A quantitative description of the activation and inhibition of CFTR by potentiators: Genistein

The revolution of personalized pharmacotherapies for cystic fibrosis: what does the future hold?

INTRODUCTION

Cystic fibrosis (CF), a potentially fatal genetic disease, is caused by loss-of-function mutations in the gene encoding for the CFTR chloride/bicarbonate channel. Modulator drugs rescuing mutant CFTR traffic and function are now in the clinic, providing unprecedented breakthrough therapies for people with CF (PwCF) carrying specific genotypes. However, several CFTR variants are unresponsive to these therapies.

AREA COVERED

We discussed several therapeutic approaches that are under development to tackle the fundamental cause of CF, including strategies targeting defective CFTR mRNA and/or protein expression and function. Alternatively, defective chloride secretion and dehydration in CF epithelia could be restored by exploiting pharmacological modulation of alternative targets, i.e., ion channels/transporters that concur with CFTR to maintain the airway surface liquid homeostasis (e.g., ENaC, TMEM16A, SLC26A4, SLC26A9, and ATP12A). Finally, we assessed progress and challenges in the development of gene-based therapies to replace or correct the mutant CFTR gene.

EXPERT OPINION

CFTR modulators are benefiting many PwCF responsive to these drugs, yielding substantial improvements in various clinical outcomes. Meanwhile, the CF therapy development pipeline continues to expand with the development of novel CFTR modulators and alternative therapeutic strategies with the ultimate goal of providing effective therapies for all PwCF in the foreseeable future.

Discovery of CFTR modulators for the treatment of cystic fibrosis

INTRODUCTION

Cystic fibrosis (CF) is a life-threatening inherited disease caused by mutations in the gene encoding the CF transmembrane conductance regulator (CFTR) protein, an anion channel expressed at the apical membrane of secretory epithelia. CF leads to multiorgan dysfunction with progressive deterioration of lung function being the major cause of untimely death. Conventional CF therapies target only symptoms and consequences downstream of the primary genetic defect and the current life expectancy and quality of life of these individuals are still very limited.

AREA COVERED

CFTR modulator drugs are novel-specialized therapies that enhance or even restore functional expression of CFTR mutants and have been approved for clinical use for individuals with specific CF genotypes. This review summarizes classical approaches used for the pre-clinical development of CFTR correctors and potentiators as well as emerging strategies aiming to accelerate modulator development and expand theratyping efforts.

EXPERT OPINION

Highly effective CFTR modulator drugs are expected to deeply modify the disease course for the majority of individuals with CF. A multitude of experimental approaches have been established to accelerate the development of novel modulators. CF patient-derived specimens are valuable cell models to predict therapeutic effectiveness of existing (and novel) modulators in a precision medicine approach.

Synthesis and Structure-activity Relationship of Aminoarylthiazole Derivatives as Potential Potentiators of the Chloride Transport Defect in Cystic Fibrosis

BACKGROUND

Cystic fibrosis (CF) is the autosomal recessive disorder most common in Caucasian populations. It is caused by mutations in the cystic fibrosis transmembrane regulator protein (CFTR). CFTR is predominantly expressed at the apical plasma membranes of the epithelial cells lining several organs, and functions as a cAMP-regulated chloride/bicarbonate channel. To address the underlying causes of cystic fibrosis, two biomolecular activities are required, namely correctors to increase CFTR levels at the cell surface, and potentiators to allow the effective opening of the CFTR channel.

OBJECTIVE

In our previous data, we demonstrated that some aminoarylthiazoles (AATs) have peculiar activity acting as correctors and as potentiator-like molecules. Curiously, a compound called 1 has been shown to be markedly active as a potentiator. Now, we have further modified its scaffold at different portions, for the identification of molecules with improved potency and effectiveness on mutant CFTR.

METHODS

Starting from this active compound, we synthesized a small library trying to improve the activity as potentiators. To extrapolate the contribution of a particular structural portion to bioactivity, we selectively modified one portion at a time.

RESULTS

Our study has provided a structure-activity relationship (SAR) on AATs and led to the identification of some compounds, with a particular ability to act as CFTR potentiators.

CONCLUSION

Two compounds 2 and 13 appear to be promising molecules and could be used for the future development of potentiators of the chloride transport defect in cystic fibrosis.

Potentiation of ABCA3 lipid transport function by ivacaftor and genistein

ABCA3 is a phospholipid transporter implicated in pulmonary surfactant homoeostasis and localized at the limiting membrane of lamellar bodies, the storage compartment for surfactant in alveolar type II cells. Mutations in ABCA3 display a common genetic cause for diseases caused by surfactant deficiency like respiratory distress in neonates and interstitial lung disease in children and adults, for which currently no causal therapy exists. In this study, we investigated the effects of ivacaftor and genistein, two potentiators of the cystic fibrosis transmembrane conductance regulator (CFTR), on ABCA3-specific lipid transport function. Wild-type (WT) and functional ABCA3 mutations N568D, F629L, G667R, T1114M and L1580P were stably expressed in A549 cells. Three-dimensional modelling predicted functional impairment for all five mutants that was confirmed by in vitro experiments (all <14% of WT functional activity). Treatment with potentiators rescued the mutants N568D (up to 114% of WT), F629L (up to 47% of WT), and G667R (up to 60% of WT), the latter variation needing higher concentrations of genistein, showing reduced affinity of the potentiator to the mutant protein. Our results present a first proof that functional ABCA3 mutations are rescued by CFTR potentiators, making them a potential therapeutical option for patients suffering from surfactant deficiency due to ABCA3 mutations.

