Measurement of halide efflux from cultured and primary airway epithelial cells using fluorescence indicators

Understanding CFTR Functionality: A Comprehensive Review of Tests and Modulator Therapy in Cystic Fibrosis

Cystic fibrosis is a genetic disorder inherited in an autosomal recessive manner. It is caused by a mutation in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene on chromosome 7, which leads to abnormal regulation of chloride and bicarbonate ions in cells that line organs like the lungs and pancreas. The CFTR protein plays a crucial role in regulating chloride ion flow, and its absence or malfunction causes the production of thick mucus that affects several organs. There are more than 2000 identified mutations that are classified into seven categories based on their dysfunction mechanisms. In this article, we have conducted a thorough examination and consolidation of the diverse array of tests essential for the quantification of CFTR functionality. Furthermore, we have engaged in a comprehensive discourse regarding the recent advancements in CFTR modulator therapy, a pivotal approach utilized for the management of cystic fibrosis, alongside its concomitant relevance in evaluating CFTR functionality.

Silicate Inhibits the Cytosolic Influx of Chloride in Protoplasts of Wheat and Affects the Chloride Transporters, TaCLC1 and TaNPF2.4/2.5

Chloride is an essential nutrient for plants, but high concentrations can be harmful. Silicon ameliorates both abiotic and biotic stresses in plants, but it is unknown if it can prevent cellular increase of chloride. Therefore, we investigated the influx of Cl⁻ ions in two wheat cultivars different in salt sensitivity, by epifluorescence microscopy and a highly Cl⁻-sensitive dye, MQAE, N-[ethoxycarbonylmethyl]-6-methoxy-quinolinium bromide, in absence and presence of potassium silicate, K₂SiO₃. The Cl⁻-influx was higher in the salt-sensitive cv. Vinjett, than in the salt-tolerant cv. S-24, and silicate pre-treatment of protoplasts inhibited the Cl⁻-influx in both cultivars, but more in the sensitive cv. Vinjett. To investigate if the Cl⁻-transporters TaCLC1 and TaNPF2.4/2.5 are affected by silicate, expression analyses by RT-qPCR were undertaken of TaCLC1 and TaNPF 2.4/2.5 transcripts in the absence and presence of 100 mM NaCl, with and without the presence of K₂SiO₃. The results show that both transporter genes were expressed in roots and shoots of wheat seedlings, but their expressions were differently affected by silicate. The TaNPF2.4/2.5 expression in leaves was markedly depressed by silicate. These findings demonstrate that less chloride accumulates in the cytosol of leaf mesophyll by Si treatment and increases salt tolerance.

Assays of CFTR Function In Vitro, Ex Vivo and In Vivo

Cystic fibrosis, a multi-organ genetic disease, is characterized by abnormal function of the cystic fibrosis transmembrane conductance regulator (CFTR) protein, a chloride channel at the apical membrane of several epithelia. In recent years, therapeutic strategies have been developed to correct the CFTR defect. To evaluate CFTR function at baseline for diagnosis, or the efficacy of CFTR-restoring therapy, reliable tests are needed to measure CFTR function, in vitro, ex vivo and in vivo. In vitro techniques either directly or indirectly measure ion fluxes; direct measurement of ion fluxes and quenching of fluorescence in cell-based assays, change in transmembrane voltage or current in patch clamp or Ussing chamber, swelling of CFTR-containing organoids by secondary water influx upon CFTR activation. Several cell or tissue types can be used. Ex vivo and in vivo assays similarly evaluate current (intestinal current measurement) and membrane potential differences (nasal potential difference), on tissues from individual patients. In the sweat test, the most frequently used in vivo evaluation of CFTR function, chloride concentration or stimulated sweat rate can be directly measured. Here, we will describe the currently available bio-assays for quantitative evaluation of CFTR function, their indications, advantages and disadvantages, and correlation with clinical outcome measures.

Liposome-based measurement of light-driven chloride transport kinetics of halorhodopsin

We report a simple and direct fluorimetric vesicle-based method for measuring the transport rate of the light-driven ions pumps as specifically applied to the chloride pump, halorhodopsin, from Natronomonas pharaonis (pHR). Previous measurements were cell-based and methods to determine average single channel permeability challenging. We used a water-in-oil emulsion method for directional pHR reconstitution into two different types of vesicles: lipid vesicles and asymmetric lipid-block copolymer vesicles. We then used stopped-flow experiments combined with fluorescence correlation spectroscopy to determine per protein Cl- transport rates. We obtained a Cl^- transport rate of 442 (±17.7) Cl^-/protein/s in egg phosphatidyl choline (PC) lipid vesicles and 413 (±26) Cl^-/protein/s in hybrid block copolymer/lipid (BCP/PC) vesicles with polybutadine-polyethylene oxide (PB₁₂PEO₈) on the outer leaflet and PC in the inner leaflet at a photon flux of 1450 photons/protein/s. Normalizing to a per photon basis, this corresponds to 0.30 (±0.07) Cl^-/photon and 0.28 (±0.04) Cl^-/photon for pure PC and BCP/PC hybrid vesicles respectively, both of which are in agreement with recently reported turnover of ~500 Cl^-/protein/s from flash photolysis experiments and with voltage-clamp measurements of 0.35 (±0.16) Cl^-/photon in pHR-expressing oocytes as well as with a pHR quantum efficiency of ~30%.

