Bimodal control of a Ca(2+)-activated Cl(-) channel by different Ca(2+) signals

Mechanisms of regulation of olfactory transduction and adaptation in the olfactory cilium

Olfactory adaptation is a fundamental process for the functioning of the olfactory system, but the underlying mechanisms regulating its occurrence in intact olfactory sensory neurons (OSNs) are not fully understood. In this work, we have combined stochastic computational modeling and a systematic pharmacological study of different signaling pathways to investigate their impact during short-term adaptation (STA). We used odorant stimulation and electroolfactogram (EOG) recordings of the olfactory epithelium treated with pharmacological blockers to study the molecular mechanisms regulating the occurrence of adaptation in OSNs. EOG responses to paired-pulses of odorants showed that inhibition of phosphodiesterases (PDEs) and phosphatases enhanced the levels of STA in the olfactory epithelium, and this effect was mimicked by blocking vesicle exocytosis and reduced by blocking cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) and vesicle endocytosis. These results suggest that G-coupled receptors (GPCRs) cycling is involved with the occurrence of STA. To gain insights on the dynamical aspects of this process, we developed a stochastic computational model. The model consists of the olfactory transduction currents mediated by the cyclic nucleotide gated (CNG) channels and calcium ion (Ca²⁺)-activated chloride (CAC) channels, and the dynamics of their respective ligands, cAMP and Ca²⁺, and it simulates the EOG results obtained under different experimental conditions through changes in the amplitude and duration of cAMP and Ca²⁺ response, two second messengers implicated with STA occurrence. The model reproduced the experimental data for each pharmacological treatment and provided a mechanistic explanation for the action of GPCR cycling in the levels of second messengers modulating the levels of STA. All together, these experimental and theoretical results indicate the existence of a mechanism of regulation of STA by signaling pathways that control GPCR cycling and tune the levels of second messengers in OSNs, and not only by CNG channel desensitization as previously thought.

Mid-range Ca2+ signalling mediated by functional coupling between store-operated Ca2+ entry and IP3-dependent Ca2+ release

The versatility and universality of Ca(2+) signals stem from the breadth of their spatial and temporal dynamics. Spatially, Ca(2+) signalling is well studied in the microdomain scale, close to a Ca(2+) channel, and at the whole-cell level. However, little is known about how local Ca(2+) signals are regulated to specifically activate spatially distant effectors without a global Ca(2+) rise. Here we show that an intricate coupling between the inositol 1,4,5 trisphosphate (IP3) receptor, SERCA pump and store-operated Ca(2+) entry (SOCE) allows for efficient mid-range Ca(2+) signalling. Ca(2+) flowing through SOCE is taken up into the ER lumen by the SERCA pump, only to be re-released by IP3Rs to activate distal Ca(2+)-activated Cl(-) channels (CaCCs). This CaCC regulation contributes to setting the membrane potential of the cell. Hence functional coupling between SOCE, SERCA and IP3R limits local Ca(2+) diffusion and funnels Ca(2+) through the ER lumen to activate a spatially separate Ca(2+) effector.

Functions of volume-sensitive and calcium-activated chloride channels

The review describes molecular and functional properties of the volume regulated anion channel and Ca(2+)-dependent Cl(-) channels belonging to the anoctamin family with emphasis on physiological importance of these channels in regulation of cell volume, cell migration, cell proliferation, and programmed cell death. Finally, we discuss the role of Cl(-) channels in various diseases.

Computational modeling of anoctamin 1 calcium-activated chloride channels as pacemaker channels in interstitial cells of Cajal

Interstitial cells of Cajal (ICC) act as pacemaker cells in the gastrointestinal tract by generating electrical slow waves to regulate rhythmic smooth muscle contractions. Intrinsic Ca(2+) oscillations in ICC appear to produce the slow waves by activating pacemaker currents, currently thought to be carried by the Ca(2+)-activated Cl(-) channel anoctamin 1 (Ano1). In this article we present a novel model of small intestinal ICC pacemaker activity that incorporates store-operated Ca(2+) entry and a new model of Ano1 current. A series of simulations were carried out with the ICC model to investigate current controversies about the reversal potential of the Ano1 Cl(-) current in ICC and to predict the characteristics of the other ion channels that are necessary to generate slow waves. The model results show that Ano1 is a plausible pacemaker channel when coupled to a store-operated Ca(2+) channel but suggest that small cyclical depolarizations may still occur in ICC in Ano1 knockout mice. The results predict that voltage-dependent Ca(2+) current is likely to be negligible during the slow wave plateau phase. The model shows that the Cl(-) equilibrium potential is an important modulator of slow wave morphology, highlighting the need for a better understanding of Cl(-) dynamics in ICC.

A novel biophysical model on calcium and voltage dual dependent gating of calcium-activated chloride channel

Ca(2+)-activated Cl(-) channels (CaCCs) are anion-selective channels and involved in physiological processes such as electrolyte/fluid secretion, smooth muscle excitability, and olfactory perception which critically depend on the Ca(2+) and voltage dual-dependent gating of channels. However, how the Ca(2+) and voltage regulate the gating of CaCCs still unclear. In this work, the authors constructed a biophysical model to illustrate the dual-dependent gating of CaCCs. For validation, we applied our model on both native CaCCs and exogenous TMEM16A which is thought to be the molecular basis of CaCCs. Our data show that the native CaCCs may share universal gating mechanism. We confirmed the assumption that by binding with the channel, Ca(2+) decreases the energy-barrier to open the channel, but not changes the voltage-sensitivity. For TMEM16A, our model indicates that the exogenous channels show different Ca(2+) dependent gating mechanism from the native ones. These results advance the understanding of intracellular Ca(2+) and membrane potential regulation in CaCCs, and shed new light on its function in aspect of physiology and pharmacology.

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.

