Respiratory tract mucin genes and mucin glycoproteins in health and disease

Role of matrix metalloproteinase-9 in lipopolysaccharide-induced mucin production in human airway epithelial cells

Matrix metalloproteinases (MMPs) have been found to be involved in the pathogenesis of inflammatory airway diseases. However, the role of MMPs in lipopolysaccharide (LPS)-induced mucin overproduction remains unclear. We explored the role of MMP-9 in LPS-induced MUC5AC production and the effect of doxycycline on MUC5AC production. The study showed that LPS induced transcription and protein expression of both MMP-9 and MUC5AC in NCI-H292 cells and in primary human epithelial cells, and the increased MUC5AC level were associated with increased MMP-9 transcripts, protein and activity. However, the increase of MUC5AC transcripts and protein were diminished after cells had been treated with doxycycline, MMP-9 siRNA or EGFR inhibitor. Doxycycline inhibited MMP-9 transcription, protein production and activity, while LPS-induced increase of MMP-9 transcription was inhibited by EGFR inhibitor, p38 MAPK and JNK inhibitor. The LPS-induced phosphorylation of p38 MAPK and JNK were inhibited by EGFR inhibitor. These results suggested that LPS-induced MUC5AC production may be partially mediated by MMP-9 activation and EGFR-p38 MAPK/JNK signaling pathway. Doxycycline may play a therapeutic role in LPS-induced mucus hypersecretion.

Mucins, mucus, and sputum

Normal airway mucus lines the epithelial surface and provides an important innate immune function by detoxifying noxious molecules and by trapping and removing pathogens and particulates from the airway via mucociliary clearance. The major macromolecular constituents of normal mucus, the mucin glycoproteins, are large, heavily glycosylated proteins with a defining feature of tandemly repeating sequences of amino acids rich in serine and threonine, the linkage sites for large carbohydrate structures. The mucins are composed of two major families: secreted mucins and membrane-associated mucins. Membrane-associated mucins have been reported to function as cell surface receptors for pathogens and to activate intracellular signaling pathways. The biochemical and cellular functions for secreted mucin glycoproteins have not been definitively assigned. In contrast to normal mucus, sputum production is the hallmark of chronic inflammatory airway diseases such as asthma, chronic bronchitis, and cystic fibrosis (CF). Sputum has altered macromolecular composition and biophysical properties which vary with disease, but unifying features are failure of mucociliary clearance, resulting in airway obstruction, and failure of innate immune properties. Mucin glycoprotein overproduction and hypersecretion are common features of chronic inflammatory airway disease, and this has been the underlying rationale to investigate the mechanisms of mucin gene regulation and mucin secretion. However, in some pathologic conditions such as CF, airway sputum contains little intact mucin and has increased content of several macromolecules including DNA, filamentous actin, lipids, and proteoglycans. This review will highlight the most recent insights on mucus biology in health and disease.

Tension amplification in molecular brushes in solutions and on substrates

Molecular bottle-brushes are highly branched macromolecules with side chains densely grafted to a long polymer backbone. The brush-like architecture allows focusing of the side-chain tension to the backbone and its amplification from the pico-Newton to nano-Newton range. The backbone tension depends on the overall molecular conformation and the surrounding environment. Here we study the relation between the tension and conformation of the molecular brushes in solutions, melts, and on substrates. In solutions, we find that the backbone tension in dense brushes with side chains attached to every backbone monomer is on the order of f(0)N(3/8) in athermal solvents, f(0)N(1/3) in theta solvents, and f(0) in poor solvents and melts, where N is the degree of polymerization of side chains, f(0) approximately equal k(B)T/b is the maximum tension in side chains, b is the Kuhn length, k(B) is Boltzmann's constant, and T is the absolute temperature. Depending on the side chain length and solvent quality, molecular brushes develop tension on the order of 10-100 pN, which is sufficient to break hydrogen bonds. Significant amplification of tension occurs upon adsorption of brushes onto a substrate. On a strongly attractive substrate, maximum tension in the brush backbone is approximately f(0)N, reaching values on the order of several nano-Newtons, which exceeds the strength of a typical covalent bond. At low grafting density and high spreading parameter, the cross-sectional profile of an adsorbed molecular brush is approximately rectangular with a thickness approximately b (A/S)1/2, where A is the Hamaker constant, and S is the spreading parameter. At a very high spreading parameter (S > A), the brush thickness saturates at monolayer approximately b. At a low spreading parameter, the cross-sectional profile of adsorbed molecular brush has a triangular tent-like shape. In the cross-over between these two opposite cases, covering a wide range of parameter space, the adsorbed molecular brush consists of two layers. Side chains in the lower layer gain surface energy due to the direct interaction with the substrate, while the second layer spreads on the top of the first layer. Scaling theory predicts that this second layer has a triangular cross-section with width R approximately N(3/5) and height h approximately N(2/5). Using self-consistent field theory we calculate the cap profile y(x) = h(1 - x2/R2)2, where x is the transverse distance from the backbone. The predicted cap shape is in excellent agreement with both computer simulation and experiment.

CREB mediates prostaglandin F2alpha-induced MUC5AC overexpression

Mucus secretion is an important protective mechanism for the luminal lining of open tubular organs, but mucin overproduction in the respiratory tract can exacerbate the inflammatory process and cause airway obstruction. Production of MUC5AC, a predominant gel-forming mucin secreted by airway epithelia, can be induced by various inflammatory mediators such as prostaglandins. The two major prostaglandins involved in inflammation are PGE(2) and PGF(2alpha). PGE(2)-induced mucin production has been well studied, but the effect of PGF(2alpha) on mucin production remains poorly understood. To elucidate the effect and underlying mechanism of PGF(2alpha) on MUC5AC production, we investigated the signal transduction of PGF(2alpha) associated with this effect using normal human tracheobronchial epithelial cells. Our results demonstrated that PGF(2alpha) induces MUC5AC overproduction via a signaling cascade involving protein kinase C, ERK, p90 ribosomal S6 protein kinase, and CREB. The regulation of PGF(2alpha)-induced MUC5AC expression by CREB was further confirmed by cAMP response element-dependent MUC5AC promoter activity and by interaction between CREB and MUC5AC promoter. The abrogation of all downstream signaling activities via suppression of each signaling molecule along the pathway indicates that a single pathway from PGF(2alpha) receptor to CREB is responsible for inducing MUC5AC overproduction. As CREB also mediates mucin overproduction induced by PGE(2) and other inflammatory mediators, our findings have important clinical implications for the management of airway mucus hypersecretion.

