Differential regulation of circadian pacemaker output by separate clock genes in Drosophila

The Never Given 2022 Pittendrigh/Aschoff Lecture: The Clock Network in the Brain-Insights From Insects

My journey into chronobiology began in 1977 with lectures and internships with Wolfgang Engelmann and Hans Erkert at the University of Tübingen in Germany. At that time, the only known animal clock gene was Period, and the location and organization of the master circadian clock in the brain was completely unknown for the model insect Drosophila melanogaster. I was thus privileged to witness and participate in the research that led us from discovering the first clock gene to identifying the clock network in the fly brain and the putative pathways linking it to behavior and physiology. This article highlights my role in these developments and also shows how the successful use of D. melanogaster for studies of circadian rhythms has contributed to the understanding of clock networks in other animals. I also report on my experiences in the German scientific system and hope that my story will be of interest to some of you.

The P-loop NTPase RUVBL2 is a conserved clock component across eukaryotes

The eukaryotic circadian clock keeps time by using a transcription-translation feedback loop, which exhibits an architecture that is conserved across a diverse range of organisms, including fungi, plants and animals1. Despite their mechanistic similarity, the molecular components of these clocks indicate a lack of common ancestry2. Our study reveals that RUVBL2, which is a P-loop NTPase enzyme previously shown to affect circadian phase and amplitude as part of mammalian clock super-complexes, influences the circadian period through its remarkably slow ATPase activity, resembling the well-characterized KaiC-based clock in cyanobacteria. A screen of RUVBL2 variants identified arrhythmic, short-period and long-period mutants that altered circadian locomotor activity rhythms following delivery by adeno-associated virus to the murine suprachiasmatic nucleus. Enzymatic assays showed that wild-type RUVBL2 hydrolyses only around 13 ATP molecules a day, a vastly reduced turnover compared with typical ATPases. Notably, physical interactions between RUVBL2 orthologues and core clock proteins in humans, Drosophila and the fungus Neurospora, along with consistent circadian phenotypes of RUVBL2-mutant orthologues across species, reinforce their clock-related function in eukaryotes. Thus, as well as establishing RUVBL2 as a common core component in eukaryotic clocks, our study supports the idea that slow ATPase activity, initially discovered in cyanobacteria, is a shared feature of eukaryotic clocks.

Correlation between circadian and photoperiodic latitudinal clines in Drosophila littoralis

Insects can survive harsh conditions, including Arctic winters, by entering a hormonally induced state of dormancy, known as diapause. Diapause is triggered by environmental cues such as shortening of the photoperiod (lengthening of the night). The time of entry into diapause depends on the latitude of the insects' habitat, and this applies even within a species: populations living at higher latitudes enter diapause earlier in the year than populations living at lower latitudes. A long-standing question in biology is whether the internal circadian clock, which governs daily behaviour and serves as a reference clock to measure night length, shows similar latitudinal adaptations. To address this question, we examined the onset of diapause and various behavioural and molecular parameters of the circadian clock in the cosmopolitan fly, Drosophila littoralis, a species distributed throughout Europe from the Black Sea (41° N) to Arctic regions (69° N). We found that all clock parameters examined showed the same correlation with latitude as the critical night length for diapause induction. We conclude that the circadian clock has adapted to the latitude and that this may result in the observed latitudinal differences in the onset of diapause.

Circadian Vesicle Capture and Phase-Delayed Activity Maximize Synaptic Neuropeptide Release by Clock Neurons

Drosophila sLNv clock neurons release the neuropeptide PDF to control circadian rhythms. Strikingly, PDF content in sLNv terminals is rhythmic with late night synaptic accumulation occurring in preparation for exocytosis from dense-core vesicles (DCVs) hours later at midmorning. The timing of this rhythm has been puzzling because (a) DCV exocytosis occurs hours after elevations in resting membrane potential (RMP) and Ca2+ levels in the soma, which have traditionally been considered surrogates for activity and peptidergic transmission, and (b) the mechanism for the daily surge in synaptic neuropeptide content is unknown. Here, a sensor for sNPF, a neuropeptide co-packaged into DCVs along with PDF, reveals native synaptic neuropeptide release requires Ca2+ influx and occurs at midmorning under control of the circadian clock. Furthermore, Ca2+ imaging demonstrates that presynaptic activity and neuropeptide release both peak at the same time of day, showing that activity and release rhythms at the presynapse are remarkably uncoupled from somatic Ca2+ and RMP rhythms in these neurons. Finally, anterograde axonal transport of DCVs is constant throughout the day and retention of circulating DCVs rhythmically boosts presynaptic neuropeptide content. This late-night vesicle capture requires the circadian clock. Thus, the robust daily rhythms of synaptic neuropeptide release that controls circadian behavior is produced by clock-induced late-night vesicle capture and midmorning activity.

Daily ultrastructural remodeling of clock neurons

In Drosophila, about 250 clock neurons in the brain form a network that orchestrates circadian rhythmicity. Among them, eight small Lateral ventral Neurons (s-LNvs) play a critical role, synchronizing the circadian ensemble via the neuropeptide Pigment-Dispersing Factor (PDF). Moreover, their neurites show daily variations in morphology, PDF levels, synaptic markers and connectivity. This process, called circadian structural plasticity, is ill-defined at the subcellular level. Here, we present 3D volumes of the s-LNv terminals generated by Serial Block-face Scanning Electron Microscopy (SBEM) at three key time points, two hours before lights-ON, two hours after lights-ON, and two hours after lights-OFF. We report a reduction in the number of neuronal varicosities at night, which reflects (and probably regulates) the cycling of the components we found therein. Indeed, in the morning we observed more presynaptic sites and increased accumulation and release of dense core vesicles. These rhythms were paralleled by periodic changes in mitochondrial structure that suggest daily modulation of their activity. We propose that circadian plasticity of the functionally relevant structures within presynaptic varicosities cyclically modulates the influence of the s-LNvs on the clock network.

Synaptic Targets of Circadian Clock Neurons Influence Core Clock Parameters

Neuronal connectivity in the circadian clock network is essential for robust endogenous timekeeping. In the Drosophila circadian clock network, four pairs of small ventral lateral neurons (sLNvs) serve as critical pacemakers. Peptidergic communication via sLNv release of the key output neuropeptide Pigment Dispersing Factor (PDF) has been well characterized. In contrast, little is known about the role of the synaptic connections that sLNvs form with downstream neurons. Connectomic analyses revealed that the sLNvs form strong synaptic connections with a group of previously uncharacterized neurons, SLP316. Here, we show that silencing synaptic output in the SLP316 neurons via tetanus toxin (TNT) expression shortens the free-running period, whereas hyper-exciting them by expressing the Na[+] channel NaChBac results in period lengthening. Under light-dark cycles, silencing SLP316 neurons also causes lower daytime activity and higher daytime sleep. Our results revealed that the main postsynaptic partners of the Drosophila pacemaker neurons are a non-clock neuronal cell type that regulates the timing of sleep and activity.

Alternative splicing of Clock transcript mediates the response of circadian clocks to temperature changes

Circadian clocks respond to temperature changes over the calendar year, allowing organisms to adjust their daily biological rhythms to optimize health and fitness. In Drosophila, seasonal adaptations are regulated by temperature-sensitive alternative splicing (AS) of period (per) and timeless (tim) genes that encode key transcriptional repressors of clock gene expression. Although Clock (Clk) gene encodes the critical activator of circadian gene expression, AS of its transcripts and its potential role in temperature regulation of clock function have not been explored. Here, we observed that Clk transcripts undergo temperature-sensitive AS. Specifically, cold temperature leads to the production of an alternative Clk transcript, hereinafter termed Clk-cold, which encodes a CLK isoform with an in-frame deletion of four amino acids proximal to the DNA binding domain. Notably, serine 13 (S13), which we found to be a CK1α-dependent phosphorylation site, is deleted in CLK-cold protein. We demonstrated that upon phosphorylation at CLK(S13), CLK-DNA interaction is reduced, thus decreasing transcriptional activity of CLK. This is in agreement with our findings that CLK occupancy at clock genes and transcriptional output are elevated at cold temperature likely due to higher amounts of CLK-cold isoforms that lack S13 residue. Finally, we showed that PER promotes CK1α-dependent phosphorylation of CLK(S13), supporting kinase-scaffolding role of repressor proteins as a conserved feature in the regulation of eukaryotic circadian clocks. This study provides insights into the complex collaboration between AS and phospho-regulation in shaping temperature responses of the circadian clock.

