CFTR induces the expression of DRA along with Cl(-)/HCO(3)(-) exchange activity in tracheal epithelial cells

Chloride/Multiple Anion Exchanger SLC26A Family: Systemic Roles of SLC26A4 in Various Organs

Solute carrier family 26 member 4 (SLC26A4) is a member of the SLC26A transporter family and is expressed in various tissues, including the airway epithelium, kidney, thyroid, and tumors. It transports various ions, including bicarbonate, chloride, iodine, and oxalate. As a multiple-ion transporter, SLC26A4 is involved in the maintenance of hearing function, renal function, blood pressure, and hormone and pH regulation. In this review, we have summarized the various functions of SLC26A4 in multiple tissues and organs. Moreover, the relationships between SLC26A4 and other channels, such as cystic fibrosis transmembrane conductance regulator, epithelial sodium channel, and sodium chloride cotransporter, are highlighted. Although the modulation of SLC26A4 is critical for recovery from malfunctions of various organs, development of specific inducers or agonists of SLC26A4 remains challenging. This review contributes to providing a better understanding of the role of SLC26A4 and development of therapeutic approaches for the SLC26A4-associated hearing loss and SLC26A4-related dysfunction of various organs.

Multiple Regulatory Signals and Components in the Modulation of Bicarbonate Transporters

Bicarbonate transporters are responsible for the appropriate flux of bicarbonate across the plasma membrane to perform various fundamental cellular functions. The functions of bicarbonate transporters, including pH regulation, cell migration, and inflammation, are highlighted in various cellular systems, encompassing their participation in both physiological and pathological processes. In this review, we focused on recently identified modulatory signaling components that regulate the expression and activity of bicarbonate transporters. Moreover, we addressed recent advances in our understanding of cooperative systems of bicarbonate transporters and channelopathies. This current review aims to provide a new, in-depth understanding of numerous human diseases associated with the dysfunction of bicarbonate transporters.

Airway Surface Liquid pH Regulation in Airway Epithelium Current Understandings and Gaps in Knowledge

Knowledge on the mechanisms of acid and base secretion in airways has progressed recently. The aim of this review is to summarize the known mechanisms of airway surface liquid (ASL) pH regulation and their implication in lung diseases. Normal ASL is slightly acidic relative to the interstitium, and defects in ASL pH regulation are associated with various respiratory diseases, such as cystic fibrosis. Basolateral bicarbonate (HCO3-) entry occurs via the electrogenic, coupled transport of sodium (Na+) and HCO3-, and, together with carbonic anhydrase enzymatic activity, provides HCO3- for apical secretion. The latter mainly involves CFTR, the apical chloride/bicarbonate exchanger pendrin and paracellular transport. Proton (H+) secretion into ASL is crucial to maintain its relative acidity compared to the blood. This is enabled by H+ apical secretion, mainly involving H+/K+ ATPase and vacuolar H+-ATPase that carry H+ against the electrochemical potential gradient. Paracellular HCO3- transport, the direction of which depends on the ASL pH value, acts as an ASL protective buffering mechanism. How the transepithelial transport of H+ and HCO3- is coordinated to tightly regulate ASL pH remains poorly understood, and should be the focus of new studies.

What Is the Impact of Diet on Nutritional Diarrhea Associated with Gut Microbiota in Weaning Piglets: A System Review

Piglets experience severe growth challenges and diarrhea after weaning due to nutritional, social, psychological, environmental, and physiological changes. Among these changes, the nutritional factor plays a key role in postweaning health. Dietary protein, fibre, starch, and electrolyte levels are highly associated with postweaning nutrition diarrhea (PWND). In this review, we mainly discuss the high protein, fibre, resistant starch, and electrolyte imbalance in diets that induce PWND, with a focus on potential mechanisms in weaned piglets.

IL-17A induces Pendrin expression and chloride-bicarbonate exchange in human bronchial epithelial cells

The epithelium plays an active role in the response to inhaled pathogens in part by responding to signals from the immune system. Epithelial responses may include changes in chemokine expression, increased mucin production and antimicrobial peptide secretion, and changes in ion transport. We previously demonstrated that interleukin-17A (IL-17A), which is critical for lung host defense against extracellular bacteria, significantly raised airway surface pH in vitro, a finding that is common to a number of inflammatory diseases. Using microarray analysis of normal human bronchial epithelial (HBE) cells treated with IL-17A, we identified the electroneutral chloride-bicarbonate exchanger Pendrin (SLC26A4) as a potential mediator of this effect. These data were verified by real-time, quantitative PCR that demonstrated a time-dependent increase in Pendrin mRNA expression in HBE cells treated with IL-17A up to 48 h. Using immunoblotting and immunofluorescence, we confirmed that Pendrin protein expression is increased in IL-17 treated HBE cells and that it is primarily localized to the mucosal surface of the cells. Functional studies using live-cell fluorescence to measure intracellular pH demonstrated that IL-17A induced chloride-bicarbonate exchange in HBE cells that was not present in the absence of IL-17A. Furthermore, HBE cells treated with short interfering RNA against Pendrin showed substantially reduced chloride-bicarbonate exchange. These data suggest that Pendrin is part of IL-17A-dependent epithelial changes and that Pendrin may therefore be a therapeutic target in IL-17A-dependent lung disease.

Translational repression of SLC26A3 by miR-494 in intestinal epithelial cells

SLC26A3 [downregulated in adenoma (DRA)] is a Cl(-)/HCO3(-) exchanger involved in electroneutral NaCl absorption in the mammalian intestine. Altered DRA expression levels are associated with infectious and inflammatory diarrheal diseases. Therefore, it is critical to understand the regulation of DRA expression. MicroRNAs (miRNAs) are endogenous, small RNAs that regulate protein expression via blocking the translation and/or promoting mRNA degradation. To investigate potential modulation of DRA expression by miRNA, five different in silico algorithms were used to predict the miRNAs that target DRA. Of these miRNAs, miR-494 was shown to have a highly conserved putative binding site in the DRA 3'-untranslated region (3'-UTR) compared with other DRA-targeting miRNAs in vertebrates. Transfection with pmirGLO dual luciferase vector containing DRA 3'-UTR (pmirGLO-3'-UTR DRA) resulted in a significant decrease in relative luciferase activity compared with empty vector. Cotransfection of the DRA 3'-UTR luciferase vector with a miR-494 mimic further decreased luciferase activity compared with cells transfected with negative control. The transfection of a miR-494 mimic into Caco-2 and T-84 cells significantly increased the expression of miR-494 and concomitantly decreased the DRA protein expression. Mutation of the seed sequences for miR-494 in 3'-UTR of DRA abrogated the effect of miR-494 on 3'-UTR. These data demonstrate a novel regulatory mechanism of DRA expression via miR-494 and indicate that targeting this microRNA may serve to be a potential therapeutic strategy for diarrheal diseases.

Pharmacological Approaches to Correcting the Ion Transport Defect in Cystic Fibrosis