Abnormal activation of calpain and protein kinase Cα promotes a constitutive release of matrix metalloproteinase 9 in peripheral blood mononuclear cells from cystic fibrosis patients

Matrix metalloproteinase 9 (MMP9) is physiologically involved in remodeling the extracellular matrix components but its abnormal release has been observed in several human pathologies. We here report that peripheral blood mononuclear cells (PBMCs), isolated from cystic fibrosis (CF) patients homozygous for F508del-cystic fibrosis transmembrane conductance regulator (CFTR), express constitutively and release at high rate MMP9 due to the alteration in their intracellular Ca(2+) homeostasis. This spontaneous and sustained MMP9 secretion may contribute to the accumulation of this protease in fluids of CF patients. Conversely, in PBMCs isolated from healthy donors, expression and secretion of MMP9 are undetectable but can be evoked, after 12 h of culture, by paracrine stimulation which also promotes an increase in [Ca(2+)]i. We also demonstrate that in both CF and control PBMCs the Ca(2+)-dependent MMP9 secretion is mediated by the concomitant activation of calpain and protein kinase Cα (PKCα), and that MMP9 expression involves extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) phosphorylation. Our results are supported by the fact that either the inhibition of Ca(2+) entry or chelation of [Ca(2+)]i as well as the inhibition of single components of the signaling pathway or the restoration of CFTR activity all promote the reduction of MMP9 secretion.

Potentiators exert distinct effects on human, murine, and Xenopus CFTR

VX-770 (Ivacaftor) has been approved for clinical usage in cystic fibrosis patients with several CFTR mutations. Yet the binding site(s) on CFTR for this compound and other small molecule potentiators are unknown. We hypothesize that insight into this question could be gained by comparing the effect of potentiators on CFTR channels from different origins, e.g., human, mouse, and Xenopus (frog). In the present study, we combined this comparative molecular pharmacology approach with that of computer-aided drug discovery to identify and characterize new potentiators of CFTR and to explore possible mechanism of action. Our results demonstrate that 1) VX-770, NPPB, GlyH-101, P1, P2, and P3 all exhibited ortholog-specific behavior in that they potentiated hCFTR, mCFTR, and xCFTR with different efficacies; 2) P1, P2, and P3 potentiated hCFTR in excised macropatches in a manner dependent on the degree of PKA-mediated stimulation; 3) P1 and P2 did not have additive effects, suggesting that these compounds might share binding sites. Also 4) using a pharmacophore modeling approach, we identified three new potentiators (IOWH-032, OSSK-2, and OSSK-3) that have structures similar to GlyH-101 and that also exhibit ortholog-specific potentiation of CFTR. These could potentially serve as lead compounds for development of new drugs for the treatment of cystic fibrosis. The ortholog-specific behavior of these compounds suggest that a comparative pharmacology approach, using cross-ortholog chimeras, may be useful for identification of binding sites on human CFTR.

How Phosphorylation and ATPase Activity Regulate Anion Flux though the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR)

The cystic fibrosis transmembrane conductance regulator (CFTR, ABCC7), mutations of which cause cystic fibrosis, belongs to the ATP-binding cassette (ABC) transporter family and works as a channel for small anions, such as chloride and bicarbonate. Anion channel activity is known to depend on phosphorylation by cAMP-dependent protein kinase A (PKA) and CFTR-ATPase activity. Whereas anion channel activity has been extensively investigated, phosphorylation and CFTR-ATPase activity are still poorly understood. Here, we show that the two processes can be measured in a label-free and non-invasive manner in real time in live cells, stably transfected with CFTR. This study reveals three key findings. (i) The major contribution (≥90%) to the total CFTR-related ATP hydrolysis rate is due to phosphorylation by PKA and the minor contribution (≤10%) to CFTR-ATPase activity. (ii) The mutant CFTR-E1371S that is still conductive, but defective in ATP hydrolysis, is not phosphorylated, suggesting that phosphorylation requires a functional nucleotide binding domain and occurs in the post-hydrolysis transition state. (iii) CFTR-ATPase activity is inversely related to CFTR anion flux. The present data are consistent with a model in which CFTR is in a closed conformation with two ATPs bound. The open conformation is induced by ATP hydrolysis and corresponds to the post-hydrolysis transition state that is stabilized by phosphorylation and binding of chloride channel potentiators.

Chlorogenic Acid Activates CFTR-Mediated Cl- Secretion in Mice and Humans: Therapeutic Implications for Chronic Rhinosinusitis

OBJECTIVES

Salubrious effects of the green coffee bean are purportedly secondary to high concentrations of chlorogenic acid. Chlorogenic acid has a molecular structure similar to bioflavonoids that activate transepithelial Cl(-) transport in sinonasal epithelia. In contrast to flavonoids, the drug is freely soluble in water. The objective of this study is to evaluate the Cl(-) secretory capability of chlorogenic acid and its potential as a therapeutic activator of mucus clearance in sinus disease.

