Ocular mucins: Purification, metabolism and functions

Mucins are present at the ocular surface in both secreted and membrane-bound forms. Mucins are produced in part by the conjunctival goblet cells, and are complemented by non-goblet secretions. This review focuses on secreted ocular mucins. They are present in the tear film, probably both in gel and soluble form, and play a role in lubrication and ocular defence. It is apparent that mucins are highly adapted to their functions. State of the art techniques for mucin purification and analysis are presented. Density gradient centrifugation, gel filtration, ion-exchange chromatography and agarose gel electrophoresis are discussed, together with methods of oligosaccharide analysis. Reagents for the detection of mucin are considered in conjunction with these methods, which we have employed in the analysis of human and canine ocular mucins. The general structure of mucins is reviewed. The biosynthesis and glycosylation of ocular mucins are not yet fully understood, and are discussed in relation to currently established concepts. The impact of disease on the nature and secretion of mucins is considered, as well as the physiological and pathological significance of mucus degradation.