Mmp1 processing of the PDF neuropeptide regulates circadian structural plasticity of pacemaker neurons

Overexpression of α-synuclein in Pigment Dispersing Factor neurons alters sleep-wake pattern by regulating lipid metabolism in Drosophila

Parkinson's disease (PD) is a complex neurodegenerative disorder, characterized by the aggregation of α-synuclein (α-syn). Current research increasingly indicates the prevalence of sleep-wake disorders in early-stage PD, although the underlying pathogenic mechanisms remain unclear. In this study, transgenic Drosophila models were utilized to observe excessive daytime sleepiness and impaired anticipation in flies overexpressing α-syn in pan-neurons and circadian clock neurons. Additionally, deficits in projection of Pigment Dispersing Factor (PDF) neuron terminals, which are involved in Drosophila sleep and circadian rhythm, were identified. An imbalance in lipid metabolism homeostasis was detected in the brains of α-syn overexpressing mutants. Ultimately, the inhibition of Sterol Regulatory Element-Binding Protein (SREBP) activity led to an improvement in the reduced daytime sleep duration phenotype. Our results suggest that lipid pathways play a role in sleep-wake disorders triggered by α-syn mutation and aggregation, thereby providing valuable insights into potential therapeutic avenues for disrupted sleep patterns associated with PD.

The Drosophila circadian clock gene cycle controls the development of clock neurons

Daily behavioral and physiological rhythms are controlled by the brain's circadian timekeeping system, a synchronized network of neurons that maintains endogenous molecular oscillations. These oscillations are based on transcriptional feedback loops of clock genes, which in Drosophila include the transcriptional activators Clock (Clk) and cycle (cyc). While the mechanisms underlying this molecular clock are very well characterized, the roles that the core clock genes play in neuronal physiology and development are much less understood. The Drosophila timekeeping center is composed of ~150 clock neurons, among which the four small ventral lateral neurons (sLNvs) are the most dominant pacemakers under constant conditions. Here, we show that downregulating the clock gene cyc specifically in the Pdf-expressing neurons leads to decreased fasciculation both in larval and adult brains. This effect is due to a developmental role of cyc, as both knocking down cyc or expressing a dominant negative form of cyc exclusively during development lead to defasciculation phenotypes in adult clock neurons. Clk downregulation also leads to developmental effects on sLNv morphology. Our results reveal a non-circadian role for cyc, shedding light on the additional functions of circadian clock genes in the development of the nervous system.

Suprachiasmatic nucleus VIPergic fibers show a circadian rhythm of expansion and retraction

In animals, overt circadian rhythms of physiology and behavior are centrally regulated by a circadian clock located in specific brain regions. In the fruit fly Drosophila and in mammals, these clocks rely on single-cell oscillators, but critical for their function as central circadian pacemakers are network properties that change dynamically throughout the circadian cycle as well as in response to environmental stimuli.1,2,3 In the fly, this plasticity involves circadian rhythms of expansion and retraction of clock neuron fibers.4,5,6,7,8,9,10,11,12,13,14 Whether these drastic structural changes are a universal property of central neuronal pacemakers is unknown. To address this question, we studied neurons of the mouse suprachiasmatic nucleus (SCN) that express vasoactive intestinal polypeptide (VIP), which are critical for the SCN to function as a central circadian pacemaker. By targeting the expression of the fluorescent protein tdTomato to these neurons and using tissue clearing techniques to visualize all SCN VIPergic neurons and their fibers, we show that, similar to clock neurons in the fly, VIPergic fibers undergo a daily rhythm of expansion and retraction, with maximal branching during the day. This rhythm is circadian, as it persists under constant environmental conditions and is present in both males and females. We propose that circadian structural remodeling of clock neurons represents a key feature of central circadian pacemakers that is likely critical to regulate network properties, the response to environmental stimuli, and the regulation of circadian outputs.

The neuropeptide pigment-dispersing factor signals independently of Bruchpilot-labelled active zones in daily remodelled terminals of Drosophila clock neurons

The small ventrolateral neurons (sLNvs) are key components of the central clock in the Drosophila brain. They signal via the neuropeptide pigment-dispersing factor (PDF) to align the molecular clockwork of different central clock neurons and to modulate downstream circuits. The dorsal terminals of the sLNvs undergo daily morphological changes that affect presynaptic sites organised by the active zone protein Bruchpilot (BRP), a homolog of mammalian ELKS proteins. However, the role of these presynaptic sites for PDF release is ill-defined. Here, we combined expansion microscopy with labelling of active zones by endogenously tagged BRP to examine the spatial correlation between PDF-containing dense-core vesicles and BRP-labelled active zones. We found that the number of BRP-labelled puncta in the sLNv terminals was similar while their density differed between Zeitgeber time (ZT) 2 and 14. The relative distance between BRP- and PDF-labelled puncta was increased in the morning, around the reported time of PDF release. Spontaneous dense-core vesicle release profiles of sLNvs in a publicly available ssTEM dataset (FAFB) consistently lacked spatial correlation to BRP-organised active zones. RNAi-mediated downregulation of brp and other active zone proteins expressed by the sLNvs did not affect PDF-dependent locomotor rhythmicity. In contrast, down-regulation of genes encoding proteins of the canonical vesicle release machinery, the dense-core vesicle-related protein CADPS, as well as PDF impaired locomotor rhythmicity. Taken together, our study suggests that PDF release from the sLNvs is independent of BRP-organised active zones, while BRP may be redistributed to active zones in a time-dependent manner.

