Role of CFTR in the colon

Gut microbiota in adults with cystic fibrosis: Implications for the severity of the CFTR gene mutation and nutritional status

BACKGROUND

Microbial dysbiosis has been linked to cystic fibrosis (CF); however, the composition of gut microbiota in adult CF patients in relation to severity of CF transmembrane conductance regulator (CFTR) gene mutation and nutritional status have not yet been explored. Study aimed to assess the gut microbiota composition in adults with CF, and its relationship with the severity of CFTR mutations, and BMI.

METHODS

Gut microbiota of 41 adults with CF, and 26 non-CF controls were compared using whole 16S rRNA gene sequencing. Differences in the microbial community between groups of patients classified according to the severity of CFTR mutations, and BMI were assessed. The alpha diversity, beta diversity, and taxa abundance were identified to reflect gut microbiota composition.

RESULTS

Results showed a significant decrease in alpha diversity of bacterial communities in CF compared to non-CF group, but no significant difference between the CF groups distinguished by the severity of CFTR mutations. However, more severe mutations were associated with the higher relative abundance of Bacteroides and Streptococcus and the lower relative abundance of Faecalibacterium and Blautia. Undernourished CF patients showed significantly lower alpha diversity compared to non-CF group and CF patients with BMI within the norm. Significant differences in the structure of the gut microbiota between CF and non-CF groups, as well as between BMI groups were also found.

CONCLUSIONS

Our research indicates that CF is associated with alterations in gut microbiota in adults. Additionally, in adult CF patients, the composition of the gut microbiota is also related to BMI.

Dietary Na+ depletion up-regulates NKCC1 expression and enhances electrogenic Cl- secretion in rat proximal colon

The corticosteroid hormone, aldosterone, markedly enhances K+ secretion throughout the colon, a mechanism critical to its role in maintaining overall K+ balance. Previous studies demonstrated that basolateral NKCC1 was up-regulated by aldosterone in the distal colon specifically to support K+ secretion-which is distinct from the more well-established role of NKCC1 in supporting luminal Cl- secretion. However, considerable segmental variability exists between proximal and distal colonic ion transport processes, especially concerning their regulation by aldosterone. Furthermore, delineating such region-specific effects has important implications for the management of various gastrointestinal pathologies. Experiments were therefore designed to determine whether aldosterone similarly up-regulates NKCC1 in the proximal colon to support K+ secretion. Using dietary Na+ depletion as a model of secondary hyperaldosteronism in rats, we found that proximal colon NKCC1 expression was indeed enhanced in Na+-depleted (i.e., hyperaldosteronemic) rats. Surprisingly, electrogenic K+ secretion was not detectable by short-circuit current (ISC) measurements in response to either basolateral bumetanide (NKCC1 inhibitor) or luminal Ba2+ (non-selective K+ channel blocker), despite enhanced K+ secretion in Na+-depleted rats, as measured by 86Rb+ fluxes. Expression of BK and IK channels was also found to be unaltered by dietary Na+ depletion. However, bumetanide-sensitive basal and agonist-stimulated Cl- secretion (ISC) were significantly enhanced by Na+ depletion, as was CFTR Cl- channel expression. These data suggest that NKCC1-dependent secretory pathways are differentially regulated by aldosterone in proximal and distal colon. Development of therapeutic strategies in treating pathologies related to aberrant colonic K+/Cl- transport-such as pseudo-obstruction or ulcerative colitis-may benefit from these findings.

Colonic Fluid and Electrolyte Transport 2022: An Update

Colonic epithelial cells are responsible for maintaining a delicate balance between luminal secretion and the absorption of fluids and ions. This review aims to discuss and update the model of colonic electrolyte secretion and absorption via the cystic fibrosis transmembrane regulator (CFTR), epithelial sodium channel (ENaC), Na-K-Cl cotransporters (NKCC1 and 2), Na-H exchangers (NHE1–4), colonic H,KATPase, and several other key components involved in multi-level transepithelial ion transport. Developments in our understanding of the activity, regulation, localization, and relationships of these ion transporters and their interactions have helped forge a more robust understanding of colonic ion movement that accounts for the colonic epithelium’s role in mucosal pH modulation, the setting of osmotic gradients pivotal for fluid retention and secretion, and cell death regulation. Deviations from homeostatic ion transport cause diarrhea, constipation, and epithelial cell death and contribute to cystic fibrosis, irritable bowel syndrome (IBS), ulcerative colitis, and cancer pathologies. Signal transduction pathways that regulate electrolyte movement and the regulatory relationships between various sensors and transporters (CFTR as a target of CaSR regulation and as a regulator of ENaC and DRA, for example) are imperative aspects of a dynamic and comprehensive model of colonic ion homeostasis.

Strategies for cystic fibrosis transmembrane conductance regulator inhibition: from molecular mechanisms to treatment for secretory diarrhoeas

Cystic fibrosis transmembrane conductance regulator (CFTR) is an unusual ABC transporter. It acts as an anion‐selective channel that drives osmotic fluid transport across many epithelia. In the gut, CFTR is crucial for maintaining fluid and acid‐base homeostasis, and its activity is tightly controlled by multiple neuro‐endocrine factors. However, microbial toxins can disrupt this intricate control mechanism and trigger protracted activation of CFTR. This results in the massive faecal water loss, metabolic acidosis and dehydration that characterize secretory diarrhoeas, a major cause of malnutrition and death of children under 5 years of age. Compounds that inhibit CFTR could improve emergency treatment of diarrhoeal disease. Drawing on recent structural and functional insight, we discuss how existing CFTR inhibitors function at the molecular and cellular level. We compare their mechanisms of action to those of inhibitors of related ABC transporters, revealing some unexpected features of drug action on CFTR. Although challenges remain, especially relating to the practical effectiveness of currently available CFTR inhibitors, we discuss how recent technological advances might help develop therapies to better address this important global health need.

Anoctamin 1/TMEM16A controls intestinal Cl- secretion induced by carbachol and cholera toxin

Calcium-activated chloride channels (CaCCs) mediate numerous physiological functions and are best known for the transport of electrolytes and water in epithelia. In the intestine, CaCC currents are considered necessary for the secretion of fluid to protect the intestinal epithelium. Although genetic ablation of ANO1/TMEM16A, a gene encoding a CaCC, reduces the carbachol-induced secretion of intestinal fluid, its mechanism of action is still unknown. Here, we confirm that ANO1 is essential for the secretion of intestinal fluid. Carbachol-induced transepithelial currents were reduced in the proximal colon of Ano1-deficient mice. Surprisingly, cholera toxin-induced and cAMP-induced fluid secretion, believed to be mediated by CFTR, were also significantly reduced in the intestine of Ano1-deficient mice. ANO1 is largely expressed in the apical membranes of intestines, as predicted for CaCCs. The Ano1-deficient colons became edematous under basal conditions and had a greater susceptibility to dextran sodium sulfate-induced colitis. However, Ano1 depletion failed to affect tumor development in a model of colorectal cancer. We thus conclude that ANO1 is necessary for cAMP- and carbachol-induced Cl⁻ secretion in the intestine, which is essential for the protection of the intestinal epithelium from colitis.

An ion channel, a membrane protein allowing ion transport, that controls the flow of chloride is needed for proper secretion of protective fluids in the intestine. Uhtaek Oh from the Korea Institute of Science & Technology in Seoul, South Korea, and colleagues showed that cells lining the intestinal surface express a calcium-activated chloride channel called anoctamin-1 (ANO1) that regulates fluid secretion in the gut. Compared to control animals, ANO1-deficient mice released less fluid into their intestines following exposure to a diarrhea-inducing toxin or to a chloride transport–stimulating signaling molecule. This fluid secretion was previously thought to be mediated via a different ion channel. The ANO1-deficient mice accumulated fluid within colonic tissues, which increased their susceptibility to colitis. The findings point to ANO1 activation as a potential therapeutic strategy for treating colitis.

Differential effects of Clostridium difficile toxins on ion secretion and cell integrity in human intestinal cells

BACKGROUND

Toxin A (TcdA), toxin B (TcdB), and binary toxin (CDT) produced by Clostridium difficile (CD) are thought to play a key role in inducing diarrhea. The aim of this study was to investigate the individual and combined roles of CD toxins in inducing enterotoxic and cytotoxic effect.

