Cystic fibrosis transmembrane conductance regulator inhibits epithelial Na+ channels carrying Liddle's syndrome mutations

Preclinical murine models for the testing of antimicrobials against Mycobacterium abscessus pulmonary infections: Current practices and recommendations

Mycobacterium abscessus, a rapidly growing nontuberculous mycobacterium, is increasingly recognized as an important pathogen of the human lung, disproportionally affecting people with cystic fibrosis (CF) and other susceptible individuals with non-CF bronchiectasis and compromised immune functions. M. abscessus infections are extremely difficult to treat due to intrinsic resistance to many antibiotics, including most anti-tuberculous drugs. Current standard-of-care chemotherapy is long, includes multiple oral and parenteral repurposed drugs, and is associated with significant toxicity. The development of more effective oral antibiotics to treat M. abscessus infections has thus emerged as a high priority. While murine models have proven instrumental in predicting the efficacy of therapeutic treatments for M. tuberculosis infections, the preclinical evaluation of drugs against M. abscessus infections has proven more challenging due to the difficulty of establishing a progressive, sustained, pulmonary infection with this pathogen in mice. To address this issue, a series of three workshops were hosted in 2023 by the Cystic Fibrosis Foundation (CFF) and the National Institute of Allergy and Infectious Diseases (NIAID) to review the current murine models of M. abscessus infections, discuss current challenges and identify priorities toward establishing validated and globally harmonized preclinical models. This paper summarizes the key points from these workshops. The hope is that the recommendations that emerged from this exercise will facilitate the implementation of informative murine models of therapeutic efficacy testing across laboratories, improve reproducibility from lab-to-lab and accelerate preclinical-to-clinical translation.

In silico analysis and theratyping of an ultra-rare CFTR genotype (W57G/A234D) in primary human rectal and nasal epithelial cells

Mutation targeted therapy in cystic fibrosis (CF) is still not eligible for all CF subjects, especially for cases carrying rare variants such as the CFTR genotype W57G/A234D (c.169T>G/c.701C>A). We performed in silico analysis of the effects of these variants on protein stability, which we functionally characterized using colonoids and reprogrammed nasal epithelial cells. The effect of mutations on cystic fibrosis transmembrane conductance regulator (CFTR) protein was analyzed by western blotting, forskolin-induced swelling (FIS), and Ussing chamber analysis. We detected a residual CFTR function that increases following treatment with the CFTR modulators VX661±VX445±VX770, correlates among models, and is associated with increased CFTR protein levels following treatment with CFTR correctors. In vivo treatment with VX770 reduced sweat chloride concentration to non-CF levels, increased the number of CFTR-dependent sweat droplets, and induced a 6% absolute increase in predicted FEV1% after 27 weeks of treatment indicating the relevance of theratyping with patient-derived cells in CF.

Efficacy and safety of inhaled ENaC inhibitor BI 1265162 in patients with cystic fibrosis: BALANCE-CF™ 1 - a randomised, Phase II study

Background

Inhibition of the epithelial sodium channel (ENaC) in cystic fibrosis (CF) airways provides a mutation-agnostic approach that could improve mucociliary clearance in all CF patients. BI 1265162 is an ENaC inhibitor with demonstrated pre-clinical efficacy and safety already demonstrated in humans.

Objective

We present results from BALANCE-CF^TM 1, a phase II, placebo-controlled, randomised, double-blind study of four dose levels of BI 1265162 versus placebo for 4 weeks on top of standard of care in adults and adolescents with CF.

Results

Initially, 28 randomised subjects (BI 1265162 200 µg twice daily n=14, placebo twice daily n=14) were assessed at an interim futility analysis. Compared with placebo, numerical changes of –0.8% (95% CI –6.6 to 4.9%) in percentage predicted forced expiratory volume in 1s (ppFEV₁) and +2.1 units (95% CI –2.4 to 6.5 units) in lung clearance index (LCI) were observed in the active group, meeting a pre-defined stopping rule; accordingly, the study was terminated. Recruitment had continued during the interim analysis and pending results; 24 patients were added across three dose levels and placebo. The final results including these patients (+1.5% ppFEV₁, 200 µg twice-daily dose versus placebo) were not supportive of relevant clinical effect. Furthermore, LCI change was not supportive, although interpretation was limited due to insufficient traces meeting quality criteria. A 9.4-point improvement in the Cystic Fibrosis Questionnaire – Revised Respiratory Domain was observed in the 200 µg twice daily dose group versus placebo. BI 1265162 up to 200 µg twice daily was safe and well-tolerated. Pharmacokinetics were similar to those in healthy volunteers.

Conclusion

BI 1265162 was safe, but did not demonstrate a potential for clinical benefit. Development has been terminated.

