Loss of Cftr function exacerbates the phenotype of Na(+) hyperabsorption in murine airways

Acute exposure to realistic simulated urban atmospheres exacerbates pulmonary phenotype in cystic fibrosis-like mice

Cystic Fibrosis (CF) is a lethal genetic disorder caused by pathogenic mutations of the CFTR gene. CF patients show a high phenotypic variability of unknown origin. In this context, the present study was therefore dedicated to investigating the effects of acute exposure to air pollution on the pulmonary morbidity of a CF-like mice model. To achieve our aim, we developed a multidisciplinary approach and designed an innovative protocol using a simulation chamber reproducing multiphasic chemical processes at the laboratory. A particular attention was paid to modulate the composition of these simulated atmospheres, in terms of concentrations of gaseous and particulate pollutants. Exposure to simulated urban atmospheres induced mucus secretion and increased inflammatory biomarkers levels, oxidative stress as well as expression of lung remodeling actors in both WT and CF-like mice. The latter were more susceptible to develop such a response. Though we could not establish direct mechanistic link between biological responses and specific components, the type of immune response induced depended on the chemical composition of the atmospheres. Overall, we demonstrated that air pollution is an important determinant of CF-like lung phenotypic variability and emphasized the added value of considering air pollution with a multi-pollutant approach.

The effectiveness of hyaluronic acid in reducing adverse effects associated with inhaled hypertonic saline therapy in patients with cystic fibrosis: A systematic review and meta-analysis

Background

Inhaled hypertonic saline (HS) is an effective mucolytic agent in patients with cystic fibrosis (CF). However, adverse events can impair the clinical utility of hypertonic saline (HS) in this patient population. In this study, we aimed to investigate the effectiveness of hyaluronic acid (HA) in reducing these adverse events.

Methods

A literature search was conducted across three electronic databases (Medline, Cochrane Central, and EMBASE) from inception through February 2023. Randomized controlled trials (RCTs) assessing the impact of hyaluronic acid (HA) in reducing adverse events in patients taking hypertonic saline (HS) for CF were included in the analysis. Outcomes of interest included cough, throat irritation, unpleasant taste, and FEV1. Evaluations were reported as risk ratios (RRs) and mean differences (MDs) with 95% confidence intervals (CI). The Cochrane Risk of Bias Tool (CRBT) was employed to assess the quality of RCTs. The New Castle Ottawa Scale was used to assess the quality of observational studies.

Results

From the 1960 articles retrieved from the initial search, five relevant studies (n=236 patients) were included in the final analysis. Compared with patients only on HS, patients with HS and HA were significantly less likely to experience cough (RR: 0.45; 95% CI, 0.28-0.72, p=0.001), throat irritation (RR: 0.43; 95% CI, 0.22-0.81, p= 0.009), and unpleasant smell (RR: 0.43; 95% CI, 0.23 - 0.80, p=0.09). In addition, patients with HS with HA had significantly less forced expiratory volume (FEV1) (MD: -2.97; 95% CI, -3.79-2.15, p=0.52) compared to patients only on HS.

Discussion

The addition of HA to HS was linked to a better tolerability profile. When HS was coupled with HA, there was a reduction in all side effects. This may permit tolerance of the medication in otherwise difficult patients, improve adherence to patients receiving frequent inhalations, and improve therapeutic outcomes.

Conclusion

The addition of HA is advantageous in CF patients who require continuous HS therapy and have previously shown poor tolerance to therapy.

30-Min Exposure to Tobacco Smoke Influences Airway Ion Transport-An In Vitro Study

Introduction: Smoking is one of the most important causes of cancer in humans. However, it has not been proven how long exposure to cigarette smoke is sufficient to induce cancerogenesis. Cigarette smoke can cause changes in ion and water transport and the maintenance of mucociliary transport. The conducted research concerned the assessment of changes in ion transport in rabbit tracheal specimens after 30 min of exposure to cigarette smoke. Materials and Methods: A modified Ussing chamber was used to measure the transepithelial electrical potential under stationary conditions (PD) and during mechanical stimulation (PDmin), and the transepithelial electrical resistance (R) in control and cigarette smoke-exposed tracheal fragments. Results: Significant changes in PD (-2.53 vs. -3.92 mV) and PDmin (-2.74 vs. -0.39 mV) were noted for the samples exposed to smoke, which can be associated with a rise in reactivity after applying a mechanical stimulus. In addition, the measured R (108 vs. 136 Ω/cm²) indicated no changes in the vitality of the samples, but an increase in their permeability to ions in the experimental conditions. Conclusions: A single 30-min exposure to cigarette smoke has been shown to be associated with increased permeability of the tracheal epithelium to ions and thus to substances emitted during smoking, which might be sufficient to create the possibility of initiating procarcinogenic processes.