TMEM16F (Anoctamin 6), an anion channel of delayed Ca(2+) activation

Members of the TMEM16 (Anoctamin) family of membrane proteins have been shown to be essential constituents of the Ca²⁺-activated Cl⁻ channel (CaCC) in many cell types. In this study, we have investigated the electrophysiological properties of mouse TMEM16F. Heterologous expression of TMEM16F in HEK293 cells resulted in plasma membrane localization and an outwardly rectifying I_Cl,Ca that was activated with a delay of several minutes. Furthermore, a significant Na⁺ current was activated, and the two permeabilities were correlated according to P_Na = 0.3 P_Cl. The current showed an EC₅₀ of 100 µM intracellular free Ca²⁺ concentration and an Eisenman type 1 anion selectivity sequence of P_SCN > P_I > P_Br > P_Cl > P_Asp. The mTMEM16F-associated I_Cl,Ca was abolished in one mutant of the putative pore region (R592E) but retained in two other mutants (K616E and R636E). The mutant K616E had a lower relative permeability to iodide, and the mutant R636E had an altered anion selectivity sequence (P_SCN = P_I = P_Br = P_Cl > P_Asp). Our data provide evidence that TMEM16F constitutes a Ca²⁺-activated anion channel or a pore-forming subunit of an anion channel with properties distinct from TMEM16A.

Activation and inhibition of TMEM16A calcium-activated chloride channels

Calcium-activated chloride channels (CaCC) encoded by family members of transmembrane proteins of unknown function 16 (TMEM16) have recently been intensely studied for functional properties as well as their physiological roles as chloride channels in various tissues. One technical hurdle in studying these channels is the well-known channel rundown that frequently impairs the precision of electrophysiological measurements for the channels. Using experimental protocols that employ fast-solution exchange, we circumvented the problem of channel rundown by normalizing the Ca²⁺-induced current to the maximally-activated current obtained within a time period in which the channel rundown was negligible. We characterized the activation of the TMEM16A-encoded CaCC (also called ANO1) by Ca²⁺, Sr²⁺, and Ba²⁺, and discovered that Mg²⁺ competes with Ca²⁺ in binding to the divalent-cation binding site without activating the channel. We also studied the permeability of the ANO1 pore for various anions and found that the anion occupancy in the pore–as revealed by the permeability ratios of these anions–appeared to be inversely correlated with the apparent affinity of the ANO1 inhibition by niflumic acid (NFA). On the other hand, the NFA inhibition was neither affected by the degree of the channel activation nor influenced by the types of divalent cations used for the channel activation. These results suggest that the NFA inhibition of ANO1 is likely mediated by altering the pore function but not through changing the channel gating. Our study provides a precise characterization of ANO1 and documents factors that can affect divalent cation activation and NFA inhibition of ANO1.

Activation of the Ano1 (TMEM16A) chloride channel by calcium is not mediated by calmodulin

Calcium-mediated activation of the TMEM16A chloride channel does not depend on changes in phosphorylation status or the calcium-binding protein calmodulin.

The Ca²⁺-activated Cl channel anoctamin-1 (Ano1; Tmem16A) plays a variety of physiological roles, including epithelial fluid secretion. Ano1 is activated by increases in intracellular Ca²⁺, but there is uncertainty whether Ca²⁺ binds directly to Ano1 or whether phosphorylation or additional Ca²⁺-binding subunits like calmodulin (CaM) are required. Here we show that CaM is not necessary for activation of Ano1 by Ca²⁺ for the following reasons. (a) Exogenous CaM has no effect on Ano1 currents in inside-out excised patches. (b) Overexpression of Ca²⁺-insensitive mutants of CaM have no effect on Ano1 currents, whereas they eliminate the current mediated by the small-conductance Ca²⁺-activated K⁺ (SK2) channel. (c) Ano1 does not coimmunoprecipitate with CaM, whereas SK2 does. Furthermore, Ano1 binds very weakly to CaM in pull-down assays. (d) Ano1 is activated in excised patches by low concentrations of Ba²⁺, which does not activate CaM. In addition, we conclude that reversible phosphorylation/dephosphorylation is not required for current activation by Ca²⁺ because the current can be repeatedly activated in excised patches in the absence of ATP or other high-energy compounds. Although Ano1 is blocked by the CaM inhibitor trifluoperazine (TFP), we propose that TFP inhibits the channel in a CaM-independent manner because TFP does not inhibit Ano1 when applied to the cytoplasmic side of excised patches. These experiments lead us to conclude that CaM is not required for activation of Ano1 by Ca²⁺. Although CaM is not required for channel opening by Ca²⁺, work of other investigators suggests that CaM may have effects in modulating the biophysical properties of the channel.

Anoctamin 1 mediates thermal pain as a heat sensor

Vertebrates can sense and avoid noxious heat that evokes pain. Many thermoTRP channels are associated with temperature sensation. TRPV1 is a representative ion channel that is activated by noxious heat. Anoctamin 1 (ANO1) is a Cl- channel activated by calcium that is highly expressed in small sensory neurons, colocalized with markers for nociceptors, and most surprisingly, activated by noxious heat over 44oC. Although ANO1 is a Cl- channel, opening of this channel leads to depolarization of sensory neurons, suggesting a role in nociception. Indeed, the functional deletion of ANO1 in sensory neurons triggers the reduction in thermal pain sensation. Thus, it seems clear that ANO1 is a heat sensor in a nociceptive pathway. Since ANO1 modulators are developed for the purpose of treating chronic diseases such as cystic fibrosis, this finding is likely to predict unwanted effects and provide a guide for better developmental strategy

Ca2+-activated Cl- channels

Ca(2+)-activated Cl(-) channels (CaCCs) are plasma membrane proteins involved in various important physiological processes. In epithelial cells, CaCC activity mediates the secretion of Cl(-) and of other anions, such as bicarbonate and thiocyanate. In smooth muscle and excitable cells of the nervous system, CaCCs have an excitatory role coupling intracellular Ca(2+) elevation to membrane depolarization. Recent studies indicate that TMEM16A (transmembrane protein 16 A or anoctamin 1) and TMEM16B (transmembrane protein 16 B or anoctamin 2) are CaCC-forming proteins. Induced expression of TMEM16A and B in null cells by transfection causes the appearance of Ca(2+)-activated Cl(-) currents similar to those described in native tissues. Furthermore, silencing of TMEM16A by RNAi causes disappearance of CaCC activity in cells from airway epithelium, biliary ducts, salivary glands, and blood vessel smooth muscle. Mice devoid of TMEM16A expression have impaired Ca(2+)-dependent Cl(-) secretion in the epithelial cells of the airways, intestine, and salivary glands. These animals also show a loss of gastrointestinal motility, a finding consistent with an important function of TMEM16A in the electrical activity of gut pacemaker cells, that is, the interstitial cells of Cajal. Identification of TMEM16 proteins will help to elucidate the molecular basis of Cl(-) transport.