Cystic fibrosis (CF) is a lethal genetic disease caused by a mutation in a membrane protein, the cystic fibrosis transmembrane conductance regulator (CFTR), which mainly (but not exclusively) functions as a chloride channel. The main clinical symptoms are chronic obstructive lung disease, which is responsible for most of the morbidity and mortality associated with CF, and pancreatic insufficiency. About 1000 mutations of the gene coding for CFTR are currently known; the most common of these, present in the great majority of the patients (Delta508) results in the deletion of a phenylalanine at position 508. In this mutation, the aberrant CFTR is not transported to the membrane but degraded in the ubiquitin-proteasome pathway. The aim of this review is to give an overview of the pharmacologic strategies currently used in attempts to overcome the ion transport defect in CF. One strategy to develop pharmacologic treatment for CF is to inhibit the breakdown of DeltaF508-CFTR by interfering with the chaperones involved in the folding of CFTR. At least in in vitro systems, this can be accomplished by sodium phenylbutyrate, or S-nitrosoglutathione (GSNO), and also by genistein or benzo[c]quinolizinium compounds. It is also possible to stimulate CFTR or its mutated forms, when present in the plasma membrane, using xanthines, genistein, and various other compounds, such as benzamidizoles and benzoxazoles, benzo[c]quinolizinium compounds or phenantrolines. Experimental results are not always unambiguous, and adverse effects have been incompletely tested. Some clinical tests have been done on sodium phenyl butyrate, GSNO and genistein, mostly in respect to other diseases, and the results demonstrate that these drugs are reasonably well tolerated. Their efficiency in the treatment of CF has not yet been demonstrated, however. An alternative strategy is to compensate for the defective chloride transport by CFTR by stimulation of other chloride channels. This can be done via purinergic receptors. A phase I study using a stable uridine triphosphate analog has recently been completed. A second alternative strategy is to attempt to maintain hydration of the airway mucus by inhibiting Na(+) uptake by the epithelial Na(+) channel using amiloride or stable analogs of amiloride. Clinical tests so far have been inconclusive. A number of other suggestions are currently being explored. The minority of patients with CF who have a stop mutation may benefit from treatment with gentamicin. The difficulties in finding a pharmacologic treatment for CF may be due to the fact that CFTR has additional functions besides chloride transport, and interfering with CFTR biosynthesis or activation implies interference with central cellular processes, which may have undesirable adverse effects.

Basolateral chloride loading by the anion exchanger type 2: role in fluid secretion by the human airway epithelial cell line Calu-3

Anion exchanger type 2 (AE2 or SLC4A2) is an electroneutral Cl(-)/HCO(3)(-) exchanger expressed at the basolateral membrane of many epithelia. It is thought to participate in fluid secretion by airway epithelia. However, the role of AE2 in fluid secretion remains uncertain, due to the lack of specific pharmacological inhibitors, and because it is electrically silent and therefore does not contribute directly to short-circuit current (I(sc)). We have studied the role of AE2 in Cl(-) and fluid secretion by the airway epithelial cell line Calu-3. After confirming expression of its mRNA and protein, a knock-down cell line called AE2-KD was generated by lentivirus-mediated RNA interference in which AE2 mRNA and protein levels were reduced 90%. Suppressing AE2 increased the expression of the cystic fibrosis transmembrane conductance regulator (CFTR) by ∼70% without affecting the levels of NKCC1 (Na(+)-K(+)-2Cl(-) cotransporter) or NBCe1 (Na(+)-nHCO(3)(-) cotransporter). cAMP agonists stimulated fluid secretion by parental Calu-3 and scrambled shRNA cells >6.5-fold. In AE2-KD cells this response was reduced by ∼70%, and the secreted fluid exhibited elevated pH and [HCO(3)(-)] as compared with the control lines. Unstimulated equivalent short-circuit current (I(eq)) was elevated in AE2-KD cells, but the incremental response to forskolin was unaffected. The modest bumetanide-induced reductions in both I(eq) and fluid secretion were more pronounced in AE2-KD cells. Basolateral Cl(-)/HCO(3)(-) exchange measured by basolateral pH-stat in cells with permeabilized apical membranes was abolished in AE2-KD monolayers, and the intracellular alkalinization resulting from basolateral Cl(-) removal was reduced by ∼80% in AE2-KD cells. These results identify AE2 as a major pathway for basolateral Cl(-) loading during cAMP-stimulated secretion of Cl(-) and fluid by Calu-3 cells, and help explain the large bumetanide-insensitive component of fluid secretion reported previously in airway submucosal glands and some other epithelia.

Functional Cftr in crypt epithelium of organotypic enteroid cultures from murine small intestine

Physiological studies of intact crypt epithelium have been limited by problems of accessibility in vivo and dedifferentiation in standard primary culture. Investigations of murine intestinal stem cells have recently yielded a primary intestinal culture in three-dimensional gel suspension that recapitulates crypt structure and epithelial differentiation (Sato T, Vries RG, Snippert HJ, van de Wetering M, Barker N, Stange DE, Van Es JH, Abo A, Kujala P, Peters PJ, Clevers H. Nature 459: 262-265, 2009). We investigated the utility of murine intestinal crypt cultures (termed "enteroids") for physiological studies of crypt epithelium by focusing on the transport activity of the cystic fibrosis transmembrane conductance regulator Cftr. Enteroids had multiple crypts with well-differentiated goblet and Paneth cells that degranulated on exposure to the muscarinic agonist carbachol. Modified growth medium provided a crypt proliferation rate, as measured by 5-ethynyl-2'-deoxyuridine labeling, which was similar to proliferation in vivo. Immunoblots demonstrated equivalent Cftr expression in comparisons of freshly isolated crypts with primary and passage 1 enteroids. Apparent enteroid differences in mRNA expression of other transporters were primarily associated with villous epithelial contamination of freshly isolated crypts. Microelectrode analysis revealed cAMP-stimulated membrane depolarization in enteroid epithelium from wild-type (WT) but not Cftr knockout (KO) mice. Morphological and microfluorimetric studies, respectively, demonstrated Cftr-dependent cell shrinkage and lower intracellular pH in WT enteroid epithelium in contrast to Cftr KO epithelium or WT epithelium treated with Cftr inhibitor 172. We conclude that crypt epithelium of murine enteroids exhibit Cftr expression and activity that recapitulates crypt epithelium in vivo. Enteroids provide a primary culture model that is suitable for physiological studies of regenerating crypt epithelium.

Requirements for ion and solute transport, and pH regulation during enamel maturation

Transcellular bicarbonate transport is suspected to be an important pathway used by ameloblasts to regulate extracellular pH and support crystal growth during enamel maturation. Proteins that play a role in amelogenesis include members of the ABC transporters (SLC gene family and CFTR). A number of carbonic anhydrases (CAs) have also been identified. The defined functions of these genes are likely interlinked during enamel mineralization. The purpose of this study is to quantify relative mRNA levels of individual SLC, Cftr, and CAs in enamel cells obtained from secretory and maturation stages on rat incisors. We also present novel data on the enamel phenotypes for two animal models, a mutant porcine (CFTR-ΔF508) and the NBCe1-null mouse. Our data show that two SLCs (AE2 and NBCe1), Cftr, and Car2, Car3, Car6, and Car12 are all significantly up-regulated at the onset of the maturation stage of amelogenesis when compared to the secretory stage. The remaining SLCs and CA gene transcripts showed negligible expression or no significant change in expression from secretory to maturation stages. The enamel of CFTR-ΔF508 adult pigs was hypomineralized and showed abnormal crystal growth. NBCe1-null mice enamel was structurally defective and had a marked decrease in mineral content relative to wild-type. These data demonstrate the importance of many non-matrix proteins to amelogenesis and that the expression levels of multiple genes regulating extracellular pH are modulated during enamel maturation in response to an increased need for pH buffering during hydroxyapatite crystal growth.

CFTR mediated chloride secretion in the avian renal proximal tubule

In primary cell cultures of the avian (Gallus gallus) renal proximal tubule parathyroid hormone and cAMP activation generate a Cl(-)-dependent short circuit current (I(SC)) response, consistent with net transepithelial Cl(-) secretion. In this study we investigated the expression and physiological function of the Na-K-2Cl (NKCC) transporter and CFTR chloride channel, both associated with Cl(-) secretion in a variety of tissues, in these proximal tubule cells. Using both RT-PCR and immunoblotting approaches, we showed that NKCC and CFTR are expressed, both in proximal tubule primary cultures and in a proximal tubule fraction of non-cultured (native tissue) fragments. We also used electrophysiological methods to assess the functional contribution of NKCC and CFTR to forskolin-activated I(SC) responses in filter grown cultured monolayers. Bumetanide (10 μM), a specific blocker of NKCC, inhibited forskolin activated I(SC) by about 40%, suggesting that basolateral uptake of Cl(-) is partially mediated by NKCC transport. In monolayers permeabilized on the basolateral side with nystatin, forskolin activated an apical Cl(-) conductance, manifested as bidirectional diffusion currents in the presence of oppositely directed Cl(-) gradients. Under these conditions the apical conductance appeared to show some bias towards apical-to-basolateral Cl(-) current. Two selective CFTR blockers, CFTR Inhibitor 172 and GlyH-101 (both at 20 μM) inhibited the forskolin activated diffusion currents by 38-68%, with GlyH-101 having a greater effect. These data support the conclusion that avian renal proximal tubules utilize an apical CFTR Cl(-) channel to mediate cAMP-activated Cl(-) secretion.

