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

The ClC-2 Chloride Channel Activator, Lubiprostone, Improves Intestinal Barrier Function in Biopsies from Crohn’s Disease but Not Ulcerative Colitis Patients

The prostone analog, lubiprostone, is approved to manage constipation-predominant irritable bowel syndrome. Lubiprostone also protects intestinal mucosal barrier function in animal models of colitis. The aim of this study was to determine if lubiprostone improves barrier properties in isolated colonic biopsies from Crohn’s disease (CD) and ulcerative colitis (UC) patients. Sigmoid colon biopsies from healthy subjects, CD and UC patients in remission, and CD patients with active disease were mounted in Ussing chambers. Tissues were treated with lubiprostone or vehicle to determine the effects on transepithelial electrical resistance (TER), FITC-dextran 4kD (FD4) permeability, and electrogenic ion transport responses to forskolin and carbachol. Localization of the tight junction protein, occludin, was determined by immunofluorescence. Lubiprostone significantly increased ion transport across control, CD and UC remission biopsies but not active CD. Lubiprostone selectively improved TER in both CD remission and active disease biopsies but not in control or UC biopsies. The improved TER was associated with increased membrane localization of occludin. Lubiprostone selectively improved barrier properties of biopsies from CD patients vs. UC and independent of an ion transport response. These data indicate that lubiprostone has potential efficacy in improving mucosal integrity in Crohn’s disease.

Molecular mechanisms underlying guanylin-induced transcellular Cl- secretion into the intestinal lumen of seawater-acclimated eels

Guanylin (GN) stimulates Cl^- secretion into the intestinal lumen of seawater-acclimated eels, but the molecular mechanisms of transepithelial Cl^- transport are still unknown. In Ussing chamber experiments, we confirmed that mucosal application of eel GN reversed intestinal serosa-negative potential difference, indicating Cl^- secretion. Serosal application of DNDS or mucosal application of DPC inhibited the GN effect, but serosal application of bumetanide had no effect. Removal of HCO₃^- from the serosal fluid also inhibited the GN effect. In intestinal sac experiments, mucosal GN stimulated luminal secretion of both Cl^- and Na⁺, which was blocked by serosal DNDS. These results suggest that Cl^- is taken up at the serosal side by DNDS-sensitive anion exchanger (AE) coupled with Na⁺-HCO₃^- cotransporter (NBC) but not by Na⁺-K⁺-2Cl^- cotransporter 1 (NKCC1), and Cl^- is secreted by unknown DPC-sensitive Cl^- channel (ClC) at the mucosal side. The transcriptomic analysis combined with qPCR showed low expression of NKCC1 gene and no upregulation of the gene after seawater transfer, while high expression of ClC2 gene and upregulation after seawater transfer. In addition, SO₄^2- transporters (apical Slc26a3/6 and basolateral Slc26a1) are also candidates for transcellular Cl^- secretion in exchange of luminal SO₄². Na⁺ secretion could occur through a paracellular route, as Na⁺-leaky claudin15 was highly expressed and upregulated after seawater transfer. High local Na⁺ concentration in the lateral interspace produced by Na⁺/K⁺-ATPase (NKA) coupled with K⁺ channels (Kir5.1b) seems to facilitate the paracellular transport. In situ hybridization confirmed the expression of the candidate genes in the epithelial enterocytes. Together with our previous results, we suggest that GN stimulates basolateral NBCela/AE2 and apical ClC2 to increase transcellular Cl^- secretion in seawater eel intestine, which differs from the involvement of apical CFTR and basolateral NKCC1 as suggested in mammals and other teleosts.

Pathophysiological role of ion channels and transporters in gastrointestinal mucosal diseases

The incidence of gastrointestinal (GI) mucosal diseases, including various types of gastritis, ulcers, inflammatory bowel disease and GI cancer, is increasing. Therefore, it is necessary to identify new therapeutic targets. Ion channels/transporters are located on cell membranes, and tight junctions (TJs) affect acid-base balance, the mucus layer, permeability, the microbiota and mucosal blood flow, which are essential for maintaining GI mucosal integrity. As ion channel/transporter dysfunction results in various GI mucosal diseases, this review focuses on understanding the contribution of ion channels/transporters to protecting the GI mucosal barrier and the relationship between GI mucosal disease and ion channels/transporters, including Cl-/HCO3- exchangers, Cl- channels, aquaporins, Na+/H+ exchangers, and K+ channels. Here, we provide novel prospects for the treatment of GI mucosal diseases.

Regulation of ClC-2 Chloride Channel Proteostasis by Molecular Chaperones: Correction of Leukodystrophy-Associated Defect

The ClC-2 channel plays a critical role in maintaining ion homeostasis in the brain and the testis. Loss-of-function mutations in the ClC-2-encoding human CLCN2 gene are linked to the white matter disease leukodystrophy. Clcn2-deficient mice display neuronal myelin vacuolation and testicular degeneration. Leukodystrophy-causing ClC-2 mutant channels are associated with anomalous proteostasis manifesting enhanced endoplasmic reticulum (ER)-associated degradation. The molecular nature of the ER quality control system for ClC-2 protein remains elusive. In mouse testicular tissues and Leydig cells, we demonstrated that endogenous ClC-2 co-existed in the same protein complex with the molecular chaperones heat shock protein 90β (Hsp90β) and heat shock cognate protein (Hsc70), as well as the associated co-chaperones Hsp70/Hsp90 organizing protein (HOP), activator of Hsp90 ATPase homolog 1 (Aha1), and FK506-binding protein 8 (FKBP8). Further biochemical analyses revealed that the Hsp90β-Hsc70 chaperone/co-chaperone system promoted mouse and human ClC-2 protein biogenesis. FKBP8 additionally facilitated membrane trafficking of ClC-2 channels. Interestingly, treatment with the Hsp90-targeting small molecule 17-allylamino-17-demethoxygeldanamycin (17-AAG) substantially boosted ClC-2 protein expression. Also, 17-AAG effectively increased both total and cell surface protein levels of leukodystrophy-causing loss-of-function ClC-2 mutant channels. Our findings highlight the therapeutic potential of 17-AAG in correcting anomalous ClC-2 proteostasis associated with leukodystrophy.

Novel roles for voltage-gated T-type Ca2+ and ClC-2 channels in phagocytosis and angiogenic factor balance identified in human iPSC-derived RPE

Human-induced pluripotent stem cell (hiPSC)-derived retinal pigment epithelium (RPE) is a powerful tool for pathophysiological studies and preclinical therapeutic screening, as well as a source for clinical cell transplantation. Thus, it must be validated for maturity and functionality to ensure correct data readouts and clinical safety. Previous studies have validated hiPSC-derived RPE as morphologically characteristic of the tissue in the human eye. However, information concerning the expression and functionality of ion channels is still limited. We screened hiPSC-derived RPE for the polarized expression of a panel of L-type (Ca_V 1.1, Ca_V 1.3) and T-type (Ca_V 3.1, Ca_V 3.3) Ca²⁺ channels, K⁺ channels (Maxi-K, Kir4.1, Kir7.1), and the Cl^- channel ClC-2 known to be expressed in native RPE. We also tested the roles of these channels in key RPE functions using specific inhibitors. In addition to confirming the native expression profiles and function of certain channels, such as L-type Ca²⁺ channels, we show for the first time that T-type Ca²⁺ channels play a role in both phagocytosis and vascular endothelial growth factor (VEGF) secretion. Moreover, we demonstrate that Maxi-K and Kir7.1 channels are involved in the polarized secretion of VEGF and pigment epithelium-derived factor (PEDF). Furthermore, we show a novel localization for ClC-2 channel on the apical side of hiPSC-derived RPE, with an overexpression at the level of fluid-filled domes, and demonstrate that it plays an important role in phagocytosis, as well as VEGF and PEDF secretion. Taken together, hiPSC-derived RPE is a powerful model for advancing fundamental knowledge of RPE functions.

