The multiple expression of Ca²⁺-activated Cl⁻ channels via homo- and hetero-dimer formation of TMEM16A splicing variants in murine portal vein

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

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

Increased TMEM16A-mediated Ca2+-activated Cl- currents in portal vein smooth muscle cells of caveolin 1-deficient mice

Ca²⁺-activated Cl^- (Cl_Ca) channels regulate membrane excitability and myogenic tone in vascular smooth muscles. TMEM16A-coding proteins are mainly responsible for functional Cl_Ca channels in vascular smooth muscles, including portal vein smooth muscles (PVSMs). Caveolae are cholesterol-rich and Ω-shaped invaginations on the plasma membrane that structurally contributes to effective and efficient signal transduction. Caveolin 1 (Cav1) accumulates in caveolae to form functional complexes among receptors, ion channels, and kinases. The present study examined the functional roles of Cav1 in the expression and activity of Cl_Ca channels in the portal vein smooth muscle cells (PVSMCs) of wild-type (WT) and Cav1-knockout (KO) mice. Contractile experiments revealed that the amplitude of spontaneous PVSM contractions was larger in Cav1-KO mice than WT mice. Under whole-cell patch-clamp configurations, Cl_Ca currents were markedly inhibited by 1 μM Ani9 (a selective TMEM16A Cl_Ca channel blocker) in WT and Cav1-KO PVSMCs. However, Ani9-sensitive Cl_Ca currents were significantly larger in Cav1-KO PVSMCs than in WT PVSMCs. Expression analyses showed that TMEM16A expression levels were higher in Cav1-KO PVSMs than in WT PVSMs. Therefore, the caveolar structure formed by Cav1 negatively regulates the expression and activity of TMEM16A-mediated Cl_Ca channels in vascular smooth muscle cells.

Downregulation of Ca2+-Activated Cl- Channel TMEM16A Mediated by Angiotensin II in Cirrhotic Portal Hypertensive Mice

Portal hypertension is defined as an increased pressure in the portal venous system and occurs as a major complication in chronic liver diseases. The pathological mechanism underlying the pathogenesis and development of portal hypertension has been extensively investigated. Vascular tone of portal vein smooth muscles (PVSMs) is regulated by the activities of several ion channels, including Ca²⁺-activated Cl^- (Cl_Ca) channels. TMEM16A is mainly responsible for Cl_Ca channel conductance in vascular smooth muscle cells, including portal vein smooth muscle cells (PVSMCs). In the present study, the functional roles of TMEM16A channels were examined using two experimental portal hypertensive models, bile duct ligation (BDL) mice with cirrhotic portal hypertension and partial portal vein ligation (PPVL) mice with non-cirrhotic portal hypertension. Expression analyses revealed that the expression of TMEM16A was downregulated in BDL-PVSMs, but not in PPVL-PVSMs. Whole-cell Cl_Ca currents were smaller in BDL-PVSMCs than in sham- and PPVL-PVSMCs. The amplitude of spontaneous contractions was smaller and the frequency was higher in BDL-PVSMs than in sham- and PPVL-PVSMs. Spontaneous contractions sensitive to a specific inhibitor of TMEM16A channels, T16A_inh-A01, were reduced in BDL-PVSMs. Furthermore, in normal PVSMs, the downregulation of TMEM16A expression was mimicked by the exposure to angiotensin II, but not to bilirubin. This study suggests that the activity of Cl_Ca channels is attenuated by the downregulation of TMEM16A expression in PVSMCs associated with cirrhotic portal hypertension, which is partly mediated by increased angiotensin II in cirrhosis.

