1 μM [Ca2+]i, whereas V2 and V5 were activated by 300 nM [Ca2+]i. Increasing the temperature to 42 °C led to activation of all ANO6 variants by 100 nM [Ca2+]i. The delay time for activation of the three variants was significantly shortened at 37 °C. Notably, the temperature-dependent Ca2+-sensitisation of ANO6 became insignificant under inside-out patch clamp, suggesting critical roles of unknown cytosolic factors. Unlike channel activity, 27 °C but not 37 °C (physiological temperature) induced the scramblase activity of ANO6 at submicromolar [Ca2+]i (300 nM), irrespective of variant type. Our results reveal a physiological ion conducting property of ANO6 at 37 °C and suggest that ANO6 channel function acts separately from its scramblase activity.

Tweety-homolog (Ttyh) Family Encodes the Pore-forming Subunits of the Swelling-dependent Volume-regulated Anion Channel (VRACswell) in the Brain

In the brain, a reduction in extracellular osmolality causes water-influx and swelling, which subsequently triggers Cl^-- and osmolytes-efflux via volume-regulated anion channel (VRAC). Although LRRC8 family has been recently proposed as the pore-forming VRAC which is activated by low cytoplasmic ionic strength but not by swelling, the molecular identity of the pore-forming swelling-dependent VRAC (VRAC_swell) remains unclear. Here we identify and characterize Tweety-homologs (TTYH1, TTYH2, TTYH3) as the major VRAC_swell in astrocytes. Gene-silencing of all Ttyh1/2/3 eliminated hypo-osmotic-solution-induced Cl^- conductance (I_Cl,swell) in cultured and hippocampal astrocytes. When heterologously expressed in HEK293T or CHO-K1 cells, each TTYH isoform showed a significant I_Cl,swell with similar aquaporin-4 dependency, pharmacological properties and glutamate permeability as I_Cl,swell observed in native astrocytes. Mutagenesis-based structure-activity analysis revealed that positively charged arginine residue at 165 in TTYH1 and 164 in TTYH2 is critical for the formation of the channel-pore. Our results demonstrate that TTYH family confers the bona fide VRAC_swell in the brain.

Contribution of Anoctamins to Cell Survival and Cell Death

Before anoctamins (TMEM16 proteins) were identified as a family of Ca2+-activated chloride channels and phospholipid scramblases, the founding member anoctamin 1 (ANO1, TMEM16A) was known as DOG1, a marker protein for gastrointestinal stromal tumors (GIST). Meanwhile, ANO1 has been examined in more detail, and the role of ANO1 in cell proliferation and the development of different types of malignomas is now well established. While ANO5, ANO7, and ANO9 may also be relevant for growth of cancers, evidence has been provided for a role of ANO6 (TMEM16F) in regulated cell death. The cellular mechanisms by which anoctamins control cell proliferation and cell death, respectively, are just emerging; however, the pronounced effects of anoctamins on intracellular Ca2+ levels are likely to play a significant role. Recent results suggest that some anoctamins control membrane exocytosis by setting Ca2+i levels near the plasma membrane, and/or by controlling the intracellular Cl- concentration. Exocytosis and increased membrane trafficking induced by ANO1 and ANO6 may enhance membrane expression of other chloride channels, such as CFTR and volume activated chloride channels (VRAC). Notably, ANO6-induced phospholipid scrambling with exposure of phosphatidylserine is pivotal for the sheddase function of disintegrin and metalloproteinase (ADAM). This may support cell death and tumorigenic activity of IL-6 by inducing IL-6 trans-signaling. The reported anticancer effects of the anthelminthic drug niclosamide are probably related to the potent inhibitory effect on ANO1, apart from inducing cell cycle arrest through the Let-7d/CDC34 axis. On the contrary, pronounced activation of ANO6 due to a large increase in intracellular calcium, activation of phospholipase A2 or lipid peroxidation, can lead to ferroptotic death of cancer cells. It therefore appears reasonable to search for both inhibitors and potent activators of TMEM16 in order to interfere with cancer growth and metastasis.