Hyaluronan fragments/CD44 mediate oxidative stress-induced MUC5B up-regulation in airway epithelium

Mucus hypersecretion with elevated MUC5B mucin production is a pathologic feature in many airway diseases associated with oxidative stress. In the present work, we evaluated MUC5B expression in airways and in primary cultures of normal human bronchial epithelial (NHBE) cells, as well as the mechanisms involved in its regulation. We found that oxidative stress generated by cigarette smoke or reactive oxygen species (ROS) induces MUC5B up-regulation in airway epithelium from smokers and in NHBE cells, respectively. We have previously shown that ROS-induced MUC5AC expression in NHBE cells is dependent on hyaluronan depolymerization and epidermal growth factor receptor (EGFR)/mitogen-activated protein kinase (MAPK) activation. Since hyaluronan fragments can activate MAPK through the hyaluronan receptor CD44, and CD44 heterodimerizes with EGFR, we tested whether ROS and/or hyaluronan fragments induce MUC5B mRNA and protein expression through CD44/EGFR. We found that ROS promotes CD44/EGFR interaction, EGFR/MAPK activation, and MUC5B up-regulation that are prevented by blocking CD44 and/or EGFR. These results were mimicked by hyaluronan fragments. In summary, our results show that oxidative stress in vivo (cigarette smoke) or in vitro (ROS) induces MUC5B up-regulation. This ROS-induced MUC5B expression requires CD44 as well as EGFR and MAPK activation. In addition, we also provide evidence that hyaluronan fragments are sufficient to induce CD44/EGFR interaction and downstream signaling that results in MUC5B up-regulation, suggesting that hyaluronan depolymerization during inflammatory responses could be directly involved in the induction of mucus hypersecretion.

Enhanced goblet cell hyperplasia in HDC knockout mice with allergic airway inflammation

BACKGROUND

Histamine is known to have immunoregulatory roles in allergic reactions through histamine receptor 1 (H1R), H2R, H3R and H4R. However, its role in goblet cell hyperplasia in the airways of asthma patients is yet to be clarified.

OBJECTIVE

This study was designed to examine the role of histamine in goblet cell hyperplasia using histamine-deficient mice (Hdc-/- mice) with allergic airway inflammation.

METHODS

Wild-type and Hdc-/- C57BL/6 mice were sensitized with ovalbumin (OVA). After a 2-week exposure to OVA, goblet cell hyperplasia was evaluated. Cell differentials and cytokines in BALF were analyzed. The mRNA levels of MUC5AC and Gob-5 gene were determined quantitatively.

RESULTS

The number of eosinophils in BALF increased in both the sensitized wild-type mice and Hdc-/- mice with OVA inhalation. In addition, the numbers of alveolar macrophages and lymphocytes in BALF increased significantly in the sensitized Hdc-/- mice with OVA inhalation compared to the wild-type mice under the same conditions. The concentrations of Interleukin-4 (IL-4), IL-5, IL-13, Interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha) and IL-2 in the BALF all increased significantly in both groups compared to those exposed to saline. In particular, the concentration of TNF-alpha in the Hdc-/- mice exposed to OVA was significantly higher than that in the wild-type mice under the same conditions. The mRNA levels of Gob-5 and MUC5AC, and the ratio of the goblet cells in the airway epithelium significantly increased in Hdc-/- mice exposed to OVA compared to wild-type mice.

CONCLUSIONS

These results suggested that histamine may play a regulatory role in goblet cell hyperplasia in allergic airway inflammation.

Suppression of prostaglandin E2-induced MUC5AC overproduction by RGS4 in the airway

The mechanism by which E-prostanoid (EP) receptor is critically involved in PGE(2)-induced mucin 5AC (MUC5AC) gene expression in the airway has been unclear. Furthermore, there have been little reports regarding the negative regulatory mechanism and/or proteins that affect PGE(2)-induced MUC5AC overproduction. In the present study, we found that PGE(2) induced MUC5AC gene expression in a dose-dependent manner (EC(50): 73.31 +/- 3.13 nM) and that the EP(2/4)-specific agonist, misoprostol, increased MUC5AC mRNA level, whereas the EP(1/3)-specific agonist, sulprostone, had no effect. Interestingly, the cAMP concentration (685.1 +/- 14.9 pM) of the EC(50) value of EP(4)-mediated cAMP production was much higher than that of EP(2) (462.33 +/- 23.79 pM), suggesting that EP(4) has higher sensitivity to PGE(2) compared with EP(2). Moreover, PGE(2)-induced Muc5ac overproduction was much increased in regulator of G protein signaling (Rgs) 4 knockout (KO) mice compared with wild-type mice at both transcriptional and translational levels, and it was dramatically suppressed in Rgs4 KO mice that had been infected with lentivirus expressing RGS4 (lenti::RGS4) compared with lentivirus expressing enhanced green fluorescent protein (lenti::eGFP). Finally, we demonstrate that PGE(2) can induce MUC5AC overproduction via the EP(4) receptor and that RGS4 may have suppressive effects in controlling MUC5AC overexpression in the airway. These findings may provide a molecular paradigm for the development of novel drugs for respiratory diseases.

The penetration of fresh undiluted sputum expectorated by cystic fibrosis patients by non-adhesive polymer nanoparticles

Highly viscoelastic and adhesive sputum has precluded efficient nanoparticle-based drug and gene delivery to the lungs of patients with cystic fibrosis (CF). We sought to determine whether nanoparticles coated with non-mucoadhesive polymers could penetrate CF sputum, and to use these "muco-inert particles" (MIPs) as non-destructive nanoprobes to characterize the sputum microstructure. Particles as large as 200 nm in diameter that were densely coated with low MW polyethylene glycol (PEG) moved through undiluted CF sputum with average speeds up to 90-fold faster than similarly-sized uncoated particles. On the other hand, the transport of both coated and uncoated 500 nm particles was strongly hindered. The local viscosity of sputum, encountered by the fastest 10% of 200 nm MIPs, was only 5-fold higher than that of water, whereas the bulk viscosity was 10,000-fold higher at low shear rates. Using measured transport rates of various sized MIPs combined with an obstruction-scaling model, we determined that the average 3D mesh spacing of CF sputum is approximately 140+/-50 nm (range: 60-300 nm). Taken together, these results demonstrate that nanoparticles up to 200 nm in diameter that do not adhere to CF sputum can move rapidly through this critical barrier by accessing pores that are filled with a low viscosity fluid. The results also offer hope that desperately needed sputum-penetrating drug- and gene-carrier nanoparticles can be developed for CF.

Chronic intermittent mechanical stress increases MUC5AC protein expression

Increased abundance of mucin secretory cells is a characteristic feature of the epithelium in asthma and other chronic airway diseases. We showed previously that the mechanical stresses of airway constriction, both in the intact mouse lung and a cell culture model, activate the epidermal growth factor receptor (EGFR), a known modulator of mucin expression in airway epithelial cells. Here we tested whether chronic, intermittent, short-duration compressive stress (30 cm H(2)O) is sufficient to increase the abundance of MUC5AC-positive cells and intracellular mucin levels in human bronchial epithelial cells cultured at an air-liquid interface. Compressive stress applied for 1 hour per day for 14 days significantly increased the percentage of cells staining positively for MUC5AC protein (22.0 +/- 3.8%, mean +/- SD) relative to unstimulated controls (8.6 +/- 2.6%), and similarly changed intracellular MUC5AC protein levels measured by Western and slot blotting. The effect of compressive stress was gradual, with significant changes in MUC5AC-positive cell numbers evident by Day 7, but required as little as 10 minutes of compressive stress daily. Daily treatment of cells with an EGFR kinase inhibitor (AG1478, 1 muM) significantly but incompletely attenuated the response to compressive stress. Complete attenuation could be accomplished by simultaneous treatment with the combination of AG1478 and a transforming growth factor (TGF)-beta(2) (1 microg/ml)-neutralizing antibody, or with anti-TGF-beta(2) alone. Our findings demonstrate that short duration episodes of mechanical stress, representative of those occurring during bronchoconstriction, are sufficient to increase goblet cell number and MUC5AC protein expression in bronchial epithelial cells in vitro. We propose that the mechanical environment present in asthma may fundamentally bias the composition of airway epithelial lining in favor of mucin secretory cells.

