Defective function of the cystic fibrosis-causing missense mutation G551D is recovered by genistein

Recommended Tool Compounds for Modifying the Cystic Fibrosis Transmembrane Conductance Regulator Channel Variants

Cystic fibrosis (CF) is a genetic disorder arising from variations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, leading to multiple organ system defects. CFTR tool compounds are molecules that can modify the activity of the CFTR channel. Especially, patients that are currently not able to benefit from approved CFTR modulators, such as patients with rare CFTR variants, benefit from further research in discovering novel tools to modulate CFTR. This Review explores the development and classification of CFTR tool compounds, including CFTR blockers (CFTRinh-172, GlyH-101), potentiators (VRT-532, Genistein), correctors (VRT-325, Corr-4a), and other approved and unapproved modulators, with detailed descriptions and discussions for each compound. The challenges and future directions in targeting rare variants and optimizing drug delivery, and the potential synergistic effects in combination therapies are outlined. CFTR modulation holds promise not only for CF treatment but also for generating CF models that contribute to CF research and potentially treating other diseases such as secretory diarrhea. Therefore, continued research on CFTR tool compounds is critical.

Therapeutic potential of phytochemicals for cystic fibrosis

The aim of this review was to review and discuss various phytochemicals that exhibit beneficial effects on mutated membrane channels, and hence, improve transmembrane conductance. These therapeutic phytochemicals may have the potential to decrease mortality and morbidity of CF patients. Four databases were searched using keywords. Relevant studies were identified, and related articles were separated. Google Scholar, as well as gray literature (i.e., information that is not produced by commercial publishers), were also checked for related articles to locate/identify additional studies. The relevant databases were searched a second time to ensure that recent studies were included. In conclusion, while curcumin, genistein, and resveratrol have demonstrated effectiveness in this regard, it should be emphasized that coumarins, quercetin, and other herbal medicines also have beneficial effects on transporter function, transmembrane conductivity, and overall channel activity. Additional in vitro and in vivo studies should be conducted on mutant CFTR to unequivocally define the mechanism by which phytochemicals alter transmembrane channel function/activity, since the results of the studies evaluated in this review have a high degree of heterogenicity and discrepancy. Finally, continued research be undertaken to clearly define the mechanism(s) of action and the therapeutic effects that therapeutic phytochemicals have on the symptoms observed in CF patients in an effort to reduce mortality and morbidity.

Elexacaftor is a CFTR potentiator and acts synergistically with ivacaftor during acute and chronic treatment

Cystic fibrosis (CF) is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR), which lead to early death due to progressive lung disease. The development of small-molecule modulators that directly interact with CFTR to aid in protein folding (“correctors”) and/or increase channel function (“potentiators”) have proven to be highly effective in the therapeutic treatment of CF. Notably, incorporation of the next-generation CFTR corrector, elexacaftor, into a triple combination therapeutic (marketed as Trikafta) has shown tremendous clinical promise in treating CF caused by F508del-CFTR. Here, we report on a newly-described role of elexacaftor as a CFTR potentiator. We explore the acute and chronic actions, pharmacology, and efficacy of elexacaftor as a CFTR potentiator in restoring function to multiple classes of CFTR mutations. We demonstrate that the potentiating action of elexacaftor exhibits multiplicative synergy with the established CFTR potentiator ivacaftor in rescuing multiple CFTR class defects, indicating that a new combination therapeutic of ivacaftor and elexacaftor could have broad impact on CF therapies.

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.

Using Genome Sequence to Enable the Design of Medicines and Chemical Probes

Rapid progress in genome sequencing technology has put us firmly into a postgenomic era. A key challenge in biomedical research is harnessing genome sequence to fulfill the promise of personalized medicine. This Review describes how genome sequencing has enabled the identification of disease-causing biomolecules and how these data have been converted into chemical probes of function, preclinical lead modalities, and ultimately U.S. Food and Drug Administration (FDA)-approved drugs. In particular, we focus on the use of oligonucleotide-based modalities to target disease-causing RNAs; small molecules that target DNA, RNA, or protein; the rational repurposing of known therapeutic modalities; and the advantages of pharmacogenetics. Lastly, we discuss the remaining challenges and opportunities in the direct utilization of genome sequence to enable design of medicines.

CFTR potentiators: from bench to bedside

One major breakthrough in cystic fibrosis research in the past decade is the development of drugs that target the root cause of the disease-dysfunctional CFTR protein. One of the compounds, Ivacaftor or Kalydeco, which has been approved for clinical use since 2012, acts by promoting the gating function of CFTR. Our recent studies have led to a gating model that features energetic coupling between nucleotide-binding domain (NBD) dimerization and gate opening/closing in CFTR's transmembrane domains (TMDs). Based on this model, we showed that ATP analogs can enhance CFTR gating by facilitating NBD dimerization, whereas Ivacaftor works by stabilizing the open channel conformation of the TMDs. This latter idea also explains the near omnipotence of Ivacaftor. Furthermore, this model identifies multiple approaches to synergistically boost the open probability of CFTR by influencing distinct molecular events that control gating conformational changes.

