Mucus glycoprotein structure, gel formation and gastrointestinal mucus function

Gastrointestinal mucus occurs as a water-insoluble gel adherent to the mucosal surfaces and as a viscous, mobile solution in the lumen. The adherent gastroduodenal mucus gel is part of the mucosal defence against acid (with HCO3-), pepsin (diffusion barrier) and mechanical damage. Rheological studies show that gastrointestinal mucus is a weak, viscoelastic gel. The size and physical properties of the isolated component glycoproteins depend critically on the methods used to obtain them. A glycoprotein preparation of Mr approximately 2 X 10(6), which possesses the gel-forming properties of the native mucus, is considered to represent the secreted covalent entity in pig gastric and small intestinal mucus. These glycoproteins have a polymeric structure of subunits joined by disulphide bridges between non-glycosylated regions of their protein cores. Glycoprotein polymerization, essential for gel formation, is deficient in gastric mucus in peptic ulcer disease. In vivo, adherent mucus gel forms a thin but continuous cover of variable thickness (rat 5-500 microns) over the gastroduodenal mucosa. Luminal pepsin rapidly dissolves this mucus cover and its continuity is maintained by fresh mucus secretion. Bile, HCl, 2 M-NaCl and ethanol (less than 40%) do not destroy mucus gel structure. Prostaglandins and carbachol increase mucus thickness, affording better protection, but it is thought that continuity of the protective mucus cover is the critical factor in its protective functions.

The MUC2 gene product: a human intestinal mucin

The MUC2 gene product is the first human secretory mucin protein core to be fully sequenced. Like the other eight human MUC genes identified to date, MUC2 is characterised by tandem and irregular repeat sequences rich in threonine and serine, the potential sites of attachment of the oligosaccharide chains. The MUC2 gene product is more than 5100 amino acids in its commonest allelic form and accounts for one fifth by weight of the mucin glycoprotein molecule (80% oligosaccharide side chains). The MUC2 product is polymerised end to end through disulphide bridges to form large secreted polymeric gel-forming mucins (Mr approximately 10(7)). The primary function of the MUC2 gene product is to provide a protective barrier between the epithelial surfaces and the gut lumen. There is decreased expression of MUC2 in colonic cancer and defective polymerisation of secreted mucin in ulcerative colitis. Elucidation of the MUC2 and other mucin gene sequences has opened the way for a full structural characterisation and an improved understanding of the structure and function of these complex mucus gel secretions.

The MUC2 gene product: a human intestinal mucin

The MUC2 gene product is the first human secretory mucin protein core to be fully sequenced. Like the other eight human MUC genes identified to date, MUC2 is characterised by tandem and irregular repeat sequences rich in threonine and serine, the potential sites of attachment of the oligosaccharide chains. The MUC2 gene product is more than 5100 amino acids in its commonest allelic form and accounts for one fifth by weight of the mucin glycoprotein molecule (80% oligosaccharide side chains). The MUC2 product is polymerised end to end through disulphide bridges to form large secreted polymeric gel-forming mucins (Mr approximately 10(7)). The primary function of the MUC2 gene product is to provide a protective barrier between the epithelial surfaces and the gut lumen. There is decreased expression of MUC2 in colonic cancer and defective polymerisation of secreted mucin in ulcerative colitis. Elucidation of the MUC2 and other mucin gene sequences has opened the way for a full structural characterisation and an improved understanding of the structure and function of these complex mucus gel secretions.

Heterogeneity in the protein cores of mucins isolated from human middle ear effusions: evidence for expression of different mucin gene products

High molecular weight mucins were isolated and purified from human middle ear effusions of children with Otitis Media with Effusion (OME) classified into three groups, (1) thick and (2) thin from anatomically normal children and (3) effusions from cleft palate patients. Amino acid analyses of the purified mucins from the three pools were similar but not identical with characteristic contents of serine threonine and proline (32%, 28%, and 38% for pools (1) (2) and (3) respectively). Proteinase resistant glycopeptide fragments corresponding to the tandem repeat domains of cloned mucin genes showed marked differences both between the three mucin pools and with the composition of the tandem repeat sequences of the cloned mucin genes expressed in the airways. Studies on the antigenic identity of middle ear mucins found an epitope likely to be present on MUC5AC, but only accounting for a maximum of 15% by weight and no reactivity was found with antibodies to MUC2 or MUC1. A polyclonal antibody raised to thick effusion mucins reacted strongly with human salivary mucin suggesting the presence of MUC5B epitopes. These studies suggest that more than one mucin gene product is secreted by the human middle ear mucosa and that there may be further mucin genes expressed by the middle ear that have yet to be cloned.

