Calcium-activated chloride channels

Bestrophin-3 (vitelliform macular dystrophy 2-like 3 protein) is essential for the cGMP-dependent calcium-activated chloride conductance in vascular smooth muscle cells

Although the biophysical fingerprints (ion selectivity, voltage-dependence, kinetics, etc) of Ca(2+)-activated Cl(-) currents are well established, their molecular identity is still controversial. Several molecular candidates have been suggested; however, none of them has been fully accepted. We have recently characterized a cGMP-dependent Ca(2+)-activated Cl(-) current with unique characteristics in smooth muscle cells. This novel current has been shown to coexist with a "classic" (cGMP-independent) Ca(2+)-activated Cl(-) current and to have characteristics distinct from those previously known for Ca(2+)-activated Cl(-) currents. Here, we suggest that a bestrophin, a product of the Best gene family, is responsible for the cGMP-dependent Ca(2+)-activated Cl(-) current based on similarities between the membrane currents produced by heterologous expressions of bestrophins and the cGMP-dependent Ca(2+)-activated Cl(-) current. This is supported by similarities in the distribution pattern of the cGMP-dependent Ca(2+)-activated Cl(-) current and bestrophin-3 (the product of Best-3 gene) expression in different smooth muscle. Furthermore, downregulation of Best-3 gene expression with small interfering RNA both in cultured cells and in vascular smooth muscle cells in vivo was associated with a significant reduction of the cGMP-dependent Ca(2+)-activated Cl(-) current, whereas the magnitude of the classic Ca(2+)-activated Cl(-) current was not affected. The majority of previous suggestions that bestrophins are a new Cl(-) channel family were based on heterologous expression in cell culture studies. Our present results demonstrate that at least 1 family member, bestrophin-3, is essential for a well-defined endogenous Ca(2+)-activated Cl(-) current in smooth muscles in the intact vascular wall.

TMEM16A, a membrane protein associated with calcium-dependent chloride channel activity

Calcium-dependent chloride channels are required for normal electrolyte and fluid secretion, olfactory perception, and neuronal and smooth muscle excitability. The molecular identity of these membrane proteins is still unclear. Treatment of bronchial epithelial cells with interleukin-4 (IL-4) causes increased calcium-dependent chloride channel activity, presumably by regulating expression of the corresponding genes. We performed a global gene expression analysis to identify membrane proteins that are regulated by IL-4. Transfection of epithelial cells with specific small interfering RNA against each of these proteins shows that TMEM16A, a member of a family of putative plasma membrane proteins with unknown function, is associated with calcium-dependent chloride current, as measured with halide-sensitive fluorescent proteins, short-circuit current, and patch-clamp techniques. Our results indicate that TMEM16A is an intrinsic constituent of the calcium-dependent chloride channel. Identification of a previously unknown family of membrane proteins associated with chloride channel function will improve our understanding of chloride transport physiopathology and allow for the development of pharmacological tools useful for basic research and drug development.

Ca2+-activated Cl- current in retinal arteriolar smooth muscle

PURPOSE

To characterize the biophysical, pharmacologic, and functional properties of the Ca(2+)-activated Cl(-) current in retinal arteriolar myocytes.

METHODS

Whole-cell perforated patch-clamp recordings were made from myocytes within intact isolated arteriolar segments. Arteriolar tone was assessed using pressure myography.

RESULTS

Depolarizing of voltage steps to -40 mV and greater activated an L-type Ca(2+) current (I(Ca(L))) that was followed by a sustained current. Large tail currents (I(tail)) were observed on stepping back to -80 mV. The sustained current and I(tail) reversed close to 0 mV in symmetrical Cl(-) concentrations. The ion selectivity sequence for I(tail) was I(-)> Cl(-)> glucuronate. Outward I(tail) was sensitive to the Cl(-) channel blockers 9-anthracene-carboxylic acid (9-AC; 1 mM), 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS; 1 mM), and disodium 4,4'-diisothiocyanatostilbene-2,2'-disulfonate (DIDS; 1 mM), but only DIDS produced a substantial (78%) block of inward tail currents at -100 mV. I(tail) was decreased in magnitude when the normal bathing medium was substituted with Ca(2+)-free solution or if I(Ca(L)) was inhibited by 1 microM nimodipine. Caffeine (10 mM) produced large transient currents that reversed close to the Cl(-) equilibrium potential and were blocked by 1 mM DIDS or 100 microM tetracaine. DIDS had no effect on basal vascular tone in pressurized arterioles but dramatically reduced the level of vasoconstriction observed in the presence of 10 nM endothelin-1.

CONCLUSIONS

Retinal arteriolar myocytes have I(Cl(Ca)), which may be activated by Ca(2+) entry through L-type Ca(2+) channels or Ca(2+) release from intracellular stores. This current appears to contribute to agonist-induced retinal vasoconstriction.

TMEM16A confers receptor-activated calcium-dependent chloride conductance

Calcium (Ca(2+))-activated chloride channels are fundamental mediators in numerous physiological processes including transepithelial secretion, cardiac and neuronal excitation, sensory transduction, smooth muscle contraction and fertilization. Despite their physiological importance, their molecular identity has remained largely unknown. Here we show that transmembrane protein 16A (TMEM16A, which we also call anoctamin 1 (ANO1)) is a bona fide Ca(2+)-activated chloride channel that is activated by intracellular Ca(2+) and Ca(2+)-mobilizing stimuli. With eight putative transmembrane domains and no apparent similarity to previously characterized channels, ANO1 defines a new family of ionic channels. The biophysical properties as well as the pharmacological profile of ANO1 are in full agreement with native Ca(2+)-activated chloride currents. ANO1 is expressed in various secretory epithelia, the retina and sensory neurons. Furthermore, knockdown of mouse Ano1 markedly reduced native Ca(2+)-activated chloride currents as well as saliva production in mice. We conclude that ANO1 is a candidate Ca(2+)-activated chloride channel that mediates receptor-activated chloride currents in diverse physiological processes.

Molecular physiology of bestrophins: multifunctional membrane proteins linked to best disease and other retinopathies

This article reviews the current state of knowledge about the bestrophins, a newly identified family of proteins that can function both as Cl(-) channels and as regulators of voltage-gated Ca(2+) channels. The founding member, human bestrophin-1 (hBest1), was identified as the gene responsible for a dominantly inherited, juvenile-onset form of macular degeneration called Best vitelliform macular dystrophy. Mutations in hBest1 have also been associated with a small fraction of adult-onset macular dystrophies. It is proposed that dysfunction of bestrophin results in abnormal fluid and ion transport by the retinal pigment epithelium, resulting in a weakened interface between the retinal pigment epithelium and photoreceptors. There is compelling evidence that bestrophins are Cl(-) channels, but bestrophins remain enigmatic because it is not clear that the Cl(-) channel function can explain Best disease. In addition to functioning as a Cl(-) channel, hBest1 also is able to regulate voltage-gated Ca(2+) channels. Some bestrophins are activated by increases in intracellular Ca(2+) concentration, but whether bestrophins are the molecular counterpart of Ca(2+)-activated Cl(-) channels remains in doubt. Bestrophins are also regulated by cell volume and may be a member of the volume-regulated anion channel family.

Hypoxic/ischemic conditions induce expression of the putative pro-death gene Clca1 via activation of extrasynaptic N-methyl-D-aspartate receptors

The stimulation of extrasynaptic N-methyl-D-aspartate (NMDA) receptors triggers cell death pathways and has been suggested to play a key role in cell degeneration and neuron loss associated with glutamate-induced excitotoxicity. In contrast, synaptic NMDA receptors promote neuronal survival. One mechanism through which extrasynaptic NMDA receptors damage neurons may involve Clca1, which encodes a putative calcium-activated chloride channel. Here we show that Clca1 expression is induced in cultured rat hippocampal neurons exposed to oxygen/glucose-free media; this induction is mediated by a signaling pathway activated by extrasynaptic NMDA receptors. Clca1 mRNA levels also increased in the gerbil hippocampus following a transient forebrain ischemia caused by bilateral carotid occlusion. Microelectrode array recordings revealed that oxygen-glucose deprivation enhances hippocampal network firing rates, which induces c-fos transcription through a signaling pathway that, in contrast to Clca1, is activated by synaptic but not extrasynaptic NMDA receptors. Thus, conditions of low oxygen/glucose lead to the activation of both extrasynaptic and synaptic NMDA receptors that regulate distinct target genes. Clca1 may be part of the genomic death program triggered by extrasynaptic NMDA receptors; it could be a marker for ischemic brain damage and a possible target for therapeutic interventions.

