Nanomolar affinity small molecule correctors of defective Delta F508-CFTR chloride channel gating

Identification of synergistic combinations of F508del cystic fibrosis transmembrane conductance regulator (CFTR) modulators

Cystic fibrosis (CF) is an inherited, life-threatening disease caused by mutations in the gene encoding cystic fibrosis transmembrane conductance regulator (CFTR), an ABC transporter-class protein and ion channel that transports ions across epithelial cell membranes. The most common mutation leads to the deletion of a single phenylalanine, and the resulting protein, F508del-CFTR, shows reduced trafficking to the membrane and defective channel gating. The ideal therapeutic approach would address both of these defects and restore channel function at the same time. We describe here the application of a combination high-throughput screening to search for synergistic modulators of F508del-CFTR. With the adapted Fischer rat thyroid-yellow fluorescent protein halide flux assay to the combination high-throughput screening platform, we identified many interesting single agents as CFTR modulators from a library of approved drugs and mechanistic probe compounds, and combinations that synergistically modulate F508del-CFTR channel function in Fischer rat thyroid cells, demonstrating the potential for combination therapeutics to address the defects that cause CF.

Optimization of a Yellow fluorescent protein-based iodide influx high-throughput screening assay for cystic fibrosis transmembrane conductance regulator (CFTR) modulators

Cystic fibrosis is an inherited, life-threatening disease associated with mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The most common mutation, F508del CFTR, is found in 90% of CF patients. The loss of a single amino acid (phenylalanine at position 508) results in malformed CFTR with defective trafficking to the plasma membrane and impaired channel function. A functional assay with cells expressing F508del CFTR has been previously described by others using genetically engineered halide-sensitive yellow fluorescent protein to screen for CFTR modulators. We adapted this yellow fluorescent protein assay to 384-well plate format with a high-throughput screening plate reader, and optimized the assay in terms of data quality, resolution, and throughput, with target-specific protocols. The optimized assay was validated with reference compounds from cystic fibrosis foundation therapeutics. On the basis of the Z-factor range (≥0.5) and the potential productivity, this assay is well suited for high-throughput screening. It was successfully used to screen for active single agent and synergistic combinations of single agent modulators of F508del CFTR from a library collection of current active pharmaceutical ingredients (supported by Cystic Fibrosis Foundation Therapeutics).

Activation of the cystic fibrosis transmembrane conductance regulator by the flavonoid quercetin: potential use as a biomarker of ΔF508 cystic fibrosis transmembrane conductance regulator rescue

Therapies to correct the ΔF508 cystic fibrosis transmembrane conductance regulator (CFTR) folding defect require sensitive methods to detect channel activity in vivo. The β₂ adrenergic receptor agonists, which provide the CFTR stimuli commonly used in nasal potential difference assays, may not overcome the channel gating defects seen in ΔF508 CFTR after plasma membrane localization. In this study, we identify an agent, quercetin, that enhances the detection of surface ΔF508 CFTR, and is suitable for nasal perfusion. A screen of flavonoids in CFBE41o⁻ cells stably transduced with ΔF508 CFTR, corrected to the cell surface with low temperature growth, revealed that quercetin stimulated an increase in the short-circuit current. This increase was dose-dependent in both Fisher rat thyroid and CFBE41o⁻ cells. High concentrations inhibited Cl⁻ conductance. In CFBE41o⁻ airway cells, quercetin (20 μg/ml) activated ΔF508 CFTR, whereas the β₂ adrenergic receptor agonist isoproterenol did not. Quercetin had limited effects on cAMP levels, but did not produce detectable phosphorylation of the isolated CFTR R-domain, suggesting an activation independent of channel phosphorylation. When perfused in the nares of Cftr(+) mice, quercetin (20 μg/ml) produced a hyperpolarization of the potential difference that was absent in Cftr(-/-) mice. Finally, quercetin-induced, dose-dependent hyperpolarization of the nasal potential difference was also seen in normal human subjects. Quercetin activates CFTR-mediated anion transport in respiratory epithelia in vitro and in vivo, and may be useful in studies intended to detect the rescue of ΔF508 CFTR by nasal potential difference.

Small molecule correctors of F508del-CFTR discovered by structure-based virtual screening

Folding correctors of F508del-CFTR were discovered by in silico structure-based screening utilizing homology models of CFTR. The intracellular segment of CFTR was modeled and three cavities were identified at inter-domain interfaces: (1) Interface between the two Nucleotide Binding Domains (NBDs); (2) Interface between NBD1 and Intracellular Loop (ICL) 4, in the region of the F508 deletion; (3) multi-domain interface between NBD1:2:ICL1:2:4. We hypothesized that compounds binding at these interfaces may improve the stability of the protein, potentially affecting the folding yield or surface stability. In silico structure-based screening was performed at the putative binding-sites and a total of 496 candidate compounds from all three sites were tested in functional assays. A total of 15 compounds, representing diverse chemotypes, were identified as F508del folding correctors. This corresponds to a 3% hit rate, ~tenfold higher than hit rates obtained in corresponding high-throughput screening campaigns. The same binding sites also yielded potentiators and, most notably, compounds with a dual corrector-potentiator activity (dual-acting). Compounds harboring both activity types may prove to be better leads for the development of CF therapeutics than either pure correctors or pure potentiators. To the best of our knowledge this is the first report of structure-based discovery of CFTR modulators.

Extended-connectivity fingerprints

Extended-connectivity fingerprints (ECFPs) are a novel class of topological fingerprints for molecular characterization. Historically, topological fingerprints were developed for substructure and similarity searching. ECFPs were developed specifically for structure-activity modeling. ECFPs are circular fingerprints with a number of useful qualities: they can be very rapidly calculated; they are not predefined and can represent an essentially infinite number of different molecular features (including stereochemical information); their features represent the presence of particular substructures, allowing easier interpretation of analysis results; and the ECFP algorithm can be tailored to generate different types of circular fingerprints, optimized for different uses. While the use of ECFPs has been widely adopted and validated, a description of their implementation has not previously been presented in the literature.

Regulatory insertion removal restores maturation, stability and function of DeltaF508 CFTR

The cystic fibrosis transmembrane conductance regulator (CFTR) epithelial anion channel is a large multidomain membrane protein that matures inefficiently during biosynthesis. Its assembly is further perturbed by the deletion of F508 from the first nucleotide-binding domain (NBD1) responsible for most cystic fibrosis. The mutant polypeptide is recognized by cellular quality control systems and is proteolyzed. CFTR NBD1 contains a 32-residue segment termed the regulatory insertion (RI) not present in other ATP-binding cassette transporters. We report here that RI deletion enabled F508 CFTR to mature and traffic to the cell surface where it mediated regulated anion efflux and exhibited robust single chloride channel activity. Long-term pulse-chase experiments showed that the mature DeltaRI/DeltaF508 had a T(1/2) of approximately 14 h in cells, similar to the wild type. RI deletion restored ATP occlusion by NBD1 of DeltaF508 CFTR and had a strong thermostabilizing influence on the channel with gating up to at least 40 degrees C. None of these effects of RI removal were achieved by deletion of only portions of RI. Discrete molecular dynamics simulations of NBD1 indicated that RI might indirectly influence the interaction of NBD1 with the rest of the protein by attenuating the coupling of the F508-containing loop with the F1-like ATP-binding core subdomain so that RI removal overcame the perturbations caused by F508 deletion. Restriction of RI to a particular conformational state may ameliorate the impact of the disease-causing mutation.

