TMEM16A, a membrane protein associated with calcium-dependent chloride channel activity

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.

Small-molecule correctors of defective DeltaF508-CFTR cellular processing identified by high-throughput screening

The most common cause of cystic fibrosis (CF) is deletion of phenylalanine 508 (DeltaF508) in the CF transmembrane conductance regulator (CFTR) chloride channel. The DeltaF508 mutation produces defects in folding, stability, and channel gating. To identify small-molecule correctors of defective cellular processing, we assayed iodide flux in DeltaF508-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 DeltaF508-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 DeltaF508-CFTR protein. Biochemical studies suggested a mechanism of action involving improved DeltaF508-CFTR folding at the ER and stability at the cell surface. The bisaminomethylbithiazoles corrected DeltaF508-CFTR in DeltaF508/DeltaF508 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 DeltaF508 mutation.

Association of TMEM16A chloride channel overexpression with airway goblet cell metaplasia

The 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.

Cytokines induce tight junction disassembly in airway cells via an EGFR-dependent MAPK/ERK1/2-pathway

Epithelial barrier permeability is altered in inflammatory respiratory disorders by a variety of noxious agents through modifications of the epithelial cell structure that possibly involve tight junction (TJ) organization. To evaluate in vitro whether pro-inflammatory cytokines involved in the pathogenesis of respiratory disorders could alter TJ organization and epithelial barrier integrity, and to characterize the signal transduction pathway involved Calu-3 airway epithelial cells were exposed to TNF-a, IL-4 and IFN-g to assess changes in: (a) TJ assembly, that is, occludin and zonula occludens (ZO)-1 expression and localization, evaluated by confocal microscopy; (b) apoptotic activity, quantified using terminal transferase deoxyuridine triphosphate nick-end labeling staining; (c) epithelial barrier integrity, detected as transmembrane electrical resistance and expressed as G(T) values; (d) epidermal growth factor receptor (EGFR)-dependent mitogenactivated protein (MAP) kinase (MAPK)/extracellular signal-regulated kinases (ERK)1/2 phosphorylation, assessed by western blotting. Exposure to cytokines for 48 h induced a noticeable downregulation of the TJ transmembrane proteins. The degree ZO-1 and occludin colocalization was 62±2% in control cultures and significantly decreased in the presence of TNF-a (47±3%), IL-4 (43±1%) and INF-g (35±3%). Although no apoptosis induction was detected following exposure to cytokines, changes in the epithelial barrier integrity were observed, with a significant enhancement in paracellular conductance. G(T) values were, respectively, 1.030±0.0, 1.300±0.04, 1.260±0.020 and 2.220±0.015 (mS/cm²)1000 in control cultures and in those exposed to TNF-a, IFN-g and IL-4. The involvement of EGFR-dependent MAPK/ERK1/2 signaling pathway in cytokine-induced damage was demonstrated by a significant increase in threonine/tyrosine phosphorylation of ERK1/2, already detectable after 5 min incubation. All these cytokine-induced changes were markedly prevented when Calu-3 cells were cultured in the presence of an EGFR inhibitor (AG1478, 1 μM) or a MAP kinase inhibitor (U0126, 25 μM). In conclusion, cytokine-induced epithelial injury includes TJ disassembly and epithelial barrier permeability alteration and involves the EGFR-dependent MAPK/ERK1/2 signaling pathway.

Thiocyanate transport in resting and IL-4-stimulated human bronchial epithelial cells: role of pendrin and anion channels

SCN(-) (thiocyanate) is an important physiological anion involved in innate defense of mucosal surfaces. SCN(-) is oxidized by H(2)O(2), 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 Ca(2+) regulatory pathways controlled SCN(-) transport through cystic fibrosis transmembrane conductance regulator and Ca(2+)-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 H(2)O(2) 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.