Reciprocal Relationship between Ca2+ Signaling and Ca2+-Gated Ion Channels as a Potential Target for Drug Discovery

Cellular Ca²⁺ signaling functions as one of the most common second messengers of various signal transduction pathways in cells and mediates a number of physiological roles in a cell-type dependent manner. Ca²⁺ signaling also regulates more general and fundamental cellular activities, including cell proliferation and apoptosis. Among ion channels, Ca²⁺-permeable channels in the plasma membrane as well as endo- and sarcoplasmic reticulum membranes play important roles in Ca²⁺ signaling by directly contributing to the influx of Ca²⁺ from extracellular spaces or its release from storage sites, respectively. Furthermore, Ca²⁺-gated ion channels in the plasma membrane often crosstalk reciprocally with Ca²⁺ signals and are central to the regulation of cellular functions. This review focuses on the physiological and pharmacological impact of i) Ca²⁺-gated ion channels as an apparatus for the conversion of cellular Ca²⁺ signals to intercellularly propagative electrical signals and ii) the opposite feedback regulation of Ca²⁺ signaling by Ca²⁺-gated ion channel activities in excitable and non-excitable cells.

Calcium-Activated Chloride Channels in Myometrial and Vascular Smooth Muscle

In smooth muscle tissues, calcium-activated chloride channels (CaCC) provide the major anionic channel. Opening of these channels leads to chloride efflux and depolarization of the myocyte membrane. In this way, activation of the channels by a rise of intracellular [Ca²⁺], from a variety of sources, produces increased excitability and can initiate action potentials and contraction or increased tone. We now have a good mechanistic understanding of how the channels are activated and regulated, due to identification of TMEM16A (ANO1) as the molecular entity of the channel, but key questions remain. In reviewing these channels and comparing two distinct smooth muscles, myometrial and vascular, we expose the differences that occur in their activation mechanisms, properties, and control. We find that the myometrium only expresses “classical,” Ca²⁺-activated, and voltage sensitive channels, whereas both tonic and phasic blood vessels express classical, and non-classical, cGMP-regulated CaCC, which are voltage insensitive. This translates to more complex activation and regulation in vascular smooth muscles, irrespective of whether they are tonic or phasic. We therefore tentatively conclude that although these channels are expressed and functionally important in all smooth muscles, they are probably not part of the mechanisms governing phasic activity. Recent knockdown studies have produced unexpected functional results, e.g. no effects on labour and delivery, and tone increasing in some but decreasing in other vascular beds, strongly suggesting that there is still much to be explored concerning CaCC in smooth muscle.

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

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

TMEM16A protein: calcium binding site and its activation mechanism

TMEM16A mediates calcium-activated transmembrane flow of chloride ion and a variety of physiological functions. The binding of cytoplasmic calcium ions of TMEM16A and the consequent conformational changes of it are the key issues to explore the relationship between its structure and function. In recent years, researchers have explored this issue through electrophysiological experiment, structure resolving, molecular dynamic simulation and other methods. The structures of TMEM16 family members resolved by cryo-Electron microscopy (cryo-EM) and X-ray crystallization provide the primarily basis for the investigation of the molecular mechanism of TMEM16A. However, the binding and activation mechanism of calcium ions in TMEM16A are still unclear and controversial. This review discusses four Ca2+ sensing sites of TMEM16A and analyze activation properties of TMEM16A by them, which will help to understand the structure-function relationship of TMEM16A and throw light on the molecular design targeting TMEM16A channel.

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

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

Calcium-Activated Chloride Channel ANO1/TMEM16A: Regulation of Expression and Signaling

Anoctamin-1 (ANO1), also known as TMEM16A, is the most studied member of anoctamin family of calcium-activated chloride channels with diverse cellular functions. ANO1 controls multiple cell functions including cell proliferation, survival, migration, contraction, secretion, and neuronal excitation. This review summarizes the current knowledge of the cellular mechanisms governing the regulation of ANO1 expression and of ANO1-mediated intracellular signaling. This includes the stimuli and mechanisms controlling ANO1 expression, agonists and processes that activate ANO1, and signal transduction mediated by ANO1. The major conclusion is that this field is poorly understood, remains highly controversial, and requires extensive and rigorous further investigation.