Glucocerebrosidase deficiency leads to neuropathology via cellular immune activation

Mutations in GBA (glucosylceramidase beta), which encodes the lysosomal enzyme glucocerebrosidase (GCase), are the strongest genetic risk factor for the neurodegenerative disorders Parkinson's disease (PD) and Lewy body dementia. Recent work has suggested that neuroinflammation may be an important factor in the risk conferred by GBA mutations. We therefore systematically tested the contributions of immune-related genes to neuropathology in a Drosophila model of GCase deficiency. We identified target immune factors via RNA-Seq and proteomics on heads from GCase-deficient flies, which revealed both increased abundance of humoral factors and increased macrophage activation. We then manipulated the identified immune factors and measured their effect on head protein aggregates, a hallmark of neurodegenerative disease. Genetic ablation of humoral (secreted) immune factors did not suppress the development of protein aggregation. By contrast, re-expressing Gba1b in activated macrophages suppressed head protein aggregation in Gba1b mutants and rescued their lifespan and behavioral deficits. Moreover, reducing the GCase substrate glucosylceramide in activated macrophages also ameliorated Gba1b mutant phenotypes. Taken together, our findings show that glucosylceramide accumulation due to GCase deficiency leads to macrophage activation, which in turn promotes the development of neuropathology.

Circadian plasticity evolves through regulatory changes in a neuropeptide gene

Many organisms, including cosmopolitan drosophilids, show circadian plasticity, varying their activity with changing dawn-dusk intervals1. How this behaviour evolves is unclear. Here we compare Drosophila melanogaster with Drosophila sechellia, an equatorial, ecological specialist that experiences minimal photoperiod variation, to investigate the mechanistic basis of circadian plasticity evolution2. D. sechellia has lost the ability to delay its evening activity peak time under long photoperiods. Screening of circadian mutants in D. melanogaster/D. sechellia hybrids identifies a contribution of the neuropeptide pigment-dispersing factor (Pdf) to this loss. Pdf exhibits species-specific temporal expression, due in part to cis-regulatory divergence. RNA interference and rescue experiments in D. melanogaster using species-specific Pdf regulatory sequences demonstrate that modulation of this neuropeptide's expression affects the degree of behavioural plasticity. The Pdf regulatory region exhibits signals of selection in D. sechellia and across populations of D. melanogaster from different latitudes. We provide evidence that plasticity confers a selective advantage for D. melanogaster at elevated latitude, whereas D. sechellia probably suffers fitness costs through reduced copulation success outside its range. Our findings highlight this neuropeptide gene as a hotspot locus for circadian plasticity evolution that might have contributed to both D. melanogaster's global distribution and D. sechellia's specialization.

The Drosophila circadian clock gene cycle controls the development of clock neurons

Daily behavioral and physiological rhythms are controlled by the brain's circadian timekeeping system, a synchronized network of neurons that maintains endogenous molecular oscillations. These oscillations are based on transcriptional feedback loops of clock genes, which in Drosophila include the transcriptional activators Clock (Clk) and cycle (cyc). While the mechanisms underlying this molecular clock are very well characterized, the roles that the core clock genes play in neuronal physiology and development are much less understood. The Drosophila timekeeping center is composed of ~150 clock neurons, among which the four small ventral lateral neurons (sLNvs) are the most dominant pacemakers under constant conditions. Here, we show that downregulating the clock gene cyc specifically in the Pdf-expressing neurons leads to decreased fasciculation both in larval and adult brains. This effect is due to a developmental role of cyc, as both knocking down cyc or expressing a dominant negative form of cyc exclusively during development lead to defasciculation phenotypes in adult clock neurons. Clk downregulation also leads to developmental effects on sLNv morphology. Our results reveal a non-circadian role for cyc, shedding light on the additional functions of circadian clock genes in the development of the nervous system.

Circadian Control of Lipid Metabolism

To ensure optimum health and performance, lipid metabolism needs to be temporally aligned to other body processes and to daily changes in the environment. Central and peripheral circadian clocks and environmental signals such as light provide internal and external time cues to the body. Importantly, each of the key organs involved in insect lipid metabolism contains a molecular clockwork which ticks with a varying degree of autonomy from the central clock in the brain. In this chapter, we review our current knowledge about peripheral clocks in the insect fat body, gut and oenocytes, and light- and circadian-driven diel patterns in lipid metabolites and lipid-related transcripts. In addition, we highlight selected neuroendocrine signaling pathways that are or may be involved in the temporal coordination and control of lipid metabolism.

Circadian clock neurons use activity-regulated gene expression for structural plasticity

Drosophila s-LNv circadian pacemaker neurons show dramatic structural plasticity, with their projections expanded at dawn and then retracted by dusk. This predictable plasticity makes s-LNvs ideal to study molecular mechanisms of plasticity. Although s-LNv plasticity is controlled by their molecular clock, changing s-LNv excitability also regulates plasticity. Here, we tested the idea that s-LNvs use activity-regulated genes to control plasticity. We found that inducing expression of either of the activity-regulated transcription factors Hr38 or Sr (orthologs of mammalian Nr4a1 and Egr1) is sufficient to rapidly expand s-LNv projections. Conversely, transiently knocking down expression of either Hr38 or sr blocks expansion of s-LNv projections at dawn. We show that Hr38 rapidly induces transcription of sif, which encodes a Rac1 GEF required for s-LNv plasticity rhythms. We conclude that the s-LNv molecular clock controls s-LNv excitability, which couples to an activity-regulated gene expression program to control s-LNv plasticity.

Tango-seq: overlaying transcriptomics on connectomics to identify neurons downstream of Drosophila clock neurons

Knowing how neural circuits change with neuronal plasticity and differ between individuals is important to fully understand behavior. Connectomes are typically assembled using electron microscopy, but this is low throughput and impractical for analyzing plasticity or mutations. Here, we modified the trans-Tango genetic circuit-tracing technique to identify neurons synaptically downstream of Drosophila s-LNv clock neurons, which show 24hr plasticity rhythms. s-LNv target neurons were labeled specifically in adult flies using a nuclear reporter gene, which facilitated their purification and then single cell sequencing. We call this Tango-seq, and it allows transcriptomic data - and thus cell identity - to be overlayed on top of anatomical data. We found that s-LNvs preferentially make synaptic connections with a subset of the CNMa+ DN1p clock neurons, and that these are likely plastic connections. We also identified synaptic connections between s-LNvs and mushroom body Kenyon cells. Tango-seq should be a useful addition to the connectomics toolkit.

Alternative splicing of clock transcript mediates the response of circadian clocks to temperature changes

Circadian clocks respond to temperature changes over the calendar year, allowing organisms to adjust their daily biological rhythms to optimize health and fitness. In Drosophila, seasonal adaptations and temperature compensation are regulated by temperature-sensitive alternative splicing (AS) of period (per) and timeless (tim) genes that encode key transcriptional repressors of clock gene expression. Although clock (clk) gene encodes the critical activator of clock gene expression, AS of its transcripts and its potential role in temperature regulation of clock function have not been explored. We therefore sought to investigate whether clk exhibits AS in response to temperature and the functional changes of the differentially spliced transcripts. We observed that clk transcripts indeed undergo temperature-sensitive AS. Specifically, cold temperature leads to the production of an alternative clk transcript, hereinafter termed clk-cold, which encodes a CLK isoform with an in-frame deletion of four amino acids proximal to the DNA binding domain. Notably, serine 13 (S13), which we found to be a CK1α-dependent phosphorylation site, is among the four amino acids deleted in CLK-cold protein. Using a combination of transgenic fly, tissue culture, and in vitro experiments, we demonstrated that upon phosphorylation at CLK(S13), CLK-DNA interaction is reduced, thus decreasing CLK occupancy at clock gene promoters. This is in agreement with our findings that CLK occupancy at clock genes and transcriptional output are elevated at cold temperature, which can be explained by the higher amounts of CLK-cold isoforms that lack S13 residue. This study provides new insights into the complex collaboration between AS and phospho-regulation in shaping temperature responses of the circadian clock.