Pyrazolylthiazole as DeltaF508-cystic fibrosis transmembrane conductance regulator correctors with improved hydrophilicity compared to bithiazoles

Deletion of phenylalanine residue 508 (DeltaF508) in the cystic fibrosis (CF) transmembrane conductance regulator protein (CFTR) is a major cause of CF. Small molecule "correctors" of defective DeltaF508-CFTR cellular processing hold promise for CF therapy. We previously identified and characterized bithiazole CF corrector 1 and s-cis-locked bithiazole 2. Herein, we report the regiodivergent synthesis of Ngamma and Nbeta isomers of thiazole-tethered pyrazoles with improved hydrophilicity compared to bithiazoles. We synthesized a focused library of 54 pyrazolylthiazoles 3, which included examples of both regioisomers 4 and 5. The thiazole-tethered pyrazoles allowed incorporation of property-modulating functionality on the pyrazole ring (ester, acid, and amide) while retaining DeltaF508-CFTR corrector activity (EC(50)) of under 1 microM. The most active pyrazolylthiazole (14h) has an experimentally determined log P of 4.1, which is 1.2 log units lower than bithiazole CF corrector 1.

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.

DeltaF508 CFTR processing correction and activity in polarized airway and non-airway cell monolayers

We examined the activity of DeltaF508 cystic fibrosis transmembrane conductance regulator (CFTR) stably expressed in polarized cystic fibrosis bronchial epithelial cells (CFBE41o(-)) human airway cells and Fisher Rat Thyroid (FRT) cells following treatment with low temperature and a panel of small molecule correctors of DeltaF508 CFTR misprocessing. Corr-4a increased DeltaF508 CFTR-dependent Cl(-) conductance in both cell types, whereas treatment with VRT-325 or VRT-640 increased activity only in FRT cells. Total currents stimulated by forskolin and genistein demonstrated similar dose/response effects to Corr-4a treatment in each cell type. When examining the relative contribution of forskolin and genistein to total stimulated current, CFBE41o(-) cells had smaller forskolin-stimulated I(sc) following either low temperature or corr-4a treatment (10-30% of the total I(sc) produced by the combination of both CFTR agonists). In contrast, forskolin consistently contributed greater than 40% of total I(sc) in DeltaF508 CFTR-expressing FRT cells corrected with low temperature, and corr-4a treatment preferentially enhanced forskolin dependent currents only in FRT cells (60% of total I(sc)). DeltaF508 CFTR cDNA transcript levels, DeltaF508 CFTR C band levels, or cAMP signaling did not account for the reduced forskolin response in CFBE41o(-) cells. Treatment with non-specific inhibitors of phosphodiesterases (papaverine) or phosphatases (endothall) did not restore DeltaF508 CFTR activation by forskolin in CFBE41o(-) cells, indicating that the Cl(-) transport defect in airway cells is distal to cAMP or its metabolism. The results identify important differences in DeltaF508 CFTR activation in polarizing epithelial models of CF, and have important implications regarding detection of rescued of DeltaF508 CFTR in vivo.

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

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

Design and synthesis of a hybrid potentiator-corrector agonist of the cystic fibrosis mutant protein DeltaF508-CFTR

A developing therapy of cystic fibrosis caused by the DeltaF508 mutation in CFTR employs correction of defective CFTR chloride channel gating by a 'potentiator' and of defective CFTR protein folding by a 'corrector'. Based on SAR data for phenylglycine-type potentiators and bithiazole correctors, we designed a hybrid molecule incorporating an enzymatic hydrolysable linker to deliver the potentiator (PG01) fragment 2 and the corrector (Corr-4a) fragment 13. The hybrid molecule 14 contained PG01-OH and Corr-4a-linker-CO(2)H moieties, linked with an ethylene glycol spacer through an ester bond. The potentiator 2 and corrector 13 fragments (after cleavage) had low micromolar potency for restoration of DeltaF508-CFTR channel gating and cellular processing, respectively. Cleavage of hybrid molecule 14 by intestinal enzymes under physiological conditions produced the active potentiator 2 and corrector fragments 13, providing proof-of-concept for small-molecule potentiator-corrector hybrids as a single drug therapy for CF caused by the DeltaF508 mutation.

Cystic fibrosis: exploiting its genetic basis in the hunt for new therapies

Cystic fibrosis (CF) is caused by mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR), an anion channel expressed in epithelial cells throughout the body. In the lungs, absence or dysfunction of CFTR results in altered epithelial salt and water transport eventuating in impaired mucociliary clearance, chronic infection and inflammation, and tissue damage. CF lung disease is the major cause of morbidity and mortality in CF despite the many therapies aimed at reducing it. However, recent technological advances combined with two decades of research driven by the discovery of the CFTR gene have resulted in the development and clinical testing of novel therapies aimed at the principal underlying defect in CF, thereby ushering in a new age of therapy for CF.

High Throughput Techniques for Discovering New Glycine Receptor Modulators and their Binding Sites

The inhibitory glycine receptor (GlyR) is a member of the Cys-loop receptor family that mediates inhibitory neurotransmission in the central nervous system. These receptors are emerging as potential drug targets for inflammatory pain, immunomodulation, spasticity and epilepsy. Antagonists that specifically inhibit particular GlyR isoforms are also required as pharmacological probes for elucidating the roles of particular GlyR isoforms in health and disease. Although a substantial number of both positive and negative GlyR modulators have been identified, very few of these are specific for the GlyR over other receptor types. Thus, the potential of known compounds as either therapeutic leads or pharmacological probes is limited. It is therefore surprising that there have been few published studies describing attempts to discover novel GlyR isoform-specific modulators. The first aim of this review is to consider various methods for efficiently screening compounds against these receptors. We conclude that an anion sensitive yellow fluorescent protein is optimal for primary screening and that automated electrophysiology of cells stably expressing GlyRs is useful for confirming hits and quantitating the actions of identified compounds. The second aim of this review is to demonstrate how these techniques are used in our laboratory for the purpose of both discovering novel GlyR-active compounds and characterizing their binding sites. We also describe a reliable, cost effective method for transfecting HEK293 cells in single wells of a 384-well plate using nanogram quantities of plasmid DNA.

