The role of the basolateral outwardly rectifying chloride channel in human airway epithelial anion secretion

Phospholipase C controls chloride-dependent short-circuit current in human bronchial epithelial cells

Chloride secretion by airway epithelial cells is primordial for water and ion homeostasis and airways surface prevention of infections. This secretion is impaired in several human diseases, including cystic fibrosis, a genetic pathology due to CFTR gene mutations leading to chloride channel defects. A potential therapeutic approach is aiming at increasing chloride secretion either by correcting the mutated CFTR itself or by stimulating non-CFTR chloride channels at the plasma membrane. Here, we studied the role of phospholipase C in regulating the transepithelial chloride secretion in human airway epithelial 16HBE14o- and CFBE cells over-expressing wild type (WT)- or F508del-CFTR. Western blot analysis shows expression of the three endogenous phospholipase C (PLC) isoforms, namely, PLCδ1, PLCγ1, and PLCβ3 in 16HBE14o- cells. In 16HBE14o- cells, we performed Ussing chamber experiments after silencing each of these PLC isoforms or using the PLC inhibitor U73122 or its inactive analogue U73343. Our results show the involvement of PLCβ3 and PLCγ1 in CFTR-dependent short-circuit current activated by forskolin, but not of PLCδ1. In CFBE-WT CFTR and corrected CFBE-F508del CFTR cells, PLCβ3 silencing also inhibits CFTR-dependent current activated by forskolin and UTP-activated calcium-dependent chloride channels (CaCC). Our study supports the importance of PLC in maintaining CFTR-dependent chloride secretion over time, getting maximal CFTR-dependent current and increasing CaCC activation in bronchial epithelial cells.

Measurement of Multi Ion Transport through Human Bronchial Epithelial Cell Line Provides an Insight into the Mechanism of Defective Water Transport in Cystic Fibrosis

We measured concentration changes of sodium, potassium, chloride ions, pH and the transepithelial potential difference by means of ion-selective electrodes, which were placed on both sides of a human bronchial epithelial 16HBE14σ cell line grown on a porous support in the presence of ion channel blockers. We found that, in the isosmotic transepithelial concentration gradient of either sodium or chloride ions, there is an electroneutral transport of the isosmotic solution of sodium chloride in both directions across the cell monolayer. The transepithelial potential difference is below 3 mV. Potassium and pH change plays a minor role in ion transport. Based on our measurements, we hypothesize that in a healthy bronchial epithelium, there is a dynamic balance between water absorption and secretion. Water absorption is caused by the action of two exchangers, Na/H and Cl/HCO₃, secreting weakly dissociated carbonic acid in exchange for well dissociated NaCl and water. The water secretion phase is triggered by an apical low volume-dependent factor opening the Cystic Fibrosis Transmembrane Regulator CFTR channel and secreting anions that are accompanied by paracellular sodium and water transport.

Chloride intracellular channel 1 (CLIC1) contributes to modulation of cyclic AMP-activated whole-cell chloride currents in human bronchial epithelial cells

Chloride channels are known to play critical physiological roles in many cell types. Here, we describe the expression of anion channels using RNA Seq in primary cultures of human bronchial epithelial cells (hBECs). Chloride intracellular channel (CLIC) family members were the most abundant chloride channel transcripts, and CLIC1 showed the highest level of expression. In addition, we characterize the chloride currents in hBECs and determine how inhibition of CLIC1 via pharmacological and molecular approaches impacts these. We demonstrate that CLIC1 is able to modulate cyclic AMP‐induced chloride currents and suggest that CLIC1 modulation could be important for chloride homeostasis in this cell type.

Functional and cytometric examination of different human lung epithelial cell types as drug transport barriers

To develop inhaled medications, various cell culture models have been used to examine the transcellular transport or cellular uptake properties of small molecules. For the reproducible high throughput screening of the inhaled drug candidates, a further verification of cell architectures as drug transport barriers can contribute to establishing appropriate in vitro cell models. In the present study, side-by-side experiments were performed to compare the structure and transport function of three lung epithelial cells (Calu-3, normal human bronchial primary cells (NHBE), and NL-20). The cells were cultured on the nucleopore membranes in the air-liquid interface (ALI) culture conditions, with cell culture medium in the basolateral side only, starting from day 1. In transport assays, paracellular transport across all three types of cells appeared to be markedly different with the NHBE or Calu-3 cells, showing low paracellular permeability and high TEER values, while the NL-20 cells showed high paracellular permeability and low TEER. Quantitative image analysis of the confocal microscope sections further confirmed that the Calu-3 cells formed intact cell monolayers in contrast to the NHBE and NL-20 cells with multilayers. Among three lung epithelial cell types, the Calu-3 cell cultures under the ALI condition showed optimal cytometric features for mimicking the biophysical characteristics of in vivo airway epithelium. Therefore, the Calu-3 cell monolayers could be used as functional cell barriers for the lung-targeted drug transport studies.

