Airway mucins are the major molecular constituents of mucus. Mucus forms the first barrier to invading organisms in the airways and is an important defense mechanism of the lung. We confirm that mucin concentrations are significantly decreased in airway secretions of subjects with cystic fibrosis (CF) who have chronic Pseudomonas aeruginosa infection. In sputum from CF subjects without a history of P. aeruginosa, we found no significant difference in the mucin concentration compared to mucus from normal controls. We demonstrate that mucins can be degraded by synthetic human neutrophil elastase (HNE) and P. aeruginosa elastase B (pseudolysin) and that degradation was inhibited by serine proteases inhibitors (diisopropyl fluorophosphates [DFP], phenylmethylsulfonyl fluoride [PMSF], and 1-chloro-3-tosylamido-7-amino-2-heptanone HCl [TLCK]). The mucin concentration in airway secretions from CF subjects is similar to that for normal subjects until there is infection by P. aeruginosa, and after that, the mucin concentration decreases dramatically. This is most likely due to degradation by serine proteases. The loss of this mucin barrier may contribute to chronic airway infection in the CF airway.
Multi-functionalized nanoparticles (NPs) have been extensively investigated for their potential in household and commercial products, and biomedical applications. Previous reports have confirmed the cellular nanotoxicity and adverse inflammatory effects on pulmonary systems induced by NPs. However, possible health hazards resulting from mucus rheological disturbances induced by NPs are underexplored. Accumulation of viscous, poorly dispersed, and less transportable mucus leading to improper mucus rheology and dysfunctional mucociliary clearance are typically found to associate with many respiratory diseases such as asthma, cystic fibrosis (CF), and COPD (Chronic Obstructive Pulmonary Disease). Whether functionalized NPs can alter mucus rheology and its operational mechanisms have not been resolved. Herein, we report that positively charged functionalized NPs can hinder mucin gel hydration and effectively induce mucin aggregation. The positively charged NPs can significantly reduce the rate of mucin matrix swelling by a maximum of 7.5 folds. These NPs significantly increase the size of aggregated mucin by approximately 30 times within 24 hrs. EGTA chelation of indigenous mucin crosslinkers (Ca2+ ions) was unable to effectively disperse NP-induced aggregated mucins. Our results have demonstrated that positively charged functionalized NPs can impede mucin gel swelling by crosslinking the matrix. This report also highlights the unexpected health risk of NP-induced change in mucus rheological properties resulting in possible mucociliary transport impairment on epithelial mucosa and related health problems. In addition, our data can serve as a prospective guideline for designing nanocarriers for airway drug delivery applications.
The polymeric mucin component of the intestinal mucus barrier changes during nematode infection to provide not only physical protection but also to directly affect pathogenic nematodes and aid expulsion. Despite this, the direct interaction of the nematodes with the mucins and the mucus barrier has not previously been addressed. We used the well-established Trichuris muris nematode model to investigate the effect on mucins of the complex mixture of immunogenic proteins secreted by the nematode called excretory/secretory products (ESPs). Different regimes of T. muris infection were used to simulate chronic (low dose) or acute (high dose) infection. Mucus/mucins isolated from mice and from the human intestinal cell line, LS174T, were treated with ESPs. We demonstrate that serine protease(s) secreted by the nematode have the ability to change the properties of the mucus barrier, making it more porous by degrading the mucin component of the mucus gel. Specifically, the serine protease(s) acted on the N-terminal polymerising domain of the major intestinal mucin Muc2, resulting in depolymerisation of Muc2 polymers. Importantly, the respiratory/gastric mucin Muc5ac, which is induced in the intestine and is critical for worm expulsion, was protected from the depolymerising effect exerted by ESPs. Furthermore, serine protease inhibitors (Serpins) which may protect the mucins, in particular Muc2, from depolymerisation, were highly expressed in mice resistant to chronic infection. Thus, we demonstrate that nematodes secrete serine protease(s) to degrade mucins within the mucus barrier, which may modify the niche of the parasite to prevent clearance from the host or facilitate efficient mating and egg laying from the posterior end of the parasite that is in intimate contact with the mucus barrier. However, during a TH2-mediated worm expulsion response, serpins, Muc5ac and increased levels of Muc2 protect the barrier from degradation by the nematode secreted protease(s).
