Expression of the cystic fibrosis phenotype in a renal amphibian epithelial cell line

Knockout of mitochondrial voltage-dependent anion channel type 3 increases reactive oxygen species (ROS) levels and alters renal sodium transport

It has been suggested that voltage-dependent anion channels (VDACs) control the release of superoxide from mitochondria. We have previously shown that reactive oxygen species (ROS) such as superoxide (O₂^̇̄) and hydrogen peroxide (H₂O₂) stimulate epithelial sodium channels (ENaCs) in sodium-transporting epithelial tissue, including cortical collecting duct (CCD) principal cells. Therefore, we hypothesized that VDACs could regulate ENaC by modulating cytosolic ROS levels. Herein, we find that VDAC3-knockout(KO) mice can maintain normal salt and water balance on low-salt and high-salt diets. However, on a high-salt diet for 2 weeks, VDAC3-KO mice had significantly higher systolic blood pressure than wildtype mice. Consistent with this observation, after a high-salt diet for 2 weeks, ENaC activity in VDAC3-KO mice was significantly higher than wildtype mice. EM analysis disclosed a significant morphological change of mitochondria in the CCD cells of VDAC3-KO mice compared with wildtype mice, which may have been caused by mitochondrial superoxide overload. Of note, compared with wildtype animals, ROS levels in VDAC3-KO animals fed a normal or high-salt diet were consistently and significantly increased in renal tubules. Both the ROS scavenger 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine (TEMPOL) and the mitochondrial ROS scavenger (2-(2,2,6,6-tetramethylpiperidin-1-oxyl-4-ylamino)-2-oxoethyl)triphenylphosphonium chloride (mito-TEMPO) could reverse the effect of high-salt on ENaC activity and systolic blood pressure in the VDAC3-KO mice. Mito-TEMPO partially correct the morphological changes in VDAC3-KO mice. Our results suggest that knocking out mitochondrial VDAC3 increases ROS, alters renal sodium transport, and leads to hypertension.

Diagnostic and prognostic significance of systemic alkyl quinolones for P. aeruginosa in cystic fibrosis: A longitudinal study

Background

Pulmonary P. aeruginosa infection is associated with poor outcomes in cystic fibrosis (CF) and early diagnosis is challenging, particularly in those who are unable to expectorate sputum. Specific P. aeruginosa 2-alkyl-4-quinolones are detectable in the sputum, plasma and urine of adults with CF, suggesting that they have potential as biomarkers for P. aeruginosa infection.

Aim

To investigate systemic 2-alkyl-4-quinolones as potential biomarkers for pulmonary P. aeruginosa infection.

Methods

A multicentre observational study of 176 adults and 68 children with CF. Cross-sectionally, comparisons were made between current P. aeruginosa infection using six 2-alkyl-4-quinolones detected in sputum, plasma and urine against hospital microbiological culture results. All participants without P. aeruginosa infection at baseline were followed up for one year to determine if 2-alkyl-4-quinolones were early biomarkers of pulmonary P. aeruginosa infection.

Results

Cross-sectional analysis: the most promising biomarker with the greatest diagnostic accuracy was 2-heptyl-4-hydroxyquinoline (HHQ). In adults, areas under the ROC curves (95% confidence intervals) for HHQ analyses were 0.82 (0.75–0.89) in sputum, 0.76 (0.69–0.82) in plasma and 0.82 (0.77–0.88) in urine. In children, the corresponding values for HHQ analyses were 0.88 (0.77–0.99) in plasma and 0.83 (0.68–0.97) in urine.

Longitudinal analysis: Ten adults and six children had a new positive respiratory culture for P. aeruginosa in follow-up. A positive plasma HHQ test at baseline was significantly associated with a new positive culture for P. aeruginosa in both adults and children in follow-up (odds ratio (OR) = 6.67;-95% CI:-1.48–30.1;-p = 0.01 and OR = 70; 95% CI: 5–956;-p < 0.001 respectively).

Conclusions

AQs measured in sputum, plasma and urine may be used to diagnose current infection with P. aeruginosa in adults and children with CF. These preliminary data show that plasma HHQ may have potential as an early biomarker of pulmonary P. aeruginosa. Further studies are necessary to evaluate if HHQ could be used in clinical practice to aid early diagnosis of P. aeruginosa infection in the future.

Prolactin stimulates sodium and chloride ion channels in A6 renal epithelial cells

Many hormonal pathways contribute to the regulation of renal epithelial sodium channel (ENaC) function, a key process for maintaining blood volume and controlling blood pressure. In the present study, we examined whether the peptide hormone prolactin (PRL) regulates ENaC function in renal epithelial cells (A6). Basolateral application of several different concentrations of PRL dramatically stimulated the transepithelial current in A6 cells, increasing both amiloride-sensitive (ENaC) and amiloride-insensitive currents. Using cell-attached patch clamp, we determined that PRL increased both the number (N) and open probability (Po) of ENaC present in the apical membrane. Inhibition of PKA with H-89 abolished the effect of PRL on amiloride-sensitive and insensitive transepithelial currents and eliminated the increase in ENaC NPo with PRL exposure. PRL also increased cAMP in A6 cells, consistent with signaling through the cAMP-dependent PKA pathway. We also identified that PRL induced activity of a 2-pS anion channel with outward rectification, electrophysiological properties consistent with ClC4 or ClC5. RT-PCR only detected ClC4, but not ClC5 transcripts. Here, we show for the first time that PRL activates sodium and chloride transport in renal epithelial cells via ENaC and ClC4.

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.

Epithelial Na(+) channel regulation by cytoplasmic and extracellular factors

Electrogenic Na(+) transport across high resistance epithelial is mediated by the epithelial Na(+) channel (ENaC). Our understanding of the mechanisms of ENaC regulation has continued to evolve over the two decades following the cloning of ENaC subunits. This review highlights many of the cellular and extracellular factors that regulate channel trafficking or gating.