A spatial model of fluid recycling in the airways of the lung

The genetic disease cystic fibrosis (CF) is a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, and results in viscous mucus and impaired mucociliary clearance leading to chronic recurring pulmonary infections. Although extensive experimental research has been conducted over the last few decades, CF lung pathophysiology remains controversial. There are two competing explanations for the observed depletion of periciliary liquid (PCL) in CF lungs. The low volume hypothesis assumes fluid hyperabsorption through surface epithelia due to an over-active epithelial Na(+) channel (ENaC), and the low secretion hypothesis assumes inspissated mucins secreted from glands due to lack of serous fluid secreted from gland acini. We present a spatial mathematical model that reflects in vivo fluid recycling via submucosal gland (SMG) secretion, and absorption through surface epithelia. We then test the model in CF conditions by increasing ENaC open probability and decreasing SMG flux while simultaneously reducing CFTR open probability. Increasing ENaC activity only results in increased fluid absorption across surface epithelia, as seen in in vitro experiments. However, combining potential CF mechanisms results in markedly less fluid absorbed while providing the largest reduction in PCL volume, suggesting that a compromise in gland fluid secretion dominates over increased ENaC activity to decrease the amount of fluid transported transcellularly in CF lungs in vivo. Model results also indicate that a spatial model is necessary for an accurate calculation of total fluid transport, as the effects of spatial gradients can be severe, particularly in close proximity to the SMGs.

The extracellular microenvironment explains variations in passive drug transport across different airway epithelial cell types

PURPOSE

We sought to identify key variables in cellular architecture and physiology that might explain observed differences in the passive transport properties of small molecule drugs across different airway epithelial cell types.

METHODS

Propranolol (PR) was selected as a weakly basic, model compound to compare the transport properties of primary (NHBE) vs. tumor-derived (Calu-3) cells. Differentiated on Transwell™ inserts, the architecture of pure vs. mixed cell co-cultures was studied with confocal microscopy followed by quantitative morphometric analysis. Cellular pharmacokinetic modeling was used to identify parameters that differentially affect PR uptake and transport across these two cell types.

RESULTS

Pure Calu-3 and NHBE cells possessed different structural and functional properties. Nevertheless, mixed Calu-3 and NHBE cell co-cultures differentiated as stable cell monolayers. After measuring the total mass of PR, the fractional areas covered by Calu-3 and NHBE cells allowed deconvoluting the transport properties of each cell type. Based on the apparent thickness of the unstirred, cell surface aqueous layer, local differences in the extracellular microenvironment explained the measured variations in passive PR uptake and permeation between Calu-3 and NHBE cells.

CONCLUSION

Mixed cell co-cultures can be used to compare the local effects of the extracellular microenvironment on drug uptake and transport across two epithelial cell types.

Ion transporting proteins of human bronchial epithelium

The electrolyte transport system across human airway epithelium followed by water movement is essential for the normal mucociliary clearance that allows the maintenance of the aseptic condition of the respiratory tract. The function of epithelial cells is to control and regulate ionic composition and volume of fluids in the airways. Various types of proteins taking part in assuring effective ions and water transport in apical and basolateral membranes of the airway epithelium have been found (e.g., CFTR, ENaC, CaCC, ORCC, potassium channels, NaKATPase, aquaporins). The paper reviews the current state of the art in the field of ion channels, transporters, and other signaling proteins identified in the human bronchial epithelium.

