Relationship between airway ion transport and a mild pulmonary disease mutation in CFTR

Roles of NHERF Family of PDZ-Binding Proteins in Regulating GPCR Functions

Multicellular organisms are equipped with an array of G-protein-coupled receptors (GPCRs) that mediate cell-cell signaling allowing them to adapt to environmental cues and ultimately survive. This is mechanistically possible through complex intracellular GPCR machinery that encompasses a vast network of proteins. Within this network, there is a group called scaffolding proteins that facilitate proper localization of signaling proteins for a quick and robust GPCR response. One protein family within this scaffolding group is the PSD-95/Dlg/ZO-1 (PDZ) family which is important for GPCR localization, internalization, recycling, and downstream signaling. Although the PDZ family of proteins regulate the functions of several receptors, this chapter focuses on a subfamily within the PDZ protein family called the Na⁺/H⁺ exchanger regulatory factors (NHERFs). Here we extensively review the predominantly characterized roles of NHERFs in renal phosphate absorption, intestinal ion regulation, cancer progression, and immune cell functions. Finally, we discuss the future perspectives and possible clinical application of targeting NHERFs in several disorders.

β2-Adrenergic receptor agonists activate CFTR in intestinal organoids and subjects with cystic fibrosis

We hypothesized that people with cystic fibrosis (CF) who express CFTR (cystic fibrosis transmembrane conductance regulator) gene mutations associated with residual function may benefit from G-protein coupled receptor (GPCR)-targeting drugs that can activate and enhance CFTR function.We used intestinal organoids to screen a GPCR-modulating compound library and identified β2-adrenergic receptor agonists as the most potent inducers of CFTR function.β2-Agonist-induced organoid swelling correlated with the CFTR genotype, and could be induced in homozygous CFTR-F508del organoids and highly differentiated primary CF airway epithelial cells after rescue of CFTR trafficking by small molecules. The in vivo response to treatment with an oral or inhaled β2-agonist (salbutamol) in CF patients with residual CFTR function was evaluated in a pilot study. 10 subjects with a R117H or A455E mutation were included and showed changes in the nasal potential difference measurement after treatment with oral salbutamol, including a significant improvement of the baseline potential difference of the nasal mucosa (+6.35 mV, p<0.05), suggesting that this treatment might be effective in vivo Furthermore, plasma that was collected after oral salbutamol treatment induced CFTR activation when administered ex vivo to organoids.This proof-of-concept study suggests that organoids can be used to identify drugs that activate CFTR function in vivo and to select route of administration.

Mutation spectrum of Egyptian children with cystic fibrosis

Objective

To know the common CFTR mutations in the Egyptian patients with cystic fibrosis as it was previously thought to be uncommon disease in Egypt.

Methods

This is a cross sectional study of 60 patients diagnosed as cystic fibrosis by sweat chloride testing. They were enrolled from the Allergy and Pulmonology Unit Children’s Hospital Cairo University. They were screened for the presence of the frequent 36 mutations in Caucasians by reverse hybridization line probe technique, using INNO-LiPACFTR19 and CFTR17 + Tn kits.

Results

Most of patients presented with classic manifestations of CF such as variable pulmonary disease and pancreatic insufficiency, and hepatomegaly with or without ascites. The mutations detected were F508 del (58 %), 2183AA/G (10 %), N1303K (6 %), I148T (4 %), W1282X (4 %), G155D (2 %), CFTRdel2-3 (21 KB) (2 %), 3199del6 (2 %), R347P (2 %). Unique to the Egyptian population are these mutations R1162X and A544E (6, 4 %) respectively they were found in our cohort study and were not reported elsewhere in the Arab population till now. There was no association between the initial clinical presentation of CF and the genotypes detected.

Conclusion

The F508 del is still the most commonly encountered mutation (58 %), however other rare mutations were identified where each ranged from (2 to 10 %).