Phase I trials showed that single and multiple doses of the inhaled ENaC inhibitor BI 1265162 are safe. In this phase II trial in patients with CF, BI 1265162 was safe, but did not demonstrate clinically relevant efficacy. The trial was terminated.https://bit.ly/3CiB8uM

Association of cystic fibrosis transmembrane conductance regulator with epithelial sodium channel subunits carrying Liddle's syndrome mutations

The association of the cystic fibrosis transmembrane conductance regulator (CFTR) and epithelial sodium channel (ENaC) in the pathophysiology of cystic fibrosis (CF) is controversial. Previously, we demonstrated a close physical association between wild-type (WT) CFTR and WT ENaC. We have also shown that the F508del CFTR fails to associate with ENaC unless the mutant protein is rescued pharmacologically or by low temperature. In this study, we present the evidence for a direct physical association between WT CFTR and ENaC subunits carrying Liddle's syndrome mutations. We show that all three ENaC subunits bearing Liddle's syndrome mutations (both point mutations and the complete truncation of the carboxy terminus), could be coimmunoprecipitated with WT CFTR. The biochemical studies were complemented by fluorescence lifetime imaging microscopy (FLIM), a distance-dependent approach that monitors protein-protein interactions between fluorescently labeled molecules. Our measurements revealed significantly increased fluorescence resonance energy transfer between CFTR and all tested ENaC combinations as compared with controls (ECFP and EYFP cotransfected cells). Our findings are consistent with the notion that CFTR and ENaC are within reach of each other even in the setting of Liddle's syndrome mutations, suggestive of a direct intermolecular interaction between these two proteins.

ENaC inhibition in cystic fibrosis: potential role in the new era of CFTR modulator therapies

Small-molecule cystic fibrosis transmembrane conductance regulator (CFTR) modulator drugs for cystic fibrosis are the first therapies since the disease was initially described by Fanconiet al. [1] in 1936 to target and partially restore the function of the CFTR Cl⁻ channel. CFTR modulator therapy is expected to have significant clinical benefits for many, but it does not result in a cure and is not appropriate or available for all patients with cystic fibrosis [2, 3]. In this review, evidence is described suggesting that inhibiting the epithelial Na⁺ channel (ENaC) responsible for the Na⁺/fluid absorption that contributes to airway surface dehydration and impaired mucociliary clearance (MCC) observed in cystic fibrosis airways may significantly improve clinical outcomes irrespective of the CFTR genotype, and may synergise with currently approved CFTR modulators to further improve clinical outcomes.

ENaC inhibition with BI 1265162 is a promising strategy to optimise outcomes in patients with CF either eligible, or ineligible, for CFTR modulator therapy. Phase II clinical trials of BI 1265162 must now show this translates into clinical benefit.https://bit.ly/2OQ1IUI

The role of inflammation in cystic fibrosis pulmonary exacerbations

INTRODUCTION

Cystic Fibrosis pulmonary exacerbations are critical events in the lives of people with CF that have deleterious effects on lung function, quality of life, and life expectancy. There are significant unmet needs in the management of exacerbations. We review here the associated inflammatory changes that underlie these events and are of interest for the development of biomarkers of exacerbation.

AREAS COVERED

Inflammatory responses in CF are abnormal and contribute to a sustained proinflammatory lung microenvironment, abundant in proinflammatory mediators and deficient in counter-regulatory mediators that terminate and resolve inflammation. There is increasing interest in these inflammatory pathways to discover novel biomarkers for pulmonary exacerbation management. In this review, we explore the inflammatory changes occurring during intravenous antibiotic therapy for exacerbation and how they may be applied as biomarkers to guide exacerbation therapy. A literature search was conducted using the PubMed database in February 2020.

EXPERT OPINION

Heterogeneity in inflammatory responses to treatment of a pulmonary exacerbation, a disease process with complex pathophysiology, limits the clinical utility of individual biomarkers. Biomarker panels may be a more successful strategy to capture informative changes within the CF population to improve pulmonary exacerbation management and outcomes.

Targeting ion channels in cystic fibrosis

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene cause a characteristic defect in epithelial ion transport that plays a central role in the pathogenesis of cystic fibrosis (CF). Hence, pharmacological correction of this ion transport defect by targeting of mutant CFTR, or alternative ion channels that may compensate for CFTR dysfunction, has long been considered as an attractive approach to a causal therapy of this life-limiting disease. The recent introduction of the CFTR potentiator ivacaftor into the therapy of a subgroup of patients with specific CFTR mutations was a major milestone and enormous stimulus for seeking effective ion transport modulators for all patients with CF. In this review, we discuss recent breakthroughs and setbacks with CFTR modulators designed to rescue mutant CFTR including the common mutation F508del. Further, we examine the alternative chloride channels TMEM16A and SLC26A9, as well as the epithelial sodium channel ENaC as alternative targets in CF lung disease, which remains the major cause of morbidity and mortality in patients with CF. Finally, we will focus on the hurdles that still need to be overcome to make effective ion transport modulation therapies available for all patients with CF irrespective of their CFTR genotype.

CFTR: cystic fibrosis and beyond

Cystic fibrosis (CF) remains the most common fatal hereditary lung disease. The discovery of the cystic fibrosis transmembrane conductance regulator (CFTR) gene 25 years ago set the stage for: 1) unravelling the molecular and cellular basis of CF lung disease; 2) the generation of animal models to study in vivo pathogenesis; and 3) the development of mutation-specific therapies that are now becoming available for a subgroup of patients with CF. This article highlights major advances in our understanding of how CFTR dysfunction causes chronic mucus obstruction, neutrophilic inflammation and bacterial infection in CF airways. Furthermore, we focus on recent breakthroughs and remaining challenges of novel therapies targeting the basic CF defect, and discuss the next steps to be taken to make disease-modifying therapies available to a larger group of patients with CF, including those carrying the most common mutation ΔF508-CFTR. Finally, we will summarise emerging evidence indicating that acquired CFTR dysfunction may be implicated in the pathogenesis of chronic obstructive pulmonary disease, suggesting that lessons learned from CF may be applicable to common airway diseases associated with mucus plugging.

