What does mucin have to do with lung disease?

Mucin glycoproteins are a major macromolecular component of mucus. Mucins are large, heavily glycosylated glycoproteins that are expressed in two major forms: the membrane-tethered mucins and the secreted mucins. In the airways, MUC1 and MUC4 are the predominant membrane-tethered mucins that are present on epithelial cell surfaces; MUC5AC, MUC5B and MUC2 are the predominant secreted mucins that contribute to the mucus gel. Although the role of MUC1 and MUC4 in the airway is not known, they may function as receptors or receptor ligands and activate intracellular signalling cascades affecting epithelial functions. Several inflammatory mediators increase expression of the secreted mucin genes, MUC5AC and MUC2. Furthermore, overexpression of MUC5AC, MUC5B and MUC2 correlates strongly with secretory cell hyperplasia and metaplasia in human and murine airways. The insights gleaned from the investigations of mucin function and gene regulation should be useful for elucidating the cellular mechanisms leading to airway remodelling and mucus obstruction.