In vivo activation of the cystic fibrosis transmembrane conductance regulator mutant deltaF508 in murine nasal epithelium

Cyclic nucleotide phosphodiesterase inhibitors as therapeutic interventions for cystic fibrosis

Cystic Fibrosis (CF) lung disease results from mutations in the CFTR anion channel that reduce anion and fluid secretion by airway epithelia. Impaired secretion compromises airway innate defence mechanisms and leads to bacterial colonization, excessive inflammation and tissue damage; thus, restoration of CFTR function is the goal of many CF therapies. CFTR channels are activated by cyclic nucleotide-dependent protein kinases. The second messengers 3'5'-cAMP and 3'5'-cGMP are hydrolysed by a large family of cyclic nucleotide phosphodiesterases that provide subcellular spatial and temporal control of cyclic nucleotide-dependent signalling. Selective inhibition of these enzymes elevates cyclic nucleotide levels, leading to activation of CFTR and other downstream effectors. Here we examine members of the PDE family that are likely to regulate CFTR-dependent ion and fluid secretion in the airways and discuss other actions of PDE inhibitors that can influence cyclic nucleotide-regulated mucociliary transport, inflammation and bronchodilation. Finally, we review PDE inhibitors and the potential benefits they could provide as CF therapeutics.

Electrochemical measurement of membrane cholesterol correlates with CFTR function and is HDAC6-dependent

BACKGROUND

Previous studies have demonstrated that CF epithelial cells exhibit increased cholesterol content at the plasma membrane compared to wild type controls as measured by electrochemical methods. Microtubule dysregulation that impacts intracellular transport has also been identified in CF cells and is reversible with histone deacetylase 6 (HDAC6) inhibition, a regulator of tubulin acetylation. The hypothesis of this study is that increased membrane cholesterol content in CF cells is dependent on HDAC6 regulation.

METHODS

Electrochemical measurement of membrane cholesterol in mouse trachea and in primary human CF bronchial epithelial cells is used to monitor CFTR correction and manipulation of cholesterol processing by HDAC6 inhibition.

RESULTS

Data demonstrate that induction of Cftr expression in an inducible CF mouse model restores tubulin acetylation levels and normalizes membrane cholesterol content. To test the relationship between tubulin acetylation, membrane cholesterol levels were measured in a CF mouse model depleted of Hdac6 expression (CF/HDA). CF/HDA mouse trachea have WT membrane cholesterol levels while CF mice have approximately two-fold increase in membrane cholesterol compared to WT consistent with previous studies. Pharmacological inhibition of HDAC6 in primary human CF bronchial epithelial cells also reduces membrane cholesterol levels.

CONCLUSIONS

This study demonstrates that elevated membrane cholesterol in CF epithelium is regulated by HDAC6 function and that the electrochemical measure of membrane cholesterol correlates with both genetic and pharmacological CFTR correction.

Improved Growth Patterns in Cystic Fibrosis Mice after Loss of Histone Deacetylase 6

Growth failure in cystic fibrosis (CF) patients has been well-documented and shown to correlate with poorer disease outcomes. This observation is also true in CF animal models, including mouse, pig, rat, and ferret. The etiology underlying growth deficits is unknown, and our previous work demonstrated reduced tubulin acetylation in CF cell models and tissue that is correctable by inhibition of histone deacetylase-6 (HDAC6). Here, we hypothesize that loss of HDAC6 will improve growth phenotype in a CF mouse model. Hdac6 knockout mice were crossed with F508del (CF) mice to generate F508del/Hdac6 (CF/HDA) mice. Growth, fat deposits, survival, and bioelectric measurements were analyzed. CF/HDA mice displayed improvements in length and weight with no correction of CFTR function. Mechanistically, Igf1 levels likely account for increased length and improvements in fertility. Weight gain is attributed to increased fat deposits potentially mediated by increased adipocyte differentiation. CF-related growth deficits can be improved via inhibition of HDAC6, further implicating it as a potential therapeutic target for CF.

PDE5 Inhibitors as Potential Tools in the Treatment of Cystic Fibrosis

Despite great advances in the understanding of the genetics and pathophysiology of cystic fibrosis (CF), there is still no cure for the disease. Using phosphodiesterase type 5 (PDE5) inhibitors, we and others have provided evidence of rescued F508del-CFTR trafficking and corrected deficient chloride transport activity. Studies using PDE5 inhibitors in mice homozygous for the clinically relevant F508del mutation have been conducted with the aim of restoring F508del-CFTR protein function. We demonstrated, by measuring transepithelial nasal potential difference in F508del mice following intraperitoneal injection of sildenafil, vardenafil, or taladafil at clinical doses are able to restore the decreased CFTR-dependent chloride transport across the nasal mucosa. Moreover, vardenafil, but not sildenafil, stimulates chloride transport through the normal CFTR protein. We developed a specific nebulizer setup for mice, with which we demonstrated, through a single inhalation of PDE5 inhibitors, local activation of CFTR protein in CF. Significant potential advantages of inhalation drug therapy over oral or intravenous routes include rapid onset of pharmacological action, reduced systemic secondary effects, and reduced effective drug doses compared to the drug delivered orally; this underlines the relevance and impact of our work for translational science. More recently, we analyzed the bronchoalveolar lavage of CF and wild-type mice for cell infiltrates and expression of pro-inflammatory cytokines and chemokines; we found that the CFTR activating effect of vardenafil, selected as a representative long-lasting PDE5 inhibitor, breaks the vicious circle of lung inflammation which plays a major role in morbi-mortality in CF. Our data highlight the potential use of PDE5 inhibitors in CF. Therapeutic approaches using clinically approved PDE5 inhibitors to address F508del-CFTR defects could speed up the development of new therapies for CF.

Mis-trafficking of bicarbonate transporters: implications to human diseasesThis paper is one of a selection of papers published in a Special Issue entitled CSBMCB 53rd Annual Meeting — Membrane Proteins in Health and Disease, and has undergone the Journal’s usual peer review process

Bicarbonate is a waste product of mitochondrial respiration and one of the main buffers in the human body. Thus, bicarbonate transporters play an essential role in maintaining acid-base balance but also during fetal development as they ensure tight regulation of cytosolic and extracellular environments. Bicarbonate transporters belong to two gene families, SLC4A and SLC26A. Proteins from these two families are widely expressed, and thus mutations in their genes result in various diseases that affect bones, pancreas, reproduction, brain, kidneys, eyes, heart, thyroid, red blood cells, and lungs. In this minireview, we discuss the current state of knowledge regarding the effect of SLC4A and SLC26A mutants, with a special emphasis on mutants that have been studied in mammalian cell lines and how they correlate with phenotypes observed in mice models.

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

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

Compartmentalized cyclic adenosine 3',5'-monophosphate at the plasma membrane clusters PDE3A and cystic fibrosis transmembrane conductance regulator into microdomains

Formation of multiple-protein macromolecular complexes at specialized subcellular microdomains increases the specificity and efficiency of signaling in cells. In this study, we demonstrate that phosphodiesterase type 3A (PDE3A) physically and functionally interacts with cystic fibrosis transmembrane conductance regulator (CFTR) channel. PDE3A inhibition generates compartmentalized cyclic adenosine 3',5'-monophosphate (cAMP), which further clusters PDE3A and CFTR into microdomains at the plasma membrane and potentiates CFTR channel function. Actin skeleton disruption reduces PDE3A-CFTR interaction and segregates PDE3A from its interacting partners, thus compromising the integrity of the CFTR-PDE3A-containing macromolecular complex. Consequently, compartmentalized cAMP signaling is lost. PDE3A inhibition no longer activates CFTR channel function in a compartmentalized manner. The physiological relevance of PDE3A-CFTR interaction was investigated using pig trachea submucosal gland secretion model. Our data show that PDE3A inhibition augments CFTR-dependent submucosal gland secretion and actin skeleton disruption decreases secretion.