MONNA, a potent and selective blocker for transmembrane protein with unknown function 16/anoctamin-1

Transmembrane protein with unknown function 16/anoctamin-1 (ANO1) is a protein widely expressed in mammalian tissues, and it has the properties of the classic calcium-activated chloride channel (CaCC). This protein has been implicated in numerous major physiological functions. However, the lack of effective and selective blockers has hindered a detailed study of the physiological functions of this channel. In this study, we have developed a potent and selective blocker for endogenous ANO1 in Xenopus laevis oocytes (xANO1) using a drug screening method we previously established (Oh et al., 2008). We have synthesized a number of anthranilic acid derivatives and have determined the correlation between biological activity and the nature and position of substituents in these derived compounds. A structure-activity relationship revealed novel chemical classes of xANO1 blockers. The derivatives contain a --NO₂ group on position 5 of a naphthyl group-substituted anthranilic acid, and they fully blocked xANO1 chloride currents with an IC₅₀ < 10 μM. The most potent blocker, N-((4-methoxy)-2-naphthyl)-5-nitroanthranilic acid (MONNA), had an IC₅₀ of 0.08 μM for xANO1. Selectivity tests revealed that other chloride channels such as bestrophin-1, chloride channel protein 2, and cystic fibrosis transmembrane conductance regulator were not appreciably blocked by 10∼30 μM MONNA. The potent and selective blockers for ANO1 identified here should permit pharmacological dissection of ANO1/CaCC function and serve as potential candidates for drug therapy of related diseases such as hypertension, cystic fibrosis, bronchitis, asthma, and hyperalgesia.

TMEM16A–TMEM16B chimaeras to investigate the structure–function relationship of calcium-activated chloride channels

TMEM16A and TMEM16B proteins are CaCCs (Ca2+-activated Cl− channels) with eight putative transmembrane segments. As shown previously, expression of TMEM16B generates CaCCs characterized by a 10-fold lower Ca2+ affinity and by faster activation and deactivation kinetics with respect to TMEM16A. To investigate the basis of the different properties, we generated chimaeric proteins in which different domains of the TMEM16A protein were replaced by the equivalent domains of TMEM16B. Replacement of the N-terminus, TMD (transmembrane domain) 1–2, the first intracellular loop and TMD3–4 did not change the channel's properties. Instead, replacement of intracellular loop 3 decreased the apparent Ca2+ affinity by nearly 8-fold with respect to wild-type TMEM16A. In contrast, the membrane currents derived from chimaeras containing TMD7–8 or the C-terminus of TMEM16B showed higher activation and deactivation rates without a change in Ca2+ sensitivity. Significantly accelerated kinetics were also found when the entire C-terminus of the TMEM16A protein (77 amino acid residues) was deleted. Our findings indicate that the third intracellular loop of TMEM16A and TMEM16B is the site involved in Ca2+-sensitivity, whereas the C-terminal part, including TMD7–8, affect the rate of transition between the open and the closed state.

Putative pore-loops of TMEM16/anoctamin channels affect channel density in cell membranes

The recently identified TMEM16/anoctamin protein family includes Ca(2+)-activated anion channels (TMEM16A, TMEM16B), a cation channel (TMEM16F) and proteins with unclear function. TMEM16 channels consist of eight putative transmembrane domains (TMs) with TM5-TM6 flanking a re-entrant loop thought to form the pore. In TMEM16A this region has also been suggested to contain residues involved in Ca(2+) binding. The role of the putative pore-loop of TMEM16 channels was investigated using a chimeric approach. Heterologous expression of either TMEM16A or TMEM16B resulted in whole-cell anion currents with very similar conduction properties but distinct kinetics and degrees of sensitivity to Ca(2+). Furthermore, whole-cell currents mediated by TMEM16A channels were ∼six times larger than TMEM16B-mediated currents. Replacement of the putative pore-loop of TMEM16A with that of TMEM16B (TMEM16A-B channels) reduced the currents by ∼six-fold, while the opposite modification (TMEM16B-A channels) produced a ∼six-fold increase in the currents. Unexpectedly, these changes were not secondary to variations in channel gating by Ca(2+) or voltage, nor were they due to changes in single-channel conductance. Instead, they depended on the number of functional channels present on the plasma membrane. Generation of additional, smaller chimeras within the putative pore-loop of TMEM16A and TMEM16B led to the identification of a region containing a non-canonical trafficking motif. Chimeras composed of the putative pore-loop of TMEM16F transplanted into the TMEM16A protein scaffold did not conduct anions or cations. These data suggest that the putative pore-loop does not form a complete, transferable pore domain. Furthermore, our data reveal an unexpected role for the putative pore-loop of TMEM16A and TMEM16B channels in the control of the whole-cell Ca(2+)-activated Cl(-) conductance.

