Physiology and pathology of tracheobronchial glands

Histology Atlas of the Developing Mouse Respiratory System From Prenatal Day 9.0 Through Postnatal Day 30

Respiratory diseases are one of the leading causes of death and disability around the world. Mice are commonly used as models of human respiratory disease. Phenotypic analysis of mice with spontaneous, congenital, inherited, or treatment-related respiratory tract abnormalities requires investigators to discriminate normal anatomic features of the respiratory system from those that have been altered by disease. Many publications describe individual aspects of normal respiratory tract development, primarily focusing on morphogenesis of the trachea and lung. However, a single reference providing detailed low- and high-magnification, high-resolution images of routine hematoxylin and eosin (H&E)-stained sections depicting all major structures of the entire developing murine respiratory system does not exist. The purpose of this atlas is to correct this deficiency by establishing one concise reference of high-resolution color photomicrographs from whole-slide scans of H&E-stained tissue sections. The atlas has detailed descriptions and well-annotated images of the developing mouse upper and lower respiratory tracts emphasizing embryonic days (E) 9.0 to 18.5 and major early postnatal events. The selected images illustrate the main structures and events at key developmental stages and thus should help investigators both confirm the chronological age of mouse embryos and distinguish normal morphology as well as structural (cellular and organ) abnormalities.

Expression of MUC-2, MUC-6, NAPE-PLD, IL-6 and IL-13 in Healthy and Metaplastic Bronchial Epithelium

Background: The normal tissue structure of the respiratory system is necessary to provide adequate protection of the airways and lungs. Prolonged exposure to trigger factors can result in adaptive mechanism activation and lead to the development of chronic pulmonary diseases or even dysplastic changes. Materials and methods: Respiratory system material with a pseudostratified ciliated epithelium was obtained from 12 patients (aged 16 to 95), and material with a stratified squamosa epithelium was obtained from six patients (aged 23 to 93). Routine staining was performed, and an immunohistochemistry was conducted for MUC-2, MUC-6, NAPE-PLD, IL-6 and IL-13. Results: Inflammatory processes were not detected in any of the specimens. A number of correlations were identified, with the most important being a strong positive correlation for IL-13 between the alveolar epithelium and alveolar macrophages and a strong positive correlation for IL-6 between the alveolar epithelium and alveolar macrophages in the stratified squamous epithelium group. We also detected a statistically significant difference in IL-6 in alveolar macrophages. Conclusions: There were no signs of dysplastic changes in either group. Increased secretion of IL-13 in the stratified squamous epithelium group shows its involvement in metaplastic changes in the bronchial epithelium. The secretion of atypical factors by hyaline cartilage demonstrates its plasticity and adaptability.

Mucus composition abnormalities in sinonasal mucosa of chronic rhinosinusitis with and without nasal polyps

Mucus secretion and its composition are vital in the maintenance of airway health, among which hypoxia-inducible factors (HIFs) are thought to be involved in the regulation of mucin synthesis and regulation. Nasal mucus composition difference between healthy individuals and chronic rhinosinusitis (CRS) patients may contribute to the pathology of chronic nasal diseases, but so far, their role has yet to be completely understood. Nasal biopsy specimens were obtained from 24 healthy subjects and 99 patients with CRS without (CRSsNP, n=36) or with (CRSwNP, n=63) nasal polyps. Immunohistochemical (IHC) and immunofluorescent (IF) staining, quantitative real-time PCR, and western blot were performed to compare the nasal mucus composition between the subjects. Areas of the serous gland and mucous gland were both significantly increased in CRSsNP patients. In CRSwNP patients, a decrease in submucosal gland density and a marked increase in goblet cells were observed. The major gel-forming mucins in the sinonasal mucosa of CRSsNP and CRSwNP are MUC5B and MUC5AC respectively. Mucous cells are found in a higher proportion in both CRSsNP and CRSwNP. The proportion of MUC5AC-positive goblet cells was increased in CRSwNP. The mRNA level of HIF-2α was significantly increased in CRS, and both HIF-1α and HIF-2α were expressed in serous cell but not mucous cell. Over secretion and altered composition of mucus are observed in sinonasal mucosa of CRS, which was mainly associated with glandular hyperplasia in CRSsNP and goblet cell hyperplasia in CRSwNP. Mucus abnormality compromised both non-specific and specific antimicrobial capabilities in the sinonasal mucosa. HIF expression may contribute to differences in mucin synthesis and serous gland regulation, which needs further investigation to understand the pathology of CRS.

Septum submucosal glands exhibit aberrant morphology and reduced mucin production in chronic rhinosinusitis

BACKGROUND

Chronic rhinosinusitis (CRS) is characterized by significant accumulation and thickening of mucus in the sinonasal cavities. One contributor of aberrant mucus production and impaired mucociliary clearance (MCC) is altered function of the sinonasal submucosal glands (SMGs), yet contributions of SMGs to upper airway disease initiation and progression remain unknown. The objective of this study was to characterize the morphology and secretory cell identities of the nasal septum SMGs in both healthy and CRS adults.

METHODS

Biopsies from adult participants with CRS without nasal polyps (CRSsNP, n = 4), CRS with nasal polyps (CRSwNP, n = 8), and non-CRS controls (n = 14) were collected from the posterior septum. Glandular morphology and mucus markers were investigated using histological techniques and high-resolution confocal microscopy.

RESULTS

Analysis revealed a significant decrease in gland density in the posterior septum of CRSsNP (28% ± 6.15%) and CRSwNP (23% ± 3.09%) compared to control participants (53% ± 1.59%, p < 0.0001). Further analysis of the CRS SMG secretory function revealed an overall decrease in Mucin 5B+ gland mucus being produced. Dilated and cystic ductal structures filled with inspissated mucus were also common to CRS glands.

CONCLUSION

Here, we describe a significant alteration in SMG structure and function in the adult CRS posterior septum suggesting reduced gland contribution to MCC. The SMGs of both the nose and sinuses may represent targets for future therapeutic approaches.

Safety And Tolerability Of Extended-Release Guaifenesin In Patients With Cough, Thickened Mucus And Chest Congestion Associated With Upper Respiratory Tract Infection

PURPOSE

An extended-release (ER) formulation of the expectorant guaifenesin has recently been launched in India for the treatment of productive cough accompanied by mucus (phlegm). Although the safety profile of ER guaifenesin marketed in the USA is well documented, there were limited safety data available in the Indian population. The aim of this study was to further elucidate the safety profile of ER guaifenesin in patients with acute upper respiratory tract infection (URTI).

