Epithelial sodium channels regulate cystic fibrosis transmembrane conductance regulator chloride channels in Xenopus oocytes

Protein interactions, calcium, phosphorylation, and cholesterol modulate CFTR cluster formation on membranes

The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel whose dysfunction leads to intracellular accumulation of chloride ions, dehydration of cell surfaces, and subsequent damage to airway and ductal organs. Beyond its function as a chloride channel, interactions between CFTR, epithelium sodium channel, and solute carrier (SLC) transporter family membrane proteins and cytoplasmic proteins, including calmodulin and Na+/H+ exchanger regulatory factor-1 (NHERF-1), coregulate ion homeostasis. CFTR has also been observed to form mesoscale membrane clusters. However, the contributions of multivalent protein and lipid interactions to cluster formation are not well understood. Using a combination of computational modeling and biochemical reconstitution assays, we demonstrate that multivalent interactions with CFTR protein binding partners, calcium, and membrane cholesterol can induce mesoscale CFTR cluster formation on model membranes. Phosphorylation of the intracellular domains of CFTR also promotes mesoscale cluster formation in the absence of calcium, indicating that multiple mechanisms can contribute to CFTR cluster formation. Our findings reveal that coupling of multivalent protein and lipid interactions promotes CFTR cluster formation consistent with membrane-associated biological phase separation.

Epithelial Na + Channels Function as Extracellular Sensors

The epithelial Na + channel (ENaC) resides on the apical surfaces of specific epithelia in vertebrates and plays a critical role in extracellular fluid homeostasis. Evidence that ENaC senses the external environment emerged well before the molecular identity of the channel was reported three decades ago. This article discusses progress toward elucidating the mechanisms through which specific external factors regulate ENaC function, highlighting insights gained from structural studies of ENaC and related family members. It also reviews our understanding of the role of ENaC regulation by the extracellular environment in physiology and disease. After familiarizing the reader with the channel's physiological roles and structure, we describe the central role protein allostery plays in ENaC's sensitivity to the external environment. We then discuss each of the extracellular factors that directly regulate the channel: proteases, cations and anions, shear stress, and other regulators specific to particular extracellular compartments. For each regulator, we discuss the initial observations that led to discovery, studies investigating molecular mechanism, and the physiological and pathophysiological implications of regulation. © 2024 American Physiological Society. Compr Physiol 14:5407-5447, 2024.

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.

The epithelial sodium channel (ENaC) as a therapeutic target for cystic fibrosis

Cystic fibrosis (CF) is a monogenic disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. CFTR dysfunction is characterized by abnormal mucociliary transport due to a dehydrated airway surface liquid (ASL) and hyperviscous mucus, among other pathologies of host defense. ASL depletion is caused by the absence of CFTR mediated chloride secretion along with continued activity of the epithelial sodium channel (ENaC) activity, which can also be affected by CFTR mediated anion conductance. Therefore, ENaC has been proposed as a therapeutic target to ameliorate ASL dehydration and improve mucus transport. Inhibition of ENaC has been shown to restore ASL hydration and enhance mucociliary transport in induced models of CF lung disease. To date, no therapy inhibiting ENaC has successfully translated to clinical efficacy, in part due to concerns regarding off-target effects, systemic exposure, durability of effect, and adverse effects. Recent efforts have been made to develop novel, rationally designed therapeutics to produce-specific, long-lasting inhibition of ENaC activity in the airways while simultaneously minimizing off target fluid transport effects, systemic exposure and side effects. Such approaches comprise next-generation small molecule direct inhibitors, indirect channel-activating protease inhibitors, synthetic peptide analogs, and oligonucleotide-based therapies. These novel therapeutics represent an exciting step forward in the development of ENaC-directed therapies for CF.

δβγ-ENaC is inhibited by CFTR but stimulated by cAMP in Xenopus laevis oocytes

The epithelial sodium channel (ENaC) and the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel critically regulate airway surface liquid by driving fluid absorption and secretion, respectively. Their functional interplay is complex and incompletely understood. ENaC is a heteromeric channel with three well-characterized subunits (α, β, and γ). In humans, an additional δ-ENaC subunit exists in lung and several other tissues, where it may replace the α-subunit to form δβγ-ENaC. Little is known about the physiological role of δβγ-ENaC and its possible interaction with CFTR. The aim of the present study was to investigate the effect of human CFTR on human δβγ-ENaC heterologously expressed in Xenopus laevis oocytes. In oocytes coexpressing δβγ-ENaC and CFTR the ENaC-mediated amiloride-sensitive whole cell current (ΔI_ami) was reduced by ~50% compared with that measured in oocytes expressing δβγ-ENaC alone. Moreover, basal level of proteolytic ENaC activation was reduced in the presence of CFTR. The inhibitory effect of CFTR on δβγ-ENaC was due to a combination of decreased average open probability (P_o) and reduced channel expression at the cell surface. Interestingly, in oocytes expressing δβγ-ENaC, increasing intracellular [cAMP] by IBMX and forskolin increased ΔI_ami by ~50%. This stimulatory effect was not observed for human and rat αβγ-ENaC and was independent of CFTR coexpression and coactivation. Experiments with a mutant channel (δβ_S520Cγ-ENaC) which can be converted to a channel with a P_o of nearly 1 suggested that cAMP activates δβγ-ENaC by increasing P_o In conclusion, our results demonstrate that δβγ-ENaC is inhibited by CFTR but activated by cAMP.

Correlation of apical fluid-regulating channel proteins with lung function in human COPD lungs

Links between epithelial ion channels and chronic obstructive pulmonary diseases (COPD) are emerging through animal model and in vitro studies. However, clinical correlations between fluid-regulating channel proteins and lung function in COPD remain to be elucidated. To quantitatively measure epithelial sodium channels (ENaC), cystic fibrosis transmembrane conductance regulator (CFTR), and aquaporin 5 (AQP5) proteins in human COPD lungs and to analyze the correlation with declining lung function, quantitative western blots were used. Spearman tests were performed to identify correlations between channel proteins and lung function. The expression of α and β ENaC subunits was augmented and inversely associated with lung function. In contrast, both total and alveolar type I (ATI) and II (ATII)-specific CFTR proteins were reduced. The expression level of CFTR proteins was associated with FEV1 positively. Abundance of AQP5 proteins and extracellular superoxide dismutase (SOD3) was decreased and correlated with spirometry test results and gas exchange positively. Furthermore, these channel proteins were significantly associated with severity of disease. Our study demonstrates that expression of ENaC, AQP5, and CFTR proteins in human COPD lungs is quantitatively associated with lung function and severity of COPD. These apically located fluid-regulating channels may thereby serve as biomarkers and potent druggable targets of COPD.