Generation of a conditional null allele for Cftr in mice

The cystic fibrosis transmembrane conductance regulator (CFTR) gene encodes a cAMP-regulated chloride channel that is important in controlling the exchange of fluid and electrolytes across epithelial cells. Mutation of CFTR can lead to cystic fibrosis (CF), the most common lethal genetic disease in Caucasians. CF is a systemic illness with multiple organ systems affected including pulmonary, gastrointestinal, pancreatic, immune, endocrine, and reproductive systems. To understand the role of CFTR in the various tissues in which it is expressed, we generated a murine conditional null allele of Cftr (Cftr(fl10)) in which loxP sites were inserted around exon 10 of the Cftr gene. The Cftr(fl10) allele was validated by generating constitutive Cftr null (Cftr(Delta10)) mice using the protamine-cre system. The Cftr(Delta10/Delta10) mice displayed almost identical phenotypes to previously published CF mouse models, including poor growth, decreased survival, intestinal obstruction, and loss of Cftr function as assessed by electrophysiology measurements on gut and nasal epithelium. Mice containing the conditional null Cftr allele will be useful in future studies to understand the role of Cftr in specific tissues and developmental time points and lead to a better understanding of CF disease.

INO-4995 therapeutic efficacy is enhanced with repeat dosing in cystic fibrosis knockout mice and human epithelia

Progressive lung damage in cystic fibrosis (CF) has been linked to inadequate airway mucosal hydration. We previously demonstrated that an inositol tetrakisphosphate analog, 1-O-octyl-2-O-butyryl-myo-inositol 3,4,5,6-tetrakisphosphate octakis(propionoxymethyl)ester (INO-4995), regulates airway secretory and absorptive processes, affecting mucosal hydration by prolonged (24 h) inhibition of Na(+) and fluid absorption in CF human nasal epithelia (CFHNE). The objectives of this study were to further assess clinical potential of INO-4995 in CF through ascertaining in vivo activity in mice with CF, determining the effects of repeated administration on potency and determining cytoplasmic half-life. Uptake and metabolism of [(3)H]INO-4995 was monitored with HPLC to calculate intracellular half-life. INO-4995 was administered in vitro repeatedly over 4 to 8 days to CFHNE. Fluid absorption was assessed by blue dextran exclusion, and basal short-circuit current was measured in Ussing chambers. INO-4995 (1-100 microg/kg) was dosed intranasally either as a single dose or once per day over 4 days to gut-corrected CF mice. [(3)H]INO-4995 was rapidly taken up by epithelial cultures and converted to the active drug, which had a half-life of 40 hours. Repeated daily application of INO-4995 to CFHNE lowered the effective concentration for inhibition of fluid absorption and amiloride-sensitive short-circuit current in cultured CFHNE, and reduced nasal potential difference to nearly control levels in gut-corrected CF mice. Ca(2+)-activated Cl(-) channel activity was also boosted in cultures. Mouse nasal levels fell from abnormal levels to within 2 muA of normal with repeated exposure to 0.8 microg/kg over 4 days. These data support further development of INO-4995 for the treatment of CF.

Mild processing defect of porcine DeltaF508-CFTR suggests that DeltaF508 pigs may not develop cystic fibrosis disease

Recent efforts have made significant progress in generating transgenic pigs with the DeltaF508-CFTR mutation to model the lung and pancreatic disease of human cystic fibrosis. However, species differences in the processing and function of human, pig and mouse DeltaF508-CFTR reported recently raise concerns about the phenotypic consequence of the gene-targeted pig model. The purpose of the present study was to characterize the DeltaF508 mutant of porcine CFTR to evaluate the severity of its processing defect. Biochemical and immunofluorescence analysis in transfected COS7 and FRT cells indicated that pig DeltaF508-CFTR efficiently targets to the plasma membrane and is present mainly as the mature glycosylated protein. Functional characterization in stably transfected FRT cells by fluorometric and electrophysiological assays supported efficient plasma membrane targeting of pig DeltaF508-CFTR. The mild cellular processing defect of pig DeltaF508-CFTR suggests that its gene-targeted pig model may not develop the lung and pancreatic phenotypes seen in CF patients.

Role of oxygen availability in CFTR expression and function

The cystic fibrosis transmembrane conductance regulator (CFTR) serves a pivotal role in normal epithelial homeostasis; its absence leads to destruction of exocrine tissues, including those of the gastrointestinal tract and lung. Acute regulation of CFTR protein in response to environmental stimuli occurs at several levels (e.g., ion channel phosphorylation, ATP hydrolysis, apical membrane recycling). However, less information is available concerning the regulatory pathways that control levels of CFTR mRNA. In the present study, we investigated regulation of CFTR mRNA during oxygen restriction, examined effects of hypoxic signaling on chloride transport across cell monolayers, and related these findings to a possible role in the pathogenesis of chronic hypoxic lung disease. CFTR mRNA, protein, and function were robustly and reversibly altered in human cells in relation to hypoxia. In mice subjected to low oxygen in vivo, CFTR mRNA expression in airways, gastrointestinal tissues, and liver was repressed. CFTR mRNA expression was also diminished in pulmonary tissues taken from hypoxemic subjects at the time of lung transplantation. Environmental factors that induce hypoxic signaling regulate CFTR mRNA and epithelial Cl(-) transport in vitro and in vivo.

Processing and function of CFTR-DeltaF508 are species-dependent

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) cause cystic fibrosis. The most common mutation, a deletion of the phenylalanine at position 508 (DeltaF508), disrupts processing of the protein. Nearly all human CFTR-DeltaF508 is retained in the endoplasmic reticulum and degraded, preventing maturation to the plasma membrane. In addition, the F508 deletion reduces the activity of single CFTR channels. Human CFTR-DeltaF508 has been extensively studied to better understand its defects. Here, we adopted a cross-species comparative approach, examining human, pig, and mouse CFTR-DeltaF508. As with human CFTR-DeltaF508, the DeltaF508 mutation reduced the single-channel activity of the pig and mouse channels. However, the mutant pig and mouse proteins were at least partially processed like their wild-type counterparts. Moreover, pig and mouse CFTR-DeltaF508 partially restored transepithelial Cl(-) transport to CF airway epithelia. Our data, combined with earlier work, suggest that there is a gradient in the severity of the CFTR-DeltaF508 processing defect, with human more severe than pig or mouse. These findings may explain some previously puzzling observations in CF mice, they have important implications for evaluation of potential therapeutics, and they suggest new strategies for discovering the mechanisms that disrupt processing of human CFTR-DeltaF508.

Olfactory epithelia exhibit progressive functional and morphological defects in CF mice

In normal nasal epithelium, the olfactory receptor neurons (ORNs) are continuously replaced through the differentiation of progenitor cells. The olfactory epithelium (OE) of the cystic fibrosis (CF) mouse appears normal at birth, yet by 6 mo of age, a marked dysmorphology of sustentacular cells and a dramatic reduction in olfactory receptor neurons are evident. Electroolfactograms revealed that the odor-evoked response in 30-day-old CF mice was reduced approximately 45%; in older CF mice, a approximately 70% reduction was observed compared with the wild type (WT) response. Consistent with studies of CF airway epithelia, Ussing chamber studies of OE isolated from CF mice showed a lack of forskolin-stimulated Cl(-) secretion and an approximately 12-fold increase in amiloride-sensitive sodium absorption compared with WT mice. We hypothesize that the marked hyperabsorption of Na(+), most likely by olfactory sustentacular cells, leads to desiccation of the surface layer in which the sensory cilia reside, followed by degeneration of the ORNs. The CF mouse thus provides a novel model to examine the mechanisms of disease-associated loss of olfactory function.

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.

