Cellular stress induces a protective sleep-like state in C. elegans

Sleep-Active Neurons: Conserved Motors of Sleep

Sleep is crucial for survival and well-being. This behavioral and physiological state has been studied in all major genetically accessible model animals, including rodents, fish, flies, and worms. Genetic and optogenetic studies have identified several neurons that control sleep, making it now possible to compare circuit mechanisms across species. The “motor” of sleep across animal species is formed by neurons that depolarize at the onset of sleep to actively induce this state by directly inhibiting wakefulness. These sleep-inducing neurons are themselves controlled by inhibitory or activating upstream pathways, which act as the “drivers” of the sleep motor: arousal inhibits “sleep-active” neurons whereas various sleep-promoting “tiredness” pathways converge onto sleep-active neurons to depolarize them. This review provides the first overview of sleep-active neurons across the major model animals. The occurrence of sleep-active neurons and their regulation by upstream pathways in both vertebrate and invertebrate species suggests that these neurons are general and ancient components that evolved early in the history of nervous systems.

Acute control of the sleep switch in Drosophila reveals a role for gap junctions in regulating behavioral responsiveness

Sleep is a dynamic process in most animals, involving distinct stages that probably perform multiple functions for the brain. Before sleep functions can be initiated, it is likely that behavioral responsiveness to the outside world needs to be reduced, even while the animal is still awake. Recent work in Drosophila has uncovered a sleep switch in the dorsal fan-shaped body (dFB) of the fly’s central brain, but it is not known whether these sleep-promoting neurons also govern the acute need to ignore salient stimuli in the environment during sleep transitions. We found that optogenetic activation of the sleep switch suppressed behavioral responsiveness to mechanical stimuli, even in awake flies, indicating a broader role for these neurons in regulating arousal. The dFB-mediated suppression mechanism and its associated neural correlates requires innexin6 expression, suggesting that the acute need to reduce sensory perception when flies fall asleep is mediated in part by electrical synapses.

Automated detection and manipulation of sleep in C. elegans reveals depolarization of a sleep-active neuron during mechanical stimulation-induced sleep deprivation

Across species, sleep is characterized by a complex architecture. Sleep deprivation is a classic method to study the consequences of sleep loss, which include alterations in the activity of sleep circuits and detrimental consequences on well being. Automating the observation and manipulation of sleep is advantageous to study its regulation and functions. Caenorhabditis elegans shows sleep behavior similar to other animals that have a nervous system. However, a method for real-time automatic sleep detection that allows sleep-specific manipulations has not been established for this model animal. Also, our understanding of how sleep deprivation affects sleep neurons in this system is incomplete. Here we describe a system for real-time automatic sleep detection of C. elegans grown in microfluidic devices based on a frame-subtraction algorithm using a dynamic threshold. As proof of principle for this setup, we used automated mechanical stimulation to perturb sleep behavior and followed the activity of the sleep-active RIS neuron. We show that our system can automatically detect sleep bouts and deprive worms of sleep. We found that mechanical stimulation generally leads to the activation of the sleep-active RIS neuron, and this stimulation-induced RIS depolarization is most prominent during sleep deprivation.

Food-Dependent Plasticity in Caenorhabditis elegans Stress-Induced Sleep Is Mediated by TOR-FOXA and TGF-β Signaling

Behavioral plasticity allows for context-dependent prioritization of competing drives, such as sleep and foraging. Despite the identification of neuropeptides and hormones implicated in dual control of sleep drive and appetite, our understanding of the mechanism underlying the conserved sleep-suppressing effect of food deprivation is limited. Caenorhabditis elegans provides an intriguing model for the dissection of sleep function and regulation as these nematodes engage a quiescence program following exposure to noxious conditions, a phenomenon known as stress-induced sleep (SIS). Here we show that food deprivation potently suppresses SIS, an effect enhanced at high population density. We present evidence that food deprivation reduces the need to sleep, protecting against the lethality associated with defective SIS. Additionally, we find that SIS is regulated by both target of rapamycin and transforming growth factor-β nutrient signaling pathways, thus identifying mechanisms coordinating sleep drive with internal and external indicators of food availability.

A C. elegans Model for the Study of RAGE-Related Neurodegeneration

The receptor for advanced glycation products (RAGE) is a cell surface, multi-ligand receptor belonging to the immunoglobulin superfamily; this receptor is implicated in a variety of maladies, via inflammatory pathways and induction of oxidative stress. Currently, RAGE is being studied using a limited number of mammalian in vivo, and some complementary in vitro, models. Here, we present a Caenorhabditis elegans model for the study of RAGE-related pathology: a transgenic strain, expressing RAGE in all neurons, was generated and subsequently tested behaviorally, developmentally, and morphologically. In addition to RAGE expression being associated with a significantly shorter lifespan, the following behavioral observations were made when RAGE-expressing worms were compared to the wild type: RAGE-expressing worms showed an impaired dopaminergic system, evaluated by measuring the fluorescent signal of GFP tagging; these worms exhibited decreased locomotion-both general and following ethanol exposure-as measured by counting body bends in adult worms; RAGE expression was also associated with impaired recovery of quiescence and pharyngeal pumping secondary to heat shock, as a significantly smaller fraction of RAGE-expressing worms engaged in these behaviors in the 2 h immediately following the heat shock. Finally, significant developmental differences were also found between the two strains: RAGE expression leads to a significantly smaller fraction of hatched eggs 24 h after laying and also to a significantly slower developmental speed overall. As evidence for the role of RAGE in a variety of neuropathologies accumulates, the use of this novel and expedient model should facilitate the elucidation of relevant underlying mechanisms and also the development of efficient therapeutic strategies.

Sleep macrostructure is modulated by positive and negative social experience in adult pet dogs

The effects of emotionally valenced events on sleep physiology are well studied in humans and laboratory rodents. However, little is known about these effects in other species, despite the fact that several sleep characteristics differ across species and thus limit the generalizability of such findings. Here we studied the effect of positive and negative social experiences on sleep macrostructure in dogs, a species proven to be a good model of human social cognition. A non-invasive polysomnography method was used to collect data from pet dogs (n = 16) participating in 3-hour-long sleep occasions. Before sleep, dogs were exposed to emotionally positive or negative social interactions (PSI or NSI) in a within-subject design. PSI consisted of petting and ball play, while NSI was a mixture of separation, threatening approach and still face test. Sleep macrostructure was markedly different between pre-treatment conditions, with a shorter sleep latency after NSI and a redistribution of the time spent in the different sleep stages. Dogs' behaviour during pre-treatments was related to the macrostructural difference between the two occasions, and was further modulated by individual variability in personality. This result provides the first direct evidence that emotional stimuli affect subsequent sleep physiology in dogs.

