Light-mediated switching of circadian pacemaker function across the neural clock circuit of drosophila

Abstract

Circadian clocks evolved as internal timekeeping mechanisms in response to the spinning of the Earth on its axis. Many vital bodily functions such as daily temperature regulation and sleep/wake cycles are tightly controlled by circadian clocks that are coordinated throughout the mammalian body by the suprachiasmatic nucleus in the brain. The molecular clock of the fruit fly Drosophila melanogaster is very similar to our own, making this insect a useful model to study our own clock. Drosophila possess a group of ‘morning’ neurons, which control the clock pacemaker at night, alternating with the ‘evening’ neurons, which control it during the day. My project aims to elucidate the signalling mechanisms that cause the switch in this pacemaker function. The morning cells are the lead pacemakers in constant darkness (DD) but more recent research indicates that the more poorly defined group of evening cells drive rhythms in continuous light. The experimental paradigms of DD and constant red light (RR) allow stable conditions within which pacemaker function is led by two distinct subsets. Spatiotemporal mapping using more specifically targeted genetic tools indicates that the PIGMENT-DISPERSING FACTOR (PDF)-expressing 4 small ventrolateral neurons (s-LNvs) per hemisphere are both sufficient and necessary as DD pacemakers while the MB122B-Gal4-driven or ChAT-Gal4-driven clock cells (5th s-LNv plus three LNds) are identified, here, as sufficient for RR pacemaker function. However, in order to fully eliminate RR rhythmicity, molecular clock function must additionally be eliminated within all PDF+ cells, suggesting that these cells also play a role in RR pacemaker function. Targeted RNA interference of PDF in the s-LNvs eliminated behavioural rhythmicity in DD, but not RR. Genetic disruptions of several different components of predicted output pathways such as the neuropeptides HUGIN (HUG), LEUCOKININ (LK), SMALL NEUROPEPTIDE F (SNPF), the neuropeptide receptors DH44 RECEPTOR 1 (DH44-R1), LK RECEPTOR (LKR) or dopaminergic ‘PPM3’ neurons indicated they were downstream of the RR pacemakers in addition to the DD pacemakers. Throughout these studies as well as published studies by others, genetic background played a role in the severity of the observed behavioural phenotypes. For example, locomotor rhythmicity persisted in some Pdf or Pdf receptor-null backgrounds but not others. In particular, an autosomal modifier of rhythmicity that co-segregated with a known polymorphisms at the tim locus was detected. The insights into light-induced pacemaker switching within the Drosophila clock raises hypotheses that can be pursued in mammalian neuronal clocks.

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ORCID for Herman Wijnen: orcid.org/0000-0002-8710-5176

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