STUDY DESIGN

Basic research.

SETTING

Laboratory.

SUBJECTS AND METHODS

Chlorogenic acid was tested on primary murine nasal septal epithelial (MNSE) (CFTR(+/+) and transgenic CFTR(-/-)) and human sinonasal epithelial (HSNE) (CFTR(+/+) and F508del/F508del) cultures under pharmacologic conditions in Ussing chambers to evaluate effects on transepithelial Cl(-) transport. Cellular cyclic adenosine monophosphate (cAMP), phosphorylation of the CFTR regulatory domain (R-D), and CFTR mRNA transcription were also measured.

RESULTS

Chlorogenic acid stimulated transepithelial Cl(-) secretion (change in short-circuit current [ΔISC = µA/cm(2)]) in MNSE (13.1 ± 0.9 vs 0.1 ± 0.1; P < .05) and HSNE (34.3 ± 0.9 vs 0.0 ± 0.1; P < .05). The drug had a long duration until peak effect at 15 to 30 minutes after application. Significant inhibition with INH-172 as well as absent stimulation in cultures lacking functional CFTR suggest effects are dependent on CFTR-mediated pathways. However, the absence of elevated cellular cAMP and phosphorylation the CFTR R-D indicates chlorogenic acid does not work through a PKA-dependent mechanism.

CONCLUSION

Chlorogenic acid is a water-soluble agent that promotes CFTR-mediated Cl(-) transport in mouse and human sinonasal epithelium. Translating activators of mucociliary transport to clinical use provides a new therapeutic approach to sinus disease. Further in vivo evaluation is planned.

Resveratrol ameliorates abnormalities of fluid and electrolyte secretion in a hypoxia-Induced model of acquired CFTR deficiency

OBJECTIVE/HYPOTHESIS

Ineffective mucociliary clearance (MCC) is a common pathophysiologic process that underlies airway inflammation and infection. A dominant fluid and electrolyte secretory pathway in the nasal airways is governed by the cystic fibrosis transmembrane conductance regulator (CFTR). Decreased transepithelial Cl(-) transport secondary to an acquired CFTR deficiency may exacerbate respiratory epithelial dysfunction by diminishing MCC and increasing mucus viscosity. The objectives of the present study are to 1) develop a model of acquired CFTR deficiency in sinonasal epithelium using hypoxia, 2) investigate whether the polyphenol resveratrol promotes CFTR-mediated anion transport, 3) explore resveratrol mechanism of action and determine therapeutic suitability for overcoming acquired CFTR defects, and 4) test the drug in the hypoxic model of acquired CFTR deficiency in preparation for a clinical trial in human sinus disease. We hypothesize that hypoxia will induce depletion of airway surface liquid (ASL) secondary to acquired CFTR deficiency and that resveratrol will restore transepithelial Cl(-) secretion and recover ASL hydration.

STUDY DESIGN

Basic science.

METHODS

Murine nasal septal (MNSE) and human sinonasal epithelial (HSNE) cultures were incubated under hypoxic conditions (1% O2 , 5% CO2 ) and transepithelial ion transport (change in short-circuit current = ΔISC ) evaluated in Ussing chambers. Resveratrol was tested using primary cells and HEK293 cells expressing human CFTR by Ussing chamber and patch clamp techniques under both phosphorylating and nonphosphorylating conditions. CFTR activation was evaluated in human explants and by murine in vivo (nasal potential difference) assessment. Cellular cyclic adenosine monophosphate (cAMP) (ELISA) and subsequent CFTR regulatory domain (R-D) phosphorylation (gel-shift assay) were also evaluated. Effects of hypoxia and resveratrol on ASL were tested using confocal laser scanning microscopy (CLSM) and micro-optical coherence tomography (µOCT).

RESULTS

Hypoxia significantly decreased ΔISC (in µA/cm(2) ) attributable to CFTR at 12 and 24 hours of exposure in both MNSE (13.55 ± 0.46 [12 hours]; 12.75 ± 0.07 [24 hours] vs. 19.23 ± 0.18 [control]; P < 0.05) and HSNE (19.55 ± 0.56 [12 hours]; 17.67 ± 1.13 [24 hours] vs. 25.49 ± 1.48 [control]; P < 0.05). We have shown that resveratrol (100 μM) enhanced CFTR-dependent Cl(-) secretion in HSNE to an extent comparable to the recently Food and Drug Administration-approved CFTR potentiator, ivacaftor. Cl(-) transport across human sinonasal explants (78.42 ± 1.75 vs. 1.75 ± 1.5 [control]; P < 0.05) and in vivo murine nasal epithelium (-4 ± 1.8 vs. -0.8 ± 1.7 mV [control]; P < 0.05) were also significantly increased by the drug. No increase in cAMP or CFTR R-D phosphorylation was detected. Inside-out patches showed increased CFTR open probability (NPo/N (N = channel number]) compared to controls in both MNSE (0.329 ± 0.116 vs. 0.119 ± 0.059 [control]; P < 0.05) and HEK293 cells (0.22 ± 0.048 vs. 0.125 ± 0.07 [control]; P < 0.05). ASL thickness was decreased under hypoxic conditions when measured by CLSM (4.19 ± 0.44 vs. 6.88 ± 0.67 [control]; P < 0.05). A 30-minute apical application of resveratrol increased ASL depth in normal epithelium (8.08 ± 1.68 vs. 6.11 ± 0.47 [control]; P < 0.05). Furthermore, hypoxia-induced abnormalities of fluid and electrolyte secretion in sinonasal epithelium were restored with resveratrol treatment (5.55 ± 0.74 vs. 3.13 ± 0.17 [control]; P < 0.05).

