Expression of the chloride channel ClC-2 in the murine small intestine epithelium

Murine mCLCA6 is an integral apical membrane protein of non-goblet cell enterocytes and co-localizes with the cystic fibrosis transmembrane conductance regulator

The CLCA family of proteins consists of a growing number of structurally and functionally diverse members with distinct expression patterns in different tissues. Several CLCA homologs have been implicated in diseases with secretory dysfunctions in the respiratory and intestinal tracts. Here we present biochemical protein characterization and details on the cellular and subcellular expression pattern of the murine mCLCA6 using specific antibodies directed against the amino- and carboxy-terminal cleavage products of mCLCA6. Computational and biochemical characterizations revealed protein processing and structural elements shared with hCLCA2 including anchorage in the apical cell membrane by a transmembrane domain in the carboxy-terminal subunit. A systematic light- and electron-microscopic immunolocalization found mCLCA6 to be associated with the microvilli of non-goblet cell enterocytes in the murine small and large intestine but in no other tissues. The expression pattern was confirmed by quantitative RT-PCR following laser-capture microdissection of relevant tissues. Confocal laser scanning microscopy colocalized the mCLCA6 protein with the cystic fibrosis transmembrane conductance regulator CFTR at the apical surface of colonic crypt cells. Together with previously published functional data, the results support a direct or indirect role of mCLCA6 in transepithelial anion conductance in the mouse intestine.

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.

Interactions of tight junctions with membrane channels and transporters

Tight junctions are unique organelles in epithelial cells. They are localized to the apico-lateral region and essential for the epithelial cell transport functions. The paracellular transport process that occurs via tight junctions is extensively studied and is intricately regulated by various extracellular and intracellular signals. Fine regulation of this transport pathway is crucial for normal epithelial cell functions. Among factors that control tight junction permeability are ions and their transporters. However, this area of research is still in its infancy and much more needs to be learned about how these molecules regulate tight junction structure and functions. In this review we have attempted to compile literature on ion transporters and channels involved in the regulation of tight junctions.

From mitochondria to disease: role of the renin-angiotensin system

Mitochondria are energy-producing organelles that conduct other key cellular tasks. Thus, mitochondrial damage may impair various aspects of tissue functioning. Mitochondria generate oxygen- and nitrogen-derived oxidants, being themselves major oxidation targets. Dysfunctional mitochondria seem to contribute to the pathophysiology of hypertension, cardiac failure, the metabolic syndrome, obesity, diabetes mellitus, renal disease, atherosclerosis, and aging. Mitochondrial proteins and metabolic intermediates participate in various cellular processes, apart from their well-known roles in energy metabolism. This emphasizes the participation of dysfunctional mitochondria in disease, notwithstanding that most evidences supporting this concept come from animal and cultured-cell studies. Mitochondrial oxidant production is altered by several factors related to vascular pathophysiology. Among these, angiotensin-II stimulates mitochondrial oxidant release leading to energy metabolism depression. By lowering mitochondrial oxidant production, angiotensin-II inhibition enhances energy production and protects mitochondrial structure. This seems to be one of the mechanisms underlying the benefits of angiotensin-II inhibition in hypertension, diabetes, and aging rodent models. If some of these findings can be reproduced in humans, they would provide a new perspective on the implications that RAS-blockade can offer as a therapeutic strategy. This review intends to present available information pointing to mitochondria as targets for therapeutic Ang-II blockade in human renal and CV disease.

Basolateral Cl channels in primary airway epithelial cultures

Salt and water absorption and secretion across the airway epithelium are important for maintaining the thin film of liquid lining the surface of the airway epithelium. Movement of Cl across the apical membrane involves the CFTR Cl channel; however, conductive pathways for Cl movement across the basolateral membrane have been little studied. Here, we determined the regulation and single-channel properties of the Cl conductance ( GCl) in airway surface epithelia using epithelial cultures from human or bovine trachea and freshly isolated ciliated cells from the human nasal epithelium. In Ussing chamber studies, a swelling-activated basolateral GCl was found, which was further stimulated by forskolin and blocked by N-phenylanthranilic acid (DPC) = sucrose > flufenamic acid = niflumic acid = glibenclamide > CdCl2 = 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) = DIDS = ZnCl2 > tamoxifen > 4,4′-dinitro-2,2′-stilbene-disulfonate disodium salt (DNDS). In whole cell patch-clamp experiments, three types of GCl were identified: 1) a voltage-activated, DIDS- (but not Cd-) blockable and osmosensitive GCl; 2) an inwardly rectifying, hyperpolarization-activated and Cd-sensitive GCl; and 3) a forskolin-activated, linear GCl, which was insensitive to Cd and DIDS. In cell-attached patch-clamp recordings, the basolateral pole of isolated ciliated cells expressed three types of Cl channels: 1) an outwardly rectifying, swelling-activated Cl channel; 2) a strongly inwardly rectifying Cl channel; and 3) a forskolin-activated, low-conductance channel. We propose that, depending on the driving force for Cl across the apical membrane, basolateral Cl channels confine Cl− secretion or support transcellular Cl− absorption.

Restoration of barrier function in injured intestinal mucosa

Mucosal repair is a complex event that immediately follows acute injury induced by ischemia and noxious luminal contents such as bile. In the small intestine, villous contraction is the initial phase of repair and is initiated by myofibroblasts that reside immediately beneath the epithelial basement membrane. Subsequent events include crawling of healthy epithelium adjacent to the wound, referred to as restitution. This is a highly regulated event involving signaling via basement membrane integrins by molecules such as focal adhesion kinase and growth factors. Interestingly, however, ex vivo studies of mammalian small intestine have revealed the importance of closure of the interepithelial tight junctions and the paracellular space. The critical role of tight junction closure is underscored by the prominent contribution of the paracellular space to measures of barrier function such as transepithelial electrical resistance. Additional roles are played by subepithelial cell populations, including neutrophils, related to their role in innate immunity. The net result of reparative mechanisms is remarkably rapid closure of mucosal wounds in mammalian tissues to prevent the onset of sepsis.

