CFTR is a conductance regulator as well as a chloride channel

Ivacaftor treatment of cystic fibrosis patients with the G551D mutation: a review of the evidence

Cystic fibrosis (CF) is a recessive disorder caused by mutations in the gene that encodes the CF transmembrane conductance regulator (CFTR) protein. CFTR protein is a chloride and bicarbonate channel that is critical for normal epithelial ion transport and hydration of epithelial surfaces. Current CF care is supportive, but recent breakthroughs have occurred with the advent of novel therapeutic strategies that assist the function of mutant CFTR proteins. The development and key clinical trial results of ivacaftor, a small molecule that targets gating defects in disease-causing CFTR mutations including G551D CFTR, are summarized in this review. The G551D mutation is reasonably common in the CF patient population and produces a CFTR protein that localizes normally to the plasma membrane, but fails to open in response to cellular cues. Ivacaftor treatment produces dramatic improvements in lung function, weight, lung disease stability, patient-reported outcomes, and CFTR biomarkers in patients with CF harboring the G551D CFTR mutation compared with placebo controls and patients with two copies of the common F508del CFTR mutation. The unprecedented success of ivacaftor treatment for the G551D CF patient population has generated excitement in the CF care community regarding the expansion of its use to other CF patient populations with primary or secondary gating defects.

Apical CFTR expression in human nasal epithelium correlates with lung disease in cystic fibrosis

Introduction

Although most individuals with cystic fibrosis (CF) develop progressive obstructive lung disease, disease severity is highly variable, even for individuals with similar CFTR mutations. Measurements of chloride transport as expression of CFTR function in nasal epithelial cells correlate with pulmonary function and suggest that F508del-CFTR is expressed at the apical membrane. However, an association between quantitative apical CFTR expression in nasal epithelium and CF disease severity is still missing.

Methods and Materials

Nasal epithelial cells from healthy individuals and individuals with CF between 12–18 years were obtained by nasal brushing. Apical CFTR expression was measured by confocal microscopy using CFTR mAb 596. Expression was compared between both groups and expression in CF nasal epithelial cells was associated with standardized pulmonary function (FEV₁%).

Results

The proportion of cells expressing apical CFTR in columnar epithelium is lower in CF compared to non-CF. The apical CFTR expression level was significantly correlated with FEV₁% in F508del homozygous subjects (r = 0.63, p = 0.012).

Conclusion

CFTR expression in nasal epithelial cells is lower in subjects with CF compared to healthy subjects. The proportion of cells expressing F508del-CFTR at the apical membrane is variable between subjects and is positively correlated with FEV₁% in F508del-CFTR homozygous subjects.

CFTR activity and mitochondrial function

Cystic Fibrosis (CF) is a frequent and lethal autosomal recessive disease, caused by mutations in the gene encoding the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR). Before the discovery of the CFTR gene, several hypotheses attempted to explain the etiology of this disease, including the possible role of a chloride channel, diverse alterations in mitochondrial functions, the overexpression of the lysosomal enzyme α-glucosidase and a deficiency in the cytosolic enzyme glucose 6-phosphate dehydrogenase. Because of the diverse mitochondrial changes found, some authors proposed that the affected gene should codify for a mitochondrial protein. Later, the CFTR cloning and the demonstration of its chloride channel activity turned the mitochondrial, lysosomal and cytosolic hypotheses obsolete. However, in recent years, using new approaches, several investigators reported similar or new alterations of mitochondrial functions in Cystic Fibrosis, thus rediscovering a possible role of mitochondria in this disease. Here, we review these CFTR-driven mitochondrial defects, including differential gene expression, alterations in oxidative phosphorylation, calcium homeostasis, oxidative stress, apoptosis and innate immune response, which might explain some characteristics of the complex CF phenotype and reveals potential new targets for therapy.

Status of fluid and electrolyte absorption in cystic fibrosis

Salt and fluid absorption is a shared function of many of the body's epithelia, but its use is highly adapted to the varied physiological roles of epithelia-lined organs. These functions vary from control of hydration of outward-facing epithelial surfaces to conservation and regulation of total body volume. In the most general context, salt and fluid absorption is driven by active Na(+) absorption. Cl(-) is absorbed passively through various available paths in response to the electrical driving force that results from active Na(+) absorption. Absorption of salt creates a concentration gradient that causes water to be absorbed passively, provided the epithelium is water permeable. Key differences notwithstanding, the transport elements used for salt and fluid absorption are broadly similar in diverse epithelia, but the regulation of these elements enables salt absorption to be tailored to very different physiological needs. Here we focus on salt absorption by exocrine glands and airway epithelia. In cystic fibrosis, salt and fluid absorption by gland duct epithelia is effectively prevented by the loss of cystic fibrosis transmembrane conductance regulator (CFTR). In airway epithelia, salt and fluid absorption persists, in the absence of CFTR-mediated Cl(-) secretion. The contrast of these tissue-specific changes in CF tissues is illustrative of how salt and fluid absorption is differentially regulated to accomplish tissue-specific physiological objectives.

The ΔF508-CFTR mutation inhibits wild-type CFTR processing and function when co-expressed in human airway epithelia and in mouse nasal mucosa

BACKGROUND

Rescue or correction of CFTR function in native epithelia is the ultimate goal of CF therapeutics development. Wild-type (WT) CFTR introduction and replacement is also of particular interest. Such therapies may be complicated by possible CFTR self-assembly into an oligomer or multimer.

RESULTS

Surprisingly, functional CFTR assays in native airway epithelia showed that the most common CFTR mutant, ΔF508-CFTR (ΔF-CFTR), inhibits WT-CFTR when both forms are co-expressed. To examine more mechanistically, both forms of CFTR were transfected transiently in varying amounts into IB3-1 CF human airway epithelial cells and HEK-293 human embryonic kidney cells null for endogenous CFTR protein expression. Increasing amounts of ΔF-CFTR inhibited WT-CFTR protein processing and function in CF human airway epithelial cells but not in heterologous HEK-293 cells. Stably expressed ΔF-CFTR in clones of the non-CF human airway epithelial cell line, CALU-3, also showed reduction in cAMP-stimulated anion secretion and in WT-CFTR processing. An ultimate test of this dominant negative-like effect of ΔF-CFTR on WT-CFTR was the parallel study of two different CF mouse models: the ΔF-CFTR mouse and the bitransgenic CFTR mouse corrected in the gut but null in the lung and airways. WT/ΔF heterozygotes had an intermediate phenotype with regard to CFTR agonist responses in in vivo nasal potential difference (NPD) recordings and in Ussing chamber recordings of short-circuit current (ISC) in vitro on primary tracheal epithelial cells isolated from the same mice. In contrast, CFTR bitransgenic +/- heterozygotes had no difference in their responses versus +/+ wild-type mice.

CONCLUSIONS

Taken altogether, these data suggest that ΔF-CFTR and WT-CFTR co-assemble into an oligomeric macromolecular complex in native epithelia and share protein processing machinery and regulation at the level of the endoplasmic reticulum (ER). As a consequence, ΔF-CFTR slows WT-CFTR protein processing and limits its expression and function in the apical membrane of native airway epithelia. Implications of these data for the relative health of CF heterozygous carriers, for CFTR protein processing in native airway epithelia, and for the relative efficacy of different CF therapeutic approaches is significant and is discussed.

