Calcium-dependent chloride channels are required for normal electrolyte and fluid secretion, olfactory perception, and neuronal and smooth muscle excitability. The molecular identity of these membrane proteins is still unclear. Treatment of bronchial epithelial cells with interleukin-4 (IL-4) causes increased calcium-dependent chloride channel activity, presumably by regulating expression of the corresponding genes. We performed a global gene expression analysis to identify membrane proteins that are regulated by IL-4. Transfection of epithelial cells with specific small interfering RNA against each of these proteins shows that TMEM16A, a member of a family of putative plasma membrane proteins with unknown function, is associated with calcium-dependent chloride current, as measured with halide-sensitive fluorescent proteins, short-circuit current, and patch-clamp techniques. Our results indicate that TMEM16A is an intrinsic constituent of the calcium-dependent chloride channel. Identification of a previously unknown family of membrane proteins associated with chloride channel function will improve our understanding of chloride transport physiopathology and allow for the development of pharmacological tools useful for basic research and drug development.

The most common cause of cystic fibrosis (CF) is deletion of phenylalanine 508 (∆F508) in the CF transmembrane conductance regulator (CFTR) chloride channel. The ∆F508 mutation produces defects in folding, stability, and channel gating. To identify small-molecule correctors of defective cellular processing, we assayed iodide flux in ∆F508-CFTR-transfected epithelial cells using a fluorescent halide indicator. Screening of 150,000 chemically diverse compounds and more than 1,500 analogs of active compounds yielded several classes of ∆F508-CFTR correctors (aminoarylthiazoles, quinazolinylaminopyrimidinones, and bisaminomethylbithiazoles) with micromolar potency that produced greater apical membrane chloride current than did low-temperature rescue. Correction was seen within 3-6 hours and persisted for more than 12 hours after washout. Functional correction was correlated with plasma membrane expression of complex-glycosylated ∆F508-CFTR protein. Biochemical studies suggested a mechanism of action involving improved ∆F508-CFTR folding at the ER and stability at the cell surface. The bisaminomethylbithiazoles corrected ∆F508-CFTR in ∆F508/∆F508 human bronchial epithelia but did not correct a different temperature-sensitive CFTR mutant (P574H-CFTR) or a dopamine receptor mutant. Small-molecule correctors may be useful in the treatment of CF caused by the ∆F508 mutation.

TMEM16 proteins, also known as anoctamins, are involved in a variety of functions that include ion transport, phospholipid scrambling, and regulation of other membrane proteins. The first two members of the family, TMEM16A (anoctamin-1, ANO1) and TMEM16B (anoctamin-2, ANO2), function as Ca2+-activated Cl- channels (CaCCs), a type of ion channel that plays important functions such as transepithelial ion transport, smooth muscle contraction, olfaction, phototransduction, nociception, and control of neuronal excitability. Genetic ablation of TMEM16A in mice causes impairment of epithelial Cl- secretion, tracheal abnormalities, and block of gastrointestinal peristalsis. TMEM16A is directly regulated by cytosolic Ca2+ as well as indirectly by its interaction with calmodulin. Other members of the anoctamin family, such as TMEM16C, TMEM16D, TMEM16F, TMEM16G, and TMEM16J, may work as phospholipid scramblases and/or ion channels. In particular, TMEM16F (ANO6) is a major contributor to the process of phosphatidylserine translocation from the inner to the outer leaflet of the plasma membrane. Intriguingly, TMEM16F is also associated with the appearance of anion/cation channels activated by very high Ca2+ concentrations. Furthermore, a TMEM16 protein expressed in Aspergillus fumigatus displays both ion channel and lipid scramblase activity. This finding suggests that dual function is an ancestral characteristic of TMEM16 proteins and that some members, such as TMEM16A and TMEM16B, have evolved to a pure channel function. Mutations in anoctamin genes (ANO3, ANO5, ANO6, and ANO10) cause various genetic diseases. These diseases suggest the involvement of anoctamins in a variety of cell functions whose link with ion transport and/or lipid scrambling needs to be clarified.

