Abstract
Purpose :
The retina is one of the most metabolically demanding tissues in the body as it is constantly exposed to oxygen and light, major oxidative stressors. To alleviate these stress conditions, cells have developed stress response pathways to maintain protein and metabolic homeostasis. This is crucial because retinal neurons are post-mitotic and terminally differentiated cells, so damage is often irreversible and leads to vision loss and blindness. Many studies have shown that the endoplasmic reticulum (ER) stress response plays a critical role in the pathology of inherited retinal degenerative diseases, such as retinitis pigmentosa (RP). Autosomal dominant RP is most commonly linked to a P23H mutation in the rhodopsin gene. This proline-to-histidine mutation results in rhodopsin misfolding, increased ER stress, and prolonged activation of the unfolded protein response (UPR), leading to photoreceptor degeneration. The ER-resident Fic enzyme has been previously shown to modulate the UPR response by catalyzing the AMPylation and deAMPylation of the BiP protein, a UPR sensor, in response to ER stress levels. Since the misfolding of rhodopsin is widely believed to induce UPR activation, we hypothesized that the absence of Fic will lead to a dysregulated UPR and more rapid and significant retinal degeneration in the RhoP23H mouse.
Methods :
To investigate how Fic regulates the UPR in the retina, we crossed the RhoP23H mouse with a novel Fic knockout mouse model. We monitored changes in both retinal structure and function over time using scanning laser ophthalmoscopy (SLO) and electroretinograms (ERGs), respectively.
Results :
To our surprise, we did not observe any exacerbated photoreceptor degeneration in the absence of Fic. Further investigation into the UPR response in the retina of these mice, at both the mRNA and protein level, is ongoing.
Conclusions :
Prior studies have shown that BiP mRNA is reduced in P23H mutant photoreceptors, and the insufficient levels of BiP may diminish the effect of the loss of Fic-mediated AMPylation. There also could be other cellular mechanisms, such as calpain activation, that could act as critical drivers in photoreceptor cell death caused by the P23H mutation in rhodopsin. Overall, our SLO and ERG results imply that the P23H mutation overrides the Fic-mediated UPR checkpoint, leaving the retina vulnerable to disease.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.