Pathological confirmation of cystic fibrosis in the fetus following prenatal diagnosis

Mucociliary Clearance is Impaired in Small Airways of Cystic Fibrosis Pigs

Rationale

Cystic fibrosis is a genetic disorder characterized by recurrent airway infections, inflammation, and progressive decline in lung function. Autopsy and spirometry data suggest that cystic fibrosis may start in the small airways which, due to the fractal nature of the airways, account for most of the airway tree surface area. However, they are not easily accessible for testing.

Objectives

Here, we tested the hypothesis that mucociliary clearance is abnormal in the small airways of newborn cystic fibrosis pigs.

Methods

Current mucociliary clearance assays are limited therefore we developed a dynamic positron emission tomography scan assay with high spatial and temporal resolution. Each study was accompanied by a high-resolution computed tomography scan that helped identify the thin outer region of the lung that contained small airways.

Measurements and Main Results

Clearance of aerosolized [ 68 Ga]macro aggregated albumin from distal airways occurred within minutes after delivery and followed a two-phase process. In cystic fibrosis pigs, both early and late clearance rates were slower. Stimulation of the cystic fibrosis airways with the purinergic agonist UTP further impaired late clearance. Only 1 cystic fibrosis pig treated with UTP out of 6 cleared more than 20% of the delivered dose.

Conclusions

These data indicate that mucociliary transport in the small airways is fast and can easily be missed if the acquisition is not fast enough. The data also indicate that mucociliary transport is impaired in small airways of cystic fibrosis pigs. This defect is exacerbated by stimulation of mucus secretions with purinergic agonists.

In Utero Mapping and Development Role of CFTR in Lung and Gastrointestinal Tract of Cystic Fibrosis Patients

In cystic fibrosis (CF) the ability of the CF transmembrane conductance regulator (CFTR) protein to mediate chloride and water transport is disrupted. While much progress has been made in CF research leading to effective treatments to improve CFTR function, including small molecule modulators, patients present with varying disease manifestations and responses to therapy. For many CF-affected organs, disease onset is known to occur during in utero development before treatments can be administered and progresses over time leading to irreversible damage to these organs. Thus, the role of functional CFTR protein, in particular, during early development needs to be further elucidated. Studies have detected CFTR proteins at very early gestational stages and revealed temporally and spatially variable CFTR expression patterns in fetuses, suggesting a potential role of CFTR in fetal development. However, the actual mechanisms of how defective CFTR in CF results in fetal morphogenetic abnormalities are yet to be established. This review aims to summarize fetal CFTR expression patterns specifically in the lung, pancreas, and gastrointestinal tract (GIT), as compared to adult patterns. Case studies of structural abnormalities in CF fetuses and newborns and the role of CFTR in fetal development will also be discussed.

Elastic mucus strands impair mucociliary clearance in cystic fibrosis pigs

SignificanceIn many lung diseases, increased amounts of and/or abnormal mucus impair mucociliary clearance, a key defense against inhaled and aspirated material. Submucosal glands lining cartilaginous airways secrete mucus strands that are pulled by cilia until they break free from the duct and sweep upward toward the larynx, carrying particulates. In cystic fibrosis (CF) pigs, progressive clearance of insufflated microdisks was repeatedly interrupted as microdisks abruptly recoiled. Aerosolizing a reducing agent to break disulfide bonds linking mucins ruptured mucus strands, freeing them from submucosal gland ducts and allowing cilia to propel them up the airways. These findings highlight the abnormally increased elasticity of CF mucus and suggest that agents that break disulfide bonds might have value in lung diseases with increased mucus.

A Developmental Role of the Cystic Fibrosis Transmembrane Conductance Regulator in Cystic Fibrosis Lung Disease Pathogenesis

The cystic fibrosis (CF) transmembrane conductance regulator (CFTR) protein is a cAMP-activated anion channel that is critical for regulating fluid and ion transport across the epithelium. This process is disrupted in CF epithelia, and patients harbouring CF-causing mutations experience reduced lung function as a result, associated with the increased rate of mortality. Much progress has been made in CF research leading to treatments that improve CFTR function, including small molecule modulators. However, clinical outcomes are not necessarily mutation-specific as individuals harboring the same genetic mutation may present with varying disease manifestations and responses to therapy. This suggests that the CFTR protein may have alternative functions that remain under-appreciated and yet can impact disease. In this mini review, we highlight some notable research implicating an important role of CFTR protein during early lung development and how mutant CFTR proteins may impact CF airway disease pathogenesis. We also discuss recent novel cell and animal models that can now be used to identify a developmental cause of CF lung disease.

PAR-2-activated secretion by airway gland serous cells: role for CFTR and inhibition by Pseudomonas aeruginosa

Airway submucosal gland serous cells are important sites of fluid secretion in conducting airways. Serous cells also express the cystic fibrosis (CF) transmembrane conductance regulator (CFTR). Protease-activated receptor 2 (PAR-2) is a G protein-coupled receptor that activates secretion from intact airway glands. We tested if and how human nasal serous cells secrete fluid in response to PAR-2 stimulation using Ca²⁺ imaging and simultaneous differential interference contrast imaging to track isosmotic cell shrinking and swelling reflecting activation of solute efflux and influx pathways, respectively. During stimulation of PAR-2, serous cells exhibited dose-dependent increases in intracellular Ca²⁺. At stimulation levels >EC₅₀ for Ca²⁺, serous cells simultaneously shrank ∼20% over ∼90 s due to KCl efflux reflecting Ca²⁺-activated Cl^- channel (CaCC, likely TMEM16A)-dependent secretion. At lower levels of PAR-2 stimulation (<EC₅₀ for Ca²⁺), shrinkage was not evident due to failure to activate CaCC. Low levels of cAMP-elevating VIP receptor (VIPR) stimulation, also insufficient to activate secretion alone, synergized with low-level PAR-2 stimulation to elicit fluid secretion dependent on both cAMP and Ca²⁺ to activate CFTR and K⁺ channels, respectively. Polarized cultures of primary serous cells also exhibited synergistic fluid secretion. Pre-exposure to Pseudomonas aeruginosa conditioned media inhibited PAR-2 activation by proteases but not peptide agonists in primary nasal serous cells, Calu-3 bronchial cells, and primary nasal ciliated cells. Disruption of synergistic CFTR-dependent PAR-2/VIPR secretion may contribute to reduced airway surface liquid in CF. Further disruption of the CFTR-independent component of PAR-2-activated secretion by P. aeruginosa may also be important to CF pathophysiology.