The neuropeptide pigment-dispersing factor signals independently of Bruchpilot-labelled active zones in daily remodelled terminals of Drosophila clock neurons

The small ventrolateral neurons (sLNvs) are key components of the central clock in the Drosophila brain. They signal via the neuropeptide pigment-dispersing factor (PDF) to align the molecular clockwork of different central clock neurons and to modulate downstream circuits. The dorsal terminals of the sLNvs undergo daily morphological changes that affect presynaptic sites organised by the active zone protein Bruchpilot (BRP), a homolog of mammalian ELKS proteins. However, the role of these presynaptic sites for PDF release is ill-defined. Here, we combined expansion microscopy with labelling of active zones by endogenously tagged BRP to examine the spatial correlation between PDF-containing dense-core vesicles and BRP-labelled active zones. We found that the number of BRP-labelled puncta in the sLNv terminals was similar while their density differed between Zeitgeber time (ZT) 2 and 14. The relative distance between BRP- and PDF-labelled puncta was increased in the morning, around the reported time of PDF release. Spontaneous dense-core vesicle release profiles of sLNvs in a publicly available ssTEM dataset (FAFB) consistently lacked spatial correlation to BRP-organised active zones. RNAi-mediated downregulation of brp and other active zone proteins expressed by the sLNvs did not affect PDF-dependent locomotor rhythmicity. In contrast, down-regulation of genes encoding proteins of the canonical vesicle release machinery, the dense-core vesicle-related protein CADPS, as well as PDF impaired locomotor rhythmicity. Taken together, our study suggests that PDF release from the sLNvs is independent of BRP-organised active zones, while BRP may be redistributed to active zones in a time-dependent manner.

The Trissin/TrissinR signaling pathway in the circadian network regulates evening activity in Drosophila melanogaster under constant dark conditions

The circadian clock in Drosophila is governed by a neural network comprising approximately 150 neurons, known as clock neurons, which are intricately interconnected by various neurotransmitters. The neuropeptides that play functional roles in these clock neurons have been identified; however, the roles of some neuropeptides, such as Trissin, remain unclear. Trissin is expressed in lateral dorsal clock neurons (LNds), while its receptor, TrissinR, is expressed in dorsal neuron 1 (DN1) and LNds. In this study, we investigated the role of the Trissin/TrissinR signaling pathway within the circadian network in Drosophila melanogaster. Analysis involving our newly generated antibody against the Trissin precursor revealed that Trissin expression in the LNds cycles in a circadian manner. Behavioral analysis further demonstrated that flies with Trissin or TrissinR knockout or knockdown showed delayed evening activity offset under constant darkness conditions. Notably, this observed delay in evening activity offset in TrissinRNAi flies was restored via the additional knockdown of Ion transport peptide (ITP), indicating that the Trissin/TrissinR signaling pathway transmits information via ITP. Therefore, this pathway may be a key regulator of the timing of evening activity offset termination, orchestrating its effects in collaboration with the neuropeptide, ITP.

Drosophila noktochor regulates night sleep via a local mushroom body circuit

We show that a sleep-regulating, Ig-domain protein (NKT) is secreted from Drosophila mushroom body (MB) α'/β' neurons to act locally on other MB cell types. Pan-neuronal or broad MB expression of membrane-tethered NKT (tNkt) protein reduced sleep, like that of an NKT null mutant, suggesting blockade of a receptor mediating endogenous NKT action. In contrast, expression in neurons requiring NKT (the MB α'/β' cells), or non-MB sleep-regulating centers, did not reduce night sleep, indicating the presence of a local MB sleep-regulating circuit consisting of communicating neural subtypes. We suggest that the leucocyte-antigen-related like (Lar) transmembrane receptor may mediate NKT action. Knockdown or overexpression of Lar in the MB increased or decreased sleep, respectively, indicating the receptor promotes wakefulness. Surprisingly, selective expression of tNkt or knockdown of Lar in MB wake-promoting cells increased rather than decreased sleep, suggesting that NKT acts on wake- as well as sleep-promoting cell types to regulate sleep.

Autophagy as a new player in the regulation of clock neurons physiology of Drosophila melanogaster

Axonal terminals of the small ventral lateral neurons (sLNvs), the circadian clock neurons of Drosophila, show daily changes in their arborization complexity, with many branches in the morning and their shrinkage during the night. This complex phenomenon is precisely regulated by several mechanisms. In the present study we describe that one of them is autophagy, a self-degradative process, also involved in changes of cell membrane size and shape. Our results showed that autophagosome formation and processing in PDF-expressing neurons (both sLNv and lLNv) are rhythmic and they have different patterns in the cell bodies and terminals. These rhythmic changes in the autophagy activity seem to be important for neuronal plasticity. We found that autophagosome cargos are different during the day and night, and more proteins involved in membrane remodeling are present in autophagosomes in the morning. In addition, we described for the first time that Atg8-positive vesicles are also present outside the sLNv terminals, which suggests that secretory autophagy might be involved in regulating the clock signaling network. Our data indicate that rhythmic autophagy in clock neurons affect the pacemaker function, through remodeling of terminal membrane and secretion of specific proteins from sLNvs.

Timed receptor tyrosine kinase signaling couples the central and a peripheral circadian clock in Drosophila

Circadian clocks impose daily periodicities to behavior, physiology, and metabolism. This control is mediated by a central clock and by peripheral clocks, which are synchronized to provide the organism with a unified time through mechanisms that are not fully understood. Here, we characterized in Drosophila the cellular and molecular mechanisms involved in coupling the central clock and the peripheral clock located in the prothoracic gland (PG), which together control the circadian rhythm of emergence of adult flies. The time signal from central clock neurons is transmitted via small neuropeptide F (sNPF) to neurons that produce the neuropeptide Prothoracicotropic Hormone (PTTH), which is then translated into daily oscillations of Ca2+ concentration and PTTH levels. PTTH signaling is required at the end of metamorphosis and transmits time information to the PG through changes in the expression of the PTTH receptor tyrosine kinase (RTK), TORSO, and of ERK phosphorylation, a key component of PTTH transduction. In addition to PTTH, we demonstrate that signaling mediated by other RTKs contributes to the rhythmicity of emergence. Interestingly, the ligand to one of these receptors (Pvf2) plays an autocrine role in the PG, which may explain why both central brain and PG clocks are required for the circadian gating of emergence. Our findings show that the coupling between the central and the PG clock is unexpectedly complex and involves several RTKs that act in concert and could serve as a paradigm to understand how circadian clocks are coordinated.

Biogenesis of circular RNAs in vitro and in vivo from the Drosophila Nk2.1 / scarecrow gene

scarecrow ( scro ) encodes a fly homolog of mammalian Nkx2.1 that is vital for early fly development as well as for optic lobe development. Interestingly, scro was reported to produce a circular RNA (circRNA). In this study, we identified 12 different scro circRNAs, which are either mono- or multi-exonic forms. The most abundant forms are circE2 carrying the second exon only and bi-exonic circE3-E4. Levels of circE2 show an age-dependent increase in adult heads, supporting a general trend of high accumulation of circRNAs in aged fly brains. Aligning sequences of introns flanking exons uncovered two pairs of intronic complementary sequences (ICSs); one pair residing in introns 1 and 2 and the other in introns 2 and 4. The first pair was demonstrated to be essential for the circE2 production in cell-based assays; furthermore, deletion of the region including potential ICS components in the intron-2 reduced in vivo production of circE2 and circE3-E4 by 80%, indicating them to be essential for the biogenesis of these isoforms. Besides the ICS, the intron regions immediately abutting exons seemed to be responsible for a basal level of circRNA formation. Moreover, the replacement of scro -ICS with those derived from laccase2 was comparably effective in scro -circRNA production, buttressing the importance of the hairpin-loop structure formed by ICS for the biogenesis of circRNA. Lastly, overexpressed scro affected outcomes of both linear and circular RNAs from the endogenous scro locus, suggesting that Scro plays a direct or indirect role in regulating expression levels of either or both forms.

