Abstract
Purpose :
The functional interaction between photoreceptor and retinal pigment epithelium (RPE) cells is essential for vision, and decreased interaction is central in age-related macular degeneration (AMD). Phagocytosis of photoreceptor outer segments (POS) by the RPE has a circadian pattern, but it is unknown whether other processes of the RPE follow a circadian rhythm. Disturbed circadian rhythm of cellular processes has been shown to cause age-related diseases. Therefore, our aim was to determine whether and which RPE processes also show a circadian rhythm.
Methods :
The physiology of mouse RPE was studied by determining the transcriptome at Zeitgeber time (ZT) 0, 2, 4, 9, 14 and 19 (n=5 per time point). Anterior segments of the eyes were removed and the RPE layer was isolated after which RNA was isolated and sequenced. Genes with a significant difference in expression between time points (DEGs, p<0.05) were identified. EnrichR pathway analysis was used to identify circadian regulated processes.
Results :
In total, 756 significant DEGs (p<0.05) were identified. EnrichR pathway enrichment resulted in 13 significantly-enriched KEGG pathways (p<0.01) including circadian rhythm and the metabolic pathway. The metabolic pathway (p= 2,82E-03) showed the highest number of DEGs, being 73. In-depth analysis of these 73 DEGs revealed 2 opposing circadian expression patterns. Genes involved in mitochondrial electron transport showed a high expression at ZT0 and low expression at ZT19, whereas the expression of genes involved in lipid metabolism (e.g. fatty acid degradation, cholesterol synthesis) were low at ZT0 and high at ZT19.
Conclusions :
We found that 13 pathways follow a circadian gene expression pattern in RPE cells. In particular, we found that genes involved in mitochondrial electron transport (i.e. ATP generation) and lipid metabolism have opposing expression patterns in RPE cells, of which increase of the latter coincides with POS phagocytosis. These data suggest that the highest energy demand of RPE cells is at another circadian time frame than when POS are phagocytosed. Moreover, because lipid metabolism plays an important role in AMD development, our finding that RPE lipid metabolism is circadian regulated may suggest that RPE cells experience the effect of AMD-associated genetic variants in these genes at particular circadian hours.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.