Ablation of C/EBP Homologous Protein Does Not Protect T17M RHO Mice from Retinal Degeneration

Sonali Nashine,Marina Gorbatyuk,Alfred S Lewin,Yogesh Bhootada,Thomas Langmann

doi:10.1371/journal.pone.0063205

Abstract

Despite the proposed link between ablation of the CHOP protein and delay of the onset of ER stress-mediated disorders including diabetes, Alzheimer Disease, and cardiac hypertrophy, the role of CHOP protein in photoreceptor cell death associated with Autosomal Dominant Retinitis Pigmentosa (ADRP) has not been investigated. T17M RHO transgenic mice carry a mutated human rhodopsin transgene, the expression of which in retina leads to protein misfolding, activation of UPR and progressive retinal degeneration. The purpose of this study is to investigate the role of CHOP protein in T17M RHO retina. Wild-type, CHOP−/−, T17M RHO and T17M RHO CHOP−/−mice were used in the study. Evaluation of the impact of CHOP ablation was performed using electroretinography (ERG), spectral-domain optical coherence tomography (SD-OCT), quantitative Real-Time PCR (qRT-PCR) and western blot analysis. Dark-adapted ERG analysis demonstrated that by 1 month, the T17M RHO CHOP−/− mice had a 70% reduction of the a-wave amplitude compared to the T17M RHO mice. The loss of function in T17M RHO CHOP−/− photoreceptors was associated with a 22–24% decline in the thickness of the outer nuclear layer. These mice had significant reduction in the expression of transcription factors, Crx and Nrl, and also in mouse Rho, and human RHO. The reduction was associated with an 8-fold elevation of the UPR marker, p-eIf2α protein and 30% down-regulation of sXbp1 protein. In addition, the histone deacetylase 1 (Hdac1) protein was 2-fold elevated in the T17M RHO CHOP−/− retina. The ablation of CHOP led to a reduction in the expression of photoreceptor-specific transcriptional factors, and both endogenous and exogenous RHO mRNA. Thus, despite its role in promoting apoptosis, CHOP protects rod photoreceptors carrying an ADRP mutation.

Highlights

Autosomal dominant forms of progressive inherited retinal degeneration, retinitis pigmentosa (RP) account for approximately 30% of all RP cases [1]
Since the T17M rhodopsin increases the pro-apoptotic CHOP protein, we investigated whether the ablation of CHOP in an Autosomal Dominant Retinitis Pigmentosa (ADRP) retina expressing mutant opsin affects the rate of retinal degeneration
When we were searching for the link leading to down-regulation of Nrl, Crx and consequent down-regulation of RHO gene expression in the T17M RHO CHOP2/2 mice, we found that critical regulation of activity, or even degradation of the endoplasmic reticulum (ER) stress– induced CHOP transcriptional factor occurs through binding of P300/Cbp-associated factor at the N-terminal domain [19]

Summary

Introduction

Autosomal dominant forms of progressive inherited retinal degeneration, retinitis pigmentosa (RP) account for approximately 30% of all RP cases [1]. Mutations in rhodopsin (RHO) are the most prevalent class identified to date, causing 25% of all ADRP cases [1]. T17M is considered a Class II RHO mutation because it is characterized by the inability of mutant opsin to form functional rhodopsin with 11cis-retinal and by opsin accumulation in the endoplasmic reticulum (ER) and Golgi apparatus. The accumulation of these mutant misfolded proteins in the ER triggers a signal transduction cascade known as the Unfolded protein Response (UPR) [4], resulting in the activation of c-Jun and apoptosis [5]

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