Sequential microarray to identify timing of molecular responses to Haemonchus contortus infection in sheep

Anthelmintics are currently the most common method of worm control. The emergence of worms with multiple-drug resistance and issues of residues in the food chain make alternative parasite control measures a priority. To develop improved and sustainable methods for controlling Haemonchus contortus such as genetic selection of resistant sheep, a better understanding of the host-parasite relationship is required. A trial was undertaken using sheep surgically implanted with abomasal fistulas to enable sequential biopsy of the abomasal mucosa during trickle infection with two strains of H. contortus. These were ivermectin-resistant CAVR and ivermectin-sensitive McMaster. From a gross parasitology perspective, this approach enabled the effect of developing immunity to be observed on both the establishment and maturation of two CAVR doses within and between groups. Since the only difference in parasite treatment between the groups was the staggering of the two CAVR doses, microarray results from biopsies taken on the same day in different groups were combined and compared between different biopsy dates to observe differential gene transcription over time. Differential gene transcription was detected by comparing transcription in our array data between different biopsy dates using a low P value screen (P<0.01) and by compiling a list of 82 immunoparasitology-related genes and examining transcription in this list with a higher P value screen (P<0.05). Our microarray data were validated in silico by comparison with intelectin 2, trefoil factor 3, calcium activated chloride channel and mucin 5 from other gene transcription studies and with phenotypic data such as the response by gammadelta T cells and immunoglobulins to H. contortus. The first four genes are involved in non-specific responses to infection and mucosal healing. These were upregulated at the early time points and intelectin 2 remained prominent throughout the trial. As the trial progressed, immunoglobulin genes became strongly upregulated. These included IgCgamma IgG2a heavy chain constant region, IGHE immunoglobulin heavy constant epsilon and IGHM immunoglobulin heavy constant mu.

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.

Scanning electron and light microscopy study of the cervical mucus in women with polycystic ovary syndrome

Two types of cervical mucus are recognized, oestrogenic and gestagenic. These are constituted by different subtypes, and their characteristics change depending on variations in the hormonal levels and on the existence of several pathologies. Our aim was to identify the ultrastructure and crystallization characteristics of the cervical mucus in women suffering from polycystic ovary syndrome, and to compare these characteristics with those of normal control women. Cervical mucus samples were taken from 10 women, 4 control group women (with normal ovulatory menstrual cycles) and 6 suffering from polycystic ovary syndrome (2 with ovulatory and 4 with anovulatory cycles). This mucus was characterized according to its ultrastructure and crystallization. The type of mucus obtained was related to the levels of oestradiol and progesterone present when the samples were taken. As regards mucus ultrastructure, differences were found between the control women and those with polycystic ovary syndrome and anovulatory menstrual cycles. Such variations were evident in the type of mesh and the average diameter of the mucus pores. Mucus crystallization in control women showed the usual oestrogenic disposition: fern-like (L, P2), rectilinear (S) or a hexagonal structure (P6). On the other hand, in women with polycystic ovary syndrome, indefinite mucus crystallizations were found, as well as crystallization patches resembling oestrogenic and gestagenic-like mucus. This study shows that the ultrastructure and crystallization characteristics of the cervical mucus in polycystic ovary syndrome women are different from those of control women. The latter would be dependent on their levels of oestradiol and progesterone.

Regulator of G-protein signaling 4 suppresses LPS-induced MUC5AC overproduction in the airway

Mucus overproduction and airway obstruction are common features in airway mucosal inflammation. The mechanism by which LPS induces MUC5AC overexpression, however, has not been fully explored. The aims of this study were twofold: first, to examine the ATP-dependent mechanism by which LPS induces MUC5AC gene expression, and second, to identify specific molecules that could suppress LPS-induced MUC5AC expression at a G-protein-coupled receptor level. Here, we suggest that LPS from Pseudomonas aeruginosa induces MUC5AC overproduction by both an ATP-dependent pathway and an ATP-independent pathway. In addition, we showed that Regulator of G-protein signaling (RGS) 4 plays as a suppressor for ATP-induced MUC5AC expression by interacting with G alpha q in a GTP-dependent manner in vivo. These results give additional insights into the molecular mechanism of negative regulation of mucin overproduction and enhance our understanding of mucus hypersecretion during airway mucosal inflammation.

Functions of ocular surface mucins in health and disease

PURPOSE OF REVIEW

The purpose of the present review is to describe new concepts on the role of mucins in the protection of corneal and conjunctival epithelia and to identify alterations of mucins in ocular surface diseases.

RECENT FINDINGS

New evidence indicates that gel-forming and cell surface-associated mucins contribute differently to the protection of the ocular surface against allergens, pathogens, extracellular molecules, abrasive stress, and drying.

SUMMARY

Mucins are high-molecular weight glycoproteins characterized by their extensive O-glycosylation. Major mucins expressed by the ocular surface epithelia include cell surface-associated mucins MUC1, MUC4, MUC16, and the gel-forming mucin MUC5AC. Recent advances using functional assays have allowed the examination of their roles in the protection of corneal and conjunctival epithelia. Alterations in mucin and mucin O-glycan biosynthesis in ocular surface disorders, including allergy, nonautoimmune dry eye, autoimmune dry eye, and infection, are presented.

Rhinovirus-induced major airway mucin production involves a novel TLR3-EGFR-dependent pathway

Mucociliary clearance is a critical innate defense system responsible for clearing up invading pathogens including bacteria and virus. Although the right amount of mucus is good, excessive mucus causes airway obstruction and tends to precipitate disease symptoms. Rhinovirus (RV) is a common cold virus that causes asthma and chronic obstructive pulmonary disease exacerbation. Mucus overproduction has been linked to the pathogenesis of RV-induced diseases and disease exacerbations. However, the molecular mechanism is not clear. In this study, using one of the major airway mucin-MUC5AC as marker, we found that both major and minor groups of RV induced mucin production in primary human epithelial cells and cell line. RV1A (a minor group of RV) could induce mucous cell metaplasia in vivo. Viral replication was needed for RV-induced mucin expression, and this induction was also dependent on TLR3, suggesting the involvement of double-stranded (ds) RNA signaling. Indeed, dsRNA alone could also induce mucin expression. TLR3-mediated mucin induction was negatively regulated by MyD88, and only partially dependent on TRIF, which suggests a departure from well-documented TLR3 signaling paradigm that mediates inflammatory and other innate defense gene inductions. In addition, TLR3 signaling activated epidermal growth factor receptor (EGFR) through inductions of the expression of EGFR ligands (transforming growth factor-alpha and amphiregulin), which in turn activated EGFR-ERK signaling and mucin expression through an autocrine/paracrine loop. This novel coupling of antiviral defense machinery (i.e., TLR3) and major epithelial proliferation/repair pathway (i.e., EGFR) might play an important role in viral-induced airway remodeling and airway disease exacerbation.