IL-4 is a potent modulator of ion transport in the human bronchial epithelium in vitro

Recent data show that proinflammatory stimuli may modify significantly ion transport in the airway epithelium and therefore the properties of the airway surface fluid. We have studied the effect of IL-4, a cytokine involved in the pathogenesis of asthma, on transepithelial ion transport in the human bronchial epithelium in vitro. Incubation of polarized bronchial epithelial cells with IL-4 for 6-48 h causes a marked inhibition of the amiloride-sensitive Na(+) channel as measured in short circuit current experiments. On the other hand, IL-4 evokes a 2-fold increase in the current activated by a cAMP analog, which reflects the activity of the cystic fibrosis transmembrane conductance regulator (CFTR). Similarly, IL-4 enhances the response to apical UTP, an agonist that activates Ca(2+)-dependent Cl(-) channels. These effects are mimicked by IL-13 and blocked by an antagonist of IL-4Ralpha. RT-PCR experiments show that IL-4 elicits a 7-fold decrease in the level of the gamma amiloride-sensitive Na(+) channel mRNA, one of the subunits of the amiloride-sensitive Na(+) channel, and an increase in CFTR mRNA. Our data suggest that IL-4 may favor the hydration of the airway surface by decreasing Na(+) absorption and increasing Cl(-) secretion. This could be required to fluidify the mucus, which is hypersecreted during inflammatory conditions. On the other hand, the modifications of ion transport could also affect the ion composition of airway surface fluid.

Rescue of the mutant CFTR chloride channel by pharmacological correctors and low temperature analyzed by gene expression profiling

The F508del mutation, the most frequent in cystic fibrosis (CF), impairs the maturation of the CFTR chloride channel. The F508del defect can be partially overcome at low temperature (27°C) or with pharmacological correctors. However, the efficacy of correctors on the mutant protein appears to be dependent on the cell expression system. We have used a bronchial epithelial cell line, CFBE41o-, to determine the efficacy of various known treatments and to discover new correctors. Compared with other cell types, CFBE41o- shows the largest response to low temperature and the lowest one to correctors such as corr-4a and VRT-325. A screening of a small-molecule library identified 9-aminoacridine and ciclopirox, which were significantly more effective than corr-4a and VRT-325. Analysis with microarrays revealed that 9-aminoacridine, ciclopirox, and low temperature, in contrast to corr-4a, cause a profound change in cell transcriptome. These data suggest that 9-aminoacridine and ciclopirox act on F508del-CFTR maturation as proteostasis regulators, a mechanism already proposed for the histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA). However, we found that 9-aminoacridine, ciclopirox, and SAHA, in contrast to corr-4a, VRT-325, and low temperature, do not increase chloride secretion in primary bronchial epithelial cells from CF patients. These conflicting data appeared to be correlated with different gene expression signatures generated by these treatments in the cell line and in primary bronchial epithelial cells. Our results suggest that F508del-CFTR correctors acting by altering the cell transcriptome may be particularly active in heterologous expression systems but markedly less effective in native epithelial cells.

Goblet Cell Hyperplasia Requires High Bicarbonate Transport To Support Mucin Release

Goblet cell hyperplasia, a feature of asthma and other respiratory diseases, is driven by the Th-2 cytokines IL-4 and IL-13. In human bronchial epithelial cells, we find that IL-4 induces the expression of many genes coding for ion channels and transporters, including TMEM16A, SLC26A4, SLC12A2, and ATP12A. At the functional level, we find that IL-4 enhances calcium- and cAMP-activated chloride/bicarbonate secretion, resulting in high bicarbonate concentration and alkaline pH in the fluid covering the apical surface of epithelia. Importantly, mucin release, elicited by purinergic stimulation, requires the presence of bicarbonate in the basolateral solution and is defective in cells derived from cystic fibrosis patients. In conclusion, our results suggest that Th-2 cytokines induce a profound change in expression and function in multiple ion channels and transporters that results in enhanced bicarbonate transport ability. This change is required as an important mechanism to favor release and clearance of mucus.

Ion channel and lipid scramblase activity associated with expression of TMEM16F/ANO6 isoforms

