Claudins in Retinal Disease: Beyond Barrier Integrity to Signaling and Therapy

The blood-retinal barrier (BRB) maintains neurovascular homeostasis by regulating solute and ion exchange between the retina and circulation. This selectivity depends on tight junctions (TJs), with claudin (Cldn) proteins forming the core structure that defines paracellular permeability. Distinct Cldn isoforms show cell-specific expression, with Cldn-5 predominating in the endothelial cells of the inner BRB and Cldn-19 is the signature Cldn in the retinal pigment epithelium forming the outer BRB. Disruption of these isoforms contributes to vascular leakage, inflammation, and neuronal loss across various ocular diseases. Cldn function in vascular homeostasis is multifaceted; barrier dysfunction does not always result from Cldn loss, as excessive expression or mislocalization, particularly of Cldn-5, can also impair BRB integrity. Cldns act as dynamic signaling hubs that respond to metabolic, oxidative, and mechanical stress and are regulated through VEGF, Wnt/β-catenin, and RhoA/ROCK pathways. This review summarizes current understanding of Cldn biology in retinal vascular regulation and highlights emerging therapeutic strategies aimed at stabilizing Cldn expression and junctional localization. Small molecules and blocking antibodies that enhance localization or prevent degradation are redefining barrier repair. Key questions remain regarding isoform specificity, inter-barrier communication, and systemic safety. Integrative omics and spatial imaging may reveal disease-specific Cldn signatures and guide molecular restoration of BRB integrity.