Corticosteroid therapy increases membrane-tethered while decreases secreted mucin expression in nasal polyps

BACKGROUND

Mucus hypersecretion is a hallmark of nasal polyposis (NP). Corticosteroids (CS) are first-line treatment for NP, decreasing their size and inflammatory component. However, their effect on mucin production is not well-understood. The aim of this (pilot) study was to investigate CS effect on mucin expression in NP.

METHODS

Patients were randomized in control (n = 9) and treatment (oral prednisone for 2 weeks and intranasal budesonide for 12 weeks; n = 23) groups. Nasal polyposis from nonasthmatic (NP; n = 13), aspirin-tolerant (NP-ATA; n = 11) and aspirin-intolerant (NP-AIA; n = 8) asthmatics were studied. Nasal polyposis biopsies were obtained before (w0) and after 2 (w2) and 12 (w12) weeks of CS treatment. Secreted (MUC5AC, MUC5B and MUC8) and membrane-tethered (MUC1, MUC4) mucins (immunohistochemistry) and goblet cells (Alcian blue-periodic acid Schiff) were quantified in both epithelium and glands. Rhinorrea and nasal obstruction were also assessed.

RESULTS

At w2, steroids increased MUC1 (from 70 to 97.5) and MUC4 (from 80 to 100) in NP-ATA patients' epithelium compared with baseline (w0). At w12, steroids decreased MUC5AC (from 40 to 5) and MUC5B (from 45 to 2.5) in NP-ATA patients' epithelium and glands, respectively, compared with baseline. No mucin presented significant changes in NP-AIA patients. MUC5AC and MUC5B expression correlated with goblet and mucous cell numbers, respectively, and MUC5AC also with rhinorrea score.

CONCLUSIONS

These results suggest: (i) CS up-regulate membrane (MUC1, MUC4) while down-regulate secreted (MUC5AC, MUC5B) mucins; (ii) there exists a link between secreted mucin expression and goblet cell hyperplasia; and (iii) NP from AIA may develop resistance to CS treatment.

Role of the coagulation system in allergic inflammation in the upper airways

Thrombin has been detected and demonstrated to play a role in the airways of patients with bronchial asthma, but its role in the upper airways including during allergic rhinitis is unknown. This study was conducted to explore whether thrombin is presence in the upper airways and, if so, whether it affects mucin secretion. Fifteen patients with allergic rhinitis were enrolled in the clinical study; primary nasal septum epithelial cells and normal bronchial epithelial cells were used for in vitro evaluation, and rats as animal models. Significant concentrations of thrombin were found in nasal secretion after allergic provocation in allergic patients, and thrombin and its agonistic receptor peptide induced significant secretion of mucin in primary nasal cells and normal bronchial epithelial cells as compared to non-stimulated cells. Increased mucosubstance secretion in septum epithelial cells was also induced after nasal instillation of thrombin in rats. Further, the anticoagulant, activated protein C, significantly inhibited thrombin-induced mucin secretion from septum epithelial cells in rats. The results of this study suggest that activation of the coagulation system occurs during the allergic response and that thrombin plays a crucial role in the regulation of mucin production in the upper airways.

MKP1 regulates the induction of MUC5AC mucin by Streptococcus pneumoniae pneumolysin by inhibiting the PAK4-JNK signaling pathway

Mucosal epithelial cells in the respiratory tract act as the first line of host innate defense against inhaled microbes by producing a range of molecules for clearance. In particular, epithelial mucins facilitate the mucociliary clearance by physically trapping the inhaled microbes. Up-regulation of mucin production thus represents an important host innate defense response against invading microbes. Excess mucin production, however, overwhelms the mucociliary clearance, resulting in defective mucosal defenses. Thus, tight regulation of mucin production is critical for maintaining an appropriate balance between beneficial and detrimental outcomes. Among various mechanisms, negative regulation plays an important role in tightly regulating mucin production. Here we show that the PAK4-JNK signaling pathway acted as a negative regulator for Streptococcus pneumoniae pneumolysin-induced MUC5AC mucin transcription. Moreover pneumolysin also selectively induced expression of MKP1 via a TLR4-dependent MyD88-TRAF6-ERK signaling pathway, which inhibited the PAK4-JNK signaling pathway, thereby leading to up-regulation of MUC5AC mucin production to maintain effective mucosal protection against S. pneumoniae infection. These studies provide novel insights into the molecular mechanisms underlying the tight regulation of mucin overproduction in the pathogenesis of airway infectious diseases and may lead to development of new therapeutic strategies.

Asthma induction in mice leads to appearance of alpha2-3- and alpha2-6-linked sialic acid residues in respiratory goblet-like cells

Allergic asthmatic inflammation in mice was induced by sensitization with ovalbumin and lipopolysaccharide from Escherichia coli and visualized in the airways of asthmatic mice by spatial and temporal changes of carbohydrates containing sialic acid residues. Immunohistochemistry was used to demonstrate binding of lectins and antibodies that detect alpha2-3- and alpha2-6-linked sialic acid residues. After sensitization and challenge, the histology of the lung changed markedly, and goblet-like cells appeared, most likely caused by Clara cell metaplasia. Normal Clara cells showed no reaction after incubation with the sialic acid detecting agents, while the goblet-like cells expressed both alpha2-3- and alpha2-6-linked sialic acid residues in the asthmatic animals. The lectins but not the antibodies reacted with intestinal goblet cells. Instead, an antibody recognizing a disialoganglioside, stained large mononuclear cells in the submucosa, indicating a difference in sialylation between goblet cells in the intestine and goblet-like cells developed from Clara cells.

cAMP-responding element-binding protein and c-Ets1 interact in the regulation of ATP-dependent MUC5AC gene expression

Exogenous ATP activates purinoreceptors on the cell surface that regulate diverse cellular functions, including mucous cell secretion in the respiratory epithelium. In this study, ATP increased MUC5AC mRNA in primary human nasal epithelial cells and in NCI-H292 pulmonary adenocarcinoma cells in vitro. ATP-induced MUC5AC mRNA was mediated by phospholipase Cbeta3. A dominant-negative mutation in the PDZ binding domain of PLCbeta3 inhibited ATP-induced MUC5AC gene expression. ATP sequentially activated the phosphorylation of Akt, ERK1/2, p38, RSK1, and cAMP-responding element-binding protein (CREB) in a protein kinase C-independent manner. ATP-induced MUC5AC mRNA levels were regulated by CREB via direct interaction with c-Ets1 on the MUC5AC gene promoter (located -938 to -930). Effects of CREB and c-Ets1 were additive. Inhibition of either CREB or c-Ets1 inhibited ATP-induced MUC5AC gene expression. Stimulation with ATP caused the direct binding of CREB and c-Ets1 to the MUC5AC promoter, increasing the phosphorylation of c-Ets1. Chromatin immunoprecipitation assays demonstrated that in the presence of ATP, both c-Ets1 and CREB bound to the MUC5AC promoter. The effects of exogenous ATP on MUC5AC gene expression are mediated by a complex regulatory cascade controlling interactions between CREB and c-Ets1 that bind to a promoter element in the MUC5AC gene enhancing MUC5AC gene transcription. ATP-dependent activation of MUC5AC gene expression via CREB-c-Ets1 may contribute to mucous cell hypersecretion associated with common respiratory disorders.

