Expression and function of epithelial anoctamins

Ano6 disruption impairs acinar cell regulatory volume decrease and protein secretion in murine submandibular salivary glands

The widely expressed Anoctamin 6 (Ano6) supports different Ca²⁺ -dependent functions, but little is known about its role in salivary glands. Mouse submandibular gland (SMG) acinar cells exhibited a robust regulatory volume decrease (RVD) following cell swelling that was reduced approximately 70% in Ano6^-/- mice. Ca²⁺ -free conditions nearly eliminated the RVD response suggesting that Ano6 is an obligatory component of the cell volume-activated, Ca²⁺ -dependent RVD pathway in salivary gland acinar cells. Ex vivo agonist-stimulated secretion of water and ions was unaffected by Ano6 disruption under both isotonic and hypotonic conditions suggesting that Ano6 does not play a major role in fluid and electrolyte secretion. In contrast, the total amount of β-adrenergic-dependent protein secretion by the SMG was significantly reduced in Ano6^-/- mice. Closer inspection of these latter results revealed that protein secretion was affected only in the female SMG by Ano6 disruption. These results indicate that Ano6 modulates the RVD response and protein secretion by salivary gland acinar cells.

ANO9 Regulated Cell Cycle in Human Esophageal Squamous Cell Carcinoma

BACKGROUND

Few studies have reported the function and activation mechanism of ANO9 in esophageal squamous cell carcinoma (ESCC). The current study aimed to investigate the role of ANO9 in the regulation of tumor progression.

METHODS

Knockdown experiments with human ESCC cell lines were performed using ANO9 siRNA, and the effects on cell proliferation, the cell cycle, apoptosis, and cellular movement were analyzed. Immunohistochemistry (IHC) analysis was performed on 57 primary tumor samples obtained from ESCC patients.

RESULTS

In an in vitro study, depletion of ANO9 reduced cell proliferation, invasion, and migration in KYSE150 and KYSE 790 cells. In the cell cycle analysis, depletion of ANO9 increased the number of cells in G₀/G₁ arrest. In addition, the knockdown of ANO9 increased apoptosis. The results of the microarray analysis indicated that various centrosome-related genes such as CEP120, CNTRL, and SPAST were up- or downregulated in ANO9-depleted KYSE150 cells. The IHC results showed that high expression of ANO9 was associated with poor prognosis.

CONCLUSIONS

The results of the current study suggest that ANO9 regulates the cell cycle via centrosome-related genes in ESCC.

Mechanisms and regulation of chloride transport in the Malpighian tubules of the larval cabbage looper Trichoplusia ni

Malpighian tubules (MTs) and the hindgut together constitute the excretory system of insects. Larvae of lepidopterans (butterflies and moths) demonstrate the so-called cryptonephric arrangement, where the distal blind end of each MT is embedded into the rectal complex. The rest of the free tubule is modified into several distinct regions that differ greatly in the transport of cations and water. However, relatively little is known about the transport of counter-anions (e.g., Cl^- and HCO₃^-) by the MTs of lepidopteran larvae. In the current study we used ion-selective microelectrodes to characterize Cl^- transport in the distinct regions of the free MT of the larval Trichoplusia ni. Firstly, we note that Cl^- transport in the MTs is sensitive to the Cl^- concentration of the bathing saline, and several regions of the MTs are capable of either secreting or reabsorbing Cl^-. In the distal ileac plexus (DIP), a region previously characterized by cellular heterogeneity and its ability to switch between cation secretion and reabsorption, principal cells (PCs) toggled between Cl^- reabsorption (in high-Cl^- saline) and Cl^- secretion (in low-Cl^- saline). In contrast, secondary cells (SCs) in the DIP secreted Cl^- regardless of saline Cl^- concentration. Mechanistically, we have detected a number of 'leak' and ligand-gated Cl^- channels (ClC) and demonstrated that Cl^- channels are involved in Cl^- secretion. Additionally, we demonstrated that the lumen-positive transepithelial potential increased in response to glycine. Using the scanning ion-selective electrode technique we demonstrated that glycine stimulated Cl^- secretion by SCs, but not by PCs. In contrast, when MTs were deprived of glycine, a decrease in Cl^- secretion, coupled with a decrease in the TEP, was observed. In contrast to the effects of glycine, an active dose of helicokinin reduced Cl^- secretion by PCs, but not by SCs. Lastly, we detected expression of chloride-bicarbonate exchangers (CBE) in all regions of the free tubule. Scans of H⁺ transport across the tubule indicated that base equivalents are likely reabsorbed across the ileac plexus. Blocking ClC or CBE led to secretion of a more basic fluid, indicating lack of base reabsorption. We suggest that the transport of Cl^- in the MTs of larval lepidopterans (i) may be correlated with the reabsorption of base, (ii) may be sensitive to Cl^- concentration in the haemolymph, and (iii) could be regulated by helicokinin and glycine.

Proteome and Secretome Dynamics of Human Retinal Pigment Epithelium in Response to Reactive Oxygen Species

Age-related macular degeneration (AMD) is the leading cause of blindness in developed countries, and is characterized by slow retinal degeneration linked to chronic reactive oxygen species (ROS) in the retinal pigmented epithelium (RPE). The molecular mechanisms leading to RPE dysfunction in response to ROS are unclear. Here, human stem cell-derived RPE samples were stressed with ROS for 1 or 3 weeks, and both intracellular and secreted proteomes were quantified by mass spectrometry. ROS increased glycolytic proteins but decreased mitochondrial complex I subunits, as well as membrane proteins required for endocytosis. RPE secreted over 1,000 proteins, many of which changed significantly due to ROS. Notably, secreted APOE is decreased 4-fold, and urotensin-II, the strongest known vasoconstrictor, doubled. Furthermore, secreted TGF-beta is increased, and its cognate signaler BMP1 decreased in the secretome. Together, our results paint a detailed molecular picture of the retinal stress response in space and time.

Control of Ion Transport by Tmem16a Expressed in Murine Intestine

Cl^– secretion by the human and murine intestinal epithelium occurs through the cystic fibrosis transmembrane conductance regulator (cftr). However, the Ca²⁺ activated Cl^– channel Tmem16a was shown to contribute to Cl^– secretion, mainly, but not exclusively, as a basolaterally located Cl^– channel that controls basolateral Ca²⁺ signaling, and thus activation of basolateral Ca²⁺ dependent Sk4 K⁺ channels. In intestinal goblet cells, Tmem16a was shown to regulated Ca²⁺ signals required for exocytosis of mucus. Because a recent report denied the existence and functional role of Tmem16a in murine intestine, we reexamined in detail expression of mRNA and protein for Tmem16a in mouse colon. In experiments using short-circuited Ussing chamber and whole cell patch-clamp techniques, we further compared ion transport in wild type (WT) colon with that in mice with intestinal epithelial specific knockout of Tmem16a. As reported earlier we fully confirm expression of Tmem16a in colonic epithelial cells and the role of Tmem16a for both Ca²⁺-dependent and cAMP-regulated ion secretion.

