Mechanisms of clock output in the Drosophila circadian pacemaker system

Thermosensation and Temperature Preference: From Molecules to Neuronal Circuits in Drosophila

Temperature has a significant effect on all physiological processes of animals. Suitable temperatures promote responsiveness, movement, metabolism, growth, and reproduction in animals, whereas extreme temperatures can cause injury or even death. Thus, thermosensation is important for survival in all animals. However, mechanisms regulating thermosensation remain unexplored, mostly because of the complexity of mammalian neural circuits. The fruit fly Drosophila melanogaster achieves a desirable body temperature through ambient temperature fluctuations, sunlight exposure, and behavioral strategies. The availability of extensive genetic tools and resources for studying Drosophila have enabled scientists to unravel the mechanisms underlying their temperature preference. Over the past 20 years, Drosophila has become an ideal model for studying temperature-related genes and circuits. This review provides a comprehensive overview of our current understanding of thermosensation and temperature preference in Drosophila. It encompasses various aspects, such as the mechanisms by which flies sense temperature, the effects of internal and external factors on temperature preference, and the adaptive strategies employed by flies in extreme-temperature environments. Understanding the regulating mechanisms of thermosensation and temperature preference in Drosophila can provide fundamental insights into the underlying molecular and neural mechanisms that control body temperature and temperature-related behavioral changes in other animals.

Organismal landscape of clock cells and circadian gene expression in Drosophila

Background

Circadian rhythms time physiological and behavioral processes to 24-hour cycles. It is generally assumed that most cells contain self-sustained circadian clocks that drive circadian rhythms in gene expression that ultimately generating circadian rhythms in physiology. While those clocks supposedly act cell autonomously, current work suggests that in Drosophila some of them can be adjusted by the brain circadian pacemaker through neuropeptides, like the Pigment Dispersing Factor (PDF). Despite these findings and the ample knowledge of the molecular clockwork, it is still unknown how circadian gene expression in Drosophila is achieved across the body.

Results

Here, we used single-cell and bulk RNAseq data to identify cells within the fly that express core-clock components. Surprisingly, we found that less than a third of the cell types in the fly express core-clock genes. Moreover, we identified Lamina wild field (Lawf) and Ponx-neuro positive (Poxn) neurons as putative new circadian neurons. In addition, we found several cell types that do not express core clock components but are highly enriched for cyclically expressed mRNAs. Strikingly, these cell types express the PDF receptor (Pdfr), suggesting that PDF drives rhythmic gene expression in many cell types in flies. Other cell types express both core circadian clock components and Pdfr, suggesting that in these cells, PDF regulates the phase of rhythmic gene expression.

Conclusions

Together, our data suggest three different mechanisms generate cyclic daily gene expression in cells and tissues: canonical endogenous canonical molecular clock, PDF signaling-driven expression, or a combination of both.

Sleep deprivation, sleep fragmentation, and social jet lag increase temperature preference in Drosophila

Despite the fact that sleep deprivation substantially affects the way animals regulate their body temperature, the specific mechanisms behind this phenomenon are not well understood. In both mammals and flies, neural circuits regulating sleep and thermoregulation overlap, suggesting an interdependence that may be relevant for sleep function. To investigate this relationship further, we exposed flies to 12 h of sleep deprivation, or 48 h of sleep fragmentation and evaluated temperature preference in a thermal gradient. Flies exposed to 12 h of sleep deprivation chose warmer temperatures after sleep deprivation. Importantly, sleep fragmentation, which prevents flies from entering deeper stages of sleep, but does not activate sleep homeostatic mechanisms nor induce impairments in short-term memory also resulted in flies choosing warmer temperatures. To identify the underlying neuronal circuits, we used RNAi to knock down the receptor for Pigment dispersing factor, a peptide that influences circadian rhythms, temperature preference and sleep. Expressing UAS-Pdfr^RNAi in subsets of clock neurons prevented sleep fragmentation from increasing temperature preference. Finally, we evaluated temperature preference after flies had undergone a social jet lag protocol which is known to disrupt clock neurons. In this protocol, flies experience a 3 h light phase delay on Friday followed by a 3 h light advance on Sunday evening. Flies exposed to social jet lag exhibited an increase in temperature preference which persisted for several days. Our findings identify specific clock neurons that are modulated by sleep disruption to increase temperature preference. Moreover, our data indicate that temperature preference may be a more sensitive indicator of sleep disruption than learning and memory.

The role of diuretic hormones (DHs) and their receptors in Drosophila

Maintaining internal homeostasis and regulating innate behaviors are essential for animal survival. In various animal species, a highly conserved neuroendocrine system integrates sensory inputs and regulates physiological responses to environmental and internal changes. Diuretic hormones 44 and 31, which are homologs of mammalian corticotropin-releasing factor (CRF) and calcitonin gene-related peptide (CGRP), respectively, control body fluid secretion in Drosophila. These neuropeptides and their receptors have multiple physiological roles, including the regulation of body-fluid secretion, sleep:wake cycle, internal nutrientsensing, and CO2-dependent response. This review discusses the physiological and behavioral roles of DH44 and DH31 signaling pathways, consisting of neuroendocrine cells that secrete DH44 or DH31 peptides and their receptor-expressing organs. Further research is needed to understand the regulatory mechanisms of the behavioral processes mediated by these neuroendocrine systems. [BMB Reports 2023; 56(4): 209-215].

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.

