Calmodulin-dependent activation of the epithelial calcium-dependent chloride channel TMEM16A

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.

Piezo1, the new actor in cell volume regulation

All animal cells control their volume through a complex set of mechanisms, both to counteract osmotic perturbations of the environment and to enable numerous vital biological processes, such as proliferation, apoptosis, and migration. The ability of cells to adjust their volume depends on the activity of ion channels and transporters which, by moving K+, Na+, and Cl- ions across the plasma membrane, generate the osmotic gradient that drives water in and out of the cell. In 2010, Patapoutian's group identified a small family of evolutionarily conserved, Ca2+-permeable mechanosensitive channels, Piezo1 and Piezo2, as essential components of the mechanically activated current that mediates mechanotransduction in vertebrates. Piezo1 is expressed in several tissues and its opening is promoted by a wide range of mechanical stimuli, including membrane stretch/deformation and osmotic stress. Piezo1-mediated Ca2+ influx is used by the cell to convert mechanical forces into cytosolic Ca2+ signals that control diverse cellular functions such as migration and cell death, both dependent on changes in cell volume and shape. The crucial role of Piezo1 in the regulation of cell volume was first demonstrated in erythrocytes, which need to reduce their volume to pass through narrow capillaries. In HEK293 cells, increased expression of Piezo1 was found to enhance the regulatory volume decrease (RVD), the process whereby the cell re-establishes its original volume after osmotic shock-induced swelling, and it does so through Ca2+-dependent modulation of the volume-regulated anion channels. More recently we reported that Piezo1 controls the RVD in glioblastoma cells via the modulation of Ca2+-activated K+ channels. To date, however, the mechanisms through which this mechanosensitive channel controls cell volume and maintains its homeostasis have been poorly investigated and are still far from being understood. The present review aims to provide a broad overview of the literature discussing the recent advances on this topic.

Function and Regulation of the Calcium-Activated Chloride Channel Anoctamin 1 (TMEM16A)

Various human tissues express the calcium-activated chloride channel Anoctamin 1 (ANO1), also known as TMEM16A. ANO1 allows the passive chloride flux that controls different physiological functions ranging from muscle contraction, fluid and hormone secretion, gastrointestinal motility, and electrical excitability. Overexpression of ANO1 is associated with pathological conditions such as hypertension and cancer. The molecular cloning of ANO1 has led to a surge in structural, functional, and physiological studies of the channel in several tissues. ANO1 is a homodimer channel harboring two pores - one in each monomer - that work independently. Each pore is activated by voltage-dependent binding of two intracellular calcium ions to a high-affinity-binding site. In addition, the binding of phosphatidylinositol 4,5-bisphosphate to sites scattered throughout the cytosolic side of the protein aids the calcium activation process. Furthermore, many pharmacological studies have established ANO1 as a target of promising compounds that could treat several illnesses. This chapter describes our current understanding of the physiological roles of ANO1 and its regulation under physiological conditions as well as new pharmacological compounds with potential therapeutic applications.

Structure and Function of Calcium-Activated Chloride Channels and Phospholipid Scramblases in the TMEM16 Family

The transmembrane protein 16 (TMEM16) family consists of Ca2+-activated chloride channels and phospholipid scramblases. Ten mammalian TMEM16 proteins, TMEM16A-K (with no TMEM16I), and several non-mammalian TMEM16 proteins, such as afTMEM16 and nhTMEM16, have been discovered. All known TMEM16 proteins are homodimeric proteins containing two subunits. Each subunit consists of ten transmembrane helices with Ca2+-binding sites and a single ion-permeation/phospholipid transport pathway. The ion-permeation pathway and the phospholipid transport pathway of TMEM16 proteins have a wide intracellular vestibule, a narrow neck, and a smaller extracellular vestibule. Interestingly, the lining wall of the ion-permeation/phospholipid transport pathway may be formed, at least partially, by membrane phospholipids, though the degree of pore-wall forming by phospholipids likely varies among TMEM16 proteins. Thus, the biophysical properties and activation mechanisms of TMEM16 proteins could differ from each other accordingly. Here we review the current understanding of the structure and function of TMEM16 molecules.

IP3R1 underlies diastolic ANO1 activation and pressure-dependent chronotropy in lymphatic collecting vessels

Pressure-dependent chronotropy of murine lymphatic collecting vessels relies on the activation of the Ca2+-activated chloride channel encoded by Anoctamin 1 (Ano1) in lymphatic muscle cells. Genetic ablation or pharmacological inhibition of ANO1 results in a significant reduction in basal contraction frequency and essentially complete loss of pressure-dependent frequency modulation by decreasing the rate of the diastolic depolarization phase of the ionic pacemaker in lymphatic muscle cells (LMCs). Oscillating Ca2+ release from sarcoendoplasmic reticulum Ca2+ channels has been hypothesized to drive ANO1 activity during diastole, but the source of Ca2+ for ANO1 activation in smooth muscle remains unclear. Here, we investigated the role of the inositol triphosphate receptor 1 (Itpr1; Ip3r1) in this process using pressure myography, Ca2+ imaging, and membrane potential recordings in LMCs of ex vivo pressurized inguinal-axillary lymphatic vessels from control or Myh11CreERT2;Ip3r1fl/fl (Ip3r1ismKO) mice. Ip3r1ismKO vessels had significant reductions in contraction frequency and tone but an increased contraction amplitude. Membrane potential recordings from LMCs of Ip3r1ismKO vessels revealed a depressed diastolic depolarization rate and an elongation of the plateau phase of the action potential (AP). Ca2+ imaging of LMCs using the genetically encoded Ca2+ sensor GCaMP6f demonstrated an elongation of the Ca2+ flash associated with an AP-driven contraction. Critically, diastolic subcellular Ca2+ transients were absent in LMCs of Ip3r1ismKO mice, demonstrating the necessity of IP3R1 activity in controlling ANO1-mediated diastolic depolarization. These findings indicate a critical role for IP3R1 in lymphatic vessel pressure-dependent chronotropy and contractile regulation.

Discovery of Fungus-Derived Nornidulin as a Novel TMEM16A Inhibitor: A Potential Therapy to Inhibit Mucus Secretion in Asthma

Introduction

Inhibition of Ca2+-activated transmembrane protein 16A (TMEM16A) Cl- channels has been proposed to alleviate mucus secretion in asthma. In this study, we identified a novel class of TMEM16A inhibitors from natural sources in airway epithelial Calu-3 cells and determine anti-asthmatic efficacy of the most potent candidate in a mouse model of asthma.

Methods

For electrophysiological analyses, IL-4-primed Calu-3 cell monolayers were mounted in Ussing chamber and treated with various fungus-derived depsidones prior to the addition of UTP, ionomycin, thapsigargin, or Eact to stimulate TMEM16A Cl- current. Ca2+-induced mucus secretion in Calu-3 cell monolayers was assessed by determining MUC5AC protein remaining in the cells using immunofluorescence staining. OVA-induced female BALB/c mice was used as an animal model of asthma. After the course of induction, cellular and mucus components in bronchoalveolar lavage were analyzed. Lungs were fixed and undergone with H&E and PAS staining for the evaluation of airway inflammation and mucus production, respectively.