Buckyball-Based Spherical Display of Crown Ethers for De Novo Custom Design of Ion Transport Selectivity

Searching for membrane-active synthetic analogues that are structurally simple yet functionally comparable to natural channel proteins has been of central research interest in the past four decades, yet custom design of the ion transport selectivity still remains a grand challenge. Here we report on a suite of buckyball-based molecular balls (MBs), enabling transmembrane ion transport selectivity to be custom designable. The modularly tunable MBm-Cn (m = 4-7; n = 6-12) structures consist of a C₆₀-fullerene core, flexible alkyl linkers Cn (i.e., C6 for n-C₆H₁₂ group), and peripherally aligned benzo-3m-crown-m ethers (i.e., m = 4 for benzo-12-crown-4) as ion-transporting units. Screening a matrix of 16 such MBs, combinatorially derived from four different crown units and four different Cn linkers, intriguingly revealed that their transport selectivity well resembles the intrinsic ion binding affinity of the respective benzo-crown units present, making custom design of the transport selectivity possible. Specifically, MB4s, containing benzo-12-crown-4 units, all are Li⁺-selective in transmembrane ion transport, with the most active MB4-C10 exhibiting an EC₅₀(Li⁺) value of 0.13 μM (corresponding to 0.13 mol % of the lipid present) while excluding all other monovalent alkali-metal ions. Likewise, the most Na⁺ selective MB5-C8 and K⁺ selective MB6-C8 demonstrate high Na⁺/K⁺ and K⁺/Na⁺ selectivity values of 13.7 and 7.8, respectively. For selectivity to Rb⁺ and Cs⁺ ions, the most active MB7-C8 displays exceptionally high transport efficiencies, with an EC₅₀(Rb⁺) value of 105 nM (0.11 mol %) and an EC₅₀(Cs⁺) value of 77 nM (0.079 mol %).

Recent Strategic Advances in CFTR Drug Discovery: An Overview

Cystic fibrosis transmembrane conductance regulator (CFTR)-rescuing drugs have already transformed cystic fibrosis (CF) from a fatal disease to a treatable chronic condition. However, new-generation drugs able to bind CFTR with higher specificity/affinity and to exert stronger therapeutic benefits and fewer side effects are still awaited. Computational methods and biosensors have become indispensable tools in the process of drug discovery for many important human pathologies. Instead, they have been used only piecemeal in CF so far, calling for their appropriate integration with well-tried CF biochemical and cell-based models to speed up the discovery of new CFTR-rescuing drugs. This review will give an overview of the available structures and computational models of CFTR and of the biosensors, biochemical and cell-based assays already used in CF-oriented studies. It will also give the reader some insights about how to integrate these tools as to improve the efficiency of the drug discovery process targeted to CFTR.

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.

Daidzein-Stimulated Increase in the Ciliary Beating Amplitude via an [Cl-]i Decrease in Ciliated Human Nasal Epithelial Cells

The effects of the isoflavone daidzein on the ciliary beat distance (CBD, which is a parameter assessing the amplitude of ciliary beating) and the ciliary beat frequency (CBF) were examined in ciliated human nasal epithelial cells (cHNECs) in primary culture. Daidzein decreased [Cl⁻]_i and enhanced CBD in cHNECs. The CBD increase that was stimulated by daidzein was mimicked by Cl⁻-free NO₃⁻ solution and bumetanide (an inhibitor of Na⁺/K⁺/2Cl⁻ cotransport), both of which decreased [Cl⁻]_i. Moreover, the CBD increase was inhibited by 5-Nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, a Cl⁻ channel blocker), which increased [Cl⁻]_i. CBF was also decreased by NPPB. The rate of [Cl⁻]_i decrease evoked by Cl⁻-free NO₃⁻ solution was enhanced by daidzein. These results suggest that daidzein activates Cl⁻ channels in cHNECs. Moreover, daidzein enhanced the microbead transport driven by beating cilia in the cell sheet of cHNECs, suggesting that an increase in CBD enhances ciliary transport. An [Cl⁻]_i decrease enhanced CBD, but not CBF, in cHNECs at 37 °C, although it enhanced both at 25 °C. Intracellular Cl⁻ affects both CBD and CBF in a temperature-dependent manner. In conclusion, daidzein, which activates Cl⁻ channels to decrease [Cl⁻]_i, stimulated CBD increase in cHNECs at 37 °C. CBD is a crucial factor that can increase ciliary transport in the airways under physiological conditions.

Polymeric chemosensor for the detection and quantification of chloride in human sweat. Application to the diagnosis of cystic fibrosis

We have developed a new extremely hydrophilic polymeric film suitable for the detection and quantification of chloride in human sweat directly on the skin. The film, or membrane, has chemically anchored 6-methoxyquinoline groups as chloride responsive fluorescent motifs. We have prepared the sensory material from a standard vinyl copolymer, by a convenient and easy solid-phase reaction. The sensory material has a water swelling percentage of 700%, facilitating an immediate detection of chloride, is reusable for at least 6 cycles and can be handled without care by unskilled persons. The initially high fluorescence of the material decreases in the presence of chloride, allowing the quantification of chloride concentration by using the colour definition of a digital picture or a fluorimeter. The suitability of the material to perform quantitative chloride analysis of human sweat by putting it in contact with the skin offers promise for its application in the sweat test used for the diagnosis of cystic fibrosis (CF).

Assessment of p.Phe508del-CFTR functional restoration in pediatric primary cystic fibrosis airway epithelial cells

BACKGROUND

Mutations in the cystic fibrosis transmembrane regulator (CFTR) gene can reduce function of the CFTR ion channel activity and impair cellular chloride secretion. The gold standard method to assess CFTR function of ion transport using the Ussing chamber requires a high number of airway epithelial cells grown at air-liquid interface, limiting the application of this method for high throughput screening of potential therapeutic compounds in primary airway epithelial cells (pAECs) featuring less common CFTR mutations. This study assessed an alternative approach, using a small scale halide assay that can be adapted for a personalized high throughput setting to analyze CFTR function of pAEC.

METHODS

Pediatric pAECs derived from children with CF (pAECCF) were established and expanded as monolayer cultures, before seeding into 96-well plates for the halide assay. Cells were then transduced with an adenoviral construct containing yellow fluorescent protein (eYFP) reporter gene, alone or in combination with either wild-type CFTR (WT-CFTR) or p.Phe508del CFTR. Four days post transduction, cells were stimulated with forskolin and genistein, and assessed for quenching of the eYFP signal following injection of iodide solution into the assay media.

RESULTS

Data showed that pAECCF can express eYFP at high efficiency following transduction with the eYFP construct. The halide assay was able to discriminate functional restoration of CFTR in pAECCF treated with either WT-CFTR construct or the positive controls syntaxin 8 and B-cell receptor-associated protein 31 shRNAs.

SIGNIFICANCE

The current study demonstrates that the halide assay can be adapted for pediatric pAECCF to evaluate restoration of CFTR function. With the ongoing development of small molecules to modulate the folding and/or activity of various mutated CFTR proteins, this halide assay presents a small-scale personalized screening platform that could assess therapeutic potential of molecules across a broad range of CFTR mutations.