Altering intracellular pH reveals the kinetic basis of intraburst gating in the CFTR Cl- channel

KEY POINTS

The cystic fibrosis transmembrane conductance regulator (CFTR), which is defective in the genetic disease cystic fibrosis (CF), forms a gated pathway for chloride movement regulated by intracellular ATP. To understand better CFTR function, we investigated the regulation of channel openings by intracellular pH. We found that short-lived channel closures during channel openings represent subtle changes in the structure of CFTR that are regulated by intracellular pH, in part, at ATP-binding site 1 formed by the nucleotide-binding domains. Our results provide a framework for future studies to understand better the regulation of channel openings, the dysfunction of CFTR in CF and the action of drugs that repair CFTR gating defects.

ABSTRACT

Cystic fibrosis transmembrane conductance regulator (CFTR) is an ATP-gated Cl^- channel defective in the genetic disease cystic fibrosis (CF). The gating behaviour of CFTR is characterized by bursts of channel openings interrupted by brief, flickery closures, separated by long closures between bursts. Entry to and exit from an open burst is controlled by the interaction of ATP with two ATP-binding sites, sites 1 and 2, in CFTR. To understand better the kinetic basis of CFTR intraburst gating, we investigated the single-channel activity of human CFTR at different intracellular pH (pH_i ) values. When compared with the control (pH_i 7.3), acidifying pH_i to 6.3 or alkalinizing pH_i to 8.3 and 8.8 caused small reductions in the open-time constant (τ_o ) of wild-type CFTR. By contrast, the fast closed-time constant (τ_cf ), which describes the short-lived closures that interrupt open bursts, was greatly increased at pH_i 5.8 and 6.3. To analyse intraburst kinetics, we used linear three-state gating schemes. All data were satisfactorily modelled by the C₁ ↔ O ↔ C₂ kinetic scheme. Changing the intracellular ATP concentration was without effect on τ_o , τ_cf and their responses to pH_i changes. However, mutations that disrupt the interaction of ATP with ATP-binding site 1, including K464A, D572N and the CF-associated mutation G1349D all abolished the prolongation of τ_cf at pH_i 6.3. Taken together, our data suggest that the regulation of CFTR intraburst gating is distinct from the ATP-dependent mechanism that controls channel opening and closing. However, our data also suggest that ATP-binding site 1 modulates intraburst gating.

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.

Cystic fibrosis transmembrane conductance regulator modulators in cystic fibrosis: current perspectives

Mutations of the CFTR gene cause cystic fibrosis (CF), the most common recessive monogenic disease worldwide. These mutations alter the synthesis, processing, function, or half-life of CFTR, the main chloride channel expressed in the apical membrane of epithelial cells in the airway, intestine, pancreas, and reproductive tract. Lung disease is the most critical manifestation of CF. It is characterized by airway obstruction, infection, and inflammation that lead to fatal tissue destruction. In spite of great advances in early and multidisciplinary medical care, and in our understanding of the pathophysiology, CF is still considerably reducing the life expectancy of patients. This review highlights the current development in pharmacological modulators of CFTR, which aim at rescuing the expression and/or function of mutated CFTR. While only Kalydeco® and Orkambi® are currently available to patients, many other families of CFTR modulators are undergoing preclinical and clinical investigations. Drug repositioning and personalized medicine are particularly detailed in this review as they represent the most promising strategies for restoring CFTR function in CF.

Natural Compounds as Therapeutic Agents in the Treatment Cystic Fibrosis

The recent FDA approval of two drugs to treat the basic defect in cystic fibrosis has given hope to patients and their families battling this devastating disease. Over many years, with heavy financial investment from Vertex Pharmaceuticals and the Cystic Fibrosis Foundation, pre-clinical evaluation of thousands of synthetic drugs resulted in the production of Kalydeco and Orkambi. Yet, despite the success of this endeavor, many other compounds have been proposed as therapeutic agents in the treatment of CF. Of note, several of these compounds are naturally occurring, and are present in spices from the grocery store and over the counter preparations in health food stores. In this short review, we look at three such compounds, genistein, curcumin, and resveratrol, and evaluate the scientific support for their use as therapeutic agents in the treatment of patients with CF.

Airway hydration and COPD

Chronic obstructive pulmonary disease (COPD) is one of the prevalent causes of worldwide mortality and encompasses two major clinical phenotypes, i.e., chronic bronchitis (CB) and emphysema. The most common cause of COPD is chronic tobacco inhalation. Research focused on the chronic bronchitic phenotype of COPD has identified several pathological processes that drive disease initiation and progression. For example, the lung's mucociliary clearance (MCC) system performs the critical task of clearing inhaled pathogens and toxic materials from the lung. MCC efficiency is dependent on: (1) the ability of apical plasma membrane ion channels such as the cystic fibrosis transmembrane conductance regulator (CFTR) and the epithelial Na(+) channel (ENaC) to maintain airway hydration; (2) ciliary beating; and (3) appropriate rates of mucin secretion. Each of these components is impaired in CB and likely contributes to the mucus stasis/accumulation seen in CB patients. This review highlights the cellular components responsible for maintaining MCC and how this process is disrupted following tobacco exposure and with CB. We shall also discuss existing therapeutic strategies for the treatment of chronic bronchitis and how components of the MCC can be used as biomarkers for the evaluation of tobacco or tobacco-like-product exposure.

