Drosophila pacemaker neurons require g protein signaling and GABAergic inputs to generate twenty-four hour behavioral rhythms

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.

Light Sampling via Throttled Visual Phototransduction Robustly Synchronizes the Drosophila Circadian Clock

The daily changes of light and dark exemplify a prominent cue for the synchronization of circadian clocks with the environment. The match between external and internal time is crucial for the fitness of organisms, and desynchronization has been linked to numerous physical and mental health problems. Organisms therefore developed complex and not fully understood mechanisms to synchronize their circadian clock to light. In mammals and in Drosophila, both the visual system and non-image-forming photoreceptors contribute to circadian clock resetting. In Drosophila, light-dependent degradation of the clock protein TIMELESS by the blue light photoreceptor Cryptochrome is considered the main mechanism for clock synchronization, although the visual system also contributes. To better understand the visual system contribution, we generated a genetic variant exhibiting extremely slow phototransduction kinetics, yet normal sensitivity. In this variant, the visual system is able to contribute its full share to circadian clock entrainment, both with regard to behavioral and molecular light synchronization. This function depends on an alternative phospholipase C-β enzyme, encoded by PLC21C, presumably playing a dedicated role in clock resetting. We show that this pathway requires the ubiquitin ligase CULLIN-3, possibly mediating CRY-independent degradation of TIMELESS during light:dark cycles. Our results suggest that the PLC21C-mediated contribution to circadian clock entrainment operates on a drastically slower timescale compared with fast, norpA-dependent visual phototransduction. Our findings are therefore consistent with the general idea that the visual system samples light over prolonged periods of time (h) in order to reliably synchronize their internal clocks with the external time.

Disrupted Glutamate Signaling in Drosophila Generates Locomotor Rhythms in Constant Light

We have used the Cambridge Protein Trap resource (CPTI) to screen for flies whose locomotor rhythms are rhythmic in constant light (LL) as a means of identifying circadian photoreception genes. From the screen of ∼150 CPTI lines, we obtained seven hits, two of which targeted the glutamate pathway, Got1 (Glutamate oxaloacetate transaminase 1) and Gs2 (Glutamine synthetase 2). We focused on these by employing available mutants and observed that variants of these genes also showed high levels of LL rhythmicity compared with controls. It was also clear that the genetic background was important with a strong interaction observed with the common and naturally occurring timeless (tim) polymorphisms, ls-tim and s-tim. The less circadian photosensitive ls-tim allele generated high levels of LL rhythmicity in combination with Got1 or Gs2, even though ls-tim and s-tim alleles do not, by themselves, generate the LL phenotype. The use of dsRNAi for both genes as well as for Gad (Glutamic acid decarboxylase) and the metabotropic glutamate receptor DmGluRA driven by clock gene promoters also revealed high levels of LL rhythmicity compared to controls. It is clear that the glutamate pathway is heavily implicated in circadian photoreception. TIM levels in Got1 and Gs2 mutants cycled and were more abundant than in controls under LL. Got1 but not Gs2 mutants showed diminished phase shifts to 10 min light pulses. Neurogenetic dissection of the LL rhythmic phenotype using the gal4/gal80 UAS bipartite system suggested that the more dorsal CRY-negative clock neurons, DNs and LNds were responsible for the LL phenotype. Immunocytochemistry using the CPTI YFP tagged insertions for the two genes revealed that the DN1s but not the DN2 and DN3s expressed Got1 and Gs2, but expression was also observed in the lateral neurons, the LNds and s-LNvs. Expression of both genes was also found in neuroglia. However, downregulation of glial Gs2 and Got1 using repo-gal4 did not generate high levels of LL rhythmicity, so it is unlikely that this phenotype is mediated by glial expression. Our results suggest a model whereby the DN1s and possibly CRY-negative LNds use glutamate signaling to supress the pacemaker s-LNvs in LL.

Genome-wide discovery of the daily transcriptome, DNA regulatory elements and transcription factor occupancy in the monarch butterfly brain

The Eastern North American monarch butterfly, Danaus plexippus, is famous for its spectacular seasonal long-distance migration. In recent years, it has also emerged as a novel system to study how animal circadian clocks keep track of time and regulate ecologically relevant daily rhythmic activities and seasonal behavioral outputs. However, unlike in Drosophila and the mouse, little work has been undertaken in the monarch to identify rhythmic genes at the genome-wide level and elucidate the regulation of their diurnal expression. Here, we used RNA-sequencing and Assay for Transposase-Accessible Chromatin (ATAC)-sequencing to profile the diurnal transcriptome, open chromatin regions, and transcription factor (TF) footprints in the brain of wild-type monarchs and of monarchs with impaired clock function, including Cryptochrome 2 (Cry2), Clock (Clk), and Cycle-like loss-of-function mutants. We identified 217 rhythmically expressed genes in the monarch brain; many of them were involved in the regulation of biological processes key to brain function, such as glucose metabolism and neurotransmission. Surprisingly, we found no significant time-of-day and genotype-dependent changes in chromatin accessibility in the brain. Instead, we found the existence of a temporal regulation of TF occupancy within open chromatin regions in the vicinity of rhythmic genes in the brains of wild-type monarchs, which is disrupted in clock deficient mutants. Together, this work identifies for the first time the rhythmic genes and modes of regulation by which diurnal transcription rhythms are regulated in the monarch brain. It also illustrates the power of ATAC-sequencing to profile genome-wide regulatory elements and TF binding in a non-model organism for which TF-specific antibodies are not yet available.