What Role Does CFTR Play in Development, Differentiation, Regeneration and Cancer?

One of the key features associated with the substantial increase in life expectancy for individuals with CF is an elevated predisposition to cancer, firmly established by recent studies involving large cohorts. With the recent advances in cystic fibrosis transmembrane conductance regulator (CFTR) modulator therapies and the increased long-term survival rate of individuals with cystic fibrosis (CF), this is a novel challenge emerging at the forefront of this disease. However, the mechanisms linking dysfunctional CFTR to carcinogenesis have yet to be unravelled. Clues to this challenging open question emerge from key findings in an increasing number of studies showing that CFTR plays a role in fundamental cellular processes such as foetal development, epithelial differentiation/polarization, and regeneration, as well as in epithelial–mesenchymal transition (EMT). Here, we provide state-of-the-art descriptions on the moonlight roles of CFTR in these processes, highlighting how they can contribute to novel therapeutic strategies. However, such roles are still largely unknown, so we need rapid progress in the elucidation of the underlying mechanisms to find the answers and thus tailor the most appropriate therapeutic approaches.

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.

Pendrin Mediates Bicarbonate Secretion and Enhances Cystic Fibrosis Transmembrane Conductance Regulator Function in Airway Surface Epithelia

Bicarbonate facilitates mucin unpacking and bacterial killing; however, its transport mechanisms in the airways are not well understood. cAMP stimulates anion efflux through the cystic fibrosis (CF) transmembrane conductance regulator (CFTR; ABCC7) anion channel, and this is defective in CF. The anion exchanger pendrin (SLC26A4) also mediates HCO₃^- efflux and is upregulated by proinflammatory cytokines. Here, we examined pendrin and CFTR expression and their contributions to HCO₃^- secretion by human nasal and bronchial epithelia. In native tissue, both proteins were most abundant at the apical pole of ciliated surface cells with little expression in submucosal glands. In well-differentiated primary nasal and bronchial cell cultures, IL-4 dramatically increased pendrin mRNA levels and apical immunostaining. Exposure to low-Cl^- apical solution caused intracellular alkalinization (ΔpH_i) that was enhanced fourfold by IL-4 pretreatment. ΔpH_i was unaffected by 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS) or CFTR inhibitor CFTR_inh-172, but was reduced by adenoviral shRNA targeting pendrin. Forskolin increased ΔpH_i, and this stimulation was prevented by CFTR_inh-172, implicating CFTR, yet forskolin only increased ΔpH_i after pendrin expression had been induced by IL-4. The dependence of ΔpH_i on pendrin suggests there is minimal electrical coupling between Cl^- and HCO₃^- fluxes and that CFTR activation increases anion exchange-mediated HCO₃^- influx. Conversely, inducing pendrin expression increased forskolin-stimulated, CFTR_inh-172-sensitive current by approximately twofold in epithelial and nonepithelial cells. We conclude that pendrin mediates most HCO₃^- secretion across airway surface epithelium during inflammation and enhances electrogenic Cl^- secretion via CFTR, as described for other SLC26A transporters.

Airway Epithelium Dysfunction in Cystic Fibrosis and COPD

Cystic fibrosis is a genetic disease caused by mutations in the CFTR gene, whereas chronic obstructive pulmonary disease (COPD) is mainly caused by environmental factors (mostly cigarette smoking) on a genetically susceptible background. Although the etiology and pathogenesis of these diseases are different, both are associated with progressive airflow obstruction, airway neutrophilic inflammation, and recurrent exacerbations, suggesting common mechanisms. The airway epithelium plays a crucial role in maintaining normal airway functions. Major molecular and morphologic changes occur in the airway epithelium in both CF and COPD, and growing evidence suggests that airway epithelial dysfunction is involved in disease initiation and progression in both diseases. Structural and functional abnormalities in both airway and alveolar epithelium have a relevant impact on alteration of host defences, immune/inflammatory response, and the repair process leading to progressive lung damage and impaired lung function. In this review, we address the evidence for a critical role of dysfunctional airway epithelial cells in chronic airway inflammation and remodelling in CF and COPD, highlighting the common mechanisms involved in the epithelial dysfunction as well as the similarities and differences of the two diseases.

Role of Sphingolipids in the Pathobiology of Lung Inflammation

Sphingolipid bioactivities in the respiratory airways and the roles of the proteins that handle them have been extensively investigated. Gas or inhaled particles or microorganisms come into contact with mucus components, epithelial cells, blood barrier, and immune surveillance within the airways. Lung structure and functionality rely on a complex interplay of polar and hydrophobic structures forming the surfactant layer and governing external-internal exchanges, such as glycerol-phospholipids sphingolipids and proteins. Sphingolipids act as important signaling mediators involved in the control of cell survival and stress response, as well as secreted molecules endowed with inflammation-regulatory activities. Most successful respiratory infection and injuries evolve in the alveolar compartment, the critical lung functional unit involved in gas exchange. Sphingolipid altered metabolism in this compartment is closely related to inflammatory reaction and ceramide increase, in particular, favors the switch to pathological hyperinflammation. This short review explores a few mechanisms underlying sphingolipid involvement in the healthy lung (surfactant production and endothelial barrier maintenance) and in a selection of lung pathologies in which the impact of sphingolipid synthesis and metabolism is most apparent, such as acute lung injury, or chronic pathologies such as cystic fibrosis and chronic obstructive pulmonary disease.