CONCLUSIONS

CFTR activation with a leading edge Cl(-) secretagogue such as resveratrol represents an innovative approach to overcoming acquired CFTR defects in sinus and nasal airway disease. This exciting new strategy bears further testing in non-CF individuals with chronic rhinosinusitis.

LEVEL OF EVIDENCE

N/A. Laryngoscope, 125:S1-S13, 2015.

Effect of genistein on basal jejunal chloride secretion in R117H CF mice is sex and route specific

Cystic fibrosis (CF) results from the loss or reduction in function of the CFTR (cystic fibrosis transmembrane conductance regulatory protein) chloride channel. The third most common CFTR mutation seen clinically is R117H. Genistein, a naturally occurring phytoestrogen, is known to stimulate CFTR function in vitro. We aimed to determine whether route of administration of genistein could mediate differential effects in R117H male and female CF mice. Mice were fed (4 weeks) or injected subcutaneously (1 week) with the following: genistein 600 mg/kg diet (600Gd); genistein-free diet (0Gd); genistein injection 600 mg/kg body weight (600Gi); dimethyl sulfoxide control (0Gi). In male R117H mice fed 600Gd, basal short circuit current (Isc) was unchanged. In 600Gd-fed female mice, there was a subgroup that demonstrated a significant increase in basal Isc (53.14±7.92 μA/cm(2), n=6, P<0.05) and a subgroup of nonresponders (12.05±6.59 μA/cm(2), n=4), compared to 0Gd controls (29.3±6.5 μA/cm(2), n=7). In R117H mice injected with 600Gi, basal Isc was unchanged in both male and female mice compared to 0Gi controls. Isc was measured in response to the following: the adenylate cyclase activator forskolin (10 μM, bilateral), bumetanide (100 μM, basolateral) to indicate the Cl(-) secretory component, and acetazolamide (100 μM, bilateral) to indicate the HCO3 (-) secretory component; however, there was no effect of genistein (diet or injection) on any of these parameters. Jejunal morphology (ie, villi length, number of goblet cells per villus, crypt depth, and number of goblet cells per crypt) in R117H mice suggested no genistein-mediated difference among the groups. Serum levels of genistein were significantly elevated, compared to respective controls, by either 600Gd (equally elevated in males and females) or 600Gi (elevated more in females versus males). These data suggest a sex-dependent increase in basal Isc of R117H mice and that the increase is also specific for route of administration.

Genistein stimulates jejunum chloride secretion via an Akt-mediated pathway in intact female mice

Background/Aims

We have previously shown that daily subcutaneous injections with the naturally occurring phytoestrogen genistein (600 mg genistein/kg body weight/day, 600G) results in a significantly increased basal intestinal chloride, Cl⁻, secretion (I_sc, a measure of transepithelial secretion) in intact C57BL/6J female mice after 1-week of treatment, compared to controls (DMSO vehicle injected). Removal of endogenous estrogen via ovariectomy (OVX) had no effect on the 600G-mediated increase in basal I_sc.

Methods

Given the estrogen-like characteristics of genistein, we compared the effects of daily estradiol (E2) injections (10 mg E2/kg body weight/day, 10E2) on basal I_sc in intact and OVX mice. In intact mice, 10E2 was without effect on basal I_sc, however, in OVX mice, 10E2 significantly increased basal I_sc (mimicked 600G). The goal of the current study was to characterize the intracellular signaling pathways responsible for mediating 600G- or 10E2-stimulated increases in basal I_sc in intact female or OVX mice.

Results

We measured total protein expression in isolated segments of jejunum using western blot from the following six groups of mice; intact or OVX with; 600G, 10E2 or control. The proteins of interest were: Akt, p-Akt, p-PDK1, p-PTEN, p-c-Raf, p-GSK-3β, rap-1 and ERK1/2. All blots were normalized to GAPDH levels (n = 6–18/group).

Conclusion

These data suggest that the presence of the endogenous sex steroid, estrogen, modifies the intracellular signaling pathway required to mediate Cl⁻ secretion when the intestine is exposed to exogenous 600G or E2. These studies may have relevance for designing pharmacological tools for women with intestinal chloride secretory dysfunctions.

Potential role of cardiac chloride channels and transporters as novel therapeutic targets

The heart and blood vessels express a range of anion currents (e.g. ICl.PKA) and symporter/antiporters (e.g. Cl(-)/HCO3(-) exchanger) that translocate chloride (Cl(-)). They have been proposed to contribute to a variety of physiological processes including cellular excitability, cell volume homeostasis and apoptosis. Additionally there is evidence that Cl(-) currents or transporters may play a role in cardiac pathophysiology. Arrhythmogenesis, the process of cardiac ischaemic preconditioning, and the adaptive remodelling process in myocardial hypertrophy and heart failure have all been linked to such channels or transporters. We have explored the possibility that selective targeting of one or more of these may provide benefit in cardiovascular disease. Existing evidence points to an emerging role of cardiac cell anion channels as potential therapeutic targets, the 'disease-specificity' of which may represent a substantial improvement on current targets. However, the limitations of current techniques hitherto applied (such as developmental compensation in gene-modified animals) and pharmacological agents (which do not at present possess sufficient selectivity for the adequate probing of function) have thus far hindered translation to the introduction of new therapy.

