Factors influencing the viscous properties of chicken tracheal mucins

Role of CFTR in airway disease

Role of CFTR in Airway Disease. Physiol. Rev. 79, Suppl.: S215-S255, 1999. - Cystic fibrosis (CF) is caused by mutations in the gene encoding the CF transmembrane conductance regulator (CFTR), which accounts for the cAMP-regulated chloride conductance of airway epithelial cells. Lung disease is the chief cause of morbidity and mortality in CF patients. This review focuses on mechanisms whereby the deletion or impairment of CFTR chloride channel function produces lung disease. It examines the major themes of the channel hypothesis of CF, which involve impaired regulation of airway surface fluid volume or composition. Available evidence indicates that the effect of CFTR deletion alters physiological functions of both surface and submucosal gland epithelia. At the airway surface, deletion of CFTR causes hyperabsorption of sodium chloride and a reduction in the periciliary salt and water content, which impairs mucociliary clearance. In submucosal glands, loss of CFTR-mediated salt and water secretion compromises the clearance of mucins and a variety of defense substances onto the airway surface. Impaired mucociliary clearance, together with CFTR-related changes in the airway surface microenvironment, leads to a progressive cycle of infection, inflammation, and declining lung function. Here, we provide the details of this pathophysiological cascade in the hope that its understanding will promote the development of new therapies for CF.

Rat gastric mucin is synthesized and secreted exclusively as filamentous oligomers

Oligomeric gastric mucin was isolated from the fundic part of the rat stomach. Previously we have shown by biochemical analysis that this oligomeric mucin consists of disulphide-linked homo-oligomers, which contain no other covalently attached proteins [Dekker, Aelmans & Strous (1991) Biochem. J. 277, 423-427]. Electron-microscopic images of the oligomeric mucin revealed a heterogenous population of long filamentous molecules of 300-3000 nm length. After reduction and carboxymethylation the monomeric mucins displayed a length distribution with a single peak at about 279 nm. Length-distribution analysis of oligomeric molecules with length up to 1000 nm revealed three subpopulations with one, two or three times the length of the monomeric mucin. The oligomers displayed small globular domains of about 15 nm, which were equally spaced along the molecule's length. As the distance between these globular domains was similar to the monomer length, these domains most likely indicate attachment sites of the monomers. These results show that the mucin monomers attached end-to-end in the oligomer. Biosynthesis of the mucin oligomers was studied by labelling of stomach explants in vitro with [35S]methionine, [3H]galactose or [35S]sulphate and subsequent immunoprecipitation of the mucin with a specific antiserum. Analysis by electrophoresis and gel filtration revealed that the oligomerization takes place by formation of disulphide bonds between the 300 kDa mucin precursors. The mucin was exclusively synthesized and secreted as fully glycosylated oligomers, as neither precursor proteins nor monomeric mucin were detected in the culture medium. A model for the biosynthesis of rat gastric mucin is proposed in which the filamentous mucin monomers are linked end-to-end by disulphide bonds.

Effect of reserpine on the histochemical and biochemical properties of rat intestinal mucin

Biochemical and histochemical parameters of intestinal mucins were examined in control and reserpine-treated rats. An assay for intestinal mucin sulfotransferase was developed and the activity shown to increase 3.4 times over control levels in rats given intraperitonal reserpine (0.5 mg/kg body wt) daily for 7 days. Histochemical staining of intestinal sections revealed an increase in sulfomucins in goblet cells of reserpine-treated rats. The effects were prominent as early as 1 day following injection, particularly in the distal third of the small intestine, and during the next 6 days these changes spread progressively to the middle and proximal thirds. After 3 days of treatment mucins were purified from each intestinal segment and compared to control mucins with respect to composition and [35S]NaSO4 incorporation. Although individual amino acid and carbohydrate molar ratios were unchanged, the total carbohydrate and sulfate content of mucins in treated animals was elevated (two to three times above control) in the middle and distal thirds of the intestine. In vivo [35S]SO4 incorporation into these mucins was also proportionaltely elevated, and was targetted to O-linked oligosaccharide side chains. These findings are consistent with an action of reserpine causing an increased production of mucin which is enriched in glycoprotein components bearing sulfated oligosaccharide chains. The relevance of these findings to the production of hypersulfated and hyperglycosylated mucins in cystic fibrosis is discussed.

Pericellular glycoconjugates of cultured fibroblasts from control and cystic fibrosis patients

1. Labeled glycoconjugates released by trypsin from cell surfaces of control and cystic fibrosis (CF) skin fibroblasts were purified and fractionated by column chromatography on Sephadex G-50 and Concanavalin A Sepharose. Based on chemical analysis and specific enzymatic digestions: (1) Glycoconjugates were characterized as O-linked glycopeptides consisting predominantly of glycosaminoglycan type and N-linked glycopeptides with glycans of complex type. Their relative proportions were similar between the two groups. (2) The N-linked glycopeptides exhibited an increased molar ratio of fucose to galactose in CF fibroblasts. (3) When pericellular glycoconjugates were metabolically labeled with [14C]glucosamine and [3H]fucose, incorporation and degradation kinetics were similar between the two groups.