ERp29 regulates epithelial sodium channel functional expression by promoting channel cleavage

The epithelial Na(+) channel (ENaC) plays a key role in the regulation of blood pressure and airway surface liquid volume. ERp29 is a 29-kDa thioredoxin-homologous endoplasmic reticulum (ER) protein that has only a single cysteine instead of the usual thioredoxin CXXC motif. Our group previously demonstrated that ERp29 promotes biogenesis of the cystic fibrosis transmembrane conductance regulator (CFTR). On the basis of similarities of CFTR and ENaC trafficking, we hypothesized that ERp29 would also regulate ENaC biogenesis and functional expression. In epithelial cells, overexpression of wild-type (wt) ERp29 increased ENaC functional expression [amiloride-sensitive short-circuit current (Isc)] in Ussing chamber experiments, as well as the abundance of the cleaved form of γ-ENaC in whole cell lysates. In contrast, siRNA-mediated depletion of ERp29 or overexpression of a mutant ERp29 lacking its single cysteine (C157S ERp29) decreased ENaC functional expression. Cells in which wt ERp29 was overexpressed had a smaller fractional increase in amiloride-sensitive Isc when trypsin was applied to the apical surface to activate uncleaved ENaC, while cells in which C157S ERp29 was overexpressed or ERp29 was depleted had a significantly greater fractional increase in amiloride-sensitive Isc in response to trypsin. Interestingly, these observations were not associated with altered expression of β-ENaC at the apical surface. Instead, ERp29 appeared to promote the interaction of β-ENaC with the Sec24D cargo recognition component of the coat complex II ER exit machinery. Together, these data support the hypothesis that ERp29 directs ENaC toward the Golgi, where it undergoes cleavage during its biogenesis and trafficking to the apical membrane.

Porosome in Cystic Fibrosis

Macromolecular structures embedded in the cell plasma membrane called ‘porosomes’, are involved in the regulated fractional release of intravesicular contents from cells during secretion. Porosomes range in size from 15 nm in neurons and astrocytes to 100-180 nm in the exocrine pancreas and neuroendocrine cells. Porosomes have been isolated from a number of cells, and their morphology, composition, and functional reconstitution well documented. The 3D contour map of the assembly of proteins within the porosome complex, and its native X-ray solution structure at sub-nm resolution has also advanced. This understanding now provides a platform to address diseases that may result from secretory defects. Water and ion binding to mucin impart hydration, critical for regulating viscosity of the mucus in the airways epithelia. Appropriate viscosity is required for the movement of mucus by the underlying cilia. Hence secretion of more viscous mucus prevents its proper transport, resulting in chronic and fatal airways disease such as cystic fibrosis (CF). CF is caused by the malfunction of CF transmembrane conductance regulator (CFTR), a chloride channel transporter, resulting in viscous mucus in the airways. Studies in mice lacking functional CFTR secrete highly viscous mucous that adhered to the epithelium. Since CFTR is known to interact with the t-SNARE protein syntaxin-1A, and with the chloride channel CLC-3, which are also components of the porosome complex, the interactions between CFTR and the porosome complex in the mucin-secreting human airway epithelial cell line Calu-3 was hypothesized and tested. Results from the study demonstrate the presence of approximately 100 nm in size porosome complex composed of 34 proteins at the cell plasma membrane in Calu-3 cells, and the association of CFTR with the complex. In comparison, the nuclear pore complex measures 120 nm and is comprised of over 500 protein molecules. The involvement of CFTR in porosome-mediated mucin secretion is hypothesized, and is currently being tested.

Proteome of the porosome complex in human airway epithelia: interaction with the cystic fibrosis transmembrane conductance regulator (CFTR)

The surface of the airways is coated with a thin film of mucus composed primarily of mucin, which is under continuous motion via ciliary action. Mucin not only serves to lubricate the airways epithelia, but also functions as a trap for foreign particles and pathogens, thereby assisting in keeping the airways clean and free of particulate matter and infections. Altered mucin secretion especially increased mucin viscosity, results in mucin stagnation due to the inability of the cilia to propel them, leading to infections and diseases such as cystic fibrosis (CF). Since porosomes have been demonstrated to be the secretory portals at the cell plasma membrane in cells, their presence, structure, and composition in the mucin-secreting human airway epithelial cell line Calu-3 expressing CF transmembrane receptor (CFTR), were investigated. Atomic force microscopy (AFM) of Calu-3 cells demonstrates the presence of approximately 100nm in diameter porosome openings at the plasma membrane surface. Electron microscopy confirms the AFM results, and tandem mass spectrometry and immunoanalysis performed on isolated Calu-3 porosomes, reveal the association of CFTR with the porosome complex. These new findings will facilitate understanding of CFTR-porosome interactions influencing mucous secretion, and provide critical insights into the etiology of CF disease.

BIOLOGICAL SIGNIFICANCE

In the present study, the porosome proteome in human airway epithelia has been determined. The interaction between the cystic fibrosis transmembrane conductance regulator (CFTR) and the porosome complex in the human airway epithelia is further demonstrated. The possible regulation by CFTR on the quality of mucus secretion via the porosome complex at the cell plasma membrane is hypothesized. These new findings will facilitate understanding of CFTR-porosome interactions influencing mucous secretion, and provide critical insights into the etiology of CF disease.

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 transport in the connecting tubule and cortical collecting duct

The central goal of this overview article is to summarize recent findings in renal epithelial transport,focusing chiefly on the connecting tubule (CNT) and the cortical collecting duct (CCD).Mammalian CCD and CNT are involved in fine-tuning of electrolyte and fluid balance through reabsorption and secretion. Specific transporters and channels mediate vectorial movements of water and solutes in these segments. Although only a small percent of the glomerular filtrate reaches the CNT and CCD, these segments are critical for water and electrolyte homeostasis since several hormones, for example, aldosterone and arginine vasopressin, exert their main effects in these nephron sites. Importantly, hormones regulate the function of the entire nephron and kidney by affecting channels and transporters in the CNT and CCD. Knowledge about the physiological and pathophysiological regulation of transport in the CNT and CCD and particular roles of specific channels/transporters has increased tremendously over the last two decades.Recent studies shed new light on several key questions concerning the regulation of renal transport.Precise distribution patterns of transport proteins in the CCD and CNT will be reviewed, and their physiological roles and mechanisms mediating ion transport in these segments will also be covered. Special emphasis will be given to pathophysiological conditions appearing as a result of abnormalities in renal transport in the CNT and CCD.