Early cystic fibrosis lung disease: Role of airway surface dehydration and lessons from preventive rehydration therapies in mice

Cystic fibrosis (CF) lung disease starts in the first months of life and remains one of the most common fatal hereditary diseases. Early therapeutic interventions may provide an opportunity to prevent irreversible lung damage and improve outcome. Airway surface dehydration is a key disease mechanism in CF, however, its role in the in vivo pathogenesis and as therapeutic target in early lung disease remains poorly understood. Mice with airway-specific overexpression of the epithelial Na(+) channel (βENaC-Tg) recapitulate airway surface dehydration and phenocopy CF lung disease. Recent studies in neonatal βENaC-Tg mice demonstrated that airway surface dehydration produces early mucus plugging in the absence of mucus hypersecretion, which triggers airway inflammation, promotes bacterial infection and causes early mortality. Preventive rehydration therapy with hypertonic saline or amiloride effectively reduced mucus plugging and mortality in neonatal βENaC-Tg mice. These results support clinical testing of preventive/early rehydration strategies in infants and young children with CF.

Does epithelial sodium channel hyperactivity contribute to cystic fibrosis lung disease?

  Airway epithelia absorb Na+ through the epithelial Na+ channel (ENaC) and secrete Cl- through the cystic fibrosis transmembrane conductance regulator (CFTR) anion channel. This balance maintains sufficient airway surface liquid hydration to permit efficient mucus clearance, which is needed to maintain sterility of the lung. Cystic fibrosis (CF) is a common autosomal recessive inherited disease caused by mutations in the CFTR gene that lead to the reduction or elimination of the CFTR protein. CF is a multi-organ disease that affects epithelia lining the intestines, lungs, pancreas, sweat ducts and vas deferens, among others. CF lungs are characterized by viscous, dehydrated mucus, persistent neutrophilia and chronic infections. ENaC is negatively regulated by CFTR and, in patients with CF, the absence of CFTR results in a double hit of reduced Cl-/HCO3- and H2O secretion as well as ENaC hyperactivity and increased Na+ and H2O absorption. Together, these effects are hypothesized to trigger mucus dehydration, resulting in a failure to clear mucus. Rehydrating CF mucus has become a recent clinical focus and yields important end-points for clinical trials. However, while ENaC hyperactivity in CF airways has been detected in vivo and in vitro, recent data have brought the role of ENaC in CF lung disease pathogenesis into question. This review will focus on our current understanding of the contribution of ENaC to CF pathogenesis.

CFTR regulates early pathogenesis of chronic obstructive lung disease in βENaC-overexpressing mice

Background

Factors determining the onset and severity of chronic obstructive pulmonary disease remain poorly understood. Previous studies demonstrated that airway surface dehydration in βENaC-overexpressing (βENaC-Tg) mice on a mixed genetic background caused either neonatal mortality or chronic obstructive lung disease suggesting that the onset of lung disease was modulated by the genetic background.

Methods

To test this hypothesis, we backcrossed βENaC-Tg mice onto two inbred strains (C57BL/6 and BALB/c) and studied effects of the genetic background on neonatal mortality, airway ion transport and airway morphology. Further, we crossed βENaC-Tg mice with CFTR-deficient mice to validate the role of CFTR in early lung disease.

Results

We demonstrate that the C57BL/6 background conferred increased CFTR-mediated Cl⁻ secretion, which was associated with decreased mucus plugging and mortality in neonatal βENaC-Tg C57BL/6 compared to βENaC-Tg BALB/c mice. Conversely, genetic deletion of CFTR increased early mucus obstruction and mortality in βENaC-Tg mice.

Conclusions

We conclude that a decrease or absence of CFTR function in airway epithelia aggravates the severity of early airway mucus obstruction and related mortality in βENaC-Tg mice. These results suggest that genetic or environmental factors that reduce CFTR activity may contribute to the onset and severity of chronic obstructive pulmonary disease and that CFTR may serve as a novel therapeutic target.

Proteases, cystic fibrosis and the epithelial sodium channel (ENaC)

Proteases perform a diverse array of biological functions. From simple peptide digestion for nutrient absorption to complex signaling cascades, proteases are found in organisms from prokaryotes to humans. In the human airway, proteases are associated with the regulation of the airway surface liquid layer, tissue remodeling, host defense and pathogenic infection and inflammation. A number of proteases are released in the airways under both physiological and pathophysiological states by both the host and invading pathogens. In airway diseases such as cystic fibrosis, proteases have been shown to be associated with increased morbidity and airway disease progression. In this review, we focus on the regulation of proteases and discuss specifically those proteases found in human airways. Attention then shifts to the epithelial sodium channel (ENaC), which is regulated by proteolytic cleavage and that is considered to be an important component of cystic fibrosis disease. Finally, we discuss bacterial proteases, in particular, those of the most prevalent bacterial pathogen found in cystic fibrosis, Pseudomonas aeruginosa.

