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

A four-oscillator model of seasonally adapted morning and evening activities in Drosophila melanogaster

The fruit fly Drosophila melanogaster exhibits two activity peaks, one in the morning and another in the evening. Because the two peaks change phase depending on the photoperiod they are exposed to, they are convenient for studying responses of the circadian clock to seasonal changes. To explain the phase determination of the two peaks, Drosophila researchers have employed the two-oscillator model, in which two oscillators control the two peaks. The two oscillators reside in different subsets of neurons in the brain, which express clock genes, the so-called clock neurons. However, the mechanism underlying the activity of the two peaks is complex and requires a new model for mechanistic exploration. Here, we hypothesize a four-oscillator model that controls the bimodal rhythms. The four oscillators that reside in different clock neurons regulate activity in the morning and evening and sleep during the midday and at night. In this way, bimodal rhythms are formed by interactions among the four oscillators (two activity and two sleep oscillators), which may judiciously explain the flexible waveform of activity rhythms under different photoperiod conditions. Although still hypothetical, this model would provide a new perspective on the seasonal adaptation of the two activity peaks.

Integration of photoperiodic and temperature cues by the circadian clock to regulate insect seasonal adaptations

Organisms adapt to unfavorable seasonal conditions to survive. These seasonal adaptations rely on the correct interpretation of environmental cues such as photoperiod, and temperature. Genetic studies in several organisms, including the genetic powerhouse Drosophila melanogaster, indicate that circadian clock components, such as period and timeless, are involved in photoperiodic-dependent seasonal adaptations, but our understanding of this process is far from complete. In particular, the role of temperature as a key factor to complement photoperiodic response is not well understood. The development of new sequencing technologies has proven extremely useful in understanding the plastic changes that the clock and other cellular components undergo in different environmental conditions, including changes in gene expression and alternative splicing. This article discusses the integration of photoperiod and temperature for seasonal biology as well as downstream molecular and cellular pathways involved in the regulation of physiological adaptations that occur with changing seasons. We focus our discussion on the current understanding of the involvement of the molecular clock and the circadian clock neuronal circuits in these adaptations in D. melanogaster.

Characterization of clock-related proteins and neuropeptides in Drosophila littoralis and their putative role in diapause

Insects from high latitudes spend the winter in a state of overwintering diapause, which is characterized by arrested reproduction, reduced food intake and metabolism, and increased life span. The main trigger to enter diapause is the decreasing day length in summer-autumn. It is thus assumed that the circadian clock acts as an internal sensor for measuring photoperiod and orchestrates appropriate seasonal changes in physiology and metabolism through various neurohormones. However, little is known about the neuronal organization of the circadian clock network and the neurosecretory system that controls diapause in high-latitude insects. We addressed this here by mapping the expression of clock proteins and neuropeptides/neurohormones in the high-latitude fly Drosophila littoralis. We found that the principal organization of both systems is similar to that in Drosophila melanogaster, but with some striking differences in neuropeptide expression levels and patterns. The small ventrolateral clock neurons that express pigment-dispersing factor (PDF) and short neuropeptide F (sNPF) and are most important for robust circadian rhythmicity in D. melanogaster virtually lack PDF and sNPF expression in D. littoralis. In contrast, dorsolateral clock neurons that express ion transport peptide in D. melanogaster additionally express allatostatin-C and appear suited to transfer day-length information to the neurosecretory system of D. littoralis. The lateral neurosecretory cells of D. littoralis contain more neuropeptides than D. melanogaster. Among them, the cells that coexpress corazonin, PDF, and diuretic hormone 44 appear most suited to control diapause. Our work sets the stage to investigate the roles of these diverse neuropeptides in regulating insect diapause.

Neural mechanisms of parasite-induced summiting behavior in 'zombie' Drosophila

For at least two centuries, scientists have been enthralled by the "zombie" behaviors induced by mind-controlling parasites. Despite this interest, the mechanistic bases of these uncanny processes have remained mostly a mystery. Here, we leverage the Entomophthora muscae-Drosophila melanogaster "zombie fly" system to reveal the mechanistic underpinnings of summit disease, a manipulated behavior evoked by many fungal parasites. Using a high-throughput approach to measure summiting, we discovered that summiting behavior is characterized by a burst of locomotion and requires the host circadian and neurosecretory systems, specifically DN1p circadian neurons, pars intercerebralis to corpora allata projecting (PI-CA) neurons and corpora allata (CA), the latter being solely responsible for juvenile hormone (JH) synthesis and release. Using a machine learning classifier to identify summiting animals in real time, we observed that PI-CA neurons and CA appeared intact in summiting animals, despite invasion of adjacent regions of the "zombie fly" brain by E. muscae cells and extensive host tissue damage in the body cavity. The blood-brain barrier of flies late in their infection was significantly permeabilized, suggesting that factors in the hemolymph may have greater access to the central nervous system during summiting. Metabolomic analysis of hemolymph from summiting flies revealed differential abundance of several compounds compared to non-summiting flies. Transfusing the hemolymph of summiting flies into non-summiting recipients induced a burst of locomotion, demonstrating that factor(s) in the hemolymph likely cause summiting behavior. Altogether, our work reveals a neuro-mechanistic model for summiting wherein fungal cells perturb the fly's hemolymph, activating a neurohormonal pathway linking clock neurons to juvenile hormone production in the CA, ultimately inducing locomotor activity in their host.