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.

Plasma membrane-localized TMEM16 proteins are indispensable for expression of CFTR

The cystic fibrosis transmembrane conductance regulator (CFTR) is the secretory chloride channel in epithelial tissues that has a central role in cystic fibrosis (CF) lung and gastrointestinal disease. A recent publication demonstrates a close association between CFTR and TMEM16A, the calcium-activated chloride channel. Thus, no CFTR chloride currents could be detected in airways and large intestine from mice lacking epithelial expression of TMEM16A. Here, we demonstrate that another plasma membrane-localized TMEM16 paralogue, TMEM16F, can compensate for the lack of TMEM16A. Using TMEM16 knockout mice, human lymphocytes, and a number of human cell lines with endogenous protein expression or heterologous expression, we demonstrate that CFTR can only function in the presence of either TMEM16A or TMEM16F. Double knockout of intestinal epithelial TMEM16A/F expression did not produce offsprings, suggesting a lethal phenotype in utero. Plasma membrane-localized TMEM16A or TMEM16F is required for exocytosis and expression of CFTR in the plasma membrane. TMEM16A/F proteins may therefore have an impact on disease severity in CF. KEY MESSAGES: • Cystic fibrosis is caused by the defective Cl- channel cystic fibrosis transmembrane conductance regulator (CFTR). • A close relationship exists between CFTR and the calcium-activated chloride channels TMEM16A/TMEM16F. • In conditional airway and intestinal knockout mice, lymphocytes from Scott disease patients and in overexpressing cells, CFTR is not functional in the absence of TMEM16A and TMEM16F. • TMEM16A and TMEM16F support membrane exocytosis and are essential for plasma membrane insertion of CFTR.

A major interspecies difference in the ionic selectivity of megakaryocyte Ca2+-activated channels sensitive to the TMEM16F inhibitor CaCCinh-A01

TMEM16F is a surface membrane protein critical for platelet procoagulant activity, which exhibits both phospholipid scramblase and ion channel activities following sustained elevation of cytosolic Ca2+. The extent to which the ionic permeability of TMEM16F is important for platelet scramblase responses remains controversial. To date, only one study has reported the electrophysiological properties of TMEM16F in cells of platelet/megakaryocyte lineage, which observed cation-selectivity within excised patch recordings from murine marrow-derived megakaryocytes. This contrasts with reports using whole-cell recordings that describe this channel as displaying either selectivity for anions or being relatively non-selective amongst the major physiological monovalent ions. We have studied TMEM16F expression and channel activity in primary rat and mouse megakaryocytes and the human erythroleukemic (HEL) cell line that exhibits megakaryocytic surface markers. Immunocytochemical analysis was consistent with surface TMEM16F expression in cells from all three species. Whole-cell recordings in the absence of K+-selective currents revealed an outwardly rectifying conductance activated by a high intracellular Ca2+ concentration in all three species. These currents appeared after 5-6 minutes and were blocked by CaCCinh-A01, properties typical of TMEM16F. Ion substitution experiments showed that the underlying conductance was predominantly Cl--permeable in rat megakaryocytes and HEL cells, yet non-selective between monovalent anions and cations in mouse megakaryocytes. In conclusion, the present study further highlights the difference in ionic selectivity of TMEM16F in platelet lineage cells of the mouse compared to other mammalian species. This provides additional support for the ionic "leak" hypothesis that the scramblase activity of TMEM16F does not rely upon its ability to conduct ions of a specific type.

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.

Role of anoctamin 5, a gene associated with gnathodiaphyseal dysplasia, in osteoblast and osteoclast differentiation

Anoctamin 5 (Ano5) mutations are responsible for gnathodiaphyseal dysplasia, a rare skeletal syndrome. Despite the close linkage of Ano5 to bone remodeling, the molecular mechanisms underlying the role of Ano5 in bone remodeling remain unknown. In this study, we investigated whether Ano5 regulates osteoblast or osteoclast differentiation to maintain normal bone remodeling. Downregulation of Ano5 expression did not affect osteoblast differentiation and mineralization, while ectopic expression of Ano5 significantly enhanced receptor activator of nuclear factor kappa B ligand (RANKL)-induced osteoclast differentiation. Furthermore, Ano5-mediated Akt phosphorylation resulted in nuclear factor of activated T-cells c1 (NFATc1) activation, indicating that Ano5 regulates osteoclast differentiation through activation of the Akt-NFATc1 signaling pathway. Thus, our results suggest a possibility that Ano5 is involved in bone remodeling through regulating the function of osteoclasts rather than that of osteoblasts.

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.

TMEM16F activation by Ca2+ triggers plasma membrane expansion and directs PD-1 trafficking

TMEM16F is a Ca2+ -gated ion channel that is required for Ca2+ -activated phosphatidylserine exposure on the surface of many eukaryotic cells. TMEM16F is widely expressed and has roles in platelet activation during blood clotting, bone formation and T cell activation. By combining microscopy and patch clamp recording we demonstrate that activation of TMEM16F by Ca2+ ionophores in Jurkat T cells triggers large-scale surface membrane expansion in parallel with phospholipid scrambling. With continued ionophore application,TMEM16F-expressing cells then undergo extensive shedding of ectosomes. The T cell co-receptor PD-1 is selectively incorporated into ectosomes. This selectivity depends on its transmembrane sequence. Surprisingly, cells lacking TMEM16F not only fail to expand surface membrane in response to elevated cytoplasmic Ca2+, but instead undergo rapid massive endocytosis with PD-1 internalisation. These results establish a new role for TMEM16F as a regulator of Ca2+ activated membrane trafficking.

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.