ENaCs and ASICs as therapeutic targets

The epithelial Na(+) channel (ENaC) and acid-sensitive ion channel (ASIC) branches of the ENaC/degenerin superfamily of cation channels have drawn increasing attention as potential therapeutic targets in a variety of diseases and conditions. Originally thought to be solely expressed in fluid absorptive epithelia and in neurons, it has become apparent that members of this family exhibit nearly ubiquitous expression. Therapeutic opportunities range from hypertension, due to the role of ENaC in maintaining whole body salt and water homeostasis, to anxiety disorders and pain associated with ASIC activity. As a physiologist intrigued by the fundamental mechanics of salt and water transport, it was natural that Dale Benos, to whom this series of reviews is dedicated, should have been at the forefront of research into the amiloride-sensitive sodium channel. The cloning of ENaC and subsequently the ASIC channels has revealed a far wider role for this channel family than was previously imagined. In this review, we will discuss the known and potential roles of ENaC and ASIC subunits in the wide variety of pathologies in which these channels have been implicated. Some of these, such as the role of ENaC in Liddle's syndrome are well established, others less so; however, all are related in that the fundamental defect is due to inappropriate channel activity.

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.

Calreticulin positively regulates the expression and function of epithelial sodium channel

Epithelial sodium channel (ENaC) is a heteromultimeric Na(+) channel at the apical membrane in the kidney, colon, and lung. Because ENaC plays a crucial role in regulating Na(+) absorption and extracellular fluid volume, its dysregulation causes severe phenotypes including hypertension, hypokalemia, and airway obstruction. Despite the importance of ENaC, its protein quality control mechanism remains less established. Here we firstly show the role of calreticulin (CRT), a lectin-like molecular chaperone in the endoplasmic reticulum (ER), on the regulation of ENaC. Overexpression and knockdown analyses clearly indicated that CRT positively affects the expression of each ENaC subunit (alpha, beta and gamma). CRT overexpression also up-regulated the cell surface expression of alpha-, beta- and gamma-ENaC. Moreover, we found that CRT directly interacts with each ENaC subunit. Although CRT knockdown did not affect the de novo synthesis of ENaC subunits, CRT overexpression decreased alpha-, beta- and gamma-ENaC expression in the detergent (RIPA)-insoluble fraction, suggesting that CRT enhanced the solubility of ENaC subunits. Consistent with the increased intracellular and cell surface expression of ENaC subunits, increased channel activity of ENaC was also observed upon overexpression of CRT. Our study thus identifies CRT as an ER chaperone that regulates ENaC expression and function.

Effects of [Ca2+]i and pH on epithelial Na+ channel activity of cultured mouse cortical collecting ducts

[Ca2+]i and pH have been demonstrated to affect Na+ transport in epithelium mediated via the apical epithelial Na+ channel (ENaC). However, it still remains unclear whether the effects of [Ca2+]i and intracellular pH (pHi) on ENaC activity are direct. In this study, inside-out recording was employed to clarify the effects of pH(i) and [Ca2+]i on ENaC activity. We found that elevation of [Ca2+]i induced a significant inhibition of ENaC open probability without altering channel conductance. The inhibitory effect was due to a direct interaction between Ca2+ and ENaC, and is dependent on [Ca2+]i. pHi also directly regulated ENaC open probability. Lower pHi (<7.0) reduced the ENaC open probability as shown in shorter opening time, and higher pH(i) (>7.0) enhanced the ENaC open probability as shown in augmented opening time. pHi did not cause any alteration in channel conductance. The effects of pHi on ENaC open probability could be summarized as an S-shaped curve around pH 7.2.

Assessment of the CFTR and ENaC association

Cystic fibrosis (CF) is one of the most common lethal genetic disorders. It results primarily from mutations in the cystic fibrosis transmembrane conductance regulator (cftr) gene. These mutations cause inadequate functioning of CFTR, which in turn leads to the severe disruption of transport function in several epithelia across various organs. Affected organs include the sweat glands, the intestine, and the reproductive system, with the most devastating consequences due to the effects of the disease on airways. Despite aggressive treatment, gradual lung failure is the major life limiting factor in patients with CF. Understanding of the exact manner by which defects in the CFTR lead to lung failure is thus critical. In the CF airway, decreased chloride secretion and increased salt absorption is observed. The decreased chloride secretion appears to be a direct consequence of defective CFTR; however, the increased salt absorption is believed to result from the failure of CFTR to restrict salt absorption through a sodium channel named the epithelial Na(+) channel, ENaC. The mechanism by which CFTR modulates the function of ENaC proteins is still obscure and somewhat controversial. In this short review we will focus on recent findings of a possible direct CFTR and ENaC association.

A synthetic prostone activates apical chloride channels in A6 epithelial cells

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

Molecular proximity of cystic fibrosis transmembrane conductance regulator and epithelial sodium channel assessed by fluorescence resonance energy transfer

We present the evidence for a direct physical association of cystic fibrosis transmembrane conductance regulator (CFTR) and epithelial sodium channel (ENaC), two major ion channels implicated in the pathophysiology of cystic fibrosis, a devastating inherited disease. We employed fluorescence resonance energy transfer, a distance-dependent imaging technique with capability to detect molecular complexes with near angstrom resolution, to estimate the proximity of CFTR and ENaC, an essential variable for possible physical interaction to occur. Fluorescence resonance energy transfer studies were complemented with a classic biochemical approach: coimmunoprecipitation. Our results place CFTR and ENaC within reach of each other, suggestive of a direct interaction between these two proteins.