Induction of Metallothionein Expression After Exposure to Conventional Cigarette Smoke but Not Electronic Cigarette (ECIG)-Generated Aerosol in Caenorhabditis elegans

Aim: With the invention of electronic cigarettes (ECIG), many questions have been raised regarding their safety as an alternative to smoking conventional cigarettes. Conventional cigarette smoke contains a variety of toxicants including heavy metals. However, ECIG-generated aerosol contains only trace amounts of metals, adding to the argument for it being a safer alternative. In response to heavy metal exposure, metallothioneins are induced in cells to help store the metal, detoxify the body, and are also known responders to oxidative stress. In an attempt to add to the evaluation of the safety of ECIGs, metallothionein expression was quantified using the nematode Caenorhabditis elegans as an assessment of stress induced cellular damage caused by exposure.

Methods: Adult nematodes were exposed to either ECIG aerosol or conventional cigarette smoke at doses of 15, 30, and 45 puffs, the equivalent of one, two, and three cigarettes, respectively. Movement, survival, and stress-induced sleep were assessed for up to 24 h after exposure. Relative expression levels for mtl-1 and mtl-2, C. elegans metallothionein genes, were analyzed after 1, 5, and 24 h post exposure using quantitative RT-PCR.

Results: Nematodes exposed to conventional cigarette smoke underwent stress-induced sleep in a dose dependent manner with animals recovering to values within the range of air control after 5 h post exposure. Those exposed to ECIG aerosol did not undergo stress-induced sleep and were indistinguishable from controls. The expression of mtl-1 increased in a dose and time dependent manner in C. elegans exposed to conventional cigarette smoke, with a maximum expression observed at 5 h post exposure of 45 puffs. No induction of mtl-2 was observed in any animals. Additionally, ECIG aerosol did not induce expression of mtl-1 and mtl-2 at levels different than those of untreated.

Conclusion: ECIG aerosol failed to induce a stress response in C. elegans. In contrast, conventional cigarette smoke induced the production of mtl-1 in a manner that correlates with the induction of stress-induced sleep suggesting a stress response to damage. The lack of cellular stress response to ECIG aerosol suggests it may be a safer alternative to conventional cigarettes.

Normal sleep bouts are not essential for C. elegans survival and FoxO is important for compensatory changes in sleep

BACKGROUND

Sleep deprivation impairs learning, causes stress, and can lead to death. Notch and JNK-1 pathways impact C. elegans sleep in complex ways; these have been hypothesized to involve compensatory sleep. C. elegans DAF-16, a FoxO transcription factor, is required for homeostatic response to decreased sleep and DAF-16 loss decreases survival after sleep bout deprivation. Here, we investigate connections between these pathways and the requirement for sleep after mechanical stress.

RESULTS

Reduced function of Notch ligand LAG-2 or JNK-1 kinase resulted in increased time in sleep bouts during development. These animals were inappropriately easy to arouse using sensory stimulation, but only during sleep bouts. This constellation of defects suggested that poor quality sleep bouts in these animals might activate homeostatic mechanisms, driving compensatory increased sleep bouts. Testing this hypothesis, we found that DAF-16 FoxO function was required for increased sleep bouts in animals with defective lag-2 and jnk-1, as loss of daf-16 reduced sleep bouts back to normal levels. However, loss of daf-16 did not suppress arousal thresholds defects. Where DAF-16 function was required differed; in lag-2 and jnk-1 animals, daf-16 function was required in neurons or muscles, respectively, suggesting that disparate tissues can drive a coordinated response to sleep need. Sleep deprivation due to mechanical stimulation can cause death in many species, including C. elegans, suggesting that sleep is essential. We found that loss of sleep bouts in C. elegans due to genetic manipulation did not impact their survival, even in animals lacking DAF-16 function. However, we found that sleep bout deprivation was often fatal when combined with the concurrent stress of mechanical stimulation.

CONCLUSIONS

Together, these results in C. elegans confirm that Notch and JNK-1 signaling are required to achieve normal sleep depth, suggest that DAF-16 is required for increased sleep bouts when signaling decreases, and that failure to enter sleep bouts is not sufficient to cause death in C. elegans, unless paired with concurrent mechanical stress. These results suggest that mechanical stress may directly contribute to death observed in previous studies of sleep deprivation and/or that sleep bouts have a uniquely restorative role in C. elegans sleep.

Energy Scarcity Promotes a Brain-wide Sleep State Modulated by Insulin Signaling in C. elegans

Neural information processing entails a high energetic cost, but its maintenance is crucial for animal survival. However, the brain’s energy conservation strategies are incompletely understood. Employing functional brain-wide imaging and quantitative behavioral assays, we describe a neuronal strategy in Caenorhabditis elegans that balances energy availability and expenditure. Upon acute food deprivation, animals exhibit a transiently elevated state of arousal, indicated by foraging behaviors and increased responsiveness to food-related cues. In contrast, long-term starvation suppresses these behaviors and biases animals to intermittent sleep episodes. Brain-wide neuronal population dynamics, which are likely energetically costly but important for behavior, are robust to starvation while animals are awake. However, during starvation-induced sleep, brain dynamics are systemically downregulated. Neuromodulation via insulin-like signaling is required to transiently maintain the animals’ arousal state upon acute food deprivation. Our data suggest that the regulation of sleep and wakefulness supports optimal energy allocation.

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Starvation shifts the behavioral strategy from exploration to intermittent sleep

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Brain-wide neuronal population dynamics are robust to starvation

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Neuromodulation via insulin signaling maintains wakefulness during short fasting

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The insulin receptor DAF-2 acts in a network of sensory neurons and interneurons

Genetic and neuronal regulation of sleep by neuropeptide VF

Sleep is an essential and phylogenetically conserved behavioral state, but it remains unclear to what extent genes identified in invertebrates also regulate vertebrate sleep. RFamide-related neuropeptides have been shown to promote invertebrate sleep, and here we report that the vertebrate hypothalamic RFamide neuropeptide VF (NPVF) regulates sleep in the zebrafish, a diurnal vertebrate. We found that NPVF signaling and npvf-expressing neurons are both necessary and sufficient to promote sleep, that mature peptides derived from the NPVF preproprotein promote sleep in a synergistic manner, and that stimulation of npvf-expressing neurons induces neuronal activity levels consistent with normal sleep. These results identify NPVF signaling and npvf-expressing neurons as a novel vertebrate sleep-promoting system and suggest that RFamide neuropeptides participate in an ancient and central aspect of sleep control.

