Claudin 5 Across the Vascular Landscape: From Blood–Tissue Barrier Regulation to Disease Mechanisms

Claudin 5 (Cldn5) is a critical tight junction protein essential for maintaining paracellular barrier integrity across endothelial and epithelial cells in barrier-forming tissues, including the blood–brain barrier and blood–retinal barrier. Cldn5 plays a central role in regulating vascular permeability, immune responses, and tissue homeostasis. The complex distribution and organ-specific regulation of Cldn5 underscore its potential as a promising therapeutic target. This review comprehensively analyzes the role of Cldn5 in endothelial and epithelial barrier function, its regulation of vascular permeability, and the discrepancies in the literature regarding its expression, regulation, and function in both physiological and pathological conditions across multiple organ systems, including the retina, brain, lung, heart, gut, kidney, liver, skin, and peripheral nerves, while emphasizing its tissue-specific expression patterns. We discuss how both reduced and excessive expressions of Cldn5 can disrupt barrier integrity and contribute to the pathogenesis of ischemic retinopathies, neuroinflammation, cardiovascular injury, and other forms of barrier dysfunction. Furthermore, we explore the dual role of Cldn5 as both a biomarker and a therapeutic target, highlighting emerging strategies such as RNA silencing, pharmacological stabilizers, and transcriptional modulators in controlling barrier leakage in disease conditions.