PATIENTS AND METHODS

A prospective, post-marketing surveillance study enrolled 552 adults with cough, thickened mucus and chest congestion due to URTI, who took ER guaifenesin 1200 mg (Mucinex®, Reckitt Benckiser; two 600 mg tablets) every 12 hrs for 7 days. Adverse events (AEs) were recorded and questionnaires administered to patients and investigators.

RESULTS

A total of 29 treatment-emergent AEs were recorded in 28/552 patients, including gastrointestinal (n = 11), nervous system (n = 8), psychiatric (n = 3), respiratory, thoracic and mediastinal (n = 2), skin and subcutaneous tissue (n = 2), and general disorders (n = 3). All AEs were mild in severity and no serious AEs or deaths occurred. The majority of both patients and investigators were either satisfied or very satisfied with improvements in treatment outcomes.

CONCLUSION

This study found that ER guaifenesin was well tolerated and had a favorable safety profile in otherwise healthy patients suffering from symptoms of cough, thickened mucus and chest congestion associated with URTI. Registered trial NCT03725085 (ClinicalTrials.gov) and CTRI/2014/07/004730 (ctri.nic.in).

FGF10 is an essential regulator of tracheal submucosal gland morphogenesis

Mucus secretion and mucociliary clearance are crucial processes required to maintain pulmonary homeostasis. In the trachea and nasal passages, mucus is secreted by submucosal glands (SMGs) that line the airway, with an additional contribution from goblet cells of the surface airway epithelium. The SMG mucus is rich in mucins and antimicrobial enzymes. Defective tracheal SMGs contribute to hyper-secretory respiratory diseases, such as cystic fibrosis, asthma, and chronic obstructive pulmonary disease, however little is known about the signals that regulate their morphogenesis and patterning. Here, we show that Fgf10 is essential for the normal development of murine tracheal SMGs, with gland development arresting at the early bud stage in the absence of FGF10 signalling. As Fgf10 knockout mice are lethal at birth, inducible knockdown of Fgf10 at late embryonic stages was used to follow postnatal gland formation, confirming the essential role of FGF10 in SMG development. In heterozygous Fgf10 mice the tracheal glands formed but with altered morphology and restricted distribution. The reduction in SMG branching in Fgf10 heterozygous mice was not rescued with time and resulted in a reduction in overall tracheal mucus secretion. Fgf10 is therefore a key signal in SMG development, influencing both the number of glands and extent of branching morphogenesis, and is likely, therefore, to play a role in aspects of SMG-dependent respiratory health.

Diversity of epithelial stem cell types in adult lung

Lung is a complex organ lined with epithelial cells. In order to maintain its homeostasis and normal functions following injuries caused by varied extraneous and intraneous insults, such as inhaled environmental pollutants and overwhelming inflammatory responses, the respiratory epithelium normally undergoes regenerations by the proliferation and differentiation of region-specific epithelial stem/progenitor cells that resided in distinct niches along the airway tree. The importance of local epithelial stem cell niches in the specification of lung stem/progenitor cells has been recently identified. Studies using cell differentiating and lineage tracing assays, in vitro and/or ex vivo models, and genetically engineered mice have suggested that these local epithelial stem/progenitor cells within spatially distinct regions along the pulmonary tree contribute to the injury repair of epithelium adjacent to their respective niches. This paper reviews recent findings in the identification and isolation of region-specific epithelial stem/progenitor cells and local niches along the airway tree and the potential link of epithelial stem cells for the development of lung cancer.

The effect of cilia and the mucociliary clearance on successful drug delivery

Nasal mucociliary clearance is one of the most important factors affecting nasal delivery of drugs and vaccines. This is also the most important physiological defense mechanism inside the nasal cavity. It removes inhaled (and delivered) particles, microbes and substances trapped in the mucus. Almost all inhaled particles are trapped in the mucus carpet and transported with a rate of 8-10 mm/h toward the pharynx. This transport is conducted by the ciliated cells, which contain about 100-250 motile cellular appendages called cilia, 0.3 µm wide and 5 µm in length that beat about 1000 times every minute or 12-15 Hz. For efficient mucociliary clearance, the interaction between the cilia and the nasal mucus needs to be well structured, where the mucus layer is a tri-layer: an upper gel layer that floats on the lower, more aqueous solution, called the periciliary liquid layer and a third layer of surfactants between these two main layers. Pharmacokinetic calculations of the mucociliary clearance show that this mechanism may account for a substantial difference in bioavailability following nasal delivery. If the formulation irritates the nasal mucosa, this mechanism will cause the irritant to be rapidly diluted, followed by increased clearance, and swallowed. The result is a much shorter duration inside the nasal cavity and therefore less nasal bioavailability.

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.

Sox2 modulates Lef-1 expression during airway submucosal gland development

Tracheobronchial submucosal glands (SMGs) are derived from one or more multipotent glandular stem cells that coalesce to form a placode in surface airway epithelium (SAE). Wnt/β-catenin-dependent induction of lymphoid enhancer factor (Lef-1) gene expression during placode formation is an early event required for SMG morphogenesis. We discovered that Sox2 expression is repressed as Lef-1 is induced within airway SMG placodes. Deletion of Lef-1 did not activate Sox2 expression in SMG placodes, demonstrating that Lef-1 activation does not directly inhibit Sox2 expression. Repression of Sox2 protein in SMG placodes occurred posttranscriptionally, since the activity of its endogenous promoter remained unchanged in SMG placodes. Thus we hypothesized that Sox2 transcriptionally represses Lef-1 expression in the SAE and that suppression of Sox2 in SMG placodes activates Wnt/β-catenin-dependent induction of Lef-1 during SMG morphogenesis. Consistent with this hypothesis, transcriptional reporter assays, ChIP analyses, and DNA-protein binding studies revealed a functional Sox2 DNA binding site in the Lef-1 promoter that is required for suppressing β-catenin-dependent transcription. In polarized primary airway epithelium, Wnt induction enhanced Lef-1 expression while also inhibiting Sox2 expression. Conditional deletion of Sox2 also enhanced Lef-1 expression in polarized primary airway epithelium, but this induction was significantly augmented by Wnt stimulation. Our findings provide the first evidence that Sox2 acts as a repressor to directly modulate Wnt-responsive transcription of the Lef-1 gene promoter. These studies support a model whereby Wnt signals and Sox2 dynamically regulate the expression of Lef-1 in airway epithelia and potentially also during SMG development.