Novel personalized therapies for cystic fibrosis: treating the basic defect in all patients

Cystic fibrosis (CF) is the most common genetic life-shortening condition in Caucasians. Despite being a multi-organ disease, CF is classically diagnosed by symptoms of acute/chronic respiratory disease, with persistent pulmonary infections and mucus plugging of the airways and failure to thrive. These multiple symptoms originate from dysfunction of the CF transmembrane conductance regulator (CFTR) protein, a channel that mediates anion transport across epithelia. Indeed, establishment of a definite CF diagnosis requires proof of CFTR dysfunction, commonly through the so-called sweat Cl(-) test. Many drug therapies, including mucolytics and antibiotics, aim to alleviate the symptoms of CF lung disease. However, new therapies to modulate defective CFTR, the basic defect underlying CF, have started to reach the clinic, and several others are in development or in clinical trials. The novelty of these therapies is that, besides targeting the basic defect underlying CF, they are mutation specific. Indeed, even this monogenic disease is influenced by a large number of different genes and biological pathways as well as by environmental factors that are difficult to assess. Accordingly, every person with CF is unique and so functional assessment of patients' tissues ex vivo is key for diagnosing and predicting the severity of this disease. Of note, such assessment will also be crucial to assess drug responses, in order to effectively treat all CF patients. It is not because it is a monogenic disorder that personalized treatment for CF is much easier than for complex disorders.

Sweat chloride as a biomarker of CFTR activity: proof of concept and ivacaftor clinical trial data

BACKGROUND

We examined data from a Phase 2 trial {NCT00457821} of ivacaftor, a CFTR potentiator, in cystic fibrosis (CF) patients with aG551D mutation to evaluate standardized approaches to sweat chloride measurement and to explore the use of sweat chloride and nasal potential difference (NPD) to estimate CFTR activity.

METHODS

Sweat chloride and NPD were secondary endpoints in this placebo-controlled, multicenter trial. Standardization of sweat collection, processing,and analysis was employed for the first time. Sweat chloride and chloride ion transport (NPD) were integrated into a model of CFTR activity.

RESULTS

Within-patient sweat chloride determinations showed sufficient precision to detect differences between dose-groups and assess ivacaftor treatment effects. Analysis of changes in sweat chloride and NPD demonstrated that patients treated with ivacaftor achieved CFTR activity equivalent to approximately 35%–40% of normal.

CONCLUSIONS

Sweat chloride is useful in multicenter trials as a biomarker of CFTR activity and to test the effect of CFTR potentiators.

Medical reversal of chronic sinusitis in a cystic fibrosis patient with ivacaftor

BACKGROUND

Chronic sinusitis is universal in cystic fibrosis (CF) and our current treatments are ineffective in reversing sinus disease. The objective of this work was to determine if increasing CF transmembrane conductance regulator (CFTR) activity by ivacaftor could treat CF sinus disease and assess its effect on primary sinus epithelial cultures.

METHODS

Case report of 1 patient with long-standing chronic sinus disease and a new diagnosis of CF with a mild mutation (P205S) and a severe mutation (G551D). We discuss clinical changes in symptoms, radiographic findings, nasal potential difference testing, and nasal pH values before and after treatment with ivacaftor. We then developed primary sinonasal epithelial cell cultures from a biopsy of the patient to determine changes in airway surface liquid (ASL) pH and ASL viscosity after ivacaftor treatment.

RESULTS

Ivacaftor treatment reversed CT findings of CF sinus disease, increased nasal voltage and pH, and resolved sinus symptoms after 10 months of therapy. Ivacaftor significantly increased ASL pH and decreased ASL viscosity in primary airway cultures.

CONCLUSION

This report documents the reversal of CF sinus disease. Based on our in vivo and in vitro results, we speculate that ivacaftor may reverse CF sinusitis by increasing ASL pH and decreasing ASL viscosity. These studies suggest that CFTR modulation may be effective in treating CF and perhaps non-CF sinusitis.

Revertant mutants modify, but do not rescue, the gating defect of the cystic fibrosis mutant G551D-CFTR

