Drosophila neuroligin 4 regulates sleep through modulating GABA transmission

Fly into tranquility: GABA's role in Drosophila sleep

Sleep is conserved across the animal kingdom, and Drosophila melanogaster is a prime model to understand its intricate circadian and homeostatic control. GABA (gamma-aminobutyric acid), the brain's main inhibitory neurotransmitter, plays a central role in sleep. This review delves into GABA's complex mechanisms of actions within Drosophila's sleep-regulating neural networks. We discuss how GABA promotes sleep, both by inhibiting circadian arousal neurons and by being a key neurotransmitter in sleep homeostatic circuits. GABA's impact on sleep is modulated by glia through astrocytic GABA recapture and metabolism. Interestingly, GABA can be coexpressed with other neurotransmitters in sleep-regulating neurons, which likely contributes to context-based sleep plasticity.

Genome-wide association in Drosophila identifies a role for Piezo and Proc-R in sleep latency

Sleep latency, the amount of time that it takes an individual to fall asleep, is a key indicator of sleep need. Sleep latency varies considerably both among and within species and is heritable, but lacks a comprehensive description of its underlying genetic network. Here we conduct a genome-wide association study of sleep latency. Using previously collected sleep and activity data on a wild-derived population of flies, we calculate sleep latency, confirming significant, heritable genetic variation for this complex trait. We identify 520 polymorphisms in 248 genes contributing to variability in sleep latency. Tests of mutations in 23 candidate genes and additional putative pan-neuronal knockdown of 9 of them implicated CG44153, Piezo, Proc-R and Rbp6 in sleep latency. Two large-effect mutations in the genes Proc-R and Piezo were further confirmed via genetic rescue. This work greatly enhances our understanding of the genetic factors that influence variation in sleep latency.

Neuroligin 2 governs synaptic morphology and function through RACK1-cofilin signaling in Drosophila

Neuroligins are transmembrane cell adhesion proteins well-known for their genetic links to autism spectrum disorders. Neuroligins can function by regulating the actin cytoskeleton, however the factors and mechanisms involved are still largely unknown. Here, using the Drosophila neuromuscular junction as a model, we reveal that F-Actin assembly at the Drosophila NMJ is controlled through Cofilin signaling mediated by an interaction between DNlg2 and RACK1, factors not previously known to work together. The deletion of DNlg2 displays disrupted RACK1-Cofilin signaling pathway with diminished actin cytoskeleton proteo-stasis at the terminal of the NMJ, aberrant NMJ structure, reduced synaptic transmission, and abnormal locomotion at the third-instar larval stage. Overexpression of wildtype and activated Cofilin in muscles are sufficient to rescue the morphological and physiological defects in dnlg2 mutants, while inactivated Cofilin is not. Since the DNlg2 paralog DNlg1 is known to regulate F-actin assembly mainly via a specific interaction with WAVE complex, our present work suggests that the orchestration of F-actin by Neuroligins is a diverse and complex process critical for neural connectivity.

Genetic atlas of hygro-and thermosensory cells in the vinegar fly Drosophila melanogaster

The ability of animals to perceive and respond to sensory information is essential for their survival in diverse environments. While much progress has been made in understanding various sensory modalities, the sense of hygrosensation, which involves the detection and response to humidity, remains poorly understood. In this study, we focused on the hygrosensory, and closely related thermosensory, systems in the vinegar fly Drosophila melanogaster to unravel the molecular profile of the cells of these senses. Using a transcriptomic analysis of over 37,000 nuclei, we identified twelve distinct clusters of cells corresponding to temperature-sensing arista neurons, humidity-sensing sacculus neurons, and support cells relating to these neurons. By examining the expression of known and novel marker genes, we validated the identity of these clusters and characterized their gene expression profiles. We found that each cell type could be characterized by a unique expression profile of ion channels, GPCR signaling molecules, synaptic vesicle cycle proteins, and cell adhesion molecules. Our findings provide valuable insights into the molecular basis of hygro- and thermosensation. Understanding the mechanisms underlying hygro- and thermosensation may shed light on the broader understanding of sensory systems and their adaptation to different environmental conditions in animals.

Dissecting neuron-specific functions of circadian genes using modified cell-specific CRISPR approaches

Circadian behavioral rhythms in Drosophila melanogaster are regulated by about 75 pairs of brain neurons. They all express the core clock genes but have distinct functions and gene expression profiles. To understand the importance of these distinct molecular programs, neuron-specific gene manipulations are essential. Although RNAi based methods are standard to manipulate gene expression in a cell-specific manner, they are often ineffective, especially in assays involving smaller numbers of neurons or weaker Gal4 drivers. We and others recently exploited a neuron-specific CRISPR-based method to mutagenize genes within circadian neurons. Here, we further explore this approach to mutagenize three well-studied clock genes: the transcription factor gene vrille, the photoreceptor gene Cryptochrome (cry), and the neuropeptide gene Pdf (pigment dispersing factor). The CRISPR-based strategy not only reproduced their known phenotypes but also assigned cry function for different light-mediated phenotypes to discrete, different subsets of clock neurons. We further tested two recently published methods for temporal regulation in adult neurons, inducible Cas9 and the auxin-inducible gene expression system. The results were not identical, but both approaches successfully showed that the adult-specific knockout of the neuropeptide Pdf reproduces the canonical loss-of-function mutant phenotypes. In summary, a CRISPR-based strategy is a highly effective, reliable, and general method to temporally manipulate gene function in specific adult neurons.

Nutritional Composition of Plant Protein Beverages on China's Online Market: A Cross-Sectional Analysis

Plant protein beverages are gaining popularity due to various reasons such as lactose intolerance, veganism and health claims. This study aimed to conduct a cross-sectional analysis of plant protein beverages sold online in China, with a focus on assessing their nutritional composition. A total of 251 kinds of plant protein beverages were analyzed, including coconut (n = 58), soy (n = 52), oats (n = 49), walnut (n = 14), almond (n = 11), peanut (n = 5), rice (n = 4), other beans (n = 5), mixed nuts (n = 5) and mixed beverages (n = 48), according to the nutrition label on the commercial package and retailer websites. The results showed that, except for soy beverages, plant protein beverages generally had low protein content, cereal beverages showed relatively high energy and carbohydrate levels, and all plant protein beverages had low sodium content. Additionally, the fortification rate of vitamins and minerals in the analyzed plant protein beverages was found to be extremely low, at only 13.1%. Given the substantial variation in the nutritional composition of plant protein beverages, consumers should pay more attention to the nutrition facts and ingredient information when choosing these beverages.

A perspective on molecular signalling dysfunction, its clinical relevance and therapeutics in autism spectrum disorder

Intellectual disability (ID) and autism spectrum disorder (ASD) are neurodevelopmental disorders that have become a primary clinical and social concern, with a prevalence of 2-3% in the population. Neuronal function and behaviour undergo significant malleability during the critical period of development that is found to be impaired in ID/ASD. Human genome sequencing studies have revealed many genetic variations associated with ASD/ID that are further verified by many approaches, including many mouse and other models. These models have facilitated the identification of fundamental mechanisms underlying the pathogenesis of ASD/ID, and several studies have proposed converging molecular pathways in ASD/ID. However, linking the mechanisms of the pathogenic genes and their molecular characteristics that lead to ID/ASD has progressed slowly, hampering the development of potential therapeutic strategies. This review discusses the possibility of recognising the common molecular causes for most ASD/ID based on studies from the available models that may enable a better therapeutic strategy to treat ID/ASD. We also reviewed the potential biomarkers to detect ASD/ID at early stages that may aid in diagnosis and initiating medical treatment, the concerns with drug failure in clinical trials, and developing therapeutic strategies that can be applied beyond a particular mutation associated with ASD/ID.