Calcium-dependent, non-apoptotic, large plasma membrane bleb formation in physiologically stimulated mast cells and basophils

Large membrane derangements in the form of non-detaching blebs or membrane protrusions occur in a variety of cell stress and physiological situations and do not always reflect apoptotic processes. They have been studied in model mast cells under conditions of cell stress, but their potential physiological relevance to mast cell function and formation in primary mast cells or basophils have not been addressed. In the current study, we examine the large, non-detaching, non-apoptotic, membrane structures that form in model and primary mast cells under conditions of stimulation that are relevant to allergy, atopy and Type IV delayed hypersensitivity reactions. We characterized the inflation kinetics, dependency of formation upon external free calcium and striking geometric consistency of formation for large plasma membrane blebs (LPMBs). We describe that immunologically stimulated LPMBs in mast cells are constrained to form in locations where dissociation of the membrane-associated cytoskeleton occurs. Mast cell LPMBs decorate with wheat germ agglutinin, suggesting that they contain plasma membrane (PM) lectins. Electrophysiological capacitance measurements support a model where LPMBs are not being formed from internal membranes newly fused into the PM, but rather arise from stretching of the existing membrane, or inflation and smoothing of a micro-ruffled PM. This study provides new insights into the physiological manifestations of LPMB in response to immunologically relevant stimuli and in the absence of cell stress, death or apoptotic pathways.

TMEM16A in Cystic Fibrosis: Activating or Inhibiting?

The inflammatory airway disease cystic fibrosis (CF) is characterized by airway obstruction due to mucus hypersecretion, airway plugging, and bronchoconstriction. The cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel is dysfunctional in CF, leading to defects in epithelial transport. Although CF pathogenesis is still disputed, activation of alternative Cl- channels is assumed to improve lung function in CF. Two suitable non-CFTR Cl- channels are present in the airway epithelium, the Ca2+ activated channel TMEM16A and SLC26A9. Activation of these channels is thought to be feasible to improve hydration of the airway mucus and to increase mucociliary clearance. Interestingly, both channels are upregulated during inflammatory lung disease. They are assumed to support fluid secretion, necessary to hydrate excess mucus and to maintain mucus clearance. During inflammation, however, TMEM16A is upregulated particularly in mucus producing cells, with only little expression in ciliated cells. Recently it was shown that knockout of TMEM16A in ciliated cells strongly compromises Cl- conductance and attenuated mucus secretion, but does not lead to a CF-like lung disease and airway plugging. Along this line, activation of TMEM16A by denufosol, a stable purinergic ligand, failed to demonstrate any benefit to CF patients in earlier studies. It rather induced adverse effects such as cough. A number of studies suggest that TMEM16A is essential for mucus secretion and possibly also for mucus production. Evidence is now provided for a crucial role of TMEM16A in fusion of mucus-filled granules with the apical plasma membrane and cellular exocytosis. This is probably due to local Ca2+ signals facilitated by TMEM16A. Taken together, TMEM16A supports fluid secretion by ciliated airway epithelial cells, but also maintains excessive mucus secretion during inflammatory airway disease. Because TMEM16A also supports airway smooth muscle contraction, inhibition rather than activation of TMEM16A might be the appropriate treatment for CF lung disease, asthma and COPD. As a number of FDA-approved and well-tolerated drugs have been shown to inhibit TMEM16A, evaluation in clinical trials appears timely.

Lipid Peroxidation Drives Renal Cyst Growth In Vitro through Activation of TMEM16A

BACKGROUND

Transepithelial chloride^- secretion, through the chloride channels cystic fibrosis transmembrane conductance regulator (CFTR) and TMEM16A (anoctamin 1), drives cyst enlargement in polycystic kidney disease (PKD). Polycystic kidneys are hypoxic, and oxidative stress activates TMEM16A. However, mechanisms for channel activation in PKD remain obscure.