Prevalence and Mechanisms of Mucus Accumulation in COVID-19 Lung Disease

Rationale: The incidence and sites of mucus accumulation and molecular regulation of mucin gene expression in coronavirus (COVID-19) lung disease have not been reported. Objectives: To characterize the incidence of mucus accumulation and the mechanisms mediating mucin hypersecretion in COVID-19 lung disease. Methods: Airway mucus and mucins were evaluated in COVID-19 autopsy lungs by Alcian blue and periodic acid-Schiff staining, immunohistochemical staining, RNA in situ hybridization, and spatial transcriptional profiling. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-infected human bronchial epithelial (HBE) cultures were used to investigate mechanisms of SARS-CoV-2-induced mucin expression and synthesis and test candidate countermeasures. Measurements and Main Results: MUC5B and variably MUC5AC RNA concentrations were increased throughout all airway regions of COVID-19 autopsy lungs, notably in the subacute/chronic disease phase after SARS-CoV-2 clearance. In the distal lung, MUC5B-dominated mucus plugging was observed in 90% of subjects with COVID-19 in both morphologically identified bronchioles and microcysts, and MUC5B accumulated in damaged alveolar spaces. SARS-CoV-2-infected HBE cultures exhibited peak titers 3 days after inoculation, whereas induction of MUC5B/MUC5AC peaked 7-14 days after inoculation. SARS-CoV-2 infection of HBE cultures induced expression of epidermal growth factor receptor (EGFR) ligands and inflammatory cytokines (e.g., IL-1α/β) associated with mucin gene regulation. Inhibiting EGFR/IL-1R pathways or administration of dexamethasone reduced SARS-CoV-2-induced mucin expression. Conclusions: SARS-CoV-2 infection is associated with a high prevalence of distal airspace mucus accumulation and increased MUC5B expression in COVID-19 autopsy lungs. HBE culture studies identified roles for EGFR and IL-1R signaling in mucin gene regulation after SARS-CoV-2 infection. These data suggest that time-sensitive mucolytic agents, specific pathway inhibitors, or corticosteroid administration may be therapeutic for COVID-19 lung disease.

Association of cystic fibrosis transmembrane conductance regulator with epithelial sodium channel subunits carrying Liddle's syndrome mutations

The association of the cystic fibrosis transmembrane conductance regulator (CFTR) and epithelial sodium channel (ENaC) in the pathophysiology of cystic fibrosis (CF) is controversial. Previously, we demonstrated a close physical association between wild-type (WT) CFTR and WT ENaC. We have also shown that the F508del CFTR fails to associate with ENaC unless the mutant protein is rescued pharmacologically or by low temperature. In this study, we present the evidence for a direct physical association between WT CFTR and ENaC subunits carrying Liddle's syndrome mutations. We show that all three ENaC subunits bearing Liddle's syndrome mutations (both point mutations and the complete truncation of the carboxy terminus), could be coimmunoprecipitated with WT CFTR. The biochemical studies were complemented by fluorescence lifetime imaging microscopy (FLIM), a distance-dependent approach that monitors protein-protein interactions between fluorescently labeled molecules. Our measurements revealed significantly increased fluorescence resonance energy transfer between CFTR and all tested ENaC combinations as compared with controls (ECFP and EYFP cotransfected cells). Our findings are consistent with the notion that CFTR and ENaC are within reach of each other even in the setting of Liddle's syndrome mutations, suggestive of a direct intermolecular interaction between these two proteins.

Optimizations of In Vitro Mucus and Cell Culture Models to Better Predict In Vivo Gene Transfer in Pathological Lung Respiratory Airways: Cystic Fibrosis as an Example

The respiratory epithelium can be affected by many diseases that could be treated using aerosol gene therapy. Among these, cystic fibrosis (CF) is a lethal inherited disease characterized by airways complications, which determine the life expectancy and the effectiveness of aerosolized treatments. Beside evaluations performed under in vivo settings, cell culture models mimicking in vivo pathophysiological conditions can provide complementary insights into the potential of gene transfer strategies. Such models must consider multiple parameters, following the rationale that proper gene transfer evaluations depend on whether they are performed under experimental conditions close to pathophysiological settings. In addition, the mucus layer, which covers the epithelial cells, constitutes a physical barrier for gene delivery, especially in diseases such as CF. Artificial mucus models featuring physical and biological properties similar to CF mucus allow determining the ability of gene transfer systems to effectively reach the underlying epithelium. In this review, we describe mucus and cellular models relevant for CF aerosol gene therapy, with a particular emphasis on mucus rheology. We strongly believe that combining multiple pathophysiological features in single complex cell culture models could help bridge the gaps between in vitro and in vivo settings, as well as viral and non-viral gene delivery strategies.

Antibacterial Effects of Bicarbonate in Media Modified to Mimic Cystic Fibrosis Sputum

Cystic fibrosis (CF) is a hereditary disease caused by mutations in the gene encoding an epithelial anion channel. In CF, Cl⁻ and HCO₃⁻ hyposecretion, together with mucin hypersecretion, leads to airway dehydration and production of viscous mucus. This habitat is ideal for colonization by pathogenic bacteria. We have recently demonstrated that HCO₃⁻ inhibits the growth and biofilm formation of Pseudomonas aeruginosa and Staphylococcus aureus when tested in laboratory culture media. Using the same bacteria our aim was to investigate the effects of HCO₃⁻ in artificial sputum medium (ASM), whose composition resembles CF mucus. Control ASM containing no NaHCO₃ was incubated in ambient air (pH 7.4 or 8.0). ASM containing NaHCO₃ (25 and 100 mM) was incubated in 5% CO₂ (pH 7.4 and 8.0, respectively). Viable P. aeruginosa and S. aureus cells were counted by colony-forming unit assay and flow cytometry after 6 h and 17 h of incubation. Biofilm formation was assessed after 48 h. The data show that HCO₃⁻ significantly decreased viable cell counts and biofilm formation in a concentration-dependent manner. These effects were due neither to extracellular alkalinization nor to altered osmolarity. These results show that HCO₃⁻ exerts direct antibacterial and antibiofilm effects on prevalent CF bacteria.

