Characterization of Ca2+-activated Cl- currents in mouse kidney inner medullary collecting duct cells

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.

Recent progress in structural studies on TMEM16A channel

The calcium-activated chloride channel, also known as TMEM16A, shows both calcium and membrane potential dependent activation. The channel is expressed broadly and contributes to a variety of physiological processes, and it is expected to be a target for the treatment of diseases such as hypertension, pain, cystic fibrosis and lung cancer. A thorough understanding of the structural characteristics of TMEM16A is important to reveal its physiological and pathological roles. Recent studies have released several Cryo-EM structures of the channel, revealed the structural basis and mechanism of the gating of the channel. This review focused on the understandings of the structural basis and molecular mechanism of the gating and permeation of TMEM16A channel, which will provide important basis for the development of drugs targeting TMEM16A.

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

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

Recent advances in TMEM16A: Structure, function, and disease

TMEM16A (also known as anoctamin 1, ANO1) is the molecular basis of the calcium-activated chloride channels, with ten transmembrane segments. Recently, atomic structures of the transmembrane domains of mouse TMEM16A (mTMEM16A) were determined by single-particle electron cryomicroscopy. This gives us a solid ground to discuss the electrophysiological properties and functions of TMEM16A. TMEM16A is reported to be dually regulated by Ca²⁺ and voltage. In addition, the dysfunction of TMEM16A has been found to be involved in many diseases including cystic fibrosis, various cancers, hypertension, and gastrointestinal motility disorders. TMEM16A is overexpressed in many cancers, including gastrointestinal stromal tumors, gastric cancer, head and neck squamous cell carcinoma (HNSCC), colon cancer, pancreatic ductal adenocarcinoma, and esophageal cancer. Furthermore, overexpression of TMEM16A is related to the occurrence, proliferation, and migration of tumor cells. To date, several studies have shown that many natural compounds and synthetic compounds have regulatory effects on TMEM16A. These small molecule compounds might be novel drugs for the treatment of diseases caused by TMEM16A dysfunction in the future. In addition, recent studies have shown that TMEM16A plays different roles in different diseases through different signal transduction pathways. This review discusses the topology, electrophysiological properties, modulators and functions of TMEM16A in mediates nociception, gastrointestinal dysfunction, hypertension, and cancer and focuses on multiple regulatory mechanisms regarding TMEM16A.

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.

Toxic effects of TiO2 nanoparticles in primary cultured rat sertoli cells are mediated via a dysregulated Ca2+ /PKC/p38 MAPK/NF-κB cascade

Although numerous studies have demonstrated that titanium dioxide nanoparticles (TiO₂ NPs) can be accumulated in various animal organs and can cause toxicity, there is currently only limited data regarding reproductive toxicity especially on the toxic mechanisms of TiO₂ NPs in Sertoli cells. In order to investigate the mechanism of reproductive toxicity, primary cultured rat Sertoli cells were exposed to 5, 15, or 30 μg/mL TiO₂ NPs for 24 h, and TiO₂ NPs internalization, expression of PKC (p-PKC) and p38 MAPK (p-p38 MAPK) as well as calcium homeostasis were examined. Our findings demonstrated that TiO₂ NPs crossed the membrane into the cytoplasm or nucleus, and significantly suppressed cell viability of primary cultured rat Sertoli cells in a concentration-dependent manner. Furthermore, immunological dysfunction caused by TiO₂ NPs was involved in the increased expression of NF-κB, TNF-α, and IL-1β, and decreased IκB expression. TiO₂ NPs significantly decreased Ca²⁺ -ATPase and Ca²⁺ /Mg²⁺ -ATPase activity and enhanced intracellular Ca²⁺ levels, and up-regulated the expression of p-PKC and p-p38 MAPK in a dose-dependent manner in primary cultured rat Sertoli cells. Taken together, these findings indicate that TiO₂ NPs may induce immunological dysfunction of primary cultured rat Sertoli cells by stimulating the Ca²⁺ /PKC/p38 MAPK cascade, which triggers NF-κB activation and ultimately induces the expression of inflammatory cytokines in primary cultured rat Sertoli cells. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 1374-1382, 2017.

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.

Hypoxia augments the calcium-activated chloride current carried by anoctamin-1 in cardiac vascular endothelial cells of neonatal mice

BACKGROUND AND PURPOSE

The molecular identity of calcium-activated chloride channels (CaCCs) in vascular endothelial cells remains unknown. This study sought to identify whether anoctamin-1 (Ano1, also known as TMEM16A) functions as a CaCC and whether hypoxia alters the biophysical properties of Ano1 in mouse cardiac vascular endothelial cells (CVECs).