TMEM16F is a membrane protein with possible dual function as an ion channel and a phospholipid scramblase. The properties of ion channels associated with TMEM16F and the link between ion channel and scramblase activity are a matter of debate. We studied the properties of four isoforms of TMEM16F generated by alternative splicing. Upregulation of three TMEM16F isoforms or silencing of endogenous TMEM16F increased and decreased, respectively, both scramblase and channel activities. Introduction of an activating mutation in TMEM16F sequence caused a marked increase in phosphatidylserine scrambling and in ion transport indicating direct involvement of the protein in both functions. TMEM16F, also known as ANO6, is a membrane protein that has been associated with phospholipid scramblase and ion channel activity. However, the characteristics of TMEM16F-dependent channels, particularly the ion selectivity, are a matter of debate. Furthermore, the direct involvement of TMEM16F in phospholipid scrambling has been questioned. We studied the properties of different TMEM16F variants generated by alternative splicing. Using whole-cell patch-clamp recordings, we found that V1, V2 and V5 variants generated membrane currents activated by very high (micromolar) intracellular Ca(2+) concentrations and positive membrane potentials. These variants showed different degrees of Ca(2+) sensitivity and kinetics of activation but similar ion permeability, characterized by a slight selectivity for Cl(-) over Na(+) . A fourth variant (V3) showing a unique carboxy-terminus was devoid of activity, in agreement with its intracellular localization. We also measured scramblase activity using the binding of annexin V to detect phosphatidylserine on the cell surface. V1, V2 and V5 variants were associated with calcium-dependent phosphatidylserine externalization. Interestingly, introduction of an activating mutation, D409G, produced a marked increase in the apparent Ca(2+) sensitivity of TMEM16F-dependent channels. In parallel, this mutation also enhanced the extent of phosphatidylserine externalization that occurred even under resting conditions. These results support the conclusion that TMEM16F proteins are directly involved in dual activity, as a phospholipid scramblase and as an ion channel.

A novel missense mutation in ANO5/TMEM16E is causative for gnathodiaphyseal dyplasia in a large Italian pedigree

Gnathodiaphyseal dysplasia (GDD) is an autosomal dominant syndrome characterized by frequent bone fractures at a young age, bowing of tubular bones and cemento-osseus lesions of the jawbones. Anoctamin 5 (ANO5) belongs to the anoctamin protein family that includes calcium-activated chloride channels. However, recent data together with our own experiments reported here add weight to the hypothesis that ANO5 may not function as calcium-activated chloride channel. By sequencing the entire ANO5 gene coding region and untranslated regions in a large Italian GDD family, we found a novel missense mutation causing the p.Thr513Ile substitution. The mutation segregates with the disease in the family and has never been described in any database as a polymorphism. To date, only two mutations on the same cysteine residue at position 356 of ANO5 amino-acid sequence have been described in GDD families. As ANO5 has also been found to be mutated in two different forms of muscular dystrophy, the finding of this third mutation in GDD adds clues to the role of ANO5 in these disorders.

Dual activity of aminoarylthiazoles on the trafficking and gating defects of the cystic fibrosis transmembrane conductance regulator chloride channel caused by cystic fibrosis mutations

A large fraction of mutations causing cystic fibrosis impair the function of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel by causing reduced channel activity (gating defect) and/or impaired exit from the endoplasmic reticulum (trafficking defect). Such defects need to be treated with separate pharmacological compounds termed potentiators and correctors, respectively. Here, we report the characterization of aminoarylthiazoles (AATs) as compounds having dual activity. Cells expressing mutant CFTR were studied with functional assays (fluorescence-based halide transport and short circuit current measurements) to assess the effect of acute and chronic treatment with compounds. We found that AATs are effective on F508del, the most frequent cystic fibrosis mutation, which is associated with both a gating and a trafficking defect. AATs are also effective on mutations like G1349D and G551D, which cause only a gating defect. Evaluation of a panel of AAT analogs identified EN277I as the most effective compound. Incubation of cells expressing mutant CFTR with EN277I caused a strong stimulation of channel activity as demonstrated by single channel recordings. Compounds with dual activity such as AATs may be useful for the development of effective drugs for the treatment of cystic fibrosis.

Ionocytes and CFTR Chloride Channel Expression in Normal and Cystic Fibrosis Nasal and Bronchial Epithelial Cells

The airway epithelium contains ionocytes, a rare cell type with high expression of Forkhead Box I1 (FOXI1) transcription factor and Cystic Fibrosis Transmembrane conductance Regulator (CFTR), a chloride channel that is defective in cystic fibrosis (CF). Our aim was to verify if ionocyte development is altered in CF and to investigate the relationship between ionocytes and CFTR-dependent chloride secretion. We collected nasal cells by brushing to determine ionocyte abundance. Nasal and bronchial cells were also expanded in vitro and reprogrammed to differentiated epithelia for morphological and functional studies. We found a relatively high (~3%) ionocyte abundance in ex vivo nasal samples, with no difference between CF and control individuals. In bronchi, ionocytes instead appeared very rarely as previously reported, thus suggesting a possible proximal-distal gradient in human airways. The difference between nasal and bronchial epithelial cells was maintained in culture, which suggests an epigenetic control of ionocyte development. In the differentiation phase of the culture procedure, we used two media that resulted in a different pattern of CFTR expression: confined to ionocytes or more broadly expressed. CFTR function was similar in both conditions, thus indicating that chloride secretion equally occurs irrespective of CFTR expression pattern.