Novel role for pendrin in orchestrating bicarbonate secretion in cystic fibrosis transmembrane conductance regulator (CFTR)-expressing airway serous cells

In most HCO(3)(-)-secreting epithelial tissues, SLC26 Cl(-)/HCO(3)(-) transporters work in concert with the cystic fibrosis transmembrane conductance regulator (CFTR) to regulate the magnitude and composition of the secreted fluid, a process that is vital for normal tissue function. By contrast, CFTR is regarded as the only exit pathway for HCO(3)(-) in the airways. Here we show that Cl(-)/HCO(3)(-) anion exchange makes a major contribution to transcellular HCO(3)(-) transport in airway serous cells. Real-time measurement of intracellular pH from polarized cultures of human Calu-3 cells demonstrated cAMP/PKA-activated Cl(-)-dependent HCO(3)(-) transport across the luminal membrane via CFTR-dependent coupled Cl(-)/HCO(3)(-) anion exchange. The pharmacological and functional profile of the luminal anion exchanger was consistent with SLC26A4 (pendrin), which was shown to be expressed by quantitative RT-PCR, Western blot, and immunofluorescence. Pendrin-mediated anion exchange activity was confirmed by shRNA pendrin knockdown (KD), which markedly reduced cAMP-activated Cl(-)/HCO(3)(-) exchange. To establish the relative roles of CFTR and pendrin in net HCO(3)(-) secretion, transepithelial liquid secretion rate and liquid pH were measured in wild type, pendrin KD, and CFTR KD cells. cAMP/PKA increased the rate and pH of the secreted fluid. Inhibiting CFTR reduced the rate of liquid secretion but not the pH, whereas decreasing pendrin activity lowered pH with little effect on volume. These results establish that CFTR predominately controls the rate of liquid secretion, whereas pendrin regulates the composition of the secreted fluid and identifies a critical role for this anion exchanger in transcellular HCO(3)(-) secretion in airway serous cells.

Modulatory effect of the SLC9A3 gene on susceptibility to infections and pulmonary function in children with cystic fibrosis

In cystic fibrosis (CF), CFTR dysfunction leads to salt and water imbalance across airway epithelia, depleted surface liquid layer, and impaired mucociliary clearance. This provides optimal conditions for chronic bacterial infections leading to excessive inflammation and progressive obstructive lung disease. We hypothesized that other epithelial channels affecting salt balance across the airways may play a role in the susceptibility to bacterial infections and modulate severity of CF lung disease. The SLC9A3 gene encoding a Na(+) /H(+) exchanger was demonstrated to be a modifier intestinal disease in a murine model of CF. We examined the potential role of SLC9A3 as a modifier of CF lung disease severity. We analyzed 11 SLC9A3 gene variants for association with age of first Pseudomonas aeruginosa infection and lung function in children with CF. The T allele of an intronic variant in the SLC9A3 gene (rs4957061) was significantly (P = 0.02) associated with earlier acquisition of Pseudomonas infection in a cohort of 1,004 pediatric patients. Analysis of lung function in a subset of these patients (752) revealed that patients homozygous for the T allele had substantially reduced lung function and accelerated rate of decline. Although the functional basis for the modulatory effects of this SLC9A3 variant on CF lung disease remains to be elucidated, altered function of the Na(+) /H(+) exchanger may further deplete the airway liquid surface, thereby enhancing susceptibility to Pseudomonas infections and worsening the severity of lung disease.

Basolateral ion transporters involved in colonic epithelial electrolyte absorption, anion secretion and cellular homeostasis

Electrolyte transporters located in the basolateral membrane of the colonic epithelium are increasingly appreciated as elaborately regulated components of specific transport functions and cellular homeostasis: During electrolyte absorption, Na(+) /K(+) ATPase, Cl⁻ conductance, Cl⁻/HCO₃⁻ exchange, K(+) /Cl⁻ cotransport and K(+) channels are candidates for basolateral Na(+) , Cl⁻ and K(+) extrusion. The process of colonic anion secretion involves basolateral Na(+) /K(+) /2Cl⁻ , and probably also Na(+) /HCO₃⁻ cotransport, as well as Na(+) /K(+) ATPase and K(+) channels to supply substrate, stabilize the membrane potential and generate driving force respectively. Together with a multitude of additional transport systems, Na(+) /H(+) exchange and Na(+) /HCO₃⁻ cotransport have been implicated in colonocyte pH(i) and volume homeostasis. The purpose of this article is to summarize recently gathered information on the molecular identity, function and regulation of the involved basolateral transport systems in native tissue. Furthermore, we discuss how these findings can help to integrate these systems into the transport function and the cellular homoeostasis of colonic epithelial cells. Finally, disturbances of basolateral electrolyte transport during disease states such as mucosal inflammation will be reviewed.

Reciprocal regulation of the primary sodium absorptive pathways in rat intestinal epithelial cells

Sodium absorption in the mammalian small intestine occurs predominantly by two primary pathways that include Na/H exchange (NHE3) and Na-glucose cotransport (SGLT1) on the brush border membrane (BBM) of villus cells. However, whether NHE3 and SGLT1 function together to regulate intestinal sodium absorption is unknown. Nontransformed small intestinal epithelial cells (IEC-18) were transfected with either NHE3 or SGLT1 small interfering RNAs (siRNAs) and were grown in confluent monolayers on transwell plates to measure the effects on Na absorption. Uptake studies were performed as well as molecular studies to determine the effects on NHE3 and SGLT1 activity. When IEC-18 monolayers were transfected with silencing NHE3 RNA, the cells demonstrated decreased NHE3 activity as well as decreased NHE3 mRNA and protein. However, in NHE3 siRNA-transected cells, SGLT1 activity, mRNA, and protein in the BBM were significantly increased. Thus, inhibition of NHE3 expression regulates the expression and function of SGLT1 in the BBM of intestinal epithelial cells. In addition, IEC-18 cells transected with silencing SGLT1 RNA demonstrated an inhibition of Na-dependent glucose uptake and a decrease in SGLT1 activity, mRNA, and protein levels. However, in these cells, Na/H exchange activity was significantly increased. Furthermore, NHE3 mRNA and protein levels were also increased. Therefore, the inhibition of SGLT1 expression stimulates the transcription and function of NHE3 and vice versa in the BBM of intestinal epithelial cells. Thus this study demonstrates that the major sodium absorptive pathways together function to regulate sodium absorption in epithelial cells.

Review article: the clinical management of congenital chloride diarrhoea

BACKGROUND

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

AIM

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Slc26a9--anion exchanger, channel and Na+ transporter

The SLC26 gene family encodes anion transporters with diverse functional attributes: (a) anion exchanger, (b) anion sensor, and (c) anion conductance (likely channel). We have cloned and studied Slc26a9, a paralogue expressed mostly in lung and stomach. Immunohistochemistry shows that Slc26a9 is present at apical and intracellular membranes of lung and stomach epithelia. Using expression in Xenopus laevis oocytes and ion-sensitive microelectrodes, we discovered that Slc26a9 has a novel function not found in any other Slc26 proteins: cation coupling. Intracellular pH and voltage measurements show that Slc26a9 is a nCl(-)-HCO(3)(-) exchanger, suggesting roles in gastric HCl secretion or pulmonary HCO(3)(-) secretion; Na(+) electrodes and uptakes reveal that Slc26a9 has a cation dependence. Single-channel measurements indicate that Slc26a9 displays discrete open and closed states. These experiments show that Slc26a9 has three discrete physiological modes: nCl(-)-HCO(3)(-) exchanger, Cl(-) channel, and Na(+)-anion cotransporter. Thus, the Slc26a9 transporter channel is uniquely suited for dynamic and tissue-specific physiology or regulation in epithelial tissues.