Results

The screening of fungus-derived depsidones revealed that nornidulin completely abolished the UTP-activated TMEM16A current in Calu-3 cell monolayers with the IC50 and a maximal effect being at ~0.8 µM and 10 µM, respectively. Neither cell viability nor barrier function was affected by nornidulin. Mechanistically, nornidulin (10 µM) suppressed Cl- currents induced by ionomycin (a Ca2+-specific ionophore), thapsigargin (an inhibitor of the endoplasmic reticulum Ca2+ ATPase), and Eact (a putative TMEM16A activator) without interfering with intracellular Ca2+ ([Ca2+]i) levels. These results suggest that nornidulin exerts its effect without changing [Ca2+]i, possibly through direct effect on TMEM16A. Interestingly, nornidulin (at 10 µM) reduced Ca2+-dependent mucus release in the Calu-3 cell monolayers. In addition, nornidulin (20 mg/kg) inhibited bronchoalveolar mucus secretion without impeding airway inflammation in ovalbumin-induced asthmatic mice.

Discussion and Conclusion

Our study revealed that nornidulin is a novel TMEM16A inhibitor that suppresses mucus secretion without compromising immunologic activity. Further development of nornidulin may provide a new remedy for asthma or other diseases associated with allergic mucus hypersecretion without causing opportunistic infections.

TMEM16A ion channel: A novel target for cancer treatment

Cancer draws attention owing to the high morbidity and mortality. It is urgent to develop safe and effective cancer therapeutics. The calcium-activated chloride channel TMEM16A is widely distributed in various tissues and regulates physiological functions. TMEM16A is abnormally expressed in several cancers and associate with tumorigenesis, metastasis, and prognosis. Knockdown or inhibition of TMEM16A in cancer cells significantly inhibits cancer development. Therefore, TMEM16A is considered as a biomarker and therapeutic target for some cancers. This work reviews the cancers associated with TMEM16A. Then, the molecular mechanism of TMEM16A overexpression in cancer was analyzed, and the possible signal transduction mechanism of TMEM16A regulating cancer development was summarized. Finally, TMEM16A inhibitors with anticancer effect and their anticancer mechanism were concluded. We hope to provide new ideas for pharmacological studies on TMEM16A in cancer.

KCNE1 does not shift TMEM16A from a Ca2+ dependent to a voltage dependent Cl- channel and is not expressed in renal proximal tubule

The TMEM16A (ANO1) Cl^- channel is activated by Ca²⁺ in a voltage-dependent manner. It is broadly expressed and was shown to be also present in renal proximal tubule (RPT). KCNQ1 is an entirely different K⁺ selective channel that forms the cardiac I_KS potassium channel together with its ß-subunit KCNE1. Surprisingly, KCNE1 has been claimed to interact with TMEM16A, and to be required for activation of TMEM16A in mouse RPT. Interaction with KCNE1 was reported to switch TMEM16A from a Ca2²⁺-dependent to a voltage-dependent ion channel. Here we demonstrate that KCNE1 is not expressed in mouse RPT. TMEM16A expressed in RPT is activated by angiotensin II and ATP in a KCNE1-independent manner. Coexpression of KCNE1 does not change TMEM16A to a voltage gated Cl^- channel and Ca²⁺-dependent regulation of TMEM16A is fully maintained in the presence of KCNE1. While overexpressed KCNE1 slightly affects Ca²⁺-dependent regulation of TMEM16A, the data provide no evidence for KCNE1 being an auxiliary functional subunit for TMEM16A.

TMEM16A/F support exocytosis but do not inhibit Notch-mediated goblet cell metaplasia of BCi-NS1.1 human airway epithelium

Cl^- channels such as the Ca²⁺ activated Cl^- channel TMEM16A and the Cl^- permeable phospholipid scramblase TMEM16F may affect the intracellular Cl^- concentration ([Cl^-]_i), which could act as an intracellular signal. Loss of airway expression of TMEM16A induced a massive expansion of the secretory cell population like goblet and club cells, causing differentiation into a secretory airway epithelium. Knockout of the Ca²⁺-activated Cl^- channel TMEM16A or the phospholipid scramblase TMEM16F leads to mucus accumulation in intestinal goblet cells and airway secretory cells. We show that both TMEM16A and TMEM16F support exocytosis and release of exocytic vesicles, respectively. Lack of TMEM16A/F expression therefore causes inhibition of mucus secretion and leads to goblet cell metaplasia. The human basal epithelial cell line BCi-NS1.1 forms a highly differentiated mucociliated airway epithelium when grown in PneumaCult™ media under an air liquid interface. The present data suggest that mucociliary differentiation requires activation of Notch signaling, but not the function of TMEM16A. Taken together, TMEM16A/F are important for exocytosis, mucus secretion and formation of extracellular vesicles (exosomes or ectosomes) but the present data do no not support a functional role of TMEM16A/F in Notch-mediated differentiation of BCi-NS1.1 cells towards a secretory epithelium.

Translational potential of targeting Anoctamin-1-Encoded Calcium-Activated chloride channels in hypertension

Calcium-activated chloride channels (CaCC) provide a depolarizing stimulus to a variety of tissues through chloride efflux in response to a rise in internal Ca²⁺ and voltage. One of these channels, Anoctamin-1 (ANO1 or TMEM16A) is now recognized to play a central role in promoting smooth muscle tone in various types of blood vessels. Its role in hypertension, and thus the therapeutic promise of targeting ANO1, is less straightforward. This review gives an overview of our current knowledge about the potential role ANO1 may play in hypertension within the systemic, portal, and pulmonary vascular systems and the importance of this information when pursuing potential treatment strategies. While the role of ANO1 is well-established in several forms of pulmonary hypertension, its contributions to both the generation of vascular tone and its role in hypertension within the systemic and portal systems are much less clear. This, combined with ANO1's various roles throughout a multitude of tissues throughout the body, command caution when targeting ANO1 as a therapeutic target and may require tissue-selective strategies.

Scramblases as Regulators of Proteolytic ADAM Function

Proteolytic ectodomain release is a key mechanism for regulating the function of many cell surface proteins. The sheddases ADAM10 and ADAM17 are the best-characterized members of the family of transmembrane disintegrin-like metalloproteinase. Constitutive proteolytic activities are low but can be abruptly upregulated via inside-out signaling triggered by diverse activating events. Emerging evidence indicates that the plasma membrane itself must be assigned a dominant role in upregulation of sheddase function. Data are discussed that tentatively identify phospholipid scramblases as central players during these events. We propose that scramblase-dependent externalization of the negatively charged phospholipid phosphatidylserine (PS) plays an important role in the final activation step of ADAM10 and ADAM17. In this manuscript, we summarize the current knowledge on the interplay of cell membrane changes, PS exposure, and proteolytic activity of transmembrane proteases as well as the potential consequences in the context of immune response, infection, and cancer. The novel concept that scramblases regulate the action of ADAM-proteases may be extendable to other functional proteins that act at the cell surface.

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.

