Mucins and mucosal protection in the gastrointestinal tract: new prospects for mucins in the pathology of gastrointestinal disease

The luminal surface of the gastrointestinal tract is covered by a viscoelastic mucous gel layer that acts as a protective barrier against the harsh luminal environment. The structural characteristics of this barrier are primary indicators of its physiological function and changes to its composition have long been identified in gastrointestinal pathologies. During the past decade significant improvements in analytical techniques coupled with detailed knowledge of the genes coding for the mucin proteins have provided exciting new insights into the role of the mucous layer and its relevance to gastrointestinal disease. The high molecular weight mucins are responsible for the viscoelastic properties of the mucous barrier. They are widely expressed in epithelial tissues and are characterised by variable number tandem repeat peptide sequences rich in serine, threonine, and proline which carry large numbers of O -linked oligosaccharide chains.1 2 At present, 12 genes have been described, shown in table 1.1 3 Secreted and membrane associated forms have been found based on their function as extracellular viscous secretions or viscoelastic polymer gels or location as membrane anchored molecules in the glycocalyx.3 4 Two clusters have been reported, the secretory mucin genes MUC2, MUC5AC, MUC5B, and MUC6 on chromosome 11p15.5, and MUC3, MUC11, and MUC12 on chromosome 7q22.3 View this table: Table 1 Mucin genes and their location in the human gastrointestinal tract Histochemical techniques for mucin detection rely on the ability to detect carbohydrate or negative charge and were widely used for classification of changes in disease.5 6 The use of lectins and anticarbohydrate antibodies has greatly improved the specific detection of mucins histochemically and biochemically.5 6 A group of mucin oligosaccharide antigens, including Tn, sialyl-Tn, T, Lewisx and Lewisy, sialyl and sulpho-Lewisx and -Lewisa, and the blood group ABH antigens, have been identified …