Bestrophin Cl- channels are highly permeable to HCO3-

Bestrophin-1 (Best1) is a Cl(-) channel that is linked to various retinopathies in both humans and dogs. Dysfunction of the Best1 Cl(-) channel has been proposed to cause retinopathy because of altered Cl(-) transport across the retinal pigment epithelium (RPE). In addition to Cl(-), many Cl(-) channels also transport HCO3(-). Because HCO3(-) is physiologically important in pH regulation and in fluid and ion transport across the RPE, we measured the permeability and conductance of bestrophins to HCO3(-) relative to Cl(-). Four human bestrophin homologs (hBest1, hBest2, hBest3, and hBest4) and mouse Best2 (mBest2) were expressed in HEK cells, and the relative HCO3(-) permeability (P HCO3/PCl) and conductance (G HCO3/GCl) were determined. P HCO3/PCl was calculated from the change in reversal potential (Erev) produced by replacing extracellular Cl(-) with HCO3(-). hBest1 was highly permeable to HCO3(-) (P HCO3)/PCl = approximately 0.44). hBest2, hBest4, and mBest2 had an even higher relative HCO3(-) permeability (P HCO3/PCl = 0.6-0.7). All four bestrophins had HCO3(-) conductances that were nearly the same as Cl(-) (G HCO3/GCl = 0.9-1.1). Extracellular Na+ did not affect the permeation of hBest1 to HCO3(-). At physiological HCO3(-) concentration, HCO3(-) was also highly conductive. The hBest1 disease-causing mutations Y85H, R92C, and W93C abolished both Cl(-) and HCO3(-) currents equally. The V78C mutation changed P HCO3/PCl and G HCO3/GCl of mBest2 channels. These results raise the possibility that disease-causing mutations in hBest1 produce disease by altering HCO3(-) homeostasis as well as Cl(-) transport in the retina.

Na+,K+,2Cl- cotransport and intracellular chloride regulation in rat primary sensory neurons: thermodynamic and kinetic aspects

Adult primary afferent neurons are depolarized by GABA throughout their entire surface, including their somata located in dorsal root ganglia (DRG). Primary afferent depolarization (PAD) mediated by GABA released from spinal interneurons determines presynaptic inhibition, a key mechanism in somatosensory processing. The depolarization is due to Cl(-) efflux through GABA(A) channels; the outward Cl(-) gradient is generated by a Na+,K+,2Cl(-) cotransporter (NKCC) as first established in amphibians. Using fluorescence imaging microscopy we measured [Cl(-)]i and cell water volume (CWV) in dissociated rat DRG cells (P0-P21) loaded with N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide and calcein, respectively. Basal [Cl(-)]i was 44.2 +/- 1.2 mM (mean +/- SE), Cl(-) equilibrium potential (E Cl) was -27.0 +/- 0.7 mV (n = 75). This [Cl(-)]i is about four times higher than electrochemical equilibrium. On isosmotic removal of external Cl(-), cells lost Cl(-) and shrank. On returning to control solution, cells reaccumulated Cl(-) and recovered CWV. Cl(-) reaccumulation had Na+-dependent (SDC) and Na+-independent (SIC) components. The SIC stabilized at [Cl(-)]i = 13.2 +/- 1.2 mM, suggesting that it was passive (E(Cl) = -60.5 +/- 3 mV). Bumetanide blocked CWV recovery and most (65%) of the SDC (IC50 = 5.7 microM), indicating that both were mediated by NKCC. Active Cl(-) uptake fell with increasing [Cl(-)]i and became negligible when [Cl(-)]i reached basal levels. The kinetics of active Cl(-) uptake suggests a negative feedback system in which intracellular Cl(-)regulates its own influx thereby keeping [Cl(-)]i constant, above electrochemical equilibrium but below the value that would attain if NKCC reached thermodynamic equilibrium.

Contributions of Voltage- and Ca2+-Activated Conductances to GABA-Induced Depolarization in Spider Mechanosensory Neurons

Activation of ionotropic γ-aminobutyric acid type A (GABAA) receptors depolarizes neurons that have high intracellular [Cl−], causing inhibition or excitation in different cell types. The depolarization often leads to inactivation of voltage-gated Na channels, but additional ionic mechanisms may also be affected. Previously, a simulated model of spider VS-3 mechanosensory neurons suggested that although voltage-activated Na+current is partially inactivated during GABA-induced depolarization, a slowly activating and inactivating component remains and may contribute to the depolarization. Here, we confirmed experimentally, by blocking Na channels prior to GABA application, that Na+current contributes to GABA-induced depolarization in VS-3 neurons. Ratiometric Ca2+imaging experiments combined with intracellular recordings revealed a significant increase in intracellular [Ca2+] when GABAAreceptors were activated, synchronous with the depolarization and probably due to Ca2+influx via low-voltage–activated (LVA) Ca channels. In contrast, GABAB-receptor activation in these neurons was previously shown to inhibit LVA current. Blockade of voltage-gated K channels delayed membrane repolarization, extending GABA-induced depolarization. However, inhibition of Ca channels significantly increased the amplitude of GABA-induced depolarization, indicating that Ca2+-activated K+current has an even stronger repolarizing effect. Regulation of intracellular [Ca2+] is important for many cellular processes and Ca2+control of K+currents may be particularly important for some functions of mechanosensory neurons, such as frequency tuning. These data show that GABAA-receptor activation participates in this regulation.

Chloride channels as drug targets

Chloride channels represent a relatively under-explored target class for drug discovery as elucidation of their identity and physiological roles has lagged behind that of many other drug targets. Chloride channels are involved in a wide range of biological functions, including epithelial fluid secretion, cell-volume regulation, neuroexcitation, smooth-muscle contraction and acidification of intracellular organelles. Mutations in several chloride channels cause human diseases, including cystic fibrosis, macular degeneration, myotonia, kidney stones, renal salt wasting and hyperekplexia. Chloride-channel modulators have potential applications in the treatment of some of these disorders, as well as in secretory diarrhoeas, polycystic kidney disease, osteoporosis and hypertension. Modulators of GABA(A) (gamma-aminobutyric acid A) receptor chloride channels are in clinical use and several small-molecule chloride-channel modulators are in preclinical development and clinical trials. Here, we discuss the broad opportunities that remain in chloride-channel-based drug discovery.

Expression of three distinct families of calcium-activated chloride channel genes in the mouse dorsal root ganglion

Objective A calcium-activated chloride current (IClCa) has been observed in medium-sized sensory neurons of the dorsal root ganglion (DRG). Axotomy of the sciatic nerve induces a similar current in the majority of medium and large diameter neurons. Our aim is to identify the molecule(s) underlying this current. Methods Using conventional and quantitative RT-PCR, we examined the expression in DRG of members of three families of genes, which have been shown to have IClCa current inducing properties. Results We showed the detection of transcripts representing several members of these families, i.e. chloride channel calcium-activated (CLCA), Bestrophin and Tweety gene families in adult DRG, in the normal state and 3 d after sciatic nerve section, a model for peripheral nerve injury. Conclusion Our analysis revealed that that mBest1 and Tweety2 appear as the best candidates to play a role in the injury-induced IClCa in DRG neurons.