Distribution of mucins and antimicrobial substances lysozyme and lactoferrin in the laryngeal subglottic region

The subglottic region of the larynx is of high clinical relevance with regard to infections and malignancies. Little is known about the distribution of mucins and antimicrobial substances in this area. In this study, we have investigated the mucin distribution in the normal subglottis of the larynx. Moreover, we analysed the expression of lysozyme and lactoferrin in this area. Therefore, the subglottic region of 34 larynges was investigated immunohistochemically with different antibodies to mucins and antimicrobial substances. The epithelium reacted positive with antibodies to mucins MUC1 (34/34), 5AC (26/34), 5B (10/34), 7 (8/34), 8 (10/34) and 16 (19/34); submucosal glands were positive to mucins MUC1 (34/34), 5B (10/34), 7 (8/34), and 16 (19/34); high columnar epithelial cells and serous parts of subepithelial seromucous glands were also positive for lysozyme (34/34) and lactoferrin (34/34). The results show that human subglottic epithelium and subepithelial submucosal glands produce a broad spectrum of mucins that is almost comparable with that in other areas of the respiratory tract. We hypothesize that the mucin diversity of the subglottis has an impact on positive functional consequences during vocal production and antimicrobial defence. This antimicrobial defence is supported by synthesis and secretion of antimicrobial substances such as lysozyme and lactoferrin. Moreover, knowledge of the observed distribution pattern of mucins in the subglottis can be a useful tool for a classification of subglottic laryngeal carcinomas.

Analysis of mucins: role in laboratory diagnosis

Mucins are high molecular weight glycoproteins with complex oligosaccharide side chains attached to the apomucin protein backbone byO-glycosidic linkage; they are found in crude mucus gels that protect epithelial surfaces in the major tracts of the body and as transmembrane proteins expressed on the apical cell surface of glandular and ductal epithelia of various organs. Changes in the sequence of glycosylation of mucins in different settings generate a variety of epitopes in the oligosaccharide side chains of mucins, including newly expressed blood-group antigens, distinguishing between normal and diseased states. Tumour-associated epitopes on mucins and their antigenicity make them suitable as immunotargets on malignant epithelial cells and their secretions, creating a surge of interest in mucins as diagnostic and prognostic markers for various diseases, and even influencing the design of mucin-based vaccines. This review discusses the emerging roles of mucins such as MUC1 and MUC4 in cancer and some other diseases, and stresses how underglycosylated and truncated mucins are exploited as markers of disease and to monitor widespread metastasis, making them useful in patient management. Furthermore the type, pattern and amount of mucin secreted in some tissues have been considered in the classification and terminology of neoplasia and in specific organs such as the pancreas. These factors have been instrumental in pathological classification, diagnosis and prognostication of neoplasia.

MUC1 mucin: a peacemaker in the lung

MUC1 is a membrane-tethered mucin expressed on the surface of epithelial cells lining mucosal surfaces. Recent studies have begun to elucidate the physiologic function of MUC1 in the airways, pointing to an antiinflammatory role that is initiated late in the course of bacterial infection and is mediated through inhibition of TLR signaling. These new findings have great potential for clinical applications in controlling excessive and prolonged lung inflammation. This review briefly summarizes the protein structural features of MUC1 relevant to its function, the discovery of its antiinflammatory properties, and potential directions for future avenues of study.

Endogenous IL-11 signaling is essential in Th2- and IL-13-induced inflammation and mucus production

IL-11 and IL-11 receptor (R)alpha are induced by Th2 cytokines. However, the role(s) of endogenous IL-11 in antigen-induced Th2 inflammation has not been fully defined. We hypothesized that IL-11, signaling via IL-11Ralpha, plays an important role in aeroallergen-induced Th2 inflammation and mucus metaplasia. To test this hypothesis, we compared the responses induced by the aeroallergen ovalbumin (OVA) in wild-type (WT) and IL-11Ralpha-null mutant mice. We also generated and defined the effects of an antagonistic IL-11 mutein on pulmonary Th2 responses. Increased levels of IgE, eosinophilic tissue and bronchoalveolar lavage (BAL) inflammation, IL-13 production, and increased mucus production and secretion were noted in OVA-sensitized and -challenged WT mice. These responses were at least partially IL-11 dependent because each was decreased in mice with null mutations of IL-11Ralpha. Importantly, the administration of the IL-11 mutein to OVA-sensitized mice before aerosol antigen challenge also caused a significant decrease in OVA-induced inflammation, mucus responses, and IL-13 production. Intraperitoneal administration of the mutein to lung-specific IL-13-overexpressing transgenic mice also reduced BAL inflammation and airway mucus elaboration. These studies demonstrate that endogenous IL-11R signaling plays an important role in antigen-induced sensitization, eosinophilic inflammation, and airway mucus production. They also demonstrate that Th2 and IL-13 responses can be regulated by interventions that manipulate IL-11 signaling in the murine lung.

Sialic acids in human health and disease

The surfaces of all vertebrate cells are decorated with a dense and complex array of sugar chains, which are mostly attached to proteins and lipids. Most soluble secreted proteins are also similarly decorated with such glycans. Sialic acids are a diverse family of sugar units with a nine-carbon backbone that are typically found attached to the outermost ends of these chains. Given their location and ubiquitous distribution, sialic acids can mediate or modulate a wide variety of physiological and pathological processes. This review considers some examples of their established and newly emerging roles in aspects of human physiology and disease.

Neutrophil elastase (NE) and NE inhibitors: canonical and noncanonical functions in lung chronic inflammatory diseases (cystic fibrosis and chronic obstructive pulmonary disease)

Proteases and antiproteases have multiple important roles both in normal homeostasis and during inflammation. Antiprotease molecules may have developed in a parallel network, consisting of "alarm" and "systemic" inhibitors. Their primary function was thought until recently to mainly prevent the potential injurious effects of excess release of proteolytic enzymes, such as neutrophil elastase (NE), from inflammatory cells. However, recently, new potential roles have been ascribed to these antiproteases. We will review "canonical" and new "noncanonical" functions for these molecules, and more particularly, those pertaining to their role in innate and adaptive immunity (antibacterial activity and biasing of the adaptive immune response).