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

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

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

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

A thermometer circuit for hot temperature adjusts Drosophila behavior to persistent heat

Small poikilotherms such as the fruit fly Drosophila depend on absolute temperature measurements to identify external conditions that are above (hot) or below (cold) their preferred range and to react accordingly. Hot and cold temperatures have a different impact on fly activity and sleep, but the circuits and mechanisms that adjust behavior to specific thermal conditions are not well understood. Here, we use patch-clamp electrophysiology to show that internal thermosensory neurons located within the fly head capsule (the AC neurons1) function as a thermometer active in the hot range. ACs exhibit sustained firing rates that scale with absolute temperature-but only for temperatures above the fly's preferred ∼25°C (i.e., "hot" temperature). We identify ACs in the fly brain connectome and demonstrate that they target a single class of circadian neurons, the LPNs.2 LPNs receive excitatory drive from ACs and respond robustly to hot stimuli, but their responses do not exclusively rely on ACs. Instead, LPNs receive independent drive from thermosensory neurons of the fly antenna via a new class of second-order projection neurons (TPN-IV). Finally, we show that silencing LPNs blocks the restructuring of daytime "siesta" sleep, which normally occurs in response to persistent heat. Our previous work described a distinct thermometer circuit for cold temperature.3 Together, the results demonstrate that the fly nervous system separately encodes and relays absolute hot and cold temperature information, show how patterns of sleep and activity can be adapted to specific temperature conditions, and illustrate how persistent drive from sensory pathways can impact behavior on extended temporal scales.

The Drosophila circadian clock circuit is a nonhierarchical network of peptidergic oscillators

The relatively simple Drosophila circadian clock circuit consists of 150 clock neurons that coordinate rhythmic behavior and physiology, which are generally classified based on neuroanatomical location. Transcriptional and connectomic studies have identified novel subdivisions of these clock neuron populations, and identified neuropeptides not previously known to be expressed in the fly clock circuit. An additional feature of fly clock neurons is daily axonal remodeling, first noted in small ventrolateral neurons, but more recently also found in additional clock neuron groups. These findings raise new questions about the functional roles of clock neuron subpopulations and daily remodeling of network architecture in regulating circadian behavior and physiology.

Endocrine cybernetics: neuropeptides as molecular switches in behavioural decisions

Plasticity in animal behaviour relies on the ability to integrate external and internal cues from the changing environment and hence modulate activity in synaptic circuits of the brain. This context-dependent neuromodulation is largely based on non-synaptic signalling with neuropeptides. Here, we describe select peptidergic systems in the Drosophila brain that act at different levels of a hierarchy to modulate behaviour and associated physiology. These systems modulate circuits in brain regions, such as the central complex and the mushroom bodies, which supervise specific behaviours. At the top level of the hierarchy there are small numbers of large peptidergic neurons that arborize widely in multiple areas of the brain to orchestrate or modulate global activity in a state and context-dependent manner. At the bottom level local peptidergic neurons provide executive neuromodulation of sensory gain and intrinsically in restricted parts of specific neuronal circuits. The orchestrating neurons receive interoceptive signals that mediate energy and sleep homeostasis, metabolic state and circadian timing, as well as external cues that affect food search, aggression or mating. Some of these cues can be triggers of conflicting behaviours such as mating versus aggression, or sleep versus feeding, and peptidergic neurons participate in circuits, enabling behaviour choices and switches.

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

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

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

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

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

Allatostatin A Signalling: Progress and New Challenges From a Paradigmatic Pleiotropic Invertebrate Neuropeptide Family

Neuropeptides have gained broad attraction in insect neuroscience and physiology, as new genetic tools are increasingly uncovering their wide-ranging pleiotropic functions with high cellular resolution. Allatostatin A (AstA) peptides constitute one of the best studied insect neuropeptide families. In insects and other panarthropods, AstA peptides qualify as brain-gut peptides and have regained attention with the discovery of their role in regulating feeding, growth, activity/sleep and learning. AstA receptor homologs are found throughout the protostomia and group with vertebrate somatostatin/galanin/kisspeptin receptors. In this review, we summarise the current knowledge on the evolution and the pleiotropic and cell-specific non-allatostatic functions of AstA. We speculate about the core functions of AstA signalling, and derive open questions and challengesfor future research on AstA and invertebrate neuropeptides in general.

The Neuronal Circuit of the Dorsal Circadian Clock Neurons in Drosophila melanogaster

Drosophila’s dorsal clock neurons (DNs) consist of four clusters (DN_1as, DN_1ps, DN₂s, and DN₃s) that largely differ in size. While the DN_1as and the DN₂s encompass only two neurons, the DN_1ps consist of ∼15 neurons, and the DN₃s comprise ∼40 neurons per brain hemisphere. In comparison to the well-characterized lateral clock neurons (LNs), the neuroanatomy and function of the DNs are still not clear. Over the past decade, numerous studies have addressed their role in the fly’s circadian system, leading to several sometimes divergent results. Nonetheless, these studies agreed that the DNs are important to fine-tune activity under light and temperature cycles and play essential roles in linking the output from the LNs to downstream neurons that control sleep and metabolism. Here, we used the Flybow system, specific split-GAL4 lines, trans-Tango, and the recently published fly connectome (called hemibrain) to describe the morphology of the DNs in greater detail, including their synaptic connections to other clock and non-clock neurons. We show that some DN groups are largely heterogenous. While certain DNs are strongly connected with the LNs, others are mainly output neurons that signal to circuits downstream of the clock. Among the latter are mushroom body neurons, central complex neurons, tubercle bulb neurons, neurosecretory cells in the pars intercerebralis, and other still unidentified partners. This heterogeneity of the DNs may explain some of the conflicting results previously found about their functionality. Most importantly, we identify two putative novel communication centers of the clock network: one fiber bundle in the superior lateral protocerebrum running toward the anterior optic tubercle and one fiber hub in the posterior lateral protocerebrum. Both are invaded by several DNs and LNs and might play an instrumental role in the clock network.