The plant mitochondrial carrier family: functional and evolutionary aspects

Mitochondria play a key role in respiration and energy production and are involved in multiple eukaryotic but also in several plant specific metabolic pathways. Solute carriers in the inner mitochondrial membrane connect the internal metabolism with that of the surrounding cell. Because of their common basic structure, these transport proteins affiliate to the mitochondrial carrier family (MCF). Generally, MCF proteins consist of six membrane spanning helices, exhibit typical conserved domains and appear as homodimers in the native membrane. Although structurally related, MCF proteins catalyze the specific transport of various substrates, such as nucleotides, amino acids, dicarboxylates, cofactors, phosphate or H(+). Recent investigations identified MCF proteins also in several other cellular compartments and therefore their localization and physiological function is not only restricted to mitochondria. MCF proteins are a characteristic feature of eukaryotes and bacterial genomes lack corresponding sequences. Therefore, the evolutionary origin of MCF proteins is most likely associated with the establishment of mitochondria. It is not clear whether the host cell, the symbiont, or the chimerical organism invented the ancient MCF sequence. Here, we try to explain the establishment of different MCF proteins and focus on the characteristics of members from plants, in particular from Arabidopsis thaliana.

Introduction to section V: assessment of CFTR function

This chapter introduces the various techniques to asses the function of CFTR. The numerous functional interactions of CFTR and cellular properties affected by CFTR will be described initially. This will be followed by sections explaining the importance of patch clamping and double electrode voltage clamp experiments in Xenopus oocytes for expression analysis of CFTR, and the Ussing chamber technique to analyze CFTR in polarized epithelia. It is concluded that examining CFTR function should occur at different levels, starting with the intact epithelium and ending with isolated CFTR proteins.

ERp29 regulates DeltaF508 and wild-type cystic fibrosis transmembrane conductance regulator (CFTR) trafficking to the plasma membrane in cystic fibrosis (CF) and non-CF epithelial cells

Sodium 4-phenylbutyrate (4PBA) improves the intracellular trafficking of ΔF508-CFTR in cystic fibrosis (CF) epithelial cells. The underlying mechanism is uncertain, but 4PBA modulates the expression of some cytosolic molecular chaperones. To identify other 4PBA-regulated proteins that might regulate ΔF508-CFTR trafficking, we performed a differential display RT-PCR screen on IB3-1 CF bronchiolar epithelial cells exposed to 4PBA. One transcript up-regulated by 4PBA encoded ERp29, a luminal resident of the endoplasmic reticulum (ER) thought to be a novel molecular chaperone. We tested the hypothesis that ERp29 is a 4PBA-regulated ER chaperone that influences ΔF508-CFTR trafficking. ERp29 mRNA and protein expression was significantly increased (∼1.5-fold) in 4PBA-treated IB3-1 cells. In Xenopus oocytes, ERp29 overexpression increased the functional expression of both wild-type and ΔF508-CFTR over 3-fold and increased wild-type cystic fibrosis transmembrane conductance regulator (CFTR) plasma membrane expression. In CFBE41o- WT-CFTR cells, expression of and short circuit currents mediated by CFTR decreased upon depletion of ERp29 as did maturation of newly synthesized CFTR. In IB3-1 cells, ΔF508-CFTR co-immunoprecipitated with endogenous ERp29, and overexpression of ERp29 led to increased ΔF508-CFTR expression at the plasma membrane. These data suggest that ERp29 is a 4PBA-regulated ER chaperone that regulates WT-CFTR biogenesis and can promote ΔF508-CFTR trafficking in CF epithelial cells.

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.

The cystic fibrosis transmembrane conductance regulator impedes proteolytic stimulation of the epithelial Na+ channel

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR) that prevent its proper folding and trafficking to the apical membrane of epithelial cells. Absence of cAMP-mediated Cl(-) secretion in CF airways causes poorly hydrated airway surfaces in CF patients, and this condition is exacerbated by excessive Na(+) absorption. The mechanistic link between missing CFTR and increased Na(+) absorption in airway epithelia has remained elusive, although substantial evidence implicates hyperactivity of the epithelial Na(+) channel (ENaC). ENaC is known to be activated by selective endoproteolysis of the extracellular domains of its α- and γ-subunits, and it was recently reported that ENaC and CFTR physically associate in mammalian cells. We confirmed this interaction in oocytes by co-immunoprecipitation and found that ENaC associated with wild-type CFTR was protected from proteolytic cleavage and stimulation of open probability. In contrast, ΔF508 CFTR, the most common mutant protein in CF patients, failed to protect ENaC from proteolytic cleavage and stimulation. In normal airway epithelial cells, ENaC was contained in the anti-CFTR immunoprecipitate. In CF airway epithelial cultures, the proportion of full-length to total α-ENaC protein signal was consistently reduced compared with normal cultures. Our results identify limiting proteolytic cleavage of ENaC as a mechanism by which CFTR down-regulates Na(+) absorption.

Voltage-dependent calcium and chloride currents in S17 bone marrow stromal cell line

The bone marrow stromal cell line S17 has been used to study hematopoiesis in vitro. In this study, we demonstrate the presence of calcium and chloride currents in cultured S17 cells. Calcium currents were of low amplitude or barely detectable (50-100 pA). Hence to amplify the currents, we have used barium as a charge carrier. Barium currents were identified based on their distinct voltage-dependence, and sensitivity to dihydropyridines. S17 cells also exhibited a slowly activating outward current without inactivation, most commonly seen when the sodium of the extracellular solution was replaced either by TEA (TEA/Cs saline) or NMDG (NMDG saline), or by addition of amiloride to the extracellular solution. This current was abolished either by 500 microM SITS (4,4'-diisothiocyanatostilbene-2-2'-disulfonic acid) or 500 microM DPC (diphenylamine-2-carboxylic acid) a cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel blocker, identifying it as a Cl(-) current. RT-PCR identified the presence of ENaC and CFTR transcripts. CFTR blockade reduced cell proliferation, suggesting that this channel plays a physiological role in regulation of S17 cell proliferation.