Deletion of Phe-508 (⌬F508) is the most common mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) causing cystic fibrosis. ⌬F508-CFTR has defects in both channel gating and endoplasmic reticulum-to-plasma membrane processing. We identified six novel classes of high affinity potentiators of defective ⌬F508-CFTR Cl ؊ channel gating by screening 100,000 diverse small molecules. Compounds were added 15 min prior to assay of iodide uptake in epithelial cells co-expressing ⌬F508-CFTR and a high sensitivity halide indicator (YFP-H148Q/I152L) in which ⌬F508-CFTR was targeted to the plasma membrane by culture at 27°C for 24 h. Thirty-two compounds with submicromolar activating potency were identified; most had tetrahydrobenzothiophene, benzofuran, pyramidinetrione, dihydropyridine, and anthraquinone core structures (360 -480 daltons). Further screening of >1000 structural analogs revealed tetrahydrobenzothiophenes that activated ⌬F508-CFTR Cl ؊ conductance reversibly with K d < 100 nM. Single-cell voltage clamp analysis showed characteristic CFTR currents after ⌬F508-CFTR activation. Activation required low concentrations of a cAMP agonist, thus mimicking the normal physiological response. A Bayesian computational model was developed using tetrahydrobenzothiophene structure-activity data, yielding insight into the physical character and structural features of active and inactive potentiators and successfully predicting the activity of structural analogs. Efficient potentiation of defective ⌬F508-CFTR gating was also demonstrated in human bronchial epithelial cells from a ⌬F508 cystic fibrosis subject after 27°C temperature rescue. In conjunction with correctors of defective ⌬F508-CFTR processing, small molecule potentiators of defective ⌬F508-CFTR gating may be useful for therapy of cystic fibrosis caused by the ⌬F508 mutation.

Key Points• Chloride channels are important for proper hydration of the airway surface.• TMEM16A protein is an important component of calcium-activated chloride channels.• Interleukin-4, a cytokine that induces mucous cell metaplasia, also upregulates calcium-dependent chloride secretion in human bronchial epithelial cells.• In bronchial epithelial cells treated with interleukin-4, we found that TMEM16A protein becomes highly expressed in goblet but not in ciliated cells.• Upregulation of TMEM16A by interleukin-4 may be important for secretion and proper expansion of mucins. AbstractThe TMEM16A protein has a potential role as a Ca 2+ -activated Cl − channel (CaCC) in airway epithelia where it may be important in the homeostasis of the airway surface fluid. We investigated the function and expression of TMEM16A in primary human bronchial epithelial cells and in a bronchial cell line (CFBE41o-). Under resting conditions, TMEM16A protein expression was relatively low. However, TMEM16A silencing with short-interfering RNAs caused a marked inhibition of CaCC activity, thus demonstrating that a low TMEM16A expression is sufficient to support Ca 2+ -dependent Cl − transport. Following treatment for 24-72 h with interleukin-4 (IL-4), a cytokine that induces mucous cell metaplasia, TMEM16A protein expression was strongly increased in approximately 50% of primary bronchial epithelial cells, with a specific localization in the apical membrane. IL-4 treatment also increased the percentage of cells expressing MUC5AC, a marker of goblet cells. Interestingly, MUC5AC was detected specifically in cells expressing TMEM16A. In particular, MUC5AC was found in 15 and 60% of TMEM16A-positive cells when epithelia were treated with IL-4 for 24 or 72 h, respectively. In contrast, ciliated cells showed expression of the cystic fibrosis transmembrane conductance regulator Cl − channel but not of TMEM16A. Our results indicate that TMEM16A protein is responsible for CaCC activity in airway epithelial cells, particularly in cells treated with IL-4, and that TMEM16A upregulation by IL-4 appears as an early event of goblet cell differentiation. These findings suggest that TMEM16A expression is particularly required under conditions of mucus hypersecretion to ensure adequate secretion of electrolytes and water.

