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Jingming Li, Yimin Zhong, Joshua J. Wang, Yun-zheng Le, Md Nawajes A. Mandal, Sarah X. Zhang; Loss Of Xbp1 In The Aging Rpe Reduces Autophagy And Exacerbates Rpe Cell Death: A Novel Mechanism For Age-related Macular Degeneration. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4784.
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Retinal pigment epithelium (RPE) dysfunction is a hallmark of age-related macular degeneration (AMD). Autophagy is a vital cellular housekeeping mechanism by eliminating malfunctional organelles and damaged proteins. X-box binding protein 1 (XBP1) is a key coordinator of unfolded protein response (UPR). In this study, we investigated the role of XBP1 in regulation of autophagic pathways in the RPE and its implication in AMD.
Albinotic RPE-specific XBP1 knockout mice were generated by using Cre/lox system. Light-induced retinal damage (LIRD) model was produced by exposure of mice to 3 klux of white fluorescent light for 6h. Human RPE (ARPE-19) cell line was used for in vitro study. Up- and down-regulation of XBP1 activity was achieved by adenovirus expressing spliced XBP1 and quinotrierixin, a specific inhibitor of XBP1 activation, or XBP1 siRNA, respectively. Expression of key autophagic genes was determined by quantitative RT-PCR, western blotting or immunohistochemistry. Autophagic flux was measured by conversion of LC3BII to LC3BI. RPE cell apoptosis was assessed TUNEL staining.
We observed a decline in XBP1 activation in aged mouse RPE when exposed to ER stress stimulators. Decreased XBP1 activation was also observed in the RPE after light damage in a LIRD model. To study the role of XBP1 in the RPE and in AMD, we generated a RPE-specific XBP1 KO mouse line. Mice deficient of XBP1 in the RPE displayed reduced LC3BI/II expression with aging, accompanied by RPE apoptosis and reduced outer nuclear layer (ONL) thickness. XBP1 KO mice exposed to light stress exhibited exaggerated photoreceptor damage, suggesting a protective role of XBP1 in the RPE. In ARPE-19 cells, over-expression of XBP1 significantly increased ATG7, ATG12, ATG5 and LC3B I/II expression. Conversely, knockdown of XBP1 decreased Beclin-1 and LC3BI/II level. Moreover, incubation with 4-HNE, an oxidative stress inducer, or nutrient deprivation, stimulated autophagic flux in ARPE-19 cells or mouse RPE/Choroid complex, which was remarkably reduced in XBP1-deficient RPE. In addition, lack of XBP1 resulted in RPE apoptosis and potentiated cell death induced by 4-HNE, which was reversed by enhancing autophagy using rapamycin.
Taken together, our data indicate that XBP1 is a key protective gene that regulates UPR and autophagy in the RPE. Declined XBP1 activation in the RPE may contribute to the pathogenesis of AMD.
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