Evidence for basolateral Cl- channels as modulators of apical Cl- secretion in pulmonary epithelia of Xenopus laevis

Pulmonary epithelia of air-breathing vertebrates are covered by a thin, fluid layer that is essential for immune defense and gas diffusion. The composition of this layer is maintained by ion transport mechanisms, including Cl(-) transport. The present study focuses on the function of basolateral Cl(-) channels in Xenopus pulmonary epithelia, since knowledge concerning this issue is limited. Therefore, Ussing chamber measurements were performed, and transepithelial short-circuit currents (I(SC)) were monitored. Basolateral application of the Cl(-) channel inhibitor N-phenylanthranilic acid (DPC) resulted in an increase of the I(SC), indicating a DPC-sensitive Cl(-) conductance. This observation was confirmed in experiments using an apical-to-basolateral Cl(-) gradient, with and without nystatin (apical side) to permeabilize the epithelia as well as by establishing an iodide gradient. The DPC-sensitive Cl(-) conductance was influenced by procedures interfering with apical Cl(-) secretion. For example, the effect of forskolin was increased when basolateral Cl(-) channels were blocked by the simultaneous application of DPC. Activation of apical Cl(-) secretion by forskolin/IBMX and subsequent DPC application resulted in a significantly reduced DPC effect. Accordingly, DPC led to an increased apical Cl(-) secretion estimated by an increased 5-nitro-2-(3-phenylpropylamino)benzoic acid-sensitive I(SC). Furthermore, inhibition of basolateral anion exchangers responsible for Cl(-) uptake resulted in a decreased DPC-sensitive current. Taken together, we have evidence concerning the function of basolateral Cl(-) channels in Xenopus pulmonary epithelium and that these channels play a significant role in mediating apical Cl(-) secretion involving a novel Cl(-) recycling mechanism across the basolateral membrane.

Regulation of CFTR chloride channel macroscopic conductance by extracellular bicarbonate

The CFTR contributes to Cl⁻ and HCO₃⁻ transport across epithelial cell apical membranes. The extracellular face of CFTR is exposed to varying concentrations of Cl⁻ and HCO₃⁻ in epithelial tissues, and there is evidence that CFTR is sensitive to changes in extracellular anion concentrations. Here we present functional evidence that extracellular Cl⁻ and HCO₃⁻ regulate anion conduction in open CFTR channels. Using cell-attached and inside-out patch-clamp recordings from constitutively active mutant E1371Q-CFTR channels, we show that voltage-dependent inhibition of CFTR currents in intact cells is significantly stronger when the extracellular solution contains HCO₃⁻ than when it contains Cl⁻. This difference appears to reflect differences in the ability of extracellular HCO₃⁻ and Cl⁻ to interact with and repel intracellular blocking anions from the pore. Strong block by endogenous cytosolic anions leading to reduced CFTR channel currents in intact cells occurs at physiologically relevant HCO₃⁻ concentrations and membrane potentials and can result in up to ∼50% inhibition of current amplitude. We propose that channel block by cytosolic anions is a previously unrecognized, physiologically relevant mechanism of channel regulation that confers on CFTR channels sensitivity to different anions in the extracellular fluid. We further suggest that this anion sensitivity represents a feedback mechanism by which CFTR-dependent anion secretion could be regulated by the composition of the secretions themselves. Implications for the mechanism and regulation of CFTR-dependent secretion in epithelial tissues are discussed.

Basolateral chloride current in human airway epithelia

Electrolyte transport by airway epithelia regulates the quantity and composition of liquid covering the airways. Previous data indicate that airway epithelia can absorb NaCl. At the apical membrane, cystic fibrosis transmembrane conductance regulator (CFTR) provides a pathway for Cl(-) absorption. However, the pathways for basolateral Cl(-) exit are not well understood. Earlier studies, predominantly in cell lines, have reported that the basolateral membrane contains a Cl(-) conductance. However, the properties have varied substantially in different epithelia. To better understand the basolateral Cl(-) conductance in airway epithelia, we studied primary cultures of well-differentiated human airway epithelia. The basolateral membrane contained a Cl(-) current that was inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). The current-voltage relationship was nearly linear, and the halide selectivity was Cl(-) > Br(-) >> I(-). Several signaling pathways increased the current, including elevation of cellular levels of cAMP, activation of protein kinase C (PKC), and reduction of pH. In contrast, increasing cell Ca(2+) and inducing cell swelling had no effect. The basolateral Cl(-) current was present in both cystic fibrosis (CF) and non-CF airway epithelia. Likewise, airway epithelia from wild-type mice and mice with disrupted genes for ClC-2 or ClC-3 all showed similar Cl(-) currents. These data suggest that the basolateral membrane of airway epithelia possesses a Cl(-) conductance that is not due to CFTR, ClC-2, or ClC-3. Its regulation by cAMP and PKC signaling pathways suggests that coordinated regulation of Cl(-) conductance in both apical and basolateral membranes may be important in controlling transepithelial Cl(-) movement.