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Intestinal current measurement versus nasal potential difference measurements for diagnosis of cystic fibrosis: a case-control study

Background

Nasal potential difference (NPD) and intestinal current measurement (ICM) are functional CFTR tests that are used as adjunctive diagnostic tools for cystic fibrosis (CF). Smoking has a systemic negative impact on CFTR function. A diagnostic comparison between NPD and ICM and the impact of smoking on both CFTR tests has not been done.

Methods

The sweat chloride test, NPD, and ICM were performed in 18 patients with CF (sweat chloride >60 mmol/l), including 6 pancreatic sufficient (PS) patients, and 13 healthy controls, including 8 smokers. The NPD CFTR response to Cl-free and isoproterenol perfusion (Δ0Cl^- + Iso) was compared to the ICM CFTR response to forskolin/IBMX, carbachol, and histamine (ΔI_{sc, forskolin/IBMX+ carbachol+histamine}).

Results

The mean NPD CFTR response and ICM CFTR response between patients with CF and healthy controls was significantly different (p <0.001), but not between patients with CF who were PS and those who were pancreatic insufficient (PI). Smokers have a decreased CFTR response measured by NPD (p = 0.049). For ICM there is a trend towards decreased CFTR response (NS). Three healthy control smokers had NPD responses within the CF-range. In contrast to NPD, there was no overlap of the ICM response between patients with CF and controls.

Conclusions

ICM is superior to NPD in distinguishing between patients with CF who have a sweat chloride > 60 mmol/l and healthy controls, including smokers. Neither NPD nor ICM differentiated between patients with CF who were PS from those who were PI. Smoking has a negative impact on CFTR function in healthy controls measured by NPD and challenges the diagnostic interpretation of NPD, but not ICM.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2466-14-156) contains supplementary material, which is available to authorized users.

Measurements of CFTR-mediated Cl- secretion in human rectal biopsies constitute a robust biomarker for Cystic Fibrosis diagnosis and prognosis

Background

Cystic Fibrosis (CF) is caused by ∼1,900 mutations in the CF transmembrane conductance regulator (CFTR) gene encoding for a cAMP-regulated chloride (Cl⁻) channel expressed in several epithelia. Clinical features are dominated by respiratory symptoms, but there is variable organ involvement thus causing diagnostic dilemmas, especially for non-classic cases.

Methodology/Principal Findings

To further establish measurement of CFTR function as a sensitive and robust biomarker for diagnosis and prognosis of CF, we herein assessed cholinergic and cAMP-CFTR-mediated Cl⁻ secretion in 524 freshly excised rectal biopsies from 118 individuals, including patients with confirmed CF clinical diagnosis (n = 51), individuals with clinical CF suspicion (n = 49) and age-matched non-CF controls (n = 18). Conclusive measurements were obtained for 96% of cases. Patients with “Classic CF”, presenting earlier onset of symptoms, pancreatic insufficiency, severe lung disease and low Shwachman-Kulczycki scores were found to lack CFTR-mediated Cl⁻ secretion (<5%). Individuals with milder CF disease presented residual CFTR-mediated Cl⁻ secretion (10–57%) and non-CF controls show CFTR-mediated Cl⁻ secretion ≥30–35% and data evidenced good correlations with various clinical parameters. Finally, comparison of these values with those in “CF suspicion” individuals allowed to confirm CF in 16/49 individuals (33%) and exclude it in 28/49 (57%). Statistical discriminant analyses showed that colonic measurements of CFTR-mediated Cl⁻ secretion are the best discriminator among Classic/Non-Classic CF and non-CF groups.

Conclusions/Significance

Determination of CFTR-mediated Cl⁻ secretion in rectal biopsies is demonstrated here to be a sensitive, reproducible and robust predictive biomarker for the diagnosis and prognosis of CF. The method also has very high potential for (pre-)clinical trials of CFTR-modulator therapies.