Glucocorticoids Distinctively Modulate the CFTR Channel with Possible Implications in Lung Development and Transition into Extrauterine Life

During fetal development, the lung is filled with fluid that is secreted by an active Cl- transport promoting lung growth. The basolateral Na⁺,K⁺,2Cl^- cotransporter (NKCC1) participates in Cl^- secretion. The apical Cl^- channels responsible for secretion are unknown but studies suggest an involvement of the cystic fibrosis transmembrane conductance regulator (CFTR). CFTR is developmentally regulated with a high expression in early fetal development and a decline in late gestation. Perinatal lung transition is triggered by hormones that stimulate alveolar Na⁺ channels resulting in fluid absorption. Little is known on how hormones affect pulmonary Cl^- channels. Since the rise of fetal cortisol levels correlates with the decrease in fetal CFTR expression, a causal relation may be assumed. The aim of this study was to analyze the influence of glucocorticoids on pulmonary Cl^- channels. Alveolar cells from fetal and adult rats, A549 cells, bronchial Calu-3 and 16HBE14o- cells, and primary rat airway cells were studied with real-time quantitative PCR and Ussing chambers. In fetal and adult alveolar cells, glucocorticoids strongly reduced Cftr expression and channel activity, which was prevented by mifepristone. In bronchial and primary airway cells CFTR mRNA expression was also reduced, whereas channel activity was increased which was prevented by LY-294002 in Calu-3 cells. Therefore, glucocorticoids strongly reduce CFTR expression while their effect on CFTR activity depends on the physiological function of the cells. Another apical Cl^- channel, anoctamin 1 showed a glucocorticoid-induced reduction of mRNA expression in alveolar cells and an increase in bronchial cells. Furthermore, voltage-gated chloride channel 5 and anoctamine 6 mRNA expression were increased in alveolar cells. NKCC1 expression was reduced by glucocorticoids in alveolar and bronchial cells alike. The results demonstrate that glucocorticoids differentially modulate pulmonary Cl^- channels and are likely causing the decline of CFTR during late gestation in preparation for perinatal lung transition.

Understanding the development of the respiratory glands

BACKGROUND

The submucosal glands (SMGs) of the respiratory system are specialized structures essential for maintaining airway homeostasis. The significance of SMGs is highlighted by their involvement in respiratory diseases such as cystic fibrosis, asthma and chronic bronchitis, where their phenotype and function are severely altered. Uncovering the normal development of the airway SMGs is essential to elucidate their role in these disorders, however, very little is known about the cellular mechanisms and intracellular signals involved in their morphogenesis.

RESULTS

This review describes in detail the embryonic developmental journey of the nasal SMGs and the postnatal development of the tracheal SMGs in the mouse. Current knowledge of the genes and signalling molecules involved in SMG organogenesis is also explored.

CONCLUSION

Here we review the temporal localisation and development of the murine respiratory glands in the hope of stimulating further research into the mechanisms required for successful SMG patterning and function.

Glycaemic regulation and insulin secretion are abnormal in cystic fibrosis pigs despite sparing of islet cell mass

Diabetes is a common and significant co-morbidity in cystic fibrosis (CF). The pathogenesis of cystic fibrosis related diabetes (CFRD) is incompletely understood. Because exocrine pancreatic disease is similar between humans and pigs with CF, the CF pig model has the potential to contribute significantly to the understanding of CFRD pathogenesis. We determined the structure of the endocrine pancreas in fetal, newborn and older CF and non-CF pigs and assessed endocrine pancreas function by intravenous glucose tolerance test (IV-GTT). In fetal pigs, pancreatic insulin and glucagon density was similar between CF and non-CF. In newborn and older pigs, the insulin and glucagon density was unchanged between CF and non-CF per total pancreatic area, but increased per remnant lobular tissue in CF reflecting exocrine pancreatic loss. Although fasting glucose levels were not different between CF and non-CF newborns, CF newborns demonstrated impaired glucose tolerance and increased glucose area under the curve during IV-GTT. Second phase insulin secretion responsiveness was impaired in CF newborn pigs and significantly lower than that observed in non-CF newborns. Older CF pigs had elevated random blood glucose levels compared with non-CF. In summary, glycaemic abnormalities and insulin secretion defects were present in newborn CF pigs and spontaneous hyperglycaemia developed over time. Functional changes in CF pig pancreas were not associated with a decline in islet cell mass. Our results suggest that functional islet abnormalities, independent of structural islet loss, contribute to the early pathogenesis of CFRD.

Pancreatic and biliary secretion are both altered in cystic fibrosis pigs

The pancreas, liver, and gallbladder are commonly involved in cystic fibrosis (CF), and acidic, dehydrated, and protein-rich secretions are characteristic findings. Pancreatic function studies in humans have been done by sampling the jejunal fluid. However, it has been difficult to separately study the function of pancreatic and biliary systems in humans with CF, because jejunal fluid contains a mixture of bile and pancreatic fluids. In contrast, pancreatic and biliary ducts open separately into the porcine intestine; therefore, biliary and pancreatic fluid can be individually analyzed in CF pigs. We studied newborn wild-type (WT) and CF pigs and found that CFTR was localized to the pancreatic ducts. We collected bile and pancreatic fluid and analyzed pancreatic enzymes with activity assays and immunoblot. Pancreatic enzyme expression was significantly decreased in CF compared with WT pigs. The volume and pH of pancreatic fluid were significantly lower and protein concentration was >5-fold higher in CF pigs. Secretin stimulation increased pancreatic fluid volume and pH in WT, but not CF, pigs. Baseline bile volume did not differ between WT and CF pigs, but volume did not increase in response to secretin in CF pigs. Bile pH was lower and protein concentration was twofold higher in CF pigs. These results indicate that pancreatic and biliary secretions are altered in CF pigs. Abnormal pancreatic and biliary secretion in CF may have important implications in disease pathogenesis.

Candidate diseases for prenatal gene therapy

Prenatal gene therapy aims to deliver genes to cells and tissues early in prenatal life, allowing correction of a genetic defect, before irreparable tissue damage has occurred. In contrast to postnatal gene therapy, prenatal application may target genes to a large population of stem cells, and the smaller fetal size allows a higher vector to target cell ratio to be achieved. Early gestation delivery may allow the development of immune tolerance to the transgenic protein, which would facilitate postnatal repeat vector administration if needed. Moreover, early delivery would avoid anti-vector immune responses which are often acquired in postnatal life. The NIH Recombinant DNA Advisory Committee considered that a candidate disease for prenatal gene therapy should pose serious morbidity and mortality risks to the fetus or neonate, and not have any effective postnatal treatment. Prenatal gene therapy would therefore be appropriate for life-threatening disorders, in which prenatal gene delivery maintains a clear advantage over cell transplantation or postnatal gene therapy. If deemed safer and more efficacious, prenatal gene therapy may be applicable for nonlethal conditions if adult gene transfer is unlikely to be of benefit. Many candidate diseases will be inherited congenital disorders such as thalassaemia or lysosomal storage disorders. However, obstetric conditions such as fetal growth restriction may also be treated using a targeted gene therapy approach. In each disease, the condition must be diagnosed prenatally, either via antenatal screening and prenatal diagnosis, for example, in the case of hemophilias, or by ultrasound assessment of the fetus, for example, congenital diaphragmatic hernia. In this chapter, we describe some examples of the candidate diseases and discuss how a prenatal gene therapy approach might work.