Cystic fibrosis (CF) is caused by dysfunction of the epithelial anion channel cystic fibrosis transmembrane conductance regulator (CFTR). One strategy to restore function to CF mutants is to suppress defects in CFTR processing and function using revertant mutations. Here, we investigate the effects of the revertant mutations G550E and 4RK (the simultaneous disruption of four arginine-framed tripeptides (AFTs): R29K, R516K, R555K and R766K) on the CF mutant G551D, which impairs severely channel gating without altering protein processing and which affects a residue in the same α-helix as G550 and R555. Both G550E and 4RK augmented strongly CFTR-mediated iodide efflux from BHK cells expressing G551D-CFTR. To learn how revertant mutations influence G551D-CFTR function, we studied protein processing and single-channel behaviour. Neither G550E nor 4RK altered the expression and maturation of G551D-CFTR protein. By contrast, both revertants had marked effects on G551D-CFTR channel gating, increasing strongly opening frequency, while 4RK also diminished noticeably the duration of channel openings. Because G551D-CFTR channel gating is ATP independent, we investigated whether revertant mutations restore ATP dependence to G551D-CFTR. Like wild-type CFTR, the activity of 4RK-G551D-CFTR varied with ATP concentration, suggesting that 4RK confers some ATP dependence on the G551D-CFTR channel. Thus, the revertant mutations G550E and 4RK alter the gating pattern and ATP dependence of G551D-CFTR without restoring single-channel activity to wild-type levels. Based on their impact on the CF mutants F508del and G551D, we conclude that G550E and 4RK have direct effects on CFTR structure, but that their action on CFTR processing and channel function is CF mutation specific.

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.

Pharmacological Approaches to Correcting the Ion Transport Defect in Cystic Fibrosis

Cystic fibrosis (CF) is a lethal genetic disease caused by a mutation in a membrane protein, the cystic fibrosis transmembrane conductance regulator (CFTR), which mainly (but not exclusively) functions as a chloride channel. The main clinical symptoms are chronic obstructive lung disease, which is responsible for most of the morbidity and mortality associated with CF, and pancreatic insufficiency. About 1000 mutations of the gene coding for CFTR are currently known; the most common of these, present in the great majority of the patients (Delta508) results in the deletion of a phenylalanine at position 508. In this mutation, the aberrant CFTR is not transported to the membrane but degraded in the ubiquitin-proteasome pathway. The aim of this review is to give an overview of the pharmacologic strategies currently used in attempts to overcome the ion transport defect in CF. One strategy to develop pharmacologic treatment for CF is to inhibit the breakdown of DeltaF508-CFTR by interfering with the chaperones involved in the folding of CFTR. At least in in vitro systems, this can be accomplished by sodium phenylbutyrate, or S-nitrosoglutathione (GSNO), and also by genistein or benzo[c]quinolizinium compounds. It is also possible to stimulate CFTR or its mutated forms, when present in the plasma membrane, using xanthines, genistein, and various other compounds, such as benzamidizoles and benzoxazoles, benzo[c]quinolizinium compounds or phenantrolines. Experimental results are not always unambiguous, and adverse effects have been incompletely tested. Some clinical tests have been done on sodium phenyl butyrate, GSNO and genistein, mostly in respect to other diseases, and the results demonstrate that these drugs are reasonably well tolerated. Their efficiency in the treatment of CF has not yet been demonstrated, however. An alternative strategy is to compensate for the defective chloride transport by CFTR by stimulation of other chloride channels. This can be done via purinergic receptors. A phase I study using a stable uridine triphosphate analog has recently been completed. A second alternative strategy is to attempt to maintain hydration of the airway mucus by inhibiting Na(+) uptake by the epithelial Na(+) channel using amiloride or stable analogs of amiloride. Clinical tests so far have been inconclusive. A number of other suggestions are currently being explored. The minority of patients with CF who have a stop mutation may benefit from treatment with gentamicin. The difficulties in finding a pharmacologic treatment for CF may be due to the fact that CFTR has additional functions besides chloride transport, and interfering with CFTR biosynthesis or activation implies interference with central cellular processes, which may have undesirable adverse effects.

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.

Stimulation of Wild-Type, F508del- and G551D-CFTR Chloride Channels by Non-Toxic Modified pyrrolo[2,3-b]pyrazine Derivatives

Cystic fibrosis (CF) is a major inherited disorder involving abnormalities of fluid and electrolyte transport in a number of different organs due to abnormal function of cystic fibrosis transmembrane conductance regulator (CFTR) protein. We recently identified a family of CFTR activators, which contains the hit: RP107 [7-n-butyl-6-(4-hydroxyphenyl)[5H]-pyrrolo[2,3-b]pyrazine]. Here, we further evaluated the effect of the chemical modifications of the RP107-OH radical on CFTR activation. The replacement of the OH radical by a fluorine atom at position 2 (RP193) or 4 (RP185) significantly decreased the toxicity of the compounds without altering the ability to activate CFTR, especially for RP193. The non-toxic compound RP193 has no effect on cAMP production but stimulates the channel activity of wild-type CFTR in stably transfected CHO cells, in human bronchial epithelial NuLi-1 cells, and in primary culture of human bronchial epithelial cells (HBEC). Whole-cell and single patch-clamp recordings showed that RP193 induced a linear, time- and voltage-independent current, which was fully inhibited by two different and selective CFTR inhibitors (CFTRinh-172 and GP_inh5a). Moreover, RP193 stimulates CFTR in temperature-rescued CuFi-1 (F508del/F508del) HBEC and in CHO cells stably expressing G551D-CFTR. This study shows that it is feasible to reduce cytotoxicity of chemical compounds without affecting their potency to activate CFTR and to rescue the class 2 F508del-CFTR and class 3 G551D-CFTR CF mutant activities.