Correction of G551D-CFTR transport defect in epithelial monolayers by genistein but not by CPX or MPB-07

1. This study compares the effect of three chemically unrelated cystic fibrosis transmembrane conductance regulator (CFTR) activators on epithelial cell monolayers expressing the G551D-CFTR mutant. 2. We measured Cl(-) transport as the amplitude of short-circuit current in response to the membrane permeable cAMP analogue 8-(4-chlorophenylthio)adenosine-3'-5'-cyclic monophosphate (CPT-cAMP) alone or in combination with a CFTR opener. The correction of G551D-CFTR defect was quantified by comparison with maximal activity elicited in cells expressing wild type CFTR. To this end we used Fisher rat thyroid (FRT) cells transfected with wild type or G551D CFTR, and primary cultures of human nasal epithelial cells. 3. In both types of epithelia, cAMP caused activation of Cl(-) transport that was inhibited by glibenclamide and not by 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid. After normalising for CFTR expression, the response of FRT-G551D epithelia was 1% that of wild type monolayers. 4. Addition of genistein (10-200 micro M), but not of 8-cyclopentyl-1,3-dipropylxanthine (CPX, 1-100 micro M) or of the benzo[c]quinolizinium MPB-07 (10-200 micro M) to FRT-G551D epithelia pre-treated with cAMP, stimulated a sustained current that at maximal genistein concentration corresponded to 30% of the response of wild type epithelia. 5. The genistein dose-response curve was bell-shaped due to inhibitory activity at the highest concentrations. The dose-dependence in G551D cells was shifted with respect to wild type CFTR so that higher genistein concentrations were required to observe activation and inhibition, respectively. 6. On human nasal epithelia the correction of G551D-CFTR defective conductance obtained with genistein was 20% that of wild type. The impressive effect of genistein suggests that it might correct the Cl(-) transport defect on G551D patients.

Antihypertensive 1,4-dihydropyridines as correctors of the cystic fibrosis transmembrane conductance regulator channel gating defect caused by cystic fibrosis mutations

Cystic fibrosis (CF) is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel gene. CF mutations like deltaF508 cause both a mistrafficking of the protein and a gating defect. Other mutations, like G551D, cause only a gating defect. Our aim was to find chemical compounds able to stimulate the activity of CFTR mutant proteins by screening a library containing approved drugs. Two thousand compounds were tested on Fischer rat thyroid cells coexpressing deltaF508-CFTR and a halide-sensitive yellow fluorescent protein (YFP) after correction of the trafficking defect by low-temperature incubation. The YFP-based screening allowed the identification of the antihypertensive 1,4-dihydropyridines (DHPs) nifedipine, nicardipine, nimodipine, isradipine, nitrendipine, felodipine, and niguldipine as compounds able to activate deltaF508-CFTR. This effect was not derived from the inhibition of voltage-dependent Ca2+ channels, the pharmacological target of antihypertensive DHPs. Indeed, methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-2(trifluoromethylphenyl)pyridine-5-carboxylate (BayK-8644), a DHP that is effective as an activator of such channels, also stimulated CFTR activity. DHPs were also effective on the G551D-CFTR mutant by inducing a 16- to 45-fold increase of the CFTR Cl- currents. DHP activity was confirmed in airway epithelial cells from patients with CF. DHPs may represent a novel class of therapeutic agents able to correct the defect caused by a set of CF mutations.