Pharmacological Approaches to Correcting the Ion Transport Defect in Cystic Fibrosis

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

Impact of the cystic fibrosis mutation F508del-CFTR on renal cyst formation and growth

In autosomal dominant polycystic kidney disease (ADPKD), cystic fibrosis transmembrane conductance regulator (CFTR), the protein product of the gene defective in cystic fibrosis (CF), plays a crucial role in fluid accumulation, which promotes cyst swelling. Several studies have identified individuals afflicted by both ADPKD and CF. Two studies suggested that CF mutations might attenuate the severity of ADPKD, whereas a third found no evidence of a protective effect. In this study, we investigated the impact of the commonest CF mutation F508del-CFTR on the formation and growth of renal cysts. As a model system, we used Madin-Darby canine kidney (MDCK) epithelial cells engineered to express wild-type and F508del human CFTR. We grew MDCK cysts in collagen gels in the presence of the cAMP agonist forskolin and measured transepithelial resistance and Cl(-) secretion with the Ussing chamber technique and assayed cell proliferation using nonpolarized MDCK cells. When compared with untransfected MDCK cells, cells expressing wild-type CFTR generated substantial numbers of large cysts, which grew markedly over time. By contrast, MDCK cells expressing F508del-CFTR formed very few tiny cysts that failed to enlarge. Interestingly, treatment of F508del-CFTR cysts with the CFTR corrector VRT-325 and the CFTR corrector-potentiator VRT-532 increased the number, but not size, of F508del-CFTR cysts, possibly because VRT-325 inhibited strongly cell proliferation. Based on its effects on transepithelial resistance, Cl(-) secretion, and cell proliferation, we conclude that the F508del-CFTR mutation disrupts cyst formation and growth by perturbing strongly fluid accumulation within the cyst lumen without compromising epithelial integrity.

Arrhythmia and sudden death associated with elevated cardiac chloride channel activity

The identification and analysis of several cationic ion channels and their associated genes have greatly improved our understanding of the molecular and cellular mechanisms of cardiac arrhythmia. Our objective in this study was to examine the involvement of anionic ion channels in cardiac arrhythmia. We used a transgenic mouse model to overexpress the human cystic fibrosis transmembrane conductance regulator (CFTR) gene, which encodes a cAMP-regulated chloride channel. We used RNase protection and in situ hybridization assays to determine the level of CFTR expression, and radiotelemetry and in vivo electrophysiological study in combination with pharmacological intervention to analyse the cardiac function. Cardiac CFTR overexpression leads to stress-related sudden death in this model. In vivo intracardiac electrophysiological studies performed in anaesthetized mice showed no significant differences in baseline conduction parameters including atrial-His bundle (AH) or His bundle-ventricular (HV) conduction intervals, atrioventricular (AV) Wenckebach or 2:1 AV block cycle length and AV nodal functional refractory period. However, following isoproterenol administration, there was marked slowing of conduction parameters, including high-grade AV block in transgenic mice, with non-sustained ventricular tachycardia easily inducible using programmed stimulation or burst pacing. Our sudden death mouse model can be a valuable tool for investigation of the role of chloride channels in arrhythmogenesis and, potentially, for future evaluation of novel anti-arrhythmic therapeutic strategies and pharmacological agents.

Visualization of flow-induced ATP release and triggering of Ca2+ waves at caveolae in vascular endothelial cells

Endothelial cells (ECs) release ATP in response to shear stress, a fluid mechanical force generated by flowing blood but, although its release has a crucial role in controlling a variety of vascular functions by activating purinergic receptors, the mechanism of ATP release has never been established. To analyze the dynamics of ATP release, we developed a novel chemiluminescence imaging method by using cell-surface-attached firefly luciferase and a CCD camera. Upon stimulation of shear stress, cultured human pulmonary artery ECs simultaneously released ATP in two different manners, a highly concentrated, localized manner and a less concentrated, diffuse manner. The localized ATP release occurred at caveolin-1-rich regions of the cell membrane, and was blocked by caveolin-1 knockdown with siRNA and the depletion of plasma membrane cholesterol with methyl-β-cyclodexrin, indicating involvement of caveolae in localized ATP release. Ca(2+) imaging with Fluo-4 combined with ATP imaging revealed that shear stress evoked an increase in intracellular Ca(2+) concentration and the subsequent Ca(2+) wave that originated from the same sites as the localized ATP release. These findings suggest that localized ATP release at caveolae triggers shear-stress-dependent Ca(2+) signaling in ECs.

Promoter hypermethylation of the CFTR gene and clinical/pathological features associated with non-small cell lung cancer

BACKGROUND AND OBJECTIVE

The exact role of the cystic fibrosis transmembrane conductance regulator (CFTR) in pathophysiology, and the mechanisms regulating its expression are poorly understood. The CFTR gene is known to be genetically or epigenetically associated with several cancers. In the present study, the methylation status of the promoter region of the CFTR gene and its expression in primary non-small cell lung cancer (NSCLC) were investigated.

METHODS

The methylation status of the promoter region of the CFTR gene in NSCLC tissue was assessed by pyrosequencing and methylation-specific PCR. Expression of the CFTR gene was analysed by real-time PCR, and CFTR gene reactivation was investigated using 5-aza-2'-deoxycytidine. The correlation between methylation of the CFTR gene and the clinical features of the patients was assessed.

RESULTS

Methylation of the CFTR gene in NSCLC was quantitatively high by pyrosequencing analysis and qualitatively frequent by methylation-specific PCR analysis. Expression of the CFTR gene was significantly lower in NSCLC compared with normal lung tissue. In addition, the demethylating agent 5-aza-2'-deoxycytidine increased CFTR gene expression. Methylation of the CFTR gene was significantly greater in squamous cell carcinomas than in adenocarcinomas. CFTR gene methylation was associated with significantly poorer survival in young patients, but not in elderly patients.

CONCLUSIONS

These findings suggest that DNA methylation may be important for downregulation of CFTR gene expression in lung cancer. Promoter hypermethylation of the CFTR gene may be an important prognostic factor in younger patients with NSCLC.

Pharmacological therapy for cystic fibrosis: from bench to bedside

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

Paracellular permeability of bronchial epithelium is controlled by CFTR

In normal airway epithelium, the cystic fibrosis transmembrane conductance regulator (CFTR) transports Cl(-) ions to the apical surface of the epithelium paralleled by the flow of water through transcellular and paracellular pathways. The hypothesis was tested whether CFTR not only regulates the transcellular but also the paracellular shunt pathway. Therefore, we performed measurements of transepithelial electrical resistance (TER) and paracellular (14)C-mannitol permeability in wtCFTR (16HBE14o(-)) and delF508-CFTR (CFBE41o(-)) expressing human bronchial epithelial cells. Under resting conditions, CFBE41o(-) cell monolayers exhibit a higher paracellular permeability and lower TER as compared to 16HBE14o(-) monolayers. Stimulation of CFTR by cAMP induces opposite effects in the two cell lines. 16HBE14o(-) monolayers show a sharp decrease of TER, in parallel with a concomitant increase of paracellular permeability. The change in paracellular permeability is mediated by a myosin II dependent mechanism because it can be blocked by the myosin light chain kinase inhibitor ML-7. In contrast, CFBE41o(-) cells respond to cAMP stimulation with a decrease of paracellular permeability, paralleled by slight increase of TER. We conclude that stimulation of wtCFTR increases vectorial transcellular salt transport and, simultaneously, the paracellular permeability allowing water to follow through the paracellular pathway. In contrast, in CF epithelium cAMP stimulation increases neither vectorial salt transport nor paracellular permeability which is likely to contribute to the CF pulmonary phenotype. Taken together, our results link CFTR dysfunction to an improper regulation of the paracellular transport route.

