Peter Chater

MUC2 mucin, the primary gel-forming component of intestinal mucus, is well researched and a model of polymerisation and post-secretory organisation has been published previously. Recently, several significant developments have been made which either introduce new ideas or challenge previous theories. New ideas include an overhaul of the MUC2 C-terminal globular structure which is proposed to harbour several previously unobserved domains, and include a site for an extra intermolecular disulphide bridge dimer between the cysteine 4379 of adjacent MUC2 C-termini. MUC2 polymers are also now thought to be secreted attached to the epithelial surface of goblet cells in the small intestine and removed following secretion via a metalloprotease meprin β-mediated cleavage of the von Willebrand D2 domain of the N-terminus. It remains unclear whether MUC2 forms intermolecular dimers, trimers, or both, at the N-termini during polymerisation, with several articles supporting either trimer or dimer formation. The presence of a firm inner mucus layer in the small intestine is similarly unclear. Considering this recent research, this review proposes an update to the previous model of MUC2 polymerisation and secretion, considers conflicting theories and data, and highlights the importance of this research to the understanding of MUC2 mucus layers in health and disease.

The human gut microbiota (HGM) are closely associated with health, development and disease. The thick intestinal mucus layer, especially in the colon, is the key barrier between the contents of the lumen and the epithelial cells, providing protection against infiltration by the microbiota as well potential pathogens. The upper layer of the colonic mucus is a niche for a subset of the microbiota which utilise the mucin glycoproteins as a nutrient source and mucin grazing by the microbiota appears to play a key role in maintaining barrier function as well as community stability. Despite the importance of mucin breakdown for gut health, the mechanisms by which gut bacteria access this complex glycoprotein are not well understood. The current model for mucin degradation involves exclusively exo-acting glycosidases that sequentially trim monosaccharides from the termini of the glycan chains to eventually allow access to the mucin peptide backbone by proteases. However, this model is in direct contrast to the Sus paradigm of glycan breakdown used by the Bacteroidetes which involves extracellular cleavage of glycans by surface located endo-acting enzymes prior to import of the oligosaccharide products. Here we describe the discovery and characterisation of endo-acting family 16 glycoside hydrolases (GH16s) from prominent mucin degrading gut bacteria that specifically target the oligosaccharide side chains of intestinal mucins from both animals and humans. These endo-acting O-glycanases display β1,4-glactosidase activity and in several cases are surface located indicating they are involved in the initial step in mucin breakdown. The data suggest a new paradigm for mucin breakdown by the microbiota and the endo-mucinases provide a potential tool to explore changes that occur in mucin structure in intestinal disorders such as inflammatory bowel disease and colon cancer.

Objectives/Hypothesis: To assess if, as previously reported in the literature, bile acids inhibit pepsin activity, resulting in pepsin having a less important role in laryngopharyngeal damage in reflux disease.Study Design: Prospective translational research study.Methods: A total of 78 patient's fasting gastric juice samples were obtained from routine endoscopy. The total bile acid (TBA) content and pepsin activity were measured using a 3α‐hydroxysteroid dehydrogenase–based kit for bile acids; and pepsin activity was measured using succinyl albumin/dimethylhaemoglobin as a substrate and the development of new N‐terminals. The ability of bile acids to effect pepsin activity was assessed with three primary bile acids, two unconjugated and one taurine conjugated, using the above N‐terminal assay.Results: Gastric juice contained median TBA of 40 (range 10–10010) μM and pepsin activity of 408 (range 27–3892)ug/ml. We used this data to inform the relative levels of pepsin and bile acids that might occur in a reflux event, and we used concentrations of bile acids between 10–100μM.Pepsin activity was pH dependent, but 28% of the activity was retained at pH 5.5. None of the bile acids showed any significant effect on pepsin activity across the pH range 2.0–6.0.Conclusions: At the levels and pH that pepsin and bile acids might occur in an LPR event, bile acids do not attenuate pepsin activity. Pepsin could be considered a damaging factor even at high pH, and it will aggravate further any damaging effects of bile acids in the refluxate. Laryngoscope, 2012