Pancreatic damage in fetal and newborn cystic fibrosis pigs involves the activation of inflammatory and remodeling pathways

Pancreatic disease has onset in utero in humans with cystic fibrosis (CF), and progresses over time to complete destruction of the organ. The exact mechanisms leading to pancreatic damage in CF are incompletely understood. Inflammatory cells are present in the pancreas of newborn pigs with CF (CF pigs) and humans, which suggests that inflammation may have a role in the destructive process. We wondered whether tissue inflammation and genes associated with inflammatory pathways were increased in the pancreas of fetal CF pigs [83 to 90 days gestation (normal pig gestation is ~114 days)] and newborn pigs. Compared with fetal pigs without CF (non-CF pigs), in fetal CF pigs, the pancreas exhibited patchy inflammation and acinar atrophy, with progression in distribution and severity in neonatal CF pigs. Large-scale transcript profiling revealed that the pancreas in fetal and newborn CF pigs exhibited significantly increased expression of proinflammatory, complement cascade, and profibrotic genes when compared with fetal and newborn non-CF pigs. Acinar cells exhibited increased apoptosis in the pancreas of fetal and newborn CF pigs. α-Smooth muscle actin and transforming growth factor β1 were increased in both fetal and newborn CF pig pancreas, suggesting activation of profibrotic pathways. Cell proliferation and mucous cell metaplasia were detected in newborn, but not fetal, CF pigs, indicating that they were not an initiator of pathogenesis but a response. Proinflammatory, complement cascade, proapoptotic, and profibrotic pathways are activated in CF pig pancreas, and likely contribute to the destructive process.

Cystic fibrosis growth retardation is not correlated with loss of Cftr in the intestinal epithelium

Maldigestion due to exocrine pancreatic insufficiency leads to intestinal malabsorption and consequent malnutrition, a mechanism proposed to cause growth retardation associated with cystic fibrosis (CF). However, although enzyme replacement therapy combined with increased caloric intake improves weight gain, the effect on stature is not significant, suggesting that growth retardation has a more complex etiology. Mouse models of CF support this, since these animals do not experience exocrine pancreatic insufficiency yet are growth impaired. Cftr absence from the intestinal epithelium has been suggested as a primary source of growth retardation in CF mice, a concept we directly tested by generating mouse models with Cftr selectively inactivated or restored in intestinal epithelium. The relationship between growth and functional characteristics of the intestines, including transepithelial electrophysiology, incidence of intestinal obstruction, and histopathology, were assessed. Absence of Cftr exclusively from intestinal epithelium resulted in loss of cAMP-stimulated short-circuit current, goblet cell hyperplasia, and occurrence of intestinal obstructions but only slight and transient impaired growth. In contrast, specifically restoring Cftr to the intestinal epithelium resulted in restoration of ion transport and completely protected against obstruction and histopathological anomalies, but growth was indistinguishable from CF mice. These results indicate that absence of Cftr in the intestinal epithelium is an important contributor to the intestinal obstruction phenotype in CF but does not correlate with the observed growth reduction in CF.

Introduction to section I: overview of approaches to study cystic fibrosis pathophysiology

Mutation of the CFTR chloride channel was identified as the genetic basis of cystic fibrosis over 20 years ago; however, correlation of the pathophysiological changes occurring in CF lung disease with the mutation of a chloride channel is ongoing. The failure of innate lung defense in CF, and the subsequent cyclical microbial colonization of airways, explains the gross anatomical changes that occur in CF pathophysiology. However, ongoing research is focused on how the lack of the CFTR channel explains the failure of innate lung defense. Hydration status of the mucus blanket is key to understanding this link, and this series of chapters details the recent progress that has been made in understanding the interplay between ion transport activity and innate lung defense, and the initiation of CF lung pathophysiology.

The antioxidant role of thiocyanate in the pathogenesis of cystic fibrosis and other inflammation-related diseases

Cystic fibrosis (CF) is a pleiotropic disease, originating from mutations in the CF transmembrane conductance regulator (CFTR). Lung injuries inflicted by recurring infection and excessive inflammation cause approximately 90% of the morbidity and mortality of CF patients. It remains unclear how CFTR mutations lead to lung illness. Although commonly known as a Cl(-) channel, CFTR also conducts thiocyanate (SCN(-)) ions, important because, in several ways, they can limit potentially harmful accumulations of hydrogen peroxide (H(2)O(2)) and hypochlorite (OCl(-)). First, lactoperoxidase (LPO) in the airways catalyzes oxidation of SCN(-) to tissue-innocuous hypothiocyanite (OSCN(-)), while consuming H(2)O(2). Second, SCN(-) even at low concentrations competes effectively with Cl(-) for myeloperoxidase (MPO) (which is released by white blood cells), thus limiting OCl(-) production by the enzyme. Third, SCN(-) can rapidly reduce OCl(-) without catalysis. Here, we show that SCN(-) and LPO protect a lung cell line from injuries caused by H(2)O(2); and that SCN(-) protects from OCl(-) made by MPO. Of relevance to inflammation in other diseases, we find that in three other tested cell types (arterial endothelial cells, a neuronal cell line, and a pancreatic beta cell line) SCN(-) at concentrations of > or =100 microM greatly attenuates the cytotoxicity of MPO. Humans naturally derive SCN(-) from edible plants, and plasma SCN(-) levels of the general population vary from 10 to 140 microM. Our findings raise the possibility that insufficient levels of antioxidant SCN(-) provide inadequate protection from OCl(-), thus worsening inflammatory diseases, and predisposing humans to diseases linked to MPO activity, including atherosclerosis, neurodegeneration, and certain cancers.