With a rich biology that includes a clock-regulated migratory behavior and a circadian clock possessing mammalian clock orthologues, the monarch butterfly is an unconventional system with broad appeal to study circadian and seasonal rhythms. While clockwork mechanisms and rhythmic behavioral outputs have been studied in this species, the rhythmic genes that regulate rhythmic daily and seasonal activities remain largely unknown. Likewise, the mechanisms regulating rhythmic gene expression have not been explored in the monarch. Here, we applied genome-wide sequencing approaches to identify genes with rhythmic diurnal expression in the monarch brain, revealing the coordination of key pathways for brain function. We also identified the monarch brain open chromatin regions and provide evidence that regulation of rhythmic gene expression does not occur through temporal regulation of chromatin opening but rather by the time-of-day dependent binding of transcription factors in cis-regulatory elements. Together, our data extend our knowledge of the molecular rhythmic pathways, which may prove important in understanding the mechanisms underlying the daily and seasonal biology of the migratory monarch butterflies.

A Symphony of Signals: Intercellular and Intracellular Signaling Mechanisms Underlying Circadian Timekeeping in Mice and Flies

The central pacemakers of circadian timekeeping systems are highly robust yet adaptable, providing the temporal coordination of rhythms in behavior and physiological processes in accordance with the demands imposed by environmental cycles. These features of the central pacemaker are achieved by a multi-oscillator network in which individual cellular oscillators are tightly coupled to the environmental day-night cycle, and to one another via intercellular coupling. In this review, we will summarize the roles of various neurotransmitters and neuropeptides in the regulation of circadian entrainment and synchrony within the mammalian and Drosophila central pacemakers. We will also describe the diverse functions of protein kinases in the relay of input signals to the core oscillator or the direct regulation of the molecular clock machinery.

A genome-wide enhancer/suppressor screen for Dube3a interacting genes in Drosophila melanogaster

The genetics underlying autism spectrum disorder (ASD) are complex. Approximately 3–5% of ASD cases arise from maternally inherited duplications of 15q11.2-q13.1, termed Duplication 15q syndrome (Dup15q). 15q11.2-q13.1 includes the gene UBE3A which is believed to underlie ASD observed in Dup15q syndrome. UBE3A is an E3 ubiquitin ligase that targets proteins for degradation and trafficking, so finding UBE3A substrates and interacting partners is critical to understanding Dup15q ASD. In this study, we take an unbiased genetics approach to identify genes that genetically interact with Dube3a, the Drosophila melanogaster homolog of UBE3A. We conducted an enhancer/suppressor screen using a rough eye phenotype produced by Dube3a overexpression with GMR-GAL4. Using the DrosDel deficiency kit, we identified 3 out of 346 deficiency lines that enhanced rough eyes when crossed to two separate Dube3a overexpression lines, and subsequently identified IA2, GABA-B-R3, and lola as single genes responsible for rough eye enhancement. Using the FlyLight GAL4 lines to express uas-Dube3a + uas-GFP in the endogenous lola pattern, we observed an increase in the GFP signal compared to uas-GFP alone, suggesting a transcriptional co-activation effect of Dube3a on the lola promoter region. These findings extend the role of Dube3a/UBE3A as a transcriptional co-activator, and reveal new Dube3a interacting genes.

Drosophila CaV2 channels harboring human migraine mutations cause synapse hyperexcitability that can be suppressed by inhibition of a Ca2+ store release pathway

Gain-of-function mutations in the human CaV2.1 gene CACNA1A cause familial hemiplegic migraine type 1 (FHM1). To characterize cellular problems potentially triggered by CaV2.1 gains of function, we engineered mutations encoding FHM1 amino-acid substitutions S218L (SL) and R192Q (RQ) into transgenes of Drosophila melanogaster CaV2/cacophony. We expressed the transgenes pan-neuronally. Phenotypes were mild for RQ-expressing animals. By contrast, single mutant SL- and complex allele RQ,SL-expressing animals showed overt phenotypes, including sharply decreased viability. By electrophysiology, SL- and RQ,SL-expressing neuromuscular junctions (NMJs) exhibited enhanced evoked discharges, supernumerary discharges, and an increase in the amplitudes and frequencies of spontaneous events. Some spontaneous events were gigantic (10-40 mV), multi-quantal events. Gigantic spontaneous events were eliminated by application of TTX-or by lowered or chelated Ca2+-suggesting that gigantic events were elicited by spontaneous nerve firing. A follow-up genetic approach revealed that some neuronal hyperexcitability phenotypes were reversed after knockdown or mutation of Drosophila homologs of phospholipase Cβ (PLCβ), IP3 receptor, or ryanodine receptor (RyR)-all factors known to mediate Ca2+ release from intracellular stores. Pharmacological inhibitors of intracellular Ca2+ store release produced similar effects. Interestingly, however, the decreased viability phenotype was not reversed by genetic impairment of intracellular Ca2+ release factors. On a cellular level, our data suggest inhibition of signaling that triggers intracellular Ca2+ release could counteract hyperexcitability induced by gains of CaV2.1 function.