Short Chain Fatty Acids Effect on Chloride Channel ClC-2 as a Possible Mechanism for Lubiprostone Intestinal Action

Lubiprostone, a 20-carbon synthetic fatty acid used for the treatment of constipation, is thought to act through an action on Cl⁻ channel ClC-2. Short chain fatty acids (SCFAs) are produced and absorbed in the distal intestine. We explore whether SCFAs affect ClC-2, re-examine a possible direct effect of lubiprostone on ClC-2, and use mice deficient in ClC-2 to stringently address the hypothesis that the epithelial effect of lubiprostone targets this anion channel. Patch-clamp whole cell recordings of ClC-2 expressed in mammalian cells are used to assay SCFA and lubiprostone effects. Using chamber measurements of ion current in mice deficient in ClC-2 or CFTR channels served to analyze the target of lubiprostone in the distal intestinal epithelium. Intracellular SCFAs had a dual action on ClC-2, partially inhibiting conduction but, importantly, facilitating the voltage activation of ClC-2. Intra- or extracellular lubiprostone had no effect on ClC-2 currents. Lubiprostone elicited a secretory current across colonic epithelia that was increased in mice deficient in ClC-2, consistent with the channel’s proposed proabsorptive function, but absent from those deficient in CFTR. Whilst SCFAs might exert a physiological effect on ClC-2 as part of their known proabsorptive effect, ClC-2 plays no part in the lubiprostone intestinal effect that appears mediated by CFTR activation.

CUL4-DDB1-CRBN E3 Ubiquitin Ligase Regulates Proteostasis of ClC-2 Chloride Channels: Implication for Aldosteronism and Leukodystrophy

Voltage-gated ClC-2 channels are essential for chloride homeostasis. Complete knockout of mouse ClC-2 leads to testicular degeneration and neuronal myelin vacuolation. Gain-of-function and loss-of-function mutations in the ClC-2-encoding human CLCN2 gene are linked to the genetic diseases aldosteronism and leukodystrophy, respectively. The protein homeostasis (proteostasis) mechanism of ClC-2 is currently unclear. Here, we aimed to identify the molecular mechanism of endoplasmic reticulum-associated degradation of ClC-2, and to explore the pathophysiological significance of disease-associated anomalous ClC-2 proteostasis. In both heterologous expression system and native neuronal and testicular cells, ClC-2 is subject to significant regulation by cullin-RING E3 ligase-mediated polyubiquitination and proteasomal degradation. The cullin 4 (CUL4)-damage-specific DNA binding protein 1 (DDB1)-cereblon (CRBN) E3 ubiquitin ligase co-exists in the same complex with and promotes the degradation of ClC-2 channels. The CRBN-targeting immunomodulatory drug lenalidomide and the cullin E3 ligase inhibitor MLN4924 promotes and attenuates, respectively, proteasomal degradation of ClC-2. Analyses of disease-related ClC-2 mutants reveal that aldosteronism and leukodystrophy are associated with opposite alterations in ClC-2 proteostasis. Modifying CUL4 E3 ligase activity with lenalidomide and MLN4924 ameliorates disease-associated ClC-2 proteostasis abnormality. Our results highlight the significant role and therapeutic potential of CUL4 E3 ubiquitin ligase in regulating ClC-2 proteostasis.

The Integral Role of Tight Junction Proteins in the Repair of Injured Intestinal Epithelium

The intestinal epithelial monolayer forms a transcellular and paracellular barrier that separates luminal contents from the interstitium. The paracellular barrier consists of a highly organized complex of intercellular junctions that is primarily regulated by apical tight junction proteins and tight junction-associated proteins. This homeostatic barrier can be lost through a multitude of injurious events that cause the disruption of the tight junction complex. Acute repair after injury leading to the reestablishment of the tight junction barrier is crucial for the return of both barrier function as well as other cellular functions, including water regulation and nutrient absorption. This review provides an overview of the tight junction complex components and how they link to other plasmalemmal proteins, such as ion channels and transporters, to induce tight junction closure during repair of acute injury. Understanding the components of interepithelial tight junctions and the mechanisms of tight junction regulation after injury is crucial for developing future therapeutic targets for patients experiencing dysregulated intestinal permeability.

Functional Role of Basolateral ClC-2 Channels in the Regulation of Duodenal Anion Secretion in Mice

BACKGROUND

Although ClC-2 channels are important in colonic Cl^- secretion, it is unclear about their roles in small intestinal anion secretion. Therefore, we sought to examine whether ClC-2 channels play important roles in anion secretion, particularly duodenal bicarbonate secretion (DBS).

METHODS

Duodenal mucosae from mice were stripped of seromuscular layers and mounted in Ussing chambers. Both duodenal short-circuit current (I_sc) and HCO₃^- secretion in vitro were simultaneously recorded. DBS in vivo was measured by a CO₂-sensitive electrode.

RESULTS

Lubiprostone, a selective ClC-2 activator, concentration-dependently increased both duodenal I_sc and DBS only when applied basolaterally, but not when applied apically. Removal of extracellular Cl^- abolished lubiprostone-induced duodenal I_sc, but did not alter HCO₃^- secretion even in the presence of DIDS, a Cl^-/HCO₃^- exchanger inhibitor. However, further addition of glibenclamide, a CFTR channel blocker, abolished lubiprostone-evoked HCO₃^- secretion. Moreover, lubiprostone-induced HCO₃^- secretion was impaired in CFTR^-/- mice compared to wild-type littermates. Luminal perfusion of duodenal lumen with lubiprostone did not alter basal DBS in vivo, but lubiprostone (i.p.) was able to induce DBS, which was also significantly inhibited by Cd²⁺, a ClC-2 channel blocker. [Ca²⁺]_cyt level, Ca²⁺-activated K⁺ channel- and cAMP-mediated duodenal I_sc, and HCO₃^- secretion were unchanged by lubiprostone.

CONCLUSIONS

We have provided the first evidence for the novel functional role of basolateral ClC-2 channels in the regulation of duodenal anion secretion.

CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease

CLC anion transporters are found in all phyla and form a gene family of eight members in mammals. Two CLC proteins, each of which completely contains an ion translocation parthway, assemble to homo- or heteromeric dimers that sometimes require accessory β-subunits for function. CLC proteins come in two flavors: anion channels and anion/proton exchangers. Structures of these two CLC protein classes are surprisingly similar. Extensive structure-function analysis identified residues involved in ion permeation, anion-proton coupling and gating and led to attractive biophysical models. In mammals, ClC-1, -2, -Ka/-Kb are plasma membrane Cl−channels, whereas ClC-3 through ClC-7 are 2Cl−/H+-exchangers in endolysosomal membranes. Biological roles of CLCs were mostly studied in mammals, but also in plants and model organisms like yeast and Caenorhabditis elegans. CLC Cl−channels have roles in the control of electrical excitability, extra- and intracellular ion homeostasis, and transepithelial transport, whereas anion/proton exchangers influence vesicular ion composition and impinge on endocytosis and lysosomal function. The surprisingly diverse roles of CLCs are highlighted by human and mouse disorders elicited by mutations in their genes. These pathologies include neurodegeneration, leukodystrophy, mental retardation, deafness, blindness, myotonia, hyperaldosteronism, renal salt loss, proteinuria, kidney stones, male infertility, and osteopetrosis. In this review, emphasis is laid on biophysical structure-function analysis and on the cell biological and organismal roles of mammalian CLCs and their role in disease.

Research and progress on ClC‑2 (Review)

Chloride channel 2 (ClC-2) is one of the nine mammalian members of the ClC family. The present review discusses the molecular properties of ClC‑2, including CLCN2, ClC‑2 promoter and the structural properties of ClC‑2 protein; physiological properties; functional properties, including the regulation of cell volume. The effects of ClC‑2 on the digestive, respiratory, circulatory, nervous and optical systems are also discussed, in addition to the mechanisms involved in the regulation of ClC‑2. The review then discusses the diseases associated with ClC‑2, including degeneration of the retina, Sjögren's syndrome, age‑related cataracts, degeneration of the testes, azoospermia, lung cancer, constipation, repair of impaired intestinal mucosa barrier, leukemia, cystic fibrosis, leukoencephalopathy, epilepsy and diabetes mellitus. It was concluded that future investigations of ClC‑2 are likely to be focused on developing specific drugs, activators and inhibitors regulating the expression of ClC‑2 to treat diseases associated with ClC‑2. The determination of CLCN2 is required to prevent and treat several diseases associated with ClC‑2.