ATP-driven Rad50 conformations regulate DNA tethering, end resection, and ATM checkpoint signaling

The Mre11-Rad50 complex is highly conserved, yet the mechanisms by which Rad50 ATP-driven states regulate the sensing, processing and signaling of DNA double-strand breaks are largely unknown. Here we design structure-based mutations in Pyrococcus furiosus Rad50 to alter protein core plasticity and residues undergoing ATP-driven movements within the catalytic domains. With this strategy we identify Rad50 separation-of-function mutants that either promote or destabilize the ATP-bound state. Crystal structures, X-ray scattering, biochemical assays, and functional analyses of mutant PfRad50 complexes show that the ATP-induced 'closed' conformation promotes DNA end binding and end tethering, while hydrolysis-induced opening is essential for DNA resection. Reducing the stability of the ATP-bound state impairs DNA repair and Tel1 (ATM) checkpoint signaling in Schizosaccharomyces pombe, double-strand break resection in Saccharomyces cerevisiae, and ATM activation by human Mre11-Rad50-Nbs1 in vitro, supporting the generality of the P. furiosus Rad50 structure-based mutational analyses. These collective results suggest that ATP-dependent Rad50 conformations switch the Mre11-Rad50 complex between DNA tethering, ATM signaling, and 5' strand resection, revealing molecular mechanisms regulating responses to DNA double-strand breaks.

In vivo readout of CFTR function: ratiometric measurement of CFTR-dependent secretion by individual, identifiable human sweat glands

To assess CFTR function in vivo, we developed a bioassay that monitors and compares CFTR-dependent and CFTR-independent sweat secretion in parallel for multiple (~50) individual, identified glands in each subject. Sweating was stimulated by intradermally injected agonists and quantified by optically measuring spherical sweat bubbles in an oil-layer that contained dispersed, water soluble dye particles that partitioned into the sweat bubbles, making them highly visible. CFTR-independent secretion (M-sweat) was stimulated with methacholine, which binds to muscarinic receptors and elevates cytosolic calcium. CFTR-dependent secretion (C-sweat) was stimulated with a β-adrenergic cocktail that elevates cytosolic cAMP while blocking muscarinic receptors. A C-sweat/M-sweat ratio was determined on a gland-by-gland basis to compensate for differences unrelated to CFTR function, such as gland size. The average ratio provides an approximately linear readout of CFTR function: the heterozygote ratio is ~0.5 the control ratio and for CF subjects the ratio is zero. During assay development, we measured C/M ratios in 6 healthy controls, 4 CF heterozygotes, 18 CF subjects and 4 subjects with 'CFTR-related' conditions. The assay discriminated all groups clearly. It also revealed consistent differences in the C/M ratio among subjects within groups. We hypothesize that these differences reflect, at least in part, levels of CFTR expression, which are known to vary widely. When C-sweat rates become very low the C/M ratio also tended to decrease; we hypothesize that this nonlinearity reflects ductal fluid absorption. We also discovered that M-sweating potentiates the subsequent C-sweat response. We then used potentiation as a surrogate for drugs that can increase CFTR-dependent secretion. This bioassay provides an additional method for assessing CFTR function in vivo, and is well suited for within-subject tests of systemic, CFTR-directed therapeutics.

Calpain inhibition promotes the rescue of F(508)del-CFTR in PBMC from cystic fibrosis patients

A basal calpain activity promotes the limited proteolysis of wild type (WT) cystic fibrosis conductance regulator (CFTR), inducing the internalization of the split channel. This process contributes to the regulation in the level of the active CFTR at the plasma membranes. In peripheral blood mononuclear cells (PBMC) from 16 healthy donors, the inhibition of calpain activity induces a 3-fold increase in the amount of active WT CFTR at the plasma membranes. Instead, in PBMC from cystic fibrosis (CF) patients, calpain activity is expressed at aberrant levels causing the massive removal of F⁵⁰⁸del-CFTR from the cell surface. In these patients, the inhibition of such abnormal proteolysis rescues physiological amounts of active mutated CFTR in 90% of the patients (25 over 28). The recovery of functional F⁵⁰⁸del-CFTR at the physiological location, in cells treated with a synthetic calpain inhibitor, indicates that F⁵⁰⁸del-CFTR folding, maturation, and trafficking operate in CF-PBMC at significant rate. Thus, an increase in the basal calpain activity seems primarily involved in the CFTR defect observed in various CF cells. Furthermore, in CF-PBMC the recovery of the scaffolding protein Na⁺/H⁺ exchanger regulatory factor 1 (NHERF-1), occurring following inhibition of the aberrant calpain activity, can contribute to rescue CFTR-functional clusters.

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.