Interleukin-17A induces bicarbonate secretion in normal human bronchial epithelial cells

The innate immune functions of human airways include mucociliary clearance and antimicrobial peptide activity. Both functions may be affected by changes in epithelial ion transport. Interleukin-17A (IL-17A), which has a receptor at the basolateral membrane of airway epithelia, is a T cell cytokine that has been shown to increase mucus secretion and antimicrobial peptide production by human bronchial epithelial (HBE) cells. Furthermore, IL-17A levels are increased in sputum from patients during pulmonary exacerbations of cystic fibrosis. Therefore, we investigated the effects of IL-17A on basal, amiloride-sensitive, and forskolin-stimulated ion transport in mature, well-differentiated HBE cells. Exposure of HBE monolayers to IL-17A for 48 h induced a novel forskolin-stimulated bicarbonate secretion in addition to forskolin-stimulated chloride secretion and resulted in alkalinization of liquid on the mucosal surface of polarized cells. IL-17A-induced bicarbonate secretion was cystic fibrosis transmembrane conductance regulator (CFTR)-dependent, mucosal chloride-dependent, partially Na(+)-dependent, and sensitive to serosal, but not mucosal, stilbene inhibition. These data suggest that IL-17A modulates epithelial bicarbonate secretion and implicate a mechanism by which airway surface liquid pH changes may be abnormal in cystic fibrosis.

Diarrhea as a cause of mortality in a mouse model of infectious colitis

Analysis of gene expression in the colons of Citrobacter rodentium-infected susceptible and resistant mice suggests that mortality is associated with impaired intestinal ion transport.

Background

Comparative characterization of genome-wide transcriptional changes during infection can help elucidate the mechanisms underlying host susceptibility. In this study, transcriptional profiling of the mouse colon was carried out in two cognate lines of mice that differ in their response to Citrobacter rodentium infection; susceptible inbred FVB/N and resistant outbred Swiss Webster mice. Gene expression in the distal colon was determined prior to infection, and at four and nine days post-inoculation using a whole mouse genome Affymetrix array.

Results

Computational analysis identified 462 probe sets more than 2-fold differentially expressed between uninoculated resistant and susceptible mice. In response to C. rodentium infection, 5,123 probe sets were differentially expressed in one or both lines of mice. Microarray data were validated by quantitative real-time RT-PCR for 35 selected genes and were found to have a 94% concordance rate. Transcripts represented by 1,547 probe sets were differentially expressed between susceptible and resistant mice regardless of infection status, a host effect. Genes associated with transport were over-represented to a greater extent than even immune response-related genes. Electrolyte analysis revealed reduction in serum levels of chloride and sodium in susceptible animals.

Conclusion

The results support the hypothesis that mortality in C. rodentium-infected susceptible mice is associated with impaired intestinal ion transport and development of fatal fluid loss and dehydration. These studies contribute to our understanding of the pathogenesis of C. rodentium and suggest novel strategies for the prevention and treatment of diarrhea associated with intestinal bacterial infections.

Molecular and functional expression of anion exchangers in cultured normal human nasal epithelial cells

Aims

Anions have an important role in the regulation of airway surface liquid (ASL) volume, viscosity and pH. However, functional localization and regulation of anion exchangers (AEs) have not been clearly described. The aim of this study was to investigate the regulation of AE mRNA expression level in accordance with mucociliary differentiation and the functional expression of AEs cultured normal human nasal epithelial (NHNE) cells.

Methods

Nasal mucosal specimens from three patients are obtained and serially cultured cells are subjected to morphological examinations, RT-PCR, Western blot analysis and immunocytochemistry. AE activity is assessed by pHi measurements.

Results

Expression of ciliated cells on the apical membrane and expression of MUC5AC, a marker of mucous differentiation, increased with time. AE2 and SLC26A4 mRNA expression decreased as mucociliary differentiation progressed, and AE4, SLC26A7 and SLC26A8 mRNA expression increased on the 14th and 28th day after confluence. Accordingly, AE4 protein expression also progressively increased. AE activity in 100 mm K⁺ buffer solutions was nearly twofold higher than that in 5 mm K⁺ buffer solutions. Moreover, only luminal AE activity increased about fourfold over the control in the presence of 5 μm forskolin. In the presence of 100 μm adenosine-5′-triphosphate (ATP) which evokes intracellular calcium signalling through activation of purinergic receptors, only luminal AE activity was again significantly increased. On the other hand, 500 μm 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid (DIDS), an inhibitor of most SLC4 and SLC26AE isoforms, nearly abolished AE activity in both luminal and basolateral membranes. We found that AE activity was affected by intracellular cAMP and calcium signalling in the luminal membrane and was DIDS-sensitive in both membranes of cultured NHNE cells.

Conclusion

Our findings through molecular and functional studies using cultured NHNE cells suggest that AEs may have an important role in the regulation of ASL.

Mechanisms of neuronal chloride accumulation in intact mouse olfactory epithelium

When olfactory receptor neurons respond to odours, a depolarizing Cl(-) efflux is a substantial part of the response. This requires that the resting neuron accumulate Cl(-) against an electrochemical gradient. In isolated olfactory receptor neurons, the Na(+)-K(+)-2Cl(-) cotransporter NKCC1 is essential for Cl(-) accumulation. However, in intact epithelium, a robust electrical olfactory response persists in mice lacking NKCC1. This response is largely due to a neuronal Cl(-) efflux. It thus appears that NKCC1 is an important part of a more complex system of Cl(-) accumulation. To identify the remaining transport proteins, we first screened by RT-PCR for 21 Cl(-) transporters in mouse nasal tissue containing olfactory mucosa. For most of the Cl(-) transporters, the presence of mRNA was demonstrated. We also investigated the effects of pharmacological block or genetic ablation of Cl(-) transporters on the olfactory field potential, the electroolfactogram (EOG). Mice lacking the common Cl(-)/HCO(3)(-) exchanger AE2 had normal EOGs. Block of NKCC cotransport with bumetanide reduced the EOG in epithelia from wild-type mice but had no effect in mice lacking NKCC1. Hydrochlorothiazide, a blocker of the Na(+)-Cl(-) cotransporter, had only a small effect. DIDS, a blocker of some KCC cotransporters and Cl(-)/HCO(3)(-) exchangers, reduced the EOG in epithelia from both wild-type and NKCC1 knockout mice. A combination of bumetanide and DIDS decreased the response more than either drug alone. However, no combination of drugs completely abolished the Cl(-) component of the response. These results support the involvement of both NKCC1 and one or more DIDS-sensitive transporters in Cl(-) accumulation in olfactory receptor neurons.

Cystic fibrosis transmembrane conductance regulator is vital to sperm fertilizing capacity and male fertility

Cystic fibrosis transmembrane conductance regulator (CFTR) is an anion channel, mutations of which cause cystic fibrosis, a disease characterized by defective Cl(-) and HCO(3)(-) transport. Although >95% of all CF male patients are infertile because of congenital bilateral absence of the vas deferens (CBAVD), the question whether CFTR mutations are involved in other forms of male infertility is under intense debates. Here we report that CFTR is detected in both human and mouse sperm. CFTR inhibitor or antibody significantly reduces the sperm capacitation, and the associated HCO(3)(-)-dependent events, including increases in intracellular pH, cAMP production and membrane hyperpolarization. The fertilizing capacity of the sperm obtained from heterozygous CFTR mutant mice is also significantly lower compared with that of the wild-type. These results suggest that CFTR in sperm may be involved in the transport of HCO(3)(-) important for sperm capacitation and that CFTR mutations with impaired CFTR function may lead to reduced sperm fertilizing capacity and male infertility other than CBAVD.

