Antihypertensive 1,4-Dihydropyridines as Correctors of the Cystic Fibrosis Transmembrane Conductance Regulator Channel Gating Defect Caused by Cystic Fibrosis Mutations

Overview of CFTR activators and their recent studies for dry eye disease: a review

The cystic fibrosis transmembrane conductance regulator (CFTR) gets activated via the cAMP signaling pathway and is present in various secretory epithelial cells, including conjunctival and corneal epithelial cells. Activation of CFTR leads to fluid secretion in both mouse and human ocular surfaces. Dry eye disease is a significant health problem for which limited therapeutic options are available. In this review, on the one hand, small molecule CFTR activators with different chemical structures are summarized, and on the other hand, the pharmacological activity test and structural optimization of small molecule CFTR activators in the treatment of dry eye are outlined. The purpose of this review is to highlight the important role of CFTR activators in the treatment of dry eye disease and their potential as a new strategy for the treatment of dry eye disease.

Using the genome to correct the ion transport defect in cystic fibrosis

KEY POINTS

Human genome information can help finding drugs for human diseases. 'Omics' allow unbiased identification of novel drug targets. High-throughput (HT) approaches provide a global view on disease mechanisms. As a monogenic disease CF has led the way in multiple 'Omic' studies. 'Multi-omics' integration will generate maximal biological significance.

ABSTRACT

Today Biomedicine faces one of its greatest challenges, i.e. treating diseases through their causative dysfunctional processes and not just their symptoms. However, we still miss a global view of mechanisms and pathways involved in pathophysiology of most diseases. In fact, disease mechanisms and pathways can be achieved by holistic studies provided by 'Omic' approaches. Cystic Fibrosis (CF), caused by mutations in the CF transmembrane conductance regulator (CFTR) gene which encodes an anion channel, is paradigmatic for monogenic disorders, namely channelopathies. A high number of 'omics studies' have focussed on CF, namely several cell-based high-throughput (HT) approaches were developed and applied towards a global mechanistic characterization of CF pathophysiology and the identification of novel and 'unbiased' drug targets. Notwithstanding, it is likely that, through the integration of all these 'layers' of large datasets into comprehensive disease maps that biological significance can be extracted so that the enormous potential of these approaches to identifying dysfunctional mechanisms and novel drugs may become a reality. Abstract figure legend Schematic overview of the 3 main approaches to discovery of new drugs/drug targets. This article is protected by copyright. All rights reserved.

Effects of Antihypertensive Drugs on Thyroid Function in Type 2 Diabetes Patients With Euthyroidism

Objective: There is little literature about whether antihypertensive drugs would affect thyroid function in patients with euthyroid type 2 diabetes, which was significant in maintaining a proper balance of thyroid function. A retrospective cohort study was conducted to evaluate the influence of antihypertensive drugs on thyroid function in patients with type 2 diabetes with euthyroidism. Design and Methods: The study involved dividing 698 patients with antihypertensive monotherapy into five groups according to the antihypertensive drugs they were treated with. Antihypertensive drugs included in this study were β-blockers, angiotensin-converting enzyme inhibitors (ACEI), angiotensin receptor blockers (ARB), and calcium channel blockers (CCB). The clinical data and thyroid function level between or within groups were compared. Multiple logistic regression analysis was conducted to evaluate the association of antihypertensive drugs with thyroid function level. Results: Selective β₁- adrenergic receptor blockers treatment was related to thyroid-stimulating hormone (TSH), increasing in patients with diabetes and euthyroidism as shown by multiple logistic regression analysis. The association existed after adjustment for confounding factors. No significant influence on thyroid function was found among other antihypertensive drugs. Conclusion: These data show the TSH-lifting effect of selective β₁-adrenergic receptor blockers in patients with type 2 diabetes with euthyroidism.

Ethical Dilemma: Elexacaftor-Tezacaftor-Ivacaftor or Lung Transplantation in Cystic Fibrosis and End-Stage Lung Disease?