Expression of conserved mucin domains by epithelial tissues in various mammalian species

Mucins are related to infectious and non-infectious diseases in Veterinary and Human Medicine. MUC1 mucin is a transmembrane glycoprotein expressed on the apical surface of human epithelia while MUC5AC is the predominant secreted mucin expressed in human gastric epithelium and goblet cells of lung and eyes. MUC5AC C-terminus cysteine rich regions and the cytoplasmic tail of MUC1 domains are conserved among several mammalian species.

OBJECTIVE

to compare the expression of MUC1 and MUC5AC mucins in mammalian epithelia. CT33 anti-MUC1 cytoplasmic tail (MUC1CT) polyclonal antibody and 45M1 anti-MUC5AC monoclonal antibody were employed. By immunohistochemistry, MUC1CT was expressed in most tissues while MUC5AC was restricted to gastric surface epithelium and goblet cells from trachea and lung. By western blot, MUC1CT showed a band at approximately 35 kDa in most tissues; MUC5AC revealed bands at >180 kDa in stomach and lung secretions from rat, cat, pig and cow. When rat MUC5AC was immunoprecipitated, a band at about 180 kDa was obtained.

Identification of pendrin as a common mediator for mucus production in bronchial asthma and chronic obstructive pulmonary disease

Excessive production of airway mucus is a cardinal feature of bronchial asthma and chronic obstructive pulmonary disease (COPD) and contributes to morbidity and mortality in these diseases. IL-13, a Th2-type cytokine, is a central mediator in the pathogenesis of bronchial asthma, including mucus overproduction. Using a genome-wide search for genes induced in airway epithelial cells in response to IL-13, we identified pendrin encoded by the SLC26A4 (PDS) gene as a molecule responsible for airway mucus production. In both asthma and COPD mouse models, pendrin was up-regulated at the apical side of airway epithelial cells in association with mucus overproduction. Pendrin induced expression of MUC5AC, a major product of mucus in asthma and COPD, in airway epithelial cells. Finally, the enforced expression of pendrin in airway epithelial cells in vivo, using a Sendai virus vector, rapidly induced mucus overproduction in the lumens of the lungs together with neutrophilic infiltration in mice. These findings collectively suggest that pendrin can induce mucus production in airway epithelial cells and may be a therapeutic target candidate for bronchial asthma and COPD.

Regulation of airway mucin gene expression

Mucins are important components that exert a variety of functions in cell-cell interaction, epidermal growth factor receptor signaling, and airways protection. In the conducting airways of the lungs, mucins are the major contributor to the viscoelastic property of mucous secretion, which is the major barrier to trapping inhaled microbial organism, particulates, and oxidative pollutants. The homeostasis of mucin production is an important feature in conducting airways for the maintenance of mucociliary function. Aberrant mucin secretion and accumulation in airway lumen are clinical hallmarks associated with various lung diseases, such as asthma, chronic obstructive pulmonary disease, cystic fibrosis, emphysema, and lung cancer. Among 20 known mucin genes identified, 11 of them have been verified at either the mRNA and/or protein level in airways. The regulation of mucin genes is complicated, as are the mediators and signaling pathways. This review summarizes the current view on the mediators, the signaling pathways, and the transcriptional units that are involved in the regulation of airway mucin gene expression. In addition, we also point out essential features of epigenetic mechanisms for the regulation of these genes.

Salivary mucins inhibit antibacterial activity of the cathelicidin-derived LL-37 peptide but not the cationic steroid CSA-13

OBJECTIVES

Cationic antimicrobial peptides (CAPs) are the effector molecules of innate immunity, similar in potency to classic antibiotics that function in the first-line of defence against infectious agents. The purpose of this study was to investigate the effects of negatively charged mucins on the antibacterial activity of the positively charged cathelicidin LL-37 peptide, its synthetic analogue WLBU2 and the antimicrobial cationic steroid CSA-13.

METHODS

Mucin, DNA, F-actin and hCAP-18/LL-37 in saliva samples were evaluated by microscopy or immunoblotting. Bacterial killing assays and determination of MICs were used to determine bactericidal activity. Binding of rhodamine-B-labelled LL-37 peptide to mucin was fluorimetrically assessed.

RESULTS

Microscopic evaluation of saliva after addition of rhodamine-B-labelled LL-37 showed localization similar to that observed after the addition of a specific mucin-binding lectin. Immunoblotting confirmed the presence of hCAP-18/LL-37 in saliva samples and LL-37 peptide bound to isolated submaxillary gland mucin-coated plates. Mucin/LL-37 binding was partially prevented by treatment of mucin with neuraminidase, indicating involvement of sialic acid moieties. Decreased LL-37 and WLBU2 antibacterial activity was observed in the presence of mucin or dialysed human saliva, whereas CSA-13 antibacterial activity was significantly resistant to inhibition by mucins.

CONCLUSIONS

This study shows that the antibacterial LL-37 peptide and its synthetic analogue WLBU2 are inhibited by salivary mucin and that the cationic steroid CSA-13 retains most of its function in the presence of an equal amount of mucin or saliva.

Progress in allergy signal research on mast cells: the role of histamine in goblet cell hyperplasia in allergic airway inflammation - a study using the Hdc knockout mouse

Although histamine is a central mediator in the immediate allergic reaction, its role in goblet cell hyperplasia in the airway of asthma is not completely understood. This study was designed to examine the role of histamine in goblet cell hyperplasia using histamine-deficient mice (Hdc(-/-) mice) with allergic airway inflammation. Wild-type and Hdc(-/-) C57BL/6 mice were sensitized with ovalbumin (OVA). After two-week exposure to OVA, goblet cell hyperplasia was evaluated. Cell differentials in BALF were analyzed. The mRNAs level of MUC5AC and Gob-5 gene were quantitatively determined. The number of eosinophils in BALF increased in both the wild-type mice and Hdc(-/-) mice; however, their ratio in Hdc(-/-) mice was significantly lower than that in the wild-type mice. The mRNA levels of Gob-5 and MUC5AC and the ratio of the goblet cells in the airway epithelium were significantly increased in Hdc(-/-) mice exposed to OVA compared to the wild-type mice under the same condition. These results suggested that histamine may play a regulatory role in goblet cell hyperplasia in allergic airway inflammation.

Proteases and cystic fibrosis

Cystic fibrosis is the most common, inherited fatal disease in Caucasians. The major cause of morbidity and mortality is chronic lung disease due to infection and inflammation in the airways leading to bronchiectasis and respiratory failure. The signature pathologic features of CF lung disease including abnormal mucus obstructing airways, chronic infection with Staphylococcus aureus, Pseudomonas aeruginosa and other gram negative bacteria, and a robust neutrophil-dominant airway inflammation, are exacerbated by unopposed proteases present at high concentrations in the ASL. There is strong evidence that proteases, particularly neutrophil elastase, contribute to the pathology of CF by impairing mucociliary clearance, interfering with innate immune functions, and perpetuating neutrophilic inflammation. The mechanisms employed by proteases to impact airway function in CF will be reviewed.