The short-term rinsing of airways by N-acetylcysteine helps expectoration: The mechanism of sodium and chloride transport

N-acetyl-L-cysteine (NAC) mucolytic and antioxidant role is well known, but the effect on epithelial ion transport has not been yet described. The aim of the study was to evaluate the short-term and prolonged influence of NAC on ion transport in the epithelium. The experiment was performed on 108 fragments of rabbit tracheae. Fragments were divided into four groups: inhibited sodium (I) and chloride (II) transport, NAC with inhibited sodium (III) and NAC with inhibited chloride (IV) transport. The changes in electrophysiological parameters were measured in stationary conditions and during mechanical-chemical stimulation after immediate (15 s) and prolonged (60 min) N-acetylcysteine administration on the tissue. Each 15-second stimulation caused repeatable changes in the electric potential of the tissue. In trachea fragments with blocked chloride ion transport, significantly lower (P <0.0001) values of electric potential following prolonged NAC effect were observed when compared to short-term NAC-stimulation. The values of resistance were constant during experiments, which reflects the vitality of the tissue. Short-term NAC administration influences sodium ion transport, which is not observed in a prolonged stimulation. The use of the NAC solution to rinse the airways is of great clinical importance due to the short and intense contact with the epithelium.

Phenotypic Characterization and Comparison of Cystic Fibrosis Rat Models Generated Using CRISPR/Cas9 Gene Editing

Animal models of cystic fibrosis (CF) are essential for investigating disease mechanisms and trialing potential therapeutics. This study generated two CF rat models using clustered regularly interspaced short palindromic repeats/clustered regularly interspaced short palindromic repeats-associated protein 9 gene editing. One rat model carries the common human Phe508del (ΔF508) CF transmembrane conductance regulator (CFTR) mutation, whereas the second is a CFTR knockout model. Phenotype was characterized using a range of functional and histologic assessments, including nasal potential difference to measure electrophysiological function in the upper airways, RNAscope in situ hybridization and quantitative PCR to assess CFTR mRNA expression in the lungs, immunohistochemistry to localize CFTR protein in the airways, and histopathologic assessments in a range of tissues. Both rat models revealed a range of CF manifestations, including reduced survival, intestinal obstruction, bioelectric defects in the nasal epithelium, histopathologic changes in the trachea, large intestine, and pancreas, and abnormalities in the development of the male reproductive tract. The CF rat models presented herein will prove useful for longitudinal assessments of pathophysiology and therapeutics.

The Cystic Fibrosis-Like Airway Surface Layer Is not a Significant Barrier for Delivery of Eluforsen to Airway Epithelial Cells

Background: Eluforsen (previously known as QR-010) is a 33-mer antisense oligonucleotide under development for oral inhalation in cystic fibrosis (CF) patients with the delta F508 mutation. Previous work has shown that eluforsen restores CF transmembrane conductance regulator (CFTR) function in vitro and in vivo. To be effective, eluforsen has first to reach its primary target, the lung epithelial cells. Therefore, it has to diffuse through the CF airway surface layer (ASL), which in CF is characterized by the presence of thick and viscous mucus, impaired mucociliary clearance, and persistent infections. The goal of this study was to assess delivery of eluforsen through CF-like ASL. Methods and Results: First, air-liquid interface studies with cultured primary airway epithelial cells revealed that eluforsen rapidly diffuses through CF-like mucus at clinically relevant doses when nebulized once or repeatedly, over a range of testing doses. Furthermore, eluforsen concentrations remained stable in CF patient sputum for at least 48 hours, and eluforsen remained intact in the presence of various inhaled CF medications for at least 24 hours. When testing biodistribution of eluforsen after orotracheal administration in vivo, no differences in lung, liver, trachea, and kidney eluforsen concentration were observed between mice with a CF-like lung phenotype (ENaC-overexpressing mice) and control wild-type (WT) littermates. Also, eluforsen was visualized in the airway epithelial cell layer of CF-like muco-obstructed mice and WT littermates. Finally, studies of eluforsen uptake and binding to bacteria prevalent in CF lungs, and diffusion through bacterial biofilms showed that eluforsen was stable and not absorbed by, or bound to bacteria. In addition, eluforsen was found to be able to penetrate Pseudomonas aeruginosa biofilms. Conclusions: The thickened and concentrated CF ASL does not constitute a significant barrier for delivery of eluforsen, and feasibility of oral inhalation of eluforsen is supported by these data.