Regulation of basolateral Cl(-) channels in airway epithelial cells: the role of nitric oxide

The presence of basolateral Cl(-) channels in airway epithelium has been reported in several studies, but little is known about their role in the regulation of anion secretion. The purpose of this study was to characterize regulation of these channels by nitric oxide (NO) in Calu-3 cells. Transepithelial measurements revealed that NO donors activated a basolateral Cl(-) conductance sensitive to 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and anthracene-9-carboxylic acid. Apical membrane permeabilization studies confirmed the basolateral localization of NO-activated Cl(-) channels. Experiments using 8-bromo cyclic guanosine monophosphate (8Br-cGMP) and selective inhibitors of soluble guanylyl cyclase and inducible NO synthase (1H-[1, 2, 4] oxadiazolol-[4, 3-a] quinoxalin-1-one [ODQ] and 1400W [N-(3-Aminomethyl)benzyl)acetamidine], respectively) demonstrated that NO activated Cl(-) channels via a cGMP-dependent pathway. Anion replacement and (36)Cl(-) flux studies showed that NO affected both Cl(-) and HCO (3) (-) secretion. Two different types of Cl(-) channels are known to be present in the basolateral membrane of epithelial cells: Zn(2+)-sensitive ClC-2 and DIDS-sensitive bestrophin channels. S-Nitrosoglutathione (GSNO) activated Cl(-) conductance in the presence of Zn(2+) ions, indicating that ClC-2 channel function was not affected by GSNO. In contrast, DIDS completely inhibited GSNO-activated Cl(-) conductance. Bestrophin immunoprecipitation studies showed that under control conditions bestrophin channels were not phosphorylated but became phosphorylated after GSNO treatment. The presence of bestrophin in airway epithelia was confirmed using immunohistochemistry. We conclude that basolateral Cl(-) channels play a major role in the NO-dependent regulation of anion secretion in Calu-3 cells.

Lubiprostone: a chloride channel activator

In January 2006 the Food and Drug Administration approved lubiprostone for the treatment of chronic constipation in men and women aged 18 and over. Lubiprostone is categorized as a prostone, a bicyclic fatty acid metabolite of prostaglandin E1. Lubiprostone activates a specific chloride channel (ClC-2) in the gastrointestinal (GI) tract to enhance intestinal fluid secretion, which increases GI transit and improves symptoms of constipation. This article reviews the role of chloride channels in the GI tract, describes the structure, function, and pharmacokinetics of lubiprostone, and discusses clinically important data on this new medication.

Alpha1-adrenoceptors down-regulate ClC-2 chloride channels in epithelial cells from the acutely denervated jejunum

Acute sympathetic denervation of the small intestine up-regulates alpha1-adrenoceptors on villus enterocytes and activation of these alpha1-adrenoceptors inhibits chloride secretion. We tested whether alpha1-adrenoceptor-mediated inhibition of chloride secretion was the result of reduced ClC-2 chloride channel expression. Phorbol myristate acetate (PMA) (a protein kinase C (PKC) activator) had no effect on ClC-2 levels. In contrast, alpha1-adrenoceptor activation significantly decreased ClC-2 protein levels in both the villus (1.58+/-0.19 to 0.75+/-0.19 arbitrary units) and crypt (1.69+/-0.15 to 0.37+/-0.23 arbitrary units) epithelial cells from the acutely denervated jejunum but not innervated controls. These data suggest that inhibition of chloride secretion following alpha1-adrenoceptor activation in the acutely denervated small intestine may be through ClC-2 down-regulation.

Epidermal growth factor stimulates chloride transport in primary cultures of weanling and adult rabbit colonocytes

OBJECTIVES

We have shown that Ca2+-dependent regulation of Cl- secretion in the mammalian colon exhibits age dependence. Because epidermal growth factor (EGF) has a well-established role in growth and can increase intracellular calcium [Ca2+]i, it is conceivable that its developmental influence may extend to the regulation of intestinal ion transport. In this study, we examined the role of EGF in the regulation of Cl- transport in the developing rabbit distal colon.

MATERIALS AND METHODS

Because serum contains growth factors, which could have confounded our studies, we first established an optimal milieu for testing EGF in primary cultures of adult rabbit distal colonocytes by culturing them for 24 h in media containing 0%, 1%, 5%, and 20% serum. Chloride transport (millimoles per second) and [Ca2+]i were measured with use of the fluorescent indicator N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide (MQAE) and Fura-2AM, respectively.

RESULTS

Serum depletion had no effect on cell number, DNA content, or basal Cl- transport, but it significantly affected cell viability. In media with 0%, 1%, or 20% serum, bethanechol, 8BrcAMP, taurodeoxycholate, and EGF stimulated Cl- transport to a similar extent. EGF maximally stimulated Cl- transport at 16.3 nmol/L and 20 minutes. Bethanechol, but not EGF, increased [Ca2+]i. EGF did not alter bethanechol-stimulated Cl- transport or [Ca2+]i. EGF acts via an EGF-receptor and mitogen activated protein kinase (MAPK) signaling pathway, since stimulation of Cl- transport was abolished by genistein, AG1478, and PD98059. Weanling and adult colonocytes, cultured in 1% serum, showed similar basal and EGF-stimulated Cl- transport.

CONCLUSIONS

EGF stimulates rabbit colonic Cl- transport via a Ca2+-independent, tyrosine kinase- and MAPK-dependent pathway, and its effects are not age dependent.