The duration of caffeine treatment plays an essential role in its effect on sleep and circadian rhythm

Sleep is regulated by the homeostatic system and the circadian clock. Caffeine intake promotes wakefulness in Drosophila. In humans, caffeine is consumed on a daily basis and hence it is important to understand the effect of prolonged caffeine intake on both circadian and homeostatic regulation of sleep. Furthermore, sleep changes with age and the impact of caffeine on age-dependent sleep fragmentation are yet to be understood. Hence in the present study, we examined the effect of short exposure to caffeine on homeostatic sleep and age-dependent sleep fragmentation in Drosophila. We further assessed the effect of prolonged exposure to caffeine on homeostatic sleep and circadian clock. The results of our study showed that short exposure to caffeine reduces sleep and food intake in mature flies. It also enhances sleep fragmentation with increasing age. However, we have not assessed the effect of caffeine on food intake in older flies. On the other hand, prolonged caffeine exposure did not exert any significant effect on the duration of sleep and food intake in mature flies. Nevertheless, prolonged caffeine ingestion decreased the morning and evening anticipatory activity in these flies indicating that it affects the circadian rhythm. These flies also exhibited phase delay in the clock gene timeless transcript oscillation and exhibited either behavioral arrhythmicity or a longer free-running period under constant darkness. In summary, the results of our studies showed that short exposure to caffeine increases the sleep fragmentation with age whereas prolonged caffeine exposure disrupts the circadian clock.

Assessing olfactory, memory, social and circadian phenotypes associated with schizophrenia in a genetic model based on Rim

Schizophrenia shows high heritability and several of the genes associated with this disorder are involved in calcium (Ca2+) signalling and synaptic function. One of these is the Rab-3 interacting molecule-1 (RIM1), which has recently been associated with schizophrenia by Genome Wide Association Studies (GWAS). However, its contribution to the pathophysiology of this disorder remains unexplored. In this work, we use Drosophila mutants of the orthologue of RIM1, Rim, to model some aspects of the classical and non-classical symptoms of schizophrenia. Rim mutants showed several behavioural features relevant to schizophrenia including social distancing and altered olfactory processing. These defects were accompanied by reduced evoked Ca2+ influx and structural changes in the presynaptic terminals sent by the primary olfactory neurons to higher processing centres. In contrast, expression of Rim-RNAi in the mushroom bodies (MBs), the main memory centre in flies, spared learning and memory suggesting a differential role of Rim in different synapses. Circadian deficits have been reported in schizophrenia. We observed circadian locomotor activity deficits in Rim mutants, revealing a role of Rim in the pacemaker ventral lateral clock neurons (LNvs). These changes were accompanied by impaired day/night remodelling of dorsal terminal synapses from a subpopulation of LNvs and impaired day/night release of the circadian neuropeptide pigment dispersing factor (PDF) from these terminals. Lastly, treatment with the commonly used antipsychotic haloperidol rescued Rim locomotor deficits to wildtype. This work characterises the role of Rim in synaptic functions underlying behaviours disrupted in schizophrenia.

Identification of a molecular basis for the juvenile sleep state

Across species, sleep in young animals is critical for normal brain maturation. The molecular determinants of early life sleep remain unknown. Through an RNAi-based screen, we identified a gene, pdm3, required for sleep maturation in Drosophila. Pdm3, a transcription factor, coordinates an early developmental program that prepares the brain to later execute high levels of juvenile adult sleep. PDM3 controls the wiring of wake-promoting dopaminergic (DA) neurites to a sleep-promoting region, and loss of PDM3 prematurely increases DA inhibition of the sleep center, abolishing the juvenile sleep state. RNA-Seq/ChIP-Seq and a subsequent modifier screen reveal that pdm3 represses expression of the synaptogenesis gene Msp300 to establish the appropriate window for DA innervation. These studies define the molecular cues governing sleep behavioral and circuit development, and suggest sleep disorders may be of neurodevelopmental origin.

Aging and the clock: Perspective from flies to humans

Endogenous circadian oscillators regulate molecular, cellular and physiological rhythms, synchronizing tissues and organ function to coordinate activity and metabolism with environmental cycles. The technological nature of modern society with round-the-clock work schedules and heavy reliance on personal electronics has precipitated a striking increase in the incidence of circadian and sleep disorders. Circadian dysfunction contributes to an increased risk for many diseases and appears to have adverse effects on aging and longevity in animal models. From invertebrate organisms to humans, the function and synchronization of the circadian system weakens with age aggravating the age-related disorders and pathologies. In this review, we highlight the impacts of circadian dysfunction on aging and longevity and the reciprocal effects of aging on circadian function with examples from Drosophila to humans underscoring the highly conserved nature of these interactions. Additionally, we review the potential for using reinforcement of the circadian system to promote healthy aging and mitigate age-related pathologies. Advancements in medicine and public health have significantly increased human life span in the past century. With the demographics of countries worldwide shifting to an older population, there is a critical need to understand the factors that shape healthy aging. Drosophila melanogaster, as a model for aging and circadian interactions, has the capacity to facilitate the rapid advancement of research in this area and provide mechanistic insights for targeted investigations in mammals.

Shaw and Shal voltage-gated potassium channels mediate circadian changes in Drosophila clock neuron excitability

As in mammals, Drosophila circadian clock neurons display rhythms of activity with higher action potential firing rates and more positive resting membrane potentials during the day. This rhythmic excitability has been widely observed but, critically, its regulation remains unresolved. We have characterized and modelled the changes underlying these electrical activity rhythms in the lateral ventral clock neurons (LNvs). We show that currents mediated by the voltage-gated potassium channels Shaw (Kv3) and Shal (Kv4) oscillate in a circadian manner. Disruption of these channels, by expression of dominant negative (DN) subunits, leads to changes in circadian locomotor activity and shortens lifespan. LNv whole-cell recordings then show that changes in Shaw and Shal currents drive changes in action potential firing rate and that these rhythms are abolished when the circadian molecular clock is stopped. A whole-cell biophysical model using Hodgkin-Huxley equations can recapitulate these changes in electrical activity. Based on this model and by using dynamic clamp to manipulate clock neurons directly, we can rescue the pharmacological block of Shaw and Shal, restore the firing rhythm, and thus demonstrate the critical importance of Shaw and Shal. Together, these findings point to a key role for Shaw and Shal in controlling circadian firing of clock neurons and show that changes in clock neuron currents can account for this. Moreover, with dynamic clamp we can switch the LNvs between morning-like and evening-like states of electrical activity. We conclude that changes in Shaw and Shal underlie the daily oscillation in LNv firing rate.

