Intracellular pH and its relationship to regulation of ion transport in normal and cystic fibrosis human nasal epithelia

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.

Proton channel blockers inhibit Duox activity independent of Hv1 effects

The NADPH oxidase reaction produces protons. In the case of the NADPH oxidase, NOX2, activity depends on secretion of these protons and is inhibited by blockade of the voltage-gated proton channel (Hv1). Duox1 and Duox2 activities similarly produce intracellular protons but synthesize hydrogen peroxide directly instead of superoxide. Hv1 contributes to acid secretion in some epithelia that express Duox. To test the hypothesis that Duox activity is also sensitive to Hv1 channel blockers, Duox was assayed in the presence of either Zn²⁺ or 5-chloro-2-guanidinobenzimidazole (ClGBI). Both compounds inhibited Duox activity in normal human bronchial epithelial cells but with an IC50 over 10-fold higher than that reported for Hv1 (IC50 Zn²⁺ = 0.68 mM; IC50 ClGBI = 0.07–0.14 mM). Homogenized HEK293T cells expressing either Duox1 or Duox2 showed similar IC50 values for ClGBI suggesting these compounds inhibit the enzymes through alternate mechanisms independent of Hv1 proton secretion. Inclusion of superoxide dismutase did not restore Duox hydrogen peroxide synthesis. Addition of nigericin to eliminate any possible transmembrane pH gradients in intracellular membrane-localized Duox did not alter activity in HEK293T homogenates. Extracellular Zn²⁺ blocked intracellular Ca²⁺ increases needed for Duox activity. Together the data suggest that Duox enzyme activities in epithelia are inhibited by compounds that block Hv1 but inhibition occurs through Hv1-independent mechanisms and support the idea that Hv1 is not required for Duox activity.

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Hv1 proton channel inhibitors block Duox in differentiated bronchial epithelial cells.

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Zinc blocks Duox activity concurrently with reduction of calcium transients.

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ClGBI, an inhibitor of Hv1, blocks Duox activity in homogenates of cells lacking Hv1.

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In differentiated bronchial epithelia, Hv1 blockers did not alter intracellular pH.

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H⁺/K⁺ ATPase inhibition acidified cytoplasm but did not block Duox activity.

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

INTRODUCTION

Cystic fibrosis is an autosomal recessive disorder caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene that codes for the CFTR anion channel. In the absence of functional CFTR, the epithelial Na⁺ channel is also dysregulated. Airway surface liquid (ASL) hydration is maintained by a balance between epithelial sodium channel (ENaC)-led Na⁺ absorption and CFTR-dependent anion secretion. This finely tuned homeostatic mechanism is required to maintain sufficient airway hydration to permit the efficient mucus clearance necessary for a sterile lung environment. In CF airways, the lack of CFTR and increased ENaC activity lead to ASL/mucus dehydration that causes mucus obstruction, neutrophilic infiltration, and chronic bacterial infection. Rehydration of ASL/mucus in CF airways can be achieved by inhibiting Na⁺ absorption with pharmacological inhibitors of ENaC. Areas covered: In this review, we discuss ENaC structure and function and its role in CF lung disease and focus on ENaC inhibition as a potential therapeutic target to rehydrate CF mucus. We also discuss the failure of the first generation of pharmacological inhibitors of ENaC and recent alternate strategies to attenuate ENaC activity in the CF lung. Expert opinion: ENaC is an attractive therapeutic target to rehydrate CF ASL that may serve as a monotherapy or function in parallel with other treatments. Given the increased number of strategies being employed to inhibit ENaC, this is an exciting and optimistic time to be in this field.

Cellular chloride and bicarbonate retention alters intracellular pH regulation in Cftr KO crypt epithelium

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR), an anion channel providing a major pathway for Cl(-) and HCO3 (-) efflux across the apical membrane of the epithelium. In the intestine, CF manifests as obstructive syndromes, dysbiosis, inflammation, and an increased risk for gastrointestinal cancer. Cftr knockout (KO) mice recapitulate CF intestinal disease, including intestinal hyperproliferation. Previous studies using Cftr KO intestinal organoids (enteroids) indicate that crypt epithelium maintains an alkaline intracellular pH (pHi). We hypothesized that Cftr has a cell-autonomous role in downregulating pHi that is incompletely compensated by acid-base regulation in its absence. Here, 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein microfluorimetry of enteroids showed that Cftr KO crypt epithelium sustains an alkaline pHi and resistance to cell acidification relative to wild-type. Quantitative real-time PCR revealed that Cftr KO enteroids exhibit downregulated transcription of base (HCO3 (-))-loading proteins and upregulation of the basolateral membrane HCO3 (-)-unloader anion exchanger 2 (Ae2). Although Cftr KO crypt epithelium had increased Ae2 expression and Ae2-mediated Cl(-)/HCO3 (-) exchange with maximized gradients, it also had increased intracellular Cl(-) concentration relative to wild-type. Pharmacological reduction of intracellular Cl(-) concentration in Cftr KO crypt epithelium normalized pHi, which was largely Ae2-dependent. We conclude that Cftr KO crypt epithelium maintains an alkaline pHi as a consequence of losing both Cl(-) and HCO3 (-) efflux, which impairs pHi regulation by Ae2. Retention of Cl(-) and an alkaline pHi in crypt epithelium may alter several cellular processes in the proliferative compartment of Cftr KO intestine.