Molecular circadian rhythms are robust in marine annelids lacking rhythmic behavior

The circadian clock controls behavior and metabolism in various organisms. However, the exact timing and strength of rhythmic phenotypes can vary significantly between individuals of the same species. This is highly relevant for rhythmically complex marine environments where organismal rhythmic diversity likely permits the occupation of different microenvironments. When investigating circadian locomotor behavior of Platynereis dumerilii, a model system for marine molecular chronobiology, we found strain-specific, high variability between individual worms. The individual patterns were maintained for several weeks. A diel head transcriptome comparison of behaviorally rhythmic versus arrhythmic wild-type worms showed that 24-h cycling of core circadian clock transcripts is identical between both behavioral phenotypes. While behaviorally arrhythmic worms showed a similar total number of cycling transcripts compared to their behaviorally rhythmic counterparts, the annotation categories of their transcripts, however, differed substantially. Consistent with their locomotor phenotype, behaviorally rhythmic worms exhibit an enrichment of cycling transcripts related to neuronal/behavioral processes. In contrast, behaviorally arrhythmic worms showed significantly increased diel cycling for metabolism- and physiology-related transcripts. The prominent role of the neuropeptide pigment-dispersing factor (PDF) in Drosophila circadian behavior prompted us to test for a possible functional involvement of Platynereis pdf. Differing from its role in Drosophila, loss of pdf impacts overall activity levels but shows only indirect effects on rhythmicity. Our results show that individuals arrhythmic in a given process can show increased rhythmicity in others. Across the Platynereis population, rhythmic phenotypes exist as a continuum, with no distinct "boundaries" between rhythmicity and arrhythmicity. We suggest that such diel rhythm breadth is an important biodiversity resource enabling the species to quickly adapt to heterogeneous or changing marine environments. In times of massive sequencing, our work also emphasizes the importance of time series and functional tests.

Autophagy as a new player in the regulation of clock neurons physiology of Drosophila melanogaster

Axonal terminals of the small ventral lateral neurons (sLNvs), the circadian clock neurons of Drosophila, show daily changes in their arborization complexity, with many branches in the morning and their shrinkage during the night. This complex phenomenon is precisely regulated by several mechanisms. In the present study we describe that one of them is autophagy, a self-degradative process, also involved in changes of cell membrane size and shape. Our results showed that autophagosome formation and processing in PDF-expressing neurons (both sLNv and lLNv) are rhythmic and they have different patterns in the cell bodies and terminals. These rhythmic changes in the autophagy activity seem to be important for neuronal plasticity. We found that autophagosome cargos are different during the day and night, and more proteins involved in membrane remodeling are present in autophagosomes in the morning. In addition, we described for the first time that Atg8-positive vesicles are also present outside the sLNv terminals, which suggests that secretory autophagy might be involved in regulating the clock signaling network. Our data indicate that rhythmic autophagy in clock neurons affect the pacemaker function, through remodeling of terminal membrane and secretion of specific proteins from sLNvs.

Polygenic adaptation from standing genetic variation allows rapid ecotype formation

Adaptive ecotype formation can be the first step to speciation, but the genetic underpinnings of this process are poorly understood. Marine midges of the genus Clunio (Diptera) have recolonized Northern European shore areas after the last glaciation. In response to local tide conditions they have formed different ecotypes with respect to timing of adult emergence, oviposition behavior and larval habitat. Genomic analysis confirms the recent establishment of these ecotypes, reflected in massive haplotype sharing between ecotypes, irrespective of whether there is ongoing gene flow or geographic isolation. QTL mapping and genome screens reveal patterns of polygenic adaptation from standing genetic variation. Ecotype-associated loci prominently include circadian clock genes, as well as genes affecting sensory perception and nervous system development, hinting to a central role of these processes in ecotype formation. Our data show that adaptive ecotype formation can occur rapidly, with ongoing gene flow and largely based on a re-assortment of existing alleles.