Regulation of G protein-coupled receptor function by Na+/H+ exchange regulatory factors

Many G protein-coupled receptors (GPCR) exert patterns of cell-specific signaling and function. Mounting evidence now supports the view that cytoplasmic adapter proteins contribute critically to this behavior. Adapter proteins recognize highly conserved motifs such as those for Src homology 3 (SH3), phosphotyrosine-binding (PTB), and postsynaptic density 95/discs-large/zona occludens (PDZ) docking sequences in candidate GPCRs. Here we review the behavior of the Na+/H+ exchange regulatory factor (NHERF) family of PDZ adapter proteins on GPCR signalling, trafficking, and function. Structural determinants of NHERF proteins that allow them to recognize targeted GPCRs are considered. NHERF1 and NHERF2 are capable also of modifying the assembled complex of accessory proteins such as β-arrestins, which have been implicated in regulating GPCR signaling. In addition, NHERF1 and NHERF2 modulate GPCR signaling by altering the G protein to which the receptor binds or affect other regulatory proteins that affect GTPase activity, protein kinase A, phospholipase C, or modify downstream signaling events. Small molecules targeting the site of NHERF1-GPCR interaction are being developed and may become important and selective drug candidates.

Assessment of CFTR function in homozygous R117H-7T subjects

BACKGROUND

R117H is a frequent missense mutation included in most CFTR mutation panels. However knowledge about the residual function of R117H-CFTR channels in cystic fibrosis-affected organs, e.g. airways, intestines and sweat glands is presently lacking.

METHODS

We evaluated clinical CF symptoms and assessed CFTR function by sweat tests, nasal potential difference and intestinal current measurements in 2 homozygous R117H individuals (7T variant).

RESULTS

The CFTR activity in airways and intestine was within the normal range. However both individuals presented with a borderline sweat test and the male patient was infertile.

CONCLUSIONS

The lack of impact of the R117H mutation on chloride secretion in intestine and nose contrasts with the ~80% loss of CFTR activity reported in patch clamp studies. Apparently CFTR activity is not rate-limiting for chloride secretion in both tissues at levels >20% of normal, or compensatory factors may operate that are absent in heterologous host cells in vitro.

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.

Airway ion transport impacts on disease presentation and severity in cystic fibrosis

OBJECTIVES

Abnormal airway ion transport is a feature of cystic fibrosis. The aim of this study was to investigate whether distinct components of ion transport are associated with the clinical expression and severity of the disease.

DESIGN AND METHODS

Univariate and multivariate analyses were used to study interaction effects between nasal potential difference parameters and clinical status, recorded at stable conditions, in 75 F508del homozygous young adults.

RESULTS

All patients demonstrated increased sodium and reduced chloride conductances. Less sodium transport abnormalities were related to better respiratory function and nutrition. Presentation with digestive symptoms at diagnosis was associated with lower chloride conductance. With an accuracy of 85% good nutritional status was linked to more preserved lung function, increasing age and more preserved chloride conductance.

CONCLUSIONS

Ion transport abnormalities have distinct clinical outcomes. Sodium conductance relates to respiratory function and nutrition; chloride conductance to nutrition and presentation with digestive symptoms at diagnosis.

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

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

Gene delivery to human sweat glands: a model for cystic fibrosis gene therapy

Gene therapy vectors are mostly studied in cultured cells, rodents, and sometimes in non-human primates, but it is useful to test them in human tissue prior to clinical trials. In this study, we investigated the possibility of using human sweat glands as a model for testing cystic fibrosis (CF) gene therapy vectors. Human sweat glands are relatively easy to obtain from skin biopsy, and can be tested for CFTR function. Using patients' sweat glands could provide a safe model to study the efficacy of CF gene therapy. As the first step to explore using sweat glands as a model for CF gene therapy, we examined various ex vivo gene delivery methods for a helper-dependent adenovirus (HD-Ad) vector. Gene delivery to sweat glands in skin organ culture was studied by topical application, intradermal injection or submerged culture. We found that transduction efficiency can be enhanced by pretreating isolated sweat glands with dispase, which suggests that the basement membrane is a critical barrier to gene delivery by adenoviral vectors. Using this approach, we showed that Cftr could be efficiently delivered to and expressed by the epithelial cells of sweat glands with our helper-dependent adenoviral vector containing cytokeratin 18 regulatory elements. Based on this study we propose that sweat glands might be used as an alternative model to study CF gene therapy in humans.