CFTR activation in human bronchial epithelial cells by novel benzoflavone and benzimidazolone compounds

Activators of the CFTR Cl- channel may be useful for therapy of cystic fibrosis. Short-circuit current (Isc) measurements were done on human bronchial epithelial cells to characterize the best flavone and benzimidazolone CFTR activators identified by lead-based combinatorial synthesis and high-throughput screening. The 7,8-benzoflavone UCcf-029 was a potent activator of Cl- transport, with activating potency (<1 microM) being much better than other flavones, such as apigenin. The benzimidazolone UCcf-853 gave similar Isc but with lower potency (5-20 microM). In combination, the effect induced by maximal UCcf-029 and UCcf-029, UCcf-853, and apigenin increased strongly with increasing basal CFTR activity: for example, Kd for activation by UCcf-029 decreased from >5 to <0.4 microM with increasing basal Isc from approximately 4 microA/cm2 to approximately 12 microA/cm2. This dependence was confirmed in permeabilized Fischer rat thyroid cells stably expressing CFTR. Our results demonstrate efficacy of novel CFTR activators in bronchial epithelia and provide evidence that activating potency depends on basal CFTR activity.

Gelsolin secretion in interleukin-4-treated bronchial epithelia and in asthmatic airways

RATIONALE

The airway surface liquid, the thin layer of liquid covering the airways, is essential for mucociliary clearance and as a barrier against microbial and other noxious agents. Proteins secreted into the airway surface liquid by epithelial and nonepithelial cells may be important in innate immunity and to improve the fluidity of mucous secretions.

OBJECTIVES

We aimed to identify proteins specifically secreted into the airway surface liquid by human bronchial epithelial cells, in resting conditions and after treatment with interleukin 4 (IL-4), a cytokine released in asthma.

METHODS AND MAIN RESULTS

By using a proteomics approach, we found that one of the most abundant proteins was gelsolin, which breaks down actin filaments. Gelsolin mRNA and protein secretion were increased threefold in the airway surface liquid of epithelia treated with IL-4. These results were confirmed at the functional level by measuring actin depolymerization using a fluorescence assay. Gelsolin protein was also upregulated in the airways of subjects with asthma.

CONCLUSIONS

Our findings indicate that gelsolin is released by epithelial cells into the airways and that its secretion is increased by IL-4 in vitro. In addition, we found that the concentration of both IL-4 and gelsolin were raised in the bronchoalveolar lavage of patients with asthma. These results suggest that gelsolin might improve the fluidity of airway surface liquid in asthma by breaking down filamentous actin that may be released in large amounts by dying cells during inflammation.

Increased expression of ATP12A proton pump in cystic fibrosis airways

Proton secretion mediated by ATP12A protein on the surface of the airway epithelium may contribute to cystic fibrosis (CF) lung disease by favoring bacterial infection and airway obstruction. We studied ATP12A in fresh bronchial samples and in cultured epithelial cells. In vivo, ATP12A expression was found almost exclusively at the apical side of nonciliated cells of airway epithelium and in submucosal glands, with much higher expression in CF samples. This could be due to bacterial infection and inflammation, since treating cultured cells with bacterial supernatants or with IL-4 (a cytokine that induces goblet cell hyperplasia) increased the expression of ATP12A in nonciliated cells. This observation was associated with upregulation and translocation of ATP1B1 protein from the basal to apical epithelial side, where it colocalizes with ATP12A. ATP12A function was evaluated by measuring the pH of the apical fluid in cultured epithelia. Under resting conditions, CF epithelia showed more acidic values. This abnormality was minimized by inhibiting ATP12A with ouabain. Following treatment with IL-4, ATP12A function was markedly increased, as indicated by strong acidification occurring under bicarbonate-free conditions. Our study reveals potentially novel aspects of ATP12A and remarks its importance as a possible therapeutic target in CF and other respiratory diseases.

TRPV4 and purinergic receptor signalling pathways are separately linked in airway epithelia to CFTR and TMEM16A chloride channels

KEY POINTS

Eact is a putative pharmacological activator of TMEM16A. Eact is strongly effective in recombinant Fischer rat thyroid (FRT) cells but not in airway epithelial cells with endogenous TMEM16A expression. Transcriptomic analysis, gene silencing and functional studies in FRT cells reveal that Eact is actually an activator of the Ca²⁺ -permeable TRPV4 channel. In airway epithelial cells TRPV4 and TMEM16A are expressed in separate cell types. Intracellular Ca²⁺ elevation by TRPV4 stimulation leads to CFTR channel activation.