The porcine lung as a potential model for cystic fibrosis

Airway disease currently causes most of the morbidity and mortality in patients with cystic fibrosis (CF). However, understanding the pathogenesis of CF lung disease and developing novel therapeutic strategies have been hampered by the limitations of current models. Although the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) has been targeted in mice, CF mice fail to develop lung or pancreatic disease like that in humans. In many respects, the anatomy, biochemistry, physiology, size, and genetics of pigs resemble those of humans. Thus pigs with a targeted CFTR gene might provide a good model for CF. Here, we review aspects of porcine airways and lung that are relevant to CF.

Lung development and adult lung diseases

Adult respiratory diseases are caused by many factors, including genetic-environmental interaction. Genetic abnormalities can impact early fetal lung development, postnatal lung maturation, as well as adult lung injury and repair. Studies suggest that abnormally developed lung structure and function may contribute as a susceptibility factor for several adult lung diseases. This review focuses on the relationship between lung development and pathogenesis of several lung diseases including COPD, cystic fibrosis (CF), and asthma. COPD with emphysema has been considered to be an accelerated involutional disease of aging smokers. However, since only a proportion (approximately 15%) of smokers get COPD with emphysema, clearly genetic susceptibility must play a significant part in determining both the age of onset and the rapidity of decline in lung function. In mice, interference with key genes either by null mutation, hypomorphism, or gain or loss of function results in phenotypes comprising either neonatal lethal respiratory distress if the structural effect is severe, or reduced alveolarization and/or early onset emphysema if the effect is milder. Reported susceptibility candidate genes are therefore discussed in some detail, including elastin, lysyl oxidase, fibrillin, the transforming growth factor-beta-Smad3 pathway, as well as extracellular matrix proteases. In the case of CF, the Cftr gene has been shown to regulate fetal lung epithelial cell differentiation and maturation. Subtle abnormalities of lung structure and function are found in clinically asymptomatic CF infants. Finally, airway remodeling due to chronic inflammation is important in infants who later acquire asthma.

Early inflammation in the airways of a cystic fibrosis foetus

In cystic fibrosis patients, inflammation is often considered to be secondary to chronic infections. In the present study, we show increased levels of pro-inflammatory proteins in the lungs of a cystic fibrosis foetus compared to the lungs of two normal foetuses. Our findings suggest therefore the existence of an early intrinsic pro-inflammatory state in cystic fibrosis airways.

Dietary n-3 fatty acids have suppressive effects on mucin upregulation in mice infected with Pseudomonas aeruginosa

Background

Mucin hypersecretion and mucus plugging in the airways are characteristic features of chronic respiratory diseases like cystic fibrosis (CF) and contribute to morbidity and mortality. In CF, Pseudomonas aeruginosa superinfections in the lung exacerbate inflammation and alter mucus properties. There is increasing evidence that n-3 polyunsaturated fatty acids (PUFAs) exhibit anti-inflammatory properties in many inflammatory diseases while n-6 PUFA arachidonic acid (AA) favors inflammatory mediators such as eicosanoids prostaglandin E2 (PGE2) and leukotriene B4 (LTB4) that may enhance inflammatory reactions. This suggests that n-3 PUFAs may have a protective effect against mucus over-production in airway diseases. Therefore, we hypothesized that n-3 PUFAs may downregulate mucins expression.

Methods

We designed an absolute real-time PCR assay to assess the effect of a 5-week diet enriched either with n-3 or n-6 PUFAs on the expression of large mucins in the lungs of mice infected by P. aeruginosa.

Results

Dietary fatty acids did not influence mucin gene expression in healthy mice. Lung infection induced an increase of the secreted gel-forming mucin Muc5b and a decrease of the membrane bound mucin Muc4. These deregulations are modulated by dietary fatty acids with a suppressive effect of n-3 PUFAs on mucin (increase of Muc5b from 19-fold up to 3.6 × 10⁵-fold for the n-3 PUFAs treated group and the control groups, respectively, 4 days post-infection and decrease of Muc4 from 15-fold up to 3.2 × 10⁴-fold for the control and the n-3 PUFAs treated groups, respectively, 4 days post-infection).

Conclusion

Our data suggest that n-3 PUFAs enriched diet represents an inexpensive strategy to prevent or treat mucin overproduction in pulmonary bacterial colonization.

Optical imaging of Ca2+-evoked fluid secretion by murine nasal submucosal gland serous acinar cells

Airway submucosal glands are sites of high expression of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel and contribute to fluid homeostasis in the lung. However, the molecular mechanisms of gland ion and fluid transport are poorly defined. Here, submucosal gland serous acinar cells were isolated from murine airway, identified by immunofluorescence and gene expression profiling, and used in physiological studies. Stimulation of isolated acinar cells with carbachol (CCh), histamine or ATP was associated with marked decreases in cell volume (20 +/- 2% within 62 +/- 5 s) that were tightly correlated with increases in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) as revealed by simultaneous DIC and fluorescent indicator dye microscopy. Simultaneous imaging of cell volume and the Cl(-)-sensitive fluorophore SPQ indicated that the 20% shrinkage was associated with a fall of [Cl(-)](i) from 65 mm to 28 mm, reflecting loss of 67% of cell Cl(-) content, accompanied by parallel efflux of K(+). Upon agonist removal, [Ca(2+)](i) relaxed and the cells swelled back to resting volume via a bumetanide-sensitive Cl(-) influx pathway, likely to be NKCC1. Accordingly, agonist-induced serous acinar cell shrinkage and swelling are caused by activation of solute efflux and influx pathways, respectively, and cell volume reflects the secretory state of these cells. In contrast, elevation of cAMP failed to elicit detectible volume responses, or enhance those induced by submaximal [CCh], because the magnitude of the changes were likely to be below the threshold of detection using optical imaging. Finally, when stimulated with cholinergic or cAMP agonists, cells from mice that lacked CFTR, as well as wild-type cells treated with a CFTR inhibitor, exhibited identical rates and magnitudes of shrinkage and Cl(-) efflux compared with control cells. These results provide insights into the molecular mechanisms of salt and water secretion by lung submucosal glands, and they suggest that while murine submucosal gland fluid secretion in response to cholinergic stimulation can originate from CFTR-expressing serous acinar cells, it is not dependent upon CFTR function.