Neurobiology and Changing Ecosystems: Mechanisms Underlying Responses to Human-Generated Environmental Impacts

Human generated environmental change profoundly affects organisms that reside across diverse ecosystems. Although nervous systems evolved to flexibly sense, respond, and adapt to environmental change, it is unclear whether the rapid rate of environmental change outpaces the adaptive capacity of complex nervous systems. Here, we explore neural systems mediating responses to, or impacted by, changing environments, such as those induced by global heating, sensory pollution, and changing habitation zones. We focus on rising temperature and accelerated changes in environments that impact sensory experience as examples of perturbations that directly or indirectly impact neural function, respectively. We also explore a mechanism involved in cross-species interactions that arises from changing habitation zones. We demonstrate that anthropogenic influences on neurons, circuits, and behaviors are widespread across taxa and require further scientific investigation to understand principles underlying neural resilience to accelerating environmental change.SIGNIFICANCE STATEMENT Neural systems evolved over hundreds of millions of years to allow organisms to sense and respond to their environments - to be receptive and responsive, yet flexible. Recent rapid, human-generated environmental changes are testing the limits of the adaptive capacity of neural systems. This presents an opportunity and an urgency to understand how neurobiological processes, including molecular, cellular, and circuit-level mechanisms, are vulnerable or resilient to changing environmental conditions. We showcase examples that range from molecular to circuit to behavioral levels of analysis across several model species, framing a broad neuroscientific approach to explore topics of neural adaptation, plasticity, and resilience. We believe this emerging scientific area is of great societal and scientific importance and will provide a unique opportunity to reexamine our understanding of neural adaptation and the mechanisms underlying neural resilience.

The circadian clock gene bmal1 is necessary for co-ordinated circatidal rhythms in the marine isopod Eurydice pulchra (Leach)

Circadian clocks in terrestrial animals are encoded by molecular feedback loops involving the negative regulators PERIOD, TIMELESS or CRYPTOCHROME2 and positive transcription factors CLOCK and BMAL1/CYCLE. The molecular basis of circatidal (~12.4 hour) or other lunar-mediated cycles (~15 day, ~29 day), widely expressed in coastal organisms, is unknown. Disrupting circadian clockworks does not appear to affect lunar-based rhythms in several organisms that inhabit the shoreline suggesting a molecular independence of the two cycles. Nevertheless, pharmacological inhibition of casein kinase 1 (CK1) that targets PERIOD stability in mammals and flies, affects both circadian and circatidal phenotypes in Eurydice pulchra (Ep), the speckled sea-louse. Here we show that these drug inhibitors of CK1 also affect the phosphorylation of EpCLK and EpBMAL1 and disrupt EpCLK-BMAL1-mediated transcription in Drosophila S2 cells, revealing a potential link between these two positive circadian regulators and circatidal behaviour. We therefore performed dsRNAi knockdown of Epbmal1 as well as the major negative regulator in Eurydice, Epcry2 in animals taken from the wild. Epcry2 and Epbmal1 knockdown disrupted Eurydice's circadian phenotypes of chromatophore dispersion, tim mRNA cycling and the circadian modulation of circatidal swimming, as expected. However, circatidal behaviour was particularly sensitive to Epbmal1 knockdown with consistent effects on the power, amplitude and rhythmicity of the circatidal swimming cycle. Thus, three Eurydice negative circadian regulators, EpCRY2, in addition to EpPER and EpTIM (from a previous study), do not appear to be required for the expression of robust circatidal behaviour, in contrast to the positive regulator EpBMAL1. We suggest a neurogenetic model whereby the positive circadian regulators EpBMAL1-CLK are shared between circadian and circatidal mechanisms in Eurydice but circatidal rhythms require a novel, as yet unknown negative regulator.

Circadian clock disruption promotes the degeneration of dopaminergic neurons in male Drosophila

Sleep and circadian rhythm disruptions are frequent comorbidities of Parkinson's disease (PD), a disorder characterized by the progressive loss of dopaminergic (DA) neurons in the substantia nigra. However, the causal role of circadian clocks in the degenerative process remains uncertain. We demonstrated here that circadian clocks regulate the rhythmicity and magnitude of the vulnerability of DA neurons to oxidative stress in male Drosophila. Circadian pacemaker neurons are presynaptic to a subset of DA neurons and rhythmically modulate their susceptibility to degeneration. The arrhythmic period (per) gene null mutation exacerbates the age-dependent loss of DA neurons and, in combination with brief oxidative stress, causes premature animal death. These findings suggest that circadian clock disruption promotes dopaminergic neurodegeneration.

In vivo characterization of the maturation steps of a pigment dispersing factor neuropeptide precursor in the Drosophila circadian pacemaker neurons

Pigment dispersing factor (PDF) is a key signaling molecule coordinating the neuronal network associated with the circadian rhythms in Drosophila. The precursor (proPDF) of the mature PDF (mPDF) consists of 2 motifs, a larger PDF-associated peptide (PAP) and PDF. Through cleavage and amidation, the proPDF is predicted to produce cleaved-PAP (cPAP) and mPDF. To delve into the in vivo mechanisms underlying proPDF maturation, we generated various mutations that eliminate putative processing sites and then analyzed the effect of each mutation on the production of cPAP and mPDF by 4 different antibodies in both ectopic and endogenous conditions. We also assessed the knockdown effects of processing enzymes on the proPDF maturation. At the functional level, circadian phenotypes were measured for all mutants and knockdown lines. As results, we confirm the roles of key enzymes and their target residues: Amontillado (Amon) for the cleavage at the consensus dibasic KR site, Silver (Svr) for the removal of C-terminal basic residues from the intermediates, PAP-KR and PDF-GK, derived from proPDF, and PHM (peptidylglycine-α-hydroxylating monooxygenase) for the amidation of PDF. Our results suggest that the C-terminal amidation occurs independently of proPDF cleavage. Moreover, the PAP domain is important for the proPDF trafficking into the secretory vesicles and a close association between cPAP and mPDF following cleavage seems required for their stability within the vesicles. These studies highlight the biological significance of individual processing steps and the roles of the PAP for the stability and function of mPDF which is essential for the circadian clockworks.

The head mesodermal cell couples FMRFamide neuropeptide signaling with rhythmic muscle contraction in C. elegans

FMRFamides are evolutionarily conserved neuropeptides that play critical roles in behavior, energy balance, and reproduction. Here, we show that FMRFamide signaling from the nervous system is critical for the rhythmic activation of a single cell of previously unknown function, the head mesodermal cell (hmc) in C. elegans. Behavioral, calcium imaging, and genetic studies reveal that release of the FLP-22 neuropeptide from the AVL neuron in response to pacemaker signaling activates hmc every 50 s through an frpr-17 G protein-coupled receptor (GPCR) and a protein kinase A signaling cascade in hmc. hmc activation results in muscle contraction through coupling by gap junctions composed of UNC-9/Innexin. hmc activation is inhibited by the neuronal release of a second FMRFamide-like neuropeptide, FLP-9, which functions through its GPCR, frpr-21, in hmc. This study reveals a function for two opposing FMRFamide signaling pathways in controlling the rhythmic activation of a target cell through volume transmission.

Pigment-dispersing factor is present in circadian clock neurons of pea aphids and may mediate photoperiodic signalling to insulin-producing cells

The neuropeptide pigment-dispersing factor (PDF) plays a pivotal role in the circadian clock of most Ecdysozoa and is additionally involved in the timing of seasonal responses of several photoperiodic species. The pea aphid, Acyrthosiphon pisum, is a paradigmatic photoperiodic species with an annual life cycle tightly coupled to the seasonal changes in day length. Nevertheless, PDF could not be identified in A. pisum so far. In the present study, we identified a PDF-coding gene that has undergone significant changes in the otherwise highly conserved insect C-terminal amino acid sequence. A newly generated aphid-specific PDF antibody stained four neurons in each hemisphere of the aphid brain that co-express the clock protein Period and have projections to the pars lateralis that are highly plastic and change their appearance in a daily and seasonal manner, resembling those of the fruit fly PDF neurons. Most intriguingly, the PDF terminals overlap with dendrites of the insulin-like peptide (ILP) positive neurosecretory cells in the pars intercerebralis and with putative terminals of Cryptochrome (CRY) positive clock neurons. Since ILP has been previously shown to be crucial for seasonal adaptations and CRY might serve as a circadian photoreceptor vital for measuring day length, our results suggest that PDF plays a critical role in aphid seasonal timing.