TMEM16A Ca2+-Activated Cl- Channel Regulates the Proliferation and Migration of Brain Capillary Endothelial Cells

The blood-brain barrier (BBB) is essential for the maintenance of homeostasis in the brain. Brain capillary endothelial cells (BCECs) comprise the BBB, and thus a delicate balance between their proliferation and death is required. Although the activity of ion channels in BCECs is involved in BBB functions, the underlying molecular mechanisms remain unclear. In the present study, the molecular components of Ca²⁺-activated Cl^- (Cl_Ca) channels and their physiological roles were examined using mouse BCECs (mBCECs) and a cell line derived from bovine BCECs, t-BBEC117. Expression analyses revealed that TMEM16A was strongly expressed in mBCECs and t-BBEC117 cells. In t-BBEC117 cells, whole-cell Cl^- currents were sensitive to the Cl_Ca channel blockers, 100 μM niflumic acid and 10 μM T16A_inh-A01, and were also reduced markedly by small-interfering RNA (siRNA) knockdown of TMEM16A. Importantly, block of Cl_Ca currents with Cl_Ca channel blockers or TMEM16A siRNA induced membrane hyperpolarization. Moreover, treatment with TMEM16A siRNA caused an increase in resting cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt). T16A_inh-A01 reduced cell viability in a concentration-dependent manner. Either Cl_Ca channel blockers or TMEM16A siRNA also curtailed cell proliferation and migration. Furthermore, Cl_Ca channel blockers attenuated the trans-endothelial permeability. In combination, these results strongly suggest that TMEM16A contributes to Cl_Ca channel conductance and can regulate both the resting membrane potential and [Ca²⁺]_cyt in BCECs. Our data also reveal how these BCECs may be involved in the maintenance of BBB functions, as both the proliferation and migration are altered following changes in channel activity. SIGNIFICANCE STATEMENT: In brain capillary endothelial cells (BCECs) of the blood-brain barrier (BBB), TMEM16A is responsible for Ca²⁺-activated Cl^- channels and can regulate both the resting membrane potential and cytosolic Ca²⁺ concentration, contributing to the proliferation and migration of BCECs. The present study provides novel information on the molecular mechanisms underlying the physiological functions of BCECs in the BBB and a novel target for therapeutic drugs for disorders associated with dysfunctions in the BBB.

Endophilin A2 regulates calcium-activated chloride channel activity via selective autophagy-mediated TMEM16A degradation

TMEM16A Ca2+-activated chloride channel (CaCC) plays an essential role in vascular homeostasis. In this study we investigated the molecular mechanisms underlying downregulation of TMEM16A CaCC activity during hypertension. In cultured basilar artery smooth muscle cells (BASMCs) isolated from 2k2c renohypertesive rats, treatment with angiotensin II (0.125-1 μM) dose-dependently increased endophilin A2 levels and decreased TMEM16A expression. Similar phenomenon was observed in basilar artery isolated from 2k2c rats. We then used whole-cell recording to examine whether endophilin A2 could regulate TMEM16A CaCC activity in BASMCs and found that knockdown of endophilin A2 significantly enhanced CaCC activity, whereas overexpression of endophilin A2 produced the opposite effect. Overexpression of endophilin A2 did not affect the TMEM16A mRNA level, but markedly decreased TMEM16A protein level in BASMCs by inducing ubiquitination and autophagy of TMEM16A. Ubiquitin-binding receptor p62 (SQSTM1) could bind to ubiquitinated TMEM16A and resulted in a process of TMEM16A proteolysis in autophagosome/lysosome. These data provide new insights into the regulation of TMEM16A CaCC activity by endophilin A2 in BASMCs, which partly explains the mechanism of angiotensin-II-induced TMEM16A inhibition during hypertension-induced vascular remodeling.