Pharmacological induction of CFTR function in patients with cystic fibrosis: mutation-specific therapy

CFTR mutations cause defects of CFTR protein production and function by different molecular mechanisms. Mutations can be classified according to the mechanisms by which they disrupt CFTR function. This understanding of the different molecular mechanisms of CFTR dysfunction provides the scientific basis for the development of targeted drugs for mutation-specific therapy of cystic fibrosis (CF). Class I mutations are nonsense mutations that result in the presence of a premature stop codon that leads to the production of unstable mRNA, or the release from the ribosome of a short, truncated protein that is not functional. Aminoglycoside antibiotics can suppress premature termination codons by disrupting translational fidelity and allowing the incorporation of an amino acid, thus permitting translation to continue to the normal termination of the transcript. Class II mutations cause impairment of CFTR processing and folding in the Golgi. As a result, the mutant CFTR is retained in the endoplasmic reticulum (ER) and eventually targeted for degradation by the quality control mechanisms. Chemical and molecular chaperones such as sodium-4-phenylbutyrate can stabilize protein structure, and allow it to escape from degradation in the ER and be transported to the cell membrane. Class III mutations disrupt the function of the regulatory domain. CFTR is resistant to phosphorylation or adenosine tri-phosphate (ATP) binding. CFTR activators such as alkylxanthines (CPX) and the flavonoid genistein can overcome affected ATP binding through direct binding to a nucleotide binding fold. In patients carrying class IV mutations, phosphorylation of CFTR results in reduced chloride transport. Increases in the overall cell surface content of these mutants might overcome the relative reduction in conductance. Alternatively, restoring native chloride pore characteristics pharmacologically might be effective. Activators of CFTR at the plasma membrane may function by promoting CFTR phosphorylation, by blocking CFTR dephosphorylation, by interacting directly with CFTR, and/or by modulation of CFTR protein-protein interactions. Class V mutations affect the splicing machinery and generate both aberrantly and correctly spliced transcripts, the levels of which vary among different patients and among different organs of the same patient. Splicing factors that promote exon inclusion or factors that promote exon skipping can promote increases of correctly spliced transcripts, depending on the molecular defect. Inconsistent results were reported regarding the required level of corrected or mutated CFTR that had to be reached in order to achieve normal function.

Novel, mechanism-based therapies for cystic fibrosis

PURPOSE OF REVIEW

Cystic fibrosis results from disruption of the biosynthesis or function of the cystic fibrosis transmembrane conductance regulator. Cystic fibrosis transmembrane conductance regulator plays a critical role in the regulation of epithelial ion transport. Restoration of cystic fibrosis transmembrane conductance regulator function should improve the cystic fibrosis phenotype.

RECENT FINDINGS

Recent investigations affording a better understanding of the mechanism of dysfunction of mutant cystic fibrosis transmembrane conductance regulators, as well as the roles of cystic fibrosis transmembrane conductance regulator in regulating epithelial ion transport, have led to development of therapeutic strategies based on repair or bypass of mutant cystic fibrosis transmembrane conductance regulator dysfunction. The former strategy, coined 'protein repair therapy,' is aimed at improving or restoring the function of mutant cystic fibrosis transmembrane conductance regulators, whereas the latter approach aims to augment epithelial ion transport to compensate for the absent function mutant cystic fibrosis transmembrane conductance regulator.

SUMMARY

Strategies to improve mutant cystic fibrosis transmembrane conductance regulator function or to bypass mutant cystic fibrosis transmembrane conductance regulator function hold great promise for development of novel therapies aimed at correcting the underlying pathophysiology of cystic fibrosis.

The role of pyocyanin in Pseudomonas aeruginosa infection

Pyocyanin (PCN) is a blue redox-active secondary metabolite that is produced by Pseudomonas aeruginosa. PCN is readily recovered in large quantities in sputum from patients with cystic fibrosis who are infected by P. aeruginosa. Despite in vitro studies demonstrating that PCN interferes with multiple cellular functions, its importance during clinical infection is uncertain. This is partially caused by the difficulty in defining the contribution of PCN among the numerous virulence factors produced by P. aeruginosa during infection. In addition, few cellular pathways that are affected by PCN are known. This review briefly highlights recent advances that might clarify the role of PCN in P. aeruginosa pathogenesis.

Dynamic activation of cystic fibrosis transmembrane conductance regulator by type 3 and type 4D phosphodiesterase inhibitors

The diseases of cystic fibrosis, chronic obstructive pulmonary disease (COPD), and chronic bronchitis are characterized by mucus-congested and inflamed airways. Anti-inflammatory agents that can simultaneously restore or enhance mucociliary clearance through cystic fibrosis transmembrane conductance regulator (CFTR) activation may represent new therapeutics in their treatment. Herein, we report the activation of CFTR-mediated chloride secretion by phosphodiesterase (PDE) 4 inhibitors in T84 monolayer using (125)I anion as tracer. In the absence of forskolin, the iodide secretion was insensitive to PDE4 inhibitor L-826,141 [4-[2-(3,4-bis-difluoromethoxyphenyl)-2-[4-(1,1,1,3,3,3-hexafluoro-2-hydroxypropan-2-yl)phenyl]-ethyl]-3-methylpyridine-1-oxide], roflumilast, or to PDE3 inhibitor trequinsin. However, these inhibitors potently augmented iodide secretion after forskolin stimulation, with efficacy coupled to the activation states of adenylyl cyclase. The iodide secretion from PDE3 or PDE4 inhibition was characterized at first by a prolonged efflux duration, followed by progressively elevated peak efflux rates at higher inhibitor concentrations. Paralleled with an increased phosphor-cAMP response element-binding protein formation, the CFTR activation dissociated from a global cAMP elevation and was blocked by H89 [N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide]. 2-(4-Fluorophenoxy)-N-[(1S)-1-(4-methoxyphenyl)ethyl]nicotinamide, a stereoselective PDE4D inhibitor, augmented iodide efflux more efficiently than its less potent (R)-isomer. The peak efflux from maximal PDE4 and PDE3 inhibition matched that from full adenylyl cyclase activation. These data suggest that PDE3 and PDE4 (mainly PDE4D) form the major cAMP diffusion barrier in T84 cells to ensure a compartmentalized CFTR signaling. Together with their potent anti-inflammatory properties, the potentially enhanced airway mucociliary clearance from CFTR activation may have contributed to the efficacy of PDE4 inhibitors in COPD and asthmatic patients. PDE4 inhibitors may represent new opportunities to combat cystic fibrosis and other respiratory diseases in future.

Phosphodiesterase 4D Forms a cAMP Diffusion Barrier at the Apical Membrane of the Airway Epithelium

We demonstrated previously that Calu-3 airway epithelial cells sense adenosine on their luminal surface through adenosine A2B receptors coupled to adenylyl cyclase. Occupancy of these receptors leads to activation of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel through protein kinase A (PKA) anchored at the apical membrane. Because luminal A2B receptor activation does not raise total cellular cAMP levels, we hypothesized that activation of phosphodiesterases (PDEs) confines cAMP generated by apical A2B receptors to a microdomain that includes the CFTR channel. Using reverse transcription-PCR, Western blotting, and activity measurements, PDE4D was identified as the major PDE species in airway epithelia. Consistent with these results, inhibitors of PDE4, but not PDE3, selectively abolished the lateral confinement of cAMP signaling in apical membrane patches during cell-attached recordings. Furthermore, stimulation of the CFTR in excised apical patches by rolipram and RS25344 indicated that PDE4 is localized in close proximity to the CFTR channel. Indeed, immunohistochemistry of human airway sections revealed that PDE4D is localized in the apical domain of the cell. PDE4 was activated after luminal adenosine exposure in a PKA-dependent manner. Because PDE4 activity is positively regulated by PKA, our results support a model whereby the PDE diffusion barrier is proportional to the degree of receptor stimulation. These findings underscore the concept that subcellular localization of individual PDE isozymes is an important mechanism for confining cAMP signaling to functional domains within cells.

