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.
Airway mucin secretion studies have focused on goblet cell responses to exogenous agonists almost to the exclusion of baseline mucin secretion (BLMS). In human bronchial epithelial cell cultures ...(HBECCs), maximal agonist-stimulated secretion exceeds baseline by ~3-fold as measured over hour-long periods, but mucin stores are discharged completely and require 24 h for full restoration. Hence, over 24 h, total baseline exceeds agonist-induced secretion by several-fold. Studies with HBECCs and mouse tracheas showed that BLMS is highly sensitive to mechanical stresses. Harvesting three consecutive 1 h baseline luminal incubations with HBECCs yielded equal rates of BLMS; however, lengthening the middle period to 72 h decreased the respective rate significantly, suggesting a stimulation of BLMS by the gentle washes of HBECC luminal surfaces. BLMS declined exponentially after washing HBECCs (t1/2 = 2.75 h), to rates approaching zero. HBECCs exposed to low perfusion rates exhibited spike-like increases in BLMS when flow was jumped 5-fold: BLMS increased >4 fold, then decreased within 5 min to a stable plateau at 1.5-2-fold over control. Higher flow jumps induced proportionally higher BLMS increases. Inducing mucous hyperplasia in HBECCs increased mucin production, BLMS and agonist-induced secretion. Mouse tracheal BLMS was ~6-fold higher during perfusion, than when flow was stopped. Munc13-2 null mouse tracheas, with their defect of accumulated cellular mucins, exhibited similar BLMS as WT, contrary to predictions of lower values. Graded mucous metaplasia induced in WT and Munc13-2 null tracheas with IL-13, caused proportional increases in BLMS, suggesting that naïve Munc13-2 mouse BLMS is elevated by increased mucin stores. We conclude that BLMS is, i a major component of mucin secretion in the lung, ii sustained by the mechanical activity of a dynamic lung, iii proportional to levels of mucin stores, and iv regulated differentially from agonist-induced mucin secretion.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The localization of Burkholderia cepacia complex (Bcc) bacteria in cystic fibrosis (CF) lungs, alone or during coinfection with Pseudomonas aeruginosa, is poorly understood. We performed ...immunohistochemistry for Bcc and P. aeruginosa bacteria on 21 coinfected or singly infected CF lungs obtained at transplantation or autopsy. Parallel in vitro experiments examined the growth of two Bcc species, Burkholderia cenocepacia and Burkholderia multivorans, in environments similar to those occupied by P. aeruginosa in the CF lung. Bcc bacteria were predominantly identified in the CF lung as single cells or small clusters within phagocytes and mucus but not as "biofilm-like structures." In contrast, P. aeruginosa was identified in biofilm-like masses, but densities appeared to be reduced during coinfection with Bcc bacteria. Based on chemical analyses of CF and non-CF respiratory secretions, a test medium was defined to study Bcc growth and interactions with P. aeruginosa in an environment mimicking the CF lung. When test medium was supplemented with alternative electron acceptors under anaerobic conditions, B. cenocepacia and B. multivorans used fermentation rather than anaerobic respiration to gain energy, consistent with the identification of fermentation products by high-performance liquid chromatography (HPLC). Both Bcc species also expressed mucinases that produced carbon sources from mucins for growth. In the presence of P. aeruginosa in vitro, both Bcc species grew anaerobically but not aerobically. We propose that Bcc bacteria (i) invade a P. aeruginosa-infected CF lung when the airway lumen is anaerobic, (ii) inhibit P. aeruginosa biofilm-like growth, and (iii) expand the host bacterial niche from mucus to also include macrophages.
Cystic fibrosis (CF) airways disease produces a mucoobstructive lung phenotype characterized by airways mucus plugging, epithelial mucous cell metaplasia/hyperplasia, chronic infection, and ...inflammation. Simultaneous biochemical and functional in vivo studies of mucin synthesis and secretion from CF airways are not available. In vitro translational models may quantitate differential CF versus normal mucin and fluid secretory responses to infectious/inflammatory stimuli.
We tested the hypothesis that CF airways exhibit defective epithelial fluid, but not mucin, secretory responses to bacterial/inflammatory host products.
Well-differentiated primary human bronchial epithelial cultures were exposed to supernatant from mucopurulent material (SMM) from human CF airways as a test of bacterial/inflammatory host product stimulus. Human bronchial epithelia (HBE) with normal CF transmembrane conductance regulator function were compared with ΔF508/ΔF508 CF HBE.
Acute (up to 60 min) SMM exposure promoted mucin secretion, but mucins were degraded by the proteolytic enzymes present in SMM. Chronic SMM exposure induced upregulation of mucin synthesis and storage and generated absolute increases in basal and stimulated mucin release in normal and CF cultures. These responses were similar in normal and CF cultures. In contrast, SMM produced a coordinated CF transmembrane conductance regulator-mediated Cl
secretory response in normal HBE, but not in CF HBE. The absence of the fluid secretory response in CF produced quantitatively more dehydrated mucus.