Xenopus oocytes as a heterologous expression system for studying ion channels with the patch-clamp technique

Oocytes from the Xenopus laevis represent one of the most widely used expression systems for functional characterization of ion channels. Their large size facilitates both injection of heterologous cRNA and subsequent electrophysiological recordings of ion channel currents. Furthermore, Xenopus oocytes translate cRNA very efficiently, resulting in the generation of a large number of ion channels in the plasma membrane. In this chapter, we outline methods for oocyte preparation and maintenance and describe procedures for patch-clamping of oocytes, with a special focus on the macropatch technique. We discuss some common problems associated with patch-clamping of oocytes and their use as an expression system for ion channels.

Small-molecule screen identifies inhibitors of a human intestinal calcium-activated chloride channel

Calcium-activated chloride channels (CaCCs) are widely expressed in mammalian tissues, including intestinal epithelia, where they facilitate fluid secretion. Potent, selective CaCC inhibitors have not been available. We established a high-throughput screen for identification of inhibitors of a human intestinal CaCC based on inhibition of ATP/carbachol-stimulated iodide influx in HT-29 cells after lentiviral infection with the yellow fluorescent halide-sensing protein YFP-H148Q/I152L. Screening of 50,000 diverse, drug-like compounds yielded six classes of putative CaCC inhibitors, two of which, 3-acyl-2-aminothiophenes and 5-aryl-2-aminothiazoles, inhibited by >95% iodide influx in HT-29 cells in response to multiple calcium-elevating agonists, including thapsigargin, without inhibition of calcium elevation, calcium-calmodulin kinase II activation, or cystic fibrosis transmembrane conductance regulator chloride channels. These compounds also inhibited calcium-dependent chloride secretion in T84 human intestinal epithelial cells. Patch-clamp analysis indicated inhibition of CaCC gating, which, together with the calcium-calmodulin data, suggests that the inhibitors target the CaCC directly. Structure-activity relationships were established from analysis of more than 1800 analogs, with IC(50) values of the best analogs down to approximately 1 muM. Small-molecule CaCC inhibitors may be useful in pharmacological dissection of CaCC functions and in reducing intestinal fluid losses in CaCC-mediated secretory diarrheas.

The product of the geneGEF1ofSaccharomyces cerevisiaetransports Cl−across the plasma membrane

Expression of GEF1 in Xenopus laevis oocytes and HEK-293 cells gave rise to a Cl- channel that remained permanently open and was blocked by nitro-2-(3-phenyl-propylamino) benzoic acid and niflumic acid. NPPB induced petite-like colonies, resembling the GEF1 knock-out. The fluorescent halide indicator SPQ was quenched in a wild-type strain, in contrast to both a GEF1 knock-out strain and yeast grown in the presence of NPPB. Immunogold and electron microscopy located Gef1p in the plasma membrane, vacuole, endoplasmic reticulum and Golgi apparatus. Eleven substitutions in five residues forming the ion channel of GEF1 were introduced; some of them (S186A, I188N, Y459D, Y459F, Y459V, I467A, I467N and F468N) did not rescue the pet phenotype, whereas F468A, A558F and A558Y formed normal colonies. All the pet mutants showed reduced O2 consumption, small mitochondria and mostly disrupted organelles. Finally, electron microscopy revealed that the plasma membrane of the mutants develop multiple foldings and highly ordered cylindrical protein-membrane complexes. All the experiments above suggest that Gef1p transports Cl- through the plasma membrane and reveal the importance of critical amino acids for the proper function of the protein as suggested by structural models. However, the mechanism of activation of the channel has yet to be defined.

Calcium-dependent fast depolarizing afterpotentials in vasopressin neurons in the rat supraoptic nucleus

Oxytocin (OT) and vasopressin (VP) synthesizing magnocellular cells (MNCs) in the supraoptic nucleus (SON) display distinct firing patterns during the physiological demands for these hormones. Depolarizing afterpotentials (DAPs) in these neurons are involved in controlling phasic bursting in VP neurons. Our whole cell recordings demonstrated a Cs(+)-resistant fast DAP (fDAP; decay tau = approximately 200 ms), which has not been previously reported, in addition to the well-known Cs(+)-sensitive slower DAP (sDAP; decay tau = approximately 2 s). Immunoidentification of recorded neurons revealed that all VP neurons, but only 20% of OT neurons, expressed the fDAP. The activation of the fDAP required influx of Ca(2+) through voltage-gated Ca(2+) channels as it was strongly suppressed in Ca(2+)-free extracellular solution or by bath application of Cd(2+). Additionally, the current underlying the fDAP (I(fDAP)) is a Ca(2+)-activated current rather than a Ca(2+) current per se as it was abolished by strongly buffering intracellular Ca(2+) with BAPTA. The I-V relationship of the I(fDAP) was linear at potentials less than -60 mV but showed pronounced outward rectification near -50 mV. I(fDAP) is sensitive to changes in extracellular Na(+) and K(+) but not Cl(-). A blocker of Ca(2+)-activated nonselective cation (CAN) currents, flufenamic acid, blocked the fDAP, suggesting the involvement of a CAN current in the generation of fDAP in VP neurons. We speculate that the two DAPs have different roles in generating after burst discharges and could play important roles in determining the distinct firing properties of VP neurons in the SON neurons.

A mathematical model of plasma membrane electrophysiology and calcium dynamics in vascular endothelial cells

Vascular endothelial cells (ECs) modulate smooth muscle cell (SMC) contractility, assisting in vascular tone regulation. Cytosolic Ca2+ concentration ([Ca2+]i) and membrane potential ( Vm) play important roles in this process by controlling EC-dependent vasoactive signals and intercellular communication. The present mathematical model integrates plasmalemma electrophysiology and Ca2+ dynamics to investigate EC responses to different stimuli and the controversial relationship between [Ca2+]i and Vm. The model contains descriptions for the intracellular balance of major ionic species and the release of Ca2+ from intracellular stores. It also expands previous formulations by including more detailed transmembrane current descriptions. The model reproduces Vm responses to volume-regulated anion channel (VRAC) blockers and extracellular K+ concentration ([K+]o) challenges, predicting 1) that Vm changes upon VRAC blockade are [K+]o dependent and 2) a biphasic response of Vm to increasing [K+]o. Simulations of agonist-induced Ca2+ mobilization replicate experiments under control and Vm hyperpolarization blockade conditions. They show that peak [Ca2+]i is governed by store Ca2+ release while Ca2+ influx (and consequently Vm) impacts more the resting and plateau [Ca2+]i. The Vm sensitivity of rest and plateau [Ca2+]i is dictated by a [Ca2+]i “buffering” system capable of masking the Vm-dependent transmembrane Ca2+ influx. The model predicts plasma membrane Ca2+-ATPase and Ca2+ permeability as main players in this process. The heterogeneous Vm impact on [Ca2+]i may elucidate conflicting reports on how Vm influences EC Ca2+. The present study forms the basis for the development of multicellular EC-SMC models that can assist in understanding vascular autoregulation in health and disease.

Potassium channels and uterine function

Ion channels are effector proteins that mediate uterine excitability throughout gestation. This review will focus primarily on the role of potassium channels in regulating myometrial tone in pregnancy and labor contractions. During gestation, potassium channels maintain the uterus in a state of quiescence by contributing to the resting membrane potential and counteracting contractile stimuli. This review summarizes the current knowledge about the significance of the potassium channels in maintaining a normal gestational period and initiating labor contractions at term.

Niflumic acid and AG-1478 reduce cigarette smoke-induced mucin synthesis: the role of hCLCA1

BACKGROUND

Cigarette smoke induces bronchial mucus secretion. However, the mechanism of this induction is still unidentified. In this study, we investigated the role of the putative calcium-activated chloride channel 1 (CLCA1) and its blocker, niflumic acid, in cigarette smoke-induced mucin synthesis both in vivo and in vitro.

METHODS AND RESULTS

Sprague-Dawley rats were exposed to cigarette smoke for 4 weeks. The CLCA1, epidermal growth factor receptor (EGFR), and MUC5AC expressions were increased in the trachea and lung tissues. Goblet-cell hyperplasia with marked mucin staining was detected in the tracheal and bronchial epithelium. In the human bronchial epithelial cell line NCI-H292, cigarette smoke solution also induced mucin production as well as the RNA and protein expressions of CLCA1, EGFR, and MUC5AC. Both in vivo and in vitro, the induction of MUC5AC and mucin synthesis were inhibited by niflumic acid, and/or a selective EGFR tyrosine kinase inhibitor, AG-1478. Niflumic acid also blocked the epidermal growth factor-induced MUC5AC and mucin staining in the NCI-H292 cell line.