Synthesis of 4-thiophen-2'-yl-1,4-dihydropyridines as potentiators of the CFTR chloride channel

The gating of the CFTR chloride channel is altered by a group of mutations that cause cystic fibrosis. This gating defect may be corrected by small molecules called potentiators. Some 1,4-dihydropyridine (DHP) derivatives, bearing a thiophen-2-yl and a furanyl ring at the 4-position of the nucleus, were prepared and tested as CFTR potentiators. In particular, we evaluated the ability of novel DHPs to enhance the activity of the rescued DeltaF508-CFTR as measured with a functional assay based on the halide-sensitive yellow fluorescent protein. Most DHPs showed an effect comparable to or better than that of the reference compound genistein. The potency was instead significantly improved, with some compounds, such as 3g, 3h, 3n, 4a, 4b, and 4d, having a half effective concentration in the submicromolar range. CoMFA analysis gave helpful suggestions to improve the activity of DHPs.

Mutation-specific potency and efficacy of cystic fibrosis transmembrane conductance regulator chloride channel potentiators

Cystic fibrosis (CF) is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel. The mutations G551D and G1349D, which affect the nucleotide-binding domains (NBDs) of CFTR protein, reduce channel activity. This defect can be corrected pharmacologically by small molecules called potentiators. CF mutations residing in the intracellular loops (ICLs), connecting the transmembrane segments of CFTR, may also reduce channel activity. We have investigated the extent of loss of function caused by ICL mutations and the sensitivity to pharmacological stimulation. We found that E193K and G970R (in ICL1 and ICL3, respectively) cause a severe loss of CFTR channel activity that can be rescued by the same potentiators that are effective on NBD mutations. We compared potency and efficacy of three different potentiators for E193K, G970R, and G551D. The 1,4-dihydropyridine felodipine and the phenylglycine PG-01 [2-[(2-1H-indol-3-yl-acetyl)-methylamino]-N-(4-isopropylphenyl)-2-phenylacetamide] were strongly effective on the three CFTR mutants. The efficacy of sulfonamide SF-01 [6-(ethylphenylsulfamoyl)-4-oxo-1,4-dihydroquinoline-3-carboxylic acid cycloheptylamide], another CFTR potentiator, was instead significantly lower than felodipine and PG-01 for the E193K and G970R mutations, and almost abolished for G551D. Furthermore, SF-01 modified the response of G551D and G970R to the other two potentiators, an effect that may be explained by an allosteric antagonistic effect. Our results indicate that CFTR potentiators correct the basic defect caused by CF mutations residing in different CFTR domains. However, there are differences among potentiators, with felodipine and PG-01 having a wider pharmacological activity, and SF-01 being more mutation specific. Our observations are useful in the prioritization and development of drugs targeting the CF basic defect.

High-content functional screen to identify proteins that correct F508del-CFTR function

Cystic Fibrosis is caused by mutations in CFTR, with a deletion of a phenylalanine at position 508 (F508del-CFTR) representing the most common mutation. The F508del-CFTR protein exhibits a trafficking defect and is retained in the endoplasmic reticulum. Here we describe the development of a high-content screen based on a functional assay to identify proteins that correct the F508del-CFTR defect. Using a HEK293 MSR GripTite cell line that stably expresses F508del-CFTR, we individually co-expressed approximately 450 unique proteins fused to the Cl(-)-sensitive YFP(H148Q/I152L) mutant. We then tested correction of F508del-CFTR function by the CI(-)/l(-) exchange method following stimulation with forskolin/IBMX/genistein, using quantitative recordings in multiple individual cells with a high-content (high-throughput) Cellomics KSR imaging system. Using this approach, we identified several known and novel proteins that corrected F508del-CFTR function, including STAT1, Endothelin 1, HspA4, SAPK substrate protein 1, AP2M1, LGALS3/galectin-3, Trk-fused gene, Caveolin 2, PAP/REG3alpha, and others. The ability of these correctors to rescue F508del-CFTR trafficking was then validated by demonstrating their enhancement of maturation (appearance of band C) and by cell surface expression of F508del-CFTR bearing HA tag at the ectodomain using confocal microscopy and flow cytometry. These data demonstrate the utility of high-content analyses for identifying proteins that correct mutant CFTR and discover new proteins that stimulate this correction. This assay can also be utilized for RNAi screens to identify inhibitory proteins that block correction of F508del-CFTR, small molecule, and peptide screens.

Potent s-cis-locked bithiazole correctors of DeltaF508 cystic fibrosis transmembrane conductance regulator cellular processing for cystic fibrosis therapy

N-(5-(2-(5-Chloro-2-methoxyphenylamino)thiazol-4-yl)-4-methylthiazol-2-yl)pivalamide 1 (compound 15Jf) was found previously to correct defective cellular processing of the cystic fibrosis protein DeltaF508-CFTR. Eight C4'-C5 C,C-bond-controlling bithiazole analogues of 1 were designed, synthesized, and evaluated to establish that constraining rotation about the bithiazole-tethering has a significant effect on corrector activity. For example, constraining the C4'-C5 bithiazole tether in the s-cis conformation [N-(2-(5-chloro-2-methoxyphenylamino)-7,8-dihydro-6 H-cyclohepta[1,2- d:3,4- d']bithiazole-2'-yl)pivalamide, 29] results in improved corrector activity. Heteroatom placement in the bithaizole core is also critical as evidenced by the decisive loss of corrector activity with s-cis constrained N-(2-(5-chloro-2-methoxyphenylamino)-5,6-dihydro-4 H-cyclohepta[1,2- d:3,4- d']bithiazole-2'-yl)pivalamide 33. In addition, computational models were utilized to examine the conformational preferences for select model systems. Following our analysis, the " s-cis-locked" cycloheptathiazolothiazole 29 was found to be the most potent bithiazole corrector, with an IC50 of approximately 450 nM.

CFTR function and prospects for therapy

Mutations in the gene coding for the cystic fibrosis transmembrane conductance regulator (CFTR) epithelial anion channel cause cystic fibrosis (CF). The multidomain integral membrane glycoprotein, a member of the adenine nucleotide-binding cassette (ABC) transporter family, conserved in metazoan salt-transporting tissues, is required to control ion and fluid homeostasis on epithelial surfaces. This review considers different therapeutic strategies that have arisen from knowledge of CFTR structure and function as well as its biosynthetic processing, intracellular trafficking, and turnover.

Correctors promote folding of the CFTR in the endoplasmic reticulum

Cystic fibrosis (CF) is most commonly caused by deletion of a residue (DeltaF508) in the CFTR (cystic fibrosis transmembrane conductance regulator) protein. The misfolded mutant protein is retained in the ER (endoplasmic reticulum) and is not trafficked to the cell surface (misprocessed mutant). Corrector molecules such as corr-2b or corr-4a are small molecules that increase the amount of functional CFTR at the cell surface. Correctors may function by stabilizing CFTR at the cell surface or by promoting folding in the ER. To test whether correctors promoted folding of CFTR in the ER, we constructed double-cysteine CFTR mutants that would be retained in the ER and only undergo cross-linking when the protein folds into a native structure. The mature form, but not the immature forms, of M348C(TM6)/T1142C(TM12) (where TM is transmembrane segment), T351C(TM6)/T1142C(TM12) and W356C(TM6)/W1145C(TM12) mutants were efficiently cross-linked. Mutations to the COPII (coatamer protein II) exit motif (Y(563)KDAD(567)) were then made in the cross-linkable cysteine mutants to prevent the mutant proteins from leaving the ER. Membranes were prepared from the mutants expressed in the absence or presence of correctors and subjected to disulfide cross-linking analysis. The presence of correctors promoted folding of the mutants as the efficiency of cross-linking increased from approx. 2-5% to 22-35%. The results suggest that correctors interact with CFTR in the ER to promote folding of the protein into a native structure.