Molecular mechanisms of activation and regulation of ANO1-Encoded Ca2+-Activated Cl- channels

Ca2+-activated Cl- channels (CaCCs) perform a multitude of functions including the control of cell excitability, regulation of cell volume and ionic homeostasis, exocrine and endocrine secretion, fertilization, amplification of olfactory sensory function, and control of smooth muscle cell contractility. CaCCs are the translated products of two members (ANO1 and ANO2, also known as TMEM16A and TMEM16B) of the Anoctamin family of genes comprising ten paralogs. This review focuses on recent progress in understanding the molecular mechanisms involved in the regulation of ANO1 by cytoplasmic Ca2+, post-translational modifications, and how the channel protein interacts with membrane lipids and protein partners. After first reviewing the basic properties of native CaCCs, we then present a brief historical perspective highlighting controversies about their molecular identity in native cells. This is followed by a summary of the fundamental biophysical and structural properties of ANO1. We specifically address whether the channel is directly activated by internal Ca2+ or indirectly through the intervention of the Ca2+-binding protein Calmodulin (CaM), and the structural domains responsible for Ca2+- and voltage-dependent gating. We then review the regulation of ANO1 by internal ATP, Calmodulin-dependent protein kinase II-(CaMKII)-mediated phosphorylation and phosphatase activity, membrane lipids such as the phospholipid phosphatidyl-(4,5)-bisphosphate (PIP2), free fatty acids and cholesterol, and the cytoskeleton. The article ends with a survey of physical and functional interactions of ANO1 with other membrane proteins such as CLCA1/2, inositol trisphosphate and ryanodine receptors in the endoplasmic reticulum, several members of the TRP channel family, and the ancillary Κ+ channel β subunits KCNE1/5.

TMEM16A/ANO1: Current Strategies and Novel Drug Approaches for Cystic Fibrosis

Cystic fibrosis (CF) is the most common of rare hereditary diseases in Caucasians, and it is estimated to affect 75,000 patients globally. CF is a complex disease due to the multiplicity of mutations found in the CF transmembrane conductance regulator (CFTR) gene causing the CFTR protein to become dysfunctional. Correctors and potentiators have demonstrated good clinical outcomes for patients with specific gene mutations; however, there are still patients for whom those treatments are not suitable and require alternative CFTR-independent strategies. Although CFTR is the main chloride channel in the lungs, others could, e.g., anoctamin-1 (ANO1 or TMEM16A), compensate for the deficiency of CFTR. This review summarizes the current knowledge on calcium-activated chloride channel (CaCC) ANO1 and presents ANO1 as an exciting target in CF.

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.

Chloride channel accessory 1 integrates chloride channel activity and mTORC1 in aging-related kidney injury

The mechanism of kidney injury in aging are not well understood. In order to identify hitherto unknown pathways of aging‐related kidney injury, we performed RNA‐Seq on kidney extracts of young and aged mice. Expression of chloride (Cl) channel accessory 1 (CLCA1) mRNA and protein was increased in the kidneys of aged mice. Immunostaining showed a marked increase in CLCLA1 expression in the proximal tubules of the kidney from aged mice. Increased kidney CLCA1 gene expression also correlated with aging in marmosets and in a human cohort. In aging mice, increased renal cortical CLCA1 content was associated with hydrogen sulfide (H₂S) deficiency, which was ameliorated by administering sodium hydrosulfide (NaHS), a source of H₂S. In order to study whether increased CLCA1 expression leads to injury phenotype and the mechanisms involved, stable transfection of proximal tubule epithelial cells overexpressing human CLCA1 (hCLCA1) was performed. Overexpression of hCLCA1 augmented Cl⁻ current via the Ca⁺⁺‐dependent Cl⁻ channel TMEM16A (anoctamin‐1) by patch‐clamp studies. hCLCA1 overexpression also increased the expression of fibronectin, a matrix protein, and induced the senescence‐associated secretory phenotype (SASP). Mechanistic studies underlying these changes showed that hCLCA1 overexpression leads to inhibition of AMPK activity and stimulation of mTORC1 as cellular signaling determinants of injury. Both TMEM16A inhibitor and NaHS reversed these signaling events and prevented changes in fibronectin and SASP. We conclude that CLCA1‐TMEM16A‐Cl⁻ current pathway is a novel mediator of kidney injury in aging that is regulated by endogenous H₂S.

The Fight against COVID-19 on the Multi-Protease Front and Surroundings: Could an Early Therapeutic Approach with Repositioning Drugs Prevent the Disease Severity?

The interaction between the membrane spike (S) protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and the transmembrane angiotensin-converting enzyme 2 (ACE2) receptor of the human epithelial host cell is the first step of infection, which has a critical role for viral pathogenesis of the current coronavirus disease-2019 (COVID-19) pandemic. Following the binding between S1 subunit and ACE2 receptor, different serine proteases, including TMPRSS2 and furin, trigger and participate in the fusion of the viral envelope with the host cell membrane. On the basis of the high virulence and pathogenicity of SARS-CoV-2, other receptors have been found involved for viral binding and invasiveness of host cells. This review comprehensively discusses the mechanisms underlying the binding of SARS-CoV2 to ACE2 and putative alternative receptors, and the role of potential co-receptors and proteases in the early stages of SARS-CoV-2 infection. Given the short therapeutic time window within which to act to avoid the devastating evolution of the disease, we focused on potential therapeutic treatments-selected mainly among repurposing drugs-able to counteract the invasive front of proteases and mild inflammatory conditions, in order to prevent severe infection. Using existing approved drugs has the advantage of rapidly proceeding to clinical trials, low cost and, consequently, immediate and worldwide availability.

Development of a QPatch-Automated Electrophysiology Assay for Identifying TMEM16A Small-Molecule Inhibitors

The calcium-activated chloride channel, TMEM16A, is involved in airway hydration and bronchoconstriction and is a promising target for respiratory disease. Drug development efforts around channels require an electrophysiology-based assay for identifying inhibitors or activators. TMEM16A has proven to be a difficult channel to record on automated electrophysiology platforms due to its propensity for rundown. We developed an automated, whole-cell, electrophysiology assay on the QPatch-48 to evaluate small-molecule inhibitors of TMEM16A. In this assay, currents remained stable for a duration of roughly 11 min, allowing for the cumulative addition of five concentrations of compounds and resulted in reproducible IC₅₀s. The absence of rundown was likely due to a low internal free-calcium level of 250 nM, which was high enough to produce large currents, but also maintained the voltage dependence of the channel. Current amplitude averaged 6 nA using the single-hole QPlate and the channel maintained outward rectification throughout the recording. Known TMEM16A inhibitors were tested and their IC₅₀s aligned with those reported in the literature using manual patch-clamp. Once established, this assay was used to validate novel TMEM16A inhibitors that were identified in our high-throughput fluorescent-based assay, as well as to assist in structure–activity relationship efforts by the chemists. Overall, we demonstrate an easy to operate, reproducible, automated electrophysiology assay using the QPatch-48 for TMEM16A drug development efforts.