Trachea: anatomy and physiology

The windpipe (trachea) is a tube of 12 cm length connecting the larynx to the principal bronchi that lead to the lungs. The main functions of the trachea comprise air flow into the lungs, mucociliary clearance, and humidification and warming of air. Mucociliary clearance is achieved by kinocilia and goblet cells in the mucosa, and by tracheal glands. The trachea develops from the endodermal lining of the foregut in interaction with the visceral mesoderm. During adult life, different types of stem cells reside in the mucosal epithelium and glandular ducts. Recently, cholinergic chemosensory cells have been described in the trachea.

Isolation of basal cells and submucosal gland duct cells from mouse trachea

The large airways are directly in contact with the environment and therefore susceptible to injury from toxins and infectious agents that we breath in. The large airways therefore require an efficient repair mechanism to protect our bodies. This repair process occurs from stem cells in the airways and isolating these stem cells from the airways is important for understanding the mechanisms of repair and regeneration. It is also important for understanding abnormal repair that can lead to airway diseases. The goal of this method is to isolate a novel stem cell population from the mouse tracheal submucosal gland ducts and to place these cells in in vitro and in vivo model systems to identify the mechanisms of repair and regeneration of the submucosal glands. This production shows methods that can be used to isolate and assay the duct and basal stem cells from the large airways.This will allow us to study diseases of the airway, such as cystic fibrosis, asthma and chronic obstructive pulmonary disease. Currently, there are no methods for isolation of submucosal gland duct cells and there are no in vivo models to study the regeneration of submucosal glands.

Cystic fibrosis and the relationship between mucin and chloride secretion by cultures of human airway gland mucous cells

We investigated how cystic fibrosis (CF) alters the relationship between Cl(-) and mucin secretion in cultures of non-CF and CF human tracheobronchial gland mucous (HTGM and CFTGM, respectively) cells. Biochemical studies showed that HTMG cells secreted typical airway mucins, and immunohistochemical studies showed that these cells expressed MUC1, MUC4, MUC5B, MUC8, MUC13, MUC16, and MUC20. Effects of cumulative doses of methacholine (MCh), phenylephrine (Phe), isoproterenol (Iso), and ATP on mucin and Cl(-) secretion were studied on HTGM and CFTGM cultures. Baseline mucin secretion was not significantly altered in CFTGM cells, and the increases in mucin secretion induced by mediators were unaltered (Iso, Phe) or slightly decreased (MCh, ATP). Across mediators, there was no correlation between the maximal increases in Cl(-) secretion and mucin secretion. In HTGM cells, the Cl(-) channel blocker, diphenylamine-2-carboxylic acid, greatly inhibited Cl(-) secretion but did not alter mucin release. In HTGM cells, mediators (10(-5) M) increased mucin secretion in the rank order ATP > Phe = Iso > MCh. They increased Cl(-) secretion in the sequence ATP > MCh ≈ Iso > Phe. The responses in Cl(-) secretion to MCh, ATP, and Phe were unaltered by CF, but the response to Iso was greatly reduced. We conclude that mucin secretion by cultures of human tracheobronchial gland cells is independent of Cl(-) secretion, at baseline, and is unaltered in CF; that the ratio of Cl(-) secretion to mucus secretion varies markedly depending on mediator; and that secretions induced by stimulation of β-adrenergic receptors will be abnormally concentrated in CF.

New pharmacotherapy for airway mucus hypersecretion in asthma and COPD: targeting intracellular signaling pathways

Airway mucus hypersecretion is a pathophysiological feature of asthma and chronic obstructive pulmonary disease (COPD). The hypersecretion is associated with phenotypic changes in the airways, notably, increases in the number of surface epithelial goblet cells (hyperplasia) and in the size of the submucosal glands (hypertrophy). The hyperplasia and hypertrophy are associated with increased production of mucin, the gel-forming component of mucus. The excess mucus production contributes to morbidity and mortality in many patients, particularly in those with more severe disease. Although current pharmacotherapy is effective in clinical management of patients with stable asthma, severe asthma is poorly treated and there is no current drug treatment for COPD. In neither disease is there specific, effective pharmacotherapy for the hypersecretion. Consequently, identification of potential drug targets for treatment of hypersecretion in asthma and COPD is warranted. The inflammatory mediators and the associated intracellular signaling pathways underlying upregulation of mucin synthesis and development of goblet cell hyperplasia are gradually being elucidated. These include Th2 cytokines (predominantly IL-9 and IL-13), and IL-1 beta, tumor necrosis factor-alpha (TNF-alpha) and cyclooxygenase (COX)-2. IL-9 may act predominantly via calcium-activated chloride channels (CLCA), IL-13 via STAT-6 and FOXA2, TNF-alpha via NF-kappaB, and IL-1 beta via COX-2. Epidermal growth factor receptor (EGF-R) signaling and FOXA2 appear to be convergent intracellular pathways for a number of inflammatory mediators, with EGF-R upregulated in the airways of asthmatic and COPD patients. Thus, preclinical studies have clearly identified a number of intracellular signaling pathways as possible targets for pharmacotherapy of airway mucus hypersecretion in asthma and COPD. Of these, the EGF-R and Th2 cytokine pathways may have the greatest potential for inhibition of excessive mucus production. However, because these targets are so often intimately involved with different aspects of airway (and systemic) homeostasis, there is potential for development of unwanted side effects with drug intervention. Thus, translation of the promising preclinical studies to the clinic will depend on development of drug moieties with low off-target activity. This may be accomplished by maximizing airway selectivity, which may be facilitated by appropriate delivery device design.

Histochemical and ultrastructural observations of respiratory epithelium and gland in yak (Bos grunniens)

Submucous glands and epithelial mucous cells of yak (Bos grunniens) respiratory tract have been studied by a variety of histochemical methods and transmission electron microscopy for differentiating and characterizing serous and mucous cells. By light microscopy, the distribution, numbers of mucous cells, volume of mucous glands (Reid index), and the ratio of mucous cell to serous cell in the bronchial tree were measured with different staining. Histochemically, a majority of mucous cells, presented in the surface epithelium of bronchi and glands, secreted neutral and acid mucosubstances, only a few sulfated mucosubstances were present. No mucus-producing cells were observed from the terminal to respiratory bronchiolar level. Ultrastructurally, serous cells in glands of the lamina propria had two distinct forms: one type filled with many round dense secretory granules, plentiful RER and few other organelles, similar to other animals; the other type contained some oval mitochondrial and distended RER, the granules resembled the former. The mucous cells in gland were similar to that of epithelium, which containing abundant secretory granules with an eccentric core. The mucous cells of the surface epithelium differ from other animals in the structure and histochemistry of their secretory granules. Analysis of the size and distribution of the secretory granules and other organelles of serous cells suggested that differences represent different phases of a secretory cycle, not various populations of cell or granules.

