Neuropeptides PDF and DH31 hierarchically regulate free-running rhythmicity in Drosophila circadian locomotor activity

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.

Conditional chemoconnectomics (cCCTomics) as a strategy for efficient and conditional targeting of chemical transmission

Dissection of neural circuitry underlying behaviors is a central theme in neurobiology. We have previously proposed the concept of chemoconnectome (CCT) to cover the entire chemical transmission between neurons and target cells in an organism and created tools for studying it (CCTomics) by targeting all genes related to the CCT in Drosophila. Here we have created lines targeting the CCT in a conditional manner after modifying GFP RNA interference, Flp-out, and CRISPR/Cas9 technologies. All three strategies have been validated to be highly effective, with the best using chromatin-peptide fused Cas9 variants and scaffold optimized sgRNAs. As a proof of principle, we conducted a comprehensive intersection analysis of CCT genes expression profiles in the clock neurons, uncovering 43 CCT genes present in clock neurons. Specific elimination of each from clock neurons revealed that loss of the neuropeptide CNMa in two posterior dorsal clock neurons (DN1ps) or its receptor (CNMaR) caused advanced morning activity, indicating a suppressive role of CNMa-CNMaR on morning anticipation, opposite to the promoting role of PDF-PDFR on morning anticipation. These results demonstrate the effectiveness of conditional CCTomics and its tools created here and establish an antagonistic relationship between CNMa-CNMaR and PDF-PDFR signaling in regulating morning anticipation.

Transcriptional profiling of Drosophila male-specific P1 (pC1) neurons

In Drosophila melanogaster, the P1 (pC1) cluster of male-specific neurons both integrates sensory cues and drives or modulates behavioral programs such as courtship, in addition to contributing to a social arousal state. The behavioral function of these neurons is linked to the genes they express, which underpin their capacity for synaptic signaling, neuromodulation, and physiology. Yet, P1 (pC1) neurons have not been fully characterized at the transcriptome level. Moreover, it is unknown how the molecular landscape of P1 (pC1) neurons acutely changes after flies engage in social behaviors, where baseline P1 (pC1) neural activity is expected to increase. To address these two gaps, we use single cell-type RNA sequencing to profile and compare the transcriptomes of P1 (pC1) neurons harvested from socially paired versus solitary male flies. Compared to control transcriptome datasets, we find that P1 (pC1) neurons are enriched in 2,665 genes, including those encoding receptors, neuropeptides, and cell-adhesion molecules (dprs/DIPs). Furthermore, courtship is characterized by changes in ~300 genes, including those previously implicated in regulating behavior (e.g. DopEcR, Octβ3R, Fife, kairos, rad). Finally, we identify a suite of genes that link conspecific courtship with the innate immune system. Together, these data serve as a molecular map for future studies of an important set of higher-order and sexually-dimorphic neurons.

Hormonal gatekeeping via the blood-brain barrier governs caste-specific behavior in ants

Here, we reveal an unanticipated role of the blood-brain barrier (BBB) in regulating complex social behavior in ants. Using scRNA-seq, we find localization in the BBB of a key hormone-degrading enzyme called juvenile hormone esterase (Jhe), and we show that this localization governs the level of juvenile hormone (JH3) entering the brain. Manipulation of the Jhe level reprograms the brain transcriptome between ant castes. Although ant Jhe is retained and functions intracellularly within the BBB, we show that Drosophila Jhe is naturally extracellular. Heterologous expression of ant Jhe into the Drosophila BBB alters behavior in fly to mimic what is seen in ants. Most strikingly, manipulation of Jhe levels in ants reprograms complex behavior between worker castes. Our study thus uncovers a remarkable, potentially conserved role of the BBB serving as a molecular gatekeeper for a neurohormonal pathway that regulates social behavior.

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].

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.

Neuropeptide diuretic hormone 31 mediates memory and sleep via distinct neural pathways in Drosophila

Memory formation and sleep regulation are critical for brain functions in animals from invertebrates to humans. Neuropeptides play a pivotal role in regulating physiological behaviors, including memory formation and sleep. However, the detailed mechanisms by which neuropeptides regulate these physiological behaviors remains unclear. Herein, we report that neuropeptide diuretic hormone 31 (DH31) positively regulates memory formation and sleep in Drosophila melanogaster. The expression of DH31 in the dorsal and ventral fan-shaped body (dFB and vFB) neurons of the central complex and ventral lateral clock neurons (LNvs) in the brain was responsive to sleep regulation. In addition, the expression of membrane-tethered DH31 in dFB neurons rescued sleep defects in Dh31 mutants, suggesting that DH31 secreted from dFB, vFB, and LNvs acts on the DH31 receptor in the dFB to regulate sleep partly in an autoregulatory feedback loop. Moreover, the expression of DH31 in octopaminergic neurons, but not in the dFB neurons, is involved in forming intermediate-term memory. Our results suggest that DH31 regulates memory formation and sleep through distinct neural pathways.