EXPERIMENTAL APPROACH

Western blot, quantitative real-time PCR, confocal imaging analysis and patch-clamp analysis combined with pharmacological approaches were used to determine whether Ano1 was expressed and functioned as CaCC in CVECs.

KEY RESULTS

Ano1 was expressed in CVECs. The biophysical properties of the current generated in the CVECs, including the Ca²⁺ and voltage dependence, outward rectification, anion selectivity and the pharmacological profile, are similar to those described for CaCCs. The density of I_Cl(Ca) detected in CVECs was significantly inhibited by T16A_inh-A01, an Ano1 inhibitor, and a pore-targeting, specific anti-Ano1 antibody, and was markedly decreased in Ano1 gene knockdown CVECs. The density of I_Cl(Ca) was significantly potentiated in CVECs exposed to hypoxia, and this hypoxia-induced increase in the density of I_Cl(Ca) was inhibited by T16A_inh-A01 or anti-Ano1 antibody. Hypoxia also increased the current density of I_Cl(Ca) in Ano1 gene knockdown CVECs.

CONCLUSIONS AND IMPLICATIONS

Ano1 formed CaCC in CVECs of neonatal mice. Hypoxia enhances Ano1-mediated I_Cl(Ca) density via increasing its expression, altering the ratio of its splicing variants, sensitivity to membrane voltage and to Ca²⁺. Ano1 may play a role in the pathophysiological processes during ischaemia in heart, and therefore, Ano1 might be a potential therapeutic target to prevent ischaemic damage.

Conditional knockout of TMEM16A/anoctamin1 abolishes the calcium-activated chloride current in mouse vomeronasal sensory neurons

TMEM16A is an essential component of Ca²⁺-activated Cl⁻ currents in mouse vomeronasal sensory neurons.

Pheromones are substances released from animals that, when detected by the vomeronasal organ of other individuals of the same species, affect their physiology and behavior. Pheromone binding to receptors on microvilli on the dendritic knobs of vomeronasal sensory neurons activates a second messenger cascade to produce an increase in intracellular Ca²⁺ concentration. Here, we used whole-cell and inside-out patch-clamp analysis to provide a functional characterization of currents activated by Ca²⁺ in isolated mouse vomeronasal sensory neurons in the absence of intracellular K⁺. In whole-cell recordings, the average current in 1.5 µM Ca²⁺ and symmetrical Cl⁻ was −382 pA at −100 mV. Ion substitution experiments and partial blockade by commonly used Cl⁻ channel blockers indicated that Ca²⁺ activates mainly anionic currents in these neurons. Recordings from inside-out patches from dendritic knobs of mouse vomeronasal sensory neurons confirmed the presence of Ca²⁺-activated Cl⁻ channels in the knobs and/or microvilli. We compared the electrophysiological properties of the native currents with those mediated by heterologously expressed TMEM16A/anoctamin1 or TMEM16B/anoctamin2 Ca²⁺-activated Cl⁻ channels, which are coexpressed in microvilli of mouse vomeronasal sensory neurons, and found a closer resemblance to those of TMEM16A. We used the Cre–loxP system to selectively knock out TMEM16A in cells expressing the olfactory marker protein, which is found in mature vomeronasal sensory neurons. Immunohistochemistry confirmed the specific ablation of TMEM16A in vomeronasal neurons. Ca²⁺-activated currents were abolished in vomeronasal sensory neurons of TMEM16A conditional knockout mice, demonstrating that TMEM16A is an essential component of Ca²⁺-activated Cl⁻ currents in mouse vomeronasal sensory neurons.

Overexpression of ANO1/TMEM16A, an arterial Ca2+-activated Cl- channel, contributes to spontaneous hypertension

Calcium-activated chloride channels (CaCCs) have been implicated in hypertension; however, the mechanism underlying their involvement is unknown. The aim of this study was to determine whether the CaCC ANO1 is involved in the pathogenesis of spontaneous hypertension. Arterial ANO1 expression and the effects on blood pressure (BP) of inhibiting ANO1 with an ANO1 inhibitor, T16(Ainh)-A01, and in vivo RNAi, were examined in spontaneously hypertensive rats (SHRs). Knockdown of ANO1 by siRNA prevented hypertensive development, and attenuation of ANO1 channel activity reduced BP in SHRs. Angiotensin II upregulated ANO1 expression in primary cultures of vascular smooth muscle cells (VSMCs). The protein level and activity of cellular ANO1 positively correlated with VSMC proliferation. Our data indicate an important role of increased ANO1 expression and activity in inducing hypertension in SHRs. It may mediate angiotensin II-dependent vascular remodeling. Our results increase the mechanistic understanding of hypertension and suggest ANO1 as a possible therapeutic target for hypertension.