Plasma choline depletion is associated with decreased peripheral blood leukocyte acetylcholine in children with cystic fibrosis

BACKGROUND

Choline is an important constituent of acetylcholine. Choline is needed for acetylcholine in the nonneuronal acetylcholine system that includes epithelial cells of the lung and intestine, endothelial cells, and immune cells. Plasma free choline concentrations are low in children with cystic fibrosis (CF), but the implications for acetylcholine are unknown.

OBJECTIVE

We determined the relation between plasma free choline and related metabolites and leukocyte acetylcholine in children with CF and in a control group of healthy children without CF.

DESIGN

This was a cross-sectional study in 34 children with CF who were pancreatic insufficient and taking pancreatic enzyme-replacement therapy and in 16 healthy children. Plasma free choline, betaine, dimethylglycine, methionine, homocysteine, and leukocyte acetylcholine concentrations were quantified by using isotope-dilution HPLC-tandem mass spectrometry.

RESULTS

Mean (±SE) plasma free choline was 9.30 ± 0.37 and 6.54 ± 0.38 μmol/L (P < 0.05) and leukocyte acetylcholine was 1.21 ± 0.016 and 0.077 ± 0.011 pmol leukocyte acetylcholine/10(6) cells (P < 0.05) in control children and children with CF, respectively. Leukocyte acetylcholine was positively correlated with plasma free choline concentration in children with CF (r = 0.412, P < 0.05) but not in control children. Plasma betaine, dimethylglycine, and methionine concentrations were also lower in children with CF than in control children (P < 0.05).

CONCLUSIONS

A low free choline and methyl status in children with CF is associated with reduced acetylcholine in leukocytes. Whether these changes are explained by a mutation in the CF transmembrane conductance regulator or disturbances in choline metabolism and the implications for immune cell dysfunction in CF are unknown. This trial was registered at clinicaltrials.gov as NCT01150136.

Normal and pathological erythrocytes studied by atomic force microscopy

Erythrocytes (red blood cells, RBCs) are the most common type of blood cells in vertebrates. Many diseases and dysfunctions directly affect their structure and function. Employing the atomic force microscope (AFM) physical, chemical, and biological/physiological properties of RBCs can be studied even under near-physiological conditions. In this chapter, we present the application of different AFM techniques to investigate and compare normal and pathological RBCs. We give a detailed description for nondestructive immobilization of whole intact RBCs and explain preparation techniques for isolated native RBC membranes. High-resolution imaging of morphological details and pathological differences are demonstrated with healthy and systemic lupus erythematosus (SLE) erythrocytes revealing substructural changes due to SLE. We also present the technique of simultaneous topography and recognition imaging, which was used to map the distribution of cystic fibrosis transmembrane conductance regulator sites on erythrocyte membranes in healthy and cystic fibrosis-positive RBCs.

Nucleotide-mediated airway clearance

A thin layer of airway surface liquid (ASL) lines the entire surface of the lung and is the first point of contact between the lung and the environment. Surfactants contained within this layer are secreted in the alveolar region and are required to maintain a low surface tension and to prevent alveolar collapse. Mucins are secreted into the ASL throughout the respiratory tract and serve to intercept inhaled pathogens, allergens and toxins. Their removal by mucociliary clearance (MCC) is facilitated by cilia beating and hydration of the ASL by active ion transport. Throughout the lung, secretion, ion transport and cilia beating are under purinergic control. Pulmonary epithelia release ATP into the ASL which acts in an autocrine fashion on P2Y(2) (ATP) receptors. The enzymatic network describes in Chap. 2 then mounts a secondary wave of signaling by surface conversion of ATP into adenosine (ADO), which induces A(2B) (ADO) receptor-mediated responses. This chapter offers a comprehensive description of MCC and the extensive ramifications of the purinergic signaling network on pulmonary surfaces.

CFTR expression regulation by the unfolded protein response

The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel and key regulator of epithelial functions. Mutations in the CFTR gene lead to reduced or dysfunctional CFTR protein and cause cystic fibrosis (CF), a generalized exocrinopathy affecting multiple organs. In the airways, loss of CFTR function leads to thickened mucus, reduced mucociliary clearance, chronic infections, and respiratory failure. Common airway disorders such as bronchitis and chronic obstructive pulmonary disease (COPD) also present CF-like symptoms such as mucus congestion and chronic inflammation without mutations in CFTR. The primary risk factors for COPD and chronic bronchitis include environmental stress insults such as pollutants and infections that often result in hypoxic conditions. Furthermore, environmental factors such as cigarette smoke and reactive oxygen species have been implicated in reduced CFTR function. Activation of cellular stress responses by these factors promotes differential, stress-associated gene expression regulation. During our investigations on the mechanisms of CFTR expression regulation, we have shown that the ER stress response, the unfolded protein response (UPR), decreases CFTR expression at the transcriptional, translational, and maturational levels. Here, we provide a detailed description of the methods we employ to study CFTR expression regulation by the UPR. Similar approaches are applicable in studies on other genes and how they are affected by the UPR.

The primary folding defect and rescue of ΔF508 CFTR emerge during translation of the mutant domain

In the vast majority of cystic fibrosis (CF) patients, deletion of residue F508 from CFTR is the cause of disease. F508 resides in the first nucleotide binding domain (NBD1) and its absence leads to CFTR misfolding and degradation. We show here that the primary folding defect arises during synthesis, as soon as NBD1 is translated. Introduction of either the I539T or G550E suppressor mutation in NBD1 partially rescues ΔF508 CFTR to the cell surface, but only I539T repaired ΔF508 NBD1. We demonstrated rescue of folding and stability of NBD1 from full-length ΔF508 CFTR expressed in cells to isolated purified domain. The co-translational rescue of ΔF508 NBD1 misfolding in CFTR by I539T advocates this domain as the most important drug target for cystic fibrosis.

An outwardly rectifying chloride channel in BeWo choriocarcinoma cell line

In this study, an outwardly rectifying chloride channel was characterized in the trophoblastic cell line BeWo, a human hormone-synthesizing cell which displays many biochemical and morphological properties similar to those reported for the human cytotrophoblast. Ion channel activity was recorded in the cell attached and inside-out configurations with standard patch-clamp technology. In most of the BeWo cells studied, the channel under symmetrical N-methyl-d-glucamine (NMDG-Cl) concentration (Na(+) free solution) in both sides of the membrane exhibited spontaneous activity, an outwardly rectifying current/voltage relationship and single-channel conductances of 15 pS and 48 pS for inwards and outwards currents, respectively. The channel has a low permeability for gluconate with a relative permeability P(gluconate)/P(Cl) of 0.23, and a higher permeability to I(-). The open probability (Po) of the channel exhibited dependence with the applied membrane potential with greater activity at positive pulses. The channel activity was inhibited by the sulphonylurea hypoglycemic agent glibenclamide (50 μM) or by diphenylamine-2-carboxylate (DPC, 500 μM) added to the cytoplasmic side of the patch whereas conductances remained unchanged. The blockade with glibenclamide and DPC was independent of the applied membrane potential. All these results are characteristic of the outwardly rectifying Cl channel (ORCC) found in other types of cells. Neither Po, conductances nor reversal potential (Er) values were affected by the absence of intracellular Ca(2+), suggesting that the channel is not sensitive to Ca(2+).

