Ocular surface hydration barrier

The **ocular surface hydration barrier** is not a single molecule or receptor but rather refers to the collective structural and biochemical features of the ocular surface that maintain moisture and protect underlying tissues. This includes several key components: - The **glycocalyx**, composed primarily of membrane-associated mucins such as MUC1, MUC4, and MUC16 expressed by corneal and conjunctival epithelial cells. These heavily glycosylated proteins extend into the tear film, providing wettability, lubrication, antiadhesive properties, and forming a physical shield against pathogens[2][5]. - **Secreted mucins** like MUC5AC from conjunctival goblet cells contribute to tear film stability and ocular lubrication. Loss of these mucins is associated with dry eye disease[4]. - The **tear film lipid layer**, which reduces evaporation. Other proteins such as **clusterin** play roles in sealing and protecting this barrier under stress conditions; decreased levels are linked with increased vulnerability in dry eye states. Clusterin binds selectively to stressed surfaces—especially via galectin LGALS3—and can physically seal damaged areas while protecting epithelial cells from further injury[1][3]. Disruption of any component can lead to increased evaporation, inflammation, infection risk, or clinical syndromes like dry eye disease. While some therapeutics aim at restoring individual elements (e.g., stimulating mucin secretion with diquafosol), there is no drug that targets an "ocular surface hydration barrier" per se because it is not a discrete molecular entity but rather an emergent property arising from multiple interacting molecules. In summary: “Ocular surface hydration barrier” describes an essential physiological function maintained by various molecules—including membrane-associated mucins (MUC1/4/16), secreted mucins (MUC5AC), clusterin protein—and does not correspond to any single canonical drug target molecule or receptor.[2][4][5]