Mechanisms of bicarbonate secretion: lessons from the airways

Early studies showed that airway cells secrete HCO(3)(-) in response to cAMP-mediated agonists and HCO(3)(-) secretion was impaired in cystic fibrosis (CF). Studies with Calu-3 cells, an airway serous model with high expression of CFTR, also show the secretion of HCO(3)(-) when cells are stimulated with cAMP-mediated agonists. Activation of basolateral membrane hIK-1 K(+) channels inhibits HCO(3)(-) secretion and stimulates Cl(-) secretion. CFTR mediates the exit of both HCO(3)(-) and Cl(-) across the apical membrane. Entry of HCO(3)(-) on a basolateral membrane NBC or Cl(-) on the NKCC determines which anion is secreted. Switching between these two secreted anions is determined by the activity of hIK-1 K(+) channels.

Function of Proton Channels in Lung Epithelia

The properties of the voltage-dependent H(+) channel have been studied in lung epithelial cells for many years, and recently HVCN1 mRNA expression has been linked directly to H(+) channel function in lung epithelium. The H(+) channel is activated by strong membrane depolarization, intracellular acidity, or extracellular alkalinity. Early on it was noted that these are surprising physiological channel characteristics when considering that lung epithelial cells have rather stable membrane potentials and a well pH-buffered intracellular milieu. This raised the question under which conditions the H(+) channel is active in lung epithelium and what is its physiological function there. Current understanding of the HVCN1 H(+) channel in lung epithelial acid secretion, its activation by an alkaline mucosal extracellular pH, and its role in the regulation of the mucosal pH of the lung has resulted in a model of mucosal pH regulation based on the parallel function of the HVCN1 H(+) channel and the CFTR HCO(3) (-) channel, which suggests that HVCN1 is a critical factor that maintains a neutral surface pH in the lung.

Function of the HVCN1 proton channel in airway epithelia and a naturally occurring mutation, M91T

Airways secrete considerable amounts of acid. In this study, we investigated the identity and the pH-dependent function of the apical H(+) channel in the airway epithelium. In pH stat recordings of confluent JME airway epithelia in Ussing chambers, Zn-sensitive acid secretion was activated at a mucosal threshold pH of approximately 7, above which it increased pH-dependently at a rate of 339 +/- 34 nmol x h(-1) x cm(-2) per pH unit. Similarly, H(+) currents measured in JME cells in patch clamp recordings were readily blocked by Zn and activated by an alkaline outside pH. Small interfering RNA-mediated knockdown of HVCN1 mRNA expression in JME cells resulted in a loss of H(+) currents in patch clamp recordings. Cloning of the open reading frame of HVCN1 from primary human airway epithelia resulted in a wild-type clone and a clone characterized by two sequential base exchanges (452T>C and 453G>A) resulting in a novel missense mutation, M91T HVCN1. Out of 95 human genomic DNA samples that were tested, we found one HVCN1 allele that was heterozygous for the M91T mutation. The activation of acid secretion in epithelia that natively expressed M91T HVCN1 required approximately 0.5 pH units more alkaline mucosal pH values compared with wild-type epithelia. Similarly, activation of H(+) currents across recombinantly expressed M91T HVCN1 required significantly larger pH gradients compared with wild-type HVCN1. This study provides both functional and molecular indications that the HVCN1 H(+) channel mediates pH-regulated acid secretion by the airway epithelium. These data indicate that apical HVCN1 represents a mechanism to acidify an alkaline airway surface liquid.

Differential activation of the HCO(3)(-) conductance through the cystic fibrosis transmembrane conductance regulator anion channel by genistein and forskolin in murine duodenum

BACKGROUND AND PURPOSE

Many cystic fibrosis (CF)-associated mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) anion channels affect CFTR-activated HCO(3)(-) transport more than Cl(-) transport. Targeting the CFTR HCO(3)(-) conductance, if possible, may therefore be of major therapeutic benefit. In the present study, we examined the effects of genistein and forskolin on duodenal mucosal HCO(3)(-) and Cl(-) secretion.

EXPERIMENTAL APPROACH

Murine duodenal mucosal HCO(3)(-) and Cl(-) secretions were examined in vitro in Ussing chambers by the pH stat and short circuit current (I(sc)) techniques.

KEY RESULTS

Genistein markedly stimulated duodenal HCO(3)(-) secretion and I(sc) in a dose-dependent manner in CFTR wild-type mice, but not in CFTR null mice. CFTR(inh)-172, a highly specific CFTR inhibitor, inhibited genistein-stimulated duodenal HCO(3)(-) secretion and I(sc) in wild-type mice. Genistein induced 59% net HCO(3)(-) increase and 123% net I(sc) increase over basal value, whereas forskolin, an activator of adenylate cyclase, induced 94% net HCO(3)(-) increase and 507% net I(sc) increase, indicating that, compared with forskolin, genistein induced a relatively high HCO(3)(-)/I(sc) ratio. Further data showed that CFTR HCO(3)(-)/Cl(-) conductance ratio was 1.05 after genistein stimulation, whereas after forskolin stimulation, the CFTR HCO(3)(-)/Cl(-) conductance ratio was 0.27.

CONCLUSIONS AND IMPLICATIONS

Genistein stimulates duodenal HCO(3)(-) and Cl(-) secretion through CFTR, and has a relatively high selectivity for the CFTR HCO(3)(-) conductance, compared with forskolin. This may indicate the feasibility of selective targeting of the HCO(3)(-) conductance of the CFTR channels.