Compositional differences between bilateral middle ear effusions in otitis media with effusion: evidence for a different etiology?

The aim of this study was to clarify the site of primary pathology in otitis media with effusion. Effusions were collected from 64 children with bilateral effusions at the time of myringotomy. The rheological properties and biochemical compositions of effusions were measured for 23 pairs of effusions, and the levels of the inflammatory mediators TNF alpha, IL-1beta, and IL-8 were measured in 41 pairs using specific enzyme-linked immunosorbent assays (ELISAs). Measurements from paired ears were compared using analysis of variance (ANOVA) tests and significant differences were found for reduced specific viscosity, mucin content, protein content, and levels of IL-8. The results demonstrate that the two ears have different immunological processes or rates of processes which might explain the significantly different rheological properties of effusions. This suggests that each ear undergoes pathological changes independently and has implications for using the opposite ear as a control in clinical trials.

Channelized detection filters

The problem of detecting objects in noisy backgrounds is addressed. We derive detection filters by training a linear classifier, using features obtained from subimages corresponding to circular channels in the Fourier domain. The classifier weights approach the prewhitening matched filter when the classifier is trained for the detection of known objects in stationary noise. A simple form of rotation invariance is attained for considerably less computation than by the direct application of multiple matched filters. The method is demonstrated for the task of detecting simulated tumors in simulated nuclear medical images.

A polydisperse linear random coil model for the quaternary structure of pig colonic mucin

The distribution of molecular weights for polymeric colonic mucus glycoprotein or "mucin" isolated and solubilised in the presence of protease inhibitors from pig colons is shown to be considerably greater than its "subunit" (thiol reduction product) and papain digested forms using the technique of size-exclusion chromatography coupled to multi-angle laser light scattering, and confirmed by sedimentation equilibrium measurements. The conformation of this mucin is probed by examining the molecular weight-intrinsic viscosity relationship in terms of the Mark-Houwink-Kuhn-Sakurada analysis for its polymeric (or "whole"), reduced and papain-digested forms: an exponent "a" of (1.1 +/- 0.1) is obtained indicating a linear random coil conformation consistent with other mucins. Size-exclusion chromatography coupled to multi-angle laser light scattering is shown to provide a relatively simple complementary technique to sedimentation equilibrium for the molecular weight distribution analysis of polydisperse materials.

Characterization of pig colonic mucins

Pig colonic mucins isolated from the adherent mucus gel in the presence of proteinase inhibitors were solubilized by homogenization and the component mucins fractionated by CsC1 density-gradient centrifugation. Polymeric and reduced pig colonic mucin were both largely excluded on Sepharose CL-2B, papain-digested colonic mucin was included. The M(r) values of polymeric, reduced and digested mucins were 5.5 x 10(6), 2.1 x 10(6) and 0.6 x 10(6) respectively. This suggests that pig colonic mucin is comprised of 2-3 subunits, each subunit containing 3-4 glycosylated regions. The intrinsic viscosities of polymeric, reduced and digested mucin were 240 ml.g-1, 100 ml.g-1 and 20 ml.g-1 respectively. Polymeric pig colonic mucin comprised 16% protein per mg of glycoprotein and was rich in serine, threonine and proline (43% of total amino acids). There were approx. 150 disulphide bridges and 53 free thiol groups per mucin polymer. A seventh of the protein content was lost on reduction. This protein was particularly rich in proline and the hydrophobic amino acids. Papain-digested pig colonic mucin contained 11% protein per mg of glycoprotein and was rich in serine, threonine, glutamate and aspartate. All types of amino acids with the exception of aspartate were lost on digestion. The amino acid analysis of the proteolytically digested regions of pig colonic mucin are markedly different to the tandem repeat regions of the human mucin genes shown to be expressed in the colon.