Thymosin α1 represents a potential potent single-molecule-based therapy for cystic fibrosis

Cystic fibrosis (CF) is caused by mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) that compromise its chloride-channel activity. The most common mutation, p.Phe508del, results in the production of a misfolded CFTR protein, which has residual channel activity but is prematurely degraded. Because of the inherent complexity of the pathogenetic mechanisms involved in CF —which include impaired chloride permeability and persistent lung inflammation—a multidrug approach is required for efficacious CF therapy. To date, no individual, drug with pleiotropic beneficial effects for CF is available. Here we report on the ability of thymosin alpha 1 (Tα1)—a naturally occurring polypeptide with an excellent safety profile in the clinic when used as an adjuvant or an immunotherapeutic agent—to rectify the multiple tissue defects in CF mice as well as in cells from subjects with the p.Phe508del mutation. Tα1 displayed two combined properties that favorably opposed CF symptomatology; namely, it reduced inflammation and increased CFTR maturation, stability and activity. By virtue of this two-pronged action, Tα1 offers a strong potential to be an efficacious single molecule-based therapeutic agent in CF.

The gating of the CFTR channel

Cystic fibrosis transmembrane conductance regulator (CFTR) is an anion channel expressed in the apical membrane of epithelia. Mutations in the CFTR gene are the cause of cystsic fibrosis. CFTR is the only ABC-protein that constitutes an ion channel pore forming subunit. CFTR gating is regulated in complex manner as phosphorylation is mandatory for channel activity and gating is directly regulated by binding of ATP to specific intracellular sites on the CFTR protein. This review covers our current understanding on the gating mechanism in CFTR and illustrates the relevance of alteration of these mechanisms in the onset of cystic fibrosis.

A novel treatment of cystic fibrosis acting on-target: cysteamine plus epigallocatechin gallate for the autophagy-dependent rescue of class II-mutated CFTR

We previously reported that the combination of two safe proteostasis regulators, cysteamine and epigallocatechin gallate (EGCG), can be used to improve deficient expression of the cystic fibrosis transmembrane conductance regulator (CFTR) in patients homozygous for the CFTR Phe508del mutation. Here we provide the proof-of-concept that this combination treatment restored CFTR function and reduced lung inflammation (P<0.001) in Phe508del/Phe508del or Phe508del/null-Cftr (but not in Cftr-null mice), provided that such mice were autophagy-competent. Primary nasal cells from patients bearing different class II CFTR mutations, either in homozygous or compound heterozygous form, responded to the treatment in vitro. We assessed individual responses to cysteamine plus EGCG in a single-centre, open-label phase-2 trial. The combination treatment decreased sweat chloride from baseline, increased both CFTR protein and function in nasal cells, restored autophagy in such cells, decreased CXCL8 and TNF-α in the sputum, and tended to improve respiratory function. These positive effects were particularly strong in patients carrying Phe508del CFTR mutations in homozygosity or heterozygosity. However, a fraction of patients bearing other CFTR mutations failed to respond to therapy. Importantly, the same patients whose primary nasal brushed cells did not respond to cysteamine plus EGCG in vitro also exhibited deficient therapeutic responses in vivo. Altogether, these results suggest that the combination treatment of cysteamine plus EGCG acts 'on-target' because it can only rescue CFTR function when autophagy is functional (in mice) and improves CFTR function when a rescuable protein is expressed (in mice and men). These results should spur the further clinical development of the combination treatment.

Restoration of CFTR function in patients with cystic fibrosis carrying the F508del-CFTR mutation

Restoration of BECN1/Beclin 1-dependent autophagy and depletion of SQSTM1/p62 by genetic manipulation or autophagy-stimulatory proteostasis regulators, such as cystamine, have positive effects on mouse models of human cystic fibrosis (CF). These measures rescue the functional expression of the most frequent pathogenic CFTR mutant, F508del, at the respiratory epithelial surface and reduce lung inflammation in Cftr^F508del homozygous mice. Cysteamine, the reduced form of cystamine, is an FDA-approved drug. Here, we report that oral treatment with cysteamine greatly reduces the mortality rate and improves the phenotype of newborn mice bearing the F508del-CFTR mutation. Cysteamine was also able to increase the plasma membrane expression of the F508del-CFTR protein in nasal epithelial cells from F508del homozygous CF patients, and these effects persisted for 24 h after cysteamine withdrawal. Importantly, this cysteamine effect after washout was further sustained by the sequential administration of epigallocatechin gallate (EGCG), a green tea flavonoid, both in vivo, in mice, and in vitro, in primary epithelial cells from CF patients. In a pilot clinical trial involving 10 F508del-CFTR homozygous CF patients, the combination of cysteamine and EGCG restored BECN1, reduced SQSTM1 levels and improved CFTR function from nasal epithelial cells in vivo, correlating with a decrease of chloride concentrations in sweat, as well as with a reduction of the abundance of TNF/TNF-alpha (tumor necrosis factor) and CXCL8 (chemokine [C-X-C motif] ligand 8) transcripts in nasal brushing and TNF and CXCL8 protein levels in the sputum. Altogether, these results suggest that optimal schedules of cysteamine plus EGCG might be used for the treatment of CF caused by the F508del-CFTR mutation.