Regulation of ENaC biogenesis by the stress response protein SERP1

Cystic fibrosis lung disease is caused by reduced Cl(-) secretion along with enhanced Na(+) absorption, leading to reduced airway surface liquid and compromised mucociliary clearance. Therapeutic strategies have been developed to activate cystic fibrosis transmembrane conductance regulator (CFTR) or to overcome enhanced Na(+) absorption by the epithelial Na(+) channel (ENaC). In a split-ubiquitin-based two-hybrid screening, we identified stress-associated ER protein 1 (SERP1)/ribosome-associated membrane protein 4 as a novel interacting partner for the ENaC β-subunit. SERP1 is induced during cell stress and interacts with the molecular chaperone calnexin, thus controlling early biogenesis of membrane proteins. ENaC activity was measured in the human airway epithelial cell lines H441 and A549 and in voltage clamp experiments with ENaC-overexpressing Xenopus oocytes. We found that expression of SERP1 strongly inhibits amiloride-sensitive Na(+) transport. SERP1 coimmunoprecipitated and colocalized with βENaC in the endoplasmic reticulum, together with the chaperone calnexin. In contrast to the inhibitory effects on ENaC, SERP1 appears to promote expression of CFTR. Taken together, SERP1 is a novel cochaperone and regulator of ENaC expression.

Regulation of endogenous ENaC functional expression by CFTR and ΔF508-CFTR in airway epithelial cells

The functional expression of the epithelial sodium channel (ENaC) appears elevated in cystic fibrosis (CF) airway epithelia, but the mechanism by which this occurs is not clear. We tested the hypothesis that the cystic fibrosis transmembrane conductance regulator (CFTR) alters the trafficking of endogenously expressed human ENaC in the CFBE41o⁻ model of CF bronchial epithelia. Functional expression of ENaC, as defined by amiloride-inhibited short-circuit current (I(sc)) in Ussing chambers, was absent under control conditions but present in CFBE41o⁻ parental and ΔF508-CFTR-overexpressing cells after treatment with 1 μM dexamethasone (Dex) for 24 h. The effect of Dex was mimicked by incubation with the glucocorticoid hydrocortisone but not with the mineralocorticoid aldosterone. Application of trypsin to the apical surface to activate uncleaved, "near-silent" ENaC caused an additional increase in amiloride-sensitive I(sc) in the Dex-treated cells and was without effect in the control cells, suggesting that Dex increased ENaC cell surface expression. In contrast, Dex treatment did not stimulate amiloride-sensitive I(sc) in CFBE41o⁻ cells that stably express wild-type (wt) CFTR. CFBE41o⁻ wt cells also had reduced expression of α- and γ-ENaC compared with parental and ΔF508-CFTR-overexpressing cells. Furthermore, application of trypsin to the apical surface of Dex-treated CFBE41o⁻ wt cells did not stimulate amiloride-sensitive I(sc), suggesting that ENaC remained absent from the surface of these cells even after Dex treatment. We also tested the effect of trafficking-corrected ΔF508-CFTR on ENaC functional expression. Incubation with 1 mM 4-phenylbutyrate synergistically increased Dex-induced ENaC functional expression in ΔF508-CFTR-overexpressing cells. These data support the hypothesis that wt CFTR can regulate the whole cell, functional, and surface expression of endogenous ENaC in airway epithelial cells and that absence of this regulation may foster ENaC hyperactivity in CF airway epithelia.

Airway surface liquid volume regulation determines different airway phenotypes in liddle compared with betaENaC-overexpressing mice

Studies in cystic fibrosis patients and mice overexpressing the epithelial Na(+) channel beta-subunit (betaENaC-Tg) suggest that raised airway Na(+) transport and airway surface liquid (ASL) depletion are central to the pathogenesis of cystic fibrosis lung disease. However, patients or mice with Liddle gain-of-function betaENaC mutations exhibit hypertension but no lung disease. To investigate this apparent paradox, we compared the airway phenotype (nasal versus tracheal) of Liddle with CFTR-null, betaENaC-Tg, and double mutant mice. In mouse nasal epithelium, the region that functionally mimics human airways, high levels of CFTR expression inhibited Liddle epithelial Nat channel (ENaC) hyperfunction. Conversely, in mouse trachea, low levels of CFTR failed to suppress Liddle ENaC hyperfunction. Indeed, Na(+) transport measured in Ussing chambers ("flooded" conditions) was raised in both Liddle and betaENaC-Tg mice. Because enhanced Na(+) transport did not correlate with lung disease in these mutant mice, measurements in tracheal cultures under physiologic "thin film" conditions and in vivo were performed. Regulation of ASL volume and ENaC-mediated Na(+) absorption were intact in Liddle but defective in betaENaC-Tg mice. We conclude that the capacity to regulate Na(+) transport and ASL volume, not absolute Na(+) transport rates in Ussing chambers, is the key physiologic function protecting airways from dehydration-induced lung disease.