Characterization of Human Dermal Fibroblasts in Fabry Disease

Fabry disease (FD) is a hereditary X-linked metabolic lysosomal storage disorder due to insufficient amounts or a complete lack of the lysosomal enzyme α-galactosidase A (α-GalA). The loss of α-GalA activity leads to an abnormal accumulation of globotriaosylcerami (Gb3) in lysosomes and other cellular components of different tissues and cell types, affecting the cell function. However, whether these biochemical alterations also modify functional processes associated to the cell mitotic ability is still unknown. The goal of the present study was to characterize lineages of human dermal fibroblasts (HDFs) of FD patients and healthy controls focusing on Gb3 accumulation, expression of chloride channels that regulate proliferation, and proliferative activity. The biochemical and functional analyses indicate the existence of quantitative differences in some but not all the parameters of cytoskeletal organization, proliferation, and differentiation processes.

Pharmaceutical Activation or Genetic Absence of ClC-2 Alters Tight Junctions During Experimental Colitis

BACKGROUND

We have previously reported that the ClC-2 chloride channel has an important role in regulation of tight junction barrier function during experimental colitis, and the pharmaceutical ClC-2 activator lubiprostone initiates intestinal barrier repair in ischemic-injured intestine. Thus, we hypothesized that pharmaceutical ClC-2 activation would have a protective and therapeutic effect in murine models of colitis, which would be absent in ClC-2 mice.

METHODS

We administered lubiprostone to wild-type or ClC-2 mice with dextran sulfate sodium (DSS) or 2, 4, 5-trinitrobenzene sulfonic acid-induced colitis. We determined the severity of colitis and assessed intestinal permeability. Selected tight junction proteins were analyzed by Western blotting and immunofluorescence/confocal microscopy, whereas proliferative and differentiated cells were examined with special staining and immunohistochemistry.

RESULTS

Oral preventive or therapeutic administration of lubiprostone significantly reduced the severity of colitis and reduced intestinal permeability in both DSS and trinitrobenzene sulfonic acid-induced colitis. Preventive treatment with lubiprostone induced significant recovery of the expression and distribution of selected sealing tight junction proteins in mice with DSS-induced colitis. In addition, lubiprostone reduced crypt proliferation and increased the number of differentiated epithelial cells. Alternatively, when lubiprostone was administered to ClC-2 mice, the protective effect against DSS colitis was limited.

CONCLUSIONS

This study suggests a central role for ClC-2 in restoration of barrier function and tight junction architecture in experimental murine colitis, which can be therapeutically targeted with lubiprostone.

ClC-2 regulation of intestinal barrier function: Translation of basic science to therapeutic target

The ClC-2 chloride channel is a member of the voltage-gated chloride channel family. ClC-2 is involved in various physiological processes, including fluid transport and secretion, regulation of cell volume and pH, maintaining the membrane potential of the cell, cell-to-cell communication, and tissue homeostasis. Recently, our laboratory has accumulated evidence indicating a critical role of ClC-2 in the regulation of intestinal barrier function by altering inter-epithelial tight junction composition. This review will detail the role of ClC-2 in intestinal barrier function during intestinal disorders, including experimental ischemia/reperfusion injury and dextran sodium sulfate (DSS)-induced inflammatory bowel disease. Details of pharmacological manipulation of ClC-2 via prostone agonists will also be provided in an effort to show the potential therapeutic relevance of ClC-2 regulation, particularly during intestinal barrier disruption.

Genetic Ablation of the ClC-2 Cl- Channel Disrupts Mouse Gastric Parietal Cell Acid Secretion

The present studies were designed to examine the effects of ClC-2 ablation on cellular morphology, parietal cell abundance, H/K ATPase expression, parietal cell ultrastructure and acid secretion using WT and ClC-2^-/- mouse stomachs. Cellular histology, morphology and proteins were examined using imaging techniques, electron microscopy and western blot. The effect of histamine on the pH of gastric contents was measured. Acid secretion was also measured using methods and secretagogues previously established to give maximal acid secretion and morphological change. Compared to WT, ClC-2^-/- gastric mucosal histological organization appeared disrupted, including dilation of gastric glands, shortening of the gastric gland region and disorganization of all cell layers. Parietal cell numbers and H/K ATPase expression were significantly reduced by 34% (P<0.05) and 53% (P<0.001) respectively and cytoplasmic tubulovesicles appeared markedly reduced on electron microscopic evaluation without evidence of canalicular expansion. In WT parietal cells, ClC-2 was apparent in a similar cellular location as the H/K ATPase by immunofluorescence and appeared associated with tubulovesicles by immunogold electron microscopy. Histamine-stimulated [H⁺] of the gastric contents was significantly (P<0.025) lower by 9.4 fold (89%) in the ClC-2^-/- mouse compared to WT. Histamine/carbachol stimulated gastric acid secretion was significantly reduced (range 84–95%, P<0.005) in ClC-2^-/- compared to WT, while pepsinogen secretion was unaffected. Genetic ablation of ClC-2 resulted in reduced gastric gland region, reduced parietal cell number, reduced H/K ATPase, reduced tubulovesicles and reduced stimulated acid secretion.

Molecular Physiology of an Extra-renal Cl(-) Uptake Mechanism for Body Fluid Cl(-) Homeostasis

The development of an ion regulatory mechanism for body fluid homeostasis was an important trait for vertebrates during the evolution from aquatic to terrestrial life. The homeostatic mechanism of Cl^- in aquatic fish appears to be similar to that of terrestrial vertebrates; however, the mechanism in non-mammalian vertebrates is poorly understood. Unlike in mammals, in which the kidney plays a central role, in most fish species, the gill is responsible for the maintenance of Cl^- homeostasis via Cl^- transport uptake mechanisms. Previous studies in zebrafish identified Na⁺-Cl^- cotransporter (NCC) 2b-expressing cells in the gills and skin as the major ionocytes responsible for Cl^- uptake, similar to distal convoluted tubular cells in mammalian kidney. However, the mechanism by which basolateral ions exit from NCC cells is still unclear.

Of the in situ hybridization signals of twelve members of the clc Cl^- channel family, only that of clc-2c exhibited an ionocyte pattern in the gill and embryonic skin. Double in situ hybridization/immunocytochemistry confirmed colocalization of apical NCC2b with basolateral CLC-2c. Acclimation to a low Cl^- environment increased mRNA expression of both clc-2c and ncc2b, and also the protein expression of CLC-2c in embryos and adult gills. Loss-of-function of clc-2c resulted in a significant decrease in whole body Cl^- content in zebrafish embryos, a phenotype similar to that of ncc2b mutants; this finding suggests a role for CLC-2c in Cl^- uptake. Translational knockdown of clc-2c stimulated ncc2b mRNA expression and vice versa, revealing cooperation between these two transporters in the context of zebrafish Cl^- homeostasis. Further comparative genomic and phylogenetic analyses revealed that zebrafish CLC-2c is a fish-specific isoform that diverged from a kidney-predominant homologue, in the same manner as NCC2b and its counterparts (NCCs).

Several lines of molecular and cellular physiological evidences demonstrated the cofunctional role of apical NCC2b and basolateral CLC-2c in the gill/skin Cl^- uptake pathway. Taking the phylogenetic evidence into consideration, fish-specific NCC2b and CLC-2c may have coevolved to perform extra-renal Cl^- uptake during the evolution of vertebrates in an aquatic environment.

Renal Effects of Cyclooxygenase Inhibition When Nitric Oxide Synthesis Is Reduced and Angiotensin II Levels Are Enhanced

The involvement of both cyclooxygenase (COX) isoforms in regulating renal function is well known but their interactions with other regulatory mechanisms, such as angiotensin II (Ang II) and nitric oxide (NO), are not well defined. This study has evaluated the relative contribution of both COX isoforms in regulating renal function when NO synthesis is reduced with and without a simultaneous increment in Ang II levels. The renal responses to a nonselective (meclofenamate) or a selective COX2 (nimesulide) inhibitor were examined in dogs pretreated with L-NAME with or without an intrarenal Ang II infusion. Meclofenamate induced a greater (P < 0.05) renal vasoconstriction than nimesulide in dogs pretreated with L-NAME. This vasoconstriction seems to be Ang II-dependent because it was reduced (P < 0.05) by captopril administration. Meclofenamate also induced a greater (P < 0.05) renal vasoconstriction than that elicited by nimesulide in dogs with reduced NO synthesis and elevated Ang II levels. The renal vasoconstriction induced by nimesulide but not that elicited by meclofenamate in dogs pretreated with L-NAME and Ang II, decreased (P < 0.05) during an extracellular volume expansion. These results demonstrate that the nonselective COX inhibition induces a greater renal vasoconstriction than that elicited by the selective COX2 inhibition when NO synthesis is reduced, and when NO synthesis is reduced and Ang II levels are elevated.