METHODS

Using tissue samples from patients with autosomal dominant PKD, embryonic kidney cultures, and an MDCK in vitro cyst model, we assessed peroxidation of plasma membrane phospholipids in human and mouse polycystic kidneys. We also used electrophysiologic Ussing chamber and patch clamp experiments to analyze activation of TMEM16A and growth of renal cysts.

RESULTS

Peroxidation of phospholipids in human and mouse kidneys as well as MDCK cysts in vitro is probably due to enhanced levels of reactive oxygen species. Lipid peroxidation correlated with increased cyst volume as shown in renal cultures and MDCK cysts in three-dimensional cultures. Reactive oxygen species and lipid peroxidation strongly activated TMEM16A, leading to depletion of calcium ion stores and store-operated calcium influx. Activation of TMEM16A- and CFTR-dependent chloride secretion strongly augmented cyst growth. Exposure to scavengers of reactive oxygen species, such as glutathione, coenzyme Q10, or idebenone (a synthetic coenzyme Q10 homolog), as well as inhibition of oxidative lipid damage by ferrostatin-1 largely reduced activation of TMEM16A. Inhibition of TMEM16A reduced proliferation and fluid secretion in vitro.

CONCLUSIONS

These findings indicate that activation of TMEM16A by lipid peroxidation drives growth of renal cysts. We propose direct inhibition of TMEM16A or inhibition of lipid peroxidation as potentially powerful therapeutic approaches to delay cyst development in PKD.

Cell Volume-Activated and Volume-Correlated Anion Channels in Mammalian Cells: Their Biophysical, Molecular, and Pharmacological Properties

There are a number of mammalian anion channel types associated with cell volume changes. These channel types are classified into two groups: volume-activated anion channels (VAACs) and volume-correlated anion channels (VCACs). VAACs can be directly activated by cell swelling and include the volume-sensitive outwardly rectifying anion channel (VSOR), which is also called the volume-regulated anion channel; the maxi-anion channel (MAC or Maxi-Cl); and the voltage-gated anion channel, chloride channel (ClC)-2. VCACs can be facultatively implicated in, although not directly activated by, cell volume changes and include the cAMP-activated cystic fibrosis transmembrane conductance regulator (CFTR) anion channel, the Ca2+-activated Cl- channel (CaCC), and the acid-sensitive (or acid-stimulated) outwardly rectifying anion channel. This article describes the phenotypical properties and activation mechanisms of both groups of anion channels, including accumulating pieces of information on the basis of recent molecular understanding. To that end, this review also highlights the molecular identities of both anion channel groups; in addition to the molecular identities of ClC-2 and CFTR, those of CaCC, VSOR, and Maxi-Cl were recently identified by applying genome-wide approaches. In the last section of this review, the most up-to-date information on the pharmacological properties of both anion channel groups, especially their half-maximal inhibitory concentrations (IC50 values) and voltage-dependent blocking, is summarized particularly from the standpoint of pharmacological distinctions among them. Future physiologic and pharmacological studies are definitely warranted for therapeutic targeting of dysfunction of VAACs and VCACs.

ATP-Gated P2X7 Receptors Require Chloride Channels To Promote Inflammation in Human Macrophages

Immune cells of myeloid origin show robust expression of ATP-gated P2X7 receptors, two-transmembrane ion channels permeable to Na⁺, K⁺, and Ca²⁺ Receptor activation promotes inflammasome activation and release of the proinflammatory cytokines IL-1β and IL-18. In this study, we show that ATP generates facilitating cationic currents in monocyte-derived human macrophages and permeabilizes the plasma membrane to polyatomic cationic dyes. We find that antagonists of PLA₂ and Cl^- channels abolish P2X7 receptor-mediated current facilitation, membrane permeabilization, blebbing, phospholipid scrambling, inflammasome activation, and IL-1β release. Our data demonstrate significant differences in the actions of ATP in murine and human macrophages and suggest that PLA₂ and Cl^- channels mediate innate immunity downstream of P2X7 receptors in human macrophages.