Thermosensitive alternative splicing senses and mediates temperature adaptation in Drosophila

Circadian rhythms are generated by the cyclic transcription, translation, and degradation of clock gene products, including timeless (tim), but how the circadian clock senses and adapts to temperature changes is not completely understood. Here, we show that temperature dramatically changes the splicing pattern of tim in Drosophila. We found that at 18°C, TIM levels are low because of the induction of two cold-specific isoforms: tim-cold and tim-short and cold. At 29°C, another isoform, tim-medium, is upregulated. Isoform switching regulates the levels and activity of TIM as each isoform has a specific function. We found that tim-short and cold encodes a protein that rescues the behavioral defects of tim⁰¹ mutants, and that flies in which tim-short and cold is abrogated have abnormal locomotor activity. In addition, miRNA-mediated control limits the expression of some of these isoforms. Finally, data that we obtained using minigenes suggest that tim alternative splicing might act as a thermometer for the circadian clock.

Evolution: Fruit Fly Clocks on the Edge

Fruit flies evolved in tropical regions under stable light-dark cycles. However, their photosensitive circadian clock had to adapt to extreme seasonal photoperiods during their colonisation of temperate regions. This was achieved by changing the neuronal expression of two key clock-related components.

Identification of putative circadian clock genes in the American horseshoe crab, Limulus polyphemus

While the American horseshoe crab, Limulus polyphemus, has robust circadian and circatidal rhythms, virtually nothing is known about the molecular basis of these rhythms in this species or any other chelicerate. In this study, next generation sequencing was used to assemble transcriptomic reads and then putative homologs of known core and accessory circadian genes were identified in these databases. Homologous transcripts were discovered for one circadian clock input gene, five core genes, 22 accessory genes, and two possible output pathways. Alignments and functional domain analyses showed generally high conservation between the putative L. polyphemus clock genes and homologs from Drosophila melanogaster and Daphnia pulex. The presence of both cry1 and cry2 in the L. polyphemus transcriptome would classify its system as an "ancestral", type 2 clock system. In addition, a novel duplication of CYCLE, and a novel triplication of PERIOD were found. Investigations are currently underway to determine if any of these "circadian" genes also participate in the molecular processes that drive the Limulus circatidal clock.

Positive geotactic behaviors induced by geomagnetic field in Drosophila

Background

Appropriate vertical movement is critical for the survival of flying animals. Although negative geotaxis (moving away from Earth) driven by gravity has been extensively studied, much less is understood concerning a static regulatory mechanism for inducing positive geotaxis (moving toward Earth).

Results

Using Drosophila melanogaster as a model organism, we showed that geomagnetic field (GMF) induces positive geotaxis and antagonizes negative gravitaxis. Remarkably, GMF acts as a sensory cue for an appetite-driven associative learning behavior through the GMF-induced positive geotaxis. This GMF-induced positive geotaxis requires the three geotaxis genes, such as cry, pyx and pdf, and the corresponding neurons residing in Johnston’s organ of the fly’s antennae.

Conclusions

These findings provide a novel concept with the neurogenetic basis on the regulation of vertical movement by GMF in the flying animals.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-016-0235-1) contains supplementary material, which is available to authorized users.

Heterogeneity of the Peripheral Circadian Systems in Drosophila melanogaster: A Review

Circadian rhythms in organisms are involved in many aspects of metabolism, physiology, and behavior. In many animals, these rhythms are produced by the circadian system consisting of a central clock located in the brain and peripheral clocks in various peripheral tissues. The oscillatory machinery and entrainment mechanism of peripheral clocks vary between different tissues and organs. The relationship between the central and peripheral clocks is also tissue-dependent. Here we review the heterogeneous nature of peripheral circadian clocks in the fruit fly Drosophila melanogaster and their dependence on the central clock, and discuss their significance in the temporal organization of physiology in peripheral tissues/organs.

Evolutionary divergence of core and post-translational circadian clock genes in the pitcher-plant mosquito, Wyeomyia smithii

BACKGROUND

Internal circadian (circa, about; dies, day) clocks enable organisms to maintain adaptive timing of their daily behavioral activities and physiological functions. Eukaryotic clocks consist of core transcription-translation feedback loops that generate a cycle and post-translational modifiers that maintain that cycle at about 24 h. We use the pitcher-plant mosquito, Wyeomyia smithii (subfamily Culicini, tribe Sabethini), to test whether evolutionary divergence of the circadian clock genes in this species, relative to other insects, has involved primarily genes in the core feedback loops or the post-translational modifiers. Heretofore, there is no reference transcriptome or genome sequence for any mosquito in the tribe Sabethini, which includes over 375 mainly circumtropical species.

METHODS

We sequenced, assembled and annotated the transcriptome of W. smithii containing nearly 95 % of conserved single-copy orthologs in animal genomes. We used the translated contigs and singletons to determine the average rates of circadian clock-gene divergence in W. smithii relative to three other mosquito genera, to Drosophila, to the butterfly, Danaus, and to the wasp, Nasonia.

RESULTS

Over 1.08 million cDNA sequence reads were obtained consisting of 432.5 million nucleotides. Their assembly produced 25,904 contigs and 54,418 singletons of which 62 % and 28 % are annotated as protein-coding genes, respectively, sharing homology with other animal proteomes.

DISCUSSION

The W. smithii transcriptome includes all nine circadian transcription-translation feedback-loop genes and all eight post-translational modifier genes we sought to identify (Fig. 1). After aligning translated W. smithii contigs and singletons from this transcriptome with other insects, we determined that there was no significant difference in the average divergence of W. smithii from the six other taxa between the core feedback-loop genes and post-translational modifiers.

CONCLUSIONS

The characterized transcriptome is sufficiently complete and of sufficient quality to have uncovered all of the insect circadian clock genes we sought to identify (Fig. 1). Relative divergence does not differ between core feedback-loop genes and post-translational modifiers of those genes in a Sabethine species (W. smithii) that has experienced a continual northward dispersal into temperate regions of progressively longer summer day lengths as compared with six other insect taxa. An associated microarray platform derived from this work will enable the investigation of functional genomics of circadian rhythmicity, photoperiodic time measurement, and diapause along a photic and seasonal geographic gradient.