Review article: duodenal bicarbonate - mucosal protection, luminal chemosensing and acid-base balance

The duodenum serves as a buffer zone between the stomach and the jejunum. Over a length of only 25 cm, large volumes of strong acid secreted by the stomach must be converted to the neutral-alkaline chyme of the hindgut lumen, generating large volumes of CO(2). The duodenal mucosa consists of epithelial cells connected by low-resistance tight junctions, forming a leaky epithelial barrier. Despite this permeability, the epithelial cells, under intense stress from luminal mineral acid and highly elevated Pco(2), maintain normal functioning. Bicarbonate ion uniquely protects the duodenal epithelial cells from acid-related injury. The specific protective mechanisms likely involve luminal bicarbonate secretion, intracellular pH buffering and interstitial buffering. Furthermore, the duodenum plays an active role in foregut acid-base homeostasis, absorbing large amounts of H(+) and CO(2). We have studied mucosal protection and acid-base balance using live-animal fluorescence ratio microimaging and by performing H(+) and CO(2) balance studies on duodenal perfusates. On the basis of these data, we have formulated novel hypotheses with regard to mucosal protection.

Mechanisms of acid and base secretion by the airway epithelium

One of the main functions of the airway epithelium is to inactivate and remove infectious particles from inhaled air and thereby prevent infection of the distal lung. This function is achieved by mucociliary and cough clearance and by antimicrobial factors present in the airway surface liquid (ASL). There are indications that airway defenses are affected by the pH of the ASL and historically, acidification of the airway surfaces has been suggested as a measure of airway disease. However, even in health, the ASL is slightly acidic, and this acidity might be part of normal airway defense. Only recently research has focused on the mechanisms responsible for acid and base secretion into the ASL. Advances resulted from research into the airway disease associated with cystic fibrosis (CF) after it was found that the CFTR Cl(-) channel conducts HCO (3) (-) and, therefore, may contribute to ASL pH. However, the acidity of the ASL indicated parallel mechanisms for H(+) secretion. Recent investigations identified several H(+) transporters in the apical membrane of the airway epithelium. These include H(+) channels and ATP-driven H(+) pumps, including a non-gastric isoform of the H(+)-K(+) ATPase and a vacuolar-type H(+) ATPase. Current knowledge of acid and base transporters and their potential roles in airway mucosal pH regulation is reviewed here.

The emerging role of PDZ adapter proteins for regulation of intestinal ion transport

In the gastrointestinal tract, CFTR, in conjunction with one or several members of the SLC26 anion exchanger family, mediates electrogenic Cl- and HCO3- secretion. Na+/H+ exchanger isoform NHE3, on the other hand, coupled to one or several of the SLC26 isoforms, mediates electroneutral NaCl absorption. The agonist-induced activation of anion secretion and inhibition of salt absorption causes secretory diarrhea. Current dogma sees the formation of a multiprotein complex of transport proteins, postsynaptic density-95/discs large/zonula occludens-1 (PDZ) adapter proteins, anchoring proteins, the cytoskeleton, and the involved protein kinases as one crucial step in the regulation of these transport processes. Data obtained in heterologous expression studies suggest an important role of these PDZ adapter proteins in trafficking, endocytic recycling, and membrane retention of the respective transmembrane proteins. This article reviews recent advances in our understanding of the role of the PDZ adapter proteins NHERF, E3KARP, PDZK1, IKEPP (NHERF-1 to NHERF-4), CAL, and Shank-2 that bind to CFTR, NHE3, and the intestinal SLC26 members in the regulation of intestinal fluid transport. Current concepts are mostly derived from heterologous expression studies and studies on their role in organ physiology are still in infancy. Recently, however, PDZ adapter protein-deficient mice and organ-specific cell lines have become available, and the first results suggest a more cell-type and possibly signal-specific role of these adapter proteins. This opens the potential for drug development targeted to PDZ domain interactions, which is, in theory, one of the most efficient antidiarrheal strategies.

SLC26A9 is expressed in gastric surface epithelial cells, mediates Cl-/HCO3- exchange, and is inhibited by NH4+

HCO3- secretion by gastric mucous cells is essential for protection against acidic injury and peptic ulcer. Herein we report the identification of an apical HCO3- transporter in gastric surface epithelial cells. Northern hybridization and RT-PCR demonstrate the expression of this transporter, also known as SLC26A9, in mouse and rat stomach and trachea (but not kidney). In situ hybridization in mouse stomach showed abundant expression of SLC26A9 in surface epithelial cells with apical localization on immunofluorescence labeling. Functional studies in HEK-293 cells demonstrated that SLC26A9 mediates Cl-/HCO3- exchange and is also capable of Cl--independent HCO3- extrusion. Unlike other anion exchangers or transport proteins reported to date, SLC26A9 activity is inhibited by ammonium (NH4+). The inhibitory effect of NH4+ on gastric HCO3- secretion was also indicated by reduced gastric juxtamucosal pH (pHjm) in rat stomach in vivo. This report is the first to describe the inhibition of HCO3- transport in vitro and the reduction of pHjm in stomach in vivo by NH4+. Given its critical localization on the apical membrane of surface epithelial cells, its ability to transport HCO3-, and its inhibition by NH4+, we propose that SLC26A9 mediates HCO3- secretion in surface epithelial cells and is essential for protection against acidic injury in the stomach. Disease states that are associated with increased ammonia (NH3)/NH4+ generation (e.g., Helicobacter pylori) may impair gastric HCO3- secretion and therefore predispose patients to peptic ulcer by inhibiting SLC26A9.

Purinergic signaling underlies CFTR control of human airway epithelial cell volume

BACKGROUND

Loss of cystic fibrosis transmembrane conductance regulator (CFTR) function in cystic fibrosis (CF) causes dysregulation of multiple ion channels, water channels, and acid-base transporters in epithelia. As such, we hypothesized that dysregulation of many critical ion channels and transporters may cause defects in human airway epithelial cell volume regulation.

METHODS

Cell volume, regulatory volume decrease, and its regulation was assessed in real-time via Coulter Counter Multisizer III-driven electronic cell sizing in non-CF, CF, and CFTR-complemented CF human airway epithelial cells. SPQ halide fluorescence assay of hypotonicity-induced chloride efflux provided indirect validation of the cell volume assays.

RESULTS

CFTR, via autocrine ATP signaling, governs human airway epithelial cell volume regulation. Non-CF cells and wild-type (WT)-CFTR-transfected CF cells had normal regulatory volume decrease (RVD) responses that were attenuated by blockade of autocrine and paracrine purinergic signaling. In contrast, parental IB3-1 CF cells or IB3-1 cells expressing CFTR mutants (DeltaF508, G551D, and S1455X) failed to RVD. CF cell RVD was rescued by agonists to P2Y G protein-coupled receptors and, more robustly, by agonists to P2X purinergic receptor channels.

CONCLUSIONS

Loss of CFTR and CFTR-driven autocrine ATP signaling may underlie defective cell volume regulation and dysregulated ion, water, and acid-base transport in CF airway epithelia.

Transport properties of the human intestinal anion exchanger DRA (down-regulated in adenoma) in transfected HEK293 cells

Electroneutral NaCl absorption in the intestine is mediated by parallel Na+/H+ and Cl-/HCO3- exchange. Mutations in the down-regulated in adenoma (DRA) gene cause congenital chloride diarrhoea but the transport characteristics of human DRA have not been studied in a heterologous human expression system. A N-terminal enhanced green fluorescence protein (EGFP)-tagged human DRA construct was therefore expressed stably in HEK293 cells. Cl-/HCO3- exchange was assessed by measuring intracellular pH and intracellular Cl- using fluorescent dyes. Expression of DRA resulted in the appearance of EGFP fluorescence and DRA immunoreactivity consistent with a location in the plasma membrane and possibly structures below the plasma membrane. DRA mediated electroneutral Cl-/HCO3- exchange but OH- was not transported and SO4(2-)/HCO3- exchange was minimal. In the presence of 5% CO2/HCO3- the apparent affinity of DRA for Cl- in transfected HEK cells was 23-36 mM, which is lower than that reported for rabbit ileal brush border membrane vesicles and for oocytes injected with human DRA. DRA was inhibited by 4 mM DIDS (45+/-11%), by 50 microM tenidap (71+/-8%) and by 100 microM glibenclamide (59+/-22% inhibition of HCO3- transport and 79+/-3% inhibition of Cl- transport). The effects of DIDS and tenidap were not additive to those of glibenclamide.