SCN− (thiocyanate) is an important physiological anion involved in innate defense of mucosal surfaces. SCN− is oxidized by H2O2, a reaction catalyzed by lactoperoxidase, to produce OSCN− (hypothiocyanite), a molecule with antimicrobial activity. Given the importance of the availability of SCN− in the airway surface fluid, we studied transepithelial SCN− transport in the human bronchial epithelium. We found evidence for at least three mechanisms for basolateral to apical SCN− flux. cAMP and Ca2+ regulatory pathways controlled SCN− transport through cystic fibrosis transmembrane conductance regulator and Ca2+-activated Cl− channels, respectively, the latter mechanism being significantly increased by treatment with IL-4. Stimulation with IL-4 also induced the strong up-regulation of an electroneutral SCN−/Cl− exchange. Global gene expression analysis with microarrays and functional studies indicated pendrin (SLC26A4) as the protein responsible for this SCN− transport. Measurements of H2O2 production at the apical surface of bronchial cells indicated that the extent of SCN− transport is important to modulate the conversion of this oxidant molecule by the lactoperoxidase system. Our studies indicate that the human bronchial epithelium expresses various SCN− transport mechanisms under resting and stimulated conditions. Defects in SCN− transport in the airways may be responsible for susceptibility to infections and/or decreased ability to scavenge oxidants.

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR) protein that reduce cAMP-stimulated Cl؊ conductance in airway and other epithelia. The purpose of this investigation was to identify new classes of potent CFTR activators. A collection of 60,000 diverse drug-like compounds was screened at 10 M together with a low concentration of forskolin (0.5 M) in Fisher rat thyroid epithelial cells co-expressing human CFTR and a green fluorescent protein-based Cl ؊ sensor. Primary screening yielded 57 strong activators (greater activity than reference compound apigenin), most of which were unrelated in chemical structure to known CFTR activators, and 284 weaker activators. Secondary analysis of the strong activators included analysis of CFTR specificity, forskolin requirement, transepithelial short-circuit current, activation kinetics, dose response, toxicity, and activation mechanism. Three compounds, the most potent being a dihydroisoquinoline, activated CFTR by elevating cellular cAMP, probably by phosphodiesterase inhibition. Fourteen compounds activated CFTR without cAMP elevation or phosphatase inhibition, suggesting direct CFTR interaction. The most potent compounds had tetrahydrocarbazol, hydroxycoumarin, and thiazolidine core structures. These compounds induced CFTR Cl

Previous studies in intact lung suggest that CFTR may play a role in cAMP-regulated fluid transport from the distal air spaces of the lung. However, the potential contribution of different epithelial cells (alveolar epithelial type I, type II, or bronchial epithelial cells) to CFTR-regulated fluid transport is unknown. In this study we determined whether the CFTR gene is expressed in human lung alveolar epithelial type II (AT II) cells and whether the CFTR chloride channel contributes to cAMP-regulated fluid transport in cultured human AT II cells. Human AT II cells were isolated and cultured on collagen I-coated Transwell membranes for 120-144 h with an air-liquid interface. The cultured cells retained typical AT II-like features based on morphologic studies. Net basal fluid transport was 0.9+/-0.1 mul.cm-2.h-1 and increased to 1.35+/-0.11 mul.cm-2.h-1 (mean+/-SE, n=18, P<0.05) by stimulation with cAMP agonists. The CFTR inhibitor, CFTRinh-172, inhibited cAMP stimulated but not basal fluid transport. In short-circuit current (Isc) studies with an apical-to-basolateral transepithelial Cl- gradient, apical application of CFTRinh-172 reversed the forskolin-induced decrease in Isc. Real time RT-PCR demonstrated CFTR transcript expression in human AT II cells at a level similar to that in airway epithelial cells. We conclude that CFTR is expressed in cultured human AT II cells and may contribute to cAMP-regulated apical-basolateral fluid transport.