Regulation of mammalian ciliary beating

Recent advances in our understanding of the structure-function relationship of motile cilia with the 9 + 2 microtubular arrangement have helped explain some of the mechanisms of ciliary beat regulation by intracellular second messengers. These second messengers include cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) as well as calcium and pH. cAMP activates protein kinase A (PKA), which is localized to the axoneme. The cAMP-dependent phosphorylation of PKA's main target, originally described as p29 in Paramecium, seems to increase ciliary beat frequency (CBF) directly. The mechanism by which cGMP increases CBF is less well defined but involves protein kinase G and possibly PKA. Protein kinase C inhibits ciliary beating. The regulation mechanisms of CBF by calcium remain somewhat controversial, favoring an immediate, direct action of calcium on ciliary beating and a second cyclic nucleotide-dependent phase. Finally, intracellular pH likely affects CBF through direct influences on dynein arms.

Ion and liquid transport across the bronchiolar epithelium

The proper homeostasis of the airway surface liquid (ASL) depends on transepithelial ion and fluid transport and is critically important for lung defence, and more specifically for mucociliary transport. In cystic fibrosis (CF), abnormal ion and fluid transport lead to depleted ASL volume resulting in mucus plugs and recurrent lung infections. Like bronchi, human bronchioles exhibit amiloride-sensitive Na(+) absorption and cyclic-AMP and Ca(2+)-activated Cl(-) secretion. However, cyclic-AMP-stimulated Cl(-) and fluid secretion appears to be quantitatively more important in bronchioles than in bronchi. In CF bronchioles, like in CF bronchi, the ASL height is reduced because of an abnormally persistent Na(+) absorption, combined with a lacking CFTR-dependent Cl(-) secretion. The precocity and severity of the bronchiolar disease in CF could be attributed in part to the more important role of CFTR-dependent Cl(-) secretion and fluid secretion, and the lack of compensatory ATP-driven Cl(-) secretion and fluid secretion, in bronchioles compared to bronchi.

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.

Soluble mediators, not cilia, determine airway surface liquid volume in normal and cystic fibrosis superficial airway epithelia

A key aspect of the lung's innate defense system is the ability of the superficial epithelium to regulate airway surface liquid (ASL) volume to maintain a 7-mum periciliary liquid layer (PCL), which is required for cilia to beat and produce mucus flow. The mechanisms whereby airway epithelia regulate ASL height to >or=7 microm are poorly understood. Using bumetanide as an inhibitor of Cl- secretion, and nystatin as an activator of Na+ absorption, we found that a coordinated "blending" of both Cl- secretion and Na+ absorption must occur to effect ASL volume homeostasis. We then investigated how ASL volume status is regulated by the underlying epithelia. Cilia were not critical to this process as (a) ASL volume was normal in cultures from patients with primary ciliary dyskinesia with immotile cilia, and (b) in normal cultures that had not yet undergone ciliogenesis. However, we found that maneuvers that mimic deposition of excess ASL onto the proximal airways, which occurs during mucociliary clearance and after glandular secretion, acutely stimulated Na+ absorption, suggesting that volume regulation was sensitive to changes in concentrations of soluble mediators in the ASL rather than alterations in ciliary beating. To investigate this hypothesis further, we added potential "soluble mediators" to the ASL. ASL volume regulation was sensitive to a channel-activating protein (CAP; trypsin) and a CAP inhibitor (aprotinin), which regulated Na+ absorption via changes in epithelial Na+ channel (ENaC) activity in both normal and cystic fibrosis cultures. ATP was also found to acutely regulate ASL volume by inducing secretion in normal and cystic fibrosis (CF) cultures, while its metabolite adenosine (ADO) evoked secretion in normal cultures but stimulated absorption in CF cultures. Interestingly, the amount of ASL/Cl- secretion elicited by ATP/ADO was influenced by the level of CAP-induced Na+ absorption, suggesting that there are important interactions between the soluble regulators which finely tune ASL volume.

Ion and Fluid Transport Properties of Small Airways in Cystic Fibrosis

RATIONALE

Small airways constitute a major site of pathology in cystic fibrosis (CF) and provide most of the surface area of the conducting airways of the lung. Little is known, however, about the impact of CF on ion and fluid transport in small (bronchiolar) airways.

OBJECTIVES

To describe the ion and fluid transport properties of CF bronchiolar epithelium.

METHODS

Primary cultures of human bronchial and bronchiolar (non-CF and CF) epithelial cells were obtained. The bioelectric properties were studied in Ussing chambers and the airway surface liquid (ASL) height was measured with confocal microscopy.

MAIN RESULTS

Primary cultures of DeltaF508 CF bronchiolar epithelial cells displayed higher transepithelial resistance than non-CF cultures, whereas baseline short circuit current and amiloride-inhibitable short circuit current were similar in both preparations. The ASL height was significantly smaller in CF compared with non-CF preparations. In the presence of amiloride, addition of forskolin increased short circuit current in non-CF but not in CF bronchiolar cultures, and the ATP-induced increase in short circuit current was lower in CF than in non-CF cultures. Non-CF bronchiolar preparations displayed larger short circuit current and fluid secretion in responses to forskolin than non-CF bronchial preparations, suggesting that CFTR-dependent Cl(-) transport may play a more important role in the regulation of fluid transport in small airways than in large airways.

CONCLUSION

In CF small airways, defective Cl(-) secretion combined with unregulated (persistent) Na(+) absorption results in ASLdepletion.