Synergistic induction of the clock protein PERIOD by insulin-like peptide and prothoracicotropic hormone in Rhodnius prolixus (Hemiptera): implications for convergence of hormone signaling pathways

We showed previously that release of the cerebral neurohormones, bombyxin (an insulin-like peptide, ILP) and prothoracicotropic hormone (PTTH) from the brain have strong circadian rhythms, driven by master clock cells in the brain. These neurohormone rhythms synchronize the photosensitive brain clock with the photosensitive peripheral clock in the cells of the prothoracic glands (PGs), in which both regulate steroidogenesis. Here, using immunohistochemistry and confocal laser scanning microscopy, we show these neurohormones likely act on clock cells in the brain and PGs by regulating expression of PERIOD (PER) protein. PER is severely reduced in the nuclei of all clock cells in continuous light, but on transfer of tissues to darkness in vitro, it is rapidly induced. A 4h pulse of either PTTH or ILPs to brain and PGs in vitro both rapidly and highly significantly induce PER in the nuclei of clock cells. Administration of both neurohormones together induces more PER than does either alone and even more than does transfer to darkness, at least in PG cells. These are clearly non-steroidogenic actions of these peptides. In the peripheral oscillators salivary gland (SG) and fat body cells, neither bombyxin nor PTTH nor darkness induced PER, but a combination of both bombyxin and PTTH induced PER. Thus, PTTH and ILPs exert synergistic actions on induction of PER in both clock cells and peripheral oscillators, implying their signaling pathways converge, but in different ways in different cell types. We infer clock cells are able to integrate light cycle information with internal signals from hormones.

Circadian genes differentially affect tolerance to ethanol in Drosophila

BACKGROUND

There is a strong relationship between circadian rhythms and ethanol (EtOH) responses. EtOH consumption has been shown to disrupt physiological and behavioral circadian rhythms in mammals (Alcohol Clin Exp Res 2005b, 29, 1550). The Drosophila central circadian pacemaker is composed of proteins encoded by the per, tim, cyc, and Clk genes. Using Drosophila mutant analysis, we asked whether these central components of the circadian clock make the equivalent contribution toward EtOH tolerance and whether rhythmicity itself is necessary for tolerance.

METHODS

We tested flies carrying mutations in core clock genes for the capacity to acquire EtOH tolerance. Tolerance was assayed by comparing the sedation curves of populations during their first and second sedation. Animals that had acquired tolerance sedated more slowly. Movement was also monitored as the flies breathe the EtOH vapor to determine if other facets of the EtOH response were affected by the mutations. Gas chromatography was used to measure internal EtOH concentration. Constant light was used to nongenetically destabilize the PER and TIM proteins.

RESULTS

A group of circadian mutations, all of which eliminate circadian rhythms, do not disrupt tolerance identically. Mutations in per, tim, and cyc completely block tolerance. However, a mutation in Clk does not interfere with tolerance. Constant light also disrupts the capacity to acquire tolerance. These lines did not differ in EtOH absorption.

CONCLUSIONS

Mutations affecting different parts of the intracellular circadian clock can block the capacity to acquire rapid EtOH tolerance. However, the role of circadian genes in EtOH tolerance is independent of their role in producing circadian rhythmicity. The interference in the capacity to acquire EtOH tolerance by some circadian mutations is not merely a downstream effect of a nonfunctional circadian clock; instead, these circadian genes play an independent role in EtOH tolerance.

Patterns of activity expressed by juvenile horseshoe crabs

Adult American horseshoe crabs, Limulus polyphemus, possess endogenous circadian and circatidal clocks controlling visual sensitivity and locomotion, respectively. The goal of this study was to determine the types of activity rhythms expressed by juvenile horseshoe crabs (n = 24) when exposed to a 14:10 light/dark cycle (LD) for 10 days, followed by 10 days of constant darkness (DD). Horseshoe crab activity was recorded with a digital time-lapse video system that used an infrared-sensitive camera so animals could be monitored at night. In LD, 15 animals expressed daily patterns of activity, 6 displayed a circatidal pattern, and the remaining 3 were arrhythmic. Of the 15 animals with daily patterns of locomotion, 7 had a significant preference (P < 0.05) for diurnal activity and 3 for nocturnal activity; the remainder did not express a significant preference for day or night activity. In DD, 13 horseshoe crabs expressed circatidal rhythms and 8 maintained a pattern of about 24 h. Although these results suggest the presence of a circadian clock influencing circatidal patterns of locomotion, these apparent circadian rhythms may actually represent the expression of just one of the two bouts of activity driven by the putative circalunidian clocks that control their tidal rhythms. Overall, these results indicate that, like adults, juvenile horseshoe crabs express both daily and tidal patterns of activity and that at least one, and maybe both, of these patterns is driven by endogenous clocks.

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.

Aging signaling pathways and circadian clock-dependent metabolic derangements

The circadian clock machinery orchestrates organism metabolism to ensure that development, survival, and reproduction are attuned to diurnal environmental variations. For unknown reasons, there is a decline in circadian rhythms with age, concomitant with declines in the overall metabolic tissue homeostasis and changes in the feeding behavior of aged organisms. This disruption of the relationship between the clock and the nutrient-sensing networks might underlie age-related diseases; overall, greater knowledge of the molecular mediators of and variations in clock networks during lifespan may shed light on the aging process and how it may be delayed. In this review we address the complex links between the circadian clock, metabolic (dys)functions, and aging in different model organisms.

Flies in the north: locomotor behavior and clock neuron organization of Drosophila montana

The circadian clock plays an important role in adaptation in time and space by synchronizing changes in physiological, developmental, and behavioral traits of organisms with daily and seasonal changes in their environment. We have studied some features of the circadian activity and clock organization in a northern Drosophila species, Drosophila montana, at both the phenotypic and the neuronal levels. In the first part of the study, we monitored the entrained and free-running locomotor activity rhythms of females in different light-dark and temperature regimes. These studies showed that D. montana flies completely lack the morning activity component typical to more southern Drosophila species in an entrained environment and that they are able to maintain their free-running locomotor activity rhythm better in constant light than in constant darkness. In the second part of the study, we traced the expression of the PDF neuropeptide and the CRY protein in the neurons of the brain in D. montana adults and found differences in the number and location of PDF- and CRY-expressing neurons compared with those described in Drosophila melanogaster. These differences could account, at least in part, for the lack of morning activity and the reduced circadian rhythmicity of D. montana flies in constant darkness, both of which are likely to be adaptive features during the long and dark winters occurring in nature.