The effect of NO-donors on chloride efflux, intracellular Ca(2+) concentration and mRNA expression of CFTR and ENaC in cystic fibrosis airway epithelial cells

Since previous studies showed that the endogenous bronchodilator, S-nitrosglutathione (GSNO), caused a marked increase in CFTR-mediated chloride (Cl(-)) efflux and improved the trafficking of CFTR to the plasma membrane, and that also the nitric oxide (NO)-donor GEA3162 had a similar, but smaller, effect on Cl(-) efflux, it was investigated whether the NO-donor properties of GSNO were relevant for its effect on Cl(-) efflux from airway epithelial cells. Hence, the effect of a number of other NO-donors, sodium nitroprusside (SNP), S-nitroso-N-acetyl-DL-penicillamine (SNAP), diethylenetriamine/nitric oxide adduct (DETA-NO), and diethylenetriamine/nitric oxide adduct (DEA-NONOate) on Cl(-) efflux from CFBE (∆F508/∆F508-CFTR) airway epithelial cells was tested. Cl(-) efflux was determined using the fluorescent N-(ethoxycarbonylmethyl)-6-methoxyquinoliniu bromide (MQAE)-technique. Possible changes in the intracellular Ca(2+) concentration were tested by the fluorescent fluo-4 method in a confocal microscope system. Like previously with GSNO, after 4 h incubation with the NO-donor, an increased Cl(-) efflux was found (in the order SNAP>DETA-NO>SNP). The effect of DEA-NONOate on Cl(-) efflux was not significant, and the compound may have (unspecific) deleterious effects on the cells. Again, as with GSNO, after a short (5 min) incubation, SNP had no significant effect on Cl(-) efflux. None of the NO-donors that had a significant effect on Cl(-) efflux caused significant changes in the intracellular Ca(2+) concentration. After 4 h preincubation, SNP caused a significant increase in the mRNA expression of CFTR. SNAP and DEA-NONOate decreased the mRNA expression of all ENaC subunits significantly. DETA-NO caused a significant decrease only in α-ENaC expression. After a short preincubation, none of the NO-donors had a significant effect, neither on the expression of CFTR, nor on that of the ENaC subunits in the presence and absence of L-cysteine. It can be concluded that the effect of GSNO on Cl(-) efflux is, at least in part, due to its properties as an NO-donor, and the effect is likely to be mediated by CFTR, not by Ca(2+)-activated Cl(-) channels.

Human heat shock protein 105/110 kDa (Hsp105/110) regulates biogenesis and quality control of misfolded cystic fibrosis transmembrane conductance regulator at multiple levels

Heat shock protein 105/110-kDa (Hsp105/110), a member of the Hsp70 super family of molecular chaperones, serves as a nucleotide exchange factor for Hsc70, independently prevents the aggregation of misfolded proteins, and functionally relates to Hsp90. We investigated the roles of human Hsp105α, the constitutively expressed isoform, in the biogenesis and quality control of the cystic fibrosis transmembrane conductance regulator (CFTR). In the endoplasmic reticulum (ER), Hsp105 facilitates CFTR quality control at an early stage in its biosynthesis but promotes CFTR post-translational folding. Deletion of Phe-508 (ΔF508), the most prevalent mutation causing cystic fibrosis, interferes with de novo folding of CFTR, impairing its export from the ER and accelerating its clearance in the ER and post-Golgi compartments. We show that Hsp105 preferentially associates with and stabilizes ΔF508 CFTR at both levels. Introduction of the Hsp105 substrate binding domain potently increases the steady state level of ΔF508 CFTR by reducing its early-stage degradation. This in turn dramatically enhances ΔF508 CFTR cell surface functional expression in cystic fibrosis airway epithelial cells. Although other Hsc70 nucleotide exchange factors such as HspBP1 and BAG-2 inhibit CFTR post-translational degradation in the ER through cochaperone CHIP, Hsp105 has a primary role promoting CFTR quality control at an earlier stage. The Hsp105-mediated multilevel regulation of ΔF508 CFTR folding and quality control provides new opportunities to understand how chaperone machinery regulates the homeostasis and functional expression of misfolded proteins in the cell. Future studies in this direction will inform therapeutics development for cystic fibrosis and other protein misfolding diseases.

FKBP38 peptidylprolyl isomerase promotes the folding of cystic fibrosis transmembrane conductance regulator in the endoplasmic reticulum

FK506-binding protein 38 (FKBP38), a membrane-anchored, tetratricopeptide repeat (TPR)-containing immunophilin, associates with nascent plasma membrane ion channels in the endoplasmic reticulum (ER). It promotes the maturation of the human ether-à-go-go-related gene (HERG) potassium channel and maintains the steady state level of the cystic fibrosis transmembrane conductance regulator (CFTR), but the underlying mechanisms remain unclear. Using a combination of steady state and pulse-chase analyses, we show that FKBP38 knockdown increases protein synthesis but inhibits the post-translational folding of CFTR, leading to reduced steady state levels of CFTR in the ER, decreased processing, and impaired cell surface functional expression in Calu-3 human airway epithelial cells. The membrane anchorage of FKBP38 is necessary for the inhibition of protein synthesis but not for CFTR post-translational folding. In contrast, the peptidylprolyl cis/trans isomerase active site is utilized to promote CFTR post-translational folding but is not important for regulation of protein synthesis. Uncoupling FKBP38 from Hsp90 by substituting a conserved lysine in the TPR domain modestly enhances CFTR maturation and further reduces its synthesis. Removing the N-terminal glutamate-rich domain (ERD) slightly enhances CFTR synthesis but reduces its maturation, suggesting that the ERD contributes to FKBP38 biological activities. Our data support a dual role for FKBP38 in regulating CFTR synthesis and post-translational folding. In contrast to earlier prediction but consistent with in vitro enzymological studies, FKBP38 peptidylprolyl cis/trans isomerase plays an important role in membrane protein biogenesis on the cytoplasmic side of the ER membrane, whose activity is negatively regulated by Hsp90 through the TPR domain.

An intramolecular transport metabolon: fusion of carbonic anhydrase II to the COOH terminus of the Cl(-)/HCO(3)(-)exchanger, AE1