Seasonal cues act through the circadian clock and pigment-dispersing factor to control EYES ABSENT and downstream physiological changes

Organisms adapt to seasonal changes in photoperiod and temperature to survive; however, the mechanisms by which these signals are integrated in the brain to alter seasonal biology are poorly understood. We previously reported that EYES ABSENT (EYA) shows higher levels in cold temperature or short photoperiod and promotes winter physiology in Drosophila. Nevertheless, how EYA senses seasonal cues is unclear. Pigment-dispersing factor (PDF) is a neuropeptide important for regulating circadian output rhythms. Interestingly, PDF has also been shown to regulate seasonality, suggesting that it may mediate the function of the circadian clock in modulating seasonal physiology. In this study, we investigated the role of EYA in mediating the function of PDF on seasonal biology. We observed that PDF abundance is lower on cold and short days as compared with warm and long days, contrary to what was previously observed for EYA. We observed that manipulating PDF signaling in eya+ fly brain neurons, where EYA and PDF receptor are co-expressed, modulates seasonal adaptations in daily activity rhythm and ovary development via EYA-dependent and EYA-independent mechanisms. At the molecular level, altering PDF signaling impacted EYA protein abundance. Specifically, we showed that protein kinase A (PKA), an effector of PDF signaling, phosphorylates EYA promoting its degradation, thus explaining the opposite responses of PDF and EYA abundance to changes in seasonal cues. In summary, our results support a model in which PDF signaling negatively modulates EYA levels to regulate seasonal physiology, linking the circadian clock to the modulation of seasonal adaptations.

Pigment-dispersing factor and CCHamide1 in the Drosophila circadian clock network

Animals possess a circadian central clock in the brain, where circadian behavioural rhythms are generated. In the fruit fly (Drosophila melanogaster), the central clock comprises a network of approximately 150 clock neurons, which is important for the maintenance of a coherent and robust rhythm. Several neuropeptides involved in the network have been identified, including Pigment-dispersing factor (PDF) and CCHamide1 (CCHa1) neuropeptides. PDF signals bidirectionally to CCHa1-positive clock neurons; thus, the clock neuron groups expressing PDF and CCHa1 interact reciprocally. However, the role of these interactions in molecular and behavioural rhythms remains elusive. In this study, we generated Pdf ⁰¹ and CCHa1^SK8 double mutants and examined their locomotor activity-related rhythms. The single mutants of Pdf ⁰¹ or CCHa1^SK8 displayed free-running rhythms under constant dark conditions, whereas approximately 98% of the double mutants were arrhythmic. In light-dark conditions, the evening activity of the double mutants was phase-advanced compared with that of the single mutants. In contrast, both the single and double mutants had diminished morning activity. These results suggest that the effects of the double mutation varied in behavioural parameters. The double and triple mutants of per ⁰¹, Pdf ⁰¹, and CCHa1^SK8 further revealed that PDF signalling plays a role in the suppression of activity during the daytime under a clock-less background. Our results provide insights into the interactions between PDF and CCHa1 signalling and their roles in activity rhythms.

A novel period mutation implicating nuclear export in temperature compensation of the Drosophila circadian clock

Circadian clocks are self-sustained molecular oscillators controlling daily changes of behavioral activity and physiology. For functional reliability and precision, the frequency of these molecular oscillations must be stable at different environmental temperatures, known as "temperature compensation." Despite being an intrinsic property of all circadian clocks, this phenomenon is not well understood at the molecular level. Here, we use behavioral and molecular approaches to characterize a novel mutation in the period (per) clock gene of Drosophila melanogaster, which alters a predicted nuclear export signal (NES) of the PER protein and affects temperature compensation. We show that this new per^I530A allele leads to progressively longer behavioral periods and clock oscillations with increasing temperature in both clock neurons and peripheral clock cells. While the mutant PER^I530A protein shows normal circadian fluctuations and post-translational modifications at cool temperatures, increasing temperatures lead to both severe amplitude dampening and hypophosphorylation of PER^I530A. We further show that PER^I530A displays reduced repressor activity at warmer temperatures, presumably because it cannot inactivate the transcription factor CLOCK (CLK), indicated by temperature-dependent altered CLK post-translational modification in per^I530A flies. With increasing temperatures, nuclear accumulation of PER^I530A within clock neurons is increased, suggesting that wild-type PER is exported out of the nucleus at warm temperatures. Downregulating the nuclear export factor CRM1 also leads to temperature-dependent changes of behavioral rhythms, suggesting that the PER NES and the nuclear export of clock proteins play an important role in temperature compensation of the Drosophila circadian clock.

Mating disrupts morning anticipation in Drosophila melanogaster females

After mating, the physiology of Drosophila females undergo several important changes, some of which are reflected in their rest-activity cycles. To explore the hypothesis that mating modifies the temporal organization of locomotor activity patterns, we recorded fly activity by a video tracking method. Monitoring rest-activity patterns under light/dark (LD) cycles indicated that mated females lose their ability to anticipate the night-day transition, in stark contrast to males and virgins. This postmating response is mediated by the activation of the sex peptide receptor (SPR) mainly on pickpocket (ppk) expressing neurons, since reducing expression of this receptor in these neurons restores the ability to anticipate the LD transition in mated females. Furthermore, we provide evidence of connectivity between ppk+ neurons and the pigment-dispersing factor (PDF)-positive ventral lateral neurons (sLNv), which play a central role in the temporal organization of daily activity. Since PDF has been associated to the generation of the morning activity peak, we hypothesized that the mating signal could modulate PDF levels. Indeed, we confirm that mated females have reduced PDF levels at the dorsal protocerebrum; moreover, SPR downregulation in ppk+ neurons mimics PDF levels observed in males. In sum, our results are consistent with a model whereby mating-triggered signals reach clock neurons in the fly central nervous system to modulate the temporal organization of circadian behavior according to the needs of the new status.

Glial control of sphingolipid levels sculpts diurnal remodeling in a circadian circuit

Structural plasticity in the brain often necessitates dramatic remodeling of neuronal processes, with attendant reorganization of the cytoskeleton and membranes. Although cytoskeletal restructuring has been studied extensively, how lipids might orchestrate structural plasticity remains unclear. We show that specific glial cells in Drosophila produce glucocerebrosidase (GBA) to locally catabolize sphingolipids. Sphingolipid accumulation drives lysosomal dysfunction, causing gba1b mutants to harbor protein aggregates that cycle across circadian time and are regulated by neural activity, the circadian clock, and sleep. Although the vast majority of membrane lipids are stable across the day, a specific subset that is highly enriched in sphingolipids cycles daily in a gba1b-dependent fashion. Remarkably, both sphingolipid biosynthesis and degradation are required for the diurnal remodeling of circadian clock neurites, which grow and shrink across the day. Thus, dynamic sphingolipid regulation by glia enables diurnal circuit remodeling and proper circadian behavior.

Death of a Protein: The Role of E3 Ubiquitin Ligases in Circadian Rhythms of Mice and Flies

Circadian clocks evolved to enable organisms to anticipate and prepare for periodic environmental changes driven by the day–night cycle. This internal timekeeping mechanism is built on autoregulatory transcription–translation feedback loops that control the rhythmic expression of core clock genes and their protein products. The levels of clock proteins rise and ebb throughout a 24-h period through their rhythmic synthesis and destruction. In the ubiquitin–proteasome system, the process of polyubiquitination, or the covalent attachment of a ubiquitin chain, marks a protein for degradation by the 26S proteasome. The process is regulated by E3 ubiquitin ligases, which recognize specific substrates for ubiquitination. In this review, we summarize the roles that known E3 ubiquitin ligases play in the circadian clocks of two popular model organisms: mice and fruit flies. We also discuss emerging evidence that implicates the N-degron pathway, an alternative proteolytic system, in the regulation of circadian rhythms. We conclude the review with our perspectives on the potential for the proteolytic and non-proteolytic functions of E3 ubiquitin ligases within the circadian clock system.

