Ocular Surface Membrane-Associated Mucins

Ocular surface epithelial cells produce and secrete mucins that form a hydrophilic barrier for protection and lubrication of the eye. This barrier, the glycocalyx, is formed by high molecular weight heavily glycosylated membrane-associated mucins (MAMs) that include MUC1, MUC4, and MUC16. These mucins extend into the tear film from the anterior surfaces of the conjunctiva and cornea, and, through interactions with galectin-3, prevent penetrance of pathogens into the eye. Due primarily to the glycosylation of the mucins, the glycocalyx also creates less friction during blinking and enables the tear film to maintain wetting of the eye. The secretory mucins include soluble MUC7 and gel-forming MUC5AC. These mucins, particularly MUC5AC, assist with removal of debris from the tear film and contribute to the hydrophilicity of the tear film. While new methodologies and cell culture models have expanded our understanding of mucin structure and function on the ocular surface, there is still a paucity of studies characterizing the glycosylation of MAMs on a normal ocular surface and a diseased ocular surface. Although studies have shown alterations in mucin production and expression in dry eye diseases, the relationship between changes in mucins and functional consequences is unclear. This review focuses on comparing what is known about MAMs in wet-surfaced epithelia of the body to what has been studied on the eye.