NHE3 inhibition activates duodenal bicarbonate secretion in the rat

We examined the effect of inhibition of Na+/H+ exchange (NHE) on duodenal bicarbonate secretion (DBS) in rats to further understand DBS regulation. DBS was measured by using the pH-stat method and by using CO2-sensitive electrodes. 5-(N,N-dimethyl)-amiloride (50 microM; DMA), a concentration that selectively inhibits the NHE isoforms NHE1 and NHE2, but not NHE3, did not affect DBS. Nevertheless, 3 mM DMA, a higher concentration that inhibits NHE1, NHE2, and NHE3, significantly increased DBS. Moreover, S1611 and S3226, both specific inhibitors of NHE3 only, or perfusion with Na+-free solutions, dose dependently increased DBS, as measured by pH-stat and CO2-sensitive electrode, without affecting intracellular pH. Coperfusion with 0.1 microM indomethacin, 0.5 mM DIDS, or 1 mM methazolamide did not affect S3226-induced DBS. Nevertheless, coperfusion with 0.1 and 0.3 mM 5-nitro-2-(3-phenylpropylamino) benzoic acid, which inhibits the cystic fibrosis transmembrane conductor regulator (CFTR), dose dependently inhibited S3226-induced DBS. In conclusion, only specific apical NHE3 inhibition increased DBS, whereas prostaglandin synthesis, Na+-HCO3- cotransporter activation, or intracellular HCO3- formation by carbonic anhydrase was not involved. Because NHE3 inhibition-increased DBS was inhibited by an anion channel inhibitor and because reciprocal CFTR regulation has been previously shown between NHE3 and apical membrane anion transporters, we speculate that NHE3 inhibition increased DBS by altering anion transporter function.

Expression and regulation of the Na+-K+-2Cl- cotransporter NKCC1 in the normal and CFTR-deficient murine colon

Defective regulation and/or reduced expression of the Na+-K+-2Cl- cotransporter NKCC1 may contribute to the severe secretory defect that is observed in cystic fibrosis, but data concerning the expression and function of NKCC1 in cystic fibrosis transmembrane conductance regulator (CFTR)-deficient cells are equivocal. We therefore investigated NKCC1 mRNA expression, Na+-K+-2Cl- cotransport activity and regulation by cAMP in crypts isolated from the proximal colon of CFTR-containing (CFTR (+/+)) and CFTR-deficient (CFTR (-/-)) mice. mRNA expression levels were determined by semiquantitative PCR, transport rates were measured fluorometrically in 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein acetomethylester (BCECF)-loaded crypts, cytoplasmic volume changes were assessed by confocal microscopy, and [Cl-]i changes were examined by N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide (MQAE) quenching. NKCC1 mRNA expression levels were not significantly reduced in CFTR (-/-) crypts compared to controls. Azosemide-sensitive NH4+ influx (used as a measure of Na+-K+-2Cl- cotransport) was 2.23 +/- 0.72 vs. 1.56 +/- 0.16 mM min-1, and increased by 63.6 % in (+/+) and 87.3 % in (-/-) crypts upon stimulation for 5 min with forskolin. After 20 min of stimulation with forskolin, the Na+-K+-2Cl- cotransport rates in (-/-) and (+/+) crypts were identical. Crypt cross-sectional area and [Cl-]i decreased only in (+/+) crypts upon stimulation. In conclusion, normal NKCC1 expression levels, somewhat reduced Na+-K+-2Cl- cotransport rates, but preserved activation by cAMP were found in colonic crypts from CFTR (-/-) mice, ruling out a severe dysfunction of the Na+-K+-2Cl- cotransporter in the CF intestine. Furthermore, these studies establish the existence of a direct, cell-volume- and [Cl-]i-independent activation of colonic NKCC1 by an increase in intracellular cAMP.

Identification of a basolateral Cl-/HCO3- exchanger specific to gastric parietal cells

The basolateral Cl(-)/HCO(3)(-) exchanger in parietal cells plays an essential role in gastric acid secretion mediated via the apical gastric H(+)-K(+)-ATPase. Here, we report the identification of a new Cl(-)/HCO(3)(-) exchanger, which shows exclusive expression in mouse stomach and kidney, with expression in the stomach limited to the basolateral membrane of gastric parietal cells. Tissue distribution studies by RT-PCR and Northern hybridizations demonstrated the exclusive expression of this transporter, also known as SLC26A7, to stomach and kidney, with the stomach expression significantly more abundant. No expression was detected in the intestine. Cellular distribution studies by RT-PCR and Northern hybridizations demonstrated predominant localization of SLC26A7 in gastric parietal cells. Immunofluorescence labeling localized this exchanger exclusively to the basolateral membrane of gastric parietal cells, and functional studies in oocytes indicated that SLC26A7 is a DIDS-sensitive Cl(-)/HCO(3)(-) exchanger that is active in both acidic and alkaline pH(i). On the basis of its unique expression pattern and function, we propose that SLC26A7 is a basolateral Cl(-)/HCO(3)(-) exchanger in gastric parietal cells and plays a major role in gastric acid secretion.

Acute regulation of the SLC26A3 congenital chloride diarrhoea anion exchanger (DRA) expressed in Xenopus oocytes

Mutations in the human SLC26A3 gene, also known as down-regulated in adenoma (hDRA), cause autosomal recessive congenital chloride-losing diarrhoea (CLD). hDRA expressed in Xenopus oocytes mediated bidirectional Cl--Cl- and Cl--HCO3- exchange. In contrast, transport of oxalate was low, and transport of sulfate and of butyrate was undetectable. Two CLD missense disease mutants of hDRA were nonfunctional in oocytes. Truncation of up to 44 C-terminal amino acids from the putatively cytoplasmic C-terminal hydrophilic domain left transport function unimpaired, but deletion of the adjacent STAS (sulfate transporter anti-sigma factor antagonist) domain abolished function. hDRA-mediated Cl- transport was insensitive to changing extracellular pH, but was inhibited by intracellular acidification and activated by NH4+ at acidifying concentrations. These regulatory responses did not require the presence of either hDRA's N-terminal cytoplasmic tail or its 44 C-terminal amino acids, but they did require more proximate residues of the C-terminal cytoplasmic domain. Although only weakly sensitive to inhibition by stilbenes, hDRA was inhibited with two orders of magnitude greater potency by the anti-inflammatory drugs niflumate and tenidap. cAMP-insensitive Cl--HCO3- exchange mediated by hDRA gained modest cAMP sensitivity when co-expressed with cystic fibrosis transmembrane conductance regulator (CFTR). Despite the absence of hDRA transcripts in human cell lines derived from CFTR patients, DRA mRNA was present at wild-type levels in proximal colon and nearly so in the distal ileum of CFTR(-/-) mice. Thus, pharmacological modulation of DRA might be a useful adjunct treatment of cystic fibrosis.

Bicarbonate transport proteins

Bicarbonate is not freely permeable to membranes. Yet, bicarbonate must be moved across membranes, as part of CO2 metabolism and to regulate cell pH. Mammalian cells ubiquitously express bicarbonate transport proteins to facilitate the transmembrane bicarbonate flux. These bicarbonate transporters, which function by different transport mechanisms, together catalyse transmembrane bicarbonate movement. Recent advances have allowed the identification of several new bicarbonate transporter genes. Bicarbonate transporters cluster into two separate families: (i) the anion exachanger (AE) family of Cl-/HCO3- exchangers is related in sequence to the NBC family of Na+/HCO3- cotransporters and the Na(+)-dependent Cl/HCO3- exchangers and (ii) some members of the SLC26a family of sulfate transporters will also transport bicarbonate but are not related in sequence to the AE/NBC family of transporters. This review summarizes our understanding of the mammalian bicarbonate transporter superfamily.