Decapentaplegic Acutely Defines the Connectivity of Central Pacemaker Neurons in Drosophila

Rhythmic rest-activity cycles are controlled by an endogenous clock. In Drosophila, this clock resides in ∼150 neurons organized in clusters whose hierarchy changes in response to environmental conditions. The concerted activity of the circadian network is necessary for the adaptive responses to synchronizing environmental stimuli. Thus far, work was devoted to unravel the logic of the coordination of different clusters focusing on neurotransmitters and neuropeptides. We further explored communication in the adult male brain through ligands belonging to the bone morphogenetic protein (BMP) pathway. Herein we show that the lateral ventral neurons (LNvs) express the small morphogen decapentaplegic (DPP). DPP expression in the large LNvs triggered a period lengthening phenotype, the downregulation of which caused reduced rhythmicity and affected anticipation at dawn and dusk, underscoring DPP per se conveys time-of-day relevant information. Surprisingly, DPP expression in the large LNvs impaired circadian remodeling of the small LNv axonal terminals, likely through local modulation of the guanine nucleotide exchange factor Trio. These findings open the provocative possibility that the BMP pathway is recruited to strengthen/reduce the connectivity among specific clusters along the day and thus modulate the contribution of the clusters to the circadian network.SIGNIFICANCE STATEMENT The circadian clock relies on the communication between groups of so-called clock neurons to coordinate physiology and behavior to the optimal times across the day, predicting and adapting to a changing environment. The circadian network relies on neurotransmitters and neuropeptides to fine-tune connectivity among clock neurons and thus give rise to a coherent output. Herein we show that decapentaplegic, a ligand belonging to the BMP retrograde signaling pathway required for coordinated growth during development, is recruited by a group of circadian neurons in the adult brain to trigger structural remodeling of terminals on a daily basis.

Drosophila Accessory Gland: A Complementary In Vivo Model to Bring New Insight to Prostate Cancer

Prostate cancer is the most common cancer in aging men. Despite recent progress, there are still few effective treatments to cure its aggressive and metastatic stages. A better understanding of the molecular mechanisms driving disease initiation and progression appears essential to support the development of more efficient therapies and improve patient care. To do so, multiple research models, such as cell culture and mouse models, have been developed over the years and have improved our comprehension of the biology of the disease. Recently, a new model has been added with the use of the Drosophila accessory gland. With a high level of conservation of major signaling pathways implicated in human disease, this functional equivalent of the prostate represents a powerful, inexpensive, and rapid in vivo model to study epithelial carcinogenesis. The purpose of this review is to quickly overview the existing prostate cancer models, including their strengths and limitations. In particular, we discuss how the Drosophila accessory gland can be integrated as a convenient complementary model by bringing new understanding in the mechanisms driving prostate epithelial tumorigenesis, from initiation to metastatic formation.

Emerging roles of MT-MMPs in embryonic development

Membrane-type matrix metalloproteinases (MT-MMPs) are cell membrane-tethered proteinases that belong to the family of the MMPs. Apart from their roles in degradation of the extracellular milieu, MT-MMPs are able to activate through proteolytic processing at the cell surface distinct molecules such as receptors, growth factors, cytokines, adhesion molecules, and other pericellular proteins. Although most of the information regarding these enzymes comes from cancer studies, our current knowledge about their contribution in distinct developmental processes occurring in the embryo is limited. In this review, we want to summarize the involvement of MT-MMPs in distinct processes during embryonic morphogenesis, including cell migration and proliferation, epithelial-mesenchymal transition, cell polarity and branching, axon growth and navigation, synapse formation, and angiogenesis. We also considered information about MT-MMP functions from studies assessed in pathological conditions and compared these data with those relevant for embryonic development.

High-Frequency Neuronal Bursting is Essential for Circadian and Sleep Behaviors in Drosophila

Circadian rhythms have been extensively studied in Drosophila; however, still little is known about how the electrical properties of clock neurons are specified. We have performed a behavioral genetic screen through the downregulation of candidate ion channels in the lateral ventral neurons (LNvs) and show that the hyperpolarization-activated cation current I_h is important for the behaviors that the LNvs influence: temporal organization of locomotor activity, analyzed in males, and sleep, analyzed in females. Using whole-cell patch clamp electrophysiology we demonstrate that small LNvs (sLNvs) are bursting neurons, and that I_h is necessary to achieve the high-frequency bursting firing pattern characteristic of both types of LNvs in females. Since firing in bursts has been associated to neuropeptide release, we hypothesized that I_h would be important for LNvs communication. Indeed, herein we demonstrate that I_h is fundamental for the recruitment of pigment dispersing factor (PDF) filled dense core vesicles (DCVs) to the terminals at the dorsal protocerebrum and for their timed release, and hence for the temporal coordination of circadian behaviors.SIGNIFICANCE STATEMENT Ion channels are transmembrane proteins with selective permeability to specific charged particles. The rich repertoire of parameters that may gate their opening state, such as voltage-sensitivity, modulation by second messengers and specific kinetics, make this protein family a determinant of neuronal identity. Ion channel structure is evolutionary conserved between vertebrates and invertebrates, making any discovery easily translatable. Through a screen to uncover ion channels with roles in circadian rhythms, we have identified the I_h channel as an important player in a subset of clock neurons of the fruit fly. We show that lateral ventral neurons (LNvs) need I_h to fire action potentials in a high-frequency bursting mode and that this is important for peptide transport and the control of behavior.

