Hypoxia Causes the Changes in Claudin–5 Expression and the Barrier Function of Endothelial Cells

Abstract

Abstract: : Purpose: Diabetic macular edema which impairs the visual acuity of patients with diabetic retinopathy is attributed to the disruption of blood–retinal barrier (BRB), and the retinal edema is enhanced by the tissue hypoxia at the pre–proliferative stage. Complex network of tight junctions between the endothelial cells is indispensable for the maintenance of BRB function. This study aims to clarify the molecular mechanisms of BRB disruption in hypoxic diabetic retina through analyses of the expression of claudin–5 which is a tight junction component of retinal vascular endothelial cells. Methods: bEND.3, a brain microvascular endothelial cell line expressing claudin–5, was cultured under normoxic (20% O₂) or hypoxic (1% O₂) condition. The expression of claudin–5 was examined by immunocytochemistry and western blotting. Transendothelial electrical resistance (TER) of a bEND.3 monolayer was measured as an index of its barrier function. Furthermore, RNAi technique for the inhibition of claudin–5 expression was used to evaluate the role of claudin–5 in the barrier function of a bEND.3 monolayer. Results: Expression and localization of claudin–5 to the cell borders of normoxic bEND.3 cells were diminished after hypoxic incubation in the time–dependent manner. In close correlation to the changes in claudin–5 expression, TER of the bEND.3 monolayer decreased by hypoxia. Furthermore, the inhibition of claudin–5 expression by RNAi resulted in the decrease in TER of the bEND.3 monolayer even under normoxic condition. Conclusions: The barrier function formed by endothelial cells was shown to be disrupted by hypoxia, depending on the changes in claudin–5 expression and localization in endothelial cells. The present data could be a valuable clue to solve the molecular mechanisms of BRB disruption in diabetic retinopathy.

Keywords: diabetic retinopathy • hypoxia