Altered ion transport by thyroid epithelia from CFTR(-/-) pigs suggests mechanisms for hypothyroidism in cystic fibrosis

Subclinical hypothyroidism has been linked to cystic fibrosis, and the cystic fibrosis transmembrane conductance regulator (CFTR) shown to be expressed in the thyroid. The thyroid epithelium secretes Cl⁻ and absorbs Na(+) in response to cAMP. Chloride secretion may provide a counter-ion for the SLC26A4 (pendrin)-mediated I⁻ secretion which is required for the first step of thyroid hormonogenesis, thyroglobulin iodination. In contrast, few models exist to explain a role for Na(+) absorption. Whether CFTR mediates the secretory Cl⁻ current in thyroid epithelium has not been directly addressed. We used thyroids from a novel pig CFTR(-/-) model, generated primary pig thyroid epithelial cell cultures (pThECs), analysed these cultures for preservation of thyroid-specific transcripts and proteins, and monitored the following parameters: (1) the Cl⁻ secretory response to the cAMP agonist, isoprenaline; and (2) the amiloride-sensitive Na(+) current. Baseline short-circuit current (I(sc)) did not differ between CFTR(+/+) and CFTR(-/-) cultures. Serosal isoprenaline increased I(sc) in CFTR(+/+), but not CFTR(-/-), monolayers. Compared with CFTR(+/+) thyroid cultures, amiloride-sensitive Na(+) absorption measured in CFTR(-/-) pThECs represented a greater fraction of the resting I(sc). However, levels of transcripts encoding epithelial sodium channel (ENaC) subunits did not differ between CFTR(+/+) and CFTR(-/-) pThECs. Immunoblot analysis verified ENaC subunit protein expression, but quantification indicated no difference in expression levels. Our studies definitively demonstrate that CFTR mediates cAMP-stimulated Cl⁻ secretion in a well-differentiated thyroid culture model and that knockout of CFTR promotes increased Na(+) absorption by a mechanism other than increased ENaC expression. These findings suggest several models for the mechanism of cystic fibrosis-associated hypothyroidism.

Therapeutic potential of cystic fibrosis transmembrane conductance regulator (CFTR) inhibitors in polycystic kidney disease

In the common genetic disorder autosomal dominant polycystic kidney disease (ADPKD), kidney function is disrupted by multiple fluid-filled epithelial cysts. Cyst growth in ADPKD involves fluid accumulation within the cyst lumen driven by cystic fibrosis transmembrane conductance regulator (CFTR)-mediated transepithelial Cl- secretion. This suggests that inhibitors of the CFTR Cl- channel might retard cyst growth. This review considers how knowledge of CFTR structure and function and its role in transepithelial salt and water movements provides insight into the mechanism of action of CFTR inhibitors. Some small molecules, termed open-channel blockers, inhibit directly the CFTR Cl- channel by physically obstructing the CFTR pore and preventing Cl- flow. By contrast, other small molecules, termed allosteric inhibitors, bind to CFTR at a site remote from the channel pore and interfere with conformational changes that open the pore. The application of high-throughput screening to CFTR drug discovery has led to the identification of new inhibitors of the CFTR Cl- channel including the thiazolidinone CFTR(inh)-172 and the glycine hydrazide GlyH-101. The demonstration that CFTR inhibitors retard cyst expansion and kidney enlargement in mouse models of ADPKD provides proof of concept for the use of small-molecule CFTR inhibitors in the treatment of ADPKD.

Mechanosensitive gating of CFTR

Cystic fibrosis transmembrane conductance regulator (CFTR) is an anion and intracellular ligand-gated channel associated with cystic fibrosis, a lethal genetic disorder common among Caucasians. Here we show that CFTR is robustly activated by membrane stretch induced by negative pressures as small as 5 mmHg at the single-channel, cellular and tissue levels. Stretch increased the product of the number of channels present and probability of being open (NPo), and also increased the unitary conductance of CFTR in cell-attached membrane patches. CFTR stretch-mediated activation appears to be an intrinsic property independent of cytosolic factors and kinase signalling. CFTR stretch-mediated activation resulted in chloride transport in Calu-3 human airway epithelial cells and mouse intestinal tissues. Our study has revealed an unexpected function of CFTR in mechanosensing, in addition to its roles as a ligand-gated anion channel and a regulator of other membrane transporters, demonstrating for the first time a mechanosensitive anion channel with a clearly defined molecular identity. Given that CFTR is often found in mechanically dynamic environments, its mechanosensitivity has important physiological implications in epithelial ion transport and cell volume regulation in vivo.

Mouse cystic fibrosis transmembrane conductance regulator forms cAMP-PKA-regulated apical chloride channels in cortical collecting duct

The cystic fibrosis transmembrane conductance regulator (CFTR) is expressed in many segments of the mammalian nephron, where it may interact with and modulate the activity of a variety of apical membrane proteins, including the renal outer medullary potassium (ROMK) K(+) channel. However, the expression of CFTR in apical cell membranes or its function as a Cl(-) channel in native renal epithelia has not been demonstrated. Here, we establish that CFTR forms protein kinase A (PKA)-activated Cl(-) channels in the apical membrane of principal cells from the cortical collecting duct obtained from mice. These Cl(-) channels were observed in cell-attached apical patches of principal cells after stimulation by forskolin/3-isobutyl-1-methylxanthine. Quiescent Cl(-) channels were present in patches excised from untreated tubules because they could be activated after exposure to Mg-ATP and the catalytic subunit of PKA. The single-channel conductance, kinetics, and anion selectivity of these Cl(-) channels were the same as those of recombinant mouse CFTR channels expressed in Xenopus laevis oocytes. The CFTR-specific closed-channel blocker CFTR(inh)-172 abolished apical Cl(-) channel activity in excised patches. Moreover, apical Cl(-) channel activity was completely absent in principal cells from transgenic mice expressing the DeltaF508 CFTR mutation but was present and unaltered in ROMK-null mice. We discuss the physiologic implications of open CFTR Cl(-) channels on salt handling by the collecting duct and on the functional CFTR-ROMK interactions in modulating the metabolic ATP-sensing of ROMK.

CFTR chloride channel in the apical compartments: spatiotemporal coupling to its interacting partners

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated chloride channel located primarily at the apical or luminal surfaces of epithelial cells in the airway, intestine, pancreas, kidney, sweat gland, as well as male reproductive tract, where it plays a crucial role in transepithelial fluid homeostasis. CFTR dysfunction can be detrimental and may result in life-threatening disorders. CFTR hypofunctioning because of genetic defects leads to cystic fibrosis, the most common lethal genetic disease in Caucasians, whereas CFTR hyperfunctioning resulting from various infections evokes secretory diarrhea, the leading cause of mortality in early childhood. Therefore, maintaining a dynamic balance between CFTR up-regulating processes and CFTR down-regulating processes is essential for maintaining fluid and body homeostasis. Accumulating evidence suggests that protein-protein interactions play a critical role in the fine-tuned regulation of CFTR function. A growing number of proteins have been reported to interact directly or indirectly with CFTR chloride channel, suggesting that CFTR might be coupled spatially and temporally to a wide variety of interacting partners including ion channels, receptors, transporters, scaffolding proteins, enzyme molecules, signaling molecules, and effectors. Most interactions occur primarily between the opposing terminal tails (amino or carboxyl) of CFTR protein and its binding partners, either directly or mediated through various PDZ scaffolding proteins. These dynamic interactions impact the channel function, as well as localization and processing of CFTR protein within cells. This article reviews the most recent progress and findings about the interactions between CFTR and its binding partners through PDZ scaffolding proteins, as well as the spatiotemporal regulation of CFTR-containing macromolecular signaling complexes in the apical compartments of polarized cells lining the secretory epithelia.

