Circadian clocks, retinogenesis and ocular health in vertebrates: new molecular insights.

Circadian clocks are cell-autonomous, molecular pacemakers regulating a wide variety of behavioural and physiological processes in accordance with the 24 â€‹h light/dark cycle. The retina contains a complex network of cell-specific clocks orchestrating many biochemical and cellular parameters to adapt retinal biology and visual function to daily changes in light intensity. The gene regulatory networks controlling proliferation, specification and differentiation of retinal precursors into the diverse retinal cell types are evolutionary conserved among vertebrates. However, how these mechanisms are interconnected with circadian clocks is not well-characterized. Here we explore the existing evidence for the regulation of retinal development by circadian clock-related pathways, throughout vertebrates. We provide evidence for the influence of clock genes, from early to final differentiation steps. In addition, we report that the clock, integrating environmental cues, modulates a number of pathological processes. We highlight its potential role in retinal diseases and its instructive function on eye growth and related disorders.

Core circadian clock genes Per1 and Per2 regulate the rhythm in photoreceptor outer segment phagocytosis.

Retinal photoreceptors undergo daily renewal of their distal outer segments, a process indispensable for maintaining retinal health. Photoreceptor outer segment (POS) phagocytosis occurs as a daily peak, roughly about 1 hour after light onset. However, the underlying cellular and molecular mechanisms which initiate this process are still unknown. Here we show that, under constant darkness, mice deficient for core circadian clock genes (Per1 and Per2) lack a daily peak in POS phagocytosis. By qPCR analysis, we found that core clock genes were rhythmic over 24 hours in both WT and Per1, Per2 double mutant whole retinas. More precise transcriptomics analysis of laser capture microdissected WT photoreceptors revealed no differentially expressed genes between time points preceding and during the peak of POS phagocytosis. In contrast, we found that microdissected WT retinal pigment epithelium (RPE) had a number of genes that were differentially expressed at the peak phagocytic time point compared to adjacent ones. We also found a number of differentially expressed genes in Per1, Per2 double mutant RPE compared to WT ones at the peak phagocytic time point. Finally, based on STRING analysis, we found a group of interacting genes that potentially drive POS phagocytosis in the RPE. This potential pathway consists of genes such as: Pacsin1, Syp, Camk2b, and Camk2d among others. Our findings indicate that Per1 and Per2 are necessary clock components for driving POS phagocytosis and suggest that this process is transcriptionally driven by the RPE.

Core-clock genes Period 1 and 2 regulate visual cascade and cell cycle components during mouse eye development.

The retinas from Period 1 (Per1) and Period 2 (Per2) double-mutant mice (Per1-/-Per2Brdm1) display abnormal blue-cone distribution associated with a reduction in cone opsin mRNA and protein levels, up to 1 year of age. To reveal the molecular mechanisms by which Per1 and Per2 control retina development, we analyzed genome-wide gene expression differences between wild-type (WT) and Per1-/-Per2Brdm1 mice across ocular developmental stages (E15, E18 and P3). All clock genes displayed changes in transcript levels along with normal eye development. RNA-Seq data show major gene expression changes between WT and mutant eyes, with the number of differentially expressed genes (DEG) increasing with developmental age. Functional annotation of the genes showed that the most significant changes in expression levels in mutant mice involve molecular pathways relating to circadian rhythm signaling at E15 and E18. At P3, the visual cascade and the cell cycle were respectively higher and lower expressed compared to WT eyes. Overall, our study provides new insights into signaling pathways -phototransduction and cell cycle- controlled by the circadian clock in the eye during development.