Regulatory interactions of N1303K-CFTR and ENaC inXenopusoocytes: evidence that chloride transport is not necessary for inhibition of ENaC

Regulatory interactions of the cystic fibrosis transmembrane conductance regulator (CFTR) and the epithelial Na+channel (ENaC) are readily apparent in Xenopus oocytes. However, the mechanism underlying these interactions remains controversial. CFTR's first nucleotide binding fold (NBD-1) may be important in these interactions, as dysfunctional CFTRs containing mutations within NBD-1, such as ΔF508 and G551D, lack such functional interactions with murine ENaC (mENaC). We hypothesized that a dysfunctional CFTR containing a non-NBD-1 mutation would retain regulatory interactions with mENaC and tested this hypothesis for N1303K-CFTR, where the mutation is located in CFTR's second nucleotide binding fold (NBD-2). cRNA for αβγ-mENaC and N1303K-CFTR was injected separately or together into Xenopus oocytes. ENaC and CFTR functional expression was assessed by two-electrode voltage clamp. Injection of N1303K (class II trafficking mutation) yielded low levels of CFTR function on activation with forskolin and 3-isobutyl-1-methylxanthine (IBMX). In coinjected oocytes, N1303K did not alter mENaC functional expression or surface expression before activation of N1303K. This is similar to our prior observations with ΔF508. However, unlike our observations with ΔF508, activation of N1303K acutely decreased mENaC functional and surface expression, and N1303K currents were enhanced by coinjection of mENaC. Furthermore, genistein only mildly enhanced the functional expression of N1303K-CFTR and did not improve regulation of ENaC by N1303K-CFTR. These data suggest that a structurally and functionally intact CFTR NBD-1 in activated CFTR can regulate mENaC surface expression independent of Cl−transport in Xenopus oocytes.

Targeted therapy for cystic fibrosis: cystic fibrosis transmembrane conductance regulator mutation-specific pharmacologic strategies

Cystic fibrosis (CF) results from the absence or dysfunction of a single protein, the CF transmembrane conductance regulator (CFTR). CFTR plays a critical role in the regulation of ion transport in a number of exocrine epithelia. Improvement or restoration of CFTR function, where it is deficient, should improve the CF phenotype. There are >1000 reported disease-causing mutations of the CFTR gene. Recent investigations have afforded a better understanding of the mechanism of dysfunction of many of these mutant CFTRs, and have allowed them to be classified according to their mechanism of dysfunction. These data, as well as an enhanced understanding of the role of CFTR in regulating epithelial ion transport, have led to the development of therapeutic strategies based on pharmacologic enhancement or repair of mutant CFTR dysfunction. The strategy, termed 'protein repair therapy', is aimed at improving the regulation of epithelial ion transport by mutant CFTRs in a mutation-specific fashion. The grouping of CFTR gene mutations, according to mechanism of dysfunction, yields some guidance as to which pharmacologic repair agents may be useful for specific CFTR mutations. Recent data has suggested that combinations of pharmacologic repair agents may be necessary to obtain clinically meaningful CFTR repair. Nevertheless, such strategies to improve mutant CFTR function hold great promise for the development of novel therapies aimed at correcting the underlying pathophysiology of CF.

Abnormal regulatory interactions of I148T-CFTR and the epithelial Na+ channel in Xenopus oocytes

The mechanisms underlying regulatory interactions of the cystic fibrosis transmembrane conductance regulator (CFTR) and the epithelial Na(+) channel (ENaC) in Xenopus oocytes are controversial. CFTR's first nucleotide binding domain (NBD-1) may be important in these interactions, because mutations within NBD-1 impair these functional interactions. We hypothesized that an abnormal CFTR containing a non-NBD-1 mutation and able to transport chloride would retain regulatory interactions with murine ENaC (mENaC). We tested this hypothesis for I148T-CFTR, where the mutation is located in CFTR's first intracellular loop. I148T-CFTR has been associated with a severe CF phenotype, perhaps because of defects in its regulation of bicarbonate transport, but it transports chloride similarly to wild-type CFTR in model systems (Choi JY, Muallem D, Kiselyov K, Lee MG, Thomas PJ, Muallem S. Nature 410: 94-97, 2001). cRNAs encoding alphabetagamma-mENaC and I148T-CFTR were injected separately or together into Xenopus oocytes. mENaC and CFTR functional expression were assessed by two-electrode voltage clamp. mENaC whole oocyte expression was determined by immunoblotting, and surface expression was quantitated by surface biotinylation. Injection of I148T-CFTR cRNA alone yielded high levels of CFTR functional expression. In coinjected oocytes, mENaC functional and surface expression was not altered by activation of I148T-CFTR with forskolin/ IBMX. Furthermore, the CFTR potentiator genistein both enhanced functional expression of I148T-CFTR and restored regulation of mENaC surface expression by activated I148T-CFTR. These data suggest that the ability to transport chloride is not a critical determinant of regulation of mENaC by activated CFTR in Xenopus oocytes and provide further evidence that I148T-CFTR is dysfunctional despite maintaining the ability to transport chloride.

