Ca(2+)-activated chloride channels go molecular

Calcium-Activated Chloride Channels in Myometrial and Vascular Smooth Muscle

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

The mechanism of ions in pulmonary hypertension

Pulmonary hypertension（PH）is a kind of hemodynamic and pathophysiological state, in which the pulmonary artery pressure (PAP) rises above a certain threshold. The main pathological manifestation is pulmonary vasoconstriction and remodelling progressively. More and more studies have found that ions play a major role in the pathogenesis of PH. Many vasoactive substances, inflammatory mediators, transcription-inducing factors, apoptosis mediators, redox substances and translation modifiers can control the concentration of ions inside and outside the cell by regulating the activity of ion channels, which can regulate vascular contraction, cell proliferation, migration, apoptosis, inflammation and other functions. We all know that there are no effective drugs to treat PH. Ions are involved in the occurrence and development of PH, so it is necessary to clarify the mechanism of ions in PH as a therapeutic target for PH. The main ions involved in PH are calcium ion (Ca2+), potassium ion (K+), sodium ion (Na+) and chloride ion (Cl-). Here, we mainly discuss the distribution of these ions and their channels in pulmonary arteries and their role in the pathogenesis of PH.

Increased intracellular Cl- concentration improves airway epithelial migration by activating the RhoA/ROCK Pathway

In the airway, Cl- is the most abundant anion and is critically involved in transepithelial transport. The correlation of the abnormal expression and activation of chloride channels (CLCs), such as cystic fibrosis transmembrane conductance regulators (CFTRs), anoctamin-1, and CLC-2, with cell migration capability suggests a relationship between defective Cl- transport and epithelial wound repair. However, whether a correlation exists between intracellular Cl- and airway wound repair capability has not been explored thus far, and the underlying mechanisms involved in this relationship are not fully defined. Methods: In this work, the alteration of intracellular chloride concentration ([Cl-]i) was measured by using a chloride-sensitive fluorescent probe (N-[ethoxycarbonylmethyl]-6-methoxyquinolium bromide). Results: We found that clamping with high [Cl-]i and 1 h of treatment with the CLC inhibitor CFTR blocker CFTRinh-172 and chloride intracellular channel inhibitor IAA94 increased intracellular Cl- concentration ([Cl-]i) in airway epithelial cells. This effect improved epithelial cell migration. In addition, increased [Cl-]i in cells promoted F-actin reorganization, decreased cell stiffness, and improved RhoA activation and LIMK1/2 phosphorylation. Treatment with the ROCK inhibitor of Y-27632 and ROCK1 siRNA significantly attenuated the effects of increased [Cl-]i on LIMK1/2 activation and cell migration. In addition, intracellular Ca2+ concentration was unaffected by [Cl-]i clamping buffers and CFTRinh-172 and IAA94. Conclusion: Taken together, these results suggested that Cl- accumulation in airway epithelial cells could activate the RhoA/ROCK/LIMK cascade to induce F-actin reorganization, down-regulate cell stiffness, and improve epithelial migration.

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

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

TMEM16A knockdown abrogates two different Ca(2+)-activated Cl (-) currents and contractility of smooth muscle in rat mesenteric small arteries

The presence of Ca²⁺-activated Cl⁻ channels (CaCCs) in vascular smooth muscle cells (SMCs) is well established. Their molecular identity is, however, elusive. Two distinct Ca²⁺-activated Cl⁻ currents (I_Cl(Ca)) were previously characterized in SMCs. We have shown that the cGMP-dependent I_Cl(Ca) depends on bestrophin expression, while the “classical” I_Cl(Ca) is not. Downregulation of bestrophins did not affect arterial contraction but inhibited the rhythmic contractions, vasomotion. In this study, we have used in vivo siRNA transfection of rat mesenteric small arteries to investigate the role of a putative CaCC, TMEM16A. Isometric force, [Ca²⁺]_i, and SMC membrane potential were measured in isolated arterial segments. I_Cl(Ca) and GTPγS-induced nonselective cation current were measured in isolated SMCs. Downregulation of TMEM16A resulted in inhibition of both the cGMP-dependent I_Cl(Ca) and the “classical” I_Cl(Ca) in SMCs. TMEM16A downregulation also reduced expression of bestrophins. TMEM16A downregulation suppressed vasomotion both in vivo and in vitro. Downregulation of TMEM16A reduced agonist (noradrenaline and vasopressin) and K⁺-induced contractions. In accordance with the depolarizing role of CaCCs, TMEM16A downregulation suppressed agonist-induced depolarization and elevation in [Ca²⁺]_i. Surprisingly, K⁺-induced depolarization was unchanged but Ca²⁺ entry was reduced. We suggested that this is due to reduced expression of the L-type Ca²⁺ channels, as observed at the mRNA level. Thus, the importance of TMEM16A for contraction is, at least in part, independent from membrane potential. This study demonstrates the significance of TMEM16A for two SMCs I_Cl(Ca) and vascular function and suggests an interaction between TMEM16A and L-type Ca²⁺ channels.

