Molecular mechanisms underlying the retinal vascular lesions in diabetic retinopathy

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Blood-retinal barrier (BRB) is known to be disrupted in diabetic retinopathy, which subsequently accelerates the progression of disease processes. However, it has not been clarified how the BRB breakdown occurs during the course of diabetic retinopathy, although the tissue hypoxia is thought to be involved as a trigger. In the present study, the mechanisms of BRB disruption in diabetic retinopathy were analyzed with reference to the retinal tissue hypoxia as well as the expression of claudin-5, a component of tight junctions, in vascular endothelial cells. In vitro, the movement of claudin-5 from the cytoplasm to the plasma membrane of cultured confluent brain-derived endothelial (bEND.3) cells was closely correlated with the increase in the transendothelial electrical resistance (TEER) which is an idex of barrier function of the cell monolayer. Inhibition of the expression of claudin-5 by RNAi resulted in a reduction of TEER indicating a critical role of claudin-5 in the barrier prope … More rty. Hypoxia (1% O_2) altered the location of claudin-5 in the plasma membrane and the level of claudin-5 protein in bEND.3 cells, and these changes were accompanied by a decrease in the TEER. In vivo, the claudin-5 molecules are expressed under normoxia in the plasma membrane of retinal microvascular endothelial cells, but were significantly reduced under hypoxic conditions. Tracer experiments revealed that the barrier function of hypoxic retinal vasculature with depressed claudin-5 expression was selectively disrupted against small molecules, which is very similar to the phenotype of claudin-5-deficient mice. These in vitro and in vivo data indicate that claudin-5 is a target molecule of hypoxia leading to the BRB disruption. Furthermore, the hypoxia-induced changes in claudin-5 expression were shown to be suppressed in the presence of an inhibitor of ubiquitin-proteasome pathway, MG-132, suggesting that the ubiquitin-proteasome pathway is involved in the hypoxia-induced breakdown of BRB. Less