Drugs that inhibit TMEM16 proteins block SARS-CoV-2 spike-induced syncytia

COVID-19 is a disease with unique characteristics that include lung thrombosis1, frequent diarrhoea2, abnormal activation of the inflammatory response3 and rapid deterioration of lung function consistent with alveolar oedema4. The pathological substrate for these findings remains unknown. Here we show that the lungs of patients with COVID-19 contain infected pneumocytes with abnormal morphology and frequent multinucleation. The generation of these syncytia results from activation of the SARS-CoV-2 spike protein at the cell plasma membrane level. On the basis of these observations, we performed two high-content microscopy-based screenings with more than 3,000 approved drugs to search for inhibitors of spike-driven syncytia. We converged on the identification of 83 drugs that inhibited spike-mediated cell fusion, several of which belonged to defined pharmacological classes. We focused our attention on effective drugs that also protected against virus replication and associated cytopathicity. One of the most effective molecules was the antihelminthic drug niclosamide, which markedly blunted calcium oscillations and membrane conductance in spike-expressing cells by suppressing the activity of TMEM16F (also known as anoctamin 6), a calcium-activated ion channel and scramblase that is responsible for exposure of phosphatidylserine on the cell surface. These findings suggest a potential mechanism for COVID-19 disease pathogenesis and support the repurposing of niclosamide for therapy.

The roles of calcium and ATP in the physiology and pathology of the exocrine pancreas

This review deals with the roles of calcium ions and ATP in the control of the normal functions of the different cell types in the exocrine pancreas as well as the roles of these molecules in the pathophysiology of acute pancreatitis. Repetitive rises in the local cytosolic calcium ion concentration in the apical part of the acinar cells not only activate exocytosis but also, via an increase in the intramitochondrial calcium ion concentration, stimulate the ATP formation that is needed to fuel the energy-requiring secretion process. However, intracellular calcium overload, resulting in a global sustained elevation of the cytosolic calcium ion concentration, has the opposite effect of decreasing mitochondrial ATP production, and this initiates processes that lead to necrosis. In the last few years it has become possible to image calcium signaling events simultaneously in acinar, stellate, and immune cells in intact lobules of the exocrine pancreas. This has disclosed processes by which these cells interact with each other, particularly in relation to the initiation and development of acute pancreatitis. By unraveling the molecular mechanisms underlying this disease, several promising therapeutic intervention sites have been identified. This provides hope that we may soon be able to effectively treat this often fatal disease.

Differential Regulation of Ca2+-Activated Cl- Channel TMEM16A Splice Variants by Membrane PI(4,5)P2

TMEM16A is a Ca²⁺-activated Cl⁻ channel that controls broad cellular processes ranging from mucus secretion to signal transduction and neuronal excitability. Recent studies have reported that membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) is an important cofactor that allosterically regulates TMEM16A channel activity. However, the detailed regulatory actions of PIP₂ in splice variants of TMEM16A remain unclear. Here, we demonstrated that the attenuation of membrane phosphoinositide levels selectively inhibited the current amplitude of the TMEM16A(ac) isoform by decreasing the slow, but not instantaneous, Cl⁻ currents, which are independent of the membrane potential and specific to PI(4,5)P₂ depletion. The attenuation of endogenous PI(4,5)P₂ levels by the activation of Danio rerio voltage-sensitive phosphatase (Dr-VSP) decreased the Cl⁻ currents of TMEM16A(ac) but not the TMEM16A(a) isoform, which was abolished by the co-expression of PIP 5-kinase type-1γ (PIPKIγ). Using the rapamycin-inducible dimerization of exogenous phosphoinositide phosphatases, we further revealed that the stimulatory effects of phosphoinositide on TMEM16A(ac) channels were similar in various membrane potentials and specific to PI(4,5)P₂, not PI4P and PI(3,4,5)P₃. Finally, we also confirmed that PI(4,5)P₂ resynthesis is essential for TMEM16A(ac) recovery from Dr-VSP-induced current inhibition. Our data demonstrate that membrane PI(4,5)P₂ selectively modulates the gating of the TMEM16A(ac) channel in an agonistic manner, which leads to the upregulation of TMEM16A(ac) functions in physiological conditions.

The Groovy TMEM16 Family: Molecular Mechanisms of Lipid Scrambling and Ion Conduction

The TMEM16 family of membrane proteins displays a remarkable functional dichotomy - while some family members function as Ca²⁺-activated anion channels, the majority of characterized TMEM16 homologs are Ca²⁺-activated lipid scramblases, which catalyze the exchange of phospholipids between the two membrane leaflets. Furthermore, some TMEM16 scramblases can also function as channels. Due to their involvement in important physiological processes, the family has been actively studied ever since their molecular identity was unraveled. In this review, we will summarize the recent advances in the field and how they influenced our view of TMEM16 family function and evolution. Structural, functional and computational studies reveal how relatively small rearrangements in the permeation pathway are responsible for the observed functional duality: while TMEM16 scramblases can adopt both ion- and lipid conductive conformations, TMEM16 channels can only populate the former. Recent data further provides the molecular details of a stepwise activation mechanism, which is initiated by Ca²⁺ binding and modulated by various cellular factors, including lipids. TMEM16 function and the surrounding membrane properties are inextricably intertwined, with the protein inducing bilayer deformations associated with scrambling, while the surrounding lipids modulate TMEM16 conformation and activity.

Ca2+ Sensitivity of Anoctamin 6/TMEM16F Is Regulated by the Putative Ca2+-Binding Reservoir at the N-Terminal Domain

Anoctamin 6/TMEM16F (ANO6) is a dual-function protein with Ca2+-activated ion channel and Ca2+-activated phospholipid scramblase activities, requiring a high intracellular Ca2+ concentration (e.g., half-maximal effective Ca2+ concentration [EC50] of [Ca2+]i > 10 μM), and strong and sustained depolarization above 0 mV. Structural comparison with Anoctamin 1/TMEM16A (ANO1), a canonical Ca2+- activated chloride channel exhibiting higher Ca2+ sensitivity (EC50 of 1 μM) than ANO6, suggested that a homologous Ca2+-transferring site in the N-terminal domain (Nt) might be responsible for the differential Ca2+ sensitivity and kinetics of activation between ANO6 and ANO1. To elucidate the role of the putative Ca2+-transferring reservoir in the Nt (Nt-CaRes), we constructed an ANO6-1-6 chimera in which Nt-CaRes was replaced with the corresponding domain of ANO1. ANO6- 1-6 showed higher sensitivity to Ca2+ than ANO6. However, neither the speed of activation nor the voltage-dependence differed between ANO6 and ANO6-1-6. Molecular dynamics simulation revealed a reduced Ca2+ interaction with Nt- CaRes in ANO6 than ANO6-1-6. Moreover, mutations on potentially Ca2+-interacting acidic amino acids in ANO6 Nt- CaRes resulted in reduced Ca2+ sensitivity, implying direct interactions of Ca2+ with these residues. Based on these results, we cautiously suggest that the net charge of Nt- CaRes is responsible for the difference in Ca2+ sensitivity between ANO1 and ANO6.