METHODS

Ion secretion and epithelial damage were evaluated in the Ussing chambers as measure of enterotoxic or cytotoxic effect, respectively, in human-derived intestinal cells.

RESULTS

When added to the mucosal side of Caco-2 cells, TcdB, but not TcdA, induced ion secretion and its effects increased in the presence of TcdA. CDT also induced ion secretion when added to either the mucosal or serosal compartment. Serosal addition of TcdB induced epithelial damage consistent with its cytotoxic effect. However, mucosal addition of TcdB had similar effects, but only in the presence of TcdA. CDT induced epithelial damage when added to the serosal side of cell monolayers, and this was associated with a late onset but prolonged effect. All data were replicated using human colon biopsies.

CONCLUSIONS

These data indicate that CD, through the combined and direct activity of its three toxins, induces integrated and synergic enterotoxic and cytotoxic effects on the intestinal epithelium.

Antidiarrheal Action of Bacillus subtilis CU1 CNCM I-2745 and Lactobacillus plantarum CNCM I-4547 in Mice

Preventive actions of probiotics as antidiarrheal agents are well documented, but their mechanisms are poorly understood. Two selected probiotics, Bacillus subtilis CU1 and Lactobacillus plantarum CNCM I-4547, were tested in mouse experimental models of diarrhea and the possible mechanisms of action were investigated. Diarrhea was induced in mice by oral castor oil administration or by i.v. injection of lipopolysaccharide (LPS) of Salmonella enteritis. The antidiarrheal drug loperamide was used as control. Fecal water excretion was quantified for 2 h and paracellular permeability and electrical parameters of the colon were assessed in Ussing chambers. The expression of colonic exchangers or channels and of Toll-like receptor 4 (TLR4) was assessed by immunohistochemistry. Prophylactic treatment with B. subtilis CU1 or with L. plantarum CNCM I-4547 reduced LPS-induced diarrhea. The reduction of water excretion was in the same range as those induced by loperamide. In the castor oil model, this effect was only observed with B. subtilis CU1. The two probiotic treatments abolished the increase in paracellular permeability induced by LPS, but not by castor oil. However, only L. plantarum CNCM I-4547 treatment decreased the colonic expression of TLR-4. After B. subtilis CU1, colonic expression of cystic fibrosis transmembrane conductance regulator (CFTR) was reduced and that of Na⁺/H⁺ exchanger 3 (NHE3) increased. B. subtilis CU1 may increase the capacity of the colon to absorb excess of water in diarrheic conditions by acting on CFTR and NHE3 expression. The two probiotics strains showed an impact on diarrhea through limitation of water excretion that may involve paracellular permeability or electrolyte transport for L. plantarum CNCM I-4547 and B. subtilis CU1 respectively.

Development of Arrayed Colonic Organoids for Screening of Secretagogues Associated with Enterotoxins

Enterotoxins increase intestinal fluid secretion through modulation of ion channels as well as activation of the enteric nervous and immune systems. Colonic organoids, also known as colonoids, are functionally and phenotypically similar to in vivo colonic epithelium and have been used to study intestinal ion transport and subsequent water flux in physiology and disease models. In conventional cultures, organoids exist as spheroids embedded within a hydrogel patty of extracellular matrix, and they form at multiple depths, impairing efficient imaging necessary to capture data from statistically relevant sample sizes. To overcome these limitations, an analytical platform with colonic organoids localized to the planar surface of a hydrogel layer was developed. The arrays of densely packed colonoids (140 μm average diameter, 4 colonoids/mm2) were generated in a 96-well plate, enabling assay of the response of hundreds of organoids so that organoid subpopulations with distinct behaviors were identifiable. Organoid cell types, monolayer polarity, and growth were similar to those embedded in hydrogel. An automated imaging and analysis platform efficiently tracked over time swelling due to forskolin and fluid movement across the cell monolayer stimulated by cholera toxin. The platform was used to screen compounds associated with the enteric nervous and immune systems for their effect on fluid movement across epithelial cells. Prostaglandin E2 promoted increased water flux in a subset of organoids that resulted in organoid swelling, confirming a role for this inflammatory mediator in diarrheal conditions but also illustrating organoid differences in response to an identical stimulus. By allowing sampling of a large number of organoids, the arrayed organoid platform permits identification of organoid subpopulations intermixed within a larger group of nonresponding organoids. This technique will enable automated, large-scale screening of the impact of drugs, toxins, and other compounds on colonic physiology.

K2P TASK-2 and KCNQ1-KCNE3 K+ channels are major players contributing to intestinal anion and fluid secretion

KEY POINTS

K⁺ channels are important in intestinal epithelium as they ensure the ionic homeostasis and electrical potential of epithelial cells during anion and fluid secretion. Intestinal epithelium cAMP-activated anion secretion depends on the activity of the (also cAMP dependent) KCNQ1-KCNE3 K⁺ channel, but the secretory process survives after genetic inactivation of the K⁺ channel in the mouse. Here we use double mutant mice to investigate which alternative K⁺ channels come into action to compensate for the absence of KCNQ1-KCNE3 K⁺ channels. Our data establish that whilst Ca²⁺ -activated K_Ca 3.1 channels are not involved, K_2P two-pore domain TASK-2 K⁺ channels are major players providing an alternative conductance to sustain the intestinal secretory process. Work with double mutant mice lacking both TASK-2 and KCNQ1-KCNE3 channels nevertheless points to yet-unidentified K⁺ channels that contribute to the robustness of the cAMP-activated anion secretion process.

ABSTRACT

Anion and fluid secretion across the intestinal epithelium, a process altered in cystic fibrosis and secretory diarrhoea, is mediated by cAMP-activated CFTR Cl^- channels and requires the simultaneous activity of basolateral K⁺ channels to maintain cellular ionic homeostasis and membrane potential. This function is fulfilled by the cAMP-activated K⁺ channel formed by the association of pore-forming KCNQ1 with its obligatory KCNE3 β-subunit. Studies using mice show sizeable cAMP-activated intestinal anion secretion in the absence of either KCNQ1 or KCNE3 suggesting that an alternative K⁺ conductance must compensate for the loss of KCNQ1-KCNE3 activity. We used double mutant mouse and pharmacological approaches to identify such a conductance. Ca²⁺ -dependent anion secretion can also be supported by Ca²⁺ -dependent K_Ca 3.1 channels after independent CFTR activation, but cAMP-dependent anion secretion is not further decreased in the combined absence of K_Ca 3.1 and KCNQ1-KCNE3 K⁺ channel activity. We show that the K_2P K⁺ channel TASK-2 is expressed in the epithelium of the small and large intestine. Tetrapentylammonium, a TASK-2 inhibitor, abolishes anion secretory current remaining in the absence of KCNQ1-KCNE3 activity. A double mutant mouse lacking both KCNQ1-KCNE3 and TASK-2 showed a much reduced cAMP-mediated anion secretion compared to that observed in the single KCNQ1-KCNE3 deficient mouse. We conclude that KCNQ1-KCNE3 and TASK-2 play major roles in the intestinal anion and fluid secretory phenotype. The persistence of an, admittedly reduced, secretory activity in the absence of these two conductances suggests that further additional K⁺ channel(s) as yet unidentified contribute to the robustness of the intestinal anion secretory process.

