Tob Regulates the Timing of Sleep Onset at Night in Drosophila .

Sleep is regulated by homeostatic sleep drive and the circadian clock. While tremendous progress has been made in elucidating the molecular components of the core circadian oscillator, the output mechanisms by which this robust oscillator generates rhythmic sleep behaviour remain poorly understood. At the cellular level, growing evidence suggests that subcircuits in the master circadian pacemaker suprachiasmatic nucleus (SCN) in mammals and in the clock network in Drosophila regulate distinct aspects of sleep. Thus, to identify novel molecules regulating the circadian timing of sleep, we conducted a large-scale screen of mouse SCN-enriched genes in Drosophila Here, we show that Tob (Transducer of ERB-B2) regulates the timing of sleep onset at night in female fruit flies. Knockdown of Tob pan-neuronally, either constitutively or conditionally, advances sleep onset at night. We show that Tob is specifically required in "evening neurons" (the LNds and the 5th s-LNv) of the clock network for proper timing of sleep onset. Tob levels cycle in a clock-dependent manner in these neurons. Silencing of these "evening" clock neurons results in an advanced sleep onset at night, similar to that seen with Tob knockdown. Finally, sharp intracellular recordings demonstrate that the amplitude and kinetics of LNd post-synaptic potentials (PSPs) cycle between day and night, and this cycling is attenuated with Tob knockdown in these cells. Our data suggest that Tob acts as a clock-output molecule in a subset of clock neurons to potentiate their activity in the evening and enable the proper timing of sleep onset at night. Significance Summary Well-timed, high quality sleep is critical for human health and function. Elucidating how sleep is regulated by the circadian clock may reveal molecular targets for intervening in this process. However, our understanding of the genetic mechanisms underlying clock regulation of sleep is limited. From a high-throughput behavioral screen in Drosophila , we identified a novel clock output gene tob ( transducer of erb-b2 ) required for proper timing of sleep onset at night. Tob levels cycle under clock control, and we show that Tob facilitates post-synaptic potentiation in a specific population of clock neurons. Our work suggests a molecular mechanism by which the clock regulates sleep onset by potentiating the function of a subset of clock neurons at night.