Gastrointestinal parasitic worm infections cause significant morbidity, affecting up to a third of the world's populationand their domestic pets and livestock. Mucus, the gel-like material that blankets the surface of the intestine, forms a protective barrier that is an important part of our innate immune system. The whipworm Trichuris is closely associated with the intestinal mucus barrier. The major structural component of this barrier, large glycoproteins known as mucins play a significant role in the expulsion of these worms in a mouse model. Using mice that get longterm chronic infections and others able to expel the worms from the intestine, we uncover a novel role for products secreted by the worms. Enzymes secreted by whipworms can disrupt the mucin network that gives mucus its viscous properties. Moreover, we unravel that worm products are unable to degrade forms of mucins present in the mucus barrier during worm expulsion, suggesting that these enzymes may be released by the worm as part of its regime to improve its niche and survival in the host. However, the host is capable of producing mucins and other protective molecules that protect the mucus barrier from degradation and are detrimental to the viability of the worm.
Purpose of review
Airway mucus plugging has long been recognized as a principal cause of death in asthma. However, molecular mechanisms of mucin overproduction and secretion have not been understood until recently. These mechanisms are reviewed together with ongoing investigations relating them to lung pathophysiology.
Of the five secreted gel-forming mucins in mammals, only MUC5AC and MUC5B are produced in significant quantities in intrapulmonary airways. MUC5B is the principal gel-forming mucin at baseline in small airways of humans and mice, and therefore likely performs most homeostatic clearance functions. MUC5AC is the principal gel-forming mucin upregulated in airway inflammation and is under negative control by forkhead box a2 and positive control by hypoxia inducible factor-1. Mucin secretion is regulated separately from production, principally by extracellular triphosphate nucleotides that bind P2Y2 receptors on the lumenal surface of airway secretory cells, generating intracellular second messengers that activate the exocytic proteins, Munc13-2 and synaptotagmin-2.
Markedly upregulated production of MUC5AC together with stimulated secretion leads to airflow obstruction in asthma. As MUC5B appears to mediate homeostatic functions, it may be possible to selectively inhibit MUC5AC production without impairing airway function. The precise roles of mucin hypersecretion in asthma symptoms such as dyspnea and cough and in physiologic phenomena such as airway hyperresponsiveness remain to be defined.
airway; asthma; mucin; mucous; mucus
Respiratory surfaces are exposed to billions of particulates and pathogens daily. A protective mucus barrier traps and eliminates them via mucociliary clearance (MCC)1,2. However, excessive mucus contributes to transient respiratory infections and to the pathogenesis of numerous respiratory diseases1. MUC5AC and MUC5B are evolutionarily conserved genes that encode structurally related mucin glycoproteins, the principal macromolecules in airway mucus1,3. Genetic variants are linked to diverse lung diseases4-6, but specific roles for MUC5AC and MUC5B in MCC, and the lasting effects of their inhibition, are unknown. Here we show that Muc5b (but not Muc5ac) is required for MCC, for controlling infections in the airways and middle ear, and for maintaining immune homeostasis in the lungs. Muc5b deficiency caused materials to accumulate in upper and lower airways. This defect led to chronic infection by multiple bacterial species, including Staphylococcus aureus, and to inflammation that failed to resolve normally7. Apoptotic macrophages accumulated, phagocytosis was impaired, and IL-23 production was reduced inMuc5b−/− mice. By contrast, in Muc5b transgenic (Tg) mice, macrophage functions improved. Existing dogma defines mucous phenotypes in asthma and chronic obstructive pulmonary disease (COPD) as driven by increased MUC5AC, with MUC5B levels either unaffected or increased in expectorated sputum1,8. However, in many patients, MUC5B production at airway surfaces decreases by as much as 90%9-11. By distinguishing a specific role for Muc5b in MCC, and by determining its impact on bacterial infections and inflammation in mice, our results provide a refined framework for designing targeted therapies to control mucin secretion and restore MCC.
Mucous cell metaplasia is induced in response to harmful insults and provides front-line protection to clear the airway of toxic substances and cellular debris. In chronic airway diseases mucous metaplasia persists and results in airway obstruction and contributes significantly to morbidity and mortality. Mucus hypersecretion involves increased expression of mucin genes, and increased mucin production and release. The past decade has seen significant advances in our understanding of the molecular mechanisms by which these events occur. Inflammation stimulates epidermal growth factor receptor activation and IL-13 to induce both Clara and ciliated cells to transition into goblet cells through the coordinated actions of FoxA2, TTF-1, SPDEF, and GABAAR. Ultimately, these steps lead to up-regulation of MUC5AC expression, and increased mucin in goblet cell granules that fuse to the plasma membrane through actions of MARCKS, SNAREs, and Munc proteins. Blockade of mucus in exacerbations of asthma and chronic obstructive pulmonary disease may affect morbidity. Development of new therapies to target mucus production and secretion are now possible given the advances in our understanding of molecular mechanisms of mucous metaplasia. We now have a greater incentive to focus on inhibition of mucus as a therapy for chronic airway diseases.