Cl- interference with the epithelial Na+ channel ENaC

The cystic fibrosis transmembrane conductance regulator (CFTR) is a protein kinase A and ATP-regulated Cl- channel that also controls the activity of other membrane transport proteins, such as the epithelial Na+ channel ENaC. Previous studies demonstrated that cytosolic domains of ENaC are critical for down-regulation of ENaC by CFTR, whereas others suggested a role of cytosolic Cl- ions. We therefore examined in detail the anion dependence of ENaC and the role of its cytosolic domains for the inhibition by CFTR and the Cl- channel CLC-0. Coexpression of rat ENaC with human CFTR or the human Cl- channel CLC-0 caused inhibition of amiloride-sensitive Na+ currents after cAMP-dependent stimulation and in the presence of a 100 mM bath Cl- concentration. After activation of CFTR by 3-isobutyl-1-methylxanthine and forskolin or expression of CLC-0, the intracellular Cl- concentration was increased in Xenopus oocytes in the presence of a high bath Cl- concentration, which inhibited ENaC without changing surface expression of alpha beta gammaENaC. In contrast, a 5 mM bath Cl- concentration reduced the cytosolic Cl- concentration and enhanced ENaC activity. ENaC was also inhibited by injection of Cl- into oocytes and in inside/out macropatches by exposure to high cytosolic Cl- concentrations. The effect of Cl- was mimicked by Br-, Br-, NO3(-), and I-. Inhibition by Cl- was reduced in trimeric channels with a truncated COOH terminus of betaENaC and gammaENaC, and it was no longer detected in dimeric alpha deltaCbeta ENaC channels. Deletion of the NH2 terminus of alpha-, beta-, or gammaENaC, mutations in the NH2-terminal phosphatidylinositol bisphosphate-binding domain of betaENaC and gammaEnaC, and activation of phospholipase C, all reduced ENaC activity but allowed for Cl(-)-dependent inhibition of the remaining ENaC current. The results confirm a role of the carboxyl terminus of betaENaC for Cl(-)-dependent inhibition of the Na+ channel, which, however, may only be part of a complex regulation of ENaC by CFTR.

CFTR: more than just a chloride channel

This review examines the cystic fibrosis transmembrane conductance regulator (CFTR) protein. After summarizing the ion channels regulated by CFTR, the review focuses on the functions of CFTR that do not relate directly to a disease mechanism based on a channelopathy. The key concept is that newly synthesized CFTR has to enter lipid vesicles which bud from the endoplasmic reticulum. This is abnormally low in DeltaF508 CFTR. Normal wild type vesicular CFTR enters a recycling pool of lipid vesicles which transiently dock with the apical membrane only for CFTR to be retrieved shortly after into a sub-apical recycling compartment. This retrieval is abnormally fast in DeltaF508 CFTR. The review discusses the relationship between this process and the difficult topic of fat metabolism and then explores the possible links between abnormal fatty acid turnover and inflammatory cascades that are abnormal in cystic fibrosis. Finally the review concentrates on the emerging functions of a protein kinase (AMP-activated kinase) which is bound near the C terminus of the CFTR protein whose functions could intergrate some of the abnormalities in lipid metabolism that result from mislocalization of CFTR in clinical disease.

SECRETION AND ABSORPTION BY COLONIC CRYPTS

The intestines play an important role in the absorption and secretion of nutrients. The colon is the final area for recapturing electrolytes and water prior to excretion, and in order to maintain this electrolyte homeostasis, a complex interaction between secretory and absorptive processes is necessary. Until recently it was thought that secretion and absorption were two distinct processes associated with either crypts or surface cells, respectively. Recently it was demonstrated that both the surface and crypt cells can perform secretory and absorptive functions and that, in fact, these functions can be going on simultaneously. This issue is important in the complexities associated with secretory diarrhea and also in attempting to develop treatment strategies for intestinal disorders. Here, we update the model of colonic secretion and absorption, discuss new issues of transporter activation, and identify some important new receptor pathways that are important modulators of the secretory and absorptive functions of the colon.

Vasopressin-stimulated CFTR Cl- currents are increased in the renal collecting duct cells of a mouse model of Liddle's syndrome

Liddle's syndrome is a genetic form of hypertension linked to Na(+) retention caused by activating mutations in the COOH terminus of the beta or gamma subunit of the epithelial sodium channel (ENaC). In this study, we used the short-circuit current (I(sc)) method to investigate the effects of deamino-8-d-arginine vasopressin (dDAVP) on Na(+) and Cl(-) fluxes in primary cultures of cortical collecting ducts (CCDs) microdissected from the kidneys of mice with Liddle's syndrome carrying a stop codon mutation, corresponding to the beta-ENaC R(566) stop mutation (L) found in the original pedigree. Compared to wild-type (+/+) CCD cells, untreated L/+ and L/L CCD cells exhibited 2.7- and 4.2-fold increases, respectively, in amiloride-sensitive (Ams) I(sc), reflecting ENaC-dependent Na(+) absorption. Short-term incubation with dDAVP caused a rapid and significant increase (approximately 2-fold) in Ams I(sc) in +/+, but not in L/+ or L/L CCD cells. In sharp contrast, dDAVP induced a greater increase in 5-nitro-2-(3-phenylpropamino)benzoate (NPPB)-inhibited apical Cl(-) currents in amiloride-treated L/L and L/+ cells than in their +/+ counterparts. I(sc) recordings performed under apical ion substituted conditions revealed that the dDAVP-stimulated apical secretion of Cl(-), which was absent in cultured CCDs lacking CFTR, was 1.8-fold greater in L/+ and 3.7-fold greater in L/L CCD cells than in their +/+ CCD counterparts. After the basal membrane had been permeabilized with nystatin and a basal-to-apical Cl(-) gradient had been imposed, dDAVP also stimulated larger Cl(-) currents across L/L and L/+ CCD layers than +/+ CCD layers. These findings demonstrate that vasopressin stimulates greater apical CFTR Cl(-) conductance in the renal CCD cells of mice with Liddle's syndrome than in wild-type mice. This effect could contribute to the enhanced NaCl reabsorption observed in the distal nephron of patients with Liddle's syndrome.