Patch-Clamping Fly Brain Neurons

The membrane potential of excitable cells, such as neurons and muscle cells, experiences a rich repertoire of dynamic changes mediated by an array of ligand- and voltage-gated ion channels. Central neurons, in particular, are fantastic computators of information, sensing, and integrating multiple subthreshold currents mediated by synaptic inputs and translating them into action potential patterns. Electrophysiology comprises a group of techniques that allow the direct measurement of electrical signals. There are many different electrophysiological approaches, but, because Drosophila neurons are small, the whole-cell patch-clamp technique is the only applicable method for recording electrical signals from individual central neurons. Here, we provide background on patch-clamp electrophysiology in Drosophila and introduce protocols for dissecting larval and adult brains, as well as for achieving whole-cell patch-clamp recordings of identified neuronal types. Patch clamping is a labor-intensive technique that requires a great deal of practice to become an expert; therefore, a steep learning curve should be anticipated. However, the instant gratification of neuronal spiking is an experience that we wish to share and disseminate, as many more Drosophila patch clampers are needed to study the electrical features of so many fly neuronal types unknown to date.

Morphology and synaptic connections of pigment-dispersing factor-immunoreactive neurons projecting to the lateral protocerebrum in the large black chafer, Holotrichia parallela

Pigment-dispersing factor (PDF) is a well-known output neuropeptide modulator of circadian pacemakers in insects. Here, we investigated PDF-immunoreactive (ir) neurons in the brain of the large black chafer Holotrichia parallela to search for circadian neural components, which are potentially involved in its circabidian rhythm. PDF-ir cells were exclusively detected near the accessory medulla (AME) as a cluster of ∼ 100 cells with almost homogeneous size. No other cells exhibited immunoreactivity. The PDF-ir cells send beaded fibers into the proximal half of the AME and ventral elongation in an anterior region between the medulla (ME) and lobula (LO). Neither the lamina, ME, LO, nor lobula plate receives PDF-ir fibers. Primary axons derived from the PDF-ir cells extend toward the contralateral hemisphere through the dorsolateral protocerebrum anterior to the calyx to connect the bilateral AME. The axons form varicose outgrowths exclusively in the lateral protocerebrum. Double labeling with antisynapsin revealed partial overlaps between PDF-ir varicosities and synapsin-ir puncta. Thus, it was assumed that the PDF-ir fibers form output synapses there. To verify this, we investigated the ultrastructure of the PDF-ir varicosities in the lateral protocerebrum by preembedding immunoelectron microscopy. The PDF-ir profiles contain small clear synaptic vesicles as well as both PDF-positive and PDF-negative dense-core vesicles, and the profiles form output synapses upon unknown profiles and receive synapses from other PDF-ir profiles. PDF neurons near the AME are considered to be prominent circadian pacemakers in the cockroach and flies. Their possible function in the circabidian rhythm was discussed based on these anatomical insights.

---Connectomic analysis of the Drosophila lateral neuron clock cells reveals the synaptic basis of functional pacemaker classes

The circadian clock orchestrates daily changes in physiology and behavior to ensure internal temporal order and optimal timing across the day. In animals, a central brain clock coordinates circadian rhythms throughout the body and is characterized by a remarkable robustness that depends on synaptic connections between constituent neurons. The clock neuron network of Drosophila, which shares network motifs with clock networks in the mammalian brain yet is built of many fewer neurons, offers a powerful model for understanding the network properties of circadian timekeeping. Here, we report an assessment of synaptic connectivity within a clock network, focusing on the critical lateral neuron (LN) clock neuron classes within the Janelia hemibrain dataset. Our results reveal that previously identified anatomical and functional subclasses of LNs represent distinct connectomic types. Moreover, we identify a small number of non-clock cell subtypes representing highly synaptically coupled nodes within the clock neuron network. This suggests that neurons lacking molecular timekeeping likely play integral roles within the circadian timekeeping network. To our knowledge, this represents the first comprehensive connectomic analysis of a circadian neuronal network.

Most organisms on Earth possess an internal timekeeping system which ensures that bodily processes such as sleep, wakefulness or digestion take place at the right time. These precise daily rhythms are kept in check by a master clock in the brain. There, thousands of neurons – some of which carrying an internal ‘molecular clock’ – connect to each other through structures known as synapses. Exactly how the resulting network is organised to support circadian timekeeping remains unclear.

To explore this question, Shafer, Gutierrez et al. focused on fruit flies, as recent efforts have systematically mapped every neuron and synaptic connection in the brain of this model organism. Analysing available data from the hemibrain connectome project at Janelia revealed that that the neurons with the most important timekeeping roles were in fact forming the fewest synapses within the network. In addition, neurons without internal molecular clocks mediated strong synaptic connections between those that did, suggesting that ‘clockless’ cells still play an integral role in circadian timekeeping.

With this research, Shafer, Gutierrez et al. provide unexpected insights into the organisation of the master body clock. Better understanding the networks that underpin circadian rhythms will help to grasp how and why these are disrupted in obesity, depression and Alzheimer’s disease.

Hsp40 overexpression in pacemaker neurons delays circadian dysfunction in a Drosophila model of Huntington's disease

Circadian disturbances are early features of neurodegenerative diseases, including Huntington's disease (HD). Emerging evidence suggests that circadian decline feeds into neurodegenerative symptoms, exacerbating them. Therefore, we asked whether known neurotoxic modifiers can suppress circadian dysfunction. We performed a screen of neurotoxicity-modifier genes to suppress circadian behavioural arrhythmicity in a Drosophila circadian HD model. The molecular chaperones Hsp40 and HSP70 emerged as significant suppressors in the circadian context, with Hsp40 being the more potent mitigator. Upon Hsp40 overexpression in the Drosophila circadian ventrolateral neurons (LNv), the behavioural rescue was associated with neuronal rescue of loss of circadian proteins from small LNv soma. Specifically, there was a restoration of the molecular clock protein Period and its oscillations in young flies and a long-lasting rescue of the output neuropeptide Pigment dispersing factor. Significantly, there was a reduction in the expanded Huntingtin inclusion load, concomitant with the appearance of a spot-like Huntingtin form. Thus, we provide evidence implicating the neuroprotective chaperone Hsp40 in circadian rehabilitation. The involvement of molecular chaperones in circadian maintenance has broader therapeutic implications for neurodegenerative diseases.

This article has an associated First Person interview with the first author of the paper.

Summary: This study shows, for the first time, a neuroprotective role of chaperone Hsp40 in suppressing circadian dysfunction associated with Huntington's disease in a Drosophila model.

An auxin-inducible, GAL4-compatible, gene expression system for Drosophila

The ability to control transgene expression, both spatially and temporally, is essential for studying model organisms. In Drosophila, spatial control is primarily provided by the GAL4/UAS system, whilst temporal control relies on a temperature-sensitive GAL80 (which inhibits GAL4) and drug-inducible systems. However, these are not ideal. Shifting temperature can impact on many physiological and behavioural traits, and the current drug-inducible systems are either leaky, toxic, incompatible with existing GAL4-driver lines, or do not generate effective levels of expression. Here, we describe the auxin-inducible gene expression system (AGES). AGES relies on the auxin-dependent degradation of a ubiquitously expressed GAL80, and therefore, is compatible with existing GAL4-driver lines. Water-soluble auxin is added to fly food at a low, non-lethal, concentration, which induces expression comparable to uninhibited GAL4 expression. The system works in both larvae and adults, providing a stringent, non-lethal, cost-effective, and convenient method for temporally controlling GAL4 activity in Drosophila.

Chloride oscillation in pacemaker neurons regulates circadian rhythms through a chloride-sensing WNK kinase signaling cascade

Central pacemaker neurons regulate circadian rhythms and undergo diurnal variation in electrical activity in mammals and flies.1,2 Circadian variation in the intracellular chloride concentration of mammalian pacemaker neurons has been proposed to influence the response to GABAergic neurotransmission through GABAA receptor chloride channels.3 However, results have been contradictory,4-9 and a recent study demonstrated circadian variation in pacemaker neuron chloride without an effect on GABA response.10 Therefore, whether and how intracellular chloride regulates circadian rhythms remains controversial. Here, we demonstrate a signaling role for intracellular chloride in the Drosophila small ventral lateral (sLNv) pacemaker neurons. In control flies, intracellular chloride increases in sLNvs over the course of the morning. Chloride transport through sodium-potassium-2-chloride (NKCC) and potassium-chloride (KCC) cotransporters is a major determinant of intracellular chloride concentrations.11Drosophila melanogaster with loss-of-function mutations in the NKCC encoded by Ncc69 have abnormally low intracellular chloride 6 h after lights on, loss of morning anticipation, and a prolonged circadian period. Loss of kcc, which is expected to increase intracellular chloride, suppresses the long-period phenotype of Ncc69 mutant flies. Activation of a chloride-inhibited kinase cascade, consisting of WNK (with no lysine [K]) kinase and its downstream substrate, Fray, is necessary and sufficient to prolong period length. Fray activation of an inwardly rectifying potassium channel, Irk1, is also required for the long-period phenotype. These results indicate that the NKCC-dependent rise in intracellular chloride in Drosophila sLNv pacemakers restrains WNK-Fray signaling and overactivation of an inwardly rectifying potassium channel to maintain normal circadian period length.

