Innovative Strategy toward Mutant CFTR Rescue in Cystic Fibrosis: Design and Synthesis of Thiadiazole Inhibitors of the E3 Ligase RNF5

In cystic fibrosis (CF), deletion of phenylalanine 508 (F508del) in the CF transmembrane conductance regulator (CFTR) is associated to misfolding and defective gating of the mutant channel. One of the most promising CF drug targets is the ubiquitin ligase RNF5, which promotes F508del-CFTR degradation. Recently, the first ever reported inhibitor of RNF5 was discovered, i.e., the 1,2,4-thiadiazol-5-ylidene inh-2. Here, we designed and synthesized a series of new analogues to explore the structure-activity relationships (SAR) of this class of compounds. SAR efforts ultimately led to compound 16, which showed a greater F508del-CFTR corrector activity than inh-2, good tolerability, and no toxic side effects. Analogue 16 increased the basal level of autophagy similar to what has been described with RNF5 silencing. Furthermore, co-treatment with 16 significantly improved the F508del-CFTR rescue induced by the triple combination elexacaftor/tezacaftor/ivacaftor in CFBE41o^- cells. These findings validate the 1,2,4-thiadiazolylidene scaffold for the discovery of novel RNF5 inhibitors and provide evidence to pursue this unprecedented strategy for the treatment of CF.

Proteostasis Regulators in Cystic Fibrosis: Current Development and Future Perspectives

In cystic fibrosis (CF), the deletion of phenylalanine 508 (F508del) in the CF transmembrane conductance regulator (CFTR) leads to misfolding and premature degradation of the mutant protein. These defects can be targeted with pharmacological agents named potentiators and correctors. During the past years, several efforts have been devoted to develop and approve new effective molecules. However, their clinical use remains limited, as they fail to fully restore F508del-CFTR biological function. Indeed, the search for CFTR correctors with different and additive mechanisms has recently increased. Among them, drugs that modulate the CFTR proteostasis environment are particularly attractive to enhance therapy effectiveness further. This Perspective focuses on reviewing the recent progress in discovering CFTR proteostasis regulators, mainly describing the design, chemical structure, and structure-activity relationships. The opportunities, challenges, and future directions in this emerging and promising field of research are discussed, as well.

Targeting of Ubiquitin E3 Ligase RNF5 as a Novel Therapeutic Strategy in Neuroectodermal Tumors

RNF5, an endoplasmic reticulum (ER) E3 ubiquitin ligase, participates to the ER-associated protein degradation guaranteeing the protein homeostasis. Depending on tumor model tested, RNF5 exerts pro- or anti-tumor activity. The aim of this study was to elucidate the controversial role of RNF5 in neuroblastoma and melanoma, two neuroectodermal tumors of infancy and adulthood, respectively. RNF5 gene levels are evaluated in publicly available datasets reporting the gene expression profile of melanoma and neuroblastoma primary tumors at diagnosis. The therapeutic effect of Analog-1, an RNF5 pharmacological activator, was investigated on in vitro and in vivo neuroblastoma and melanoma models. In both neuroblastoma and melanoma patients the high expression of RNF5 correlated with a better prognostic outcome. Treatment of neuroblastoma and melanoma cell lines with Analog-1 reduced cell viability by impairing the glutamine availability and energy metabolism through inhibition of F1Fo ATP-synthase activity. This latter event led to a marked increase in oxidative stress, which, in turn, caused cell death. Similarly, neuroblastoma- and melanoma-bearing mice treated with Analog-1 showed a significant delay of tumor growth in comparison to those treated with vehicle only. These findings validate RNF5 as an innovative drug target and support the development of Analog-1 in early phase clinical trials for neuroblastoma and melanoma patients.