Cystic fibrosis (CF) is caused by mutations in the cystic fibrosis transmembrane conductance regulator gene (CFTR). Novel, highly effective, modulator therapies correcting and potentiating CFTR function are changing the course of this disease. We present an ethical dilemma involving an 11-year-old child with CF and end-stage lung disease. Shortly after starting treatment with elexacaftor-tezacaftor-ivacaftor, the family received notification that a matched donor lung had been allocated. Clinical decision-making in this case is challenging as definitive data to medically support one treatment option over the other are limited. A survey of CF center team members was conducted for the purpose of this article. Ethical principles that may guide us in these situations are discussed. Overall, results of the survey present a lack of agreement as to the best approach in this situation. Physicians, when compared with other team members, are more likely to provide a specific recommendation vs presenting the information to the family and letting them decide (OR, 4.0; 95% CI, 1.2-12.8; P = .021). A shared decision-making model, stressing our moral obligation as physicians to respect autonomy by appreciating family values, while offering to participate in the decision-making process and ensuring nonmaleficence, is presented. In summary, CFTR modulators affect the outcomes of CF disease and influence clinical decision-making. The current lack of data on long-term outcomes, in young patients with CF receiving effective modulator therapy, should not preclude CF team participation in decision-making. Shared decision-making, which is focused on respecting autonomy, is our preferred approach in these situations.

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.

Current development of CFTR potentiators in the last decade

Cystic fibrosis (CF) is a genetic disorder produced by the loss of function of CFTR, a main chloride channel involved in transepithelial salt and water transport. CFTR function can be rescued by small molecules called "potentiators" which increase gating activity of CFTR on epithelial surfaces. High throughput screening (HTS) assays allowed the identification of new chemical entities endowed with potentiator properties, further improved through medicinal chemistry optimization. In this review, the most relevant classes of CFTR potentiators developed in the last decade were explored, focusing on structure-activity relationships (SAR) of the different chemical entities, as a useful tool for the improvement of their pharmacological activity.

Recent Strategic Advances in CFTR Drug Discovery: An Overview

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

Innovative Therapies for Cystic Fibrosis: The Road from Treatment to Cure

Cystic fibrosis (CF), a life-threatening multiorgan genetic disease, is facing a new era of research and development using innovative gene-directed personalized therapies. The priority organ to cure is the lung, which suffers recurrent and chronic bacterial infection and inflammation since infancy, representing the main cause of morbidity and precocious mortality of these individuals. After the disappointing failure of gene-replacement approaches using gene therapy vectors, no single drug is presently available to repair all the CF gene defects. The impressive number of different CF gene mutations is now tackled with different chemical and biotechnological tools tailored to the specific molecular derangements, thanks to the extensive knowledge acquired over many years on the mechanisms of CF cell and organ pathology. This review provides an overview and recalls both the successes and limitations of the different experimental approaches, such as high-throughput screening on chemical libraries to discover CF gene correctors and potentiators, dual-acting compounds, read-through molecules, splicing defect repairing tools, cystic fibrosis transmembrane conductance regulator (CFTR) "amplifiers," CFTR interactome modulators and the first gene editing attempts.

Molecular basis of cystic fibrosis: from bench to bedside

Cystic fibrosis (CF), is an autosomal recessive disease affecting different organs. The lung disease, characterized by recurrent and chronic bacterial infection and inflammation since infancy, is the main cause of morbidity and precocious mortality of these individuals. The innovative therapies directed to repair the defective CF gene should account for the presence of more than 200 disease-causing mutations of the CF transmembrane conductance regulator (CFTR) gene. The review will recall the different experimental approaches in discovering CFTR protein targeted molecules, such as the high throughput screening on chemical libraries to discover correctors and potentiators of CFTR protein, dual-acting compounds, read-through molecules, splicing defects repairing tools, CFTR "amplifiers".