Epithelial carbonic anhydrases facilitate PCO2 and pH regulation in rat duodenal mucosa

The duodenum is the site of mixing of massive amounts of gastric H+ with secreted HCO3-, generating CO2 and H2O accompanied by the neutralization of H+. We examined the role of membrane-bound and soluble carbonic anhydrases (CA) by which H+ is neutralized, CO2 is absorbed, and HCO3- is secreted. Rat duodena were perfused with solutions of different pH and PCO2 with or without a cell-permeant CA inhibitor methazolamide (MTZ) or impermeant CA inhibitors. Flow-through pH and PCO2 electrodes simultaneously measured perfusate and effluent pH and PCO2. High CO2 (34.7 kPa) perfusion increased net CO2 loss from the perfusate compared with controls (pH 6.4 saline, PCO2 approximately 0) accompanied by portal venous (PV) acidification and PCO2 increase. Impermeant CA inhibitors abolished net perfusate CO2 loss and increased net HCO3- gain, whereas all CA inhibitors inhibited PV acidification and PCO2 increase. The changes in luminal and PV pH and [CO2] were also inhibited by the Na+-H+ exchanger-1 (NHE1) inhibitor dimethylamiloride, but not by the NHE3 inhibitor S3226. Luminal acid decreased total CO2 output and increased H+ loss with PV acidification and PCO2 increase, all inhibited by all CA inhibitors. During perfusion of a 30% CO2 buffer, loss of CO2 from the lumen was CA dependent as was transepithelial transport of perfused 13CO2. H+ and CO2 loss from the perfusate were accompanied by increases of PV H+ and tracer CO2, but unchanged PV total CO2, consistent with CA-dependent transmucosal H+ and CO2 movement. Inhibition of membrane-bound CAs augments the apparent rate of net basal HCO3- secretion. Luminal H+ traverses the apical membrane as CO2, is converted back to cytosolic H+, which is extruded via NHE1. Membrane-bound and cytosolic CAs cooperatively facilitate secretion of HCO3- into the lumen and CO2 diffusion into duodenal mucosa, serving as important acid-base regulators.

Apical ammonium inhibition of cAMP-stimulated secretion in T84 cells is bicarbonate dependent

Normal human colonic luminal (NH(4)(+)) concentration ([NH(4)(+)]) ranges from approximately 10 to 100 mM. However, the nature of the effects of NH(4)(+) on transport, as well as NH(4)(+) transport itself, in colonic epithelium is poorly understood. We elucidate here the effects of apical NH(4)(+) on cAMP-stimulated Cl(-) secretion in colonic T84 cells. In HEPES-buffered solutions, 10 mM apical NH(4)(+) had no significant effect on cAMP-stimulated current. In contrast, 10 mM apical NH(4)(+) reduced current within 5 min to 61 +/- 4% in the presence of 25 mM HCO(3)(-). Current inhibition was not simply due to an increase in extracellular K(+)-like cations, in that the current magnitude was 95 +/- 5% with 10 mM apical K(+) and 46 +/- 3% with 10 mM apical NH(4)(+) relative to that with 5 mM apical K(+). We previously demonstrated that inhibition of Cl(-) secretion by basolateral NH(4)(+) occurs in HCO(3)(-)-free conditions and exhibits anomalous mole fraction behavior. In contrast, apical NH(4)(+) inhibition of current in HCO(3)(-) buffer did not show anomalous mole fraction behavior and followed the absolute [NH(4)(+)] in K(+)-NH(4)(+) mixtures, where K(+) concentration + [NH(4)(+)] = 10 mM. The apical NH(4)(+) inhibitory effect was not prevented by 100 microM methazolamide, suggesting no role for apical carbonic anhydrase. However, apical NH(4)(+) inhibition of current was prevented by 10 min of pretreatment of the apical surface with 500 microM DIDS, 100 microM 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS), or 25 microM niflumic acid, suggesting a role for NH(4)(+) action through an apical anion exchanger. mRNA and protein for the apical anion exchangers SLC26A3 [downregulated in adenoma (DRA)] and SLC26A6 [putative anion transporter (PAT1)] were detected in T84 cells by RT-PCR and Northern and Western blots. DRA and PAT1 appear to associate with CFTR in the apical membrane. We conclude that the HCO(3)(-) dependence of apical NH(4)(+) inhibition of secretion is due to the action of NH(4)(+) on an apical anion exchanger.

Rapid effects of dexamethasone on intracellular pH and Na+/H+ exchanger activity in human bronchial epithelial cells

Glucocorticoids have been shown to produce rapid nongenomic responses in airway epithelia. By using an intracellular pH (pH(i)) spectrofluorescence imaging system and the NH4Cl acid-loading technique, we have shown that the synthetic glucocorticoid,dexamethasone, accelerated intracellular pH recovery after an acid load in a human bronchial epithelial cell line (16HBE14o- cells). Exposure to NH4Cl (20 mm) elicited an intracellular acidification, followed by a pH(i) recovery. Inhibition of the Na+/H+ exchanger decreased the steady-state pH(i) and antagonized the dexamethasone stimulation of pH(i) regulation. The rapid effect of dexamethasone on pH(i) was neither affected by the inhibitor of transcription, cycloheximide, nor by the classical glucocorticoid and mineralocorticoid receptors antagonists, RU486 and spironolactone, respectively. The dexamethasone effect on pH(i) regulation was reduced by inhibitors of adenylate cyclase, cAMP-dependent protein kinase and mitogenactivated protein kinase (ERK1/2). By using a PepTag assay system and Western blotting, we have shown that dexamethasone stimulated cAMP-dependent protein kinase and mitogen-activated protein kinase activities. Taken together our results provide evidence for the rapid stimulation of Na+/H+ exchange activity by glucocorticoids in bronchial epithelial cells via a nongenomic mechanism involving cAMP-dependent protein kinase and mitogen-activated protein kinase ERK1/2 pathways.