Discovery of putative oocyte quality markers by comparative ExacTag proteomics

PURPOSE

Identification of the biomarkers of oocyte quality, and developmental and reprogramming potential is of importance to assisted reproductive technology in humans and animals.

EXPERIMENTAL DESIGN

PerkinElmer ExacTag™ Kit was used to label differentially proteins in pig oocyte extracts (oocyte proteome) and pig oocyte-conditioned in vitro maturation media (oocyte secretome) obtained with high- and low-quality oocytes.

RESULTS

We identified 16 major proteins in the oocyte proteome that were expressed differentially in high- versus low-quality oocytes. More abundant proteins in the high-quality oocyte proteome included kelch-like ECH-associated protein 1 (an adaptor for ubiquitin-ligase CUL3), nuclear export factor CRM1 and ataxia-telangiectasia mutated protein kinase. Dystrophin (DMD) was more abundant in low-quality oocytes. In the secretome, we identified 110 proteins, including DMD and cystic fibrosis transmembrane conductance regulator, two proteins implicated in muscular dystrophy and cystic fibrosis, respectively. Monoubiquitin was identified in the low-quality-oocyte secretome.

CONCLUSIONS AND CLINICAL IMPLICATIONS

A direct, quantitative proteomic analysis of small oocyte protein samples can identify potential markers of oocyte quality without the need for a large amount of total protein. This approach will be applied to discovery of non-invasive biomarkers of oocyte quality in assisted human reproduction and in large animal embryo transfer programs.

CFTR delivery to 25% of surface epithelial cells restores normal rates of mucus transport to human cystic fibrosis airway epithelium

Delivering CFTR to ciliated cells of cystic fibrosis (CF) patients fully restores ion and fluid transport to the lumenal surface of airway epithelium and returns mucus transport rates to those of non-CF airways.

Dysfunction of CFTR in cystic fibrosis (CF) airway epithelium perturbs the normal regulation of ion transport, leading to a reduced volume of airway surface liquid (ASL), mucus dehydration, decreased mucus transport, and mucus plugging of the airways. CFTR is normally expressed in ciliated epithelial cells of the surface and submucosal gland ductal epithelium and submucosal gland acinar cells. Critical questions for the development of gene transfer strategies for CF airway disease are what airway regions require CFTR function and how many epithelial cells require CFTR expression to restore normal ASL volume regulation and mucus transport to CF airway epithelium? An in vitro model of human CF ciliated surface airway epithelium (CF HAE) was used to test whether a human parainfluenza virus (PIV) vector engineered to express CFTR (PIVCFTR) could deliver sufficient CFTR to CF HAE to restore mucus transport, thus correcting the CF phenotype. PIVCFTR delivered CFTR to >60% of airway surface epithelial cells and expressed CFTR protein in CF HAE approximately 100-fold over endogenous levels in non-CF HAE. This efficiency of CFTR delivery fully corrected the basic bioelectric defects of Cl⁻ and Na⁺ epithelial ion transport and restored ASL volume regulation and mucus transport to levels approaching those of non-CF HAE. To determine the numbers of CF HAE surface epithelial cells required to express CFTR for restoration of mucus transport to normal levels, different amounts of PIVCFTR were used to express CFTR in 3%–65% of the surface epithelial cells of CF HAE and correlated to increasing ASL volumes and mucus transport rates. These data demonstrate for the first time, to our knowledge, that restoration of normal mucus transport rates in CF HAE was achieved after CFTR delivery to 25% of surface epithelial cells. In vivo experimentation in appropriate models will be required to determine what level of mucus transport will afford clinical benefit to CF patients, but we predict that a future goal for corrective gene transfer to the CF human airways in vivo would attempt to target at least 25% of surface epithelial cells to achieve mucus transport rates comparable to those in non-CF airways.

The ciliated epithelium that lines the conducting airways of the lung normally functions to transport hydrated mucus secretions out of the airways to maintain respiratory sterility. Cystic fibrosis (CF) lung disease results from reduced airway surface hydration leading to decreased mucus clearance that precipitates bacterial infection and progressive obstructive lung disease. CF is a genetic disease, and the mutant protein is a chloride ion channel (CFTR) that normally regulates ion and fluid transport on the airway surface. Restoration of corrected CFTR function to the airway epithelium of CF patients by delivering a new CFTR gene to airway epithelial cells has long been envisioned as a therapeutic strategy for CF lung disease. Towards this goal, we use a novel viral vector to deliver CFTR to a culture model that represents the ciliated airway epithelium of CF patients and show that this strategy restores airway surface hydration and mucus transport to levels of that in non-CF individuals. This study demonstrates efficient and efficacious CFTR delivery to CF ciliated airway epithelium and that CFTR delivered to approximately 25% of the surface epithelial cells restores normal levels of airway surface hydration and mucus transport. These studies serve as a benchmark for the efficiency of CFTR gene delivery to CF airways for future CF gene therapy studies in vivo.

Regulated sodium transport in the renal connecting tubule (CNT) via the epithelial sodium channel (ENaC)

The aldosterone-sensitive distal nephron (ASDN) includes the late distal convoluted tubule 2, the connecting tubule (CNT) and the collecting duct. The appropriate regulation of sodium (Na(+)) absorption in the ASDN is essential to precisely match urinary Na(+) excretion to dietary Na(+) intake whilst taking extra-renal Na(+) losses into account. There is increasing evidence that Na(+) transport in the CNT is of particular importance for the maintenance of body Na(+) balance and for the long-term control of extra-cellular fluid volume and arterial blood pressure. Na(+) transport in the CNT critically depends on the activity and abundance of the amiloride-sensitive epithelial sodium channel (ENaC) in the luminal membrane of the CNT cells. As a rate-limiting step for transepithelial Na(+) transport, ENaC is the main target of hormones (e.g. aldosterone, angiotensin II, vasopressin and insulin/insulin-like growth factor 1) to adjust transepithelial Na(+) transport in this tubular segment. In this review, we highlight the structural and functional properties of the CNT that contribute to the high Na(+) transport capacity of this segment. Moreover, we discuss some aspects of the complex pathways and molecular mechanisms involved in ENaC regulation by hormones, kinases, proteases and associated proteins that control its function. Whilst cultured cells and heterologous expression systems have greatly advanced our knowledge about some of these regulatory mechanisms, future studies will have to determine the relative importance of the various pathways in the native tubule and in particular in the CNT.