Functional regulation of ClC-3 in the migration of vascular smooth muscle cells

Migration of vascular smooth muscle cells (VSMCs) into neointima contributes to atherosclerosis and restenosis. This migration requires coordinated plasmalemmal fluxes of water and ions. Here, we show that aortic VSMC migration depends on the regulation of transmembrane Cl(-) flux by ClC-3, a Cl(-) channel/transporter. The contribution of ClC-3 to plasmalemmal Cl(-) current was studied in VSMCs by electrophysiological recordings. Cl(-) current was negligible in cells perfused with 0 [Ca(2+)]. Raising intracellular [Ca(2+)] to 0.5 μM activated a Cl(-) current (I(Cl.Ca)), approximately half of which was eliminated on inhibition by KN-93 of calmodulin-dependent protein kinase II. I(Cl.Ca) was also halved by inositol-3,4,5,6-tetrakisphosphate, a cellular signal with the biological function of specifically preventing calmodulin-dependent protein kinase II from activating I(Cl.Ca). Gene disruption of ClC-3 reduced I(Cl.Ca) by 50%. Moreover, I(Cl.Ca) in the ClC-3 null VSMCs was not affected by either KN-93 or inositol-3,4,5,6-tetrakisphosphate. We conclude that I(Cl.Ca) is composed of 2 components, one is ClC-3 independent whereas the other is ClC-3 dependent, activated by calmodulin-dependent protein kinase II and inhibited by inositol-3,4,5,6-tetrakisphosphate. We also assayed VSMC migration in transwell assays. Migration was halved in ClC-3 null cells versus wild-type cells. In addition, inhibition of ClC-3 by niflumic acid, KN-93, or inositol-3,4,5,6-tetrakisphosphate each reduced cell migration in wild-type cells but not in ClC-3 null cells. These cell-signaling roles of ClC-3 in VSMC migration suggest new therapeutic approaches to vascular remodeling diseases.

Dynamic modulation of ANO1/TMEM16A HCO3(-) permeability by Ca2+/calmodulin

Anoctamin 1 (ANO1)/transmembrane protein 16A (TMEM16A) is a calcium-activated anion channel that may play a role in HCO(3)(-) secretion in epithelial cells. Here, we report that the anion selectivity of ANO1 is dynamically regulated by the Ca(2+)/calmodulin complex. Whole-cell current measurements in HEK 293T cells indicated that ANO1 becomes highly permeable to HCO(3)(-) at high [Ca(2+)](i). Interestingly, this result was not observed in excised patches, indicating the involvement of cytosolic factors in this process. Further studies revealed that the direct association between ANO1 and calmodulin at high [Ca(2+)](i) is responsible for changes in anion permeability. Calmodulin physically interacted with ANO1 in a [Ca(2+)](i)-dependent manner, and addition of recombinant calmodulin to the cytosolic side of excised patches reversibly increased P(HCO3)/P(Cl). In addition, the high [Ca(2+)](i)-induced increase in HCO(3)(-) permeability was reproduced in mouse submandibular gland acinar cells, in which ANO1 plays a critical role in fluid secretion. These results indicate that the HCO(3)(-) permeability of ANO1 can be dynamically modulated and that ANO1 may play an important role in cellular HCO(3)(-) transport, especially in transepithelial HCO(3)(-) secretion.

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.

Chronic hypoxia-induced upregulation of Ca2+-activated Cl- channel in pulmonary arterial myocytes: a mechanism contributing to enhanced vasoreactivity

Chronic hypoxic pulmonary hypertension (CHPH) is associated with altered expression and function of cation channels in pulmonary arterial smooth muscle cells (PASMCs), but little is known for anion channels. The Ca(2+)-activated Cl(-) channel (CaCC), recently identified as TMEM16A, plays important roles in pulmonary vascular function. The present study sought to determine the effects of chronic hypoxia (CH) on the expression and function of CaCCs in PASMCs, and their contributions to the vascular hyperreactivity in CHPH. Male Wistar rats were exposed to room air or 10% O(2) for 3–4 weeks to generate CHPH. CaCC current (I(CI.Ca)) elicited by caffeine-induced Ca(2+) release or by depolarization at a constant high [Ca(2+)](i) (500 or 750 nm) was significantly larger in PASMCs of CH rats compared to controls. The enhanced I(CI.Ca)) density in CH PASMCs was unrelated to changes in amplitude of Ca(2+) release, Ca(2+)-dependent activation, voltage-dependent properties or calcineurin-dependent modulation of CaCCs, but was associated with increased TMEM16A mRNA and protein expression. Maximal contraction induced by serotonin, an important mediator of CHPH, was potentiated in endothelium-denuded pulmonary arteries of CH rats. The enhanced contractile response was prevented by the CaCC blockers niflumic acid and T16A(inh)-A01, or by the L-type Ca(2+) channel antagonist nifedipine. The effects of niflumic acid and nifedipine were non-additive. Our results demonstrate for the first time that CH increases I(CI.Ca) density, which is attributable to an upregulation of TMEM16A expression in PASMCs. The augmented CaCC activity in PASMCs may potentiate membrane depolarization and L-type channel activation in response to vasoconstrictors and enhance pulmonary vasoreactivity in CHPH.

δ-Opioid receptor agonist SNC80 induces central antinociception mediated by Ca2+-activated Cl- channels

Objectives

The aim of this study was to determine whether Ca2+-activated Cl- channels (CaCCs) are involved in central antinociception induced by the activation of µ-, δ- and κ-opioid receptors.

Methods

The nociceptive threshold for thermal stimulation was measured using the tail-flick test in Swiss mice. The drugs were administered via the intracerebroventricular route. Probabilities values of P &lt; 0.05 were considered to be statistically significant (analysis of variance/Bonferroni test).

Key findings

The results demonstrate that exposure to the CaCC blocker niflumic acid (2, 4 and 8 µg) partially reverses the central antinociception induced by the δ-opioid receptor agonist SNC80 ((+)-4-[(αR)-α-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide; 4 µg). In contrast, niflumic acid did not modify the antinociceptive effect of the µ-opioid receptor agonist [D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (0.5 µg) or κ-opioid receptor agonist bremazocine (4 µg).

Conclusions

These data provide evidence for the involvement of CaCCs in δ-opioid receptor-induced central antinociception resulting from receptor activation by the agonist SNC80. CaCC activation does not appear to be involved when µ- and κ-opioid receptors are activated.