TMEM16A and TMEM16B channel proteins generate Ca2+-activated Cl- current and regulate melatonin secretion in rat pineal glands

Pinealocytes regulate circadian rhythm by synthesizing and secreting melatonin. These cells generate action potentials; however, the contribution of specific ion channels to melatonin secretion from pinealocytes remains unclear. In this study, the involvement and molecular identity of Ca²⁺-activated Cl^- (Cl_Ca) channels in the regulation of melatonin secretion were examined in rat pineal glands. Treatment with the Cl_Ca channel blockers, niflumic acid or T16A_inh-A01, significantly reduced melatonin secretion in pineal glands. After pineal K⁺ currents were totally blocked under whole-cell patch clamp conditions, depolarization and subsequent repolarization induced a slowly activating outward current and a substantial inward tail current, respectively. Both of these current changes were dependent on intracellular Ca²⁺ concentration and inhibited by niflumic acid and T16A_inh-A01. Quantitative real-time PCR, Western blotting, and immunocytochemical analyses revealed that TMEM16A and TMEM16B were highly expressed in pineal glands. siRNA knockdown of TMEM16A and/or TMEM16B showed that both channels contribute to Cl_Ca currents in pinealocytes. Conversely, co-expression of TMEM16A and TMEM16B channels or the expression of this tandem channel in HEK293 cells mimicked the electrophysiological characteristics of Cl_Ca currents in pinealocytes. Moreover, bimolecular fluorescence complementation, FRET, and co-immunoprecipitation experiments suggested that TMEM16A and TMEM16B can form heteromeric channels, as well as homomeric channels. In conclusion, pineal Cl_Ca channels are composed of TMEM16A and TMEM16B subunits, and these fluxes regulate melatonin secretion in pineal glands.

Negative News: Cl− and HCO3− in the Vascular Wall

Cl− and HCO3− are the most prevalent membrane-permeable anions in the intra- and extracellular spaces of the vascular wall. Outwardly directed electrochemical gradients for Cl− and HCO3− permit anion channel opening to depolarize vascular smooth muscle and endothelial cells. Transporters and channels for Cl− and HCO3− also modify vascular contractility and structure independently of membrane potential. Transport of HCO3− regulates intracellular pH and thereby modifies the activity of enzymes, ion channels, and receptors. There is also evidence that Cl− and HCO3− transport proteins affect gene expression and protein trafficking. Considering the extensive implications of Cl− and HCO3− in the vascular wall, it is critical to understand how these ions are transported under physiological conditions and how disturbances in their transport can contribute to disease development. Recently, sensing mechanisms for Cl− and HCO3− have been identified in the vascular wall where they modify ion transport and vasomotor function, for instance, during metabolic disturbances. This review discusses current evidence that transport (e.g., via NKCC1, NBCn1, Ca2+-activated Cl− channels, volume-regulated anion channels, and CFTR) and sensing (e.g., via WNK and RPTPγ) of Cl− and HCO3− influence cardiovascular health and disease.

Recent advances in therapeutic strategies that focus on the regulation of ion channel expression

A number of different ion channel types are involved in cell signaling networks, and homeostatic regulatory mechanisms contribute to the control of ion channel expression. Profiling of global gene expression using microarray technology has recently provided novel insights into the molecular mechanisms underlying the homeostatic and pathological control of ion channel expression. It has demonstrated that the dysregulation of ion channel expression is associated with the pathogenesis of neural, cardiovascular, and immune diseases as well as cancers. In addition to the transcriptional, translational, and post-translational regulation of ion channels, potentially important evidence on the mechanisms controlling ion channel expression has recently been accumulated. The regulation of alternative pre-mRNA splicing is therefore a novel therapeutic strategy for the treatment of dominant-negative splicing disorders. Epigenetic modification plays a key role in various pathological conditions through the regulation of pluripotency genes. Inhibitors of pre-mRNA splicing and histone deacetyalase/methyltransferase have potential as potent therapeutic drugs for cancers and autoimmune and inflammatory diseases. Moreover, membrane-anchoring proteins, lysosomal and proteasomal degradation-related molecules, auxiliary subunits, and pharmacological agents alter the protein folding, membrane trafficking, and post-translational modifications of ion channels, and are linked to expression-defect channelopathies. In this review, we focused on recent insights into the transcriptional, spliceosomal, epigenetic, and proteasomal regulation of ion channel expression: Ca(2+) channels (TRPC/TRPV/TRPM/TRPA/Orai), K(+) channels (voltage-gated, KV/Ca(2+)-activated, KCa/two-pore domain, K2P/inward-rectifier, Kir), and Ca(2+)-activated Cl(-) channels (TMEM16A/TMEM16B). Furthermore, this review highlights expression of these ion channels in expression-defect channelopathies.

