Threonine Enhances Sleep Drive via a GABAergic Pathway in Drosophila

Department of Biological SciencesAmino acids are often used as sleep-inducing supplements, yet the neural basis underlying sleep regulation remains unclear. Here we employed Drosophila as a genetic model to demonstrate that threonine facilitates sleep onset via a specific GABAergic pathway. Feeding wild-type flies with sucrose supplemented with individual amino acids differentially affected their sleep behaviors. Glycine, a co-agonist for the N-methyl-D-aspartate receptor, lengthened the average duration of sleep bouts and thus improved the sleep quality, consistent with its effects on human sleep. On the other hand, threonine markedly increased the daily amount of sleep and shortened latency to sleep onset in a dose-dependent manner. Threonine-fed flies also fell asleep faster than control-fed flies when their sleep was disturbed in midnight which implicates the SPET is regulated in time of day independent manner. Circadian clock components are reported to have intimate relationship with sleep behavior. However, our genetic ablation of clock component revealed that the sleep-promoting effects of a threonine (SPET) is independent of clock components. GABA-transaminase (GABAT) is a mitochondrial enzyme that metabolizes GABA in glial cells so that it results in increased GABA in brain in the absence of this enzyme. Genetic ablation or pharmacological inhibition of GABA-transaminase masked SPET. Pharmacological inhibition of GABA reuptake by feeding nipecotic acid (NipA) also abolished SPET. A transcriptional reporter for intracellular Ca2+ levels revealed that a threonine diet led to excitation of a specific subset of GABAergic neurons, whereas a conditional blockade of the synaptic transmission in GABAergic neurons suppressed SPET. Transgenic RNA interference of GABAB receptor in neurons suppressed SPET whereas RNA interference of GABAB in glia fully sustains it. It further implicated metabotropic GABA receptors in the neural output pathway of SPET. Finally, we have elevated the endogenous threonine levels by genetic down-regulation of threonine metabolizing enzyme. Hypomorphic mutants of threonine 3-dehydrogenase (CG5955) had elevated threonine levels and showed shorter time for latency to sleep in both natural and sleep-disturbed condition. Pan-neuronal knock down of CG5955 by RNA interference was sufficient for enhancing sleep drive. Taken together, these findings reveal a neural mechanism underlying how animals adaptively adjust their sleep behaviors based on a specific diet and define a novel sleep-regulatory pathway that intimately links essential threonine metabolism to the control of sleep drive. Given genetic elevation of endogenous threonine levels facilitates sleep onset, threonine can be considered as an endogenous sleep enhancer.ope

Sleep-promoting effects of threonine link amino acid metabolism in Drosophila neuron to GABAergic control of sleep drive

Emerging evidence indicates the role of amino acid metabolism in sleep regulation. Here we demonstrate sleep-promoting effects of dietary threonine (SPET) in Drosophila. Dietary threonine markedly increased daily sleep amount and decreased the latency to sleep onset in a dose-dependent manner. High levels of synaptic GABA or pharmacological activation of metabotropic GABA receptors (GABAB-R) suppressed SPET. By contrast, synaptic blockade of GABAergic neurons or transgenic depletion of GABAB-R in the ellipsoid body R2 neurons enhanced sleep drive non-additively with SPET. Dietary threonine reduced GABA levels, weakened metabotropic GABA responses in R2 neurons, and ameliorated memory deficits in plasticity mutants. Moreover, genetic elevation of neuronal threonine levels was sufficient for facilitating sleep onset. Taken together, these data define threonine as a physiologically relevant, sleep-promoting molecule that may intimately link neuronal metabolism of amino acids to GABAergic control of sleep drive via the neuronal substrate of sleep homeostasis

The voltage-gated potassium channel Shaker promotes sleep via thermosensitive GABA transmission