Cystic fibrosis transmembrane conductance regulator gene mutation and lung cancer risk

The cystic fibrosis transmembrane conductance regulator (CFTR) holds an important role in retaining lung function, but its association with lung cancer is unclear. A case-control study was conducted to determine the possible associations of the genetic variants in the CFTR gene with lung cancer risk. Genotypes of the most common deletion DeltaF508, one functional SNP, and eight tag SNPs in the CFTR gene were determined in 574 lung cancer patients and 679 controls. A logistic regression model, adjusting for known risk factors, was used to evaluate the association of each variant with lung cancer risk, as confirmation haplotype and sub-haplotype analyses were performed. DeltaF508 deletion and genotypes with minor alleles in one tag SNP, rs10487372, and one functional SNP, rs213950, were inversely associated with lung cancer risk. The results of haplotype and sub-haplotype analyses were consistent with single variant analysis, all pointing to deletion DeltaF508 being the key variant for significant haplotypes and sub-haplotypes. Individuals with 'deletion-T' (DeltaF508/rs10487372) haplotype had a 68% reduced risk for lung cancer compared to common haplotype 'no-deletion-C' (OR=0.32; 95% CI=0.15-0.68; p=0.01). Genetic variations in the CFTR gene might modulate the risk of lung cancer. This study, for the first time, provides evidence of a protective role of the CFTR deletion carrier in the etiology of lung cancer.

Syntaxin 6 and CAL mediate the degradation of the cystic fibrosis transmembrane conductance regulator

STX6 is a new component of the CAL complex that regulates the abundance and function of CFTR at the post-ER level. Our results suggest a therapeutic role of STX6 in enhancing rescued ΔF508-CFTR.

The PDZ domain–containing protein CAL mediates lysosomal trafficking and degradation of CFTR. Here we demonstrate the involvement of a CAL-binding SNARE protein syntaxin 6 (STX6) in this process. Overexpression of STX6, which colocalizes and coimmunoprecipitates with CAL, dramatically reduces the steady-state level and stability of CFTR. Conversely, overexpression of a STX6 dominant-negative mutant increases CFTR. Silencing endogenous STX6 increases CFTR but has no effect on ΔTRL-CFTR, which cannot bind to CAL. Silencing CAL eliminates the effect of STX6 on CFTR. Both results suggest a dependence of CAL on STX6 function. Consistent with its Golgi localization, STX6 does not bind to ER-localized ΔF508-CFTR. Silencing STX6 has no effect on ΔF508-CFTR expression. However, overexpression of STX6 coimmunoprecipitates with and reduces temperature-rescued ΔF508-CFTR that escapes ER degradation. Conversely, silencing STX6 enhances the effect of low temperature in rescuing ΔF508-CFTR. Finally, in human bronchial epithelial cells, silencing endogenous STX6 leads to increases in protein levels and Cl⁻ currents of both wild-type and temperature-rescued CFTR. We have identified STX6 as a new component of the CAL complex that regulates the abundance and function of CFTR at the post-ER level. Our results suggest a therapeutic role of STX6 in enhancing rescued ΔF508-CFTR.

The H-loop in the second nucleotide-binding domain of the cystic fibrosis transmembrane conductance regulator is required for efficient chloride channel closing

The cystic fibrosis transmembrane conductance regulator (CFTR) is an ATP-binding cassette (ABC) transporter that functions as a cAMP-activated chloride channel. The recent model of CFTR gating predicts that the ATP binding to both nucleotide-binding domains (NBD1 and NBD2) of CFTR is required for the opening of the channel, while the ATP hydrolysis at NBD2 induces subsequent channel closing. In most ABC proteins, efficient hydrolysis of ATP requires the presence of the invariant histidine residue within the H-loop located in the C-terminal part of the NBD. However, the contribution of the corresponding region (H-loop) of NBD2 to the CFTR channel gating has not been examined so far. Here we report that the alanine substitution of the conserved dipeptide HR motif (HR-->AA) in the H-loop of NBD2 leads to prolonged open states of CFTR channel, indicating that the H-loop is required for efficient channel closing. On the other hand, the HR-->AA substitution lead to the substantial decrease of CFTR-mediated current density (pA/pF) in transfected HEK 293 cells, as recorded in the whole-cell patch-clamp analysis. These results suggest that the H-loop of NBD2, apart from being required for CFTR channel closing, may be involved in regulating CFTR trafficking to the cell surface.

CFTR knockdown stimulates lipid synthesis and transport in intestinal Caco-2/15 cells

Cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel highly expressed in epithelial cells of the gastrointestinal tract. Mutations in the CFTR gene cause cystic fibrosis (CF), a disease characterized by pancreatic insufficiency, fat malabsorption, and steatorrhea. Despite the administration of pancreatic enzymes to normalize malabsorption, CF patients still experienced lipid fecal loss, nutritional deficiencies, and abnormalities in serum lipid profile, suggesting the presence of intrinsic defects in the intestinal handling of nutrients. The objective of the present study was to assess the impact of CFTR gene knockdown on intracellular lipid metabolism of the intestinal Caco-2/15 cell line. Partial CFTR gene inactivation led to cellular lipid accretion of phospholipids, triglycerides, and cholesteryl esters. Likewise, secretion of these lipid fractions was significantly increased following CFTR gene manipulation. As expected from these findings, the output of triglyceride-rich lipoproteins showed the same increasing pattern. Investigation of the mechanisms underlying these changes revealed that CFTR knockdown resulted in raised levels of apolipoproteins in cells and media and microsomal transfer protein activity, two important factors for the efficient assembly and secretion of lipoproteins. Similarly, scrutiny of the enzymatic monoacylglycerol acyltransferase and diacylglycerol acyltransferase, which exhibit dynamic function in triacylglycerol resynthesis and chylomicron formation in enterocytes, revealed a significant augmentation in their activity. Conversely, cholesterol uptake mediated by Niemann-Pick C1 like 1, Scavenger Receptor Class B Type I, and ATP-binding cassette G8 remains unaffected by genetic modification of CFTR. Collectively, these results highlight the role played by CFTR in intestinal handling of lipids and may suggest that factors other than defective CFTR are responsible for the abnormal intracellular events leading to fat malabsorption in CF patients.