Disease-associated missense mutations in bestrophin-1 affect cellular trafficking and anion conductance

Bestrophin-1, an integral membrane protein encoded by the BEST1 gene, is localized predominantly to the basolateral membrane of the retinal pigment epithelium. Mutations in the BEST1 gene have been associated with Best vitelliforme macular dystrophy (BMD), a central retinopathy with autosomal dominant inheritance and variable penetrance. Over 120 disease-causing mutations are known, the majority of which result in amino acid substitutions within four mutational hot-spot regions in the highly conserved N-terminal half of the protein. Although initially thought to impair Cl⁻ channel function, the molecular pathology of BEST1 mutations is still controversial. We have analyzed the subcellular localization of 13 disease-associated BEST1 mutant proteins in polarized MDCK II cells, an established model of apical to basolateral protein sorting. Immunostaining demonstrated that nine of the 13 mutant proteins failed to integrate into the cell membrane. The defective proteins were predominantly retained in the cytoplasm, whereas wild-type bestrophin-1 revealed cell membrane localization. Functional analysis of I⁻ fluxes in HEK-293 cells showed that all mutants exhibited a significant reduction in anion conductance. Our data indicate that defective intracellular trafficking could be a common cause of BMD accompanied by impaired anion conductance, representing a loss of anion channel function that is probably due to mistargeting of mutant protein.

Course of visual decline in relation to the Best1 genotype in vitelliform macular dystrophy

PURPOSE

To describe the disease course in patients with vitelliform macular dystrophy (VMD) with a Best1 mutation and to determine the association between Best1 genotype and visual prognosis.

DESIGN

Consecutive case series.

PARTICIPANTS

Fifty-three patients with VMD with Best1 mutations from 27 Dutch families, aged 11 to 87 years.

METHODS

Best-corrected visual acuity (VA), fundus appearance, and Arden ratio on the electro-oculogram (EOG) during clinical follow-up were assessed from medical records. Mutation analysis of the Best1 gene was performed on DNA samples using denaturing high-pressure liquid chromatography and direct sequencing.

MAIN OUTCOME MEASURES

Cumulative lifetime risk of visual decline below 0.5, 0.3, and 0.1 for the entire group and stratified for genotype.

RESULTS

Median age of onset of visual symptoms was 33 years (range: 2-78). The cumulative risk of VA below 0.5 (20/40) was 50% at 55 years and 75% at 66 years. The cumulative risk of decline less than 0.3 (20/63) was 50% by age 66 years and 75% by age 74 years. Two patients progressed to VA less than 0.1 (20/200). Fourteen different mutations were found. Most patients (96%) had missense mutations; the Thr6Pro, Ala10Val, and Tyr227Asn mutations were most common. Visual decline was significantly faster in patients with an Ala10Val mutation than either the Thr6Pro or the Tyr227Asn mutation (P=0.001).

CONCLUSIONS

Age of onset of visual symptoms varies greatly among patients with VMD. All patients show a gradual decrease in VA, and most progress to visual impairment at a relatively late age. Our data suggest a phenotype-genotype correlation, because the Ala10Val mutation has a more rapid disease progression than other common mutations.

FINANCIAL DISCLOSURE(S)

The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Calcium-activated chloride currents in olfactory sensory neurons from mice lacking bestrophin-2

Olfactory sensory neurons use a chloride-based signal amplification mechanism to detect odorants. The binding of odorants to receptors in the cilia of olfactory sensory neurons activates a transduction cascade that involves the opening of cyclic nucleotide-gated channels and the entry of Ca(2+) into the cilia. Ca(2+) activates a Cl(-) current that produces an efflux of Cl(-) ions and amplifies the depolarization. The molecular identity of Ca(2+)-activated Cl(-) channels is still elusive, although some bestrophins have been shown to function as Ca(2+)-activated Cl(-) channels when expressed in heterologous systems. In the olfactory epithelium, bestrophin-2 (Best2) has been indicated as a candidate for being a molecular component of the olfactory Ca(2+)-activated Cl(-) channel. In this study, we have analysed mice lacking Best2. We compared the electrophysiological responses of the olfactory epithelium to odorant stimulation, as well as the properties of Ca(2+)-activated Cl(-) currents in wild-type (WT) and knockout (KO) mice for Best2. Our results confirm that Best2 is expressed in the cilia of olfactory sensory neurons, while odorant responses and Ca(2+)-activated Cl(-) currents were not significantly different between WT and KO mice. Thus, Best2 does not appear to be the main molecular component of the olfactory channel. Further studies are required to determine the function of Best2 in the cilia of olfactory sensory neurons.