Anoctamin-1 Cl(-) channels in nociception: activation by an N-aroylaminothiazole and capsaicin and inhibition by T16A[inh]-A01

Background

Anoctamin 1 (ANO1 or TMEM16A) Ca²⁺-gated Cl⁻ channels of nociceptor neurons are emerging as important molecular components of peripheral pain transduction. At physiological intracellular Cl⁻ concentrations ([Cl⁻]_i) sensory neuronal Cl⁻ channels are excitatory. The ability of sensory neuronal ANO1 to trigger action potentials and subsequent nocifensive (pain) responses were examined by direct activation with an N-aroylaminothiazole. ANO1 channels are also activated by intracellular Ca²⁺ ([Ca²⁺]_i) from sensory neuronal TRPV1 (transient-receptor-potential vallinoid 1) ion channels and other noxicant receptors. Thus, sensory neuronal ANO1 can facilitate TRPV1 triggering of action potentials, resulting in enhanced nociception. This was investigated by reducing ANO1 facilitation of TRPV1 effects with: (1) T16A[inh]-A01 ANO1-inhibitor reagent at physiological [Cl⁻]_i and (2) by lowering sensory neuronal [Cl⁻]_i to switch ANO1 to be inhibitory.

Results

ANO1 effects on action potential firing of mouse dorsal root ganglia (DRG) neurons in vitro and mouse nocifensive behaviors in vivo were examined with an N-aroylaminothiazole ANO1-activator (E-act), a TRPV1-activator (capsaicin) and an ANO1-inhibitor (T16A[inh]-A01). At physiological [Cl⁻]_i (40 mM), E-act (10 µM) increased current sizes (in voltage-clamp) and action potential firing (in current-clamp) recorded in DRG neurons using whole-cell electrophysiology. To not disrupt TRPV1 carried-Ca²⁺ activation of ANO1 in DRG neurons, ANO1 modulation of capsaicin-induced action potentials was measured by perforated-patch (Amphotericin–B) current-clamp technique. Subsequently, at physiological [Cl⁻]_i, capsaicin (15 µM)-induced action potential firing was diminished by co-application with T16A[inh]-A01 (20 µM). Under conditions of low [Cl⁻]_i (10 mM), ANO1 actions were reversed. Specifically, E-act did not trigger action potentials; however, capsaicin-induced action potential firing was inhibited by co-application of E-act, but was unaffected by co-application of T16A[inh]-A01. Nocifensive responses of mice hind paws were dramatically induced by subcutaneous injections of E-act (5 mM) or capsaicin (50 µM). The nocifensive responses were attenuated by co-injection with T16A[inh]-A01 (1.3 mM).

Conclusions

An ANO1-activator (E-act) induced [Cl⁻]_i-dependent sensory neuronal action potentials and mouse nocifensive behaviors; thus, direct ANO1 activation can induce pain perception. ANO1-inhibition attenuated capsaicin-triggering of action potentials and capsaicin-induced nocifensive behaviors. These results indicate ANO1 channels are involved with TRPV1 actions in sensory neurons and inhibition of ANO1 could be a novel means of inducing analgesia.