Role of Spdef in the Regulation of Muc5b Expression in the Airways of Naive and Mucoobstructed Mice

Understanding how expression of airway secretory mucins MUC5B and MUC5AC is regulated in health and disease is important to elucidating the pathogenesis of mucoobstructive respiratory diseases. The transcription factor SPDEF (sterile α-motif pointed domain epithelial specific transcription factor) is a key regulator of MUC5AC, but its role in regulating MUC5B in health and in mucoobstructive lung diseases is unknown. Characterization of Spdef-deficient mice upper and lower airways demonstrated region-specific, Spdef-dependent regulation of basal Muc5b expression. Neonatal Spdef-deficient mice exhibited reductions in BAL Muc5ac and Muc5b. Adult Spdef-deficient mice partially phenocopied Muc5b-deficient mice as they exhibited reduced Muc5b in nasopharyngeal and airway epithelia but not in olfactory Bowman glands, 75% incidence of nasopharyngeal hair/mucus plugs, and mild bacterial otitis media, without defective mucociliary clearance in the nasopharynx. In contrast, tracheal mucociliary clearance was reduced in Spdef-deficient mice in the absence of lung disease. To evaluate the role of Spdef in the development and persistence of Muc5b-predominant mucoobstructive lung disease, Spdef-deficient mice were crossed with Scnn1b-transgenic (Scnn1b-Tg) mice, which exhibit airway surface dehydration-induced airway mucus obstruction and inflammation. Spdef-deficient Scnn1b-Tg mice exhibited reduced Muc5ac, but not Muc5b, expression and BAL content. Airway mucus obstruction was not decreased in Spdef-deficient Scnn1b-Tg mice, consistent with Muc5b-dominant Scnn1b disease, but increased airway neutrophilia was observed compared with Spdef-sufficient Scnn1b-Tg mice. Collectively, these results indicate that Spdef regulates baseline Muc5b expression in respiratory epithelia but does not contribute to Muc5b regulation in a mouse model of Muc5b-predominant mucus obstruction caused by airway dehydration.

Lipid Nanoparticle-Delivered Chemically Modified mRNA Restores Chloride Secretion in Cystic Fibrosis

The promise of gene therapy for the treatment of cystic fibrosis has yet to be fully clinically realized despite years of effort toward correcting the underlying genetic defect in the cystic fibrosis transmembrane conductance regulator (CFTR). mRNA therapy via nanoparticle delivery represents a powerful technology for the transfer of genetic material to cells with large, widespread populations, such as airway epithelia. We deployed a clinically relevant lipid-based nanoparticle (LNP) for packaging and delivery of large chemically modified CFTR mRNA (cmCFTR) to patient-derived bronchial epithelial cells, resulting in an increase in membrane-localized CFTR and rescue of its primary function as a chloride channel. Furthermore, nasal application of LNP-cmCFTR restored CFTR-mediated chloride secretion to conductive airway epithelia in CFTR knockout mice for at least 14 days. On day 3 post-transfection, CFTR activity peaked, recovering up to 55% of the net chloride efflux characteristic of healthy mice. This magnitude of response is superior to liposomal CFTR DNA delivery and is comparable with outcomes observed in the currently approved drug ivacaftor. LNP-cmRNA-based systems represent a powerful platform technology for correction of cystic fibrosis and other monogenic disorders.

Ion channels of the lung and their role in disease pathogenesis

Maintenance of normal epithelial ion and water transport in the lungs includes providing a thin layer of surface liquid that coats the conducting airways. This airway surface liquid is critical for normal lung function in a number of ways but, perhaps most importantly, is required for normal mucociliary clearance and bacterial removal. Preservation of the appropriate level of hydration, pH, and viscosity for the airway surface liquid requires the proper regulation and function of a battery of different types of ion channels and transporters. Here we discuss how alterations in ion channel/transporter function often lead to lung pathologies.

Lung disease phenotypes caused by overexpression of combinations of α-, β-, and γ-subunits of the epithelial sodium channel in mouse airways

The epithelial Na⁺ channel (ENaC) regulates airway surface hydration. In mouse airways, ENaC is composed of three subunits, α, β, and γ, which are differentially expressed (α > β > γ). Airway-targeted overexpression of the β subunit results in Na⁺ hyperabsorption, causing airway surface dehydration, hyperconcentrated mucus with delayed clearance, lung inflammation, and perinatal mortality. Notably, mice overexpressing the α- or γ-subunit do not exhibit airway Na⁺ hyperabsorption or lung pathology. To test whether overexpression of multiple ENaC subunits produced Na⁺ transport and disease severity exceeding that of βENaC-Tg mice, we generated double (αβ, αγ, βγ) and triple (αβγ) transgenic mice and characterized their lung phenotypes. Double αγENaC-Tg mice were indistinguishable from WT littermates. In contrast, double βγENaC-Tg mice exhibited airway Na⁺ absorption greater than that of βENaC-Tg mice, which was paralleled by worse survival, decreased mucociliary clearance, and more severe lung pathology. Double αβENaC-Tg mice exhibited Na⁺ transport rates comparable to those of βENaC-Tg littermates. However, αβENaC-Tg mice had poorer survival and developed severe parenchymal consolidation. In situ hybridization (RNAscope) analysis revealed both alveolar and airway αENaC-Tg overexpression. Triple αβγENaC-Tg mice were born in Mendelian proportions but died within the first day of life, and the small sample size prevented analyses of cause(s) of death. Cumulatively, these results indicate that overexpression of βENaC is rate limiting for generation of pathological airway surface dehydration. Notably, airway co-overexpression of β- and γENaC had additive effects on Na⁺ transport and disease severity, suggesting dose dependency of these two variables.