Macromolecular complexes of cystic fibrosis transmembrane conductance regulator and its interacting partners

The cystic fibrosis transmembrane conductance regulator (CFTR) is the product of the gene mutated in patients with cystic fibrosis (CF). CFTR is a cAMP-regulated chloride channel localized primarily at the apical or luminal surfaces of epithelial cells lining the airway, gut, exocrine glands, etc., where it is responsible for transepithelial salt and water transport. CFTR chloride channel belongs to the superfamily of the ATP-binding cassette (ABC) transporters, which bind ATP and use the energy to drive the transport of a wide variety of substrates across extra- and intracellular membranes. A growing number of proteins have been reported to interact directly or indirectly with CFTR chloride channel, suggesting that CFTR might regulate the activities of other ion channels, receptors, or transporters, in addition to its role as a chloride conductor. The molecular assembly of CFTR with these interacting proteins is of great interest and importance because several human diseases are attributed to altered regulation of CFTR, among which cystic fibrosis is the most serious one. Most interactions primarily occur between the opposing terminal tails (N- or C-) of CFTR and its binding partners, either directly or mediated through various PDZ domain-containing proteins. These dynamic interactions impact the channel function as well as the localization and processing of CFTR protein within cells. This review focuses on the recent developments in defining the assembly of CFTR-containing complexes in the plasma membrane and its interacting proteins.

Processing of CFTR: traversing the cellular maze--how much CFTR needs to go through to avoid cystic fibrosis?

Biosynthesis of the cystic fibrosis transmembrane conductance regulator (CFTR), like other proteins aimed at the cell surface, involves transport through a series of membranous compartments, the first of which is the endoplasmic reticulum (ER), where CFTR encounters the appropriate environment for folding, oligomerization, maturation, and export from the ER. After exiting the ER, CFTR has to traffic through complex pathways until it reaches the cell surface. Although not yet fully understood, the fine details of these pathways are starting to emerge, partially through identification of an increasing number of CFTR-interacting proteins (CIPs) and the clarification of their roles in CFTR trafficking and function. These aspects of CFTR biogenesis/degradation and by membrane traffic and CIPs are discussed in this review. Following this description of complex pathways and multiple checkpoints to which CFTR is subjected in the cell, the basic question remains of how much CFTR has to overcome these barriers and be functionally expressed at the plasma membrane to avoid CF. This question is also discussed here.

CFTR Cl- channel function in native human colon correlates with the genotype and phenotype in cystic fibrosis

BACKGROUND & AIMS

Cystic fibrosis (CF) is caused by over 1000 mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene and presents with a widely variable phenotype. Genotype-phenotype studies identified CFTR mutations that were associated with pancreatic sufficiency (PS). Residual Cl- channel function was shown for selected PS mutations in heterologous cells. However, the functional consequences of most CFTR mutations in native epithelia are not well established.

METHODS

To elucidate the relationships between epithelial CFTR function, CFTR genotype, and patient phenotype, we measured cyclic adenosine monophosphate (cAMP)-mediated Cl- secretion in rectal biopsy specimens from 45 CF patients who had at least 1 non-DeltaF508 mutation carrying a wide spectrum of CFTR mutations. We compared CFTR genotypes and clinical manifestations of CF patients who expressed residual CFTR-mediated Cl- secretion with patients in whom Cl- secretion was absent.

RESULTS

Residual anion secretion was detected in 40% of CF patients, and was associated with later disease onset (P < 0.0001), higher frequency of PS (P < 0.0001), and less severe lung disease (P < 0.05). Clinical outcomes correlated with the magnitude of residual CFTR activity, which was in the range of approximately 12%-54% of controls.

CONCLUSIONS

Specific CFTR mutations confer residual CFTR function to rectal epithelia, which is related closely to a mild disease phenotype. Quantification of rectal CFTR-mediated Cl- secretion may be a sensitive test to predict the prognosis of CF disease and identify CF patients who would benefit from therapeutic strategies that would increase residual CFTR activity.

