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.

Akkermansia muciniphila is a human microbial symbiont residing in the mucosal layer of the large intestine. Its main carbon source is the highly heterogeneous mucin glycoprotein and A. muciniphila uses an array of Carbohydrate-active enzymes and sulfatases to access this complex energy source. Here we describe the biochemical characterisation of fifty-four glycoside hydrolases, eleven sulfatases, and one polysaccharide lyase from A. muciniphila to provide a holistic understanding of the carbohydrate-degrading activities. The results provide an extensive insight into the sequence of O-glycan degradation and how A. muciniphila can access this structurally variable substrate. One of the most outstanding elements of this work was the demonstration that these enzymes can act synergistically to degrade the O-glycans on the mucin polypeptide to completion, down to the core GalNAc. Additionally, human breast milk oligosaccharide, ganglioside, and globoside glycan structures were included in the study to understand the full degradative capability of A. muciniphila .

Lactopontin (LPN) is a bioactive milk protein of increasing interest for nutritional applications. While traditionally sourced from bovine milk, precision fermentation now provides an alternative means of producing recombinant human lactopontin (rhLPN). This study compared the gastrointestinal digestion and intestinal interaction characteristics of rhLPN and commercially available bmLPN within a representative nutritional matrix using complementary in vitro gastrointestinal models. Protein digestion was initially characterised using the INFOGEST static digestion protocol, followed by evaluation in the MuCo-Absorb+ model, which combines simulated gastrointestinal digestion with a mucus-covered differentiated Caco-2 epithelial interface to assess post-digestive epithelial interaction and transepithelial transport. Digestion products were characterised by Western blotting, SDS-PAGE, size-exclusion HPLC and LC-MS/MS, while epithelial compatibility was assessed by transepithelial electrical resistance (TEER) and cell viability. Within both models, rhLPN and bmLPN exhibited highly comparable digestion behaviour and peptide molecular weight distributions across all concentrations and manufacturing scales evaluated. Differences in peptide molecular weight distributions were observed between the INFOGEST and MuCo-Absorb+ models, demonstrating that digestion model configuration had a greater influence on digestion outcome than protein source, concentration or manufacturing scale. No intact lactopontin was detected in the basolateral compartment by Western blotting, RP-HPLC or LC-MS/MS. However, low-molecular-weight peptides derived from the nutritional matrix were detected following transepithelial transport. This pattern is consistent with loss of the intact integrin-binding region during intestinal digestion. Digestion products from both protein sources maintained epithelial barrier integrity and cell viability throughout the transport experiment. Collectively, these findings demonstrate that precision-fermented rhLPN undergoes gastrointestinal processing and intestinal interaction comparable to commercially available bmLPN, supporting its application as an alternative source of dietary lactopontin while highlighting the importance of model selection when evaluating the digestive fate of bioactive food proteins.