Anion exchanger 1 (AE1) is the plasma membrane Cl(-)/HCO(3)(-) exchanger of erythrocytes. Carbonic anhydrases (CA) provide substrate for AE1 by catalyzing the reaction, H(2)O + CO(2) ↔ HCO(3)(-) + H(+). The physical complex of CAII with AE1 has been proposed to maximize anion exchange activity. To examine the effect of CAII catalysis on AE1 transport rate, we fused either CAII-wild type or catalytically inactive CAII-V143Y to the cytoplasmic COOH terminus of AE1 to form AE1.CAII and AE1.CAII-V143Y, respectively. When expressed in transfected human embryonic kidney 293 cells, AE1.CAII had a similar Cl(-)/HCO(3)(-) exchange activity to AE1 alone, as assessed by the flux of H(+) equivalents (87 ± 4% vs. AE1) or rate of change of intracellular Cl(-) concentration (93 ± 4% vs. AE1), suggesting that CAII does not activate AE1. In contrast, AE1.CAII-V143Y displayed transport rates for H(+) equivalents and Cl(-) of 55 ± 2% and of 40 ± 2%, versus AE1. Fusion of CAII to AE1 therefore reduces anion transport activity, but this reduction is compensated for during Cl(-)/HCO(3)(-) exchange by the presence of catalytically active CAII. Overexpression of free CAII-V143Y acts in a dominant negative manner to reduce AE1-mediated HCO(3)(-) transport by displacement of endogenous CAII-wild type from its binding site on AE1. To examine whether AE1.CAII bound endogenous CAII, we coexpressed CAII-V143Y along with AE1 or AE1.CAII. The bicarbonate transport activity of AE1 was inhibited by CAII-V143Y, whereas the activity of AE1.CAII was unaffected by CAII-V143Y, suggesting impaired transport activity upon displacement of functional CAII from AE1 but not AE1.CAII. Taken together, these data suggest that association of functional CAII with AE1 increases Cl(-)/HCO(3)(-) exchange activity, consistent with the HCO(3)(-) transport metabolon model.

Sensitivity of chloride efflux vs. transepithelial measurements in mixed CF and normal airway epithelial cell populations

BACKGROUND/AIMS

While the Cl(-) efflux assays are relatively straightforward, their ability to assess the efficacy of phenotypic correction in cystic fibrosis (CF) tissue or cells may be limited. Accurate assessment of therapeutic efficacy, i.e., correlating wild type CF transmembrane conductance regulator (CFTR) levels with phenotypic correction in tissue or individual cells, requires a sensitive assay.

METHODS

Radioactive chloride ((36)Cl) efflux was compared to Ussing chamber analysis for measuring cAMP-dependent Cl(-) transport in mixtures of human normal (16HBE14o-) and cystic fibrosis (CF) (CFTE29o- or CFBE41o-, respectively) airway epithelial cells. Cell mixtures with decreasing amounts of 16HBE14o- cells were evaluated.

RESULTS

Efflux and Ussing chamber studies on mixed populations of normal and CF airway epithelial cells showed that, as the number of CF cells within the population was progressively increased, the cAMP-dependent Cl(-) decreased. The (36)Cl efflux assay was effective for measuring Cl(-) transport when ≥ 25% of the cells were normal. If < 25% of the cells were phenotypically wild-type (wt), the (36)Cl efflux assay was no longer reliable. Polarized CFBE41o- cells, also homozygous for the ΔF508 mutation, were used in the Ussing chamber studies. Ussing analysis detected cAMP-dependent Cl(-) currents in mixtures with ≥1% wild-type cells indicating that Ussing analysis is more sensitive than (36)Cl efflux analysis for detection of functional CFTR.

CONCLUSIONS

Assessment of CFTR function by Ussing analysis is more sensitive than (36)Cl efflux analysis. Ussing analysis indicates that cell mixtures containing 10% 16HBE14o- cells showed 40-50% of normal cAMP-dependent Cl(-) transport that drops off exponentially between 10-1% wild-type cells.

Effect of duramycin on chloride transport and intracellular calcium concentration in cystic fibrosis and non-cystic fibrosis epithelia

The lantibiotic duramycin (Moli1901, Lancovutide) has been suggested as a drug of choice in the treatment for cystic fibrosis (CF). It has been proposed that duramycin may stimulate chloride secretion through Ca²(+) -activated Cl⁻ channels (CaCC). We investigated whether duramycin exhibited any effect on Cl⁻ efflux and intracellular Ca²(+) concentration ([Ca²(+)](i)) in CF and non-CF epithelial cells. Duramycin did stimulate Cl⁻ efflux from CF bronchial epithelial cells (CFBE) in a narrow concentration range (around 1 μM). However, 100 and 250 μM of duramycin inhibited Cl⁻ efflux from CFBE cells. An inhibitor of the CF transmembrane conductance regulator (CFTR(inh)₋₁₇₂) and a blocker of the capacitative Ca²(+) entry, gadolinium chloride, inhibited the duramycin-induced Cl⁻ efflux. No effect on Cl⁻ efflux was observed in non-CF human bronchial epithelial cells (16HBE), human airway submucosal gland cell line, human pancreatic epithelial cells, CF airway submucosal gland epithelial cells, and CF pancreatic cells. The [Ca²(+)](i) was increased by 3 μM duramycin in 16HBE cells, but decreased after 1, and 3 μM of duramycin in CFBE cells. The results suggest that the mechanism responsible for the stimulation of Cl⁻ efflux by duramycin is mainly related to unspecific changes of the cell membrane or its components rather than to effects on CaCC.

Direct evidence of chloride ion efflux in ischaemic and pharmacological preconditioning of cultured cardiomyocytes

AIMS

We have previously shown that reduction of ischaemic cell swelling via enhanced cell volume regulation is a key mechanism of ischaemic preconditioning (IPC) in cardiomyocytes. We have also shown that pharmacological blockade of Cl(-) channels abolishes cardioprotection achieved by IPC in Langendorff-perfused hearts and freshly isolated cardiomyocytes, thus suggesting that Cl(-) plays a key role in IPC cardioprotection. However, direct evidence of Cl(-) channel activation resulting in transsarcolemmal Cl(-) efflux by IPC had been lacking. To address this issue, 24 h cultured rabbit cardiomyocytes were loaded with 5 mM 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ), a specific fluorescence indicator that is quenched by Cl(-) so that cellular efflux of Cl(-) results in an increase in SPQ fluorescence.

METHODS AND RESULTS

After stabilization for 10 min, cardiomyocytes were preconditioned either with 10 min simulated ischaemia/10 min simulated reperfusion or with 10 min treatment with 1 microM N(6)-2-(4-aminophenyl)ethyladenosine (APNEA). IPC and APNEA significantly (P < 0.001) reduced the intracellular Cl(-) concentration ([Cl(-)](i)) to 31.9 +/- 3.2 mM (mean +/- SEM) and 32.5 +/- 2.8 mM, respectively, from an initial [Cl(-)](i) (pooled stabilization 61.5 +/- 7.1 mM). [Cl(-)](i) did not change in control (non-preconditioned) cardiomyocytes (control 58.1 +/- 1.9 mM and control + vehicle 62.6 +/- 4.9 mM, P = 0.98 and 0.99 vs. pooled pre-treatment baseline, respectively). Inhibition of Cl(-) channels with 50 microM indanyloxyacetic acid 94 completely blocked preconditioning-induced Cl(-) efflux. Thus, a net Cl(-) efflux of 29.6 and 29.0 mM was triggered by IPC and APNEA.