Molecular Identities and ATP Release Activities of Two Types of Volume-Regulatory Anion Channels, VSOR and Maxi-Cl

An elaborate volume regulation system based on interplay of ion channels and transporters was evolved to cope with constant osmotic challenges caused by intensive metabolism, transport and other physiological/pathophysiological events. In animal cells, two types of anion channels are directly activated by cell swelling and involved in the regulatory volume decrease (RVD): volume-sensitive outwardly rectifying anion channel (VSOR), also called volume-regulated anion channel (VRAC), and Maxi-Cl which is the most major type of maxi-anion channel (MAC). These two channels have very different biophysical profiles and exhibit opposite dependence on intracellular ATP. After several decades of verifying many false-positive candidates for VSOR and Maxi-Cl, LRRC8 family proteins emerged as major VSOR components, and SLCO2A1 protein as a core of Maxi-Cl. Still, neither of these proteins alone can fully reproduce the native channel phenotypes suggesting existence of missing components. Although both VSOR and Maxi-Cl have pores wide enough to accommodate bulky ATP^4- and MgATP^2- anions, evidence accumulated hitherto, based on pharmacological and gene silencing experiments, suggests that Maxi-Cl, but not VSOR, serves as one of the major pathways for the release of ATP from swollen and ischemic/hypoxic cells. Relations of VSOR and Maxi-Cl with diseases and their selective pharmacology are the topics promoted by recent advance in molecular identification of the two volume-activated, volume-regulatory anion channels.

Regulation and Function of TMEM16F in Renal Podocytes

The Ca²⁺-activated phospholipid scramblase and ion channel TMEM16F is expressed in podocytes of renal glomeruli. Podocytes are specialized cells that form interdigitating foot processes as an essential component of the glomerular filter. These cells, which participate in generation of the primary urine, are often affected during primary glomerular diseases, such as glomerulonephritis and secondary hypertensive or diabetic nephropathy, which always leads to proteinuria. Because the function of podocytes is known to be controlled by intracellular Ca²⁺ signaling, it is important to know about the role of Ca²⁺-activated TMEM16F in these cells. To that end, we generated an inducible TMEM16F knockdown in the podocyte cell line AB8, and produced a conditional mouse model with knockout of TMEM16F in podocytes and renal epithelial cells of the nephron. We found that knockdown of TMEM16F did not produce proteinuria or any obvious phenotypic changes. Knockdown of TMEM16F affected cell death of tubular epithelial cells but not of glomerular podocytes when analyzed in TUNEL assays. Surprisingly, and in contrast to other cell types, TMEM16F did not control intracellular Ca²⁺ signaling and was not responsible for Ca²⁺-activated whole cell currents in podocytes. TMEM16F levels in podocytes were enhanced after inhibition of the endolysosomal pathway and after treatment with angiotensin II. Renal knockout of TMEM16F did not compromise renal morphology and serum electrolytes. Taken together, in contrast to other cell types, such as platelets, bone cells, and immune cells, TMEM16F shows little effect on basal properties of podocytes and does not appear to be essential for renal function.

Known structures and unknown mechanisms of TMEM16 scramblases and channels

Falzone et al. interpret the mechanisms underlying the activity of TMEM16 family members from recent structural and functional work.

The TMEM16 family of membrane proteins is composed of both Ca²⁺-gated Cl⁻ channels and Ca²⁺-dependent phospholipid scramblases. The functional diversity of TMEM16s underlies their involvement in numerous signal transduction pathways that connect changes in cytosolic Ca²⁺ levels to cellular signaling networks. Indeed, defects in the function of several TMEM16s cause a variety of genetic disorders, highlighting their fundamental pathophysiological importance. Here, we review how our mechanistic understanding of TMEM16 function has been shaped by recent functional and structural work. Remarkably, the recent determination of near-atomic-resolution structures of TMEM16 proteins of both functional persuasions has revealed how relatively minimal rearrangements in the substrate translocation pathway are sufficient to precipitate the dramatic functional differences that characterize the family. These structures, when interpreted in the light of extensive functional analysis, point to an unusual mechanism for Ca²⁺-dependent activation of TMEM16 proteins in which substrate permeation is regulated by a combination of conformational rearrangements and electrostatics. These breakthroughs pave the way to elucidate the mechanistic bases of ion and lipid transport by the TMEM16 proteins and unravel the molecular links between these transport activities and their function in human pathophysiology.