TMEM16A Mediates Mucus Production in Human Airway Epithelial Cells

TMEM16A is a Ca²⁺-activated chloride channel that was shown to enhance production and secretion of mucus in inflamed airways. It is, however, not clear whether TMEM16A directly supports mucus production, or whether mucin and TMEM16A are upregulated independently during inflammatory airway diseases such as asthma and cystic fibrosis (CF). We examined this question using BCi-NS1 cells, a human airway basal cell line that maintains multipotent differentiation capacity, and the two human airway epithelial cell lines, Calu-3 and CFBE. The data demonstrate that exposure of airway epithelial cells to IL-8 and IL-13, two cytokines known to be enhanced in CF and asthma, respectively, leads to an increase in mucus production. Expression of MUC5AC was fully dependent on expression of TMEM16A, as shown by siRNA knockdown of TMEM16A. In addition, different inhibitors of TMEM16A attenuated IL-13-induced mucus production. Interestingly, in CFBE cells expressing F508 delCFTR, IL-13 was unable to upregulate membrane expression of TMEM16A or Ca²⁺-activated whole cell currents. The regulator of TMEM16A, CLCA1, strongly augmented both Ca²⁺- and cAMP-activated Cl^- currents in cells expressing wtCFTR but failed to augment membrane expression of TMEM16A in F508 delCFTR-expressing CFBE cells. The data confirm the functional relationship between CFTR and TMEM16A and suggest an impaired upregulation of TMEM16A by IL-13 or CLCA1 in cells expressing the most frequent CF-causing mutation F508 delCFTR.

Allosteric modulation of alternatively spliced Ca2+-activated Cl- channels TMEM16A by PI(4,5)P2 and CaMKII

Transmembrane 16A (TMEM16A, anoctamin1), 1 of 10 TMEM16 family proteins, is a Cl- channel activated by intracellular Ca2+ and membrane voltage. This channel is also regulated by the membrane phospholipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]. We find that two splice variants of TMEM16A show different sensitivity to endogenous PI(4,5)P2 degradation, where TMEM16A(ac) displays higher channel activity and more current inhibition by PI(4,5)P2 depletion than TMEM16A(a). These two channel isoforms differ in the alternative splicing of the c-segment (exon 13). The current amplitude and PI(4,5)P2 sensitivity of both TMEM16A(ac) and (a) are significantly strengthened by decreased free cytosolic ATP and by conditions that decrease phosphorylation by Ca2+/calmodulin-dependent protein kinase II (CaMKII). Noise analysis suggests that the augmentation of currents is due to a rise of single-channel current (i), but not of channel number (N) or open probability (PO). Mutagenesis points to arginine 486 in the first intracellular loop as a putative binding site for PI(4,5)P2, and to serine 673 in the third intracellular loop as a site for regulatory channel phosphorylation that modulates the action of PI(4,5)P2 In silico simulation suggests how phosphorylation of S673 allosterically and differently changes the structure of the distant PI(4,5)P2-binding site between channel splice variants with and without the c-segment exon. In sum, our study reveals the following: differential regulation of alternatively spliced TMEM16A(ac) and (a) by plasma membrane PI(4,5)P2, modification of these effects by channel phosphorylation, identification of the molecular sites, and mechanistic explanation by in silico simulation.

Endothelial Dysfunction Following Enhanced TMEM16A Activity in Human Pulmonary Arteries

Endothelial dysfunction is one of the hallmarks of different vascular diseases, including pulmonary arterial hypertension (PAH). Ion channelome changes have long been connected to vascular remodeling in PAH, yet only recently has the focus shifted towards Ca²⁺-activated Cl⁻ channels (CaCC). The most prominent member of the CaCC TMEM16A has been shown to contribute to the pathogenesis of idiopathic PAH (IPAH) in pulmonary arterial smooth muscle cells, however its role in the homeostasis of healthy human pulmonary arterial endothelial cells (PAECs) and in the development of endothelial dysfunction remains underrepresented. Here we report enhanced TMEM16A activity in IPAH PAECs by whole-cell patch-clamp recordings. Using adenoviral-mediated TMEM16A increase in healthy primary human PAECs in vitro and in human pulmonary arteries ex vivo, we demonstrate the functional consequences of the augmented TMEM16A activity: alterations of Ca²⁺ dynamics and eNOS activity as well as decreased NO production, PAECs proliferation, wound healing, tube formation and acetylcholine-mediated relaxation of human pulmonary arteries. We propose that the ERK1/2 pathway is specifically affected by elevated TMEM16A activity, leading to these pathological changes. With this work we introduce increased TMEM16A activity in the cell membrane of human PAECs for the development of endothelial dysfunction in PAH.

Regulation of the Ca2+-activated chloride channel Anoctamin-1 (TMEM16A) by Ca2+-induced interaction with FKBP12 and calcineurin

Chloride fluxes through the calcium-gated chloride channel Anoctamin-1 (TMEM16A) control blood pressure, secretion of saliva, mucin, insulin, and melatonin, gastrointestinal motility, sperm capacitation and motility, and pain sensation. Calcium activates a myriad of regulatory proteins but how these proteins affect TMEM16A activity is unresolved. Here we show by co-immunoprecipitation that increasing intracellular calcium with ionomycin or by activating sphingosine-1-phosphate receptors, induces coupling of calcium/calmodulin-dependent phosphatase calcineurin and prolyl isomerase FK506-binding protein 12 (FKBP12) to TMEM16A in HEK-293 cells. Application of drugs that target either calcineurin (cyclosporine A) or FKBP12 (tacrolimus known as FK506 and sirolimus known as rapamycin) caused a decrease in TMEM16A activity. In addition, FK506 and BAPTA-AM prevented co-immunoprecipitation between FKBP12 and TMEM16A. FK506 rendered the channel insensitive to cyclosporine A without altering its apparent calcium sensitivity whereas zero intracellular calcium blocked the effect of FK506. Rapamycin decreased TMEM16A activity in cells pre-treated with cyclosporine A or FK506. These results suggest the formation of a TMEM16A-FKBP12-calcineurin complex that regulates channel function. We conclude that upon a cytosolic calcium increase the TMEM16A-FKPB12-calcineurin trimers are assembled. Such hetero-oligomerization enhances TMEM16A channel activity but is not mandatory for activation by calcium.