Explaining calcium-dependent gating of anoctamin-1 chloride channels requires a revised topology

RATIONALE

Ca2+ -activated Cl channels play pivotal roles in the cardiovascular system. They regulate vascular smooth muscle tone and participate in cardiac action potential repolarization in some species. Ca2+ -activated Cl channels were recently discovered to be encoded by members of the anoctamin (Ano, also called Tmem16) superfamily, but the mechanisms of Ano1 gating by Ca2+ remain enigmatic.

OBJECTIVE

The objective was to identify regions of Ano1 involved in channel gating by Ca2+.

METHODS AND RESULTS

The Ca2+ sensitivity of Ano1 was estimated from rates of current activation, and deactivation in excised patches rapidly switched between zero and high Ca2+ on the cytoplasmic side. Mutation of glutamates E702 and E705 dramatically altered Ca2+ sensitivity. E702 and E705 are predicted to be in an extracellular loop, but antigenic epitopes introduced into this loop are not accessible to extracellular antibodies, suggesting this loop is intracellular. Cytoplasmically applied membrane-impermeant sulfhydryl reagents alter the Ca2+ sensitivity of Ano1 E702C and E705C as expected if E702 and E705 are intracellular. Substituted cysteine accessibility mutagenesis of the putative re-entrant loop suggests that E702 and E705 are located adjacent to the Cl conduction pathway.

CONCLUSIONS

We propose an alternative model of Ano1 topology based on mutagenesis, epitope accessibility, and cysteine-scanning accessibility. These data contradict the popular re-entrant loop model by showing that the putative fourth extracellular loop (ECL 4) is intracellular and may contain a Ca2+ binding site. These studies provide new perspectives on regulation of Ano1 by Ca2+.

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.

ANOs 3-7 in the anoctamin/Tmem16 Cl- channel family are intracellular proteins

Ca(2+)-activated Cl(-) channels (CaCCs) participate in numerous physiological functions such as neuronal excitability, sensory transduction, and transepithelial fluid transport. Recently, it was shown that heterologously expressed anoctamins ANO1 and ANO2 generate currents that resemble native CaCCs. The anoctamin family (also called Tmem16) consists of 10 members, but it is not known whether all members of the family are CaCCs. Expression of ANOs 3-7 in HEK293 cells did not generate Cl(-) currents activated by intracellular Ca(2+), as determined by whole cell patch clamp electrophysiology. With the use of confocal imaging, only ANO1 and ANO2 traffic to the plasma membrane when expressed heterologously. Furthermore, endogenously expressed ANO7 in the human prostate is predominantly intracellular. We took a chimeric approach to identify regions critical for channel trafficking and function. However, none of the chimeras of ANO1 and ANO5/7 that we made trafficked to the plasma membrane. Our results suggest that intracellular anoctamins may be endoplasmic reticulum proteins, although it remains unknown whether these family members are CaCCs. Determining the role of anoctamin family members in ion transport will be critical to understanding their functions in physiology and disease.

Expression and function of epithelial anoctamins

Endogenous Ca(2+)-activated Cl(-) currents (CaCCs) are abundant and present in very different cell types. Very good evidence has been provided that endogenous CaCC is produced by anoctamin 1 (Ano1) and Ano2. Insight into the physiological role of anoctamins has been provided for Ano1, Ano2 and Ano6; however, the physiological role of the other seven members of the anoctamin family remains obscure. Anoctamins 1 and 2 may operate as individual Ca(2+)-sensitive channel proteins or may require accessory subunits for complete function. We find that overexpressed Ano1 has properties resembling all those of endogenous CaCCs, although with some noticeable biophysical and regulatory differences when compared with endogenous channels. Apart from Ano1 and Ano2, expression of Ano6 also produces a Cl(-) conductance. Depending on the cellular background, Ano6 currents may have variable properties. Anoctamins 1 and 6 are frequent in epithelial cells, often coexpressed together with Ano8, Ano9 and Ano10. Most available data on anoctamins were obtained from mouse tissues and from cultured cells, which may not be representative of native human tissues.

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.

Voltage- and calcium-dependent gating of TMEM16A/Ano1 chloride channels are physically coupled by the first intracellular loop

Ca(2+)-activated Cl(-) channels (CaCCs) are exceptionally well adapted to subserve diverse physiological roles, from epithelial fluid transport to sensory transduction, because their gating is cooperatively controlled by the interplay between ionotropic and metabotropic signals. A molecular understanding of the dual regulation of CaCCs by voltage and Ca(2+) has recently become possible with the discovery that Ano1 (TMEM16a) is an essential subunit of CaCCs. Ano1 can be gated by Ca(2+) or by voltage in the absence of Ca(2+), but Ca(2+)- and voltage-dependent gating are very closely coupled. Here we identify a region in the first intracellular loop that is crucial for both Ca(2+) and voltage sensing. Deleting (448)EAVK in the first intracellular loop dramatically decreases apparent Ca(2+) affinity. In contrast, mutating the adjacent amino acids (444)EEEE abolishes intrinsic voltage dependence without altering the apparent Ca(2+)affinity. Voltage-dependent gating of Ano1 measured in the presence of intracellular Ca(2+) was facilitated by anions with high permeability or by an increase in [Cl(-)](e). Our data show that the transition between closed and open states is governed by Ca(2+) in a voltage-dependent manner and suggest that anions allosterically modulate Ca(2+)-binding affinity. This mechanism provides a unified explanation of CaCC channel gating by voltage and ligand that has long been enigmatic.

TMEM16A protein: a new identity for Ca(2+)-dependent Cl⁻ channels

Ca(+)-dependent Cl⁻ channels (CaCCs) play a variety of physiological roles in different organs and tissues, including transepithelial Cl⁻ secretion, smooth muscle contraction, regulation of neuronal excitability, and transduction of sensory stimuli. The recent identification of TMEM16A protein as an important component of CaCCs should allow a better understanding of their physiological role, structure-function relationship, and regulatory mechanisms.