CONCLUSION

These findings provide the first direct evidence of activation of sarcolemmal Cl(-) channels by ischaemic and pharmacological preconditioning in cardiomyocytes.

Genetically encoded optical sensors for monitoring of intracellular chloride and chloride-selective channel activity

This review briefly discusses the main approaches for monitoring chloride (Cl⁻), the most abundant physiological anion. Noninvasive monitoring of intracellular Cl⁻ ([Cl⁻]i) is a challenging task owing to two main difficulties: (i) the low transmembrane ratio for Cl⁻, approximately 10:1; and (ii) the small driving force for Cl⁻, as the Cl⁻ reversal potential (E_Cl) is usually close to the resting potential of the cells. Thus, for reliable monitoring of intracellular Cl⁻, one has to use highly sensitive probes. From several methods for intracellular Cl⁻ analysis, genetically encoded chloride indicators represent the most promising tools. Recent achievements in the development of genetically encoded chloride probes are based on the fact that yellow fluorescent protein (YFP) exhibits Cl⁻-sensitivity. YFP-based probes have been successfully used for quantitative analysis of Cl⁻ transport in different cells and for high-throughput screening of modulators of Cl⁻-selective channels. Development of a ratiometric genetically encoded probe, Clomeleon, has provided a tool for noninvasive estimation of intracellular Cl⁻ concentrations. While the sensitivity of this protein to Cl⁻ is low (EC₅₀ about 160 mM), it has been successfully used for monitoring intracellular Cl⁻ in different cell types. Recently a CFP–YFP-based probe with a relatively high sensitivity to Cl⁻ (EC₅₀ about 30 mM) has been developed. This construct, termed Cl-Sensor, allows ratiometric monitoring using the fluorescence excitation ratio. Of particular interest are genetically encoded probes for monitoring of ion channel distribution and activity. A new molecular probe has been constructed by introducing into the cytoplasmic domain of the Cl⁻-selective glycine receptor (GlyR) channel the CFP–YFP-based Cl-Sensor. This construct, termed BioSensor-GlyR, has been successfully expressed in cell lines. The new genetically encoded chloride probes offer means of screening pharmacological agents, analysis of Cl⁻ homeostasis and functions of Cl⁻-selective channels under different physiological and pathological conditions.

Characteristics of Cl- uptake in rat alveolar type I cells

Although Cl- transport in fetal lung is important for fluid secretion and normal lung development, the role of Cl- transport in adult lung is not well understood. In physiological studies, the cystic fibrosis transmembrane regulator (CFTR) plays a role in fluid absorption in the distal air spaces of adult lung, and alveolar type II cells cultured for 5 days have the capacity to transport Cl-. Although both alveolar type I and type II cells express CFTR, it has previously not been known whether type I cells transport Cl-. We studied Cl- uptake in isolated type I cells directly, using either radioisotopic tracers or halide-sensitive fluorescent indicators. By both methods, type I cells take up Cl-. In the presence of beta-adrenergic agonist stimulation, Cl- uptake can be inhibited by CFTR antagonists. Type I cells express both the Cl-/HCO3- anion exchanger AE2 and the voltage-gated Cl- channels CLC5 and CLC2. Inhibitors of AE2 also block Cl- uptake in type I cells. Together, these results demonstrate that type I cells are capable of Cl- uptake and suggest that the effects seen in whole lung studies establishing the importance of Cl- movement in alveolar fluid clearance may be, in part, the result of Cl- transport across type I cells.

Azithromycin increases chloride efflux from cystic fibrosis airway epithelial cells

It was investigated whether azithromycin (AZM) stimulates chloride (Cl-) efflux from cystic fibrosis (CF) and non-CF airway epithelial cells, possibly secondary to up-regulation of the multidrug resistance protein (MDR). CF and non-CF human airway epithelial cell lines (CFBE and 16HBE) were treated with 0.4, 4, and 40 microg/mL AZM for 4 days. Cl- efflux was explored in the presence or absence of specific inhibitors of CFTR and alternative Cl- channels. Six CF patients received AZM (500 mg daily) for 6 months. The percentage of predicted forced vital capacity (FVC%), forced expiratory volume (FEV1%), and the number of acute exacerbations were compared before and after treatment. Nasal biopsies were taken before and after treatment, and mRNA expression of MDR and CFTR was determined by in situ hybridization. A significant dose-dependent increase of Cl- efflux from CFBE cells (but not from 16HBE cells) was observed after AZM treatment. A CFTR inhibitor significantly reduced AZM-stimulated Cl- efflux from CFBE cells. A significant improvement in FEV1%, and fewer exacerbations were observed. AZM treatment did not affect mRNA expression of MDR and CFTR. The stimulation of Cl- efflux could be part of the explanation for the clinical improvement seen among the patients.

Cystic fibrosis transmembrane conductance regulator and caveolin-1 regulate epithelial cell internalization of Pseudomonas aeruginosa

Patients with cystic fibrosis (CF) exhibit defective innate immunity and are susceptible to chronic lung infection with Pseudomonas aeruginosa. To investigate the molecular bases for the hypersusceptibility of CF patients to P. aeruginosa, we used the IB3-1 cell line with two defective CF transmembrane conductance regulator (CFTR) genes (DeltaF508/W1282X) to generate isogenic stable, clonal lung epithelial cells expressing wild-type (WT)-CFTR with an NH(2)-terminal green fluorescent protein (GFP) tag. GFP-CFTR exhibited posttranslational modification, subcellular localization, and anion transport function typical of WT-CFTR. P. aeruginosa internalization, a component of effective innate immunity, required functional CFTR and caveolin-1, as shown by: 1) direct correlation between GFP-CFTR expression levels and P. aeruginosa internalization; 2) enhanced P. aeruginosa internalization by aminoglycoside-induced read through of the CFTR W1282X allele in IB3-1 cells; 3) decreased P. aeruginosa internalization following siRNA knockdown of GFP-CFTR or caveolin-1; and 4) spatial association of P. aeruginosa with GFP-CFTR and caveolin-1 at the cell surface. P. aeruginosa internalization also required free lateral diffusion of GFP-CFTR, allowing for bacterial coclustering with GFP-CFTR and caveolin-1 at the plasma membrane. Thus efficient initiation of innate immunity to P. aeruginosa requires formation of an epithelial "internalization platform" involving both caveolin-1 and functional, laterally mobile CFTR.