Electronic supplementary material

The online version of this article (doi:10.1186/s12990-015-0061-y) contains supplementary material, which is available to authorized users.

The bestrophin- and TMEM16A-associated Ca(2+)- activated Cl(–) channels in vascular smooth muscles

The presence of Ca²⁺-activated Cl^– currents (I_Cl(Ca)) in vascular smooth muscle cells (VSMCs) is well established. I_Cl(Ca) are supposedly important for arterial contraction by linking changes in [Ca²⁺]_i and membrane depolarization. Bestrophins and some members of the TMEM16 protein family were recently associated with I_Cl(Ca). Two distinct I_Cl(Ca) are characterized in VSMCs; the cGMP-dependent I_Cl(Ca) dependent upon bestrophin expression and the ‘classical’ Ca²⁺-activated Cl^– current, which is bestrophin-independent. Interestingly, TMEM16A is essential for both the cGMP-dependent and the classical I_Cl(Ca). Furthermore, TMEM16A has a role in arterial contraction while bestrophins do not. TMEM16A’s role in the contractile response cannot be explained however only by a simple suppression of the depolarization by Cl^– channels. It is suggested that TMEM16A expression modulates voltage-gated Ca²⁺ influx in a voltage-independent manner and recent studies also demonstrate a complex role of TMEM16A in modulating other membrane proteins.

Molecular and functional significance of Ca(2+)-activated Cl(-) channels in pulmonary arterial smooth muscle

Increased peripheral resistance of small distal pulmonary arteries is a hallmark signature of pulmonary hypertension (PH) and is believed to be the consequence of enhanced vasoconstriction to agonists, thickening of the arterial wall due to remodeling, and increased thrombosis. The elevation in arterial tone in PH is attributable, at least in part, to smooth muscle cells of PH patients being more depolarized and displaying higher intracellular Ca(2+) levels than cells from normal subjects. It is now clear that downregulation of voltage-dependent K(+) channels (e.g., Kv1.5) and increased expression and activity of voltage-dependent (Cav1.2) and voltage-independent (e.g., canonical and vanilloid transient receptor potential [TRPC and TRPV]) Ca(2+) channels play an important role in the functional remodeling of pulmonary arteries in PH. This review focuses on an anion-permeable channel that is now considered a novel excitatory mechanism in the systemic and pulmonary circulations. It is permeable to Cl(-) and is activated by a rise in intracellular Ca(2+) concentration (Ca(2+)-activated Cl(-) channel, or CaCC). The first section outlines the biophysical and pharmacological properties of the channel and ends with a description of the molecular candidate genes postulated to encode for CaCCs, with particular emphasis on the bestrophin and the newly discovered TMEM16 and anoctamin families of genes. The second section provides a review of the various sources of Ca(2+) activating CaCCs, which include stimulation by mobilization from intracellular Ca(2+) stores and Ca(2+) entry through voltage-dependent and voltage-independent Ca(2+) channels. The third and final section summarizes recent findings that suggest a potentially important role for CaCCs and the gene TMEM16A in PH.

New light on ion channel imaging by total internal reflection fluorescence (TIRF) microscopy

Ion channels play pivotal roles in a wide variety of cellular functions; therefore, their physiological characteristics, pharmacological responses, and molecular structures have been extensively investigated. However, the mobility of an ion channel itself in the cell membrane has not been examined in as much detail. A total internal reflection fluorescence (TIRF) microscope allows fluorophores to be imaged in a restricted region within an evanescent field of less than 200 nm from the interface of the coverslip and plasma membrane in living cells. Thus the TIRF microscope is useful for selectively visualizing the plasmalemmal surface and subplasmalemmal zone. In this review, we focused on a single-molecule analysis of the dynamic movement of ion channels in the plasma membrane using TIRF microscopy. We also described two single-molecule imaging techniques under TIRF microscopy: fluorescence resonance energy transfer (FRET) for the identification of molecules that interact with ion channels, and subunit counting for the determination of subunit stoichiometry in a functional channel. TIRF imaging can also be used to analyze spatiotemporal Ca(2+) events in the subplasmalemma. Single-molecule analyses of ion channels and localized Ca(2+) signals based on TIRF imaging provide beneficial pharmacological and physiological information concerning the functions of ion channels.

