Glycobiology of the ocular surface: mucins and lectins

Abstract

Glycosylation is an important and common form of posttranscriptional modification of proteins in cells. During the last decade, a vast array of biological functions has been ascribed to glycans because of a rapid evolution in glycomic technologies. Glycogenes that are highly expressed at the human ocular surface include families of glycosyltransferases, proteoglycans, and glycan degradation proteins, as well as mucins and carbohydrate-binding proteins, such as the galectins. On the apical glycocalyx, mucin O-glycans promote boundary lubrication, prevent bacterial adhesion and endocytic activity, and maintain epithelial barrier function through interactions with galectins. The emerging roles attributed to glycans are contributing to the appreciation of their biological capabilities at the ocular surface.

Figure 1

Proposed model for cell surface O-glycans preventing endocytosis in human corneal epithelial cells. Exposed mucosal surfaces limit constitutive endocytosis under physiological conditions to prevent uptake of macromolecules and pathogens. Expression of C1galt1 results in mucin O-glycan biosynthesis and formation of cell surface lattices through interaction with galectin-3 multimers. Abrogation of C1galt1 decreases the availability of mucin carbohydrate ligands in the glycocalyx, causing disruption of galectin-glycan complexes, and triggering clathrin-dependent endocytosis of plasma membrane proteins and extracellular components.