Astrocytic GABA transporter controls sleep by modulating GABAergic signaling in Drosophila circadian neurons

A precise balance between sleep and wakefulness is essential to sustain a good quality of life and optimal brain function. GABA is known to play a key and conserved role in sleep control, and GABAergic tone should, therefore, be tightly controlled in sleep circuits. Here, we examined the role of the astrocytic GABA transporter (GAT) in sleep regulation using Drosophila melanogaster. We found that a hypomorphic gat mutation (gat33-1) increased sleep amount, decreased sleep latency, and increased sleep consolidation at night. Interestingly, sleep defects were suppressed when gat33-1 was combined with a mutation disrupting wide-awake (wake), a gene that regulates the cell-surface levels of the GABAA receptor resistance to dieldrin (RDL) in the wake-promoting large ventral lateral neurons (l-LNvs). Moreover, RNAi knockdown of rdl and its modulators dnlg4 and wake in these circadian neurons also suppressed gat33-1 sleep phenotypes. Brain immunohistochemistry showed that GAT-expressing astrocytes were located near RDL-positive l-LNv cell bodies and dendritic processes. We concluded that astrocytic GAT decreases GABAergic tone and RDL activation in arousal-promoting LNvs, thus determining proper sleep amount and quality in Drosophila.

Synaptic dysfunction connects autism spectrum disorder and sleep disturbances: A perspective from studies in model organisms

Sleep disturbances (SD) accompany many neurodevelopmental disorders, suggesting SD is a transdiagnostic process that can account for behavioral deficits and influence underlying neuropathogenesis. Autism Spectrum Disorder (ASD) comprises a complex set of neurodevelopmental conditions characterized by challenges in social interaction, communication, and restricted, repetitive behaviors. Diagnosis of ASD is based primarily on behavioral criteria, and there are no drugs that target core symptoms. Among the co-occurring conditions associated with ASD, SD are one of the most prevalent. SD often arises before the onset of other ASD symptoms. Sleep interventions improve not only sleep but also daytime behaviors in children with ASD. Here, we examine sleep phenotypes in multiple model systems relevant to ASD, e.g., mice, zebrafish, fruit flies and worms. Given the functions of sleep in promoting brain connectivity, neural plasticity, emotional regulation and social behavior, all of which are of critical importance in ASD pathogenesis, we propose that synaptic dysfunction is a major mechanism that connects ASD and SD. Common molecular targets in this interplay that are involved in synaptic function might be a novel avenue for therapy of individuals with ASD experiencing SD. Such therapy would be expected to improve not only sleep but also other ASD symptoms.

Bisphenol a affects neurodevelopmental gene expression, cognitive function, and neuromuscular synaptic morphology in Drosophila melanogaster

Bisphenol A (BPA) is an environmentally prevalent endocrine disrupting chemical that can impact human health and may be an environmental risk factor for neurodevelopmental disorders. BPA has been associated with behavioral impairment in children and a variety of neurodevelopmental phenotypes in model organisms. We used Drosophila melanogaster to explore the consequences of developmental BPA exposure on gene expression, cognitive function, and synapse development. Our transcriptome analysis indicated neurodevelopmentally relevant genes were predominantly downregulated by BPA. Among the misregulated genes were those with roles in learning, memory, and synapse development, as well as orthologs of human genes associated with neurodevelopmental and neuropsychiatric disorders. To examine how gene expression data corresponded to behavioral and cellular phenotypes, we first used a predator-response behavioral paradigm and found that BPA disrupts visual perception. Further analysis using conditioned courtship suppression showed that BPA impairs associative learning. Finally, we examined synapse morphology within the larval neuromuscular junction and found that BPA significantly increased the number of axonal branches. Given that our findings align with studies of BPA in mammalian model organisms, this data indicates that BPA impairs neurodevelopmental pathways that are functionally conserved from invertebrates to mammals. Further, because Drosophila do not possess classic estrogen receptors or estrogen, this research suggests that BPA can impact neurodevelopment by molecular mechanisms distinct from its role as an estrogen mimic.

Neuroligin 3 from common cutworm enhances the GABA-induced current of recombinant SlRDL1 channel

BACKGROUND

Neuroligin (NLG) protein is a nerve cell adhesion molecule and plays a key role in the precision apposition of presynaptic domains on inhibitory and excitatory synapses. Existing studies mainly focused on the function of NLG3 against the excitatory channel. However, the interaction between insect NLG3 and ionotropic GABA receptor, which is the main inhibitory channel, remains unclear. In this study, the Nlg3 of common cutworm (CCW), Spodoptera litura Fabricius, one important agricultural Lepidopteron, is selected to explore its function in the inhibitory channel.

RESULTS

The SlNlg3 was obtained and the SlNLG3 contains the characteristic features including transmembrane domain, PDZ-binding motif and type-B carboxylesterases signature 2 motif. The SlNlg3 messenger RNA (mRNA) was most abundant in midgut, and exhibited multiple expression patterns in different developmental stages and tissues or body parts. Compared with the single injection of SlRDL1, the median effective concentration value of GABA in activating currents was smaller in Xenopus laevis oocytes co-injected with SlRDL1 and SlNlg3. In addition, SlNlg3 could enhance the GABA-induced current of homomeric SlRDL1 channel from -391.86 ± 15.41 to -2152.51 ± 30.09 nA. DsSlNlg3 depressed the expression level of SlNlg3 mRNA more than 64.29% at 6 h. After exposure to median lethal dose of fluralaner, the mortality of CCW injected with dsSlNlg3 was significantly decreased by 13.34% and 30.00% at 24 and 48 h, respectively, compared to injection of dsEGFP.

CONCLUSION

NLG3 should have physiological function on ionotropic GABA receptor in vitro, which provided a favorable foundation for further research on the physiological function of Nlg gene in Lepidopteron. © 2021 Society of Chemical Industry.

Rare NRXN1 missense variants identified in autism interfered protein degradation and Drosophila sleeping

NRXN1 is involved in synaptogenesis and have been implicated in Autism spectrum disorders. However, many rare inherited missense variants of NRXN1 have not been thoroughly evaluated. Here, functional analyses in vitro and in Drosophila of three NRXN1 missense mutations, Y282H, L893V, and I1135V identified in ASD patients in our previous study were performed. Our results showed these three mutations interfered protein degradation compared with NRXN1-WT protein. Expressing human NRXN1 in Drosophila could lead to abnormal circadian rhythm and sleep behavior, and three mutated proteins caused milder phenotypes, indicating the mutations may change the function of NRXN1 slightly. These findings highlight the functional role of rare NRXN1 missense variants identified in autism patients, and provide clues for us to better understand the pathogenesis of abnormal circadian rhythm and sleep behavior of other organisms, including humans.