Chronic inflammation in the cystic fibrosis lung: alterations in inter- and intracellular signaling

A vicious cycle of airway obstruction, infection, and inflammation continues to cause most of the morbidity and mortality in cystic fibrosis (CF). Mutations that result in decreased expression or function of the membrane Cl(-) channel, cystic fibrosis transmembrane regulator (CFTR), result in a decrease in the volume (and hence the depth) of liquid on the airway surface, impaired ciliary function, and dehydrated glandular secretions. In turn, these abnormalities contribute to a milieu, which promotes chronic infection with a limited but unique spectrum of microorganisms. Defects in CFTR also perturb regulation of several intracellular signaling pathways including signal transducers and activator of transcription, I-kappaB and nuclear factor-kappa B, and low molecular weight GTPases. Together, these abnormalities result in excessive production of NF-kappaB dependent cytokines such as interleukin (IL)-1, tumor necrosis factor (TNF), IL-6, and IL-8. There are decreased responses to interferon gamma and transforming growth factor beta leading to decreased production of iNOS and NO. Abnormalities of lipid mediators and decreased secretion of counter/regulatory cytokines have also been reported. Together, these effects combine to create a chronic inflammatory process, which damages and obstructs the airways, and eventually claims the life of the patient.

Infections in chronic lung diseases

Chronic lung diseases are prevalent worldwide and cause significant mortality and suffering. This article discusses infections that occur in three chronic lung diseases: chronic obstructive pulmonary disease, bronchiectasis, and cystic fibrosis. Rather than discussing the role of infections as etiology of these diseases, this article focuses on infections that occur in the background of established chronic lung disease.

Reproducibility of nasal potential difference measurements in cystic fibrosis

BACKGROUND

Nasal potential difference (NPD) measurement has been advocated as a diagnostic tool for cystic fibrosis (CF) patients and as a method for assessing the response to new therapies. The purpose of this study was to examine the reproducibility of NPD measurements performed in a single center.

METHODS

A total of 68 CF patients with a mean (+/- SD) age of 16 +/- 8 years (age range, 6 to 52 years) underwent NPD measurements on at least two occasions.

RESULTS

A total of 25 patients with classic CF (mean age, 21 +/- 8 years) and 43 patients with nonclassic CF (mean age, 14 +/- 8 years) underwent sweat tests and NPD measurements. The mean sweat chloride values were 102 +/- 18 and 54 +/- 14 mEq/L, respectively, for classic CF and nonclassic CF groups. All patients underwent repeat NPD measurements. The basal NPD and the response to amiloride (DeltaAmil) and response to Cl(-) free and isoproterenol (DeltaCl(-) free + iso) were very similar in both measurements. In the classic CF group, the basal potential difference values were -40 +/- 12 vs -39 +/- 11 mV (p = 0.57), respectively, for the first and second measurements; 27 +/- 9 vs 26 +/- 10 mV (p = 0.55), respectively, for DeltaAmil; and 2.1 +/- 3.8 vs 0.4 +/- 2.9 mV (p = 0.07), respectively, for DeltaCl(-) free + iso. In the nonclassic CF group, the values were -32 +/- 13 vs -28 +/- 10 mV (p = 0.008), respectively; 19 +/- 10 vs 17 +/- 8 mV (p = 0.388), respectively; and -3.2 +/- 4.6 vs -3.3 +/- 4.4 mV (p = 0.876), respectively.

CONCLUSION

When performed in a single center, NPD is a reproducible test for CF patients and thus may be a useful outcome measurement for assessment of the efficacy of new treatments.

Upregulated expression of ENaC in human CF nasal epithelium

Cystic fibrosis (CF) is characterised by the absence of CFTR function resulting in a reduced Cl(-) secretion and an increase in Na+ absorption. This Na+ hyperabsorption is mediated by the human amiloride-sensitive epithelial sodium channel (ENaC), but the underlying mechanisms are still unknown. After demonstrating functional differences of the Na+ absorption in CF and non-CF epithelia in Ussing chamber experiments with human primary cultures, we compared ENaC sequences from CF and non-CF human nasal tissue (hnENaC), investigated the mRNA transcription levels via real-time PCR and studied the protein expression in Western blot analyses. We found no differences in the sequences of CF and non-CF hnENaC, but identified some polymorphisms. The real-time experiments revealed an enhanced mRNA amount of all three hnENaC subunits in CF tissue. By comparing the two groups on the protein level, we observed differences in the abundance of the Na+ channel. While the alpha- and beta-hnENaC protein amount was increased in CF tissue the gamma-hnENaC was decreased. We conclude that the Na+ hyperabsorption in CF is not caused by mutations in hnENaC, but by an increase in the transcription of the hnENaC subunits. This could be induced by a disturbed regulation of the channel in CF.

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.

Interregulation of proton-gated Na(+) channel 3 and cystic fibrosis transmembrane conductance regulator

Proton-gated Na(+) channels (ASIC) are new members of the epithelial sodium channel/degenerin gene family. ASIC3 mRNA has been detected in the homogenate of pulmonary tissues. However, whether ASIC3 is expressed in the apical membranes of lung epithelial cells and whether it regulates cystic fibrosis transmembrane conductance regulator (CFTR) function are not known at the present time. Using reverse transcription-PCR, we found that the ASIC3 mRNA was expressed in the human airway mucosal gland (Calu-3) and human airway epithelial (16HBE14o) cells. Indirect immunofluorescence microscopy revealed that ASIC3 was co-segregated with CFTR in the apical membranes of Calu-3 cells. Proton-gated, amiloride-sensitive short circuit Na(+) currents were recorded across Calu-3 monolayers mounted in an Ussing chamber. In whole-cell patch clamp studies, activation of CFTR channels with cAMP reduced proton-gated Na(+) current in Calu-3 cells from -154 +/- 28 to -33 +/- 16 pA (n = 5, p < 0.05) at -100 mV. On the other hand, cAMP-activated CFTR activity was significantly inhibited following constitutive activation of putative ASIC3 at pH 6.0. Immunoassays showed that both ASIC3 and CFTR proteins were expressed and co-immunoprecipitated mutually in Calu-3 cells. Similar results were obtained in human embryonic kidney 293T cells following transient co-transfection of ASIC3 and CFTR. Our results indicate that putative CFTR and ASIC3 channels functionally interact with each other, possibly via an intermolecular association. Because acidic luminal fluid in the cystic fibrosis airway and lung tends to stimulate ASIC3 channel expression and activity, the interaction of ASIC3 and CFTR may contribute to defective salt and fluid transepithelial transport in the cystic fibrotic pulmonary system.

