X-Box Binding Protein 1 Is Essential for the Anti-Oxidant Defense and Cell Survival in the Retinal Pigment Epithelium

Yimin Zhong,Md Nawajes A Mandal,Robert E Anderson,Julie-Thu A Tran,Qiang Yu,Chen Chen,Anisse Saadi,Joshua J Wang,Jingming Li,Sarah X Zhang,Yun-Zheng Le,Neeraj Vij

doi:10.1371/journal.pone.0038616

Abstract

Damage to the retinal pigment epithelium (RPE) is an early event in the pathogenesis of age-related macular degeneration (AMD). X-box binding protein 1 (XBP1) is a key transcription factor that regulates endoplasmic reticulum (ER) homeostasis and cell survival. This study aimed to delineate the role of endogenous XBP1 in the RPE. Our results show that in a rat model of light-induced retinal degeneration, XBP1 activation was suppressed in the RPE/choroid complex, accompanied by decreased anti-oxidant genes and increased oxidative stress. Knockdown of XBP1 by siRNA resulted in reduced expression of SOD1, SOD2, catalase, and glutathione synthase and sensitized RPE cells to oxidative damage. Using Cre/LoxP system, we generated a mouse line that lacks XBP1 only in RPE cells. Compared to wildtype littermates, RPE-XBP1 KO mice expressed less SOD1, SOD2, and catalase in the RPE, and had increased oxidative stress. At age 3 months and older, these mice exhibited apoptosis of RPE cells, decreased number of cone photoreceptors, shortened photoreceptor outer segment, reduced ONL thickness, and deficit in retinal function. Electron microscopy showed abnormal ultrastructure, Bruch's membrane thickening, and disrupted basal membrane infolding in XBP1-deficient RPE. These results indicate that XBP1 is an important gene involved in regulation of the anti-oxidant defense in the RPE, and that impaired activation of XBP1 may contribute to RPE dysfunction and cell death during retinal degeneration and AMD.

Highlights

Age-related macular degeneration (AMD) is the leading cause of blindness in the elderly
A recent study showed that several ER stress markers such as glucose-regulated protein-78 (GRP78), phosphorylated PKR-like endoplasmic reticulum kinase (PERK), and phosphorylated eukaryotic initiation factor 2a were elevated in the rat retina after light damage, indicating an induction of ER stress [23]
We found that the level of spliced X-box binding protein 1 (XBP1) in the retina was significantly increased after light stress, coincident with enhanced expression of retinal GRP78 and C/EBP homologous protein (CHOP), an ER stress-associated pro-apoptotic gene (Fig. 1A)

Summary

Introduction

Age-related macular degeneration (AMD) is the leading cause of blindness in the elderly. The dry form of AMD, characterized by depigmentation of the retinal pigment epithelial cells (RPE), loss of RPE cells, and drusen formation, presents in 80–90% of the AMD patients. The pathogenic mechanisms of AMD are not fully understood, compelling evidence suggests that decreased anti-oxidant defense with age in highly metabolically active retinal cells is a key etiological factor for AMD. The activity of catalase, an anti-oxidant enzyme, was decreased with age [3]. These changes suggest that the RPE in the elderly may be more susceptible to oxidative damage. In human retinas with AMD, apoptotic photoreceptors were found clustered near the area of RPE atrophy, suggesting that loss of RPE cell may proceed photoreceptor apoptosis during disease development [8]. Down-regulation of SOD2, a major anti-oxidant enzyme in the mitochondria, in the RPE by a subretinal injection of an AAV-

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