Non-canonical Phototransduction Mediates Synchronization of the Drosophila melanogaster Circadian Clock and Retinal Light Responses

The daily light-dark cycles represent a key signal for synchronizing circadian clocks. Both insects and mammals possess dedicated "circadian" photoreceptors but also utilize the visual system for clock resetting. In Drosophila, circadian clock resetting is achieved by the blue-light photoreceptor cryptochrome (CRY), which is expressed within subsets of the brain clock neurons. In addition, rhodopsin-expressing photoreceptor cells contribute to light synchronization. Light resets the molecular clock by CRY-dependent degradation of the clock protein Timeless (TIM), although in specific subsets of key circadian pacemaker neurons, including the small ventral lateral neurons (s-LNvs), TIM and Period (PER) oscillations can be synchronized by light independent of CRY and canonical visual Rhodopsin phototransduction. Here, we show that at least three of the seven Drosophila rhodopsins can utilize an alternative transduction mechanism involving the same α-subunit of the heterotrimeric G protein operating in canonical visual phototransduction (Gq). Surprisingly, in mutants lacking the canonical phospholipase C-β (PLC-β) encoded by the no receptor potential A (norpA) gene, we uncovered a novel transduction pathway using a different PLC-β encoded by the Plc21C gene. This novel pathway is important for behavioral clock resetting to semi-natural light-dark cycles and mediates light-dependent molecular synchronization within the s-LNv clock neurons. The same pathway appears to be responsible for norpA-independent light responses in the compound eye. We show that Rhodopsin 5 (Rh5) and Rh6, present in the R8 subset of retinal photoreceptor cells, drive both the long-term circadian and rapid light responses in the eye.

cAMPr: A single-wavelength fluorescent sensor for cyclic AMP

Genetically encoded fluorescent sensors enable cell-specific measurements of ions and small molecules in real time. Cyclic adenosine monophosphate (cAMP) is one of the most important signaling molecules in virtually all cell types and organisms. We describe cAMPr, a new single-wavelength cAMP sensor. We developed cAMPr in bacteria and embryonic stem cells and validated the sensor in mammalian neurons in vitro and in Drosophila circadian pacemaker neurons in intact brains. Comparison with other single-wavelength cAMP sensors showed that cAMPr improved the quantitative detection of cAMP abundance. In addition, cAMPr is compatible with both single-photon and two-photon imaging. This enabled us to use cAMPr together with the red fluorescent Ca²⁺ sensor RCaMP1h to simultaneously monitor Ca²⁺ and cAMP in Drosophila brains. Thus, cAMPr is a new and versatile genetically encoded cAMP sensor.

Circadian Rhythm Neuropeptides in Drosophila: Signals for Normal Circadian Function and Circadian Neurodegenerative Disease

Circadian rhythm is a ubiquitous phenomenon in many organisms ranging from prokaryotes to eukaryotes. During more than four decades, the intrinsic and exogenous regulations of circadian rhythm have been studied. This review summarizes the core endogenous oscillation in Drosophila and then focuses on the neuropeptides, neurotransmitters and hormones that mediate its outputs and integration in Drosophila and the links between several of these (pigment dispersing factor (PDF) and insulin-like peptides) and neurodegenerative disease. These signaling molecules convey important network connectivity and signaling information for normal circadian function, but PDF and insulin-like peptides can also convey signals that lead to apoptosis, enhanced neurodegeneration and cognitive decline in flies carrying circadian mutations or in a senescent state.

SlgA, encoded by the homolog of the human schizophrenia-associated gene PRODH, acts in clock neurons to regulate Drosophila aggression

Mutations in the proline dehydrogenase gene PRODH are linked to behavioral alterations in schizophrenia and as part of DiGeorge and velo-cardio-facial syndromes, but the role of PRODH in their etiology remains unclear. Here, we establish a Drosophila model to study the role of PRODH in behavioral disorders. We determine the distribution of the Drosophila PRODH homolog slgA in the brain and show that knockdown and overexpression of human PRODH and slgA in the lateral neurons ventral (LNv) lead to altered aggressive behavior. SlgA acts in an isoform-specific manner and is regulated by casein kinase II (CkII). Our data suggest that these effects are, at least partially, due to effects on mitochondrial function. We thus show that precise regulation of proline metabolism is essential to drive normal behavior and we identify Drosophila aggression as a model behavior relevant for the study of the mechanisms that are impaired in neuropsychiatric disorders.

Editors' choice: A Drosophila model to study the role of PRODH, a schizophrenia-associated gene, in behavioral disorders.

Functional PDF Signaling in the Drosophila Circadian Neural Circuit Is Gated by Ral A-Dependent Modulation

The neuropeptide PDF promotes the normal sequencing of circadian behavioral rhythms in Drosophila, but its signaling mechanisms are not well understood. We report daily rhythmicity in responsiveness to PDF in critical pacemakers called small LNvs. There is a daily change in potency, as great as 10-fold higher, around dawn. The rhythm persists in constant darkness and does not require endogenous ligand (PDF) signaling or rhythmic receptor gene transcription. Furthermore, rhythmic responsiveness reflects the properties of the pacemaker cell type, not the receptor. Dopamine responsiveness also cycles, in phase with that of PDF, in the same pacemakers, but does not cycle in large LNv. The activity of RalA GTPase in s-LNv regulates PDF responsiveness and behavioral locomotor rhythms. Additionally, cell-autonomous PDF signaling reversed the circadian behavioral effects of lowered RalA activity. Thus, RalA activity confers high PDF responsiveness, providing a daily gate around the dawn hours to promote functional PDF signaling.