New insights into cystic fibrosis: molecular switches that regulate CFTR

Cystic fibrosis transmembrane conductance regulator (CFTR), a Cl(-)-selective ion channel, is a prototypic member of the ATP-binding cassette transporter superfamily that is expressed in several organs. In these organs, CFTR assembles into large, dynamic macromolecular complexes that contain signalling molecules, kinases, transport proteins, PDZ-domain-containing proteins, myosin motors, Rab GTPases, and SNAREs. Understanding how these complexes regulate the intracellular trafficking and activity of CFTR provides a unique insight into the aetiology of cystic fibrosis and other diseases.

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.

Differential effects of Hsc70 and Hsp70 on the intracellular trafficking and functional expression of epithelial sodium channels

The members of the cytoplasmic 70-kDa heat shock protein family are involved in appropriate folding and trafficking of newly synthesized proteins in the cell. Hsc70, which is expressed constitutively, and Hsp70, the expression of which is stress- and heat shock-induced, are often considered to have similar cellular functions in this regard, but there are suggestions that the intracellular functions of these homologous but not identical proteins may differ. We tested the hypothesis that Hsc70 and Hsp70 would have differential effects on the expression of the epithelial sodium channel (ENaC). In Xenopus oocytes, overexpression of human Hsc70 decreased the functional (defined as amiloride-sensitive whole-oocyte current) and surface expression of murine ENaC (mENaC) in a concentration-dependent fashion. In contrast, coinjection of a moderate amount of Hsp70 cRNA (10 ng) increased the functional and surface expression of mENaC, whereas a higher amount of coinjected Hsp70 cRNA (30 ng) decreased mENaC functional and surface expression. The increase in mENaC functional expression with coinjection of 10 ng of Hsp70 cRNA was antagonized by the additional coinjection of Hsc70 cRNA in a concentration-dependent fashion. These data are consistent with Hsc70 and Hsp70 having differential and antagonistic effects with regard to the intracellular trafficking of mENaC in oocytes, which may have an impact on our understanding and potential treatment of diseases of aberrant ion channel trafficking.

Cytosolic potassium controls CFTR deactivation in human sweat duct

Absorptive epithelial cells must admit large quantities of salt (NaCl) during the transport process. How these cells avoid swelling to protect functional integrity in the face of massive salt influx is a fundamental, unresolved problem. A special preparation of the human sweat duct provides critical insights into this crucial issue. We now show that negative feedback control of apical salt influx by regulating the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel activity is key to this protection. As part of this control process, we report a new physiological role of K(+) in intracellular signaling and provide the first direct evidence of acute in vivo regulation of CFTR dephosphorylation activity. We show that cytosolic K(+) concentration ([K(+)](c)) declines as a function of increasing cellular NaCl content at the onset of absorptive activity. Declining [K(+)](c) cause parallel deactivation of CFTR by dephosphorylation, thereby limiting apical influx of Cl(-) (and its co-ion Na(+)) until [K(+)](c) is stabilized. We surmise that [K(+)](c) stabilizes when Na(+) influx decreases to a level equal to its efflux through the basolateral Na(+)-K(+) pump thereby preventing disruptive changes in cell volume.

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.

Novel, mechanism-based therapies for cystic fibrosis

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

ClC-5 chloride channel alters expression of the epithelial sodium channel (ENaC)

ClC-5 chloride channels and epithelial sodium channels (ENaC) are present in many cell types including airway and retinal epithelia. Since ENaC activity is known to be affected by chloride transport, we co-injected Xenopus oocytes with cRNAs encoding ENaC and ClC-5 to investigate whether channel currents are impacted by heterologous co-expression of these proteins. ClC-5 currents were not detectably affected by co-expression with ENaC, whereas amiloride-sensitive ENaC currents were significantly lower compared to control oocytes expressing ENaC alone. Co-expression of ENaC with cRNA sequences encoding non-conducting fragments of ClC-5 revealed that the amino acid sequence region between positions 347 and 647 was sufficient for inhibition of ENaC currents. Co-expression of ENaC and another transport protein, the sodium dicarboxylate co-transporter (NaDC-1), did not affect ENaC currents. To test whether the inhibitory effects of ClC-5 were specific for ENaC, ClC-5 was also co-expressed with CFTR. CFTR currents were also inhibited by co-expression with ClC-5, whereas ClC-5 currents were unaffected. Western blot analysis of biotinylated oocyte surface membranes revealed that the co-expression of ClC-5 with ENaC, CFTR, or NaDC-1 decreased the abundance of these proteins at the surface membrane. We conclude that overexpression of ClC-5, specifically amino acids 347-647, can alter the normal translation or trafficking of ENaC and other ion transport proteins by a mechanism that is independent of the chloride conductance of ClC-5.

CFTR fails to inhibit the epithelial sodium channel ENaC expressed in Xenopus laevis oocytes

The cystic fibrosis transmembrane conductance regulator (CFTR) plays a crucial role in regulating fluid secretion by the airways, intestines, sweat glands and other epithelial tissues. It is well established that the CFTR is a cAMP-activated, nucleotide-dependent anion channel, but additional functions are often attributed to it, including regulation of the epithelial sodium channel (ENaC). The absence of CFTR-dependent ENaC inhibition and the resulting sodium hyperabsorption were postulated to be a major electrolyte transport abnormality in cystic fibrosis (CF)-affected epithelia. Several ex vivo studies, including those that used the Xenopus oocyte expression system, have reported ENaC inhibition by activated CFTR, but contradictory results have also been obtained. Because CFTR-ENaC interactions have important implications in the pathogenesis of CF, the present investigation was undertaken by our three independent laboratories to resolve whether CFTR regulates ENaC in oocytes and to clarify potential sources of previously reported dissimilar observations. Using different experimental protocols and a wide range of channel expression levels, we found no evidence that activated CFTR regulates ENaC when oocyte membrane potential was carefully clamped. We determined that an apparent CFTR-dependent ENaC inhibition could be observed when resistance in series with the oocyte membrane was not low enough or the feedback voltage gain was not high enough. We suggest that the inhibitory effect of CFTR on ENaC reported in some earlier oocyte studies could be attributed to problems arising from high levels of channel expression and suboptimal recording conditions, that is, large series resistance and/or insufficient feedback voltage gain.