The Jellyfish Cassiopea Exhibits a Sleep-like State

Do all animals sleep? Sleep has been observed in many vertebrates, and there is a growing body of evidence for sleep-like states in arthropods and nematodes [1-5]. Here we show that sleep is also present in Cnidaria [6-8], an earlier-branching metazoan lineage. Cnidaria and Ctenophora are the first metazoan phyla to evolve tissue-level organization and differentiated cell types, such as neurons and muscle [9-15]. In Cnidaria, neurons are organized into a non-centralized radially symmetric nerve net [11, 13, 15-17] that nevertheless shares fundamental properties with the vertebrate nervous system: action potentials, synaptic transmission, neuropeptides, and neurotransmitters [15-20]. It was reported that cnidarian soft corals [21] and box jellyfish [22, 23] exhibit periods of quiescence, a pre-requisite for sleep-like states, prompting us to ask whether sleep is present in Cnidaria. Within Cnidaria, the upside-down jellyfish Cassiopea spp. displays a quantifiable pulsing behavior, allowing us to perform long-term behavioral tracking. Monitoring of Cassiopea pulsing activity for consecutive days and nights revealed behavioral quiescence at night that is rapidly reversible, as well as a delayed response to stimulation in the quiescent state. When deprived of nighttime quiescence, Cassiopea exhibited decreased activity and reduced responsiveness to a sensory stimulus during the subsequent day, consistent with homeostatic regulation of the quiescent state. Together, these results indicate that Cassiopea has a sleep-like state, supporting the hypothesis that sleep arose early in the metazoan lineage, prior to the emergence of a centralized nervous system.

Genome-Wide Screen for Genes Involved in Caenorhabditis elegans Developmentally Timed Sleep

In Caenorhabditis elegans, Notch signaling regulates developmentally timed sleep during the transition from L4 larval stage to adulthood (L4/A) . To identify core sleep pathways and to find genes acting downstream of Notch signaling, we undertook the first genome-wide, classical genetic screen focused on C. elegans developmentally timed sleep. To increase screen efficiency, we first looked for mutations that suppressed inappropriate anachronistic sleep in adult hsp::osm-11 animals overexpressing the Notch coligand OSM-11 after heat shock. We retained suppressor lines that also had defects in L4/A developmentally timed sleep, without heat shock overexpression of the Notch coligand. Sixteen suppressor lines with defects in developmentally timed sleep were identified. One line carried a new allele of goa-1; loss of GOA-1 Gαo decreased C. elegans sleep. Another line carried a new allele of gpb-2, encoding a Gβ5 protein; Gβ5 proteins have not been previously implicated in sleep. In other scenarios, Gβ5 GPB-2 acts with regulators of G protein signaling (RGS proteins) EAT-16 and EGL-10 to terminate either EGL-30 Gαq signaling or GOA-1 Gαo signaling, respectively. We found that loss of Gβ5 GPB-2 or RGS EAT-16 decreased L4/A sleep. By contrast, EGL-10 loss had no impact. Instead, loss of RGS-1 and RGS-2 increased sleep. Combined, our results suggest that, in the context of L4/A sleep, GPB-2 predominantly acts with EAT-16 RGS to inhibit EGL-30 Gαq signaling. These results confirm the importance of G protein signaling in sleep and demonstrate that these core sleep pathways function genetically downstream of the Notch signaling events promoting sleep.

Molecular Mechanisms of Sleep Homeostasis in Flies and Mammals

Sleep is homeostatically regulated with sleep pressure accumulating with the increasing duration of prior wakefulness. Yet, a clear understanding of the molecular components of the homeostat, as well as the molecular and cellular processes they sense and control to regulate sleep intensity and duration, remain a mystery. Here, we will discuss the cellular and molecular basis of sleep homeostasis, first focusing on the best homeostatic sleep marker in vertebrates, slow wave activity; second, moving to the molecular genetic analysis of sleep homeostasis in the fruit fly Drosophila; and, finally, discussing more systemic aspects of sleep homeostasis.

Stress-Induced Sleep After Exposure to Ultraviolet Light Is Promoted by p53 in Caenorhabditis elegans

Stress-induced sleep (SIS) in Caenorhabditis elegans is important for restoration of cellular homeostasis and is a useful model to study the function and regulation of sleep. SIS is triggered when epidermal growth factor (EGF) activates the ALA neuron, which then releases neuropeptides to promote sleep. To further understand this behavior, we established a new model of SIS using irradiation by ultraviolet C (UVC) light. While UVC irradiation requires ALA signaling and leads to a sleep state similar to that induced by heat and other stressors, it does not induce the proteostatic stress seen with heat exposure. Based on the known genotoxic effects of UVC irradiation, we tested two genes, atl-1 and cep-1, which encode proteins that act in the DNA damage response pathway. Loss-of-function mutants of atl-1 had no defect in UVC-induced SIS but a partial loss-of-function mutant of cep-1, gk138, had decreased movement quiescence following UVC irradiation. Germline ablation experiments and tissue-specific RNA interference experiments showed that cep-1 is required somatically in neurons for its effect on SIS. The cep-1(gk138) mutant suppressed body movement quiescence controlled by EGF, indicating that CEP-1 acts downstream or in parallel to ALA activation to promote quiescence in response to ultraviolet light.

A global brain state underlies C. elegans sleep behavior

How the brain effectively switches between and maintains global states, such as sleep and wakefulness, is not yet understood. We used brainwide functional imaging at single-cell resolution to show that during the developmental stage of lethargus, the Caenorhabditis elegans brain is predisposed to global quiescence, characterized by systemic down-regulation of neuronal activity. Only a few specific neurons are exempt from this effect. In the absence of external arousing cues, this quiescent brain state arises by the convergence of neuronal activities toward a fixed-point attractor embedded in an otherwise dynamic neural state space. We observed efficient spontaneous and sensory-evoked exits from quiescence. Our data support the hypothesis that during global states such as sleep, neuronal networks are drawn to a baseline mode and can be effectively reactivated by signaling from arousing circuits.

Transcriptional Profiling of Cholinergic Neurons From Basal Forebrain Identifies Changes in Expression of Genes Between Sleep and Wake

Study objective

To assess differences in gene expression in cholinergic basal forebrain cells between sleeping and sleep-deprived mice sacrificed at the same time of day.