Front-runners for pharmacotherapeutic correction of the airway ion transport defect in cystic fibrosis

Although cystic fibrosis (CF) patients display multiorgan dysfunction (e.g. pancreas, gut, and lung) it is lung disease that is the leading cause of premature death in these patients. CF lung disease is characterized by persistent pulmonary infection and mucus plugging of the airways initiated by the failure of solute transport across the airway epithelium. Many drug therapies aim to alleviate the secondary characteristics of CF lung disease; however, new therapies in development are targeted at correcting the ion transport deficiency of CF. The goal is to hydrate airway surfaces by stimulating secretion (through activation of the CF transmembrane conductance regulator and calcium-activated chloride channels), and/or inhibiting absorption (through the epithelial sodium channel) thereby stimulating healthy mucociliary clearance. If mucociliary clearance can be stimulated sufficiently from an early age, then there is the possibility that secondary lung infection may be eradicated from the syndrome of CF disease.

Activation of the unfolded protein response by deltaF508 CFTR

Environmental insults and misfolded proteins cause endoplasmic reticulum (ER) stress and activate the unfolded protein response (UPR). The UPR decreases endogenous cystic fibrosis transmembrane conductance regulator (CFTR) mRNA levels and protein maturation efficiency. Herein, we investigated the effects of the folding-deficient deltaF508 CFTR on ER stress induction and UPR activation. For these studies, we developed and characterized stable clones of Calu3deltaF cells that express different levels of endogenous wild-type (WT) and recombinant deltaF508 CFTR. We also present a novel RT-PCR-based assay for differential quantification of wild-type CFTR mRNA in the presence of deltaF508 CFTR message. The assay is based on a TaqMan minor groove binding (MGB) probe that recognizes a specific TTT sequence (encoding phenylalanine at position 508 in human CFTR). The MGB probe is extremely specific and sensitive to changes in WT CFTR message levels. In RNA samples that contain both WT and deltaF508 CFTR mRNAs, measurement of WT CFTR mRNA levels (using the MGB probe) and total CFTR mRNA (using commercial primers) allowed us to calculate deltaF508 CFTR mRNA levels. The results indicate that overexpression of deltaF508 CFTR causes ER stress and activates the UPR. UPR activation precedes a marked decrease in endogenous WT CFTR mRNA expression. Furthermore, polarized airway epithelial cell lines are important tools in cystic fibrosis research, and herein we provide an airway epithelial model to study the biogenesis and function of WT and deltaF508 CFTR expressed within the same cell.

F413C and A531V but not R894X myotonia congenita mutations cause defective endoplasmic reticulum export of the muscle-specific chloride channel CLC-1

In northern Finland myotonia congenita is caused by three main mutations in the ClC-1 chloride channel. We studied the molecular basis of these mutations (1238T>G/F413C, 1592C>T/A531V, and 2680C>T/R894X). The mutated cDNAs were expressed either in L6 myotubes or in isolated rat myofibers using recombinant Semliki Forest virus. Experiments in L6 cells indicated that A531V and R894X proteins suffered from stability problems in these cells. Analysis in myofibers indicated that the A531V protein was totally retained in the endoplasmic reticulum (ER), whereas the export of the F413C protein was severely reduced. The C-terminal nonsense mutant (R894X), however, was normally transported to the Golgi elements in the myofibers. Defective export or reduced stability of the mutated proteins may thus be reasons for the myotonic symptoms.

Identification of natural coumarin compounds that rescue defective DeltaF508-CFTR chloride channel gating

1. Deletion of phenylalanine at position 508 (DeltaF508) of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel is the most common mutation causing cystic fibrosis (CF). Effective pharmacological therapy of CF caused by the DeltaF508-CFTR mutation requires the rescue of both intracellular processing and channel gating defects. 2. We identified a class of natural coumarin compounds that can correct the defective DeltaF508-CFTR chloride channel gating by screening a collection of 386 single natural compounds from Chinese medicinal herbs. Screening was performed with an iodide influx assay in Fischer rat thyroid epithelial cells coexpressing DeltaF508-CFTR and an iodide-sensitive fluorescent indicator (YFP-H148Q/I152L). 3. Dose-dependent potentiation of defective DeltaF508-CFTR chloride channel gating by five coumarin compounds was demonstrated by the fluorescent iodide influx assay and confirmed by an Ussing chamber short-circuit current assay. Activation was fully abolished by the specific CFTR inhibitor CFTR(inh)-172. Two potent compounds, namely imperatorin and osthole, have activation K(d) values of approximately 10 micromol/L, as determined by the short-circuit current assay. The active coumarin compounds do not elevate intracellular cAMP levels. Activation of DeltaF508-CFTR by the coumarin compounds requires cAMP agonist, suggesting direct interaction with the mutant CFTR molecule. Kinetics analysis indicated rapid activation of DeltaF508-CFTR by the coumarin compounds, with half-maximal activation of < 5 min. The activating effect was fully reversed for all five active compounds 45 min after washout. 4. In conclusion, the natural coumarin DeltaF508-CFTR activators may represent a new class of natural lead compounds for the development of pharmacological therapies for CF caused by the DeltaF508 mutation.

Enhanced cell-surface stability of rescued DeltaF508 cystic fibrosis transmembrane conductance regulator (CFTR) by pharmacological chaperones

Misfolded proteins destined for the cell surface are recognized and degraded by the ERAD [ER (endoplasmic reticulum) associated degradation] pathway. TS (temperature-sensitive) mutants at the permissive temperature escape ERAD and reach the cell surface. In this present paper, we examined a TS mutant of the CFTR [CF (cystic fibrosis) transmembrane conductance regulator], CFTR DeltaF508, and analysed its cell-surface trafficking after rescue [rDeltaF508 (rescued DeltaF508) CFTR]. We show that rDeltaF508 CFTR endocytosis is 6-fold more rapid (approximately 30% per 2.5 min) than WT (wild-type, approximately 5% per 2.5 min) CFTR at 37 degrees C in polarized airway epithelial cells (CFBE41o-). We also investigated rDeltaF508 CFTR endocytosis under two further conditions: in culture at the permissive temperature (27 degrees C) and following treatment with pharmacological chaperones. At low temperature, rDeltaF508 CFTR endocytosis slowed to WT rates (20% per 10 min), indicating that the cell-surface trafficking defect of rDeltaF508 CFTR is TS. Furthermore, rDeltaF508 CFTR is stabilized at the lower temperature; its half-life increases from <2 h at 37 degrees C to >8 h at 27 degrees C. Pharmacological chaperone treatment at 37 degrees C corrected the rDeltaF508 CFTR internalization defect, slowing endocytosis from approximately 30% per 2.5 min to approximately 5% per 2.5 min, and doubled DeltaF508 surface half-life from 2 to 4 h. These effects are DeltaF508 CFTR-specific, as pharmacological chaperones did not affect WT CFTR or transferrin receptor internalization rates. The results indicate that small molecular correctors may reproduce the effect of incubation at the permissive temperature, not only by rescuing DeltaF508 CFTR from ERAD, but also by enhancing its cell-surface stability.