An expanded biological repertoire for Ins(3,4,5,6)P4 through its modulation of ClC-3 function

Ins(3,4,5,6)P(4) inhibits plasma membrane Cl(-) flux in secretory epithelia [1]. However, in most other mammalian cells, receptor-dependent elevation of Ins(3,4,5,6)P(4) levels is an "orphan" response that lacks biological significance [2]. We set out to identify Cl(-) channel(s) and/or transporter(s) that are regulated by Ins(3,4,5,6)P4 in vivo. Several candidates [3-5] were excluded through biophysical criteria, electrophysiological analysis, and confocal immunofluorescence microscopy. Then, we heterologously expressed ClC-3 in the plasma membrane of HEK293-tsA201 cells; whole-cell patch-clamp analysis showed Ins(3,4,5,6)P4 to inhibit Cl(-) conductance through ClC-3. Next, we heterologously expressed ClC-3 in the early endosomal compartment of BHK cells; by fluorescence ratio imaging of endocytosed FITC-transferrin, we recorded intra-endosomal pH, an in situ biosensor for Cl(-) flux across endosomal membranes [6]. A cell-permeant, bioactivatable Ins(3,4,5,6)P4 analog elevated endosomal pH from 6.1 to 6.6, reflecting inhibition of ClC-3. Finally, Ins(3,4,5,6)P(4) inhibited endogenous ClC-3 conductance in postsynaptic membranes of neonatal hippocampal neurones. Among other ClC-3 functions that could be regulated by Ins(3,4,5,6)P4 are tumor cell migration [7], apoptosis [8], and inflammatory responses [9]. Ins(3,4,5,6)P4 is a ubiquitous cellular signal with diverse biological actions.

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 cloning of TMEM16A as a calcium-activated chloride channel subunit

Calcium-activated chloride channels (CaCCs) are major regulators of sensory transduction, epithelial secretion, and smooth muscle contraction. Other crucial roles of CaCCs include action potential generation in Characean algae and prevention of polyspermia in frog egg membrane. None of the known molecular candidates share properties characteristic of most CaCCs in native cells. Using Axolotl oocytes as an expression system, we have identified TMEM16A as the Xenopus oocyte CaCC. The TMEM16 family of "transmembrane proteins with unknown function" is conserved among eukaryotes, with family members linked to tracheomalacia (mouse TMEM16A), gnathodiaphyseal dysplasia (human TMEM16E), aberrant X segregation (a Drosophila TMEM16 family member), and increased sodium tolerance (yeast TMEM16). Moreover, mouse TMEM16A and TMEM16B yield CaCCs in Axolotl oocytes and mammalian HEK293 cells and recapitulate the broad CaCC expression. The identification of this new family of ion channels may help the development of CaCC modulators for treating diseases including hypertension and cystic fibrosis.

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.

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.

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.

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.

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 anion-selective pore of the bestrophins, a family of chloride channels associated with retinal degeneration

Mutations in human bestrophin-1 (VMD2) are genetically linked to a juvenile form of macular degeneration and autosomal dominant vitreoretinochoroidopathy. Recently, it has been proposed that bestrophins are Cl- channels and that the putative second transmembrane domain participates in forming the bestrophin pore. However, the structural determinants of Cl- ion permeation through the channel pore are not known. Here we systematically replaced every amino acid in mouse bestrophin-2 (mBest2) between positions 69 and 104 with cysteine. We then measured the effects on the relative permeability and conductance of the channel to Cl- and SCN- (thiocyanate) and determined the accessibility of the cysteine-substituted amino acids to extracellularly applied, membrane-impermeant sulfhydryl reagents. Unlike K+ channels, the amino acids forming the mBest2 selectivity filter are not discretely localized but are distributed over approximately 20 amino acids within the transmembrane domain. Cysteine-substituted amino acids in the selectivity filter are easily accessible to extracellularly applied sulfhydryl reagents and select for anionic sulfhydryl reagents over cationic ones. Understanding the structure of the anion conduction pathway of bestrophins provides insights into how mutations produce channel dysfunction and may provide important information for development of therapeutic strategies for treating macular degeneration.

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.