mucus; goblet cell; airway epithelium; asthma; COPD
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-α 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.
mucin; airway epithelium; TLR3; rhinovirus
Mucins are gel-forming proteins that are responsible for the characteristic viscoelastic properties of mucus. Mucin overproduction is a hallmark of asthma, but the cellular requirements for airway mucin production are poorly understood. The endoplasmic reticulum (ER) protein anterior gradient homolog 2 (AGR2) is required for production of the intestinal mucin MUC2, but its role in the production of the airway mucins MUC5AC and MUC5B is not established. Microarray data were analyzed to examine the relationship between AGR2 and MUC5AC expression in asthma. Immunofluorescence was used to localize AGR2 in airway cells. Coimmunoprecipitation was used to identify AGR2-immature MUC5AC complexes. Agr2−/− mice were used to determine the role of AGR2 in allergic airway disease. AGR2 localized to the ER of MUC5AC- and MUC5B-producing airway cells and formed a complex with immature MUC5AC. AGR2 expression increased together with MUC5AC expression in airway epithelium from “Th2-high” asthmatics. Allergen-challenged Agr2−/− mice had greater than 50% reductions in MUC5AC and MUC5B proteins compared with allergen-challenged wild-type mice. Impaired mucin production in Agr2−/− mice was accompanied by an increase in the proportion of mucins contained within the ER and by evidence of ER stress in airway epithelium. This study shows that AGR2 increases with mucin overproduction in individuals with asthma and in mouse models of allergic airway disease. AGR2 interacts with immature mucin in the ER and loss of AGR2 impairs allergen-induced MUC5AC and MUC5B overproduction.
asthma; airway epithelium; mucus; endoplasmic reticulum stress; protein disulfide isomerase
The mucus lining of the gastrointestinal tract serves as a protective barrier over the epithelial surface that must be crossed by invading bacteria seeking entry into the mucosa. The gel-forming component of mucus is mucin, a large polymeric glycoprotein. The present study examined the growth of Yersinia enterocolitica (with and without its virulence plasmid) in purified rabbit small intestinal mucin and the ability of bacteria to degrade mucin. Both virulent and nonvirulent organisms showed enhanced growth in mucin-supplemented media compared with unsupplemented media, but only at 37 degrees C and not at 25 degrees C. The effects of mucin were not specific because medium supplemented with bovine serum albumin also enhanced bacterial growth at 37 degrees C. Purified mucin was broken down into lower-molecular-weight components (assessed by monitoring its elution profile on a Sepharose CL-2B column) by plasmid-bearing Y. enterocolitica but not by plasmid-cured organisms. Culturing virulent Y. enterocolitica at 25 degrees C completely suppressed its capacity to degrade mucin, suggesting that this activity depends on plasmid expression. These results were confirmed in similar studies with purified rabbit colonic mucin. Mucin-degrading activity could be demonstrated in spent culture media from virulent Y. enterocolitica incubated at 37 degrees C but not in bacterial membrane preparations. Changes in the elution profiles of small intestinal and colonic mucins exposed to plasmid-bearing Y. enterocolitica at 37 degrees C were consistent with proteolytic depolymerization. The ability to grow well in mucin may help Y. enterocolitica to colonize the intestine, while the production of a mucin-degrading enzyme(s) by plasmid-bearing organisms may assist pathogenic strains to solubilize and penetrate the mucus gel layer.
Background: Mucus plugging of the airways is invariably seen in cases of fatal asthma, mucus production is associated with asthma attacks, and the area of submucosal glands is increased in asthma. Mediators secreted from mast cells and neutrophils can stimulate mucous gland secretion. A study was undertaken to count the mast cells and neutrophils in submucosal glands and to relate cell numbers to the presence of mucus in the airway lumen.
Methods: Cartilaginous airways obtained at necropsy from cases of fatal asthma (n=8), non-fatal asthma (n=8), and control cases (n=8) were examined. Contiguous transverse sections were stained for mast cell tryptase and neutrophil elastase, and with Periodic Acid Schiff solution to identify mucus. Mucous gland area, lumen area, and the percentage of the relaxed lumen area occupied by mucus (mucus occupying ratio, MOR) were measured. Mast cells (intact and degranulated) and neutrophils per area of submucosal gland were calculated.