Regulation of TMEM16A by CK2 and Its Role in Cellular Proliferation

Casein kinase 2 (CK2) is a highly ubiquitous and conserved serine/threonine kinase that forms a tetramer consisting of a catalytic subunit (CK2α) and a regulatory subunit (CK2β). Despite being ubiquitous, CK2 is commonly found at higher expression levels in cancer cells, where it inhibits apoptosis, and supports cell migration and proliferation. The Ca²⁺-activated chloride channel TMEM16A shows similar effects in cancer cells: TMEM16A increases cell proliferation and migration and is highly expressed in squamous cell carcinoma of the head and neck (HNSCC) as well as other malignant tumors. A microscopy-based high-throughput screening was performed to identify proteins that regulate TMEM16A. Within this screen, CK2 was found to be required for proper membrane expression of TMEM16A. small interfering (si) RNA-knockdown of CK2 reduced plasma membrane expression of TMEM16A and inhibited TMEM16A whole cell currents in (cystic fibrosis bronchial epithelial) CFBE airway epithelial cells and in the head and neck cancer cell lines Cal33 and BHY. Inhibitors of CK2, such as TBB and the preclinical compound CX4549 (silmitasertib), also blocked membrane expression of TMEM16A and Ca²⁺-activated whole cell currents. siRNA-knockout of CK2 and its pharmacological inhibition, as well as knockdown or inhibition of TMEM16A by either niclosamide or Ani9, attenuated cell proliferation. Simultaneous inhibition of CK2 and TMEM16A strongly potentiated inhibition of cell proliferation. Although membrane expression of TMEM16A is reduced by inhibition of CK2, our data suggest that the antiproliferative effects by inhibition of CK2 are mostly independent of TMEM16A. Simultaneous inhibition of TMEM16A by niclosamide and inhibition of CK2 by silmitasertib was additive with respect to blocking cell proliferation, while cytotoxicity was reduced when compared to solely blockade of CK2. Therefore, parallel blockade TMEM16A by niclosamide may assist with anticancer therapy by silmitasertib.

Molecular mechanism of TMEM16A regulation: role of CaMKII and PP1/PP2A

This study explored the mechanism by which Ca²⁺-activated Cl^- channels (CaCCs) encoded by the Tmem16a gene are regulated by calmodulin-dependent protein kinase II (CaMKII) and protein phosphatases 1 (PP1) and 2A (PP2A). Ca²⁺-activated Cl^- currents (I_ClCa) were recorded from HEK-293 cells expressing mouse TMEM16A. I_ClCa were evoked using a pipette solution in which free Ca²⁺ concentration was clamped to 500 nM, in the presence (5 mM) or absence of ATP. With 5 mM ATP, I_ClCa decayed to <50% of the initial current magnitude within 10 min after seal rupture. I_ClCa rundown seen with ATP-containing pipette solution was greatly diminished by omitting ATP. I_ClCa recorded after 20 min of cell dialysis with 0 ATP were more than twofold larger than those recorded with 5 mM ATP. Intracellular application of autocamtide-2-related inhibitory peptide (5 µM) or KN-93 (10 µM), two specific CaMKII inhibitors, produced a similar attenuation of TMEM16A rundown. In contrast, internal application of okadaic acid (30 nM) or cantharidin (100 nM), two nonselective PP1 and PP2A blockers, promoted the rundown of TMEM16A in cells dialyzed with 0 ATP. Mutating serine 528 of TMEM16A to an alanine led to a similar inhibition of TMEM16A rundown to that exerted by either one of the two CaMKII inhibitors tested, which was not observed for three putative CaMKII consensus sites for phosphorylation (T273, T622, and S730). Our results suggest that TMEM16A-mediated CaCCs are regulated by CaMKII and PP1/PP2A. Our data also suggest that serine 528 of TMEM16A is an important contributor to the regulation of I_ClCa by CaMKII.

Molecular basis of PIP2-dependent regulation of the Ca2+-activated chloride channel TMEM16A

The calcium-activated chloride channel (CaCC) TMEM16A plays crucial roles in regulating neuronal excitability, smooth muscle contraction, fluid secretion and gut motility. While opening of TMEM16A requires binding of intracellular Ca2+, prolonged Ca2+-dependent activation results in channel desensitization or rundown, the mechanism of which is unclear. Here we show that phosphatidylinositol (4,5)-bisphosphate (PIP2) regulates TMEM16A channel activation and desensitization via binding to a putative binding site at the cytosolic interface of transmembrane segments (TMs) 3-5. We further demonstrate that the ion-conducting pore of TMEM16A is constituted of two functionally distinct modules: a Ca2+-binding module formed by TMs 6-8 and a PIP2-binding regulatory module formed by TMs 3-5, which mediate channel activation and desensitization, respectively. PIP2 dissociation from the regulatory module results in ion-conducting pore collapse and subsequent channel desensitization. Our findings thus provide key insights into the mechanistic understanding of TMEM16 channel gating and lipid-dependent regulation.

Phosphatidylinositol 4,5-bisphosphate (PIP2) and Ca2+ are both required to open the Cl- channel TMEM16A

Transmembrane member 16A (TMEM16A) is a widely expressed Ca²⁺-activated Cl^- channel with various physiological functions ranging from mucosal secretion to regulating smooth muscle contraction. Understanding how TMEM16A controls these physiological processes and how its dysregulation may cause disease requires a detailed understanding of how cellular processes and second messengers alter TMEM16A channel gating. Here we assessed the regulation of TMEM16A gating by recording Ca²⁺-evoked Cl^- currents conducted by endogenous TMEM16A channels expressed in Xenopus laevis oocytes, using the inside-out configuration of the patch clamp technique. During continuous application of Ca²⁺, we found that TMEM16A-conducted currents decay shortly after patch excision. Such current rundown is common among channels regulated by phosphatidylinositol 4,5-bisphosphate (PIP₂). Thus, we sought to investigate a possible role of PIP₂ in TMEM16A gating. Consistently, synthetic PIP₂ rescued the current after rundown, and the application of PIP₂ modulating agents altered the speed kinetics of TMEM16A current rundown. First, two PIP₂ sequestering agents, neomycin and anti-PIP₂, applied to the intracellular surface of excised patches sped up TMEM16A current rundown to nearly twice as fast. Conversely, rephosphorylation of phosphatidylinositol (PI) derivatives into PIP₂ using Mg-ATP or inhibiting dephosphorylation of PIP₂ using β-glycerophosphate slowed rundown by nearly 3-fold. Our results reveal that TMEM16A regulation is more complicated than it initially appeared; not only is Ca²⁺ necessary to signal TMEM16a opening, but PIP₂ is also required. These findings improve our understanding of how the dysregulation of these pathways may lead to disease and suggest that targeting these pathways could have utility for potential therapies.

Calcium-activated chloride channels: Potential targets for antinociceptive therapy

The molecular identity of calcium-activated chloride channels (CaCCs) was clarified only some ten years ago when it was linked to the family of "transmembrane proteins of unknown function 16″ (TMEM16). Since then, numerous studies have been conducted both to define their role in physiology and identify their biophysical functions. For the latter, the ultrastructural description of mouse TMEM16 A was a breakthrough. CaCCs were functionally described in a number of different tissues including first-order sensory neurons. The activating rise in intracellular calcium concentration can be caused by an influx of calcium through other calcium permeable ion channels. Calcium release from intracellular stores, mediated by G-protein coupled receptors, also leads to CaCC activation. Prominent inflammatory mediators like bradykinin or serotonin stimulate CaCCs via such a mechanism. The (patho) physiological function of these ion channels renders them promising targets for antinociceptive treatment.

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.

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.