Defective respiratory amiloride-sensitive sodium transport predisposes to pulmonary oedema and delays its resolution in mice

Pulmonary oedema results from an imbalance between the forces driving fluid into the airspace and the biological mechanisms for its removal. In mice lacking the alpha-subunit of the amiloride-sensitive sodium channel (alphaENaC(-/-)), impaired sodium transport-mediated lung liquid clearance at birth results in neonatal death. Transgenic expression of alphaENaC driven by a cytomegalovirus (CMV) promoter (alphaENaC(-/-)Tg+) rescues the lethal pulmonary phenotype, but only partially restores respiratory sodium transport in vitro. To test whether this may also be true in vivo, and to assess the functional consequences of this defect on experimental pulmonary oedema, we measured respiratory transepithelial potential difference (PD) and alveolar fluid clearance (AFC), and quantified pulmonary oedema during experimental acute lung injury in these mice. Both respiratory PD and AFC were roughly 50% lower (P < 0.01) in alphaENaC(-/-)Tg+ than in control mice. This impairment was associated with a significantly larger increase of the wet/dry lung weight ratio in alphaENaC(-/-)Tg+ than in control mice, both after exposure to hyperoxia and thiourea. Moreover, the rate of resolution of thiourea-induced pulmonary oedema was more than three times slower (P < 0.001) in alphaENaC(-/-)Tg+ mice. alphaENaC(-/-)Tg+ mice represent the first model of a constitutively impaired respiratory transepithelial sodium transport, and provide direct evidence that this impairment facilitates pulmonary oedema in conscious freely moving animals. These data in mice strengthen indirect evidence provided by clinical studies, suggesting that defective respiratory transepithelial sodium transport may also facilitate pulmonary oedema in humans.

Role of Cftr genotype in the response to chronic Pseudomonas aeruginosa lung infection in mice

Patients with cystic fibrosis have a lesion in the cystic fibrosis transmembrane conductance regulator gene (CFTR), which is associated with abnormal regulation of other ion channels, abnormal glycosylation of secreted and cell surface molecules, and vulnerability to bacterial infection and inflammation in the lung usually leading to the death of these patients. The exact mechanism(s) by which mutation in CFTR leads to lung infection and inflammation is not clear. Mice bearing different mutations in the murine homolog to CFTR (Cftr) (R117H, S489X, Y122X, and DeltaF508, all backcrossed to the C57BL/6J background) were compared with respect to growth and in their ability to respond to lung infection elicited with Pseudomonas aeruginosa-laden agarose beads. Body weights of mice bearing mutations in Cftr were significantly smaller than wild-type mice at most ages. The inflammatory responses to P. aeruginosa-laden agarose beads were comparable in mice of all four Cftr mutant genotypes with respect to absolute and relative cell counts in bronchoalveolar lavage fluid, and cytokine levels (TNF-alpha, IL-1beta, IL-6, macrophage inflammatory protein-2, and keratinocyte chemoattractant) and eicosanoid levels (PGE2 and LTB4) in epithelial lining fluid: the few small differences observed occurred only between cystic fibrosis mice bearing the S489X mutation and those bearing the knockout mutation Y122X. Thus we cannot implicate either misprocessing of CFTR or failure of CFTR to reach the plasma membrane in the genesis of the excess inflammatory response of CF mice. Therefore, it appears that any functional defect in CFTR produces comparable inflammatory responses to lung infections with P. aeruginosa.

CFTR involvement in nasal potential differences in mice and pigs studied using a thiazolidinone CFTR inhibitor

Nasal potential difference (PD) measurements have been used to demonstrate defective CFTR function in cystic fibrosis (CF) and to evaluate potential CF therapies. We used the selective thiazolidinone CFTR inhibitor CFTR(inh)-172 to define the involvement of CFTR in nasal PD changes in mice and pigs. In normal mice infused intranasally with a physiological saline solution containing amiloride, nasal PD was -4.7 +/- 0.7 mV, hyperpolarizing by 15 +/- 1 mV after a low-Cl- solution, and a further 3.9 +/- 0.5 mV after forskolin. CFTR(inh)-172 produced 1.1 +/- 0.9- and 4.3 +/- 0.7-mV depolarizations when added after low Cl- and forskolin, respectively. Systemically administered CFTR(inh)-172 reduced the forskolin-induced hyperpolarization from 4.7 +/- 0.4 to 0.9 +/- 0.1 mV but did not reduce the low Cl(-)-induced hyperpolarization. Nasal PD was -12 +/- 1 mV in CF mice after amiloride, changing by <0.5 mV after low Cl- or forskolin. In pigs, nasal PD was -14 +/- 3 mV after amiloride, hyperpolarizing by 13 +/- 2 mV after low Cl- and a further 9 +/- 1 mV after forskolin. CFTR(inh)-172 and glibenclamide did not affect nasal PD in pigs. Our results suggest that cAMP-dependent nasal PDs in mice primarily involve CFTR-mediated Cl- conductance, whereas cAMP-independent PDs are produced by a different, but CFTR-dependent, Cl- channel. In pigs, CFTR may not be responsible for Cl- channel-dependent nasal PDs. These results have important implications for interpreting nasal PDs in terms of CFTR function in animal models of CFTR activation and inhibition.

Curcumin, a major constituent of turmeric, corrects cystic fibrosis defects

Cystic fibrosis is caused by mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR). The most common mutation, DeltaF508, results in the production of a misfolded CFTR protein that is retained in the endoplasmic reticulum and targeted for degradation. Curcumin is a nontoxic Ca-adenosine triphosphatase pump inhibitor that can be administered to humans safely. Oral administration of curcumin to homozygous DeltaF508 CFTR mice in doses comparable, on a weight-per-weight basis, to those well tolerated by humans corrected these animals' characteristic nasal potential difference defect. These effects were not observed in mice homozygous for a complete knockout of the CFTR gene. Curcumin also induced the functional appearance of DeltaF508 CFTR protein in the plasma membranes of transfected baby hamster kidney cells. Thus, curcumin treatment may be able to correct defects associated with the homozygous expression of DeltaF508 CFTR.

Pharmacotherapy of the ion transport defect in cystic fibrosis: role of purinergic receptor agonists and other potential therapeutics

Cystic fibrosis (CF), is an autosomal recessive disease frequently seen in the Caucasian population. It is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. CF is characterized by enhanced airway Na(+) absorption, mediated by epithelial Na(+) channels (ENaC), and deficient Cl(-) transport. In addition, other mechanisms may contribute to the pathophysiological changes in the CF lung, such as defective regulation of HCO(3)(-) secretion. In other epithelial tissues, epithelial Na(+) conductance is either increased (intestine) or decreased (sweat duct) in CF. CFTR is a cyclic AMP-regulated epithelial Cl(-) channel, and appears to control the activity of several other transport proteins. Accordingly, defective epithelial ion transport in CF is likely to be a combination of defective Cl(-) channel function and impaired regulator function of CFTR, which in turn is linked to impaired mucociliary clearance and development of chronic lung disease. As the clinical course of CF is determined primarily by progressive lung disease, novel pharmacological strategies for the treatment of CF focus on correction of the ion transport defect in the airways. In recent years, it has been demonstrated that activation of purinergic receptors in airway epithelia by extracellular nucleotides (adenosine triphosphate/uridine triphosphate) has beneficial effects on mucus clearance in CF. Activation of the dominant class of metabotropic purinergic receptors, P2Y(2) receptors, appears to have a 2-fold benefit on ion transport in CF airways; excessive Na(+) absorption is attenuated, most likely by inhibition of the ENaC and, simultaneously, an alternative Ca(2+)-dependent Cl(-) channel is activated that may compensate for the CFTR Cl(-) channel defect. Thus activation of P2Y(2) receptors is expected to lead to improved hydration of the airway surface liquid in CF. Furthermore, purinergic activation has been shown to promote other components of mucociliary clearance such as ciliary beat frequency and mucus secretion. Clinical trials are under way to test the effect of synthetic purinergic compounds, such as the P2Y(2) receptor agonist INS37217, on the progression of lung disease in patients with CF. Administration of these compounds alone, or in combination with other drugs that inhibit accelerated Na(+) transport and help recover or increase residual activity of mutant CFTR, is most promising as successful therapy to counteract the ion transport defect in the airways of CF patients.