Modulation of epithelial sodium channel trafficking and function by sodium 4-phenylbutyrate in human nasal epithelial cells

Sodium 4-phenylbutyrate (4-PBA) has been shown to correct the cellular trafficking of several mutant or nonmutant plasma membrane proteins such as cystic fibrosis transmembrane conductance regulator through the expression of 70-kDa heat shock proteins. The objective of the study was to determine whether 4-PBA may influence the functional expression of epithelial sodium channels (ENaC) in human nasal epithelial cells (HNEC). Using primary cultures of HNEC, we demonstrate that 4-PBA (5 mm for 6 h) markedly stimulated amiloride-sensitive sodium channel activity and that this was related to an increased abundance of alpha-, beta-, and gamma-ENaC subunits in the apical membrane. The increase in ENaC cell surface expression (i) was due to insertion of newly ENaC subunits as determined by brefeldin A experiments and (ii) was not associated with cell surface retention of ENaC subunits because endocytosis of ENaC subunits was unchanged. In addition, we find that ENaC co-immunoprecipitated with the heat shock protein constitutively expressed Hsc70, that has been reported to modulate ENaC trafficking, and that 4-PBA decreased Hsc70 protein level. Finally, we report that in cystic fibrosis HNEC obtained from two cystic fibrosis patients, 4-PBA increased functional expression of ENaC as demonstrated by the increase in amiloride-sensitive sodium transport and in alpha-, beta-, and gamma-ENaC subunit expression in the apical membrane. Our results suggest that in HNEC, 4-PBA increases the functional expression of ENaC through the insertion of new alpha-, beta-, and gamma-ENaC subunits into the apical membrane and also suggest that 4-PBA could modify ENaC trafficking by reducing Hsc70 protein expression.

CFTR stabilizes ENaC at the plasma membrane

CFTR was reported to regulate ENaC channel opening, decreasing ENaC activity in airways and increasing it in sweat ducts. We generated MDCK-I cell lines stably expressing tagged alphabetagammaENaC+CFTR or ENaC alone, and developed an assay to quantify cell-surface half-life of ENaC. Surprisingly, we found that co-expressed CFTR stabilizes ENaC at the plasma membrane, suggesting that CFTR regulates ENaC stability, not just opening.

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.

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.

Stimulation of Xenopus P2Y1 receptor activates CFTR in A6 cells

Nucleotide binding to purinergic P2Y receptors contributes to the regulation of a variety of physiological functions in renal epithelial cells. Here, we investigate the regulatory mechanism of the P2Y1 receptor agonist 2-methylthioadenosine diphosphate (2-MeSADP) on Cl- transport in A6 cells, a commonly used model of the distal section of the Xenopus laevis nephron. Protein and mRNA expression analysis together with functional measurements demonstrated the basolateral location of the Xenopus P2Y1 receptor. 2-MeSADP increased intracellular [Ca2+] and cAMP and Cl- efflux, responses that were all inhibited by the specific P2Y1 receptor antagonist MRS 2179. Cl- efflux was also inhibited by the cystic fibrosis transmembrane conductance regulator (CFTR) blocker glibenclamide. Inhibition of either protein kinase A (PKA) or the binding between A-kinase-anchoring proteins (AKAPs) and the regulatory PKA RII subunit blocked the 2-MeSADP-induced activation of CFTR, suggesting that PKA mediates P2Y1 receptor regulation of CFTR through one or more AKAPs. Further, the truncation of the PDZ1 domain of the scaffolding protein Na+/H+ exchanger regulatory factor-2 (NHERF-2) inhibited 2-MeSADP-dependent stimulation of Cl- efflux, suggesting the involvement of this scaffolding protein. Activation or inhibition of PKC had no effect per se on basal Cl- efflux but potentiated or reduced the 2-MeSADP-dependent stimulation of Cl- efflux, respectively. These data suggest that the X laevis P2Y1 receptor in A6 cells can increase both cAMP/PKA and Ca2+/PKC intracellular levels and that the PKC pathway is involved in CFTR activation via potentiation of the PKA pathway.

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.

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.

Pharmacotherapy of the ion transport defect in cystic fibrosis: role of purinergic receptor agonists and other potential therapeutics

Cystic fibrosis (CF), is an autosomal recessive disease frequently seen in the Caucasian population. It is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. CF is characterized by enhanced airway Na(+) absorption, mediated by epithelial Na(+) channels (ENaC), and deficient Cl(-) transport. In addition, other mechanisms may contribute to the pathophysiological changes in the CF lung, such as defective regulation of HCO(3)(-) secretion. In other epithelial tissues, epithelial Na(+) conductance is either increased (intestine) or decreased (sweat duct) in CF. CFTR is a cyclic AMP-regulated epithelial Cl(-) channel, and appears to control the activity of several other transport proteins. Accordingly, defective epithelial ion transport in CF is likely to be a combination of defective Cl(-) channel function and impaired regulator function of CFTR, which in turn is linked to impaired mucociliary clearance and development of chronic lung disease. As the clinical course of CF is determined primarily by progressive lung disease, novel pharmacological strategies for the treatment of CF focus on correction of the ion transport defect in the airways. In recent years, it has been demonstrated that activation of purinergic receptors in airway epithelia by extracellular nucleotides (adenosine triphosphate/uridine triphosphate) has beneficial effects on mucus clearance in CF. Activation of the dominant class of metabotropic purinergic receptors, P2Y(2) receptors, appears to have a 2-fold benefit on ion transport in CF airways; excessive Na(+) absorption is attenuated, most likely by inhibition of the ENaC and, simultaneously, an alternative Ca(2+)-dependent Cl(-) channel is activated that may compensate for the CFTR Cl(-) channel defect. Thus activation of P2Y(2) receptors is expected to lead to improved hydration of the airway surface liquid in CF. Furthermore, purinergic activation has been shown to promote other components of mucociliary clearance such as ciliary beat frequency and mucus secretion. Clinical trials are under way to test the effect of synthetic purinergic compounds, such as the P2Y(2) receptor agonist INS37217, on the progression of lung disease in patients with CF. Administration of these compounds alone, or in combination with other drugs that inhibit accelerated Na(+) transport and help recover or increase residual activity of mutant CFTR, is most promising as successful therapy to counteract the ion transport defect in the airways of CF patients.