Enhancement of leukemia-like phenotypes in Drosophila mxc mutant larvae due to activation of the RAS-MAP kinase cascade possibly via down-regulation of DE-cadherin

Loss of mxc gene function in mature hemocytes of Drosophila mxc^mbn1 mutant results in malignant hyperplasia in larval hematopoietic tissues termed lymph glands (LGs) owing to over-proliferation of immature cells. This is a useful model for genetic analyses of leukemia progression. To identify other mutations that deteriorate the hyperplasia, we aimed to investigate whether hyper-activation of common signaling cascade enabled to enhance the phenotypes. Ectopic expression of the constitutively active forms of MAPK signaling factors in the mutant increased the hyperplasia and the number of circulating hemocytes, resulting in the production of LG fragments. The LG phenotype was related to the reduced DE-cadherin level in the mutants. Depletion of Drosophila MCRIP, involved in MAPK-induced silencing of cadherin gene expression, exhibited a similar enhancement of the mxc^mbn1 phenotypes. Furthermore, expression of MMP1 proteinase that cleaves the extracellular matrix proteins increased in the mutant larvae harboring MAPK cascade activation. Depletion of Mmp1 and that of pnt (required for Mmp1 expression) suppressed the LG hyperplasia. Hence, we speculated that reduction in DE-cadherin level by either down-regulation of MCRIP or up-regulation of MMP1 was involved in the progression of the tumor phenotype. Our findings can contribute to understanding the mechanism underlying human leukemia progression.

AKH-FOXO pathway regulates starvation-induced sleep loss through remodeling of the small ventral lateral neuron dorsal projections

Starvation caused by adverse feeding stresses or food shortages has been reported to result in sleep loss in animals. However, how the starvation signal interacts with the central nervous system is still unknown. Here, the adipokinetic hormone (AKH)-Fork head Box-O (FOXO) pathway is shown to respond to energy change and adjust the sleep of Drosophila through remodeling of the s-LNv (small ventral lateral neurons) dorsal projections. Our results show that starvation prevents flies from going to sleep after the first light-dark transition. The LNvs are required for starvation-induced sleep loss through extension of the pigment dispersing factor (PDF)-containing s-LNv dorsal projections. Further studies reveal that loss of AKH or AKHR (akh receptor) function blocks starvation-induced extension of s-LNv dorsal projections and rescues sleep suppression during food deprivation. FOXO, which has been reported to regulate synapse plasticity of neurons, acts as starvation response factor downstream of AKH, and down regulation of FOXO level considerably alleviates the influence of starvation on s-LNv dorsal projections and sleep. Taking together, our results outline the transduction pathways between starvation signal and sleep, and reveal a novel functional site for sleep regulation.

Circadian Clocks: Structural Plasticity on the Input Side

Key Drosophila clock neurons remodel their axonal arborization on a daily basis. The current view is that remodelling is part of the control of clock neuron output but new data support a major role in modulating sensory inputs.

Coupling Neuropeptide Levels to Structural Plasticity in Drosophila Clock Neurons

We have previously reported that pigment dispersing factor (PDF) neurons, which are essential in the control of rest-activity cycles in Drosophila, undergo circadian remodeling of their axonal projections, a phenomenon called circadian structural plasticity. Axonal arborizations display higher complexity during the day and become simpler at night, and this remodeling involves changes in the degree of connectivity. This phenomenon depends on the clock present within the ventrolateral neurons (LNvs) as well as in glia. In this work, we characterize in detail the contribution of the PDF neuropeptide to structural plasticity at different times across the day. Using diverse genetic strategies to temporally restrict its downregulation, we demonstrate that even subtle alterations to PDF cycling at the dorsal protocerebrum correlate with impaired remodeling, underscoring its relevance for the characteristic morning spread; PDF released from the small LNvs (sLNvs) and the large LNvs (lLNvs) contribute to the process. Moreover, forced depolarization recruits activity-dependent mechanisms to mediate growth only at night, overcoming the restriction imposed by the clock on membrane excitability. Interestingly, the active process of terminal remodeling requires PDF receptor (PDFR) signaling acting locally through the cyclic-nucleotide-gated channel ion channel subunit A (CNGA). Thus, clock-dependent PDF signaling shapes the connectivity of these essential clock neurons on daily basis.

Sites of Circadian Clock Neuron Plasticity Mediate Sensory Integration and Entrainment

Networks of circadian timekeeping in the brain display marked daily changes in neuronal morphology. In Drosophila melanogaster, the striking daily structural remodeling of the dorsal medial termini of the small ventral lateral neurons has long been hypothesized to mediate endogenous circadian timekeeping. To test this model, we have specifically abrogated these sites of daily neuronal remodeling through the reprogramming of neural development and assessed the effects on circadian timekeeping and clock outputs. Remarkably, the loss of these sites has no measurable effects on endogenous circadian timekeeping or on any of the major output functions of the small ventral lateral neurons. Rather, their loss reduces sites of glutamatergic sensory neurotransmission that normally encodes naturalistic time cues from the environment. These results support an alternative model: structural plasticity in critical clock neurons is the basis for proper integration of light and temperature and gates sensory inputs into circadian clock neuron networks.

