Limited ATF4 Expression in Degenerating Retinas with Ongoing ER Stress Promotes Photoreceptor Survival in a Mouse Model of Autosomal Dominant Retinitis Pigmentosa.

T17M rhodopsin expression in rod photoreceptors leads to severe retinal degeneration and is associated with the activation of ER stress related Unfolded Protein Response (UPR) signaling. Here, we show a novel role of a UPR transcription factor, ATF4, in photoreceptor cellular pathology. We demonstrated a pro-death role for ATF4 overexpression during autosomal dominant retinitis pigmentosa (ADRP). Based on our results in ATF4 knockout mice and adeno-associated viral (AAV) delivery of ATF4 to the retina, we validated a novel therapeutic approach targeting ATF4 over the course of retinal degeneration. In T17M rhodopsin retinas, we observed ATF4 overexpression concomitantly with reduction of p62 and elevation of p53 levels. These molecular alterations, together with increased CHOP and caspase-3/7 activity, possibly contributed to the mechanism of photoreceptor cell loss. Conversely, ATF4 knockdown retarded retinal degeneration in 1-month-old T17M Rhodopsin mice and promoted photoreceptor survival, as measured by scotopic and photopic ERGs and photoreceptor nuclei row counts. Similarly, ATF4 knockdown also markedly delayed retinal degeneration in 3-month-old ADRP animals. This delay was accompanied by a dramatic decrease in UPR signaling, the launching of anti-oxidant defense, initiation of autophagy, and improvement of rhodopsin biosynthesis which together perhaps combat the cellular stress associated with T17M rhodopsin. Our data indicate that augmented ATF4 signals during retinal degeneration plays a cytotoxic role by triggering photoreceptor cell death. Future ADRP therapy regulating ATF4 expression can be developed to treat retinal degenerative disorders associated with activated UPR.

Up-regulation of activating transcription factor 4 induces severe loss of dopamine nigral neurons in a rat model of Parkinson's disease.

Activating transcription factor 4 (ATF4) is a member of the PERK signaling pathway, which directly binds endoplasmic reticulum stress target genes and plays a crucial role in both adaptations to stress and activation of apoptosis. Previous publications demonstrated conflicting evidence on the role of ATF4 in the pathogenesis of neurodegenerative disorders. In this study, we used recombinant adeno-associate virus (rAAV)-mediated gene transfer to investigate if the sustained up-regulation of ATF4 launches a pro-survival or pro-death trend in the dopamine (DA) cells of the substantia nigra pars compacta in a rat model of Parkinson-like neurodegeneration induced by human alpha-synuclein (αS) overexpression. We showed that ATF4 does not protect nigral DA neurons against an αS-induced pathology. Moreover, the rAAV-mediated overexpression of ATF4 resulted in severe nigra-striatal degeneration via activation of caspases 3/7.

Unfolded protein response is activated in aged retinas.

An unfolded protein response (UPR) in addition to oxidative stress and the inflammatory response is known to be activated in age-related ocular disorders, such as macular degeneration, diabetic retinopathy, glaucoma, and cataracts. Therefore, we aimed to investigate whether healthy aged retinas display UPR hallmarks, in order to establish a baseline for the activated UPR markers for age-related ocular diseases. Using western blotting, we determined that the hallmarks of the UPR PERK arm, phosphorylated (p) eIF2a, ATF4, and GADD34, were significantly altered in aged vs. young rat retinas. The cleaved pATF6 (50) and CHOP proteins were dramatically upregulated in the aged rodent retinas, indicating the activation of the ATF6 UPR arm. The UPR activation was associated with a drop in rhodopsin expression and in the NRF2 and HO1 levels, suggesting a decline in the anti-oxidant defense in aged retinas. Moreover, we observed down-regulation of anti-inflammatory IL-10 and IL-13 and upregulation of pro-inflammatory RANTES in the healthy aged retinas, as measured using the Bio-plex assay. Our results suggest that cellular homeostasis in normal aged retinas is compromised, resulting in the concomitant activation of the UPR, oxidative stress, and inflammatory signaling. This knowledge brings us closer to understanding the cellular mechanisms of the age-related retinopathies and ocular disorders characterized by an ongoing UPR, and highlight the UPR signaling molecules that should be validated as potential therapeutic targets.

Targeting Caspase-12 to Preserve Vision in Mice With Inherited Retinal Degeneration.

PURPOSE: The unfolded protein response is known to contribute to the inherited retinal pathology observed in T17M rhodopsin (T17M) mice. Recently it has been demonstrated that the endoplasmic reticulum stress-associated caspase-12 is activated during progression of retinal degeneration in different animal models. Therefore, we wanted to explore the role of caspase-12 in the mechanism of retinopathy in T17M mice and determine if inhibiting apoptosis in this way is a viable approach for halting retinal degeneration. METHODS: One, two-, and three-month-old C57BL6/J, caspase-12-/-, T17M, and T17M caspase-12-/- mice were analyzed by scotopic ERG, spectral-domain optical coherence tomography (SD-OCT), histology, quantitative (q)RT-PCR, and Western blot of retinal RNA and protein extracts. Calpain and caspase-3/7 activity assays were measured in postnatal (P) day 30 retinal extracts. RESULTS: Caspase-12 ablation significantly prevented a decline in the a- and b-wave ERG amplitudes in T17M mice during three months, increasing the amplitudes from 232% to 212% and from 160% to 138%, respectively, as compared to T17M retinas. The SD-OCT results and photoreceptor row counts demonstrated preservation of retinal structural integrity and postponed photoreceptor cell death. The delay in photoreceptor cell death was due to significant decreases in the activity of caspase-3/7 and calpain, which correlated with an increase in calpastatin expression. CONCLUSIONS: We validated caspase-12 as a therapeutic target, ablation of which significantly protects T17M photoreceptors from deterioration. Although the inhibition of apoptotic activity alone was not sufficient to rescue T17M photoreceptors, in combination with other nonapoptotic targets, caspase-12 could be used to treat inherited retinopathy.

Modulation of the rate of retinal degeneration in T17M RHO mice by reprogramming the unfolded protein response.

The goal of this study is to validate whether reprogramming of the UPR via modulation of pro-apoptotic caspase-7 and CHOP proteins could be an effective approach to slow down the rate of retinal degeneration in ADRP mice. In order to pursue our goal we created the T17M RHO CASP7 and T17M RHO CHOP mice to study the impact of the CASP7 or CHOP ablations in T17M RHO retina by ERG, SD-OCT, histology and western blot analysis. The scotopic ERG demonstrated that the ablation of the CASP7 in T17M RHO retina leads to significant preservation of the function of photoreceptors compared to control. Surprisingly, the ablation of pro-apoptotic CHOP protein in T17M RHO mice led to a more severe form of retinal degeneration. Results of the SD-OCT and histology were in agreement with the ERG data. The further analysis demonstrated that the preservation of the structure and function or the acceleration of the onset of the T17M RHO photoreceptor degeneration occurred via reprogramming of the UPR. In addition, the CASP7 ablation leads to the inhibition of cJUN mediated apoptosis, while the ablation of CHOP induces an increase in the HDAC. Thus, manipulation with the UPR requires careful examination in order to achieve a therapeutic effect.

Ablation of C/EBP homologous protein does not protect T17M RHO mice from retinal degeneration.

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.