Communication between circadian clusters: The key to a plastic network

Drosophila melanogaster is a model organism that has been instrumental in understanding the circadian clock at different levels. A range of studies on the anatomical and neurochemical properties of clock neurons in the fly led to a model of interacting neural circuits that control circadian behavior. Here we focus on recent research on the dynamics of the multiple communication pathways between clock neurons, and, particularly, on how the circadian timekeeping system responds to changes in environmental conditions. It is increasingly clear that the fly clock employs multiple signalling cues, such as neuropeptides, fast neurotransmitters, and other signalling molecules, in the dynamic interplay between neuronal clusters. These neuronal groups seem to interact in a plastic fashion, e.g., rearranging their hierarchy in response to changing environmental conditions. A picture is emerging supporting that these dynamic mechanisms are in place to provide an optimal balance between flexibility and an extraordinary accuracy.

A single-cross, RNA interference-based genetic tool for examining the long-term maintenance of homeostatic plasticity

Homeostatic synaptic plasticity (HSP) helps neurons and synapses maintain physiologically appropriate levels of output. The fruit fly Drosophila melanogaster larval neuromuscular junction (NMJ) is a valuable model for studying HSP. Here we introduce a genetic tool that allows fruit fly researchers to examine the lifelong maintenance of HSP with a single cross. The tool is a fruit fly stock that combines the GAL4/UAS expression system with RNA interference (RNAi)-based knock down of a glutamate receptor subunit gene. With this stock, we uncover important new information about the maintenance of HSP. We address an open question about the role that presynaptic Ca_V2-type Ca²⁺ channels play in NMJ homeostasis. Published experiments have demonstrated that hypomorphic missense mutations in the Ca_V2 α1a subunit gene cacophony (cac) can impair homeostatic plasticity at the NMJ. Here we report that reducing cac expression levels by RNAi is not sufficient to impair homeostatic plasticity. The presence of wild-type channels appears to support HSP—even when total Ca_V2 function is severely reduced. We also conduct an RNAi- and electrophysiology-based screen to identify new factors required for sustained homeostatic signaling throughout development. We uncover novel roles in HSP for Drosophila homologs of Cysteine string protein (CSP) and Phospholipase Cβ (Plc21C). We characterize those roles through follow-up genetic tests. We discuss how CSP, Plc21C, and associated factors could modulate presynaptic Ca_V2 function, presynaptic Ca²⁺ handling, or other signaling processes crucial for sustained homeostatic regulation of NMJ function throughout development. Our findings expand the scope of signaling pathways and processes that contribute to the durable strength of the NMJ.

A neuron-glia interaction involving GABA transaminase contributes to sleep loss in sleepless mutants

Sleep is an essential process and yet mechanisms underlying it are not well understood. Loss of the Drosophila quiver/sleepless (qvr/sss) gene increases neuronal excitability and diminishes daily sleep, providing an excellent model for exploring the underpinnings of sleep regulation. Here, we used a proteomic approach to identify proteins altered in sss brains. We report that loss of sleepless post-transcriptionally elevates the CG7433 protein, a mitochondrial γ-aminobutyric acid transaminase (GABAT), and reduces GABA in fly brains. Loss of GABAT increases daily sleep and improves sleep consolidation, indicating that GABAT promotes wakefulness. Importantly, disruption of the GABAT gene completely suppresses the sleep phenotype of sss mutants, demonstrating that GABAT is required for loss of sleep in sss mutants. While SSS acts in distinct populations of neurons, GABAT acts in glia to reduce sleep in sss flies. Our results identify a novel mechanism of interaction between neurons and glia that is important for the regulation of sleep.

A single pair of neurons links sleep to memory consolidation in Drosophila melanogaster

Sleep promotes memory consolidation in humans and many other species, but the physiological and anatomical relationships between sleep and memory remain unclear. Here, we show the dorsal paired medial (DPM) neurons, which are required for memory consolidation in Drosophila, are sleep-promoting inhibitory neurons. DPMs increase sleep via release of GABA onto wake-promoting mushroom body (MB) α'/β' neurons. Functional imaging demonstrates that DPM activation evokes robust increases in chloride in MB neurons, but is unable to cause detectable increases in calcium or cAMP. Downregulation of α'/β' GABAA and GABABR3 receptors results in sleep loss, suggesting these receptors are the sleep-relevant targets of DPM-mediated inhibition. Regulation of sleep by neurons necessary for consolidation suggests that these brain processes may be functionally interrelated via their shared anatomy. These findings have important implications for the mechanistic relationship between sleep and memory consolidation, arguing for a significant role of inhibitory neurotransmission in regulating these processes.

Patch-clamp electrophysiology in Drosophila circadian pacemaker neurons

Circadian clocks modulate the action potential firing frequency of pacemaker neurons. This daily variation in membrane excitability has been described in multiple species: from mollusks to fruit flies and mammals. Here, we provide an overview of the Drosophila pacemaker neural network, how circadian clocks drive neuronal activity within this network and we will present electrophysiological methods that we have applied to directly measure neuronal activity and reveal signal transduction pathways.