ABSTRACT

TMEM16A is a Ca²⁺ -activated Cl^- channel expressed in airway epithelial cells, particularly under conditions of mucus hypersecretion. To investigate the role of TMEM16A, we used Eact, a putative TMEM16A pharmacological activator. However, in contrast to purinergic stimulation, we found little effect of Eact on bronchial epithelial cells under conditions of high TMEM16A expression. We hypothesized that Eact is an indirect activator of TMEM16A. By a combination of approaches, including short-circuit current recordings, bulk and single cell RNA sequencing, intracellular Ca²⁺ imaging and RNA interference, we found that Eact is actually an activator of the Ca²⁺ -permeable TRPV4 channel and that the modest effect of this compound in bronchial epithelial cells is due to a separate expression of TMEM16A and TRPV4 in different cell types. Importantly, we found that TRPV4 stimulation induced activation of the CFTR Cl^- channel. Our study reveals the existence of separate Ca²⁺ signalling pathways linked to different Cl^- secretory processes.

Functional human CFTR produced by a stable minichromosome

Artificial chromosomes have been claimed to be the ideal vector for gene therapy, but their use has been hampered by an inability to produce stable and well designed molecules. We have used a structurally defined minichromosome to clone the human cystic fybrosis transmembrane conductance regulator (CFTR) locus. To guarantee the presence of the proper regulatory elements, we used the 320 kb yeast artificial chromosome (YAC) 37AB12 with the intact CFTR gene and upstream sequences. The resulting minichromosome was analyzed for the presence of the entire CFTR gene and for its functional activity by molecular and functional methods. We have identified clones showing the presence of both the transcript and the CFTR protein. Moreover, in the same clones, a chloride secretory response to cAMP was detected. Mitotic and molecular stability after prolonged growth without selection demonstrated that the constructs were stable. This is the first example of a structurally known minichromosome made to contain an active therapeutic gene.

Two CFTR mutations within codon 970 differently impact on the chloride channel functionality

Pharmacological rescue of mutant cystic fibrosis transmembrane conductance regulator (CFTR) in cystic fibrosis (CF) depends on the specific defect caused by different mutation classes. We asked whether a patient with the rare p.Gly970Asp (c.2909G>A) mutation could benefit from CFTR pharmacotherapy since a similar missense mutant p.Gly970Arg (c.2908G>C) was previously found to be sensitive to potentiators in vitro but not in vivo. By complementary DNA transfection, we found that both mutations are associated with defective CFTR function amenable to pharmacological treatment. However, analysis of messenger RNA (mRNA) from patient's cells revealed that c.2908G>C impairs RNA splicing whereas c.2909G>A does not perturb splicing and leads to the expected p.Gly970Asp mutation. In agreement with these results, nasal epithelial cells from the p.Gly970Asp patient showed significant improvement of CFTR function upon pharmacological treatment. Our results underline the importance of controlling the effect of CF mutation at the mRNA level to determine if the pharmacotherapy of CFTR basic defect is appropriate.

IL-12 can target human lung adenocarcinoma cells and normal bronchial epithelial cells surrounding tumor lesions

Background

Non small cell lung cancer (NSCLC) is a leading cause of cancer death. We have shown previously that IL-12rb2 KO mice develop spontaneously lung adenocarcinomas or bronchioalveolar carcinomas.

Aim of the study was to investigate i) IL-12Rβ2 expression in human primary lung adenocarcinomas and in their counterparts, i.e. normal bronchial epithelial cells (NBEC), ii) the direct anti-tumor activity of IL-12 on lung adenocarcinoma cells in vitro and vivo, and the mechanisms involved, and iii) IL-12 activity on NBEC.

Methodology/Principal Findings

Stage I lung adenocarcinomas showed significantly (P = 0.012) higher frequency of IL-12Rβ2 expressing samples than stage II/III tumors. IL-12 treatment of IL-12R⁺ neoplastic cells isolated from primary adenocarcinoma (n = 6) inhibited angiogenesis in vitro through down-regulation of different pro-angiogenic genes (e.g. IL-6, VEGF-C, VEGF-D, and laminin-5), as assessed by chorioallantoic membrane (CAM) assay and PCR array. In order to perform in vivo studies, the Calu6 NSCLC cell line was transfected with the IL-12RB2 containing plasmid (Calu6/β2). Similar to that observed in primary tumors, IL-12 treatment of Calu6/β2⁺ cells inhibited angiogenesis in vitro. Tumors formed by Calu6/β2 cells in SCID/NOD mice, inoculated subcutaneously or orthotopically, were significantly smaller following IL-12 vs PBS treatment due to inhibition of angiogenesis, and of IL-6 and VEGF-C production. Explanted tumors were studied by histology, immuno-histochemistry and PCR array. NBEC cells were isolated and cultured from lung specimens of non neoplastic origin. NBEC expressed IL-12R and released constitutively tumor promoting cytokines (e.g. IL-6 and CCL2). Treatment of NBEC with IL-12 down-regulated production of these cytokines.