CONCLUSION

Both EGFR and niflumic acid-sensitive chloride channels (probably CLCA1) are dependently affecting the mucin production as a part of a single complex signaling pathway. CLCA1 may be a key signaling member that can be targeted with pharmacologic interventions to treat mucus hypersecretion.

2-APB Induces Instability in Rat Left Atrial Mechanical Activity

Atrial contractile abnormalities are common clinical disorders but few pharmacological models can reliably produce such abnormalities in isolated atrial muscle. Since sarcoplasmic reticulum (SR) calcium leak may underlie these contractile irregularities, we investigated whether 2-aminoethoxydiphenyl borate (2-APB), a calcium leak-inducer, affects mechanical function in isolated, superfused rat left atria. Exposing left atria paced at 3 Hz to >10 microM 2-APB produced sporadic mechanical events that occurred in the absence of pacing stimulus. Prolonging atrial diastole in the presence of 2-APB produced spontaneous mechanical activity (SMA) defined as numerous mechanical events occurring in the absence of pacing stimulus. SMA depends on atrial sodium and chloride gradients as decreasing superfusate concentration of either ion suppressed SMA. Increasing superfusate potassium to produce an EK of approximately -74mV reversed SMA, revealing possible membrane potential sensitivity. Mechanical function decreased with time in left atria treated with 2-APB and low sodium or the anion transport inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) compared with atria exposed to low sodium or DIDS alone, suggesting 2-APB may decrease left atrial SR activator calcium. Thus, 2-APB produces instability in regular left atrial mechanical activity that may require forward-mode sodium-calcium exchange and chloride channel activities. This data identify a new model for studying atrial contractile abnormalities.

Voltage-dependent calcium channel CaV1.3 subunits regulate the light peak of the electroretinogram

In response to light, the mouse retinal pigment epithelium (RPE) generates a series of slow changes in potential that are referred to as the c-wave, fast oscillation (FO), and light peak (LP) of the electroretinogram (ERG). The LP is generated by a depolarization of the basolateral RPE plasma membrane by the activation of a calcium-sensitive chloride conductance. We have previously shown that the LP is reduced in both mice and rats by nimodipine, which blocks voltage-dependent calcium channels (VDCCs) and is abnormal in lethargic mice, carrying a null mutation in the calcium channel beta(4) subunit. To define the alpha(1) subunit involved in this process, we examined mice lacking Ca(V)1.3. In comparison with wild-type (WT) control littermates, LPs were reduced in Ca(V)1.3(-/-) mice. This pattern matched closely with that previously noted in lethargic mice, confirming a role for VDCCs in regulating the signaling pathway that culminates in LP generation. These abnormalities do not reflect a defect in rod photoreceptor activity, which provides the input to the RPE to generate the c-wave, FO, and LP, because ERG a-waves were comparable in WT and Ca(V)1.3(-/-) littermates. Our results identify Ca(V)1.3 as the principal pore-forming subunit of VDCCs involved in stimulating the ERG LP.

Calcium-dependent chloride conductance in epithelia: is there a contribution by Bestrophin?

Although known for more than 20 years, the molecular identity of epithelial Ca(2+)-activated Cl(-) channels remains obscure. Previous candidate proteins did not hold initial promises, and thus, new hope is put into the recently identified family of bestrophin proteins, as they reflect many of the properties found for native channels. Mutations in the bestrophin gene cause an autosomal form of macular dystrophy of the retina. Bestrophin 1 is assumed to form the basolateral Ca(2+)-activated Cl(-) channel in the retinal pigment epithelium of the eye. Other data suggest that bestrophin is a regulator of voltage gated Ca(2+) channels. Structural information on bestrophins is available and a Cl(-) selective filter has been localized to the second transmembrane domain of bestrophin. It is possible that bestrophins function as physiologically regulated Cl(-) channels only in association with additional subunits and auxiliary proteins. Little is known about expression of bestrophin in gland acinar cells, which show a pronounced Ca(2+)-activated Cl(-) secretion. In airways and intestinal epithelia, bestrophins could be particularly important in diseases such as cystic fibrosis and secretory diarrhea.

Functional and molecular characterization of the fluid secretion mechanism in human parotid acinar cells

The strategies available for treating salivary gland hypofunction are limited because relatively little is known about the secretion process in humans. An initial microarray screen detected ion transport proteins generally accepted to be critically involved in salivation. We tested for the activity of some of these proteins, as well as for specific cell properties required to support fluid secretion. The resting membrane potential of human acinar cells was near -51 mV, while the intracellular [Cl-] was approximately 62 mM, about fourfold higher than expected if Cl ions were passively distributed. Active Cl- uptake mechanisms included a bumetanide-sensitive Na+ -K+ -2Cl- cotransporter and paired DIDS-sensitive Cl-/HCO3- and EIPA-sensitive Na+/H+ exchangers that correlated with expression of NKCC1, AE2, and NHE1 transcripts, respectively. Intracellular Ca2+ stimulated a niflumic acid-sensitive Cl- current with properties similar to the Ca2+ -gated Cl channel BEST2. In addition, intracellular Ca2+ stimulated a paxilline-sensitive and voltage-dependent, large-conductance K channel and a clotrimazole-sensitive, intermediate-conductance K channel, consistent with the detection of transcripts for KCNMA1 and KCNN4, respectively. Our results demonstrate that the ion transport mechanisms in human parotid glands are equivalent to those in the mouse, confirming that animal models provide valuable systems for testing therapies to prevent salivary gland dysfunction.

Stimulation of Ca2+-gated Cl- currents by the calcium-dependent K+ channel modulators NS1619 [1,3-dihydro-1-[2-hydroxy-5-(trifluoromethyl)phenyl]-5-(trifluoromethyl)-2H-benzimidazol-2-one] and isopimaric acid

Because chloride (Cl(-)) channel blockers such as niflumic acid enhance large-conductance Ca(2+)-activated potassium channels (BK(Ca)), the aim of this study was to determine whether there is a reciprocal modification of Ca(2+)-activated chloride Cl(-) currents (I(ClCa)) by two selective activators of BK(Ca). Single smooth muscle cells were isolated by enzymatic digestion from murine portal vein and rabbit pulmonary artery. The BK(Ca) activators NS1619 [1,3-dihydro-1-[2-hydroxy-5-(trifluoromethyl-)phenyl]-5-(trifluoromethyl)-2H-benzimidazol-2-one] and isopimaric acid (IpA) augmented macroscopic I(ClCa) elicited by pipette solutions containing [Ca(2+)](i) > 100 nM without any alteration in current kinetics. Enhanced currents recorded in the presence of NS1619 or IpA reversed at the theoretical Cl(-) equilibrium potential, which was shifted by approximately -40 mV upon replacement of the external anion with the more permeable thiocyanate anion. NS1619 increased the sensitivity of calcium-activated chloride channel (Cl(Ca)) to Ca(2+) (approximately 100 nM at +60 mV) and induced a leftward shift in their voltage dependence (approximately 80 mV with 1 micro Ca(2+)). Single-channel experiments revealed that NS1619 increased the number of open channels times the open probability of small-conductance (1.8-3.1 pS) Cl(Ca) without any alteration in their unitary amplitude or number of observable unitary levels of activity. These data, in addition to the established stimulatory effects of niflumic acid on BK(Ca), show that there is similarity in the pharmacology of calcium-activated chloride and potassium channels. Although nonspecific interactions are possible, one alternative hypothesis is that the channel underlying vascular I(ClCa) shares some structural similarity to the BK(Ca) or that the latter K(+) channel physically interacts with Cl(Ca).

