Reversible disruption of tight junction complexes in the rat blood‐brain barrier, following transitory focal astrocyte loss

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Abstract
Breakdown of the blood‐brain barrier is a feature of acute and chronic neurodegenerative changes, yet the relationship between astrocytes and the mature barrier remains unclear. We studied this role of astrocytes in vivo using a gliotoxin and evaluated changes in three vascular tight junction markers. Male Fisher F344 rats given systemic 3‐chloropropanediol showed astrocytic loss in the inferior colliculus from 12–24 h until the lesion was repopulated 8–28 days later. Within 6 h of astrocyte loss, microvessels in this area began to demonstrate a loss of the normal paracellular localization of the transmembrane proteins occludin and claudin‐5 and cytoplasmic zonula occludens‐1, which correlated with focal vascular leak of dextran (10 kDa) and fibrinogen. Platelet endothelial adhesion molecule‐1 staining revealed that there was no loss of the endothelial lining. Between 4–8 days, severe downregulation of tight junction protein expression was observed, which subsequently returned over the same time period as astrocytes repopulated the lesion. Unexpectedly, dextran and fibrinogen leak from vessels had ceased at 6 days, well before the return of occludin and claudin‐5 to appropriate paracellular domains. Control nonvulnerable cortical tissue showed no change in astrocyte morphology and tight junction expression over the same time course. Our data supports a primary role for astrocytic contact in the expression of occludin, claudin‐5, and zonula occludens‐1 in the mature brain vasculature in vivo. However, barrier integrity to dextran (10 kDa) and fibrinogen can be restored in the absence of astrocytes and tight junction proteins (occludin, claudin‐5, and zonula occludens‐1).