Curcumin and genistein additively potentiate G551D-CFTR

BACKGROUND

The G551D mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) is a common cause of cystic fibrosis (CF). G551D-CFTR is characterized by an extremely low open probability despite its normal trafficking to the plasma membrane. Numerous small molecules have been shown to increase the activity of G551D-CFTR presumably by binding to the CFTR protein.

METHODS

We investigated the effect of curcumin, genistein and their combined application on G551D-CFTR activity using the patch clamp technique.

RESULTS

Curcumin increased G551D-CFTR whole-cell and single-channel currents less than genistein did at their maximally effective concentrations. However, curcumin further increased the channel activity of G551D-CFTR that had been already maximally potentiated by genistein, up to ~50% of the WT-CFTR level. In addition, the combined application of genistein and curcumin over a lower concentration range synergistically rescued the gating defect of G551D-CFTR.

CONCLUSIONS

The additive effects between curcumin and genistein not only support the hypothesis that multiple mechanisms are involved in the action of CFTR potentiators, but also pose pharmaceutical implications in the development of drugs for CF pharmacotherapy.

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.

Genistein: a natural isoflavone with a potential for treatment of genetic diseases

Genistein [4′,5,7-trihydroxyisoflavone or 5,7-dihydroxy-3-(4-hydroxyphenyl)-4H-1-benzopyran-4-one] is a natural isoflavone occurring in many plants known to possess various biological activities, ranging from phyto-oestrogenic to antioxidative actions. Recent studies indicated that this isoflavone can also be considered as a drug for as yet untreatable genetic diseases. In the present review, we discuss a plausible use of genistein in treatment of two genetic disorders: CF (cystic fibrosis) and MPS (mucopolysaccharidosis). Although various biological actions of genistein are employed in these two cases, in vitro studies, tests on animal models and pilot clinical trials suggest that this plant-derived compound might be a real hope for patients suffering from severe inherited disorders with relatively complicated pathomechanisms, including those affecting the central nervous system.

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.

Rescue of CF airway epithelial cell function in vitro by a CFTR potentiator, VX-770

Cystic fibrosis (CF) is a fatal genetic disease caused by mutations in the gene encoding the CF transmembrane conductance regulator (CFTR), a protein kinase A (PKA)-activated epithelial anion channel involved in salt and fluid transport in multiple organs, including the lung. Most CF mutations either reduce the number of CFTR channels at the cell surface (e.g., synthesis or processing mutations) or impair channel function (e.g., gating or conductance mutations) or both. There are currently no approved therapies that target CFTR. Here we describe the in vitro pharmacology of VX-770, an orally bioavailable CFTR potentiator in clinical development for the treatment of CF. In recombinant cells VX-770 increased CFTR channel open probability (P(o)) in both the F508del processing mutation and the G551D gating mutation. VX-770 also increased Cl(-) secretion in cultured human CF bronchial epithelia (HBE) carrying the G551D gating mutation on one allele and the F508del processing mutation on the other allele by approximately 10-fold, to approximately 50% of that observed in HBE isolated from individuals without CF. Furthermore, VX-770 reduced excessive Na(+) and fluid absorption to prevent dehydration of the apical surface and increased cilia beating in these epithelial cultures. These results support the hypothesis that pharmacological agents that restore or increase CFTR function can rescue epithelial cell function in human CF airway.

Differential activation of the HCO(3)(-) conductance through the cystic fibrosis transmembrane conductance regulator anion channel by genistein and forskolin in murine duodenum

BACKGROUND AND PURPOSE

Many cystic fibrosis (CF)-associated mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) anion channels affect CFTR-activated HCO(3)(-) transport more than Cl(-) transport. Targeting the CFTR HCO(3)(-) conductance, if possible, may therefore be of major therapeutic benefit. In the present study, we examined the effects of genistein and forskolin on duodenal mucosal HCO(3)(-) and Cl(-) secretion.

EXPERIMENTAL APPROACH

Murine duodenal mucosal HCO(3)(-) and Cl(-) secretions were examined in vitro in Ussing chambers by the pH stat and short circuit current (I(sc)) techniques.

KEY RESULTS

Genistein markedly stimulated duodenal HCO(3)(-) secretion and I(sc) in a dose-dependent manner in CFTR wild-type mice, but not in CFTR null mice. CFTR(inh)-172, a highly specific CFTR inhibitor, inhibited genistein-stimulated duodenal HCO(3)(-) secretion and I(sc) in wild-type mice. Genistein induced 59% net HCO(3)(-) increase and 123% net I(sc) increase over basal value, whereas forskolin, an activator of adenylate cyclase, induced 94% net HCO(3)(-) increase and 507% net I(sc) increase, indicating that, compared with forskolin, genistein induced a relatively high HCO(3)(-)/I(sc) ratio. Further data showed that CFTR HCO(3)(-)/Cl(-) conductance ratio was 1.05 after genistein stimulation, whereas after forskolin stimulation, the CFTR HCO(3)(-)/Cl(-) conductance ratio was 0.27.

CONCLUSIONS AND IMPLICATIONS

Genistein stimulates duodenal HCO(3)(-) and Cl(-) secretion through CFTR, and has a relatively high selectivity for the CFTR HCO(3)(-) conductance, compared with forskolin. This may indicate the feasibility of selective targeting of the HCO(3)(-) conductance of the CFTR channels.