Multiple effects of anthracene-9-carboxylic acid on the TMEM16B/anoctamin2 calcium-activated chloride channel

Ca(2+)-activated Cl(-) currents (CaCCs) play important roles in many physiological processes. Recent studies have shown that TMEM16A/anoctamin1 and TMEM16B/anoctamin2 constitute CaCCs in several cell types. Here we have investigated for the first time the extracellular effects of the Cl(-) channel blocker anthracene-9-carboxylic acid (A9C) and of its non-charged analogue anthracene-9-methanol (A9M) on TMEM16B expressed in HEK 293T cells, using the whole-cell patch-clamp technique. A9C caused a voltage-dependent block of outward currents and inhibited a larger fraction of the current as depolarization increased, whereas the non-charged A9M produced a small, not voltage dependent block of outward currents. A similar voltage-dependent block by A9C was measured both when TMEM16B was activated by 1.5 and 13μM Ca(2+). However, in the presence of 1.5μM Ca(2+) (but not in 13μM Ca(2+)), A9C also induced a strong potentiation of tail currents measured at -100mV after depolarizing voltages, as well as a prolongation of the deactivation kinetics. On the contrary, A9M did not produce potentiation of tail currents, showing that the negative charge is required for potentiation. Our results provide the first evidence that A9C has multiple effects on TMEM16B and that the negative charge of A9C is necessary both for voltage-dependent block and for potentiation. Future studies are required to identify the molecular mechanisms underlying these complex effects of A9C on TMEM16B. Understanding these mechanisms will contribute to the elucidation of the structure and functional properties of TMEM16B channels.

Structure and function of TMEM16 proteins (anoctamins)

TMEM16 proteins, also known as anoctamins, are involved in a variety of functions that include ion transport, phospholipid scrambling, and regulation of other membrane proteins. The first two members of the family, TMEM16A (anoctamin-1, ANO1) and TMEM16B (anoctamin-2, ANO2), function as Ca2+-activated Cl- channels (CaCCs), a type of ion channel that plays important functions such as transepithelial ion transport, smooth muscle contraction, olfaction, phototransduction, nociception, and control of neuronal excitability. Genetic ablation of TMEM16A in mice causes impairment of epithelial Cl- secretion, tracheal abnormalities, and block of gastrointestinal peristalsis. TMEM16A is directly regulated by cytosolic Ca2+ as well as indirectly by its interaction with calmodulin. Other members of the anoctamin family, such as TMEM16C, TMEM16D, TMEM16F, TMEM16G, and TMEM16J, may work as phospholipid scramblases and/or ion channels. In particular, TMEM16F (ANO6) is a major contributor to the process of phosphatidylserine translocation from the inner to the outer leaflet of the plasma membrane. Intriguingly, TMEM16F is also associated with the appearance of anion/cation channels activated by very high Ca2+ concentrations. Furthermore, a TMEM16 protein expressed in Aspergillus fumigatus displays both ion channel and lipid scramblase activity. This finding suggests that dual function is an ancestral characteristic of TMEM16 proteins and that some members, such as TMEM16A and TMEM16B, have evolved to a pure channel function. Mutations in anoctamin genes (ANO3, ANO5, ANO6, and ANO10) cause various genetic diseases. These diseases suggest the involvement of anoctamins in a variety of cell functions whose link with ion transport and/or lipid scrambling needs to be clarified.

The calcium-activated chloride channel Anoctamin 1 contributes to the regulation of renal function

The role of calcium-activated chloride channels for renal function is unknown. By immunohistochemistry we demonstrate dominant expression of the recently identified calcium-activated chloride channels, Anoctamin 1 (Ano1, TMEM16A) in human and mouse proximal tubular epithelial (PTE) cells, with some expression in podocytes and other tubular segments. Ano1-null mice had proteinuria and numerous large reabsorption vesicles in PTE cells. Selective knockout of Ano1 in podocytes (Ano1-/-/Nphs2-Cre) did not impair renal function, whereas tubular knockout in Ano1-/-/Ksp-Cre mice increased urine protein excretion and decreased urine electrolyte concentrations. Purinergic stimulation activated calcium-dependent chloride currents in isolated proximal tubule epithelial cells from wild-type but not from Ano1-/-/Ksp-Cre mice. Ano1 currents were activated by acidic pH, suggesting parallel stimulation of Ano1 chloride secretion with activation of the proton-ATPase. Lack of calcium-dependent chloride secretion in cells from Ano1-/-/Ksp-Cre mice was paralleled by attenuated proton secretion and reduced endosomal acidification, which compromised proximal tubular albumin uptake. Tubular knockout of Ano1 enhanced serum renin and aldosterone concentrations, probably leading to enhanced compensatory distal tubular reabsorption, thus maintaining normal blood pressure levels. Thus, Ano1 has a role in proximal tubular proton secretion and protein reabsorption. The results correspond to regulation of the proton-ATPase by the Ano1-homolog Ist2 in yeast.