Anoctamin-6 regulates ADAM sheddase function

ADAM17, a prominent member of the "Disintegrin and Metalloproteinase" (ADAM) family, controls vital cellular functions through cleavage of transmembrane substrates including TGF-alpha, Amphiregulin (AREG) and TNF-Receptor 1 (TNFR1). We recently presented evidence that surface exposure of phosphatidylserine (PS) is pivotal for ADAM17 to exert sheddase activity. Anoctamin-6 (ANO6) has Ca2+-dependent phospholipid scramblase activity and it followed that the functions of ANO6 and ADAM17 might be linked. We report that overexpression of ANO6 in HEK293T cells led to increased Ca2+-mediated PS-exposure that was indeed accompanied by enhanced release of AREG and TGF-alpha. The effect was not observed when cells were treated with the PKC-dependent ADAM17 activator PMA. Transformation of cells with a constitutively active ANO6 mutant led to spontaneous PS-exposure and to the release of ADAM17-substrates in the absence of any stimuli. Inhibitor experiments indicated that ANO6-mediated enhancement of substrate cleavage simultaneously broadened the spectrum of participating metalloproteinases. In complementary experiments, siRNA-mediated downregulation of ANO6 was shown to decrease ionophore-mediated release of TNFR1 in human umbilical vein endothelial cells (HUVECs). We conclude that ANO6, by virtue of its scramblase activity, may play a role as an important regulator of the ADAM-network in the plasma membrane.

Comprehensive behavioral analyses of anoctamin1/TMEM16A-conditional knockout mice

AIMS

Anoctamin-1 (TMEM16A) is a calcium-activated chloride channel that is involved in numerous physiological conditions. Its role has been identified in electrophysiological and histological studies of genetic knockout animals. Recent cellular localization studies have shown that anoctamin-1 is co-expressed with presynaptic proteins, therefore its role in presynaptic terminals has been suggested. However, behavioral studies are lacking because conventional knockouts of anoctamin-1 are lethal after birth. In this study, we explored the role of anoctamin-1 in presynaptic terminals by analyzing the behavior of mice with conditional knockouts of anoctamin-1 in synapsin1-expressing cells.

MAIN METHODS

Using a synapsin1-Cre system, we selectively ablated anoctamin-1 in synapsin1 expressing cells. The mice were used in the behavioral experiments when they were between 6 and 9 months of age.

KEY FINDINGS

The mice with the conditional knockout of anoctamin-1 in synapsin1-expressing cells displayed impaired social behavior. In addition, the mice showed depressive-like behavior and decreased weight. However, these animals displayed normal locomotor activity, cognitive function, and motor coordination.

SIGNIFICANCE

These results suggested that anoctamin-1 is involved in psychiatric behavior because of its role in the regulation of synaptic transmission in presynaptic terminals.

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.

Gain of function of TMEM16E/ANO5 scrambling activity caused by a mutation associated with gnathodiaphyseal dysplasia

Mutations in the human TMEM16E (ANO5) gene are associated both with the bone disease gnathodiaphyseal dysplasia (GDD; OMIM: 166260) and muscle dystrophies (OMIM: 611307, 613319). However, the physiological function of TMEM16E has remained unclear. We show here that human TMEM16E, when overexpressed in mammalian cell lines, displayed partial plasma membrane localization and gave rise to phospholipid scrambling (PLS) as well as non-selective ionic currents with slow time-dependent activation at highly depolarized membrane potentials. While the activity of wild-type TMEM16E depended on elevated cytosolic Ca2+ levels, a mutant form carrying the GDD-causing T513I substitution showed PLS and large time-dependent ion currents even at low cytosolic Ca2+ concentrations. Contrarily, mutation of the homologous position in the Ca2+-activated Cl- channel TMEM16B paralog hardly affected its function. In summary, these data provide the first direct demonstration of Ca2+-dependent PLS activity for TMEM16E and suggest a gain-of-function phenotype related to a GDD mutation.

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.

Involvement of TRPV1-ANO1 Interactions in Pain-Enhancing Mechanisms

Primary sensory neurons detect potentially dangerous environmental situations via many "sensor" proteins located on the plasma membrane. Although receptor-type cation channels are thought to be the major sensors in sensory neurons, anion channels are also important players in the peripheral nervous system. Recently, we showed that transient receptor potential vanilloid 1 (TRPV1) interacts with anoctamin 1 (ANO1, also called TMEM16A) in primary sensory neurons and that this interaction enhanced TRPV1-mediated pain sensation. In that study, we induced ANO1 currents by application of capsaicin to small DRG neurons and showed that ANO1-dependent depolarization following TRPV1 activation could evoke more action potentials. Furthermore, capsaicin-evoked pain-related behaviors in mice were strongly inhibited by a selective ANO1 blocker. Together these findings indicate that selective ANO1 inhibition can reduce pain sensation. We also investigated non-specific inhibitory effects on ion channel activities to control ion dynamics via the TRPV1-ANO1 complex. We found that 4-isopropylcyclohexanol (4-iPr-CyH-OH) had an analgesic effect on burning pain sensations through its inhibition of TRPV1 and ANO1 together. Additionally, 4-iPr-CyH-OH did not have clear agonistic effects on TRPV1, TRPA1, and ANO1 activity individually. These results indicate that 4-iPr-CyH-OH could function globally to mediate TRP-ANO1 complex functions to reduce skin hypersensitivity and could form the basis for novel analgesic agents.

Anoctamin 9/TMEM16J is a cation channel activated by cAMP/PKA signal

Anoctamins (ANOs) are multifunctional membrane proteins that consist of 10 homologs. ANO1 (TMEM16A) and ANO2 (TMEM16B) are anion channels activated by intracellular calcium that meditate numerous physiological functions. ANO6 is a scramblase that redistributes phospholipids across the cell membrane. The other homologs are not well characterized. We found ANO9/TMEM16J is a cation channel activated by a cAMP-dependent protein kinase A (PKA). Intracellular cAMP-activated robust currents in whole cells expressing ANO9, which were inhibited by a PKA blocker. A cholera toxin that persistently stimulated adenylate cyclase activated ANO9 as did the application of PKA. The cAMP-induced ANO9 currents were permeable to cations. The cAMP-dependent ANO9 currents were augmented by intracellular Ca²⁺. Ano9 transcripts were predominant in the intestines. Human intestinal SW480 cells expressed high levels of Ano9 transcripts and showed PKA inhibitor-reversible cAMP-dependent currents. We conclude that ANO9 is a cation channel activated by a cAMP/PKA pathway and could play a role in intestine function.