Results: Compared with controls, the cases of fatal asthma had increased mucous gland area, MOR, percentage of degranulated mast cells, and numbers of neutrophils in the submucosal glands (p<0.05). In cases of non-fatal asthma the MOR and the numbers of mast cells and neutrophils in the submucosal glands were increased (p<0.05). When all cases were pooled together, the MOR correlated with the total number of mast cells (r=0.55, p=0.005) and with the number of degranulated mast cells in the submucosal glands (r=0.51, p=0.013), but not with the number of neutrophils (r=0.21, p=0.121).
Conclusion: These results show that mucous gland area, MOR, and mucous gland inflammation are increased in asthma and that degranulation of mast cells may contribute to secretion of mucus into the lumen in cases of fatal asthma.
Infection with respiratory viruses, including rhinoviruses, influenza virus, and respiratory syncytial virus, exacerbates asthma, which is associated with processes such as airway inflammation, airway hyperresponsiveness, and mucus hypersecretion. In patients with viral infections and with infection-induced asthma exacerbation, inflammatory mediators and substances, including interleukins (ILs), leukotrienes and histamine, have been identified in the airway secretions, serum, plasma, and urine. Viral infections induce an accumulation of inflammatory cells in the airway mucosa and submucosa, including neutrophils, lymphocytes and eosinophils. Viral infections also enhance the production of inflammatory mediators and substances in airway epithelial cells, mast cells, and other inflammatory cells, such as IL-1, IL-6, IL-8, GM-CSF, RANTES, histamine, and intercellular adhesion molecule-1. Viral infections affect the barrier function of the airway epithelial cells and vascular endothelial cells. Recent reports have demonstrated augmented viral production mediated by an impaired interferon response in the airway epithelial cells of asthma patients. Several drugs used for the treatment of bronchial asthma reduce viral and pro-inflammatory cytokine release from airway epithelial cells infected with viruses. Here, I review the literature on the pathogenesis of the viral infection-induced exacerbation of asthma and on the modulation of viral infection-induced airway inflammation.
Background and aims: Mucus released from goblet cells is important in intestinal mucosal defence, and mucin glycoproteins are thought to be major components of mucus. Recently, we identified and cloned another component of human colonic mucus, IgG Fc binding protein (FcγBP). FcγBP is immunologically distinct from known Fcγ receptors and its structure contains repeated cysteine rich unit sequences resembling those present in mucins. In this work, we assessed the tissue distribution of FcγBP, its binding activity in various body fluids, and its ability to inhibit complement mediated haemolysis.
Methods: Immunohistochemical localisation of FcγBP, using monoclonal antibodies against FcγBP (K9 or K17) and labelled IgG, was conducted in various mucin producing tissues: colon, small intestine, stomach, gall bladder, cystic duct, choledochus, bronchus, submandibular gland, conjunctiva, and cervix uteri. The binding activity of FcγBP in mucus extracted from colon, gastric juice, bile, nasal discharges, saliva, sputum, and tears was also examined by immunodotblot and immunoprecipitation using these monoclonal antibodies. Inhibition of complement mediated haemolysis by FcγBP was investigated using sheep red blood cells (SRBC) and anti-SRBC IgG.
Results: The immunohistochemical study revealed that mucin secreting cells in the colon, small intestine, gall bladder, cystic duct, choledochus, bronchus, submandibular gland, and cervix uteri contained FcγBP, and immunodotblot and immunoprecipitation analysis using IgG and monoclonal antibodies demonstrated that the fluids secreted by these cells were capable of binding IgG. Mucin producing cells of the conjunctiva did not express FcγBP molecules or bind to IgG. The surface mucus cells in the stomach were variably positive for FcγBP. Perhaps because of proteolytic degradation, FcγBP in gut lavage fluid did not have IgG binding activity, although this activity was present in the mucus covering the colon. FcγBP suppressed complement mediated haemolysis of SRBC.
Conclusions: FcγBP is widely expressed on mucosal surfaces and in external secretions. It is functionally intact in several fluids. These findings lend support to the concept that FcγBP is an important component of mucosal immunological defences.
IgG Fc binding protein; mucin; mucus; goblet cell; mucosal immunity
Acute exacerbations in allergic asthmatics may lead to impaired ability to clear mucus from the airways, a key factor in asthma morbidity.
The purpose of this study was to determine the effect of inhaled house dust mite challenge on regional deposition of inhaled particles and mucociliary clearance (MCC) in allergic asthmatics.
We used gamma scintigraphy (inhalation of 99mTc -sulfur colloid particles) to measure regional particle deposition and MCC in allergic asthmatics (n=12) 4hr following an inhaled dust mite allergen challenge (Dermatophagoides farinae extract; PDmax = fall in FEV1 of 10%) for comparison to baseline non-challenge measures.