Ae4 (Slc4a9) is an electroneutral monovalent cation-dependent Cl-/HCO3- exchanger

Ae4 (Slc4a9) belongs to the Slc4a family of Cl(-)/HCO3 (-) exchangers and Na(+)-HCO3 (-) cotransporters, but its ion transport cycle is poorly understood. In this study, we find that native Ae4 activity in mouse salivary gland acinar cells supports Na(+)-dependent Cl(-)/HCO3 (-) exchange that is comparable with that obtained upon heterologous expression of mouse Ae4 and human AE4 in CHO-K1 cells. Additionally, whole cell recordings and ion concentration measurements demonstrate that Na(+) is transported by Ae4 in the same direction as HCO3 (-) (and opposite to that of Cl(-)) and that ion transport is not associated with changes in membrane potential. We also find that Ae4 can mediate Na(+)-HCO3 (-) cotransport-like activity under Cl(-)-free conditions. However, whole cell recordings show that this apparent Na(+)-HCO3 (-) cotransport activity is in fact electroneutral HCO3 (-)/Na(+)-HCO3 (-) exchange. Although the Ae4 anion exchanger is thought to regulate intracellular Cl(-) concentration in exocrine gland acinar cells, our thermodynamic calculations predict that the intracellular Na(+), Cl(-), and HCO3 (-) concentrations required for Ae4-mediated Cl(-) influx differ markedly from those reported for acinar secretory cells at rest or under sustained stimulation. Given that K(+) ions share many properties with Na(+) ions and reach intracellular concentrations of 140-150 mM (essentially the same as extracellular [Na(+)]), we hypothesize that Ae4 could mediate K(+)-dependent Cl(-)/HCO3 (-) exchange. Indeed, we find that Ae4 mediates Cl(-)/HCO3 (-) exchange activity in the presence of K(+) as well as Cs(+), Li(+), and Rb(+) In summary, our results strongly suggest that Ae4 is an electroneutral Cl(-)/nonselective cation-HCO3 (-) exchanger. We postulate that the physiological role of Ae4 in secretory cells is to promote Cl(-) influx in exchange for K(+)(Na(+)) and HCO3 (-) ions.

The altered gut microbiota in adults with cystic fibrosis

BACKGROUND

Cystic Fibrosis (CF) is an autosomal recessive disease that affects the function of a number of organs, principally the lungs, but also the gastrointestinal tract. The manifestations of cystic fibrosis transmembrane conductance regulator (CFTR) dysfunction in the gastrointestinal tract, as well as frequent antibiotic exposure, undoubtedly disrupts the gut microbiota. To analyse the effects of CF and its management on the microbiome, we compared the gut microbiota of 43 individuals with CF during a period of stability, to that of 69 non-CF controls using 454-pyrosequencing of the 16S rRNA gene. The impact of clinical parameters, including antibiotic therapy, on the results was also assessed.

RESULTS

The CF-associated microbiome had reduced microbial diversity, an increase in Firmicutes and a reduction in Bacteroidetes compared to the non-CF controls. While the greatest number of differences in taxonomic abundances of the intestinal microbiota was observed between individuals with CF and the healthy controls, gut microbiota differences were also reported between people with CF when grouped by clinical parameters including % predicted FEV₁ (measure of lung dysfunction) and the number of intravenous (IV) antibiotic courses in the previous 12 months. Notably, CF individuals presenting with severe lung dysfunction (% predicted FEV₁ ≤ 40%) had significantly (p < 0.05) reduced gut microbiota diversity relative to those presenting with mild or moderate dysfunction. A significant negative correlation (-0.383, Simpson's Diversity Index) was also observed between the number of IV antibiotic courses and gut microbiota diversity.

CONCLUSIONS

This is one of the largest single-centre studies on gut microbiota in stable adults with CF and demonstrates the significantly altered gut microbiota, including reduced microbial diversity seen in CF patients compared to healthy controls. The data show the impact that CF and it's management have on gut microbiota, presenting the opportunity to develop CF specific probiotics to minimise microbiota alterations.

PGE2 is a direct and robust mediator of anion/fluid secretion by human intestinal epithelial cells

Intestinal epithelial cells (IECs) play an indispensable role in maintaining body fluid balance partly through their ability to regulate anion/fluid secretion. Yet in various inflammatory gastrointestinal diseases, over-secretion of anions results in symptoms such as severe diarrhoea. Endogenous mediators, such as vasoactive intestinal peptide or prostaglandin E₂ (PGE₂), regulate intestinal anion/fluid secretion, but their direct effect on purified human IECs has never been described in detail. Based on a previously described intestinal organoid swelling model, we established a 3D-scanner-assisted quantification method to evaluate the anion/fluid secretory response of cultured human IECs. Among various endogenous secretagogues, we found that PGE₂ had the lowest EC₅₀ value with regard to the induction of swelling of the jejunal and colonic organoids. This PGE₂-mediated swelling response was dependent on environmental Cl^- concentrations as well as on several channels and transporters as shown by a series of chemical inhibitor studies. The concomitant presence of various inflammatory cytokines with PGE₂ failed to modulate the PGE₂-mediated organoid swelling response. Therefore, the present study features PGE₂ as a direct and robust mediator of anion/fluid secretion by IECs in the human intestine.

Hypoxia/Reoxygenation Effects on Ion Transport across Rat Colonic Epithelium

Ischemia causes severe damage in the gastrointestinal tract. Therefore, it is interesting to study how the barrier and transport functions of intestinal epithelium change under hypoxia and subsequent reoxygenation. For this purpose we simulated hypoxia and reoxygenation on mucosa-submucosa preparations from rat distal colon in Ussing chambers and on isolated crypts. Hypoxia (N₂ gassing for 15 min) induced a triphasic change in short-circuit current (I_sc): a transient decrease, an increase and finally a long-lasting fall below the initial baseline. During the subsequent reoxygenation phase, I_sc slightly rose to values above the initial baseline. Tissue conductance (G_t) showed a biphasic increase during both the hypoxia and the reoxygenation phases. Omission of Cl⁻ or preincubation of the tissue with transport inhibitors revealed that the observed changes in I_sc represented changes in Cl⁻ secretion. The radical scavenger trolox C reduced the I_sc response during hypoxia, but failed to prevent the rise of I_sc during reoxygenation. All changes in I_sc were Ca²⁺-dependent. Fura-2 experiments at loaded isolated colonic crypts revealed a slow increase of the cytosolic Ca²⁺ concentration during hypoxia and the reoxygenation phase, mainly caused by an influx of extracellular Ca²⁺. Surprisingly, no changes could be detected in the fluorescence of the superoxide anion-sensitive dye mitosox or the thiol-sensitive dye thiol tracker, suggesting a relative high capacity of the colonic epithelium (with its low O₂ partial pressure even under physiological conditions) to deal with enhanced radical production during hypoxia/reoxygenation.

Effects of multivalent histamine supported on gold nanoparticles: activation of histamine receptors by derivatized histamine at subnanomolar concentrations

Colloidal gold nanoparticles with a functionalized ligand shell were synthesized and used as new histamine receptor agonists. Mercaptoundecanoic acid moieties were attached to the surface of the nanoparticles and derivatized with native histamine. The multivalent presentation of the immobilized ligands carried by the gold nanoparticles resulted in extremely low activation concentrations for histamine receptors on rat colonic epithelium. As a functional read-out system, chloride secretion resulting from stimulation of neuronal and epithelial histamine H1 and H2 receptors was measured in Ussing chamber experiments. These responses were strictly attributed to the histamine entities as histamine-free particles Au-MUDOLS or the monovalent ligand AcS-MUDA-HA proved to be ineffective. The vitality of the tissues used was not impaired by the nanoparticles.

A whole-genome RNAi screen uncovers a novel role for human potassium channels in cell killing by the parasite Entamoeba histolytica

The parasite Entamoeba histolytica kills human cells resulting in ulceration, inflammation and invasion of the colonic epithelium. We used the cytotoxic properties of ameba to select a genome-wide RNAi library to reveal novel host factors that control susceptibility to amebic killing. We identified 281 candidate susceptibility genes and bioinformatics analyses revealed that ion transporters were significantly enriched among susceptibility genes. Potassium (K⁺) channels were the most common transporter identified. Their importance was further supported by colon biopsy of humans with amebiasis that demonstrated suppressed K⁺ channel expression. Inhibition of human K⁺ channels by genetic silencing, pharmacologic inhibitors and with excess K⁺ protected diverse cell types from E. histolytica-induced death. Contact with E. histolytica parasites triggered K⁺ channel activation and K⁺ efflux by intestinal epithelial cells, which preceded cell killing. Specific inhibition of Ca²⁺-dependent K⁺ channels was highly effective in preventing amebic cytotoxicity in intestinal epithelial cells and macrophages. Blockade of K⁺ efflux also inhibited caspase-1 activation, IL-1β secretion and pyroptotic death in THP-1 macrophages. We concluded that K⁺ channels are host mediators of amebic cytotoxicity in multiple cells types and of inflammasome activation in macrophages.