A comparison of linaclotide and lubiprostone dosing regimens on ion transport responses in human colonic mucosa

Linaclotide, a synthetic guanylyl cyclase C (GC-C) agonist, and the prostone analog, Lubiprostone, are approved to manage chronic idiopathic constipation and constipation-predominant irritable bowel syndrome. Lubiprostone also protects intestinal mucosal barrier function in ischemia. GC-C signaling regulates local fluid balance and other components of intestinal mucosal homeostasis including epithelial barrier function. The aim of this study was to compare if select dosing regimens differentially affect linaclotide and lubiprostone modulation of ion transport and barrier properties of normal human colonic mucosa. Normal sigmoid colon biopsies from healthy subjects were mounted in Ussing chambers. Tissues were treated with linaclotide, lubiprostone, or vehicle to determine effects on short-circuit current (I_sc). Subsequent I_sc responses to the cAMP agonist, forskolin, and the calcium agonist, carbachol, were also measured to assess if either drug caused desensitization. Barrier properties were assessed by measuring transepithelial electrical resistance. I_sc responses to linaclotide and lubiprostone were significantly higher than vehicle control when administered bilaterally or to the mucosal side only. Single versus cumulative concentrations of linaclotide showed differences in efficacy while cumulative but not single dosing caused desensitization to forskolin. Lubiprostone reduced forskolin responses under all conditions. Linaclotide and lubiprostone exerted a positive effect on TER that was dependent on the dosing regimen. Linaclotide and lubiprostone increase ion transport responses across normal human colon but linaclotide displays increased sensitivity to the dosing regimen used. These findings may have implications for dosing protocols of these agents in patients with constipation.

Basolateral sorting of chloride channel 2 is mediated by interactions between a dileucine motif and the clathrin adaptor AP-1

ClC-2 is a ubiquitous chloride channel that regulates cell volume, ion transport, and acid-base balance. Mice knocked out for ClC-2 are blind and sterile. Basolateral localization of ClC-2 in epithelia is mediated by the interaction of a dileucine motif with a highly conserved pocket in the γ1-σ1A hemicomplex of AP-1.

In spite of the many key cellular functions of chloride channels, the mechanisms that mediate their subcellular localization are largely unknown. ClC-2 is a ubiquitous chloride channel usually localized to the basolateral domain of epithelia that regulates cell volume, ion transport, and acid–base balance; mice knocked out for ClC-2 are blind and sterile. Previous work suggested that CLC-2 is sorted basolaterally by TIFS⁸¹²LL, a dileucine motif in CLC-2's C-terminal domain. However, our in silico modeling of ClC-2 suggested that this motif was buried within the channel's dimerization interface and identified two cytoplasmically exposed dileucine motifs, ESMI⁶²³LL and QVVA⁶³⁵LL, as candidate sorting signals. Alanine mutagenesis and trafficking assays support a scenario in which ESMI⁶²³LL acts as the authentic basolateral signal of ClC-2. Silencing experiments and yeast three-hybrid assays demonstrated that both ubiquitous (AP-1A) and epithelium-specific (AP-1B) forms of the tetrameric clathrin adaptor AP-1 are capable of carrying out basolateral sorting of ClC-2 through interactions of ESMI⁶²³LL with a highly conserved pocket in their γ1-σ1A hemicomplex.

Chloride channelopathies of ClC-2

Chloride channels (ClCs) have gained worldwide interest because of their molecular diversity, widespread distribution in mammalian tissues and organs, and their link to various human diseases. Nine different ClCs have been molecularly identified and functionally characterized in mammals. ClC-2 is one of nine mammalian members of the ClC family. It possesses unique biophysical characteristics, pharmacological properties, and molecular features that distinguish it from other ClC family members. ClC-2 has wide organ/tissue distribution and is ubiquitously expressed. Published studies consistently point to a high degree of conservation of ClC-2 function and regulation across various species from nematodes to humans over vast evolutionary time spans. ClC-2 has been intensively and extensively studied over the past two decades, leading to the accumulation of a plethora of information to advance our understanding of its pathophysiological functions; however, many controversies still exist. It is necessary to analyze the research findings, and integrate different views to have a better understanding of ClC-2. This review focuses on ClC-2 only, providing an analytical overview of the available literature. Nearly every aspect of ClC-2 is discussed in the review: molecular features, biophysical characteristics, pharmacological properties, cellular function, regulation of expression and function, and channelopathies.

Cell biology and physiology of CLC chloride channels and transporters

Proteins of the CLC gene family assemble to homo- or sometimes heterodimers and either function as Cl(-) channels or as Cl(-)/H(+)-exchangers. CLC proteins are present in all phyla. Detailed structural information is available from crystal structures of bacterial and algal CLCs. Mammals express nine CLC genes, four of which encode Cl(-) channels and five 2Cl(-)/H(+)-exchangers. Two accessory β-subunits are known: (1) barttin and (2) Ostm1. ClC-Ka and ClC-Kb Cl(-) channels need barttin, whereas Ostm1 is required for the function of the lysosomal ClC-7 2Cl(-)/H(+)-exchanger. ClC-1, -2, -Ka and -Kb Cl(-) channels reside in the plasma membrane and function in the control of electrical excitability of muscles or neurons, in extra- and intracellular ion homeostasis, and in transepithelial transport. The mainly endosomal/lysosomal Cl(-)/H(+)-exchangers ClC-3 to ClC-7 may facilitate vesicular acidification by shunting currents of proton pumps and increase vesicular Cl(-) concentration. ClC-3 is also present on synaptic vesicles, whereas ClC-4 and -5 can reach the plasma membrane to some extent. ClC-7/Ostm1 is coinserted with the vesicular H(+)-ATPase into the acid-secreting ruffled border membrane of osteoclasts. Mice or humans lacking ClC-7 or Ostm1 display osteopetrosis and lysosomal storage disease. Disruption of the endosomal ClC-5 Cl(-)/H(+)-exchanger leads to proteinuria and Dent's disease. Mouse models in which ClC-5 or ClC-7 is converted to uncoupled Cl(-) conductors suggest an important role of vesicular Cl(-) accumulation in these pathologies. The important functions of CLC Cl(-) channels were also revealed by human diseases and mouse models, with phenotypes including myotonia, renal loss of salt and water, deafness, blindness, leukodystrophy, and male infertility.

Chloride channel ClC-2 is a key factor in the development of DSS-induced murine colitis

BACKGROUND

Previously, it was shown that the chloride channel ClC-2 modulates intestinal tight junction (TJ) barrier function. The aim of the present study was to investigate the role of ClC-2 in epithelial barrier function and recovery in the event of epithelial injury.

METHODS

The role of ClC-2 was investigated in TJ barrier function in dextran sodium sulfate (DSS)-induced colitis in ClC-2 knockout mice and ClC-2 knockdown intestinal Caco-2 cells. Barrier function was measured electrophysiologically and by transepithelial mannitol fluxes. Selected TJ and associated proteins were Western blotted, cytokines were measured using quantitative PCR, and human colonic biopsies were examined with immunohistochemistry.

RESULTS

ClC-2 mice had a higher disease activity index, higher histological scores, and increased paracellular permeability compared with wild-type mice when treated with DSS. DSS-treated ClC-2 mice had increased claudin-2 expression, greater loss of occludin in the membrane, increased association of occludin with caveolin-1, and significantly increased tumor necrosis factor-α and interleukin-1β messenger RNA. ClC-2 knockdown in human intestinal Caco-2 cells resulted in a greater loss of epithelial resistance in the event of epithelial injury. The restoration of colonic barrier function after DSS colitis was delayed in ClC-2 mice. In human colonic biopsies, the protein and messenger RNA expression of ClC-2 was found to be reduced in patients with ulcerative colitis.