The human corneal endothelium: new insights into electrophysiology and ion channels

The corneal endothelium is a monolayer that mediates the flux of solutes and water across the posterior corneal surface. Thereby, it plays an essential role to maintain the transparency of the cornea. Unlike the epithelium, the human endothelium is an amitotic cell layer with a critical cell density and the risk of corneal decompensation. The number of endothelial cells subsequently decreases with age. Moreover, the endothelial cell loss is accelerated after various impairments such as surgical trauma (e.g. cataract extraction) and following corneal transplantation. This cell loss is associated with programmed cell death (apoptosis) and changed ion channel activity. However, little is known about the electrophysiology and ion channel expression (in particular Ca2+ channels) in corneal endothelial cells. This article reviews our current knowledge about the electrophysiology of the corneal endothelium. It highlights ion channel expression, which may have a major role in corneal cell physiology and pathological events. A better understanding of the (electro)physiological function of the cornea may lead to the development of clinical relevant new therapeutic and preventive measures.

Depolarization evoked by acetylcholine in mesenteric arteries of hypertensive rats attenuates endothelium-dependent hyperpolarizing factor

OBJECTIVE

During blockade of endothelium-dependent hyperpolarizing factor (EDHF), acetylcholine evoked larger and faster depolarization in mesenteric arteries of spontaneously hypertensive rats (SHR) than normotensive Wistar-Kyoto (WKY) rats. We studied the mechanism underlying this response and its role in the attenuation of EDHF.

METHODS

Electrophysiology, computational modelling and myography were used to study changes in membrane potential and effects on contractility.

RESULTS

The large acetylcholine-evoked depolarization in SHR was accompanied by contraction, but this was not seen in WKY rats. The depolarization depended on release of intracellular Ca2+ but was unaffected by nonselective cation channel inhibitors, gadolinium, lanthanum or amiloride. The depolarization was significantly reduced by the Ca2+-dependent Cl- channel inhibitors, niflumic acid or flufenamic acid, or alterations in Cl- gradients using bumetanide (Na/K/Cl transporter inhibitor) or external Cl- replacement with isethionate. These drugs altered the time course of EDHF-evoked hyperpolarizations in SHR, making them indistinguishable from those in WKY rats. EDHF-induced relaxation was less sensitive to acetylcholine in SHR than in WKY rats, but this difference was eliminated following artery pretreatment with bumetanide. Computational modelling in which the SHR fast depolarizing response was selectively modulated mimicked physiologically acquired results obtained in SHR and WKY rats during Cl- -channel blockade.

CONCLUSIONS

Acetylcholine evokes a fast depolarization in SHR but not in WKY rats, mediated by the opening of Ca2+-dependent Cl- channels. The depolarization is responsible for a constriction that reduces EDHF-mediated relaxation. Data suggest that Ca2+-dependent Cl- channels may provide a novel therapeutic target for improvement of endothelial dysfunction during hypertension.

The sodium-coupled neutral amino acid transporter SNAT2 mediates an anion leak conductance that is differentially inhibited by transported substrates

The sodium-coupled neutral amino acid transporter SNAT2 mediates cellular uptake of glutamine and other small, neutral amino acids. Here, we report the existence of a leak anion pathway associated with SNAT2. The leak anion conductance was increased by, but did not require the presence of, extracellular sodium. The transported substrates L-alanine, L-glutamine, and alpha-(methylamino)isobutyrate inhibited the anion leak conductance, each with different potency. A transporter with the mutation H-304A did not catalyze alanine transport but still catalyzed anion leak current, demonstrating that substrate transport is not required for anion current inhibition. Both the substrate and Na+ were able to bind to the SNAT2H-304A transporter normally. The selectivity sequence of the SNAT2H-304A anion conductance was SCN->>NO3->I->Br->Cl->Mes-. Anion flux mediated by the more hydrophobic anion SCN- was not saturable, whereas nitrate flux demonstrated saturation kinetics with an apparent Km of 29 mM. SNAT2, which belongs to the SLC38 family of transporters, has to be added to the growing number of secondary, Na+-coupled transporters catalyzing substrate-gated or leak anion conductances. Therefore, we can speculate that such anion-conducting pathways are general features of Na+-transporting systems.

CLC chloride channels and transporters: a biophysical and physiological perspective

Chloride-transporting proteins play fundamental roles in many tissues in the plasma membrane as well as in intracellular membranes. They have received increasing attention in the last years because crucial, and often unexpected and novel, physiological functions have been disclosed with gene-targeting approaches, X-ray crystallography, and biophysical analysis. CLC proteins form a gene family that comprises nine members in mammals, at least four of which are involved in human genetic diseases. The X-ray structure of the bacterial CLC homolog, ClC-ec1, revealed a complex fold and confirmed the anticipated homodimeric double-barreled architecture of CLC-proteins with two separate Cl-ion transport pathways, one in each subunit. Four of the mammalian CLC proteins, ClC-1, ClC-2, ClC-Ka, and ClC-Kb, are chloride ion channels that fulfill their functional roles-stabilization of the membrane potential, transepithelial salt transport, and ion homeostasisin the plasma membrane. The other five CLC proteins are predominantly expressed in intracellular organelles like endosomes and lysosomes, where they are probably important for a proper luminal acidification, in concert with the V-type H+-ATPase. Surprisingly, ClC-4, ClC-5, and probably also ClC-3, are not Cl- ion channels but exhibit significant Cl-/H+ antiporter activity, as does the bacterial homolog ClC-ec1 and the plant homolog AtCLCa. The physiological significance of the Cl-/H+ antiport activity remains to be established.

Overlapping pharmacology of Ca2+-activated Cl- and K+ channels

Research into Ca2+-activated Cl- channels is hampered by the inability to decipher their molecular identity and the fact that all extant Cl- channel blockers have effects on other ion channels. Most notably, Cl- channel blockers such as the fenamates (e.g. niflumic acid and flufenamic acid) activate Ca2+-dependent K+ channels, although other pharmacological overlaps have been discovered. In this article, we highlight the complex pharmacology of Ca2+-activated Cl- channels and the caveats associated with using these blockers--a necessary requirement because many researchers use Cl- channel blockers as probes for Cl- channel activity. Moreover, we discuss the argument for a common structural motif between Ca2+-activated Cl- channels and Ca2+-dependent K+ channels, which has led to the possibility that the molecular identity of Cl- channels will be revealed by research in this new direction, in addition to the use of existing candidates such as the CLCA, Bestrophin and tweety genes.

Single Cl- channels activated by Ca2+ in Drosophila S2 cells are mediated by bestrophins

Mutations in human bestrophin-1 (VMD2) are genetically linked to several forms of retinal degeneration but the underlying mechanisms are unknown. Bestrophin-1 (hBest1) has been proposed to be a Cl⁻ channel involved in ion and fluid transport by the retinal pigment epithelium (RPE). To date, however, bestrophin currents have only been described in overexpression systems and not in any native cells. To test whether bestrophins function as Ca²⁺-activated Cl⁻ (CaC) channels physiologically, we used interfering RNA (RNAi) in the Drosophila S2 cell line. S2 cells express four bestrophins (dbest1–4) and have an endogenous CaC current. The CaC current is abolished by several RNAi constructs to dbest1 and dbest2, but not dbest3 or dbest4. The endogenous CaC current was mimicked by expression of dbest1 in HEK cells, and the rectification and relative permeability of the current were altered by replacing F81 with cysteine. Single channel analysis of the S2 bestrophin currents revealed an ∼2-pS single channel with fast gating kinetics and linear current–voltage relationship. A similar channel was observed in CHO cells transfected with dbest1, but no such channel was seen in S2 cells treated with RNAi to dbest1. This provides definitive evidence that bestrophins are components of native CaC channels at the plasma membrane.

Niflumic acid inhibits chloride conductance of rat skeletal muscle by directly inhibiting the CLC-1 channel and by increasing intracellular calcium

BACKGROUND AND PURPOSE

Given the crucial role of the skeletal muscle chloride conductance (gCl), supported by the voltage-gated chloride channel CLC-1, in controlling muscle excitability, the availability of ligands modulating CLC-1 are of potential medical as well as toxicological importance. Here, we focused our attention on niflumic acid (NFA), a molecule belonging to the fenamates group of non-steroidal anti-inflammatory drugs (NSAID).