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.

Phosphatase inhibitors increase the open probability of ENaC in A6 cells

We studied the cellular phosphatase inhibitors okadaic acid (OKA), calyculin A, and microcystin on the epithelial sodium channel (ENaC) in A6 renal cells. OKA increased the amiloride-sensitive current after approximately 30 min with maximal stimulation at 1-2 h. Fluctuation analysis of cell-attached patches containing a large number of ENaC yielded power spectra with corner frequencies in untreated cells almost two times as large as in cells pretreated for 30 min with OKA, implying an increase in single channel open probability (P(o)) that doubled after OKA. Single channel analysis showed that, in cells pretreated with OKA, P(o) and mean open time approximately doubled. Two other phosphatase inhibitors, calyculin A and microcystin, had similar effects on P(o) and mean open time. An analog of OKA, okadaone, that does not inhibit phosphatases had no effect. Pretreatment with 10 nM OKA, which blocks protein phosphatase 2A (PP2A) but not PP1 in mammalian cells, had no effect even though both phosphatases are present in A6 cells. Several proteins were differentially phosphorylated after OKA, but ENaC subunit phosphorylation did not increase. We conclude that, in A6 cells, there is an OKA-sensitive phosphatase that suppresses ENaC activity by altering the phosphorylation of a regulatory molecule associated with the channel.

Angiotensin II evokes calcium-mediated signaling events in isolated dog pancreatic epithelial cells

INTRODUCTION

Calcium-activated chloride conductance has been identified in normal pancreatic duct cells. Recent in vitro evidence suggests that angiotensin II (AngII) stimulates pancreatic secretion in both cystic fibrosis (CFPAC) and transformed pancreatic cells.

AIMS

To investigate calcium-mediated stimulatory effects of AngII in both nontransformed dog pancreatic duct epithelial (DPDE) and CFPAC cells.

METHODS

Western blots were performed in both cells seeking AngII receptors. In additional studies, DPDE and CFPAC cells were grown on vitrogen-coated glass cover slips and loaded with Indo-1-AM dye. Cells were placed in a confocal microscope's perfusion chamber and perfused with 100 microM AngII or ATP (control). Cells were excited with UV light, and intracellular calcium ([Ca+2]i) was read using fluorescence emission at 405 and 530 nm. Finally, single channels in the DPDE cells were examined using cell-attached patch clamps. Current amplitude histograms provided estimates of the conductance and open probability of channels.

RESULTS

Western blots demonstrated presence of both AT and AT AngII receptors in DPDE and CFPAC cells; the density of AT receptors appeared lower than that of AT receptors. Basal intracellular calcium concentrations did not differ between DPDE (109 +/- 11 nM) and CFPAC (103 +/- 8 nM) cells. AngII significantly increased measured intracellular calcium concentrations in both DPDE (909 +/- 98 nM) and CFPAC (879 +/- 207 nM) cells, as did ATP (DPDE = 1722 +/- 228 nM; CFPAC = 1522 +/- 245 nM). In the patch clamp studies, a variety of different channels were observed; they appeared to be an 11pS nonselective cation (NSC) channel, a 4.6pS Na+ channel, a 3pS anion channel, and an 8pS chloride channel. The latter channel had characteristics similar to cystic fibrosis transmembrane conductance regulator (CFTR). Apical or basolateral application of AngII activated both the 11pS NSC and the 3pS channels.

CONCLUSION

In nontransformed DPDE and CFPAC cells, specific AngII receptors mediate increases in [Ca ]. The latter effect of AngII may elicit activation of calcium-mediated chloride channels, suggesting a role for AngII as an alternative mediator of pancreatic ductal secretion.

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.

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 role of SGK1 in hormone-regulated sodium transport

Ion transport in epithelia is regulated by a variety of hormonal and nonhormonal factors, including mineralocorticoids, insulin, shear stress and osmotic pressure. In mammals, the mineralocorticoid aldosterone is the principal regulator of sodium homeostasis and hence is central to the control of extracellular fluid volume and blood pressure. Aldosterone acts through a member of the nuclear receptor superfamily, the mineralocorticoid receptor (MR), to control the transcriptional activity of specific target genes. Recently, a serine/threonine kinase, SGK1 (serum and glucocorticoid-regulated kinase isoform 1) was identified as a candidate mediator of aldosterone action in the colon and distal nephron. The aldosterone-activated MR increases SGK1 gene transcription and SGK1, in turn, strongly stimulates the activity of the epithelial sodium channel (ENaC). Interestingly, other factors appear to regulate SGK1 gene expression and kinase activity. Insulin, for example, stimulates SGK1 activity (but not gene transcription) through its effects on phosphatidylinositol-3-kinase and osmotic shock appears to stimulate both SGK1 activity and gene transcription. Hence, SGK1 might integrate the effects of multiple hormonal and nonhormonal regulators of Na(+) transport in tight epithelia and thereby play a key role in volume homeostasis. It is interesting to speculate that SGK1 might be implicated in medical conditions, such as the insulin resistance syndrome, hypertension and congestive heart failure.