Location of release sites and calcium-activated chloride channels relative to calcium channels at the photoreceptor ribbon synapse

Vesicle release from photoreceptor ribbon synapses is regulated by L-type Ca(2+) channels, which are in turn regulated by Cl(-) moving through calcium-activated chloride [Cl(Ca)] channels. We assessed the proximity of Ca(2+) channels to release sites and Cl(Ca) channels in synaptic terminals of salamander photoreceptors by comparing fast (BAPTA) and slow (EGTA) intracellular Ca(2+) buffers. BAPTA did not fully block synaptic release, indicating some release sites are <100 nm from Ca(2+) channels. Comparing Cl(Ca) currents with predicted Ca(2+) diffusion profiles suggested that Cl(Ca) and Ca(2+) channels average a few hundred nanometers apart, but the inability of BAPTA to block Cl(Ca) currents completely suggested some channels are much closer together. Diffuse immunolabeling of terminals with an antibody to the putative Cl(Ca) channel TMEM16A supports the idea that Cl(Ca) channels are dispersed throughout the presynaptic terminal, in contrast with clustering of Ca(2+) channels near ribbons. Cl(Ca) currents evoked by intracellular calcium ion concentration ([Ca(2+)](i)) elevation through flash photolysis of DM-nitrophen exhibited EC(50) values of 556 and 377 nM with Hill slopes of 1.8 and 2.4 in rods and cones, respectively. These relationships were used to estimate average submembrane [Ca(2+)](i) in photoreceptor terminals. Consistent with control of exocytosis by [Ca(2+)] nanodomains near Ca(2+) channels, average submembrane [Ca(2+)](i) remained below the vesicle release threshold (∼ 400 nM) over much of the physiological voltage range for cones. Positioning Ca(2+) channels near release sites may improve fidelity in converting voltage changes to synaptic release. A diffuse distribution of Cl(Ca) channels may allow Ca(2+) influx at one site to influence relatively distant Ca(2+) channels.

Characterization of the oligomeric structure of the Ca(2+)-activated Cl- channel Ano1/TMEM16A

Members of the Anoctamin (Ano)/TMEM16A family have recently been identified as essential subunits of the Ca(2+)-activated chloride channel (CaCC). For example, Ano1 is highly expressed in multiple tissues including airway epithelia, where it acts as an apical conduit for transepithelial Cl(-) secretion and helps regulate lung liquid homeostasis and mucus clearance. However, little is known about the oligomerization of this protein in the plasma membrane. Thus, utilizing mCherry- and eGFP-tagged Ano1 constructs, we conducted biochemical and Förster resonance energy transfer (FRET)-based experiments to determine the quaternary structure of Ano1. FRET and co-immunoprecipitation studies revealed that tagged Ano1 subunits directly associated before they reached the plasma membrane. This association was not altered by changes in cytosolic Ca(2+), suggesting that this is a fixed interaction. To determine the oligomeric structure of Ano1, we performed chemical cross-linking, non-denaturing PAGE, and electromobility shift assays, which revealed that Ano1 exists as a dimer. These data are the first to probe the quaternary structure of Ano1. Understanding the oligomeric nature of Ano1 is an essential step in the development of therapeutic drugs that could be useful in the treatment of cystic fibrosis.

Murine vasa recta pericyte chloride conductance is controlled by calcium, depolarization, and kinase activity

We used the whole cell patch-clamp technique to investigate the regulation of descending vasa recta (DVR) pericyte Ca(2+)-dependent Cl(-) currents (CaCC) by cytoplasmic Ca(2+) concentration ([Ca](CYT)), voltage, and kinase activity. Murine CaCC increased with voltage and electrode Ca(2+) concentration. The current saturated at [Ca](CYT) of ∼1,000 nM and exhibited an EC(50) for Ca(2+) of ∼500 nM, independent of depolarization potential. Activation time constants were between 100 and 200 ms, independent of electrode Ca(2+). Repolarization-related tail currents elicited by stepping from +100 mV to varying test potentials exhibited deactivation time constants of 50-200 ms that increased with voltage when electrode [Ca](CYT) was 1,000 nM. The calmodulin inhibitor N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7, 30 μM) blocked CaCC. The myosin light chain kinase blockers 1-(5-iodonaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine hydrochloride (ML-7, 1-50 μM) and 1-(5-chloronaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine hydrochloride (ML-9, 10 μM) were similarly effective. Resting pericytes were hyperpolarized by ML-7. Pericytes exposed to ANG II (10 nM) depolarized from a baseline of -50 ± 6 to -29 ± 3 mV and were repolarized to -63 ± 7 mV by exposure to 50 μM ML-7. The Ca(2+)/calmodulin-dependent kinase inhibitor KN-93 reduced pericyte CaCC only when it was present in the electrode and extracellular buffer from the time of membrane break-in. We conclude that murine DVR pericytes are modulated by [Ca](CYT), membrane potential, and phosphorylation events, suggesting that Ca(2+)-dependent Cl(-) conductance may be a target for regulation of vasoactivity and medullary blood flow in vivo.

Chloride channels: often enigmatic, rarely predictable

Until recently, anion (Cl(-)) channels have received considerably less attention than cation channels. One reason for this may be that many Cl(-) channels perform functions that might be considered cell-biological, like fluid secretion and cell volume regulation, whereas cation channels have historically been associated with cellular excitability, which typically happens more rapidly. In this review, we discuss the recent explosion of interest in Cl(-) channels, with special emphasis on new and often surprising developments over the past five years. This is exemplified by the findings that more than half of the ClC family members are antiporters, and not channels, as was previously thought, and that bestrophins, previously prime candidates for Ca(2+)-activated Cl(-) channels, have been supplanted by the newly discovered anoctamins and now hold a tenuous position in the Cl(-) channel world.