Ca(2+) -independent effects of BAPTA and EGTA on single-channel Cl(-) currents in brown adipocytes

The Cl(-) channels of brown adipocytes electrophysiologically resemble outwardly rectifying Cl(-) channels (ORCC). To study tentative Ca(2+) regulation of these channels, we attempted to control Ca(2+) levels at the cytoplasmic side of the inside-out membrane patches with Ca(2+)-chelating agents. However, we found that the commonly used Ca(2+)-chelators EGTA and BAPTA by themselves influenced the Cl(-) channel currents, unrelated to their calcium chelating effects. Consequently, in this report we delineate effects of Ca(2+)-chelators (acting from the cytoplasmic side) on the single Cl(-) channel currents in patch-clamp experiments. Using fixed (1-2 mM) concentrations of chelators, two types of Cl(-) channels were identified, as discriminated by their reaction to the Ca(2+)-chelators and by their conductance: true-blockage channels (31 pS) and quasi-blockage channels (52 pS). In true-blockage channels, EGTA and BAPTA inhibited channel activity in a classical flickery type manner. In quasi-blockage channels, chelators significantly shortened the duration of individual openings, as in a flickering block, but the overall channel activity tended to increase. This dual effect of mean open time decrease accompanied by a tendency of open probability to increase we termed a quasi-blockage. Despite the complications due to the chelators as such, we could detect a moderate inhibitory effect of Ca(2+). The anionic classical Cl(-) channel blockers DIDS and SITS could mimic the true/quasi blockage of EGTA and BAPTA. It was concluded that at least in this experimental system, standard techniques for Ca(2+) level control in themselves could fundamentally affect the behaviour of Cl(-) channels.

Chloride channels in normal and cystic fibrosis human erythrocyte membrane

Electrophysiological studies on human RBCs have been difficult due to fragility and small size of cells, and little is known of ionic conductive pathways present in the RBC membrane in health and disease. We report on anionic channels in cells of healthy donors (control) and cystic fibrosis (CF) patients. Anion channel activity (8-12 pS, linear) was induced in cell-attached configuration by forskolin (50 microM) and in excised inside-out configuration by PKA (100 nM) and ATP (1 mM) but control and CF RBCs differed by their respective kinetics and gating properties. These channels were permeable to ATP (100 mM, symmetrical Tris-ATP). These data suggest either the existence of two different anionic channel types or regulation of a single channel type either by the CFTR (cystic fibrosis transmembrane regulator) protein or by different cytosolic factors. Another anionic channel type displaying outward rectification (approximately 80 pS, outward conductance) was present in 30% of CF cell patches but was not observed in normal cell patches. The frequently recorded activity of this channel in CF patches suggests a down-regulation in normal RBCs.

Basolateral Cl channels in primary airway epithelial cultures

Salt and water absorption and secretion across the airway epithelium are important for maintaining the thin film of liquid lining the surface of the airway epithelium. Movement of Cl across the apical membrane involves the CFTR Cl channel; however, conductive pathways for Cl movement across the basolateral membrane have been little studied. Here, we determined the regulation and single-channel properties of the Cl conductance ( GCl) in airway surface epithelia using epithelial cultures from human or bovine trachea and freshly isolated ciliated cells from the human nasal epithelium. In Ussing chamber studies, a swelling-activated basolateral GCl was found, which was further stimulated by forskolin and blocked by N-phenylanthranilic acid (DPC) = sucrose > flufenamic acid = niflumic acid = glibenclamide > CdCl2 = 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) = DIDS = ZnCl2 > tamoxifen > 4,4′-dinitro-2,2′-stilbene-disulfonate disodium salt (DNDS). In whole cell patch-clamp experiments, three types of GCl were identified: 1) a voltage-activated, DIDS- (but not Cd-) blockable and osmosensitive GCl; 2) an inwardly rectifying, hyperpolarization-activated and Cd-sensitive GCl; and 3) a forskolin-activated, linear GCl, which was insensitive to Cd and DIDS. In cell-attached patch-clamp recordings, the basolateral pole of isolated ciliated cells expressed three types of Cl channels: 1) an outwardly rectifying, swelling-activated Cl channel; 2) a strongly inwardly rectifying Cl channel; and 3) a forskolin-activated, low-conductance channel. We propose that, depending on the driving force for Cl across the apical membrane, basolateral Cl channels confine Cl− secretion or support transcellular Cl− absorption.