A novel TMEM16A splice variant lacking the dimerization domain contributes to calcium-activated chloride secretion in human sweat gland epithelial cells

Sweating is an important physiological process to regulate body temperature in humans, and various disorders are associated with dysregulated sweat formation. Primary sweat secretion in human eccrine sweat glands involves Ca(2+) -activated Cl(-) channels (CaCC). Recently, members of the TMEM16 family were identified as CaCCs in various secretory epithelia; however, their molecular identity in sweat glands remained elusive. Here, we investigated the function of TMEM16A in sweat glands. Gene expression analysis revealed that TMEM16A is expressed in human NCL-SG3 sweat gland cells as well as in isolated human eccrine sweat gland biopsy samples. Sweat gland cells express several previously described TMEM16A splice variants, as well as one novel splice variant, TMEM16A(acΔe3) lacking the TMEM16A-dimerization domain. Chloride flux assays using halide-sensitive YFP revealed that TMEM16A is functionally involved in Ca(2+) -dependent Cl(-) secretion in NCL-SG3 cells. Recombinant expression in NCL-SG3 cells showed that TMEM16A(acΔe3) is forming a functional CaCC, with basal and Ca(2+) -activated Cl(-) permeability distinct from canonical TMEM16A(ac). Our results suggest that various TMEM16A isoforms contribute to sweat gland-specific Cl(-) secretion providing opportunities to develop sweat gland-specific therapeutics for treatment of sweating disorders.

Downregulation of Ca2+-activated Cl- channel TMEM16A by the inhibition of histone deacetylase in TMEM16A-expressing cancer cells

The Ca(2+)-activated Cl(-) channel transmembrane proteins with unknown function 16 A (TMEM16A; also known as anoctamin 1 or discovered on gastrointestinal stromal tumor 1) plays an important role in facilitating the cell growth and metastasis of TMEM16A-expressing cancer cells. Histone deacetylase (HDAC) inhibitors (HDACi) are useful agents for cancer therapy, but it remains unclear whether ion channels are epigenetically regulated by them. Using real-time polymerase chain reaction, Western blot analysis, and whole-cell patch-clamp assays, we found a significant decrease in TMEM16A expression and its functional activity was induced by the vorinostat, a pan-HDACi in TMEM16A-expressing human cancer cell lines, the prostatic cancer cell line PC-3, and the breast cancer cell line YMB-1. TMEM16A downregulation was not induced by the chemotherapy drug paclitaxel in either cell type. Pharmacologic blockade of HDAC3 by 1 μM T247 [N-(2-aminophenyl)-4-[1-(2-thiophen-3-ylethyl)-1H-[1],[2],[3]triazol-4-yl]benzamide], a HDAC3-selective HDACi, elicited a large decrease in TMEM16A expression and functional activity in both cell types, and pharmacologic blockade of HDAC2 by AATB [4-(acetylamino)-N-[2-amino-5-(2-thienyl)phenyl]-benzamide; 300 nM] elicited partial inhibition of TMEM16A expression (∼40%) in both. Pharmacologic blockade of HDAC1 or HDAC6 did not elicit any significant change in TMEM16A expression, respectively. In addition, inhibition of HDAC3 induced by small interfering RNA elicited a large decrease in TMEM16A transcripts in both cell types. Taken together, in malignancies with a frequent gene amplification of TMEM16A, HDAC3 inhibition may exert suppressive effects on cancer cell viability via downregulation of TMEM16A.