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.

Control of Epithelial Ion Transport by Cl− and PDZ Proteins

Inhibition of epithelial Na+ channels (ENaC) by the cystic fibrosis transmembrane conductance regulator (CFTR) has been demonstrated previously. Recent studies suggested a role of cytosolic Cl- for the interaction of CFTR with ENaC, when studied in Xenopus oocytes. In the present study we demonstrate that the Na+ / H+ -exchanger regulator factor (NHERF) controls expression of CFTR in mouse collecting duct cells. Inhibition of NHERF largely attenuates CFTR expression, which is paralleled by enhanced Ca(2+) -dependent Cl- secretion and augmented Na+ absorption by the ENaC. It is further demonstrated that epithelial Na+ absorption and ENaC are inhibited by cytosolic Cl- and that stimulation by secretagogues enhances the intracellular Cl- concentration. Thus, the data provide a clue to the question, how epithelial cells can operate as both absorptive and secretory units: Increase in intracellular Cl- during activation of secretion will inhibit ENaC and switch epithelial transport from salt absorption to Cl- secretion.

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.

New concepts of the pathogenesis of cystic fibrosis lung disease

Although there has been impressive progress in the elucidation of the genetic and molecular basis of cystic fibrosis (CF), the pathogenesis of CF lung disease remains obscure. The elucidation of the pathogenesis of CF lung disease requires both a full description of normal innate airway defence and how absent function of the cystic fibrosis transmembrane regulator protein (CFTR) adversely perturbs this activity. Recent data have linked the abnormal ion transport properties of CF airway epithelia to depleted airway surface liquid (ASL) volume, reflecting the combined defects of accelerated Na+ transport and the failure to secrete Cl-. Depletion of a specific compartment of the ASL, i.e. the periciliary liquid (PCL), appears to abrogate both cilia-dependent and cough clearance. Subsequent to PCL depletion, mucus adheres to airway surfaces and persistent mucin secretion generates the formation of "thickened" mucus plaques and plugs, which become the nidus for bacterial infection. The paucity of liquid in these plaques/plugs, and the hypoxia in this environment, appear to promote biofilm bacterial infection. Therapeutic agents that restore airway surface liquid volume, i.e. blockers of Na+ transport, initiators of Cl- transport and osmolytes, are reviewed, as are strategies that may be required to use volume-restoring agents safely in patients with cystic fibrosis.

State of the art: why do the lungs of patients with cystic fibrosis become infected and why can't they clear the infection?

Cystic Fibrosis (CF) lung disease, which is characterized by airway obstruction, chronic bacterial infection, and an excessive inflammatory response, is responsible for most of the morbidity and mortality. Early in life, CF patients become infected with a limited spectrum of bacteria, especially P. aeruginosa. New data now indicate that decreased depth of periciliary fluid and abnormal hydration of mucus, which impede mucociliary clearance, contribute to initial infection. Diminished production of the antibacterial molecule nitric oxide, increased bacterial binding sites (e.g., asialo GM-1) on CF airway epithelial cells, and adaptations made by the bacteria to the airway microenvironment, including the production of virulence factors and the ability to organize into a biofilm, contribute to susceptibility to initial bacterial infection. Once the patient is infected, an overzealous inflammatory response in the CF lung likely contributes to the host's inability to eradicate infection. In response to increased IL-8 and leukotriene B₄ production, neutrophils infiltrate the lung where they release mediators, such as elastase, that further inhibit host defenses, cripple opsonophagocytosis, impair mucociliary clearance, and damage airway wall architecture. The combination of these events favors the persistence of bacteria in the airway. Until a cure is discovered, further investigations into therapies that relieve obstruction, control infection, and attenuate inflammation offer the best hope of limiting damage to host tissues and prolonging survival.

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.

Where have all the Na+ channels gone? In search of functional ENaC in exocrine pancreas

Many epithelia express specific Na(+) channels (ENaC) together with the cystic fibrosis regulator (CFTR) Cl(-) channels. Pancreatic ducts secrete HCO(3)(-)-rich fluid and express CFTR. However, the question whether they possess ENaC has not been consistently addressed. The aim of the present study was to investigate if pancreatic ducts express functional ENaC. Membrane voltages (V) of ducts isolated from rat pancreas were measured with microelectrodes or whole-cell patch-clamp technique. Amiloride and benzamil given from bath or luminal sides did not hyperpolarize V. Lowering of extracellular Na(+) concentrations had effects that were not consistent with a simple Na(+) conductance, but rather with a Na(+)/Ca(2+) exchange. Acute or long-lasting treatment of pancreatic ducts with mineralocorticoids had no effect on V of unstimulated or secretin-stimulated preparations. Furthermore, pre-treatment of animals with glucocorticoids had no effect on pancreatic fluid secretion evoked from ducts, or from acini. Hence, our study shows that pancreas especially pancreatic ducts do not express functional ENaC.

Cystic fibrosis transmembrane conductance regulator-dependent up-regulation of Kir1.1 (ROMK) renal K+ channels by the epithelial sodium channel

The epithelial sodium channel (ENaC) and the secretory potassium channel (Kir1.1/ROMK) are expressed in the apical membrane of renal collecting duct principal cells where they provide the rate-limiting steps for Na(+) absorption and K(+) secretion. The cystic fibrosis transmembrane conductance regulator (CFTR) is thought to regulate the function of both ENaC and Kir1.1. We hypothesized that CFTR may provide a regulatory link between ENaC and Kir1.1. In Xenopus laevis oocytes co-expressing both ENaC and CFTR, the CFTR currents were 3-fold larger than those in oocytes expressing CFTR alone due to an increased expression of CFTR in the plasma membrane. ENaC was also able to increase Kir1.1 currents through an increase in surface expression, but only in the presence of CFTR. In the absence of CFTR, co-expression of ENaC was without effect on Kir1.1. ENaC-mediated CFTR-dependent up-regulation of Kir1.1 was reduced with a Liddle's syndrome mutant of ENaC. Furthermore, ENaC co-expressed with CFTR was without effect on the closely related K(+) channel, Kir4.1. We conclude that ENaC up-regulates Kir1.1 in a CFTR-dependent manner. CFTR may therefore provide the mechanistic link that mediates the coordinated up-regulation of Kir1.1 during the stimulation of ENaC by hormones such as aldosterone or antidiuretic hormone.

