Circadian rhythm refers to the 24-hour fluctuations in gene expression that allow organisms to respond to the environmental changes that result with the Earth’s rotation. These challenges include sleep-wake and feeding-fasting cycles. Disruptions to the circadian rhythms can lead to diseases including metabolic disorders. Molecularly, the circadian clock is a transcriptional-translational feedback loop that produces the oscillations and is present in most mammalian cells. The transcriptional-translational feedback loop starts with the heterodimer CLOCK and BMAL1. This then leads to the transcription of other clock-controlled genes including CRY1-2, PER1-2 and REV-ERBa and b.

This project focuses on the relationship between REV-ERBa and SUMO3 in the liver. The nuclear hormone receptor REV-ERB is an important part of the circadian clock by repressing BMAL1. The activity of clock genes is also regulated by post-translational modifications including SUMOylation. SUMO3 modification regulates BMAL1 protein degradation. This led us to question whether there are other ways in which REV-ERB can take part in the regulation of the clock. Our hypothesis is that REV-ERB can repress certain PTMs.

To test this hypothesis, mice were injected with either AAV.GFP or AAV.CRE, and liver samples were collected at different timepoints. RNA extraction followed by RTqPCR allowed us to generate four different graphs to show the mRNA expression of Rev-erba, Rev-erbb, Npas2, and Sumo3. Graphs of Rev-erba and Rev-erbb expression confirmed that the dKO was successful with their respective expression ablated. Npas2, a REV-ERB target gene, is derepressed, again confirming the success of our experimental setup. Sumo3mRNA expression followed the pattern of other REV-ERB targets like Npas2, which led us to further investigate. It was determined through ChIP-seq that REV-ERB and the NCoR co-repressive complex binds at the SUMO3 gene body, suggesting that indeed REV-ERB is capable of repressing SUMO3 transcription.

 From these experiments, we can conclude that REV-ERBa transcriptionally represses SUMO3. Future work is needed to determine if REV-ERBa regulation of SUMO3 influences Bmal1 and if SUMOylation of other proteins is controlled by REV-ERBs.