Activity-dependent regulation of astrocyte GAT levels during synaptogenesis

Astrocytic uptake of GABA through GABA transporters (GATs) is an important mechanism regulating excitatory/inhibitory balance in the nervous system; however, mechanisms by which astrocytes regulate GAT levels are undefined. We found that at mid-pupal stages the Drosophila melanogaster CNS neuropil was devoid of astrocyte membranes and synapses. Astrocyte membranes subsequently infiltrated the neuropil coordinately with synaptogenesis, and astrocyte ablation reduced synapse numbers by half, indicating that Drosophila astrocytes are pro-synaptogenic. Shortly after synapses formed in earnest, GAT was upregulated in astrocytes. Ablation or silencing of GABAergic neurons or disruption of metabotropic GABA receptor 1 and 2 (GABA(B)R1/2) signaling in astrocytes led to a decrease in astrocytic GAT. Notably, developmental depletion of astrocytic GABA(B)R1/2 signaling suppressed mechanosensory-induced seizure activity in mutants with hyperexcitable neurons. These data reveal that astrocytes actively modulate GAT expression via metabotropic GABA receptor signaling and highlight the importance of precise regulation of astrocytic GAT in modulation of seizure activity.

Differentially timed extracellular signals synchronize pacemaker neuron clocks

Circadian pacemaker neurons in Drosophila are regulated by two synchronizing signals that are released at opposite times of day, generating a rhythm in intracellular cyclic AMP.

Synchronized neuronal activity is vital for complex processes like behavior. Circadian pacemaker neurons offer an unusual opportunity to study synchrony as their molecular clocks oscillate in phase over an extended timeframe (24 h). To identify where, when, and how synchronizing signals are perceived, we first studied the minimal clock neural circuit in Drosophila larvae, manipulating either the four master pacemaker neurons (LN_vs) or two dorsal clock neurons (DN₁s). Unexpectedly, we found that the PDF Receptor (PdfR) is required in both LN_vs and DN₁s to maintain synchronized LN_v clocks. We also found that glutamate is a second synchronizing signal that is released from DN₁s and perceived in LN_vs via the metabotropic glutamate receptor (mGluRA). Because simultaneously reducing Pdfr and mGluRA expression in LN_vs severely dampened Timeless clock protein oscillations, we conclude that the master pacemaker LN_vs require extracellular signals to function normally. These two synchronizing signals are released at opposite times of day and drive cAMP oscillations in LN_vs. Finally we found that PdfR and mGluRA also help synchronize Timeless oscillations in adult s-LN_vs. We propose that differentially timed signals that drive cAMP oscillations and synchronize pacemaker neurons in circadian neural circuits will be conserved across species.

Circadian molecular clocks are essential for daily cycles in animal behavior and we have a good understanding of how these clocks work in individual pacemaker neurons. However, the accuracy of these individual clocks is meaningless unless they are synchronized with one another. In this study we show that synchronizing the principal pacemaker LN_v neurons in Drosophila larvae require two extracellular signals that are received at opposite times of day: namely, the neuropeptide PDF released from LN_vs themselves at dawn and glutamate released from dorsal clock neurons at dusk. LN_vs perceive both PDF and glutamate via G-protein coupled receptors that increase or decrease intracellular cAMP, respectively. The alternating phases of PDF and glutamate release generate oscillations in intracellular cyclic AMP. In addition to maintaining synchrony between LN_vs, this rhythm is also required for molecular clock oscillations in individual larval LN_vs. We show that disruption of PDF and glutamate signaling also reduces synchrony in adult LN_vs. This impairs the oscillations of clock proteins and flies have delayed onset of sleep. Our data highlight the importance of intercellular signaling in ensuring synchrony between clock neurons within the circadian network. Our findings help extend the conservation of clock properties between Drosophila and mammals beyond clock genes to include clock circuitry.

Suprachiasmatic nucleus function and circadian entrainment are modulated by G protein-coupled inwardly rectifying (GIRK) channels

G protein signalling within the central circadian oscillator, the suprachiasmatic nucleus (SCN), is essential for conveying time-of-day information. We sought to determine whether G protein-coupled inwardly rectifying potassium channels (GIRKs) modulate SCN physiology and circadian behaviour. We show that GIRK current and GIRK2 protein expression are greater during the day. Pharmacological inhibition of GIRKs and genetic loss of GIRK2 depolarized the day-time resting membrane potential of SCN neurons compared to controls. Behaviourally, GIRK2 knockout (KO) mice failed to shorten free running period in response to wheel access in constant darkness and entrained more rapidly to a 6 h advance of a 12 h:12 h light-dark (LD) cycle than wild-type (WT) littermate controls. We next examined whether these effects were due to disrupted signalling of neuropeptide Y (NPY), which is known to mediate non-photic phase shifts, attenuate photic phase shifts and activate GIRKs. Indeed, GIRK2 KO SCN slices had significantly fewer silent cells in response to NPY, likely contributing to the absence of NPY-induced phase advances of PER2::LUC rhythms in organotypic SCN cultures from GIRK2 KO mice. Finally, GIRK channel activation is sufficient to cause a non-photic-like phase advance of PER2::LUC rhythms on a Per2(Luc+/-) background. These results suggest that rhythmic regulation of GIRK2 protein and channel function in the SCN contributes to day-time resting membrane potential, providing a mechanism for the fine tuning responses to non-photic and photic stimuli. Further investigation could provide insight into disorders with circadian disruption comorbidities such as epilepsy and addiction, in which GIRK channels have been implicated.