Targeting the E1 ubiquitin-activating enzyme (UBA1) improves elexacaftor/tezacaftor/ivacaftor efficacy towards F508del and rare misfolded CFTR mutants

The advent of Trikafta (Kaftrio in Europe) (a triple-combination therapy based on two correctors—elexacaftor/tezacaftor—and the potentiator ivacaftor) has represented a revolution for the treatment of patients with cystic fibrosis (CF) carrying the most common misfolding mutation, F508del-CFTR. This therapy has proved to be of great efficacy in people homozygous for F508del-CFTR and is also useful in individuals with a single F508del allele. Nevertheless, the efficacy of this therapy needs to be improved, especially in light of the extent of its use in patients with rare class II CFTR mutations. Using CFBE41o- cells expressing F508del-CFTR, we provide mechanistic evidence that targeting the E1 ubiquitin-activating enzyme (UBA1) by TAK-243, a small molecule in clinical trials for other diseases, boosts the rescue of F508del-CFTR induced by CFTR correctors. Moreover, TAK-243 significantly increases the F508del-CFTR short-circuit current induced by elexacaftor/tezacaftor/ivacaftor in differentiated human primary airway epithelial cells, a gold standard for the pre-clinical evaluation of patients’ responsiveness to pharmacological treatments. This new combinatory approach also leads to an improvement in CFTR conductance on cells expressing other rare CF-causing mutations, including N1303K, for which Trikafta is not approved. These findings show that Trikafta therapy can be improved by the addition of a drug targeting the misfolding detection machinery at the beginning of the ubiquitination cascade and may pave the way for an extension of Trikafta to low/non-responding rare misfolded CFTR mutants.

Partial Rescue of F508del-CFTR Stability and Trafficking Defects by Double Corrector Treatment

Deletion of phenylalanine at position 508 (F508del) in the CFTR chloride channel is the most frequent mutation in cystic fibrosis (CF) patients. F508del impairs the stability and folding of the CFTR protein, thus resulting in mistrafficking and premature degradation. F508del-CFTR defects can be overcome with small molecules termed correctors. We investigated the efficacy and properties of VX-445, a newly developed corrector, which is one of the three active principles present in a drug (Trikafta^®/Kaftrio^®) recently approved for the treatment of CF patients with F508del mutation. We found that VX-445, particularly in combination with type I (VX-809, VX-661) and type II (corr-4a) correctors, elicits a large rescue of F508del-CFTR function. In particular, in primary bronchial epithelial cells of CF patients, the maximal rescue obtained with corrector combinations including VX-445 was close to 60–70% of CFTR function in non-CF cells. Despite this high efficacy, analysis of ubiquitylation, resistance to thermoaggregation, protein half-life, and subcellular localization revealed that corrector combinations did not fully normalize F508del-CFTR behavior. Our study indicates that it is still possible to further improve mutant CFTR rescue with the development of corrector combinations having maximal effects on mutant CFTR structural and functional properties.

A high-content drug screening strategy to identify protein level modulators for genetic diseases: A proof-of-principle in autosomal dominant leukodystrophy

In genetic diseases, the most prevalent mechanism of pathogenicity is an altered expression of dosage-sensitive genes. Drugs that restore physiological levels of these genes should be effective in treating the associated conditions. We developed a screening strategy, based on a bicistronic dual-reporter vector, for identifying compounds that modulate protein levels, and used it in a pharmacological screening approach. To provide a proof-of-principle, we chose autosomal dominant leukodystrophy (ADLD), an ultra-rare adult-onset neurodegenerative disorder caused by lamin B1 (LMNB1) overexpression. We used a stable Chinese hamster ovary (CHO) cell line that simultaneously expresses an AcGFP reporter fused to LMNB1 and a Ds-Red normalizer. Using high-content imaging analysis, we screened a library of 717 biologically active compounds and approved drugs, and identified alvespimycin, an HSP90 inhibitor, as a positive hit. We confirmed that alvespimycin can reduce LMNB1 levels by 30%-80% in five different cell lines (fibroblasts, NIH3T3, CHO, COS-7, and rat primary glial cells). In ADLD fibroblasts, alvespimycin reduced cytoplasmic LMNB1 by about 50%. We propose this approach for effectively identifying potential drugs for treating genetic diseases associated with deletions/duplications and paving the way toward Phase II clinical trials.