Role of IKK and ERK pathways in intrinsic inflammation of cystic fibrosis airways

In cystic fibrosis (CF) patients, pulmonary inflammation is a major cause of morbidity and mortality and may precede bacterial colonization. The aim of the present study was to investigate the molecular mechanisms underlying intrinsic inflammation in cystic fibrosis airways. Using different cystic fibrosis cell models, we first demonstrated that, beside a high constitutive nuclear factor of kappaB (NF-kappaB) activity, CF cells showed a higher activator protein-1 (AP-1) activity as compared to their respective control cells. Gene expression profiles, confirmed by RT-PCR and ELISA, showed over-expression of numerous NF-kappaB and AP-1-dependent pro-inflammatory genes in CF cells in comparison with control cells. Activation of NF-kappaB was correlated with higher inhibitor of kappaB kinase (IKK) activity. In addition, Bio-plex phosphoprotein assays revealed higher extracellular signal-regulated kinase (ERK) phosphorylation in CFT-2 cells. Inhibition of this kinase strongly decreased expression of pro-inflammatory genes coding for growth-regulated proteins (Gro-alpha, Gro-beta and Gro-gamma) and interleukins (IL-1beta, IL-6 and IL-8). Moreover, inhibition of secreted interleukin-1beta (IL-1beta) and basic fibroblast growth factor (bFGF) with neutralizing antibodies reduced pro-inflammatory gene expression. Our data thus demonstrated for the first time that the absence of functional cystic fibrosis transmembrane conductance regulator (CFTR) at the plasma membrane leads to an intrinsic AP-1, in addition to NF-kappaB, activity and consequently to a pro-inflammatory state sustained through autocrine factors such as IL-1beta and bFGF.

Developmental paradigm for early features of cystic fibrosis

Cystic fibrosis (CF) is a progressive disease in which the lung is perceived to be normal at birth and is injured by recurrent infection. However, there is increasing evidence that the lung is functionally and structurally abnormal prior to the appearance of clinical infection. The cystic fibrosis transmembrane regulator (CFTR) is highly expressed in fetal tissues, and this review examines the role of CFTR in regulatory cascades during lung development. Early changes in the CF lung are examined from a perspective of disrupted fetal development, explaining a number of paradoxes seen with the disease.

Cystic fibrosis and airway submucosal glands

The chronic pulmonary infections and inflammation associated with cystic fibrosis (CF) are responsible for almost all the morbidity and mortality of this disease. Our understanding of the mechanisms that underlie the very early stages of CF lung disease, that result directly from mutations in the CF gene, is relatively poor. However, the demonstration that the predominant sites of expression of the CF gene in normal lungs are the submucosal glands, together with the histological observations showing that hyperplasia of these glands and mucin occlusion of the gland ducts are the earliest signs of disease in the CF lung, suggest that malfunction of the submucosal glands may be an important factor contributing to the early pathophysiology of CF lung disease. This review describes the function of submucosal glands in normal lungs, and the way in which their function may be disrupted in CF and may thus contribute to the early stages of CF lung disease.

Liquid secretion properties of airway submucosal glands

The tracheobronchial submucosal glands secrete liquid that is important for hydrating airway surfaces, supporting mucociliary transport, and serving as a fluid matrix for numerous secreted macromolecules including the gel-forming mucins. This review details the essential structural elements of airway glands and summarizes what is currently known regarding the ion transport processes responsible for producing the liquid component of gland secretion. Liquid secretion most likely arises from serous cells and is principally under neural control with muscarinic agonists, substance P, and vasoactive intestinal peptide (VIP) functioning as effective secretogogues. Liquid secretion is driven by the active transepithelial secretion of both Cl(-) and HCO(3)(-) and at least a portion of this process is mediated by the cystic fibrosis transmembrane conductance regulator (CFTR), which is highly expressed in glands. The potential role of submucosal glands in cystic fibrosis lung disease is discussed.

A small molecule CFTR inhibitor produces cystic fibrosis-like submucosal gland fluid secretions in normal airways

Airway submucosal glands have been proposed as a primary site for initiating and sustaining airway disease in cystic fibrosis (CF). However, it has been difficult to define the role of CFTR in gland fluid secretion because of concerns in interpreting experiments on diseased CF human airways subjected to chronic infection and inflammation. Here, we test the role of CFTR in gland fluid secretion by using a selective CFTR inhibitor (CFTRinh-172) in pig and human airways. Measurements of single-gland fluid secretion rates showed inhibition of both cholinergic and cAMP-stimulated fluid secretion by CFTRinh-172. Secreted fluid [Na+] and [Cl-] measured by fluorescence ratio imaging were 101 and 116 mM, respectively, and not significantly altered by secretory agonists or CFTR inhibition. Gland fluid pH was 7.1 and reduced by 0.4 units after CFTR inhibition. Gland fluid viscosity, determined by photobleaching of FITC-dextran, was threefold increased in pilocarpine-stimulated gland fluid after CFTR inhibition, and protein concentration was increased from 12 to 20 mg/ml. Our data provide strong evidence that gland fluid secretion is CFTR-dependent. The relatively hyper-viscous and acidic fluid secretions produced by acute CFTR inhibition support a role for defective gland function in CF lung disease and provide a rational basis for pharmacological creation of a large animal model of CF.

Pathophysiology and management of pulmonary infections in cystic fibrosis

This comprehensive State of the Art review summarizes the current published knowledge base regarding the pathophysiology and microbiology of pulmonary disease in cystic fibrosis (CF). The molecular basis of CF lung disease including the impact of defective cystic fibrosis transmembrane regulator (CFTR) protein function on airway physiology, mucociliary clearance, and establishment of Pseudomonas aeruginosa infection is described. An extensive review of the microbiology of CF lung disease with particular reference to infection with P. aeruginosa is provided. Other pathogens commonly associated with CF lung disease including Staphylococcal aureus, Burkholderia cepacia, Stenotrophomonas maltophilia, Achromobacter xylosoxidans and atypical mycobacteria are also described. Clinical presentation and assessment of CF lung disease including diagnostic microbiology and other measures of pulmonary health are reviewed. Current recommendations for management of CF lung disease are provided. An extensive review of antipseudomonal therapies in the settings of treatment for early P. aeruginosa infection, maintenance for patients with chronic P. aeruginosa infection, and treatment of exacerbation in pulmonary symptoms, as well as antibiotic therapies for other CF respiratory pathogens, are included. In addition, the article discusses infection control policies, therapies to optimize airway clearance and reduce inflammation, and potential future therapies.