UBR4/POE facilitates secretory trafficking to maintain circadian clock synchrony

Ubiquitin ligases control the degradation of core clock proteins to govern the speed and resetting properties of the circadian pacemaker. However, few studies have addressed their potential to regulate other cellular events within clock neurons beyond clock protein turnover. Here, we report that the ubiquitin ligase, UBR4/POE, strengthens the central pacemaker by facilitating neuropeptide trafficking in clock neurons and promoting network synchrony. Ubr4-deficient mice are resistant to jetlag, whereas poe knockdown flies are prone to arrhythmicity, behaviors reflective of the reduced axonal trafficking of circadian neuropeptides. At the cellular level, Ubr4 ablation impairs the export of secreted proteins from the Golgi apparatus by reducing the expression of Coronin 7, which is required for budding of Golgi-derived transport vesicles. In summary, UBR4/POE fulfills a conserved and unexpected role in the vesicular trafficking of neuropeptides, a function that has important implications for circadian clock synchrony and circuit-level signal processing.

Although ubiquitin ligases are known to control clock protein degradation, their other roles in clock neurons are unclear. Here the authors report that UBR4 promotes export of neuropeptides from the Golgi for axonal trafficking, which is important for circadian clock synchrony in mice and flies.

The lateral posterior clock neurons (LPN) of Drosophila melanogaster express three neuropeptides and have multiple connections within the circadian clock network and beyond

Drosophila's lateral posterior neurons (LPNs) belong to a small group of circadian clock neurons that is so far not characterized in detail. Thanks to a new highly specific split-Gal4 line, here we describe LPNs' morphology in fine detail, their synaptic connections, daily bimodal expression of neuropeptides, and propose a putative role of this cluster in controlling daily activity and sleep patterns. We found that the three LPNs are heterogeneous. Two of the neurons with similar morphology arborize in the superior medial and lateral protocerebrum and most likely promote sleep. One unique, possibly wakefulness-promoting, neuron with wider arborizations extends from the superior lateral protocerebrum toward the anterior optic tubercle. Both LPN types exhibit manifold connections with the other circadian clock neurons, especially with those that control the flies' morning and evening activity (M- and E-neurons, respectively). In addition, they form synaptic connections with neurons of the mushroom bodies, the fan-shaped body, and with many additional still unidentified neurons. We found that both LPN types rhythmically express three neuropeptides, Allostatin A, Allostatin C, and Diuretic Hormone 31 with maxima in the morning and the evening. The three LPN neuropeptides may, furthermore, signal to the insect hormonal center in the pars intercerebralis and contribute to rhythmic modulation of metabolism, feeding, and reproduction. We discuss our findings in the light of anatomical details gained by the recently published hemibrain of a single female fly on the electron microscopic level and of previous functional studies concerning the LPN. This article is protected by copyright. All rights reserved.

Different transcriptional levels of Corazonin, Elevenin, and PDF according to the body color of the two-spotted cricket, Gryllus bimaculatus

Body color in insects changes according to the living environment and physiological stresses possibly involved in endocrine factors. To date, 3 predominant bioactive peptides, Corazonin, Elevenin, and pigment-dispersing factor (PDF), have been illuminated to be involved in the body color in insects and crustaceans. Here, we examined the possibilities that these 3 factors would contribute to body color changes via melanization in the two-spotted cricket, Gryllus bimaculatus, whose body color changes according to population density drastically. Quantitative analyses revealed the higher transcriptional levels of Corazonin and Elevenin in the crowded-conditioned crickets, whereas the transcriptional level of PDF was higher in the isolated-conditioned crickets. However, the body color was not changed by knockdown of Corazonin, Elevenin, and PDF by RNA interference. The present data indicated that coloration mechanisms in G. bimaculatus is differently controlled from the previous observation in Locusta migratoria, a closely related orthopteran species.

Perception of Daily Time: Insights from the Fruit Flies

We create mental maps of the space that surrounds us; our brains also compute time—in particular, the time of day. Visual, thermal, social, and other cues tune the clock-like timekeeper. Consequently, the internal clock synchronizes with the external day-night cycles. In fact, daylength itself varies, causing the change of seasons and forcing our brain clock to accommodate layers of plasticity. However, the core of the clock, i.e., its molecular underpinnings, are highly resistant to perturbations, while the way animals adapt to the daily and annual time shows tremendous biological diversity. How can this be achieved? In this review, we will focus on 75 pairs of clock neurons in the Drosophila brain to understand how a small neural network perceives and responds to the time of the day, and the time of the year.

The E3 ubiquitin ligase adaptor Tango10 links the core circadian clock to neuropeptide and behavioral rhythms

Circadian transcriptional timekeepers in pacemaker neurons drive profound daily rhythms in sleep and wake. Here we reveal a molecular pathway that links core transcriptional oscillators to neuronal and behavioral rhythms. Using two independent genetic screens, we identified mutants of Transport and Golgi organization 10 (Tango10) with poor behavioral rhythmicity. Tango10 expression in pacemaker neurons expressing the neuropeptide PIGMENT-DISPERSING FACTOR (PDF) is required for robust rhythms. Loss of Tango10 results in elevated PDF accumulation in nerve terminals even in mutants lacking a functional core clock. TANGO10 protein itself is rhythmically expressed in PDF terminals. Mass spectrometry of TANGO10 complexes reveals interactions with the E3 ubiquitin ligase CULLIN 3 (CUL3). CUL3 depletion phenocopies Tango10 mutant effects on PDF even in the absence of the core clock gene timeless Patch clamp electrophysiology in Tango10 mutant neurons demonstrates elevated spontaneous firing potentially due to reduced voltage-gated Shaker-like potassium currents. We propose that Tango10/Cul3 transduces molecular oscillations from the core clock to neuropeptide release important for behavioral rhythms.

Hugin + neurons provide a link between sleep homeostat and circadian clock neurons

Sleep is controlled by homeostatic mechanisms, which drive sleep after wakefulness, and a circadian clock, which confers the 24-h rhythm of sleep. These processes interact with each other to control the timing of sleep in a daily cycle as well as following sleep deprivation. However, the mechanisms by which they interact are poorly understood. We show here that hugin ⁺ neurons, previously identified as neurons that function downstream of the clock to regulate rhythms of locomotor activity, are also targets of the sleep homeostat. Sleep deprivation decreases activity of hugin ⁺ neurons, likely to suppress circadian-driven activity during recovery sleep, and ablation of hugin ⁺ neurons promotes sleep increases generated by activation of the homeostatic sleep locus, the dorsal fan-shaped body (dFB). Also, mutations in peptides produced by the hugin ⁺ locus increase recovery sleep following deprivation. Transsynaptic mapping reveals that hugin ⁺ neurons feed back onto central clock neurons, which also show decreased activity upon sleep loss, in a Hugin peptide-dependent fashion. We propose that hugin ⁺ neurons integrate circadian and sleep signals to modulate circadian circuitry and regulate the timing of sleep.