GABA(B) receptors play an essential role in maintaining sleep during the second half of the night in Drosophila melanogaster

GABAergic signalling is important for normal sleep in humans and flies. Here we advance the current understanding of GABAergic modulation of daily sleep patterns by focusing on the role of slow metabotropic GABAB receptors in the fruit fly Drosophila melanogaster. We asked whether GABAB-R2 receptors are regulatory elements in sleep regulation in addition to the already identified fast ionotropic Rdl GABAA receptors. By immunocytochemical and reporter-based techniques we show that the pigment dispersing factor (PDF)-positive ventrolateral clock neurons (LNv) express GABAB-R2 receptors. Downregulation of GABAB-R2 receptors in the large PDF neurons (l-LNv) by RNAi reduced sleep maintenance in the second half of the night, whereas sleep latency at the beginning of the night that was previously shown to depend on ionotropic Rdl GABAA receptors remained unaltered. Our results confirm the role of the l-LNv neurons as an important part of the sleep circuit in D. melanogaster and also identify the GABAB-R2 receptors as the thus far missing component in GABA-signalling that is essential for sleep maintenance. Despite the significant effects on sleep, we did not observe any changes in circadian behaviour in flies with downregulated GABAB-R2 receptors, indicating that the regulation of sleep maintenance via l-LNv neurons is independent of their function in the circadian clock circuit.

Regulation of circadian locomotor rhythm by neuropeptide Y-like system in Drosophila melanogaster

Circadian rhythms in behaviour and physiology exist widely in animals, plants, fungi and cyanobacteria. Although much work has been carried out to characterize the endogenous clock circuit, the output signals coupling the circadian pacemaker to behaviour and physiology remain elusive. Here, we show that neuropeptide F (NPF), a homologue of mammalian neuropeptide Y, and its G protein-coupled receptor NPFR1 regulate the locomotor rhythm in Drosophila melanogaster. Flies with loss of function in NPF or NPFR1 were unable to ramp up their activity before lights off under light : dark (LD) cycles, and oscillations in npf/NPF and npfr1/NPFR1 were found to correlate temporally with the locomotor rhythm. Furthermore, NPF is expressed in clock neurones including dorsolateral neurones (LNd s) and ventrolateral neurones (LNv s), whereas NPFR1 is expressed in dorsal neurone 1 (DN1) and LNd s. These results show that NPF signalling is involved in the circadian locomotor rhythm in LD cycles.

GW182 controls Drosophila circadian behavior and PDF-receptor signaling

The neuropeptide PDF is crucial for Drosophila circadian behavior: it keeps circadian neurons synchronized. Here, we identify GW182 as a key regulator of PDF signaling. Indeed, GW182 downregulation results in phenotypes similar to those of Pdf and Pdf-receptor (Pdfr) mutants. gw182 genetically interacts with Pdfr and cAMP signaling, which is essential for PDFR function. GW182 mediates miRNA-dependent gene silencing through its interaction with AGO1. Consistently, GW182's AGO1 interaction domain is required for GW182's circadian function. Moreover, our results indicate that GW182 modulates PDFR signaling by silencing the expression of the cAMP phosphodiesterase DUNCE. Importantly, this repression is under photic control, and GW182 activity level--which is limiting in circadian neurons--influences the responses of the circadian neural network to light. We propose that GW182's gene silencing activity functions as a rheostat for PDFR signaling and thus profoundly impacts the circadian neural network and its response to environmental inputs.

Intracellular signaling in neurons: unraveling specificity, compensatory mechanisms and essential gene function

Understanding how unique signaling outputs are generated in neurons using a limited set of intracellular signaling mechanisms has been a challenge. A combination of genetics and cell imaging, with tools developed to measure signaling outputs, has shown that the restricted presence of a signaling attenuator visibly alters the axonal range of the output and can be correlated with different behavioral outputs. Another question of interest is regarding the extent of genetic plasticity possible in the context of a single behavioral change. Recent molecular and genetic studies support the presence of parallel pathways that can compensate for the primary defect both at the level of physiology and behavior.

Balance of activity between LN(v)s and glutamatergic dorsal clock neurons promotes robust circadian rhythms in Drosophila

Circadian rhythms offer an excellent opportunity to dissect the neural circuits underlying innate behavior because the genes and neurons involved are relatively well understood. We first sought to understand how Drosophila clock neurons interact in the simple circuit that generates circadian rhythms in larval light avoidance. We used genetics to manipulate two groups of clock neurons, increasing or reducing excitability, stopping their molecular clocks, and blocking neurotransmitter release and reception. Our results revealed that lateral neurons (LN(v)s) promote and dorsal clock neurons (DN(1)s) inhibit light avoidance, these neurons probably signal at different times of day, and both signals are required for rhythmic behavior. We found that similar principles apply in the more complex adult circadian circuit that generates locomotor rhythms. Thus, the changing balance in activity between clock neurons with opposing behavioral effects generates robust circadian behavior and probably helps organisms transition between discrete behavioral states, such as sleep and wakefulness.

Immunocytochemical localization of amines and GABA in the optic lobe of the butterfly, Papilio xuthus

Butterflies have sophisticated color vision. While the spectral organization of the compound eye has been well characterized in the Japanese yellow swallowtail butterfly, Papilio xuthus, neural mechanisms underlying its color vision are largely unexplored. Towards a better understanding of signal processing in the visual system of P. xuthus, we used immunocytochemical techniques to analyze the distribution of transmitter candidates, namely, histamine, serotonin, tyramine and γ-aminobutyric acid (GABA). Photoreceptor terminals in the lamina and medulla exhibited histamine immunoreactivity as demonstrated in other insects. The anti-histamine antiserum also labeled a few large medulla neurons. Medulla intrinsic neurons and centrifugal neurons projecting to the lamina showed serotonin immunoreactivity. Tyramine immunostaining was detected in a subset of large monopolar cells (LMCs) in the lamina, transmedullary neurons projecting to the lobula plate, and cell bodies surrounding the first optic chiasma. An anti-GABA antiserum labeled a subset of LMCs and populations of columnar and tangential neurons surrounding the medulla. Each of the four antisera also labeled a few centrifugal neurons that innervate the lobula complex from the central brain, suggesting that they have neuromodulatory roles. A distinctive feature we found in this study is the possibility that tyramine and GABA act as transmitters in LMCs of P. xuthus, which has not been reported in any other insects so far.