Bioactivity-guided fractionation of an antidiarrheal Chinese herb Rhodiola kirilowii (Regel) Maxim reveals (-)-epicatechin-3-gallate and (-)-epigallocatechin-3-gallate as inhibitors of cystic fibrosis transmembrane conductance regulator

Cystic fibrosis transmembrane conductance regulator (CFTR) is the principal apical route for transepithelial fluid transport induced by enterotoxin. Inhibition of CFTR has been confirmed as a pharmaceutical approach for the treatment of secretory diarrhea. Many traditional Chinese herbal medicines, like Rhodiola kirilowii (Regel) Maxim, have long been used for the treatment of secretory diarrhea. However, the active ingredients responsible for their therapeutic effectiveness remain unknown. The purpose of this study is to identify CFTR inhibitors from Rhodiola kirilowii (Regel) Maxim via bioactivity-directed isolation strategy. We first identified fractions of Rhodiola kirilowii (Regel) Maxim that inhibited CFTR Cl^- channel activity. Further bioactivity-directed fractionation led to the identification of (-)–epigallocatechin-3-gallate (EGCG) as CFTR Cl^- channel inhibitor. Analysis of 5 commercially available EGCG analogs including (+)–catechins (C), (-)–epicatechin (EC), (-)–epigallocatechin (EGC), (-)–epicatechin-3-gallate (ECG) and EGCG revealed that ECG also had CFTR inhibitory activity. EGCG dose-dependently and reversibly inhibited CFTR Cl^- channel activity in transfected FRT cells with an IC₅₀ value around 100 μM. In ex vivo studies, EGCG and ECG inhibited CFTR-mediated short-circuit currents in isolated rat colonic mucosa in a dose-dependent manner. In an intestinal closed-loop model in mice, intraluminal application of EGCG (10 μg) and ECG (10 μg) significantly reduced cholera toxin-induced intestinal fluid secretion. CFTR Cl^- channel is a molecular target of natural compounds EGCG and ECG. CFTR inhibition may account, at least in part, for the antidiarrheal activity of Rhodiola kirilowii (Regel) Maxim. EGCG and ECG could be new lead compounds for development of CFTR-related diseases such as secretory diarrhea.

A fluid secretion pathway unmasked by acinar-specific Tmem16A gene ablation in the adult mouse salivary gland

Activation of an apical Ca(2+)-activated Cl(-) channel (CaCC) triggers the secretion of saliva. It was previously demonstrated that CaCC-mediated Cl(-) current and Cl(-) efflux are absent in the acinar cells of systemic Tmem16A (Tmem16A Cl(-) channel) null mice, but salivation was not assessed in fully developed glands because Tmem16A null mice die within a few days after birth. To test the role of Tmem16A in adult salivary glands, we generated conditional knockout mice lacking Tmem16A in acinar cells (Tmem16A(-/-)). Ca(2+)-dependent salivation was abolished in Tmem16A(-/-) mice, demonstrating that Tmem16A is obligatory for Ca(2+)-mediated fluid secretion. However, the amount of saliva secreted by Tmem16A(-/-) mice in response to the β-adrenergic receptor agonist isoproterenol (IPR) was comparable to that seen in controls, indicating that Tmem16A does not significantly contribute to cAMP-induced secretion. Furthermore, IPR-stimulated secretion was unaffected in mice lacking Cftr (Cftr(∆F508/∆F508)) or ClC-2 (Clcn2(-/-)) Cl(-) channels. The time course for activation of IPR-stimulated fluid secretion closely correlated with that of the IPR-induced cell volume increase, suggesting that acinar swelling may activate a volume-sensitive Cl(-) channel. Indeed, Cl(-) channel blockers abolished fluid secretion, indicating that Cl(-) channel activity is critical for IPR-stimulated secretion. These data suggest that β-adrenergic-induced, cAMP-dependent fluid secretion involves a volume-regulated anion channel. In summary, our results using acinar-specific Tmem16A(-/-) mice identify Tmem16A as the Cl(-) channel essential for muscarinic, Ca(2+)-dependent fluid secretion in adult mouse salivary glands.

Intestinal current measurement versus nasal potential difference measurements for diagnosis of cystic fibrosis: a case-control study

Background

Nasal potential difference (NPD) and intestinal current measurement (ICM) are functional CFTR tests that are used as adjunctive diagnostic tools for cystic fibrosis (CF). Smoking has a systemic negative impact on CFTR function. A diagnostic comparison between NPD and ICM and the impact of smoking on both CFTR tests has not been done.

Methods

The sweat chloride test, NPD, and ICM were performed in 18 patients with CF (sweat chloride >60 mmol/l), including 6 pancreatic sufficient (PS) patients, and 13 healthy controls, including 8 smokers. The NPD CFTR response to Cl-free and isoproterenol perfusion (Δ0Cl^- + Iso) was compared to the ICM CFTR response to forskolin/IBMX, carbachol, and histamine (ΔI_{sc, forskolin/IBMX+ carbachol+histamine}).

Results

The mean NPD CFTR response and ICM CFTR response between patients with CF and healthy controls was significantly different (p <0.001), but not between patients with CF who were PS and those who were pancreatic insufficient (PI). Smokers have a decreased CFTR response measured by NPD (p = 0.049). For ICM there is a trend towards decreased CFTR response (NS). Three healthy control smokers had NPD responses within the CF-range. In contrast to NPD, there was no overlap of the ICM response between patients with CF and controls.

Conclusions

ICM is superior to NPD in distinguishing between patients with CF who have a sweat chloride > 60 mmol/l and healthy controls, including smokers. Neither NPD nor ICM differentiated between patients with CF who were PS from those who were PI. Smoking has a negative impact on CFTR function in healthy controls measured by NPD and challenges the diagnostic interpretation of NPD, but not ICM.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2466-14-156) contains supplementary material, which is available to authorized users.

Identification of resveratrol oligomers as inhibitors of cystic fibrosis transmembrane conductance regulator by high-throughput screening of natural products from chinese medicinal plants

Inhibitors of cystic fibrosis transmembrane conductance regulator (CFTR) have been widely used for characterizing CFTR function in epithelial fluid transport and in diseases such as secretory diarrhea, polycystic kidney disease and cystic fibrosis. Few small molecule CFTR inhibitors have been discovered so far from combinatorial compound library. In the present study, we used a high throughput screening (HTS)-based natural product discovery strategy to identify new CFTR inhibitors from Chinese medicinal herbs. By screening 40,000 small molecule fractions from 500 herbal plants, we identified 42 positive fractions from 5 herbs and isolated two compounds that inhibited CFTR conductance from Chinese wild grapevine (Vitis amurensis Rupr). Mass spectrometry (MS) and nuclear magnetic resonance (NMR) studies determined the two active compounds as trans-ε-viniferin (TV) and r-2-viniferin (RV), respectively. Both compounds dose-dependently blocked CFTR-mediated iodide influx with IC₅₀ around 20 μM. Further analysis by excised inside-out patch-clamp indicated strong inhibition of protein kinase A (PKA)-activated CFTR chloride currents by TV and RV. In ex vivo studies, TV and RV inhibited CFTR-mediated short-circuit Cl⁻ currents in isolated rat colonic mucosa in a dose-dependent manner. In a closed-loop mouse model, intraluminal applications of TV (2.5 μg) and RV (4.5 μg) significantly reduced cholera toxin–induced intestinal fluid secretion. The present study identified two resveratrol oligomers as new CFTR inhibitors and validates our high-throughput screening method for discovery of bioactive compounds from natural products with complex chemical ingredients such as herbal plants.