Regulation of basolateral Cl(-) channels in airway epithelial cells: the role of nitric oxide

The presence of basolateral Cl(-) channels in airway epithelium has been reported in several studies, but little is known about their role in the regulation of anion secretion. The purpose of this study was to characterize regulation of these channels by nitric oxide (NO) in Calu-3 cells. Transepithelial measurements revealed that NO donors activated a basolateral Cl(-) conductance sensitive to 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and anthracene-9-carboxylic acid. Apical membrane permeabilization studies confirmed the basolateral localization of NO-activated Cl(-) channels. Experiments using 8-bromo cyclic guanosine monophosphate (8Br-cGMP) and selective inhibitors of soluble guanylyl cyclase and inducible NO synthase (1H-[1, 2, 4] oxadiazolol-[4, 3-a] quinoxalin-1-one [ODQ] and 1400W [N-(3-Aminomethyl)benzyl)acetamidine], respectively) demonstrated that NO activated Cl(-) channels via a cGMP-dependent pathway. Anion replacement and (36)Cl(-) flux studies showed that NO affected both Cl(-) and HCO (3) (-) secretion. Two different types of Cl(-) channels are known to be present in the basolateral membrane of epithelial cells: Zn(2+)-sensitive ClC-2 and DIDS-sensitive bestrophin channels. S-Nitrosoglutathione (GSNO) activated Cl(-) conductance in the presence of Zn(2+) ions, indicating that ClC-2 channel function was not affected by GSNO. In contrast, DIDS completely inhibited GSNO-activated Cl(-) conductance. Bestrophin immunoprecipitation studies showed that under control conditions bestrophin channels were not phosphorylated but became phosphorylated after GSNO treatment. The presence of bestrophin in airway epithelia was confirmed using immunohistochemistry. We conclude that basolateral Cl(-) channels play a major role in the NO-dependent regulation of anion secretion in Calu-3 cells.

Expression of the chloride channel CLC-K in human airway epithelial cells

Airway submucosal gland function is severely disrupted in cystic fibrosis (CF), as a result of genetic mutation of the cystic fibrosis transmembrane conductance regulator (CFTR), an apical membrane Cl(-) channel. To identify other Cl(-) channel types that could potentially substitute for lost CFTR function in these cells, we investigated the functional and molecular expression of Cl(-) channels in Calu-3 cells, a human cell line model of the submucosal gland serous cell. Whole cell patch clamp recording from these cells identified outwardly rectified, pH- and calcium-sensitive Cl(-) currents that resemble those previously ascribed to ClC-K type chloride channels. Using reverse transcription polymerase chain reaction, we identified expression of mRNA for ClC-2, ClC-3, ClC-4, ClC-5, ClC-6, ClC-7, ClC-Ka, and ClC-Kb, as well as the common ClC-K channel beta subunit barttin. Western blotting confirmed that Calu-3 cells express both ClC-K and barttin protein. Thus, Calu-3 cells express multiple members of the ClC family of Cl(-) channels that, if also expressed in native submucosal gland serous cells within the CF lung, could perhaps act to partially substitute lost CFTR function. Furthermore, this work represents the first evidence for functional ClC-K chloride channel expression within the lung.

Cystic fibrosis and airway submucosal glands

The chronic pulmonary infections and inflammation associated with cystic fibrosis (CF) are responsible for almost all the morbidity and mortality of this disease. Our understanding of the mechanisms that underlie the very early stages of CF lung disease, that result directly from mutations in the CF gene, is relatively poor. However, the demonstration that the predominant sites of expression of the CF gene in normal lungs are the submucosal glands, together with the histological observations showing that hyperplasia of these glands and mucin occlusion of the gland ducts are the earliest signs of disease in the CF lung, suggest that malfunction of the submucosal glands may be an important factor contributing to the early pathophysiology of CF lung disease. This review describes the function of submucosal glands in normal lungs, and the way in which their function may be disrupted in CF and may thus contribute to the early stages of CF lung disease.

The role of bestrophin in airway epithelial ion transport

The purpose of this study was to identify Cl- channels in the basolateral membrane of airway epithelial cells at the molecular level. We have focused on a new family of Cl- channels, bestrophins, which have previously been identified in retinal pigment epithelium. RT-PCR, Western blot and confocal microscopy studies revealed the presence of bestrophin in airway epithelial cells. Decreasing bestrophin expression using siRNA resulted in diminished 36Cl- flux. These studies also showed that bestrophin regulation is similar to that of native basolateral Cl- channels. The data indicate that the presence of a functional bestrophin may contribute to the basolateral cell conductance in airway epithelial cells.