A novel microscopy-based assay identifies extended synaptotagmin-1 (ESYT1) as a positive regulator of anoctamin 1 traffic

An attractive possibility to treat Cystic Fibrosis (CF), a severe condition caused by dysfunctional CFTR, an epithelial anion channel, is through the activation of alternative (non-CFTR) anion channels. Anoctamin 1 (ANO1) was demonstrated to be a Ca²⁺-activated chloride channel (CaCC) and thus of high potential to replace CFTR. Despite that ANO1 is expressed in human lung CF tissue, it is present at the cell surface at very low levels. In addition, little is known about regulation of ANO1 traffic, namely which factors promote its plasma membrane (PM) localization. Here, we generated a novel cellular model, expressing an inducible 3HA-ANO1-eGFP construct, and validated its usage as a microscopy tool to monitor for ANO1 traffic. We demonstrate the robustness and specificity of this cell-based assay, by the identification of siRNAs acting both as ANO1 traffic enhancer and inhibitor, targeting respectively COPB1 and ESYT1 (extended synaptotagmin-1), the latter involved in coupling of the endoplasmic reticulum to the PM at specific microdomains. We further show that knockdown of ESYT1 (and family members ESYT2 and ESYT3) significantly decreased ANO1 current density. This ANO1 cell-based assay constitutes an important tool to be further used in high-throughput screens and drug discovery of high relevance for CF and cancer.

Ion channels of the lung and their role in disease pathogenesis

Maintenance of normal epithelial ion and water transport in the lungs includes providing a thin layer of surface liquid that coats the conducting airways. This airway surface liquid is critical for normal lung function in a number of ways but, perhaps most importantly, is required for normal mucociliary clearance and bacterial removal. Preservation of the appropriate level of hydration, pH, and viscosity for the airway surface liquid requires the proper regulation and function of a battery of different types of ion channels and transporters. Here we discuss how alterations in ion channel/transporter function often lead to lung pathologies.

Cell-specific mechanisms of TMEM16A Ca2+-activated chloride channel in cancer

TMEM16A (known as anoctamin 1) Ca2+-activated chloride channel is overexpressed in many tumors. TMEM16A overexpression can be caused by gene amplification in many tumors harboring 11q13 amplification. TMEM16A expression is also controlled in many cancer cells via transcriptional regulation, epigenetic regulation and microRNAs. In addition, TMEM16A activates different signaling pathways in different cancers, e.g. the EGFR and CAMKII signaling in breast cancer, the p38 and ERK1/2 signaling in hepatoma, the Ras-Raf-MEK-ERK1/2 signaling in head and neck squamous cell carcinoma and bladder cancer, and the NFκB signaling in glioma. Furthermore, TMEM16A overexpression has been reported to promote, inhibit, or produce no effects on cell proliferation and migration in different cancer cells. Since TMEM16A exerts different roles in different cancer cells via activation of distinct signaling pathways, we try to develop the idea that TMEM16A regulates cancer cell proliferation and migration in a cell-dependent mechanism. The cell-specific role of TMEM16A may depend on the cellular environment that is predetermined by TMEM16A overexpression mechanisms specific for a particular cancer type. TMEM16A may exert its cell-specific role via its associated protein networks, phosphorylation by different kinases, and involvement of different signaling pathways. In addition, we discuss the role of TMEM16A channel activity in cancer, and its clinical use as a prognostic and predictive marker in different cancers. This review highlights the cell-type specific mechanisms of TMEM16A in cancer, and envisions the promising use of TMEM16A inhibitors as a potential treatment for TMEM16A-overexpressing cancers.

EAVK segment "c" sequence confers Ca2+-dependent changes to the kinetics of full-length human Ano1

Anoctamin1 (Ano1 and TMEM16A) is a calcium-activated chloride channel specifically expressed in the interstitial cells of Cajal (ICC) of the gastrointestinal tract muscularis propria. Ano1 is necessary for normal electrical slow waves and ICC proliferation. The full-length human Ano1 sequence includes an additional exon, exon "0," at the NH₂ terminus. Ano1 with exon 0 [Ano1₍₀₎] had a lower EC₅₀ for intracellular calcium ([Ca²⁺]_i) and faster chloride current (I_Cl) kinetics. The Ano1 alternative splice variant with segment "c" encoding exon 13 expresses on the first intracellular loop four additional amino acid residues, EAVK, which alter I_Cl at low [Ca²⁺]_i Exon 13 is expressed in 75-100% of Ano1 transcripts in most human tissues but only 25% in the human stomach. Our aim was to determine the effect of EAVK deletion on Ano1₍₀₎I_Cl parameters. By voltage-clamp electrophysiology, we examined I_Cl in HEK293 cells transiently expressing Ano1₍₀₎ with or without the EAVK sequence [Ano1₍₀₎ΔEAVK]. The EC₅₀ values of activating and deactivating I_Cl for [Ca²⁺]_i were 438 ± 7 and 493 ± 9 nM for Ano1₍₀₎ but higher for Ano1₍₀₎ΔEAVK at 746 ± 47 and 761 ± 26 nM, respectively. Meanwhile, the EC₅₀ values for the ratio of instantaneous to steady-state I_Cl were not different between variants. Congruently, the time constant of activation was slower for Ano1₍₀₎ΔEAVK than Ano1₍₀₎ currents at intermediate [Ca²⁺]_i These results suggest that EAVK decreases the calcium sensitivity of Ano1₍₀₎ current activation and deactivation by slowing activation kinetics. Differential expression of EAVK in the human stomach may function as a switch to increase sensitivity to [Ca²⁺]_i via faster gating of Ano1.NEW & NOTEWORTHY Calcium-activated chloride channel anoctamin1 (Ano1) is necessary for normal slow waves in the gastrointestinal interstitial cells of Cajal. Exon 0 encodes the NH₂ terminus of full-length human Ano1 [Ano1₍₀₎], while exon 13 encodes residues EAVK on its first intracellular loop. Splice variants lack EAVK more often in the stomach than other tissues. Ano1₍₀₎ without EAVK [Ano1₍₀₎ΔEAVK] has reduced sensitivity for intracellular calcium, attributable to slower kinetics. Differential expression of EAVK may function as a calcium-sensitive switch in the human stomach.

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.

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.

Increased TMEM16A Involved in Alveolar Fluid Clearance After Lipopolysaccharide Stimulation

Transmembrane protein 16A (TMEM16A) regulates a wide variety of cellular activities, including epithelial fluid secretion and maintenance of ion homeostasis. Lipopolysaccharide (LPS), an outer membrane component of Gram-negative bacteria, is one of the major causes of acute lung injury (ALI). In this study, we investigated the effects of LPS on the expression of TMEM16A in LA795 cells and mouse lung tissue and the potential mechanism.