Polyphenol cytotoxicity induced by the bacterial toxin pyocyanin: role of NQO1

Pyocyanin is an important bacterial redox-active toxin produced by the opportunistic human pathogen Pseudomonas aeruginosa. The bacterium is a cause of serious infections of the respiratory tract, particularly for those with cystic fibrosis and for those with burn injuries. Pyocyanin induces oxidative stress and causes cells to become prematurely senescent, which compromises tissue remodeling and wound repair. A diverse range of antioxidants have been found useful in preventing oxidant-induced cellular senescence, including quercetin, a common dietary polyphenol. This study evaluated the effectiveness of three common polyphenols (quercetin, (+)-catechin, and (-)-epicatechin) as potential inhibitors of pyocyanin-induced senescence. Whereas at the lowest concentration the polyphenols maintained cellular replicative capacity, in the presence of pyocyanin they unexpectedly displayed concentration-dependent cytotoxicity with a rank order of quercetin>epicatechin>>catechin. On oxidation, polyphenols with B-ring catechol functionality form toxic alkylating quinones that are normally inactivated by cellular antioxidant defense and redox maintenance systems, including reduction by ascorbate and NAD(P)H:quinone oxidoreductase 1 (NQO1). Pyocyanin inhibited cellular NQO1 activity at low micromolar concentrations, but the presence of exogenous ascorbate eliminated pyocyanin-induced polyphenol cytotoxicity. These data indicate that pyocyanin compromises cellular redox maintenance systems, leaving cells susceptible to the adverse effects of otherwise nontoxic redox-active compounds.

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.

Cystic fibrosis

Cystic fibrosis is the most common lethal genetic disease in white populations. The outlook for patients with the disease has improved steadily over many years, largely as a result of earlier diagnosis, more aggressive therapy, and provision of care in specialised centres. Researchers now have a more complete understanding of the molecular-biological defect that underlies cystic fibrosis, which is leading to new approaches to treatment. One of these treatments, hypertonic saline, is already in use, whereas others are in advanced stages of development. We review clinical care for cystic fibrosis and discuss recent advances in the understanding of its pathogenesis, implementation of screening of neonates, and development of therapies aimed at treating the basic defect.

Molecular modeling of the heterodimer of human CFTR's nucleotide-binding domains using a protein-protein docking approach

We have presented a new protein-protein docking approach to model heterodimeric structures based on the conformations of the monomeric units. The conventional modeling method relies on superimposing two monomeric structures onto the crystal structure of a homologous protein dimer. The resulting structure may exhibit severe backbone clashes at the dimeric interface depending on the backbone dissimilarity between the target and template proteins. Our method overcomes the backbone clashing problem and requires no a priori knowledge of the dimeric structure of a homologous protein. Here we used human Cystic Fibrosis Transmembrane conductance Regulator (CFTR), a chloride channel whose dysfunction causes cystic fibrosis, for illustration. The two intracellular nucleotide-binding domains (NBDs) of CFTR control the opening and closing of the channel. Yet, the structure of the CFTR's NBD1-NBD2 complex has not been experimentally determined. Thus, correct modeling of this heterodimeric structure is valuable for understanding CFTR functions and would have potential applications for drug design for cystic fibrosis treatment. Based on the crystal structure of human CFTR's NBD1, we constructed a model of the NBD1-NBD2 complex. The constructed model is consistent with the dimeric mode observed in the crystal structures of other ABC transporters. To verify our structural model, an ATP substrate was docked into the nucleotide-binding site. The predicted binding mode shows consistency with related crystallographic findings and CFTR functional studies. Finally, genistein, an agent that enhances CFTR activity, though the mechanism for such enhancement is unclear, was docked to the model. Our predictions agreed with genistein's bell-shaped dose-response relationship. Potential mutagenesis experiments were proposed for understanding the potentiation mechanism of genistein and for providing insightful information for drug design targeting at CFTR. The method used in this study can be applied to modeling studies of other dimeric protein structures.

Activation of the CFTR Cl- channel by trimethoxyflavone in vitro and in vivo

The flavone apigenin has been previously selected as a potent pharmacological activator of the CFTR Cl(-) channel, however, its utility for the activation of CFTR in vivo is expected to be limited because flavonoids are readily metabolized. We therefore investigated the poorly metabolizable methylether of apigenin, 5,7,4'-trimethoxyflavone (TMF) as a CFTR activator using transepithelial short-circuit current measurements, whole cell and single cell patch clamp techniques, and nasal potential difference (PD) measurements. Transepithelial Cl(-) secretion by Calu-3 epithelia was stimulated by TMF with a halfmaximal concentration of 64+/-5 microM to 55+/-15% of maximal currents achieved by subsequent addition of cAMP agonist forskolin (10 microM). In forskolin-prestimulated tissues, TMF showed small effects and stimulated Cl(-) secretion by an additional 6%. Single channel and whole cell patch clamp techniques were used to verify these effects and identify CFTR as the target of TMF. TMF increased the open probability of silent CFTR (to 0.31+/-0.06) but showed small effects once CFTR had been prestimulated with forskolin. In nasal PD measurements in humans, perfusion of TMF onto the nasal mucosa activated nasal PD by -9.5+/-1.1 mV, which was 69% of the effect of TMF+isoproterenol (-13.8+/-3.9 mV). These data show that TMF is an activator of CFTR in both in vitro and in vivo assays that targets mainly the unstimulated CFTR.