Contribution of TMEM16F to pyroptotic cell death

Pyroptosis is a highly inflammatory form of programmed cell death that is caused by infection with intracellular pathogens and activation of canonical or noncanonical inflammasomes. The purinergic receptor P2X₇ is activated by the noncanonical inflammasome and contributes essentially to pyroptotic cell death. The Ca²⁺ activated phospholipid scramblase and ion channel TMEM16F has been shown earlier to control cellular effects downstream of purinergic P2X₇ receptors that ultimately lead to cell death. As pyroptotic cell death is accompanied by an increases in intracellular Ca²⁺, we asked whether TMEM16F is activated during pyroptosis. The N-terminal cleavage product of gasdermin D (GD-N) is an executioner of pyroptosis by forming large plasma membrane pores. Expression of GD-N enhanced basal Ca²⁺ levels and induced cell death. We observed that GD-N induced cell death in HEK293 and HAP1 cells, which was depending on expression of endogenous TMEM16F. GD-N activated large whole cell currents that were suppressed by knockdown or inhibition of TMEM16F. The results suggest that whole cell currents induced by the pore forming domain of gasdermin-D, are at least in part due to activation of TMEM16F. Knockdown of other TMEM16 paralogues expressed in HAP1 cells suggest TMEM16F as a crucial element during pyroptosis and excluded a role of other TMEM16 proteins. Thus TMEM16F supports pyroptosis and other forms of inflammatory cell death such as ferroptosis. Its potent inhibition by tannic acid may be part of the anti-inflammatory effects of flavonoids.

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.

Understanding taurine CNS activity using alternative zebrafish models

Taurine is a highly abundant "amino acid" in the brain. Despite the potential neuroactive role of taurine in vertebrates has long been recognized, the underlying molecular mechanisms related to its pleiotropic effects in the brain remain poorly understood. Due to the genetic tractability, rich behavioral repertoire, neurochemical conservation, and small size, the zebrafish (Danio rerio) has emerged as a powerful candidate for neuropsychopharmacology investigation and in vivo drug screening. Here, we summarize the main physiological roles of taurine in mammals, including neuromodulation, osmoregulation, membrane stabilization, and antioxidant action. In this context, we also highlight how zebrafish models of brain disorders may present interesting approaches to assess molecular mechanisms underlying positive effects of taurine in the brain. Finally, we outline recent advances in zebrafish drug screening that significantly improve neuropsychiatric translational researches and small molecule screens.

Comparative Effects of Chloride Channel Inhibitors on LRRC8/VRAC-Mediated Chloride Conductance