Interaction of polyacrylates with porcine pepsin and the gastric mucus barrier: a mechanism for mucosal protection

1. The mechanism of interaction of the polyacrylates, carbopols with the mucus barrier in vivo has been investigated in vitro. 2. Carbopol caused a dramatic increase in the viscosity of porcine gastric mucin solutions that was up to 19-fold greater than that of the sum of the individual polymers. 3. The mucin-carbopol interaction was stable after an initial 30 min period for up to 36 h at 25 degrees C or 37 degrees C. It was reduced by increasing the temperature from 20 degrees C to 45 degrees C, was unaffected by pH and ionic strength, but was enhanced by Ca2+. 4. The magnitude of the interaction between mucin and carbopol depended on the polymeric structure of the mucin and the molecular size and level of cross-linking of the carbopol. 5. The interactions were reversible and increased with increasing carbopol and mucin concentration. The dramatic increase in viscosity can be explained in terms of space filling by the mucin molecules leading to predominantly carbopol-carbopol interactions. 6. Carbopol 934P inhibits pepsin hydrolysis and therefore has potential as a mucosal protective agent in vivo.

Preliminary characterization of mucin from effusions of cleft palate patients

Middle ear effusions from children undergoing myringotomy were classified into three groups-cleft palate, thick (mucoid), and thin (serous). Mucin was purified from each of the three groups using CsCI equilibrium density gradient centrifugation. Analysis of the cleft palate mucin on Sepharose CL-2B showed it was excluded and therefore of large molecular weight. It could be broken down into smaller glycopeptide units by proteolysis and these glycopeptides had, based on elution position, a larger hydrodynamic size than those from the thick mucin. Intrinsic viscosity measurements demonstrated that the intact mucins could be ranked in order of molecular space occupancy; cleft palate > thick > thin. Amino acid analysis showed the cleft palate mucin to have an amino acid composition similar to other mucins, with serine, threonine, and proline constituting 41% by weight of the protein core. Thiol analysis gave evidence of a possible difference in polymerization between the three mucins, in that thin (the smallest mucin) contained the lowest number of thiols. This preliminary analysis of cleft palate mucin suggests a mucin with larger glycopeptide units forming an intact mucin of larger hydrodynamic size than either thick or thin middle ear mucins from anatomically normal children.

Culture of human middle ear mucosal explants; mucin production

Middle ear mucosal biopsies could be maintained in culture for up to 7 days, the longest time attempted in this study. Mucin biosynthesis and secretion were measured by incorporation of 14C-glucosamine. Three peaks of radioactivity were present when the dialysed medium was chromatographed. Peak I which accounted for about 10% of the total radioactivity had properties characteristic of mucin. The other two peaks were not characteristic of mucins. Labelled macromolecules excluded on Sepharose 2B were also present in the tissue. Autoradiography of the explants showed that the labelled glucosamine was concentrated in the epithelial layer. Morphometry demonstrated that 1-2% of the epithelial cell volume consisted of goblet cells. The proportionate incorporation of radioactivity into macromolecules increased with increasing epithelial cell volume. This system will allow assessment of factors implicated in the pathogenesis of otitis media with effusion and the study of the action of pharmacological agents on biosynthesis and secretion.

Otitis media with effusion: components which contribute to the viscous properties

Middle ear effusions from children undergoing myringotomy were classified into thick (mucoid) and thin (serous) on the basis of their flow properties. Their composition was analysed and their rheological properties measured. The viscosity of the effusions was measured using a Contraves low shear viscometer and expressed as specific viscosity per mg/ml of non-dialysable solids present. In order to measure the effusion viscosity it was necessary to solubilize the effusion by mild homogenisation in a phosphate buffer pH 6.7 containing a cocktail of proteolytic inhibitors. The viscosity of mucoid effusions was significantly greater than that of the serous effusions. There was a small but measurable amount of proteolytic activity in the effusions, range 0.05-1.79 micrograms/mg of non-dialysable solids. This proteolytic activity was not significantly different between the thick and thin effusions and was therefore unlikely to explain the difference in viscosity. Analysis of the constituents of the effusions showed that glycoprotein and DNA but not protein nor lipid were significantly higher in the mucoid effusions compared to the serous effusions. The viscosity of the effusions correlated with the glycoprotein concentration but not with the protein or lipid concentration. Under certain circumstances the DNA concentration did correlate with the viscosity of the effusion. However, digestion with a proteinase free DNase did not reduce the viscosity of the effusion. These results demonstrate that classifying effusions as thick and thin based on visual inspection and flow properties is valid and that the only constituent present in the effusions that determines viscosity is mucin.

