Mucins in contact lens wear and dry eye conditions.

Ocular mucins are thought to play integral roles in ocular surface lubrication, anchoring of the aqueous, stabilizing the lipid components of the tear film, eliminating foreign bodies and pathogens, and with potential involvement in cell cycle mediation and apoptotic activity of ocular surface epithelia. Ocular mucins are of secreted and membrane-associated types. Secreted mucins may be of large gel-forming type or small soluble mucins (e.g., MUC5AC and MUC7). Membrane-associated mucins such as MUCs 1 and 4 are a major component of the glycocalyx. They are thought to render structural support to the microplicae and mediate epithelial cell cycle and apoptotic activity. The alterations in ocular mucins with contact lens wear are unclear. Recent work shows mucin expression may be up-regulated during the early years of contact lens wear, and with long-term lens wear, mucin expression may return to normal levels or sub-normal levels, although this is not well understood. Further, the polar nature of mucins may be associated with their affinity for contact lens surfaces making them a component of contact lens deposition. This has potential implications in the wettability and tolerability of contact lenses, and may be impacted by surface coatings, polymer characteristics, or care solutions. Conjunctival mucin gene expression and secretion may be deficient in several ocular surface disorders associated with dry eye. Deficiency and alterations in glycosylation characteristics of MUC5AC and MUC2 have been reported in both Sjögren and non-Sjögren dry eye types. Decreased binding of the membrane-associated mucin MUC16 to the conjunctival epithelium has been reported in Sjögren dry eye while MUC1 alterations have been reported in Sjögren and non-Sjögren dry eye states. In view of the mucin involvement in dry eye conditions, stimulation of mucus secretion pathways may hold promise in the pharmaceutical treatment of dry eye.