Chloride channels play an essential role in a variety of physiological functions and in human diseases. Historically, the field of chloride channels has long been neglected owing to the lack of powerful selective pharmacological agents that are needed to overcome the technical challenge of characterizing the molecular identities of these channels. Recently, members of the LRRC8 family have been shown to be essential for generating the volume-regulated anion channel (VRAC) current, a chloride conductance that governs the regulatory volume decrease (RVD) process. The inhibitory effects of six commonly used chloride channel inhibitors on VRAC/LRRC8-mediated chloride transport were tested in wild-type HEK-293 cells expressing LRRC8 proteins and devoid of other types of chloride channels (CFTR and ANO1/2). We explored the effectiveness of the inhibitors using the patch-clamp whole-cell approach and fluorescence-based quantification of cellular volume changes during hypotonic challenge. Both DCPIB and NFA inhibited VRAC current in a whole-cell configuration, with IC₅₀ values of 5 ± 1 μM and 55 ± 2 μM, respectively. Surprisingly, GlyH-101 and PPQ-102, two CFTR inhibitors, also inhibited VRAC conductance at concentrations in the range of their current use, with IC₅₀ values of 10 ± 1 μM and 20 ± 1 μM, respectively. T16A_inh-A01, a so-called specific inhibitor of calcium-activated Cl^- conductance, blocked the chloride current triggered by hypo-osmotic challenge, with an IC₅₀ of 6 ± 1 μM. Moreover, RVD following hypotonic challenge was dramatically reduced by these inhibitors. CFTR_inh-172 was the only inhibitor that had almost no effect on VRAC/LRRC8-mediated chloride conductance. All inhibitors tested except CFTR_inh-172 inhibited VRAC/LRRC8-mediated chloride conductance and cellular volume changes during hypotonic challenge. These results shed light on the apparent lack of chloride channel inhibitors specificity and raise the question of how these inhibitors actually block chloride conductances.

Differential effects of anoctamins on intracellular calcium signals

The Ca²⁺-activated Cl^- channel TMEM16A [anoctamin (ANO)1] is homologous to yeast Ist2 and has been shown to tether the cortical endoplasmic reticulum (ER) to the plasma membrane. We therefore examined whether ANO1 and other members of the ANO family affect intracellular Ca²⁺ ([Ca²⁺]_i) signals. It is shown that expression of ANO1 augments Ca²⁺ store release upon stimulation of GPCRs, whereas knockdown of ANO1, or lack of Ano1 expression in Ano1^-/- animals, as shown in an earlier report, inhibits Ca²⁺ release. ANO6, and -10 show similar effects, whereas expression of ANO4, -8, and -9 attenuate filling of the ER store. The impact of ANO1 and -4 were examined in more detail. ANO1 colocalized and interacted with IP₃R, whereas ANO4 colocalized with SERCA Ca²⁺ pumps and interacted with ORAI-1 channels, respectively. ANO1 Cl currents were rapidly activated exclusively through Ca²⁺ store release, and remained untouched by influx of extracellular Ca²⁺ In contrast expression of ANO4 caused a delayed activation of membrane-localized ANO6 channels, solely through store-operated Ca²⁺ entry via ORAI. Ca²⁺ signals were inhibited by knocking down expression of endogenous ANO1, -5, -6, and -10 and were also reduced in epithelial cells from Ano10^-/- mice. The data suggest that ANOs affect compartmentalized [Ca²⁺]_i signals, which may explain some of the cellular defects related to ANO mutations.-Cabrita, I., Benedetto, R., Fonseca, A., Wanitchakool, P., Sirianant, L., Skryabin, B. V., Schenk, L. K., Pavenstädt, H., Schreiber, R., Kunzelmann, K. Differential effects of anoctamins on intracellular calcium signals.

Ca2+ signals, cell membrane disintegration, and activation of TMEM16F during necroptosis

Activated receptor-interacting protein kinase 3 (RIPK3) and mixed lineage kinase domain like (MLKL) are essential components of the necroptotic pathway. Phosphorylated MLKL (pMLKL) is thought to induce membrane leakage, leading to cell swelling and disintegration of the cell membrane. However, the molecular identity of the necroptotic membrane pore remains unclear, and the role of pMLKL for membrane permeabilization is currently disputed. We observed earlier that the phospholipid scramblase and ion channel TMEM16F/anoctamin 6 cause large membrane currents, cell swelling, and cell death when activated by a strong increase in intracellular Ca2+. We, therefore, asked whether TMEM16F is also central to necroptotic cell death and other cellular events during necroptosis. Necroptosis was induced by TNFα, smac mimetic, and Z-VAD (TSZ) in NIH3T3 fibroblasts and the four additional cell lines HT29, 16HBE, H441, and L929. Time-dependent changes in intracellular Ca2+, cell morphology, and membrane currents were recorded. TSZ induced a small and only transient oscillatory rise in intracellular Ca2+, which was paralleled by the activation of outwardly rectifying Cl- currents, which were typical for TMEM16F/ANO6. Ca2+ oscillations were due to Ca2+ release from endoplasmic reticulum, and were independent of extracellular Ca2+. The initial TSZ-induced cell swelling was followed by cell shrinkage. Using typical channel blockers and siRNA-knockdown, the Cl- currents were shown to be due to the activation of ANO6. However, the knockdown of ANO6 or inhibitors of ANO6 did not inhibit necroptotic cell death. The present data demonstrate the activation of ANO6 during necroptosis, which, however, is not essential for cell death.