Expression of mucin 3 and mucin 5AC in arthritic synovial tissue

OBJECTIVE

Rheumatoid arthritis (RA) is a chronic inflammatory disease that is characterized by hypertrophy of the synovial tissue, leukocyte infiltration, angiogenesis, and ultimately joint destruction. Mucins (MUCs) are a family of heavily glycosylated proteins that protect epithelial membranes and are used as ligands for cell adhesion. MUC gene expression has been found to be altered in many cancers and inflammatory states. This study was undertaken to examine its expression in synovial tissue (ST) and role in arthritis.

METHODS

We performed immunohistochemistry, Western blotting, and reverse transcriptase-polymerase chain reaction to determine expression patterns of MUC1, MUC2, MUC3, and MUC5AC in RA, osteoarthritic (OA), and normal human ST.

RESULTS

MUC3 was expressed in synovial lining cells, macrophages, and fibroblasts. Significantly more RA (n=12) and OA (n=13) synovial lining cells expressed MUC3 than did normal synovial lining cells (n=7) (22% and 24% versus 0.4%, respectively; P<0.05). Additionally, macrophages in RA and OA ST expressed significantly more MUC3 than did macrophages in normal ST (50% and 51% versus 10%, respectively; P<0.05). MUC5AC was expressed at low levels in synovial lining cells, macrophages, and endothelial cells in RA and OA ST, and was barely expressed in normal ST. MUC1 and MUC2 proteins were not detected in ST. Messenger RNA (mRNA) for MUC3 and MUC5AC was detected in ST, and mRNA for MUC3 was detected in cultured ST fibroblasts.

CONCLUSION

These data demonstrate up-regulated MUC expression by ST cells and suggest a novel role of MUC3 and MUC5AC in the pathogenesis of arthritis.

Cigarette smoke activates NF kappa B and induces Muc5b expression in mouse middle ear cells

OBJECTIVES

Cigarette smoke exposure is a significant risk factor in the development of otitis media (OM). Nuclear factor kappa B (NF-kappa B) is a ubiquitous transcription factor known to mediate cigarette smoke effects on gene regulation in multiple cell types. The MUC5B mucin gene contains several putative NF-kappa B sites in its promoter and is the predominant mucin expressed in human OM. We hypothesized that in vitro stimulation of a recently developed model system, murine middle ear epithelial cells (MEEC), with cigarette smoke condensate (CSC) activates NF-kappa B and subsequently induces Muc5b gene expression.

METHODS

Luciferase reporter assays, electromobility shift assays (EMSA), and quantitative microplate transcription factor assays (TFA) were performed to evaluate NF-kappaB activation with CSC in immortalized murine MEEC (mMEEC). Reverse transcriptase polymerase chain reaction (RT-PCR) assays and quantitative real time RT-PCR were performed to determine whether time course CSC stimulation upregulates Muc5b mRNA levels in differentiated mMEEC. Luciferase reporter assays were performed to determine whether CSC activates the Muc5b promoter.

RESULTS

Reporter assays, EMSA, and TFA demonstrated three- to five-fold dose-dependent activation of NF-kappa B with CSC in mMEEC. CSC stimulation likewise increased Muc5b mRNA abundance and induced reporter activity 1.8- to 4.8-fold in plasmids containing -556 and -255 base pairs upstream of the Muc5b transcriptional start site in mMEEC.

CONCLUSIONS

CSC activates NF-kappaB in immortalized MEEC. Furthermore, this activation correlates with CSC-induced Muc5b promoter activation and gene expression. Taken together, these results hint that much as in lung cells, the activation of mucins by cigarette smoke is mediated in part by NF-kappa B.

Structure and Function of the Cell Surface (Tethered) Mucins

Cell surface mucins are large transmembrane glycoproteins involved in diverse functions ranging from shielding the airway epithelium against pathogenic infection to regulating cellular signaling and transcription. Although hampered by the relatively recent characterization of cell surface mucins and the difficulties inherent in working with molecules of their size, numerous studies have placed the tethered mucins in the thick of normal and diseased lung physiology. This review focuses on the three best-characterized cell surface mucins expressed in the respiratory tract: MUC1, MUC4, and MUC16.

The airway epithelium is central to the pathogenesis of asthma

Asthma is an inflammatory disorder principally involving the conducting airways and characterised by infiltration of the airway wall with a range of inflammatory cells driven in large part by activation of Th2-type lymphocytes, mast cells and eosinophils. However a key component of asthma is the structural change that involves all of the elements of the airway wall. Here evidence is presented to suggest that the airway epithelium in asthma is fundamentally abnormal with increased susceptibility to environmental injury and impaired repair associated with activation of the epithelial-mesenchymal trophic unit (EMTU). In addition to adopting an activated phenotype, the barrier function of the epithelium is impaired through defective tight junction formation thereby facilitating penetration of potentially toxic or damaging environmental insults. Activated and repairing epithelial cells generate a range of growth factors that are involved in the early life origins of this disease as well as its progression in the form of mucous metaplasia and airway wall remodeling. By placing the epithelium at the forefront of asthma pathogenesis, different approaches to treatment can be devised focused more on protecting vulnerable airways against environmental injury rather than focusing on suppressing airway inflammation or manipulating the immune response.

Advances in mechanisms of repair and remodelling in acute lung injury

BACKGROUND

Acute respiratory distress syndrome (ARDS) is the most severe manifestation of acute lung injury (ALI). In patients who survive the acute injury the process of repair and remodelling may be an independent risk factor determining morbidity and mortality. This review explores recent advances in the field of fibroproliferative ARDS/ALI, with a special emphasis on (a) the primary contributing factors with a focus on cellular and soluble factors, and (b) mechanisms involved in repair and remodelling as they pertain to the importance of cell death, re-population, and matrix deposition.

DISCUSSION

Factors influencing progression to fibroproliferative ARDS vs. resolution and reconstitution of the normal pulmonary parenchymal architecture are poorly understood. Determinants of persistent injury and abnormal repair and remodelling may be profoundly affected by both environmental and genetic factors. Moreover, cumulative evidence suggests that acute inflammation and fibrosis may be in part independent and interactive processes that are autonomously regulated and thus amenable to individual and specific therapy.

CONCLUSIONS

Although our current understanding of these processes is limited by the inability to accurately replicate the complex human physiology in laboratory settings, it has recently become apparent that the process of repair and remodelling begins early in the course of ARDS/ALI and may be determined by the type of pulmonary injury. Understanding the mechanisms leading to and regulating fibroproliferative changes may contribute to the development of novel early therapeutic interventions in ARDS/ALI patients.