Interactions between the circadian clock and metabolism: there are good times and bad times

An endogenous circadian (∼24 h) clock regulates rhythmic processes of physiology, metabolism and behavior in most living organisms. While able to free-run under constant conditions, the circadian clock is coupled to day : night cycles to increase its amplitude and align the phase of circadian rhythms to the right time of the day. Disruptions of the circadian clock are correlated with brain dysfunctions, cardiovascular diseases and metabolic disorders. In this review, we focus on the interactions between the circadian clock and metabolism. We discuss recent findings on circadian clock regulation of feeding behavior and rhythmic expression of metabolic genes, and present evidence of metabolic input to the circadian clock. We emphasize how misalignment of circadian clocks within the body and with environmental cycles or daily schedules leads to the increasing prevalence of metabolic syndromes in modern society.

Circadian regulation of glutathione levels and biosynthesis in Drosophila melanogaster

Circadian clocks generate daily rhythms in neuronal, physiological, and metabolic functions. Previous studies in mammals reported daily fluctuations in levels of the major endogenous antioxidant, glutathione (GSH), but the molecular mechanisms that govern such fluctuations remained unknown. To address this question, we used the model species Drosophila, which has a rich arsenal of genetic tools. Previously, we showed that loss of the circadian clock increased oxidative damage and caused neurodegenerative changes in the brain, while enhanced GSH production in neuronal tissue conferred beneficial effects on fly survivorship under normal and stress conditions. In the current study we report that the GSH concentrations in fly heads fluctuate in a circadian clock-dependent manner. We further demonstrate a rhythm in activity of glutamate cysteine ligase (GCL), the rate-limiting enzyme in glutathione biosynthesis. Significant rhythms were also observed for mRNA levels of genes encoding the catalytic (Gclc) and modulatory (Gclm) subunits comprising the GCL holoenzyme. Furthermore, we found that the expression of a glutathione S-transferase, GstD1, which utilizes GSH in cellular detoxification, significantly fluctuated during the circadian day. To directly address the role of the clock in regulating GSH-related rhythms, the expression levels of the GCL subunits and GstD1, as well as GCL activity and GSH production were evaluated in flies with a null mutation in the clock genes cycle and period. The rhythms observed in control flies were not evident in the clock mutants, thus linking glutathione production and utilization to the circadian system. Together, these data suggest that the circadian system modulates pathways involved in production and utilization of glutathione.

Cellular requirements for LARK in the Drosophila circadian system

RNA-binding proteins mediate posttranscriptional functions in the circadian systems of multiple species. A conserved RNA recognition motif (RRM) protein encoded by the lark gene is postulated to serve circadian output and molecular oscillator functions in Drosophila and mammals, respectively. In no species, however, has LARK been eliminated, in vivo, to determine the consequences for circadian timing. The present study utilized RNA interference (RNAi) techniques in Drosophila to decrease LARK levels in clock neurons and other cell types in order to evaluate the circadian functions of the protein. Knockdown of LARK in timeless (TIM)- or pigment dispersing factor (PDF)-containing clock cells caused a significant number of flies to exhibit arrhythmic locomotor activity, demonstrating a requirement for the protein in pacemaker cells. There was no obvious effect on PER protein cycling in lark interference (RNAi) flies, but a knockdown within the PDF neurons was associated with increased PDF immunoreactivity at the dorsal termini of the small ventral lateral neuronal (s-LNv) projections, suggesting an effect on neuropeptide release. The expression of lark RNAi in multiple neurosecretory cell populations demonstrated that LARK is required within pacemaker and nonpacemaker cells for the manifestation of normal locomotor activity rhythms. Interestingly, decreased LARK function in the prothoracic gland (PG), a peripheral organ containing a clock required for the circadian control of eclosion, was associated with weak population eclosion rhythms or arrhythmicity.

Is the sex communication of two pyralid moths, Plodia interpunctella and Ephestia kuehniella, under circadian clock regulation?

Females of the Indian meal moth, Plodia interpunctella, and females of the Mediterranean flour month, Ephestia kuehniella (both Lepidoptera: Pyralidae), exhibit daily rhythms in calling behavior. The peak in P. interpunctella calling occurs at dusk, whereas E. kuehniella calls preferentially at dawn. This behavior turned arrhythmic in P. interpunctella females in constant darkness (DD) and remained arrhythmic in constant light (LL), whereas E. kuehniella females showed a persistent rhythm in DD and suppression of the behavior in LL, indicating regulation by a circadian clock mechanism. The rhythm of male locomotor activity corresponded well with the sexual activity of females, reaching the peak at dusk in P. interpunctella and at dawn in E. kuehniella. An immunohistochemical study of the pheromone biosynthesis activating neuropeptide, corazonin, and pigment dispersing factor revealed distinct sets of neurons in the brain-subesophageal complex and in the neurohemal organs of the 2 species.

Daily Rhythms of PERIOD protein in the eyestalk of the American lobster, Homarus americanus

The daily rhythm of PERIOD protein (PER) expression is an integral component of the circadian clock, which is found among a broad range of animal species including fruit flies, marine mollusks and even humans. The use of antibodies directed against PER has provided a helpful tool in the discovery of PER homologues and the labeling of putative pacemaker cells, especially in animals for which an annotated genome is not readily available. In this study, DrosophilaPER antibodies were used to probe for PER in the American lobster, Homarus americanus. This species exhibits robust endogenous circadian rhythms but the circadian clock has yet to be located or characterized. PER was detected in the eyestalks of the lobster but not in the brain. Furthermore, a significant effect of the LD cycle on daily PER abundance was identified, and PER was significantly more abundant at mid dark than in early light or mid light hours. Our results suggest that PER is a part of the molecular machinery of the circadian clock located in the eyestalk of the lobster.