A subset of DN1p neurons integrates thermosensory inputs to promote wakefulness via CNMa signaling

Sleep is an essential and evolutionarily conserved behavior that is modulated by many environmental factors. Ambient temperature shifting usually occurs during climatic or seasonal change or travel from high-latitude area to low-latitude area that affects animal physiology. Increasing ambient temperature modulates sleep in both humans and Drosophila. Although several thermosensory molecules and neurons have been identified, the neural mechanisms that integrate temperature sensation into the sleep neural circuit remain poorly understood. Here, we reveal that prolonged increasing of ambient temperature induces a reversible sleep reduction and impaired sleep consolidation in Drosophila via activating the internal thermosensory anterior cells (ACs). ACs form synaptic contacts with a subset of posterior dorsal neuron 1 (DN1p) neurons and release acetylcholine to promote wakefulness. Furthermore, we identify that this subset of DN1ps promotes wakefulness by releasing CNMamide (CNMa) neuropeptides to inhibit the Dh44-positive pars intercerebralis (PI) neurons through CNMa receptors. Our study demonstrates that the AC-DN1p-PI neural circuit is responsible for integrating thermosensory inputs into the sleep neural circuit. Moreover, we identify the CNMa signaling pathway as a newly recognized wakefulness-promoting DN1 pathway.

Functional mosaic organization of neuroligins in neuronal circuits

Complex brain circuitry with feedforward and feedback systems regulates neuronal activity, enabling neural networks to process and drive the entire spectrum of cognitive, behavioral, sensory, and motor functions. Simultaneous orchestration of distinct cells and interconnected neural circuits is underpinned by hundreds of synaptic adhesion molecules that span synaptic junctions. Dysfunction of a single molecule or molecular interaction at synapses can lead to disrupted circuit activity and brain disorders. Neuroligins, a family of cell adhesion molecules, were first identified as postsynaptic-binding partners of presynaptic neurexins and are essential for synapse specification and maturation. Here, we review recent advances in our understanding of how this family of adhesion molecules controls neuronal circuit assembly by acting in a synapse-specific manner.

Abnormal Social Interactions in a Drosophila Mutant of an Autism Candidate Gene: Neuroligin 3

Social interactions are typically impaired in neuropsychiatric disorders such as autism, for which the genetic underpinnings are very complex. Social interactions can be modeled by analysis of behaviors, including social spacing, sociability, and aggression, in simpler organisms such as Drosophila melanogaster. Here, we examined the effects of mutants of the autism-related gene neuroligin 3 (nlg3) on fly social and non-social behaviors. Startled-induced negative geotaxis is affected by a loss of function nlg3 mutation. Social space and aggression are also altered in a sex- and social-experience-specific manner in nlg3 mutant flies. In light of the conserved roles that neuroligins play in social behavior, our results offer insight into the regulation of social behavior in other organisms, including humans.

Neurexin and Neuroligins Maintain the Balance of Ghost and Satellite Boutons at the Drosophila Neuromuscular Junction

Neurexins and neuroligins are common synaptic adhesion molecules that are associated with autism and interact with each other in the synaptic cleft. The Drosophila neuromuscular junction (NMJ) bouton is a well-known model system in neuroscience, and ghost and satellite boutons, respectively, indicate the poor development and overgrowth of the NMJ boutons. However, the Drosophila neurexin (DNrx) and Drosophila neuroligins (DNlgs) are mainly observed in type Ib boutons, indicating the ultrastructural and developmental phenotypes of the Drosophila NMJ. Here, we identified the ultrastructural and developmental features of ghost and satellite boutons by utilizing dneurexin (dnrx) and dneuroligins (dnlgs) fly mutants and other associated fly strains. Ghost boutons contain synaptic vesicles with multiple diameters but very rarely contain T-bar structures and swollen or thin subsynaptic reticulum (SSR) membranes. The muscle cell membrane is invaginated at different sites, stretches to the ghost bouton from different directions, forms several layers that enwrap the ghost bouton, and then branches into the complex SSR. Satellite boutons share a common SSR membrane and present either a typical profile in which a main bouton is encircled by small boutons or two atypical profiles in which the small boutons are grouped together or distributed in beads without a main bouton. Electron and confocal microscopy data showed that dnrx, dnlg1, dnlg2, dnlg3, and dnlg4 mutations led to ghost boutons; the overexpression of dnrx, dnlg1, dnlg2, dnlg3, and dnlg4 led to satellite boutons; and the dnlg2;dnlg3 double mutation also led to satellite boutons. These results suggested that DNrx and DNlgs jointly maintain the development and function of NMJ boutons by regulating the balance of ghost and satellite boutons in Drosophila.

Sleep as a translationally-relevant endpoint in studies of autism spectrum disorder (ASD)

Sleep has numerous advantages for aligning clinical and preclinical (basic neuroscience) studies of neuropsychiatric illness. Sleep has high translational relevance, because the same endpoints can be studied in humans and laboratory animals. In addition, sleep experiments are conducive to continuous data collection over long periods (hours/days/weeks) and can be based on highly objective neurophysiological measures. Here, we provide a translationally-oriented review on what is currently known about sleep in the context of autism spectrum disorder (ASD), including ASD-related conditions, thought to have genetic, environmental, or mixed etiologies. In humans, ASD is frequently associated with comorbid medical conditions including sleep disorders. Animal models used in the study of ASD frequently recapitulate dysregulation of sleep and biological (diurnal, circadian) rhythms, suggesting common pathophysiologies across species. As our understanding of the neurobiology of ASD and sleep each become more refined, it is conceivable that sleep-derived metrics may offer more powerful biomarkers of altered neurophysiology in ASD than the behavioral tests currently used in humans or lab animals. As such, the study of sleep in animal models for ASD may enable fundamentally new insights on the condition and represent a basis for strategies that enable the development of more effective therapeutics.

Differences in gene regulation in a tephritid model of prezygotic reproductive isolation

The two tephritid fruit fly pests, Bactrocera tryoni and Bactrocera neohumeralis, are unusually well suited to the study of the genetics of reproductive isolating mechanisms. Sequence difference between the species is no greater than between a pair of conspecific Drosophila melanogaster populations. The two species exist in close sympatry, yet do not hybridize in the field, apparently kept separate by a strong premating isolation mechanism involving the time of day at which mating occurs. This spurred us to search for key genes for which time of day expression is regulated differently between the species. Using replicated, quantitative transcriptomes from head tissues of males of the two species, sampled in the day and night, we identified 141 transcripts whose abundance showed a significant interaction between species and time of day, indicating a difference in gene regulation. The brain transcripts showing this interaction were enriched for genes with a neurone function and 90% of these were more abundant at night than day in B. tryoni. Features of the expression patterns suggest that there may be a difference in the regulation of sleep-wake cycles between the species. In particular several genes, which in D. melanogaster are expressed in circadian pacemaker cells, are promising candidates to further explore the genetic differentiation involved in this prezygotic reproductive isolation mechanism.

Intellectual disability and autism spectrum disorders 'on the fly': insights from Drosophila

Intellectual disability (ID) and autism spectrum disorders (ASD) are frequently co-occurring neurodevelopmental disorders and affect 2-3% of the population. Rapid advances in exome and genome sequencing have increased the number of known implicated genes by threefold, to more than a thousand. The main challenges in the field are now to understand the various pathomechanisms associated with this bewildering number of genetic disorders, to identify new genes and to establish causality of variants in still-undiagnosed cases, and to work towards causal treatment options that so far are available only for a few metabolic conditions. To meet these challenges, the research community needs highly efficient model systems. With an increasing number of relevant assays and rapidly developing novel methodologies, the fruit fly Drosophila melanogaster is ideally positioned to change gear in ID and ASD research. The aim of this Review is to summarize some of the exciting work that already has drawn attention to Drosophila as a model for these disorders. We highlight well-established ID- and ASD-relevant fly phenotypes at the (sub)cellular, brain and behavioral levels, and discuss strategies of how this extraordinarily efficient and versatile model can contribute to 'next generation' medical genomics and to a better understanding of these disorders.