PKA mediates constitutive activation of CFTR in human sweat duct

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channels are constitutively activated in sweat ducts. Since phosphorylation-dependent and -independent mechanisms can activate CFTR, we sought to determine the actual mechanism responsible for constitutive activation of these channels in vivo. We show that the constitutively activated CFTR Cl⁻ conductance (gCFTR) in the apical membrane is completely deactivated following α-toxin permeabilization of the basolateral membrane. We investigated whether such inhibition of gCFTR following permeabilization is due to the loss of cytoplasmic glutamate or due to dephosphorylation of CFTR by an endogenous phosphatase in the absence of kinase activity (due to the loss of kinase agonist cAMP, cGMP or GTP through α-toxin pores). In order to distinguish between these two possibilities, we examined the effect of inhibiting the endogenous phosphatase activity with okadaic acid (10⁻⁸ M) on the permeabilization-induced deactivation of gCFTR. We show that okadaic acid (1) inhibits an endogenous phosphatase responsible for dephosphorylating cAMP but not cGMP or G protein-activated CFTR and (2) prevents deactivation of CFTR following permeabilization of the basolateral membrane. These results indicate that distinctly different phosphatases may be responsible for dephosphorylating different kinase-specific sites on CFTR. We conclude that the phosphorylation by PKA alone appears to be primarily responsible for constitutive activation of gCFTR in vivo.

An immunocytochemical assay to detect human CFTR expression following gene transfer

BACKGROUND

To assess gene therapy treatment for cystic fibrosis (CF) in clinical trials it is essential to develop robust assays that can accurately detect transgene expression in human airway epithelial cells. Our aim was to develop a reproducible immunocytochemical assay for human CFTR protein which can measure both endogenous CFTR levels and augmented CFTR expression after gene delivery.

METHODS

We characterised an antibody (G449) which satisfied the criteria for use in clinical trials. We optimised our immunocytochemistry method and identified G449 dilutions at which endogenous CFTR levels were negligible in CF samples, thus enhancing detection of transgenic CFTR protein. After developing a transfection technique for brushed human nasal epithelial cells, we transfected non-CF and CF cells with a clinically relevant CpG-free plasmid encoding human CFTR.

RESULTS

The optimised immunocytochemistry method gave improved discrimination between CF and non-CF samples. Transfection of a CFTR expression vector into primary nasal epithelial cells resulted in detectable RNA and protein expression. CFTR protein was present in 0.05-10% of non-CF cells and 0.02-0.8% of CF cells.

CONCLUSION

We have developed a sensitive, clinically relevant immunocytochemical assay for CFTR protein and have used it to detect transgene-expressed CFTR in transfected human primary airway epithelial cells.

Src Signaling Links Mediators of Inflammation to Cx43 Gap Junction Channels in Primary and Transformed CFTR-Expressing Airway Cells

Dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) is associated with recurrent pulmonary infections and inflammation. We previously reported that tumor necrosis factor (TNF)-alpha decreases gap junction connectivity in cell lines derived from the airway epithelium of non-cystic fibrosis (non-CF) subjects, a mechanism that was defective in cells derived from CF patients, and identified the tyrosine kinase c-Src as a possible bridge between TNF-alpha and Cx43. To examine whether this modulation also takes place in primary epithelial cells, the functional expression of Cx43 was studied in non-CF and CF airway cells, obtained from surgical polypectomies and turbinectomies, which were grown either on culture dishes or permeable filters. Expression of Cx43 was detected by immunofluorescence on cells grown under both culture conditions. Non-CF and CF airway cells also showed intercellular diffusion of Lucifer Yellow. Dye coupling was rapidly abolished in non-CF cells in the presence of TNF-alpha, lipopolysaccharide and lysophosphatidic acid, and could be prevented by tyrphostin47, an inhibitor of Src tyrosine kinases. This down-regulation, however, was not detected in CF airway cells. These data indicate that CFTR dysfunction is associated with altered Src signaling, resulting in the persistence of gap junction connectivity in primary and transformed CF airway cells.

Glutathione dysregulation and the etiology and progression of human diseases

Glutathione (GSH) plays an important role in a multitude of cellular processes, including cell differentiation, proliferation, and apoptosis, and as a result, disturbances in GSH homeostasis are implicated in the etiology and/or progression of a number of human diseases, including cancer, diseases of aging, cystic fibrosis, and cardiovascular, inflammatory, immune, metabolic, and neurodegenerative diseases. Owing to the pleiotropic effects of GSH on cell functions, it has been quite difficult to define the role of GSH in the onset and/or the expression of human diseases, although significant progress is being made. GSH levels, turnover rates, and/or oxidation state can be compromised by inherited or acquired defects in the enzymes, transporters, signaling molecules, or transcription factors that are involved in its homeostasis, or from exposure to reactive chemicals or metabolic intermediates. GSH deficiency or a decrease in the GSH/glutathione disulfide ratio manifests itself largely through an increased susceptibility to oxidative stress, and the resulting damage is thought to be involved in diseases, such as cancer, Parkinson's disease, and Alzheimer's disease. In addition, imbalances in GSH levels affect immune system function, and are thought to play a role in the aging process. Just as low intracellular GSH levels decrease cellular antioxidant capacity, elevated GSH levels generally increase antioxidant capacity and resistance to oxidative stress, and this is observed in many cancer cells. The higher GSH levels in some tumor cells are also typically associated with higher levels of GSH-related enzymes and transporters. Although neither the mechanism nor the implications of these changes are well defined, the high GSH content makes cancer cells chemoresistant, which is a major factor that limits drug treatment. The present report highlights and integrates the growing connections between imbalances in GSH homeostasis and a multitude of human diseases.

A comprehensive guide to the ROMK potassium channel: form and function in health and disease

The discovery of the renal outer medullary K+ channel (ROMK, K(ir)1.1), the founding member of the inward-rectifying K+ channel (K(ir)) family, by Ho and Hebert in 1993 revolutionized our understanding of potassium channel biology and renal potassium handling. Because of the central role that ROMK plays in the regulation of salt and potassium homeostasis, considerable efforts have been invested in understanding the underlying molecular mechanisms. Here we provide a comprehensive guide to ROMK, spanning from the physiology in the kidney to the organization and regulation by intracellular factors to the structural basis of its function at the atomic level.

SLC26A9 is a constitutively active, CFTR-regulated anion conductance in human bronchial epithelia

Human bronchial epithelial (HBE) cells exhibit constitutive anion secretion that is absent in cells from cystic fibrosis (CF) patients. The identity of this conductance is unknown, but SLC26A9, a member of the SLC26 family of CF transmembrane conductance regulator (CFTR)-interacting transporters, is found in the human airway and exhibits chloride channel behavior. We sought differences in the properties of SLC26A9 and CFTR expressed in HEK 293 (HEK) cells as a fingerprint to identify HBE apical anion conductances. HEK cells expressing SLC26A9 displayed a constitutive chloride current that was inhibited by the CFTR blocker GlyH-101 (71 +/- 4%, 50 microM) and exhibited a near-linear current-voltage (I-V) relation during block, while GlyH-101-inhibited wild-type (wt)CFTR exhibited a strong inward-rectified (IR) I-V relation. We tested polarized HBE cells endogenously expressing either wt or DeltaF508-CFTR for similar activity. After electrical isolation of the apical membrane using basolateral alpha-toxin permeabilization, wtCFTR monolayers displayed constitutive chloride currents that were inhibited by GlyH-101 (68 +/- 6%) while maintaining a near-linear I-V relation. In the absence of blocker, the addition of forskolin stimulated a current increase having a linear I-V; GlyH-101 blocked 69 +/- 7% of the current and shifted the I-V relation IR, consistent with CFTR activation. HEK cells coexpressing SLC26A9 and wtCFTR displayed similar properties, as well as forskolin-stimulated currents that exceeded the sum of those in cells separately expressing SLC26A9 or wtCFTR, and an I-V relation during GlyH-101 inhibition that was moderately IR, indicating that SLC26A9 contributed to the stimulated current. HBE cells from CF patients expressed SLC26A9 mRNA, but no constitutive chloride currents. HEK cells coexpressing SLC26A9 with DeltaF508-CFTR also failed to exhibit SLC26A9 current. We conclude that SLC26A9 functions as an anion conductance in the apical membranes of HBE cells, it contributes to transepithelial chloride currents under basal and cAMP/protein kinase A-stimulated conditions, and its activity in HBE cells requires functional CFTR.