Adenosine receptors and phosphodiesterase inhibitors stimulate Cl- secretion in Calu-3 cells

We investigated cystic fibrosis transmembrane conductance regulator (CFTR) activation by clinically used phosphodiesterase inhibitors (PDEis) in Calu-3 cell monolayers alone and in combination with A2B adenosine receptor stimulation. This receptor pathway has previously been shown to activate wild-type and mutant CFTR molecules. Several PDEis, including milrinone, cilostazol (Pletal), papaverine, rolipram, and sildenafil (Viagra), produced a short circuit current (Isc) that was glibenclamide-sensitive, achieving 20-85% of forskolin-stimulated Isc. Papaverine, cilostazol, and rolipram also augmented both the magnitude and the duration of Isc following low dose stimulation of adenosine receptors with Ado (0.1-1.0 microM, P < 0.01). Subsequent studies demonstrated that very low concentrations of cilostazol or papaverine (approximately 1/2 peak serum concentrations) were sufficient to activate Isc, and both agents markedly augmented Ado-stimulated Isc (1 microM, P < 0.01). Our results provide evidence that select PDEis, at concentrations achieved as part of systemic therapies, can activate CFTR-dependent Isc in Calu-3 cell monolayers. These studies also indicate that PDEis have the capacity to augment an endogenous CFTR-activating pathway in an "in vivo"-like model system, and supports future investigations of these agents relevant to cystic fibrosis.

Transcriptional adaptation to cystic fibrosis transmembrane conductance regulator deficiency

Cystic fibrosis, the most commonly inherited lethal pulmonary disorder in Caucasians, is caused by mutations in the cystic fibrosis transmembrane conductance regulator gene (CFTR). To identify genomic responses to the presence or absence of CFTR in pulmonary tissues in vivo, microarray analyses of lung mRNAs were performed on whole lung tissue from mice lacking (CFTR(-)) or expressing mouse CFTR (CFTR(+)). Whereas the histology of lungs from CFTR(-) and CFTR(+) mice was indistinguishable, statistically significant increases in the relative abundance of 29 and decreases in 25 RNAs were identified by RNA microarray analysis. Of RNAs whose expression was consistently altered by the absence of CFTR, functional classes of genes influencing gene transcription, inflammation, intracellular trafficking, signal transduction, and ion transport were identified. RNAs encoding the transcription factor CCAAT enhancer-binding protein (CEBP) delta and interleukin (IL) 1beta, both known to regulate CFTR expression, were induced, perhaps indicating adaptation to the lack of CFTR. RNAs mediating lung inflammation including calgranulin-S100 family members, IL-1beta and IL-4, were increased. Likewise, expression of several membrane transport proteins that interact directly with CFTR were increased, suggesting that CFTR-protein complexes initiate genomic responses. Absence of CFTR influenced the expression of genes modulating diverse pulmonary cell functions that may ameliorate or contribute to the pathogenesis of CF.

Therapeutic approaches to repair defects in deltaF508 CFTR folding and cellular targeting

The deltaF508 mutation in the cystic fibrosis transmembrane regulator (CFTR) gene is the most common mutation in CF. The mutant CFTR protein is defective with respect to multiple functions including cAMP-regulated chloride conductance, nucleotide transport, and regulatory actions on other ion channels. Since the deltaF508 protein is also temperature-sensitive and unstable during translation and folding in the endoplasmic reticulum (ER), most of the nascent chains are targeted for premature proteolysis from the ER. This paper focuses on the events that occur in the ER during folding and reviews potential targets for therapeutic intervention.

Normal function of the cystic fibrosis conductance regulator protein can be associated with homozygous (Delta)F508 mutation

Cystic fibrosis (CF) is caused by mutations of the gene encoding for the CFTR (CF transmembrane conductance regulator) protein. The most frequent mutation, the (Delta)F508 mutation, results in a defective cAMP-regulated chloride transport in the epithelial cells. The spectrum of clinical manifestations in patients bearing homozygous (Delta)F508 mutations can vary considerably, suggesting that, in the patients with a mild disease, CFTR could be partly functional. To test this hypothesis, we explored in nasal ciliated epithelial cells (NCC) of 9 control subjects and 23 (Delta)F508 homozygous patients the anion conductive pathway by a halide sensitive fluorescent dye assay SPQ (6-methoxy-N-3'-sulfopropylquinolinium) and the CFTR transcript levels by RT-PCR. As 50% represented the lowest fraction of the control subjects NCC demonstrating a cAMP-dependent conductance, a CF patient was considered as "cAMP responder" if at least 50% of the NCC tested displayed a cAMP-dependent conductive pathway. According to these criteria, 8 of the 23 patients were considered as cAMP responders. They had a significantly less severe disease considering the respiratory function and infectious status. The amount of CFTR mRNA did not differ between the control subjects and the patients. No statistical correlation could be found between the transcript level and the expression of a cAMP conductive pathway. This cAMP-dependent Cl(-) conductance detected in homozygous NCC could be due to a residual CFTR activity and may explain the mild phenotypes observed in some (Delta)F508 homozygous patients.

The human DnaJ homologue (Hdj)-1/heat-shock protein (Hsp) 40 co-chaperone is required for the in vivo stabilization of the cystic fibrosis transmembrane conductance regulator by Hsp70

The CFTR (cystic fibrosis transmembrane conductance regulator) gene, defective in cystic fibrosis, codes for a polytopic apical membrane protein functioning as a chloride channel. Wild-type (wt) CFTR matures inefficiently and CFTR with a deletion of Phe-508 (F508del), the most frequent mutation, is substantially retained as a core-glycosylated intermediate in the endoplasmic reticulum (ER), probably due to misfolding that is recognized by the cellular quality control machinery involving molecular chaperones. Here, we overexpressed the heat-shock protein (Hsp) 70 chaperone in vivo and observed no changes in degradation rate of the core-glycosylated form, nor in the efficiency of its conversion into the fully glycosylated form, for either wt- or F508del-CFTR, contrary to previous in vitro studies on the affect of heat-shock cognate (Hsc) 70 on part of the first nucleotide-binding domain of CFTR. Co-transfection of Hsp70 with its co-chaperone human DnaJ homologue (Hdj)-1/Hsp40, however, stabilizes the immature form of wt-CFTR, but not of F508del-CFTR, suggesting that these chaperones act on a wt-specific conformation. As the efficiency of conversion into the fully glycosylated form is not increased under Hsp70/Hdj-1 overexpression, the lack of these two chaperones does not seem to be critical for CFTR maturation and ER retention. The effects of 4-phenylbutyrate and deoxyspergualin, described previously to interfere with Hsp70 binding, were also tested upon CFTR degradation and processing. The sole effect observed was destabilization of F508del-CFTR.

In vivo microdialysis for determination of nasal liquid ion composition

Airway surface liquid (ASL) contains substances important in mucociliary clearance and airway defense. Little is known about substance concentrations in ASL because of its small volume and sampling difficulties. We used in vivo microdialysis (IVMD) to sample liquid lining the nasal cavity without net volume removal and incorporated into IVMD a potential difference (PD) electrode to assess airway integrity. The cystic fibrosis (CF) mouse nasal epithelia exhibit ion transport defects identical to those in CF human airways and, thus, are a good model for CF disease. We determined that nasal liquid [Na+] (107 +/- 4 mM normal; 111 +/- 9 mM CF) and [Cl-] (120 +/- 6 mM normal; 122 +/- 4 mM CF) did not differ between genotypes. The nasal liquid [K+] (8.7 +/- 0.4 mM) was significantly less in normal than in CF mice (16.6 +/- 4 mM). IVMD accurately samples nasal liquid for ionic composition. The ionic composition of nasal liquid in the normal and CF mice is similar.