In responders (n=9 PDmax dose), lung function returned to pre-challenge values by 3 hours but was significantly decreased at 6 and 24 hours in 3 of the responders (i.e. late phase response) and induced sputum eosinophils were increased at 24 hours post-challenge (p < 0.05). Responders showed enhanced bronchial airway deposition of inhaled particles (p < 0.05) and slowed clearance from the central lung zone (p < 0.01) at 4 hrs post-challenge compared to baseline (no allergen challenge) that was predicted by the PDmax allergen concentration (r = − 0.70, p < 0.05). The fall in lung function at 24 hours post challenge correlated with reduced MCC from the central lung zone (r = − 0.78, p < 0.02) and PDmax. Non-responders (n=3) had no change in lung function, regional deposition or MCC post-challenge vs. baseline.
Conclusions and clinical relevance
These data suggest that regional deposition and clearance of inhaled particles may be sensitive for detecting mild airway obstruction associated with early and late-phase allergen-induced effects on mucus secretions. The study was listed on clinicaltrials.gov (NCT00448851).
dust-mite allergen; particle inhalation; airway deposition; mucus
Goblet cell metaplasia that causes mucus hypersecretion and obstruction in the airway lumen could be life threatening in asthma and chronic obstructive pulmonary disease patients. Inflammatory cytokines such as IL-13 mediate the transformation of airway ciliary epithelial cells to mucin-secreting goblet cells in acute as well as chronic airway inflammatory diseases. However, no effective and specific pharmacologic treatment is currently available. Here, we investigated the mechanisms by which aldose reductase (AR) regulates the mucus cell metaplasia in vitro and in vivo.
Metaplasia in primary human small airway epithelial cells (SAEC) was induced by a Th2 cytokine, IL-13, without or with AR inhibitor, fidarestat. After 48 h of incubation with IL-13 a large number of SAEC were transformed into goblet cells as determined by periodic acid-schiff (PAS)-staining and immunohistochemistry using antibodies against Mucin5AC. Further, IL-13 significantly increased the expression of Mucin5AC at mRNA and protein levels. These changes were significantly prevented by treatment of the SAEC with AR inhibitor. AR inhibition also decreased IL-13-induced expression of Muc5AC, Muc5B, and SPDEF, and phosphorylation of JAK-1, ERK1/2 and STAT-6. In a mouse model of ragweed pollen extract (RWE)-induced allergic asthma treatment with fidarestat prevented the expression of IL-13, phosphorylation of STAT-6 and transformation of epithelial cells to goblet cells in the lung. Additionally, while the AR-null mice were resistant, wild-type mice showed goblet cell metaplasia after challenge with RWE.
The results show that exposure of SAEC to IL-13 caused goblet cell metaplasia, which was significantly prevented by AR inhibition. Administration of fidarestat to mice prevented RWE-induced goblet cell metaplasia and AR null mice were largely resistant to allergen induced changes in the lung. Thus our results indicate that AR inhibitors such as fidarestat could be developed as therapeutic agents to prevent goblet cell metaplasia in asthma and related pathologies.
Mucus secretions have played a central role in the evolution of multicellular organisms, enabling adaptation to widely differing environments. In vertebrates, mucus covers and protects the epithelial cells in the respiratory, gastrointestinal, urogenital, visual, and auditory systems, amphibian's epidermis, and the gills in fishes. Deregulation of mucus production and/or composition has important consequences for human health. For example, mucus obstruction of small airways is observed in chronic airway diseases, including chronic obstructive pulmonary disease, asthma, and cystic fibrosis. The major protein component in the mucus is a family of large, disulfide-bonded glycoproteins known as gel-forming mucins. These proteins are accumulated in large, regulated secretory granules (the mucin granules) that occupy most of the apical cytoplasm of specialized cells known as mucous/goblet cells. Since mucin oligomers have contour dimensions larger than the mucin granule average diameter, the question arises how these highly hydrophilic macromolecules are organized within these organelles. I review here the intraluminal organization of the mucin granule in view of our knowledge on the structure, biosynthesis, and biophysical properties of gel-forming mucins, and novel imaging studies in living mucous/goblet cells. The emerging concept is that the mucin granule lumen comprises a partially condensed matrix meshwork embedded in a fluid phase where proteins slowly diffuse.
granule matrix; mucin granules; mucins; secretory granules; secretion
Airway mucus hypersecretion is a key pathophysiological feature in number of lung diseases. Cigarette smoke/nicotine and allergens are strong stimulators of airway mucus; however, the mechanism of mucus modulation is unclear.
Characterize the pathway by which cigarette smoke/nicotine regulates airway mucus and identify agents that decrease airway mucus.
