Involvement of posttranscriptional regulation of <i>Clock</i> in the emergence of circadian clock oscillation during mouse development

Abstract

Circadian clock oscillation emerges in mouse embryo in the later developmental stages. Although circadian clock development is closely correlated with cellular differentiation, the mechanisms of its emergence during mammalian development are not well understood. Here, we demonstrate an essential role of the posttranscriptional regulation of <i>Clock</i> subsequent to the cellular differentiation for the emergence of circadian clock oscillation in mouse fetal hearts and mouse embryonic stem cells (ESCs). In mouse fetal hearts, no apparent oscillation of cell-autonomous molecular clock was detectable around E10, whereas oscillation was clearly visible in E18 hearts. Temporal RNA-sequencing analysis using mouse fetal hearts reveals many fewer rhythmic genes in E10-12 hearts (63, no core circadian genes) than in E17-19 hearts (483 genes), suggesting the lack of functional circadian transcriptional/translational feedback loops (TTFLs) of core circadian genes in E10 mouse fetal hearts. In both ESCs and E10 embryos, CLOCK protein was absent despite the expression of <i>Clock</i> mRNA, which we showed was due to <i>Dicer/Dgcr8</i>-dependent translational suppression of CLOCK. The CLOCK protein is required for the discernible molecular oscillation in differentiated cells, and the posttranscriptional regulation of <i>Clock</i> plays a role in setting the timing for the emergence of the circadian clock oscillation during mammalian development.