Identification, Structure-Activity Relationship, and Biological Characterization of 2,3,4,5-Tetrahydro-1H-pyrido[4,3-b]indoles as a Novel Class of CFTR Potentiators

Cystic fibrosis (CF) is a life-threatening autosomal recessive disease, caused by mutations in the CF transmembrane conductance regulator (CFTR) chloride channel. CFTR modulators have been reported to address the basic defects associated with CF-causing mutations, partially restoring the CFTR function in terms of protein processing and/or channel gating. Small-molecule compounds, called potentiators, are known to ameliorate the gating defect. In this study, we describe the identification of the 2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indole core as a novel chemotype of potentiators. In-depth structure-activity relationship studies led to the discovery of enantiomerically pure 39 endowed with a good efficacy in rescuing the gating defect of F508del- and G551D-CFTR and a promising in vitro druglike profile. The in vivo characterization of γ-carboline 39 showed considerable exposure levels and good oral bioavailability, with detectable distribution to the lungs after oral administration to rats. Overall, these findings may represent an encouraging starting point to further expand this chemical class, adding a new chemotype to the existing classes of CFTR potentiators.

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.

High-Content Screening Identifies Vanilloids as a Novel Class of Inhibitors of NET Formation

Neutrophils migrate to sites of infection where they phagocytose, degranulate, and/or, in the presence of appropriate stimuli, release decondensed chromatin strands (called neutrophil extracellular traps, NETs) for trapping and possibly killing microorganisms. NET formation is characterized by marked morphological cell changes, in particular within the nucleus. Lytic NET formation can be observed in neutrophils undergoing cell death, which is referred to as NETosis. Dysregulation of NET production and/or degradation can exert pathogenic effects, contributing to the pathogenesis of various diseases, including cystic fibrosis, autoimmune diseases and inflammatory conditions. By employing a phenotypic assay based on high-content imaging and analysis, we screened a library of biologically active compounds and identified vanilloids as a novel class of chemical compounds able to hinder NETosis induction and NET release. Vanilloids also markedly decrease cytosolic ROS production. The identification of novel vanilloid NET inhibitors, able to stop excessive or aberrant NET production might offer new therapeutic options for those disorders displaying NET overproduction.

The Autophagy Inhibitor Spautin-1 Antagonizes Rescue of Mutant CFTR Through an Autophagy-Independent and USP13-Mediated Mechanism

The mutation F508del, responsible for a majority of cystic fibrosis cases, provokes the instability and misfolding of the CFTR chloride channel. Pharmacological recovery of F508del-CFTR may be obtained with small molecules called correctors. However, treatment with a single corrector in vivo and in vitro only leads to a partial rescue, a consequence of cell quality control systems that still detect F508del-CFTR as a defective protein causing its degradation. We tested the effect of spautin-1 on F508del-CFTR since it is an inhibitor of USP10 deubiquitinase and of autophagy, a target and a biological process that have been associated with cystic fibrosis and mutant CFTR. We found that short-term treatment of cells with spautin-1 downregulates the function and expression of F508del-CFTR despite the presence of corrector VX-809, a finding obtained in multiple cell models and assays. In contrast, spautin-1 was ineffective on wild type CFTR. Silencing and upregulation of USP13 (another target of spautin-1) but not of USP10, had opposite effects on F508del-CFTR expression/function. In contrast, modulation of autophagy with known activators or inhibitors did not affect F508del-CFTR. Our results identify spautin-1 as a novel chemical probe to investigate the molecular mechanisms that prevent full rescue of mutant CFTR.

Chemically modified hCFTR mRNAs recuperate lung function in a mouse model of cystic fibrosis

Gene therapy has always been a promising therapeutic approach for Cystic Fibrosis (CF). However, numerous trials using DNA or viral vectors encoding the correct protein resulted in a general low efficacy. In the last years, chemically modified messenger RNA (cmRNA) has been proven to be a highly potent, pulmonary drug. Consequently, we first explored the expression, function and immunogenicity of human (h)CFTR encoded by cmRNA^hCFTR in vitro and ex vivo, quantified the expression by flow cytometry, determined its function using a YFP based assay and checked the immune response in human whole blood. Similarly, we examined the function of cmRNA^hCFTR in vivo after intratracheal (i.t.) or intravenous (i.v.) injection of the assembled cmRNA^hCFTR together with Chitosan-coated PLGA (poly-D, L-lactide-co-glycolide 75:25 (Resomer RG 752 H)) nanoparticles (NPs) by FlexiVent. The amount of expression of human hCFTR encoded by cmRNA^hCFTR was quantified by hCFTR ELISA, and cmRNA^hCFTR values were assessed by RT-qPCR. Thereby, we observed a significant improvement of lung function, especially in regards to FEV_0.1, suggesting NP-cmRNA^hCFTR as promising therapeutic option for CF patients independent of their CFTR genotype.