Stimulation of murine intestinal secretion by daily genistein injections: gender-dependent differences

BACKGROUND/AIMS

The effect of daily injections with genistein (naturally occurring phytoestrogen) on intestinal chloride (Cl(-)) secretion was measured with Ussing chamber short circuit current (I(sc), μA/cm(2)), in C57BL/6J male and female mice, using 600 mg/kg genistein/day (600G), 300 mg/kg genistein/day (300G), 150 mg/kg genistein/day (150G) or genistein-free vehicle control (0G) for 1- or 2-weeks.

METHODS AND RESULTS

Injecting with 600G elicited significant increases in basal I(sc) in females after 1-week (ñ70 μA/cm(2), n=15, p < 0.05) and in males after 2-weeks (ñ80 μA/cm(2), n=5, p < 0.05) compared to their 0G counterparts. Chloride-free ringer significantly reduced basal I(sc) by 65% in 600G males and 72% in 600G females, suggesting that Cl(-) was the major anion comprising the genistein-stimulated secretion. The forskolin-stimulated (10 μM) I(sc) was significantly inhibited by the CFTR chloride channel inhibitors, glibenclamide (500 μM) and CFTR(inh)-172 (100 μM) in 600G males and females, suggesting some contribution by genistein-dependent CFTR-mediated Cl(-) secretion. We found no associated changes in intestinal morphology, nor change in total CFTR protein with 600G. There was a 5% increase in apical/subapical ratio in 600G males compared to controls (no change in females).

CONCLUSION

These data suggest that male and female mice both exhibit increased Cl- secretion with 600G, however, the mechanisms mediating this are gender-dependent.

Resveratrol has salutary effects on mucociliary transport and inflammation in sinonasal epithelium

OBJECTIVE/HYPOTHESIS

Therapeutic agents that enhance mucociliary transport (via stimulation of transepithelial Cl- secretion) and inhibit inflammation could provide considerable advantages over conventional treatments for chronic rhinosinusitis (CRS). The objectives of the present study were to investigate whether the polyphenolic compound resveratrol promotes transepithelial Cl- transport and inhibits KC/IL-8 secretion in sinonasal epithelium.

STUDY DESIGN

In vitro and in vivo study.

METHODS

Transepithelial Cl- transport was investigated in primary murine nasal septal (MNSE) and human sinonasal epithelial (HSNE) cultures. In vivo activity was also measured using the murine nasal potential difference assay. CFTR R-domain phosphorylation and cAMP levels were examined as a test of cAMP/PKA-dependent activation. In vitro LPS-induced KC/IL-8 secretion was quantified and compared to a panel of intranasal steroids.

RESULTS

Resveratrol(100 μM) significantly increased CFTR-mediated Cl- transport (change in short-circuit current, ΔI(SC) ) in both MNSE (13.51 ± 0.77 vs. 4.4 ± 0.66 [control]; P < .05) and HSNE (12.28 ± 1.08 vs. 0.69 ± 0.32 [control]; P < .05). Cl- secretion across in vivo murine nasal epithelium was also enhanced (-4 ± 1.8 vs. -0.8 ± 1.7mV [control], P < .05). There was no increase in cellular cAMP or CFTR R-domain phosphorylation detected. Resveratrol also significantly inhibited KC/IL-8 secretion in a dose-dependent fashion (pg/mL) in MNSE (181 ± 39[100 μM) vs. 94 ± 16 [200 μM] vs. 16 ± 22 [500 μM] vs. 1195 ± 355 [LPS control]; P < .001). The compound robustly abrogated KC/IL-8 secretion when compared to ciclesonide (765 ± 139), triamcinolone (561 ± 124), and budesonide (742 ± 428), but had similar activity to fluticasone proprionate (65 ± 47). Similar effects were demonstrated in HSNE (975 ± 244 [100 μM] vs. 1825 ± 144 [LPS control]; P < .001) with inhibition comparable to fluticasone proprionate (785 ± 277).

CONCLUSIONS

These in vitro and in vivo findings indicate resveratrol is a potent Cl- secretagogue and anti-inflammatory agent. Future clinical trials for CRS are warranted.

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.

Model of the cAMP activation of chloride transport by CFTR channel and the mechanism of potentiators

Mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) cause cystic fibrosis, a hereditary lethal disease. CFTR is a chloride channel expressed in the apical membrane of epithelia. It is activated by cAMP dependent phosphorylation and gated by the binding of ATP. The impaired chloride transport of some types of cystic fibrosis mutations could be pharmacologically solved by the use of chemical compounds called potentiators. Here it is undertaken the construction of a model of the CFTR activation pathways, and the possible modification produced by a potentiator application. The model yields a novel mechanism for the potentiator action, describing the activatory and inhibitory activities on two different positions in the CFTR activation pathway.

On the measurement of the functional properties of the CFTR

A number of methods are currently employed to assess the functional properties of CFTR channels and their response to pharmacological potentiators, correction of the defective CFTR trafficking, and vectorial introduction of new proteins. Here we review the most common methods used to assess CFTR channel function. The suitability of each technique to various experimental conditions is discussed.