4'-Methyl-4,5'-bithiazole-based correctors of defective delta F508-CFTR cellular processing

The synthesis and Delta F508-CFTR corrector activity of a 148-member methylbithiazole-based library are reported. Synthetic routes were devised and optimized to generate methylbithiazole analogs in four steps. Corrector potency and efficacy were assayed using epithelial cells expressing human Delta F508-CFTR. These structure-activity data establish that the bithiazole substructure plays a critical function; eight novel methylbithiazole correctors were identified with low micromolar potencies.

The mechanism of cystic fibrosis transmembrane conductance regulator transcriptional repression during the unfolded protein response

The unfolded protein response (UPR) aids cellular recovery by increasing the capacity and decreasing the protein load of the endoplasmic reticulum (ER). Although the main pathways of the UPR are known, the mechanisms of UPR-associated transcriptional repression have not been explored in mammalian cells. Previous studies indicate that endogenous cystic fibrosis transmembrane conductance regulator (CFTR) mRNA levels and protein maturation efficiency decrease when the UPR is activated. In the present study, we demonstrate that inhibition of CFTR expression under ER stress leads to reduced cAMP-activated chloride secretion in epithelial monolayers, an indication of diminished CFTR function. Moreover, ER stress and the UPR obliterate endogenous DeltaF508 CFTR mRNA expression in CFPAC-1 cells without affecting recombinant DeltaF508 CFTR mRNA levels or mRNA half-life. These results emphasize that transcriptional repression of CFTR under ER stress, in concert with decreased CFTR maturation efficiency, leads to diminished function. Using human CFTR promoter reporter constructs, we confined the ER stress-associated CFTR transcriptional repression to the minimal promoter. Chromatin immunoprecipitation assays established the binding of the UPR-activated ATF6 transcription factor to this region during ER stress, which links the repression to the UPR. Methylation-specific PCR (MSP) revealed hypermethylation of CpG sites inside and in the vicinity of the MAZ transcription factor binding region of CFTR, demonstrating methylation-dependent repression. Using pharmacological inhibitors, we show that both DNA methylation and histone deacetylation contribute to CFTR transcriptional inhibition. These studies provide novel insight into the mechanism of gene repression during the mammalian UPR.

Chloride channels as drug targets

Chloride channels represent a relatively under-explored target class for drug discovery as elucidation of their identity and physiological roles has lagged behind that of many other drug targets. Chloride channels are involved in a wide range of biological functions, including epithelial fluid secretion, cell-volume regulation, neuroexcitation, smooth-muscle contraction and acidification of intracellular organelles. Mutations in several chloride channels cause human diseases, including cystic fibrosis, macular degeneration, myotonia, kidney stones, renal salt wasting and hyperekplexia. Chloride-channel modulators have potential applications in the treatment of some of these disorders, as well as in secretory diarrhoeas, polycystic kidney disease, osteoporosis and hypertension. Modulators of GABA(A) (gamma-aminobutyric acid A) receptor chloride channels are in clinical use and several small-molecule chloride-channel modulators are in preclinical development and clinical trials. Here, we discuss the broad opportunities that remain in chloride-channel-based drug discovery.

Genetically encoded chloride indicator with improved sensitivity

Chloride (Cl) is the most abundant physiological anion. Abnormalities in Cl regulation are instrumental in the development of several important diseases including motor disorders and epilepsy. Because of difficulties in the spectroscopic measurement of Cl in live tissues there is little knowledge available regarding the mechanisms of regulation of intracellular Cl concentration. Several years ago, a CFP-YFP based ratiometric Cl indicator (Clomeleon) was introduced [Kuner, T., Augustine, G.J. A genetically encoded ratiometric indicator for chloride: capturing chloride transients in cultured hippocampal neurons. Neuron 2000; 27: 447-59]. This construct with relatively low sensitivity to Cl (K(app) approximately 160 mM) allows ratiometric monitoring of Cl using fluorescence emission ratio. Here, we propose a new CFP-YFP-based construct (Cl-sensor) with relatively high sensitivity to Cl (K(app) approximately 30 mM) due to triple YFP mutant. The construct also exhibits good pH sensitivity with pK(alpha) ranging from 7.1 to 8.0 pH units at different Cl concentrations. Using Cl-sensor we determined non-invasively the distribution of [Cl](i) in cultured CHO cells, in neurons of primary hippocampal cultures and in photoreceptors of rat retina. This genetically encoded indicator offers a means for monitoring Cl and pH under different physiological conditions and high-throughput screening of pharmacological agents.

Low temperature induces the delivery of mature and immature CFTR to the plasma membrane

Deletion of phenylalanine 508 (DeltaF508) is the most prevalent disease-causing mutation resulting in retention of the immature CFTR in the endoplasmic reticulum. The most common strategy to induce the delivery of DeltaF508-CFTR to the surface of cells is by reducing the incubation temperature ( approximately 28 degrees C). Cell surface biotinylation of HEK293T cells grown at 37 degrees C for 48h, confirmed the presence of mature wild-type CFTR, but not DeltaF508-CFTR at the cell surface. On the other hand, cells incubated at 28 degrees C for 16h showed both mature and immature DeltaF508-CFTR at their surface. The trafficking of immature DeltaF508-CFTR, but not mature DeltaF508-CFTR, to the cell surface occurred at low temperature even upon addition of BFA, suggesting the involvement of a Golgi-independent pathway. These results suggest that low temperature induces the appearance of a mix population of mature and immature CFTR molecules at the plasma membrane through distinct pathways.