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.

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.

Influence of an L-type SALMFamide neuropeptide on locomotory performance and muscle physiology in the sea cucumber Apostichopus japonicus

Neuropeptides in the SALMFamide family serve as muscle relaxants in echinoderms and may affect locomotion, as the motor behavior in sea cucumbers involves alternating contraction and extension of the body wall, which is under the control of longitudinal muscle. We evaluated the effect of an L-type SALMFamide neuropeptide (LSA) on locomotory performance of Apostichopus japonicus. We also investigated the metabolites of longitudinal muscle tissue using ultra performance liquid chromatography and quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS) to assess the potential physiological mechanisms underlying the effect of LSA. The hourly distance, cumulative duration and number of steps moved significantly increased in sea cucumbers in the fourth hour after injection with LSA. Also, the treatment enhanced the mean and maximum velocity by 9.8% and 17.8%, respectively, and increased the average stride by 12.4%. Levels of 27 metabolites in longitudinal muscle changed after LSA administration, and the increased concentration of pantothenic acid, arachidonic acid and lysophosphatidylethanolamine, and the altered phosphatidylethanolamine/phosphatidylcholine ratio are potential physiological mechanisms that could explain the observed effect of LSA on locomotor behavior in A. japonicus.

Drosophila clock cells use multiple mechanisms to transmit time-of-day signals in the brain

Regulation of circadian behavior and physiology by the Drosophila brain clock requires communication from central clock neurons to downstream output regions, but the mechanism by which clock cells regulate downstream targets is not known. We show here that the pars intercerebralis (PI), previously identified as a target of the morning cells in the clock network, also receives input from evening cells. We determined that morning and evening clock neurons have time-of-day-dependent connectivity to the PI, which is regulated by specific peptides as well as by fast neurotransmitters. Interestingly, PI cells that secrete the peptide DH44, and control rest:activity rhythms, are inhibited by clock inputs while insulin-producing cells (IPCs) are activated, indicating that the same clock cells can use different mechanisms to drive cycling in output neurons. Inputs of morning cells to IPCs are relevant for the circadian rhythm of feeding, reinforcing the role of the PI as a circadian relay that controls multiple behavioral outputs. Our findings provide mechanisms by which clock neurons signal to nonclock cells to drive rhythms of behavior.

Integration of Circadian Clock Information in the Drosophila Circadian Neuronal Network

Circadian clocks are biochemical time-keeping machines that synchronize animal behavior and physiology with planetary rhythms. In Drosophila, the core components of the clock comprise a transcription/translation feedback loop and are expressed in seven neuronal clusters in the brain. Although it is increasingly evident that the clocks in each of the neuronal clusters are regulated differently, how these clocks communicate with each other across the circadian neuronal network is less clear. Here, we review the latest evidence that describes the physical connectivity of the circadian neuronal network . Using small ventral lateral neurons as a starting point, we summarize how one clock may communicate with another, highlighting the signaling pathways that are both upstream and downstream of these clocks. We propose that additional efforts are required to understand how temporal information generated in each circadian neuron is integrated across a neuronal circuit to regulate rhythmic behavior.

Pigment Dispersing Factors and Their Cognate Receptors in a Crustacean Model, With New Insights Into Distinct Neurons and Their Functions

Pigment dispersing factors (PDFs, or PDHs in crustaceans) form a structurally related group of neuropeptides found throughout the Ecdysozoa and were first discovered as pigmentary effector hormones in crustaceans. In insects PDFs fulfill crucial neuromodulatory roles, most notably as output regulators of the circadian system, underscoring their central position in physiological and behavioral organization of arthropods. Intriguingly, decapod crustaceans express multiple isoforms of PDH originating from separate genes, yet their differential functions are still to be determined. Here, we functionally define two PDH receptors in the crab Carcinus maenas and show them to be selectively activated by four PDH isoforms: PDHR 43673 was activated by PDH-1 and PDH-2 at low nanomolar doses whilst PDHR 41189 was activated by PDH-3 and an extended 20 residue e-PDH. Detailed examination of the anatomical distribution of all four peptides and their cognate receptors indicate that they likely perform different functions as secreted hormones and/or neuromodulators, with PDH-1 and its receptor 43,673 implicated in an authentic hormonal axis. PDH-2, PDH-3, and e-PDH were limited to non-neurohemal interneuronal sites in the CNS; PDHR 41189 was largely restricted to the nervous system suggesting a neuromodulatory function. Notably PDH-3 and e-PDH were without chromatophore dispersing activity. This is the first report which functionally defines a PDHR in an endocrine system in a crustacean and to indicate this and other putative roles of this physiologically pivotal peptide group in these organisms. Thus, our findings present opportunities to further examine the endocrine and circadian machinery in this important arthropod phylum.

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.