TMEM16A/anoctamin 1 protein mediates calcium-activated chloride currents in pulmonary arterial smooth muscle cells

Calcium-activated chloride channels (CaCCs) play important roles in several physiological processes. In vascular smooth muscle, activation of these ion channels by agonist-induced Ca(2+) release results in membrane depolarization and vasoconstriction. The molecular identity of vascular CaCCs is not fully defined. Here we present evidence that TMEM16A (or anoctamin 1), a member of the transmembrane 16 (TMEM16) protein family, forms CaCCs in pulmonary artery smooth muscle cells (PASMCs). Patch-clamp analysis in freshly isolated PASMCs revealed strongly outward-rectifying, slowly activating Ca(2+)-activated Cl(-) currents sharing a high degree of similarity with heterologous TMEM16A currents. TMEM16A mRNA was identified in rat and human pulmonary arteries and various other vascular smooth muscle cell types. Further analyses revealed that several TMEM16A splice variants were detected in rat PASMCs and that TMEM16F and TMEM16K were also expressed in these cells, while TMEM16B, TMEM16D and TMEM16E were all at least 50 times less abundantly expressed and the remaining TMEM16 family members were absent. Downregulation of TMEM16A gene expression in primary cultures of rat PASMCs, with small interfering RNAs, was accompanied by almost total loss of whole-cell CaCC currents. Based on these results, we propose that TMEM16A is the major constituent of the vascular calcium-activated chloride channel in rat pulmonary artery smooth muscle.

The TMEM16 protein family: a new class of chloride channels?

Cl(-) channels play important roles in many physiological processes, including transepithelial ion absorption and secretion, smooth and skeletal muscle contraction, neuronal excitability, sensory perception, and cell volume regulation. The molecular identity of many types of Cl(-) channels is still unknown. Recently, three research groups have arrived independently at the identification of TMEM16A (also known as anoctamin-1) as a membrane protein strongly related to the activity of Ca(2+)-activated Cl(-) channels (CaCCs). Site-specific mutagenesis of TMEM16A alters the properties of the channels, thus suggesting that TMEM16A forms, at least in part, the CaCC. TMEM16A is a member of a family that includes nine other membrane proteins. All TMEM16 proteins have a similar structure, with eight putative transmembrane domains and cytosolic amino- and carboxy-termini. TMEM16B expression also evokes the appearance of CaCCs, but with biophysical characteristics (voltage dependence, unitary conductance) different from those associated to TMEM16A. The roles of the other TMEM16 proteins are still unknown. The study of TMEM16 proteins may lead to identification of novel molecular mechanisms underlying ion transport and channel gating by voltage and Ca(2+).

Endogenous transport systems in the Xenopus laevis oocyte plasma membrane

Oocytes of the South African clawed frog Xenopus laevis are widely used as a heterologous expression system for the characterization of transport systems such as passive and active membrane transporters, receptors and a whole plethora of other membrane proteins originally derived from animal or plant tissues. The large size of the oocytes and the high degree of expression of exogenous mRNA or cDNA makes them an optimal tool, when compared with other expression systems such as yeast, Escherichia coli or eukaryotic cell lines, for the expression and functional characterization of membrane proteins. This easy to handle expression system is becoming increasingly attractive for pharmacological research. Commercially available automated systems that microinject mRNA into the oocytes and perform electrophysiological measurements fully automatically allow for a mass screening of new computer designed drugs to target membrane transport proteins. Yet, the oocytes possess a large variety of endogenous membrane transporters and it is absolutely mandatory to distinguish the endogenous transporters from the heterologous, expressed transport systems. Here, we review briefly the endogenous membrane transport systems of the oocytes.

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.

TMEM16 proteins: the long awaited calcium-activated chloride channels?

Currents mediated by calcium-activated chloride channels (CaCCs), observed for the first time in Xenopus oocytes, have been recorded in many cells and tissues ranging from different types of neurons to epithelial and muscle cells. CaCCs play a role in the regulation of excitability in neurons including sensory receptors. In addition, they are crucial mediators of chloride movements in epithelial cells where their activity regulates electrolyte and fluid transport. The roles of CaCCs, particularly in epithelia, are briefly reviewed with emphasis on their function in secretory epithelia. The recent identification by three independent groups, using different strategies, of TMEM16A as the molecular counterpart of the CaCC is discussed. TMEM16A is part of a family that has 10 other members in mice. The discovery of the potential TMEM16 anion channel activity opens the way for the molecular investigation of the role of these anion channels in specific cells and in organ physiology and pathophysiology. The identification of TMEM16A protein as a CaCC chloride channel molecule represents a great triumph of scientific perseverance and ingenuity. The varied approaches used by the three independent research groups also augur well for the solidity of the discovery.

Loss of TMEM16A causes a defect in epithelial Ca2+-dependent chloride transport

Molecular identification of the Ca(2+)-dependent chloride channel TMEM16A (ANO1) provided a fundamental step in understanding Ca(2+)-dependent Cl(-) secretion in epithelia. TMEM16A is an intrinsic constituent of Ca(2+)-dependent Cl(-) channels in cultured epithelia and may control salivary output, but its physiological role in native epithelial tissues remains largely obscure. Here, we demonstrate that Cl(-) secretion in native epithelia activated by Ca(2+)-dependent agonists is missing in mice lacking expression of TMEM16A. Ca(2+)-dependent Cl(-) transport was missing or largely reduced in isolated tracheal and colonic epithelia, as well as hepatocytes and acinar cells from pancreatic and submandibular glands of TMEM16A(-/-) animals. Measurement of particle transport on the surface of tracheas ex vivo indicated largely reduced mucociliary clearance in TMEM16A(-/-) mice. These results clearly demonstrate the broad physiological role of TMEM16A(-/-) for Ca(2+)-dependent Cl(-) secretion and provide the basis for novel treatments in cystic fibrosis, infectious diarrhea, and Sjöegren syndrome.