Cultures of human tracheal gland cells of mucous or serous phenotype

There are two main epithelial cell types in the secretory tubules of mammalian glands: serous and mucous. The former is believed to secrete predominantly water and antimicrobials, the latter mucins. Primary cultures of human airway gland epithelium have been available for almost 20 yr, but they are poorly differentiated and lack clear features of either serous or mucous cells. In this study, by varying growth supports and media, we have produced cultures from human airway glands that in terms of their ultrastructure and secretory products resemble either mucous or serous cells. Of four types of porous-bottomed insert tested, polycarbonate filters (Transwells) most strongly promoted the mucous phenotype. Coupled with the addition of epidermal growth factor (EGF), this growth support produced “mucous” cells that contained the large electron-lucent granules characteristic of native mucous cells, but lacked the small electron-dense granules characteristic of serous cells. Furthermore, they showed high levels of mucin secretion and low levels of release of lactoferrin and lysozyme (markers of native serous cells). By contrast, growth on polyethylene terephthalate filters (Cyclopore) in medium lacking EGF produced “serous” cells in which small electron-dense granules replaced the electron-lucent ones, and the cells had high levels of lactoferrin and lysozyme but low levels of mucins. Measurements of transepithelial resistance and short-circuit current showed that both “serous” and “mucous” cell cultures possessed tight junctions, had become polarized, and were actively secreting Cl.

Normal mouse intestinal mucus release requires cystic fibrosis transmembrane regulator-dependent bicarbonate secretion

The mechanisms underlying mucus-associated pathologies in cystic fibrosis (CF) remain obscure. However, recent studies indicate that CF transmembrane conductance regulator (CFTR) is required for bicarbonate (HCO3-) transport and that HCO3- is critical for normal mucus formation. We therefore investigated the role of HCO3- in mucus secretion using mouse small intestine segments ex vivo. Basal rates of mucus release in the presence or absence of HCO3- were similar. However, in the absence of HCO3-, mucus release stimulated by either PGE2 or 5-hydroxytryptamine (5-HT) was approximately half that stimulated by these molecules in the presence of HCO3-. Inhibition of HCO3- and fluid transport markedly reduced stimulated mucus release. However, neither absence of HCO3- nor inhibition of HCO3- transport affected fluid secretion rates, indicating that the effect of HCO3- removal on mucus release was not due to decreased fluid secretion. In a mouse model of CF (mice homozygous for the most common human CFTR mutation), intestinal mucus release was minimal when stimulated with either PGE2 or 5-HT in the presence or absence of HCO3-. These data suggest that normal mucus release requires concurrent HCO3- secretion and that the characteristically aggregated mucus observed in mucin-secreting organs in individuals with CF may be a consequence of defective HCO3- transport.

ACUTE BRONCHIAL OBSTRUCTION IN SHEEP: HISTOPATHOLOGY AND GLAND CYTOKINE EXPRESSION

An ovine model of smoke inhalation and burn (S+B) injury models the pathophysiology of these injuries in humans. This study examines the degree of airway obstruction, associated histopathology, and bronchial gland cell expression of cytokines during the first 24 hours after S+B injury in sheep. Changes in the mean degree of obstruction were limited to the bronchial airways, showing significant increases in obstruction with time, P<.05. At 4 hours after injury, the obstructive material was predominantly mucus, with neutrophils clustered around and within gland acini. At 8 to 24 hours, bronchial obstruction was characterized by increased inflammatory cell accumulation. Immunohistochemical results showed that gland cells constitutively express and secrete interleukin (IL)-1beta, and that after injury there is an increase in the percentage of gland cells staining for IL-1alpha, IL-8, and tumor necrosis factor (TNF)-alpha, P<.05.

Airway epithelial cells: current concepts and challenges

The adult human bronchial tree is covered with a continuous layer of epithelial cells that play a critical role in maintaining the conduit for air, and which are central to the defenses of the lung against inhaled environmental concomitants. The epithelial sheet functions as an interdependent unit with the other lung components. Importantly, the structure and/or function of airway epithelium is deranged in major lung disorders, including chronic obstructive pulmonary disease, asthma, and bronchogenic carcinoma. Investigations regarding the airway epithelium have led to many advances over the past few decades, but new developments in genetics and stem cell/progenitor cell biology have opened the door to understanding how the airway epithelium is developed and maintained, and how it responds to environmental stress. This article provides an overview of the current state of knowledge regarding airway epithelial stem/progenitor cells, gene expression, cell-cell interactions, and less frequent cell types, and discusses the challenges for future areas of investigation regarding the airway epithelium in health and disease.

The glandular stem/progenitor cell niche in airway development and repair

Airway submucosal glands (SMGs) are major secretory structures that lie beneath the epithelium of the cartilaginous airway. These glands are believed to play important roles in normal lung function and airway innate immunity by secreting antibacterial factors, mucus, and fluid into the airway lumen. Recent studies have suggested that SMGs may additionally serve as a protective niche for adult epithelial stem/progenitor cells of the proximal airways. As in the case of other adult stem cell niches, SMGs are believed to provide the localized environmental signals required to both maintain and mobilize stem/progenitor cells, in the setting of normal cellular turnover or injury. Aberrant proliferation and differentiation of glandular stem/progenitor cells may be associated with several hypersecretory lung diseases, including chronic bronchitis, asthma, and cystic fibrosis. To better understand the molecular mechanisms that regulate the specification and proliferation of glandular stem/progenitor cells in lung diseases associated with SMG hypertrophy and hyperplasia, researchers have begun to search for the molecular signals and cell types responsible for establishing the glandular stem/progenitor cell niche, and to dissect how these determinants of the niche change in the setting of proximal airway injury and repair. Such studies have revealed certain similarities between stem/progenitor cell niches of the distal conducting airways and the SMGs of the proximal airways.