RESULT

We detected the expression of TMEM16A in LA795 cells and mouse lung tissue by RT-PCR, Western blot, and RNA interference techniques. TMEM16A expression was significantly increased by LPS stimulation in LA795 cells and in mouse lung tissue. Moreover, the LPS-induced TMEM16A expression enhancement in lung tissue was much more prominent in the alveolar epithelial region than in bigger airway epithelial cells. The typical TMEM16A current was recorded, and LPS treatment significantly enhances the current amplitude in LA795 cells. TMEM16A shRNA or TMEM16A inhibitor (T16Ainh-A01) did not affect alveolar fluid clearance (AFC), while co-application of T16Ainh-A01 induced a stronger AFC inhibition than LPS alone. LPS notably and synchronously enhanced Akt phosphorylation (p-Akt) and TMEM16A expression in a time-dependent manner in LA795 cells. Taken together, our results suggest that TMEM16A maybe plays an important role in pathological conditions of LPS-induced ALI as a protective protein.

Regulatory Roles of Anoctamin-6 in Human Trabecular Meshwork Cells

Purpose

Trabecular meshwork (TM) cell volume is a determinant of aqueous humor outflow resistance, and thereby IOP. Regulation of TM cell volume depends on chloride ion (Cl-) release through swelling-activated channels (ICl,Swell), whose pore is formed by LRRC8 proteins. Chloride ion release through swelling-activated channels has been reported to be regulated by calcium-activated anoctamins, but this finding is controversial. Particularly uncertain has been the effect of anoctamin Ano6, reported as a Ca2+-activated Cl- (CaCC) or cation channel in other cells. The current study tested whether anoctamin activity modifies volume regulation of primary TM cell cultures and cell lines.

Methods

Gene expression was studied with quantitative PCR, supplemented by reverse-transcriptase PCR and Western immunoblots. Currents were measured by ruptured whole-cell patch clamping and volume by electronic cell sizing.

Results

Primary TM cell cultures and the TM5 and GTM3 cell lines expressed Ano6 3 to 4 orders of magnitude higher than the other anoctamin CaCCs (Ano1 and Ano2). Ionomycin increased cell Ca2+ and activated macroscopic currents conforming to CaCCs in other cells, but displayed significantly more positive mean reversal potentials (+5 to +12 mV) than those displayed by ICl,Swell (-14 to -21 mV) in the same cells. Nonselective CaCC inhibitors (tannic acid>CaCCinh-A01) and transient Ano6 knockdown strongly inhibited ionomycin-activated currents, ICl,Swell and the regulatory volume response to hyposmotic swelling.

Conclusions

Ionomycin activates CaCCs associated with net cation movement in TM cells. These currents, ICl,Swell, and cell volume are regulated by Ano6. The findings suggest a novel clinically-relevant approach for altering cell volume, and thereby outflow resistance, by targeting Ano6.

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.

Anoctamin 6 Contributes to Cl- Secretion in Accessory Cholera Enterotoxin (Ace)-stimulated Diarrhea: AN ESSENTIAL ROLE FOR PHOSPHATIDYLINOSITOL 4,5-BISPHOSPHATE (PIP2) SIGNALING IN CHOLERA

Accessory cholera enterotoxin (Ace) of Vibrio cholerae has been shown to contribute to diarrhea. However, the signaling mechanism and specific type of Cl- channel activated by Ace are still unknown. We have shown here that the recombinant Ace protein induced ICl of apical plasma membrane, which was inhibited by classical CaCC blockers. Surprisingly, an Ace-elicited rise of current was neither affected by ANO1 (TMEM16A)-specific inhibitor T16A(inh)-AO1(TAO1) nor by the cystic fibrosis transmembrane conductance regulator (CFTR) blocker, CFTR inh-172. Ace stimulated whole-cell current in Caco-2 cells. However, the apical ICl was attenuated by knockdown of ANO6 (TMEM16F). This impaired phenotype was restored by re-expression of ANO6 in Caco-2 cells. Whole-cell patch clamp recordings of ANO currents in HEK293 cells transiently expressing mouse ANO1-mCherry or ANO6-GFP confirmed that Ace induced Cl- secretion. Application of Ace produced ANO6 but not the ANO1 currents. Ace was not able to induce a [Ca2+]i rise in Caco-2 cells, but cellular abundance of phosphatidylinositol 4,5-bisphosphate (PIP2) increased. Identification of the PIP2-binding motif at the N-terminal sequence among human and mouse ANO6 variants along with binding of PIP2 directly to ANO6 in HEK293 cells indicate likely PIP2 regulation of ANO6. The biophysical and pharmacological properties of Ace stimulated Cl- current along with intestinal fluid accumulation, and binding of PIP2 to the proximal KR motif of channel proteins, whose mutagenesis correlates with altered binding of PIP2, is comparable with ANO6 stimulation. We conclude that ANO6 is predominantly expressed in intestinal epithelia, where it contributes secretory diarrhea by Ace stimulation in a calcium-independent mechanism of RhoA-ROCK-PIP2 signaling.

Role of Quercetin in Modulating Chloride Transport in the Intestine

Epithelial chloride channels provide the pathways for fluid secretion in the intestine. Cystic fibrosis transmembrane conductance regulator (CFTR) and calcium-activated chloride channels (CaCCs) are the main chloride channels in the luminal membrane of enterocytes. These transmembrane proteins play important roles in many physiological processes. In this study, we have identified a flavonoid quercetin as a modulator of CaCC chloride channel activity. Fluorescence quenching assay showed that quercetin activated Cl⁻ transport in a dose-dependent manner, with EC₅₀ ~37 μM. Short-circuit current analysis confirmed that quercetin activated CaCC-mediated Cl⁻ currents in HT-29 cells that can be abolished by CaCC_inh-A01. Ex vivo studies indicated that application of quercetin to mouse ileum and colon on serosal side resulted in activation of CFTR and CaCC-mediated Cl⁻ currents. Notably, we found that quercetin exhibited inhibitory effect against ANO1 chloride channel activity in ANO1-expressing FRT cells and decreased mouse intestinal motility. Quercetin-stimulated short-circuit currents in mouse ileum was multi-component, which included elevation of Ca²⁺ concentration through L-type calcium channel and activation of basolateral NKCC, Na⁺/K⁺-ATPase, and K⁺ channels. In vivo studies further revealed that quercetin promoted fluid secretion in mouse ileum. The modulatory effect of quercetin on CaCC chloirde channels may therefore represent a potential therapeutic strategy for treating CaCC-related diseases like constipation, secretory diarrhea and hypertension. The inverse effects of quercetin on CaCCs provided evidence that ANO1 and intestinal epithelial CaCCs are different calcium-activated chloride channels.