Recovery of mucosal barrier function in ischemic porcine ileum and colon is stimulated by a novel agonist of the ClC-2 chloride channel, lubiprostone

Previous studies utilizing an ex vivo porcine model of intestinal ischemic injury demonstrated that prostaglandin (PG)E(2) stimulates repair of mucosal barrier function via a mechanism involving Cl(-) secretion and reductions in paracellular permeability. Further experiments revealed that the signaling mechanism for PGE(2)-induced mucosal recovery was mediated via type-2 Cl(-) channels (ClC-2). Therefore, the objective of the present study was to directly investigate the role of ClC-2 in mucosal repair by evaluating mucosal recovery in ischemia-injured intestinal mucosa treated with the selective ClC-2 agonist lubiprostone. Ischemia-injured porcine ileal mucosa was mounted in Ussing chambers, and short-circuit current (I(sc)) and transepithelial electrical resistance (TER) were measured in response to lubiprostone. Application of 0.01-1 microM lubiprostone to ischemia-injured mucosa induced concentration-dependent increases in TER, with 1 microM lubiprostone stimulating a twofold increase in TER (DeltaTER = 26 Omega.cm(2); P < 0.01). However, lubiprostone (1 microM) stimulated higher elevations in TER despite lower I(sc) responses compared with the nonselective secretory agonist PGE(2) (1 microM). Furthermore, lubiprostone significantly (P < 0.05) reduced mucosal-to-serosal fluxes of (3)H-labeled mannitol to levels comparable to those of normal control tissues and restored occludin localization to tight junctions. Activation of ClC-2 with the selective agonist lubiprostone stimulated elevations in TER and reductions in mannitol flux in ischemia-injured intestine associated with structural changes in tight junctions. Prostones such as lubiprostone may provide a selective and novel pharmacological mechanism of accelerating recovery of acutely injured intestine compared with the nonselective action of prostaglandins such as PGE(2).

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.

Acute denervation alters the epithelial response to adrenoceptor activation through an increase in alpha1-adrenoceptor expression on villus enterocytes

Loss of sympathetic input due to intestinal denervation results in hypersensitivity and increased intestinal secretion. It is unknown whether denervation-induced alterations in intestinal epithelial physiology are the result of changes in adrenoceptors on enterocytes (ENTs). The purpose of this study was to examine adrenoceptor distribution and pharmacology on small intestinal ENTs following acute intestinal denervation. Lewis rats underwent small bowel transplantation (SBT) or sham operation and proximal small intestinal segments were harvested 1, 2 and 4 weeks postoperatively. Intestinal electrolyte movement was assessed using short-circuit current (Isc) measurements of stripped epithelial sheets following stimulation with phenylephrine (PE), an alpha(1)-adrenoceptor agonist. The presence of adrenoceptor subtypes on separated villus and crypt ENTs was assessed using flow cytometry. Alpha(1)-adrenoceptors were found on approximately 27% of jejunal villus ENTs, but not crypt ENTs, following acute extrinsic denervation. ENTs from the Lewis rat have few beta-adrenoceptors. Alpha(1)-adrenoceptor stimulation of acutely denervated intestinal epithelial sheets decreased Isc by -13.45%. This effect was mediated by a reduction in chloride (Cl(-)) secretion; the absence of Cl(-) reversed the Isc to +13.79%. In conclusion, loss of sympathetic innervation to the gastrointestinal epithelium causes acute upregulation of alpha(1)-adrenoceptors on villus ENTs, leading to inhibition of Cl(-) secretion at the villus tip. The increase in adrenoceptors may reflect a compensatory mechanism to combat the increased secretory state of the bowel due to the loss of the sympathetic innervation and tonic control over intestinal secretion.

Expression of the chloride channel CLC-K in human airway epithelial cells

Airway submucosal gland function is severely disrupted in cystic fibrosis (CF), as a result of genetic mutation of the cystic fibrosis transmembrane conductance regulator (CFTR), an apical membrane Cl(-) channel. To identify other Cl(-) channel types that could potentially substitute for lost CFTR function in these cells, we investigated the functional and molecular expression of Cl(-) channels in Calu-3 cells, a human cell line model of the submucosal gland serous cell. Whole cell patch clamp recording from these cells identified outwardly rectified, pH- and calcium-sensitive Cl(-) currents that resemble those previously ascribed to ClC-K type chloride channels. Using reverse transcription polymerase chain reaction, we identified expression of mRNA for ClC-2, ClC-3, ClC-4, ClC-5, ClC-6, ClC-7, ClC-Ka, and ClC-Kb, as well as the common ClC-K channel beta subunit barttin. Western blotting confirmed that Calu-3 cells express both ClC-K and barttin protein. Thus, Calu-3 cells express multiple members of the ClC family of Cl(-) channels that, if also expressed in native submucosal gland serous cells within the CF lung, could perhaps act to partially substitute lost CFTR function. Furthermore, this work represents the first evidence for functional ClC-K chloride channel expression within the lung.

Effect of a selective chloride channel activator, lubiprostone, on gastrointestinal transit, gastric sensory, and motor functions in healthy volunteers

Chloride channels modulate gastrointestinal neuromuscular functions in vitro. Lubiprostone, a selective type 2 chloride channel (ClC-2) activator, induces intestinal secretion and has been shown to relieve constipation in clinical trials; however, the effects of lubiprostone on gastric function and whole gut transit in humans are unclear. Our aim was to compare the effects of the selective ClC-2 activator lubiprostone on maximum tolerated volume (MTV) of a meal, postprandial symptoms, gastric volumes, and gastrointestinal and colonic transit in humans. We performed a randomized, parallel-group, double-blind, placebo-controlled study evaluating the effects of lubiprostone (24 microg bid) in 30 healthy volunteers. Validated methods were used: scintigraphic gastrointestinal and colonic transit, SPECT to measure gastric volumes, and the nutrient drink ("satiation") test to measure MTV and postprandial symptoms. Lubiprostone accelerated small bowel and colonic transit, increased fasting gastric volume, and retarded gastric emptying. MTV values were reduced compared with placebo; however, the MTV was within the normal range for healthy adults in 13 of 14 participants, and there was no significant change compared with baseline measurements. Lubiprostone had no significant effect on postprandial gastric volume or aggregate symptoms but did decrease fullness 30 min after the fully satiating meal. Thus the ClC-2 activator lubiprostone accelerates small intestinal and colonic transit, which confers potential in the treatment of constipation.