Methods

Tg(ChAT-eGFP)86Gsat mice expressing enhanced green fluorescent protein (eGFP) under control of the choline acetyltransferase (Chat) promoter were utilized to guide laser capture of cholinergic cells in basal forebrain. Messenger RNA expression levels in these cells were profiled using microarrays. Gene expression in eGFP(+) neurons was compared (1) to that in eGFP(-) neurons and to adjacent white matter, (2) between 7:00 am (lights on) and 7:00 pm (lights off), (3) between sleep-deprived and sleeping animals at 0, 3, 6, and 9 hours from lights on.

Results

There was a marked enrichment of ChAT and other markers of cholinergic neurons in eGFP(+) cells. Comparison of gene expression in these eGFP(+) neurons between 7:00 am and 7:00 pm revealed expected differences in the expression of clock genes (Arntl2, Per1, Per2, Dbp, Nr1d1) as well as mGluR3. Comparison of expression between spontaneous sleep and sleep-deprived groups sacrificed at the same time of day revealed a number of transcripts (n = 55) that had higher expression in sleep deprivation compared to sleep. Genes upregulated in sleep deprivation predominantly were from the protein folding pathway (25 transcripts, including chaperones). Among 42 transcripts upregulated in sleep was the cold-inducible RNA-binding protein.

Conclusions

Cholinergic cell signatures were characterized. Whether the identified genes are changing as a consequence of differences in behavioral state or as part of the molecular regulatory mechanism remains to be determined.

Circadian Rhythms and Sleep in Drosophila melanogaster

The advantages of the model organism Drosophila melanogaster, including low genetic redundancy, functional simplicity, and the ability to conduct large-scale genetic screens, have been essential for understanding the molecular nature of circadian (∼24 hr) rhythms, and continue to be valuable in discovering novel regulators of circadian rhythms and sleep. In this review, we discuss the current understanding of these interrelated biological processes in Drosophila and the wider implications of this research. Clock genes period and timeless were first discovered in large-scale Drosophila genetic screens developed in the 1970s. Feedback of period and timeless on their own transcription forms the core of the molecular clock, and accurately timed expression, localization, post-transcriptional modification, and function of these genes is thought to be critical for maintaining the circadian cycle. Regulators, including several phosphatases and kinases, act on different steps of this feedback loop to ensure strong and accurately timed rhythms. Approximately 150 neurons in the fly brain that contain the core components of the molecular clock act together to translate this intracellular cycling into rhythmic behavior. We discuss how different groups of clock neurons serve different functions in allowing clocks to entrain to environmental cues, driving behavioral outputs at different times of day, and allowing flexible behavioral responses in different environmental conditions. The neuropeptide PDF provides an important signal thought to synchronize clock neurons, although the details of how PDF accomplishes this function are still being explored. Secreted signals from clock neurons also influence rhythms in other tissues. SLEEP is, in part, regulated by the circadian clock, which ensures appropriate timing of sleep, but the amount and quality of sleep are also determined by other mechanisms that ensure a homeostatic balance between sleep and wake. Flies have been useful for identifying a large set of genes, molecules, and neuroanatomic loci important for regulating sleep amount. Conserved aspects of sleep regulation in flies and mammals include wake-promoting roles for catecholamine neurotransmitters and involvement of hypothalamus-like regions, although other neuroanatomic regions implicated in sleep in flies have less clear parallels. Sleep is also subject to regulation by factors such as food availability, stress, and social environment. We are beginning to understand how the identified molecules and neurons interact with each other, and with the environment, to regulate sleep. Drosophila researchers can also take advantage of increasing mechanistic understanding of other behaviors, such as learning and memory, courtship, and aggression, to understand how sleep loss impacts these behaviors. Flies thus remain a valuable tool for both discovery of novel molecules and deep mechanistic understanding of sleep and circadian rhythms.

Food responsiveness regulates episodic behavioral states in Caenorhabditis elegans

Animals optimize survival and reproduction in part through control of behavioral states, which depend on an organism's internal and external environments. In the nematode Caenorhabditis elegans a variety of behavioral states have been described, including roaming, dwelling, quiescence, and episodic swimming. These states have been considered in isolation under varied experimental conditions, making it difficult to establish a unified picture of how they are regulated. Using long-term imaging, we examined C. elegans episodic behavioral states under varied mechanical and nutritional environments. We found that animals alternate between high-activity (active) and low-activity (sedentary) episodes in any mechanical environment, while the incidence of episodes and their behavioral composition depend on food levels. During active episodes, worms primarily roam, as characterized by continuous whole body movement. During sedentary episodes, animals exhibit dwelling (slower movements confined to the anterior half of the body) and quiescence (a complete lack of movement). Roaming, dwelling, and quiescent states are manifest not only through locomotory characteristics but also in pharyngeal pumping (feeding) and in egg-laying behaviors. Next, we analyzed the genetic basis of behavioral states. We found that modulation of behavioral states depends on neuropeptides and insulin-like signaling in the nervous system. Sensory neurons and the Foraging homolog EGL-4 regulate behavior through control of active/sedentary episodes. Optogenetic stimulation of dopaminergic and serotonergic neurons induced dwelling, implicating dopamine as a dwell-promoting neurotransmitter. Our findings provide a more unified description of behavioral states and suggest that perception of nutrition is a conserved mechanism for regulating animal behavior.NEW & NOTEWORTHY One strategy by which animals adapt to their internal states and external environments is by adopting behavioral states. The roundworm Caenorhabditis elegans is an attractive model for investigating how behavioral states are genetically and neuronally controlled. Here we describe the hierarchical organization of behavioral states characterized by locomotory activity, feeding, and egg-laying. We show that decisions to engage in these behaviors are controlled by the nervous system through insulin-like signaling and the perception of food.

The Lipocalin LPR-1 Cooperates with LIN-3/EGF Signaling To Maintain Narrow Tube Integrity in Caenorhabditis elegans

Lipocalins are secreted cup-shaped glycoproteins that bind sterols, fatty acids, and other lipophilic molecules. Lipocalins have been implicated in a wide array of processes related to lipophilic cargo transport, sequestration, and signaling, and several are used as biomarkers for human disease, but the functions of most lipocalins remain poorly understood. Here we show that the Caenorhabditis elegans lipocalin LPR-1 is required to maintain apical membrane integrity and a continuous lumen in two narrow unicellular tubes, the excretory duct and pore, during a period of rapid lumen elongation. LPR-1 fusion protein is expressed by the duct and pore and accumulates both intracellularly and in apical extracellular compartments, but it can also function cell nonautonomously when provided from outside of the excretory system. lpr-1 mutant defects can be rescued by increased signaling through the epidermal growth factor (EGF)-Ras-extracellular signal regulated kinase (ERK) pathway, which promotes the more elongated duct vs. less elongated pore tube fate. Spatial and temporal rescue experiments indicate that Ras signaling acts within the duct and pore tubes during or prior to cell fate determination to bypass the requirement for LPR-1 lpr-1 mutations did not disrupt LIN-3/EGF-dependent duct-fate specification, prevent functioning of any specific LIN-3/EGF isoform, or alter LET-23/EGFR localization, and reduced signaling did not phenocopy or enhance lpr-1 mutant defects. These data suggest that LPR-1 protects lumen integrity through a LIN-3/EGF-independent mechanism, but that increased signaling upregulates some target(s) that can compensate for lpr-1 absence.