S-adenosyl-L-homocysteine hydrolase is necessary for aldosterone-induced activity of epithelial Na(+) channels

The A6 cell line was used to study the role of S-adenosyl-L-homocysteine hydrolase (SAHHase) in the aldosterone-induced activation of the epithelial Na(+) channel (ENaC). Because aldosterone increases methylation of several different molecules, and because this methylation is associated with increased Na(+) reabsorption, we tested the hypothesis that aldosterone increases the expression and activity of SAHHase protein. The rationale for this work is that general methylation may be promoted by activation of SAHHase, the only enzyme known to metabolize SAH, a potent end-product inhibitor of methylation. Although aldosterone increased SAHHase activity, steroid did not affect SAHHase expression. Antisense SAHHase oligonucleotide decreased SAHHase expression and activity. Moreover, this oligonucleotide, as well as a pharmacological inhibitor of SAHHase, decreased aldosterone-induced activity of ENaC via a decrease in ENaC open probability. The kinetics of ENaC in cells treated with antisense plus aldosterone were similar to those reported previously for the channel in the absence of steroid. This is the first report showing that active SAHHase, in part, increases ENaC open probability by reducing the transition rate from open states in response to aldosterone. Thus aldosterone-induced SAHHase activity plays a critical role in shifting ENaC from a gating mode with short open and closed times to one with longer open and closed times.

cAMP-dependent fluid secretion in rat inner medullary collecting ducts

We used an unambiguous in vitro method to determine if inner medullary collecting ducts (IMCD) have intrinsic capacities to absorb and secrete solutes and fluid in an isotonic medium. IMCD(1), IMCD(2), and IMCD(3) were dissected from kidneys of young Sprague-Dawley rats. 8-Bromo-3',5'-cyclic monophosphate (8-BrcAMP) stimulated lumen formation and progressive dilation in all IMCD subsegments; lumen formation was greatest in IMCD(1.) Benzamil potentiated the rate of lumen expansion in response to 8-BrcAMP. Fluid entered tubule lumens by transcellular secretion rather than simple translocation of intracellular fluid. Secreted lumen solutes were osmometrically active. Inhibition of protein kinase A with H-89 and Rp diastereomer of adenosine 3',5'-cyclic monophosphorothioate blocked fluid secretion. The rate of lumen expansion was reduced by the selective addition of ouabain, barium, diphenyl-2-carboxylate, bumetanide, glybenclamide, or DIDS, or reduction of extracellular Cl(-). We conclude that IMCD absorb and secrete electrolytes and fluid in vitro and that secretion is accelerated by cAMP. We suggest that salt and fluid secretion by the terminal portions of the renal collecting system may have a role in modulating the composition and volume of the final urine.

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

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

Methylation increases the open probability of the epithelial sodium channel in A6 epithelia

We used single channel methods on A6 renal cells to study the regulation by methylation reactions of epithelial sodium channels. 3-Deazaadenosine (3-DZA), a methyltransferase blocker, produced a 5-fold decrease in sodium transport and a 6-fold decrease in apical sodium channel activity by decreasing channel open probability (P(o)). 3-Deazaadenosine also blocked the increase in channel open probability associated with addition of aldosterone. Sodium channel activity in excised "inside-out" patches usually decreased within 1-2 min; in the presence of S-adenosyl-l-methionine (AdoMet), activity persisted for 5-8 min. Sodium channel mean time open (t(open)) before and after patch excision was higher in the presence of AdoMet than in untreated excised patches but less than t(open) in cell-attached patches. Sodium channel activity in excised patches exposed to both AdoMet and GTP usually remained stable for more than 10 min, and P(o) and the number of active channels per patch were close to values in cell-attached patches from untreated cells. These findings suggest that a methylation reaction contributes to the activity of epithelial sodium channels in A6 cells and is directed to some regulatory element closely connected with the channel, whose activity also depends on the presence of intracellular GTP.

Epithelial sodium channels regulate cystic fibrosis transmembrane conductance regulator chloride 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 epithelial Na(+) channel (ENaC) currents in many epithelial and nonepithelial cells. Because modulation of net NaCl reabsorption has important implications in extracellular fluid volume homeostasis and airway fluid volume and composition, we investigated whether this regulation was reciprocal by examining whether ENaC regulates CFTR. Co-expression of human (h) CFTR and mouse (m) alphabetagammaENaC in Xenopus oocytes resulted in a significant, 3.7-fold increase in whole-cell hCFTR Cl(-) conductance compared with oocytes expressing hCFTR alone. The forskolin/3-isobutyl-1-methylxanthine-stimulated whole-cell conductance in hCFTR-mENaC co-injected oocytes was amiloride-insensitive, indicating an inhibition of mENaC following hCFTR activation, and it was blocked by DPC (diphenylamine-2-carboxylic acid) and was DIDS (4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid)-insensitive. Enhanced hCFTR Cl(-) conductance was also observed when either the alpha- or beta-subunit of mENaC was co-expressed with hCFTR, but this was not seen when CFTR was co-expressed with the gamma-subunit of mENaC. Single Cl(-) channel analyses showed that both CFTR Cl(-) channel open probability and the number of CFTR Cl(-) channels detected per patch increased when hCFTR was co-expressed with alphabetagammamENaC. We conclude that in addition to acting as a regulator of ENaC, CFTR activity is regulated by ENaC.

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

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

Mineralocorticoid action

The physiology of mineralocorticoid action, particularly with respect to epithelial sodium transport, is well defined. A full understanding of the molecular basis of mineralocorticoid action has however proven to be more elusive. In the last decade insights into structural and functional aspects of the mineralocorticoid receptor combined with emerging details of the components of the mediators of the sodium flux has resulted in a clearer picture. This review focuses on two aspects of these new developments; the mineralocorticoid receptor and putative aldosterone induced proteins.