Mucolysis of the colonic mucus barrier by faecal proteinases: inhibition by interacting polyacrylate

1. Mucolytic (mucus solubilizing) activity in human faeces has been characterized with both purified human and pig colonic mucin and shown to be mediated by proteolysis. 2. Mucolytic activity was demonstrated by: (i) a drop in mucin viscosity; (ii) a substantial reduction in mucin size, from polymer to degraded subunit, as assessed by Sepharose CL-2B gel filtration; (iii) formation of new N-terminal peptides. 3. Mucolytic activity was also followed in faecal extracts by its proteolytic activity using standard succinyl albumin substrate. Proteolysis extended over the pH range 4.5-11.0. Proteolysis was inhibited at pH 7.5 by soybean trypsin inhibitor and phenylmethanesulphonyl fluoride, suggesting the presence of serine proteinases. 4. The polyacrylate carbomer (934P) inhibited both mucolysis of pig colonic mucin and proteolysis of succinyl albumin. 5. Interaction between the polyacrylate (carbomer 934P) and purified human and pig colonic mucin was demonstrated by a marked synergistic increase in solution viscosity (360% above control). 6. The results demonstrate the presence of a mucolytic activity in the human colonic lumen that has the potential to degrade the mucus barrier, and that polyacrylates inhibit this mucolysis and interact to strengthen the colonic mucus barrier. Polyacrylates may therefore have therapeutic potential in inflammatory bowel disease where luminal proteolytic activity can be raised.

Pepsins and the mucus barrier in peptic ulcer disease

Studies show that the gastroduodenal mucosal barrier is damaged by pepsin under conditions in which it is resistant to acid alone. The continuous layer of adherent mucus gel provides a diffusion barrier to luminal pepsin, preventing its access to the underlying epithelium. Pepsin has mucolytic activity and will progressively digest the adherent mucus layer at its luminal surface, although normally this is balanced by secretion of new mucus to maintain a continuous barrier. In peptic ulcer disease the proportion of peptic activity in gastric juice attributable to pepsin type 1 is significantly raised (four to five-fold). Pepsin 1 has increased mucolytic activity compared with the major component, pepsin 3, both at the optimal pH of 2 (twofold increase in activity) and at higher pH values up to pH 5 (sixfold increase in activity at pH 4). Structural studies show that the gel forming polymeric mucin of the antral adherent mucus barrier is deficient in peptic ulcer disease. This breakdown of the mucus barrier in peptic ulcer patients can be attributed to the increased pepsin activity of gastric juice seen in this disease, although other explanations are also possible. The increased pepsin activity of gastric juice in peptic ulcer patients is compatible with the concept 'no acid, no pepsin, no ulcer'.

The role of mucus in the protection of the gastroduodenal mucosa

There is good evidence that the adherent mucus plays an important role in the protection of gastroduodenal mucosa from the endogenous aggressors acid and pepsin. Adherent mucus provides a stable unstirred layer which supports surface neutralization of acid by mucosal bicarbonate output and acts as a permeability barrier to luminal pepsin. The adherent mucus layer is continuous. True thickness of the mucus layer and its continuity can only be observed on unfixed sections of mucosa, since histological fixatives and preparation for electron microscopy can cause dehydration and shrinkage of the mucus gel. The structure of adherent gastric mucus is deficient in patients with peptic ulcer disease because of decreased polymerization of the component glycoproteins. This impairment of the mucus barrier is associated with raised amounts of pepsin 1, which digests the mucus layer more aggressively than the major pepsin, pepsin 3, under conditions that pertain both in the stomach (pH 2) and duodenum (pH 4-5). Adherent mucus does not appear to offer much protection against exogenous damaging agents, e.g. alcohol and aspirin. These agents permeate the mucus barrier, damaging the underlying epithelium. The subsequent epithelial repair process is protected by a gelatinous coat over ten times thicker and distinct from the normal adherent mucus layer. Our recent studies show this gelatinous coat to be primarily a fibrin-based gel with mucus and necrotic cells.