Thymosin α-1 does not correct F508del-CFTR in cystic fibrosis airway epithelia

In cystic fibrosis (CF), deletion of phenylalanine 508 (F508del) in the cystic fibrosis transmembrane conductance regulator (CFTR) anion channel causes misfolding and premature degradation. Considering the numerous effects of the F508del mutation on the assembly and processing of CFTR protein, combination therapy with several pharmacological correctors is likely to be required to treat CF patients. Recently, it has been reported that thymosin α-1 (Tα-1) has multiple beneficial effects that could lead to a single-molecule-based therapy for CF patients with F508del. Such effects include suppression of inflammation, improvement in F508del-CFTR maturation and gating, and stimulation of chloride secretion through the calcium-activated chloride channel (CaCC). Given the importance of such a drug, we aimed to characterize the underlying molecular mechanisms of action of Tα-1. In-depth analysis of Tα-1 effects was performed using well-established microfluorimetric, biochemical, and electrophysiological techniques on epithelial cell lines and primary bronchial epithelial cells from CF patients. The studies, which were conducted in 2 independent laboratories with identical outcome, demonstrated that Tα-1 is devoid of activity on mutant CFTR as well as on CaCC. Although Tα-1 may still be useful as an antiinflammatory agent, its ability to target defective anion transport in CF remains to be further investigated.

RNF5, DAB2 and Friends: Novel Drug Targets for Cystic Fibrosis

BACKGROUND

Deletion of phenylalanine 508 is the most frequent mutation causing cystic fibrosis. It causes multiple defects: 1) misfolding of the protein causing retention at the ER (processing defect); 2) reduced channel activity (gating defect); 3) reduced plasma membrane residency time due to increased internalization rate and defective recycling.

METHODS

Druggability of F508del-CFTR was demonstrated by several studies. Correctors are molecules able to improve maturation and trafficking to the membrane of F508del- CFTR. Correctors could act as pharmacological chaperones or as proteostasis regulators. Pharmacological chaperones act directly on mutant CFTR, while proteostasis regulators modify the proteostasis environment leading to beneficial effects on CFTR maturation.

RESULTS

The use of a single compound is not sufficient to promote a therapeutically relevant F508del-CFTR rescue. Drug therapy for CF will require combinations of correctors exploiting different mechanisms of action, i.e. pharmacological chaperones combined together or with a proteostasis regulator.

CONCLUSION

Development of more effective CF drugs could therefore rely on a better understanding of the molecular events underlying CFTR processing/degradation. This review will focus on most promising pathways and related targets for the development of novel CF pharmacotherapies.

High-throughput screening identifies FAU protein as a regulator of mutant cystic fibrosis transmembrane conductance regulator channel

In cystic fibrosis, deletion of phenylalanine 508 (F508del) in the cystic fibrosis transmembrane conductance regulator (CFTR) anion channel causes misfolding and premature degradation. One possible approach to reducing the detrimental health effects of cystic fibrosis could be the identification of proteins whose suppression rescues F508del-CFTR function in bronchial epithelial cells. However, searches for these potential targets have not yet been conducted, particularly in a relevant airway background using a functional readout. To identify proteins associated with F508del-CFTR processing, we used a high-throughput functional assay to screen an siRNA library targeting 6,650 different cellular proteins. We identified 37 proteins whose silencing significantly rescued F508del-CFTR activity, as indicated by enhanced anion transport through the plasma membrane. These proteins included FAU, UBE2I, UBA52, MLLT6, UBA2, CHD4, PLXNA1, and TRIM24, among others. We focused our attention on FAU, a poorly characterized protein with unknown function. FAU knockdown increased the plasma membrane targeting and function of F508del-CFTR, but not of wild-type CFTR. Investigation into the mechanism of action revealed a preferential physical interaction of FAU with mutant CFTR, leading to its degradation. FAU and other proteins identified in our screening may offer a therapeutically relevant panel of drug targets to correct basic defects in F508del-CFTR processing.