Anoctamins/TMEM16 Proteins: Chloride Channels Flirting with Lipids and Extracellular Vesicles

Anoctamin (ANO)/TMEM16 proteins exhibit diverse functions in cells throughout the body and are implicated in several human diseases. Although the founding members ANO1 (TMEM16A) and ANO2 (TMEM16B) are Ca²⁺-activated Cl^- channels, most ANO paralogs are Ca²⁺-dependent phospholipid scramblases that serve as channels facilitating the movement (scrambling) of phospholipids between leaflets of the membrane bilayer. Phospholipid scrambling significantly alters the physical properties of the membrane and its landscape and has vast downstream signaling consequences. In particular, phosphatidylserine exposed on the external leaflet of the plasma membrane functions as a ligand for receptors vital for cell-cell communication. A major consequence of Ca²⁺-dependent scrambling is the release of extracellular vesicles that function as intercellular messengers by delivering signaling proteins and noncoding RNAs to alter target cell function. We discuss the physiological implications of Ca²⁺-dependent phospholipid scrambling, the extracellular vesicles associated with this activity, and the roles of ANOs in these processes.

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.

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.

Identification and characterization of ANO9 in stage II and III colorectal carcinoma

BACKGROUND AND OBJECTIVES

The precise role and potential underlying mechanisms of anoctamin 9 (ANO9) remain largely unknown. This study aims to characterize the role and oncogenic mechanisms of ANO9 in stage II and III colorectal cancer (CRC).

METHODS

We examined the expression of ANO9 in colorectal cancerous tissues and cells using real-time quantitative PCR and immunohistochemistry, respectively. Multiple cellular and molecular approaches such as gene transfection, CCK-8 assay, flow cytometry, and invasion assay were also performed to explore its oncogenic mechanisms. Furthermore, the clinical significance of ANO9 in clinical CRC specimens was assessed by clinical correlation and survival analyses.

RESULTS

Lower expression of ANO9 messenger RNA (mRNA) was frequently detected both in CRC tissues with recurrence and metastasis-derived cell lines. Compared with matched nontumorous tissues, lower expression of ANO9 protein was observed in tumors, which was significantly correlated with tumorigenesis (p < 0.05). In vitro functional studies showed that ANO9 contributed to tumor cell proliferation, apoptosis, and invasion. Moreover, investigation of clinical CRC specimens showed that ANO9 were markedly overexpressed in metastatic tissue compared with primary tissue. Decreased expression of ANO9 was correlated with poor prognostic outcomes.

CONCLUSIONS

This study highlighted the role of ANO9 in progression and metastasis of stage II and III CRC. These findings suggested that up-regulation of ANO9, as a metastasis-related gene, could be a novel approach for inhibiting CRC progression.

Exome sequencing in pooled DNA samples to identify maternal pre-eclampsia risk variants

Pre-eclampsia is a common pregnancy disorder that is a major cause for maternal and perinatal mortality and morbidity. Variants predisposing to pre-eclampsia might be under negative evolutionary selection that is likely to keep their population frequencies low. We exome sequenced samples from a hundred Finnish pre-eclamptic women in pools of ten to screen for low-frequency, large-effect risk variants for pre-eclampsia. After filtering and additional genotyping steps, we selected 28 low-frequency missense, nonsense and splice site variants that were enriched in the pre-eclampsia pools compared to reference data, and genotyped the variants in 1353 pre-eclamptic and 699 non-pre-eclamptic women to test the association of them with pre-eclampsia and quantitative traits relevant for the disease. Genotypes from the SISu project (n = 6118 exome sequenced Finnish samples) were included in the binary trait association analysis as a population reference to increase statistical power. In these analyses, none of the variants tested reached genome-wide significance. In conclusion, the genetic risk for pre-eclampsia is likely complex even in a population isolate like Finland, and larger sample sizes will be necessary to detect risk variants.

Role of Ca(2+) in the Stability and Function of TMEM16F and 16K

There are 10 transmembrane protein (TMEM) 16-family proteins in humans and mice. Among them, TMEM16F acts as a Ca(2+)-dependent phospholipid scramblase at the plasma membrane. However, how Ca(2+) activates TMEM16F's phospholipid-scramblase activity has not been elucidated. Here we found that in the presence of Ca(2+), TMEM16K (whose function is unknown) directly binds Ca(2+) to form a stable complex that can be detected by blue-native polyacrylamide gel electrophoresis. In the absence of Ca(2+), TMEM16K and TMEM16F aggregated, suggesting that their structure is stabilized by Ca(2+). Comprehensive mutagenesis of acidic residues in TMEM16K's cytoplasmic and transmembrane regions identified five residues that are critical for binding Ca(2+). These residues were well conserved between TMEM16F and 16K, and point mutations of these residues in TMEM16F reduced its ability to support Ca(2+)-dependent phospholipid scrambling. Our results suggest that Ca(2+) binds TMEM16F directly and induces conformational changes that support its stability and function.

Getting to the Outer Leaflet: Physiology of Phosphatidylserine Exposure at the Plasma Membrane

Phosphatidylserine (PS) is a major component of membrane bilayers whose change in distribution between inner and outer leaflets is an important physiological signal. Normally, members of the type IV P-type ATPases spend metabolic energy to create an asymmetric distribution of phospholipids between the two leaflets, with PS confined to the cytoplasmic membrane leaflet. On occasion, membrane enzymes, known as scramblases, are activated to facilitate transbilayer migration of lipids, including PS. Recently, two proteins required for such randomization have been identified: TMEM16F, a scramblase regulated by elevated intracellular Ca(2+), and XKR8, a caspase-sensitive protein required for PS exposure in apoptotic cells. Once exposed at the cell surface, PS regulates biochemical reactions involved in blood coagulation, and bone mineralization, and also regulates a variety of cell-cell interactions. Exposed on the surface of apoptotic cells, PS controls their recognition and engulfment by other cells. This process is exploited by parasites to invade their host, and in specialized form is used to maintain photoreceptors in the eye and modify synaptic connections in the brain. This review discusses what is known about the mechanism of PS exposure at the surface of the plasma membrane of cells, how actors in the extracellular milieu sense surface exposed PS, and how this recognition is translated to downstream consequences of PS exposure.