Chloride conductance of CFTR facilitates basal Cl-/HCO3- exchange in the villous epithelium of intact murine duodenum

Villi of the proximal duodenum are situated for direct exposure to gastric acid chyme. However, little is known about active bicarbonate secretion across villi that maintains the protective alkaline mucus barrier, a process that may be compromised in cystic fibrosis (CF), i.e., in the absence of a functional CF transmembrane conductance regulator (CFTR) anion channel. We investigated Cl(-)/HCO(3)(-) exchange activity across the apical membrane of epithelial cells located at the midregion of villi in intact duodenal mucosa from wild-type (WT) and CF mice using the pH-sensitive dye BCECF. Under basal conditions, the Cl(-)/HCO(3)(-) exchange rate was reduced by approximately 35% in CF compared with WT villous epithelium. Cl(-)/HCO(3)(-) exchange in WT and CF villi responded similarly to inhibitors of anion exchange, and membrane depolarization enhanced rates of Cl(-)(out)/HCO(3)(-)(in) exchange in both epithelia. In anion substitution studies, anion(in)/HCO(3)(-)(out) exchange rates were greater in WT epithelium using Cl(-) or NO(3)(-), but decreased to the level of the CF epithelium using the CFTR-impermeant anion, SO(4)(2-). Similarly, treatment of WT epithelium with the CFTR-selective blocker glybenclamide decreased the Cl(-)/HCO(3)(-) exchange rate to the level of CF epithelium. The mRNA expression of Slc26a3 (downregulated in adenoma) and Slc26a6 (putative anion exchanger-1) was similar between WT and CF duodena. From these studies of murine duodenum, we conclude 1) characteristics of Cl(-)/HCO(3)(-) exchange in the villous epithelium are most consistent with Slc26a6 activity, and 2) Cl(-) channel activity of CFTR facilitates apical membrane Cl(-)(in)/HCO(3)(-)(out) exchange by providing a Cl(-) "leak" under basal conditions.

Cystic fibrosis gene mutation reduces epithelial cell acidification and injury in acid-perfused mouse duodenum

BACKGROUND & AIMS

Dysfunction of the cystic fibrosis transmembrane regulator (CFTR) is associated with diminished duodenal HCO3- secretion, despite a reported lack of clinical duodenal ulceration in affected subjects. We hypothesized that duodenal epithelial cells expressing a mutant CFTR have enhanced resistance to acid-induced injury. To test this hypothesis, we measured duodenal epithelial cell intracellular pH (pHi), injury, and acid back-diffusion in response to a luminal acid challenge in transgenic mice.

METHODS

A murine colony was established for the CFTR DeltaF508 (DeltaF) mutation. Epithelial cell pH i was measured by microscopy with a trapped, fluorescent pH-sensitive dye in living C57BL/6 and DeltaF/DeltaF, +/DeltaF, and +/+ mice. In vivo confocal microscopy confirmed the localization of the dye in the cytoplasm of the epithelial cells. Epithelial injury was measured fluorometrically using propidium iodide. Duodenal epithelial bicarbonate secretion and proton permeability were measured by back-titration. Bicarbonate secretion and acid back-diffusion were measured in a perfused duodenal loop.

RESULTS

Basal and post-acid exposure bicarbonate secretion were reduced in DeltaF/DeltaF mice, although acid back-diffusion was similar to controls. Epithelial pHi of CFTR DeltaF/DeltaF mice during luminal acid exposure was significantly higher than pHi in +/DeltaF, +/+, or C57BL/6 mice. Acid-related epithelial injury was markedly less in DeltaF/DeltaF mice in comparison with the other groups.

CONCLUSIONS

Increased cellular buffering power of the epithelial cells of DeltaF/DeltaF mice likely protects against acidification and injury during acid exposure. We speculate that this protective mechanism partially underlies the perceived relative lack of peptic ulceration in patients affected by cystic fibrosis.

Regulation of human airway ciliary beat frequency by intracellular pH

pHi affects a number of cellular functions, but the influence of pHi on mammalian ciliary beat frequency (CBF) is not known. CBF and pHi of single human tracheobronchial epithelial cells in submerged culture were measured simultaneously using video microscopy (for CBF) and epifluorescence microscopy with the pH-sensitive dye BCECF. Baseline CBF and pHi values in bicarbonate-free medium were 7.2 +/- 0.2 Hz and 7.49 +/- 0.02, respectively (n = 63). Alkalization by ammonium pre-pulse to pHi 7.78 +/- 0.02 resulted in a 2.2 +/- 0.1 Hz CBF increase (P < 0.05). Following removal of NH4Cl, pHi decreased to 7.24 +/- 0.02 and CBF to 5.8 +/- 0.1 Hz (P < 0.05). Removal of extracellular CO2 to change pHi resulted in similar CBF changes. Pre-activation of cAMP-dependent protein kinase (10 microM forskolin), broad inhibition of protein kinases (100 microM H-7), inhibition of PKA (10 microM H-89), nor inhibition of phosphatases (10 microM cyclosporin + 1.5 microM okadaic acid) changed pHi-mediated changes in CBF, nor were they due to [Ca2+]i changes. CBF of basolaterally permeabilized human tracheobronchial cells, re-differentiated at the air-liquid interface, was 3.9 +/- 0.3, 5.7 +/- 0.4, 7.0 +/- 0.3 and 7.3 +/- 0.3 Hz at basolateral i.e., intracellular pH of 6.8, 7.2, 7.6 and 8.0, respectively (n = 18). Thus, intracellular alkalization stimulates, while intracellular acidification attenuates human airway CBF. Since phosphorylation and [Ca2+]i changes did not seem to mediate pHi-induced CBF changes, pHi may directly act on the ciliary motile machinery.