Fruit fly research in China

Served as a model organism over a century, fruit fly has significantly pushed forward the development of global scientific research, including in China. The high similarity in genomic features between fruit fly and human enables this tiny insect to benefit the biomedical studies of human diseases. In the past decades, Chinese biologists have used fruit fly to make numerous achievements on understanding the fundamental questions in many diverse areas of biology. Here, we review some of the recent fruit fly studies in China, and mainly focus on those studies in the fields of stem cell biology, cancer therapy and regeneration medicine, neurological disorders and epigenetics.

Scale-Free Dynamics of the Mouse Wakefulness and Sleep Electroencephalogram Quantified Using Wavelet-Leaders

Scale-free analysis of brain activity reveals a complexity of synchronous neuronal firing which is different from that assessed using classic rhythmic quantifications such as spectral analysis of the electroencephalogram (EEG). In humans, scale-free activity of the EEG depends on the behavioral state and reflects cognitive processes. We aimed to verify if fractal patterns of the mouse EEG also show variations with behavioral states and topography, and to identify molecular determinants of brain scale-free activity using the ‘multifractal formalism’ (Wavelet-Leaders). We found that scale-free activity was more anti-persistent (i.e., more different between time scales) during wakefulness, less anti-persistent (i.e., less different between time scales) during non-rapid eye movement sleep, and generally intermediate during rapid eye movement sleep. The scale-invariance of the frontal/motor cerebral cortex was generally more anti-persistent than that of the posterior cortex, and scale-invariance during wakefulness was strongly modulated by time of day and the absence of the synaptic protein Neuroligin-1. Our results expose that the complexity of the scale-free pattern of organized neuronal firing depends on behavioral state in mice, and that patterns expressed during wakefulness are modulated by one synaptic component.

Circadian rhythm and sleep-wake systems share the dynamic extracellular synaptic milieu

The mammalian circadian and sleep-wake systems are closely aligned through their coordinated regulation of daily activity patterns. Although they differ in their anatomical organization and physiological processes, they utilize overlapping regulatory mechanisms that include an assortment of proteins and molecules interacting within the extracellular space. These extracellular factors include proteases that interact with soluble proteins, membrane-attached receptors and the extracellular matrix; and cell adhesion molecules that can form complex scaffolds connecting adjacent neurons, astrocytes and their respective intracellular cytoskeletal elements. Astrocytes also participate in the dynamic regulation of both systems through modulating neuronal appositions, the extracellular space and/or through release of gliotransmitters that can further contribute to the extracellular signaling processes. Together, these extracellular elements create a system that integrates rapid neurotransmitter signaling across longer time scales and thereby adjust neuronal signaling to reflect the daily fluctuations fundamental to both systems. Here we review what is known about these extracellular processes, focusing specifically on areas of overlap between the two systems. We also highlight questions that still need to be addressed. Although we know many of the extracellular players, far more research is needed to understand the mechanisms through which they modulate the circadian and sleep-wake systems.

Proteolytic cleavage is required for functional neuroligin 2 maturation and trafficking in Drosophila

Neuroligins (Nlgs) are transmembrane cell adhesion molecules playing essential roles in synapse development and function. Genetic mutations in neuroligin genes have been linked with some neurodevelopmental disorders such as autism. These mutated Nlgs are mostly retained in the endoplasmic reticulum (ER). However, the mechanisms underlying normal Nlg maturation and trafficking have remained largely unknown. Here, we found that Drosophila neuroligin 2 (DNlg2) undergoes proteolytic cleavage in the ER in a variety of Drosophila tissues throughout developmental stages. A region encompassing Y642-T698 is required for this process. The immature non-cleavable DNlg2 is retained in the ER and non-functional. The C-terminal fragment of DNlg2 instead of the full-length or non-cleavable DNlg2 is able to rescue neuromuscular junction defects and GluRIIB reduction induced by dnlg2 deletion. Intriguingly, the autism-associated R598C mutation in DNlg2 leads to similar marked defects in DNlg2 proteolytic process and ER export, revealing a potential role of the improper Nlg cleavage in autism pathogenesis. Collectively, our findings uncover a specific mechanism that controls DNlg2 maturation and trafficking via proteolytic cleavage in the ER, suggesting that the perturbed proteolytic cleavage of Nlgs likely contributes to autism disorder.

The neurobiological basis of sleep: Insights from Drosophila

Sleep is a biological enigma that has raised numerous questions about the inner workings of the brain. The fundamental question of why our nervous systems have evolved to require sleep remains a topic of ongoing scientific deliberation. This question is largely being addressed by research using animal models of sleep. Drosophila melanogaster, also known as the common fruit fly, exhibits a sleep state that shares common features with many other species. Drosophila sleep studies have unearthed an immense wealth of knowledge about the neuroscience of sleep. Given the breadth of findings published on Drosophila sleep, it is important to consider how all of this information might come together to generate a more holistic understanding of sleep. This review provides a comprehensive summary of the neurobiology of Drosophila sleep and explores the broader insights and implications of how sleep is regulated across species and why it is necessary for the brain.

Sleep/Wake Physiology and Quantitative Electroencephalogram Analysis of the Neuroligin-3 Knockout Rat Model of Autism Spectrum Disorder

Study Objectives

Neuroligin-3 (NLGN3) is one of the many genes associated with autism spectrum disorder (ASD). Sleep dysfunction is highly prevalent in ASD, but has not been rigorously examined in ASD models. Here, we evaluated sleep/wake physiology and behavioral phenotypes of rats with genetic ablation of Nlgn3.

Methods

Male Nlgn3 knockout (KO) and wild-type (WT) rats were assessed using a test battery for ASD-related behaviors and also implanted with telemeters to record the electroencephalogram (EEG), electromyogram, body temperature, and locomotor activity. 24-h EEG recordings were analyzed for sleep/wake states and spectral composition.

Results

Nlgn3 KO rats were hyperactive, exhibited excessive chewing behavior, and had impaired prepulse inhibition to an auditory startle stimulus. KO rats also spent less time in non-rapid eye movement (NREM) sleep, more time in rapid eye movement (REM) sleep, exhibited elevated theta power (4-9 Hz) during wakefulness and REM, and elevated delta power (0.5-4 Hz) during NREM. Beta (12-30 Hz) power and gamma (30-50 Hz) power were suppressed across all vigilance states.

Conclusions

The sleep disruptions in Nlgn3 KO rats are consistent with observations of sleep disturbances in ASD patients. The EEG provides objective measures of brain function to complement rodent behavioral analyses and therefore may be a useful tool to study ASD.

Unraveling the Evolutionary Determinants of Sleep

Despite decades of intense study, the functions of sleep are still shrouded in mystery. The difficulty in understanding these functions can be at least partly attributed to the varied manifestations of sleep in different animals. Daily sleep duration can range from 4-20 hrs among mammals, and sleep can manifest throughout the brain, or it can alternate over time between cerebral hemispheres, depending on the species. Ecological factors are likely to have shaped these and other sleep behaviors during evolution by altering the properties of conserved arousal circuits in the brain. Nonetheless, core functions of sleep are likely to have arisen early and to have persisted to the present day in diverse organisms. This review will discuss the evolutionary forces that may be responsible for phylogenetic differences in sleep and the potential core functions that sleep fulfills.