Cystic Fibrosis Transmembrane Conductance Regulator Differentially Regulates Human and Mouse Epithelial Sodium Channels in Xenopus Oocytes

The cystic fibrosis transmembrane conductance regulator (CFTR), in addition to its well defined Cl- channel properties, regulates other ion channels. CFTR inhibits murine or rat epithelial Na+ channel (mENaC or rENaC) currents in many epithelial and non-epithelial cells, whereas murine or rat ENaC increases CFTR functional expression. These regulatory interactions are reproduced in Xenopus oocytes where both the open probability and surface expression of wild type CFTR Cl- channels are increased when CFTR is co-expressed with alphabetagamma mENaC, and conversely the activity of mENaC is inhibited after wild type CFTR activation. Using the Xenopus oocyte expression system, differences in functional regulatory interactions were observed when CFTR was co-expressed with either alphabetagamma mENaC or alphabetagamma human ENaC (hENaC). Co-expression of CFTR and alphabetagamma mENaC or hENaC resulted in an approximately 3-fold increase in CFTR Cl- current compared with oocytes expressing CFTR alone. Oocytes co-injected with both CFTR and mENaC or hENaC expressed an amiloride-sensitive whole cell current that was decreased compared with that observed with the injection of mENaC or hENaC alone before CFTR activation with forskolin/3-isobutyl-1-methylxanthine. CFTR activation resulted in a further 50% decrease in mENaC-mediated currents, an approximately 20% decrease in alpha-T663-hENaC-mediated currents, and essentially no change in alpha-A663-hENaC-mediated currents. Changes in ENaC functional expression correlated with ENaC surface expression by oocyte surface biotinylation experiments. Assessment of regulatory interactions between CFTR and chimeric mouse/human ENaCs suggest that the 20 C-terminal amino acid residues of alpha ENaC confer species specificity regarding ENaC inhibition by activated CFTR.

Genistein improves regulatory interactions between G551D-cystic fibrosis transmembrane conductance regulator and the epithelial sodium channel in Xenopus oocytes

The cystic fibrosis transmembrane conductance regulator (CFTR) in addition to its well defined Cl(-) channel properties regulates other ion channels. CFTR inhibits epithelial Na(+) channel (ENaC) currents in many epithelial and non-epithelial cells, whereas the presence of ENaC increases CFTR functional expression. This interregulation is reproduced in Xenopus oocytes where both the open probability and surface expression of wild type CFTR Cl(-) channels are increased when CFTR is co-expressed with alphabetagamma-mouse ENaC (mENaC) and conversely when the activity of mENaC is inhibited after wild type CFTR activation. Using the Xenopus oocyte expression system, different functional regulatory interactions were observed between G551D-CFTR and alphabetagamma-mENaC. The co-expression of G551D-CFTR and alphabetagamma-mENaC resulted in a 5-fold increase in G551D-CFTR Cl(-) current compared with oocytes expressing G551D-CFTR alone. Oocytes co-injected with both G551D-CFTR and ENaC expressed an amiloride-sensitive whole cell current that was similar to that observed before and after G551D-CFTR activation with forskolin/isobutylmethylxanthine. Treatment with genistein both enhanced the functional expression of G551D-CFTR and improved regulatory interactions between G551D-CFTR and ENaC. These data suggest that genistein may be useful in patients with cystic fibrosis and the G551D-CFTR mutation.

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

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

Role of CFTR in the colon

In contrast to the airways, the defects in colonic function in cystic fibrosis (CF) patients are closely related to the defect in CFTR. The gastrointestinal phenotype of CF transgenic mice closely resembles the phenotype in CF patients, which clearly indicates the crucial role of CFTR in colonic Cl- secretion and the absence of an effective compensation. In the colon, stimulation of CFTR Cl- channels involves cAMP- or cGMP-dependent phosphorylation. Exocytosis is not involved. Activation of CFTR leads to coactivation of basolateral KVLQT1-type K+ channels and inhibition of luminal Na+ channels (ENaC). In contrast to cultured cells, Ca2+ does not activate luminal Cl- channels in intact enterocytes. It activates basolateral SK4-type K+ channels and luminal K+ channels, which provide additional driving force for Cl- exit. The magnitude of Cl- secretion, however, completely depends on the presence of at least a residual CFTR function in the luminal membrane. These findings have been clearly demonstrated by Ussing chamber experiments in colon epithelium biopsies of CF and normal individuals: Colonic Cl- secretion in CF patients is variable and reflects the genotype; a complete defect of CFTR is paralleled by the absence of Cl- secretion and unmasks Ca(2+)-regulated K+ channels in the luminal membrane; overabsorption of Na+ in CF reflects the absence of ENaC inhibition by CFTR; and the functional status of CF colon can be mimicked by the complete suppression of cAMP stimulation in enterocytes of healthy individuals.