Dynamic changes in mucus thickness and ion secretion during Citrobacter rodentium infection and clearance

Citrobacter rodentium is an attaching and effacing pathogen used as a murine model for enteropathogenic Escherichia coli. The mucus layers are a complex matrix of molecules, and mucus swelling, hydration and permeability are affected by many factors, including ion composition. Here, we used the C. rodentium model to investigate mucus dynamics during infection. By measuring the mucus layer thickness in tissue explants during infection, we demonstrated that the thickness changes dynamically during the course of infection and that its thickest stage coincides with the start of a decrease of bacterial density at day 14 after infection. Although quantitative PCR analysis demonstrated that mucin mRNA increases during early infection, the increased mucus layer thickness late in infection was not explained by increased mRNA levels. Proteomic analysis of mucus did not demonstrate the appearance of additional mucins, but revealed an increased number of proteins involved in defense responses. Ussing chamber-based electrical measurements demonstrated that ion secretion was dynamically altered during the infection phases. Furthermore, the bicarbonate ion channel Bestrophin-2 mRNA nominally increased, whereas the Cftr mRNA decreased during the late infection clearance phase. Microscopy of Muc2 immunostained tissues suggested that the inner striated mucus layer present in the healthy colon was scarce during the time point of most severe infection (10 days post infection), but then expanded, albeit with a less structured appearance, during the expulsion phase. Together with previously published literature, the data implies a model for clearance where a change in secretion allows reformation of the mucus layer, displacing the pathogen to the outer mucus layer, where it is then outcompeted by the returning commensal flora. In conclusion, mucus and ion secretion are dynamically altered during the C. rodentium infection cycle.

Lineage-specific expression of bestrophin-2 and bestrophin-4 in human intestinal epithelial cells

Intestinal epithelial cells (IECs) regulate the absorption and secretion of anions, such as HCO3^- or Cl^-. Bestrophin genes represent a newly identified group of calcium-activated Cl^- channels (CaCCs). Studies have suggested that, among the four human bestrophin-family genes, bestrophin-2 (BEST2) and bestrophin-4 (BEST4) might be expressed within the intestinal tissue. Consistently, a study showed that BEST2 is expressed by human colonic goblet cells. However, their precise expression pattern along the gastrointestinal tract, or the lineage specificity of the cells expressing these genes, remains largely unknown. Here, we show that BEST2 and BEST4 are expressed in vivo, each in a distinct, lineage-specific manner, in human IECs. While BEST2 was expressed exclusively in colonic goblet cells, BEST4 was expressed in the absorptive cells of both the small intestine and the colon. In addition, we found that BEST2 expression is significantly down-regulated in the active lesions of ulcerative colitis, where goblet cells were depleted, suggesting that BEST2 expression is restricted to goblet cells under both normal and pathologic conditions. Consistently, the induction of goblet cell differentiation by a Notch inhibitor, LY411575, significantly up-regulated the expression of not BEST4 but BEST2 in MUC2-positive HT-29 cells. Conversely, the induction of absorptive cell differentiation up-regulated the expression of BEST4 in villin-positive Caco-2 cells. In addition, we found that the up- or down-regulation of Notch activity leads to the preferential expression of either BEST4 or BEST2, respectively, in LS174T cells. These results collectively confirmed that BEST2 and BEST4 could be added to the lineage-specific genes of humans IECs due to their abilities to clearly identify goblet cells of colonic origin and a distinct subset of absorptive cells, respectively.

Multicenter intestinal current measurements in rectal biopsies from CF and non-CF subjects to monitor CFTR function

Intestinal current measurements (ICM) from rectal biopsies are a sensitive means to detect cystic fibrosis transmembrane conductance regulator (CFTR) function, but have not been optimized for multicenter use. We piloted multicenter standard operating procedures (SOPs) to detect CFTR activity by ICM and examined key questions for use in clinical trials. SOPs for ICM using human rectal biopsies were developed across three centers and used to characterize ion transport from non-CF and CF subjects (two severe CFTR mutations). All data were centrally evaluated by a blinded interpreter. SOPs were then used across four centers to examine the effect of cold storage on CFTR currents and compare CFTR currents in biopsies from one subject studied simultaneously either at two sites (24 hours post-biopsy) or when biopsies were obtained by either forceps or suction. Rectal biopsies from 44 non-CF and 17 CF subjects were analyzed. Mean differences (µA/cm²; 95% confidence intervals) between CF and non-CF were forskolin/IBMX=102.6(128.0 to 81.1), carbachol=96.3(118.7 to 73.9), forskolin/IBMX+carbachol=200.9(243.1 to 158.6), and bumetanide=-44.6 (-33.7 to -55.6) (P<0.005, CF vs non-CF for all parameters). Receiver Operating Characteristic curves indicated that each parameter discriminated CF from non-CF subjects (area under the curve of 0.94-0.98). CFTR dependent currents following 18-24 hours of cold storage for forskolin/IBMX, carbachol, and forskolin/IBMX+carbachol stimulation (n=17 non-CF subjects) were 44%, 47.5%, and 47.3%, respectively of those in fresh biopsies. CFTR-dependent currents from biopsies studied after cold storage at two sites simultaneously demonstrated moderate correlation (n=14 non-CF subjects, Pearson correlation coefficients 0.389, 0.484, and 0.533). Similar CFTR dependent currents were detected from fresh biopsies obtained by either forceps or suction (within-subject comparisons, n=22 biopsies from three non-CF subjects). Multicenter ICM is a feasible CFTR outcome measure that discriminates CF from non-CF ion transport, offers unique advantages over other CFTR bioassays, and warrants further development as a potential CFTR biomarker.

ATP-sensitive K(+) channels in rat colonic epithelium

ATP-sensitive K(+) (KATP) channels couple the metabolic state of a cell to its electrical activity. They consist of a hetero-octameric complex with pore-forming Kir6.x (Kir6.1, Kir6.2) and regulatory sulfonylurea receptor (SUR) subunits. Functional data indicate that KATP channels contribute to epithelial K(+) currents at colonic epithelia. However, their molecular identity and their properties are largely unknown. Therefore, changes in short-circuit current (I sc) induced by the KATP channel opener pinacidil (5 10(-4) mol l(-1)) were measured in Ussing chambers under control conditions and in the presence of different blockers of KATP channels. The channel subunits expressed by the colonic epithelium were identified by immunohistochemistry and by RT-PCR. The K(+) channel opener, when administered at the serosal side, induced an increase in I sc consistent with the induction of transepithelial Cl(-) secretion after activation of basolateral K(+) channels, whereas mucosal administration of pinacidil resulted in a negative I sc. The increase in I sc evoked by serosal pinacidil was inhibited by serosal administration of glibenclamide (5 10(-4) mol l(-1)) and gliclazide (10(-6) mol l(-1)), but was resistant even against a high concentration (10(-2) mol l(-1)) of tolbutamide. In contrast, none of these inhibitors (administered at the mucosal side) reduced significantly the negative I sc induced by mucosal pinacidil. Instead, pinacidil inhibited Cl(-) currents across apical Cl(-) channels in basolaterally depolarized epithelia indicating that the negative I sc induced by mucosal pinacidil is due to a transient inhibition of Cl(-) secretion. In mRNA prepared from isolated colonic crypts, messenger RNA for both pore-forming subunits, Kir6.1 and Kir6.2, and two regulatory subunits (SUR1 and SUR2B) was found. Expression within the colonic epithelium was confirmed for these subunits by immunohistochemistry. In consequence, KATP channels are present in the basolateral membrane of the colonic epithelium; their exact subunit composition, however, has still to be revealed.

Contribution of adenosine A(2B) receptors in Clostridium difficile intoxication and infection

Clostridium difficile toxins A (TcdA) and B (TcdB) induce a pronounced systemic and intestinal inflammatory response. A(2B) adenosine receptors (A(2B)ARs) are the predominant adenosine receptors in the intestinal epithelium. We investigated whether A(2B)ARs are upregulated in human intestinal cells by TcdA or TcdB and whether blockade of A(2B)ARs can ameliorate C. difficile TcdA-induced enteritis and alter the outcome of C. difficile infection (CDI). Adenosine receptor subtype (A(1), A(2A), A(2B), and A(3)) mRNAs were assayed in HCT-8 cells. Ileal loops from wild-type rabbits and mice and A(2B)AR(-/-) mice were treated with TcdA, with or without the selective A(2B)AR antagonist ATL692 or PSB1115. A murine model of CDI was used to determine the effect of A(2B)AR deletion or blockade with the orally available agent ATL801, on clinical outcome, histopathology and intestinal interleukin-6 (IL-6) expression from infection. TcdA and TcdB upregulated A(2B)AR gene expression in HCT-8 cells. ATL692 decreased TcdA-induced secretion and epithelial injury in rabbit ileum. Deletion of A(2B)ARs reduced secretion and histopathology in TcdA-challenged mouse ileum. Deletion or blockade of A(2B)ARs reduced histopathology, IL-6 expression, weight loss, diarrhea, and mortality in C. difficile-infected mice. A(2B)ARs mediate C. difficile toxin-induced enteritis and disease. Inhibition of A(2B)AR activation may be a potential strategy to limit morbidity and mortality from CDI.