Exposure of phosphatidylserine on the cell surface

Phosphatidylserine (PtdSer) is a phospholipid that is abundant in eukaryotic plasma membranes. An ATP-dependent enzyme called flippase normally keeps PtdSer inside the cell, but PtdSer is exposed by the action of scramblase on the cell's surface in biological processes such as apoptosis and platelet activation. Once exposed to the cell surface, PtdSer acts as an 'eat me' signal on dead cells, and creates a scaffold for blood-clotting factors on activated platelets. The molecular identities of the flippase and scramblase that work at plasma membranes have long eluded researchers. Indeed, their identity as well as the mechanism of the PtdSer exposure to the cell surface has only recently been revealed. Here, we describe how PtdSer is exposed in apoptotic cells and in activated platelets, and discuss PtdSer exposure in other biological processes.

Anoctamin 2 identified as an autoimmune target in multiple sclerosis

Multiple sclerosis (MS) is the most common chronic inflammatory disease of the central nervous system and also is regarded as an autoimmune condition. However, the antigenic targets of the autoimmune response in MS have not yet been deciphered. In an effort to mine the autoantibody repertoire within MS, we profiled 2,169 plasma samples from MS cases and population-based controls using bead arrays built with 384 human protein fragments selected from an initial screening with 11,520 antigens. Our data revealed prominently increased autoantibody reactivity against the chloride-channel protein anoctamin 2 (ANO2) in MS cases compared with controls. This finding was corroborated in independent assays with alternative protein constructs and by epitope mapping with peptides covering the identified region of ANO2. Additionally, we found a strong interaction between the presence of ANO2 autoantibodies and the HLA complex MS-associated DRB1*15 allele, reinforcing a potential role for ANO2 autoreactivity in MS etiopathogenesis. Furthermore, immunofluorescence analysis in human MS brain tissue showed ANO2 expression as small cellular aggregates near and inside MS lesions. Thus this study represents one of the largest efforts to characterize the autoantibody repertoire within MS. The findings presented here demonstrate that an ANO2 autoimmune subphenotype may exist in MS and lay the groundwork for further studies focusing on the pathogenic role of ANO2 autoantibodies in MS.

Generation and functional characterization of epithelial cells with stable expression of SLC26A9 Cl- channels

Recent studies identified the SLC26A9 Cl(-) channel as a modifier and potential therapeutic target in cystic fibrosis (CF). However, understanding of the regulation of SLC26A9 in epithelia remains limited and cellular models with stable expression for biochemical and functional studies are missing. We, therefore, generated Fisher rat thyroid (FRT) epithelial cells with stable expression of HA-tagged SLC26A9 via retroviral transfection and characterized SLC26A9 expression and function using Western blotting, immunolocalization, whole cell patch-clamp, and transepithelial bioelectric studies in Ussing chambers. We demonstrate stable expression of SLC26A9 in transfected FRT (SLC26A9-FRT) cells on the mRNA and protein level. Immunolocalization and Western blotting detected SLC26A9 in different intracellular compartments and to a lesser extent at the cell surface. Whole cell patch-clamp recordings demonstrated significantly increased constitutive Cl(-) currents in SLC26A9-FRT compared with control-transduced FRT (Control-FRT) cells (P < 0.01). Similar, transepithelial measurements showed that the basal short circuit current was significantly increased in SLC26A9-FRT vs. Control-FRT cell monolayers (P < 0.01). SLC26A9-mediated Cl(-) currents were increased by cAMP-dependent stimulation (IBMX and forskolin) and inhibited by GlyH-101, niflumic acid, DIDS, and 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), as well as RNAi knockdown of WNK1 implicated in epithelial osmoregulation. Our results support that these novel epithelial cells with stable expression of SLC26A9 will be a useful model for studies of pharmacological regulation including the identification of activators of SLC26A9 Cl(-) channels that may compensate deficient cystic fibrosis transmembrane regulator (CFTR)-mediated Cl(-) secretion and serve as an alternative therapeutic target in patients with CF and potentially other muco-obstructive lung diseases.

Modulating Ca²⁺ signals: a common theme for TMEM16, Ist2, and TMC

Since the discovery of TMEM16A (anoctamin 1, ANO1) as Ca(2+)-activated Cl(-) channel, the protein was found to serve different physiological functions, depending on the type of tissue. Subsequent reports on other members of the anoctamin family demonstrated a broad range of yet poorly understood properties. Compromised anoctamin function is causing a wide range of diseases, such as hearing loss (ANO2), bleeding disorder (ANO6), ataxia and dystonia (ANO3, 10), persistent borrelia and mycobacteria infection (ANO10), skeletal syndromes like gnathodiaphyseal dysplasia and limb girdle muscle dystrophy (ANO5), and cancer (ANO1, 6, 7). Animal models demonstrate CF-like airway disease, asthma, and intestinal hyposecretion (ANO1). Although present data indicate that ANO1 is a Ca(2+)-activated Cl(-) channel, it remains unclear whether all anoctamins form plasma membrane-localized or intracellular chloride channels. We find Ca(2+)-activated Cl(-) currents appearing by expression of most anoctamin paralogs, including the Nectria haematococca homologue nhTMEM16 and the yeast homologue Ist2. As recent studies show a role of anoctamins, Ist2, and the related transmembrane channel-like (TMC) proteins for intracellular Ca(2+) signaling, we will discuss the role of these proteins in generating compartmentalized Ca(2+) signals, which may give a hint as to the broad range of cellular functions of anoctamins.

Genetic disruption of Ano5 in mice does not recapitulate human ANO5-deficient muscular dystrophy

BACKGROUND

Anoctamin 5 (ANO5) is a member of a conserved gene family (TMEM16), which codes for proteins predicted to have eight transmembrane domains and putative Ca(2+)-activated chloride channel (CaCC) activity. It was recently reported that mutations in this gene result in the development of limb girdle muscular dystrophy type 2L (LGMD2L), Miyoshi myopathy type 3 (MMD3), or gnathodiaphyseal dysplasia 1 (GDD1). Currently, there is a lack of animal models for the study of the physiological function of Ano5 and the disease pathology in its absence.

RESULTS

Here, we report the generation and characterization of the first Ano5-knockout (KO) mice. Our data demonstrate that the KO mice did not present overt skeletal or cardiac muscle pathology at rest conditions from birth up to 18 months of age. There were no significant differences in force production or force deficit following repeated eccentric contractions between wild type (WT) and KO mice. Although cardiac hypertrophy developed similarly in both KO and WT mice after daily isoproterenol (ISO, 100 mg/kg) treatment via intraperitoneal injection for 2 weeks, they were functionally indiscernible. However, microarray analysis identified the genes involved in lipid metabolism, and complement pathways were altered in the KO skeletal muscle.

CONCLUSIONS

Taken together, these data provide the evidence to show that genetic ablation of Ano5 in C57BL/6J mice does not cause overt pathology in skeletal and cardiac muscles, but Ano5 deficiency may lead to altered lipid metabolism and inflammation signaling.