NADPH oxidase-dependent acid production in airway epithelial cells

The purpose of this study was to determine the role of NADPH oxidase in H(+) secretion by airway epithelia. In whole cell patch clamp recordings primary human tracheal epithelial cells (hTE) and the human serous gland cell line Calu-3 expressed a functionally similar zinc-blockable plasma membrane H(+) conductance. However, the rate of H(+) secretion of confluent epithelial monolayers measured in Ussing chambers was 9-fold larger in hTE compared with Calu-3. In hTE H(+) secretion was blocked by mucosal ZnCl(2) and the NADPH oxidase blockers acetovanillone and 4-(2-aminoethyl)benzenesulfonyl fluoride (AEBSF), whereas these same blockers had no effect in Calu-3. We determined levels of transcripts for the NADPH oxidase transmembrane isoforms (Nox1 through -5, Duox1 and -2, and p22(phox)) and found Duox1, -2, and p22(phox) to be highly expressed in hTE, as well as the intracellular subunits p40(phox), p47(phox), and p67(phox). In contrast, Calu-3 lacked transcripts for Duox1, p40(phox), and p47(phox). Anti-Duox antibody staining resulted in prominent apical staining in hTE but no significant staining in Calu-3. When treated with amiloride to block the Na(+)/H(+) exchanger, intracellular pH in hTE acidified at significantly higher rates than in Calu-3, and treatment with AEBSF blocked acidification. These data suggest a role for an apically located Duox-based NADPH oxidase during intracellular H(+) production and H(+) secretion, but not in H(+) conduction.

Characterization of the adenosinetriphosphatase and transport activities of purified cystic fibrosis transmembrane conductance regulator

The cystic fibrosis transmembrane conductance regulator (CFTR) functions in vivo as a cAMP-activated chloride channel. A member of the ATP-binding cassette superfamily of membrane transporters, CFTR contains two transmembrane domains (TMDs), two nucleotide-binding domains (NBDs), and a regulatory (R) domain. It is presumed that CFTR couples ATP hydrolysis to channel gating, and as a first step in addressing this issue directly, we have established conditions for purification of biochemical quantities of human CFTR expressed in Sf9 insect cells. Use of an 8-azido[alpha-(32)P]ATP-binding and vanadate-trapping assay allowed us to devise conditions to preserve CFTR function during purification of a C-terminal His(10)-tagged variant after solubilization with lysophosphatidylglycerol (1%) and diheptanoylphosphatidylcholine (0.3%) in the presence of excess phospholipid. Study of purified and reconstituted CFTR showed that it binds nucleotide with an efficiency comparable to that of P-glycoprotein and that it hydrolyzes ATP at rates sufficient to account for presumed in vivo activity [V(max) of 58 +/- 5 nmol min(-1) (mg of protein)(-1), K(M)(MgATP) of 0.15 mM]. In further work, we found that neither nucleotide binding nor ATPase activity was altered by phosphorylation (using protein kinase A) or dephosphorylation (with protein phosphatase 2B); we also observed inhibition (approximately 40%) of ATP hydrolysis by reduced glutathione but not by DTT. To evaluate CFTR function as an anion channel, we introduced an in vitro macroscopic assay based on the equilibrium exchange of proteoliposome-entrapped radioactive tracers. This revealed a CFTR-dependent transport of (125)I that could be inhibited by known chloride channel blockers; no significant CFTR-dependent transport of [alpha-(32)P]ATP was observed. We conclude that heterologous expression of CFTR in Sf9 cells can support manufacture and purification of fully functional CFTR. This should aid in further biochemical characterization of this important molecule.

Abnormal surface liquid pH regulation by cultured cystic fibrosis bronchial epithelium

Cystic fibrosis (CF) transmembrane conductance regulator (CFTR)-dependent airway epithelial bicarbonate transport is hypothesized to participate in airway surface liquid pH regulation and contribute to lung defense. We measured pH and ionic composition in apical surface liquid (ASL) on polarized normal (NL) and CF primary bronchial epithelial cell cultures under basal conditions, after cAMP stimulation, and after challenge with luminal acid loads. Under basal conditions, CF epithelia acidified ASL more rapidly than NL epithelia. Two ASL pH regulatory paths that contributed to basal pH were identified in the apical membrane of airway epithelia, and their activities were measured. We detected a ouabain-sensitive (nongastric) H+,K+-ATPase that acidified ASL, but its activity was not different in NL and CF cultures. We also detected the following evidence for a CFTR-dependent HCO3- secretory pathway that was defective in CF: (i). ASL [HCO3-] was higher in NL than CF ASL; (ii). activating CFTR with forskolin/3-isobutyl-1-methylxanthine alkalinized NL ASL but acidified CF ASL; and (iii). NL airway epithelia more rapidly and effectively alkalinized ASL in response to a luminal acid challenge than CF epithelia. We conclude that cultured human CF bronchial epithelial pHASL is abnormally regulated under basal conditions because of absent CFTR-dependent HCO3- secretion and that this defect can lead to an impaired capacity to respond to airway conditions associated with acidification of ASL.