CONCLUSIONS

ClC-2 plays a critical role in experimental colitis in that its absence increases disease activity, reduces barrier function and recovery, and perturbs TJs. Furthermore, ClC-2 expression is markedly reduced in the colon of human patients with ulcerative colitis.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Irritable bowel syndrome: methods, mechanisms, and pathophysiology. The confluence of increased permeability, inflammation, and pain in irritable bowel syndrome

Irritable bowel syndrome (IBS) is one of the most common gastrointestinal ailments among those seeking health care for gastrointestinal disorders. Despite its prevalence, IBS pathophysiology is still not completely understood. Continued elucidation of IBS etiological mechanisms will lead to a greater appreciation of possible therapeutic targets. In the past decade, there has been increasing focus on the possible connection between increased intestinal mucosal permeability, inflammation, and visceral hypersensitivity. Increased permeability in subsets of IBS patients has been observed and the possible mechanisms underlying this defect are just beginning to be understood. The objectives of this review are to summarize the role of the healthy intestinal epithelium as a barrier between the lumen and the rest of the body with a focus on tight junctions; to examine the lines of evidence that suggest that different triggers lead to increased intestinal mucosal permeability and disruption of tight junctions in IBS patients; and to explore how this increased permeability may elicit immune responses that affect afferent nerves, resulting in the pain associated with IBS.

Gastro protective properties of the novel prostone SPI-8811 against acid-injured porcine mucosa

AIM: To evaluate the protective properties of novel prostone ClC-2 agonist SPI-8811 in porcine model of gastric acid injury.

METHODS: Porcine gastric mucosa was mounted in Ussing chambers and injured by bathing mucosal tissues in an HCl Ringer’s solution (pH = 1.5) with or without SP1-8811 (1 μmol/L), cystic fibrosis transmembrane conductance regulator (CFTR) inhibitor (inhibitor 172, 10 μmol/L, apical) and ClC-2 inhibitor ZnCl₂, 300 μmol/L, apical), on the apical surface of tissues. Transepithelial resistance and mucosal-to-serosal ³H-mannitol fluxes were measured over a 90-min period. Tissues were analyzed by morph metric techniques, Immunofluorescence and by western blots.

RESULTS: Compared with control tissues, acid exposure decreased transepithelial electrical resistance (TER) and increased ³H-mannitol flux. Pretreatment of gastric mucosa with SPI-8811 was protective against acid-induced decreases in TER (TER, 50 Ω.cm²vs 100 Ω.cm²) and abolished increases in flux (³H-mannitol flux, 0.10 μmol/L.cm²vs 0.04 μmol/L.cm²). Evidence of histological damage in the presence of acid was markedly attenuated by SPI-0811. Immunofluorescence and western analysis for occludin revealed enhanced localization to the region of the tight junction (TJ) after treatment with SPI-8811. Pretreatment with the ClC-2 inhibitor ZnCl₂, but not the selective CFTR inhibitor 172, attenuated SPI-8811-mediated mucosal protection, suggesting a role for ClC-2. Prostone may serve both protective and reparative roles in injured tissues.

CONCLUSION: ClC-2 agonist SPI-8811 stimulated enhancement of mucosal barrier function by protecting TJ protein occludin in porcine gastric mucosa and thus protected the gastric acid injury in porcine stomach.

Prolactin and dexamethasone regulate second messenger-stimulated cl(-) secretion in mammary epithelia

Mammary gland ion transport is essential for lactation and is regulated by prolactin and glucocorticoids. This study delineates the roles of prolactin receptors (PRLR) and long-term prolactin and dexamethasone (P-D)-mediation of [Ca²⁺]_i and Cl⁻ transport in HC-11 cells. P-D (24 h) suppressed ATP-induced [Ca²⁺]_i. This may be due to decreased Ca²⁺ entry since P-D decreased transient receptor potential channel 3 (TRPC3) but not secretory pathway Ca²⁺-ATPase 2 (SPCA2) mRNA. ATP increased Cl⁻ transport, measured by iodide (I⁻) efflux, in control and P-D-treated cells. P-D enhanced I⁻ efflux response to cAMP secretagogues without altering Cl⁻ channels or NKCC cotransporter expression. HC-11 cells contain only the long form of PRLR (PRLR-L). Since the short isoform, PRLR-S, is mammopoietic, we determined if transfecting PRLR-S (rs) altered PRLR-L-mediated Ca²⁺ and Cl⁻ transport. Untreated rs cells showed an attenuated [Ca²⁺]_i response to ATP with no further response to P-D, in contrast to vector-transfected (vtc) controls. P-D inhibited TRPC3 in rs and vtc cells but increased SPCA2 only in rs cells. As in wild-type, cAMP-stimulated Cl⁻ transport, in P-D-treated vtc and rs cells. In summary, 24 h P-D acts via PRLR-L to attenuate ATP-induced [Ca²⁺]_i and increase cAMP-activated Cl⁻ transport. PRLR-S fine-tunes these responses underscoring its mammopoietic action.

Severe defects in absorptive ion transport in distal colons of mice that lack ClC-2 channels

BACKGROUND & AIMS

The fluid secretion model predicts that intestinal obstruction disorders can be alleviated by promoting epithelial Cl(-) secretion. The adenosine 3',5'-cyclic monophosphate (cAMP)-activated anion channel CFTR mediates Cl(-)-dependent fluid secretion in the intestine. Although the role of the ClC-2 channel has not been determined in the intestine, this voltage-gated Cl(-) channel might compensate for the secretory defects observed in patients with cystic fibrosis and other chronic constipation disorders. We investigated whether mice that lack ClC-2 channels (Clcn2(-/-)) have defects in intestinal ion transport.

METHODS

Immunolocalization and immunoblot analyses were used to determine the cellular localization and the amount of ClC-2 expressed in mouse early distal colon (EDC) and late distal colon (LDC). Colon sheets from wild-type and Clcn2(-/-) littermates were mounted in Ussing chambers to determine transepithelial bioelectrical parameters and Na(+), K(+), and Cl(-) fluxes.

RESULTS

Expression of ClC-2 was higher in the basolateral membrane of surface cells in the EDC compared with the LDC, with little expression in crypts. Neither cAMP nor Ca(2+)-induced secretion of Cl(-) was affected in the EDC or LDC of Clcn2(-/-) mice, whereas the amiloride-sensitive short-circuit current was increased approximately 3-fold in Clcn2(-/-) EDC compared with control littermates. Conversely, electroneutral Na(+), K(+), and Cl(-) absorption was dramatically reduced in colons of Clcn2(-/-) mice.

CONCLUSIONS

Basolateral ClC-2 channels are required for colonic electroneutral absorption of NaCl and KCl. The increase in the amiloride-sensitive short-circuit current in Clcn2(-/-) mice revealed a compensatory mechanism that is activated in the colons of mice that lack the ClC-2 channel.

Chloride channel ClC-2 modulates tight junction barrier function via intracellular trafficking of occludin

Previously, we have demonstrated that the chloride channel ClC-2 modulates intestinal mucosal barrier function. In the present study, we investigated the role of ClC-2 in epithelial barrier development and maintenance in Caco-2 cells. During early monolayer formation, silencing of ClC-2 with small interfering (si)RNA led to a significant delay in the development of transepithelial resistance (TER) and disruption of occludin localization. Proteomic analysis employing liquid chromatography-mass spectrometry /mass spectrometry revealed association of ClC-2 with key proteins involved in intracellular trafficking, including caveolin-1 and Rab5. In ClC-2 siRNA-treated cells, occludin colocalization with caveolin-1 was diffuse and in the subapical region. Subapically distributed occludin in ClC-2 siRNA-treated cells showed marked colocalization with Rab5. To study the link between ClC-2 and trafficking of occludin in confluent epithelial monolayers, a Caco-2 cell clone expressing ClC-2 short hairpin (sh)RNA was established. Disruption of caveolae with methyl-β-cyclodextrin (MβCD) caused a marked drop in TER and profound redistribution of caveolin-1-occludin coimmunofluorescence in ClC-2 shRNA cells. In ClC-2 shRNA cells, focal aggregations of Rab5-occludin coimmunofluorescence were present within the cytoplasm. Wortmannin caused an acute fall in TER in ClC-2 shRNA cells and subapical, diffuse redistribution of Rab5-occludin coimmunofluorescence in ClC-2 shRNA cells. An endocytosis and recycling assay for occludin revealed higher basal rate of endocytosis of occludin in ClC-2 shRNA cells. Wortmannin significantly reduced the rate of recycling of occludin in ClC-2 shRNA cells. These data clearly indicate that ClC-2 plays an important role in the modulation of tight junctions by influencing caveolar trafficking of the tight junction protein occludin.