Control of apical membrane chloride permeability in the renal A6 cell line by nucleotides

1. The effect of extracellular nucleotides applied on the apical side of polarised A6 cells grown on permeant filters was investigated by measuring the changes in (i) the 36Cl efflux through the apical membranes, (ii) the intracellular chloride concentrations (aCli, measured with N-(6-methoxyquinolyl) acetoethyl ester, MQAE), (iii) ICl, the short-circuit current in the absence of Na+ transport and (iv) the characteristics of the apical chloride channels using a patch-clamp approach. 2. ATP or UTP (0.1-500 microM) transiently stimulated ICl. The sequence of purinergic agonist potencies was UTP = ATP > ADP >> the P2X-selective agonist beta,gamma-methylene ATP = the P2Y-selective agonist 2-methylthioATP. Suramin (100 microM) as the P2Y antagonist Reactive Blue 2 (10 microM) had no effect on the UTP (or ATP)-stimulated current. These findings are consistent with the presence of P2Y2-like receptors located on the apical membranes of A6 cells. Apical application of adenosine also transiently increased ICl. This effect was blocked by theophylline while the UTP-stimulated ICl was not. The existence of a second receptor, of the P1 type is proposed. 3. ATP (or UTP)-stimulated ICl was blocked by apical application of 200 microM N-phenylanthranilic acid (DPC) or 100 microM niflumic acid while 100 microM glibenclamide was ineffective. 4. Ionomycin and thapsigargin both transiently stimulated ICl; the nucleotide stimulation of ICl was not suppressed by pre-treatment with these agents. Chlorpromazin (50 microM), a Ca2+-calmodulin inhibitor strongly inhibited the stimulation of ICl induced either by apical UTP or by ionomycin application. BAPTA-AM pre-treatment of A6 cells blocked the UTP-stimulated ICl. Niflumic acid also blocked the ionomycin stimulated ICl. 5. A fourfold increase in 36Cl effluxes through the apical membranes was observed after ATP or UTP application. These increases of the apical chloride permeability could also be observed when following aCli changes. Apical application of DPC (1 mM) or 5-nitro-2(3-phenylpropylamino)benzoic acid (NPPB; 500 microM) produced an incomplete inhibition of 36Cl effluxes through the apical membranes in ATP-stimulated and in untreated monolayers. 6. In single channel patch-clamp experiments, an apical chloride channel with a unitary single channel conductance of 7.3 +/- 0.6 pS (n = 12) was usually observed. ATP application induced the activation of one or more of these channels within a few minutes. 7. These results indicate that multiple purinergic receptor subtypes are present in the apical membranes of A6 cells and that nucleotides can act as modulators of Cl- secretion in renal cells.

CFTR-mediated inhibition of epithelial Na+ conductance in human colon is defective in cystic fibrosis

Cystic fibrosis (CF) patients show characteristic defects in epithelial ion transport, such as failure in cAMP-dependent Cl- secretion. Because the cystic fibrosis transmembrane conductance regulator (CFTR) also functions as a downregulator of epithelial Na+ channels (ENaC), enhanced Na+ conductance was found in the airways of CF patients. Here, we examined whether enhanced epithelial Na+ conductance is also present in the colonic epithelium of CF patients and examined the underlying mechanisms. Thus transepithelial voltages were measured, and equivalent short-circuit currents (I(sc-eq)) were determined by means of a novel type of Ussing chamber. Non-CF tissues demonstrated cAMP-dependent Cl- secretion that was absent in biopsies of CF patients. Correspondingly, Isc-eq was inhibited in non-CF but not in CF epithelia when synthesis of endogenous prostaglandins was blocked by indomethacin. In the presence of indomethacin, a larger portion of amiloride-sensitive Isc-eq was detected in CF tissues, suggesting enhanced ENaC conductance in colonic mucosa of CF patients. Increase of intracellular cAMP by forskolin and IBMX inhibited amiloride-sensitive ENaC currents in non-CF tissues but not in CF biopsies. Therefore, enhanced epithelial Na+ conductance is present in the CF colon and is probably due to missing downregulation by CFTR.

The cystic fibrosis transmembrane conductance regulator and its function in epithelial transport

CF is a well characterized disease affecting a variety of epithelial tissues. Impaired function of the cAMP activated CFTR Cl- channel appears to be the basic defect detectable in epithelial and non-epithelial cells derived from CF patients. Apart from cAMP-dependent Cl- channels also Ca2+ and volume activated Cl- currents may be changed in the presence of CFTR mutations. This is supported by recent additional findings showing that different intracellular messengers converge on the CFTR Cl- channel. Analysis of the ion transport in CF airways and intestinal epithelium identified additional defects in Na+ transport. It became clear recently that mutations of CFTR may also affect the activity of other membrane conductances including epithelial Na+ channels, KvLQT-1 K+ channels and aquaporins (Fig. 7). Several additional, initially unexpected effects of CFTR on cellular functions, such as exocytosis, mucin secretion and regulation of the intracellular pH were reported during the past. Taken together, these results clearly indicate that CFTR not only acts as a cAMP regulated Cl- channel, but may fulfill several other cellular functions, particularly by regulating other membrane conductances. Failure in CFTR dependent regulation of these membrane conductances is likely to contribute to the defects observed in CF. Currently, no general concept is available that can explain how CFTR controls this variety of cellular functions. Further studies will have to verify whether direct protein interaction, specific effects on membrane turnover, changes of the intracellular ion concentration or additional proteins are involved in these regulatory loops. At the end of this review one cannot share the provocative and reassuring title "CFTR!" of a review written a few years ago [114]. Today one might rather finish with the statement "CFTR?".

Antiidiotypic antibody recognizes an amiloride binding domain within the alpha subunit of the epithelial Na+ channel

We previously raised an antibody (RA6.3) by an antiidiotypic approach which was designed to be directed against an amiloride binding domain on the epithelial Na+ channel (ENaC). This antibody mimicked amiloride in that it inhibited transepithelial Na+ transport across A6 cell monolayers. RA6.3 recognized a 72-kDa polypeptide in A6 epithelia treated with tunicamycin, consistent with the size of nonglycosylated Xenopus laevis alphaENaC. RA6.3 specifically recognized an amiloride binding domain within the alpha-subunit of mouse and bovine ENaC. The deduced amino acid sequence of RA6.3 was used to generate a three-dimensional model structure of the antibody. The combining site of RA6.3 was epitope mapped using a novel computer-based strategy. Organic residues that potentially interact with the RA6.3 combining site were identified by data base screening using the program LUDI. Selected residues docked to the antibody in a manner corresponding to the ordered linear array of amino acid residues within an amiloride binding domain on the alpha-subunit of ENaC. A synthetic peptide spanning this domain inhibited the binding of RA6.3 to alphaENaC. This analysis provided a novel approach to develop models of antibody-antigen interaction as well as a molecular perspective of RA6.3 binding to an amiloride binding domain within alphaENaC.