Standardized procedure for measurement of nasal potential difference: an outcome measure in multicenter cystic fibrosis clinical trials

Patients with cystic fibrosis (CF) can be discriminated from healthy subjects by measurement of the nasal potential difference, which has become a useful outcome measure for therapies directed toward correcting defective electrolyte transport in CF. A standard operating procedure was developed by a CF Foundation clinical trials network, to be followed by all sites performing collaborative studies. Key variables in the measurement included type of voltmeter, exploring probe, reference electrodes, and solutions used to assess both sodium transport and chloride conductance. Eight sites submitted data on 3-8 normal and 4-5 CF subjects. Baseline voltage, an index of sodium transport, was -18.2 +/- 8.3 mV (mean +/- SD) for normals, and -45.3 +/- 11.4 mV for CF patients. There was no CFTR-mediated chloride secretion in CF subjects, as evidenced by the lack of response to perfusion with zero chloride + beta agonist solutions (+3.2 +/- 3.5 mV) vs. that in normals (-23.7 +/- 10.2 mV). The standardized nasal potential difference measurement minimizes variability between operators and study sites. Valid and consistent results can be attained with trained operators and attention to technical details. These data demonstrate the procedure to be sufficient for multicenter studies in the CF Foundation network.

Cystic fibrosis transmembrane conductance regulator (CFTR)-mediated residual chloride secretion does not protect against early chronic Pseudomonas aeruginosa infection in F508del homozygous cystic fibrosis patients

Cystic fibrosis (CF) disease severity is characterized by a broad variability that has been attributed, in addition to the CF transmembrane conductance regulator (CFTR) genotype, to modulating factors such as CFTR-mediated residual chloride (Cl-) secretion. Moreover, CFTR has been suggested to function as a receptor for Pseudomonas aeruginosa (PA). In this study, we investigated whether or not the presence of residual Cl- secretion protects against early chronic PA colonization of patients' airways. Excluding influences on the phenotype caused by different CFTR mutations, we evaluated a cohort of F508del homozygous individuals with respect to the correlation between residual Cl- secretion and the age of onset of PA colonization as an important marker of clinical phenotype. A group with early chronic PA colonization before the age of 7 y (n = 14) was compared with a cohort that had no initial PA detection at least until the age of 13 y (n = 10). We determined the Cl- transport properties by using the intestinal current measurement in rectal suction biopsies. Residual Cl- secretion, most likely due to the CFTR Cl- channel, was observed in 63% of subjects, more frequently in early chronically PA colonized than among late or not colonized patients. These results demonstrate the presence of some active F508del-CFTR in the apical cell membrane and imply that factors other than the CFTR-mediated residual Cl- secretion determine the age of onset of PA colonization.

Nasal airway ion transport and lung function in young people with cystic fibrosis

There is strong evidence that abnormal airway ion transport is the primary defect that initiates the pathophysiology of lung disease in cystic fibrosis (CF). To examine the relationship between airway ion transport abnormality and severity of lung disease, we measured nasal potential difference in 51 young people with CF using a validated modified technique. There was no correlation between any component of the ion transport measurement and clinical condition (respiratory function, chest radiograph score, or Shwachman clinical score). Thirty subjects, homozygous for the DeltaF508 mutation, were divided into those above and those below average respiratory function for their age. There was no significant difference in any of the ion transport parameters between those with above and below average pulmonary function. Of the 51 subjects, 10 had significant hyperpolarization after perfusion with a zero Cl- solution (> 5 mV). This Cl- secretory capacity did not correlate with above average lung function. These data do not support the assertion that the extent of lung disease in CF reflects the degree of ion transport abnormality. We suggest that although an ion transport abnormality initiates lung disease, other factors (e.g., environmental and genetic modifiers) are more influential in determining disease severity.