Up-regulated cystic fibrosis transmembrane conductance regulator after anal stenosis in rats

BACKGROUND

The pathophysiology of fecal incontinence from fecal impaction and rectal distension is poorly understood. We hypothesize that fecal impaction elicits up-regulation of cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-activated mucosal chloride channel.

METHODS

The anus was ligated to produce 75% stenosis in rats. Controls received ligation without inducing stenosis. 24 to 48 hours after ligation the colon was removed. Mucosal short-circuit current was measured by Ussing chamber. Western blot analysis was used to detect CFTR expression in the colonic mucosa. Ligated rats failed to defecate, whereas control rats stooled normally.

RESULTS

Ligated colons were markedly stool filled and dilated. Water content of feces was significantly increased to 66.5% ± 1.1% (P < .01, n = 12) 24 hours after ligation, vs controls (49.5 ± 5.2%, n = 12). Baseline short-circuit current was significantly increased in the distal (78.8 ± 7.4 μA/cm(2), n = 8, P < .01) and mid colon (24.5 ± 2.5 μA/cm(2), n = 8, P < .05) 24 hours after ligation, compared to control rats (12.5 ± 3.2 μA/cm2, n = 8). CFTR expression was significantly increased 24 hours after ligation in the mid and distal colon.

CONCLUSION

We observe that fecal impaction from anal ligation induces early compensatory up-regulation of CFTR, altering function from net absorption to net secretion in the mid and distal colon.

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.

Stimulation of Airway and Intestinal Mucosal Secretion by Natural Coumarin CFTR Activators

Mutations of cystic fibrosis (CF) transmembrane conductance regulator (CFTR) cause lethal hereditary disease CF that involves extensive destruction and dysfunction of serous epithelium. Possible pharmacological therapy includes correction of defective intracellular processing and abnormal channel gating. In a previous study, we identified five natural coumarin potentiators of ΔF508-CFTR including osthole, imperatorin, isopsoralen, praeruptorin A, and scoparone. The present study was designed to determine the activity of these coumarine compounds on CFTR activity in animal tissues as a primary evaluation of their therapeutic potential. In the present study, we analyzed the affinity of these coumarin potentiators in activating wild-type CFTR and found that they are all potent activators. Osthole showed the highest affinity with K_d values <50 nmol/L as determined by Ussing chamber short-circuit current assay. Stimulation of rat colonic mucosal secretion by osthole was tested by the Ussing chamber short-circuit current assay. Osthole reached maximal activation of colonic Cl⁻ secretion at 5 μmol/L. Stimulation of mouse tracheal mucosal secretion was analyzed by optical measurement of single gland secretion. Fluid secretion rate of tracheal single submucosal gland stimulated by osthole at 10 μmol/L was three-fold more rapid than that in negative control. In both cases the stimulated secretions were fully abolished by CFTR_inh-172. In conclusion, the effective stimulation of Cl^– and fluid secretion in colonic and tracheal mucosa by osthole suggested the therapeutic potential of natural coumarin compounds for the treatment of CF and other CFTR-related diseases.

Traditional Chinese formula, lubricating gut pill, stimulates cAMP-dependent CI(−) secretion across rat distal colonic mucosa

AIM OF THE STUDY

Lubricating gut pill (LGP), a traditional Chinese formula, had been conformed to improve the loperamide-induced rat constipation by stimulation of Cl(-) secretion, but its mechanism has not been fully explored. Thus, the purpose of this study was to identify the action sites of LGP-stimulated Cl(-) secretion across rat distal colonic mucosa.

MATERIALS AND METHODS

Rat distal colonic mucosa was mounted in Ussing chambers and short circuit current (I(SC)), apical Cl(-) current and basolateral K(+) current were recorded. Intracellular cyclic adenosine monophosphate (cAMP) content and protein kinase A (PKA) activity were determined with ELISA kit and the non-radioactive PepTag test, respectively.

RESULTS

LGP at 800μg/ml elicited a sustained increase in Cl(-) secretory response, which was inhibited by CFTR(inh)172, a cystic fibrosis transmembrane conductance regulator (CFTR) inhibitor. Permeabilizing apical membrane with nystatin revealed that LGP-stimulated basolateral K(+) current was significantly inhibited by KCNQ1 K(+) channel inhibitor chromanol 293B. LGP-stimulated I(SC) was markedly reduced by pretreatment with cis-N-[2-phenylcyclopentyl]-azacyclotridec-1-en-2amine (MDL-12,330A) and N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H-89), but not with inhibitors of Ca(2+)-dependent signaling pathway. Treatment of tissue with LGP resulted in an increase in intracellular cAMP level and the activation in protein kinase A. The E-prostanoid(4) (EP)(4) receptor antagonist L-161,982 completely eliminated LGP-induced response.

CONCLUSIONS

The results showed that LGP enhances Cl(-) and fluid secretion via prostanoid receptor signaling and also cAMP and protein kinase A pathway, subsequently triggering the activation of apical Cl(-) channels mostly CFTR and basolateral cAMP-dependent K(+) channel.

Effects of carbon monoxide on ion transport across rat distal colon

The aim of the present study was to investigate whether carbon monoxide (CO) induces changes in ion transport across the distal colon of rats and to study the mechanisms involved. In Ussing chamber experiments, tricarbonyldichlororuthenium(II) dimer (CORM-2), a CO donor, evoked a concentration-dependent increase in short-circuit current (I(sc)). A maximal response was achieved at a concentration of 2.5·10(-4) mol/l. Repeated application of CORM-2 resulted in a pronounced desensitization of the tissue. Anion substitution experiments suggest that a secretion of Cl(-) and HCO(3)(-) underlie the CORM-2-induced current. Glibenclamide, a blocker of the apical cystic fibrosis transmembrane regulator channel, inhibited the I(sc) induced by the CO donor. Similarly, bumetanide, a blocker of the basolateral Na(+)-K(+)-2Cl(-) cotransporter, combined with 4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulfonic acid sodium salt, an inhibitor of the basolateral Cl(-)/HCO(3)(-) exchanger, inhibited the CORM-2-induced I(sc). Membrane permeabilization experiments indicated an activation of basolateral K(+) and apical Cl(-) channels by CORM-2. A partial inhibition by the neurotoxin, tetrodotoxin, suggests the involvement of secretomotor neurons in this response. In imaging experiments at fura-2-loaded colonic crypts, CORM-2 induced an increase of the cytosolic Ca(2+) concentration. This increase depended on the influx of extracellular Ca(2+), but not on the release of Ca(2+) from intracellular stores. Both enzymes for CO production, heme oxygenase I and II, are expressed in the colon as observed immunohistochemically and by RT-PCR. Consequently, endogenous CO might be a physiological modulator of colonic ion transport.

Measurement of ion transport function in rectal biopsies

Cystic fibrosis (CF) is caused by mutations in the gene encoding for the cystic fibrosis transmembrane conductance regulator (CFTR). CFTR functions as an anion channel and is known to interact with a number of other cellular proteins involved in ion transport. To date more than 1,800 mutations are known, most of which result in various degrees of impaired transport function of the gene product. Due to the high inter-individual variability of disease onset and progression, CF still is a diagnostic challenge. Implemented almost 20 years ago, the measurement of electrolyte transport function of rectal biopsies is a useful ex vivo tool to diagnose CF. In this chapter we will review the different approaches to perform ion transport measurements and try to highlight the advantages and limitations of these techniques.