Epithelial sodium channel regulated by aldosterone-induced protein sgk

Sodium homeostasis in terrestrial and freshwater vertebrates is controlled by the corticosteroid hormones, principally aldosterone, which stimulate electrogenic Na+ absorption in tight epithelia. Although aldosterone is known to increase apical membrane Na+ permeability in target cells through changes in gene transcription, the mechanistic basis of this effect remains poorly understood. The predominant early effect of aldosterone is to increase the activity of the epithelial sodium channel (ENaC), although ENaC mRNA and protein levels do not change initially. Rather, the open probability and/or number of channels in the apical membrane are greatly increased by unknown modulators. To identify hormone-stimulated gene products that modulate ENaC activity, a subtracted cDNA library was generated from A6 cells, a stable cell line of renal distal nephron origin, and the effect of candidates on ENaC activity was tested in a coexpression assay. We report here the identification of sgk (serum and glucocorticoid-regulated kinase), a member of the serine-threonine kinase family, as an aldosterone-induced regulator of ENaC activity. sgk mRNA and protein were strongly and rapidly hormone stimulated both in A6 cells and in rat kidney. Furthermore, sgk stimulated ENaC activity approximately 7-fold when they were coexpressed in Xenopus laevis oocytes. These data suggest that sgk plays a central role in aldosterone regulation of Na+ absorption and thus in the control of extracellular fluid volume, blood pressure, and sodium homeostasis.

S-adenosyl-L-homocysteine hydrolase regulates aldosterone-induced Na+ transport

Aldosterone-induced Na+ reabsorption, in part, is regulated by a critical methyl esterification; however, the signal transduction pathway regulating this methylation remains unclear. The A6 cell line was used as a model epithelia to investigate regulation of aldosterone-induced Na+ transport by S-adenosyl-L-homocysteine hydrolase (SAHHase), the only enzyme in vertebrates known to catabolize S-adenosyl-L-homocysteine (SAH), an end product inhibitor of methyl esterification. Sodium reabsorption was decreased within 2 h by 3-deazaadenosine, a competitive inhibitor of SAHHase, with a half inhibitory concentration between 40 and 50 microM. Aldosterone increased SAH catabolism by activating SAHHase. Increased SAH catabolism was associated with a concomitant increase in S-adenosylmethionine catabolism. Moreover, SAH decreased substrate methylation. Antisense oligonucleotide complementary to SAHHase mRNA decreased SAHHase activity and Na+ current by approximately 50%. Overexpression of SAHHase increased SAHHase activity and dependent substrate methyl esterification. Whereas basal Na+ current was not affected by overexpression of SAHHase, aldosterone-induced current in SAHHase-overexpressing cells was significantly potentiated. These results demonstrate that aldosterone induction of SAHHase activity is necessary for a concomitant relief of the methylation reaction from end product inhibition by SAH and the subsequent increase in Na+ reabsorption. Thus, regulation of SAHHase activity is a control point for aldosterone signal transduction, but SAHHase is not an aldosterone-induced protein.

ATP release mechanisms in primary cultures of epithelia derived from the cysts of polycystic kidneys

Autosomal dominant polycystic kidney disease (ADPKD) cyst enlargement is exacerbated by accumulation of fluid within the lumen of the cyst. Extracellular nucleotides and nucleosides stimulate fluid and chloride (Cl-) secretion across epithelia and are potent autocrine and paracrine agonists within tissues. This study tests the hypothesis that ATP may be released by ADPKD epithelial cells. Once released, extracellular nucleotides and their metabolites may become "trapped" in the cyst lumen. As a consequence, extracellular ATP may augment ADPKD cyst enlargement through stimulation of salt and water secretion across ADPKD epithelia that encapsulate ADPKD cysts. To test this hypothesis, bioluminescence detection assays of ATP released from primary cultures of human ADPKD epithelial cells were compared with non-ADPKD human epithelial primary cultures. ADPKD cultures release comparable or greater amounts of ATP than non-ADPKD cultures derived from proximal tubule or cortex. ATP release in both ADPKD and non-ADPKD primary epithelial monolayers was directed largely into the apical medium; however, basolateral-directed ATP release under basal and stimulated conditions was also observed. Hypotonicity potentiated ATP release into the apical and basolateral medium in a reversible manner. Reconstitution of isotonic conditions with specific osmoles or inhibition with mechanosensitive ion channel blockers dampened hypotonicity-induced ATP release. "Flash-frozen" cyst fluids from ADPKD cysts, harvested from multiple donor kidneys, were screened by luminometry. A subset of cyst fluids contained as much as 0.5 to 10 microM ATP, doses sufficient to stimulate purinergic receptors. Taken together, these results show that ADPKD and non-ADPKD human epithelial primary cultures release ATP under basal and stimulated conditions and that ATP is released in vitro and into the cyst fluid by cystic epithelial cells in concentrations sufficient to stimulate ATP receptors. It is hypothesized that extracellular nucleotide release and signaling may contribute detrimentally to the gradual expansion of cyst fluid volume that is a hallmark of ADPKD.