Disruptive effects of anion secretion inhibitors on airway mucus morphology in isolated perfused pig lung

Since anion secretion inhibitors reproduce important aspects of cystic fibrosis (CF) lung disease, the effects of these antagonists on airway mucus morphology were assessed in isolated perfused pig lungs. Maximal inhibitory concentrations of bumetanide and dimethylamiloride, which respectively block Cl- and HCO3- secretion in porcine airways, induced the formation of dense 'plastered' mucus on the airway surface, depletion of periciliary fluid and collapse of cilia. This effect was more pronounced when lungs were also exposed to bethanechol to stimulate submucosal gland secretion, when plastered mucus covered > 98 % of the airway surface. Bethanechol also reduced gland duct mucin content in the absence, but not presence, of the anion secretion inhibitors. Anion secretion inhibitors did not induce measurable increases in goblet cell degranulation. We conclude that inhibition of anion and liquid secretion in porcine lungs disrupts the normal morphology of airway surface mucus, providing further evidence that impaired anion secretion alone could account for critical aspects of CF lung disease.

Role of airway surface liquid and submucosal glands in cystic fibrosis lung disease

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR) protein, an epithelial chloride channel expressed in the airways, pancreas, testis, and other tissues. A central question is how defective CFTR function in CF leads to chronic lung infection and deterioration of lung function. Several mechanisms have been proposed to explain lung disease in CF, including abnormal airway surface liquid (ASL) properties, defective airway submucosal gland function, altered inflammatory response, defective organellar acidification, loss of CFTR regulation of plasma membrane ion transporters, and others. This review focuses on the physiology of the ASL and submucosal glands with regard to their proposed role in CF lung disease. Experimental evidence for defective ASL properties and gland function in CF is reviewed, and deficiencies in understanding ASL/gland physiology are identified as areas for further investigation. New model systems and measurement technologies are being developed to make progress in establishing lung disease mechanisms in CF, which should facilitate mechanism-based design of therapies for CF.

The relationship between insulin secretion, the insulin-like growth factor axis and growth in children with cystic fibrosis

OBJECTIVE

Cystic fibrosis-related diabetes mellitus (CFRD) is an increasingly common complication of cystic fibrosis. CFRD is preceded by a progressive decline in insulin secretion but there is no accepted definition of the prediabetic state in CFRD. This prediabetic state appears to have adverse effects on clinical status, nutrition and lung function, but there is no direct evidence that the impaired glucose homeostasis is the cause of these deteriorations. This study examined the prevalence of glucose intolerance and impaired insulin secretion in a population of children with CF without CFRD. Severe CF lung disease is often associated with poor weight gain and slower growth but the mechanism for this is still unclear. The relationships between the current state of glucose homeostasis, insulin secretion and the insulin-like growth factor axis, height velocity, nutrition status and lung function were therefore studied.

DESIGN AND PATIENTS

Eighteen children with cystic fibrosis aged 9.5-15 years had oral glucose tolerance tests and 14 of these also had intravenous glucose tolerance tests (four refused). Blood samples were collected for insulin, C-peptide, glucose, HbA1c, insulin-like growth factor (IGF)-I, IGF-II, IGF-binding protein (IGFBP)-1 and IGFBP-3. Data on height, weight, puberty status, clinical score (Shwachman score) and lung function were recorded. Height velocity, height and weight standard deviation scores (SDS) were calculated using WHO/CDC data.

RESULTS

The mean height SDS (-0.52 +/- 0.17) was less than the normal population (P = 0.007) and the mean height velocity was 4.6 +/- 0.5 cm/year, 39% with a height velocity less than the third percentile for age. The weight SDS and body mass index (BMI) were similar to the normal population. Four children had impaired glucose tolerance. The first-phase insulin response (FPIR) was below the first percentile of normal population values in nine (65%). Impaired FPIR or impaired glucose tolerance did not correlate with the Shwachman score, nutritional status or pulmonary function. There was a significant positive correlation between insulin secretion (area under the curve) and height velocity (P = 0.001) and serum IGFBP-3 levels (P = 0.001).

CONCLUSIONS

Impaired glucose tolerance was present in 20% of children with cystic fibrosis. Impaired insulin secretion was common (65%) even in children with normal glucose tolerance. The mean height SDS for the group was low and the height velocity was abnormally slow in 39%, yet nutritional status as measured by BMI was appropriate for age. Relative insulin deficiency rather than nutritional deprivation or poor clinical status thus appears to be implicated in the poor linear growth of these children with relatively stable lung disease. This was a small study and firm conclusions on this chronic suppurative disease as to the cause of poor growth are not possible. The causes of poor growth are likely to be complex; nevertheless, the apparent decrease in insulin secretion combined with the expected increased demands on insulin production during pubertal growth raises the question as to whether insulin therapy should be considered in children with cystic fibrosis before the onset of cystic fibrosis-related diabetes mellitus.

Lentiviral vector gene transfer into fetal rhesus monkeys (Macaca mulatta): lung-targeting approaches

We previously reported the efficiency of gene transfer in fetal monkeys using retroviral vectors and an intraperitoneal (IP) approach. Here, we explored intrapulmonary administration to determine whether gene transfer can be limited to the developing lung. The HIV-1-derived lentiviral vector (VSV-G pseudotyped; 1 x 10(7) infectious particles/fetus), using the enhanced green fluorescent protein (EGFP) as a reporter, was directly injected into fetal lung with ultrasound guidance (n=4; 55 or 70 days gestation; term 165+/-10 days). Fetuses were monitored sonographically, fetal/maternal blood samples collected during gestation, and four of four healthy newborns were delivered at term. All lung lobes were positive for the transgene (< or = 1%) when assessed by PCR, and transgene expression was observed by direct fluorescence microscopy and flow cytometry. The results of this study show the following: (1) successful gene transfer in fetal monkeys using an intrapulmonary approach; (2) less transduction of non-pulmonary tissues with gene transfer at 70 days gestation compared with 55 days gestation or use of an IP approach; (3) that the pulmonary epithelium was EGFP-positive by immunohistochemistry; and (4) no evidence of transplacental transport of vector sequences or antibody responses in the dams. The results of these investigations indicate the efficiency of fetal gene transfer by intrapulmonary delivery, and emphasize the importance of the fetal monkey as a preclinical model system for exploring in utero genetic treatment strategies for pulmonary disorders.