A plant-derived hydrolysable tannin inhibits CFTR chloride channel: a potential treatment of diarrhea

PURPOSE

The present study examined the effects and mechanisms of actions of penta-m-digalloyl-glucose (PDG), a hydrolysable tannin extracted from Chinese gallnut, on cystic fibrosis transmembrane conductance regulator protein (CFTR).

MATERIALS AND METHODS

Fisher rat thyroid cells stably expressing human CFTR (FRT cells) and human intestinal T84 cells were used as cell models to investigate the effects of PDG on chloride secretion using short-circuit current analysis. The mechanisms by which PDG affected chloride secretion were also examined. Finally, in vivo antidiarrheal efficacy and effects of PDG on intestinal fluid absorption were evaluated in mouse closed-loop models.

RESULTS

In FRT cells, apical chloride current induced by forskolin, CPT-cAMP and apigenin were reversibly inhibited by PDG (IC50 approximately 10microM) without effects on intracellular cAMP content and cell viability. Similarly, in T84 cells, PDG effectively inhibited chloride secretion induced by forskolin and cholera toxin. However, it had no effect on calcium-induced chloride secretion. In mice, a single intraluminal injection of PDG (0.6 mg/kg) reduced cholera toxin-induced intestinal fluid secretion by 75% with no effect on intestinal fluid absorption.

CONCLUSIONS

PDG represents a new class of CFTR inhibitors. Further development of this class of compounds may provide a new therapeutic intervention for diarrhea.

Bestrophin-2 mediates bicarbonate transport by goblet cells in mouse colon

Anion transport by the colonic mucosa maintains the hydration and pH of the colonic lumen, and its disruption causes a variety of diarrheal diseases. Cholinergic agonists raise cytosolic Ca2+ levels and stimulate anion secretion, but the mechanisms underlying this effect remain unclear. Cholinergic stimulation of anion secretion may occur via activation of Ca2+-activated Cl- channels (CaCCs) or an increase in the Cl- driving force through CFTR after activation of Ca2+-dependent K+ channels. Here we investigated the role of a candidate CaCC protein, bestrophin-2 (Best2), using Best2-/- mice. Cholinergic stimulation of anion current was greatly reduced in Best2-/- mice, consistent with our proposed role for Best2 as a CaCC. However, immunostaining revealed Best2 localized to the basolateral membrane of mucin-secreting colonic goblet cells, not the apical membrane of Cl--secreting enterocytes. In addition, in the absence of HCO3-, cholinergic-activated current was identical in control and Best2-/- tissue preparations, which suggests that most of the Best2 current was carried by HCO3-. These data delineate an alternative model of cholinergic regulation of colonic anion secretion in which goblet cells play a critical role in HCO3- homeostasis. We therefore propose that Best2 is a HCO3- channel that works in concert with a Cl:HCO3- exchanger in the apical membrane to affect transcellular HCO3- transport. Furthermore, previous models implicating CFTR in cholinergic Cl- secretion may be explained by substantial downregulation of Best2 in Cftr-/- mice.

Review article: the clinical management of congenital chloride diarrhoea

BACKGROUND

Congenital chloride diarrhoea in a newborn is a medical emergency, requiring early diagnostics and treatment to prevent severe dehydration and infant mortality. While most of the 250 cases reported arise from Finland, Poland and Arab countries, single cases with this autosomal recessive disorder appear worldwide. Such congenital chloride diarrhoea rarity makes diagnosis difficult. Life-long salt substitution with NaCl and KCl stabilizes fluid, electrolyte and acid-base balance diagnosis. When properly treated, the long-term outcome is favourable.

AIM

To summarize data on congenital chloride diarrhoea diagnosis, pathophysiology and treatment, and to provide guidelines for both acute and long-term management of congenital chloride diarrhoea.

METHODS

Data are based on MEDLINE search for 'chloride diarrhoea', in addition to clinical experience in the treatment of the largest known series of patients.

RESULTS

Treatment of congenital chloride diarrhoea involves (i) life-long salt substitution; (ii) management of acute dehydration and hypokalaemia during gastroenteritis or other infections; and (iii) recognition and treatment of other manifestations of the disease, such as intestinal inflammation, renal impairment and male subfertility.

CONCLUSIONS

This review summarizes data on congenital chloride diarrhoea and provides guidelines for treatment. After being a mostly paediatric problem, adult patients constitute a rare challenge for gastroenterologists worldwide.

Actions of hydrogen sulphide on ion transport across rat distal colon

BACKGROUND AND PURPOSE

The aim of this study was to identify the actions of H(2)S on ion transport across rat distal colon.

EXPERIMENTAL APPROACH

Changes in short-circuit current (Isc) induced by the H(2)S-donor, NaHS, were measured in Ussing chambers. Cytosolic Ca(2+) concentration was evaluated using fura-2.

KEY RESULTS

NaHS concentration-dependently induced a change in Isc, that was only partially inhibited by the neurotoxin, tetrodotoxin. Lower concentrations (< or =10(-3) mol.L(-1)) of NaHS induced a monophasic increase in Isc, whereas higher concentrations induced an additional, secondary fall of Isc, before a third phase when Isc rose again. Blockers of H(2)S-producing enzymes (expression demonstrated immunohistochemically) decreased basal Isc, suggesting that endogenous production of H(2)S contributes to spontaneous anion secretion. The positive Isc phases induced by NaHS were due to Cl(-) secretion as shown by anion substitution and transport inhibitor experiments, whereas the transient negative Isc induced by higher concentrations of the H(2)S-donor was inhibited by mucosal tetrapentylammonium suggesting a transient K(+) secretion. When applied from the serosal side, glibenclamide, an inhibitor of ATP-sensitive K(+) channels, and tetrapentylammonium, a blocker of Ca(2+)-dependent K(+) channels, suppressed NaHS-induced Cl(-) secretion suggesting different types of K(+) channels are stimulated by the H(2)S-donor. NaHS-induced increase in cytosolic Ca(2+) concentration was confirmed in isolated, fura-2-loaded colonic crypts. This response was not dependent on extracellular Ca(2+), but was inhibited by blockers of intracellular Ca(2+) channels present on Ca(2+) storage organelles.

CONCLUSIONS AND IMPLICATIONS

H(2)S induces colonic ion secretion by stimulation of apical as well as basolateral epithelial K(+) channels.

Actions of hydrogen sulphide on ion transport across rat distal colon

BACKGROUND AND PURPOSE

The aim of this study was to identify the actions of H(2)S on ion transport across rat distal colon.

EXPERIMENTAL APPROACH

Changes in short-circuit current (Isc) induced by the H(2)S-donor, NaHS, were measured in Ussing chambers. Cytosolic Ca(2+) concentration was evaluated using fura-2.

KEY RESULTS

NaHS concentration-dependently induced a change in Isc, that was only partially inhibited by the neurotoxin, tetrodotoxin. Lower concentrations (< or =10(-3) mol.L(-1)) of NaHS induced a monophasic increase in Isc, whereas higher concentrations induced an additional, secondary fall of Isc, before a third phase when Isc rose again. Blockers of H(2)S-producing enzymes (expression demonstrated immunohistochemically) decreased basal Isc, suggesting that endogenous production of H(2)S contributes to spontaneous anion secretion. The positive Isc phases induced by NaHS were due to Cl(-) secretion as shown by anion substitution and transport inhibitor experiments, whereas the transient negative Isc induced by higher concentrations of the H(2)S-donor was inhibited by mucosal tetrapentylammonium suggesting a transient K(+) secretion. When applied from the serosal side, glibenclamide, an inhibitor of ATP-sensitive K(+) channels, and tetrapentylammonium, a blocker of Ca(2+)-dependent K(+) channels, suppressed NaHS-induced Cl(-) secretion suggesting different types of K(+) channels are stimulated by the H(2)S-donor. NaHS-induced increase in cytosolic Ca(2+) concentration was confirmed in isolated, fura-2-loaded colonic crypts. This response was not dependent on extracellular Ca(2+), but was inhibited by blockers of intracellular Ca(2+) channels present on Ca(2+) storage organelles.