Matrine in association with FD‑2 stimulates F508del‑cystic fibrosis transmembrane conductance regulator activity in the presence of corrector VX809

Cystic fibrosis is caused by mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) gene, and the predominant mutation is termed Phe508del (F508del). Therapy for F508del‑CFTR patients is based on the use of Orkambi®, a combination of VX809 and VX770. However, though Orkambi leads to an improvement in the lung function of patients, a progressive reduction in its efficacy has been observed. In order to overcome this effect, the aim of the present study was to investigate the role of matrine and the in‑house compound FD‑2 in increasing the action of VX809 and VX770. Fischer rat thyroid cells overexpressing F508del‑CFTR were treated with matrine, VX809 (corrector) and/or with a number of potentiators (VX770, FD‑1 and FD‑2). The results demonstrated that matrine was able to stimulate CFTR activity and, in association with FD‑2, increased the functionality of the channel in the presence of VX809. Based on these results, it may be hypothesized that FD‑2 may be a novel and more effective potentiator compared with VX770.

CFTR pharmacology

CFTR protein is an ion channel regulated by cAMP-dependent phosphorylation and expressed in many types of epithelial cells. CFTR-mediated chloride and bicarbonate secretion play an important role in the respiratory and gastrointestinal systems. Pharmacological modulators of CFTR represent promising drugs for a variety of diseases. In particular, correctors and potentiators may restore the activity of CFTR in cystic fibrosis patients. Potentiators are also potentially useful to improve mucociliary clearance in patients with chronic obstructive pulmonary disease. On the other hand, CFTR inhibitors may be useful to block fluid and electrolyte loss in secretory diarrhea and slow down the progression of polycystic kidney disease.

Accessible heavier s-block dihydropyridines: structural elucidation and reactivity of isolable molecular hydride sources

Transmetallation of lithiodihydropyridines with Group 1 alkoxides provides facile access to reactive MH (M = Na, K) sources, which show significant structural diversity due in part to the distinct ways that Na/K engage with the σ (green) and π (red) donor systems of the DHP ligands.

Roscovitine is a proteostasis regulator that corrects the trafficking defect of F508del-CFTR by a CDK-independent mechanism

BACKGROUND AND PURPOSE

The most common mutation in cystic fibrosis (CF), F508del, causes defects in trafficking, channel gating and endocytosis of the CF transmembrane conductance regulator (CFTR) protein. Because CF is an orphan disease, therapeutic strategies aimed at improving mutant CFTR functions are needed to target the root cause of CF.

EXPERIMENTAL APPROACH

Human CF airway epithelial cells were treated with roscovitine 100 μM for 2 h before CFTR maturation, expression and activity were examined. The mechanism of action of roscovitine was explored by recording the effect of depleting endoplasmic reticulum (ER) Ca(2+) on the F508del-CFTR/calnexin interaction and by measuring proteasome activity.

KEY RESULTS

Of the cyclin-dependent kinase (CDK) inhibitors investigated, roscovitine was found to restore the cell surface expression and defective channel function of F508del-CFTR in human CF airway epithelial cells. Neither olomoucine nor (S)-CR8, two very efficient CDK inhibitors, corrected F508del-CFTR trafficking demonstrating that the correcting effect of roscovitine was independent of CDK inhibition. Competition studies with inhibitors of the ER quality control (ERQC) indicated that roscovitine acts on the calnexin pathway and on the degradation machinery. Roscovitine was shown (i) to partially inhibit the interaction between F508del-CFTR and calnexin by depleting ER Ca(2+) and (ii) to directly inhibit the proteasome activity in a Ca(2+) -independent manner.

CONCLUSIONS AND IMPLICATIONS

Roscovitine is able to correct the defective function of F508del-CFTR by preventing the ability of the ERQC to interact with and degrade F508del-CFTR via two synergistic but CDK-independent mechanisms. Roscovitine has potential as a pharmacological therapy for CF.