Analysis of the NK2 homeobox gene ceh-24 reveals sublateral motor neuron control of left-right turning during sleep

Sleep is a behavior that is found in all animals that have a nervous system and that have been studied carefully. In Caenorhabditis elegans larvae, sleep is associated with a turning behavior, called flipping, in which animals rotate 180° about their longitudinal axis. However, the molecular and neural substrates of this enigmatic behavior are not known. Here, we identified the conserved NK-2 homeobox gene ceh-24 to be crucially required for flipping. ceh-24 is required for the formation of processes and for cholinergic function of sublateral motor neurons, which separately innervate the four body muscle quadrants. Knockdown of cholinergic function in a subset of these sublateral neurons, the SIAs, abolishes flipping. The SIAs depolarize during flipping and their optogenetic activation induces flipping in a fraction of events. Thus, we identified the sublateral SIA neurons to control the three-dimensional movements of flipping. These neurons may also control other types of motion.

DOI:http://dx.doi.org/10.7554/eLife.24846.001

Although sleeping individuals do not move voluntarily, they are not completely immobile. Both people and animals regularly change position in their sleep, but it is not known why these movements occur or what regulates them.

One of the simplest animals known to require sleep is the nematode worm Caenorhabditis elegans, which is often used by researchers to study the molecular basis of behavior. In common with more complex animals, worms go to sleep lying on either their left or right side and then switch periodically between the two. This “flipping” behavior is typically not seen outside of sleep.

By screening worms with mutations in different genes, Schwarz and Bringmann identified one mutant that does not flip during sleep. The mutant lacked a gene called ceh-24, which is normally active in a set of four neurons known as SIAs. These are a type of motor neuron; that is, neurons that control the contraction of muscles.

The body wall muscles of C. elegans run along the length of its body and are organized into “quadrants” that each cover a quarter of the worm. Schwarz and Bringmann show that unlike other C. elegans motor neurons, SIA neurons control each quadrant separately. By activating specific SIA neurons the worms can contract the muscles on each side of the body independently, and thereby flip from one side to the other.

Further investigation revealed that the SIA motor neurons can also control other types of complex movement. Additional experiments are now needed to determine how the neurons support these behaviors. Another challenge will be to work out the purpose of posture changes during sleep for C. elegans and other animals.

DOI:http://dx.doi.org/10.7554/eLife.24846.002

Sleep and Development in Genetically Tractable Model Organisms

Sleep is widely recognized as essential, but without a clear singular function. Inadequate sleep impairs cognition, metabolism, immune function, and many other processes. Work in genetic model systems has greatly expanded our understanding of basic sleep neurobiology as well as introduced new concepts for why we sleep. Among these is an idea with its roots in human work nearly 50 years old: sleep in early life is crucial for normal brain maturation. Nearly all known species that sleep do so more while immature, and this increased sleep coincides with a period of exuberant synaptogenesis and massive neural circuit remodeling. Adequate sleep also appears critical for normal neurodevelopmental progression. This article describes recent findings regarding molecular and circuit mechanisms of sleep, with a focus on development and the insights garnered from models amenable to detailed genetic analyses.

A mechanism for sickness sleep: lessons from invertebrates

During health, animal sleep is regulated by an internal clock and by the duration of prior wakefulness. During sickness, sleep is regulated by cytokines released from either peripheral cells or from cells within the nervous system. These cytokines regulate central nervous system neurons to induce sleep. Recent research in the invertebrates Caenorhabditis elegans and Drosophila melanogaster has led to new insights into the mechanism of sleep during sickness. Sickness is triggered by exposure to environments such as infection, heat, or ultraviolet light irradiation, all of which cause cellular stress. Epidermal growth factor is released from stressed cells and signals to activate central neuroendocrine cell(s). These neuron(s) release neuropeptides including those containing an amidated arginine(R)-phenylalanine(F) motif at their C-termini (RFamide peptides). Importantly, mechanisms regulating sickness sleep are partially distinct from those regulating healthy sleep. We will here review key findings that have elucidated the central neuroendocrine mechanism of sleep during sickness. Adaptive mechanisms employed in the control of sickness sleep may play a role in correcting cellular homeostasis after various insults. We speculate that these mechanisms may play a maladaptive role in human pathological conditions such as in the fatigue and anorexia associated with autoimmune diseases, with major depression, and with unexplained chronic fatigue.

The RFamide receptor DMSR-1 regulates stress-induced sleep in C. elegans

In response to environments that cause cellular stress, animals engage in sleep behavior that facilitates recovery from the stress. In Caenorhabditis elegans, stress-induced sleep(SIS) is regulated by cytokine activation of the ALA neuron, which releases FLP-13 neuropeptides characterized by an amidated arginine-phenylalanine (RFamide) C-terminus motif. By performing an unbiased genetic screen for mutants that impair the somnogenic effects of FLP-13 neuropeptides, we identified the gene dmsr-1, which encodes a G-protein coupled receptor similar to an insect RFamide receptor. DMSR-1 is activated by FLP-13 peptides in cell culture, is required for SIS in vivo, is expressed non-synaptically in several wake-promoting neurons, and likely couples to a Gi/o heterotrimeric G-protein. Our data expand our understanding of how a single neuroendocrine cell coordinates an organism-wide behavioral response, and suggest that similar signaling principles may function in other organisms to regulate sleep during sickness.

DOI:http://dx.doi.org/10.7554/eLife.19837.001

People often feel fatigued and sleepy when they are sick. Other animals also show signs of sleepiness when ill – they stop eating, move less, and are less responsive to changes in their environment. Sickness-induced sleep helps both people and other animals to recover, and many scientists believe that this type of sleep is different than nightly sleep.