EXPERIMENTAL APPROACH

Rat muscle Cl(-) conductance (gCl) and heterologously expressed CLC-1 currents were evaluated by means of current-clamp (using two-microelectrodes) and patch-clamp techniques, respectively. Fura-2 fluorescence was used to determine intracellular calcium concentration, [Ca(2+)](i), in native muscle fibres.

KEY RESULTS

NFA inhibited native gCl with an IC(50) of 42 muM and blocked CLC-1 by interacting with an intracellular binding site. Additionally, NFA increased basal [Ca(2+)](i) in myofibres by promoting a mitochondrial calcium efflux that was not dependent on cyclooxygenase or CLC-1. A structure-activity study revealed that the molecular conditions that mediate the two effects are different. Pretreatment with the Ca-dependent protein kinase C (PKC) inhibitor chelerythrine partially inhibited the NFA effect. Therefore, in addition to direct channel block, NFA also inhibits gCl indirectly by promoting PKC activation.

CONCLUSIONS AND IMPLICATIONS

These cellular effects of NFA on skeletal muscle demonstrate that it is possible to modify CLC-1 and consequently gCl directly by interacting with channel proteins and indirectly by interfering with the calcium-dependent regulation of the channel. The effect of NFA on mitochondrial calcium stores suggests that NSAIDs, widely used drugs, could have potentially dangerous side-effects.

Development of synthetic membrane transporters for anions

The development of low molecular weight anion transporters is an emerging topic in supramolecular chemistry. The major focus of this tutorial review is on synthetic chloride transport systems that operate in vesicle and cell membranes. The transporters alter transmembrane concentration gradients, and thus they have applications as reagents for cell biology research and as potential chemotherapeutic agents. The molecular designs include monomolecular channels, self-assembled channels and mobile carriers. Also discussed are the experimental assays that measure transport rates across model bilayer membranes.

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

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

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

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

Dopamine modulation of Ca(2+) dependent Cl(-) current regulates ciliary beat frequency controlling locomotion in Tritonia diomedea

The physiological mechanisms controlling ciliary beating remain largely unknown. Evidence exists supporting both hormonal control of ciliary beating and control via direct innervation. In the present study we investigated nervous control of cilia based locomotion in the nudibranch mollusc, Tritonia diomedea. Ciliated pedal epithelial (CPE) cells acting as locomotory effectors may be electrically excitable. To explore this possibility we characterized the cells' electrical properties, and found that CPE cells have large voltage dependent whole cell currents with two components. First, there is a fast activating outward Cl(-) current that is both voltage and Ca(2+) influx dependent (I(Cl(Ca))). I(Cl(Ca)) is sensitive to DIDS and 9-AC, and resembles currents of Ca(2+)-activated Cl(-) channels (CaCC). Ca(2+) dependence also suggests the presence of voltage-gated Ca(2+) channels; however, we were unable to detect these currents. The second current, a voltage dependent proton current (I(H)), activates very slowly and is sensitive to both Zn(2+) and changes in pH. In addition we identify a new cilio-excitatory substance in Tritonia, viz., dopamine. Dopamine, in the 10 mumol l(-1)-1 mmol l(-1) range, significantly increases ciliary beat frequency (CBF). We also found dopamine and Tritonia Pedal Peptide (TPep-NLS) selectively suppress I(Cl(Ca)) in CPE cells, demonstrating a link between CBF excitation and I(Cl(Ca)). It appears that dopamine and TPep-NLS inhibit I(Cl(Ca)) not through changing [Ca(2+)](in), but directly by an unknown mechanism. Coupling of I(Cl(Ca)) and CBF is further supported by our finding that DIDS and zero [Cl(-)](out) both increase CBF, mimicking dopamine and TPep-NLS excitation. These results suggest that dopamine and TPep-NLS act to inhibit I(Cl(Ca)), initiating and prolonging Ca(2+) influx, and activating CBF excitation.

Bestrophin-2 is a candidate calcium-activated chloride channel involved in olfactory transduction

Ca-activated Cl channels are an important component of olfactory transduction. Odor binding to olfactory receptors in the cilia of olfactory sensory neurons (OSNs) leads to an increase of intraciliary Ca concentration by Ca entry through cyclic nucleotide-gated (CNG) channels. Ca activates a Cl channel that leads to an efflux of Cl from the cilia, contributing to the amplification of the OSN depolarization. The molecular identity of this Cl channel remains elusive. Recent evidence has indicated that bestrophins are able to form Ca-activated Cl channels in heterologous systems. Here we have analyzed the expression of bestrophins in the mouse olfactory epithelium and demonstrated that only mouse bestrophin-2 (mBest2) was expressed. Single-cell RT-PCR showed that mBest2 was expressed in OSNs but not in supporting cells. Immunohistochemistry revealed that mBest2 was expressed on the cilia of OSNs, the site of olfactory transduction, and colocalized with the main CNGA2 channel subunit. Electrophysiological properties of Ca-activated Cl currents from native channels in dendritic knob/cilia of mouse OSNs were compared with those induced by the expression of mBest2 in HEK-293 cells. We found the same anion permeability sequence, small estimated single-channel conductances, a Ca sensitivity difference of one order of magnitude, and the same side-specific blockage of the two Cl channel blockers commonly used to inhibit the odorant-induced Ca-activated Cl current in OSNs, niflumic acid, and 4-acetamido-4'-isothiocyanato-stilben-2,2'-disulfonate (SITS). Therefore, our data suggest that mBest2 is a good candidate for being a molecular component of the olfactory Ca-activated Cl channel.

Molecular expression and functional involvement of the bovine calcium-activated chloride channel 1 (bCLCA1) in apical HCO3- permeability of bovine corneal endothelium

Corneal endothelium secretes HCO(3)(-) from basolateral (stroma) to apical (anterior chamber) compartments. Apical HCO(3)(-) permeability can be enhanced by increasing [Ca(2+)](i). We hypothesized that the bovine calcium-activated chloride channel 1 (bCLCA1), shown previously by PCR screening to be expressed in corneal endothelium, is involved in Ca(2+) activated apical HCO(3)(-) permeability. bCLCA1 expression in cultured bovine corneal endothelial cells (CBCEC) was examined by in situ hybridization analysis, immunoblotting, immunofluorescence and confocal microscopy. Rabbit polyclonal antibodies were generated using a 14 aa polypeptide (417-430) from the predicted sequence of bCLCA1. The small interference RNA (siRNA) knock down technique was used to evaluate the functional involvement of bCLCA1 in apical HCO(3)(-) permeability. In situ hybridization confirmed prominent bCLCA1-specific mRNA expression in CBCEC. bCLCA1 antiserum detected the heterologously expressed bCLCA1 in HEK293 cells and a 90kDa band in CBCEC, which was absent when using the pre-immune serum or antigen absorption of serum. Immunofluoresence staining with anti-bCLCA1 antibody and confocal microscopy indicates an apical membrane location in CBCEC. In CBCEC transfected with bCLCA1 specific siRNA, bCLCA1 expression was reduced by 80%, while transfection with siControl scrambled sequence had no effect. Increasing [Ca(i)(2+)] by application of ATPgammaS or cyclopiazonic acid (CPA) increased apical HCO(3)(-) permeability in siControl transfected CBCEC, while having no effect on apical HCO(3)(-) permeability in bCLCA1 specific siRNA transfected cells. Baseline HCO(3)(-) permeability, however, was not different between controls and siRNA treated cells. We conclude that the calcium-activated chloride channel (bCLCA1) is expressed in bovine corneal endothelial cells and can contribute to Ca(2+) dependent apical HCO(3)(-) permeability, but not resting permeability, across the corneal endothelium.

