Roles of Post-transcriptional Regulation in Circadian Rhythms of Clock Gene and Clock-controlled Gene

Abstract

A circadian rhythm is defined as the periodic oscillation with a period of 24 hours observed in a wide-range of living organisms. The underlying mechanism of such molecular rhythmicity is a transcriptional-translational feedback loop (TTFL). It is believed that the transcriptional regulation and the translational regulation are sufficient to generate the daily oscillation of clock genes, and numbers of mathematical models successfully support the idea; however, another mechanism is required for understanding how to fine-tune circadian rhythms. In this study, using mathematical models, I demonstrate the roles of untranslated region (UTR)-mediated post-transcriptional regulation in the circadian rhythms of a clock-controlled gene and a clock gene. In the first chapter, I will describe the post-transcriptional regulation of one clock-controlled gene participated in the biosynthesis of melatonin. The circadian oscillation of melatonin is regulated by Arylalkylamine N-acetyltransferase (AANAT). The transcriptional and post-transcriptional regulatory processes governing rodent AANAT production have been clarified. However, the basis for species-specific AANAT expression is not clearly understood. I introduce a mathematical model for the rhythmic expression of rodent AANAT mRNA and protein that includes post-transcriptional mRNA degradation and internal ribosomal entry site (IRES)-mediated translation. My model yields results consistent with the previous studies and describes the expression of bovine AANAT mRNA and protein by incorporating post-transcriptional modifications. The results suggest that the post-transcriptional regulation by RNA-binding proteins is important factors in species-specific AANAT expression. In the second chapter, I will discuss the post-transcriptional regulation of one clock gene and its mathematical modeling. Mouse Period3 (mPer3) is one of the paralogs of Period gene and its function is important in peripheral clocks and sleep physiology. mPer3 mRNA displays a circadian oscillation as well as a circadian phase-dependent stability, while the stability regulators still remain unknown. I identify three proteins – heterogeneous nuclear ribonucleoprotein (hnRNP) K, polypyrimidine tract-binding protein (PTB), and hnRNP D – that bind to mPer3 mRNA 3'-UTR. I show that hnRNP K is a stabilizer that increases the amplitude of circadian mPer3 mRNA oscillation and hnRNP D is a destabilizer that decreases it, while PTB exhibits no effect on mPer3 mRNA expression. The experiments describe their cytoplasmic roles for the mRNA stability regulation and the circadian amplitude formation. Moreover, my mathematical model suggests a mechanism how post-transcriptional mRNA stability modulation provides not only the flexibility of oscillation amplitude, but also the robustness of the period and the phase, for circadian mPer3 expression.