TMEM16B induces chloride currents activated by calcium in mammalian cells

Ca(2+)-activated Cl(-) channels play important physiological roles in various cell types, but their molecular identity is still unclear. Recently, members of the protein family named transmembrane 16 (TMEM16) have been suggested to function as Ca(2+)-activated Cl(-) channels. Here, we report the functional properties of mouse TMEM16B (mTMEM16B) expressed in human embryonic kidney (HEK) 293T cells, measured both in the whole-cell configuration and in inside-out excised patches. In whole cell, a current induced by mTMEM16B was activated by intracellular Ca(2+) diffusing from the patch pipette, released from intracellular stores through activation of a G-protein-coupled receptor, or photoreleased from caged Ca(2+) inside the cell. In inside-out membrane patches, a current was rapidly activated by bath application of controlled Ca(2+) concentrations, indicating that mTMEM16B is directly gated by Ca(2+). Both in the whole-cell and in the inside-out configurations, the Ca(2+)-induced current was anion selective, blocked by the Cl(-) channel blocker niflumic acid, and displayed a Ca(2+)-dependent rectification. In inside-out patches, Ca(2+) concentration for half-maximal current activation decreased from 4.9 microM at -50 mV to 3.3 microM at +50 mV, while the Hill coefficient was >2. In inside-out patches, currents showed a reversible current decrease at -50 mV in the presence of a constant high Ca(2+) concentration and, moreover, an irreversible rundown, not observed in whole-cell recordings, indicating that some unknown modulator was lost upon patch excision. Our results demonstrate that mTMEM16B functions as a Ca(2+)-activated Cl(-) channel when expressed in HEK 293T cells.

Human ClCa1 modulates anionic conduction of calcium-dependent chloride currents

Proteins of the CLCA gene family including the human ClCa1 (hClCa1) have been suggested to constitute a new family of chloride channels mediating Ca(2+)-dependent Cl- currents. The present study examines the relationship between the hClCa1 protein and Ca(2+)-dependent Cl- currents using heterologous expression of hClCa1 in HEK293 and NCIH522 cell lines and whole cell recordings. By contrast to previous reports claiming the absence of Cl- currents in HEK293 cells, we find that HEK293 and NCIH522 cell lines express constitutive Ca(2+)-dependent Cl- currents and show that hClCa1 increases the amplitude of Ca(2+)-dependent Cl- currents in those cells. We further show that hClCa1 does not modify the permeability sequence but increases the Cl- conductance while decreasing the G(SCN-)/G(Cl-) conductance ratio from approximately 2-3 to approximately 1. We use an Eyring rate theory (two barriers, one site channel) model and show that the effect of hClCa1 on the anionic channel can be simulated by its action on lowering the first and the second energy barriers. We conclude that hClCa1 does not form Ca(2+)-dependent Cl- channels per se or enhance the trafficking/insertion of constitutive channels in the HEK293 and NCIH522 expression systems. Rather, hClCa1 elevates the single channel conductance of endogenous Ca(2+)-dependent Cl- channels by lowering the energy barriers for ion translocation through the pore.

Four Ca2+ ions activate TRPM2 channels by binding in deep crevices near the pore but intracellularly of the gate

TRPM2 is a tetrameric Ca(2+)-permeable channel involved in immunocyte respiratory burst and in postischaemic neuronal death. In whole cells, TRPM2 activity requires intracellular ADP ribose (ADPR) and intra- or extracellular Ca(2+), but the mechanism and the binding sites for Ca(2+) activation remain unknown. Here we study TRPM2 gating in inside-out patches while directly controlling intracellular ligand concentrations. Concentration jump experiments at various voltages and Ca(2+) dependence of steady-state single-channel gating kinetics provide unprecedented insight into the molecular mechanism of Ca(2+) activation. In patches excised from Xenopus laevis oocytes expressing human TRPM2, coapplication of intracellular ADPR and Ca(2+) activated approximately 50-pS nonselective cation channels; K(1/2) for ADPR was approximately 1 microM at saturating Ca(2+). Intracellular Ca(2+) dependence of TRPM2 steady-state opening and closing rates (at saturating [ADPR] and low extracellular Ca(2+)) reveals that Ca(2+) activation is a consequence of tighter binding of Ca(2+) in the open rather than in the closed channel conformation. Four Ca(2+) ions activate TRPM2 with a Monod-Wymann-Changeux mechanism: each binding event increases the open-closed equilibrium constant approximately 33-fold, producing altogether 10(6)-fold activation. Experiments in the presence of 1 mM of free Ca(2+) on the extracellular side clearly show that closed channels do not sense extracellular Ca(2+), but once channels have opened Ca(2+) entering passively through the pore slows channel closure by keeping the "activating sites" saturated, despite rapid continuous Ca(2+)-free wash of the intracellular channel surface. This effect of extracellular Ca(2+) on gating is gradually lost at progressively depolarized membrane potentials, where the driving force for Ca(2+) influx is diminished. Thus, the activating sites lie intracellularly from the gate, but in a shielded crevice near the pore entrance. Our results suggest that in intact cells that contain micromolar ADPR a single brief puff of Ca(2+) likely triggers prolonged, self-sustained TRPM2 activity.

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.

Expression, localization, and functional properties of Bestrophin 3 channel isolated from mouse heart

Bestrophins are a novel family of proteins that encode calcium-activated chloride channels. In this study we establish that Bestrophin transcripts are expressed in the mouse and human heart. Native mBest3 protein expression and localization in heart was demonstrated by using a specific polyclonal mBest3 antibody. Immunostaining of isolated cardiac myocytes indicates that mBest3 is present at the membrane. Using the patch-clamp technique, we characterized the biophysical and pharmacological properties of mBest3 cloned from heart. Whole cell chloride currents were evoked in both HEK293 and COS-7 cells expressing mBest3 by elevation of intracellular calcium. mBest3 currents displayed a K(D) for Ca(2+) of approximately 175 nM. The calcium-activated chloride current was found to be time and voltage independent and displayed slight outward rectification. The anion permeability sequence of the channel was SCN(-)>I(-)>Cl(-), and the current was inhibited by niflumic acid and DIDS in the micromolar range. In addition, we generated a site-specific mutation (F80L) in the putative pore region of mBest3 that significantly altered the ion conduction and pharmacology of this channel. Our functional and mutational studies examining the biophysical properties of mBest3 indicate that it functions as a pore-forming chloride channel that is activated by physiological levels of calcium. This study reports novel findings regarding the molecular expression, tissue localization, and functional properties of mBest3 cloned from heart.