Postnatal airway growth in cystic fibrosis piglets

Mutations in the gene encoding the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) anion channel cause CF. The leading cause of death in the CF population is lung disease. Increasing evidence suggests that in utero airway development is CFTR-dependent and that developmental abnormalities may contribute to CF lung disease. However, relatively little is known about postnatal CF airway growth, largely because such studies are limited in humans. Therefore, we examined airway growth and lung volume in a porcine model of CF. We hypothesized that CF pigs would have abnormal postnatal airway growth. To test this hypothesis, we performed CT-based airway and lung volume measurements in 3-wk-old non-CF and CF pigs. We found that 3-wk-old CF pigs had tracheas of reduced caliber and irregular shape. Their bronchial lumens were reduced in size proximally but not distally, were irregularly shaped, and had reduced distensibility. Our data suggest that lack of CFTR results in aberrant postnatal airway growth and development, which could contribute to CF lung disease pathogenesis.NEW & NOTEWORTHY This CT scan-based study of airway morphometry in the cystic fibrosis (CF) postnatal period is unique, as analogous studies in humans are greatly limited for ethical and technical reasons. Findings such as reduced airway lumen area and irregular caliber suggest that airway growth and development are CF transmembrane conductance regulator-dependent and that airway growth defects may contribute to CF lung disease pathogenesis.

Single-cell RNA sequencing identifies diverse roles of epithelial cells in idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a lethal interstitial lung disease characterized by airway remodeling, inflammation, alveolar destruction, and fibrosis. We utilized single-cell RNA sequencing (scRNA-seq) to identify epithelial cell types and associated biological processes involved in the pathogenesis of IPF. Transcriptomic analysis of normal human lung epithelial cells defined gene expression patterns associated with highly differentiated alveolar type 2 (AT2) cells, indicated by enrichment of RNAs critical for surfactant homeostasis. In contrast, scRNA-seq of IPF cells identified 3 distinct subsets of epithelial cell types with characteristics of conducting airway basal and goblet cells and an additional atypical transitional cell that contributes to pathological processes in IPF. Individual IPF cells frequently coexpressed alveolar type 1 (AT1), AT2, and conducting airway selective markers, demonstrating "indeterminate" states of differentiation not seen in normal lung development. Pathway analysis predicted aberrant activation of canonical signaling via TGF-β, HIPPO/YAP, P53, WNT, and AKT/PI3K. Immunofluorescence confocal microscopy identified the disruption of alveolar structure and loss of the normal proximal-peripheral differentiation of pulmonary epithelial cells. scRNA-seq analyses identified loss of normal epithelial cell identities and unique contributions of epithelial cells to the pathogenesis of IPF. The present study provides a rich data source to further explore lung health and disease.

Barriers to inhaled gene therapy of obstructive lung diseases: A review

Knowledge of genetic origins of obstructive lung diseases has made inhaled gene therapy an attractive alternative to the current standards of care that are limited to managing disease symptoms. Initial lung gene therapy clinical trials occurred in the early 1990s following the discovery of the genetic defect responsible for cystic fibrosis (CF), a monogenic disorder. However, despite over two decades of intensive effort, gene therapy has yet to help patients with CF or any other obstructive lung disease. The slow progress is due in part to poor understanding of the biological barriers to inhaled gene therapy. Encouragingly, clinical trials have shown that inhaled gene therapy with various viral vectors and non-viral gene vectors is well tolerated by patients, and continued research has provided valuable lessons and resources that may lead to future success of this therapeutic strategy. In this review, we first introduce representative obstructive lung diseases and examine limitations of currently available therapeutic options. We then review key components for successful execution of inhaled gene therapy, including gene delivery systems, primary physiological barriers and strategies to overcome them, and advances in preclinical disease models with which the most promising systems may be identified for human clinical trials.

Role of epithelial sodium channels in the regulation of lung fluid homeostasis

In utero, fetal lung epithelial cells actively secrete Cl(-) ions into the lung air spaces while Na(+) ions follow passively to maintain electroneutrality. This process, driven by an electrochemical gradient generated by the Na(+)-K(+)-ATPase, is responsible for the secretion of fetal fluid that is essential for normal lung development. Shortly before birth, a significant upregulation of amiloride-sensitive epithelial channels (ENaCs) on the apical side of the lung epithelial cells results in upregulation of active Na(+) transport. This process is critical for the reabsorption of fetal lung fluid and the establishment of optimum gas exchange. In the adult lung, active Na(+) reabsorption across distal lung epithelial cells limits the degree of alveolar edema in patients with acute lung injury and cardiogenic edema. Cl(-) ions are transported either paracellularly or transcellularly to preserve electroneutrality. An increase in Cl(-) secretion across the distal lung epithelium has been reported following an acute increase in left atrial pressure and may result in pulmonary edema. In contrast, airway epithelial cells secrete Cl(-) through apical cystic fibrosis transmembrane conductance regulator and Ca(2+)-activated Cl(-) channels and absorb Na(+). Thus the coordinated action of Cl(-) secretion and Na(+) absorption is essential for maintenance of the volume of epithelial lining fluid that, in turn, maximizes mucociliary clearance and facilitates clearance of bacteria and debris from the lungs. Any factor that interferes with Na(+) or Cl(-) transport or dramatically upregulates ENaC activity in airway epithelial cells has been associated with lung diseases such as cystic fibrosis or chronic obstructive lung disease. In this review we focus on the role of the ENaC, the mechanisms involved in ENaC regulation, and how ENaC dysregulation can lead to lung pathology.