On the measurement of the functional properties of the CFTR

A number of methods are currently employed to assess the functional properties of CFTR channels and their response to pharmacological potentiators, correction of the defective CFTR trafficking, and vectorial introduction of new proteins. Here we review the most common methods used to assess CFTR channel function. The suitability of each technique to various experimental conditions is discussed.

ATP-induced P2X7-associated uptake of large molecules involves distinct mechanisms for cations and anions in macrophages

Macrophages express the P2X(7) receptor and other nucleotide (P2) receptors, and display the phenomenon of extracellular ATP (ATP(e))-induced P2X(7)-dependent membrane permeabilization, which occurs through a poorly understood mechanism. We used patch-clamp recordings, cytoplasmic Ca(2+) measurements and fluorescent dye uptake assays to compare P2X(7)-associated transport phenomena of macrophages and HEK-293 cells transfected with P2X(7) receptors (HEK-P2X(7) cells). Both cell types showed inward currents, increase of free cytoplasmic Ca(2+) concentration and the uptake of cationic dyes upon exposure to ATP(e), as previously described. However, in contrast to the macrophages, HEK-P2X(7) cells did not take up anionic dyes and did not display the 440 pS channels (Z pores) under cell-attached patch-clamping conditions. In addition, the transport mechanism of anionic dyes displayed by macrophages was also able to support dye efflux and, once activated at 37 degrees C, it remained active at 4 degrees C, whereas uptake of cationic dyes was temperature-dependent and unidirectional. Our results indicate that the mechanism of ATP(e)-induced dye uptake, usually called a ;permeabilization phenomenon' and associated with a ;permeabilization pore' can be ascribed to at least two distinct mechanisms in macrophages: a diffusional pathway, possibly associated with the 440 pS Z pores, and a cation uptake mechanism that is not diffusional and should be ascribed to an, as yet, unidentified transport mechanism.

Time resolved secretion of chloride from a monolayer of mucin-secreting epithelial cells

Short-circuit current (Isc) measurement is used to quantify transepithelial ion flux. This technique provides a direct measure of net charge transport across a cell monolayer. Isc however, lacks chemical selectivity. Chemically resolved ion fluxes may be much greater than Isc, and differ in different biological processes. This work describes a novel experimental approach and deconvolution method to obtain temporally resolved ion fluxes at epithelial cell monolayers. HT29-Cl.16E cells, a sub clone of the human colonic cancer cell line HT29 was used as a model cell line to validate this approach in the context of epithelial transport studies. This cell line is known to secrete chloride in response to purinergic stimulation. Changes in chloride concentration after stimulation with 1 mM ATP plus 50 nM phorbol-myristate acetate (PMA) are recorded with a chloride ion-selective electrode (ISE) at a short distance (approximately 50 microm) from the monolayer. The recorded concentrations are transformed to corresponding chloride flux across the monolayer using a deconvolution algorithm for extracellular mass transport based on minimization of the shape error function (Nair and Gratzl in Anal Chem 77:2875-2888, 2005). Simultaneous voltage clamp yields the associated net electrical charge flux (Isc). The dynamics of Cl(-) flux did correlate with that of the electrical flux, but was found to be greater in amplitude. This suggests that Cl(-) may not be the only ion secreted. The method of simultaneously assessing ionic and electrical fluxes with a temporal resolution of seconds provides unique information about the dynamics of solute fluxes across the apical membrane.

Detection of cystic fibrosis transmembrane conductance regulator activity in early-phase clinical trials

Advances in our understanding of cystic fibrosis pathogenesis have led to strategies directed toward treatment of underlying causes of the disease rather than treatments of disease-related symptoms. To expedite evaluation of these emerging therapies, early-phase clinical trials require extension of in vivo cystic fibrosis transmembrane conductance regulator (CFTR)-detecting assays to multicenter trial formats, including nasal potential difference and sweat chloride measurements. Both of these techniques can be used to fulfill diagnostic criteria for the disease, and can discriminate various levels of CFTR function. Full realization of these assays in multicenter clinical trials requires identification of sources of nonbiological intra- and intersite variability, and careful attention to study design and statistical analysis of study-generated data. In this review, we discuss several issues important to the performance of these assays, including efforts to identify and address aspects that can contribute to inconsistent and/or potentially erroneous results. Adjunctive means of detecting CFTR including mRNA expression, immunocytochemical localization, and other methods are also discussed. Recommendations are presented to advance our understanding of these biomarkers and to improve their capacity to predict cystic fibrosis outcomes.

Chloride transport in NCL-SG3 sweat gland cells: Channels involved

The aim of the study was to assess whether NCL-SG3, the only immortalized sweat gland cell line available, can be used as an in vitro model to study chloride ion transport in cultured sweat gland cells. Cl(-) efflux was measured using the MQAE dye fluorescence technique after stimulating the cells with different agonists. A significant stimulation of chloride efflux was achieved with the calcium ionophore A23187 resulting in an efflux rate of 0.9 mM/s. Both ATP and UTP activated chloride efflux in these cells, with the ATP response being larger. IBMX and forskolin stimulation did not induce a rate of chloride efflux above the basal level. Immunocytochemistry showed no detectable CFTR in NCL-SG3 cells. This finding was confirmed with flow cytometry analysis. Niflumic acid (20 and 100 microM NFA) and 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulfonic acid (H2DIDS) (100 ìM) decreased the rate of ATP-stimulated chloride efflux significantly (0.40 and 0.31 mM/s with NFA, 0.37 mM/s with H2DIDS). Gadolinium (20 ìM) had no effect on the chloride transport rate. In conclusion, the NCL-SG3 cells retain some of the aspects of human sweat gland epithelium, such as the ability to form cell-cell contacts. The CFTR protein is neither functional nor expressed in cultured NCL-SG3 sweat gland cells. Ca(2+)-activated chloride conductance is confirmed and the putative Ca(2+)-activated chloride channel (CaCC) is further characterized in term of its pharmacological sensitivity. The NCL-SG3 sweat gland cell line can be used to investigate the characteristics of the CaCC and to identify the channel.