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.

Epithelial sodium channel silencing as a strategy to correct the airway surface fluid deficit in cystic fibrosis

In the respiratory system, Na(+) absorption and Cl(-) secretion are balanced to maintain an appropriate airway surface fluid (ASF) volume and ensure efficient mucociliary clearance. In cystic fibrosis (CF), this equilibrium is disrupted by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, resulting in the absence of functional CFTR-dependent Cl(-) secretion. The consequences of defective Cl(-) transport are worsened by the persistence of Na(+) absorption, which contributes to airway surface dehydration. We asked whether normal ASF can be restored to an equal extent by recovering Cl(-) secretion from mutated CFTR or by reducing Na(+) absorption. This is highly relevant in the selection of the best strategy for the treatment of patients with CF. We analyzed the ASF thickness of primary cultured bronchial CF and non-CF epithelia after silencing the epithelial Na(+) channel (ENaC) with specific short, interfering RNAs (siRNAs) and after the pharmacological stimulation of CFTR. Our results indicate that (1) single siRNAs complementary to ENaC subunits are sufficient to reduce ENaC transcripts, Na(+) channel activity, and fluid transport, but only silencing both the α and β ENaC subunits at the same time leads to an increase of ASF (from nearly 7 µm to more than 9 µm); (2) the ASF thickness obtained in this way is about half that measured after maximal CFTR stimulation in non-CF epithelia (10-14 µm); and (3) the pharmacological rescue of mutant CFTR increases the ASF to the same extent as ENaC silencing. Our results indicate that CFTR rescue and ENaC silencing both produce a significant and long-lasting increase of airway hydration in vitro.

Non-canonical translation start sites in the TMEM16A chloride channel

TMEM16A is a plasma membrane protein with voltage- and calcium-dependent chloride channel activity. The role of the various TMEM16A domains in expression and function is poorly known. In a previous study, we found that replacing the first ATG of the TMEM16A coding sequence with a nonsense codon (M1X mutation), to force translation from the second ATG localized at position 117, only had minor functional consequences. Therefore, we concluded that this region is dispensable for TMEM16A processing and channel activity. We have now removed the first 116 codons from the TMEM16A coding sequence. Surprisingly, the expression of the resulting mutant, Δ(1–116), resulted in complete loss of activity. We hypothesized that, in the mutant M1X, translation may start at a position before the second ATG, using a non-canonical start codon. Therefore, we placed an HA-epitope at position 89 in the M1X mutant. We found, by western blot analysis, that the HA-epitope can be detected, thus demonstrating that translation starts from an upstream non-ATG codon. We truncated the N-terminus of TMEM16A at different sites while keeping the HA-epitope. We found that stepwise shortening of TMEM16A caused an in parallel stepwise decrease in TMEM16A expression and function. Our results indicate that indeed the N-terminus of TMEM16A is important for its activity. The use of an alternative start codon appears to occur in a naturally-occurring TMEM16A isoform that is particularly expressed in human testis. Future experiments will need to address the role of normal and alternative amino-terminus in TMEM16A structure and function.

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TMEM16A is a membrane protein with chloride channel activity.

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Two ATG codons are present at positions 1 and 117 of the TMEM16A coding sequence.

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Deletion of the first ATG, in contrast to removal of the first 116 codons, does not abolish TMEM16A function.

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Without the first ATG, TMEM16A translation begins from a non-canonical start codon.

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.