Nitric Oxide Deficiency Contributes to Impairment of Airway Relaxation in Cystic Fibrosis Mice

The pulmonary disease of cystic fibrosis (CF) is characterized by persistent airway obstruction, which has been attributed to chronic endobronchial infection and inflammation. The levels of exhaled nitric oxide (NO) are reduced in CF patients, which could contribute to bronchial obstruction through dysregulated constriction of airway smooth muscle. Because airway epithelium from CF mice has been shown to have reduced expression of inducible NO synthase, we examined airway responsiveness and relaxation in isolated tracheas of CF mice. Airway relaxation as measured by percent relaxation of precontracted tracheal segments to electrical field stimulation (EFS) and substance P, a nonadrenergic, noncholinergic substance, was significantly impaired in CF mice. The airway relaxation in response to prostaglandin E2 was similar in CF and non-CF animals. Treatment with the NO synthase inhibitor NG-nitro-L-arginine methylester reduced tracheal relaxation induced by EFS in wild-type animals but had virtually no effect in the CF mice. Conversely, exogenous NO and L-arginine, a NO substrate, reversed the relaxation defect in CF airway. We conclude that the relative absence of NO compromises airways relaxation in CF, and may contribute to the bronchial obstruction seen in the disease.

New models of the tracheal airway define the glandular contribution to airway surface fluid and electrolyte composition

Antibacterial defenses in the airway are dependent on multifactorial influences that determine the composition of both fluid and/or electrolytes at the surface of the airway and the secretory products that aid in bacterial killing and clearance. In cystic fibrosis (CF), these mechanisms of airway protection may be defective, leading to increased colonization with Pseudomonas aeruginosa. Submucosal glands, a predominant site of cystic fibrosis transmembrane conductance regulator (CFTR) protein expression in the airway, have been hypothesized to play an important role in protection of the airway. Furthermore, recent studies have suggested that the salt concentration at the airway surface may be a key factor in regulating the activity of antibacterial substances in the airway. To explore these issues, we have used a new model of the ferret tracheal airway to evaluate the contribution of submucosal glands in regulating airway surface fluid and electrolyte composition. Using tracheal xenograft models with and without submucosal glands, we have characterized several aspects of airway physiology that may be important in defining antibacterial properties. These endpoints included the contribution of submucosal glands in defining bioelectric properties of the surface airway epithelium, airway surface fluid (ASF) chloride composition, ASF volumes, and secretion of the antibacterial factor lysozyme. Findings from these studies demonstrate a significantly elevated secreted fluid volume (Vs) and chloride concentration ([Cl](s)) in ASF from airways with submucosal glands (Vs = 47 +/- 4 microl; [Cl](s) = 128 +/- 5 mM), as compared with xenograft airways without glands (Vs = 36 +/- 2 microl; [Cl](s) = 103 +/- 6 mM). Furthermore, a temperature labile factor secreted by submucosal glands appears to alter the baseline activation of 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and/or diphenylamine-2-carboxylic acid-sensitive chloride channels in the surface airway epithelium. Lastly, the lysozyme content of tracheal airways with submucosal glands was 8.5-fold higher than were airways without glands. These studies demonstrate that submucosal glands affect both the ionic composition and bioelectric properties of the airway and suggest that models evaluating antibacterial properties of the airway in CF should take into account the contribution of glands in airway physiology.

Cystic fibrosis revisited

Cystic fibrosis is a pleiotropic disease whose primary defect is thought to be abnormal chloride conductance. Despite intensive study, the role of the protein in the airway and the mechanism for its direct participation in the disease pathology remain unclear. This paper reviews CFTR's cell regulatory functions and data supporting the role of CFTR in secretory epithelial cell development. A hypothesis for CF pathophysiology based on secretory cell differentiation is proposed.

CFTR modulates lung secretory cell proliferation and differentiation

We have permanently reversed the lethal phenotype in the cystic fibrosis (CF) transmembrane conductance regulator (CFTR)-deficient (knockout) mouse after in utero gene therapy with an adenovirus containing the cftr gene. The gene transfer targeted somatic stem cells in the developing lung and intestine, and these epithelial surfaces demonstrated permanent developmental changes after treatment. The survival statistics from the progeny of heterozygote-heterozygote matings after in utero cftr gene treatment demonstrated an increased mortality in the homozygous normal pups, indicating that overexpression during development was detrimental. The lungs of these pups revealed accelerated secretory cell proliferation and differentiation. The extent of proliferation and differentiation in the secretory cells of the lung parenchyma after in utero transfer of the cftr gene was evaluated with morphometric and biochemical analyses. These studies provide further support of the regulatory role of the cftr gene in the development of the secretory epithelium.

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.

Localization of cystic fibrosis transmembrane conductance regulator mRNA in human fetal lung tissue by in situ hybridization

The fetal pulmonary epithelium secretes fluid. Cl transport is presumed to provide the driving force for net fluid secretion, although the cellular mechanisms have not been well identified in the fetus. The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP- and nucleoside triphosphate-regulated Cl channel; mutations in CFTR cause cystic fibrosis. We hypothesized that if CFTR is involved in fetal lung fluid transport, the fetal pulmonary epithelium should express CFTR mRNA. We used the technique of in situ hybridization with 3H-anti-sense and, as a control, 3H-sense CFTR cRNA probes to localize CFTR mRNA in human fetal lung tissue and cultured lung explants and determine when in gestation it is expressed. Epithelial cells of both first and second trimester lung tissues expressed CFTR mRNA. A decreasing gradient of CFTR mRNA expression was present from the proximal to the distal pulmonary epithelium. Cultured second trimester lung tissue explants expressed more CFTR mRNA than the uncultured starting tissue, suggesting CFTR gene expression increased during the five days in culture. Furthermore, alveolar type II cells in cultured explants expressed CFTR mRNA, suggesting that these cells are Cl-secretory and may be involved in lung fluid transport. These data confirm that CFTR mRNA is expressed in the human fetal pulmonary epithelium, consistent with the Cl-secretory properties of the fetal lung.