Electrophysiological evidence for the presence of cystic fibrosis transmembrane conductance regulator (CFTR) in mouse sperm

Mammalian sperm must undergo a maturational process, named capacitation, in the female reproductive tract to fertilize the egg. Sperm capacitation is regulated by a cAMP/protein kinase A (PKA) pathway and involves increases in intracellular Ca(2+), pH, Cl(-), protein tyrosine phosphorylation, and in mouse and some other mammals a membrane potential hyperpolarization. The cystic fibrosis transmembrane conductance regulator (CFTR), a Cl(-) channel modulated by cAMP/PKA and ATP, was detected in mammalian sperm and proposed to modulate capacitation. Our whole-cell patch-clamp recordings from testicular mouse sperm now reveal a Cl(-) selective component to membrane current that is ATP-dependent, stimulated by cAMP, cGMP, and genistein (a CFTR agonist, at low concentrations), and inhibited by DPC and CFTR(inh) -172, two well-known CFTR antagonists. Furthermore, the Cl(-) current component activated by cAMP and inhibited by CFTR(inh) -172 is absent in recordings on testicular sperm from mice possessing the CFTR ΔF508 loss-of-function mutation, indicating that CFTR is responsible for this component. A Cl(-) selective like current component displaying CFTR characteristics was also found in wild type epididymal sperm bearing the cytoplasmatic droplet. Capacitated sperm treated with CFTR(inh) -172 undergo a shape change, suggesting that CFTR is involved in cell volume regulation. These findings indicate that functional CFTR channels are present in mouse sperm and their biophysical properties are consistent with their proposed participation in capacitation.

Anoctamin 1 (Tmem16A) Ca2+-activated chloride channel stoichiometrically interacts with an ezrin-radixin-moesin network

The newly discovered Ca(2+)-activated Cl(-) channel (CaCC), Anoctamin 1 (Ano1 or TMEM16A), has been implicated in vital physiological functions including epithelial fluid secretion, gut motility, and smooth muscle tone. Overexpression of Ano1 in HEK cells or Xenopus oocytes is sufficient to generate Ca(2+)-activated Cl(-) currents, but the details of channel composition and the regulatory factors that control channel biology are incompletely understood. We used a highly sensitive quantitative SILAC proteomics approach to obtain insights into stoichiometric protein networks associated with the Ano1 channel. These studies provide a comprehensive footprint of putative Ano1 regulatory networks. We find that Ano1 associates with the signaling/scaffolding proteins ezrin, radixin, moesin, and RhoA, which link the plasma membrane to the cytoskeleton with very high stoichiometry. Ano1, ezrin, and moesin/radixin colocalize apically in salivary gland epithelial cells, and overexpression of moesin and Ano1 in HEK cells alters the subcellular localization of both proteins. Moreover, interfering RNA for moesin modifies Ano1 current without affecting its surface expression level. Another network associated with Ano1 includes the SNARE and SM proteins VAMP3, syntaxins 2 and -4, and syntaxin-binding proteins munc18b and munc18c, which are integral to translocation of vesicles to the plasma membrane. A number of other regulatory proteins, including GTPases, Ca(2+)-binding proteins, kinases, and lipid-interacting proteins are enriched in the Ano1 complex. These data provide stoichiometrically prioritized information about mechanisms regulating Ano1 function and trafficking to polarized domains of the plasma membrane.

International Union of Basic and Clinical Pharmacology. LXXXV: calcium-activated chloride channels

Calcium-activated chloride channels (CaCCs) are widely expressed in various tissues and implicated in physiological processes such as sensory transduction, epithelial secretion, and smooth muscle contraction. Transmembrane proteins with unknown function 16 (TMEM16A) has recently been identified as a major component of CaCCs. Detailed molecular analysis of TMEM16A will be needed to understand its structure-function relationships. The role this channel plays in physiological systems remains to be established and is currently a subject of intense investigation.

