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Lana M Pollock, Jing Xie, Brent A Bell, Mariya Ali, Alecia Cutler, Bela Anand-Apte; Retinoic acid signaling regulates tight junction protein expression in blood-retinal barrier maintenance. Invest. Ophthalmol. Vis. Sci. 2017;58(8):600.
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The blood-retinal barrier (BRB) mediates movement of molecules from the blood to the inner retina, protecting the retinal neural tissue from potentially harmful molecules and maintaining retinal homeostasis. Tight junctions between retinal capillary endothelial cells in the inner BRB and between RPE cells in the outer BRB are essential to barrier function. Retinoic acid (RA) has been shown to regulate development of the blood brain barrier (BBB), but its role in the BRB has not yet been explored. The purpose of this study was to investigate the role of RA signaling in regulation of tight junctions in BRB maintenance.
To visualize the BRB in vivo, we utilized the transgenic Tg(l-fabp:DBP-EGFP) zebrafish model that expresses vitamin D binding protein (a member of the albumin gene family) tagged to GFP. This model displays the integrity of the BRB with GFP-tagged protein localized within the retinal vasculature by 3 days post-fertilization. Breakdown of the BRB is visualized as “leaking” of GFP outside the vasculature. To disrupt RA signaling, transgenic embryos and adults were treated with varying concentrations of DEAB and BMS493, antagonists of retinal dehydrogenase and the RA receptor, respectively. Leakage from the retinal vasculature was visualized under a fluorescent microscope. Tight junction protein expression was assessed by immunolabeling of the hyaloid vessels and RPE in RA-inhibitor-treated fish.
Treatment with DEAB or BMS493 resulted in a non-functional disrupted BRB in up to 95% of embryos and adults, and expression of tight junction proteins ZO1 and claudin-5 was disrupted in the inner and outer BRB. Co-treatment with 0.05 µM all-trans retinoic acid effectively rescued BRB integrity. Degradation of RA is catalyzed by Cyp26 enzymes. To determine whether Cyp26 enzymes could also regulate BRB breakdown, we utilized a transgenic zebrafish, Tg(hsp:cyp26a1), which expresses Cyp26a1 driven by a heat shock promoter. Tg(hsp:cyp26a1) fish were crossed with the Tg(l-fabp:DBP-EGFP) line and the resulting embryos were heat shocked to induce overexpression of Cyp26a1. The BRB was also disrupted in heat-shocked Tg(hsp:cyp26a1) fish.
Our results clearly demonstrate that RA signaling is essential for BRB tight junction maintenance and we hypothesize that this pathway may play a role in diseases with disruption of the BRB such as diabetic retinopathy.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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