A review of recent developments and applications of morphometry/stereology in lung research

Design-based stereology is the gold standard of morphometry in lung research. Here, we analyze the current use of morphometric and stereological methods in lung research and provide an overview on recent methodological developments and biological observations made by the use of stereology. Based on this analysis we hope to provide useful recommendations for a good stereological practice to further the use of advanced and unbiased stereological methods.

miR-17 overexpression in cystic fibrosis airway epithelial cells decreases interleukin-8 production

Interleukin (IL)-8 levels are higher than normal in cystic fibrosis (CF) airways, causing neutrophil infiltration and non-resolving inflammation. Overexpression of microRNAs that target IL-8 expression in airway epithelial cells may represent a therapeutic strategy for cystic fibrosis. IL-8 protein and mRNA were measured in cystic fibrosis and non-cystic fibrosis bronchoalveolar lavage fluid and bronchial brushings (n=20 per group). miRNAs decreased in the cystic fibrosis lung and predicted to target IL-8 mRNA were quantified in βENaC-transgenic, cystic fibrosis transmembrane conductance regulator (Cftr)-/- and wild-type mice, primary cystic fibrosis and non-cystic fibrosis bronchial epithelial cells and a range of cystic fibrosis versus non-cystic fibrosis airway epithelial cell lines or cells stimulated with lipopolysaccharide, Pseudomonas-conditioned medium or cystic fibrosis bronchoalveolar lavage fluid. The effect of miRNA overexpression on IL-8 protein production was measured. miR-17 regulates IL-8 and its expression was decreased in adult cystic fibrosis bronchial brushings, βENaC-transgenic mice and bronchial epithelial cells chronically stimulated with Pseudomonas-conditioned medium. Overexpression of miR-17 inhibited basal and agonist-induced IL-8 protein production in F508del-CFTR homozygous CFTE29o(-) tracheal, CFBE41o(-) and/or IB3 bronchial epithelial cells. These results implicate defective CFTR, inflammation, neutrophilia and mucus overproduction in regulation of miR-17. Modulating miR-17 expression in cystic fibrosis bronchial epithelial cells may be a novel anti-inflammatory strategy for cystic fibrosis and other chronic inflammatory airway diseases.

Sonographic evidence of abnormal tracheal cartilage ring structure in cystic fibrosis

OBJECTIVES/HYPOTHESIS

Tracheal cartilage ring structural abnormalities have been reported in cystic fibrosis (CF) mice and pigs. Whether similar findings are present in humans with CF is unknown. We hypothesized that tracheal cartilage ring shape and size would be different in people with CF.

STUDY DESIGN

Tracheal cartilage ring size and shape were measured in adults with (n = 21) and without CF (n = 18).

METHODS

Ultrasonography was used in human subjects to noninvasively assess tracheal cartilage ring structure in both the sagittal and the transverse planes. Tracheal cartilage ring thickness was also determined from histological sections obtained from newborn non-CF and CF pigs. These values were compared with human data.

RESULTS

Human CF tracheas had a greater width and were less circular in shape compared to non-CF subjects. CF tracheal cartilage rings had a greater midline cross-sectional area and were thicker compared to non-CF rings. Maximal tracheal cartilage ring thickness was also greater in both newborn CF pigs and human adults with CF, compared to non-CF controls.

CONCLUSIONS

Our findings demonstrate that structural differences exist in tracheal cartilage rings in adults with CF. Comparison with newborn CF pig data suggests that some of these changes may be congenital in nature.

LEVEL OF EVIDENCE

3b

Cigarette smoke exposure reveals a novel role for the MEK/ERK1/2 MAPK pathway in regulation of CFTR

BACKGROUND

Cystic fibrosis transmembrane conductance regulator plays a key role in maintenance of lung fluid homeostasis. Cigarette smoke decreases CFTR expression in the lung but neither the mechanisms leading to CFTR loss, nor potential ways to prevent its loss have been identified to date.

METHODS

The molecular mechanisms leading to down-regulation of CFTR by cigarette smoke were determined using pharmacologic inhibitors and silencing ribonucleic acids (RNAs).

RESULTS

Using human bronchial epithelial cells, here we show that cigarette smoke induces degradation of CFTR that is attenuated by lysosomal inhibitors, but not proteasome inhibitors. Cigarette smoke can activate multiple signaling pathways in airway epithelial cells, including the MEK/Erk1/2 MAPK (MEK: mitogen-activated protein kinase/ERK kinase Erk1/2: extracellular signal-regulated kinase 1/2 MAPK: Mitogen-activated protein kinase) pathway regulating cell survival. Interestingly, pharmacological inhibition of the MEK/Erk1/2 MAPK pathway prevented the loss of plasma membrane CFTR upon cigarette smoke exposure. Similarly, decreased expression of Erk1/2 using silencing RNAs prevented the suppression of CFTR protein by cigarette smoke. Conversely, specific inhibitors of the c-Jun N-terminal kinase (JNK) or p38 MAPK pathways had no effect on CFTR decrease after cigarette smoke exposure. In addition, inhibition of the MEK/Erk1/2 MAPK pathway prevented the reduction of the airway surface liquid observed upon cigarette smoke exposure of primary human airway epithelial cells. Finally, addition of the antioxidant N-acetylcysteine inhibited activation of Erk1/2 by cigarette smoke and precluded the cigarette smoke-induced decrease of CFTR.

CONCLUSIONS

These results show that the MEK/Erk1/2 MAPK pathway regulates plasma membrane CFTR in human airway cells.