Molecular and circuit mechanisms mediating circadian clock output in the Drosophila brain

A central question in the circadian biology field concerns the mechanisms that translate ~24-hr oscillations of the molecular clock into overt rhythms. Drosophila melanogaster is a powerful system that provided the first understanding of how molecular clocks are generated and is now illuminating the neural basis of circadian behavior. The identity of ~150 clock neurons in the Drosophila brain and their roles in shaping circadian rhythms of locomotor activity have been described before. This review summarizes mechanisms that transmit time-of-day signals from the clock, within the clock network as well as downstream of it. We also discuss the identification of functional multisynaptic circuits between clock neurons and output neurons that regulate locomotor activity.

Splice variants of DOMINO control Drosophila circadian behavior and pacemaker neuron maintenance

Circadian clocks control daily rhythms in behavior and physiology. In Drosophila, the small ventral lateral neurons (sLN_vs) expressing PIGMENT DISPERSING FACTOR (PDF) are the master pacemaker neurons generating locomotor rhythms. Despite the importance of sLN_vs and PDF in circadian behavior, little is known about factors that control sLN_vs maintenance and PDF accumulation. Here, we identify the Drosophila SWI2/SNF2 protein DOMINO (DOM) as a key regulator of circadian behavior. Depletion of DOM in circadian neurons eliminates morning anticipatory activity under light dark cycle and impairs behavioral rhythmicity in constant darkness. Interestingly, the two major splice variants of DOM, DOM-A and DOM-B have distinct circadian functions. DOM-A depletion mainly leads to arrhythmic behavior, while DOM-B knockdown lengthens circadian period without affecting the circadian rhythmicity. Both DOM-A and DOM-B bind to the promoter regions of key pacemaker genes period and timeless, and regulate their protein expression. However, we identify that only DOM-A is required for the maintenance of sLNvs and transcription of pdf. Lastly, constitutive activation of PDF-receptor signaling rescued the arrhythmia and period lengthening of DOM downregulation. Taken together, our findings reveal that two splice variants of DOM play distinct roles in circadian rhythms through regulating abundance of pacemaker proteins and sLN_vs maintenance.

Circadian rhythms are critical for timing of animal bodily functions. In flies, sLN_vs are the master pacemaker neurons regulating locomotor rhythms, which release the neuropeptide PDF. Little is known about factors that control sLN_vs maintenance and PDF accumulation. Here, we identified the Drosophila chromatin remodeler DOMINO (DOM) as a new regulator of circadian behavior. Depletion of DOM in circadian neurons impaired behavioral rhythmicity in constant darkness. Interestingly, two splice variants of DOM have distinct functions. DOM-A depletion mainly led to arrhythmia, while DOM-B knockdown lengthened circadian period. Furthermore, we found DOM-A is critical for the maintenance of sLN_vs and transcription of pdf. Our findings reveal that DOM splice variants play distinct roles in rhythms through different mechanisms.

miR-263b Controls Circadian Behavior and the Structural Plasticity of Pacemaker Neurons by Regulating the LIM-Only Protein Beadex

: Circadian clocks drive rhythmic physiology and behavior to allow adaption to daily environmental changes. In Drosophila, the small ventral lateral neurons (sLNvs) are primary pacemakers that control circadian rhythms. Circadian changes are observed in the dorsal axonal projections of the sLNvs, but their physiological importance and the underlying mechanism are unclear. Here, we identified miR-263b as an important regulator of circadian rhythms and structural plasticity of sLNvs in Drosophila. Depletion of miR-263b (miR-263b^KO) in flies dramatically impaired locomotor rhythms under constant darkness. Indeed, miR-263b is required for the structural plasticity of sLNvs. miR-263b regulates circadian rhythms through inhibition of expression of the LIM-only protein Beadex (Bx). Consistently, overexpression of Bx or loss-of-function mutation (Bx^hdpR26) phenocopied miR-263b^KO and miR-263b overexpression in behavior and molecular characteristics. In addition, mutating the miR-263b binding sites in the Bx 3' UTR using CRISPR/Cas9 recapitulated the circadian phenotypes of miR-263b^KO flies. Together, these results establish miR-263b as an important regulator of circadian locomotor behavior and structural plasticity.

miR-210 controls the evening phase of circadian locomotor rhythms through repression of Fasciclin 2

Circadian clocks control the timing of animal behavioral and physiological rhythms. Fruit flies anticipate daily environmental changes and exhibit two peaks of locomotor activity around dawn and dusk. microRNAs are small non-coding RNAs that play important roles in post-transcriptional regulation. Here we identify Drosophila miR-210 as a critical regulator of circadian rhythms. Under light-dark conditions, flies lacking miR-210 (miR-210KO) exhibit a dramatic 2 hrs phase advance of evening anticipatory behavior. However, circadian rhythms and molecular pacemaker function are intact in miR-210KO flies under constant darkness. Furthermore, we identify that miR-210 determines the evening phase of activity through repression of the cell adhesion molecule Fasciclin 2 (Fas2). Ablation of the miR-210 binding site within the 3' UTR of Fas2 (Fas2ΔmiR-210) by CRISPR-Cas9 advances the evening phase as in miR-210KO. Indeed, miR-210 genetically interacts with Fas2. Moreover, Fas2 abundance is significantly increased in the optic lobe of miR-210KO. In addition, overexpression of Fas2 in the miR-210 expressing cells recapitulates the phase advance behavior phenotype of miR-210KO. Together, these results reveal a novel mechanism by which miR-210 regulates circadian locomotor behavior.