The anoctamin family: TMEM16A and TMEM16B as calcium-activated chloride channels

The Ca(2+)-activated Cl(-) channels (CaCCs) are involved in a variety of physiological functions, such as transepithelial anion transport, smooth muscle contraction and olfaction. Recently, the question of the molecular identity of CaCCs has apparently been resolved with the identification of TMEM16A protein (also known as anoctamin-1). Expression of TMEM16A is associated with the appearance of Ca(2+)- and voltage-dependent Cl(-) currents with properties similar to those of native CaCCs. The putative structure of TMEM16A consists of eight transmembrane domains, with both the amino- and the carboxy-terminus protruding in the cytosol. TMEM16A is also characterized by the existence of different protein variants generated by alternative splicing. A close paralogue of TMEM16A, TMEM16B (anoctamin-2), is also associated with CaCC activity, although with different properties. The TMEM16B-dependent channels require higher intracellular Ca(2+) concentrations and have faster activation and deactivation kinetics. Expression of other anoctamins is devoid of detectable channel activity. These proteins, such as TMEM16F (anoctamin-6), may have different functions.

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.

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.

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.

Alternative splicing at a NAGNAG acceptor site as a novel phenotype modifier

Approximately 30% of alleles causing genetic disorders generate premature termination codons (PTCs), which are usually associated with severe phenotypes. However, bypassing the deleterious stop codon can lead to a mild disease outcome. Splicing at NAGNAG tandem splice sites has been reported to result in insertion or deletion (indel) of three nucleotides. We identified such a mechanism as the origin of the mild to asymptomatic phenotype observed in cystic fibrosis patients homozygous for the E831X mutation (2623G>T) in the CFTR gene. Analyses performed on nasal epithelial cell mRNA detected three distinct isoforms, a considerably more complex situation than expected for a single nucleotide substitution. Structure-function studies and in silico analyses provided the first experimental evidence of an indel of a stop codon by alternative splicing at a NAGNAG acceptor site. In addition to contributing to proteome plasticity, alternative splicing at a NAGNAG tandem site can thus remove a disease-causing UAG stop codon. This molecular study reveals a naturally occurring mechanism where the effect of either modifier genes or epigenetic factors could be suspected. This finding is of importance for genetic counseling as well as for deciding appropriate therapeutic strategies.

Mild disease outcomes associated with premature termination codons can result from at least three different mechanisms, but none of these mechanisms explain the mild phenotype observed in some patients. Subtle differences in alternative transcripts have recently been reported at NAGNAG tandem acceptor motifs, which can be detected in 30% of human genes. We provide the first experimental evidence of premature termination codon removal by alternative splicing at a NAGNAG acceptor splice site. Our study emphasizes the biological significance of such alternative splicing in the context of disease-causing mutations and defines a new phenotype-modifying mechanism that buffers nonsense mutations.

Influence of cell background on pharmacological rescue of mutant CFTR

Cystic fibrosis (CF) is caused by mutations in the CFTR chloride channel. Deletion of phenylalanine 508 (F508del), the most frequent CF mutation, impairs the maturation and gating of the CFTR protein. Such defects may be corrected in vitro by pharmacological modulators named as correctors and potentiators, respectively. We have evaluated a panel of correctors and potentiators derived from various sources to assess potency, efficacy, and mechanism of action. For this purpose, we have used functional and biochemical assays on two different cell expression systems, Fischer rat thyroid (FRT) and A549 cells. The order of potency and efficacy of potentiators was similar in the two cell types considered, with phenylglycine PG-01 and isoxazole UCCF-152 being the most potent and least potent, respectively. Most potentiators were also effective on two mutations, G551D and G1349D, that cause a purely gating defect. In contrast, corrector effect was strongly affected by cell background, with the extreme case of many compounds working in one cell type only. Our findings are in favor of a direct action of potentiators on CFTR, possibly at a common binding site. In contrast, most correctors seem to work indirectly with various mechanisms of action. Combinations of correctors acting at different levels may lead to additive F508del-CFTR rescue.

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.

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.

Block of CFTR-dependent chloride currents by inhibitors of multidrug resistance-associated proteins

The cystic fibrosis transmembrane conductance regulator (CFTR) is a membrane protein that belongs to the same family as multidrug resistance-associated proteins whose main function is to expel xenobiotics and physiological organic anions from the cell interior. Despite the overall structural similarity with these membrane proteins, CFTR is not an active transporter but is instead a Cl- channel. We have tested the ability of known inhibitors of multidrug resistance-associated proteins to affect CFTR Cl- currents. We have found that sulfinpyrazone, probenecid, and benzbromarone are also inhibitors of CFTR activity, with a mechanism involving blockage of the channel pore.