IL-4 is a potent modulator of ion transport in the human bronchial epithelium in vitro

Recent data show that proinflammatory stimuli may modify significantly ion transport in the airway epithelium and therefore the properties of the airway surface fluid. We have studied the effect of IL-4, a cytokine involved in the pathogenesis of asthma, on transepithelial ion transport in the human bronchial epithelium in vitro. Incubation of polarized bronchial epithelial cells with IL-4 for 6-48 h causes a marked inhibition of the amiloride-sensitive Na(+) channel as measured in short circuit current experiments. On the other hand, IL-4 evokes a 2-fold increase in the current activated by a cAMP analog, which reflects the activity of the cystic fibrosis transmembrane conductance regulator (CFTR). Similarly, IL-4 enhances the response to apical UTP, an agonist that activates Ca(2+)-dependent Cl(-) channels. These effects are mimicked by IL-13 and blocked by an antagonist of IL-4Ralpha. RT-PCR experiments show that IL-4 elicits a 7-fold decrease in the level of the gamma amiloride-sensitive Na(+) channel mRNA, one of the subunits of the amiloride-sensitive Na(+) channel, and an increase in CFTR mRNA. Our data suggest that IL-4 may favor the hydration of the airway surface by decreasing Na(+) absorption and increasing Cl(-) secretion. This could be required to fluidify the mucus, which is hypersecreted during inflammatory conditions. On the other hand, the modifications of ion transport could also affect the ion composition of airway surface fluid.

Asymptomatic cystic fibrosis diagnosed in an adult evaluated for hematuria

We report a case of cystic fibrosis in an asymptomatic man evaluated for hematuria with infertility. The ever-broadening spectrum of atypical adult presentations of cystic fibrosis should prompt physicians to have a lower threshold for ordering genetic screening for cystic fibrosis transmembrane regulator (CFTR) gene mutations.

A(2) adenosine receptors regulate CFTR through PKA and PLA(2)

We investigated adenosine (Ado) activation of the cystic fibrosis transmembrane conductance regulator (CFTR) in vitro and in vivo. A(2B) Ado receptors were identified in Calu-3, IB-3-1, COS-7, and primary human airway cells. Ado elevated cAMP in Calu-3, IB-3-1, and COS-7 cells and activated protein kinase A-dependent halide efflux in Calu-3 cells. Ado promoted arachidonic acid release from Calu-3 cells, and phospholipase A(2) (PLA(2)) inhibition blocked Ado-activated halide efflux in Calu-3 and COS-7 cells expressing CFTR. Forskolin- and beta(2)-adrenergic receptor-stimulated efflux were not affected by the same treatment. Cytoplasmic PLA(2) (cPLA(2)) was identified in Calu-3, IB-3-1, and COS-7 cells, but cPLA(2) inhibition did not affect Ado-stimulated cAMP concentrations. In cftr(+) and cftr(-/-) mice, Ado stimulated nasal Cl(-) secretion that was CFTR dependent and sensitive to A(2) receptor and PLA(2) blockade. In COS-7 cells transiently expressing DeltaF508 CFTR, Ado activated halide efflux. Ado also activated G551D CFTR-dependent halide efflux when combined with arachidonic acid and phosphodiesterase inhibition. In conclusion, PLA(2) and protein kinase A both contribute to A(2) receptor activation of CFTR, and components of this signaling pathway can augment wild-type and mutant CFTR activity.

Chloride conductance and genetic background modulate the cystic fibrosis phenotype of Delta F508 homozygous twins and siblings

To investigate the impact of chloride (Cl(-)) permeability, mediated by residual activity of the cystic fibrosis transmembrane conductance regulator (CFTR) or by other Cl(-) channels, on the manifestations of cystic fibrosis (CF), we determined Cl(-) transport properties of the respiratory and intestinal tracts in Delta F508 homozygous twins and siblings. In the majority of patients, cAMP and/or Ca(2+)-regulated Cl(-) conductance was detected in the airways and intestine. Our finding of cAMP-mediated Cl(-) conductance suggests that, in vivo, at least some Delta F508 CFTR can reach the plasma membrane and affect Cl(-) permeability. In respiratory tissue, the expression of basal CFTR-mediated Cl(-) conductance, demonstrated by 30% of Delta F508 homozygotes, was identified as a positive predictor of milder CF disease. In intestinal tissue, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid-insensitive (DIDS-insensitive) Cl(-) secretion, which is indicative of functional CFTR channels, correlated with a milder phenotype, whereas DIDS-sensitive Cl(-) secretion was observed mainly in more severely affected patients. The more concordant Cl(-) secretory patterns within monozygous twins compared with dizygous pairs imply that genes other than CFTR significantly influence the manifestation of the basic defect.

Pharmacotherapy of the ion transport defect in cystic fibrosis

1. More than 1300 different mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) cause cystic fibrosis (CF), a disease characterized by deficient epithelial Cl- secretion and enhanced Na+ absorption. The clinical course of the disease is determined by the progressive lung disease. Thus, novel approaches in pharmacotherapy are based primarily on correction of the ion transport defect in the airways. 2. The current therapeutic strategies try to counteract the deficiency in Cl- secretion and the enhanced Na+ absorption. A number of compounds have been identified, such as genistein and xanthine derivatives, which directly activate mutant CFTR. Other compounds may activate alternative Ca2+-activated Cl- channels or basolateral K+ channels, which supply the driving force for Cl- secretion. Apart from that, Na+ channel blockers, such as phenamil and benzamil, are being explored, which counteract the hyperabsorption of NaCl in CF airways. 3. Clinical trials are under way using purinergic compounds such as the P2Y(2) receptor agonist INS365. Activation of P2Y(2) receptors has been found to both activate Cl- secretion and inhibit Na+ absorption. 4. The ultimate goal is to recover Cl- channel activity of mutant CFTR by either enhancing synthesis and expression of the protein or by activating silent CFTR Cl- channels. Strategies combining these drugs with compounds facilitating Cl- secretion and inhibiting Na+ absorption in vivo may have the best chance to counteract the ion transport defect in cystic fibrosis.

Examining basal chloride transport using the nasal potential difference response in a murine model

Epithelia of humans and mice with cystic fibrosis are unable to secrete chloride in response to a chloride gradient or to cAMP-elevating agents. Bioelectrical properties measured using the nasal transepithelial potential difference (TEPD) assay are believed to reflect these cystic fibrosis transmembrane conductance regulator (CFTR)-dependent chloride transport defects. Although the response to forskolin is CFTR mediated, the mechanisms responsible for the response to a chloride gradient are unknown. TEPD measurements performed on inbred mice were used to compare the responses to low chloride and forskolin in vivo. Both responses show little correlation between or within inbred strains of mice, suggesting they are mediated through partially distinct mechanisms. In addition, these responses were assayed in the presence of several chloride channel inhibitors, including DIDS, diphenylamine-2-carboxylate, glibenclamide, and 5-nitro-2-(3-phenylpropylamino)-benzoic acid, and a protein kinase A inhibitor, the Rp diastereomer of adenosine 3',5'-cyclic monophosphothioate (Rp-cAMPS). The responses to low chloride and forskolin demonstrate significantly different pharmacological profiles to both DIDS and Rp-cAMPS, indicating that channels in addition to CFTR contribute to the low chloride response.

Erythrocyte membrane ATP binding cassette (ABC) proteins: MRP1 and CFTR as well as CD39 (ecto-apyrase) involved in RBC ATP transport and elevated blood plasma ATP of cystic fibrosis

In addition to the better-known roles of the erythrocyte in the transport of oxygen and carbon dioxide, the concept that the red blood cell is involved in the transport and release of ATP has been evolving (J. Luthje, Blut 59, 367, 1989; G. R. Bergfeld and T. Forrester, Cardiovasc. Res. 26, 40, 1992; M. L. Ellsworth et al., Am. J. Physiol. 269, H2155, 1995; R. S. Sprague et al., Am. J. Physiol. 275, H1726, 1998). Membrane proteins involved in the release of ATP from erythrocytes now appear to include members of the ATP binding cassette (ABC) family (C. F. Higgins, Annu. Rev. Cell Biol. 8, 67, 1992; C. F. Higgins, Cell 82, 693, 1995). In addition to defining physiologically the presence of ABC proteins in RBCs, accumulating gel electrophoretic evidence suggests that the cystic fibrosis transmembrane conductance regulator (CFTR) and the multidrug resistance-associated protein (MRP1), respectively, constitute significant proteins in the red blood cell membrane. As such, this finding makes the mature erythrocyte compartment a major mammalian repository of these important ABC proteins. Because of its relative structural simplicity and ready accessibility, the erythrocyte offers an ideal system to explore details of the physiological functions of ABC proteins. Moreover, the presence of different ABC proteins in a single membrane implies that interaction among these proteins and with other membrane proteins may be the norm and not the exception in terms of modulation of their functions.