Speed control: cogs and gears that drive the circadian clock

In most organisms, an intrinsic circadian (~24-h) timekeeping system drives rhythms of physiology and behavior. Within cells that contain a circadian clock, specific transcriptional activators and repressors reciprocally regulate each other to generate a basic molecular oscillator. A mismatch of the period generated by this oscillator with the external environment creates circadian disruption, which can have adverse effects on neural function. Although several clock genes have been extensively characterized, a fundamental question remains: how do these genes work together to generate a ~24-h period? Period-altering mutations in clock genes can affect any of multiple regulated steps in the molecular oscillator. In this review, we examine the regulatory mechanisms that contribute to setting the pace of the circadian oscillator.

The circadian neuropeptide PDF signals preferentially through a specific adenylate cyclase isoform AC3 in M pacemakers of Drosophila

To synchronize a network of pacemakers in the Drosophila brain, a neuropeptide receptor specifically associates with adenylate cyclase 3 to create a “circadian signalosome.”

The neuropeptide Pigment Dispersing Factor (PDF) is essential for normal circadian function in Drosophila. It synchronizes the phases of M pacemakers, while in E pacemakers it decelerates their cycling and supports their amplitude. The PDF receptor (PDF-R) is present in both M and subsets of E cells. Activation of PDF-R stimulates cAMP increases in vitro and in M cells in vivo. The present study asks: What is the identity of downstream signaling components that are associated with PDF receptor in specific circadian pacemaker neurons? Using live imaging of intact fly brains and transgenic RNAi, we show that adenylate cyclase AC3 underlies PDF signaling in M cells. Genetic disruptions of AC3 specifically disrupt PDF responses: they do not affect other Gs-coupled GPCR signaling in M cells, they can be rescued, and they do not represent developmental alterations. Knockdown of the Drosophila AKAP-like scaffolding protein Nervy also reduces PDF responses. Flies with AC3 alterations show behavioral syndromes consistent with known roles of M pacemakers as mediated by PDF. Surprisingly, disruption of AC3 does not alter PDF responses in E cells—the PDF-R(+) LNd. Within M pacemakers, PDF-R couples preferentially to a single AC, but PDF-R association with a different AC(s) is needed to explain PDF signaling in the E pacemakers. Thus critical pathways of circadian synchronization are mediated by highly specific second messenger components. These findings support a hypothesis that PDF signaling components within target cells are sequestered into “circadian signalosomes,” whose compositions differ between E and M pacemaker cell types.

In the fruit fly Drosophila melanogaster, the neuropeptide Pigment Dispersing Factor (PDF) supports circadian function by synchronizing two types of pacemaker cells, M cells and E cells. The PDF receptor (PDF-R) is a G protein coupled receptor (GPCR) whose activation stimulates adenylate cyclase (AC), thereby elevating levels of the second messenger cAMP in many different pacemakers including M cells. Drosophila contains at least 12 genes that encode potential ACs. In this study, we identify the AC downstream of the PDF receptor specifically in M cells and show that PDF signals preferentially through AC3. However, other GPCRs in the very same cells do not rely on AC3. A different scaffolding protein also influences PDF responses in M cells, suggesting that signaling components are spatially grouped to allow for coupling of specific receptors with downstream components. Remarkably, in E pacemakers, AC3 disruptions have no effect. These findings suggest that distinct PDF circadian signals exist in M versus in E pacemakers, and more generally, we propose a mechanism to differentiate signaling pathways that use common components.

Distribution of metabotropic receptors of serotonin, dopamine, GABA, glutamate, and short neuropeptide F in the central complex of Drosophila

The central complex is a prominent set of midline neuropils in the insect brain, known to be a higher locomotor control center that integrates visual inputs and modulates motor outputs. It is composed of four major neuropil structures, the ellipsoid body (EB), fan-shaped body (FB), noduli (NO), and protocerebral bridge (PB). In Drosophila different types of central complex neurons have been shown to express multiple neuropeptides and neurotransmitters; however, the distribution of corresponding receptors is not known. Here, we have mapped metabotropic, G-protein-coupled receptors (GPCRs) of several neurotransmitters to neurons of the central complex. By combining immunocytochemistry with GAL4 driven green fluorescent protein, we examined the distribution patterns of six different GPCRs: two serotonin receptor subtypes (5-HT(1B) and 5-HT(7)), a dopamine receptor (DopR), the metabotropic GABA(B) receptor (GABA(B)R), the metabotropic glutamate receptor (DmGluR(A)) and a short neuropeptide F receptor (sNPFR1). Five of the six GPCRs were mapped to different neurons in the EB (sNPFR1 was not seen). Different layers of the FB express DopR, GABA(B)R, DmGluR(A,) and sNPFR1, whereas only GABA(B)R and DmGluR(A) were localized to the PB. Finally, strong expression of DopR and DmGluR(A) was detected in the NO. In most cases the distribution patterns of the GPCRs matched the expression of markers for their respective ligands. In some nonmatching regions it is likely that other types of dopamine and serotonin receptors or ionotropic GABA and glutamate receptors are expressed. Our data suggest that chemical signaling and signal modulation are diverse and highly complex in the different compartments and circuits of the Drosophila central complex. The information provided here, on receptor distribution, will be very useful for future analysis of functional circuits in the central complex, based on targeted interference with receptor expression.