The functioning of mammalian ClC-2 chloride channel in Saccharomyces cerevisiae cells requires an increased level of Kha1p

The mammalian chloride channel ClC-2 is a member of the CLC voltage-gated chloride channels family. This broadly expressed protein shows diverse cellular locations and despite numerous studies, its precise function is poorly understood. Disruption of ClC-2-encoding gene in mouse leads to retinal and testicular degeneration and mutations in CLC2 (gene encoding the ClC-2 channel) are associated with idiopathic generalized epilepsies. ClC-2 may also be responsible for Cl- transport in mouse salivary glands. The only CLC homologue of the yeast Saccharomyces cerevisiae, Gef1p, exhibits CLC activity. We expressed the mammalian ClC-2 protein in S. cerevisiae devoid of Gef1p in an attempt to identify yeast proteins influencing the functioning of ClC-2. The presence of such proteins in yeast could indicate the existence of their homologues in mammalian cells and would greatly aid their identification. Expression of ClC-2 in yeast required optimization of the sequence context of the AUG translation initiation codon. After obtaining an efficient translation, we found that rat ClC-2 cannot directly substitute for yeast Gef1p. Functional substitution for Gef1p was, however, achieved in the presence of an increased level of intact or C-terminally truncated yeast Kha1 protein. Based on the deduced amino acid sequence, the Kha1 protein can be classified as a Na+/H+ transporter since it has a large N-terminal domain similar to the family of NHEs (Na+/H+ exchangers). This suggests that the Kha1p may take part in the regulation of intracellular cation homoeostasis and pH control. We have established that Kha1p is localized in the same cellular compartment as Gef1p and yeast-expressed ClC-2: the Golgi apparatus. We propose that Kha1p may aid ClC-2-dependent suppression of the Deltagef1-associated growth defects by keeping the Golgi apparatus pH in a range suitable for ClC-2 activity. The approach employed in the present study may be of general applicability to the characterization of poorly understood proteins by their functional expression in yeast.

Association between Hsp90 and the ClC-2 chloride channel upregulates channel function

The voltage-dependent ClC-2 chloride channel has been implicated in a variety of physiological functions, including fluid transport across specific epithelia. ClC-2 is activated by hyperpolarization, weakly acidic external pH, intracellular Cl−, and cell swelling. To add more insight into the mechanisms involved in ClC-2 regulation, we searched for associated proteins that may influence ClC-2 activity. With the use of immunoprecipitation of ClC-2 from human embryonic kidney-293 cells stably expressing the channel, followed by electrophoretic separation of coimmunoprecipitated proteins and mass spectrometry identification, Hsp70 and Hsp90 were unmasked as possible ClC-2 interacting partners. Association of Hsp90 with ClC-2 was confirmed in mouse brain. Inhibition of Hsp90 by two specific inhibitors, geldanamycin or radicicol, did not affect total amounts of ClC-2 but did reduce plasma membrane channel abundance. Functional experiments using the whole cell configuration of the patch-clamp technique showed that inhibition of Hsp90 reduced ClC-2 current amplitude and impaired the intracellular Cl− concentration [Cl−]-dependent rightward shift of the fractional conductance. Geldanamycin and radicicol increased both the slow and fast activation time constants in a chloride-dependent manner. Heat shock treatment had the opposite effect. These results indicate that association of Hsp90 with ClC-2 results in greater channel activity due to increased cell surface channel expression, facilitation of channel opening, and enhanced channel sensitivity to intracellular [Cl−]. This association may have important pathophysiological consequences, enabling increased ClC-2 activity in response to cellular stresses such as elevated temperature, ischemia, or oxidative reagents.

Inhibition of ClC-2 chloride channels by a peptide component or components of scorpion venom

ClC chloride channels play essential roles in membrane excitability and maintenance of osmotic balance. Despite the recent crystallization of two bacterial ClC-like proteins, the gating mechanism for these channels remains unclear. In this study we tested scorpion venom for the presence of novel peptide inhibitors of ClC channels, which might be useful tools for dissecting the mechanisms underlying ClC channel gating. Recently, it has been shown that a peptide component of venom from the scorpion L. quinquestriatus hebraeus inhibits the CFTR chloride channel from the intracellular side. Using two-electrode voltage clamp we studied the effect of scorpion venom on ClC-0, -1, and -2, and found both dose- and voltage-dependent inhibition only of ClC-2. Comparison of voltage-dependence of inhibition by venom to that of known pore blockers revealed opposite voltage dependencies, suggesting different mechanisms of inhibition. Kinetic data show that venom induced slower activation kinetics compared to pre-venom records, suggesting that the active component(s) of venom may function as a gating modifier at ClC-2. Trypsinization abolished the inhibitory activity of venom, suggesting that the component(s) of scorpion venom that inhibits ClC-2 is a peptide.

Chloride channel diseases resulting from impaired transepithelial transport or vesicular function

The transport of anions across cellular membranes is crucial for various functions, including the control of electrical excitability of muscle and nerve, transport of salt and water across epithelia, and the regulation of cell volume or the acidification and ionic homeostasis of intracellular organelles. Given this broad range of functions, it is perhaps not surprising that mutations in Cl- channels lead to a large spectrum of diseases. These diverse pathologies include the muscle disorder myotonia, cystic fibrosis, renal salt loss in Bartter syndrome, kidney stones, deafness, and the bone disease osteopetrosis. This review will focus on diseases related to transepithelial transport and on disorders involving vesicular Cl- channels.

Novel, mechanism-based therapies for cystic fibrosis

PURPOSE OF REVIEW

Cystic fibrosis results from disruption of the biosynthesis or function of the cystic fibrosis transmembrane conductance regulator. Cystic fibrosis transmembrane conductance regulator plays a critical role in the regulation of epithelial ion transport. Restoration of cystic fibrosis transmembrane conductance regulator function should improve the cystic fibrosis phenotype.

RECENT FINDINGS

Recent investigations affording a better understanding of the mechanism of dysfunction of mutant cystic fibrosis transmembrane conductance regulators, as well as the roles of cystic fibrosis transmembrane conductance regulator in regulating epithelial ion transport, have led to development of therapeutic strategies based on repair or bypass of mutant cystic fibrosis transmembrane conductance regulator dysfunction. The former strategy, coined 'protein repair therapy,' is aimed at improving or restoring the function of mutant cystic fibrosis transmembrane conductance regulators, whereas the latter approach aims to augment epithelial ion transport to compensate for the absent function mutant cystic fibrosis transmembrane conductance regulator.

SUMMARY

Strategies to improve mutant cystic fibrosis transmembrane conductance regulator function or to bypass mutant cystic fibrosis transmembrane conductance regulator function hold great promise for development of novel therapies aimed at correcting the underlying pathophysiology of cystic fibrosis.