Increased sulfation of glycoconjugates by cultured nasal epithelial cells from patients with cystic fibrosis

Cystic fibrosis (CF) respiratory epithelia exhibit abnormal anion transport that may be linked to abnormal lung defense. In these studies, we investigated whether primary cultures of CF respiratory epithelial cells regulate abnormally the sulfate content of high molecular weight glycoconjugates (HMG) participating in airways' mucosal defense. HMG, including glycosaminoglycans and mucin-type glycoproteins released spontaneously into medium and HMG released from cell surfaces by trypsin, were metabolically labeled with 35SO4- and [6-3H]-glucosamine (GlcN) or 35SO4- and [3H]serine. All three classes of HMG from CF cells exhibited 35S/3H labeling ratios 1.5-4-fold greater than HMG from normal or disease control cells. Differences for labeling ratios of HMG from CF cells were shown to be the consequence of increased 35SO4- incorporation rather than decreased peptide synthesis and release or HMG glycosylation. The buoyant density of CF mucin-type HMG also was increased, consistent with increased sulfation. These observations suggest that oversulfation of a spectrum of HMG is a genetically determined characteristic of CF epithelial cells and may play an important pathophysiological role by altering the properties of mucous secretions and/or the interactions between selected bacteria and HMG at the airways' surface.

Increased sulfate uptake in skin fibroblasts isolated from cystic fibrosis patients

Sulfate uptake into skin fibroblasts from patients with cystic fibrosis is increased. Sulfate transport studies were carried out in skin fibroblasts isolated from age/sex matched cystic fibrosis and normal subjects. Sulfate transport occurred mainly via a carrier-mediated proton-stimulated S04(2)-/Cl-exchange. The capacity (Vmax) of the uptake system operating at physiological concentrations of sulfate was stimulated in cystic fibrosis, but the affinity of the carrier for sulfate was not altered.

Role of directional Ca2+ effect on reduced viscosities of mucus secretions from chicken trachea in vitro

Reduced viscosities of fibrillar and gelatinous type mucins produced in response to high submucosal Ca2+ and low luminal Ca2+ effects were significantly higher than those of corresponding types of normal mucins. The increased reduced viscosity of experimental mucin samples was due to their aggregation with unique low molecular weight (mr 325,000 and 46,200) sulphate-rich components. The Ca2+ appeared to exert two opposing effects on viscosity properties of mucins; whereas Ca2+-dependent complexes between different types of mucins appeared to be a selective phenomenon between sulphate-rich mucins and components. Ester sulphate residue content rather than N-acetylneuraminic acid residue content of these mucins and low molecular weight components showed a very good correlation with their reduced viscosity and Ca2+-binding values.

Respiratory mucous secretions in patients with cystic fibrosis: relationship between levels of highly sulfated mucin component and severity of the disease

The tracheobronchial secretions from cystic fibrosis patients contained higher levels of protein, DNA and sialic acid than the tracheobronchial secretions from healthy donors. In contrast, the neutral hexose content in CF secretions was strikingly lower than in secretions from normal subjects. The levels of neutral hexose and sialic acid in the CF secretions were found to increase with increasing severity of the disease. The alterations in the levels of these chemical parameters in the secretions of patients with increased disease severity are as a result of increased levels of the mucin content of the secretions, especially of the highly sulfated mucin component. Since mucins are considered, to a large extent, responsible for the viscoelastic properties of the secretions, the enhanced levels of the highly sulfated mucin component in the secretions of the patients with increased disease severity, may contribute to altered rheological properties and hence decreased mucociliary transport of the secretions.

Directional Ca2+ effect on stimulation of mucin secretion from chicken trachea in vitro

Chicken tracheal mucosa in vitro transported and incorporated radioactive precursors into mucins, which were secreted at a steady rate into the tracheal lumen. Secretion of mucins labelled with (35)S and (3)H after pulse-labelling of the mucosal layer with Na(2) (35)SO(4) and d-[1-(3)H]glucosamine as precursors was an energy-dependent process, as it was strongly inhibited by the action of respiratory-chain inhibitors, an uncoupler of oxidative phosphorylation, a metabolic blocker and a temperature shift from 41 degrees C to 5 degrees C. On the other hand, both cholinergic and parasympathomimetic agents considerably increased the secretion of dual-radiolabelled mucins when applied on the submucosal side of the trachea. The effect of Ca(2+) was directional, since only high submucosal (3.6 or 18mm) or low luminal (zero or 0.18mm) Ca(2+) massively enhanced the secretion of radiolabelled mucin compared with the mucin output measured under physiological Ca(2+) conditions (1.8mm). Whereas application of ionophore A23187 on either side of the trachea significantly increased mucin output, its presence in the appropriate tracheal compartment and under appropriate Ca(2+) conditions further accentuated the output of radiolabelled mucins. Addition of acetylcholine under appropriate conditions also had an additive effect on the Ca(2+)-stimulated secretion of mucins. Ca(2+) stimulation of mucin secretion appears to be dependent on the metabolic integrity of the mucosal cells. Mucins secreted in response to high submucosal and low luminal [Ca(2+)] appear to consist of a number of different types of glycoproteins, as judged from their ion-exchange-chromatographic behaviour.