Our study reveals the interplay between regulation of mucin and fluid secretion rates in inflamed versus noninflamed conditions and why a hyperconcentrated mucus is produced in CF 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.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
ATP in airway surface liquid (ASL) controls mucociliary clearance functions via the activation of airway epithelial purinergic receptors. However, abnormally elevated ATP levels have been reported in ...inflamed airways, suggesting that excessive ATP in ASL contributes to airway inflammation. Despite these observations, little is known about the mechanisms of ATP accumulation in the ASL covering inflamed airways. In this study, links between cystic fibrosis (CF)-associated airway inflammation and airway epithelial ATP release were investigated. Primary human bronchial epithelial (HBE) cells isolated from CF lungs exhibited enhanced IL-8 secretion after 6 to 11 days, but not 28 to 35 days, in culture, compared with normal HBE cells. Hypotonic cell swelling-promoted ATP release was increased in 6- to 11-day-old CF HBE cells compared with non-CF HBE cells, but returned to normal values after 28 to 35 days in culture. The exposure of non-CF HBE cells to airway secretions isolated from CF lungs, namely, sterile supernatants of mucopurulent material (SMM), also caused enhanced IL-8 secretion and increased ATP release. The SMM-induced increase in ATP release was sensitive to Ca(2+) chelation and vesicle trafficking/exocytosis inhibitors, but not to pannexin inhibition. Transcript levels of the vesicular nucleotide transporter, but not pannexin 1, were up-regulated after SMM exposure. SMM-treated cultures displayed increased basal mucin secretion, but mucin secretion was not enhanced in response to hypotonic challenge after the exposure of cells to either vehicle or SMM. We propose that CF airway inflammation up-regulates the capacity of airway epithelia to release ATP via Ca(2+)-dependent vesicular mechanisms not associated with mucin granule secretion.
Adenosine triphosphate (ATP) and its metabolite adenosine regulate airway mucociliary clearance via activation of purinoceptors. In this study, we investigated the contribution of goblet cells to ...airway epithelial ATP release. Primary human bronchial epithelial (HBE) cultures, typically dominated by ciliated cells, were induced to develop goblet cell metaplasia by infection with respiratory syncytial virus (RSV) or treatment with IL-13. Under resting conditions, goblet-cell metaplastic cultures displayed enhanced mucin secretion accompanied by increased rates of ATP release and mucosal surface adenosine accumulation as compared with nonmetaplastic control HBE cultures. Intracellular calcium chelation 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetraacetoxymethyl ester or disruption of the secretory pathways (nocodazole, brefeldin A, and N-ethylmaleimide) decreased mucin secretion and ATP release in goblet-cell metaplastic HBE cultures. Conversely, stimuli that triggered calcium-regulated mucin secretion (e.g., ionomycin or UTP) increased luminal ATP release and adenyl purine accumulation in control and goblet-cell metaplastic HBE cultures. Goblet cell-associated ATP release was not blocked by the connexin/pannexin hemichannel inhibitor carbenoxolone, suggesting direct nucleotide release from goblet cell vesicles rather than the hemichannel insertion. Collectively, our data demonstrate that nucleotide release is increased by goblet cell metaplasia, reflecting, at least in part, a mechanism tightly associated with goblet cell mucin secretion. Increased goblet cell nucleotide release and resultant adenosine accumulation provide compensatory mechanisms to hydrate mucins by paracrine stimulation of ciliated cell ion and water secretion and maintain mucociliary clearance, and to modulate inflammatory responses.
MARCKS (myristoylated alanine-rich C kinase substrate) is postulated to regulate the passage of secretory granules through cortical actin in the early phase of exocytosis. There are, however, three ...proposed mechanisms of action, all of which were derived from studies using synthetic peptides representing either the central phosphorylation site domain or the upstream, NH2-terminal domain: it tethers actin to the plasma membrane and/or to secretory granules, and/or it sequesters PIP2. Using MARCKS-null mice, we probed for a loss of function secretory phenotype in mast cells harvested from embryonic livers and maturated in vivo embryonic hepatic-derived mast cells (eHMCs). Both wild-type (WT) and MARCKS-null eHMCs exhibited full exocytic responses upon FcϵRI receptor activation with DNP-BSA (2,4-dinitrophenyl-BSA), whether they were in suspension or adherent. The secretory responses of MARCKS-null eHMCs were consistently higher than those of WT cells, but the differences had sporadic statistical significance. The MARCKS-null cells exhibited faster secretory kinetics, however, achieving the plateau phase of the response with a t½ ∼2.5-fold faster. Hence, MARCKS appears to be a nonessential regulatory protein in mast cell exocytosis but exerts a negative modulation. Surprisingly, the MARCKS NH2-terminal peptide, MANS, which has been reported to inhibit mucin secretion from airway goblet cells (Li Y, Martin LD, Spizz G, Adler KB. J Biol Chem 276: 40982-40990, 2001), inhibited hexosaminidase secretion from WT and MARCKS-null eHMCs, leading us to reexamine its effects on mucin secretion. Results from studies using peptide inhibitors with human bronchial epithelial cells and with binding assays using purified mucins suggested that MANS inhibited the mucin binding assay, rather than the secretory response.
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