Chloride channels in the small intestinal cell line IEC-18

Small intestinal crypt cells play a critical role in modulating Cl- secretion during digestion. The types of Cl- channels mediating Cl- secretion in the small intestine was investigated using the intestinal epithelial cell line, IEC-18, which was derived from rat small intestine crypt cells. In initial radioisotope efflux studies, exposure to forskolin, ionomycin or a decrease in extracellular osmolarity significantly increased 36Cl efflux as compared to control cells. Whole cell patch clamp techniques were subsequently used to examine in more detail the swelling-, Ca2+-, and cAMP-activated Cl- conductance. Decreasing the extracellular osmolarity from 290 to 200 mOsm activated a large outwardly rectifying Cl- current that was voltage-independent and had an anion selectivity of I- > Cl-. Increasing cytosolic Ca2+ by ionomycin activated whole cell Cl- currents, which were also outwardly rectifying but were voltage-dependent. The increase in intracellular Ca2+ levels with ionomycin was confirmed with fura-2 loaded IEC-18 cells. A third type of whole cell Cl- current was observed after increases in intracellular cAMP induced by forskolin. These cAMP-activated Cl- currents have properties consistent with cystic fibrosis transmembrane regulator (CFTR) Cl- channels, as the currents were blocked by glibenclamide or NPPB but insensitive to DIDS. In addition, the current-voltage relationship was linear and had an anion selectivity of Cl- > I-. Confocal immunofluorescence studies and Western blots with two different anti-CFTR antibodies confirmed the expression of CFTR. These results suggest that small intestinal crypt cells express multiple types of Cl- channels, which may all contribute to net Cl- secretion.

ClC-2 chloride secretion mediates prostaglandin-induced recovery of barrier function in ischemia-injured porcine ileum

BACKGROUND & AIMS

Ischemia results in the breakdown of the intestinal barrier, predisposing patients to sepsis and multiple organ failure. Prostaglandins play a critical role in mediating recovery of barrier function in ischemia-injured intestine through a mechanism involving stimulation of Cl - secretion. In the present study, we investigated the contributory role of individual Cl - channels in the recovery of barrier function in ischemia-injured porcine ileum.

METHODS

Ischemia-injured porcine ileal mucosa was mounted in Ussing chambers. Short-circuit current (Isc) and transepithelial resistance (TER) were measured in response to prostaglandin E 2 (PGE 2 ) and pharmacologic inhibitors of epithelial Cl - channels. Immunoassays were used to assess the expression and localization of ion channels.

RESULTS

Application of PGE 2 to ischemia-injured ileal mucosa stimulated increases in Isc, an indicator of Cl - secretion, that was followed by marked increases in TER, an indicator of barrier function recovery. In vitro studies revealed that although PGE 2 induced Cl - secretion via at least 3 distinct secretory pathways, recovery of barrier function was initiated by Cl - secretion via ClC-2 Cl - channels co-expressed with occludin and localized to tight junctions within restituting epithelium. Intravenous administration of furosemide to pigs subjected to 1 hour of ileal ischemia impaired recovery of barrier function, as evidenced by decreased TER and increased mucosal-to-serosal 3 H-mannitol flux after a 2-hour reperfusion/recovery period, confirming an important role for Cl - secretory pathways in vivo.

CONCLUSIONS

ClC-2-mediated intestinal Cl - secretion restores TER in ischemia-injured intestine. These data may provide the basis for targeted pharmacologic therapy for diseases associated with impaired barrier function.

SPI-0211 activates T84 cell chloride transport and recombinant human ClC-2 chloride currents

The purpose of this study was to determine the mechanism of action of SPI-0211 (lubiprostone), a novel bicyclic fatty acid in development for the treatment of bowel dysfunction. Adult rabbit intestine was shown to contain mRNA for ClC-2 using RT-PCR, Northern blot analysis, and in situ hybridization. T84 cells grown to confluence on permeable supports were shown to express ClC-2 channel protein in the apical membrane. SPI-0211 increased electrogenic Cl- transport across the apical membrane of T84 cells, with an EC50 of approximately 18 nM measured by short-circuit current (Isc) after permeabilization of the basolateral membrane with nystatin. SPI-0211 effects on Cl- currents were also measured by whole cell patch clamp using the human embryonic kidney (HEK)-293 cell line stably transfected with either recombinant human ClC-2 or recombinant human cystic fibrosis transmembrane regulator (CFTR). In these studies, SPI-0211 activated ClC-2 Cl- currents in a concentration-dependent manner, with an EC50 of approximately 17 nM, and had no effect in nontransfected HEK-293 cells. In contrast, SPI-0211 had no effect on CFTR Cl- channel currents measured in CFTR-transfected HEK-293 cells. Activation of ClC-2 by SPI-0211 was independent of PKA. Together, these studies demonstrate that SPI-0211 is a potent activator of ClC-2 Cl- channels and suggest a physiologically relevant role for ClC-2 Cl- channels in intestinal Cl- transport after SPI-0211 administration.

Cholera toxin induces expression of ion channels and carriers in rat small intestinal mucosa

Cholera toxin causes cyclic adenosine monophosphate (cAMP)-induced electrolyte and water secretion in the small intestine. The toxin-induced change in gene expression in rat small intestine was evaluated with microarray technique and the results were confirmed by semiquantitative polymerase chain reaction (PCR). The transporter CNT2 for nucleosides was upregulated between 6 and 18 h after challenge, whereas the level of GLUT1 transporter for glucose became elevated at 6 h. Both changes probably facilitate uptake of these nutrients in the gut. At 18 h, the major chloride channel in the villus, ClC2, was upregulated. Aquaporin 8 was downregulated at 6 h and two mucin-producing genes were upregulated 18 h after toxin challenge. The expression was back to normal after 72 h, which is the turnover time for intestinal epithelial cells.

