Mucins

Mucins are a family of high molecular weight glycoproteins produced by epithelial tissues, notably goblet cells, that constitute the major components of mucus. They form hydrated, viscoelastic gels that coat mucosal surfaces and provide lubrication and protection. A characteristic feature is extensive O-linked glycosylation on serine and threonine residues within tandem repeat regions, which confers water-binding capacity and resistance to proteolysis. Mucins can be secreted into luminal spaces or anchored to the cell surface as membrane-associated mucins.

Classification of mucins includes secreted gel-forming mucins, such as MUC2, MUC5AC, MUC5B, and MUC6, which polymerize to create mucus networks that trap microbes and particulates. Membrane-associated mucins, including MUC1, MUC4, MUC16, and MUC17, extend from the cell surface and participate in signaling, hydration, and the protection of the epithelial boundary.

Biological roles of mucins encompass protection against mechanical damage and microbial invasion, maintenance of mucus hydration and viscosity, and modulation of host–microbe interactions through their glycans, which can act as decoy receptors. Mucin expression and glycosylation are dynamic, responding to inflammation, infection, and hormonal regulation. Proteolytic cleavage and shedding can regulate mucin function and surface signaling.

Clinical relevance of mucins arises from altered expression or glycosylation in disease. Abnormal mucin production is linked to cystic fibrosis, chronic obstructive pulmonary disease, inflammatory bowel diseases, and various cancers. Specific mucins serve as diagnostic or prognostic markers, such as MUC16 (CA125) in certain cancers.