Abstract
Purpose :
Human retinal rod and cone photoreceptors are constantly exposed to environmental light. In both normal aging and many of the retinal diseases causing night blindness (e.g. retinitis pigmentosa, rod dystrophy), rod death by apoptosis often precedes cone death. However, the mechanisms of increased resistance of cones to light-induced damage and their genetic bases are still unclear. To address this issue, we used a unique mouse model with spontaneous mutation in the gene encoding the rod transcription factor Nr2e3 (rd7) which results in the upregulation of a subset of cone genes in its hybrid rods.
Methods :
rd7 mice and their littermate controls (WT) were generated on a mixed C57Bl/6Jx129SvE genetic background and were homozygous for the Leu-450 isoform of retinoid isomerase RPE65. The fast RPE visual cycle in these animals increases the susceptibility of rods to light damage caused by accumulation of toxic all-trans-retinal upon photobleaching. To determine the threshold of retinal degeneration, 3.5-month-old mice were exposed to white light of three intensities (7.5, 15, or 40 kLux) for 5–8 h. Retinal function and morphology were analyzed a week after the irradiation from in vivo ERG and retinal sections, respectively. Rod dark adaptation was also assessed by ERG.
Results :
ERG recordings revealed that the threshold of photoreceptor degeneration is increased by ∼10-fold in rd7 animals (40 kLux, 8 h) compared to WT controls (7.5 kLux, 5 h). In WT mice, 8-h exposure to intermediate (15 kLux) light caused substantial thinning of the outer nuclear layer in central dorsal (by 67%), peripheral dorsal (29%), and central ventral (23%) retinal areas, correlating to their dramatically reduced ERG responses. In contrast, no morphological and physiological evidence of photoreceptor degeneration was found in rd7 mice under the same experimental conditions. These striking differences were likely not due to altered RPE visual cycle in rd7 mice because the recovery of both maximal ERG response amplitude and photosensitivity of mutant rods during their dark adaptation following >90% pigment bleach was surprisingly normal.
Conclusions :
Expression of a subset of cone-specific genes in hybrid rods of rd7 mice makes them resistant to light-induced damage in a visual cycle-independent manner. The RNA-Seq analysis seeking to resolve molecular determinants of this phenomenon is currently underway.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.