Mucins and Mucus

Mucosal surfaces are exposed to the external environment and pathogens and are therefore protected by a secreted layer of mucus rich in mucin glycoproteins, which are the main components of mucus. Mucus provides physical protection and hydration, excludes pathogens, and is a reservoir for antimicrobial molecules. Underlying mucus further protection is provided by epithelial cell surface mucins, which limit microbial adherence and regulate growth and apoptosis. Differentiation of the cells that produce mucins, and expression of mucins and proteins involved in mucin biosynthesis, is regulated by innate and adaptive immunity. Experimental deficiencies in mucins lead to infectious and inflammatory diseases, and mucin gene polymorphisms are associated with disease. Many chronic mucosal inflammatory diseases are characterized by mucus hypersecretion driven by immune and microbial factors, which contributes to pathology and is a target for therapy. In this chapter we describe the nature of this mucosal barrier, its regulation and function, and its involvement in human disease, with particular emphasis on the mucin component of the barrier.