Transcriptional and Translational Mechanisms Controlling Circadian Rhythms in Drosophila: A Dissertation

Abstract

Circadian rhythms are self-sustained 24-hour period oscillations present in most organisms, from bacteria to human. They can be synchronized to external cues, thus allowing organisms to anticipate environmental variations and optimize their performance in nature. In Drosophila, the molecular pacemaker consists of two interlocked transcriptional feedback loops. CLOCK/CYCLE (CLK/CYC) sits in the center and drives rhythmic transcription of period (per), timeless (tim), vrille (vri) and PAR domain protein 1 (Pdp1). PER and TIM negatively feedback on CLK/CYC transcriptional activity, forming one loop, while VRI and PDP1 form the other by regulating Clk transcription negatively and positively, respectively. Posttranscriptional and posttranslational regulations also contribute to circadian rhythms. Although much has been learned about these feedback loops, we are still far from understanding how stable 24-hour period rhythms are generated. My thesis work was to determine by which molecular mechanisms kayak-α (kay-α) and Ataxin-2 (Atx2) regulate Drosophila circadian behavior. Both genes are required for the precision of circadian rhythms since knocking down either gene in circadian pacemaker neurons results in long period phenotype. The work on kay-α constitutes the first half of my thesis. We found that the transcription factor KAY-α can bind to VRI and inhibit VRI’s repression on the Clk promoter. Interestingly, KAY-α can also repress CLK’s transcriptional activity on its target genes (e.g., per and tim). Therefore, KAY-α is proposed to bring precision and stability to the molecular pacemaker by regulating both transcriptional loops. The second half of my thesis focuses on ATX2, an RNA binding protein whose mammalian homolog has been implicated in neurodegenerative diseases. We found that ATX2 is required for PER accumulation in circadian pacemaker neurons. It forms a complex with TWENTY-FOUR (TYF)—a crucial activator of PER translation—and promotes TYF’s interaction with Poly(A)-binding protein. This work reveals the role of ATX2 in the control of circadian rhythms as an activator of PER translation, in contrast to its well-established role as a repressor of translation. It also further demonstrates the importance of translational regulation on circadian rhythms. Finally, it may help understanding how ATX2 causes neuronal degeneration in human diseases.