Identification of a Novel EF-Loop in the N-terminus of TRPM2 Channel Involved in Calcium Sensitivity

As an oxidative stress sensor, transient receptor potential melastatin 2 (TRPM2) channel is involved in many physiological and pathological processes including warmth sensing, ischemia injury, inflammatory diseases and diabetes. Intracellular calcium is critical for TRPM2 channel activation and the IQ-like motif in the N-terminus has been shown to be important by mediating calmodulin binding. Sequence analysis predicted two potential EF-loops in the N-terminus of TRPM2. Site-directed mutagenesis combining with functional assay showed that substitution with alanine of several residues, most of which are conserved in the typical EF-loop, including D267, D278, D288, and E298 dramatically reduced TRPM2 channel currents. By further changing the charges or side chain length of these conserved residues, our results indicate that the negative charge of D267 and the side chain length of D278 are critical for calcium-induced TRPM2 channel activation. G272I mutation also dramatically reduced the channel currents, suggesting that this site is critical for calcium-induced TRPM2 channel activation. Furthermore, D267A mutant dramatically reduced the currents induced by calcium alone compared with that by ADPR, indicating that D267 residue in D267–D278 motif is the most important site for calcium sensitivity of TRPM2. In addition, inside-out recordings showed that mutations at D267, G272, D278, and E298 had no effect on single-channel conductance. Taken together, our data indicate that D267–D278 motif in the N-terminus as a novel EF-loop is critical for calcium-induced TRPM2 channel activation.

Calcium-Activated Cl- Channel: Insights on the Molecular Identity in Epithelial Tissues

Calcium-activated chloride secretion in epithelial tissues has been described for many years. However, the molecular identity of the channel responsible for the Ca2+-activated Cl− secretion in epithelial tissues has remained a mystery. More recently, TMEM16A has been identified as a new putative Ca2+-activated Cl− channel (CaCC). The primary goal of this article will be to review the characterization of TMEM16A, as it relates to the physical structure of the channel, as well as important residues that confer voltage and Ca2+-sensitivity of the channel. This review will also discuss the role of TMEM16A in epithelial physiology and potential associated-pathophysiology. This will include discussion of developed knockout models that have provided much needed insight on the functional localization of TMEM16A in several epithelial tissues. Finally, this review will examine the implications of the identification of TMEM16A as it pertains to potential novel therapies in several pathologies.

The potent insulin secretagogue effect of betulinic acid is mediated by potassium and chloride channels

Betulinic acid (BA) has been described as an insulin secretagogue which may explain its potent antihyperglycemic effect; however, the exact role of BA as an insulinogenic agent is not clear. The aim of this study was to investigate the mechanism of BA on calcium influx and static insulin secretion in pancreatic islets isolated from euglycemic rats. We found that BA triggers calcium influx by a mechanism dependent on ATP-dependent potassium channels and L-type voltage-dependent calcium channels. Additionally, the voltage-dependent and calcium-dependent chloride channels are also involved in the mechanism of BA, probably due to an indirect stimulation of calcium entry and increased intracellular calcium. Additionally, the downstream activation of PKC, which is necessary for the effect of BA on calcium influx, is involved in the full stimulatory response of the triterpene. BA stimulated the static secretion of insulin in pancreatic islets, indicating that the abrupt calcium influx may be a key step in its secretagogue effect. As such, BA stimulates insulin secretion through the activation of electrophysiological mechanisms, such as the closure of potassium channels and opening of calcium and chloride channels, inducing cellular depolarization associated with metabolic-biochemical effects, in turn activating PKC and ensuring the secretion of insulin.

Bioinformatic characterization of the Anoctamin Superfamily of Ca2+-activated ion channels and lipid scramblases

Our laboratory has developed bioinformatic strategies for identifying distant phylogenetic relationships and characterizing families and superfamilies of transport proteins. Results using these tools suggest that the Anoctamin Superfamily of cation and anion channels, as well as lipid scramblases, includes three functionally characterized families: the Anoctamin (ANO), Transmembrane Channel (TMC) and Ca²⁺-permeable Stress-gated Cation Channel (CSC) families; as well as four families of functionally uncharacterized proteins, which we refer to as the Anoctamin-like (ANO-L), Transmembrane Channel-like (TMC-L), and CSC-like (CSC-L1 and CSC-L2) families. We have constructed protein clusters and trees showing the relative relationships among the seven families. Topological analyses suggest that the members of these families have essentially the same topologies. Comparative examination of these homologous families provides insight into possible mechanisms of action, indicates the currently recognized organismal distributions of these proteins, and suggests drug design potential for the disease-related channel proteins.

Compartmentalized crosstalk of CFTR and TMEM16A (ANO1) through EPAC1 and ADCY1

Airway epithelial cells express both Ca²⁺ activated TMEM16A/ANO1 and cAMP activated CFTR anion channels. Previous work suggested a significant crosstalk of intracellular Ca²⁺ and cAMP signaling pathways, leading to activation of both chloride channels. We demonstrate that in airway epithelial cells, stimulation of purinergic or muscarinic G-protein coupled receptors (GPCRs) activates TMEM16A and CFTR. Additional expression of G_q/11 and phospholipase C coupled GPCRs strongly enhanced the crosstalk between Ca²⁺- and cAMP-dependent signaling. Knockdown of endogenous GRCRs attenuated crosstalk and functional coupling between TMEM16A and CFTR. The number of receptors did not affect expression or membrane localization of TMEM16A or CFTR, but controlled assembly of the local signalosome. GPCRs translocate Ca²⁺-sensitive adenylate cyclase type 1 (ADCY1) and exchange protein directly activated by cAMP (EPAC1) to particular plasma membrane domains containing GPCRs, CFTR and TMEM16A, thereby producing compartmentalized Ca²⁺ and cAMP signals and significant crosstalk. While biosynthesis and membrane trafficking of CFTR requires a functional Golgi apparatus, maturation and membrane trafficking of TMEM16A may occur independent of the Golgi. Because Ca²⁺ activated TMEM16A currents are only transient, continuous Cl^- secretion by airway epithelial cells requires CFTR. The present data also explain why receptor-dependent activation of TMEM16A is more efficient than direct stimulation by Ca²⁺.

The association between the expression of PAR2 and TMEM16A and neuropathic pain

Chronic constriction injury (CCI) of the sciatic nerve may induce dorsal root ganglion (DRG) neuronal hyperexcitability and behaviorally expressed hyperalgesia. CCI is a model of neuropathic pain. To investigate the association between the expression of protease activated receptor 2 (PAR2), TMEM16A and neuropathic pain, the expression of PAR2 and TMEM16A proteins in the DRG neurons of rats following CCI of the sciatic nerve was investigated. Following the creation of the CCI model, the thermal withdrawal latency (TWL) was examined by a hot plate test. An immunofluorescence assay and western blot assay were performed to determine the expression of PAR2 and TMEM16A proteins in the ipsilateral L_4–6 DRG neurons. The concentration of inositol 1,4,5-triphosphate (IP₃) in the L_4–6 DRG was determined by ELISA. In the CCI-D7 (7 days after CCI) and CCI-D14 (14 days after CCI) treatment groups, the TWL of rats was significantly shorter than that in the sham operated group (P<0.01; n=12). The expression of PAR2 and TMEM16A proteins in the CCI-D7 and CCI-D14 groups were significantly upregulated compared with the sham operated group (P<0.05; n=12). Additionally, it was revealed that PAR2 and TMEM16A were co-expressed in DRG neurons. It was also observed that IP₃ significantly increased in the CCI-D7 and CCI-D14 groups compared with the sham operation group (P<0.05; n=6) as PAR2 and TMEM16A also increased. These findings suggest that the upregulation of PAR2 and TMEM16A in DRG neurons, the co-expression of the two proteins and increasing IP₃ are critical to the development of neuropathic pain.