Genes and neural circuits coordinately regulate animal sleep. However, it remains elusive how these endogenous factors shape sleep upon environmental changes. Here, we demonstrate that Shaker (Sh)-expressing GABAergic neurons projecting onto dorsal fan-shaped body (dFSB) regulate temperature-adaptive sleep behaviors in Drosophila. Loss of Sh function suppressed sleep at low temperature whereas light and high temperature cooperatively gated Sh effects on sleep. Sh depletion in GABAergic neurons partially phenocopied Sh mutants. Furthermore, the ionotropic GABA receptor, Resistant to dieldrin (Rdl), in dFSB neurons acted downstream of Sh and antagonized its sleep-promoting effects. In fact, Rdl inhibited the intracellular cAMP signaling of constitutively active dopaminergic synapses onto dFSB at low temperature. High temperature silenced GABAergic synapses onto dFSB, thereby potentiating the wake-promoting dopamine transmission. We propose that temperature-dependent switching between these two synaptic transmission modalities may adaptively tune the neural property of dFSB neurons to temperature shifts and reorganize sleep architecture for animal fitness. Ji-hyung Kim and Yoonhee Ki et al. show that low temperatures suppress sleep in Drosophila by increasing GABA transmission in Shaker-expressing GABAergic neurons projecting onto the dorsal fan-shaped body, while high temperatures potentiate dopamine-induced arousal by reducing GABA transmission. This study highlights a role for Shaker in sleep modulation via a temperature-dependent switch in GABA signaling

A Drosophila JAK-STAT Pathway Mediates Sleep-promoting Effects of Threonine

Amino acids are newly emerging as sleep supplements yet their neural and molecular bases underlying sleep regulation still remain elusive. Here we demonstrate sleep-promoting effects of threonine in Drosophila genetic models. Feeding individual amino acids to wild-type flies differentially affected baseline sleep behaviors. Glycine, one of ligands for NMDA receptor, lengthened the average duration of sleep bouts and thus improved sleep quality consistent with its effects on human sleep. In contrast, threonine increased sleep quantity and shortened sleep latency in a dose-dependent manner. We reasoned threonine effects on sleep behaviors might involve metabolic pathways as previous studies have showed that threonine blocks the formation of fatty liver in mammals and that metabolic mutant flies display altered sleep homeostasis. Indeed, loss-of-function mutation in a Drosophila leptin-like cytokine unpaired 3 (upd3) gene desensitized sleep-promoting effects of threonine. Upd3 depletion in fat body, a Drosophila tissue analogous to mammalian adipose tissue and liver, was sufficient to phenocopy upd3 mutant flies. In addition, pan-neuronal depletion of a UPD3 receptor domeless (dome) caused resistance to threonine effects on baseline sleep. Given that DOME is an upstream activator of JAK-STAT signaling, our data suggest that a specific JAK-STAT pathway may physiologically link the metabolic organ to a sleep-relevant neural circuitry in brain to mediate threonine-dependent sleep effects

Dietary threonine enhances sleep drive via a circadian clock-independent GABAergic pathway

Amino acids are often used as sleep-inducing supplements; however, the neural basis underlying their sleep regulation remains unclear. Here we demonstrate sleep-promoting effects of dietary threonine (SPET) in Drosophila. Dietary threonine markedly increased the daily amount of sleep and shortened latency to sleep onset in a dose-dependent manner. SPET did not require the functionality of circadian clock genes or pacemaker neurons. By contrast, constitutively high levels of synaptic GABA or silencing of the GABA transmission, likely via the metabotropic GABA receptor, masked SPET. Dietary threonine activated a subset of GABAergic neurons, modulated GABA signaling in a homeostatic sleep locus, and ameliorated behavioral deficits in memory mutants. Moreover, genetic elevation of endogenous threonine levels in neurons was sufficient for enhancing sleep drive. Taken together, these data support the physiological relevance of SPET and define threonine as a sleep-promoting molecule that may intimately link neuronal metabolism of amino acids to sleep regulation

Warming Up Your Tick-Tock: Temperature-Dependent Regulation of Circadian Clocks

Circadian clocks are endogenous time-keeping mechanisms to adaptively coordinate animal behaviors and physiology with daily environmental changes. So far many circadian studies in model organisms have identified evolutionarily conserved molecular frames of circadian clock genes in the context of transcription-translation feedback loops. The molecular clockwork drives cell-autonomously cycling gene expression with similar to 24-hour periodicity, which is fundamental to circadian rhythms. Light and temperature are two of the most potent external time cues to reset the circadian phase of the internal clocks, yet relatively little is known about temperature-relevant clock regulation. In this review, we describe recent findings on temperature-dependent clock mechanisms in homeothermic mammals as compared with poikilothermic Drosophila at molecular, neural, and organismal levels. We propose thermodynamic transitions in RNA secondary structures might have been potent substrates for the molecular evolution of temperature-relevant post-transcriptional mechanisms. Future works should thus validate the potential involvement of specific post-transcriptional steps in temperature-dependent plasticity of circadian clocks.close0