The alpha2beta1 isoform of guanylyl cyclase mediates plasma membrane localized nitric oxide signalling

Nitric oxide (NO) is a mediator of copious biological processes, in many cases through the production of cGMP from the enzyme nitric oxide-sensitive guanylyl cyclase. Natriuretic peptides also elevate cGMP, often with distinct biological effects, raising the issue of how specificity is achieved. Here we show that a recently described alpha(2)beta(1) isoform of guanylyl cyclase is expressed in a number of epithelia, where it is localized to the apical plasma membrane. We measured the functional properties of the alpha(2)beta(1) isoform by utilizing the NO-dependent activation of the ion channel cystic fibrosis transmembrane conductance regulator (CFTR), which occurs by phosphorylation via the membrane-bound type II isoform of cGMP-dependent protein kinase. We found that cGMP generated by NO activation of the alpha(2)beta(1) isoform of guanylyl cyclase is an exceptionally efficient mediator of nitric oxide action on membrane targets, activating CFTR far more effectively than the cytoplasmically located alpha(1)beta(1) guanylyl cyclase isoform. Targeting the alpha(1)beta(1) isoform of guanylyl cyclase to the membrane also dramatically enhanced the effects of nitric oxide on CFTR within the membrane. This was not due to increased enzymatic activity of guanylyl cyclase in a membrane location, but to production of a localised membrane pool of cGMP by membrane-localized NO-dependent guanylyl cyclase that was resistant to degradation by phosphodiesterases. Selective effects of cGMP produced from this enzyme in response to NO are directed at membrane targets and suggest that drugs selectively activating or inhibiting this alpha(2)beta(1) isoform of guanylyl cyclase may have unique pharmacological properties.

Clinical trials in cystic fibrosis

In patients with cystic fibrosis (CF), clinical trials are of paramount importance. Here, the current status of drug development in CF is discussed and future directions highlighted. Methods for pre-clinical testing of drugs with potential activity in CF patients including relevant animal models are described. Study design options for phase II and phase III studies involving CF patients are provided, including required patient numbers, safety issues and surrogate end point parameters for drugs, tested for different disease manifestations. Finally, regulatory issues for licensing new therapies for CF patients are discussed, including new directives of the European Union and the structure of a European clinical trial network for clinical studies involving CF patients is proposed.

Evidence for nitrite reductase activity in intact mouse Leydig tumor cells

Nitric oxide (NO) supposedly derived via L-arginine-NO synthase (NOS) pathway has been implicated in inhibiting steroidogenesis by binding the heme moiety of steroidogenic enzymes. Previously, nitrite, and to a lesser extent nitrate ions inhibited steroidogenesis via NO by hitherto unknown reduction mechanism. Recently, a putative mammalian nitrite reductase activity ascribed to complex III of mitochondrial respiratory chain complexes (MRCC) has been reported, where MRCC inhibitors reduced NO production from nitrite variably. We thus studied the effects of MRCC inhibitors on testosterone production in mouse Leydig tumor cells (MLTC-1) without (basal) or with human chorionic gonadotropin (hCG) stimulation. In stimulated MLTC-1, MRCC inhibitors decreased testosterone production, order being: complex III (antimycin A and myxothiazol) > complex I (rotenone) > complex II (thenoyltrifluoroacetone), while cAMP production increased inversely. In unstimulated MLTC-1, MRCC inhibitors in same order, increased basal testosterone production, which correlated inversely with the percentage inhibition of NO production, with one exception; while antimycin A did not inhibit NO production in the nitrite reductase study mentioned above, it increased basal testosterone production in the present study. While MLTC-1 expressed mRNA for endothelial and neuronal, but not inducible NOS, various stimulators and inhibitors of L-arginine-NOS pathway had no effect on basal testosterone production in MLTC-1 or fresh Balb/c Leydig cells. Moreover, hCG increased nitrate uptake into MLTC-1, which suggests the gonadotropin aids nitrite and nitrate ions in their steroidogenesis inhibitory activity. In conclusion, this study supports the existence of a surrogate mammalian nitrite reductase and the dormancy of L-arginine-NOS pathway in MLTC-1.

Ex vivo biochemical analysis of CFTR in human rectal biopsies

This report describes the first biosynthetic analysis of the cystic fibrosis transmembrane conductance regulator (CFTR) in freshly excised human rectal biopsies. Expression of functional CFTR was assessed by intestinal current measurement (ICM) prior to biosynthetic studies. Several structural features of CFTR are found to be comparable to those established in CFTR-expressing cell lines. Interestingly, maturation of CFTR increases substantially in tissue incubated at 26 degrees C. Our data provide a solid basis for future studies on the characterisation of CFTR in pathological cases.

Curcumin does not stimulate cAMP-mediated chloride transport in cystic fibrosis airway epithelial cells

It has been suggested that curcumin and other sarcoplasmic/endoplasmic reticulum Ca(2+)-pump inhibitors could correct the defect in the most common mutation (DeltaF508) in cystic fibrosis (CF), and restore normal chloride transport. In the present study, the effect of curcumin was tested on baby hamster kidney (BHK) cells transfected with DeltaF508-CFTR, a CF airway epithelial cell line (CFBE), and cells isolated from the nasal epithelium of CF-patient homozygous for the DeltaF508-mutation. Curcumin had a small effect on basal (non-CFTR-mediated) chloride efflux in CFBE and CF nasal epithelial cells, but did not increase the net cAMP-activated (CFTR-mediated) chloride efflux. Curcumin caused a small increase in net cAMP-activated chloride efflux from DeltaF508-CFTR BHK cells. Immunocytochemical analysis failed to show significant movement of DeltaF508-CFTR to the plasma membrane in DeltaF508-CFTR BHK cells or CFBE cells. It is concluded that it is unlikely that curcumin has a significant positive effect on CFTR-mediated chloride transport in airway epithelial cells.