IL-13 and gamma-aminobutyric acid receptors (GABAARs) are implicated in airway mucus. We examined the role of IL-13 and GABAARs in nicotine-induced mucus formation in normal human bronchial epithelial (NHBE) and A549 cells, and secondhand cigarette smoke and/or ovalbumin-induced mucus formation in vivo.
Nicotine promotes mucus formation in NHBE cells; however, the nicotine-induced mucus formation is independent of IL-13 but sensitive to the GABAAR antagonist picrotoxin (PIC). Airway epithelial cells express α7/α9/α10 nicotinic acetylcholine receptors (nAChRs) and specific inhibition or knockdown of α7- but not α9/α10-nAChRs abrogates mucus formation in response to nicotine and IL-13. Moreover, addition of acetylcholine or inhibition of its degradation increases mucus in NHBE cells. Nicotinic but not muscarinic receptor antagonists block allergen or nicotine/cigarette smoke-induced airway mucus formation in NHBE cells and/or in mouse airways.
Nicotine-induced airway mucus formation is independent of IL-13 and α7-nAChRs are critical in airway mucous cell metaplasia/hyperplasia and mucus production in response to various pro-mucoid agents, including IL-13. In the absence of nicotine, acetylcholine may be the biological ligand for α7-nAChRs to trigger airway mucus formation. α7-nAChRs are downstream of IL-13 but upstream of GABAARα2 in the MUC5AC pathway. Acetylcholine and α-7-nAChRs may serve as therapeutic targets to control airway mucus.
cigarette smoke; nicotine; nicotinic acetylcholine receptors; gamma-aminobutyric acid receptors; acetylcholine; airway mucus
The pathogenesis of mucoinfective lung disease in cystic fibrosis (CF) patients likely
involves poor mucus clearance. A recent model of mucus clearance predicts that mucus flow
depends on the relative mucin concentration of the mucus layer compared with that of the
periciliary layer; however, mucin concentrations have been difficult to measure in CF
secretions. Here, we have shown that the concentration of mucin in CF sputum is low when
measured by immunologically based techniques, and mass spectrometric analyses of CF mucins
revealed mucin cleavage at antibody recognition sites. Using physical size exclusion
chromatography/differential refractometry (SEC/dRI) techniques, we determined that mucin
concentrations in CF secretions were higher than those in normal secretions. Measurements
of partial osmotic pressures revealed that the partial osmotic pressure of CF sputum and
the retained mucus in excised CF lungs were substantially greater than the partial osmotic
pressure of normal secretions. Our data reveal that mucin concentration cannot be
accurately measured immunologically in proteolytically active CF secretions; mucins are
hyperconcentrated in CF secretions; and CF secretion osmotic pressures predict mucus
layer–dependent osmotic compression of the periciliary liquid layer in CF lungs.
Consequently, mucin hypersecretion likely produces mucus stasis, which contributes to key
infectious and inflammatory components of CF lung disease.
Luminal accumulation of viscous, poorly hydrated, and less transportable mucus has been associated with altered mucus rheology and reduced mucociliary clearance. These symptoms are some of the cardinal clinical manifestations found throughout major respiratory diseases as well as gastrointestinal and digestive disorders. Applications of current mucolytics may yield short-term improvements but are continuously challenged by undesirable side-effects. While nanoparticles (NPs) can interact with mucin polymers, whether functionalized NPs can rectify mucus rheology is unknown. Herein, we report that carboxyl-functionalized NPs (24 nm and 120 nm) dramatically reduced mucin gel size and accelerated mucin matrix hydration rate (diffusivity). Our results suggest that carboxyl-functionalized NPs disperse mucin gels possibly by enhancing network hydration. This report highlights the prospective usages of carboxyl-functionalized NPs as a novel mucus dispersant or mucolytic agent in adjusting mucus rheological properties and improving mucociliary transport to relieve clinical symptoms of patients suffering from relevant diseases.
