Mucin

At least 20 human mucin genes have been distinguished by cDNA cloning — MUC1, MUC2, MUC3A, MUC3B, MUC4, MUC5AC, MUC5B, MUC6, MUC7, MUC8, MUC12, MUC13, MUC15, MUC16, MUC17, MUC19, and MUC20.[6] A recently identified human mucin gene is MUC21, previously known as chromosome 6 open reading frame 205. The highly polymorphic[7] MUC22 is located near MUC21. The major secreted airway mucins are MUC5AC and MUC5B, while MUC2 is secreted mostly in the intestine but also in the airway.

Mature mucins are composed of two distinct regions:

• The amino- and carboxy-terminal regions are very lightly glycosylated, but rich in cysteines. The cysteine residues participate in establishing disulfide linkages within and among mucin monomers.
• A large central region formed of multiple tandem repeats of 10 to 80 residue sequences in which up to half of the amino acids are serine or threonine. This area becomes saturated with hundreds of O-linked oligosaccharides. N-linked oligosaccharides are also found on mucins, but in less abundance than O-linked sugars.

Mucins have been found to have important functions in defense against bacterial and fungal infections. MUC5B, the predominate mucin in the mouth and female genital tract, has been shown to significantly reduce attachment and biofilm formation of Streptococcus mutans, a bacteria with the potential to form cavities.[8] Unusually, MUC5B does not kill the bacteria but rather maintains it in the planktonic (non-biofilm) phase, thus maintaining a diverse and healthy oral microbiome.[8] Similar effects of MUC5B and other mucins have been demonstrated with other pathogens, such as Candida albicans, Helicobacter pylori, and even HIV.[9][10] In the mouth, mucins can also recruit anti-microbial proteins such as statherins and histatine 1, which further reduces risk of infection.[10]

Mucin genes encode mucin monomers that are synthesized as rod-shape apomucin cores that are post-translationally modified by exceptionally abundant glycosylation.

The dense "sugar coating" of mucins gives them considerable water-holding capacity and also makes them resistant to proteolysis, which may be important in maintaining mucosal barriers.

Mucins are secreted as massive aggregates of proteins with molecular masses of roughly 1 to 10 million Da. Within these aggregates, monomers are linked to one another mostly by non-covalent interactions, although intermolecular disulfide bonds may also play a role in this process.

Upon stimulation, MARCKS (myristylated alanine-rich C kinase substrate) protein coordinates the secretion of mucin from mucin-filled vesicles within the specialized epithelial cells.[11] Fusion of the vesicles to the plasma membrane causes release of the mucin, which as it exchanges Ca²⁺ for Na⁺ expands up to 600 fold. The result is a viscoelastic product of interwoven molecules which, combined with other secretions (e.g., from the airway epithelium and the submucosal glands in the respiratory system), is called mucus.[12] [13]

Increased mucin production occurs in many adenocarcinomas, including cancers of the pancreas, lung, breast, ovary, colon and other tissues. Mucins are also overexpressed in lung diseases such as asthma, bronchitis, chronic obstructive pulmonary disease (COPD) or cystic fibrosis.[14] Two membrane mucins, MUC1 and MUC4 have been extensively studied in relation to their pathological implication in the disease process.[15][16][17] Mucins are under investigation as possible diagnostic markers for malignancies and other disease processes in which they are most commonly over- or mis-expressed.

Abnormal deposits of mucin are responsible for the non-pitting facial edema seen in untreated hypothyroidism. This edema is seen in the pretibial area as well.[18]

• Bovine submaxillary mucin coatings
• Verotoxin-producing Escherichia coli

• Mucins at the US National Library of Medicine Medical Subject Headings (MeSH)
• "Mucin" at Dorland's Medical Dictionary