Activation of Cl- channel and Na+/K+/2Cl- cotransporter in renal epithelial A6 cells by flavonoids: genistein, daidzein, and apigenin

The present study investigates regulation of Cl- channels and Na+/K+/2Cl- cotransporter in a renal epithelial cell line, A6, by flavones: genistein [an inhibitor of protein tyrosine kinases (PTK)], daidzein (an inactive compound of genistein), and apigenin [an inhibitor of mitogen-activated protein (MAP) kinase]. Genistein and daidzein activated Cl- channels. Genistein and apigenin had a stimulatory effect on the bumetanide-sensitive Na+/K+/2Cl- cotransporter. Other PTK inhibitors, tyrphostin A23, lavendustin A, and herbimycin A, which do not contain a structure flavone, had no stimulatory action on Cl- channels or the Na+/K+/2Cl- cotransporter. These observations conclude that (i) genistein activates a Cl- channel and the Na+/K+/2Cl- cotransporter; and (ii) the stimulatory action is not mediated through its inhibitory action on protein tyrosine kinase, but rather the structure of flavone itself plays a crucial role in stimulatory regulation of Cl- channels and Na+/K+/2Cl- cotransporter.

ABC transporter-facilitated ATP conductive transport

The concept that the cystic fibrosis (CF) transmembrane conductance regulator, the protein product of the CF gene, can conduct larger multivalent anions such as ATP as well as Cl- is controversial. In this review, I examine briefly past findings that resulted in controversy. It is not the goal of this review to revisit these disparate findings in detail. Rather, I focus intently on more recent studies, current studies in progress, and possible future directions that arose from the controversy and that may reconcile this issue. Important questions and hypotheses are raised as to the physiological roles that ATP-binding cassette (ABC) transporter-facilitated ATP transport and signaling may play in the control of epithelial cell function. Perhaps the identification of key biological paradigms for ABC transporter-mediated extracellular nucleotide signaling may unify and guide the CF research community and other research groups interested in ABC transporters toward understanding why ABC transporters facilitate ATP transport.

CFTR is a conductance regulator as well as a chloride channel

CFTR Is a Conductance Regulator as well as a Chloride Channel. Physiol. Rev. 79, Suppl.: S145-S166, 1999. - Cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the ATP-binding cassette (ABC) transporter gene family. Although CFTR has the structure of a transporter that transports substrates across the membrane in a nonconductive manner, CFTR also has the intrinsic ability to conduct Cl- at much higher rates, a function unique to CFTR among this family of ABC transporters. Because Cl- transport was shown to be lost in cystic fibrosis (CF) epithelia long before the cloning of the CF gene and CFTR, CFTR Cl- channel function was considered to be paramount. Another equally valid perspective of CFTR, however, derives from its membership in a family of transporters that transports a multitude of different substances from chemotherapeutic drugs, to amino acids, to glutathione conjugates, to small peptides in a nonconductive manner. Moreover, at least two members of this ABC transporter family (mdr-1, SUR) can regulate other ion channels in the membrane. More simply, ABC transporters can regulate somehow the function of other cellular proteins or cellular functions. This review focuses on a plethora of studies showing that CFTR also regulates other ion channel proteins. It is the hope of the authors that the reader will take with him or her the message that CFTR is a conductance regulator as well as a Cl- channel.

Bioluminescence detection of ATP release mechanisms in epithelia

Autocrine and paracrine release of and extracellular signaling by ATP is a ubiquitous cell biological and physiological process. Despite this knowledge, the mechanisms and physiological roles of cellular ATP release are unknown. We tested the hypothesis that epithelia release ATP under basal and stimulated conditions by using a newly designed and highly sensitive assay for bioluminescence detection of ATP released from polarized epithelial monolayers. This bioluminescence assay measures ATP released from cystic fibrosis (CF) and non-CF human epithelial monolayers in a reduced serum medium through catalysis of the luciferase-luciferin reaction, yielding a photon of light collected by a luminometer. This novel assay measures ATP released into the apical or basolateral medium surrounding epithelia. Of relevance to CF, CF epithelia fail to release ATP across the apical membrane under basal conditions. Moreover, hypotonicity is an extracellular signal that stimulates ATP release into both compartments of non-CF epithelia in a reversible manner; the response to hypotonicity is also lost in CF epithelia. The bioluminescence detection assay for ATP released from epithelia and other cells will be useful in the study of extracellular nucleotide signaling in physiological and pathophysiological paradigms. Taken together, these results suggest that extracellular ATP may be a constant regulator of epithelial cell function under basal conditions and an autocrine regulator of cell volume under hypotonic conditions, two functions that may be lost in CF and contribute to CF pathophysiology.

Membrane trafficking of the cystic fibrosis gene product, cystic fibrosis transmembrane conductance regulator, tagged with green fluorescent protein in madin-darby canine kidney cells

The mechanism by which cAMP stimulates cystic fibrosis transmembrane conductance regulator (CFTR)-mediated chloride (Cl-) secretion is cell type-specific. By using Madin-Darby canine kidney (MDCK) type I epithelial cells as a model, we tested the hypothesis that cAMP stimulates Cl- secretion by stimulating CFTR Cl- channel trafficking from an intracellular pool to the apical plasma membrane. To this end, we generated a green fluorescent protein (GFP)-CFTR expression vector in which GFP was linked to the N terminus of CFTR. GFP did not alter CFTR function in whole cell patch-clamp or planar lipid bilayer experiments. In stably transfected MDCK type I cells, GFP-CFTR localization was substratum-dependent. In cells grown on glass coverslips, GFP-CFTR was polarized to the basolateral membrane, whereas in cells grown on permeable supports, GFP-CFTR was polarized to the apical membrane. Quantitative confocal fluorescence microscopy and surface biotinylation experiments demonstrated that cAMP did not stimulate detectable GFP-CFTR translocation from an intracellular pool to the apical membrane or regulate GFP-CFTR endocytosis. Disruption of the microtubular cytoskeleton with colchicine did not affect cAMP-stimulated Cl- secretion or GFP-CFTR expression in the apical membrane. We conclude that cAMP stimulates CFTR-mediated Cl- secretion in MDCK type I cells by activating channels resident in the apical plasma membrane.