Searching for combinations of small-molecule correctors to restore f508del-cystic fibrosis transmembrane conductance regulator function and processing

The mutated protein F508del-cystic fibrosis transmembrane conductance regulator (CFTR) failed to traffic properly as a result of its retention in the endoplasmic reticulum and functions as a chloride (Cl(-)) channel with abnormal gating and endocytosis. Small chemicals (called correctors) individually restore F508del-CFTR trafficking and Cl(-) transport function, but recent findings indicate that synergistic pharmacology should be considered to address CFTR defects more clearly. We studied the function and maturation of F508del-CFTR expressed in HeLa cells using a combination of five correctors [miglustat, IsoLAB (1,4-dideoxy-2-hydroxymethyl-1,4-imino-l-threitol), Corr4a (N-[2-(5-chloro-2-methoxy-phenylamino)-4'-methyl-[4,5']bithiazolyl-2'-yl]-benzamide), VX-809 [3-(6-(1-(2,2-difluorobenzo[d][1,3]dioxol-5-yl)cyclopropanecarboxamido)-3-methylpyridin-2-yl)benzoic acid], and suberoylamilide hydroxamic acid (SAHA)]. Using the whole-cell patch-clamp technique, the current density recorded in response to CFTR activators (forskolin + genistein) was significantly increased in the presence of the following combinations: VX-809 + IsoLAB; VX-809 + miglustat + SAHA; VX-809 + miglustat + IsoLAB; VX-809 + IsoLAB + SAHA; VX-809 + miglustat + IsoLAB + SAHA. These combinations restored the activity of F508del-CFTR but with a differential effect on the appearance of mature c-band of F508del-CFTR proteins. Focusing on the VX-809 + IsoLAB cocktail, we recorded a level of correction higher at 37°C versus room temperature, but without amelioration of the thermal instability of CFTR. The level of functional rescue with VX-809 + IsoLAB after 4 hours of incubation was maximal and similar to that obtained in optimal conditions of use for each compound (i.e., 24 hours for VX-809 + 4 hours for IsoLAB). Finally, we compared the stimulation of F508del-CFTR by forskolin or forskolin + VX-770 [N-(2,4-di-tert-butyl-5-hydroxyphenyl)-4-oxo-1,4-dihydroquinoline-3-carboxamide] with cells corrected by VX-809 + IsoLAB. Our results open new perspectives for the development of a synergistic polypharmacology to rescue F508del-CFTR and show the importance of temperature on the effect of correctors and on the level of correction, suggesting that optimized combination of correctors could lead to a better rescue of F508del-CFTR function.

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.

Managing the underlying cause of cystic fibrosis: a future role for potentiators and correctors

Cystic fibrosis (CF), a severe genetic disease, is caused by mutations that alter the structure and function of CFTR, a plasma membrane channel permeable to chloride and bicarbonate. Defective anion transport in CF irreversibly damages the lungs, pancreas, liver, and other organs. CF mutations cause loss of CFTR function in multiple ways. In particular, class 3 mutations such as p.Gly551Asp strongly decrease the time spent by CFTR in the open state (gating defect). Instead, class 2 mutations impair the maturation of CFTR protein and its transport from the endoplasmic reticulum to the plasma membrane (trafficking defect). The deletion of phenylalanine 508 (p.Phe508del), the most frequent mutation among CF patients (70-90 %), destabilizes the CFTR protein, thus causing both a trafficking and a gating defect. These two defects can be overcome with drug-like molecules generically called correctors and potentiators, respectively. The potentiator Kalydeco™ (also known as Ivacaftor or VX-770), developed by Vertex Pharmaceuticals, has been recently approved by the US FDA and the European Medicines Agency (EMA) for the treatment of CF patients carrying at least one CFTR allele with the p.Gly551Asp mutation (2-5 % of all patients). In contrast, the corrector VX-809, which significantly improves p.Phe508del-CFTR trafficking in vitro, is still under study in clinical trials. Because of multiple defects caused by the p.Phe508del mutation, it is probable that rescue of the mutant protein will require combined treatment with correctors having different mechanisms of action. This review evaluates the status of experimental and clinical research in pharmacotherapy for the CF basic defect.

