[PhD Thesis Presentation] -Haytham Mohamed Aly Mohamed- Post-transcriptional Regulation of Circadian Rhythm: Involvement of the CCR4-NOT Complex

Abstract

Circadian clocks are an endogenous internal timekeeping mechanism that drive the rhythmic expression of genes, controlling the 24 hr oscillatory pattern in behavior and physiology. These cell-autonomous clocks synchronize to external factors primarily light; allowing organisms to anticipate, adapt, and coordinate their biology to the daily light/dark cycle. Post-transcriptional mechanisms have recently been shown to play an essential role in regulating mRNA and protein oscillations in a time-dependent manner. mRNA stability/decay control through poly(A) tail length modulation is one such mechanism. Poly (A) tail shortening results in mRNA destabilization, subsequent decay, and translational repression. The major deadenylase complex in the cytoplasm is the CCR4-NOT complex, which is essential for regulating gene homeostasis by modulating RNA metabolism on multiple fronts primarily mRNA decay. In this thesis, we examine the role of the CCR4-NOT complex in regulating circadian clocks by focusing on CNOT1, the scaffold protein. Cnot1 exhibits a constantly high expression in the mouse superchiasmatic nucleus (SCN) as well as a rhythmic protein and mRNA pattern in the mouse liver with peak expression at the early morning. Cnot1 deficiency in mice results in elongation of circadian period and alteration in mRNA and protein expression patterns of various clock genes, mainly Per2. The recruitment of CNOT1 to Per2 mRNA is mediated through Zfp36L1 (BRF1), which itself oscillates in antiphase with Per2 mRNA. Upon BRF1 knockdown, Per2 is stabilized. Taken together, this suggests that CNOT1 plays a role in tuning and regulating the mammalian circadian clock and circadian behavior.