Fbxl4 Serves as a Clock Output Molecule that Regulates Sleep through Promotion of Rhythmic Degradation of the GABAA Receptor

The timing of sleep is tightly governed by the circadian clock, which contains a negative transcriptional feedback loop and synchronizes the physiology and behavior of most animals to daily environmental oscillations. However, how the circadian clock determines the timing of sleep is largely unclear. In vertebrates and invertebrates, the status of sleep and wakefulness is modulated by the electrical activity of pacemaker neurons that are circadian regulated and suppressed by inhibitory GABAergic inputs. Here, we showed that Drosophila GABA_A receptors undergo rhythmic degradation in arousal-promoting large ventral lateral neurons (lLNvs) and their expression level in lLNvs displays a daily oscillation. We also demonstrated that the E3 ligase Fbxl4 promotes GABA_A receptor ubiquitination and degradation and revealed that the transcription of fbxl4 in lLNvs is CLOCK dependent. Finally, we demonstrated that Fbxl4 regulates the timing of sleep through rhythmically reducing GABA sensitivity to modulate the excitability of lLNvs. Our study uncovered a critical molecular linkage between the circadian clock and the electrical activity of pacemaker neurons and demonstrated that CLOCK-dependent Fbxl4 expression rhythmically downregulates GABA_A receptor level to increase the activity of pacemaker neurons and promote wakefulness.

Proteolytic maturation of Drosophila Neuroligin 3 by tumor necrosis factor α-converting enzyme in the nervous system

BACKGROUND

The functions of autism-associated Neuroligins (Nlgs) are modulated by their post-translational modifications, such as proteolytic cleavage. A previous study has shown that there are different endogenous forms of DNlg3 in Drosophila, indicating it may undergo proteolytic processing. However, the molecular mechanism underlying DNlg3 proteolytic processing is unknown. Here, we report a novel proteolytic mechanism that is essential for DNlg3 maturation and function in the nervous system.

METHODS

Molecular cloning, cell culture, immunohistochemistry, western blotting and genetic studies were employed to map the DNlg3 cleavage region, identify the protease and characterize the cleavage manner. Behavior analysis, immunohistochemistry and genetic manipulations were employed to study the functions of different DNlg3 forms in the nervous system and neuromuscular junction (NMJs).

RESULTS

Tumor necrosis factor α-converting enzyme (TACE) cleaved DNlg3 exclusively at its extracellular acetylcholinesterase-like domain to generate the N-terminal fragment and the short membrane-anchored fragment (sDNlg3). DNlg3 was constitutively processed in an activity-independent manner. Interestingly, DNlg3 was cleaved intracellularly in the Golgi apparatus before it arrived at the cell surface, a unique cleavage mechanism that is distinct from 'conventional' ectodomain shedding of membrane proteins, including rodent Nlg1. Genetic studies showed that sDNlg3 was essential for maintaining proper locomotor activity in Drosophila.

CONCLUSIONS

Our results revealed a unique cleavage mechanism of DNlg3 and a neuron-specific role for DNlg3 maturation which is important in locomotor activity.

GENERAL SIGNIFICANCE

Our study provides a new insight into a cleavage mechanism of Nlgs maturation in the nervous system.

Identification of Genes that Maintain Behavioral and Structural Plasticity during Sleep Loss

Although patients with primary insomnia experience sleep disruption, they are able to maintain normal performance on a variety of cognitive tasks. This observation suggests that insomnia may be a condition where predisposing factors simultaneously increase the risk for insomnia and also mitigate against the deleterious consequences of waking. To gain insight into processes that might regulate sleep and buffer neuronal circuits during sleep loss, we manipulated three genes, fat facet (faf), highwire (hiw) and the GABA receptor Resistance to dieldrin (Rdl), that were differentially modulated in a Drosophila model of insomnia. Our results indicate that increasing faf and decreasing hiw or Rdl within wake-promoting large ventral lateral clock neurons (lLNvs) induces sleep loss. As expected, sleep loss induced by decreasing hiw in the lLNvs results in deficits in short-term memory and increases of synaptic growth. However, sleep loss induced by knocking down Rdl in the lLNvs protects flies from sleep-loss induced deficits in short-term memory and increases in synaptic markers. Surprisingly, decreasing hiw and Rdl within the Mushroom Bodies (MBs) protects against the negative effects of sleep deprivation (SD) as indicated by the absence of a subsequent homeostatic response, or deficits in short-term memory. Together these results indicate that specific genes are able to disrupt sleep and protect against the negative consequences of waking in a circuit dependent manner.

Sleep homeostasis regulated by 5HT2b receptor in a small subset of neurons in the dorsal fan-shaped body of drosophila

Our understanding of the molecular mechanisms underlying sleep homeostasis is limited. We have taken a systematic approach to study neural signaling by the transmitter 5-hydroxytryptamine (5-HT) in drosophila. We have generated knockout and knockin lines for Trh, the 5-HT synthesizing enzyme and all five 5-HT receptors, making it possible for us to determine their expression patterns and to investigate their functional roles. Loss of the Trh, 5HT1a or 5HT2b gene decreased sleep time whereas loss of the Trh or 5HT2b gene diminished sleep rebound after sleep deprivation. 5HT2b expression in a small subset of, probably a single pair of, neurons in the dorsal fan-shaped body (dFB) is functionally essential: elimination of the 5HT2b gene from these neurons led to loss of sleep homeostasis. Genetic ablation of 5HT2b neurons in the dFB decreased sleep and impaired sleep homeostasis. Our results have shown that serotonergic signaling in specific neurons is required for the regulation of sleep homeostasis.

Neuroligin 4 regulates synaptic growth via the bone morphogenetic protein (BMP) signaling pathway at the Drosophila neuromuscular junction

The neuroligin (Nlg) family of neural cell adhesion molecules is thought to be required for synapse formation and development and has been linked to the development of autism spectrum disorders in humans. In Drosophila melanogaster, mutations in the neuroligin 1–3 genes have been reported to induce synapse developmental defects at neuromuscular junctions (NMJs), but the role of neuroligin 4 (dnlg4) in synapse development has not been determined. Here, we report that the Drosophila neuroligin 4 (DNlg4) is different from DNlg1–3 in that it presynaptically regulates NMJ synapse development. Loss of dnlg4 results in reduced growth of NMJs with fewer synaptic boutons. The morphological defects caused by dnlg4 mutant are associated with a corresponding decrease in synaptic transmission efficacy. All of these defects could only be rescued when DNlg4 was expressed in the presynapse of NMJs. To understand the basis of DNlg4 function, we looked for genetic interactions and found connections with the components of the bone morphogenetic protein (BMP) signaling pathway. Immunostaining and Western blot analyses demonstrated that the regulation of NMJ growth by DNlg4 was due to the positive modulation of BMP signaling by DNlg4. Specifically, BMP type I receptor thickvein (Tkv) abundance was reduced in dnlg4 mutants, and immunoprecipitation assays showed that DNlg4 and Tkv physically interacted in vivo. Our study demonstrates that DNlg4 presynaptically regulates neuromuscular synaptic growth via the BMP signaling pathway by modulating Tkv.

Drosophila Studies on Autism Spectrum Disorders

In the past decade, numerous genes associated with autism spectrum disorders (ASDs) have been identified. These genes encode key regulators of synaptogenesis, synaptic function, and synaptic plasticity. Drosophila is a prominent model system for ASD studies to define novel genes linked to ASDs and decipher their molecular roles in synaptogenesis, synaptic function, synaptic plasticity, and neural circuit assembly and consolidation. Here, we review Drosophila studies on ASD genes that regulate synaptogenesis, synaptic function, and synaptic plasticity through modulating chromatin remodeling, transcription, protein synthesis and degradation, cytoskeleton dynamics, and synaptic scaffolding.