Additional disruption of the ClC-2 Cl(-) channel does not exacerbate the cystic fibrosis phenotype of cystic fibrosis transmembrane conductance regulator mouse models

Cystic fibrosis is a fatal inherited disease that is caused by mutations in the gene encoding a cAMP-activated chloride channel, the cystic fibrosis transmembrane conductance regulator (CFTR). It has been suggested that the cystic fibrosis phenotype might be modulated by the presence of other Cl(-) channels that are coexpressed with CFTR in some epithelial cells. Because the broadly expressed plasma membrane Cl(-) channel, ClC-2, is present in the tissues whose function is compromised in cystic fibrosis, we generated mice with a disruption of both Cl(-) channel genes. No morphological changes in their intestine, lung, or pancreas, tissues affected by cystic fibrosis, were observed in these mice. The mortality was not increased over that observed with a complete lack of functional CFTR. Surprisingly, mice expressing mutant CFTR (deletion of phenylalanine 508), survived longer when ClC-2 was disrupted additionally. Currents across colonic epithelia were investigated in Ussing chamber experiments. The disruption of ClC-2, in addition to CFTR, did not decrease Cl(-) secretion. Colon expressing wild-type CFTR even secreted more Cl(-) when ClC-2 was disrupted, although CFTR transcript levels were unchanged. It is concluded that ClC-2 is unlikely to be a candidate rescue channel in cystic fibrosis. Our data are consistent with a model in which ClC-2 is located in the basolateral membrane.

Independence of apical Cl-/HCO3- exchange and anion conductance in duodenal HCO3- secretion

Reduced gastrointestinal HCO3- secretion contributes to malabsorption and obstructive syndromes in cystic fibrosis. The apical HCO3- transport pathways in these organs have not been defined. We therefore assessed the involvement of apical Cl-/HCO3- exchangers and anion conductances in basal and cAMP-stimulated duodenal HCO3- secretion. Muscle-stripped rat and rabbit proximal duodena were mounted in Ussing chambers, and electrical parameters, HCO3- secretion rates, and 36Cl-, 22Na+, and 3H+ mannitol fluxes were assessed. mRNA expression levels were measured by a quantitative PCR technique. Removal of Cl- from or addition of 1 mM DIDS to the luminal perfusate markedly decreased basal HCO3- secretion but did not influence the HCO3- secretory response to 8-bromo-cAMP, which was inhibited by luminal 5-nitro-2-(3-phenylpropylamino)-benzoate. Bidirectional 22Na+ and 36Cl- flux measurements demonstrated an inhibition rather than a stimulation of apical anion exchange during cAMP-stimulated HCO3- secretion. The ratio of Cl- to HCO3- in the anion secretory response was compatible with both Cl- and HCO3- being secreted via the CFTR anion channel. CFTR expression was very high in the duodenal mucosa of both species. We conclude that in rat and rabbit duodena, an apical Cl-/HCO3- exchanger mediates a significant part of basal HCO3- secretion but is not involved in the HCO3- secretory response to cAMP analogs. The inhibitor profile, the strong predominance of Cl- over HCO3- in the anion secretory response, and the high duodenal CFTR expression levels suggest that a major portion of cAMP-stimulated duodenal HCO3- secretion is directly mediated by CFTR.

Modulation of Sp1 and Sp3 in lung epithelial cells regulates ClC-2 chloride channel expression

ClC-2 is a pH- and voltage-activated chloride channel, which is highly expressed in fetal airways and downregulated at birth. The ClC-2 promoter contains consensus binding sites within the first 237 bp, which bind transcription factors Sp1 and Sp3(1). This study directly links Sp1 and Sp3 with ClC-2 protein expression by demonstrating: (i) induction of ClC-2 protein by transient overexpression of each transcription factor in adult rat Type II cells, which have low levels of ClC-2; and (ii) reduction of ClC-2 expression by incubation with a competitive inhibitor of Sp1 and Sp3 in fetal rat Type II cells, which have high levels of endogenous ClC-2. Endogenous fetal lung Sp1 is differentially expressed as two major species of 105 kD and 95 kD. Although low-level expression of Sp1 in adult cells is almost exclusively the 105-kD species, overexpression of Sp1 results in increased expression of the 95-kD band. These experiments suggest that the mechanism for postnatal reduction of ClC-2 expression in lung epithelia is based on decreased interaction of Sp1 and Sp3 with the ClC-2 promoter.

Evidence for a functional interaction between the ClC-2 chloride channel and the retrograde motor dynein complex

The ClC-2 chloride channel has been implicated in essential physiological functions. Analyses of ClC-2 knock-out mice suggest that ClC-2 expression in retinal pigment epithelia and Sertoli cells normally supports the viability of photoreceptor cells and male germ cells, respectively. Further, other studies suggest that ClC-2 expression in neurons may modify inhibitory synaptic transmission via the gamma-aminobutyric acid, type A receptor. However, complete understanding of the physiological functions of ClC-2 requires elucidation of the molecular basis for its regulation. Using cell imaging and biochemical and electrophysiological techniques, we show that expression of ClC-2 at the cell surface may be regulated via an interaction with the dynein motor complex. Mass spectrometry and Western blot analysis of eluate from a ClC-2 affinity matrix showed that heavy and intermediate chains of dynein bind ClC-2 in vitro. The dynein intermediate chain co-immunoprecipitates with ClC-2 from hippocampal membranes suggesting that they also interact in vivo. Disruption of dynein motor function perturbs ClC-2 localization and increases the functional expression of ClC-2 in the plasma membranes of COS7 cells. Thus, cell surface expression of ClC-2 may be regulated by dynein motor activity. This work is the first to demonstrate an in vivo interaction between an ion channel and the dynein motor complex.

ClC-2 in guinea pig colon: mRNA, immunolabeling, and functional evidence for surface epithelium localization

The principal function of the colon in fluid homeostasis is the absorption of NaCl and water. Apical membrane Na(+) channels, Na(+)/H(+) and Cl(-)/HCO exchangers, have all been postulated to mediate NaCl entry into colonocytes. The identity of the basolateral exit pathway for Cl(-) is unknown. We have previously demonstrated the presence of the ClC-2 transcript in the guinea pig intestine. Now we explore in more detail, the tissue and cellular distribution of chloride channel ClC-2 in the distal colon by in situ hybridization and immunohistochemistry. The patch-clamp technique was used to characterize Cl(-) currents in isolated surface epithelial cells from guinea pig distal colon and these were compared with those mediated by recombinant guinea pig (gp)ClC-2. ClC-2 mRNA and protein were found in the surface epithelium of the distal colon. Immunolocalization revealed that, in addition to some intracellular labeling, ClC-2 was present in the basolateral membranes but absent from the apical pole of colonocytes. Isolated surface epithelial cells exhibited hyperpolarization-activated chloride currents showing a Cl(-) > I(-) permeability and Cd(2+) sensitivity. These characteristics, as well as some details of the kinetics of activation and deactivation, were very similar to those of recombinant gpClC-2 measured in parallel experiments. The presence of active ClC-2 type currents in surface colonic epithelium, coupled to a basolateral location for ClC-2 in the distal colon, suggests a role for ClC-2 channel in mediating basolateral membrane exit of Cl(-) as an essential step in a NaCl absorption process.