Small-molecule screen identifies inhibitors of a human intestinal calcium-activated chloride channel

Calcium-activated chloride channels (CaCCs) are widely expressed in mammalian tissues, including intestinal epithelia, where they facilitate fluid secretion. Potent, selective CaCC inhibitors have not been available. We established a high-throughput screen for identification of inhibitors of a human intestinal CaCC based on inhibition of ATP/carbachol-stimulated iodide influx in HT-29 cells after lentiviral infection with the yellow fluorescent halide-sensing protein YFP-H148Q/I152L. Screening of 50,000 diverse, drug-like compounds yielded six classes of putative CaCC inhibitors, two of which, 3-acyl-2-aminothiophenes and 5-aryl-2-aminothiazoles, inhibited by >95% iodide influx in HT-29 cells in response to multiple calcium-elevating agonists, including thapsigargin, without inhibition of calcium elevation, calcium-calmodulin kinase II activation, or cystic fibrosis transmembrane conductance regulator chloride channels. These compounds also inhibited calcium-dependent chloride secretion in T84 human intestinal epithelial cells. Patch-clamp analysis indicated inhibition of CaCC gating, which, together with the calcium-calmodulin data, suggests that the inhibitors target the CaCC directly. Structure-activity relationships were established from analysis of more than 1800 analogs, with IC(50) values of the best analogs down to approximately 1 muM. Small-molecule CaCC inhibitors may be useful in pharmacological dissection of CaCC functions and in reducing intestinal fluid losses in CaCC-mediated secretory diarrheas.

What have we learned from mouse models for cystic fibrosis?

Genetically modified mouse strains are important research tools for the study of numerous human diseases. These models provide us with differentiated tissues, which are not often available from human sources. Furthermore, they allow for testing the effects of genetic manipulation and experimental therapeutics on physiology and pathology. Their importance relies on the assumption that biological processes in the mouse very closely resemble those in humans. Cystic fibrosis (CF) is the most common lethal genetic disease in the Caucasian population. CF is a monogenic disease whose phenotype variability is also attributed to genetic variation in other genes, the so-called modifier genes. Modulation of such modifier genes could be a therapeutic strategy to treat CF. CF mice models have been essential not only for understanding the disease better, but also for the discovery of modifier genes and testing of chemical compounds developed to repair the main protein dysfunction in CF, the CF transmembrane conductance regulator. Mice were also indispensable in gene therapy trials and for the study of CF and non-CF lung response to bacterial infections and inflammation challenges, although no spontaneous lung disease is developed in these mice. In this review, mouse models and their most important contribution to the understanding and management of CF will be presented and discussed.

Cystic Fibrosis Transmembrane Conductance Regulator-independent Phagosomal Acidification in Macrophages

It was reported recently that the cystic fibrosis transmembrane conductance regulator (CFTR) is required for acidification of phagosomes in alveolar macrophages (Di, A., Brown, M. E., Deriy, L. V., Li, C., Szeto, F. L., Chen, Y., Huang, P., Tong, J., Naren, A. P., Bindokas, V., Palfrey, H. C., and Nelson, D. J. (2006) Nat. Cell Biol. 8, 933-944). Here we determined whether the CFTR chloride channel is a generalized pathway for chloride entry into phagosomes in macrophages and whether mutations in CFTR could contribute to alveolar macrophage dysfunction. The pH of mature phagolysosomes in macrophages was measured by fluorescence ratio imaging using a zymosan conjugate containing Oregon Green(R) 488 and tetramethylrhodamine. Acidification of phagolysosomes in J774A.1 macrophages (pH approximately 5.1 at 45 min), murine alveolar macrophages (pH approximately 5.3), and human alveolar macrophages (pH approximately 5.3) was insensitive to CFTR inhibition by the thiazolidinone CFTR(inh)-172. Acidification of phagolysosomes in alveolar macrophages isolated from mice homozygous for DeltaF508-CFTR, the most common mutation in cystic fibrosis, was not different compared with that in alveolar macrophages isolated from wild-type mice. We also measured the kinetics of phagosomal acidification in J774A.1 and murine alveolar macrophages using a zymosan conjugate containing fluorescein and tetramethylrhodamine. Phagosomal acidification began within 3 min of zymosan binding and was complete within approximately 15 min of internalization. The rate of phagosomal acidification in J774A.1 cells was not slowed by CFTR(inh)-172 and was not different in alveolar macrophages from wild-type versus DeltaF508-CFTR mice. Our data indicate that phagolysosomal acidification in macrophages is not dependent on CFTR channel activity and do not support a proposed mechanism for cystic fibrosis lung disease involving defective phagosomal acidification and bacterial killing in alveolar macrophages.

Altered biogenesis of deltaF508-CFTR following treatment with doxorubicin

Cystic fibrosis (CF) is caused by mutations to the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The most common of these mutations is deletion of a phenylalanine residue at position 508 (Delta F508), which accounts for approximately 70% of all CF alleles. This mutation interferes with the biogenesis and maturation of Delta F508-CFTR to the plasma membrane. However, Delta F508-CFTR can partially function upon proper localization. Thus, pharmacological correction of Delta F508-CFTR maturation holds promise in CF therapy. Our previous studies indicate that a single non-cytotoxic dose of the anthracycline doxorubicin (Dox) significantly increase Delta F508-CFTR-associated chloride secretion in MDCK cells by increasing the expression of this protein at the apical plasma membrane. We report here that Dox alters the biogenesis of Delta F508-CFTR. Treatment with Dox increases the resistance of Delta F508-CFTR to trypsin digestion, possibly by expediting protein folding. Further, treatment with Dox reduces the amount of polyubiquitinated Delta F508-CFTR in cells and prolongs the half-life of this protein. Concomitantly, treatment with Dox decreases the association of Delta F508-CFTR with HSP70 but does not alter the expression of major HSP70 family members. Based on these results, we propose that Dox expedites the folding and maturation of Delta F508-CFTR by acting as a pharmacological chaperone, which consequently promotes the functional expression of this protein in MDCK cells.

Molecular targeting of CFTR as a therapeutic approach to cystic fibrosis

One of the major challenges facing the pharmaceutical field is the identification of novel, 'druggable' targets common to distinct diseases that, despite their clinical diversity, share the same basic molecular defect(s) - thus, being termed 'horizontal diseases'. Membrane proteins constitute one of the largest families in the human genome and, given their major roles in cells and organisms, they are relevant to common human disorders such as cardiovascular disease and cancer, but also to rare genetic conditions such as cystic fibrosis (CF). Here, we review therapeutic approaches to correcting the basic defect in CF, which is caused mainly by the intracellular retention of a misfolded protein, and focus on various recent drug-discovery strategies for this important and paradigmatic disease. These strategies have possible applications in many membrane protein disorders, including other channelopathies. The mechanisms of action of potent and specific compounds, representing promising drug leads for CF pharmacotherapy, are explained and discussed.

Cystic Fibrosis: The Mechanisms of Pathogenesis of an Inherited Lung Disorder

Cystic fibrosis patients exhibit lung disease consistent with a failure of innate airway defense mechanisms. The link between abnormal ion transport and disease initiation and progression is not fully understood, but airway mucus dehydration seems paramount in the initiation of CF lung disease. New therapies are currently in development that target the ion transport defects in CF with the intention of rehydrating airway surfaces.