The involvement of potassium channel ORK1 in short-term memory and sleep in Drosophila

BACKGROUND

The sleep and cognitive dysfunction are common in major depressive disorders (MDDs). Recently, the 2-pore domain potassium channel twik-related K(+) channel 1 (TREK-1) has been identified to be closely related to the etiology of MDD. However, whether TREK-1 is involved in the regulation of sleep and cognition is still unknown.

METHODS

The present study tried to dissect the role of outwardly rectifying K+ channel-1 (ORK1) (TREK-1 homolog in Drosophila) in sleep and cognition in Drosophila. The mutant and over-expressed lines of ork1 were generated using Drosophila genetics. Sleep analysis and short-term memory experiments were used to test sleep time and short-term memory of the mutant and over-expressed ORK1 lines, respectively.

RESULTS

Our results showed that the learning index of ork1 mutant lines was increased compared with the wild type. However, ork1 mutant could obviously decrease sleep time in Drosophila. Contrary to the ork1 mutant lines, we also found that ORK1 over-expression could increase sleep time and decreased learning index in Drosophila.

CONCLUSION

Results from this study suggest that ORK1 might play an important role in the regulation of sleep time and short-term memory in Drosophila.

Neuroligin 3 R451C mutation alters electroencephalography spectral activity in an animal model of autism spectrum disorders

Human studies demonstrate that sleep impairment is a concurrent comorbidity of autism spectrum disorders (ASD), but its etiology remains largely uncertain. One of the prominent theories of ASD suggests that an imbalance in synaptic excitation/inhibition may contribute to various aspects of ASD, including sleep impairments. Following the identification of Nlgn3^R451C mutation in patients with ASD, its effects on synaptic transmission and social behaviours have been examined extensively in the mouse model. However, the contributory role of this mutation to sleep impairments in ASD remains unknown. In this study, we showed that Nlgn3^R451C knock-in mice, an established genetic model for ASD, exhibited normal duration and distribution of sleep/wake states but significantly altered electroencephalography (EEG) power spectral profiles for wake and sleep.

Expression and modulation of neuroligin and neurexin in the olfactory organ of the cotton leaf worm Spodoptera littoralis

Carboxylesterases are enzymes widely distributed within living organisms. In insects, they have been mainly involved in dietary metabolism and detoxification function. Interestingly, several members of this family called carboxylesterase-like adhesion molecules (CLAMs) have lost their catalytic properties and are mainly involved in neuro/developmental functions. CLAMs include gliotactins, neurotactins, glutactins, and neuroligins. The latter have for binding partner the neurexin. In insects, the function of these proteins has been mainly studied in Drosophila central nervous system or neuromuscular junction. Some studies suggested a role of neuroligins and neurexin in sensory processing but CLAM expression within sensory systems has not been investigated. Here, we reported the identification of 5 putative CLAMs expressed in the olfactory system of the model pest insect Spodoptera littoralis. One neuroligin, Slnlg4-yll and its putative binding partner neurexin SlnrxI were the most expressed in the antennae and were surprisingly associated with olfactory sensilla. In addition, both transcripts were upregulated in male antennae after mating, known to modulate the sensitivity of the peripheral olfactory system in S. littoralis, suggesting that these molecules could be involved in sensory plasticity.

Neurexin Restricts Axonal Branching in Columns by Promoting Ephrin Clustering

Columnar restriction of neurites is critical for forming nonoverlapping receptive fields and preserving spatial sensory information from the periphery in both vertebrate and invertebrate nervous systems, but the underlying molecular mechanisms remain largely unknown. Here, we demonstrate that Drosophila homolog of α-neurexin (DNrx) plays an essential role in columnar restriction during L4 axon branching. Depletion of DNrx from L4 neurons resulted in misprojection of L4 axonal branches into neighboring columns due to impaired ephrin clustering. The proper ephrin clustering requires its interaction with the intracellular region of DNrx. Furthermore, we find that Drosophila neuroligin 4 (DNlg4) in Tm2 neurons binds to DNrx and initiates DNrx clustering in L4 neurons, which subsequently induces ephrin clustering. Our study demonstrates that DNrx promotes ephrin clustering and reveals that ephrin/Eph signaling from adjacent L4 neurons restricts axonal branches of L4 neurons in columns.

Neuroligins Nlg2 and Nlg4 Affect Social Behavior in Drosophila melanogaster

The genome of Drosophila melanogaster includes homologs to approximately one-third of the currently known human disease genes. Flies and humans share many biological processes, including the principles of information processing by excitable neurons, synaptic transmission, and the chemical signals involved in intercellular communication. Studies on the molecular and behavioral impact of genetic risk factors of human neuro-developmental disorders [autism spectrum disorders (ASDs), schizophrenia, attention deficit hyperactivity disorders, and Tourette syndrome] increasingly use the well-studied social behavior of D. melanogaster, an organism that is amenable to a large variety of genetic manipulations. Neuroligins (Nlgs) are a family of phylogenetically conserved postsynaptic adhesion molecules present (among others) in nematodes, insects, and mammals. Impaired function of Nlgs (particularly of Nlg 3 and 4) has been associated with ASDs in humans and impaired social and communication behavior in mice. Making use of a set of behavioral and social assays, we, here, analyzed the impact of two Drosophila Nlgs, Dnlg2 and Dnlg4, which are differentially expressed at excitatory and inhibitory central nervous synapses, respectively. Both Nlgs seem to be associated with diurnal activity and social behavior. Even though deficiencies in Dnlg2 and Dnlg4 appeared to have no effects on sensory or motor systems, they differentially impacted on social interactions, suggesting that social behavior is distinctly regulated by these Nlgs.

Control of sleep by a network of cell cycle genes

Sleep is essential for health and cognition, but the molecular and neural mechanisms of sleep regulation are not well understood. We recently reported the identification of TARANIS (TARA) as a sleep-promoting factor that acts in a previously unknown arousal center in Drosophila. tara mutants exhibit a dose-dependent reduction in sleep amount of up to ∼60%. TARA and its mammalian homologs, the Trip-Br (Transcriptional Regulators Interacting with PHD zinc fingers and/or Bromodomains) family of proteins, are primarily known as transcriptional coregulators involved in cell cycle progression, and contain a conserved Cyclin-A (CycA) binding homology domain. We found that tara and CycA synergistically promote sleep, and CycA levels are reduced in tara mutants. Additional data demonstrated that Cyclin-dependent kinase 1 (Cdk1) antagonizes tara and CycA to promote wakefulness. Moreover, we identified a subset of CycA expressing neurons in the pars lateralis, a brain region proposed to be analogous to the mammalian hypothalamus, as an arousal center. In this Extra View article, we report further characterization of tara mutants and provide an extended discussion of our findings and future directions within the framework of a working model, in which a network of cell cycle genes, tara, CycA, and Cdk1, interact in an arousal center to regulate sleep.