Pharmacological treatment of the biochemical defect in cystic fibrosis airways

The understanding of the biochemical defect in cystic fibrosis (CF) has advanced considerably since discovery of the CF gene in 1989 and characterization of its product. Studies showing that the abnormality in chloride flux could be corrected by transfection of wild-type cystic fibrosis transmembrane conductance regulator (CFTR) complimentary deoxyribonucleic acid (cDNA) have led to gene therapy trials on both sides of the Atlantic. However, gene therapy as a treatment for CF has yet to be realized. Pharmacological manipulation of the biochemical defect may provide an alternative or complementary approach to treatment. This review will discuss pharmacological agents in development which could correct the abnormal ion movement. The mechanisms of action of these pharmacological agents can be divided broadly into drugs which affect the most common CF mutation, deltaF508, which increase trafficking of the mutant CF protein to the apical membrane; drugs which increase chloride secretion; and drugs which reduce sodium reabsorption across the apical membrane. Treatment options for cystic fibrosis have developed rapidly since discovery of the cystic fibrosis gene over a decade ago. The targeting of specific therapies for particular cystic fibrosis genotypes and the use of combination treatments of chloride channel openers with sodium channel blockers are likely to be key advances in the next decade.

Therapeutic strategies to correct malfunction of CFTR

Cystic fibrosis (CF) is a systemic autosomal recessive inherited disorder that results from mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Although the gene was cloned 11 years ago, there still is no definitive treatment to correct the functional deficit. Current treatment strategies focus on pancreatic enzyme replacement and control of pulmonary inflammation and infection. This review examines novel strategies still in preclinical development or phase 1 clinical trials. Gene therapy is an evolving area of study that offers the potential for a cure for cystic fibrosis. CF lung disease is a significant barrier to effective gene delivery and transfer, but new vectors show promise in overcoming these limitations. There are also new pharmacological therapies aimed at correcting defects in CFTR processing and function. These are tailored to the specific class of mutation but may offer therapeutic benefit to many patients. They include phenylbutyrate, flavonoids, aminoglycosides and xanthines.

Type I, II, III, IV, and V cystic fibrosis transmembrane conductance regulator defects and opportunities for therapy

Recent advances in cellular and molecular biology have furthered the understanding of several genetic diseases, including cystic fibrosis. Mutations that cause cystic fibrosis are now understood in terms of the specific molecular consequences to the cystic fibrosis transmembrane conductance regulator (CFTR) protein expression and function. This knowledge has spawned interest in the development of therapies aimed directly at correcting the defective CFTR itself. In this article, we review the molecular defect underlying each recognized class of CFTR mutation and the potential therapies currently under investigation. Opportunities for protein-repair therapy appear to be vast and range from naturally occurring compounds, such as isoflavonoids, to pharmaceuticals already in clinical use, including aminoglycoside antibiotics, butyrate analogues, phosphodiesterase inhibitors, and adenosine nucleotides. Future therapies may resemble designer compounds like benzo[c]quinoliziniums or take the form of small peptide replacements. Given the heterogeneity and progressive nature of cystic fibrosis, however, optimal benefit from protein-repair therapy will most likely require the initiation of combined therapies early in the course of disease to avoid irreparable organ damage.

Future pharmacological treatment of cystic fibrosis

Cystic fibrosis (CF) is an autosomal recessive disorder that is caused by over 850 different mutations in the CF gene. It is useful to group these mutations according to the defect that results in the CFTR mRNA or protein. New pharmacological treatments targeted towards specific mutations that are relatively common are being developed. Class I mutations do not produce CFTR protein because of a premature stop signal in the CFTR DNA. These null mutations can be corrected by certain aminoglycosides which cause the aberrant stop signal to be skipped. Mutations leading to a CFTR protein that attains an unstable structure shortly after translation in the endoplasmic reticulum form class II. Class II mutations can be restored to the protein trafficking pathway by manipulation of chaperone protein/CFTR interactions with chemical chaperones or drugs that affect gene regulation such as the butyrates. Production of a CFTR with reduced Cl(-) transport on the basis of abnormal regulation of the chloride channel is the basis of class III. Genistein can overcome this block in regulation. Mutations that partially reduce chloride conductance through CFTR (class IV) can be stimulated with milrinone, which is a phosphodiesterase inhibitor. Finally, mutations that lead to a severe reduction in normal CFTR protein form class V. Increased levels of CFTR could be generated with the butyrates or supplemented with gene therapy. Although most of the reported mutations in CFTR are rare and unclassified, it may be possible to use genotype-phenotype correlations to determine the best approach.

In vivo alterations of IFN regulatory factor-1 and PIAS1 protein levels in cystic fibrosis epithelium

Inducible nitric oxide synthase-2 (NOS2) expression has been shown to be reduced in cystic fibrosis (CF) epithelial cells. Reduced NOS2 expression is unexpected, given the inflammatory nature of CF airway disease, and is an indication that cell-signaling mechanisms necessary for proper NOS2 regulation are probably altered in CF epithelium. Therefore, we examined the expression levels of regulatory factors necessary for NOS2 expression in CF epithelium and showed that IFN regulatory factor-1 (IRF-1) is necessary for full NOS2 expression. Mice lacking IRF-1 expression have diminished epithelial NOS2 expression, as well as reduced NO-dependent chloride transport across the nasal epithelia. Furthermore, IRF-1 protein expression is reduced in nasal and intestinal epithelial cells from CF mice, suggesting a possible mechanism for the CF-related reduction of epithelial NOS2 expression. Active signal transducer and activator of transcription-1 (Stat1) is necessary for both NOS2 and IRF-1 expression. We found that protein levels of Stat1 were increased in CF cells, but that the active phosphorylated form of Stat1 was bound to the protein inhibitor of activated Stat1 (PIAS1). We propose that increased levels of PIAS1 diminish certain cell-signaling pathways, resulting in reduced IRF-1 and NOS2 expression in CF epithelial cells.

Identification of transport abnormalities in duodenal mucosa and duodenal enterocytes from patients with cystic fibrosis

BACKGROUND & AIMS

The duodenum is a cystic fibrosis transmembrane conductance regulator (CFTR)-expressing epithelium with high bicarbonate secretory capacity. We aimed to define the role of CFTR in human duodenal epithelial bicarbonate secretion in normal (NL) subjects and patients with cystic fibrosis (CF).

METHODS

Endoscopic biopsy specimens of the duodenal bulb were obtained from 9 CF patients and 16 volunteers. Tissues were mounted in modified Ussing chambers. Bicarbonate secretion and short-circuit current (Isc) were quantitated under basal conditions and in response to dibutyryl adenosine 3',5'-cyclic monophosphate (db-cAMP), carbachol, and the heat-stable toxin of Escherichia coli (STa). Duodenocytes were also isolated and loaded with the pH-sensitive fluoroprobe BCECF/AM, and intracellular pH (pH(i)) was measured at rest and after intracellular acidification and alkalinization.

RESULTS

Basal HCO(3)(-) secretion and Isc were significantly lower in the CF vs. NL duodenal mucosa. In contrast to NL, db-cAMP failed to alter either HCO(3)(-) or Isc in CF tissues. However, in CF, carbachol resulted in an electroneutral HCO(3)(-) secretion, whereas STa induced electrogenic HCO(3)(-) secretion that was similar to NL. In CF and NL duodenocytes, basal pH(i) and recovery from an acid load were comparable, but pH(i) recovery after an alkaline load in CF duodenocytes was Cl(-) dependent, whereas in NL duodenocytes it was Cl(-) independent.