Discovery of novel potent ΔF508-CFTR correctors that target the nucleotide binding domain

The deletion of Phe508 (ΔF508) in the first nucleotide binding domain (NBD1) of CFTR is the most common mutation associated with cystic fibrosis. The ΔF508-CFTR mutant is recognized as improperly folded and targeted for proteasomal degradation. Based on molecular dynamics simulation results, we hypothesized that interaction between ΔF508-NBD1 and housekeeping proteins prevents ΔF508-CFTR delivery to the plasma membrane. Based on this assumption we applied structure-based virtual screening to identify new low-molecular-weight compounds that should bind to ΔF508-NBD1 and act as protein–protein interaction inhibitors. Using different functional assays for CFTR activity, we demonstrated that in silico-selected compounds induced functional expression of ΔF508-CFTR in transfected HeLa cells, human bronchial CF cells in primary culture, and in the nasal epithelium of homozygous ΔF508-CFTR mice. The proposed compounds disrupt keratin8-ΔF508-CFTR interaction in ΔF508-CFTR HeLa cells. Structural analysis of ΔF508-NBD1 in the presence of these compounds suggests their binding to NBD1. We conclude that our strategy leads to the discovery of new compounds that are among the most potent correctors of ΔF508-CFTR trafficking defect known to date.

Functional Rescue of F508del-CFTR Using Small Molecule Correctors

High-throughput screens for small molecules that are effective in “correcting” the functional expression of F508del-CFTR have yielded several promising hits. Two such compounds are currently in clinical trial. Despite this success, it is clear that further advances will be required in order to restore 50% or greater of wild-type CFTR function to the airways of patients harboring the F508del-CFTR protein. Progress will be enhanced by our better understanding of the molecular and cellular defects caused by the F508del mutation, present in 90% of CF patients. The goal of this chapter is to review the current understanding of defects caused by F508del in the CFTR protein and in CFTR-mediated interactions important for its biosynthesis, trafficking, channel function, and stability at the cell surface. Finally, we will discuss the gaps in our knowledge regarding the mechanism of action of existing correctors, the unmet need to discover compounds which restore proper CFTR structure and function in CF affected tissues and new strategies for therapy development.

Asymmetric 4-aryl-1,4-dihydropyridines potentiate mutant cystic fibrosis transmembrane conductance regulator (CFTR)

Some of the genetic mutations that cause cystic fibrosis (CF) impair the gating of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) ion channel. This defect can be corrected with pharmacological tools (potentiators) that belong to various chemical families, including the 1,4-dihydropyridines (DHPs). A small set of asymmetric 4-aryl-DHPs was synthesized, and each racemic couple was tested in a functional assay carried out on cells expressing the G1349D, ΔF508, and G551D mutants. The most active racemates were subjected to chiral separation by HPLC, and the pure enantiomers were tested to evaluate any gains in activity. Although three enantiomers demonstrated high potency (K(d) values less than 0.09, 0.1, and 0.5 μM in G1349D, ΔF508, and G551D, respectively), in general, the screening of pure enantiomers did not produce a great diversity in potency values. It is probable that the degree of DHP asymmetry considered in our analysis is still insufficient with respect to that allowed in a putative DHP binding site in CFTR, so that the site could equally accommodate both enantiomers.

Pharmacological therapy for cystic fibrosis: from bench to bedside

With knowledge of the molecular behaviour of the cystic fibrosis transmembrane conductance regulator (CFTR), its physiological role and dysfunction in cystic fibrosis (CF), therapeutic strategies are now being developed that target the root cause of CF rather than disease symptoms. Here, we review progress towards the development of rational new therapies for CF. We highlight the discovery of small molecules that rescue the cell surface expression and defective channel gating of CF mutants, termed CFTR correctors and CFTR potentiators, respectively. We draw attention to alternative approaches to restore epithelial ion transport to CF epithelia, including inhibitors of the epithelial Na(+) channel (ENaC) and activators of the Ca(2+)-activated Cl(-) channel TMEM16A. The expertise required to translate small molecules identified in the laboratory to drugs for CF patients depends on our ability to coordinate drug development at an international level and our ability to provide pertinent biological information using suitable disease models.