Blocking synaptic transmission with tetanus toxin light chain reveals modes of neurotransmission in the PDF-positive circadian clock neurons of Drosophila melanogaster

Circadian locomotor rhythms of Drosophila melanogaster are controlled by a neuronal circuit composed of approximately 150 clock neurons that are roughly classified into seven groups. In the circuit, a group of neurons expressing pigment-dispersing factor (PDF) play an important role in organizing the pacemaking system. Recent studies imply that unknown chemical neurotransmitter(s) (UNT) other than PDF is also expressed in the PDF-positive neurons. To explore its role in the circadian pacemaker, we examined the circadian locomotor rhythms of pdf-Gal4/UAS-TNT transgenic flies in which chemical synaptic transmission in PDF-positive neurons was blocked by expressed tetanus toxin light chain (TNT). In constant darkness (DD), the flies showed a free-running rhythm, which was similar to that of wild-type flies but significantly different from pdf null mutants. Under constant light conditions (LL), however, they often showed complex rhythms with a short period and a long period component. The UNT is thus likely involved in the synaptic transmission in the clock network and its release caused by LL leads to arrhythmicity. Immunocytochemistry revealed that LL induced phase separation in TIMELESS (TIM) cycling among some of the PDF-positive and PDF-negative clock neurons in the transgenic flies. These results suggest that both PDF and UNT play important roles in the Drosophila circadian clock, and activation of PDF pathway alone by LL leads to the complex locomotor rhythm through desynchronized oscillation among some of the clock neurons.

Effects of neuropeptides on feeding initiation in larvae of the silkworm, Bombyx mori

In insects, especially phytophagous insects, feeding behavior occurs at a regular frequency. Although a number of physiological studies have revealed various causal factors leading to feeding behavior in insects, little has been demonstrated regarding the regulatory mechanisms underlying insect feeding behavior. To confirm the presence of an endocrinological regulatory mechanism in feeding behavior, we tested the effects of several biologically active peptides on silkworm, Bombyx mori larvae feeding behaviors. To evaluate the effects of the biologically active peptides, we measured the period of latency to the first bite following sample injection into starved Bombyx larvae. Of the chemically synthesized peptides tested, myosuppressin exhibited a prolonged latency, indicating that myosuppressin is a possible inhibitory peptide in Bombyx larvae. In contrast, injections of tachykinin and short neuropeptide F, which are members of the structurally related RF-amide peptide family, had a shorter latency period, indicating that these two peptides are possible stimulatory peptides. In addition, the present study suggests that this bioassay will be advantageous for screening for peptides that regulate insect feeding behavior.

Pigment-Dispersing Factor Affects Nocturnal Activity Rhythms, Photic Entrainment, and the Free-Running Period of the Circadian Clock in the Cricket Gryllus bimaculatus

Pigment-dispersing factor (PDF) is a neuropeptide widely distributed in insect brains and plays important roles in the circadian system. In this study, we used RNA interference to study the role of the pigment-dispersing factor ( pdf) gene in regulating circadian locomotor rhythms in the cricket, Gryllus bimaculatus. Injections of pdf double-stranded RNA (ds pdf) effectively knocked down the pdf mRNA and PDF peptide levels. The treated crickets maintained the rhythm both under light-dark cycles (LD) and constant darkness (DD). However, they showed rhythms with reduced nocturnal activity with prominent peaks at lights-on and lights-off. Entrainability of ds pdf-injected crickets was higher than control crickets as they required fewer cycles to resynchronize to the LD cycles shifted by 6 h. The free-running periods of the ds pdf-injected crickets were shorter than those of control crickets in DD. These results suggest that PDF is not essential for the rhythm generation but involved in control of the nocturnality, photic entrainment, and fine tuning of the free-running period of the circadian clock.

Cryptochrome-Positive and -Negative Clock Neurons in Drosophila Entrain Differentially to Light and Temperature

The blue-light photoreceptive protein Cryptochrome (CRY) plays an important role in the light synchronization of the Drosophila circadian clock. Previously, we found that among the approximately 150 clock neurons, many but not all neurons express CRY. We speculated that the CRY-positive pacemaker neurons may be especially important for light entrainment, whereas the CRY-negative neurons may be important for other environmental cues, for example, temperature. To investigate this hypothesis, we tested the entrainability of the clock neurons to out-of-phase light and temperature cycles. When light-dark or light-dim light cycles were shifted by 12 h with respect to temperature cycles, behavioral rhythms of wild-type flies were re-entrained by the light cycles. In this condition, we found that TIMELESS (TIM) level was strongly influenced by the temperature cycles in many CRY-negative clock neurons, suggesting that the CRY-negative neurons have higher sensitivity to temperature. Under the same conditions, cry-null mutants entrained to the temperature cycles or very slowly re-entrained to light-dark cycles. Our results suggest that there are 2 types of clock neurons having differential sensitivities to light and temperature, and CRY is a key component for the preferential entrainment to light.

Precursor structure, distribution and possible functions of pigment-dispersing hormone (PDH) in the terrestrial isopod Armadillidium vulgare (Latreille)

Pigment-dispersing hormone (PDH) is an 18 amino acid neuropeptide that induces pigment migration in Decapoda and serves as a circadian neurotransmitter in the locomotor activity rhythm in Drosophila. In this study, a cDNA encoding PDH was cloned from adult brains of the pill bug, Armadillidium vulgare (Av). The cDNA comprising 529 bp encodes a peptide (AvPDH) that consists of a putative 26 amino acid signal peptide, and a 34 amino acid PDH-precursor-related peptide containing an 18 amino acid mature peptide. The peptide shows a high sequence identity (55-77%) to crustacean β-PDHs and insect PDFs. The tissue-specific expression pattern was examined by reverse transcription PCR. The transcript is expressed in the brain strongly and ventral nerve cord weakly, but the signal was not detected in the intestinal tract. A similar expression profile appeared in Western blot analyses. Western blot analyses with timed samples showed more intense expression of PDH-like antigen at night. PDH-like immunohistochemical reactivity (PDH-ir) was detected in the optic lobe, anteromedian protocerebrum, accessory lobe, tritocerebrum, and suboesophageal ganglion but the reactivity was faint or nil in the pseudofrontal organ (sinus gland). These results were substantiated by in situ hybridization. Co-localization using anti-Gryllus bimaculatus (Gb)-PDF, anti-Bombyx mori (Bm)-CLK, and anti-Bm-CYC showed a co-localization of these antigens in the optic lobe and SOG. The results provide the first structural and immunocytochemical identification of PDH neurons in terrestrial isopods, and the co-localization of PDH with CLK and CYC supports its possible involvement in circadian clock. A day/night rhythm of PDH content is also a new feature.