CONCLUSIONS

These findings implicate CFTR in NL duodenal alkaline transport and its absence in CF. Although duodenal bicarbonate secretion is impaired in CF tissues, alternate pathway(s) likely exist that can be activated by carbachol and STa.

Cystic fibrosis F508del patients have apically localized CFTR in a reduced number of airway cells

Present state of knowledge, mostly based on heterologous expression studies, indicates that the cystic fibrosis transmembrane conductance regulator (CFTR) protein bearing the F508del mutation is misprocessed and mislocalized in the cytoplasm, unable to reach the cell surface. Recently, however, it was described that protein levels and localization are similar between F508del and wild-type CFTR in airway and intestinal tissues, but not in the sweat glands. In this study, we used immunocytochemistry with three different anti-CFTR antibodies to investigate endogenous CFTR expression and localization in nasal epithelial cells from F508del homozygous patients, F508del carriers, and non-CF individuals. On average, 300 cells were observed per individual. No significant differences were observed for cell type distributions among CF, carrier, and non-CF samples; epithelial cells made up approximately 80% to 95% of all cells present. CFTR was detected mostly in the apical region (AR) of the tall columnar epithelial (TCE) cells, ciliated or nonciliated. By confocal microscopy analysis, we show that the CFTR apical region-staining does not overlap with either anti-calnexin (endoplasmic reticulum), anti-p58 (Golgi), or anti-tubulin (cilia) stainings. The median from results with three antibodies indicate that the apical localization of CFTR happens in 22% of TCE cells from F508del homozygous patients with CF (n = 12), in 42% of cells from F508del carriers (n = 20), and in 56% of cells from healthy individuals (n = 12). Statistical analysis indicates that differences are significant among all groups studied and for the three antibodies (p < 0.05). These results confirm the presence of CFTR in the apical region of airway cells from F508del homozygous patients; however, they also reveal that the number of cells in which this occurs is significantly lower than in F508del carriers and much lower than in healthy individuals. These findings may have an impact on the design of novel pharmacological strategies aimed at circumventing the CF defect caused by the F508del mutation.

A pilot study of the effect of gentamicin on nasal potential difference measurements in cystic fibrosis patients carrying stop mutations

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene containing a premature termination signal are expected to produce little or no CFTR chloride channels. It has been shown in vitro, that aminoglycoside antibiotics can increase the frequency of erroneous insertion of nonsense codons hence permitting the translation of CFTR alleles carrying missense mutations to continue reading to the end of the gene. This led to the appearance of functional CFTR channels at the apical plasma membrane. The aim of this research was to determine if topical application of gentamicin to the nasal epithelium of patients with cystic fibrosis (CF) carrying stop mutations can express, in vivo, functional CFTR channels. Nine CF patients carrying stop mutations (mean age 23 +/- 11 yr, range 12 to 46 yr) received gentamicin drops (0.3%, 3 mg/ml) three times daily intranasally for a total of 14 d. Nasal potential difference (PD) was measured before and after the treatment. Before gentamicin application all the patients had abnormal nasal PD typical of CF. After gentamicin treatment, significant repolarization of the nasal epithelium representing chloride transport was increased from -1 +/- 1 mV to -10 +/- 11 mV (p < 0. 001). In conclusion, gentamicin may influence the underlying chloride transport abnormality in patients with CF carrying stop mutations.

Cystic fibrosis transmembrane conductance regulator-dependent regulation of epithelial inducible nitric oxide synthase expression

Recent evidence has shown that the inducible form of nitric oxide (NO) synthase (NOS2) has reduced expression in airway epithelia of patients with cystic fibrosis (CF) despite the presence of chronic inflammation. The goal of this paper is to determine whether NOS2 expression is regulated by the presence of functional CF transmembrane conductance regulator (CFTR). Using a human trachea epithelial cell line in which CFTR activity is blocked by the overexpression of the CFTR regulatory domain, we found that loss of CFTR activity reduces NOS2 messenger RNA expression as determined by reverse transcriptase/polymerase chain reaction and reduces overall NO production compared with mock-transfected controls. An in vivo model using mice lacking CFTR expression (cftr -/-), wild-type mice (cftr +/+), and cftr -/- mice that have had human CFTR introduced to the intestinal epithelium using the fatty acid binding protein (FABP) promoter (FABP-hcftr) was also examined. Electrical characterization confirmed that FABP-hcftr mice had corrected electrophysiologic properties compared with cftr -/- mice in the ileum, but FABP-hcftr nasal transepithelial potential difference measurements were identical to cftr -/- values showing specific intestinal correction. NOS2-specific immunostaining revealed that NOS2 expression is evident in sections of ileum and nasal epithelium of cftr +/+ mice but is absent in both tissues in cftr -/- mice. FABP-hcftr mice, however, show strong NOS2 staining in epithelial cells of the ileum but reduced staining in the nasal epithelium, suggesting a CFTR-related influence in the regulation of NOS2 expression in epithelial cells.

A cyclic nucleotide PDE5 inhibitor corrects defective mucin secretion in submandibular cells containing antibody directed against the cystic fibrosis transmembrane conductance regulator protein

A selective cyclic nucleotide PDE5 inhibitor corrected the defective mucin secretion response to the beta-agonist isoproterenol in submandibular acinar cells inhibited by antibody directed against the cystic fibrosis transmembrane conductance regulator. The PDE5 inhibitor was as effective as cpt-cyclic AMP or a selective PDE4 inhibitor. However, the PDE5 inhibitor had no effect on basal or isoproterenol-stimulated cyclic AMP levels and did not stimulate mucin secretion. The results showing, for the first time, correction of the CFTR mucin secretion defect by a PDE5 inhibitor, which may involve cyclic GMP, will have a major impact in development of a rational drug treatment for cystic fibrosis.

Update on clinical trials in the treatment of pulmonary disease in patients with cystic fibrosis

Cystic fibrosis is a congenital disease resulting from an abnormality of the cystic fibrosis transmembrane conductance regulator (CFTR) gene. A defect in ion transport leads to poor clearance of viscoelastic secretions and a susceptibility to bacterial infection. This initiates a self-perpetuating cycle of infection and inflammation that accounts for the chronic endobronchial sepsis and pulmonary damage observed in patients with cystic fibrosis. Recent studies have attempted to correct the gene defect, enhance the expression and function of the CFTR protein and correct the ion transport defect. Improving the rheological properties of airway secretions, enhancing host defence and controlling inflammation are the other key strategies.

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

The patch-clamp technique was used to investigate the effects of the isoflavone genistein on disease-causing mutations (G551D and DeltaF508) of the cystic fibrosis transmembrane conductance regulator (CFTR). In HeLa cells recombinantly expressing the trafficking-competent G551D-CFTR, the forskolin-stimulated Cl currents were small, and average open probability of G551D-CFTR was P(o) = 0.047 +/- 0.019. Addition of genistein activated Cl currents approximately 10-fold, and the P(o) of G551D-CFTR increased to 0.49 +/- 0.12, which is a P(o) similar to wild-type CFTR. In cystic fibrosis (CF) epithelial cells homozygous for the trafficking-impaired DeltaF508 mutation, forskolin and genistein activated Cl currents only after 4-phenylbutyrate treatment. These data suggested that genistein activated CFTR mutants that were present in the cell membrane. Therefore, we tested the effects of genistein in CF patients with the G551D mutation in nasal potential difference (PD) measurements in vivo. The perfusion of the nasal mucosa of G551D CF patients with isoproterenol had no effect; however, genistein stimulated Cl-dependent nasal PD by, on average, -2.4 +/- 0.6 mV, which corresponds to 16.9% of the responses (to beta-adrenergic stimulation) found in healthy subjects.