Conclusions

This study demonstrates that IL-12 inhibits directly the growth of human lung adenocarcinoma and targets the adjacent NBEC. These novel anti-tumor activities of IL-12 add to the well known immune-modulatory properties of the cytokine and may provide a rational basis for the development of a clinical trial.

A minimal isoform of the TMEM16A protein associated with chloride channel activity

TMEM16A protein, also known as anoctamin-1, has been recently identified as an essential component of Ca(2+)-activated Cl(-) channels. We previously reported the existence of different TMEM16A isoforms generated by alternative splicing. In the present study, we have determined the functional properties of a minimal TMEM16A protein. This isoform, called TMEM16A(0), has a significantly shortened amino-terminus and lacks three alternative segments localized in the intracellular regions of the protein (total length: 840 amino acids). TMEM16A(0) expression is associated with Ca(2+)-activated Cl(-) channel activity as measured by three different functional assays based on the halide-sensitive yellow fluorescent protein, short-circuit current recordings, and patch-clamp technique. However, compared to a longer isoform, TMEM16(abc) (total length: 982 amino acids), TMEM16A(0) completely lacks voltage-dependent activation. Furthermore, TMEM16A(0) and TMEM16A(abc) have similar but not identical responses to extracellular anion replacement, thus suggesting a difference in ion selectivity and conductance. Our results indicate that TMEM16A(0) has the basic domains required for anion transport and Ca(2+)-sensitivity. However, the absence of alternative segments, which are present in more complex isoforms of TMEM16A, modifies the channel gating and ion transport ability.

Discovery of a picomolar potency pharmacological corrector of the mutant CFTR chloride channel

F508del, the most frequent mutation causing cystic fibrosis (CF), results in mistrafficking and premature degradation of the CFTR chloride channel. Small molecules named correctors may rescue F508del-CFTR and therefore represent promising drugs to target the basic defect in CF. We screened a carefully designed chemical library to find F508del-CFTR correctors. The initial active compound resulting from the primary screening underwent extensive chemical optimization. The final compound, ARN23765, showed an extremely high potency in bronchial epithelial cells from F508del homozygous patients, with an EC₅₀ of 38 picomolar, which is more than 5000-fold lower compared to presently available corrector drugs. ARN23765 also showed high efficacy, synergy with other types of correctors, and compatibility with chronic VX-770 potentiator. Besides being a promising drug, particularly suited for drug combinations, ARN23765 represents a high-affinity probe for CFTR structure-function studies.

Epithelial sodium channel inhibition in primary human bronchial epithelia by transfected siRNA

Na(+) absorption and Cl(-) secretion are in equilibrium to maintain an appropriate airway surface fluid volume and ensure appropriate mucociliary clearance. In cystic fibrosis, this equilibrium is disrupted by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene resulting in the absence of functional CFTR protein, which in turn results in deficient cAMP-dependent Cl(-) secretion and predominant Na(+) absorption. It has been suggested that down-regulation of the epithelial sodium channel, ENaC, might help to restore airway hydration and reverse the airway phenotype in patients with cystic fibrosis. We used an siRNA approach to analyze the possibility of down-regulating ENaC function in bronchial epithelia and examine the resulting effects on fluid transport. siRNA sequences complementary to each of the three ENaC subunits have been used to establish whether single subunit down-regulation is enough to reduce Na(+) absorption. Transfection was performed by exposure to siRNA for 24 hours at the time of cell seeding on permeable support. By using primary human bronchial epithelial cells we demonstrate that (1) siRNA sequences complementary to ENaC subunits are able to reduce ENaC transcripts and Na(+) channel activity by 50 to 70%, (2) transepithelial fluid absorption decreases, and (3) these functional effects last at least 8 days. A decrease in ENaC mRNA results in a significant reduction of ENaC protein function and fluid absorption through the bronchial epithelium, indicating that an RNA interference approach may improve the airway hydration status in patients with cystic fibrosis.