Methoxsalen stimulates electrogenic Cl- secretion in the mouse jejunum

We used the short-circuit current (I(sc)) and patch-clamp techniques to investigate the effects of methoxsalen (MTX) on the electrogenic Cl- secretion of the mouse jejunum. MTX stimulated a sustained increase in Isc that was dose dependent. Bumetanide inhibited MTX-stimulated Isc in a dose-dependent manner consistent with activation of Cl- secretion. MTX failed to stimulate I(sc) following maximal activation of the cAMP pathway by forskolin, but did increase Isc after a submaximal dose of forskolin. Glibenclamide, a blocker of the cystic fibrosis transmembrane conductance regulator (CFTR), reduced the MTX-stimulated increase of Isc by 59 +/- 6%. The cAMP-dependent K+ channel blocker 293B did not alter the MTX-activated I(sc); however, clotrimazole, an intermediate Ca2(+)-activated K+ channel (IK(Ca)) blocker, reduced the MTX-stimulated I(sc). MTX did not alter Na(+)-glucose cotransport across the mouse jejunum. In cell-attached membrane patches, MTX increased the open probability of the basolateral IK(Ca) channel of isolated crypts. These data suggest that the CFTR and IK(Ca) channels participate in the MTX-activated, sustained Cl- secretory response of the mouse jejunum.

Loss of hyperpolarization-activated Cl(-) current in salivary acinar cells from Clcn2 knockout mice

ClC-2 is localized to the apical membranes of secretory epithelia where it has been hypothesized to play a role in fluid secretion. Although ClC-2 is clearly the inwardly rectifying anion channel in several tissues, the molecular identity of the hyperpolarization-activated Cl(-) current in other organs, including the salivary gland, is currently unknown. To determine the nature of the hyperpolarization-activated Cl(-) current and to examine the role of ClC-2 in salivary gland function, a mouse line containing a targeted disruption of the Clcn2 gene was generated. The resulting homozygous Clcn2(-/-) mice lacked detectable hyperpolarization-activated chloride currents in parotid acinar cells and, as described previously, displayed postnatal degeneration of the retina and testis. The magnitude and biophysical characteristics of the volume- and calcium-activated chloride currents in these cells were unaffected by the absence of ClC-2. Although ClC-2 appears to contribute to fluid secretion in some cell types, both the initial and sustained salivary flow rates were normal in Clcn2(-/-) mice following in vivo stimulation with pilocarpine, a cholinergic agonist. In addition, the electrolytes and protein contents of the mature secretions were normal. Because ClC-2 has been postulated to contribute to cell volume control, we also examined regulatory volume decrease following cell swelling. However, parotid acinar cells from Clcn2(-/-) mice recovered volume with similar efficiency to wild-type littermates. These data demonstrate that ClC-2 is the hyperpolarization-activated Cl(-) channel in salivary acinar cells but is not essential for maximum chloride flux during stimulated secretion of saliva or acinar cell volume regulation.

Molecular structure and physiological function of chloride channels

Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.

Distribution of ClC-2 chloride channel in rat and human epithelial tissues

The ubiquitous ClC-2 Cl(-) channel is thought to contribute to epithelial Cl(-) secretion, but the distribution of the ClC-2 protein in human epithelia has not been investigated. We have studied the distribution of ClC-2 in adult human and rat intestine and airways by immunoblotting and confocal microscopy. In the rat, ClC-2 was present in the lateral membranes of villus enterocytes and was predominant at the basolateral membranes of luminal colon enterocytes. The expression pattern of ClC-2 in the human intestine differed significantly, because ClC-2 was mainly detected in a supranuclear compartment of colon cells. We found significant expression of ClC-2 at the apex of ciliated cells in both rat and human airways. These results show that the distribution of ClC-2 in airways is consistent with participation of ClC-2 channels in Cl(-) secretion and indicate that extrapolation of results from studies of ClC-2 function in rat intestine to human intestine is not straightforward.

Fluid and electrolyte transport in the small intestine

The small intestine is in a dynamic state of secretion and absorption, the sum of which results in net absorption. Secretion is principally the result of chloride and bicarbonate extrusion through apical chloride channels after the activation of the second messengers cAMP, cGMP, and calcium. In addition to the cystic fibrosis transmembrane conductance regulator, several other candidate chloride channels have been identified and proposed to play a role in intestinal secretion, including the calcium-dependent chloride channel hCLCA1. Pathways leading to the negative control of secretion have been described that use cellular messengers, including inositol (3,4,5,6) tetrakisphosphate and phosphatidylinositol 3-kinase, which may act via basolateral potassium channels. The control of ion transport can also be viewed in terms of the enteric nervous system. The reflex neural pathways involved in enterotoxin-induced secretion have been substantiated and shown to involve 5-hydroxytryptamine, substance P, and the neurokinin 1 and 2 receptors in the sensory arm, and vasoactive intestinal peptide in the secretomotor efferents. Absorption of glucose in addition to active cotransport with sodium via the Na/glucose cotransporter protein has also been shown to occur passively through a carrier-mediated mechanism, using the membrane protein glucose transporter protein 2.