Autophagy within the mushroom body protects from synapse aging in a non-cell autonomous manner

Macroautophagy is an evolutionarily conserved cellular maintenance program, meant to protect the brain from premature aging and neurodegeneration. How neuronal autophagy, usually loosing efficacy with age, intersects with neuronal processes mediating brain maintenance remains to be explored. Here, we show that impairing autophagy in the Drosophila learning center (mushroom body, MB) but not in other brain regions triggered changes normally restricted to aged brains: impaired associative olfactory memory as well as a brain-wide ultrastructural increase of presynaptic active zones (metaplasticity), a state non-compatible with memory formation. Mechanistically, decreasing autophagy within the MBs reduced expression of an NPY-family neuropeptide, and interfering with autocrine NPY signaling of the MBs provoked similar brain-wide metaplastic changes. Our results in an exemplary fashion show that autophagy-regulated signaling emanating from a higher brain integration center can execute high-level control over other brain regions to steer life-strategy decisions such as whether or not to form memories.

Responses to Intermittent Light Stimulation Late in the Night Phase Before Dawn

The circadian clock is comprised of two oscillators that independently track sunset (evening) and sunrise (morning), though little is known about how light responses differ in each. Here, we quantified the morning oscillator’s responses to 19 separate pulse trains, collecting observations from over 1300 Drosophila at ZT23. Our results show that the advances in activity onset produced by these protocols depended on the tempo of light administration even when total exposure was conserved across a 15-min window. Moreover, patterns of stimulation previously shown to optimize the evening oscillator’s delay resetting at ZT13 (an hour after dusk) were equally effective for the M oscillator at ZT23 (an hour before dawn), though the morning oscillator was by comparison more photosensitive and could benefit from a greater number of fractionation strategies that better converted light into phase-shifting drive. These data continue to build the case that the reading frames for the pacemaker’s time-of-day estimates at dusk and dawn are not uniform and suggest that the “photologic” for the evening versus morning oscillator’s resetting might be dissociable.

The CCHamide1 Neuropeptide Expressed in the Anterior Dorsal Neuron 1 Conveys a Circadian Signal to the Ventral Lateral Neurons in Drosophila melanogaster

The fruit fly Drosophila melanogaster possesses approximately 150 brain clock neurons that control circadian behavioral rhythms. Even though individual clock neurons have self-sustaining oscillators, they interact and synchronize with each other through a network. However, little is known regarding the factors responsible for these network interactions. In this study, we investigated the role of CCHamide1 (CCHa1), a neuropeptide expressed in the anterior dorsal neuron 1 (DN1a), in intercellular communication of the clock neurons. We observed that CCHa1 connects the DN1a clock neurons to the ventral lateral clock neurons (LNv) via the CCHa1 receptor, which is a homolog of the gastrin-releasing peptide receptor playing a role in circadian intercellular communications in mammals. CCHa1 knockout or knockdown flies have a generally low activity level with a special reduction of morning activity. In addition, they exhibit advanced morning activity under light-dark cycles and delayed activity under constant dark conditions, which correlates with an advance/delay of PAR domain Protein 1 (PDP1) oscillations in the small-LNv (s-LNv) neurons that control morning activity. The terminals of the s-LNv neurons show rather high levels of Pigment-dispersing factor (PDF) in the evening, when PDF is low in control flies, suggesting that the knockdown of CCHa1 leads to increased PDF release; PDF signals the other clock neurons and evidently increases the amplitude of their PDP1 cycling. A previous study showed that high-amplitude PDP1 cycling increases the siesta of the flies, and indeed, CCHa1 knockout or knockdown flies exhibit a longer siesta than control flies. The DN1a neurons are known to be receptive to PDF signaling from the s-LNv neurons; thus, our results suggest that the DN1a and s-LNv clock neurons are reciprocally coupled via the neuropeptides CCHa1 and PDF, and this interaction fine-tunes the timing of activity and sleep.

Emerging roles for microRNA in the regulation of Drosophila circadian clock

Background

The circadian clock, which operates within an approximately 24-h period, is closely linked to the survival and fitness of almost all living organisms. The circadian clock is generated through a negative transcription-translation feedback loop. microRNAs (miRNAs) are small non-coding RNAs comprised of approximately 22 nucleotides that post-transcriptionally regulate target mRNA by either inducing mRNA degradation or inhibiting translation.