A minimal isoform of the TMEM16A protein associated with chloride channel activity

TMEM16A protein, also known as anoctamin-1, has been recently identified as an essential component of Ca(2+)-activated Cl(-) channels. We previously reported the existence of different TMEM16A isoforms generated by alternative splicing. In the present study, we have determined the functional properties of a minimal TMEM16A protein. This isoform, called TMEM16A(0), has a significantly shortened amino-terminus and lacks three alternative segments localized in the intracellular regions of the protein (total length: 840 amino acids). TMEM16A(0) expression is associated with Ca(2+)-activated Cl(-) channel activity as measured by three different functional assays based on the halide-sensitive yellow fluorescent protein, short-circuit current recordings, and patch-clamp technique. However, compared to a longer isoform, TMEM16(abc) (total length: 982 amino acids), TMEM16A(0) completely lacks voltage-dependent activation. Furthermore, TMEM16A(0) and TMEM16A(abc) have similar but not identical responses to extracellular anion replacement, thus suggesting a difference in ion selectivity and conductance. Our results indicate that TMEM16A(0) has the basic domains required for anion transport and Ca(2+)-sensitivity. However, the absence of alternative segments, which are present in more complex isoforms of TMEM16A, modifies the channel gating and ion transport ability.

High extracellular Ca2+ stimulates Ca2+-activated Cl- currents in frog parathyroid cells through the mediation of arachidonic acid cascade

Elevation of extracellular Ca(2+) concentration induces intracellular Ca(2+) signaling in parathyroid cells. The response is due to stimulation of the phospholipase C/Ca(2+) pathways, but the direct mechanism responsible for the rise of intracellular Ca(2+) concentration has remained elusive. Here, we describe the electrophysiological property associated with intracellular Ca(2+) signaling in frog parathyroid cells and show that Ca(2+)-activated Cl(-) channels are activated by intracellular Ca(2+) increase through an inositol 1,4,5-trisphophate (IP(3))-independent pathway. High extracellular Ca(2+) induced an outwardly-rectifying conductance in a dose-dependent manner (EC(50) ∼6 mM). The conductance was composed of an instantaneous time-independent component and a slowly activating time-dependent component and displayed a deactivating inward tail current. Extracellular Ca(2+)-induced and Ca(2+) dialysis-induced currents reversed at the equilibrium potential of Cl(-) and were inhibited by niflumic acid (a specific blocker of Ca(2+)-activated Cl(-) channel). Gramicidin-perforated whole-cell recording displayed the shift of the reversal potential in extracellular Ca(2+)-induced current, suggesting the change of intracellular Cl(-) concentration in a few minutes. Extracellular Ca(2+)-induced currents displayed a moderate dependency on guanosine triphosphate (GTP). All blockers for phospholipase C, diacylglycerol (DAG) lipase, monoacylglycerol (MAG) lipase and lipoxygenase inhibited extracellular Ca(2+)-induced current. IP(3) dialysis failed to induce conductance increase, but 2-arachidonoylglycerol (2-AG), arachidonic acid and 12S-hydroperoxy-5Z,8Z,10E,14Z-eicosatetraenoic acid (12(S)-HPETE) dialysis increased the conductance identical to extracellular Ca(2+)-induced conductance. These results indicate that high extracellular Ca(2+) raises intracellular Ca(2+) concentration through the DAG lipase/lipoxygenase pathway, resulting in the activation of Cl(-) conductance.

Regulation of TMEM16A chloride channel properties by alternative splicing

Expression of TMEM16A protein is associated with the activity of Ca(2+)-activated Cl(-) channels. TMEM16A primary transcript undergoes alternative splicing. thus resulting in the generation of multiple isoforms. We have determined the pattern of splicing and assessed the functional properties of the corresponding TMEM16A variants. We found three alternative exons, 6b, 13, and 15, coding for segments of 22, 4, and 26 amino acids, respectively, which are differently spliced in human organs. By patch clamp experiments on transfected cells, we found that skipping of exon 6b changes the Ca(2+) sensitivity by nearly 4-fold, resulting in Cl(-) currents requiring lower Ca(2+) concentrations to be activated. At the membrane potential of 80 mV, the apparent half-effective concentration decreases from 350 to 90 nm when the segment corresponding to exon 6b is excluded. Skipping of exon 13 instead strongly reduces the characteristic time-dependent activation observed for Ca(2+)-activated Cl(-) channels at positive membrane potentials. This effect was also obtained by deleting only the second pair of amino acids corresponding to exon 13. Alternative splicing appears as an important mechanism to regulate the voltage and Ca(2+) dependence of the TMEM16A-dependent Cl(-) channels in a tissue-specific manner.