New horizons in the treatment of cystic fibrosis

Cystic fibrosis (CF) is a lethal, recessive, genetic disease affecting approximately 1 in 2500 live births among Caucasians. The CF gene codes for a cAMP/PKA-dependent, ATP-requiring, membrane chloride ion channel, generally found in the apical membranes of many secreting epithelia and known as CFTR (cystic fibrosis transmembrane conductance regulator). There are currently over 1700 known mutations affecting CFTR, many of which give rise to a disease phenotype. Around 75% of CF alleles contain the ΔF508 mutation in which a triplet codon has been lost, leading to a missing phenylalanine at position 508 in the protein. This altered protein fails to be trafficked to the correct location in the cell and is generally destroyed by the proteasome. The small amount that does reach the correct location functions poorly. Clearly the cohort of patients with at least one ΔF508 allele are a major target for therapeutic intervention. It is now over two decades since the CF gene was discovered and during this time the properties of CFTR have been intensely investigated. At long last there appears to be progress with the pharmaco-therapeutic approach. Ongoing clinical trials have produced fascinating results in which clinical benefit appears to have been achieved. To arrive at this point ingenious ways have been devised to screen very large chemical libraries for one of two properties: (i) agents promoting trafficking of mutant CFTR to, and insertion into the membrane, and known as correctors or (ii) agents which activate appropriately located mutant CFTR, known as potentiators. The best compounds emerging from these programmes are then used as chemical scaffolds to synthesize other compounds with appropriate pharmaceutical properties, hopefully with their pharmacological activity maintained or even enhanced. In summary, this approach attempts to make the mutant CFTR function in place of the real CFTR. A major function of CFTR in healthy airways is to maintain an adequate airway surface liquid (ASL) layer. In CF the position is further confounded since epithelial sodium channels (ENaC) are no longer regulated and transport salt and water out of the airways to exacerbate the lack of ASL. Thus an additional possibility for treatment of CF is to use agents that inhibit ENaC either alone or as adjuncts to CFTR correctors and/or potentiators. Yet a further way in which a pharmacological approach to CF can be considered is to recruit alternative chloride channels, such as calcium-activated chloride channel (CaCC), to act as surrogates for CFTR. A number of P2Y(2) receptor agonists have been investigated that operate by increasing Ca(2+)(i) which in turn activates CaCC. Some of these compounds are currently in clinical trials. The knowledge base surrounding the structure and function of CFTR that has accumulated in the last 20 years is impressive. Translational research feeding from this is now yielding compounds that provide real prospects for a pharmacotherapy for this disease.

Targeting F508del-CFTR to develop rational new therapies for cystic fibrosis

The mutation F508del is the commonest cause of the genetic disease cystic fibrosis (CF). CF disrupts the function of many organs in the body, most notably the lungs, by perturbing salt and water transport across epithelial surfaces. F508del causes harm in two principal ways. First, the mutation prevents delivery of the cystic fibrosis transmembrane conductance regulator (CFTR) to its correct cellular location, the apical (lumen-facing) membrane of epithelial cells. Second, F508del perturbs the Cl(-) channel function of CFTR by disrupting channel gating. Here, we discuss the development of rational new therapies for CF that target F508del-CFTR. We highlight how structural studies provide new insight into the role of F508 in the regulation of channel gating by cycles of ATP binding and hydrolysis. We emphasize the use of high-throughput screening to identify lead compounds for therapy development. These compounds include CFTR correctors that restore the expression of F508del-CFTR at the apical membrane of epithelial cells and CFTR potentiators that rescue the F508del-CFTR gating defect. Initial results from clinical trials of CFTR correctors and potentiators augur well for the development of small molecule therapies that target the root cause of CF: mutations in CFTR.