A macromolecular complex of beta 2 adrenergic receptor, CFTR, and ezrin/radixin/moesin-binding phosphoprotein 50 is regulated by PKA

It has been demonstrated previously that both the cystic fibrosis transmembrane conductance regulator (CFTR) and beta(2) adrenergic receptor (beta(2)AR) can bind ezrinradixinmoesin-binding phosphoprotein 50 (EBP50, also referred to as NHERF) through their PDZ motifs. Here, we show that beta(2) is the major adrenergic receptor isoform expressed in airway epithelia and that it colocalizes with CFTR at the apical membrane. beta(2)AR stimulation increases CFTR activity, in airway epithelial cells, that is glybenclamide sensitive. Deletion of the PDZ motif from CFTR uncouples the channel from the receptor both physically and functionally. This uncoupling is specific to the beta(2)AR receptor and does not affect CFTR coupling to other receptors (e.g., adenosine receptor pathway). Biochemical studies demonstrate the existence of a macromolecular complex involving CFTR-EBP50-beta(2)AR through PDZ-based interactions. Assembly of the complex is regulated by PKA-dependent phosphorylation. Deleting the regulatory domain of CFTR abolishes PKA regulation of complex assembly. This report summarizes a macromolecular signaling complex involving CFTR, the implications of which may be relevant to CFTR-dysfunction diseases.

High morbidity and mortality in cystic fibrosis patients compound heterozygous for 3905insT and deltaF508

Genotype-phenotype association in cystic fibrosis (CF) is difficult because of heterogeneous disease expression. The genotype-phenotype correlation for the 3905insT mutation in comparison to deltaF508 was studied here. Thirty CF patients compound heterozygous for 3905insT were compared to clinical presentation of matched patients homozygous for deltaF508 (1960-1997). Sweat tests, age at diagnosis, at death and at onset of Pseudomonas aeruginosa colonization were analysed. Chrispin-Norman scores and pulmonary function forced expiratory volume in one second (FEV1) determined severity of lung disease. Twenty-five of the patients with 3905insT had deltaF508 as a second mutation and five had another rare mutation. At the age of 15 yrs, 60% of patients with 3905insT had an FEV1 < 60% predicted in comparison to 25% of patients with deltaF508 (p<0.05). Age at death and cumulative survival rate was significantly lower (p<0.05) in the 3905insT than in the deltaF508 group (20.3 and 24.0 yrs, respectively). Age at onset of P. aeruginosa colonization was not different in the study groups. Sweat chloride concentrations were lower in patients homozygous for deltaF508 (105.63+/-15.3 mmol L(-1)) than in patients with 3905insT (119.9+/-22.1 mmol x L(-1)) (p<0.05). Patients compound heterozygous for 3905insT have similar high morbidity and mortality to patients homozygous for deltaF508.

Nasal potential difference measurements in patients with atypical cystic fibrosis

The diagnosis of cystic fibrosis (CF) is based on characteristic clinical and laboratory findings. However, a subgroup of patients present with an atypical phenotype that comprises partial CF phenotype, borderline sweat tests and one or even no common cystic fibrosis transmembrane conductance regulator (CFTR) mutations. The aim of this study was to evaluate the role of nasal potential difference (PD) measurements in the diagnosis of CF patients with an atypical presentation and in a population of patients suspected to have CF. Nasal PD was measured in 162 patients from four different groups: patients with classical CF (n = 31), atypical phenotype (n = 11), controls (n = 50), and patients with questionable CF (n = 70). The parameter, or combination of nasal PD parameters was calculated in order to best discriminate all CF patients (including atypical CF) from the non-CF group. The patients with atypical CF disease had intermediate values of PD measurements between the CF and non-CF groups. The best discriminate model that assigned all atypical CF patients as CF used: e(response to chloride-free and isoproterenol/response to amiloride) with a cut-off >0.70 to predict a CF diagnosis. When this model was applied to the group of 70 patients with questionable CF, 24 patients had abnormal PD similar to the atypical CF group. These patients had higher levels of sweat chloride concentration and increased rate of CFTR mutations. Nasal potential difference is useful in diagnosis of patients with atypical cystic fibrosis. Taking into account both the sodium and chloride transport elements of the potential difference allows for better differentiation between atypical cystic fibrosis and noncystic fibrosis patients. This calculation may assist in the diagnostic work-up of patients whose diagnosis is questionable.