Signalling through pigment dispersing hormone-like peptides in invertebrates

During recent decades, several research teams engaged in unraveling the molecular structure and the physiological significance of pigment dispersing hormone-like peptides, particularly with respect to colour change and biological rhythms. In this review, we first summarise the entire history of pigment dispersing hormone-like peptide research, thus providing a stepping stone for those who are curious about this growing area of interest. Next, we try to bring order in the plethora of experimental data on the molecular structure of the various peptides and receptors and also discuss immunolocalization, time-related expression and suggested functions in crustaceans, insects and nematodes. In addition, a brief comparison with the vertebrate system is made.

Transcription factors in light and circadian clock signaling networks revealed by genomewide mapping of direct targets for neurospora white collar complex

Light signaling pathways and circadian clocks are inextricably linked and have profound effects on behavior in most organisms. Here, we used chromatin immunoprecipitation (ChIP) sequencing to uncover direct targets of the Neurospora crassa circadian regulator White Collar Complex (WCC). The WCC is a blue-light receptor and the key transcription factor of the circadian oscillator. It controls a transcriptional network that regulates ∼20% of all genes, generating daily rhythms and responses to light. We found that in response to light, WCC binds to hundreds of genomic regions, including the promoters of previously identified clock- and light-regulated genes. We show that WCC directly controls the expression of 24 transcription factor genes, including the clock-controlled adv-1 gene, which controls a circadian output pathway required for daily rhythms in development. Our findings provide links between the key circadian activator and effectors in downstream regulatory pathways.

Persistence of morning anticipation behavior and high amplitude morning startle response following functional loss of small ventral lateral neurons in Drosophila

Light-activated large ventral lateral clock neurons (large LNv) modulate behavioral arousal and sleep in Drosophila while their counterparts, the small LNv (s-LNv) are important for circadian behavior. Recently, it has been proposed that the pattern of day-night locomotor behavioral activity is mediated by two anatomically distinct oscillators composed of a morning oscillator in the small LNv and an evening oscillator in the lateral dorsal neurons and an undefined number of dorsal pacemaker neurons. This contrasts with a circuit described by network models which are not as anatomically constrained. By selectively ablating the small LNv while sparing the large LNv, we tested the relative importance of the small and large LNv for regulating morning behavior of animals living in standard light/dark cycles. Behavioral anticipation of the onset of morning and the high amplitude morning startle response which coincides with light onset are preserved in small LNv functionally-ablated animals. However, the amplitude of the morning behavioral peak is severely attenuated in these animals during the transition from regular light/dark cycles to constant darkness, providing further support that small LNv are necessary for circadian behavior. The large LNv, in combination with the network of other circadian neurons, in the absence of functional small LNv are sufficient for the morning anticipation and the high amplitude light-activated morning startle response.

PDF receptor expression reveals direct interactions between circadian oscillators in Drosophila

Daily rhythms of behavior are controlled by a circuit of circadian pacemaking neurons. In Drosophila, 150 pacemakers participate in this network, and recent observations suggest that the network is divisible into M and E oscillators, which normally interact and synchronize. Sixteen oscillator neurons (the small and large lateral neurons [LNvs]) express a neuropeptide called pigment-dispersing factor (PDF) whose signaling is often equated with M oscillator output. Given the significance of PDF signaling to numerous aspects of behavioral and molecular rhythms, determining precisely where and how signaling via the PDF receptor (PDFR) occurs is now a central question in the field. Here we show that GAL4-mediated rescue of pdfr phenotypes using a UAS-PDFR transgene is insufficient to provide complete behavioral rescue. In contrast, we describe a approximately 70-kB PDF receptor (pdfr) transgene that does rescue the entire pdfr circadian behavioral phenotype. The transgene is widely but heterogeneously expressed among pacemakers, and also among a limited number of non-pacemakers. Our results support an important hypothesis: the small LNv cells directly target a subset of the other crucial pacemaker neurons cells. Furthermore, expression of the transgene confirms an autocrine feedback signaling by PDF back to PDF-expressing cells. Finally, the results present an unexpected PDF receptor site: the large LNv cells appear to target a population of non-neuronal cells that resides at the base of the eye.

Circadian plasticity in photoreceptor cells controls visual coding efficiency in Drosophila melanogaster

In the fly Drosophila melanogaster, neuronal plasticity of synaptic terminals in the first optic neuropil, or lamina, depends on early visual experience within a critical period after eclosion [1]. The current study revealed two additional and parallel mechanisms involved in this type of synaptic terminal plasticity. First, an endogenous circadian rhythm causes daily oscillations in the volume of photoreceptor cell terminals. Second, daily visual experience precisely modulates the circadian time course and amplitude of the volume oscillations that the photoreceptor-cell terminals undergo. Both mechanisms are separable in their molecular basis. We suggest that the described neuronal plasticity in Drosophila ensures continuous optimal performance of the visual system over the course of a 24 h-day. Moreover, the sensory system of Drosophila cannot only account for predictable, but also for acute, environmental changes. The volumetric changes in the synaptic terminals of photoreceptor cells are accompanied by circadian and light-induced changes of presynaptic ribbons as well as extensions of epithelial glial cells into the photoreceptor terminals, suggesting that the architecture of the lamina is altered by both visual exposure and the circadian clock. Clock-mutant analysis and the rescue of PER protein rhythmicity exclusively in all R1-6 cells revealed that photoreceptor-cell plasticity is autonomous and sufficient to control visual behavior. The strength of a visually guided behavior, the optomotor turning response, co-varies with synaptic-terminal volume oscillations of photoreceptor cells when elicited at low light levels. Our results show that behaviorally relevant adaptive processing of visual information is performed, in part, at the level of visual input level.