The cytosolic termini of the beta- and gamma-ENaC subunits are involved in the functional interactions between cystic fibrosis transmembrane conductance regulator and epithelial sodium channel

Epithelial sodium channel (ENaC) and cystic fibrosis transmembrane conductance regulator (CFTR) are co-localized in the apical membrane of many epithelia. These channels are essential for electrolyte and water secretion and/or reabsorption. In cystic fibrosis airway epithelia, a hyperactivated epithelial Na(+) conductance operates in parallel with defective Cl(-) secretion. Several groups have shown that CFTR down-regulates ENaC activity, but the mechanisms and the regulation of CFTR by ENaC are unknown. To test the hypothesis that ENaC and CFTR regulate each other, and to identify the region(s) of ENaC involved in the interaction between CFTR and ENaC, rENaC and its mutants were co-expressed with CFTR in Xenopus oocytes. Whole cell macroscopic sodium currents revealed that wild type (wt) alphabetagamma-rENaC-induced Na(+) current was inhibited by co-expression of CFTR, and further inhibited when CFTR was activated with a cAMP-raising mixture (CKT). Conversely, alphabetagamma-rENaC stimulated CFTR-mediated Cl(-) currents up to approximately 6-fold. Deletion mutations in the intracellular tails of the three rENaC subunits suggested that the carboxyl terminus of the beta subunit was required both for the down-regulation of ENaC by activated CFTR and the up-regulation of CFTR by ENaC. However, both the carboxyl terminus of the beta subunit and the amino terminus of the gamma subunit were essential for the down-regulation of rENaC by unstimulated CFTR. Interestingly, down-regulation of rENaC by activated CFTR was Cl(-)-dependent, while stimulation of CFTR by rENaC was not dependent on either cytoplasmic Na(+) or a depolarized membrane potential. In summary, there appear to be at least two different sites in ENaC involved in the intermolecular interaction between CFTR and ENaC.

Regulation of the epithelial sodium channel (ENaC) by accessory proteins

The epithelial sodium channel (ENaC) plays a key role in the regulation of fluid absorption in the kidney, lung, colon and exocrine glands, and in the regulation of blood pressure. Abnormal functioning of ENaC is associated with several human diseases, including pseudohypoaldosteronism type I, Liddle's syndrome, pulmonary edema, and cystic fibrosis. ENaC is regulated by several hormones, ions and accessory proteins. This review focuses on the regulation of ENaC by recently described accessory proteins, mainly Nedd4, syntaxin 1A, CFTR, sgk, K-Ras2A and Cap-1.

Gating of amiloride-sensitive Na(+) channels: subunit-subunit interactions and inhibition by the cystic fibrosis transmembrane conductance regulator

In search of the structural basis for gating of amiloride-sensitive Na(+) channels, kinetic properties of single homo and heterooligomeric ENaCs formed by the subunits with individual truncated cytoplasmic domains were studied in a cell-free planar lipid bilayer reconstitution system. Our results identify the N-terminus of the alpha-subunit as a major determinant of kinetic behavior of both homooligomeric and heterooligomeric ENaCs, although the carboxy-terminal domains of beta- and gamma-ENaC subunits play important role(s) in modulation of the kinetics of heterooligomeric channels. We also found that the cystic fibrosis transmembrane conductance regulator (CFTR) inhibits amiloride-sensitive channels, at least in part, by modulating their gating. Comparison of these data suggests that the modulatory effects of the beta- and gamma-ENaC subunits, and of the CFTR, may involve the same, or closely related, mechanism(s); namely, "locking" the heterooligomeric channels in their closed state. These mechanisms, however, do not completely override the gating mechanism of the alpha-channel.

Feedback inhibition of epithelial Na(+) channels in Xenopus oocytes does not require G(0) or G(i2) proteins

Regulation of amiloride-sensitive epithelial Na(+) channels (ENaC) is a prerequisite for coordination of electrolyte transport in epithelia. Downregulation of Na(+) conductance occurs when the intracellular Na(+) concentration is increased during reabsorption of electrolytes, known as feedback inhibition. Recent studies have demonstrated the involvement of alphaG(0) and alphaG(i2) proteins in the feedback control of ENaC in mouse salivary duct cells. In this report, we demonstrate that Na(+) feedback inhibition is also present in Xenopus oocytes after expression of rat alpha,beta, gamma-ENaC. Interfering with intracellular alphaG(0) or alphaG(i2) signaling by coexpression of either constitutively active alphaG(0)/alphaG(i2) or dominant negative alphaG(0)/alphaG(i2) and by coinjecting sense or antisense oligonucleotides for alphaG(0) had no impact on Na(+) feedback. Moreover, no evidence for involvement of the intracellular G protein cascade was found in experiments in which a regulator of G protein signaling (RGS3) or beta-adrenergic receptor kinase (betaARK) was coexpressed together with alpha,beta, gamma-ENaC. Although some experiments suggest the presence of an intracellular Na(+) receptor, we may conclude that Na(+) feedback in Xenopus oocytes is different from that described for salivary duct cells in that it does not require G protein signaling.