Results

In recent years, miRNAs have been found to play important roles in the regulation of the circadian clock, especially in Drosophila. In this review, we will use fruit flies as an example, and summarize the progress achieved in the study of miRNA-mediated clock regulation. Three main aspects of the circadian clock, namely, the free-running period, locomotion phase, and circadian amplitude, are discussed in detail in the context of how miRNAs are involved in these regulations. In addition, approaches regarding the discovery of circadian-related miRNAs and their targets are also discussed.

Conclusions

Research in the last decade suggests that miRNA-mediated post-transcriptional regulation is crucial to the generation and maintenance of a robust circadian clock in animals. In flies, miRNAs are known to modulate circadian rhythmicity and the free-running period, as well as circadian outputs. Further characterization of miRNAs, especially in the circadian input, will be a vital step toward a more comprehensive understanding of the functions underlying miRNA-control of the circadian clock.

Fragile X Mental Retardation Protein Regulates Activity-Dependent Membrane Trafficking and Trans-Synaptic Signaling Mediating Synaptic Remodeling

Fragile X syndrome (FXS) is the leading monogenic cause of autism and intellectual disability. The disease arises through loss of fragile X mental retardation protein (FMRP), which normally exhibits peak expression levels in early-use critical periods, and is required for activity-dependent synaptic remodeling during this transient developmental window. FMRP canonically binds mRNA to repress protein translation, with targets that regulate cytoskeleton dynamics, membrane trafficking, and trans-synaptic signaling. We focus here on recent advances emerging in these three areas from the Drosophila disease model. In the well-characterized central brain mushroom body (MB) olfactory learning/memory circuit, FMRP is required for activity-dependent synaptic remodeling of projection neurons innervating the MB calyx, with function tightly restricted to an early-use critical period. FMRP loss is phenocopied by conditional removal of FMRP only during this critical period, and rescued by FMRP conditional expression only during this critical period. Consistent with FXS hyperexcitation, FMRP loss defects are phenocopied by heightened sensory experience and targeted optogenetic hyperexcitation during this critical period. FMRP binds mRNA encoding Drosophila ESCRTIII core component Shrub (human CHMP4 homolog) to restrict Shrub translation in an activity-dependent mechanism only during this same critical period. Shrub mediates endosomal membrane trafficking, and perturbing Shrub expression is known to interfere with neuronal process pruning. Consistently, FMRP loss and Shrub overexpression targeted to projection neurons similarly causes endosomal membrane trafficking defects within synaptic boutons, and genetic reduction of Shrub strikingly rescues Drosophila FXS model defects. In parallel work on the well-characterized giant fiber (GF) circuit, FMRP limits iontophoretic dye loading into central interneurons, demonstrating an FMRP role controlling core neuronal properties through the activity-dependent repression of translation. In the well-characterized Drosophila neuromuscular junction (NMJ) model, developmental synaptogenesis and activity-dependent synaptic remodeling both require extracellular matrix metalloproteinase (MMP) enzymes interacting with the heparan sulfate proteoglycan (HSPG) glypican dally-like protein (Dlp) to restrict trans-synaptic Wnt signaling, with FXS synaptogenic defects alleviated by both MMP and HSPG reduction. This new mechanistic axis spanning from activity to FMRP to HSPG-dependent MMP regulation modulates activity-dependent synaptogenesis. We discuss future directions for these mechanisms, and intersecting research priorities for FMRP in glial and signaling interactions.

Neuronal and Glial Clocks Underlying Structural Remodeling of Pacemaker Neurons in Drosophila

A number of years ago we reported that ventral Lateral Neurons (LNvs), which are essential in the control of rest-activity cycles in Drosophila, undergo circadian remodeling of their axonal projections. This structural plasticity gives rise to changes in the degree of connectivity, which could provide a means of transmitting time of day information. Thus far, work from different laboratories has shown that circadian remodeling of adult projections relies on activity-dependent and -independent mechanisms. In terms of clock- dependent mechanisms, several neuronal types undergoing circadian remodeling hinted to a differential effect of clock genes; while per mutants exhibited poorly developed axonal terminals giving rise to low complexity arbors, tim mutants displayed a characteristic hyper branching phenotype, suggesting these genes could be playing additional roles to those ascribed to core clock function. To shed light onto this possibility we altered clock gene levels through RNAi- mediated downregulation and expression of a dominant negative form exclusively in the adult LNvs. These experiments confirmed that the LNv clock is necessary to drive the remodeling process. We next explored the contribution of glia to the structural plasticity of the small LNvs through acute disruption of their internal clock. Interestingly, impaired glial clocks also abolished circadian structural remodeling, without affecting other clock-controlled outputs. Taken together our data shows that both neuronal and glial clocks are recruited to define the architecture of the LNv projections along the day, thus enabling a precise reconfiguration of the circadian network.