Novel conserved hydrolase domain in the CLCA family of alleged calcium-activated chloride channels

Advanced protein structure prediction methods combined with structure modeling show that the mammalian proteins, described until now as calcium-activated chloride channels (CLCAs), appear in fact to be membrane anchored metal-dependent hydrolases, possibly proteases. A metallohydrolase structural domain was predicted, unexpectedly, in the CLCA sequences. The well-conserved active site in the modeled structure of this hydrolase domain allows the prediction of catalytic action similar to that of metalloproteases. A number of protein structure prediction methods suggest the overall fold of the N-terminal hydrolase domain to be most similar to that of zinc metalloproteases (zincins), notably matrixins. This is confirmed by analysis of the three-dimensional structure model of the predicted CLCA1 hydrolase domain built using the known structure of the MMP-11 catalytic domain. Fragments of CLCA1 corresponding to the modeled hydrolase domain were expressed in Escherichia coli, and the resulting proteins were readily refolded into monomeric soluble protein, indicating formation of stable independent domains. The homology model was used to predict putative substrate sequences. Homologs of mammalian CLCA genes were detected in the genomes of a vast array of multicellular animals: lower vertebrates, tunicates, insects, crustaceans, echinoderms, and flatworms. The hydrolase prediction is discussed in the context of published experimentally determined effects of CLCA proteins on chloride conductance. Altered proteolytic processing of full-length CLCA1 containing a mutation abolishing the predicted hydrolase activity is shown as initial experimental evidence for a role of the hydrolase domain in processing of mature full-length CLCA1. The hydrolase prediction together with the presented experimental data add to doubts about the function of CLCAs as chloride channels and strengthen the hypothesis of channel-activating and/or channel-accessory roles.

The light peak of the electroretinogram is dependent on voltage-gated calcium channels and antagonized by bestrophin (best-1)

Mutations in VMD2, encoding bestrophin (best-1), cause Best vitelliform macular dystrophy (BMD), adult-onset vitelliform macular dystrophy (AVMD), and autosomal dominant vitreoretinochoroidopathy (ADVIRC). BMD is distinguished from AVMD by a diminished electrooculogram light peak (LP) in the absence of changes in the flash electroretinogram. Although the LP is thought to be generated by best-1, we find enhanced LP luminance responsiveness with normal amplitude in Vmd2-/- mice and no differences in cellular Cl- currents in comparison to Vmd2+/+ littermates. The putative Ca2+ sensitivity of best-1, and our recent observation that best-1 alters the kinetics of voltage-dependent Ca2+ channels (VDCC), led us to examine the role of VDCCs in the LP. Nimodipine diminished the LP, leading us to survey VDCC beta-subunit mutant mice. Lethargic mice, which harbor a loss of function mutation in the beta4 subunit of VDCCs, exhibited a significant shift in LP luminance response, establishing a role for Ca2+ in LP generation. When stimulated with ATP, which increases [Ca++]I, retinal pigment epithelial cells derived from Vmd2-/- mice exhibited a fivefold greater response than Vmd2+/+ littermates, indicating that best-1 can suppress the rise in [Ca2+]I associated with the LP. We conclude that VDCCs regulated by a beta4 subunit are required to generate the LP and that best-1 antagonizes the LP luminance response potentially via its ability to modulate VDCC function. Furthermore, we suggest that the loss of vision associated with BMD is not caused by the same pathologic process as the diminished LP, but rather is caused by as yet unidentified effects of best-1 on other cellular processes.

Calmodulin contributes to gating control in olfactory calcium-activated chloride channels

In sensory neurons of the peripheral nervous system, receptor potentials can be amplified by depolarizing Cl currents. In mammalian olfactory sensory neurons (OSNs), this anion-based signal amplification results from the sequential activation of two distinct types of transduction channels: cAMP-gated Ca channels and Ca-activated Cl channels. The Cl current increases the initial receptor current about 10-fold and leads to the excitation of the neuron. Here we examine the activation mechanism of the Ca-dependent Cl channel. We focus on calmodulin, which is known to mediate Ca effects on various ion channels. We show that the cell line Odora, which is derived from OSN precursor cells in the rat olfactory epithelium, expresses Ca-activated Cl channels. Single-channel conductance, ion selectivity, voltage dependence, sensitivity to niflumic acid, and Ca sensitivity match between Odora channels and OSN channels. Transfection of Odora cells with CaM mutants reduces the Ca sensitivity of the Cl channels. This result points to the participation of calmodulin in the gating process of Ca-ativated Cl channels, and helps to understand how signal amplification works in the olfactory sensory cilia. Calmodulin was previously shown to mediate feedback inhibition of cAMP-synthesis and of the cAMP-gated Ca channels in OSNs. Our results suggest that calmodulin may also be instrumental in the generation of the excitatory Cl current. It appears to play a pivotal role in the peripheral signal processing of olfactory sensory information. Moreover, recent results from other peripheral neurons, as well as from smooth muscle cells, indicate that the calmodulin-controlled, anion-based signal amplification operates in various cell types where it converts Ca signals into membrane depolarization.

Effect of cyclosporin and tacrolimus (FK506) on the antigen-induced mediator release, membrane potential and 86Rb+/K+ and Ca2+ fluxes in the RBL-2H3 cell line

The immunosuppressants cyclosporin A (CsA) and tacrolimus (FK506) inhibit the activation by antigen of T-lymphocytes as well as mast cells. The mechanism of their action on mast cells has yet to be elucidated. We, therefore, assessed their effect on antigen-induced histamine and beta-hexosaminidase release, membrane potential changes (bis-oxonol fluorescent probe), 86RB+ (marker for K+)-efflux, the intracellular free calcium concentration ([Ca2+]i in single cells) and 45Ca2+ uptake (CsA only) in RBL-2H3 cells, a mucosal-type mast cell line, passively sensitized with monoclonal mouse IgE antibody. Antigen addition induced depolarization within 1-2 min, followed by slower repolarization, reaching a steady state (approximately 90% repolarization) after 7-9 min. CsA and FK506 each dose-dependently inhibited antigen-induced histamine and beta-hexosaminidase secretion and the membrane repolarization phase, with similar IC50s for both actions, approximately 20 nM for CsA and approximately 2 nM for FK506. Antigen-induced 86Rb+-efflux was also significantly inhibited. Antigen-evoked increase in [Ca2+]i (area under the curve, AUC) was reduced by 35% and 52% in the presence of CsA or FK506 (1 microM each), respectively. However, 45Ca2+-uptake was not inhibited by CsA. These results suggest that both CsA and FK506 may inhibit mediator release from mast cells via blocking two interrelated processes, which are involved in the secretory process: 1. Membrane repolarization phase, which is essential for optimal mediator secretion and is mediated by a Ca2+-sensitive K+-efflux, yet to be further characterized, and (2) Increase in [Ca2+]i, probably via reduction of Ca(+2)-release from intracellular stores, [Ca2+]s.

The electro-oculogram

The retinal pigment epithelium (RPE) lying distal to the retina regulates the extracellular environment and provides metabolic support to the outer retina. RPE abnormalities are closely associated with retinal death and it has been claimed several of the most important diseases causing blindness are degenerations of the RPE. Therefore, the study of the RPE is important in Ophthalmology. Although visualisation of the RPE is part of clinical investigations, there are a limited number of methods which have been used to investigate RPE function. One of the most important is a study of the current generated by the RPE. In this it is similar to other secretory epithelia. The RPE current is large and varies as retinal activity alters. It is also affected by drugs and disease. The RPE currents can be studied in cell culture, in animal experimentation but also in clinical situations. The object of this review is to summarise this work, to relate it to the molecular membrane mechanisms of the RPE and to possible mechanisms of disease states.