Studies of sickness-induced sleep have made use of a simple worm with a simple nervous system. In this worm, a single nerve cell releases chemicals that cause the worm to fall asleep in response to illness. Animals exposed to one of these chemicals, called FLP-13, fall asleep even when they are not sick. As such, scientists would like to know which cells in the nervous system FLP-13 interacts with, what receptor the cells use to recognize this chemical, and whether it turns on cells that induce sleep or turns off the cells that cause wakefulness.

Now, Iannacone et al. show that FLP-13 likely causes sleep by turning down activity in the cells in the nervous system that promote wakefulness. The experiments sifted through genetic mutations to determine which ones cause the worms not to fall asleep when FLP-13 is released. This revealed that worms with a mutation that causes them to lack a receptor protein called DMSR-1 do not become sleepy in response to FLP-13. This suggests that DMSR-1 must be essential for FLP-13 to trigger sleep. About 10% of cells in the worm’s nervous system have the DMSR-1 receptor. Some of these neurons tell the worm to move forward or to forage around for food. The experiments also showed that FLP-13 is probably not the only chemical that interacts with the DMSR-1 receptor, but the identities of these other chemicals remain unknown.

Additional experiments are now needed to determine if sickness-induced sleepiness in humans and other mammals is triggered by a similar mechanism. If it is, then drugs might be developed to treat people experiencing fatigue associated with sickness as well as other unexplained cases of fatigue.

DOI:http://dx.doi.org/10.7554/eLife.19837.002

The Sleep in Caenorhabditis elegans: What We Know Until Now

Sleep, as one of the most important requirements of our brain, has a mystical nature. Despite long-standing studies, the molecular mechanisms and physiological properties of sleep have not been defined well as the complexity of the mammals' brain make it difficult to investigate the mechanisms and properties of sleep. Although some features of sleep have changed during evolution, its existence in such a simple animal, Caenorhabditis elegans, not only signifies the importance of sleep in even simple animals, but also allows the scientist to assess the core mechanism and biological events in an uncomplicated organism. This article reviews the information which exists about the characteristics of sleep in C. elegans, its circadian rhythm, the neurons and neurotransmitters responsible for each state, and the signaling molecules involved. Although much still remains to be resolved about the sleep of C. elegans, the available knowledge helps the scientists to recognize the properties better of this mysterious function of the brain.

Acute Sleep Deprivation Blocks Short- and Long-Term Operant Memory in Aplysia

STUDY OBJECTIVES

Insufficient sleep in individuals appears increasingly common due to the demands of modern work schedules and technology use. Consequently, there is a growing need to understand the interactions between sleep deprivation and memory. The current study determined the effects of acute sleep deprivation on short and long-term associative memory using the marine mollusk Aplysia californica, a relatively simple model system well known for studies of learning and memory.

METHODS

Aplysia were sleep deprived for 9 hours using context changes and tactile stimulation either prior to or after training for the operant learning paradigm, learning that food is inedible (LFI). The effects of sleep deprivation on short-term (STM) and long-term memory (LTM) were assessed.

RESULTS

Acute sleep deprivation prior to LFI training impaired the induction of STM and LTM with persistent effects lasting at least 24 h. Sleep deprivation immediately after training blocked the consolidation of LTM. However, sleep deprivation following the period of molecular consolidation did not affect memory recall. Memory impairments were independent of handling-induced stress, as daytime handled control animals demonstrated no memory deficits. Additional training immediately after sleep deprivation failed to rescue the induction of memory, but additional training alleviated the persistent impairment in memory induction when training occurred 24 h following sleep deprivation.

CONCLUSIONS

Acute sleep deprivation inhibited the induction and consolidation, but not the recall of memory. These behavioral studies establish Aplysia as an effective model system for studying the interactions between sleep and memory formation.

rBmαTX14 Increases the Life Span and Promotes the Locomotion of Caenorhabditis Elegans

The scorpion has been extensively used in various pharmacological profiles or as food supplies. The exploration of scorpion venom has been reported due to the presence of recombinant peptides. rBmαTX14 is an α-neurotoxin extracted from the venom gland of the East Asian scorpion Buthus martensii Karsch and can affect ion channel conductance. Here, we investigated the functions of rBmαTX14 using the Caenorhabditis elegans model. Using western blot analysis, rBmαTX14 was shown to be expressed both in the cytoplasm and inclusion bodies in the E.coli Rosetta (DE3) strain. Circular dichroism spectroscopy analysis demonstrated that purified rBmαTX14 retained its biological structures. Next, feeding nematodes with E.coli Rosetta (DE3) expressing rBmαTX14 caused extension of the life span and promoted the locomotion of the nematodes. In addition, we identified several genes that play various roles in the life span and locomotion of C. elegans through microarray analysis and quantitative real-time PCR. Furthermore, if the amino acid site H₁₅ of rBmαTX14 was mutated, rBmαTX14 no longer promoted the C. elegans life span. In conclusion, the results not only demonstrated the functions and mechanism of rBmαTX14 in C. elegans, but also provided the new sight in the utility of recombinant peptides from scorpion venom.

C. elegans Stress-Induced Sleep Emerges from the Collective Action of Multiple Neuropeptides

The genetic basis of sleep regulation remains poorly understood. In C. elegans, cellular stress induces sleep through epidermal growth factor (EGF)-dependent activation of the EGF receptor in the ALA neuron. The downstream mechanism by which this neuron promotes sleep is unknown. Single-cell RNA sequencing of ALA reveals that the most highly expressed, ALA-enriched genes encode neuropeptides. Here we have systematically investigated the four most highly enriched neuropeptides: flp-7, nlp-8, flp-24, and flp-13. When individually removed by null mutation, these peptides had little or no effect on stress-induced sleep. However, stress-induced sleep was abolished in nlp-8; flp-24; flp-13 triple-mutant animals, indicating that these neuropeptides work collectively in controlling stress-induced sleep. We tested the effect of overexpression of these neuropeptide genes on five behaviors modulated during sleep-pharyngeal pumping, defecation, locomotion, head movement, and avoidance response to an aversive stimulus-and we found that, if individually overexpressed, each of three neuropeptides (nlp-8, flp-24, or flp-13) induced a different suite of sleep-associated behaviors. These overexpression results raise the possibility that individual components of sleep might be specified by individual neuropeptides or combinations of neuropeptides.