Munc13-2-/- baseline secretion defect reveals source of oligomeric mucins in mouse airways

Since the airways of control mouse lungs contain few alcian blue/periodic acid-Schiff's (AB/PAS)+ staining 'goblet' cells in the absence of an inflammatory stimulus such as allergen sensitization, it was surprising to find that the lungs of mice deficient for the exocytic priming protein Munc13-2 stain prominently with AB/PAS under control conditions. Purinergic agonists (ATP/UTP) stimulated release of accumulated mucins in the Munc13-2-deficient airways, suggesting that the other airway isoform, Munc13-4, supports agonist-regulated secretion. Notably, however, not all of the mucins in Munc13-2-deficient airways were secreted, suggesting a strict Munc13-2 priming requirement for a population of secretory granules. AB/PAS+ staining of Munc13-2-deficient airways was not caused by an inflammatory, metaplastic-like response: bronchial-alveolar lavage leucocyte numbers, Muc5ac and Muc5b mRNA levels, and Clara cell ultrastructure (except for increased secretory granule numbers) were all normal. A Muc5b-specific antibody indicated the presence of this mucin in Clara cells of wildtype (WT) control mice, and increased amounts in Munc13-2-deficient mice. Munc13-2 therefore appears to prime a regulated, baseline secretory pathway, such that Clara cell Muc5b, normally secreted soon after synthesis, accumulates in the gene-deficient animals, making them stain AB/PAS+. The defective priming phenotype is widespread, as goblet cells of several mucosal tissues appear engorged and Clara cells accumulated Clara cell secretory protein (CCSP) in Munc13-2-deficient mice. Additionally, because in the human airways, MUC5AC localizes to the surface epithelium and MUC5B to submucosal glands, the finding that Muc5b is secreted by Clara cells under control conditions may indicate that it is also secreted tonically from human bronchiolar Clara cells.

Mechanisms of cigarette smoke-induced COPD: insights from animal models

Cigarette smoke-induced animal models of chronic obstructive pulmonary disease support the protease-antiprotease hypothesis of emphysema, although which cells and proteases are the crucial actors remains controversial. Inhibition of either serine or metalloproteases produces significant protection against emphysema, but inhibition is invariably accompanied by decreases in the inflammatory response to cigarette smoke, suggesting that these inhibitors do more than just prevent matrix degradation. Direct anti-inflammatory interventions are also effective against the development of emphysema, as are antioxidant strategies; the latter again decrease smoke-induced inflammation. There is increasing evidence for autoimmunity, perhaps directed against matrix components, as a driving force in emphysema. There is intriguing but controversial animal model evidence that failure to repair/failure of lung maintenance also plays a role in the pathogenesis of emphysema. Cigarette smoke produces small airway remodeling in laboratory animals, possibly by direct induction of fibrogenic growth factors in the airway wall, and also produces pulmonary hypertension, at least in part through direct upregulation of vasoactive mediators in the intrapulmonary arteries. Smoke exposure causes goblet cell metaplasia and excess mucus production in the small airways and proximal trachea, but these changes are not good models of either chronic bronchitis or acute exacerbations. Emphysema, small airway remodeling, pulmonary hypertension, and mucus production appear to be at least partially independent processes that may require different therapeutic approaches.

Glucocorticoid actions on airway epithelial responses in immunity: functional outcomes and molecular targets

Research on the biology of airway epithelium in the last decades has progressively uncovered the many roles of this cell type during the immune response. Far from the early view of the epithelial layer simply as a passive barrier, the airway epithelium is now considered a central player in mucosal immunity, providing innate mechanisms of first-line host defense as well as facilitating adaptive immune responses. Alterations of the epithelial phenotype are primarily involved in the pathogenesis of allergic airways disease, particularly in severe asthma. Appreciation of the epithelium as target of glucocorticoid therapy has also grown, because of studies defining the pathways and mediators affected by glucocorticoids, and studies illustrating the relevance of the control of the response from epithelium in the overall efficacy of topical and systemic therapy with glucocorticoids. Studies of the mechanism of action of glucocorticoids within the biology of the immune response of the epithelium have uncovered mechanisms of gene regulation involving both transcriptional and posttranscriptional events. The view of epithelium as therapeutic target therefore has plenty of room to evolve, as new knowledge on the role of epithelium in immunity is established and novel pathways mediating glucocorticoid regulation are elucidated.

Epithelium dysfunction in asthma

Although asthma is an inflammatory disorder of the conducting airways involving T(H)2-type T cells, there is increasing evidence for an important role played by the epithelium in orchestrating the inflammatory response by interacting with multiple environmental factors to produce a chronic wound scenario involving tissue injury and aberrant repair. Part of this abnormal response is the consequence of impaired barrier function caused by a primary disruption of epithelial tight junctions that allows inhaled substances to pass more easily into the airway wall to interact with immune and inflammatory cells. Aberrant communication between the damaged and stressed epithelium leads to the generation of growth factors that interact with the underlying mesenchyme to promote airway remodeling responses and a more chronic and persistent inflammatory phenotype. Disordered epithelial function with reduced antioxidant defense and impaired capacity to produce primary IFNs may also account for asthmatic susceptibility to air pollution and respiratory virus infection, respectively. Considering asthma as a disease of impaired barrier function opens new opportunities for therapeutic intervention or prevention by agents that could increase the airways resistance to the inhaled environment rather than suppressing the immune or inflammatory response.

FGF10 maintains distal lung bud epithelium and excessive signaling leads to progenitor state arrest, distalization, and goblet cell metaplasia

BACKGROUND

Interaction with the surrounding mesenchyme is necessary for development of endodermal organs, and Fibroblast growth factors have recently emerged as mesenchymal-expressed morphogens that direct endodermal morphogenesis. The fibroblast growth factor 10 (Fgf10) null mouse is characterized by the absence of lung bud development. Previous studies have shown that this requirement for Fgf10 is due in part to its role as a chemotactic factor during branching morphogenesis. In other endodermal organs Fgf10 also plays a role in regulating differentiation.

RESULTS

Through gain-of-function analysis, we here find that FGF10 inhibits differentiation of the lung epithelium and promotes distalization of the embryonic lung. Ectopic expression of FGF10 in the lung epithelium caused impaired lung development and perinatal lethality in a transgenic mouse model. Lung lobes were enlarged due to increased interlobular distance and hyperplasia of the airway epithelium. Differentiation of bronchial and alveolar cell lineages was inhibited. The transgenic epithelium consisted predominantly of proliferating progenitor-like cells expressing Pro-surfactant protein C, TTF1, PEA3 and Clusterin similarly to immature distal tip cells. Strikingly, goblet cells developed within this arrested epithelium leading to goblet cell hyperplasia.

CONCLUSION

We conclude that FGF10 inhibits terminal differentiation in the embryonic lung and maintains the distal epithelium, and that excessive levels of FGF10 leads to metaplastic differentiation of goblet cells similar to that seen in chronic inflammatory diseases.

Architecture of the large membrane-bound mucins

Epithelial membrane-bound mucins are high molecular mass glycoproteins that may be also secreted or released into the extracellular environment. The genomic and multi-domain organizations of human large epithelial membrane-bound mucins are reviewed here with the purpose to clarify the literature on the subject with the help of mouse sequences. This family of complex molecules contains at least MUC3A, MUC12, MUC17, all organized in a cluster of genes, MUC4 and likely MUC16. In addition, we discuss the splicing events reported for these mucins with an emphasis on the human mucin MUC4.