Bestrophin 1 promotes epithelial-to-mesenchymal transition of renal collecting duct cells

Bestrophin 1 (Best1) controls intracellular Ca(2+) concentration, induces Ca(2+)-activated Cl(-) conductance, and increases proliferation of colon carcinoma cells. Here, we show that expression of Best1 in mouse renal collecting duct (CD) cells causes i) an increase in cell proliferation, ii) a loss of amiloride-sensitive Na(+) absorption, iii) induction of Ca(2+)-dependent Cl(-) conductance (CaCC), and iv) epithelial-to-mesenchymal transition. During conditions of high proliferation or when we exposed CD cells to serum or TGF-beta1, we observed upregulation of Best1, increased CaCC, redistribution of the epithelial-to-mesenchymal transition marker beta-catenin, and upregulation of vimentin. In contrast, suppression of Best1 by RNAi inhibited proliferation, reduced CaCC, and downregulated markers of EMT. CaCC and expression of Best1 were independent of the cell cycle but clearly correlated to cell proliferation and cell density. During renal inflammation in LPS-treated mice or after unilateral ureteral obstruction, we observed transient upregulation of Best1. These data indicate that repression of cell proliferation, CaCC, and expression of Best1 occurs during mesenchymal-to-epithelial transition once CD cells polarize and terminally differentiate. These results may suggest a role for Best1 in renal fibrosis and tissue repair.

Claudin-16 affects transcellular Cl- secretion in MDCK cells

Claudin-16 (paracellin-1) is a tight junction protein localized mainly in the thick ascending limb of Henle's loop and also in the distal nephron. Its defect causes familial hypomagnesaemia with hypercalciuria and nephrocalcinosis. This had been taken as an indication that claudin-16 conveys paracellular Mg(2+) and Ca(2+) transport; however, evidence is still conflicting. We studied paracellular ion permeabilities as well as effects of claudin-16 on the driving forces for passive ion movement. MDCK-C7 cells were stably transfected with wild-type (wt) and mutant (R146T, T233R) claudin-16. Results indicated that paracellular permeability to Mg(2+) but not to Ca(2+) is increased in cells transfected with wt compared to mutant claudin-16 and control cells. Increased basolateral Mg(2+) concentration activated a transcellular Cl(-) current which was greatly enhanced in cells transfected with wt and T233R claudin-16, as compared to R146T claudin-16-transfected or control cells. This current was triggered by the basolateral calcium-sensing receptor causing Ca(2+) release from internal stores, thus activating apical Ca(2+)-sensitive Cl(-) channels and basolateral Ca(2+)-sensitive K(+) channels. Immunohistochemical data suggest that the Cl(-) channel involved is bestrophin. We conclude that claudin-16 itself possesses only moderate paracellular Mg(2+) permeability but governs transcellular Cl(-) currents by interaction with apical Ca(2+)-activated Cl(-) channels, presumably bestrophin. As the transepithelial voltage generated by such a current alters the driving force for all ions, this may be the major mechanism to regulate Mg(2+) and Ca(2+) absorption in the kidney.

Anoctamin/TMEM16 family members are Ca2+-activated Cl- channels

Ca(2+)-activated Cl- channels (CaCCs) perform many important functions in cell physiology including secretion of fluids from acinar cells of secretory glands, amplification of olfactory transduction, regulation of cardiac and neuronal excitability, mediation of the fast block to polyspermy in amphibian oocytes, and regulation of vascular tone. Although a number of proteins have been proposed to be responsible for CaCC currents, the anoctamin family (ANO, also known as TMEM16) exhibits characteristics most similar to those expected for the classical CaCC. Interestingly, this family of proteins has previously attracted the interest of both developmental and cancer biologists. Some members of this family are up-regulated in a number of tumours and functional deficiency in others is linked to developmental defects.

A hyperpolarization-activated, cyclic nucleotide-gated, (Ih-like) cationic current and HCN gene expression in renal inner medullary collecting duct cells

The cation conductancein primary cultures of rat renal inner medullary collecting duct was studied using perforated-patch and conventional whole cell clamp techniques. Hyperpolarizations beyond -60 mV induced a time-dependent inward nonselective cationic current (I(vti)) that resembles the well-known hyperpolarization-activated, cyclic nucleotide-gated I(h) and I(f) currents. I(vti) showed a half-maximal activation around -102 mV with a slope factor of 25 mV. It had a higher conductance (but, at its reversal potential, not a higher permeability) for K(+) than for Na(+) (gK(+)/gNa(+) = 1.5), was modulated by cAMP and blocked by external Cd(2+) (but not Cs(+) or ZD-7288), and potentiated by a high extracellular K(+) concentration. We explored the expression of the I(h) channel genes (HCN1 to -4) by RT-PCR. The presence of transcripts corresponding to the HCN1, -2, and -4 genes was observed in both the cultured cells and kidney inner medulla. Western blot analysis with HCN2 antibody showed labeling of approximately 90- and approximately 120-kDa proteins in samples from inner medulla and cultured cells. Immunocytochemical analysis of cell cultures and inner medulla showed the presence of HCN immunoreactivity partially colocalized with the Na(+)-K(+)-ATPase at the basolateral membrane of collecting duct cells. This is the first evidence of an I(h)-like cationic current and HCN immunoreactivity in either kidney or any other nonexcitable mammalian cells.