In human airways diseases, including cystic fibrosis (CF) and chronic obstructive pulmonary disease (COPD), host defense is compromised and airways inflammation and infection often result. Mucus clearance and trapping of inhaled pathogens constitute key elements of host defense. Clearance rates are governed by mucus viscous and elastic moduli at physiological driving frequencies, whereas transport of trapped pathogens in mucus layers is governed by diffusivity. There is a clear need for simple and effective clinical biomarkers of airways disease that correlate with these properties. We tested the hypothesis that mucus solids concentration, indexed as weight percent solids (wt%), is such a biomarker. Passive microbead rheology was employed to determine both diffusive and viscoelastic properties of mucus harvested from human bronchial epithelial (HBE) cultures. Guided by sputum from healthy (1.5–2.5 wt%) and diseased (COPD, CF; 5 wt%) subjects, mucus samples were generated in vitro to mimic in vivo physiology, including intermediate range wt% to represent disease progression. Analyses of microbead datasets showed mucus diffusive properties and viscoelastic moduli scale robustly with wt%. Importantly, prominent changes in both biophysical properties arose at ∼4 wt%, consistent with a gel transition (from a more viscous-dominated solution to a more elastic-dominated gel). These findings have significant implications for: (1) penetration of cilia into the mucus layer and effectiveness of mucus transport; and (2) diffusion vs. immobilization of micro-scale particles relevant to mucus barrier properties. These data provide compelling evidence for mucus solids concentration as a baseline clinical biomarker of mucus barrier and clearance functions.
Mucus clearance is the primary defense mechanism that protects airways from inhaled infectious and toxic agents. In the current Gel-on-Liquid mucus clearance model mucus gel is propelled on top of a “watery” periciliary layer surrounding the cilia. However, this model fails to explain the formation of distinct mucus layer in health or why mucus clearance fails in disease. We propose a Gel-on-Brush model in which the periciliary layer is occupied by membrane spanning mucins and mucopolysaccharides densely tethered to the airway surface. This brush prevents mucus penetration into the periciliary space and causes mucus to form a distinct layer. The relative osmotic moduli of the mucus and periciliary brush layers explain both the stability of mucus clearance in health and its failure in airway disease.
The use of intravenous (IV) furosemide is common practice in patients under mechanical ventilation (MV), but its effects on respiratory mucus are largely unknown. Furosemide can affect respiratory mucus either directly through inhibition of the NaK(Cl)2 co-transporter on the basolateral surface of airway epithelium or indirectly through increased diuresis and dehydration. We investigated the physical properties and transportability of respiratory mucus obtained from 26 patients under MV distributed in two groups, furosemide (n = 12) and control (n = 14). Mucus collection was done at 0, 1, 2, 3 and 4 hours. The rheological properties of mucus were studied with a microrheometer, and in vitro mucociliary transport (MCT) (frog palate), contact angle (CA) and cough clearance (CC) (simulated cough machine) were measured. After the administration of furosemide, MCT decreased by 17 ± 19%, 24 ± 11%, 18 ± 16% and 18 ± 13% at 1, 2, 3 and 4 hours respectively, P < 0.001 compared with control. In contrast, no significant changes were observed in the control group. The remaining parameters did not change significantly in either group. Our results support the hypothesis that IV furosemide might acutely impair MCT in patients under MV.
furosemide; mechanical ventilation; mucociliary transport; mucus rheology
Parental, particularly maternal, smoking increases the risk of childhood allergic asthma and infection. Similarly, in a murine allergic asthma model, prenatal plus early postnatal exposure to secondhand cigarette smoke (SS) exacerbates airway hyperreactivity and Th2 responses in the lung. However, the mechanism and contribution of prenatal versus early postnatal SS exposure on allergic asthma remains unresolved. To identify the effects of prenatal and/or early postnatal SS on allergic asthma, BALB/c dams and their offspring were exposed gestationally and/or 8–10 weeks post-birth to filtered air or SS. Prenatal, but not postnatal SS strongly increased methacholine and allergen (Aspergillus)-induced airway resistance, Th2-cytokines levels and atopy, and activated the Th2 polarizing pathway GATA3/Lck/ERK1/2/STAT6. Either prenatal and/or early postnatal SS downregulated the Th1-specific transcription factor T-bet and, surprisingly, in spite of high levels of IL-4/IL-13, dramatically blocked the allergen-induced mucous cell metaplasia, airway mucus formation, and the expression of mucus-related genes/proteins: Muc5ac, GABAA-receptors, and SPDEF. Given that SS/nicotine exposure of normal adult mice promotes mucus formation, the results suggest that fetal and neonatal lung are highly sensitive to cigarette smoke. Thus, while the gestational SS promotes Th2 polarization/allergic asthma, it may also impair and/or delay the development of fetal and neonatal lung, affecting mucociliary clearance and Th1 responses. Together, this may explain the increased susceptibility of children from smoking parents to allergic asthma and childhood respiratory infections.
Environmental (secondhand) tobacco smoke; airways hyperreactivity; allergic asthma; Th2 polarization; airway mucus
Asthma is a chronic inflammatory disease of the airways associated with structural changes and airway remodeling. Interleukin (IL)-9 has pleiotropic effects on both inflammatory cells and airway structural cells, which are involved in asthma pathogenesis. We evaluated the effects of IL-9 blockade on chronic airway inflammation.