Lubiprostone targets prostanoid EP₄ receptors in ovine airways

BACKGROUND AND PURPOSE

Lubiprostone, a prostaglandin E₁ derivative, is reported to activate ClC-2 chloride channels located in the apical membranes of a number of transporting epithelia. Lack of functioning CFTR chloride channels in epithelia is responsible for the genetic disease cystic fibrosis, therefore, surrogate channels that can operate independently of CFTR are of interest. This study explores the target receptor(s) for lubiprostone in airway epithelium.

EXPERIMENTAL APPROACH

All experiments were performed on the ventral tracheal epithelium of sheep. Epithelia were used to measure anion secretion from the apical surface as short circuit current or as fluid secretion from individual airway submucosal glands, using an optical method.

KEY RESULTS

The EP₄ antagonists L-161982 and GW627368 inhibited short circuit current responses to lubiprostone, while EP₁(,)₂(&)₃ receptor antagonists were without effect. Similarly, lubiprostone induced secretion in airway submucosal glands was inhibited by L-161982. L-161982 effectively competed with lubiprostone with a K(d) value of 0.058 µM, close to its value for binding to human EP₄ receptors (0.024 µM). The selective EP₄ agonist L-902688 and lubiprostone behaved similarly with respect to EP₄ receptor antagonists. Results of experiments with H89, a protein kinase A inhibitor, were consistent with lubiprostone acting through a G(s) -protein coupled EP₄ receptor/cAMP cascade.

CONCLUSIONS AND IMPLICATIONS

Lubiprostone-induced short-circuit currents and submucosal gland secretions were inhibited by selective EP₄ receptor antagonists. The results suggest EP₄ receptor activation by lubiprostone triggers cAMP production necessary for CFTR activation and the secretory responses, a possibility precluded in CF tissues.

Lubiprostone reverses the inhibitory action of morphine on mucosal secretion in human small intestine

BACKGROUND AND AIMS

Treatments with morphine or opioid agonists cause constipation. Lubiprostone is approved for treatment of adult idiopathic constipation and constipation-predominant IBS in adult women. We tested whether lubiprostone can reverse morphine-suppression of mucosal secretion in human intestine and explored the mechanism of action.

METHODS

Fresh segments of jejunum discarded during Roux-En-Y gastric bypass surgeries were used. Changes in short-circuit current (ΔIsc) were recorded in Ussing flux chambers as a marker for electrogenic chloride secretion during pharmacological interactions between morphine, prostaglandin receptor antagonists, chloride channel blockers and lubiprostone.

RESULTS

Morphine suppressed basal Isc. Lubiprostone reversed morphine suppression of basal Isc. Lubiprostone, applied to the mucosa in concentrations ranging from 3 nM to 30 μM, evoked increases in Isc in concentration-dependent manner when applied to the mucosal side of muscle-stripped preparations. Blockade of enteric nerves did not change stimulation of Isc by lubiprostone. Removal of chloride or application of bumetanide or NPPB suppressed or abolished responses to lubiprostone. Antagonists acting at CFTR channels and prostaglandin EP(4) receptors, but not at E(1), EP(1-3) receptors, partially suppressed stimulation of Isc by lubiprostone.

CONCLUSIONS

Antisecretory action of morphine results from suppression of excitability of secretomotor neurons in the enteric nervous system. Lubiprostone, which does not affect enteric neurons directly, bypasses the action of morphine by directly opening mucosal chloride channels.

Update on the mechanisms of gastric acid secretion

Acid-related disorders represent a major healthcare concern. In recent years, our understanding of the physiologic processes underlying gastric acid secretion has improved notably. The identity of several apical ion transport proteins, which are necessary for acid secretion to take place, has been resolved. The recent developments have uncovered potential therapeutic targets for the treatment of acid-related disorders. This brief review provides an update on the mechanisms of gastric acid secretion, with a particular focus on apical ion transport.

Lubiprostone: evaluation of the newest medication for the treatment of adult women with constipation-predominant irritable bowel syndrome

Irritable bowel syndrome (IBS) is a chronic disorder that affects primarily female patients and is thought also to afflict approximately 7%-10% of the population of the Western World. Although bowel habits may change over the course of years, patients with IBS are characterized according to their predominant bowel habit, constipation (IBS-C), diarrhea (IBS-D), or mixed type (IBS-M), and treatments are focused toward the predominant symptom. Current treatments for IBS-C have included fiber, antispasmodics, osmotic and stimulant laxatives, and the now severely limited 5-HT(4) agonist tegaserod. No one agent has been universally successful in the treatment of this bothersome syndrome and the search for new agents continues. Lubiprostone (Amitiza(®)), a novel compound, is a member of a new class of agents called prostones and was approved for the treatment of chronic idiopathic constipation in 2006 at a dose of 24 μg twice daily and then in 2008 for the treatment of IBS-C in women only at a dose of 8 μg twice daily. Its purported mechanism is as a type 2 chloride channel activator, but recent evidence suggests that it may also work at the cystic fibrosis transport receptor. This article will compare the newly proposed mechanism of action of this compound to the purported mechanism and review the structure, pharmacology, safety, efficacy, and tolerability of this new therapeutic option. Clinical trial data leading to the approval of this agent for the treatment of IBS-C and the gender-based understanding of IBS, as well as this agent's place among existing and emerging therapies, will be examined.

Toxin mediated diarrhea in the 21 century: the pathophysiology of intestinal ion transport in the course of ETEC, V. cholerae and rotavirus infection

An estimated 4 billion episodes of diarrhea occur each year. As a result, 2–3 million children and 0.5–1 million adults succumb to the consequences of this major healthcare concern. The majority of these deaths can be attributed to toxin mediated diarrhea by infectious agents, such as E. coli, V. cholerae or Rotavirus. Our understanding of the pathophysiological processes underlying these infectious diseases has notably improved over the last years. This review will focus on the cellular mechanism of action of the most common enterotoxins and the latest specific therapeutic approaches that have been developed to contain their lethal effects.

ClC-2 regulates mucosal barrier function associated with structural changes to the villus and epithelial tight junction

We have previously shown an important role of the chloride channel ClC-2 in orchestrating repair of tight junctions in ischemia-injured mucosa. In this study, we examined the role of ClC-2 in regulating barrier function of normal murine intestinal mucosa. Ex vivo, ClC-2-/- ileal mucosa mounted in Ussing chambers had significantly higher transepithelial electrical resistance (TER) and reduced [(3)H]mannitol mucosal-to-serosal flux compared with wild-type (WT) mouse mucosa. We also noted that ileum from ClC-2-/- mice had a significantly reduced in vivo [(3)H]mannitol blood-to-lumen clearance compared with WT animals. By scanning electron microscopy, flat leaflike villi were found to have tapering, rounded apical tips in ClC-2-/- mucosa. By transmission electron microscopy, the apical intercellular tight junctions in ClC-2-/- intestine revealed lateral membranes that were less well defined but closely aligned compared with electron-dense and closely apposed tight junctions in WT mucosa. The width of apical tight junctions was significantly reduced in ClC-2-/- intestine. Such an alteration in tight junction ultrastructure was also noted in the testicular tissue from ClC-2-/- mice. The ClC-2-/- intestinal mucosa had reduced expression of phospho-myosin light chain (MLC), and inhibition of myosin light chain kinase (MLCK) in WT mucosa partially increased TER toward the TER in ClC-2-/- intestine. Contrary to our prior work on the reparative role of ClC-2 in injured mucosa, this study indicates that ClC-2 reduces barrier function in normal mucosa. The mechanisms underlying these differing roles are not entirely clear, although ultrastructural morphology of tight junctions and MLCK appear to be important to the function of ClC-2 in normal mucosa.