Pathogenesis of Bronchiectasis

Cystic fibrosis (CF) typically follows a more severe clinical course than non-CF bronchiectasis. Despite this recognized difference, the underpinnings of respiratory biology support a common pathogeneses of the anatomic deformations of bronchiectasis. This article reviews the observed manifestations among the related diseases of bronchiectasis and CF and discusses some of their similarities and differences. As more details of the mechanisms of bronchiectasis are unveiled, more parallels among the seemingly disparate causes of CF and non-CF bronchiectasis are recognized. With these insights, more opportunities to halt the vicious circle have become apparent.

The role of airway secretions in COPD: pathophysiology, epidemiology and pharmacotherapeutic options

Often considered an aggravating but otherwise benign component of chronic obstructive pulmonary disease (COPD), airway mucus hypersecretion is now recognised as a potential risk factor for an accelerated loss of lung function in COPD and is a key pathophysiological feature in many patients, particularly those prone to respiratory tract infection. Consequently, it is important to develop drugs that inhibit mucus hypersecretion in these susceptible patients. Conventional therapy including anticholinergics, beta2-adrenoceoptor agonists, alone or in combination with corticosteroids, mucolytics and macrolide antibiotics are not entirely or consistently effective in inhibiting airway mucus hypersecretion in COPD. Novel pharmacotherapeutic targets are being investigated, including inhibitors of nerve activity (e.g., BK(Ca) channel activators), tachykinin receptor antagonists, epoxygenase inducers (e.g., benzafibrate), inhibitors of mucin exocytosis (e.g., anti-MARCKS peptide and Munc-18B blockers), inhibitors of mucin synthesis and goblet cell hyperplasia (e.g., EGF receptor tyrosine kinase inhibitors, p38 MAP kinase inhibitors, MEK/ERK inhibitors, hCACL2 blockers and retinoic acid receptor-alpha antagonists), inducers of goblet cell apoptosis (e.g., Bax inducers or Bcl-2 inhibitors), and purinoceptor P(2Y2) antagonists to inhibit mucin secretion or P(2Y2) agonists to hydrate secretions. However, real and theoretical differences delineate the mucus hypersecretory phenotype in COPD from that in other hypersecretory diseases of the airways. More information is required on these differences to identify therapeutic targets pertinent to COPD which, in turn, should lead to rational design of anti-hypersecretory drugs for specific treatment of airway mucus hypersecretion in COPD.

Effect of corticosteroids and neuropeptides on the expression of defensins in bovine tracheal epithelial cells

Susceptibility to bacterial pneumonia in cattle is enhanced by stressors such as transportation, weaning, and commingling, which trigger a physiologic stress response resulting in elevated levels of endogenous corticosteroids and catecholamines. To determine the effect of neuroendocrine mediators on the expression of innate defense peptides in the lung, bovine tracheal epithelial cells were exposed to dexamethasone, catecholamines, acetylcholine, or substance P, and then beta-defensin expression was quantified using real-time reverse transcription-PCR. Basal expression of tracheal antimicrobial peptide (TAP) mRNA was not affected by any of the mediators tested. However, induction of TAP expression by lipopolysaccharide was significantly inhibited by pretreatment with dexamethasone. Bronchial biopsy specimens from dexamethasone-treated calves had significantly lower expression of TAP and lingual antimicrobial peptide (LAP) mRNA than saline-treated controls following 48 h of treatment. Lipopolysaccharide-elicited neutrophil recruitment was enhanced in the lungs of dexamethasone-treated calves compared to saline-treated controls. These findings indicate that modulation of epithelial antimicrobial peptide expression is one mechanism through which corticosteroids and stress may impair innate pulmonary defenses.

Local administration of 2% trimecaine affects the content of fucosylated glycoconjugates in goblet cells in rabbit tracheal epithelium

The proportion of fucosylated glycoconjugate-containing rabbit tracheal goblet cells after intratracheal application of trimecaine was studied to evaluate its possible unfavourable effects. This lapine model is comparable with diagnostic findings in humans because airway epithelia in humans and rabbits are similar; tracheal epithelium is also practically identical to bronchial epithelium in both species. Local trimecaine anaesthesia caused a proportional decrease in percentage of the tracheal goblet cells containing both alpha(1-2)- and alpha(1-6)-, alpha(1-3)- and alpha(1-4)-fucosylated glycoconjugates as revealed 10 min postexposure using lectin histochemistry. In previous studies, only mild ultrastructural damage to the airway's epithelium was revealed, but a conspicuous decrease in sialylated glycoconjugate-containing tracheal goblet cells and the dominance of acidic sulphated glycoconjugates were observed as after-effects of the same treatment. Glycoconjugate changes can influence the inner environment of airways (e.g. viscoelastic properties of the airways' mucus and mucosal barrier functions) and thus the patient's defence barriers in airways may be weakened. Concurrently, the histochemical properties of goblet cells can be altered in bronchoscopic specimens. Since trimecaine is widely used as local anaesthesia in airways in bronchoscopy, it is necessary to heed these aforementioned effects.

Expression of the chloride channel CLC-K in human airway epithelial cells

Airway submucosal gland function is severely disrupted in cystic fibrosis (CF), as a result of genetic mutation of the cystic fibrosis transmembrane conductance regulator (CFTR), an apical membrane Cl(-) channel. To identify other Cl(-) channel types that could potentially substitute for lost CFTR function in these cells, we investigated the functional and molecular expression of Cl(-) channels in Calu-3 cells, a human cell line model of the submucosal gland serous cell. Whole cell patch clamp recording from these cells identified outwardly rectified, pH- and calcium-sensitive Cl(-) currents that resemble those previously ascribed to ClC-K type chloride channels. Using reverse transcription polymerase chain reaction, we identified expression of mRNA for ClC-2, ClC-3, ClC-4, ClC-5, ClC-6, ClC-7, ClC-Ka, and ClC-Kb, as well as the common ClC-K channel beta subunit barttin. Western blotting confirmed that Calu-3 cells express both ClC-K and barttin protein. Thus, Calu-3 cells express multiple members of the ClC family of Cl(-) channels that, if also expressed in native submucosal gland serous cells within the CF lung, could perhaps act to partially substitute lost CFTR function. Furthermore, this work represents the first evidence for functional ClC-K chloride channel expression within the lung.