Acute airway inflammation was induced in Balb/c mice using aerosolized ovalbumin (OVA), whereas chronic asthma was induced by OVA exposure for 5 weeks with anti-IL-9 or isotype-matched antibody (Ab) treatment during the OVA challenge. Inflammatory cells in bronchoalveolar lavage fluid (BALF) were counted and lung tissues were stained to detect cellular infiltration, mucus deposition, and collagen accumulation. The levels of interferon (IFN)-γ, IL-4, IL-5, IL-9, IL-17, and immunoglobulin E (IgE) in BALF were measured using enzyme linked immunosorbent assays, and profiles of inflammatory cells and subsets of T helper (Th) cells were analyzed using flow cytometry.
IL-9, IL-17, and IFN-γ levels were significantly increased in the chronic group compared to the acute asthma group. However, the number of IL-9-positive cells was not affected, with a decrease in Th17 cells in OVA-challenged caspase-1 knockout mice. Numbers of eosinophils, neutrophils, B cells, mast cells, and Th17 cells decreased after administration of anti-IL-9 Ab. Total IgE, IL-5, IL-9, and IL-17 levels were also lower in the anti-IL-9 group.
Our results suggest that anti-IL-9 Ab treatment inhibits pulmonary infiltration of inflammatory cells and cytokine production, especially IL-17. These results provide a basis for the use of an anti-IL-9 Ab to combat IL-17-mediated airway inflammation.
Interleukin-9; allergic asthma; T helper 17; anti-interleukin-9 antibody
Mucus secretions typically protect exposed surfaces of the eyes and respiratory, gastrointestinal and female reproductive tracts from foreign entities, including pathogens and environmental ultrafine particles. We hypothesized that excess exposure to some foreign particles, however, may cause disruption of the mucus barrier. Many synthetic nanoparticles are likely to be mucoadhesive due to hydrophobic, electrostatic or hydrogen bonding interactions. We therefore sought to determine whether mucoadhesive particles (MAP) could alter the mucus microstructure, thereby allowing other foreign particles to more easily penetrate mucus. We engineered muco-inert probe particles 1 µm in diameter, whose diffusion in mucus is limited only by steric obstruction from the mucus mesh, and used them to measure possible MAP-induced changes to the microstructure of fresh human cervicovaginal mucus. We found that a 0.24% w/v concentration of 200 nm MAP in mucus induced a ∼10-fold increase in the average effective diffusivity of the probe particles, and a 2- to 3-fold increase in the fraction capable of penetrating physiologically thick mucus layers. The same concentration of muco-inert particles, and a low concentration (0.0006% w/v) of MAP, had no detectable effect on probe particle penetration rates. Using an obstruction-scaling model, we determined that the higher MAP dose increased the average mesh spacing (“pore” size) of mucus from 380 nm to 470 nm. The bulk viscoelasticity of mucus was unaffected by MAP exposure, suggesting MAP may not directly impair mucus clearance or its function as a lubricant, both of which depend critically on the bulk rheological properties of mucus. Our findings suggest mucoadhesive nanoparticles can substantially alter the microstructure of mucus, highlighting the potential of mucoadhesive environmental or engineered nanoparticles to disrupt mucus barriers and cause greater exposure to foreign particles, including pathogens and other potentially toxic nanomaterials.
Allergic asthma is caused by inhaled allergens and it is characterized by airway eosinophilia as well as mucus hypersecretion which can lead to airflow obstruction. Despite the association of increased IL-6 levels with human atopic asthma, the contribution of IL-6 to the development of allergic airway inflammation triggered by inhaled allergens remains unclear. In this study, we examined the role of IL-6 in a mouse model of allergic airway inflammation induced by direct airway exposure to extracts of Aspergillus fumigatus, a common allergen in humans. We show here that inhaled A. fumigatus extracts rapidly triggers the production of IL-6 in the airways. IL-6 appears to be dispensable for the recruitment of eosinophils to the lung during the development of allergic airway inflammation. However, IL-6 is essential for mucus hypersecretion by airway epithelial cells triggered in response to inhaled A. fumigatus antigens. Impaired mucus production caused by IL-6 deficiency correlates with a severe reduction in the levels of IL-13, a major inducer of mucin glycoproteins. Thus, IL-6 is a key regulator of specific hallmark features of allergic airway inflammation, and it could be a potential target for pulmonary diseases that are associated with goblet cell metaplasia and mucus hypersecretion.
Lung; Allergy; Fungal; Rodent; Cytokines