Reciprocal protein kinase A regulatory interactions between cystic fibrosis transmembrane conductance regulator and Na+/H+ exchanger isoform 3 in a renal polarized epithelial cell model

Although Cystic fibrosis transmembrane conductance regulator (CFTR) has been shown to regulate the activity of NHE3, the potential reciprocal interaction of NHE3 to modulate the protein kinase A (PKA)-dependent regulation of CFTR in epithelial cells is still unknown. In the present work, we describe experiments to define the interactions between CFTR and NHE3 with the regulatory, scaffolding protein, NHERF that organize their PKA-dependent regulation in a renal epithelial cell line that expresses endogenous CFTR. The expression of rat NHE3 significantly decreased PKA-dependent activation of CFTR without altering CFTR expression, and this decrease was prevented by mutation of either of the two rat NHE3 PKA target serines to alanine (S552A or S605A). Inhibition of CFTR expression by antisense treatment resulted in an acute decrease in PKA-dependent regulation of NHE3 activity. CFTR, NHE3, and ezrin were recognized by NHERF-2 but not NHERF-1 in glutathione S-transferase pull-down experiments. Ezrin may function as a protein kinase A anchoring protein (AKAP) in this signaling complex, because blocking the binding of PKA to an AKAP by incubation with the S-Ht31 peptide inhibited the PKA-dependent regulation of CFTR in the absence of NHE3. In the A6-NHE3 cells S-Ht31 blocked the PKA regulation of NHE3 whereas it now failed to affect the regulation of CFTR. We conclude that CFTR and NHE3 reciprocally interact via a shared regulatory complex comprised of NHERF-2, ezrin, and PKA.

Molecular structure and physiological function of chloride channels

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

Genistein restores functional interactions between Delta F508-CFTR and ENaC in Xenopus oocytes

The cystic fibrosis transmembrane conductance regulator (CFTR), in addition to its Cl(-) channel properties, has regulatory interactions with other epithelial ion channels including the epithelial Na(+) channel (ENaC). Both the open probability and surface expression of wild type CFTR Cl(-) channels are increased significantly when CFTR is co-expressed in Xenopus oocytes with alphabetagamma-ENaC, and conversely, the activity of ENaC is inhibited following wild type CFTR activation. Using the Xenopus oocyte expression system, a lack of functional regulatory interactions between DeltaF508-CFTR and ENaC was observed following activation of DeltaF508-CFTR by forskolin and isobutylmethylxanthine (IBMX). Whole cell currents in oocytes expressing ENaC alone decreased in response to genistein but increased in response to a combination of forskolin and IBMX followed by genistein. In contrast, ENaC currents in oocytes co-expressing ENaC and DeltaF508-CFTR remained stable following stimulation with forskolin/IBMX/genistein. Furthermore, co-expression of DeltaF508-CFTR with ENaC enhanced the forskolin/IBMX/genistein-mediated activation of DeltaF508-CFTR. Our data suggest that genistein restores regulatory interactions between DeltaF508-CFTR and ENaC and that combinations of protein repair agents, such as 4-phenylbutyrate and genistein, may be necessary to restore DeltaF508-CFTR function in vivo.

The cystic fibrosis transmembrane conductance regulator (CFTR) inhibits ENaC through an increase in the intracellular Cl- concentration

Activation of the CFTR Cl- channel inhibits epithelial Na+ channels (ENaC), according to studies on epithelial cells and overexpressing recombinant cells. Here we demonstrate that ENaC is inhibited during stimulation of the cystic fibrosis transmembrance conductance regulator (CFTR) in Xenopus oocytes, independent of the experimental set-up and the magnitude of the whole-cell current. Inhibition of ENaC is augmented at higher CFTR Cl- currents. Similar to CFTR, ClC-0 Cl- currents also inhibit ENaC, as well as high extracellular Na+ and Cl- in partially permeabilized oocytes. Thus, inhibition of ENaC is not specific to CFTR and seems to be mediated by Cl-.

Mechanism of action of the calcium-sensing receptor in human antral gastrin cells

BACKGROUND AND AIMS

Human G cells express the calcium-sensing receptor and respond to extracellular calcium by releasing gastrin. However, the receptor on G cells is insensitive to serum calcium levels. We investigated whether this is a result of differential regulation of signaling pathways compared with parathyroid or calcitonin cells.

METHODS

Gastrin release from primary cultures of human antral epithelial cells enriched for G cells (35%) was measured by radioimmunoassay. G cells were stimulated by increasing extracellular calcium concentration for 1 hour in the presence or absence of antagonists of specific intracellular signaling pathways. Intracellular calcium levels were monitored to evaluate the effect of the antagonists on calcium influx.

RESULTS

Inhibition of phospholipase C decreased calcium-stimulated gastrin release, but blockers of adenylate cyclase, phospholipase A(2), or mitogen-activated protein kinase had no effect. Inhibition of protein kinase C, nonselective cation channels, and phosphodiesterase increased basal and calcium-stimulated gastrin release while decreasing calcium influx. These data were consistent with basally active phosphodiesterase.

CONCLUSIONS

The calcium-sensing receptor on the G cell activates phospholipase C and opens nonselective cation channels, resulting in an influx of extracellular calcium. Protein kinase C isozymes expressed by the G cells play multiple roles regulating both gastrin secretion and phosphodiesterase activity.

Non-specific activation of the epithelial sodium channel by the CFTR chloride channel

The genetic disease cystic fibrosis is caused by mutation of the gene coding for the cystic fibrosis transmembrane conductance regulator (CFTR). Controversial studies reported regulation of the epithelial sodium channel (ENaC) by CFTR. We found that uptake of (22)Na(+) through ENaC is modulated by activation of CFTR in oocytes, coexpressing CFTR and ENaC, depending on extracellular chloride concentration. Furthermore we found that the effect of CFTR activation could be mimicked by other chloride channels. Voltage- and patch-clamp measurements, however, showed neither stimulation nor inhibition of ENaC-mediated conductance by activated CFTR. We conclude that the observed modulation of (22)Na(+) uptake by activated CFTR is due to the effect of CFTR-mediated chloride conductance on the membrane potential. These findings argue against the notion of a specific influence of CFTR on ENaC and emphasize the chloride channel function of CFTR.