Mucin glycosylation changes in cystic fibrosis lung disease are not manifest in submucosal gland secretions

SMG (submucosal gland) secretions are a major component of the airway surface liquid, are associated with innate immunity in the lung, and have been reported to be altered in lung disease. Changes in lung mucosal glycosylation have been reported in CF (cystic fibrosis), which may be responsible for differential bacterial binding to glycosylated components in the lung mucosa and hence increased pre-disposition to pulmonary infection. Glycoproteomic analysis was performed on SMG secretions collected from explanted bronchial tissue of subjects with severe lung disease, with and without CF, and controls without lung disease. Mucins MUC5B and MUC5AC were shown to be the dominant high-molecular-mass glycoprotein components, with a minor non-mucin glycoprotein component, gp-340, also present. Oligosaccharides containing blood-group determinants corresponding to subjects' blood type were abundant on MUC5B/MUC5AC, as were Lewis-type epitopes and their sialylated analogues, which are ligands for pathogens and leucocytes. No significant differences were found in the glycosylation of MUC5B/MUC5AC or gp-340 between CF and non-CF subjects with severe lung disease, implying that CF does not influence SMG secretion mucin glycosylation in end-stage lung disease. There were also no significant differences found in the glycosylation of these components in severe lung disease compared with non-diseased lungs. This suggests that previously reported changes in the glycosylation of respiratory glycoconjugates in CF, and other pulmonary conditions, are not due to the glycosylation of components in SMG secretions, but may involve other secretions, responses or extracellular factors.

Regulation of mucin expression: mechanistic aspects and implications for cancer and inflammatory diseases

Mucins are large multifunctional glycoproteins whose primary functions are to protect and lubricate the surfaces of epithelial tissues lining ducts and lumens within the human body. Several lines of evidence also support the involvement of mucins in more complex biological processes such as epithelial cell renewal and differentiation, cell signaling, and cell adhesion. Recent studies have uncovered the role of select mucins in the pathogenesis of cancer, underscoring the importance of a detailed knowledge about mucin biology. Under normal physiological conditions, the production of mucins is optimally maintained by a host of elaborate and coordinated regulatory mechanisms, thereby affording a well-defined pattern of tissue-, time-, and developmental state-specific distribution. However, mucin homeostasis may be disrupted by the action of environmental and/or intrinsic factors that affect cellular integrity. This results in an altered cell behavior that often culminates into a variety of pathological conditions. Deregulated mucin production has indeed been associated with numerous types of cancers and inflammatory disorders. It is, therefore, crucial to comprehend the underlying basis of molecular mechanisms controlling mucin production in order to design and implement adequate therapeutic strategies for combating these diseases. Herein, we discuss some physiologically relevant regulatory aspects of mucin production, with a particular emphasis on aberrations that pertain to pathological situations. Our views of the achievements, the conceptual and technical limitations, as well as the future challenges associated with studies of mucin regulation are exposed.

Treatment of airway mucus hypersecretion

Airway mucus hypersecretion is now recognized as a key pathophysiological feature in many patients with asthma, chronic obstructive pulmonary disease (COPD) and cystic fibrosis. Consequently, it is important to develop drugs that inhibit mucus hypersecretion in these susceptible patients. Conventional therapies, including anticholinergics, ss2-adrenoceptor agonists, corticosteroids, mucolytics and macrolide antibiotics, have variable efficacy in inhibiting airway mucus hypersecretion, and are less effective in COPD than in asthma. Novel pharmacotherapeutic targets are being investigated, including inhibitors of nerve activity (e.g. large conductance calcium-activated potassium, BKCa, channel activators), tachykinin receptor antagonists, epoxygenase inducers (e.g. benzafibrate), inhibitors of mucin exocytosis (e.g. anti-myristoylated alanine-rich C kinase substrate (MARCKS), peptide and Munc-18B blockers), inhibitors of mucin synthesis and goblet cell hyperplasia (e.g. epidermal growth factor (EGF), receptor tyrosine kinase inhibitors, p38 mitogen-activated protein (MAP), kinase inhibitors, MAP kinase kinase/extracellular signal-regulated kinase (MEK/ERK), inhibitors, human calcium-activated chloride (hCACL2), channel blockers and retinoic acid receptor-a antagonists), inducers of goblet cell apoptosis (e.g. Bax inducers or Bcl-2 inhibitors), and purinoceptor P(2Y2) antagonists to inhibit mucin secretion or P(2Y2) agonists to hydrate secretions. However, real and theoretical differences delineate the mucus hypersecretory phenotype in asthma from that in COPD. More information is required on these differences to identify specific therapeutic targets which, in turn, should lead to rational design of anti-hypersecretory drugs for treatment of airway mucus hypersecretion in asthma and COPD.

Airway glandular development and stem cells

Submucosal glands in the lung play important roles in several hypersecretory lung disease processes, including chronic bronchitis, asthma, and cystic fibrosis. In this context, submucosal glands undergo abnormal growth and differentiation through processes that are poorly understood. To better understand the pathophysiological mechanisms that lead to submucosal gland hypertrophy and hyperplasia in the adult human lung, efforts have been made to dissect the molecular signals and cell types responsible for normal submucosal gland development in the airway. Such studies have revealed a close relationship between progenitor?stem cell phenotypes in the surface airway epithelia and submucosal glands, and thus it has been suggested that submucosal glands serve as a protective niche for surface airway epithelial stem cells. Furthermore, the pluripotent progenitor cells that exist in the surface airway epithelium, which have the capacity to differentiate into ciliated, secretory, intermediate, and basal cells, also have a developmental capacity for submucosal glands. This putative adult stem cell compartment of the airway epithelium has been the focus of research attempting to identify molecular markers for signaling pathways that control stem cell phenotypes and their capacity for proliferation and differentiation following airway injury.

Intercellular growth factor signaling and the development of mouse tracheal submucosal glands

To provide a genetic framework for investigating changes in airway submucosal gland function in human respiratory disease, we have investigated their counterparts in normal and mutant mice. We describe their morphogenesis in relation to the expression of genes encoding conserved intercellular signaling pathways. Submucosal glands are severely reduced in number and size in mice heterozygous for Fgf10. Glands are completely absent in mice lacking Ectodysplasin (Eda) and Edaradd (Eda receptor adaptor protein), members of the tumor necrosis (TNF) superfamily of signaling factors. Furthermore, components of the Eda and closely related pathways are transcribed throughout the respiratory system in the adult mouse. Finally, the temporal and spatial pattern of Bmp4 expression suggests that it may control submucosal gland development and homeostasis. Taken together, our observations have important implications for the better understanding of the submucosal gland remodeling that occurs in human respiratory disease.