Automated analysis of sleep control via a single neuron active at sleep onset in C. elegans

Longitudinal analyses are crucial for understanding long-term processes such as development and behavioral rhythms. For a complete understanding of such processes, both organism-level observations as well as single-cell observations are necessary. Sleep is an example for a long-term process that is under developmental control. This behavioral state is induced by conserved sleep-active neurons, but little is known about how sleep neurons control the physiology of an animal systemically. In the nematode C. elegans, sleep induction crucially requires the single RIS interneuron to actively induce a developmentally regulated sleep behavior. Here, we used RIS-induced sleep as an example of how longitudinal analyses can be automated. We developed methods to analyze both behavior and neural activity in larva across the sleep-wake cycle. To image behavior, we used an improved DIC contrast to extract the head and detect the nose. To image neural activity, we used GCaMP3 expression in a small number of neurons including RIS combined with a neuron discrimination algorithm. Thus, we present a comprehensive platform for automatically analyzing behavior and neural activity in C. elegans exemplified by using RIS-induced sleep during C. elegans development.

A hydrodynamic mechanism for attraction of undulatory microswimmers to surfaces (bordertaxis)

Although small nematodes significantly impact human and animal health, agriculture, and ecology, little is known about the role of hydrodynamics in their life cycles. Using the nematode Caenorhabditis elegans as a model undulatory microswimmer, we have observed that animals are attracted to and swim along surfaces. The attraction to surfaces does not require mechanosensory neuron function. In dilute swarms, swimmers aggregate near surfaces. Using resistive force-based theory, symmetry arguments, and direct hydrodynamic simulations, we demonstrate that forces resulting from the interaction between the swimmer-induced flow field and a nearby surface cause a short-range hydrodynamic torque that stirs the swimmers towards the surface. When combined with steric forces, this causes locomotion along the surface. This surface attraction may affect nematode mate and food finding behaviour and, in the case of parasitic nematodes, may facilitate host penetration. Surface attraction must be accounted for when studying animals' responses to various stimuli, and suggests means of controlling undulatory microswimmers.

Sleep-active neuron specification and sleep induction require FLP-11 neuropeptides to systemically induce sleep

Sleep is an essential behavioral state. It is induced by conserved sleep-active neurons that express GABA. However, little is known about how sleep neuron function is determined and how sleep neurons change physiology and behavior systemically. Here, we investigated sleep in Caenorhabditis elegans, which is induced by the single sleep-active neuron RIS. We found that the transcription factor LIM-6, which specifies GABAergic function, in parallel determines sleep neuron function through the expression of APTF-1, which specifies the expression of FLP-11 neuropeptides. Surprisingly FLP-11, and not GABA, is the major component that determines the sleep-promoting function of RIS. FLP-11 is constantly expressed in RIS. At sleep onset RIS depolarizes and releases FLP-11 to induce a systemic sleep state.

Distinct Mechanisms Underlie Quiescence during Two Caenorhabditis elegans Sleep-Like States

Electrophysiological recordings have enabled identification of physiologically distinct yet behaviorally similar states of mammalian sleep. In contrast, sleep in nonmammals has generally been identified behaviorally and therefore regarded as a physiologically uniform state characterized by quiescence of feeding and locomotion, reduced responsiveness, and rapid reversibility. The nematode Caenorhabditis elegans displays sleep-like quiescent behavior under two conditions: developmentally timed quiescence (DTQ) occurs during larval transitions, and stress-induced quiescence (SIQ) occurs in response to exposure to cellular stressors. Behaviorally, DTQ and SIQ appear identical. Here, we use optogenetic manipulations of neuronal and muscular activity, pharmacology, and genetic perturbations to uncover circuit and molecular mechanisms of DTQ and SIQ. We find that locomotion quiescence induced by DTQ- and SIQ-associated neuropeptides occurs via their action on the nervous system, although their neuronal target(s) and/or molecular mechanisms likely differ. Feeding quiescence during DTQ results from a loss of pharyngeal muscle excitability, whereas feeding quiescence during SIQ results from a loss of excitability in the nervous system. Together these results indicate that, as in mammals, quiescence is subserved by different mechanisms during distinct sleep-like states in C. elegans.

A Conserved GEF for Rho-Family GTPases Acts in an EGF Signaling Pathway to Promote Sleep-like Quiescence in Caenorhabditis elegans

Sleep is evolutionarily conserved and required for organism homeostasis and survival. Despite this importance, the molecular and cellular mechanisms underlying sleep are not well understood. Caenorhabditis elegans exhibits sleep-like behavioral quiescence and thus provides a valuable, simple model system for the study of cellular and molecular regulators of this process. In C. elegans, epidermal growth factor receptor (EGFR) signaling is required in the neurosecretory neuron ALA to promote sleep-like behavioral quiescence after cellular stress. We describe a novel role for VAV-1, a conserved guanine nucleotide exchange factor (GEF) for Rho-family GTPases, in regulation of sleep-like behavioral quiescence. VAV-1, in a GEF-dependent manner, acts in ALA to suppress locomotion and feeding during sleep-like behavioral quiescence in response to cellular stress. Additionally, VAV-1 activity is required for EGF-induced sleep-like quiescence and normal levels of EGFR and secretory dense core vesicles in ALA. Importantly, the role of VAV-1 in promoting cellular stress-induced behavioral quiescence is vital for organism health because VAV-1 is required for normal survival after cellular stress.

Call it Worm Sleep

The nematode Caenorhabditis elegans stops feeding and moving during a larval transition stage called lethargus and following exposure to cellular stressors. These behaviors have been termed 'sleep-like states'. We argue that these behaviors should instead be called sleep. Sleep during lethargus is similar to sleep regulated by circadian timers in insects and mammals, and sleep in response to cellular stress is similar to sleep induced by sickness in other animals. Sleep in mammals and Drosophila shows molecular and functional conservation with C. elegans sleep. The simple neuroanatomy and powerful genetic tools of C. elegans have yielded insights into sleep regulation and hold great promise for future research into sleep regulation and function.

The Yin and Yang of Sleep and Attention

Sleep is not a single state, but a complex set of brain processes that supports several physiological needs. Sleep deprivation is known to affect attention in many animals, suggesting that a key function of sleep is to regulate attention. Conversely, tasks that require more attention drive sleep need and sleep intensity. Attention involves the ability to filter incoming stimuli based on their relative salience, and this is likely to require coordinated synaptic activity across the brain. This capacity may have only become possible with the evolution of related neural mechanisms that support two key sleep functions: stimulus suppression and synaptic plasticity. We argue here that sleep and attention may have coevolved as brain states that regulate each other.