Evidence that systemic gentamicin suppresses premature stop mutations in patients with cystic fibrosis

Here we report the effects of gentamicin treatment on cystic fibrosis transmembrane regulator (CFTR) production and function in CF airway cells and patients with CF with premature stop mutations. Using immunocytochemical and functional [6-methoxy-N- (3-sulfopropyl) quinolinium (SPQ)-based] techniques, ex vivo exposure of airway cells from stop mutation CF patients led to the identification of surface-localized CFTR in a dose-dependent fashion. Next, five patients with CF with stop mutations and five CF control subjects were treated with parenteral gentamicin for 1 wk, and underwent repeated in vivo measures of CFTR function (nasal potential difference [PD] measurements and sweat chloride [Cl(-)] testing). During the treatment period, the number of nasal PD readings in the direction of Cl(-) secretion was increased approximately 3-fold in the stop mutation patient group compared with controls (p < 0.001), and four of five stop mutation patients with CF had at least one reading during gentamicin treatment with a Cl(-) secretory response of more than -5 mV (hyperpolarized). A response of this magnitude was not seen in any of the CF control subjects (p < 0.05). In an independent series of experiments designed to test the ability of repeat nasal PDs to detect wild-type CFTR function, evidence of Cl(-) secretion was seen in 88% of control (non-CF) nasal PDs, and 71% were more than -5 mV hyperpolarized. Together, these results suggest that gentamicin treatment can suppress premature stop mutations in airway cells from patients with CF, and produce small increases in CFTR Cl(-) conductance (as measured by the nasal PD) in vivo.

R-domain interactions with distal regions of CFTR lead to phosphorylation and activation

Cystic fibrosis is caused by the aberrant function of the cystic fibrosis transmembrane conductance regulator (CFTR) protein. We examined whether intramolecular binding interactions involving the regulatory (R) domain contributed to CFTR regulation and function. When the R-domain (amino acids 596-836) was coexpressed with Delta1-836 CFTR (a carboxyl hemi-CFTR beginning immediately after the R-domain), strong binding between the two polypeptides was exhibited. The R-domain that co-immunoprecipitated with Delta1-836 exhibited a slower mobility on SDS-PAGE that resulted from phosphorylation of the protein. A larger CFTR polypeptide that included the R-domain (M837X) also exhibited a phosphorylation-dependent mobility shift when coexpressed with Delta1-836. Moreover, coexpression of M837X and Delta1-836 led to enhanced halide permeability in living cells. The activity, unlike in full-length CFTR, was present without forskolin activation, but still sensitive to the PKA inhibitor, Rp-8-CPT-cAMPS. This PKA inhibition of activity was found to be dependent on the carboxy region of the R-domain, amino acids 723-836. Our results indicate that the R-domain binds CFTR residues after amino acid 836 and that this binding facilitates phosphorylation and CFTR activation. We have also characterized a subdomain within CFTR (residues 723-837) that is necessary for PKA-dependent constitutive activation. Finally, these experiments demonstrate that constitutive CFTR activity can be accomplished by at least two mechanisms: (1) direct modulation of the R-domain to abrogate PKA regulation and (2) modifications that increase R-domain susceptibility to steady-state phosphorylation through PKA.

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.

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.

Role of CFTR in airway disease

Role of CFTR in Airway Disease. Physiol. Rev. 79, Suppl.: S215-S255, 1999. - Cystic fibrosis (CF) is caused by mutations in the gene encoding the CF transmembrane conductance regulator (CFTR), which accounts for the cAMP-regulated chloride conductance of airway epithelial cells. Lung disease is the chief cause of morbidity and mortality in CF patients. This review focuses on mechanisms whereby the deletion or impairment of CFTR chloride channel function produces lung disease. It examines the major themes of the channel hypothesis of CF, which involve impaired regulation of airway surface fluid volume or composition. Available evidence indicates that the effect of CFTR deletion alters physiological functions of both surface and submucosal gland epithelia. At the airway surface, deletion of CFTR causes hyperabsorption of sodium chloride and a reduction in the periciliary salt and water content, which impairs mucociliary clearance. In submucosal glands, loss of CFTR-mediated salt and water secretion compromises the clearance of mucins and a variety of defense substances onto the airway surface. Impaired mucociliary clearance, together with CFTR-related changes in the airway surface microenvironment, leads to a progressive cycle of infection, inflammation, and declining lung function. Here, we provide the details of this pathophysiological cascade in the hope that its understanding will promote the development of new therapies for CF.