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.

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.

Mechanism of G551D-CFTR (cystic fibrosis transmembrane conductance regulator) potentiation by a high affinity ATP analog

Cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel gated by ATP binding and hydrolysis at its nucleotide binding domains (NBD). The NBDs dimerize in a head-to-tail configuration, forming two ATP binding pockets (ABP) with the ATP molecules buried at the dimer interface. Previous studies have indicated that ABP2, formed by the Walker A and B motifs of NBD2 and the signature sequence of NBD1, is the site critical for the ATP-dependent opening of CFTR. The G551D mutation in ABP2, the third most common cystic fibrosis-associated mutation, abolishes ATP-dependent gating, resulting in an open probability that is approximately 100-fold lower than that of wild-type channels. Interestingly, we found that the ATP analog N6-(2-phenylethyl)-ATP (P-ATP) increases G551D currents mainly by increasing the open time of the channel. This effect is reduced when P-ATP is applied together with ATP, suggesting a competition between ATP and P-ATP for a common binding site. Introducing mutations that lower the nucleotide binding affinity at ABP2 did not alter significantly the effects of P-ATP on G551D-CFTR, whereas an equivalent mutation at ABP1 (consisting of the Walker A and B motifs of NBD1 and the signature sequence of NBD2) dramatically decreased the potency of P-ATP, indicating that ABP1 is the site where P-ATP binds to increase the activity of G551D-CFTR. These results substantiate the idea that nucleotide binding at ABP1 stabilizes the open channel conformation. Our observation that P-ATP enhances the G551D activity by binding at ABP1 implicates that ABP1 can potentially be a target for drugs to bind and increase the channel activity.

Detection of cystic fibrosis transmembrane conductance regulator activity in early-phase clinical trials

Advances in our understanding of cystic fibrosis pathogenesis have led to strategies directed toward treatment of underlying causes of the disease rather than treatments of disease-related symptoms. To expedite evaluation of these emerging therapies, early-phase clinical trials require extension of in vivo cystic fibrosis transmembrane conductance regulator (CFTR)-detecting assays to multicenter trial formats, including nasal potential difference and sweat chloride measurements. Both of these techniques can be used to fulfill diagnostic criteria for the disease, and can discriminate various levels of CFTR function. Full realization of these assays in multicenter clinical trials requires identification of sources of nonbiological intra- and intersite variability, and careful attention to study design and statistical analysis of study-generated data. In this review, we discuss several issues important to the performance of these assays, including efforts to identify and address aspects that can contribute to inconsistent and/or potentially erroneous results. Adjunctive means of detecting CFTR including mRNA expression, immunocytochemical localization, and other methods are also discussed. Recommendations are presented to advance our understanding of these biomarkers and to improve their capacity to predict cystic fibrosis outcomes.

Cystic fibrosis transmembrane regulator protein mutations: 'class' opportunity for novel drug innovation

Cystic fibrosis (CF) is the most common autosomal, recessive, life-span shortening disease in Caucasians. Since discovery of the gene for CF (cystic fibrosis transmembrane conductance regulator [CFTR]) in 1989, knowledge of the molecular function of this gene and its interactions has offered new therapeutic targets. New therapeutics aimed at improving mutant CFTR protein function, also known as 'protein repair therapy,' have been proposed but are yet to be successful in clinical trials. Some of the most exciting efforts involve a new field known as small molecule discovery, which entails the identification, evaluation, and optimization of small organic compounds that can alter the function of a selected gene target or cell phenotype. More than 1300 CFTR mutations have been identified. Many of the more common mutations have been organized into five broad classes based on the fate of the mutant CFTR protein. In each of these mutation classes, interventions have been able to restore some level of CFTR function in vitro. While these 'repairs' have yet to be demonstrated clinically, some early clinical trials are underway. Questions regarding the amount of CFTR correction needed, delivery methods, and optimal therapeutic combinations, however, remain outstanding.