Molecular pathogenesis of alpha-1-antitrypsin deficiency-associated liver disease: a meeting review

In recent years, we have witnessed several important paradigm shifts in understanding the molecular basis of liver disease in alpha-1-antitrypsin (AT) deficiency. These shifts have become possible as a result of a number of advances in research on the cell biology of aggregation-prone mutant proteins and in research on the pathobiological mechanisms of liver disease in general. Late-breaking research in these areas was the subject of an AASLD/Alpha-1 Foundation Single Topic Conference in Atlanta, Georgia, on January 26 to 28, 2006. The conference was titled "Alpha-1-Antitrypsin Deficiency and Other Liver Diseases Caused by Aggregated Proteins." Investigators from all over the world, representing a broad array of scientific disciplines and perspectives, discussed the pathobiology of AT deficiency, mechanisms of cell injury in diseases associated with aggregation-prone proteins, pathways by which cells respond to protein aggregation and mislocalization, and mechanisms of liver injury in general and in diseases related to AT deficiency. A session of the meeting was devoted to novel therapeutic strategies being developed for AT deficiency as well as to strategies either in development or already being applied to the class of diseases associated with mutant proteins.

Structure-Activity Relationship of 1,4-Dihydropyridines as Potentiators of the Cystic Fibrosis Transmembrane Conductance Regulator Chloride Channel

Mutations occurring in the CFTR gene, encoding for the cystic fibrosis transmembrane conductance regulator chloride channel, cause cystic fibrosis (CF). Mutations belonging to class II, such as DeltaPhe508, give rise to a protein with both a defective maturation and altered channel gating. Mutations belonging to class III, such as G551D and G1349D, cause only a gating defect. We have previously identified antihypertensive 1,4-dihydropyridines (DHPs), a class of drugs that block voltage-dependent Ca(2+) channels, as effective potentiators of CFTR gating, able to correct the defective activity of CFTR mutants (Mol Pharmacol 68:1736-1746, 2005). However, optimization of potency for CFTR versus Ca(2+) channels is required to design selective compounds for CFTR pharmacotherapy. In the present study, we have established DHP structure-activity relationship for both CFTR potentiation and Ca(2+) channel inhibition using cell-based assays for both types of channels. A panel of 333 felodipine analogs was studied to understand the effect of various substitutions and modifications in the DHP scaffold. Our results show that alkyl substitutions at the para position of the 4-phenyl ring lead to compounds with very low activity on Ca(2+) channels and strong effect as potentiators on the DeltaPhe508, G551D, and G1349D CFTR mutants.

Functional Analysis of Mutations in the Putative Binding Site for Cystic Fibrosis Transmembrane Conductance Regulator Potentiators

An increasing number of compounds able to potentiate the activity of mutants of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel have been identified by high throughput screening or by individual search of derivatives of known active compounds. Several lines of evidence suggest that most CFTR potentiators act through the same mechanism, probably by binding to the nucleotide binding domains to promote the activity of the protein and then, with lower affinity, to an inhibitory site. With the aim of identifying the activating binding site, we recently modeled the nucleotide binding domain dimer and predicted a common binding site for potentiators in its interface. To validate this model experimentally, we mutated some of the residues involved in the putative binding site, i.e. Arg(553), Ala(554), and Val(1293). The activity of CFTR potentiators was measured as apical membrane currents on polarized cells stably expressing wild type or mutated proteins. CFTR activity was elicited by application of a membrane-permeable cAMP analogue followed by increasing concentrations of potentiators. We found that all three mutants responded to cAMP, although the affinity of R553Q was higher than that of wild type CFTR. In R553Q and V1293G mutants, the dissociation constant of potentiators for the activating site was increased, whereas the dissociation constant for the inhibitory site was reduced. Our results show that the mutated residues are part of the activating binding site for potentiators, as suggested by the molecular model. In addition, these results suggest that the activating and inhibitory sites are not independent of each other.

Cell-based imaging of sodium iodide symporter activity with the yellow fluorescent protein variant YFP-H148Q/I152L

The sodium iodide symporter (NIS) mediates iodide (I−) transport in the thyroid gland and other tissues and is of increasing importance as a therapeutic target and nuclear imaging reporter. NIS activity in vitro is currently measured with radiotracers and electrophysiological techniques. We report on the development of a novel live cell imaging assay of NIS activity using the I−-sensitive and genetically encodable yellow fluorescent protein (YFP) variant YFP-H148Q/I152L. In FRTL-5 thyrocytes stably expressing YFP-H148Q/I152L, I− induced a rapid and reversible decrease in cellular fluorescence characterized by 1) high affinity for extracellular I− (35 μM), 2) inhibition by the NIS inhibitor perchlorate, 3) extracellular Na+ dependence, and 4) TSH dependence, suggesting that fluorescence changes are due to I− influx via NIS. Individual cells within a population of FRTL-5 cells exhibited a 3.5-fold variation in the rate of NIS-mediated I− influx, illustrating the utility of YFP-H148Q/I152L to detect cell-to-cell difference in NIS activity. I− also caused a perchlorate-sensitive decrease in YFP-H148Q/I152L fluorescence in COS-7 cells expressing NIS but not in cells lacking NIS. These results demonstrate that YFP-H148Q/I152L is a sensitive biosensor of NIS-mediated I− uptake in thyroid cells and in nonthyroidal cells following gene transfer and suggest that fluorescence detection of cellular I− may be a useful tool by which to study the pathophysiology and pharmacology of NIS.

Direct interaction with filamins modulates the stability and plasma membrane expression of CFTR

The role of the cystic fibrosis transmembrane conductance regulator (CFTR) as a cAMP-dependent chloride channel on the apical membrane of epithelia is well established. However, the processes by which CFTR is regulated on the cell surface are not clear. Here we report the identification of a protein-protein interaction between CFTR and the cytoskeletal filamin proteins. Using proteomic approaches, we identified filamins as proteins that associate with the extreme CFTR N terminus. Furthermore, we identified a disease-causing missense mutation in CFTR, serine 13 to phenylalanine (S13F), which disrupted this interaction. In cells, filamins tethered plasma membrane CFTR to the underlying actin network. This interaction stabilized CFTR at the cell surface and regulated the plasma membrane dynamics and confinement of the channel. In the absence of filamin binding, CFTR was internalized from the cell surface, where it prematurely accumulated in lysosomes and was ultimately degraded. Our data demonstrate what we believe to be a previously unrecognized role for the CFTR N terminus in the regulation of the plasma membrane stability and metabolic stability of CFTR. In addition, we elucidate the molecular defect associated with the S13F mutation.