Polarized distribution of HCO3- transport in human normal and cystic fibrosis nasal epithelia

The polarized distribution of HCO3- transport was investigated in human nasal epithelial cells from normal and cystic fibrosis (CF) tissues. To test for HCO3- transport via conductive versus electroneutral Cl-/HCO3- exchange (anion exchange, AE) pathways, nasal cells were loaded with the pH probe 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein and mounted in a bilateral perfusion chamber. In normal, but not CF, epithelia, replacing mucosal Cl- with gluconate caused intracellular pH (pHi) to increase, and the initial rates (Delta pH min-1) of this increase were modestly augmented (approximately 26 %) when normal cells were pretreated with forskolin (10 microM). Recovery from this alkaline shift was dependent on mucosal Cl-, was insensitive to the AE inhibitor 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulfonic acid (H2DIDS; 1.5 mM), but was sensitive to the cystic fibrosis transmembrane conductance regulator (CFTR) channel inhibitor diphenylamine-2-carboxylate (DPC; 100 microM). In contrast, removal of serosal Cl- caused pHi to alkalinize in both normal and CF epithelia. Recovery from this alkaline challenge was dependent on serosal Cl- and blocked by H2DIDS. Additional studies showed that serosally applied Ba2+ (5.0 mM) in normal, but not CF, cells induced influx of HCO3- across the apical membrane that was reversibly blocked by mucosal DPC. In a final series of studies, normal and CF cells acutely alkaline loaded by replacing bilateral Krebs bicarbonate Ringer (KBR) with Hepes-buffered Ringer solution exhibited basolateral, but not apical, recovery from an alkaline challenge that was dependent on Cl-, independent of Na+ and blocked by H2DIDS. We conclude that: (1) normal, but not CF, nasal epithelia have a constitutively active DPC-sensitive HCO3- influx/efflux pathway across the apical membrane of cells, consistent with the movement of HCO3- via CFTR; and (2) both normal and CF nasal epithelia have Na+-independent, H2DIDS-sensitive AE at their basolateral domain.

Regulation of the epithelial sodium channel by accessory proteins

The epithelial sodium channel (ENaC) is of fundamental importance in the control of sodium fluxes in epithelial cells. Modulation of sodium reabsorption through the distal nephron ENaC is an important component in the overall control of sodium balance, blood volume and thereby of blood pressure. This is clearly demonstrated by rare genetic disorders of sodium-channel activity (Liddle's syndrome and pseudohypoaldosteronism type 1), associated with contrasting effects on blood pressure. The mineralocorticoid aldosterone is a well-established modulator of sodium-channel activity. Considerable insight has now been gained into the intracellular signalling pathways linking aldosterone-mediated changes in gene transcription with changes in ion transport. Activating pathways include aldosterone-induced proteins and especially the serum- and glucocorticoid-inducible kinase (SGK) and the small G-protein, K-Ras 2A. Targeting of the ENaC for endocytosis and degradation is now emerging as a major mechanism for the down-regulation of channel activity. Several proteins acting in concert are an intrinsic part of this process but Nedd4 (neural precursor cell expressed developmentally down-regulated 4) is of central importance. Other mechanisms known to interact with ENaC and affect sodium transport include channel-activating protease 1 (CAP-1), a membrane-anchored protein, and the cystic fibrosis transmembrane regulator. The implications of research on accessory factors controlling ENaC activity are wide-ranging. Understanding cellular mechanisms controlling ENaC activity may provide a more detailed insight not only of ion-channel abnormalities in cystic fibrosis but also of the link between abnormal renal sodium transport and essential hypertension.

CFTR and bicarbonate secretion by [correction of to] epithelial cells

Defective HCO(3)(-) and fluid secretion are hallmarks of the pathophysiology of the pancreas of cystic fibrosis patients. Recently, impaired HCO(3)(-) secretion has been shown in most tissues known to express the cystic fibrosis transmembrane conductance regulator (CFTR). New results suggest that CFTR plays an important role in the transcellular secretion of HCO(3)(-).

SLC26A3 mutations in congenital chloride diarrhea

Congenital chloride diarrhea (CLD) is an autosomal recessive disorder of intestinal electrolyte absorption. It is characterized by persistent secretory diarrhea resulting in polyhydramnios and prematurity prenatally, and dehydration, hypoelectrolytemia, hyperbilirubinemia, abdominal distention, and failure to thrive immediately after birth. CLD is caused by mutations in the solute carrier family 26, member 3 gene (SLC26A3, alias CLD or DRA), which encodes a Na+-independent Cl-/HCO3- (or OH-) exchanger. SLC26A3 is a member of the SLC26 sulfate permease/anion transporter family and it is expressed mainly in the apical brush border of intestinal epithelium. The only extraintestinal tissues showing SLC26A3 expression are eccrine sweat glands and seminal vesicles. A wide variety of different mutations in the SLC26A3 gene have been associated with CLD with no apparent evidence of phenotype-genotype correlation. The clinical course of CLD, however, is variable and may rather depend on environmental factors and compensatory mechanisms than mutations. In this report, we present a summary of all published and two novel SLC26A3 mutations and polymorphisms, and review them in the context of their functional consequences and clinical implications.

The functional and physical relationship between the DRA bicarbonate transporter and carbonic anhydrase II

COOH-terminal cytoplasmic tails of chloride/bicarbonate anion exchangers (AE) bind cytosolic carbonic anhydrase II (CAII) to form a bicarbonate transport metabolon, a membrane protein complex that accelerates transmembrane bicarbonate flux. To determine whether interaction with CAII affects the downregulated in adenoma (DRA) chloride/bicarbonate exchanger, anion exchange activity of DRA-transfected HEK-293 cells was monitored by following changes in intracellular pH associated with bicarbonate transport. DRA-mediated bicarbonate transport activity of 18 +/- 1 mM H+ equivalents/min was inhibited 53 +/- 2% by 100 mM of the CAII inhibitor, acetazolamide, but was unaffected by the membrane-impermeant carbonic anhydrase inhibitor, 1-[5-sulfamoyl-1,3,4-thiadiazol-2-yl-(aminosulfonyl-4-phenyl)]-2,6-dimethyl-4-phenyl-pyridinium perchlorate. Compared with AE1, the COOH-terminal tail of DRA interacted weakly with CAII. Overexpression of a functionally inactive CAII mutant, V143Y, reduced AE1 transport activity by 61 +/- 4% without effect on DRA transport activity (105 +/- 7% transport activity relative to DRA alone). We conclude that cytosolic CAII is required for full DRA-mediated bicarbonate transport. However, DRA differs from other bicarbonate transport proteins because its transport activity is not stimulated by direct interaction with CAII.

Airways in cystic fibrosis are acidified: detection by exhaled breath condensate

BACKGROUND

The loss of cystic fibrosis transmembrane conductance regulator (CFTR) mediated chloride conductance does not fully explain the diverse pathologies evident in patients with cystic fibrosis (CF). Bicarbonate (HCO(3)(-)) secretion is also impaired in CFTR expressing tissues and CFTR is thought to regulate HCO(3)(-) secretion at the apical membrane of epithelial cells. We hypothesised that the epithelial lining fluid (ELF) of patients with CF would be acidified and that this may be worsened during an infective exacerbation due to the increased inflammatory burden.

METHODS

pH and nitrite levels in exhaled breath condensate (EBC) from 12 healthy non-smoking controls and 30 patients with CF (11 of whom were in an infective exacerbation) were measured. A further nine patients were studied before and after intravenous antibiotic treatment for an exacerbation of CF.

RESULTS

The pH of EBC was significantly lower in patients with stable CF than in controls (5.88 (0.32) v 6.15 (0.16), p=0.017), and was further reduced in CF patients with an exacerbation (5.32 (0.38), p=0.001) compared with stable CF patients. EBC pH increased significantly following antibiotic treatment from 5.27 (0.42) to 5.71 (0.42), p=0.049). Nitrite levels in EBC were increased in CF patients with an exacerbation compared with control subjects (4.4 (4.0) micro m v 1.6 (1.6) micro m p=0.047). No correlation was found between EBC pH and nitrite levels.

CONCLUSIONS

These findings support the hypothesis that airway acidification occurs in CF. This acidity is in part a function of inflammation as the pH of the EBC of patients increased significantly with treatment of an exacerbation, although not to control levels. Acidic pH of the ELF may play a role in the pathophysiology of CF lung disease and requires further investigation.