Wnt-responsive element controls Lef-1 promoter expression during submucosal gland morphogenesis

Regulated expression of lymphoid enhancer factor 1 (Lef-1) plays an obligatory role in the transcriptional control of epithelial bud formation during airway submucosal gland and mammary gland development. However, regions of the Lef-1 promoter required for spatial and temporal regulation during glandular development have yet to be defined. We hypothesized that a previously reported 110-bp Wnt-responsive element (WRE) in the Lef-1 promoter, which can be induced by Wnt-3a/beta-catenin signals, may also play a role in regulating Lef-1 expression during airway and mammary gland development. Here we show that the Lef-1 promoter is also responsive to Wnt-1 signals in both airway and mammary epithelial cell lines. To better understand the importance of the WRE in dynamically regulating Lef-1 promoter activation in these two types of epithelia in vivo, we utilized LacZ reporter transgenic mice to evaluate the significance of Wnt-responsive sequences in the Lef-1 promoter during glandular bud formation. A 2.5-kb Lef-1 promoter fragment partially reproduced endogenous Lef-1 expression patterns in a subset of cell types involved in both mammary gland and submucosal glandular bud development. Interestingly, removal of the 110-bp WRE from the Lef-1 promoter ablated expression in nasal and tracheal submucosal glandular buds while having no significant effect on developmental expression in mammary glandular buds. These findings suggest that Wnt regulation of the Lef-1 promoter at the WRE may play an important role during airway submucosal glandular bud formation.

The effects of intravenously administered methylxanthines on the proportion of goblet cells containing fucosylated glycoconjugates in rabbit tracheal epithelium

The proportion of goblet cells containing various fucosylated glycoconjugates was evaluated with the use of lectin histochemistry in rabbit tracheal epithelium at 15 and 30 min after intravenous administration of either aminophylline (Syntophyllin) or a mixture of etophylline and theophylline (Oxantil). Methylxanthine derivatives are nonspecific inhibitors of phosphodiesterases that are used to treat bronchial asthma; the proportion of fucosylated glycoconjugates strongly affects rheologic properties of respiratory tract mucus. It is concluded that administration of Syntophyllin dramatically lowered the proportion of goblet cells containing fucosylated glycoconjugates in rabbit tracheal epithelium, especially at 30 min after exposure. This decrease was strongest in the levels of alpha(1-2)-fucosylated glycoconjugates. Therefore, Syntophyllin substantially altered the composition and viscoelastic properties of mucus of the upper respiratory tract. The vasodilator Oxantil exerted less pronounced changes in the proportion of goblet cells, but the strongest effect was again found in the levels of alpha(1-2)-fucosylated glycoconjugates.

Pulmonary mucus: Pediatric perspective

Airway mucus hypersecretion is a clinical feature of a number of childhood diseases, including asthma and bronchitis-associated conditions. However, compared with adults, there is relatively scarce information concerning mucus pathophysiology in respiratory diseases in children. The available evidence indicates many similarities between adult and childhood respiratory hypersecretory conditions, including goblet-cell hyperplasia and submucosal gland hypertrophy, and airway mucus plugging in asthma. Consequently, it is likely that treatments that are effective in adults would be effective in children. Numerous therapeutic targets are linked to the pathophysiology of airway mucus hypersecretion in experimental models and adults with respiratory disease. Whether or not these same targets are relevant in children is for the most part unclear. These targets include the inflammatory cells mediating the inflammatory response that generates the hypersecretory phenotype, and highly specific cellular elements such as epidermal growth factor receptor tyrosine kinase and calcium-activated chloride (CACL) channels. Identification of these factors is linked with the development of different classes of pharmacotherapeutic molecules directed at these targets. Compounds with a broader spectrum of anti-inflammatory activity are likely to be more effective than compounds with restricted activity. However, certain highly specific targets, such as human CACL1 channels, appear to be strongly associated with the development of an airway hypersecretory phenotype. Data from current clinical trials in adults with blockers of these specific targets are awaited with great interest. The hope is that, if effective, pediatric trials with these compounds could be initiated with a view to alleviation of the clinical impact of airway mucus hypersecretion in children. A significant challenge to the therapeutic progression of these new compounds is effective delivery to the airways in children, with the research effort into development of new compounds matched by advances in inhaler design.

Innate secretory immunity in response to laboratory stressors that evoke distinct patterns of cardiac autonomic activity

OBJECTIVE

Most infections begin at mucosal surfaces. These surfaces are covered by the secretory proteins of the exocrine glands (eg, the salivary, respiratory, and gastrointestinal glands), which provide a first line of innate defense. The release of these secretory proteins is under neuroendocrine control and thus, in theory, sensitive to modulation by psychosocial stress. This was empirically tested by measuring the salivary secretion of cystatin S, lactoferrin, alpha-amylase, the mucins MUC5B and MUC7, and total salivary protein in response to stressors known to evoke distinct patterns of cardiac autonomic activity.

METHODS

Thirty-two undergraduate volunteers were each subjected to two laboratory stressors and a control condition. Stressors were an active coping memory test and a passive coping video presentation showing surgical procedures. In the control condition participants viewed a didactic video presentation.

RESULTS

The stressors evoked the expected distinct patterns of cardiac autonomic activity. The memory test produced a strong increase in sympathetic activity (evidenced by a shortened preejection period), and a decrease in cardiac parasympathetic activity (evidenced by a decrease in heart rate variability). This active coping response was associated with an enhanced secretion ( microg/min, controlling for salivary flow rate) of MUC7, lactoferrin, alpha-amylase, and total salivary protein. Conversely, the surgical video produced an increase in cardiac vagal tone and a modest increase in sympathetic activity. This passive coping response was associated with an enhanced secretion of all proteins studied. These secretory responses were generally larger than the secretory responses during the active coping memory test. Correlation analyses indicated that for both stressors autonomic and cardiovascular reactivity was positively associated with an enhanced and prolonged secretory activity.

CONCLUSIONS

Stress-induced modulation of innate secretory immunity may be a contributing factor in the observed relationship between stress and susceptibility to infectious diseases. We further propose a more differentiated approach to acute stress by distinguishing among stressors with distinct autonomic nervous system effects.