FRPR-4 Is a G-Protein Coupled Neuropeptide Receptor That Regulates Behavioral Quiescence and Posture in Caenorhabditis elegans

Neuropeptides signal through G-protein coupled receptors (GPCRs) to regulate a broad array of animal behaviors and physiological processes. The Caenorhabditis elegans genome encodes approximately 100 predicted neuropeptide receptor GPCRs, but in vivo roles for only a few have been identified. We describe here a role for the GPCR FRPR-4 in the regulation of behavioral quiescence and locomotive posture. FRPR-4 is activated in cell culture by several neuropeptides with an amidated isoleucine-arginine-phenylalanine (IRF) motif or an amidated valine-arginine-phenylalanine (VRF) motif at their carboxy termini, including those encoded by the gene flp-13. Loss of frpr-4 function results in a minor feeding quiescence defect after heat-induced cellular stress. Overexpression of frpr-4 induces quiescence of locomotion and feeding as well as an exaggerated body bend posture. The exaggerated body bend posture requires the gene flp-13. While frpr-4 is expressed broadly, selective overexpression of frpr-4 in the proprioceptive DVA neurons results in exaggerated body bends that require flp-13 in the ALA neuron. Our results suggest that FLP-13 and other neuropeptides signal through FRPR-4 and other receptors to regulate locomotion posture and behavioral quiescence.

Neural Regulatory Pathways of Feeding and Fat in Caenorhabditis elegans

The compact nervous system of Caenorhabditis elegans and its genetic tractability are features that make this organism highly suitable for investigating energy balance in an animal system. Here, we focus on molecular components and organizational principles emerging from the investigation of pathways that largely originate in the nervous system and regulate feeding behavior but also peripheral fat regulation through neuroendocrine signaling. We provide an overview of studies aimed at understanding how C. elegans integrate internal and external cues in feeding behavior. We highlight some of the similarities and differences in energy balance between C. elegans and mammals. We also provide our perspective on unresolved issues, both conceptual and technical, that we believe have hampered critical evaluation of findings relevant to fat regulation in C. elegans.

Synapse-Assembly Proteins Maintain Synaptic Vesicle Cluster Stability and Regulate Synaptic Vesicle Transport in Caenorhabditis elegans

The functional integrity of neurons requires the bidirectional active transport of synaptic vesicles (SVs) in axons. The kinesin motor KIF1A transports SVs from somas to stable SV clusters at synapses, while dynein moves them in the opposite direction. However, it is unclear how SV transport is regulated and how SVs at clusters interact with motor proteins. We addressed these questions by isolating a rare temperature-sensitive allele of Caenorhabditis elegans unc-104 (KIF1A) that allowed us to manipulate SV levels in axons and dendrites. Growth at 20° and 14° resulted in locomotion rates that were ∼3 and 50% of wild type, respectively, with similar effects on axonal SV levels. Corresponding with the loss of SVs from axons, mutants grown at 14° and 20° showed a 10- and 24-fold dynein-dependent accumulation of SVs in their dendrites. Mutants grown at 14° and switched to 25° showed an abrupt irreversible 50% decrease in locomotion and a 50% loss of SVs from the synaptic region 12-hr post-shift, with no further decreases at later time points, suggesting that the remaining clustered SVs are stable and resistant to retrograde removal by dynein. The data further showed that the synapse-assembly proteins SYD-1, SYD-2, and SAD-1 protected SV clusters from degradation by motor proteins. In syd-1, syd-2, and sad-1 mutants, SVs accumulate in an UNC-104-dependent manner in the distal axon region that normally lacks SVs. In addition to their roles in SV cluster stability, all three proteins also regulate SV transport.

FMRFamide signaling promotes stress-induced sleep in Drosophila

Enhanced sleep in response to cellular stress is a conserved adaptive behavior across multiple species, but the mechanism of this process is poorly understood. Drosophila melanogaster increases sleep following exposure to septic or aseptic injury, and Caenorhabditis elegans displays sleep-like quiescence following exposure to high temperatures that stress cells. We show here that, similar to C. elegans, Drosophila responds to heat stress with an increase in sleep. In contrast to Drosophila infection-induced sleep, heat-induced sleep is not sensitive to the time-of-day of the heat pulse. Moreover, the sleep response to heat stress does not require Relish, the NFκB transcription factor that is necessary for infection-induced sleep, indicating that sleep is induced by multiple mechanisms from different stress modalities. We identify a sleep-regulating role for a signaling pathway involving FMRFamide neuropeptides and their receptor FR. Animals mutant for either FMRFamide or for the FMRFamide receptor (FR) have a reduced recovery sleep in response to heat stress. FR mutants, in addition, show reduced sleep responses following infection with Serratia marcescens, and succumb to infection at a faster rate than wild-type controls. Together, these findings support the hypothesis that FMRFamide and its receptor promote an adaptive increase in sleep following stress. Because an FMRFamide-like neuropeptide plays a similar role in C. elegans, we propose that FRMFamide neuropeptide signaling is an ancient regulator of recovery sleep which occurs in response to cellular stress.

Neuronal responses to stress and injury in C. elegans

The nervous system plays critical roles in the stress response. Animals can survive and function under harsh conditions, and resist and recover from injuries because neurons perceive and respond to various stressors through specific regulatory mechanisms. Caenorhabditis elegans has served as an excellent model to discover fundamental mechanisms underlying the neuronal response to stress. The basic physiological processes that C. elegans exhibits under stress conditions are similar to those observed in higher organisms. Many molecular pathways activated by environmental and cellular stresses are also conserved. In this review, we summarize major findings in examining neuronal responses to hypoxia, oxidative stress, osmotic stress, and traumatic injury. These studies from C. elegans have provided novel insights into our understanding of neuronal responses to stress at the molecular, cellular, and circuit levels.

FMRFamide-like FLP-13 neuropeptides promote quiescence following heat stress in Caenorhabditis elegans

Among the most important decisions an animal makes is whether to engage in active movement and feeding behavior or to become quiescent. The molecular signaling mechanisms underlying this decision remain largely unknown. The nematode Caenorhabditis elegans displays sleep-like quiescence following exposures that result in cellular stress. The neurosecretory ALA neuron is required for this stress-induced recovery quiescence, but the mechanisms by which ALA induces quiescence have been unknown. We report here that quiescence induced by heat stress requires ALA depolarization and release of FMRFamide-like neuropeptides encoded by the flp-13 gene. Optogenetic activation of ALA reduces feeding and locomotion in a FLP-13-dependent manner. Overexpression of flp-13 is sufficient to induce quiescent behavior during normally active periods. We have here identified a major biological role for FMRFamide-like neuropeptides in nematodes, and we suggest that they may function in a similar capacity in other organisms.