The chloride channel ClC-4 co-localizes with cystic fibrosis transmembrane conductance regulator and may mediate chloride flux across the apical membrane of intestinal epithelia

Cystic fibrosis (CF) causing mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) lead to mislocalization of CFTR protein from the brush border membrane of epithelial tissues and/or its dysfunction as a chloride channel. In initial reports, it was proposed that certain channels from the ClC family of chloride channels may provide compensatory or alternative pathways for epithelial chloride secretion in tissues from cystic fibrosis patients. In the present work, we provide the first evidence that ClC-4 protein is functionally expressed on the surface of the intestinal epithelium and hence, is appropriately localized to act as a therapeutic target in this CF-affected tissue. We show using confocal and electron microscopy that ClC-4 co-localizes with CFTR in the brush border membrane of the epithelium lining intestinal crypts in mouse and human tissues. In Caco-2 cells, a cell line thought to model human enterocytes, ClC-4 protein is expressed on the cell surface and also partially co-localizes with EEA1 and transferrin, marker molecules of early and recycling endosomes, respectively. Hence, like CFTR, ClC-4 may cycle between the plasma membrane and endosomal compartment. Furthermore, we show that ClC-4 functions as a chloride channel on the surface of these epithelial cells as antisense ClC-4 cDNA expression reduced the amplitude of endogenous chloride currents by 50%. These studies provide the first evidence that ClC-4 is endogenously expressed and may be functional in the brush border membrane of enterocytes and hence should be considered as a candidate channel to provide an alternative pathway for chloride secretion in the gastrointestinal tract of CF patients.

Electrolyte transport in the mammalian colon: mechanisms and implications for disease

The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a feces with very little water and salt content. In addition, the epithelium does secret mucus, bicarbonate, and KCl. Polarized distribution of transport proteins in both luminal and basolateral membranes enables efficient salt transport in both directions, probably even within an individual cell. Meanwhile, most of the participating transport proteins have been identified, and their function has been studied in detail. Absorption of NaCl is a rather steady process that is controlled by steroid hormones regulating the expression of epithelial Na(+) channels (ENaC), the Na(+)-K(+)-ATPase, and additional modulating factors such as the serum- and glucocorticoid-regulated kinase SGK. Acute regulation of absorption may occur by a Na(+) feedback mechanism and the cystic fibrosis transmembrane conductance regulator (CFTR). Cl(-) secretion in the adult colon relies on luminal CFTR, which is a cAMP-regulated Cl(-) channel and a regulator of other transport proteins. As a consequence, mutations in CFTR result in both impaired Cl(-) secretion and enhanced Na(+) absorption in the colon of cystic fibrosis (CF) patients. Ca(2+)- and cAMP-activated basolateral K(+) channels support both secretion and absorption of electrolytes and work in concert with additional regulatory proteins, which determine their functional and pharmacological profile. Knowledge of the mechanisms of electrolyte transport in the colon enables the development of new strategies for the treatment of CF and secretory diarrhea. It will also lead to a better understanding of the pathophysiological events during inflammatory bowel disease and development of colonic carcinoma.

Regulation of anion secretion by nitric oxide in human airway epithelial cells

Nitric oxide (NO) is continuously produced and released in human airways, but the biological significance of this process is unknown. In this study, we have used Calu-3 cells to investigate the effects of NO on transepithelial anion secretion. An inhibitor of NO synthase, NG-nitro-L-arginine methyl ester, reduced short- circuit current (I(sc)), whereas an NO donor, S-nitrosoglutathione (GSNO), increased I(sc), with an EC50 approximately 1.2 microM. The NO-activated current was inhibited by diphenylamine-2-carboxylate, clotrimazole, and charybdotoxin. Selective permeabilization of cell membranes indicated that NO activated both apical anion channels and basolateral potassium channels. An inhibitor of soluble guanylate cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, prevented activation of I(sc) by NO but not by 8-bromo-cGMP, suggesting that NO acts via a cGMP-dependent pathway. Sequential treatment of cells with forskolin and GSNO or 1-ethyl-2-benzimidazolinone and GSNO showed additive effects of these chemicals on I(sc). Interestingly, GSNO elevated intracellular Ca2+ concentration ([Ca2+]i) but had no effect on I(sc) activated by thapsigargin. These results show that NO activates transepithelial anion secretion via a cGMP-dependent pathway that involves cross talk between NO and [Ca2+]i.

Ion permeation and selectivity in ClC-type chloride channels

Voltage-gated anion channels are present in almost every living cell and have many physiological functions. Recently, a novel gene family encoding voltage-gated chloride channels, the ClC family, was identified. The knowledge of primary amino acid sequences has allowed for the study of these anion channels in heterologous expression systems and made possible the combination of site-directed mutagenesis and high-resolution electrophysiological measurements as a means of gaining insights into the molecular basis of channel function. This review focuses on one particular aspect of chloride channel function, the selective transport of anions through biological membranes. I will describe recent experiments using a combination of cellular electrophysiology, molecular genetics, and recombinant DNA technology to study the molecular basis of ion permeation and selection in ClC-type chloride channels. These novel tools have provided new insights into basic mechanisms underlying the function of these biologically important channels.

ClC-2 contributes to native chloride secretion by a human intestinal cell line, Caco-2

It has been previously determined that ClC-2, a member of the ClC chloride channel superfamily, is expressed in certain epithelial tissues. These findings fueled speculation that ClC-2 can compensate for impaired chloride transport in epithelial tissues affected by cystic fibrosis and lacking the cystic fibrosis transmembrane conductance regulator. However, direct evidence linking ClC-2 channel expression to epithelial chloride secretion was lacking. In the present studies, we show that ClC-2 transcripts and protein are present endogenously in the Caco-2 cell line, a cell line that models the human small intestine. Using an antisense strategy we show that ClC-2 contributes to native chloride currents in Caco-2 cells measured by patch clamp electrophysiology. Antisense ClC-2-transfected monolayers of Caco-2 cells exhibited less chloride secretion (monitored as iodide efflux) than did mock transfected monolayers, providing the first direct molecular evidence that ClC-2 can contribute to chloride secretion by the human intestinal epithelium. Further, examination of ClC-2 localization by confocal microscopy revealed that ClC-2 contributes to secretion from a unique location in this epithelium, from the apical aspect of the tight junction complex. Hence, these studies provide the necessary rationale for considering ClC-2 as a possible therapeutic target for diseases affecting intestinal chloride secretion such as cystic fibrosis.