Targeted therapy for cystic fibrosis: cystic fibrosis transmembrane conductance regulator mutation-specific pharmacologic strategies

Cystic fibrosis (CF) results from the absence or dysfunction of a single protein, the CF transmembrane conductance regulator (CFTR). CFTR plays a critical role in the regulation of ion transport in a number of exocrine epithelia. Improvement or restoration of CFTR function, where it is deficient, should improve the CF phenotype. There are >1000 reported disease-causing mutations of the CFTR gene. Recent investigations have afforded a better understanding of the mechanism of dysfunction of many of these mutant CFTRs, and have allowed them to be classified according to their mechanism of dysfunction. These data, as well as an enhanced understanding of the role of CFTR in regulating epithelial ion transport, have led to the development of therapeutic strategies based on pharmacologic enhancement or repair of mutant CFTR dysfunction. The strategy, termed 'protein repair therapy', is aimed at improving the regulation of epithelial ion transport by mutant CFTRs in a mutation-specific fashion. The grouping of CFTR gene mutations, according to mechanism of dysfunction, yields some guidance as to which pharmacologic repair agents may be useful for specific CFTR mutations. Recent data has suggested that combinations of pharmacologic repair agents may be necessary to obtain clinically meaningful CFTR repair. Nevertheless, such strategies to improve mutant CFTR function hold great promise for the development of novel therapies aimed at correcting the underlying pathophysiology of CF.

Discovery of pyrrolo[2,3-b]pyrazines derivatives as submicromolar affinity activators of wild type, G551D, and F508del cystic fibrosis transmembrane conductance regulator chloride channels

The cystic fibrosis transmembrane conductance regulator (CFTR) represents the main Cl(-) channel in the apical membrane of epithelial cells for cAMP-dependent Cl(-) secretion. Here we report on the synthesis and screening of a small library of 6-phenylpyrrolo[2,3-b]pyrazines (named RP derivatives) evaluated as activators of wild-type CFTR, G551D-CFTR, and F508del-CFTR Cl(-) channels. Iodide efflux and whole-cell patch-clamp recordings analysis identified RP107 [7-n-butyl-6-(4-hydroxyphenyl)[5H]-pyrrolo[2,3-b]pyrazine] as a submicromolar activator of wild-type (WT)-CFTR [human airway epithelial Calu-3 and WT-CFTR-Chinese hamster ovary (CHO) cells], G551D-CFTR (G551D-CFTR-CHO cells), and F508del-CFTR (in temperature-corrected human airway epithelial F508del/F508del CF15 cells). The structural analog RP108 [7-n-butyl-6-(4-chlorophenyl)[5H]pyrrolo[2,3-b]pyrazine], contrary to RP107, was a less potent activator only at micromolar concentrations. RP107 and RP108 did not have any effect on the cellular cAMP level. Activation was potentiated by low concentration of forskolin and inhibited by glibenclamide and CFTR(inh)-172 [3-[(3-trifluoromethyl)phenyl]-5-[(4-carboxyphenyl-)methylene]-2-thioxo-4-thiazolidinone]but not by calixarene or DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid). Finally, we found significant stimulation of short circuit current (I(sc)) by RP107 (EC(50) = 89 nM) and RP108 (EC(50) = 103 microM) on colon of Cftr(+)(/)(+) but not of Cftr(-/-) mice mounted in Ussing chamber. Stimulation of I(sc) was inhibited by glibenclamide but not affected by DIDS. These results show that RP107 stimulates wild-type CFTR and mutated CFTR, with submicromolar affinity by a cAMP-independent mechanism. Our preliminary structure-activity relationship study identified 4-hydroxyphenyl and 7-n-butyl as determinants required for activation of CFTR. The potency of these agents indicates that compounds in this class may be of therapeutic benefit in CFTR-related diseases, including cystic fibrosis.

Rescue of ΔF508-CFTR trafficking and gating in human cystic fibrosis airway primary cultures by small molecules

Cystic fibrosis (CF) is a fatal genetic disease caused by mutations in cftr, a gene encoding a PKA-regulated Cl−channel. The most common mutation results in a deletion of phenylalanine at position 508 (ΔF508-CFTR) that impairs protein folding, trafficking, and channel gating in epithelial cells. In the airway, these defects alter salt and fluid transport, leading to chronic infection, inflammation, and loss of lung function. There are no drugs that specifically target mutant CFTR, and optimal treatment of CF may require repair of both the folding and gating defects. Here, we describe two classes of novel, potent small molecules identified from screening compound libraries that restore the function of ΔF508-CFTR in both recombinant cells and cultures of human bronchial epithelia isolated from CF patients. The first class partially corrects the trafficking defect by facilitating exit from the endoplasmic reticulum and restores ΔF508-CFTR-mediated Cl−transport to more than 10% of that observed in non-CF human bronchial epithelial cultures, a level expected to result in a clinical benefit in CF patients. The second class of compounds potentiates cAMP-mediated gating of ΔF508-CFTR and achieves single-channel activity similar to wild-type CFTR. The CFTR-activating effects of the two mechanisms are additive and support the rationale of a drug discovery strategy based on rescue of the basic genetic defect responsible for CF.

Sulfamoyl-4-oxoquinoline-3-carboxamides: novel potentiators of defective DeltaF508-cystic fibrosis transmembrane conductance regulator chloride channel gating

The synthesis of a small collection of sulfamoyl-4-oxoquinoline-3-carboxamides is described for use as correctors of defective gating of the DeltaF508-cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel. Several compounds with submicromolar potency were obtained. N-Ethyl 6-(ethylphenylsulfamoyl)-4-oxo-1,4-dihydroquinoline-3-carboxamide (7b) was found to be the most effective sulfonamide corrector of defective DeltaF508-CFTR gating.

Failure of cAMP agonists to activate rescued deltaF508 CFTR in CFBE41o- airway epithelial monolayers

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cyclic AMP-regulated chloride channel. Mutations in the CFTR gene result in cystic fibrosis (CF). The most common mutation, deltaF508, results in endoplasmic reticulum-associated degradation (ERAD) of CFTR. DeltaF508 CFTR has been described as a temperature-sensitive mutation that can be rescued following growth at 27 degrees C. In order to study the processing and function of wild-type and rescued deltaF508 CFTR at the cell surface under non-polarized and polarized conditions, we developed stable cell lines expressing deltaF508 or wild-type CFTR. CFBE41o- is a human airway epithelial cell line capable of forming high resistance, polarized monolayers when cultured on permeable supports, while HeLa cells are normally grown under non-polarizing conditions. Immunoprecipitation, cell surface biotinylation, immunofluorescence, and functional assays confirmed the presence of deltaF508 CFTR at the cell surface in both cell lines after incubating the cells for 48 h at 27 degrees C. However, stimulators of wild-type CFTR such as forskolin, beta2-adrenergic or A2B-adenosine receptor agonists failed to activate rescued deltaF508 CFTR in CFBE41o- monolayers. Rescued deltaF508 CFTR could be stimulated with genistein independent of pretreatment with cAMP signalling agonists. Interestingly, rescued deltaF508 CFTR in HeLa cells could be efficiently stimulated with either forskolin or genistein to promote Cl- transport. These results indicate that deltaF508 CFTR, when rescued in CFBE41o- human airway epithelial cells, is poorly responsive to signalling pathways known to regulate wild-type CFTR. Furthermore, the differences in rescue and activation of deltaF508 CFTR in the two cell lines suggest that cell-type specific differences in deltaF508 CFTR processing are likely to complicate efforts to identify potentiators and/or correctors of the deltaF508 defect.

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.