Molecular Genetic Analysis of Circadian Timekeeping in Drosophila

Abstract

A genetic screen for mutants that alter circadian rhythms in Drosophila identified the first clock gene - the period (per) gene. The per gene is a central player within a transcriptional feedback loop that represents the core mechanism for keeping circadian time in Drosophila and other animals. The per feedback loop, or core loop, is interlocked with the Clock (Clk) feedback loop, but whether the Clk feedback loop contributes to circadian timekeeping is not known. A series of distinct molecular events are thought to control transcriptional feedback in the core loop. The time it takes to complete these events should take much less than 24h, thus delays must be imposed at different steps within the core loop. As new clock genes are identified, the molecular mechanisms responsible for these delays have been revealed in ever-increasing detail, and provide an in depth accounting of how transcriptional feedback loops keep circadian time. The phase of these feedback loops shift to maintain synchrony with environmental cycles, the most reliable of which is light. Although a great deal is known about cell-autonomous mechanisms of light-induced phase shifting by CRYPTOCHROME (CRY), much less is known about non-cell autonomous mechanisms. CRY mediates phase shifts through an uncharacterized mechanism in certain brain oscillator neurons, and carries out a dual role as a photoreceptor and transcription factor in other tissues. Here I will review how transcriptional feedback loops function to keep time in Drosophila, how they impose delays to maintain a 24h cycle, and how they maintain synchrony with environmental light:dark cycles. The transcriptional feedback loops that keep time in Drosophila are well conserved in other animals, thus what we learn about these loops in Drosophila should continue to provide insight into the operation of analogous transcriptional feedback loops in other animals.

Origin of the Animal Circadian Clock: Diurnal and Light-Entrained Gene Expression in the Sponge Amphimedon queenslandica

“…Broadly, circadian clocks comprise three main modules: (i) sensors that perceive the entrainment signals from the environment; (ii) molecular oscillators of transcriptionaltranslational feedback loops that maintain the clock pacing and transmit rhythmic signals to downstream components; and (iii) clock-controlled genes (CCGs) that coordinate circadian responses within cells (Harmer et al, 2001;Hardin, 2011;Reitzel et al, 2013;Takahashi, 2017). In animals, our understanding of the clock general circuitry largely draws from work on mammal and insect model species, which has revealed that the topology of the molecular oscillator is conserved across these divergent bilaterian lineages (Hardin, 2011;Takahashi, 2017).…”

“…In animals, our understanding of the clock general circuitry largely draws from work on mammal and insect model species, which has revealed that the topology of the molecular oscillator is conserved across these divergent bilaterian lineages (Hardin, 2011;Takahashi, 2017). Specifically, two interconnected regulatory loops are organized around a pair of basic helix-loop-helix-Per-ARNTSim (bHLH-PAS) transcription factors, Clock and Cycle (Bmal1), that activate the transcription of other clock elements.…”

Measuring individual locomotor rhythms in honey bees, paper wasps and similar sized insects

“…At the molecular level, a set of proteins (e.g. period, clock, cryptochrome) cycle as a result of negative feedback loops formed by the interactions of these proteins at the transcriptional and translational levels (Bell-Pedersen et al, 2005;Hardin, 2011). At the level of the organism, various pacemakers in the brain, as well as the periphery, work together to integrate multiple predictable changes of the environment (e.g.…”

Fly cryptochrome and the visual system

“…Light signals are received through the blue-light photoreceptor CRYPTO-CHROME (dCRY), the expression of which is under clock control. dCRY associates with dTIM in a light-dependent manner and promotes its proteasome-mediated degradation (1). Cryptochromes are flavoproteins highly similar to photolyases, from which they have probably evolved, but across evolution they have lost or reduced the photolyase activity and gained roles in signaling (2).…”