GENERAL SIGNIFICANCE

The MEK/Erk1/2 MAPK pathway should be considered as a target for strategies to maintain/restore CFTR expression in the lung of smokers.

CFTR and lung homeostasis

CFTR is a cAMP-activated chloride and bicarbonate channel that is critical for lung homeostasis. Decreases in CFTR expression have dire consequences in cystic fibrosis (CF) and have been suggested to be a component of the lung pathology in chronic obstructive pulmonary disease. Decreases or loss of channel function often lead to mucus stasis, chronic bacterial infections, and the accompanying chronic inflammatory responses that promote progressive lung destruction, and, eventually in CF, lung failure. Here we discuss CFTR's functional role airway surface liquid hydration and pH, in regulation of other channels such as the epithelial sodium channel, and in regulating inflammatory responses in the lung.

Rescuing ΔF508 CFTR with trimethylangelicin, a dual-acting corrector and potentiator

Since the discovery of the cystic fibrosis (CF) gene that encodes the CF transmembrane conductance regulator (CFTR) in 1989, there has been considerable progress in understanding the molecular defects associated with different mutations in the CFTR protein. Small molecule "potentiators" have led the way as a drug therapeutic approach for correcting channel gating mutations such as the G551D mutation. Therapies for correcting the most common folding mutation in CFTR, ΔF508, however, have proven to be much more challenging. The protein-folding problem appears to be associated with both nucleotide binding domain (NBD) instability and domain interface interactions that are caused by the loss of the phenylalanine residue in NBD 1. Given the inherent complexity in the sequential folding pathway for this very large multidomain protein, it has been suggested that correcting the proper folding, anion channel function, and cell surface stability of the ΔF508 CFTR protein will require a multidrug approach to fix each of these compounding problems. Here we discuss a recent publication (Favia M, Mancini MT, Bezzerri V, Guerra L, Laselva O, Abbattiscianni AC, Debellis L, Reshkin SJ, Gambari R, Cabrini G, Casavola V. Am J Physiol Lung Cell Mol Physiol 307: L48-L61, 2014), however, that offers hope that single drug therapies are still possible.

The role of neutrophils in cystic fibrosis

PURPOSE OF REVIEW

Neutrophils are known to dominate the pulmonary inflammatory process observed in cystic fibrosis (CF). An enduring paradox is how these large numbers of neutrophils fail to eradicate colonizing bacteria. Major advances in our understanding of neutrophil dysfunction in CF and its effect on the innate immune system are leading to advances in our understanding of the pathophysiology and leading directly to new therapies.

RECENT FINDINGS

New mechanisms of neutrophil dysfunction have been described in CF including disabled cystic fibrosis transmembrane conductance regulator recruitment to phagosomes and novel mechanisms of protease-induced neutrophil dysfunction. Neutrophil elastase has been shown to be present in the airway very early in life in CF patients, and appears a biomarker of disease progression, predicting lung function decline and bronchiectasis. Elastase has also been shown to induce a pro-inflammatory state of senescence in bronchial epithelial cells in vitro and potentially in vivo. Inhibitors of neutrophil elastase are now entering clinical trials with promising results. New avenues of CF therapeutics are being explored including novel macrolides, CXCR2 antagonists and exogenous opsonins.

SUMMARY

This article reviews the past 12 months of research that contributes to our understanding of the role of neutrophils and immune dysfunction in CF.

Characterization of defects in ion transport and tissue development in cystic fibrosis transmembrane conductance regulator (CFTR)-knockout rats

Animal models for cystic fibrosis (CF) have contributed significantly to our understanding of disease pathogenesis. Here we describe development and characterization of the first cystic fibrosis rat, in which the cystic fibrosis transmembrane conductance regulator gene (CFTR) was knocked out using a pair of zinc finger endonucleases (ZFN). The disrupted Cftr gene carries a 16 base pair deletion in exon 3, resulting in loss of CFTR protein expression. Breeding of heterozygous (CFTR^+/−) rats resulted in Mendelian distribution of wild-type, heterozygous, and homozygous (CFTR^−/−) pups. Nasal potential difference and transepithelial short circuit current measurements established a robust CF bioelectric phenotype, similar in many respects to that seen in CF patients. Young CFTR^−/− rats exhibited histological abnormalities in the ileum and increased intracellular mucus in the proximal nasal septa. By six weeks of age, CFTR^−/− males lacked the vas deferens bilaterally. Airway surface liquid and periciliary liquid depth were reduced, and submucosal gland size was abnormal in CFTR^−/− animals. Use of ZFN based gene disruption successfully generated a CF animal model that recapitulates many aspects of human disease, and may be useful for modeling other CF genotypes, including CFTR processing defects, premature truncation alleles, and channel gating abnormalities.

Early cystic fibrosis lung disease: Role of airway surface dehydration and lessons from preventive rehydration therapies in mice

Cystic fibrosis (CF) lung disease starts in the first months of life and remains one of the most common fatal hereditary diseases. Early therapeutic interventions may provide an opportunity to prevent irreversible lung damage and improve outcome. Airway surface dehydration is a key disease mechanism in CF, however, its role in the in vivo pathogenesis and as therapeutic target in early lung disease remains poorly understood. Mice with airway-specific overexpression of the epithelial Na(+) channel (βENaC-Tg) recapitulate airway surface dehydration and phenocopy CF lung disease. Recent studies in neonatal βENaC-Tg mice demonstrated that airway surface dehydration produces early mucus plugging in the absence of mucus hypersecretion, which triggers airway inflammation, promotes bacterial infection and causes early mortality. Preventive rehydration therapy with hypertonic saline or amiloride effectively reduced mucus plugging and mortality in neonatal βENaC-Tg mice. These results support clinical testing of preventive/early rehydration strategies in infants and young children with CF.