Epac1 mediates protein kinase A-independent mechanism of forskolin-activated intestinal chloride secretion

Intestinal Cl⁻ secretion is stimulated by cyclic AMP (cAMP) and intracellular calcium ([Ca²⁺]_i). Recent studies show that protein kinase A (PKA) and the exchange protein directly activated by cAMP (Epac) are downstream targets of cAMP. Therefore, we tested whether both PKA and Epac are involved in forskolin (FSK)/cAMP-stimulated Cl⁻ secretion. Human intestinal T84 cells and mouse small intestine were used for short circuit current (I_sc) measurement in response to agonist-stimulated Cl⁻ secretion. FSK-stimulated Cl⁻ secretion was completely inhibited by the additive effects of the PKA inhibitor, H89 (1 µM), and the [Ca²⁺]_i chelator, 1,2-bis-(o-aminophenoxy)-ethane-N,N,N’,N’-tetraacetic acid, tetraacetoxymethyl ester (BAPTA-AM; 25 µM). Both FSK and the Epac activator 8-pCPT-2’-O-Me-cAMP (50 µM) elevated [Ca²⁺]_i, activated Ras-related protein 2, and induced Cl⁻ secretion in intact or basolateral membrane–permeabilized T84 cells and mouse ileal sheets. The effects of 8-pCPT-2’-O-Me-cAMP were completely abolished by BAPTA-AM, but not by H89. In contrast, T84 cells with silenced Epac1 had a reduced I_sc response to FSK, and this response was completely inhibited by H89, but not by the phospholipase C inhibitor U73122 or BAPTA-AM. The stimulatory effect of 8-pCPT-2’-O-Me-cAMP on Cl⁻ secretion was not abolished by cystic fibrosis transmembrane conductance (CFTR) inhibitor 172 or glibenclamide, suggesting that CFTR channels are not involved. This was confirmed by lack of effect of 8-pCPT-2’-O-Me-cAMP on whole cell patch clamp recordings of CFTR currents in Chinese hamster ovary cells transiently expressing the human CFTR channel. Furthermore, biophysical characterization of the Epac1-dependent Cl⁻ conductance of T84 cells mounted in Ussing chambers suggested that this conductance was hyperpolarization activated, inwardly rectifying, and displayed a Cl⁻>Br⁻>I⁻ permeability sequence. These results led us to conclude that the Epac-Rap-PLC-[Ca²⁺]_i signaling pathway is involved in cAMP-stimulated Cl⁻ secretion, which is carried by a novel, previously undescribed Cl⁻ channel.

Enteric nervous system: sensory physiology, diarrhea and constipation

PURPOSE OF REVIEW

The enteric nervous system integrates secretion and motility into homeostatic patterns of behavior susceptible to disorder. Progress in understanding mechanosensory detection in these processes, disordered enteric nervous system integration in diarrhea and constipation and pharmacotherapy is summarized.

RECENT FINDINGS

Most neurons in the enteric nervous system discharge in response to distortion. Drugs acting directly to open chloride conductance channels in the mucosal epithelium are therapeutic options for constipation.

SUMMARY

Mechanoreception is required for negative feedback control. At issue is identification of the neurons that fulfil the requirement for mechanoreception. Understanding secretomotor neurons is basic to understanding neurogenic secretory diarrhea and constipation and therapeutic strategies. A strategy for treatment of chronic constipation is development of agents that act directly to open Cl channels, which thereby increases the liquidity of the luminal contents. Lubiprostone, a recently Food and Drug Administration-approved drug, increases intraluminal liquidity by opening Cl channels. The future for the drug is clouded by controversy over whether its action is directly at one or the other of chloride channel type 2 (ClC-2) or cystic fibrosis transmembrane conductance regulator (CFTR) chloride channels or both and whether action reflects involvement of G protein-coupled prostaglandin receptors expressed by mucosal epithelial cells.

Ion transport in the small intestine

PURPOSE OF REVIEW

The 2009 review on small intestinal ion transport, in this series, focused on recent advances in duodenal bicarbonate secretion, the importance of scaffolding proteins and the pathophysiology of inflammation-associated diarrhea. The current review focuses on advances in ion-coupled solute transport, the dynamic role of the paracellular pathway in transepithelial-fluid transport and of elucidating the cellular basis of diarrheas associated with enteric infections.

RECENT FINDINGS

In understanding the cellular pathophysiology underlying diarrheal diseases, there is increased focus on the role of altering Na absorptive mechanisms as well as the role of the paracellular pathway. This is not to minimize the role of Cl-secretory pathways, especially cystic fibrosis transmembrane conductance regulator (CFTR), which continues to have pleiotropic roles in modulating other transporters. The Na-glucose cotransporter (SGLT) was the first transporter ever to be cloned. Twenty-one years later, with another first, the crystal structure of the related Na-galactose transporter has been described and opens new avenues to understand structure-function relationships and intelligent drug design for transporters.

SUMMARY

Progress continues to be made on integrating information obtained from reductionist models into more complex in-vivo animal models and where possible in human studies. Recognition of the coordinated regulation of cellular Na absorptive and Cl-secretory pathways together with the paracellular route in health and disease will help develop a more holistic picture of the multifaceted nature of small intestinal ion transport.

Revisiting the parietal cell

The parietal cell is responsible for secreting concentrated hydrochloric acid into the gastric lumen. To fulfill this task, it is equipped with a broad variety of functionally coupled apical and basolateral ion transport proteins. The concerted scientific effort over the last years by a variety of researchers has provided us with the molecular identity of many of these transport mechanisms, thereby contributing to the clarification of persistent controversies in the field. This article will briefly review the current model of parietal cell physiology and ion transport in particular and will update the existing models of apical and basolateral transport in the parietal cell.

Lubiprostone stimulates duodenal bicarbonate secretion in rats

BACKGROUND

Lubiprostone, a bicyclic fatty acid, is used for the treatment of chronic constipation. No published study has addressed the effect of lubiprostone on intestinal ion secretion in vivo.

AIM

The aim of this study was to test the hypothesis that lubiprostone augments duodenal HCO(3) (-) secretion (DBS).

METHODS

Rat proximal duodenal loops were perfused with pH 7.0 Krebs, control vehicle (medium-chain triglycerides), or lubiprostone (0.1-10 microM). We measured DBS with flow-through pH and CO(2) electrodes, perfusate [Cl(-)] with a Cl(-) electrode, and water flux using a non-absorbable ferrocyanide marker. Some rats were pretreated with a potent, selective CFTR antagonist, CFTR(inh)-172 (1 mg/kg, ip), 1 h before experiments.

RESULTS

Perfusion of lubiprostone concentration dependently increased DBS, whereas net Cl(-) output and net water output were only increased at 0.1 microM, compared with vehicle. CFTR(inh)-172 reduced lubiprostone (10 microM)-induced DBS increase, whereas net Cl(-) output was also unchanged. Nevertheless, CFTR(inh)-172 reduced basal net water output, which was reversed by lubiprostone. Furthermore, lubiprostone-induced DBS was inhibited by EP4 receptor antagonist, not by an EP1/2 receptor antagonist or by indomethacin pretreatment.

CONCLUSIONS

In this first study of the effect of lubiprostone on intestinal ion secretion in vivo, lubiprostone stimulated CFTR-dependent DBS without changing net Cl(-) secretion. This effect supports the hypothesis that Cl(-) secreted by CFTR is recycled across the apical membrane by anion exchangers. Recovery of water output during CFTR inhibition suggests that lubiprostone may improve the intestinal phenotype in CF patients. Furthermore, increased DBS suggests that lubiprostone may protect the duodenum from acid-induced injury via EP4 receptor activation.

Activation of intestinal Cl- secretion by lubiprostone requires the cystic fibrosis transmembrane conductance regulator

BACKGROUND & AIMS

Lubiprostone alleviates constipation by stimulating intestinal fluid secretion, purportedly through activation of ClC-2-type Cl(-) channels. Intestinal obstruction is also a recurrent cause of distress in cystic fibrosis (CF) patients, caused by loss of CF transmembrane conductance regulator (CFTR) Cl(-) channel activity. Because ClC-2 recruitment might be beneficial to CF patients, we investigated lubiprostone's mode of action.

METHODS

Cl(-) transport was measured in an Ussing chamber, in 3 model systems: (1) T84 colonocytes, (2) intestinal epithelium of wild-type and CF mice, and (3) intestinal epithelium of CF patients and controls.

RESULTS

In T84 monolayers, lubiprostone induced a robust secretory response. Selective permeabilization of the basolateral plasma membrane revealed that lubiprostone activated an apical Cl(-) conductance. The lubiprostone response was attenuated by H89, an inhibitor of the cAMP-dependent protein kinase, and lubiprostone precluded responsiveness to the cAMP agonist forskolin. CFTR blockage by CFTRinh172, but not ClC-2 blockage by CdCl(2), inhibited the lubiprostone response. Lubiprostone induced a CdCl(2)-insensitive secretory response in mouse intestine, but failed to induce intestinal Cl(-) secretion in Cftr-null mice. Correspondingly, lubiprostone induced a secretory response in human intestinal epithelium, but not in tissue of CF patients. The EP(4)-type prostanoid receptor antagonist L-161,982 blocked the lubiprostone response in all 3 models studied. In T84 cells, lubiprostone induced a rise in cAMP levels that was sensitive to EP(4)-receptor blockage.