Cellular defects by deletion of ANO10 are due to deregulated local calcium signaling

TMEM16K (ANO10) belongs to a family of ion channels and phospholipid scramblases. Mutations in ANO10 cause neurological and immunological defects, and abrogated ion transport. Here we show that Ano10 knockout in epithelial cells leads to defective ion transport, attenuated volume regulation and deranged Ca²⁺ signaling. Intestinal epithelial cells from Ano10 null mice are reduced in size and demonstrate an almost abolished spontaneous and TNFα-induced apoptosis. Similar defects were found in mouse peritoneal Ano10 null macrophages and in human THP1 macrophages with reduced ANO10 expression. A cell cycle dependent colocalization of Ano10 with acetylated tubulin, centrioles, and a submembranous tubulin containing compartment was observed in Fisher rat thyroid cells. Axs, the Drosophila ortholog of ANO10 is known for its role in mitotic spindle formation and association with the endoplasmic reticulum and Ca²⁺ signaling. We therefore propose that mutations in ANO10 cause cellular defects and genetic disorders through deranged local Ca²⁺ signaling.

Cl- channels in apoptosis

A remarkable feature of apoptosis is the initial massive cell shrinkage, which requires opening of ion channels to allow release of K+, Cl-, and organic osmolytes to drive osmotic water movement and cell shrinkage. This article focuses on the role of the Cl- channels LRRC8, TMEM16/anoctamin, and cystic fibrosis transmembrane conductance regulator (CFTR) in cellular apoptosis. LRRC8A-E has been identified as a volume-regulated anion channel expressed in many cell types. It was shown to be required for regulatory and apoptotic volume decrease (RVD, AVD) in cultured cell lines. Its presence also determines sensitivity towards cytostatic drugs such as cisplatin. Recent data point to a molecular and functional relationship of LRRC8A and anoctamins (ANOs). ANO6, 9, and 10 (TMEM16F, J, and K) augment apoptotic Cl- currents and AVD, but it remains unclear whether these anoctamins operate as Cl- channels or as regulators of other apoptotic Cl- channels, such as LRRC8. CFTR has been known for its proapoptotic effects for some time, and this effect may be based on glutathione release from the cell and increase in cytosolic reactive oxygen species (ROS). Although we find that CFTR is activated by cell swelling, it is possible that CFTR serves RVD/AVD through accumulation of ROS and activation of independent membrane channels such as ANO6. Thus activation of ANO6 will support cell shrinkage and induce additional apoptotic events, such as membrane phospholipid scrambling.