Molecular cloning and characterization of spurt, a human novel gene that is retinoic acid-inducible and encodes a secretory protein specific in upper respiratory tracts

Retinoids, such as all-trans-retinoic acid, play an essential role in the regulation of airway epithelial cell growth, differentiation, and gene expression. Using cDNA microarray, we identified a clone, DD4, that contains the cDNA of a novel gene, spurt (secretory protein in upper respiratory tracts) that was significantly induced by all-trans-retinoic acid in primary cultured human tracheobroncheal epithelia. Two alternatively spliced spurt transcripts of 1090 and 1035 base pairs exist that contain the same open reading frame expressing a 256-amino acid peptide. The full-length spurt cDNA sequence spans a genomic DNA fragment of 7,313 bp, and the gene is located on chromosome 20q11.21. spurt mRNA is notably expressed at high levels in human nasal, tracheal, and lung tissues. In situ hybridization demonstrated that spurt message is often present in secretory cell types. The human spurt gene product is a secretory protein that contains a distinct signal peptide sequence in its first 19 amino acids. Mono-specific antibodies were generated to characterize spurt expression. Our data demonstrate that spurt is secreted onto the apical side of primary human airway epithelial cultures and is present in clinical sputum samples. spurt gene expression is higher in sputum and tissue samples obtained from patients with chronic obstructive lung disease. Our results provide the cloning and characterization of this tissue-specific novel gene and its possible relationship with airway diseases.

The effects of intravenously administered methylxanthine preparations on the glycoconjugate composition of goblet cells in rabbit tracheal epithelium

Effects of methylxanthine derivatives, which are inhibitors of phosphodiesterases I-IV used against bronchial asthma, on the composition of glycoconjugates in goblet cells were evaluated in tracheal epithelium of rabbits at 15 and 30 min after intravenous administration of aminophylline (Syntophyllin) and a mixture of etophylline and theophylline (Oxantil), respectively. Percentages of tracheal goblet cells containing neutral, total acidic, sulphated acidic, and sialylated acidic glycoconjugates were assessed using both conventional and lectin histochemistry. No significant changes were found in both experimental groups at 15 min after exposure. A significant decrease in percentage of alpha(2-3)-sialylated glycoconjugate-containing goblet cells occurred at 30 min after administration of Syntophyllin only. It is concluded that the mucus composition of tracheal goblet cells has been affected by the bronchospasmolytic drug Syntophyllin but not by the vasodilator drug Oxantil.

Effects of ion transport inhibitors on MCh-mediated secretion from porcine airway submucosal glands

Submucosal glands secrete macromolecules and liquid that are essential for normal airway function. To determine the mechanisms responsible for airway gland secretion and the interaction between gland secretion and epithelial ion transport, studies were performed in porcine tracheal epithelia by using the hillocks and Ussing techniques. No significant baseline gland fluid flux (J(G)) was measured by the hillocks technique after 3 min, and the epithelia had an average potential difference of 7.5 +/- 0.5 mV (lumen negative) with a short-circuit current of 73 +/- 4 microA/cm(2), as measured by the Ussing technique. The secretagogue methacholine induced concentration-dependent increases in J(G) after 3 min from 0.003 microl. min(-1). cm(-2) at 0.1 microM to 0.41 +/- 0.04 microl. min(-1). cm(-2) at 1,000 microM, with a 0.9 +/- 0.1 mV hyperpolarization of the epithelium at 1,000 microM. When the epithelium was pretreated for 3 min with the sodium channel blocker amiloride, the methacholine (1,000 microM)-induced J(G) increased to 0.67 +/- 0.09 microl. min(-1). cm(-2), and the hyperpolarization increased to 2.2 +/- 0.5 mV over the amiloride-pretreated level. When pretreated for 3 min with the chloride channel blocker diphenylamine-2-carboxylic acid, the methacholine (1,000 microM)-induced J(G) was inhibited to 0.20 +/- 0.06 microl. min(-1). cm(-2), and the methacholine-induced hyperpolarization was abolished. These data indicate that, in porcine airways, methacholine-induced J(G) may be increased by inhibition of sodium absorption and decreased by inhibition of chloride secretion.

Mucin apoprotein expression in COPD

Mucins, which are complex glycoproteins that provide the viscoelastic properties of mucus that are essential for the protection of the airways, are characterized by a variable-number tandem repeats (VNTR) region that may undergo alternate splicing during transcription. Such transcripts may yield multiple proteins via diverse post-translational modifications involving glycosylation (within each VNTR). Fifteen distinct mucin genes have been identified, with several mapping to chromosomal clusters (ie, 7q22 and 11p15.5), possibly having evolved by gene duplication. The deduced protein sequences can be subdivided into both membrane-associated mucins and secreted mucins. Membrane-associated mucins consist of cytoplasmic, transmembrane, and extracellular domains. The membrane-associated mucins MUC1, MUC4, and MUC11 have been localized to the lung. In addition to VNTRs, secreted mucins possess repeated cysteine-rich D-domains (which are important in polymerization). Secreted mucins that are localized to the lung include MUC2 (in cells with and without secretory granules), MUC5AC (in surface and submucosal mucous cells), MUC5B and MUC8 (in submucosal mucous cells), and MUC7 (in submucosal serous cells). Currently, little is known about the regulation of mucins in COPD patients. Recent studies with acrolein and cigarette smoke have suggested that MUC5AC is inducible (accompanied by epidermal growth factor [EGF] ligand formation and the activation of EGF receptor-dependent pathways), whereas MUC5B is constitutively expressed (increasing through gland enlargement). Similarly, little is known about the genetic determinants that control mucus hypersecretion, but preliminary findings in animal models suggest that intrastrain differences in acrolein-induced mucin formation are amenable to genetic analysis. As our understanding of the functional genomics of mucin biology increases, further clinical targets and therapeutic strategies are likely to emerge.

Mucoactive drugs for asthma and COPD: any place in therapy?

Airway mucus hypersecretion is a clinical and pathophysiological feature of a number of severe respiratory conditions, including asthma and chronic obstructive pulmonary disease (COPD). The importance of mucus hypersecretion to the morbidity and mortality of asthma is acknowledged, whereas in COPD it appears to affect only certain groups of patients, particularly the elderly and those prone to chest infections. Treatment with compounds that alter mucus is perceived as a therapeutic option, in particular in continental Europe, and numerous compounds have been developed and are available for clinical use worldwide. However, acceptance (or otherwise) of these drugs in guidelines for management of asthma or COPD has been hampered by lack of information from well designed clinical trials. In addition, the mechanism of action of most of these drugs is unknown and is it likely that any beneficial effects are due to activities other than, or in addition to, effects on mucus. Current information indicates that the most effective use of mucolytic drugs is long-term therapy for reduction of exacerbations of COPD. Cost-effective treatment would be in patients with poor lung function who have frequent or prolonged exacerbations or are repeatedly admitted to hospital.