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.

Cellular mechanisms for apical ATP effects on intracellular pH in human bronchial epithelium

The effect of external ATP on intracellular pH (pH(i)) was investigated using a pH imaging system in a human bronchial epithelial cell line (16HBE14o-) loaded with BCECF-AM. The steady-state pH(i) of 16HBE14o- epithelial monolayers was 7.137 +/- 0.027 (n = 46). Apical addition of ATP (10(-4) M) to epithelial monolayers induced a rapid and sustained pH(i) decrease of 0.164 +/- 0.024 pH units (n = 17; P < 0.001). The intracellular acidification was rapidly reversed upon removal of external ATP. In contrast, the non-hydrolysable ATP analogue AMP-PNP did not produce any significant change in pH(i). Inhibition of purinoreceptors by suramin did not affect the acidification induced by apical ATP. Inhibition of Na+-H+ exchange by apical Na+ removal or addition of amiloride (0.5 mM) reduced the apical ATP-induced pH(i) decrease, suggesting the involvement of a Na+-H+ exchanger or surface pH effects on the ATP-induced pH(i) response. Inhibitors of proton channels such as ZnCl2 (10(-4) M) also partially inhibited the ATP response. The pH(i) response to ATP was dependent on the external pH (pH(o)), with increasing acidification produced at lower pH(o) values. Neither the basal pH(i) nor the ATP-induced intracellular acidification was affected by thapsigargin (a Ca2+-ATPase inhibitor), chelerythrine chloride (a protein kinase C (PKC) inhibitor), RpcAMP (a protein kinase A (PKA) inhibitor) or PMA (a PKC activator). Therefore, the intracellular acidification of human bronchial epithelial cells induced by apical ATP does not involve signalling via Ca2+, PKC or PKA nor binding to a purinoreceptor. We interpret the effect of ATP to produce an intracellular acidification as a three step process: activation of H+ channels, inhibition of Na+-H+ exchange and influx of protonated ATP.

Acid secretion and proton conductance in human airway epithelium

Acid secretion and proton conductive pathways across primary human airway surface epithelial cultures were investigated with the pH stat method in Ussing chambers and by single cell patch clamping. Cultures showed a basal proton secretion of 0.17 +/- 0.04 micromol.h(-1).cm(-2), and mucosal pH equilibrated at 6.85 +/- 0.26. Addition of histamine or ATP to the mucosal medium increased proton secretion by 0.27 +/- 0.09 and 0.24 +/- 0.09 micromol.h(-1).cm(-2), respectively. Addition of mast cells to the mucosal medium of airway cultures similarly activated proton secretion. Stimulated proton secretion was similar in cultures bathed mucosally with either NaCl Ringer or ion-free mannitol solutions. Proton secretion was potently blocked by mucosal ZnCl(2) and was unaffected by mucosal bafilomycin A(1), Sch-28080, or ouabain. Mucosal amiloride blocked proton secretion in tissues that showed large amiloride-sensitive potentials. Proton secretion was sensitive to the application of transepithelial current and showed outward rectification. In whole cell patch-clamp recordings a strongly outward-rectifying, zinc-sensitive, depolarization-activated proton conductance was identified with an average chord conductance of 9.2 +/- 3.8 pS/pF (at 0 mV and a pH 5.3-to-pH 7.3 gradient). We suggest that inflammatory processes activate proton secretion by the airway epithelium and acidify the airway surface liquid.

Properties of cytotoxic peptide-formed ion channels

Cytotoxic peptides are relatively small cationic molecules such as those found 1) in venoms, e.g., melittin in bee, scorpion toxins in scorpion, pilosulin 1 in jumper ant, and lycotoxin I and II in wolf spider; 2) in skin secretions (e.g., magainin I and II from Xenopus laevis, dermaseptin from frog, antimicrobials from carp) and cells of the immune system (e.g., insect, scorpion, and mammalian defensins and cryptdins); 3) as autocytotoxicity peptides, e.g., amylin cytotoxic to pancreatic beta-cells, prion peptide fragment 106-126 [PrP-(106-126)], and amyloid beta-protein (AbetaP) cytotoxic to neurons; and 4) as designed synthetic peptides based on the sequences and properties of naturally occurring cytotoxic peptides. The small cytotoxic peptides are composed of beta-sheets, e.g., mammalian defensins, AbetaP, amylin, and PrP-(106-126), whereas the larger cytotoxic peptides have several domains composed of both alpha-helices and beta-sheets stabilized by cysteine bonds, e.g., scorpion toxins, scorpion, and insect defensins. Electrophysiological and molecular biology techniques indicate that these structures modify cell membranes via 1) interaction with intrinsic ion transport proteins and/or 2) formation of ion channels. These two nonexclusive mechanisms of action lead to changes in second messenger systems that further augment the abnormal electrical activity and distortion of the signal transduction causing cell death.

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?".