Neurexin regulates nighttime sleep by modulating synaptic transmission

Neurexins are cell adhesion molecules involved in synaptic formation and synaptic transmission. Mutations in neurexin genes are linked to autism spectrum disorders (ASDs), which are frequently associated with sleep problems. However, the role of neurexin-mediated synaptic transmission in sleep regulation is unclear. Here, we show that lack of the Drosophila α-neurexin homolog significantly reduces the quantity and quality of nighttime sleep and impairs sleep homeostasis. We report that neurexin expression in Drosophila mushroom body (MB) αβ neurons is essential for nighttime sleep. We demonstrate that reduced nighttime sleep in neurexin mutants is due to impaired αβ neuronal output, and show that neurexin functionally couples calcium channels (Cac) to regulate synaptic transmission. Finally, we determine that αβ surface (αβ_s) neurons release both acetylcholine and short neuropeptide F (sNPF), whereas αβ core (αβ_c) neurons release sNPF to promote nighttime sleep. Our findings reveal that neurexin regulates nighttime sleep by mediating the synaptic transmission of αβ neurons. This study elucidates the role of synaptic transmission in sleep regulation, and might offer insights into the mechanism of sleep disturbances in patients with autism disorders.

Cell adhesion molecules and sleep

Cell adhesion molecules (CAMs) play essential roles in the central nervous system, where some families are involved in synaptic development and function. These synaptic adhesion molecules (SAMs) are involved in the regulation of synaptic plasticity, and the formation of neuronal networks. Recent findings from studies examining the consequences of sleep loss suggest that these molecules are candidates to act in sleep regulation. This review highlights the experimental data that lead to the identification of SAMs as potential sleep regulators, and discusses results supporting that specific SAMs are involved in different aspects of sleep regulation. Further, some potential mechanisms by which SAMs may act to regulate sleep are outlined, and the proposition that these molecules may serve as molecular machinery in the two sleep regulatory processes, the circadian and homeostatic components, is presented. Together, the data argue that SAMs regulate the neuronal plasticity that underlies sleep and wakefulness.

Neurexin, Neuroligin and Wishful Thinking coordinate synaptic cytoarchitecture and growth at neuromuscular junctions

Trans-synaptic interactions involving Neurexins and Neuroligins are thought to promote adhesive interactions for precise alignment of the pre- and postsynaptic compartments and organize synaptic macromolecular complexes across species. In Drosophila, while Neurexin (Dnrx) and Neuroligins (Dnlg) are emerging as central organizing molecules at synapses, very little is known of the spectrum of proteins that might be recruited to the Dnrx/Dnlg trans-synaptic interface for organization and growth of the synapses. Using full length and truncated forms of Dnrx and Dnlg1 together with cell biological analyses and genetic interactions, we report novel functions of Dnrx and Dnlg1 in clustering of pre- and postsynaptic proteins, coordination of synaptic growth and ultrastructural organization. We show that Dnrx and Dnlg1 extracellular and intracellular regions are required for proper synaptic growth and localization of Dnlg1 and Dnrx, respectively. dnrx and dnlg1 single and double mutants display altered subcellular distribution of Discs large (Dlg), which is the homolog of mammalian post-synaptic density protein, PSD95. dnrx and dnlg1 mutants also display ultrastructural defects ranging from abnormal active zones, misformed pre- and post-synaptic areas with underdeveloped subsynaptic reticulum. Interestingly, dnrx and dnlg1 mutants have reduced levels of the Bone Morphogenetic Protein (BMP) receptor Wishful thinking (Wit), and Dnrx and Dnlg1 are required for proper localization and stability of Wit. In addition, the synaptic overgrowth phenotype resulting from the overexpression of Dnrx fails to manifest in wit mutants. Phenotypic analyses of dnrx/wit and dnlg1/wit mutants indicate that Dnrx/Dnlg1/Wit coordinate synaptic growth and architecture at the NMJ. Our findings also demonstrate that loss of Dnrx and Dnlg1 leads to decreased levels of the BMP co-receptor, Thickveins and the downstream effector phosphorylated Mad at the Neuromuscular Junction (NMJ) synapses indicating that Dnrx/Dnlg1 regulate components of the BMP signaling pathway. Together our findings reveal that Dnrx/Dnlg are at the core of a highly orchestrated process that combines adhesive and signaling mechanisms to ensure proper synaptic organization and growth during NMJ development.

Unraveling the Neurobiology of Sleep and Sleep Disorders Using Drosophila

Sleep disorders in humans are increasingly appreciated to be not only widespread but also detrimental to multiple facets of physical and mental health. Recent work has begun to shed light on the mechanistic basis of sleep disorders like insomnia, restless legs syndrome, narcolepsy, and a host of others, but a more detailed genetic and molecular understanding of how sleep goes awry is lacking. Over the past 15 years, studies in Drosophila have yielded new insights into basic questions regarding sleep function and regulation. More recently, powerful genetic approaches in the fly have been applied toward studying primary human sleep disorders and other disease states associated with dysregulated sleep. In this review, we discuss the contribution of Drosophila to the landscape of sleep biology, examining not only fundamental advances in sleep neurobiology but also how flies have begun to inform pathological sleep states in humans.

Inability to activate Rac1-dependent forgetting contributes to behavioral inflexibility in mutants of multiple autism-risk genes

The etiology of autism is so complicated because it involves the effects of variants of several hundred risk genes along with the contribution of environmental factors. Therefore, it has been challenging to identify the causal paths that lead to the core autistic symptoms such as social deficit, repetitive behaviors, and behavioral inflexibility. As an alternative approach, extensive efforts have been devoted to identifying the convergence of the targets and functions of the autism-risk genes to facilitate mapping out causal paths. In this study, we used a reversal-learning task to measure behavioral flexibility in Drosophila and determined the effects of loss-of-function mutations in multiple autism-risk gene homologs in flies. Mutations of five autism-risk genes with diversified molecular functions all led to a similar phenotype of behavioral inflexibility indicated by impaired reversal-learning. These reversal-learning defects resulted from the inability to forget or rather, specifically, to activate Rac1 (Ras-related C3 botulinum toxin substrate 1)-dependent forgetting. Thus, behavior-evoked activation of Rac1-dependent forgetting has a converging function for autism-risk genes.

Acetylcholine from Visual Circuits Modulates the Activity of Arousal Neurons in Drosophila

Drosophila melanogaster's large lateral ventral neurons (lLNvs) are part of both the circadian and sleep-arousal neuronal circuits. In the past, electrophysiological analysis revealed that lLNvs fire action potentials (APs) in bursting or tonic modes and that the proportion of neurons firing in those specific patterns varies circadianly. Here, we provide evidence that lLNvs fire in bursts both during the day and at night and that the frequency of bursting is what is modulated in a circadian fashion. Moreover, we show that lLNvs AP firing is not only under cell autonomous control, but is also modulated by the network, and in the process we develop a novel preparation to assess this. We demonstrate that lLNv bursting mode relies on a cholinergic input because application of nicotinic acetylcholine receptor antagonists impairs this firing pattern. Finally, we found that bursting of lLNvs depends on an input from visual circuits that includes the cholinergic L2 monopolar neurons from the lamina. Our work sheds light on the physiological properties of lLNvs and on a neuronal circuit that may provide visual information to these important arousal neurons.

SIGNIFICANCE STATEMENT

Circadian rhythms are important for organisms to be able to anticipate daily changes in environmental conditions to adjust physiology and behavior accordingly. These rhythms depend on an endogenous mechanism that operates in dedicated neurons. In the fruit fly, the large lateral ventral neurons (lLNvs) are part of both the circadian and sleep-arousal neuronal circuits. Here, we provide new details about the firing properties of these neurons and demonstrate that they depend, not only on cell-autonomous mechanisms, but also on a specific neurotransmitter derived from visual circuits. Our work sheds light on the physiological properties of lLNvs and on a neuronal circuit that may provide visual information to these important arousal neurons.