Mechanisms of the noxious inflammatory cycle in cystic fibrosis

Multiple evidences indicate that inflammation is an event occurring prior to infection in patients with cystic fibrosis. The self-perpetuating inflammatory cycle may play a pathogenic part in this disease. The role of the NF-κB pathway in enhanced production of inflammatory mediators is well documented. The pathophysiologic mechanisms through which the intrinsic inflammatory response develops remain unclear. The unfolded mutated protein cystic fibrosis transmembrane conductance regulator (CFTRΔF508), accounting for this pathology, is retained in the endoplasmic reticulum (ER), induces a stress, and modifies calcium homeostasis. Furthermore, CFTR is implicated in the transport of glutathione, the major antioxidant element in cells. CFTR mutations can alter redox homeostasis and induce an oxidative stress. The disturbance of the redox balance may evoke NF-κB activation and, in addition, promote apoptosis. In this review, we examine the hypotheses of the integrated pathogenic processes leading to the intrinsic inflammatory response in cystic fibrosis.

CFTR structure and function: is there a role in the kidney?

Cystic fibrosis (CF) is a lethal autosomal recessive genetic disease caused by mutations in the CF transmembrane conductance regulator (CFTR). Mutations in the CFTR gene may result in a defective protein processing that leads to changes in function and regulation of this chloride channel. Despite of the expression of CFTR in the kidney, patients with CF do not present major renal dysfunction, but it is known that both the urinary excretion of proteins and renal capacity to concentrate and dilute urine are altered in these patients. CFTR mRNA is expressed in all nephron segments of rat and human, and this abundance is more prominent in renal cortex and outer medulla renal areas. CFTR protein was detected in apical surface of both proximal and distal tubules of rat kidney but not in the outer medullary collecting ducts. Studies have demonstrated that CFTR does not only transport Cl- but also ATP. ATP transport by CFTR could be involved in the control of other ion transporters such as Na+ (ENaC) and K+ (renal outer medullary potassium) channels, especially in TAL and CCD. In the kidney, CFTR also might be involved in the endocytosis of low-molecular-weight proteins by proximal tubules. This review is focused on the CFTR function and structure, its role in the renal physiology, and its modulation by hormones involved in the control of extracellular fluid volume.

Cystic fibrosis transmembrane conductance regulator gene polymorphisms in patients with primary sclerosing cholangitis

BACKGROUND/AIMS

Primary sclerosing cholangitis (PSC) is a progressive cholestatic disease commonly associated with inflammatory bowel disease (IBD) and characterized by fibrosing inflammatory destruction of bile ducts. The histological features in the liver of PSC patients are similar to those observed in cystic fibrosis (CF). Our aim was to study whether variants in the CFTR gene are associated with the occurrence and/or evolution of PSC.

METHODS

PSC patients (n=140) were genotyped for F508del, the TGmTn variants, and four additional polymorphic loci (1001+11 C>T, M470V, T854T and Q1463Q), and compared to 136 matched healthy controls.

RESULTS

The 1540G-allele, encoding V470, was less frequent in PSC (52%) than in controls (64%, p=0.003), and was associated with protection against PSC in individuals without IBD (OR 0.25, 95% CI 0.12-0.52, p=0.0002). Also TG11-T7 was less frequent in PSC (53%) than in controls (61%, p=0.04), this haplotype was associated with reduced risk for PSC (OR 0.34, 95% CI 0.17-0.70, p=0.003) in individuals without IBD.

CONCLUSIONS

In this cohort of PSC patients, several CFTR-variants affecting the functional properties of the CFTR protein seem to offer protection against the development of PSC, confirming our hypothesis that CFTR might be implicated in the pathogenesis of PSC.

Adenosine receptors, cystic fibrosis, and airway hydration

Adenosine (Ado) regulates diverse cellular functions in the lung through its local production, release, metabolism, and subsequent stimulation of G-protein-coupled P1 purinergic receptors. The A(2B) adenosine receptor (A(2B)AR) is the predominant P1 purinergic receptor isoform expressed in surface airway epithelia, and Ado is an important regulator of airway surface liquid (ASL) volume through its activation of the cystic fibrosis transmembrane conductance regulator (CFTR). Through a delicate balance between sodium (Na(+)) absorption and chloride (Cl(-)) secretion, the ASL volume is optimized to promote ciliary activity and mucociliary clearance, effectively removing inhaled particulates. When CFTR is dysfunctional, the Ado/A(2B)AR regulatory system fails to optimize the ASL volume, leading to its depletion and interruption of mucociliary clearance. In cystic fibrosis (CF), loss of CFTR function and resultant mucus stasis leaves the lower airways susceptible to mucus obstruction, chronic bacterial infection, relentless inflammation, and eventually panbronchiectasis. Adenosine triphosphate (ATP) also regulates transepithelial Cl(-) conductance, but through a separate system that relies on stimulation of P2Y(2) purinergic receptors, mobilization of intracellular calcium, and activation of calcium-activated chloride channels (CaCCs). These pathways remain functional in CF, and may serve a protective role in the disease. In this chapter, we will review our current understanding of how Ado and related nucleotides regulate CFTR and Cl(-) conductance in the human airway, including the regulation of additional intracellular and extracellular signaling pathways that provide important links between ion transport and inflammation relevant to the disease.

cAMP-dependent chloride secretion mediates tubule enlargement and cyst formation by cultured mammalian collecting duct cells

Polycystic kidney diseases result from disruption of the genetically defined program that controls the size and geometry of renal tubules. Cysts which frequently arise from the collecting duct (CD) result from cell proliferation and fluid secretion. From mCCD(cl1) cells, a differentiated mouse CD cell line, we isolated a clonal subpopulation (mCCD-N21) that retains morphogenetic capacity. When grown in three-dimensional gels, mCCD-N21 cells formed highly organized tubular structures consisting of a palisade of polarized epithelial cells surrounding a cylindrical lumen. Subsequent addition of cAMP-elevating agents (forskolin or cholera toxin) or of membrane-permeable cAMP analogs (CPT-cAMP) resulted in rapid and progressive dilatation of existing tubules, leading to the formation of cystlike structures. When grown on filters, mCCD-N21 cells exhibited a high transepithelial resistance as well as aldosterone- and/or vasopressin-induced amiloride-sensitive and -insensitive current. The latter was in part inhibited by Na(+)-K(+)-2Cl(-) cotransporter (bumetanide) and chloride channel (NPPB) inhibitors. Real-time PCR analysis confirmed the expression of NKCC1, the ubiquitous Na(+)-K(+)-2Cl(-) cotransporter and cystic fibrosis transmembrane regulator (CFTR) in mCCD-N21 cells. Tubule enlargement and cyst formation were prevented by inhibitors of Na(+)-K(+)-2Cl(-) cotransporters (bumetanide or ethacrynic acid) or CFTR (NPPB or CFTR inhibitor-172). These results further support the notion that cAMP signaling plays a key role in renal cyst formation, at least in part by promoting chloride-driven fluid secretion. This new in vitro model of tubule-to-cyst conversion affords a unique opportunity for investigating the molecular mechanisms that govern the architecture of epithelial tubes, as well as for dissecting the pathophysiological processes underlying cystic kidney diseases.