Reciprocal cholinergic and GABAergic modulation of the small ventrolateral pacemaker neurons of Drosophila's circadian clock neuron network

The relatively simple clock neuron network of Drosophila is a valuable model system for the neuronal basis of circadian timekeeping. Unfortunately, many key neuronal classes of this network are inaccessible to electrophysiological analysis. We have therefore adopted the use of genetically encoded sensors to address the physiology of the fly's circadian clock network. Using genetically encoded Ca(2+) and cAMP sensors, we have investigated the physiological responses of two specific classes of clock neuron, the large and small ventrolateral neurons (l- and s-LN(v)s), to two neurotransmitters implicated in their modulation: acetylcholine (ACh) and γ-aminobutyric acid (GABA). Live imaging of l-LN(v) cAMP and Ca(2+) dynamics in response to cholinergic agonist and GABA application were well aligned with published electrophysiological data, indicating that our sensors were capable of faithfully reporting acute physiological responses to these transmitters within single adult clock neuron soma. We extended these live imaging methods to s-LN(v)s, critical neuronal pacemakers whose physiological properties in the adult brain are largely unknown. Our s-LN(v) experiments revealed the predicted excitatory responses to bath-applied cholinergic agonists and the predicted inhibitory effects of GABA and established that the antagonism of ACh and GABA extends to their effects on cAMP signaling. These data support recently published but physiologically untested models of s-LN(v) modulation and lead to the prediction that cholinergic and GABAergic inputs to s-LN(v)s will have opposing effects on the phase and/or period of the molecular clock within these critical pacemaker neurons.

Molecular and genetic analysis of the Drosophila model of fragile X syndrome

The Drosophila genome contains most genes known to be involved in heritable disease. The extraordinary genetic malleability of Drosophila, coupled to sophisticated imaging, electrophysiology, and behavioral paradigms, has paved the way for insightful mechanistic studies on the causes of developmental and neurological disease as well as many possible interventions. Here, we focus on one of the most advanced examples of Drosophila genetic disease modeling, the Drosophila model of Fragile X Syndrome, which for the past decade has provided key advances into the molecular, cellular, and behavioral defects underlying this devastating disorder. We discuss the multitude of RNAs and proteins that interact with the disease-causing FMR1 gene product, whose function is conserved from Drosophila to human. In turn, we consider FMR1 mechanistic relationships in non-neuronal tissues (germ cells and embryos), peripheral motor and sensory circuits, and central brain circuits involved in circadian clock activity and learning/memory.

G(o) activation is required for both appetitive and aversive memory acquisition in Drosophila

Heterotrimeric G(o) is an abundant brain protein required for negatively reinforced short-term associative olfactory memory in Drosophila. G(o) is the only known substrate of the S1 subunit of pertussis toxin (PTX) in fly, and acute expression of PTX within the mushroom body neurons (MB) induces a reversible deficit in associative olfactory memory. We demonstrate here that the induction of PTX within the α/β and γ lobe MB neurons leads to impaired memory acquisition without affecting memory stability. The induction of PTX within these MB neurons also leads to a significant defect in an optimized positively reinforced short-term memory paradigm; however, this PTX-induced learning deficit is noticeably less severe than found with the negatively reinforced paradigm. Both negatively and positively reinforced memory phenotypes are rescued by the constitutive expression of G(o)α transgenes bearing the Cys(351)Ile mutation. Since this mutation renders the G(o) molecule insensitive to PTX, the results isolate the effect of PTX on both forms of olfactory associative learning to the inhibition of the G(o) activation.

A comparative review of short and long neuropeptide F signaling in invertebrates: Any similarities to vertebrate neuropeptide Y signaling?

Neuropeptides referred to as neuropeptide F (NPF) and short neuropeptide F (sNPF) have been identified in numerous invertebrate species. Sequence information has expanded tremendously due to recent genome sequencing and EST projects. Analysis of sequences of the peptides and prepropeptides strongly suggest that NPFs and sNPFs are not closely related. However, the NPFs are likely to be ancestrally related to the vertebrate family of neuropeptide Y (NPY) peptides. Peptide diversification may have been accomplished by different mechanisms in NPFs and sNPFs; in the former by gene duplications followed by diversification and in the sNPFs by internal duplications resulting in paracopies of peptides. We discuss the distribution and functions of NPFs and their receptors in several model invertebrates. Signaling with sNPF, however, has been investigated mainly in insects, especially in Drosophila. Both in invertebrates and in mammals NPF/NPY play roles in feeding, metabolism, reproduction and stress responses. Several other NPF functions have been studied in Drosophila that may be shared with mammals. In Drosophila sNPFs are widely distributed in numerous neurons of the CNS and some gut endocrines and their functions may be truly pleiotropic. Peptide distribution and experiments suggest roles of sNPF in feeding and growth, stress responses, modulation of locomotion and olfactory inputs, hormone release, as well as learning and memory. Available data indicate that NPF and sNPF signaling systems are distinct and not likely to play redundant roles.