Air trapping and airflow obstruction in newborn cystic fibrosis piglets

RATIONALE

Air trapping and airflow obstruction are being increasingly identified in infants with cystic fibrosis. These findings are commonly attributed to airway infection, inflammation, and mucus buildup.

OBJECTIVES

To learn if air trapping and airflow obstruction are present before the onset of airway infection and inflammation in cystic fibrosis.

METHODS

On the day they are born, piglets with cystic fibrosis lack airway infection and inflammation. Therefore, we used newborn wild-type piglets and piglets with cystic fibrosis to assess air trapping, airway size, and lung volume with inspiratory and expiratory X-ray computed tomography scans. Micro-computed tomography scanning was used to assess more distal airway sizes. Airway resistance was determined with a mechanical ventilator. Mean linear intercept and alveolar surface area were determined using stereologic methods.

MEASUREMENTS AND MAIN RESULTS

On the day they were born, piglets with cystic fibrosis exhibited air trapping more frequently than wild-type piglets (75% vs. 12.5%, respectively). Moreover, newborn piglets with cystic fibrosis had increased airway resistance that was accompanied by luminal size reduction in the trachea, mainstem bronchi, and proximal airways. In contrast, mean linear intercept length, alveolar surface area, and lung volume were similar between both genotypes.

CONCLUSIONS

The presence of air trapping, airflow obstruction, and airway size reduction in newborn piglets with cystic fibrosis before the onset of airway infection, inflammation, and mucus accumulation indicates that cystic fibrosis impacts airway development. Our findings suggest that early airflow obstruction and air trapping in infants with cystic fibrosis might, in part, be caused by congenital airway abnormalities.

Regulation of ion transport by oxidants

Ion channels perform a variety of cellular functions in lung epithelia. Oxidant- and antioxidant-mediated mechanisms (that is, redox regulation) of ion channels are areas of intense research. Significant progress has been made in our understanding of redox regulation of ion channels since the last Experimental Biology report in 2003. Advancements include: 1) identification of nonphagocytic NADPH oxidases as sources of regulated reactive species (RS) production in epithelia, 2) an understanding that excessive treatment with antioxidants can result in greater oxidative stress, and 3) characterization of novel RS signaling pathways that converge upon ion channel regulation. These advancements, as discussed at the 2013 Experimental Biology Meeting in Boston, MA, impact our understanding of oxidative stress in the lung, and, in particular, illustrate that the redox state has profound effects on ion channel and cellular function.

Cigarette smoke and CFTR: implications in the pathogenesis of COPD

Chronic obstructive pulmonary disease (COPD) is a progressive respiratory disorder consisting of chronic bronchitis and/or emphysema. COPD patients suffer from chronic infections and display exaggerated inflammatory responses and a progressive decline in respiratory function. The respiratory symptoms of COPD are similar to those seen in cystic fibrosis (CF), although the molecular basis of the two disorders differs. CF is a genetic disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene encoding a chloride and bicarbonate channel (CFTR), leading to CFTR dysfunction. The majority of COPD cases result from chronic oxidative insults such as cigarette smoke. Interestingly, environmental stresses including cigarette smoke, hypoxia, and chronic inflammation have also been implicated in reduced CFTR function, and this suggests a common mechanism that may contribute to both the CF and COPD. Therefore, improving CFTR function may offer an excellent opportunity for the development of a common treatment for CF and COPD. In this article, we review what is known about the CF respiratory phenotype and discuss how diminished CFTR expression-associated ion transport defects may contribute to some of the pathological changes seen in COPD.

Does epithelial sodium channel hyperactivity contribute to cystic fibrosis lung disease?

  Airway epithelia absorb Na+ through the epithelial Na+ channel (ENaC) and secrete Cl- through the cystic fibrosis transmembrane conductance regulator (CFTR) anion channel. This balance maintains sufficient airway surface liquid hydration to permit efficient mucus clearance, which is needed to maintain sterility of the lung. Cystic fibrosis (CF) is a common autosomal recessive inherited disease caused by mutations in the CFTR gene that lead to the reduction or elimination of the CFTR protein. CF is a multi-organ disease that affects epithelia lining the intestines, lungs, pancreas, sweat ducts and vas deferens, among others. CF lungs are characterized by viscous, dehydrated mucus, persistent neutrophilia and chronic infections. ENaC is negatively regulated by CFTR and, in patients with CF, the absence of CFTR results in a double hit of reduced Cl-/HCO3- and H2O secretion as well as ENaC hyperactivity and increased Na+ and H2O absorption. Together, these effects are hypothesized to trigger mucus dehydration, resulting in a failure to clear mucus. Rehydrating CF mucus has become a recent clinical focus and yields important end-points for clinical trials. However, while ENaC hyperactivity in CF airways has been detected in vivo and in vitro, recent data have brought the role of ENaC in CF lung disease pathogenesis into question. This review will focus on our current understanding of the contribution of ENaC to CF pathogenesis.