Looking chloride channels straight in the eye: bestrophins, lipofuscinosis, and retinal degeneration

Recent evidence suggests that Cl(-) ion channels are important for retinal integrity. Bestrophin Cl(-) channel mutations in humans are genetically linked to a juvenile form of macular degeneration, and disruption of some ClC Cl(-) channels in mice leads to retinal degeneration. In both cases, accumulation of lipofuscin pigment is a key feature of the cellular degeneration. Because Cl(-) channels regulate the ionic environment inside organelles in the endosomal-lysosomal pathway, retinal degeneration may result from defects in lysosomal trafficking or function.

Distribution of cGMP-dependent and cGMP-independent Ca(2+)-activated Cl(-) conductances in smooth muscle cells from different vascular beds and colon

In the present patch-clamp study we have, for the first time, shown the tissue distribution of a recently characterized cGMP-dependent Ca(2+)-activated Cl(-) conductance [18] in smooth muscle cells freshly isolated from different regions: aorta, pulmonary artery, tail artery, femoral artery, femoral vein, middle cerebral artery, renal artery, portal vein, superior mesenteric artery, mesenteric small artery and colon. The cGMP-dependent Cl(-) conductance has properties distinct from those of the 'classical' Ca(2+)-activated Cl(-) conductances; their different sensitivities to niflumic acid and zinc were here utilized to distinguish them. They were found to be co-expressed in different patterns in smooth muscle cells of different origins. The cGMP-dependent conductance was greater in myocytes from cerebral artery and femoral vein and was greater in the renal artery, aorta, mesenteric small artery, femoral artery and the superior mesenteric artery. The presence of the cGMP-dependent Ca(2+)-activated Cl(-) current in smooth muscle cells isolated from the colon demonstrates that this conductance is not limited to the vasculature. The 'classical' Ca(2+)-activated Cl(-) conductance was strongly expressed in smooth muscle cells from the portal vein and the tail artery, and noticeably higher in the pulmonary artery.

Calcium-activated chloride channels

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

[Function of bestrophin]

Clarification of the function of bestrophin, the gene product of VMD2, establishes a basis for the understanding of the pathomechanisms leading to Best's vitelliform macular degeneration. Studies of heterologously expressed bestrophin showed that bestrophin can function as a Cl(-) channel. All four known bestrophins were found to display Cl(-) channel activity. A loss in Cl(-) channel function would elegantly explain the development of the leading symptom for Best's disease, the reduction of the light peak amplitude in the patient's electro-oculogram. However, there are still gaps in the chain of evidence demonstrating that bestrophin is a Cl(-) channel, and this hypothesis is inconsistent with newly published follow-up observations. In an alternative hypothesis bestrophin appears as a regulator of voltage-dependent Ca(2+) channels assuming an indirect involvement of bestrophin in the generation of the light peak. Further studies on either bestrophin-deficient mice or transgenic mice will show that either one of the hypotheses is right or maybe both will be proven correct, showing bestrophin as a Cl(-) channel and Ca(2+) channel regulator.

Determinants of anion permeation in the second transmembrane domain of the mouse bestrophin-2 chloride channel

Bestrophins have been proposed to constitute a new family of Cl channels that are activated by cytosolic Ca. We showed previously that mutation of serine-79 to cysteine in mouse bestrophin-2 (mBest2) altered the relative permeability and conductance to SCN. In this paper, we have overexpressed various mutant constructs of mBest2 in HEK-293 cells to explore the contributions to anion selectivity of serine-79 and other amino acids (V78, F80, G83, F84, V86, and T87) located in the putative second transmembrane domain (TMD2). Residues selected for mutagenesis were distributed throughout TMD2, but mutations at all positions changed the selectivity. The effects on selectivity were rather modest. Replacement of residues 78, 79, 80, 83, 84, 86, or 87 with cysteine had similar effects: the permeability of the channel to SCN relative to Cl (P_SCN/P_Cl) was decreased three- to fourfold and the relative SCN conductance (G_SCN/G_Cl) was increased five- to tenfold. Side chains at positions 78 and 80 appeared to be situated close to the permeant anion, because the electrostatic charge at these positions affected permeation in specific ways. The effects of charged sulfhydryl-reactive MTS reagents were the opposite in the V78C and F80C mutants and the effects were partially mimicked by substitution of F80 with charged amino acids. In S79T, switching from Cl to SCN caused slow changes in G_SCN/G_Cl (τ = 16.6 s), suggesting that SCN binding to the channel altered channel gating as well as conductance. The data in this paper and other data support a model in which TMD2 plays an important role in forming the bestrophin pore. We suggest that the major determinant in anion permeation involves partitioning of the permeant anion into an aqueous pore whose structural features are rather flexible. Furthermore, anion permeation and gating may be linked.