Discovery of α-Aminoazaheterocycle-Methylglyoxal Adducts as a New Class of High-Affinity Inhibitors of 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 nontoxic alpha-aminoazaheterocycle-methylglyoxal adducts, inhibitors of wild-type (WT) CFTR and G551D-, G1349D-, and F508del-CFTR Cl(-) channels. In whole-cell patch-clamp experiments of Chinese hamster ovary (CHO) cells expressing WT-CFTR, we recorded rapid and reversible inhibition of forskolin-activated CFTR currents in the presence of the adducts 5a and 8a,b at 10 pM concentrations. Using iodide efflux experiments, we compared concentration-dependent inhibition of CFTR with glibenclamide (IC(50) = 14.7 microM), 3-[(3-trifluoromethyl)phenyl]-5-[(4-carboxyphenyl-)methylene]-2-thioxo-4-thiazolidinone (CFTR(inh)-172) (IC(50) = 1.2 microM), and alpha-aminoazaheterocycle-methylglyoxal adducts and identified compounds 5a (IC(50) = 71 pM), 8a,b (IC(50) = 2.5 nM), and 7a,b (IC(50) = 3.4 nM) as the most potent inhibitors of WT-CFTR channels. Similar ranges of inhibition were also found when these compounds were evaluated on CFTR channels with the cystic fibrosis mutations F508del (in temperature-corrected human airway epithelial F508del/F508del CF15 cells)-, G551D-, and G1349D-CFTR (expressed in CHO and COS-7 cells). No effect of compound 5a was detected on the volume-regulated or calcium-regulated iodide efflux. Picomolar inhibition of WT-CFTR with adduct 5a was also found using a 6-methoxy-N-(3-sulfopropyl)-quinolinium fluorescent probe applied to the human tracheobronchial epithelial cell line 16HBE14o-. Finally, we found comparable inhibition by 5a or by CFTR(inh)-172 of forskolin-dependent short-circuit currents in mouse colon. To the best of our knowledge, these new nontoxic alpha-aminoazaheterocycle-methylglyoxal adducts represent the most potent compounds reported to inhibit CFTR chloride channels.

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.

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.

Regulatory interactions of N1303K-CFTR and ENaC inXenopusoocytes: evidence that chloride transport is not necessary for inhibition of ENaC

Regulatory interactions of the cystic fibrosis transmembrane conductance regulator (CFTR) and the epithelial Na+channel (ENaC) are readily apparent in Xenopus oocytes. However, the mechanism underlying these interactions remains controversial. CFTR's first nucleotide binding fold (NBD-1) may be important in these interactions, as dysfunctional CFTRs containing mutations within NBD-1, such as ΔF508 and G551D, lack such functional interactions with murine ENaC (mENaC). We hypothesized that a dysfunctional CFTR containing a non-NBD-1 mutation would retain regulatory interactions with mENaC and tested this hypothesis for N1303K-CFTR, where the mutation is located in CFTR's second nucleotide binding fold (NBD-2). cRNA for αβγ-mENaC and N1303K-CFTR was injected separately or together into Xenopus oocytes. ENaC and CFTR functional expression was assessed by two-electrode voltage clamp. Injection of N1303K (class II trafficking mutation) yielded low levels of CFTR function on activation with forskolin and 3-isobutyl-1-methylxanthine (IBMX). In coinjected oocytes, N1303K did not alter mENaC functional expression or surface expression before activation of N1303K. This is similar to our prior observations with ΔF508. However, unlike our observations with ΔF508, activation of N1303K acutely decreased mENaC functional and surface expression, and N1303K currents were enhanced by coinjection of mENaC. Furthermore, genistein only mildly enhanced the functional expression of N1303K-CFTR and did not improve regulation of ENaC by N1303K-CFTR. These data suggest that a structurally and functionally intact CFTR NBD-1 in activated CFTR can regulate mENaC surface expression independent of Cl−transport in Xenopus oocytes.

G551D and G1349D, two CF-associated mutations in the signature sequences of CFTR, exhibit distinct gating defects

Mutations in the gene encoding cystic fibrosis transmembrane conductance regulator (CFTR) result in cystic fibrosis (CF). CFTR is a chloride channel that is regulated by phosphorylation and gated by ATP binding and hydrolysis at its nucleotide binding domains (NBDs). G551D-CFTR, the third most common CF-associated mutation, has been characterized as having a lower open probability (Po) than wild-type (WT) channels. Patients carrying the G551D mutation present a severe clinical phenotype. On the other hand, G1349D, also a mutant with gating dysfunction, is associated with a milder clinical phenotype. Residues G551 and G1349 are located at equivalent positions in the highly conserved signature sequence of each NBD. The physiological importance of these residues lies in the fact that the signature sequence of one NBD and the Walker A and B motifs from the other NBD form the ATP-binding pocket (ABP1 and ABP2, named after the location of the Walker A motif) once the two NBDs dimerize. Our studies show distinct gating characteristics for these mutants. The G551D mutation completely eliminates the ability of ATP to increase the channel activity, and the observed activity is approximately 100-fold smaller than WT-CFTR. G551D-CFTR does not respond to ADP, AMP-PNP, or changes in [Mg(2+)]. The low activity of G551D-CFTR likely represents the rare ATP-independent gating events seen with WT channels long after the removal of ATP. G1349D-CFTR maintains ATP dependence, albeit with a Po approximately 10-fold lower than WT. Interestingly, compared to WT results, the ATP dose-response relationship of G1349D-CFTR is less steep and shows a higher apparent affinity for ATP. G1349D data could be well described by a gating model that predicts that binding of ATP at ABP1 hinders channel opening. Thus, our data provide a quantitative explanation at the single-channel level for different phenotypes presented by patients carrying these two mutations. In addition, these results support the idea that CFTR's two ABPs play distinct functional roles in gating.

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.