CONCLUSIONS AND IMPLICATIONS

H(2)S induces colonic ion secretion by stimulation of apical as well as basolateral epithelial K(+) channels.

Intestinal epithelial CD98: an oligomeric and multifunctional protein

The intestinal epithelial cell-surface molecule, CD98 is a type II membrane glycoprotein. Molecular orientation studies have demonstrated that the C-terminal tail of human CD98 (hCD98), which contains a PDZ-binding domain, is extracellular. In intestinal epithelial cells, CD98 is covalently linked to an amino-acid transporter with which it forms a heterodimer. This heterodimer associates with beta(1)-integrin and intercellular adhesion molecular 1 (ICAM-1) to form a macromolecular complex in the basolateral membranes of polarized intestinal epithelial cells. This review focuses on the multifunctional roles of CD98, including involvement in extracellular signaling, adhesion/polarity, and amino-acid transporter expression in intestinal epithelia. A role for CD98 in intestinal inflammation, such as Intestinal Bowel Disease (IBD), is also proposed.

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.

Sites of action of hydrogen peroxide on ion transport across rat distal colon

BACKGROUND AND PURPOSE

The aim of this study was the identification of the mechanism of oxidant-induced intestinal secretion.

EXPERIMENTAL APPROACH

The action of H2O2 on ion transport across rat distal colon was evaluated in Ussing chambers. Changes in cytosolic Ca2+ concentration were measured using fura-2.

KEY RESULTS

H2O2 concentration-dependently induced an increase in short-circuit current (Isc), which was due to a stimulation of Cl(-) secretion. The effect of H2O2 was dependent on the presence of serosal Ca2+. It was inhibited after emptying of intracellular Ca2+ stores by cyclopiazonic acid or blockade of ryanodine receptors by ruthenium red, whereas a blocker of inositol 1,4,5-trisphosphate receptors was less effective. Fura 2-experiments confirmed an increase in the cytosolic Ca2+ concentration in the presence of H2O2. Measurements of Cl- currents across the apical membrane at basolaterally depolarized epithelia revealed the activation of a glibenclamide-sensitive, SITS-resistant Cl- conductance by the oxidant. The activation of this conductance was inhibited after blockade of protein kinases with staurosporine. When the apical membrane was permeabilized with nystatin, two sites of action of H2O2 were identified at the basolateral membrane. The oxidant stimulated a basolateral tetrapentylammonium-sensitive K+ conductance and increased the current generated by the Na+-K+ pump. Pretreatment of the tissues with H2O2 reduced the action of subsequently administered Ca2+-, cAMP- and cGMP-dependent secretagogues demonstrating a long-term downregulation after the initial secretory response evoked by the oxidant.

CONCLUSIONS AND IMPLICATIONS

H2O2 affects colonic anion secretion by action sites at both the apical, as well as the basolateral membrane.

Anion secretion evoked by Pasteurella multocida toxin across rat colon

Stimulation of muscarinic receptors is known to have a biphasic effect on colonic Cl(-) secretion: a short-lasting activation, which is followed by a long-lasting inhibition. In order to find out, which role Gq proteins play in both processes, Pasteurella multocida toxin was used, a known activator of G alpha q. This toxin (1.5 microg/ml) had a dual action on short-circuit current (Isc) across rat distal colon: it stimulated transiently Isc and subsequently down-regulated the Isc evoked by Ca2+-dependent secretagogues such as acetylcholine or ATP. The inactive mutant (P. multocida toxin C1165S), which does not stimulate G alpha q), was ineffective. Cl(-) dependence and sensitivity against bumetanide, a blocker of the Na+-K+-2Cl(-) cotransporter, confirmed that the increase in Isc evoked by the toxin represented Cl(-) secretion. The effect of P. multocida toxin was suppressed by YM-254890 (10(-7) M), a blocker of G alpha q. Experiments with apically permeabilized tissues revealed that the secretory response to P. multocida toxin was concomitant with an increase in basolateral K+ conductance as it is observed for other agonists inducing Ca2+-dependent anion secretion. Consequently, these results suggest that Gq proteins are not only involved in the activation of secretion, e.g. after stimulation of muscarinic or purinergic receptors, but also play a central role in the long-term down-regulation of intestinal secretion after activation of these types of receptors.

Ca2+-induced Cl- efflux at rat distal colonic epithelium

With the aid of the halide-sensitive dye 6-methoxy-N-ethylquinolinium iodide (MEQ), changes in intracellular Cl(-) concentration were measured to characterize the role of Ca(2+)-dependent Cl(-) channels at the rat distal colon. In order to avoid indirect effects of secretagogues mediated by changes in the driving force for Cl(-) exit (i.e., mediated by opening of Ca(2+)-dependent K(+) channels), all experiments were performed under depolarized conditions, i.e., in the presence of high extracellular K(+) concentrations. The Ca(2+)-dependent secretagogue carbachol induced a stilbene-sensitive Cl(-) efflux, which was mimicked by the Ca(2+) ionophore ionomycin. Surprisingly, the activation of Ca(2+)-dependent Cl(-) efflux was resistant against blockers of classical Ca(2+) signaling pathways such as phospholipase C, protein kinase C and calmodulin. Hence, alternative pathways must be involved in the signaling cascade. One possible signaling molecule seems to be nitric oxide (NO) as the NO donor sodium nitroprusside could induce Cl(- )efflux. Vice versa, the NO synthase inhibitor N-omega-monomethyl-arginine (L: -NMMA) reduced the carbachol-induced Cl(- )efflux. This indicates that NO may be involved in part of the signaling cascade. In order to test the ability of the epithelium to produce NO, the expression of different isoforms of NO synthase was verified by immunohistochemistry. In addition, the cytoskeleton seems to play a role in the activation of Ca(2+)-dependent Cl(-) channels. Inhibitors of microtubule association such as nocodazole and colchicine as well as jasplakinolide, a drug that enhances actin polymerization, inhibited the carbachol-induced Cl(-) efflux. Consequently, the activation of apical Cl(-) channels by muscarinic receptor stimulation differs in signal transduction from the classical phospholipase C/protein kinase C way.

Palmatine, a protoberberine alkaloid, inhibits both Ca(2+)- and cAMP-activated Cl(-) secretion in isolated rat distal colon

BACKGROUND AND PURPOSE

The protoberberine alkaloid berberine has been reported to inhibit colonic Cl(-) secretion. However, it is not known if other protoberberine alkaloids share these effects. We have therefore selected another protoberberine alkaloid, palmatine, to assess its effects on active ion transport across rat colonic epithelium.

EXPERIMENTAL APPROACH

Rat colonic mucosa was mounted in Ussing chambers and short circuit current (I (SC)), apical Cl(-) current and basolateral K(+) current were recorded. Intracellular cAMP content was determined by an enzyme immunoassay. Intracellular Ca(2+) concentration was measured with Fura-2 AM.

KEY RESULTS

Palmatine inhibited carbachol-induced Ca(2+)-activated Cl(-) secretion and the carbachol-induced increase of intracellular Ca(2+) concentration. Palmatine also inhibited cAMP-activated Cl(-) secretion induced by prostaglandin E(2) (PGE(2)) or forskolin. Palmatine prevented the elevation of intracellular cAMP by forskolin. Determination of apical Cl(-) currents showed that palmatine suppressed the forskolin-stimulated, apical cAMP-activated Cl(-) current but not the carbachol-stimulated apical Ca(2+)-activated Cl(-) current. Following permeabilization of apical membranes with nystatin, we found that palmatine inhibited a carbachol-stimulated basolateral K(+) current that was sensitive to charybdotoxin and resistant to chromanol 293B. However, the forskolin-stimulated basolateral K(+) current inhibited by palmatine was specifically blocked by chromanol 293B and not by charybdotoxin.

CONCLUSIONS AND IMPLICATIONS

Palmatine attenuated Ca(2+)-activated Cl(-) secretion through inhibiting basolateral charybdotoxin-sensitive, SK4 K(+) channels, whereas it inhibited cAMP-activated Cl(-) secretion by inhibiting apical CFTR Cl(-) channels and basolateral chromanol 293B-sensitive, KvLQT1 K(+) channels.