Relationship between TMEM16A/anoctamin 1 and LRRC8A

TMEM16A/anoctamin 1/ANO1 and VRAC/LRRC8 are independent chloride channels activated either by increase in intracellular Ca(2+) or cell swelling, respectively. In previous studies, we observed overlapping properties for both types of channels. (i) TMEM16A/ANO1 and LRRC8 are inhibited by identical compounds, (ii) the volume-regulated anion channel VRAC requires compartmentalized Ca(2+) increase to be fully activated, (iii) anoctamins are activated by cell swelling, (iv) both channels have a role for apoptotic cell death, (v) both channels are possibly located in lipid rafts/caveolae like structures, and (vi) VRAC and anoctamin 1 currents are not additive when each are fully activated. In the present study, we demonstrate in different cell types that loss of LRRC8A expression not only inhibited VRAC, but also attenuated Ca(2+) activated Cl(-) currents. Moreover, expression of LRRC8A enhanced Ca(2+) activated Cl(-) currents, and both LRRC8A and ANO1 could be coimmunoprecipitated. We found that LRRC8A becomes accessible to biotinylation upon exposure to hypotonic bath solution, while membrane capacitance was not enhanced. When intracellular Ca(2+) was increased in ANO1-expressing cells, the membrane capacitance was enhanced and increased binding of FM4-64 to the membrane was observed. As this was not seen in cells lacking ANO1 expression, a role of ANO1 for exocytosis was suggested. We propose that ANO1 and LRRC8A are activated in parallel. Thus, ionomycin or purinergic stimulation will not only activate ANO1 but also LRRC8 currents. Cell swelling will not only activate LRRC8/VRAC, but also stimulate ANO1 currents by enhancing compartmentalized Ca(2+) increase and/or through swelling induced autocrine release of ATP.

Ion channels in regulated cell death

Activation of ion channels and pores are essential steps during regulated cell death. Channels and pores participate in execution of apoptosis, necroptosis and other forms of caspase-independent cell death. Within the program of regulated cell death, these channels are strategically located. Ion channels can shrink cells and drive them towards apoptosis, resulting in silent, i.e. immunologically unrecognized cell death. Alternatively, activation of channels can induce cell swelling, disintegration of the cell membrane, and highly immunogenic necrotic cell death. The underlying cell death pathways are not strictly separated as identical stimuli may induce cell shrinkage and apoptosis when applied at low strength, but may also cause cell swelling at pronounced stimulation, resulting in regulated necrosis. Nevertheless, the precise role of ion channels during regulated cell death is far from being understood, as identical channels may support regulated death in some cell types, but may cause cell proliferation, cancer development, and metastasis in others. Along this line, the phospholipid scramblase and Cl(-)/nonselective channel anoctamin 6 (ANO6) shows interesting features, as it participates in apoptotic cell death during lower levels of activation, thereby inducing cell shrinkage. At strong activation, e.g. by stimulation of purinergic P2Y7 receptors, it participates in pore formation, causes massive membrane blebbing, cell swelling, and membrane disintegration. The LRRC8 proteins deserve much attention as they were found to have a major role in volume regulation, apoptotic cell shrinkage and resistance towards anticancer drugs.

Ani9, A Novel Potent Small-Molecule ANO1 Inhibitor with Negligible Effect on ANO2

Anoctamin1 (ANO1)/transmembrane protein 16A (TMEM16A), a calcium-activated chloride channel (CaCC), is involved in many physiological functions such as fluid secretion, smooth muscle contraction, nociception and cancer progression. To date, only a few ANO1 inhibitors have been described, and these have low potency and selectivity for ANO1. Here, we performed a high-throughput screening to identify highly potent and selective small molecule inhibitors of ANO1. Three novel ANO1 inhibitors were discovered from screening of 54,400 synthetic small molecules, and they were found to fully block ANO1 channel activity with an IC₅₀ < 3 μM. Electrophysiological analysis revealed that the most potent inhibitor, 2-(4-chloro-2-methylphenoxy)-N-[(2-methoxyphenyl)methylideneamino]-acetamide (Ani9), completely inhibited ANO1 chloride current with submicromolar potency. Notably, unlike previous small-molecule ANO1 inhibitors identified to date, Ani9 displayed high selectivity for ANO1 as compared to ANO2, which shares a high amino acid homology to ANO1. In addition, Ani9 did not affect the intracellular calcium signaling and CFTR chloride channel activity. Our results suggest that Ani9 may be a useful pharmacological tool for studying ANO1 and a potential development candidate for drug therapy of cancer, hypertension, pain, diarrhea and asthma.