Solubilizing mutations used to crystallize one CFTR domain attenuate the trafficking and channel defects caused by the major cystic fibrosis mutation

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR) Cl(-) channel. F508del, the most frequent CF-causing mutation, disrupts both the processing and function of CFTR. Recently, the crystal structure of the first nucleotide-binding domain of CFTR bearing F508del (F508del-NBD1) was elucidated. Although F508del-NBD1 shows only minor conformational changes relative to that of wild-type NBD1, additional mutations (F494N/Q637R or F429S/F494N/Q637R) were required for domain solubility and crystallization. Here we show that these solubilizing mutations in cis with F508del partially rescue the trafficking defect of full-length F508del-CFTR and attenuate its gating defect. We interpret these data to suggest that the solubilizing mutations utilized to facilitate F508del-NBD1 production also assist folding of full-length F508del-CFTR protein. Thus, the available crystal structure of F508del-NBD1 might correspond to a partially corrected conformation of this domain.

The role of the UPS in cystic fibrosis

CF is an inherited autosomal recessive disease whose lethality arises from malfunction of CFTR, a single chloride (Cl^-) ion channel protein. CF patients harbor mutations in the CFTR gene that lead to misfolding of the resulting CFTR protein, rendering it inactive and mislocalized. Hundreds of CF-related mutations have been identified, many of which abrogate CFTR folding in the endoplasmic reticulum (ER). More than 70% of patients harbor the ΔF508 CFTR mutation that causes misfolding of the CFTR proteins. Consequently, mutant CFTR is unable to reach the apical plasma membrane of epithelial cells that line the lungs and gut, and is instead targeted for degradation by the UPS. Proteins located in both the cytoplasm and ER membrane are believed to identify misfolded CFTR for UPS-mediated degradation. The aberrantly folded CFTR protein then undergoes polyubiquitylation, carried out by an E1-E2-E3 ubiquitin ligase system, leading to degradation by the 26S proteasome. This ubiquitin-dependent loss of misfolded CFTR protein can be inhibited by the application of ‘corrector’ drugs that aid CFTR folding, shielding it from the UPS machinery. Corrector molecules elevate cellular CFTR protein levels by protecting the protein from degradation and aiding folding, promoting its maturation and localization to the apical plasma membrane. Combinatory application of corrector drugs with activator molecules that enhance CFTR Cl^- ion channel activity offers significant potential for treatment of CF patients.

Publication history: Republished from Current BioData's Targeted Proteins database (TPdb; ).

Characterization of a 7,8-benzoflavone double effect on CFTR Cl(-) channel activity

The human cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the superfamily of adenosine triphosphate (ATP)-binding cassette (ABC) transporter ATPases. This protein forms a Cl(-) channel with a complex regulation; gene mutations cause cystic fibrosis disease. We investigated the interaction between the protein and the flavone UCCF-029 using the patch-clamp technique in the excised inside-out configuration in order to study the molecular mechanism of action for this potentiator on completely phosphorylated channel (25 U/ml protein kinase A) and a relatively low level of ATP (0.3 mM: ). Low concentrations of UCCF-029 (<50 nM: ) increase the open probability (p (o)), favoring the channel transition to an activated state, while high UCCF-029 (>50 nM: ) levels determine inhibition of the CFTR by a reduction of the total open time. Our data suggest that this drug can potentiate CFTR by binding to a specific site on the nucleotide binding domain, promoting dimer formation. The response of CFTR to variable concentrations of ATP is not modified by application of the potentiator UCCF-029 at either low, activatory, concentration or high, inhibitory, levels. Hence, we conclude that the potentiator may not interfere with binding of ATP but probably acts at an independent site in the protein, interacting directly with CFTR to modulate channel activity.

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.

An Extract from the Medicinal Plant Phyllanthus acidus and Its Isolated Compounds Induce Airway Chloride Secretion: A Potential Treatment for Cystic Fibrosis

According to previous reports, flavonoids and nutraceuticals correct defective electrolyte transport in cystic fibrosis (CF) airways. Traditional medicinal plants from China and Thailand contain phytoflavonoids and other bioactive compounds. We examined herbal extracts of the common Thai medicinal euphorbiaceous plant Phyllanthus acidus for their potential effects on epithelial transport. Functional assays by Ussing chamber, patch-clamping, double-electrode voltage-clamp and Ca2+ imaging demonstrate activation of Cl- secretion and inhibition of Na+ absorption by P. acidus. No cytotoxic effects of P. acidus could be detected. Mucosal application of P. acidus to native mouse trachea suggested transient and steady-state activation of Cl- secretion by increasing both intracellular Ca2+ and cAMP. These effects were mimicked by a mix of the isolated components adenosine, kaempferol, and hypogallic acid. Additional experiments in human airway cells and CF transmembrane conductance regulator (CFTR)-expressing BHK cells and Xenopus laevis oocytes confirm the results obtained in native tissues. Cl- secretion was also induced in tracheas of CF mice homozygous for Phe508del-CFTR and in Phe508del-CFTR homozygous human airway epithelial cells. Taken together, P. acidus corrects defective electrolyte transport in CF airways by parallel mechanisms including 1) increasing the intracellular levels of second messengers cAMP and Ca2+, thereby activating Ca2+-dependent Cl- channels and residual CFTR-Cl- conductance; 2) stimulating basolateral K+ channels; 3) redistributing cellular localization of CFTR; 4) directly activating CFTR; and 5) inhibiting ENaC through activation of CFTR. These combinatorial effects on epithelial transport may provide a novel complementary nutraceutical treatment for the CF lung disease.

Cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel and Na-K-Cl cotransporter NKCC1 isoform mediate the vasorelaxant action of genistein in isolated rat aorta

The soy phytoestrogen genistein is a potent vasorelaxant, but its mechanism of action is poorly understood. Here, we used endothelium-denuded rat aorta to investigate the role of the cyclic AMP(cAMP)-activated, cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel, and its associated Na-K-Cl cotransporter NKCC1. Isolated, endothelium-denuded rat aorta was contracted with phenylephrine 1 microM, and the vasorelaxant responses to genistein were investigated under conditions where CFTR was inhibited by DPC (diphenylamine-2-carboxylic acid) or glibenclamide (n=6 for compound). Both compounds fully antagonized the vasorelaxant responses to genistein, with IC50=57+/-18 microM and 42+/-11 microM for DPC and glibenclamide respectively. H-89, a selective protein kinase A (PKA) inhibitor, blocked the vasorelaxant responses to genistein. Finally, the NKCC1 inhibitor, bumetanide fully antagonized the vasorelaxant responses to genistein against phenylephrine- or KCl-induced contractions, with IC50=2.0+/-0.2 microM and 1.6+/-0.5 microM, respectively (n=6 for condition). These results strongly suggest that CFTR opening is involved in the vasorelaxant action of genistein, and that cAMP-dependent CFTR phosphorylation and chloride entry via the NKCC1 cotransporter are required for genistein action.

Selective inhibition of endoplasmic reticulum-associated degradation rescues DeltaF508-cystic fibrosis transmembrane regulator and suppresses interleukin-8 levels: therapeutic implications

Endoplasmic reticulum (ER)-associated degradation (ERAD) is the major quality control pathway of the cell. The most common disease-causing protein folding mutation, DeltaF508-cystic fibrosis transmembrane regulator (CFTR), is destroyed by ERAD to cause cystic fibrosis (CF). p97/valosin-containing protein (VCP) physically interacts with gp78/autocrine motility factor receptor to couple ubiquitination, retrotranslocation, and proteasome degradation of misfolded proteins. We show here that p97/VCP and gp78 form complexes with CFTR during translocation from the ER for degradation by the cytosolic proteasome. Interference in the VCP-CFTR complex promoted accumulation of immature CFTR in the ER and partial rescue of functional chloride channels to the cell surface. Moreover, under these conditions, interleukin-8 (IL8), the expression of which is regulated by the proteasome, was reduced. Inhibition of the proteasome with bortezomib (PS-341/Velcade) also rescued CFTR, but with less efficiency, and suppressed NFkappaB-mediated IL8 activation. The inhibition of the major stress-inducible transcription factor CHOP (CCAAT/enhancer-binding protein homologous protein)/GADD153 together with bortezomib was most effective in repressing NFkappaB-mediated IL8 activation compared with interference of VCP, MLN-273 (proteasome inhibitor), or 4-phenylbutyrate (histone deacetylase inhibitor). Immunoprecipitation of DeltaF508-CFTR from primary CF bronchial epithelial cells confirmed the interaction with VCP and associated chaperones in CF. We conclude that VCP is an integral component of ERAD and cellular stress pathways induced by the unfolded protein response and may be central to the efficacy of CF drugs that target the ubiquitin-proteasome pathway.

Forskolin-induced cell shrinkage and apical translocation of functional enhanced green fluorescent protein-human alphaENaC in H441 lung epithelial cell monolayers

Elevation of intracellular cAMP increases fluid re-absorption in the lung by raising amiloride-sensitive Na+ transport through the apically localized epithelial, amiloride-sensitive Na+ channel (ENaC). However, the signaling pathways mediating this response are still not fully understood. We show that inhibition of protein-tyrosine kinase (PTK) with Genistein and protein kinase A (PKA) with KT5720, decreased forskolin-stimulated amiloride-sensitive short circuit current (I(sc)) across H441 adult human lung epithelial cell monolayers. KT5720 also decreased basal I(sc). Stable expression of green fluorescent protein (GFP)-labeled human alphaENaC in H441 cells was used to investigate dynamic changes in the cellular localization of this protein in response to forskolin. Reverse transcription-PCR and immunoblotting analysis revealed two clones expressing a truncated (alphaC3-5) and full-length (alphaC3-3) EGFP-halphaENaC protein. Only the alphaC3-3 clone displayed dome formation and exhibited a 50% increase in basal and forskolin-stimulated amiloride-sensitive I(sc) indicating that the full-length protein was required for functional activity. Apical surface biotinylation and real-time confocal microscopy demonstrated that EGFP-halphaENaC (alphaC3-3) translocated to the apical membrane in response to forskolin in a Brefeldin A-sensitive manner. This effect was completely inhibited by Genistein but only partially inhibited by KT5720. Forskolin also induced a reduction in the height of cells within alphaC3-3 monolayers, indicative of cell shrinkage. This effect was inhibited by KT5720 but not by Genistein or Brefeldin A. These data show that forskolin activates PKA-sensitive cell shrinkage in adult human H441 lung epithelial cell monolayers, which induces a PTK-sensitive translocation of EGFP-halphaENaC subunits to the apical membrane and increases amiloride-sensitive Na+ transport.