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Jingyu Yao, Lin Jia, David N. Zacks; Circadian-rhythm and Dark-light Control of Basal Autophagy in Mouse Photoreceptor and RPE Cells. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4328.
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To characterize the circadian and dark-light control of the basal level of autophagy in mouse photoreceptor and retinal pigment epithelium (RPE) cells.
Wild-type C57BL/6 mice were bred and reared under standard cycling 12-hour light / 12 hour dark conditions. At age 2 months, retina and RPE were collected from 2 groups of animals. In group1, animals were sacrificed every 3 hours over a 24 hour period. In group 2, at 3 hours after the standard cyclic lights turned on, animals were dark adapted and sacrificed at 1, 4 and 8 hours after being moved to the dark. The basal levels of the autophagy proteins LC3, Atg5, beclin1 and p62 and the time course of LC3-I to LC3-II conversion were evaluated by western blot and immunohistochemistry (IHC).
Basal levels of autophagy in both photoreceptor and RPE cells followed a circadian rhythm. In both cell types we observe two peaks of autophagy, one at 7 hours after the onset of the light period and one at 7 hours after the onset of the dark period. The lowest autophagy activity occurred shortly (about 1 hour) after the shift from dark to light and again from light to dark. Intra-retinal LC3 staining was detected primarily in the photoreceptor inner segment, with a punctate versus diffuse staining pattern mimicked the time course of peak LC3-I to LC3-II conversion seen on western blot. Mice in group 2 also had a sharp decrease in autophagy shortly after being transferred from the light to the dark, with a subsequent time-dependent increase in LC3-II conversion the longer they were kept in the dark.
Autophagy in mouse rod photoreceptor and RPE cells follows a bimodal circadian rhythm with one peak at 7 hours after the onset of the light period and one at 7 hours after the onset of the dark period. This study helps define the basal changes in autophagy required for normal photoreceptor and RPE homeostasis. Further work is needed to understand the mechanism underling the circadian rhythm and the dark-light control of autophagy in these cells and the intracellular targets of the autophagy degradation pathway.
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