Cystic fibrosis transmembrane conductance regulator (CFTR) nucleotide-binding domain 1 (NBD-1) and CFTR truncated within NBD-1 target to the epithelial plasma membrane and increase anion permeability

The cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the traffic ATPase family that includes multiple proteins characterized by (1) ATP binding, (2) conserved transmembrane (TM) motifs and nucleotide binding domains (NBDs), and (3) molecular transport of small molecules across the cell membrane. While CFTR NBD-1 mediates ATP binding and hydrolysis, the membrane topology and function of this domain in living eukaryotic cells remains uncertain. In these studies, we have expressed wild-type CFTR NBD-1 (amino acids 433-586) or NBD-1 containing the DeltaF508 mutation transiently in COS-7 cells and established that the domain is situated across the plasma membrane by four independent assays; namely, extracellular chymotrypsin digestion, surface protein biotinylation, confocal immunofluorescent microscopy, and functional measurements of cell membrane anion permeability. Functional studies indicate that basal halide permeability is enhanced above control conditions following wild-type or DeltaF508 NBD-1 expression in three different epithelial cell lines. Furthermore, when clinically relevant CFTR proteins truncated within NBD-1 (R553X or G542X) are expressed, surface localization and enhanced halide permeability are again established. Together, these findings suggest that isolated CFTR NBD-1 (with or without the DeltaF508 mutation) is capable of targeting the epithelial cell membrane and enhancing cellular halide permeability. Furthermore, CFTR truncated at position 553 or 542 and possessing the majority of NBD-1 demonstrates surface localization and also confers increased halide permeability. These findings indicate that targeting to the plasma membrane and assumption of a transmembrane configuration are innate properties of the CFTR NBD-1. The results also support the notion that components of the halide-selective pore of CFTR reside within NBD-1.

A pilot clinical trial of oral sodium 4-phenylbutyrate (Buphenyl) in deltaF508-homozygous cystic fibrosis patients: partial restoration of nasal epithelial CFTR function

Sodium 4-phenylbutyrate (Buphenyl, 4PBA) is a new FDA approved drug for management of urea cycle disorders. We have previously presented data suggesting that 4PBA, at clinically achievable concentrations, induces CFTR channel function on the plasma membrane of deltaF508-expressing cystic fibrosis (CF) airway epithelial cells in vitro (Rubenstein, R. C., and P. L. Zeitlin, 1997. J. Clin. Invest. 100:2457-2463). We hypothesized that 4PBA would induce epithelial CFTR function in vivo in individuals homozygous for deltaF508-CFTR. A randomized, double-blind, placebo-controlled trial in 18 deltaF508-homozygous patients with CF was performed with the maximum approved adult dose of 4PBA, 19 grams p.o. divided t.i.d., given for 1 wk. Nasal potential difference (NPD) response patterns and sweat chloride concentrations were determined before and after study drug treatment, and 4PBA and metabolites were assayed in plasma and urine at the end of study drug treatment. Subjects in the 4PBA group demonstrated small, but statistically significant improvements of the NPD response to perfusion of an isoproterenol/amiloride/chloride-free solution; this measure reflects epithelial CFTR function and is highly discriminatory between patients with and without CF. Subjects who had received 4PBA did not demonstrate significantly reduced sweat chloride concentrations or alterations in the amiloride-sensitive NPD. Side effects due to drug therapy were minimal and comparable in the two groups. These data are consistent with 4PBA therapy inducing CFTR function in the nasal epithelia of deltaF508-homozygous CF patients.