Mechanism of the inhibition of Ca2+-activated Cl- currents by phosphorylation in pulmonary arterial smooth muscle cells

The aim of the present study was to provide a mechanistic insight into how phosphatase activity influences calcium-activated chloride channels in rabbit pulmonary artery myocytes. Calcium-dependent Cl⁻ currents (I_ClCa) were evoked by pipette solutions containing concentrations between 20 and 1000 nM Ca²⁺ and the calcium and voltage dependence was determined. Under control conditions with pipette solutions containing ATP and 500 nM Ca²⁺, I_ClCa was evoked immediately upon membrane rupture but then exhibited marked rundown to ∼20% of initial values. In contrast, when phosphorylation was prohibited by using pipette solutions containing adenosine 5′-(β,γ-imido)-triphosphate (AMP-PNP) or with ATP omitted, the rundown was severely impaired, and after 20 min dialysis, I_ClCa was ∼100% of initial levels. I_ClCa recorded with AMP-PNP–containing pipette solutions were significantly larger than control currents and had faster kinetics at positive potentials and slower deactivation kinetics at negative potentials. The marked increase in I_ClCa was due to a negative shift in the voltage dependence of activation and not due to an increase in the apparent binding affinity for Ca²⁺. Mathematical simulations were carried out based on gating schemes involving voltage-independent binding of three Ca²⁺, each binding step resulting in channel opening at fixed calcium but progressively greater “on” rates, and voltage-dependent closing steps (“off” rates). Our model reproduced well the Ca²⁺ and voltage dependence of I_ClCa as well as its kinetic properties. The impact of global phosphorylation could be well mimicked by alterations in the magnitude, voltage dependence, and state of the gating variable of the channel closure rates. These data reveal that the phosphorylation status of the Ca²⁺-activated Cl⁻ channel complex influences current generation dramatically through one or more critical voltage-dependent steps.

Bestrophin-1 enables Ca2+-activated Cl- conductance in epithelia

Epithelial cells express calcium-activated Cl(-) channels of unknown molecular identity. These Cl(-) channels play a central role in diseases such as secretory diarrhea, polycystic kidney disease, and cystic fibrosis. The family of bestrophins has been suggested to form calcium-activated Cl(-) channels. Here, we demonstrate molecular and functional expression of bestrophin-1 (BEST1) in mouse and human airways, colon, and kidney. Endogenous calcium-activated whole cell Cl(-) currents coincide with endogenous expression of the Vmd2 gene product BEST1 in murine and human epithelial cells, whereas calcium-activated Cl(-) currents are absent in epithelial tissues lacking BEST1 expression. Blocking expression of BEST1 with short interfering RNA or applying an anti-BEST1 antibody to a patch pipette suppressed ATP-induced whole cell Cl(-) currents. Calcium-dependent Cl(-) currents were activated by ATP in HEK293 cells expressing BEST1. Thus, BEST1 may form the Ca2+-activated Cl(-) current, or it may be a component of a Cl(-) channel complex in epithelial tissues.

Cell cycle-dependent activity of the volume- and Ca2+-activated anion currents in Ehrlich lettre ascites cells

Recent evidence implicates the volume-regulated anion current (VRAC) and other anion currents in control or modulation of cell cycle progression; however, the precise involvement of anion channels in this process is unclear. Here, Cl- currents in Ehrlich Lettre Ascites (ELA) cells were monitored during cell cycle progression, under three conditions: (i) after osmotic swelling (i.e., VRAC), (ii) after an increase in the free intracellular Ca2+ concentration (i.e., the Ca2+-activated Cl- current, CaCC), and (iii) under steady-state isotonic conditions. The maximal swelling-activated VRAC current decreased in G1 and increased in early S phase, compared to that in G0. The isotonic steady-state current, which seems to be predominantly VRAC, also decreased in G1, and increased again in early S phase, to a level similar to that in G0. In contrast, the maximal CaCC current (500 nM free Ca2+ in the pipette), was unaltered from G0 to G1, but decreased in early S phase. A novel high-affinity anion channel inhibitor, the acidic di-aryl-urea NS3728, which inhibited both VRAC and CaCC, attenuated ELA cell growth, suggesting a possible mechanistic link between cell cycle progression and cell cycle-dependent changes in the capacity for conductive Cl- transport. It is suggested that in ELA cells, entrance into the S phase requires an increase in VRAC activity and/or an increased potential for regulatory volume decrease (RVD), and at the same time a decrease in CaCC magnitude.