A molecular mechanism for aberrant CFTR-dependent HCO(3)(-) transport in cystic fibrosis

Aberrant HCO(3)(-) transport is a hallmark of cystic fibrosis (CF) and is associated with aberrant Cl(-)-dependent HCO(3)(-) transport by the cystic fibrosis transmembrane conductance regulator (CFTR). We show here that HCO(3)(-) current by CFTR cannot account for CFTR-activated HCO(3)(-) transport and that CFTR does not activate AE1-AE4. In contrast, CFTR markedly activates Cl(-) and OH(-)/HCO(3)(-) transport by members of the SLC26 family DRA, SLC26A6 and pendrin. Most notably, the SLC26s are electrogenic transporters with isoform-specific stoichiometries. DRA activity occurred at a Cl(-)/HCO(3)(-) ratio > or =2. SLC26A6 activity is voltage regulated and occurred at HCO(3)(-)/Cl(-) > or =2. The physiological significance of these findings is demonstrated by interaction of CFTR and DRA in the mouse pancreas and an altered activation of DRA by the R117H and G551D mutants of CFTR. These findings provide a molecular mechanism for epithelial HCO(3)(-) transport (one SLC26 transporter-electrogenic transport; two SLC26 transporters with opposite stoichiometry in the same membrane domain-electroneutral transport), the CF-associated aberrant HCO(3)(-) transport, and reveal a new function of CFTR with clinical implications for CF and congenital chloride diarrhea.

Upregulation of CFTR expression but not SLC26A3 and SLC9A3 in ulcerative colitis

In inflamed colonic mucosa, the equilibrium between absorptive and secretory functions for electrolyte and salt transport is disturbed. We compared the expression of three major mediators of the intestinal salt transport between healthy and inflamed colonic mucosa to understand the pathophysiology of diarrhea in inflammatory bowel disease. Expression levels of the cystic fibrosis transmembrane regulator (CFTR) (Cl- channel), SLC26A3 (Cl-/HCO exchanger) and SLC9A3 (Na+/H+ exchanger) mRNAs were measured by real-time quantitative RT-PCR in peroperative colonic samples from controls (n = 4) and patients with ulcerative colitis (n = 10). Several samples were obtained from each individual. Tissue samples were divided into three subgroups according to their histological degree of inflammation. Expression of CFTR and SLC26A3 proteins were determined by immunohistochemistry and Western blotting from the same samples, respectively. Increased expression of CFTR mRNA was observed in all three groups of affected tissue samples, most pronounced in mildly inflamed colonic mucosa (5-fold increase in expression; P < 0.001). The expression of the CFTR protein was detected from health and inflamed colon tissue. Although the expression of the SLC26A3 mRNA was significantly decreased in severe ulcerative colitis (P < 0.05), the SLC26A3 protein levels remained unchanged in all groups. The expression of SLC9A3 mRNA was significantly changed between the mild and severe groups. Intestinal inflammation modulates the expression of three major mediators of intestinal salt transport and may contribute to diarrhea in ulcerative colitis both by increasing transepithelial Cl- secretion and by inhibiting the epithelial NaCl absorption.

Targeting of carbonic anhydrase IV to plasma membranes is altered in cultured human pancreatic duct cells expressing a mutated (deltaF508) CFTR

Human pancreatic duct cells secrete HCO3- ions mediated by a Cl-/HCO3- exchanger and a HCO3- channel that may be a carbonic anhydrase IV (CA IV) in a channel-like conformation. This secretion is regulated by CFTR (Cystic Fibrosis Transmembrane conductance Regulator). In CF cells homozygous for the deltaF508 mutation, the defect in targeting of CFTR to plasma membranes leads to a disruption in the secretion of Cl- and HCO3 ions along with a defective targeting of other proteins. In this study, we analyzed the targeting of membrane CA IV in the human pancreatic duct cell line CFPAC-1, which expresses a deltaF508 CFTR, and in the same cells transfected with the wild-type CFTR (CFPAC-PLJ-CFTR6) or with the vector alone (CFPAC-PLJ6). The experiments were conducted on cells in the stationary phase the polarized state of which was checked by the distribution of occludin and actin. We show that both cell lines express a 35-kDa CA IV at comparable levels. Analysis of fractions of plasma membranes purified on a Percoll gradient evidenced lower levels of CA IV (8-fold) in the CFPAC-1 than in the CFPAC-PLJ-CFTR6 cells. Quantitative analyses showed that 6- to 10-fold fewer cells in the CFPAC-1 cell line exhibited membrane CA IV-immunoreactivity than in the CFPAC-PLJ-CFTR6 cell line. Taken together, these results suggest that the targeting of CA IV to apical plasma membranes is impaired in CFPAC-1 cells. CA IV/gamma-adaptin double labeling demonstrated the presence of CA IV in the trans-Golgi network (TGN) of numerous CFPAC-1 cells, indicating that trafficking was disrupted on the exit face of the TGN. The retargeting of CA IV observed in CFPAC-PLJ-CFTR6 cells points to a relationship between the traffic of CFTR and CA IV. On the basis of these observations, we propose that the absence of CA IV in apical plasma membranes due to the impairment in targeting in cells expressing a deltaAF508 CFTR largely contributes to the disruption in HCO3- secretion in CF epithelia.

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.

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

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

Identification of an apical Cl(-)/HCO3(-) exchanger in the small intestine

HCO3(-) secretion is the most important defense mechanism against acid injury in the duodenum. However, the identity of the transporter(s) mediating apical HCO3(-) secretion in the duodenum remains unknown. A family of anion exchangers, which include downregulated in adenoma (DRA or SLC26A3), pendrin (PDS or SLC26A4), and the putative anion transporter (PAT1 or SLC26A6) has recently been identified. DRA and pendrin mediate Cl(-)/base exchange; however, the functional identity and distribution of PAT1 (SLC26A6) is not known. In these studies, we investigated the functional identity, tissue distribution, and membrane localization of PAT1. Expression studies in Xenopus oocytes demonstrated that PAT1 functions in Cl(-)/HCO3(-) exchange mode. Tissue distribution studies indicated that the expression of PAT1 is highly abundant in the small intestine but is low in the colon, a pattern opposite that of DRA. PAT1 was also abundantly detected in stomach and heart. Immunoblot analysis studies identified PAT1 as a approximately 90 kDa protein in the duodenum. Immunohistochemical studies localized PAT1 to the brush border membranes of the villus cells of the duodenum. We propose that PAT1 is an apical Cl(-)/HCO3(-) exchanger in the small intestine.

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.

Downregulated in adenoma and putative anion transporter are regulated by CFTR in cultured pancreatic duct cells

The mechanism of the pancreatic ductal HCO secretion defect in cystic fibrosis (CF) is not well defined. However, a lack of apical Cl(-)/HCO exchange may exist in CF. To test this hypothesis, we examined the expression of Cl(-)/HCO exchangers in cultured pancreatic duct epithelial cells with physiological features prototypical of CF [CFPAC-1 cells lacking a functional CF transmembrane conductance regulator (CFTR)] or normal duct cells (CFPAC-1 cells transfected with functional wild-type CFTR, CFPAC-WT). Cl(-)/HCO exchange activity, assayed with the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein in cells grown on coverslips, increased about twofold in cells transfected with functional CFTR. This correlated with increased apical (36)Cl influx in cells expressing functional CFTR and grown on permeable support. Northern hybridizations indicated the induction of downregulated in adenoma (DRA) in cells expressing functional CFTR. The expression of putative anion transporter PAT1 also increased significantly in cells expressing functional CFTR. DRA was detected at high levels in native mouse pancreas by Northern hybridization and localized to the apical domain of the duct cells by immunohistochemical studies. In conclusion, CFTR upregulates DRA and PAT1 expression in cultured pancreatic duct cells. We propose that the pancreatic HCO secretion defect in CF patients is partly due to the downregulation of apical Cl(-)/HCO exchange activity mediated by DRA (and possibly PAT1).