Substituted 2-Acylaminocycloalkylthiophene-3-carboxylic Acid Arylamides as Inhibitors of the Calcium-Activated Chloride Channel Transmembrane Protein 16A (TMEM16A)

Transmembrane protein 16A (TMEM16A), also called anoctamin 1 (ANO1), is a calcium-activated chloride channel expressed widely mammalian cells, including epithelia, vascular smooth muscle tissue, electrically excitable cells, and some tumors. TMEM16A inhibitors have been proposed for treatment of disorders of epithelial fluid and mucus secretion, hypertension, asthma, and possibly cancer. Herein we report, by screening, the discovery of 2-acylaminocycloalkylthiophene-3-carboxylic acid arylamides (AACTs) as inhibitors of TMEM16A and analysis of 48 synthesized analogs (10ab-10bw) of the original AACT compound (10aa). Structure-activity studies indicated the importance of benzene substituted as 2- or 4-methyl, or 4-fluoro, and defined the significance of thiophene substituents and size of the cycloalkylthiophene core. The most potent compound (10bm), which contains an unusual bromodifluoroacetamide at the thiophene 2-position, had IC50 of ∼30 nM, ∼3.6-fold more potent than the most potent previously reported TMEM16A inhibitor 4 (Ani9), and >10-fold improved metabolic stability. Direct and reversible inhibition of TMEM16A by 10bm was demonstrated by patch-clamp analysis. AACTs may be useful as pharmacological tools to study TMEM16A function and as potential drug development candidates.

Molecular, biophysical, and pharmacological properties of calcium-activated chloride channels

Calcium-activated chloride channels (CaCCs) are a family of anionic transmembrane ion channels. They are mainly responsible for the movement of Cl^- and other anions across the biological membranes, and they are widely expressed in different tissues. Since the Cl^- flow into or out of the cell plays a crucial role in hyperpolarizing or depolarizing the cells, respectively, the impact of intracellular Ca²⁺ concentration on these channels is attracting a lot of attentions. After summarizing the molecular, biophysical, and pharmacological properties of CaCCs, the role of CaCCs in normal cellular functions will be discussed, and I will emphasize how dysregulation of CaCCs in pathological conditions can account for different diseases. A better understanding of CaCCs and a pivotal regulatory role of Ca²⁺ can shed more light on the therapeutic strategies for different neurological disorders that arise from chloride dysregulation, such as asthma, cystic fibrosis, and neuropathic pain.

CK2 is a key regulator of SLC4A2-mediated Cl-/HCO3- exchange in human airway epithelia

Transepithelial bicarbonate secretion by human airway submucosal glands and surface epithelial cells is crucial to maintain the pH-sensitive innate defence mechanisms of the lung. cAMP agonists stimulate HCO₃^- secretion via coordinated increases in basolateral HCO₃^- influx and accumulation, as well as CFTR-dependent HCO₃^- efflux at the luminal membrane of airway epithelial cells. Here, we investigated the regulation of a basolateral located, DIDS-sensitive, Cl^-/HCO₃^- exchanger, anion exchanger 2 (AE2; SLC4A2) which is postulated to act as an acid loader, and therefore potential regulator of HCO₃^- secretion, in human airway epithelial cells. Using intracellular pH measurements performed on Calu-3 cells, we demonstrate that the activity of the basolateral Cl^-/HCO₃^- exchanger was significantly downregulated by cAMP agonists, via a PKA-independent mechanism and also required Ca²⁺ and calmodulin under resting conditions. AE2 contains potential phosphorylation sites by a calmodulin substrate, protein kinase CK2, and we demonstrated that AE2 activity was reduced in the presence of CK2 inhibition. Moreover, CK2 inhibition abolished the activity of AE2 in primary human nasal epithelia. Studies performed on mouse AE2 transfected into HEK-293T cells confirmed almost identical Ca²⁺/calmodulin and CK2 regulation to that observed in Calu-3 and primary human nasal cells. Furthermore, mouse AE2 activity was reduced by genetic knockout of CK2, an effect which was rescued by exogenous CK2 expression. Together, these findings are the first to demonstrate that CK2 is a key regulator of Cl^--dependent HCO₃^- export at the serosal membrane of human airway epithelial cells.

TMEM16A/ANO1 suppression improves response to antibody-mediated targeted therapy of EGFR and HER2/ERBB2

TMEM16A, a Ca²⁺ -activated Cl^- channel, contributes to tumor growth in breast cancer and head and neck squamous cell carcinoma (HNSCC). Here, we investigated whether TMEM16A influences the response to EGFR/HER family-targeting biological therapies. Inhibition of TMEM16A Cl^- channel activity in breast cancer cells with HER2 amplification induced a loss of viability. Cells resistant to trastuzumab, a monoclonal antibody targeting HER2, showed an increase in TMEM16A expression and heightened sensitivity to Cl^- channel inhibition. Treatment of HNSCC cells with cetuximab, a monoclonal antibody targeting EGFR, and simultaneous TMEM16A suppression led to a pronounced loss of viability. Biochemical analyses of cells subjected to TMEM16A inhibitors or expressing chloride-deficient forms of TMEM16A provide further evidence that TMEM16A channel function may play a role in regulating EGFR/HER2 signaling. These data demonstrate that TMEM16A regulates EGFR and HER2 in growth and survival pathways. Furthermore, in the absence of TMEM16A cotargeting, tumor cells may acquire resistance to EGFR/HER inhibitors. Finally, targeting TMEM16A improves response to biological therapies targeting EGFR/HER family members.

New Insights on the Regulation of Ca2+ -Activated Chloride Channel TMEM16A

TMEM16A, also known as anoctamin 1, is a recently identified Ca²⁺ -activated chloride channel and the first member of a 10-member TMEM16 family. TMEM16A dysfunction is implicated in many diseases such as cancer, hypertension, and cystic fibrosis. TMEM16A channels are well known to be dually regulated by voltage and Ca²⁺ . In addition, recent studies have revealed that TMEM16A channels are regulated by many molecules such as calmodulin, protons, cholesterol, and phosphoinositides, and a diverse range of stimuli such as thermal and mechanical stimuli. A better understanding of the regulatory mechanisms of TMEM16A is important to understand its physiological and pathological role. Recently, the crystal structure of a TMEM16 family member from the fungus Nectria haematococcaten (nhTMEM16) is discovered, and provides valuable information for studying the structure and function of TMEM16A. In this review, we discuss the structure and function of TMEM16A channels based on the crystal structure of nhTMEM16A and focus on the regulatory mechanisms of TMEM16A channels. J. Cell. Physiol. 232: 707-716, 2017. © 2016 Wiley Periodicals, Inc.