Direct effects of 9-anthracene compounds on cystic fibrosis transmembrane conductance regulator gating

Anthracene-9-carboxylic acid (9-AC) has been reported to show both potentiation and inhibitory effects on guinea-pig cardiac cAMP-activated chloride channels via two different binding sites, and inhibition of Mg(2+)-sensitive protein phosphatases has been proposed for the mechanism of 9-AC potentiation effect. In this study, we examined the effects of 9-AC on wild-type and mutant human cystic fibrosis transmembrane conductance regulator (CFTR) chloride channels expressed in NIH3T3 or CHO cells. 9-AC inhibits whole-cell CFTR current in a voltage-dependent manner, whereas the potentiation effect is not affected by membrane potentials. Anthracene-9-methanol, an electro-neutral 9-AC analog, fails to block CFTR, but shows a nearly identical potentiation effect, corroborating the idea that two chemically distinct sites are responsible, respectively, for potentiation and inhibitory actions of 9-AC. 9-AC also enhances the activity of deltaR-CFTR, a constitutively active CFTR mutant whose R-domain is removed. In excised inside-out patches, 9-AC increases Po by prolonging the mean burst durations and shortening the interburst durations. We therefore conclude that two different 9-AC binding sites for potentiation and inhibitory effects on CFTR channels are located outside of the R-domain. We also speculate that 9-AC potentiates CFTR activity by directly affecting CFTR gating.

Calcium-activated chloride channels

Calcium-activated chloride channels (CaCCs) play important roles in cellular physiology, including epithelial secretion of electrolytes and water, sensory transduction, regulation of neuronal and cardiac excitability, and regulation of vascular tone. This review discusses the physiological roles of these channels, their mechanisms of regulation and activation, and the mechanisms of anion selectivity and conduction. Despite the fact that CaCCs are so broadly expressed in cells and play such important functions, understanding these channels has been limited by the absence of specific blockers and the fact that the molecular identities of CaCCs remains in question. Recent status of the pharmacology and molecular identification of CaCCs is evaluated.

Exploration of the basolateral chloride channels in the renal tubule using

Chloride channels located on the basolateral membrane are known to be involved in chloride absorption in several parts of the renal tubule, and particularly in the thick ascending limb and distal convoluted tubule. The data available suggest that the ClC-K channels play the major role in this process. We provide here a description of the electrophysiological properties of these channels, still very incomplete at this stage, and we attempt to compare ClC-Ks to three chloride channels that we have identified in the basolateral membrane of microdissected fragments of the mouse renal tubule using the patch-clamp technique. Based on anion selectivity and dependence on external pH and calcium shown by the ClC-Ks, we propose candidate ClC-K1 and ClC-K2 in native tissue. We also discuss the possibility that chloride channels that do not belong to the ClC family may also be involved in the absorption of chloride across the cortical thick ascending limb.

Kinetics and regulation of a Ca2+-activated Cl- conductance in mouse renal inner medullary collecting duct cells

Using the whole cell patch-clamp technique, a Ca2+-activated Cl- conductance (CaCC) was transiently activated by extracellular ATP (100 microM) in primary cultures of mouse inner medullary collecting duct (IMCD) cells and in the mouse IMCD-K2 cell line. ATP also transiently increased intracellular Ca2+ concentration ([Ca2+]i) from 100 nM to peak values of approximately 750 nM in mIMCD-K2 cells, with a time course similar to the ATP-induced activation and decay of the CaCC. Removal of extracellular Ca2+ had no major effect on the peak Cl- conductance or the increase in [Ca2+]i induced by ATP, suggesting that Ca2+ released from intracellular stores directly activates the CaCC. In mIMCD-K2 cells, a rectifying time- and voltage-dependent current was observed when [Ca2+]i was fixed via the patch pipette to between 100 and 500 nM. Maximal activation occurred at approximately 1 microM [Ca2+]i, with currents losing any kinetics and displaying a linear current-voltage relationship. From Ca2+-dose-response curves, an EC50 value of approximately 650 nM at -80 mV was obtained, suggesting that under physiological conditions the CaCC would be near fully activated by mucosal nucleotides. Noise analysis of whole cell currents in mIMCD-K2 cells suggests a single-channel conductance of 6-8 pS and a density of approximately 5,000 channels/cell. In conclusion, the CaCC in mouse IMCD cells is a low-conductance, nucleotide-sensitive Cl- channel, whose activity is tightly coupled to changes in [Ca2+]i over the normal physiological range.