Brain plasticity in Diptera and Hymenoptera

To mediate different types of behaviour, nervous systems need to coordinate the proper operation of their neural circuits as well as short- and long-term alterations that occur within those circuits. The latter ultimately devolve upon specific changes in neuronal structures, membrane properties and synaptic connections that are all examples of plasticity. This reorganization of the adult nervous system is shaped by internal and external influences both during development and adult maturation. In adults, behavioural experience is a major driving force of neuronal plasticity studied particularly in sensory systems. The range of adaptation depends on features that are important to a particular species, and is therefore specific, so that learning is essential for foraging in honeybees, while regenerative capacities are important in hemimetabolous insects with long appendages. Experience is usually effective during a critical period in early adult life, when neural function becomes tuned to future conditions in an insect's life. Tuning occur at all levels, in synaptic circuits, neuropile volumes, and behaviour. There are many examples, and this review incorporates only a select few, mainly those from Diptera and Hymenoptera.

Altered LARK expression perturbs development and physiology of the Drosophila PDF clock neurons

The LARK RNA-binding protein (RBP) has well documented roles in the circadian systems of Drosophila and mammals. Recent studies have demonstrated that the Drosophila LARK RBP is associated with many mRNA targets, in vivo, including those that regulate either neurophysiology or development of the nervous system. In the present study, we have employed conditional expression techniques to distinguish developmental and physiological functions of LARK for a defined class of neurons: the Pigment-Dispersing Factor (PDF)-containing LNv clock neurons. We found that increased LARK expression during development dramatically alters the small LNv class of neurons with no obvious effects on the large LNv cells. Conversely, conditional expression of LARK at the adult stage results in altered clock protein rhythms and circadian locomotor activity, even though neural morphology is normal in such animals. Electrophysiological analyses at the larval neuromuscular junction indicate a role for LARK in regulating neuronal excitability. Altogether, our results demonstrate that LARK activity is critical for neuronal development and physiology.

Cloning of neuropeptide-like precursor 1 in the gray flesh fly and peptide identification and expression

The neuropeptide-like precursor 1 (NPLP1) was first identified in a peptidomics experiment on Drosophila melanogaster. Limited data on this novel neuropeptide precursor suggest a role in the regulation of ecdysis in holometabolous larvae. In this study, we characterized the NPLP1 precursor in the gray flesh fly, Neobellieria bullata, which is an excellent model for physiological assays and hence to discover the role of the NPLP1 peptides. Antisera against three of the D. melanogaster NPLP1 neuropeptides stained an identical set of neurons in the central nervous system of N. bullata compared to D. melanogaster. A novel approach was applied to identify the N. bullata NPLP1 orthologs. Using a combination of affinity chromatography, mass spectrometry, cDNA cloning and RACE experiments, we obtained almost the complete coding sequence of the NPLP1 mRNA. Three encoded NPLP1 peptides were identified in central nervous system extracts by mass spectrometry. Neither doses of 25-250pmol of synthetic Neb-MGYamide and Neb-PQNamide peptides, nor the NPLP1 antisera did affect the speed of retraction, contraction and tanning in the pupariation bioassay.

Comparative analysis of Pdf-mediated circadian behaviors between Drosophila melanogaster and D. virilis

A group of small ventrolateral neurons (s-LN(v)'s) are the principal pacemaker for circadian locomotor rhythmicity of Drosophila melanogaster, and the pigment-dispersing factor (Pdf) neuropeptide plays an essential role as a clock messenger within these neurons. In our comparative studies on Pdf-associated circadian rhythms, we found that daily locomotor activity patterns of D. virilis were significantly different from those of D. melanogaster. Activities of D. virilis adults were mainly restricted to the photophase under light:dark cycles and subsequently became arrhythmic or weakly rhythmic in constant conditions. Such activity patterns resemble those of Pdf(01) mutant of D. melanogaster. Intriguingly, endogenous D. virilis Pdf (DvPdf) expression was not detected in the s-LN(v)-like neurons in the adult brains, implying that the Pdf(01)-like behavioral phenotypes of D. virilis are attributed in part to the lack of DvPdf in the s-LN(v)-like neurons. Heterologous transgenic analysis showed that cis-regulatory elements of the DvPdf transgene are capable of directing their expression in all endogenous Pdf neurons including s-LN(v)'s, as well as in non-Pdf clock neurons (LN(d)'s and fifth s-LN(v)) in a D. melanogaster host. Together these findings suggest a significant difference in the regulatory mechanisms of Pdf transcription between the two species and such a difference is causally associated with species-specific establishment of daily locomotor activity patterns.

Neurotransmitter-Mediated Collective Rhythms in Grouped Drosophila Circadian Clocks

Over the past decades, fly Drosophila melanogaster has being used as a premier model organism to study molecular and genetic bases of circadian rhythms. Here the authors propose a multicellular heterogeneous model for which the network of Drosophila circadian oscillators consists of two groups, the self-sustained lateral neurons (LNs) communicating to each other and the damped dorsal neurons (DNs) receiving neurotransmitters only from the LNs without interaction within this group. By simulating different experimental conditions, the authors find that the proposed model, except for being capable of reproducing some known experimental results well, also can predict some interesting phenomena: 1) The DNs need neuronal projections from the LNs to be rhythmic and to synchronize; 2) the effect of communication on mean amplitude and mean period of two oscillatory groups is different; 3) communication delay can facilitate the network synchronization of the LNs; and 4) only the LNs lose rhythmicity under constant light conditions. These results reveal the mechanism of an integrated pacemaker that would govern behavioral and physiological rhythmicity of the model organism.