Cystic fibrosis: a new target for 4-Imidazo[2,1-b]thiazole-1,4-dihydropyridines

The pharmacology of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel has attracted significant interest in recent years with the aim to search for rational new therapies for diseases caused by CFTR malfunction. Mutations that abolish the function of CFTR cause the life-threatening genetic disease cystic fibrosis (CF). The most common cause of CF is the deletion of phenylalanine 508 (ΔF508) in the CFTR chloride channel. Felodipine, nifedipine, and other antihypertensive 1,4-dihydropyridines (1,4-DHPs) that block L-type Ca(2+) channels are also effective potentiators of CFTR gating, able to correct the defective activity of ΔF508 and other CFTR mutants ( Mol. Pharmacol. 2005 , 68 , 1736 ). For this purpose, we evaluated the ability of the previously and newly synthesized 4-imidazo[2,1-b]thiazoles-1,4-dihydropyridines without vascular activity and inotropic and/or chronotropic cardiac effects ( J. Med. Chem. 2008 , 51 , 1592 ) to enhance the activity of ΔF508-CFTR. Our studies indicate compounds 17, 18, 20, 21, 38, and 39 as 1,4-DHPs with an interesting profile of activity.

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.

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).

Modulation of cystic fibrosis transmembrane conductance regulator (CFTR) activity and genistein binding by cytosolic pH

Potentiators are molecules that increase the activity of the cystic fibrosis transmembrane conductance regulator (CFTR). Some potentiators can also inhibit CFTR at higher concentrations. The activating binding site is thought to be located at the interface of the dimer formed by the two nucleotide-binding domains. We have hypothesized that if binding of potentiators involves titratable residues forming salt bridges, then modifications of cytosolic pH (pH(i)) would alter the binding affinity. Here, we analyzed the effect of pH(i) on CFTR activation and on the binding of genistein, a well known CFTR potentiator. We found that pH(i) does modify CFTR maximum current (I(m)) and half-activation concentration (K(d)): I(m) = 127.7, 185.5, and 231.8 μA/cm(2) and K(d) = 32.7, 56.6 and 71.9 μm at pH 6, 7.35, and 8, respectively. We also found that the genistein apparent dissociation constant for activation (K(a)) increased at alkaline pH(i), near cysteine pK (K(a) = 1.83, 1.81 and 4.99 μm at pH(i) 6, 7.35, and 8, respectively), suggesting the involvement of cysteines in the binding site. Mutations of cysteine residues predicted to be within (Cys-491) or outside (Cys-1344) the potentiator-binding site showed that Cys-491 is responsible for the sensitivity of potentiator binding to alkaline pH(i). Effects of pH(i) on inhibition by high genistein doses were also analyzed. Our results extend previous data about multiple effects of pH(i) on CFTR activity and demonstrate that binding of potentiators involves salt bridge formation with amino acids of nucleotide-binding domain 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.

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.

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.

Anti-inflammatory effect of miglustat in bronchial epithelial cells

The role of CFTR deficiency in promoting inflammation remains unclear. Perez et al. [A. Perez, A.C. Issler, C.U. Cotton, T.J. Kelley, A.S. Verkman and P.B. Davis, CFTR inhibition mimics the cystic fibrosis inflammatory profile. Am J Physiol Lung Cell Mol Physiol 2007; 292:L383-L395.] recently demonstrated that the inhibition of function of w/t CFTR produces an inflammatory profile that resembles that observed in CF patients, whereas we found that correction of F508del-CFTR function with MPB-07 down-modulates the inflammatory response to P. aeruginosa in CF bronchial cells [M.C. Dechecchi, E. Nicolis, V. Bezzerri, A. Vella, M. Colombatti, B.M. Assael, et al., MPB-07 reduces the inflammatory response to Pseudomonas aeruginosa in cystic fibrosis bronchial cells. Am J Respir Cell Mol Biol 2007; 36, 615-624.]. Since both evidence support a link between CFTR function and inflammation, we extended our investigation to other F508del-CFTR correctors, such as miglustat (Norez, 2006), an approved drug for Gaucher disease, in comparison with the galactose analogue NB-DGJ. We report here that miglustat but not NB-DGJ restores F508del-CFTR function in CF bronchial epithelial IB3-1 and CuFi-1 cells. Miglustat and NB-DGJ reduce the inflammatory response to P. aeruginosa in both CF and non-CF bronchial cells, indicating that the anti-inflammatory effect is independent of the correction of F508del-CFTR function. Miglustat also inhibits the inflammatory response induced by the supernatant of mucopurulent material obtained from the lower airway tract of cystic fibrosis patients with chronic bacterial colonization (Ribeiro, 2005). Both compounds do not interfere with the adherence of P. aeruginosa to the cells and reduce the expression of IL-8 not only after challenge with P. aeruginosa but also after exposure to TNF alpha or IL-1 beta, suggesting an effect on transduction proteins downstream and in common with different receptors for pathogens. Finally, miglustat has no major effects on overall binding activity of transcription factors NF-kappaBNF-kB and AP-1. Since miglustat is an approved drug, it could be investigated as a novel anti-inflammatory molecule to ameliorate lung inflammation in CF patients.