Effect of cytosolic pH on epithelial Na+ channel in normal and cystic fibrosis sweat ducts

The activities of cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel and the amiloride-sensitive epithelial Na(+) channel (ENaC) are acutely coordinated in the sweat duct. However, the mechanisms responsible for cross-talk between these ion channels are unknown. Previous studies indicated that luminal pH of sweat ducts varies over 3 pH units and that the cytoplasmic pH affects both CFTR and ENaC. Therefore, using basolaterally alpha-toxin-permeabilized apical membrane preparations of sweat ducts as an experimental system, we tested the hypothesis that the cytosolic pH may mediate the cross-talk between CFTR and ENaC. We showed that while luminal pH had no effect, cytosolic pH acutely affected ENaC activity. That is, acidic pH inhibited, while basic pH activated, ENaC. pH regulation of ENaC appears to be independent of CFTR or endogenous kinase activities because basic pH independently stimulated ENaC (1) in normal ducts even when CFTR was deactivated, (2) in CF ducts that lack CFTR in the plasma membranes and (3) after blocking endogenous kinase activity with staurosporine. Considering the evidence of Na(+)/H(+) exchange (NHE) activity as shown by the expression of mRNA and function of NHE in the basolateral membrane of the sweat duct, we postulate that changes in cytosolic Na(+) ([Na(+)]( i )) may alter cytosolic pH (pH( i )) as salt loads into the cell during electrolyte absorption. These changes may play a role in coordinating the activities of ENaC and CFTR during transepithelial salt transport.

Glutathione and its transporters in ocular surface defense

Glutathione (GSH) is an abundant antioxidant ubiquitous in nearly all cell types. Deficiency of GSH has been linked to ocular disease and viral infection. Other established vital roles of GSH include detoxification and immunoprotection. Endogenous GSH plays a protagonist's role in safeguarding active transport processes compartmentalized at the interface between conjunctival mucosa and the tear film. Optimal electrokinetic transport across the conjunctival epithelium requires the mucosal presence of GSH. Glutathione is the most abundant known endogenous antioxidant molecule in tear fluid, mainly derived from conjunctival secretion. Conjunctival GSH transport, a major kinetic component of GSH turnover, occurs through multiple functionally distinct mechanisms. Cell membrane potential regulates conjunctival GSH efflux, while conjunctival GSH uptake requires extracellular Na(+). Significant modulation of GSH, its constituent amino acids, and functions of associated transporters occurs in the conjunctival epithelium with viral inflammatory disease. Topical conjunctival delivery of GSH, its metabolic precursors, or pharmacologic stimulation of endogenous conjunctival GSH secretion carry potential in alleviating viral-inflammatory conjunctivitis.

Determination of CFTR densities in erythrocyte plasma membranes using recognition imaging

CFTR (cystic fibrosis transmembrane conductance regulator) is a cAMP-regulated chloride (Cl(-)) channel that plays an important role in salt and fluid movement across epithelia. Cystic fibrosis (CF), the most common genetic disease among Caucasians, is caused by mutations in the gene encoding CFTR. The most predominant mutation, F508del, disturbs CFTR protein trafficking, resulting in a reduced number of CFTR in the plasma membrane. Recent studies indicate that CFTR is not only found in epithelia but also in human erythrocytes. Although considerable attempts have been made to quantify CFTR in cells, conclusions on numbers of CFTR molecules localized in the plasma membrane have been drawn indirectly. AFM has the power to provide the needed information, since both sub-molecular spatial resolution and direct protein recognition via antibody-antigen interaction can be observed. We performed a quantification study of the CFTR copies in erythrocyte membranes at the single molecule level, and compared the difference between healthy donors and CF patients. We detected that the number of CFTR molecules is reduced by 70% in erythrocytes of cystic fibrosis patients.

TMEM16A confers receptor-activated calcium-dependent chloride conductance

Calcium (Ca(2+))-activated chloride channels are fundamental mediators in numerous physiological processes including transepithelial secretion, cardiac and neuronal excitation, sensory transduction, smooth muscle contraction and fertilization. Despite their physiological importance, their molecular identity has remained largely unknown. Here we show that transmembrane protein 16A (TMEM16A, which we also call anoctamin 1 (ANO1)) is a bona fide Ca(2+)-activated chloride channel that is activated by intracellular Ca(2+) and Ca(2+)-mobilizing stimuli. With eight putative transmembrane domains and no apparent similarity to previously characterized channels, ANO1 defines a new family of ionic channels. The biophysical properties as well as the pharmacological profile of ANO1 are in full agreement with native Ca(2+)-activated chloride currents. ANO1 is expressed in various secretory epithelia, the retina and sensory neurons. Furthermore, knockdown of mouse Ano1 markedly reduced native Ca(2+)-activated chloride currents as well as saliva production in mice. We conclude that ANO1 is a candidate Ca(2+)-activated chloride channel that mediates receptor-activated chloride currents in diverse physiological processes.

Molecular physiology of bestrophins: multifunctional membrane proteins linked to best disease and other retinopathies

This article reviews the current state of knowledge about the bestrophins, a newly identified family of proteins that can function both as Cl(-) channels and as regulators of voltage-gated Ca(2+) channels. The founding member, human bestrophin-1 (hBest1), was identified as the gene responsible for a dominantly inherited, juvenile-onset form of macular degeneration called Best vitelliform macular dystrophy. Mutations in hBest1 have also been associated with a small fraction of adult-onset macular dystrophies. It is proposed that dysfunction of bestrophin results in abnormal fluid and ion transport by the retinal pigment epithelium, resulting in a weakened interface between the retinal pigment epithelium and photoreceptors. There is compelling evidence that bestrophins are Cl(-) channels, but bestrophins remain enigmatic because it is not clear that the Cl(-) channel function can explain Best disease. In addition to functioning as a Cl(-) channel, hBest1 also is able to regulate voltage-gated Ca(2+) channels. Some bestrophins are activated by increases in intracellular Ca(2+) concentration, but whether bestrophins are the molecular counterpart of Ca(2+)-activated Cl(-) channels remains in doubt. Bestrophins are also regulated by cell volume and may be a member of the volume-regulated anion channel family.

Effects of chronic hypoxia on electrogenic transport and transport-related oxygen consumption in rat distal colon

The distal colon epithelium of rats submitted to chronic hypoxia shows higher short-circuit current (Isc) which, unlike non-hypoxic rat epithelium, has an amiloride-sensitive component despite low serum aldosterone levels. Isc and oxygen consumption (QO2) were simultaneously measured in mucosae from rats submitted to 0.5 atm for 10 days and from control rats in a modified Ussing chamber. Hypoxia increased Isc but not QO2. The slope of the regression line between Isc and QO2 reduction after ouabain addition was decreased in epithelia from hypoxic rats (P = 0.03). Chloride secretion blockade reduced Isc and QO2 in both groups, while sodium channel blockade did so only in the hypoxic group. Dual blockade in hypoxic rat epithelia caused correlated (P = 0.0025) additive decreases in Isc and QO2. Presented results suggest that chronic hypoxia induces an improved coupling between QO2 and electrogenic ion transport, and electrogenic sodium absorption despite low aldosterone levels.