Anoctamin 1 (Ano1) is required for glucose-induced membrane potential oscillations and insulin secretion by murine β-cells

Anions such as Cl(-) and HCO3 (-) are well known to play an important role in glucose-stimulated insulin secretion (GSIS). In this study, we demonstrate that glucose-induced Cl(-) efflux from β-cells is mediated by the Ca(2+)-activated Cl(-) channel anoctamin 1 (Ano1). Ano1 expression in rat β-cells is demonstrated by reverse transcriptase-polymerase chain reaction, western blotting, and immunohistochemistry. Typical Ano1 currents are observed in whole-cell and inside-out patches in the presence of intracellular Ca(++): at 1 μM, the Cl(-) current is outwardly rectifying, and at 2 μM, it becomes almost linear. The relative permeabilities of monovalent anions are NO3 (-) (1.83 ± 0.10) > Br(-) (1.42 ± 0.07) > Cl(-) (1.0). A linear single-channel current-voltage relationship shows a conductance of 8.37 pS. These currents are nearly abolished by blocking Ano1 antibodies or by the inhibitors 2-(5-ethyl-4-hydroxy-6-methylpyrimidin-2-ylthio)-N-(4-(4-methoxyphenyl)thiazol-2-yl)acetamide (T-AO1) and tannic acid (TA). These inhibitors induce a strong decrease of 16.7-mM glucose-stimulated action potential rate (at least 87 % on dispersed cells) and a partial membrane repolarization with T-AO1. They abolish or strongly inhibit the GSIS increment at 8.3 mM and at 16.7 mM glucose. Blocking Ano1 antibodies also abolish the 16.7-mM GSIS increment. Combined treatment with bumetanide and acetazolamide in low Cl(-) and HCO3 (-) media provokes a 65 % reduction in action potential (AP) amplitude and a 15-mV AP peak repolarization. Although the mechanism triggering Ano1 opening remains to be established, the present data demonstrate that Ano1 is required to sustain glucose-stimulated membrane potential oscillations and insulin secretion.

Anoctamin 1 (Ano1) is required for glucose-induced membrane potential oscillations and insulin secretion by murine β-cells

Anions such as Cl⁻ and HCO₃⁻ are well known to play an important role in glucose-stimulated insulin secretion (GSIS). In this study, we demonstrate that glucose-induced Cl⁻ efflux from β-cells is mediated by the Ca²⁺-activated Cl⁻ channel anoctamin 1 (Ano1). Ano1 expression in rat β-cells is demonstrated by reverse transcriptase–polymerase chain reaction, western blotting, and immunohistochemistry. Typical Ano1 currents are observed in whole-cell and inside-out patches in the presence of intracellular Ca⁺⁺: at 1 μM, the Cl⁻ current is outwardly rectifying, and at 2 μM, it becomes almost linear. The relative permeabilities of monovalent anions are NO₃⁻ (1.83 ± 0.10) > Br⁻ (1.42 ± 0.07) > Cl⁻ (1.0). A linear single-channel current–voltage relationship shows a conductance of 8.37 pS. These currents are nearly abolished by blocking Ano1 antibodies or by the inhibitors 2-(5-ethyl-4-hydroxy-6-methylpyrimidin-2-ylthio)-N-(4-(4-methoxyphenyl)thiazol-2-yl)acetamide (T-AO1) and tannic acid (TA). These inhibitors induce a strong decrease of 16.7-mM glucose-stimulated action potential rate (at least 87 % on dispersed cells) and a partial membrane repolarization with T-AO1. They abolish or strongly inhibit the GSIS increment at 8.3 mM and at 16.7 mM glucose. Blocking Ano1 antibodies also abolish the 16.7-mM GSIS increment. Combined treatment with bumetanide and acetazolamide in low Cl⁻ and HCO₃⁻ media provokes a 65 % reduction in action potential (AP) amplitude and a 15-mV AP peak repolarization. Although the mechanism triggering Ano1 opening remains to be established, the present data demonstrate that Ano1 is required to sustain glucose-stimulated membrane potential oscillations and insulin secretion.

Novel Roles for Chloride Channels, Exchangers, and Regulators in Chronic Inflammatory Airway Diseases

Chloride transport proteins play critical roles in inflammatory airway diseases, contributing to the detrimental aspects of mucus overproduction, mucus secretion, and airway constriction. However, they also play crucial roles in contributing to the innate immune properties of mucus and mucociliary clearance. In this review, we focus on the emerging novel roles for a chloride channel regulator (CLCA1), a calcium-activated chloride channel (TMEM16A), and two chloride exchangers (SLC26A4/pendrin and SLC26A9) in chronic inflammatory airway diseases.

A novel exon in the human Ca2+-activated Cl- channel Ano1 imparts greater sensitivity to intracellular Ca2

Anoctamin 1 (Ano1; TMEM16A) is a Ca(2+)-activated Cl(-) channel (CACC) expressed in interstitial cells of Cajal. The mechanisms by which Ca(2+) regulates Ano1 are incompletely understood. In the gastrointestinal tract, Ano1 is required for normal slow wave activity and is involved in regulating cell proliferation. Splice variants of Ano1 have varying electrophysiological properties and altered expression in disease states. Recently, we identified a transcript for human Ano1 containing a novel exon-"exon 0" upstream of and in frame with exon 1. The electrophysiological properties of this longer Ano1 isoform are unknown. Our aim was to determine the functional contribution of the newly identified exon to the Ca(2+) sensitivity and electrophysiological properties of Ano1. Constructs with [Ano1(+0)] or without [Ano1(-0)] the newly identified exon were transfected into human embryonic kidney-293 cells. Voltage-clamp electrophysiology was used to determine voltage- and time-dependent parameters of whole cell Cl(-) currents between isoforms with varying concentrations of intracellular Ca(2+), extracellular anions, or Cl(-) channel inhibitors. We found that exon 0 did not change voltage sensitivity and had no impact on the relative permeability of Ano1 to most anions. Ano1(+0) exhibited greater changes in current density but lesser changes in kinetics than Ano1(-0) in response to varying intracellular Ca(2+). The CACC inhibitor niflumic acid inhibited current with greater efficacy and higher potency against Ano1(+0) compared with Ano1(-0). Likewise, the Ano1 inhibitor T16Ainh-A01 reduced Ano1(+0) more than Ano1(-0). In conclusion, human Ano1 containing exon 0 imparts its Cl(-) current with greater sensitivity to intracellular Ca(2+) and CACC inhibitors.