Proton pump activity of mitochondria-rich cells. The interpretation of external proton-concentration gradients

We have hypothesized that a major role of the apical H(+)-pump in mitochondria-rich (MR) cells of amphibian skin is to energize active uptake of Cl- via an apical Cl-/HCO3(-)-exchanger. The activity of the H+ pump was studied by monitoring mucosal [H+]-profiles with a pH-sensitive microelectrode. With gluconate as mucosal anion, pH adjacent to the cornified cell layer was 0.98 +/- 0.07 (mean +/- SEM) pH-units below that of the lightly buffered bulk solution (pH = 7.40). The average distance at which the pH-gradient is dissipated was 382 +/- 18 microns, corresponding to an estimated "unstirred layer" thickness of 329 +/- 29 microns. Mucosal acidification was dependent on serosal pCO2, and abolished after depression of cellular energy metabolism, confirming that mucosal acidification results from active transport of H+. The [H+] was practically similar adjacent to all cells and independent of whether the microelectrode tip was positioned near an MR-cell or a principal cell. To evaluate [H+]-profiles created by a multitude of MR-cells, a mathematical model is proposed which assumes that the H+ distribution is governed by steady diffusion from a number of point sources defining a set of particular solutions to Laplace's equation. Model calculations predicted that with a physiological density of MR cells, the [H+] profile would be governed by so many sources that their individual contributions could not be experimentally resolved. The flux equation was integrated to provide a general mathematical expression for an external standing [H+]-gradient in the unstirred layer. This case was treated as free diffusion of protons and proton-loaded buffer molecules carrying away the protons extruded by the pump into the unstirred layer; the expression derived was used for estimating stationary proton-fluxes. The external [H+]-gradient depended on the mucosal anion such as to indicate that base (HCO3-) is excreted in exchange not only for Cl-, but also for Br- and I-, indicating that the active fluxes of these anions can be attributed to mitochondria-rich cells.

HCO3-dependent pHi regulation in tracheal epithelial cells

Regulation of intracellular pH (pHi) was studied in cultured bovine tracheal epithelial cells using microspectrofluorimetry of the fluorescent indicator 2',7'-biscarboxyethyl- 5(6)-carboxyfluorescein (BCECF). The cells, which were grown on coverslips and superfused in a chamber on the stage of a microscope, were acidified by NH4Cl-prepulses, and pHi recovery was measured (in DeltapH/min) at approximately pHi 6.7. In HCO3-free solutions the recovery rate was 0.14 pH/min, and addition of amiloride or Na-free solution reduced this rate to 0.02-0.03 pH/min. In HCO3/CO2-buffered Ringer's, the rate of recovery was 0.32 pH/min, and amiloride or Na-free reduced the rate to 0.08-0.10 pH/min. This residual Na-independent and HCO3-dependent pHi recovery was studied by using inhibitors of HCO3 and H transporters. Bafilomycin (inhibits H-ATPases) at 100 nM did not significantly affect pHi recovery, while 100 microM SCH28080 (inhibits H,K-ATPase) had a variable inhibitory effect (25-75%), indicating that a gastric-like H, K-ATPase, but not electrogenic H pump, may contribute in a minor way to the recovery from acidification. Cl-free solution and 500 microM H2DIDS (dihydro-4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, blocks anion exchange and the outwardly rectifying Cl channel, ORCC), both blocked apparent anion exchange activity, but had no effect on the recovery; 100 microM DNDS (4-4''-dinitro-2-2'-stilbenedisulfonate blocks the ORCC but not the cystic fibrosis transmembrane conductance regulator, CFTR) had no effect on pHi recovery; DPC (diphenylamine carboxylate, blocks the CFTR and the ORCC) caused a complete and reversible inhibition of the recovery. When [K] was increased ten fold to depolarize the cell's membrane potential, the magnitude of the pHi recovery (though not the rate) was enhanced. Thus, the HCO3-dependent, Na- and Cl-independent, DPC-blockable pHi recovery may be largely due to an influx of HCO3 via CFTR Cl channels. Under physiological conditions, when the electrochemical gradient for HCO3 is likely to be outwardly rather than inwardly directed, the CFTR (or another HCO3-permeable channel) may mediate HCO3 secretion and contribute to regulation of pH of the periciliary fluid.

Cross-talk between ATP-regulated K+ channels and Na+ transport via cellular metabolism in frog skin principal cells

Isolated frog skin epithelium, mounted in an Ussing chamber and bathed in standard NaCl Ringer solution, recycles K+ across the basolateral membrane of principal cells through an inward-rectifier K+ channel (Kir) operating in parallel with a Na+-K+-ATPase pump. Here we report on the metabolic control of the Kir channel using patch clamping, short-circuit current measurement and enzymatic determination of cellular (ATP (ATPi). 2. The constitutively active Kir channel in the basolateral membrane has the characteristics of an ATP-regulated K+ channel and is now classed as a KATP channel. In excised inside-out patches the open probability (Po) of KATP channels was reduced by ATPi with half-maximum inhibition at an ATPi concentration of 50 microM. 3. ATPi measured (under normal Na+ transport conditions) with luciferin-luciferase was 1.50 +/- 0.23 mM (mean +/- S.E.M.; range, 0.4-3.3 mM n = 11). Thus the KATP channel would be expected to be inactive in intact cells if ATPi was the sole regulator of channel activity. KATP channels which were inactivated by 1 mM ATPi in excised patches could be reactivated by addition of 100 microM ADP on the cytosolic side. When added alone, ADP blocks this channel with half-maximal inhibition at [ADPi] > 5 mM. 4. Sulphonylureas inhibit single KATP channels in cell-attached patches as well as the total basolateral K+ current measured in frog skin epithelia perforated with nystatin on the apical side. 5. Na+-K+-ATPase activity is a major determinant of cytosolic ATP. Blocking the pump activity with ouabain produced a time-dependent increase in ATPi and reduced the open probability of KATP channels in cell-attached membranes. 6. We conclude that the ratio of ATP/ADP is an important metabolic coupling factor between the rate of Na+-K+ pumping and K+ recycling.