Changes in ion transport in inflammatory disease

Ion transport is essential for maintenance of transmembranous and transcellular electric potential, fluid transport and cellular volume. Disturbance of ion transport has been associated with cellular dysfunction, intra and extracellular edema and abnormalities of epithelial surface liquid volume. There is increasing evidence that conditions characterized by an intense local or systemic inflammatory response are associated with abnormal ion transport. This abnormal ion transport has been involved in the pathogenesis of conditions like hypovolemia due to fluid losses, hyponatremia and hypokalemia in diarrhoeal diseases, electrolyte abnormalities in pyelonephritis of early infancy, septicemia induced pulmonary edema, and in hypersecretion and edema induced by inflammatory reactions of the mucosa of the upper respiratory tract. Components of membranous ion transport systems, which have been shown to undergo a change in function during an inflammatory response include the sodium potassium ATPase, the epithelial sodium channel, the Cystic Fibrosis Transmembrane Conductance Regulator and calcium activated chloride channels and the sodium potassium chloride co-transporter. Inflammatory mediators, which influence ion transport are tumor necrosis factor, gamma interferon, interleukins, transforming growth factor, leukotrienes and bradykinin. They trigger the release of specific messengers like prostaglandins, nitric oxide and histamine which alter ion transport system function through specific receptors, intracellular second messengers and protein kinases. This review summarizes data on in vivo measurements of changes in ion transport in acute inflammatory conditions and in vitro studies, which have explored the underlying mechanisms. Potential interventions directed at a correction of the observed abnormalities are discussed.

Genetic segregation of airway disease traits despite redundancy of calcium-activated chloride channel family members

Complex airway diseases such as asthma and chronic obstructive pulmonary disease exhibit stereotyped traits (especially airway hyperreactivity and mucous cell metaplasia) that are variably expressed in each patient. Here, we used a mouse model for virus-induced long-term expression of these traits to determine whether individual traits can be genetically segregated and thereby linked to separate determinants. We showed that an F2 intercross population derived from susceptible and nonsusceptible mouse strains can manifest individual phenotypic extremes that exhibit one or the other disease trait. Functional genomic analysis of these extremes further indicated that a member of the calcium-activated chloride channel (CLCA) gene family designated mClca3 was inducible with mucous cell metaplasia but not airway hyperreactivity. In confirmation of this finding, we found that mClca3 gene transfer to mouse airway epithelium was sufficient to induce mucous cell metaplasia but not airway hyperreactivity. However, newly developed mClca3(-/-) mice exhibited the same degree of mucous cell metaplasia and airway hyperreactivity as wild-type mice. Bioinformatic analysis of the Clca locus led to the identification of mClca5, and gene transfer indicated that mClca5 also selectively drives mucous cell metaplasia. Thus, in addition to the capacity of CLCA family members to exhibit diverse functional activities, there is also preserved function so that more than one family member mediates mucous cell metaplasia. Nonetheless, Clca expression appears to be a selective determinant of mucous cell metaplasia so that shared homologies between CLCA family members may still represent a useful target for focused therapeutic intervention in hypersecretory airway disease.

Morituri te salutant? Olfactory signal transduction and the role of phosphoinositides

During the past 150 years, researchers have investigated the cellular, physiological, and molecular mechanisms underlying the sense of smell. Based on these efforts, a conclusive model of olfactory signal transduction in the vertebrate's nose is now available, spanning from G-protein-mediated odorant receptors to ion channels, which are linked by a cyclic adenosine 3',5'-monophosphate-mediated signal transduction cascade. Here we review some historical milestones in the chronology of olfactory research, particularly emphasising the role of cyclic nucleotides and inositol trisphosphate as alternative second messengers in olfactory cells. We will describe the functional anatomy of the nose, outline the cellular composition of the olfactory epithelium, and describe the discovery of the molecular backbone of the olfactory signal transduction cascade. We then summarize our current model, in which cyclic adenosine monophosphate is the sole excitatory second messenger in olfactory sensory neurons. Finally, a possible significance of microvillous olfactory epithelial cells and inositol trisphosphate in olfaction will be discussed.

Membrane electrical activity elicits inositol 1,4,5-trisphosphate-dependent slow Ca2+ signals through a Gbetagamma/phosphatidylinositol 3-kinase gamma pathway in skeletal myotubes

Tetanic electrical stimulation of myotubes evokes a ryanodine receptor-related fast calcium signal, during the stimulation, followed by a phospholipase C/inositol 1,4,5-trisphosphate-dependent slow calcium signal few seconds after stimulus end. L-type calcium channels (Cav 1.1, dihydropyridine receptors) acting as voltage sensors activate an unknown signaling pathway involved in phospholipase C activation. We demonstrated that both G protein and phosphatidylinositol 3-kinase were activated by electrical stimulation, and both the inositol 1,4,5-trisphosphate rise and slow calcium signal induced by electrical stimulation were blocked by pertussis toxin, by a Gbetagamma scavenger peptide, and by phosphatidylinositol 3-kinase inhibitors. Immunofluorescence using anti-phosphatidylinositol 3-kinase gamma antibodies showed a clear location in striations within the cytoplasm, consistent with a position near the I band region of the sarcomere. The time course of phosphatidylinositol 3-kinase activation, monitored in single living cells using a pleckstrin homology domain fused to green fluorescent protein, was compatible with sequential phospholipase Cgamma1 activation as confirmed by phosphorylation assays for the enzyme. Co-transfection of a dominant negative form of phosphatidylinositol 3-kinase gamma inhibited the phosphatidylinositol 3-kinase activity as well as the slow calcium signal. We conclude that Gbetagamma/phosphatidylinositol 3-kinase gamma signaling pathway is involved in phospholipase C activation and the generation of the slow calcium signal induced by tetanic stimulation. We postulate that membrane potential fluctuations in skeletal muscle cells can activate a pertussis toxin-sensitive G protein, phosphatidylinositol 3-kinase, phospholipase C pathway toward modulation of long term, activity-dependent plastic changes.

Transient outward potassium current and Ca2+ homeostasis in the heart: beyond the action potential

The present review deals with Ca2+-independent, K+-carried transient outward current (Ito), an important determinant of the early repolarization phase of the myocardial action potential. The density of total Ito and of its fast and slow components (I(to,f) and I(to,s), respectively), as well as the expression of their molecular correlates (pore-forming protein isoforms Kv4.3/4.2 and Kv1.4, respectively), vary during postnatal development and aging across species and regions of the heart. Changes in Ito may also occur in disease conditions, which may affect the profile of cardiac repolarization and vulnerability to arrhythmias, and also influence excitation-contraction coupling. Decreased Ito density, observed in immature and aging myocardium, as well as during several types of cardiomyopathy and heart failure, may be associated with action potential prolongation, which favors Ca2+ influx during membrane depolarization and limits voltage-dependent Ca2+ efflux via the Na+/Ca2+ exchanger. Both effects contribute to increasing sarcoplasmic reticulum (SR) Ca2+ content (the main source of contraction-activating Ca2+ in mammalian myocardium), which, in addition to the increased Ca2+ influx, should enhance the amount of Ca2+ released by the SR during systole. This change usually takes place under conditions in which SR function is depressed, and may be adaptive since it provides partial compensation for SR deficiency, although possibly at the cost of asynchronous SR Ca2+ release and greater propensity to triggered arrhythmias. Thus, Ito modulation appears to be an additional mechanism by which excitation-contraction coupling in myocardial cells is indirectly regulated.

Calcium-activated chloride channels in müller cells acutely isolated from tiger salamander retina

Ca(2+)-activated chloride channels were identified with whole-cell patch-clamp recording techniques in salamander retinal Müller cells. Cl(Ca) channels were activated by membrane depolarizations that elicited Ca2+ influx or the application of the Ca2+ ionophore, ionomycin. The Ca channel blocker, Cd2+, abolished the Cl(Ca) channel tail currents. Increasing the duration of the depolarizing pulse resulted in enhancement of the Cl(Ca) channel tail current. Repetitive depolarizations with rapid pulses to +20 mV produced a buildup of I(Cl(Ca)), which reversed at 0 mV in symmetrical [Cl-] and at -40 mV when intracellular [Cl-] was reduced to 10% of the external concentration. I(Cl(Ca)) was blocked by the Cl channel blocker niflumic acid, while niflumic acid had no effect on voltage-gated Ca channels. These results offer the first demonstration of Cl(Ca) channels in a nonastrocytic glial cell and expand our understanding of the functional capacities of retinal glial cells.