CONCLUSIONS

Lubiprostone enhances intestinal Cl(-) and fluid secretion via prostanoid receptor signaling, triggering activation of CFTR. Therefore, it is of limited use for treatment of CF-related intestinal disease.

Lubiprostone stimulates secretion from tracheal submucosal glands of sheep, pigs, and humans

Lubiprostone, a putative ClC-2 chloride channel opener, has been investigated for its effects on airway epithelia (tracheas). Lubiprostone is shown to increase submucosal gland secretion in pigs, sheep, and humans and to increase short-circuit current (SCC) in the surface epithelium of pigs and sheep. Use of appropriate blocking agents and ion-substitution experiments shows anion secretion is the driving force for fluid formation in both glands and surface epithelium. From SCC concentration-response relations, it is shown that for apical lubiprostone K(d) = 10.5 nM with a Hill slope of 1.08, suggesting a single type of binding site and, from the speed of the response, close to the apical surface, confirmed the rapid blockade by Cd ions. Responses to lubiprostone were reversible and repeatable, responses being significantly larger with ventral compared with dorsal epithelium. Submucosal gland secretion rates following basolateral lubiprostone were, respectively, 0.2, 0.5, and 0.8 nl gl(-1) min(-1) in humans, sheep, and pigs. These rates dwarf any contribution surface secretion adds to the accumulation of surface liquid under the influence of lubiprostone. Lubiprostone stimulated gland secretion in two out of four human cystic fibrosis (CF) tissues and in two of three disease controls, chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis (COPD/IPF), but in neither type of tissue was the increase significant. Lubiprostone was able to increase gland secretion rates in normal human tissue in the continuing presence of a high forskolin concentration. Lubiprostone had no spasmogenic activity on trachealis muscle, making it a potential agent for increasing airway secretion that may have therapeutic utility.

Comparison of the chloride channel activator lubiprostone and the oral laxative Polyethylene Glycol 3350 on mucosal barrier repair in ischemic-injured porcine intestine

AIM: To investigate the effects of lubiprostone and Polyethylene Glycol 3350 (PEG) on mucosal barrier repair in ischemic-injured porcine intestine.

METHODS: Ileum from 6 piglets (approximately 15 kg body weight) was subjected to ischemic conditions by occluding the local mesenteric circulation for 45 min in vivo. Ileal tissues from each pig were then harvested and mounted in Ussing chambers and bathed in oxygenated Ringer’s solution in vitro. Intestinal barrier function was assessed by measuring transepithelial electrical resistance (TER) and mucosal-to-serosal fluxes of ³H-mannitol and ¹⁴C-inulin. Statistical analyses of data collected over a 120-min time course included 2-way ANOVA for the effects of time and treatment on indices of barrier function.

RESULTS: Application of 1 μmol/L lubiprostone to the mucosal surface of ischemic-injured ileum in vitro induced significant elevations in TER compared to non-treated tissue. Lubiprostone also reduced mucosal-to-serosal fluxes of ³H-mannitol and ¹⁴C-inulin. Alternatively, application of a polyethylene laxative (PEG, 20 mmol/L) to the mucosal surface of ischemic tissues significantly increased flux of ³H-mannitol and ¹⁴C-inulin.

CONCLUSION: This experiment demonstrates that lubiprostone stimulates recovery of barrier function in ischemic intestinal tissues whereas the PEG laxative had deleterious effects on mucosal repair. These results suggest that, unlike osmotic laxatives, lubiprostone stimulates repair of the injured intestinal barrier.

ClC-2 is required for rapid restoration of epithelial tight junctions in ischemic-injured murine jejunum

BACKGROUND AND AIMS

Involvement of the epithelial chloride channel ClC-2 has been implicated in barrier recovery following ischemic injury, possibly via a mechanism involving ClC-2 localization to the tight junction. The present study investigated mechanisms of intestinal barrier repair following ischemic injury in ClC-2(-/-) mice.

METHODS

Wild type, ClC-2 heterozygous and ClC-2(-/-) murine jejunal mucosa was subjected to complete ischemia, after which recovery of barrier function was monitored by measuring in vivo blood-to-lumen clearance of (3)H-mannitol. Tissues were examined by light and electron microscopy. The role of ClC-2 in re-assembly of the tight junction during barrier recovery was studied by immunoblotting, immunolocalization and immunoprecipitation.

RESULTS

Following ischemic injury, ClC-2(-/-) mice had impaired barrier recovery compared to wild type mice, defined by increases in epithelial paracellular permeability independent of epithelial restitution. The recovering ClC-2(-/-) mucosa also had evidence of ultrastructural paracellular defects. The tight junction proteins occludin and claudin-1 shifted significantly to the detergent soluble membrane fraction during post-ischemic recovery in ClC-2(-/-) mice whereas wild type mice had a greater proportion of junctional proteins in the detergent insoluble fraction. Occludin was co-immunoprecipitated with ClC-2 in uninjured wild type mucosa, and the association between occludin and ClC-2 was re-established during ischemic recovery. Based on immunofluorescence studies, re-localization of occludin from diffuse sub-apical areas to apical tight junctions was impaired in ClC-2(-/-) mice.

CONCLUSIONS

These data demonstrate a pivotal role of ClC-2 in recovery of the intestinal epithelium barrier by anchoring assembly of tight junctions following ischemic injury.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.

A synthetic prostone activates apical chloride channels in A6 epithelial cells

The bicyclic fatty acid lubiprostone (formerly known as SPI-0211) activates two types of anion channels in A6 cells. Both channel types are rarely, if ever, observed in untreated cells. The first channel type was activated at low concentrations of lubiprostone (<100 nM) in >80% of cell-attached patches and had a unit conductance of approximately 3-4 pS. The second channel type required higher concentrations (>100 nM) of lubiprostone to activate, was observed in approximately 30% of patches, and had a unit conductance of 8-9 pS. The properties of the first type of channel were consistent with ClC-2 and the second with CFTR. ClC-2's unit current strongly inwardly rectified that could be best fit by models of the channel with multiple energy barrier and multiple anion binding sites in the conductance pore. The open probability and mean open time of ClC-2 was voltage dependent, decreasing dramatically as the patches were depolarized. The order of anion selectivity for ClC-2 was Cl > Br > NO(3) > I > SCN, where SCN is thiocyanate. ClC-2 was a "double-barreled" channel favoring even numbers of levels over odd numbers as if the channel protein had two conductance pathways that opened independently of one another. The channel could be, at least, partially blocked by glibenclamide. The properties of the channel in A6 cells were indistinguishable from ClC-2 channels stably transfected in HEK293 cells. CFTR in the patches had a selectivity of Cl > Br >> NO(3) congruent with SCN congruent with I. It outwardly rectified as expected for a single-site anion channel. Because of its properties, ClC-2 is uniquely suitable to promote anion secretion with little anion reabsorption. CFTR, on the other hand, could promote either reabsorption or secretion depending on the anion driving forces.

Alterations in the Cytoplasmic Domain of CLCN2 Result in Altered Gating Kinetics

Mutations in the human ClC-2 Cl(-) channel have been described to influence its function dramatically. To test for naturally occurring gene variants in a human population and their functionality, all 24 CLCN2 exons from a Central African population were sequenced. Six single amino acid exchanges in the intracellular N-terminus (P48R, R68H), in the pore domain (G199A), or in the intracellular C-terminus (R646Q, R725W, R747H) were identified at low frequency. Heterologous expression of these polymorphisms in Xenopus laevis oocytes demonstrated their functional significance as determined by two-electrode voltage-clamp. The polymorphisms R68H, R725W, and R747H exhibited faster voltage-stimulated gating as compared to the wild type channel, resulting in higher steady state currents of R725W. Probably due to decreased surface expression P48R, R68H, and R646Q mutants generated lower currents than the wild type channels. The inward currents of the mutated channels R725W, R747H, and G199A failed to increase during hypotonic swelling, a defect paralleled by impaired swelling-accelerated voltage-gating in one mutant (G199A). In conclusion, the Africans' gene pool comprises CLCN2 gene variants in the N-terminus, the C-terminus or the pore domain that affect surface expression and voltage- or cell-swelling-stimulated channel gating.