The microtubule associated protein tau suppresses the axonal distribution of PDF neuropeptide and mitochondria in circadian clock neurons

Disrupted circadian rhythms is a prominent feature of multiple neurodegenerative diseases. Yet mechanisms linking Tau to rhythmic behavior remain unclear. Here we find that expression of a phosphomimetic human Tau mutant (TauE14) in Drosophila circadian pacemaker neurons disrupts free-running rhythmicity. While cell number and oscillations of the core clock protein PERIOD are unaffected in the small LNv (sLNv) neurons important for free running rhythms, we observe a near complete loss of the major LNv neuropeptide pigment dispersing factor (PDF) in the dorsal axonal projections of the sLNvs. This was accompanied by a ~ 50% reduction in the area of the dorsal terminals and a modest decrease in cell body PDF levels. Expression of wild-type Tau also reduced axonal PDF levels but to a lesser extent than TauE14. TauE14 also induces a complete loss of mitochondria from these sLNv projections. However, mitochondria were increased in sLNv cell bodies in TauE14 flies. These results suggest that TauE14 disrupts axonal transport of neuropeptides and mitochondria in circadian pacemaker neurons, providing a mechanism by which Tau can disrupt circadian behavior prior to cell loss.

Gap junction protein Innexin2 modulates the period of free-running rhythms in Drosophila melanogaster

A neuronal circuit of ∼150 neurons modulates rhythmic activity-rest behavior of Drosophila melanogaster. While it is known that coherent ∼24-hr rhythms in locomotion are brought about when 7 distinct neuronal clusters function as a network due to chemical communication amongst them, there are no reports of communication via electrical synapses made up of gap junctions. Here, we report that gap junction proteins, Innexins play crucial roles in determining the intrinsic period of activity-rest rhythms in flies. We show the presence of Innexin2 in the ventral lateral neurons, wherein RNAi-based knockdown of its expression slows down the speed of activity-rest rhythm along with alterations in the oscillation of a core-clock protein PERIOD and the output molecule pigment dispersing factor. Specifically disrupting the channel-forming ability of Innexin2 causes period lengthening, suggesting that Innexin2 may function as hemichannels or gap junctions in the clock circuit.

Clock-controlled arylalkylamine N-acetyltransferase (aaNAT) regulates circadian rhythms of locomotor activity in the American cockroach, Periplaneta americana, via melatonin/MT2-like receptor

Melatonin (MEL) orchestrates daily and seasonal rhythms (eg, locomotion, sleep/wake cycles, and migration among other rhythms) in diverse organisms. We investigated the effects of pharmacological doses (0.03-1 mM) of exogenous MEL intake in the cockroach, Periplaneta americana, on locomotor activity. As per os MEL concentration increased, cockroach locomotor rhythm in light-dark (LD) cycles became more synchronized. The ratio of night activity to 24-h activity increased and the acrophase (peak) slightly advanced. MEL application also influenced total activity bouts in the free-running rhythm. Since MEL slightly influenced τ in the free-running rhythms, it is not a central element of the circadian pacemaker but must influence mutual coupling of multi-oscillatory system components. Arylalkylamine N-acetyltransferase (aaNAT) regulates enzymatic production of MEL. aaNAT activities vary in circadian rhythms, and the immunoreactive aaNAT (aaNAT-ir) is colocalized with the key clock proteins cycle (CYC)-ir and pigment-dispersing factor (PDF)-ir These are elements of the central pacemaker and its output pathway as well as other circadian landmarks such as the anterior and posterior optic commissures (AOC and POC, respectively). It also partially shares immunohistochemical reactivity with PER-ir and DBT-ir neurons. We analyzed the role of Pamericana aaNAT1 (PaaaNAT1) (AB106562.1) by injecting dsRNA^aaNAT1 . qPCR showed a decrease in accumulations of mRNAs encoding PaaaNAT1. The injections led to arrhythmicity in LD cycles and the arrhythmicity persisted in constant dark (DD). Continuous administration of MEL resynchronized the rhythm after arrhythmicity was induced by dsRNA^aaNAT1 injection, suggesting that PaaaNAT is the key regulator of the circadian system in the cockroach via MEL production. PaaaNAT1 contains putative E-box regions which may explain its tight circadian control. The receptor that mediates MEL function is most likely similar to the mammalian MT2, because injecting the competitive MT2 antagonist luzindole blocked MEL function, and MEL injection after luzindole treatment restored MT function. Human MT2-ir was localized in the circadian neurons in the cockroach brain and subesophageal ganglion. We infer that MEL and its synthesizing enzyme, aaNAT, constitute at least one circadian output pathway of locomotor activity either as a distinct route or in association with PDF system.

Expression of the foraging gene in adult Drosophila melanogaster

The foraging gene in Drosophila melanogaster, which encodes a cGMP-dependent protein kinase, is a highly conserved, complex gene with multiple pleiotropic behavioral and physiological functions in both the larval and adult fly. Adult foraging expression is less well characterized than in the larva. We characterized foraging expression in the brain, gastric system, and reproductive systems using a T2A-Gal4 gene-trap allele. In the brain, foraging expression appears to be restricted to multiple sub-types of glia. This glial-specific cellular localization of foraging was supported by single-cell transcriptomic atlases of the adult brain. foraging is extensively expressed in most cell types in the gastric and reproductive systems. We then mapped multiple cis-regulatory elements responsible for parts of the observed expression patterns by a nested cloned promoter-Gal4 analysis. The mapped cis-regulatory elements were consistently modular when comparing the larval and adult expression patterns. These new data using the T2A-Gal4 gene-trap and cloned foraging promoter fusion GAL4's are discussed with respect to previous work using an anti-FOR antibody, which we show here to be non-specific. Future studies of foraging's function will consider roles for glial subtypes and peripheral tissues (gastric and reproductive systems) in foraging's pleiotropic behavioral and physiological effects.

CLOCKWORK ORANGE promotes CLOCK-CYCLE activation via the putative Drosophila ortholog of CLOCK INTERACTING PROTEIN CIRCADIAN

The Drosophila circadian clock is driven by a transcriptional feedback loop in which CLOCK-CYCLE (CLK-CYC) binds E-boxes to transcribe genes encoding the PERIOD-TIMELESS (PER-TIM) repressor, which releases CLK-CYC from E-boxes to inhibit transcription. CLOCKWORK ORANGE (CWO) reinforces PER-TIM repression by binding E-boxes to maintain PER-TIM bound CLK-CYC off DNA, but also promotes CLK-CYC transcription through an unknown mechanism. To determine how CWO activates CLK-CYC transcription, we identified CWO target genes that are upregulated in the absence of CWO repression, conserved in mammals, and preferentially expressed in brain pacemaker neurons. Among the genes identified was a putative ortholog of mouse Clock Interacting Protein Circadian (Cipc), which represses CLOCK-BMAL1 transcription. Reducing or eliminating Drosophila Cipc expression shortens period, while overexpressing Cipc lengthens period, which is consistent with previous work showing that Drosophila Cipc represses CLK-CYC transcription in S2 cells. Cipc represses CLK-CYC transcription in vivo, but not uniformly, as per is strongly repressed, tim less so, and vri hardly at all. Long period rhythms in cwo mutant flies are largely rescued when Cipc expression is reduced or eliminated, indicating that increased Cipc expression mediates the period lengthening of cwo mutants. Consistent with this behavioral rescue, eliminating Cipc rescues the decreased CLK-CYC transcription in cwo mutant flies, where per is strongly rescued, tim is moderately rescued, and vri shows little rescue. These results suggest a mechanism for CWO-dependent CLK-CYC activation: CWO inhibition of CIPC repression promotes CLK-CYC transcription. This mechanism may be conserved since cwo and Cipc perform analogous roles in the mammalian circadian clock.

Sleep drive reconfigures wake-promoting clock circuitry to regulate adaptive behavior

Circadian rhythms help animals synchronize motivated behaviors to match environmental demands. Recent evidence indicates that clock neurons influence the timing of behavior by differentially altering the activity of a distributed network of downstream neurons. Downstream circuits can be remodeled by Hebbian plasticity, synaptic scaling, and, under some circumstances, activity-dependent addition of cell surface receptors; the role of this receptor respecification phenomena is not well studied. We demonstrate that high sleep pressure quickly reprograms the wake-promoting large ventrolateral clock neurons to express the pigment dispersing factor receptor (PDFR). The addition of this signaling input into the circuit is associated with increased waking and early mating success. The respecification of PDFR in both young and adult large ventrolateral neurons requires 2 dopamine (DA) receptors and activation of the transcriptional regulator nejire (cAMP response element-binding protein [CREBBP]). These data identify receptor respecification as an important mechanism to sculpt circuit function to match sleep levels with demand.