From the genetic architecture to synaptic plasticity in autism spectrum disorder

Genetics studies of autism spectrum disorder (ASD) have identified several risk genes that are key regulators of synaptic plasticity. Indeed, many of the risk genes that have been linked to these disorders encode synaptic scaffolding proteins, receptors, cell adhesion molecules or proteins that are involved in chromatin remodelling, transcription, protein synthesis or degradation, or actin cytoskeleton dynamics. Changes in any of these proteins can increase or decrease synaptic strength or number and, ultimately, neuronal connectivity in the brain. In addition, when deleterious mutations occur, inefficient genetic buffering and impaired synaptic homeostasis may increase an individual's risk for ASD.

Genome of the house fly, Musca domestica L., a global vector of diseases with adaptations to a septic environment

Background

Adult house flies, Musca domestica L., are mechanical vectors of more than 100 devastating diseases that have severe consequences for human and animal health. House fly larvae play a vital role as decomposers of animal wastes, and thus live in intimate association with many animal pathogens.

Results

We have sequenced and analyzed the genome of the house fly using DNA from female flies. The sequenced genome is 691 Mb. Compared with Drosophila melanogaster, the genome contains a rich resource of shared and novel protein coding genes, a significantly higher amount of repetitive elements, and substantial increases in copy number and diversity of both the recognition and effector components of the immune system, consistent with life in a pathogen-rich environment. There are 146 P450 genes, plus 11 pseudogenes, in M. domestica, representing a significant increase relative to D. melanogaster and suggesting the presence of enhanced detoxification in house flies. Relative to D. melanogaster, M. domestica has also evolved an expanded repertoire of chemoreceptors and odorant binding proteins, many associated with gustation.

Conclusions

This represents the first genome sequence of an insect that lives in intimate association with abundant animal pathogens. The house fly genome provides a rich resource for enabling work on innovative methods of insect control, for understanding the mechanisms of insecticide resistance, genetic adaptation to high pathogen loads, and for exploring the basic biology of this important pest. The genome of this species will also serve as a close out-group to Drosophila in comparative genomic studies.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-014-0466-3) contains supplementary material, which is available to authorized users.

Exaggerated Nighttime Sleep and Defective Sleep Homeostasis in a Drosophila Knock-In Model of Human Epilepsy

Despite an established link between epilepsy and sleep behavior, it remains unclear how specific epileptogenic mutations affect sleep and subsequently influence seizure susceptibility. Recently, Sun et al. (2012) created a fly knock-in model of human generalized epilepsy with febrile seizures plus (GEFS+), a wide-spectrum disorder characterized by fever-associated seizing in childhood and lifelong affliction. GEFS+ flies carry a disease-causing mutation in their voltage-gated sodium channel (VGSC) gene and display semidominant heat-induced seizing, likely due to reduced GABAergic inhibitory activity at high temperature. Here, we show that at room temperature the GEFS+ mutation dominantly modifies sleep, with mutants exhibiting rapid sleep onset at dusk and increased nighttime sleep as compared to controls. These characteristics of GEFS+ sleep were observed regardless of sex, mating status, and genetic background. GEFS+ mutant sleep phenotypes were more resistant to pharmacologic reduction of GABA transmission by carbamazepine (CBZ) than controls, and were mitigated by reducing GABAA receptor expression specifically in wake-promoting pigment dispersing factor (PDF) neurons. These findings are consistent with increased GABAergic transmission to PDF neurons being mainly responsible for the enhanced nighttime sleep of GEFS+ mutants. Additionally, analyses under other light conditions suggested that the GEFS+ mutation led to reduced buffering of behavioral responses to light on and off stimuli, which contributed to characteristic GEFS+ sleep phenotypes. We further found that GEFS+ mutants had normal circadian rhythms in free-running dark conditions. Interestingly, the mutants lacked a homeostatic rebound following mechanical sleep deprivation, and whereas deprivation treatment increased heat-induced seizure susceptibility in control flies, it unexpectedly reduced seizure activity in GEFS+ mutants. Our study has revealed the sleep architecture of a Drosophila VGSC mutant that harbors a human GEFS+ mutation, and provided unique insight into the relationship between sleep and epilepsy.

Neurexin-1 regulates sleep and synaptic plasticity in Drosophila melanogaster

Neurexins are cell adhesion molecules that are important for synaptic plasticity and homeostasis, although links to sleep have not yet been investigated. We examined the effects of neurexin-1 perturbation on sleep in Drosophila, showing that neurexin-1 nulls displayed fragmented sleep and altered circadian rhythm. Conversely, the over-expression of neurexin-1 could increase and consolidate night-time sleep. This was not solely due to developmental effects as it could be induced acutely in adulthood, and was coupled with evidence of synaptic growth. The timing of over-expression could differentially impact sleep patterns, with specific night-time effects. These results show that neurexin-1 was dynamically involved in synaptic plasticity and sleep in Drosophila. Neurexin-1 and a number of its binding partners have been repeatedly associated with mental health disorders, including autism spectrum disorders, schizophrenia and Tourette syndrome, all of which are also linked to altered sleep patterns. How and when plasticity-related proteins such as neurexin-1 function during sleep can provide vital information on the interaction between synaptic homeostasis and sleep, paving the way for more informed treatments of human disorders.

The Neurexin/N-Ethylmaleimide-sensitive Factor (NSF) Interaction Regulates Short Term Synaptic Depression

Although Neurexins, which are cell adhesion molecules localized predominantly to the presynaptic terminals, are known to regulate synapse formation and synaptic transmission, their roles in the regulation of synaptic vesicle release during repetitive nerve stimulation are unknown. Here, we show that nrx mutant synapses exhibit rapid short term synaptic depression upon tetanic nerve stimulation. Moreover, we demonstrate that the intracellular region of NRX is essential for synaptic vesicle release upon tetanic nerve stimulation. Using a yeast two-hybrid screen, we find that the intracellular region of NRX interacts with N-ethylmaleimide-sensitive factor (NSF), an enzyme that mediates soluble NSF attachment protein receptor (SNARE) complex disassembly and plays an important role in synaptic vesicle release. We further map the binding sites of each molecule and demonstrate that the NRX/NSF interaction is critical for both the distribution of NSF at the presynaptic terminals and SNARE complex disassembly. Our results reveal a previously unknown role of NRX in the regulation of short term synaptic depression upon tetanic nerve stimulation and provide new mechanistic insights into the role of NRX in synaptic vesicle release.

The interplay between synaptic activity and neuroligin function in the CNS

Neuroligins (NLs) are postsynaptic transmembrane cell-adhesion proteins that play a key role in the regulation of excitatory and inhibitory synapses. Previous in vitro and in vivo studies have suggested that NLs contribute to synapse formation and synaptic transmission. Consistent with their localization, NL1 and NL3 selectively affect excitatory synapses, whereas NL2 specifically affects inhibitory synapses. Deletions or mutations in NL genes have been found in patients with autism spectrum disorders or mental retardations, and mice harboring the reported NL deletions or mutations exhibit autism-related behaviors and synapse dysfunction. Conversely, synaptic activity can regulate the phosphorylation, expression, and cleavage of NLs, which, in turn, can influence synaptic activity. Thus, in clinical research, identifying the relationship between NLs and synapse function is critical. In this review, we primarily discuss how NLs and synaptic activity influence each other.