Cl transport in complemented CF bronchial epithelial cells correlates with CFTR mRNA expression levels

Little is known about the relationship between CF transmembrane conductance regulator (CFTR) gene expression and the corresponding transport of Cl. The phenotypic characteristics of polarized DeltaF508 homozygote CF bronchial epithelial (CFBE41o-) cells were evaluated following transfection with episomal expression vector containing either full-length (6.2kb) wild type (wt) and (4.7kb) DeltaF508CFTR cDNA. Forskolin-stimulated Cl secretion in two clones expressing the full-length wild type CFTR was assessed; clone c7-6.2wt gave 13.4+/-2.5 microA/cm(2) and clone c10-6.2wt showed 41.3+/-25.3 microA/cm(2). Another clone (c4-4.7DeltaF) complemented with the DeltaF508 CFTR cDNA showed high and stable expression of vector-derived DeltaF508 CFTR mRNA and a small cAMP-stimulated Cl current (4.7+/-0.7 microA/cm(2)) indicating DeltaF508CFTR trafficking to the plasma membrane at physiological temperatures. Vector-driven CFTR mRNA levels were 5-fold (c7-6.2wt), 14-fold (c10-6.2wt), and 27-fold (c7-4.7DeltaF) higher than observed in normal bronchial epithelial cells (16HBE14o-) endogenously expressing wtCFTR. Assessment of CFTR mRNA levels and CFTR function showed that cAMP-stimulated CFTR Cl currents were 33%, 167% and 24%, respectively, of those in 16HBE14o- cells. The data suggest that transgene expression needs to be significantly higher than endogenously expressed CFTR to restore functional wtCFTR Cl transport to levels sufficient to reverse CF pathology.

The DeltaF508-CFTR mutation results in increased biofilm formation by Pseudomonas aeruginosa by increasing iron availability

Enhanced antibiotic resistance of Pseudomonas aeruginosa in the cystic fibrosis (CF) lung is thought to be due to the formation of biofilms. However, there is no information on the antibiotic resistance of P. aeruginosa biofilms grown on human airway epithelial cells or on the effects of airway cells on biofilm formation by P. aeruginosa. Thus we developed a coculture model and report that airway cells increase the resistance of P. aeruginosa to tobramycin (Tb) by >25-fold compared with P. aeruginosa grown on abiotic surfaces. Therefore, the concentration of Tb required to kill P. aeruginosa biofilms on airway cells is 10-fold higher than the concentration achievable in the lungs of CF patients. In addition, CF airway cells expressing DeltaF508-CFTR significantly enhanced P. aeruginosa biofilm formation, and DeltaF508 rescue with wild-type CFTR reduced biofilm formation. Iron (Fe) content of the airway in CF is elevated, and Fe is known to enhance P. aeruginosa growth. Thus we investigated whether enhanced biofilm formation on DeltaF508-CFTR cells was due to increased Fe release by airway cells. We found that airway cells expressing DeltaF508-CFTR released more Fe than cells rescued with WT-CFTR. Moreover, Fe chelation reduced biofilm formation on airway cells, whereas Fe supplementation enhanced biofilm formation on airway cells expressing WT-CFTR. These data demonstrate that human airway epithelial cells promote the formation of P. aeruginosa biofilms with a dramatically increased antibiotic resistance. The DeltaF508-CFTR mutation enhances biofilm formation, in part, by increasing Fe release into the apical medium.

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.

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.

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.

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.

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.