Effects and mechanisms of matrix metalloproteinase2 on neural differentiation of induced pluripotent stem cells

Induced pluripotent stem cells (iPSCs) possess the potential to differentiate into neural lineage cells. Matrix metalloproteinase 2 (MMP2), an endopeptidase in the extracellular matrix, has been shown to protect neural cells from injury. However, the mechanisms and effects of MMP2 on neural differentiation of iPSCs remain poorly understood. Here, we demonstrated a role for MMP2 in the differentiation of iPSCs to neurons via the AKT pathway. Treatment of iPSCs with MMP2 promoted their proliferation and differentiation into neural stem cells (NSCs), and then into neurons. The transcript and protein expression of Nestin and microtubule-associated protein 2 (MAP2) increased. Moreover, MMP2 markedly induced the expression of phospho-AKT (pAKT) during these differentiation stages. Consistently, silencing MMP2 using siRNA attenuated the expression of Nestin, MAP2 and pAKT, compared with the control group. In addition, the increasing levels of Nestin, MAP2 and pAKT in the MMP2 group were declined by pretreatment with the phosphoinositide 3-kinase (PI3K)/AKT inhibitor, LY294002. Furthermore, the study detected that TrkA and TrkB were perhaps the potential receptors for these effects of MMP2 on neural differentiation through PI3K/AKT signaling pathway. Taken together, these results suggest that MMP2 induces the differentiation of iPSCs into neurons by regulating the AKT signaling pathway.

Neuronal activity drives FMRP- and HSPG-dependent matrix metalloproteinase function required for rapid synaptogenesis

Matrix metalloproteinase (MMP) functions modulate synapse formation and activity-dependent plasticity. Aberrant MMP activity is implicated in fragile X syndrome (FXS), a disease caused by the loss of the RNA-binding protein FMRP and characterized by neurological dysfunction and intellectual disability. Gene expression studies in Drosophila suggest that Mmps cooperate with the heparan sulfate proteoglycan (HSPG) glypican co-receptor Dally-like protein (Dlp) to restrict trans-synaptic Wnt signaling and that synaptogenic defects in the fly model of FXS are alleviated by either inhibition of Mmp or genetic reduction of Dlp. We used the Drosophila neuromuscular junction (NMJ) glutamatergic synapse to test activity-dependent Dlp and Mmp intersections in the context of FXS. We found that rapid, activity-dependent synaptic bouton formation depended on secreted Mmp1. Acute neuronal stimulation reduced the abundance of Mmp2 but increased that of both Mmp1 and Dlp, as well as enhanced the colocalization of Dlp and Mmp1 at the synapse. Dlp function promoted Mmp1 abundance, localization, and proteolytic activity around synapses. Dlp glycosaminoglycan (GAG) chains mediated this functional interaction with Mmp1. In the FXS fly model, activity-dependent increases in Mmp1 abundance and activity were lost but were restored by reducing the amount of synaptic Dlp. The data suggest that neuronal activity-induced, HSPG-dependent Mmp regulation drives activity-dependent synaptogenesis and that this is impaired in FXS. Thus, exploring this mechanism further may reveal therapeutic targets that have the potential to restore synaptogenesis in FXS patients.

VRILLE Controls PDF Neuropeptide Accumulation and Arborization Rhythms in Small Ventrolateral Neurons to Drive Rhythmic Behavior in Drosophila

In Drosophila, the circadian clock is comprised of transcriptional feedback loops that control rhythmic gene expression responsible for daily rhythms in physiology, metabolism, and behavior. The core feedback loop, which employs CLOCK-CYCLE (CLK-CYC) activators and PERIOD-TIMELESS (PER-TIM) repressors to drive rhythmic transcription peaking at dusk, is required for circadian timekeeping and overt behavioral rhythms. CLK-CYC also activates an interlocked feedback loop, which uses the PAR DOMAIN PROTEIN 1ε (PDP1ε) activator and the VRILLE (VRI) repressor to drive rhythmic transcription peaking at dawn. Although Pdp1ε mutants disrupt activity rhythms without eliminating clock function, whether vri is required for clock function and/or output is not known. Using a conditionally inactivatable transgene to rescue vri developmental lethality, we show that clock function persists after vri inactivation but that activity rhythms are abolished. The inactivation of vri disrupts multiple output pathways thought to be important for activity rhythms, including PDF accumulation and arborization rhythms in the small ventrolateral neuron (sLN_v) dorsal projection. These results demonstrate that vri acts as a key regulator of clock output and suggest that the primary function of the interlocked feedback loop in Drosophila is to drive rhythmic transcription required for overt rhythms.

A novel Drosophila injury model reveals severed axons are cleared through a Draper/MMP-1 signaling cascade

Neural injury triggers swift responses from glia, including glial migration and phagocytic clearance of damaged neurons. The transcriptional programs governing these complex innate glial immune responses are still unclear. Here, we describe a novel injury assay in adult Drosophila that elicits widespread glial responses in the ventral nerve cord (VNC). We profiled injury-induced changes in VNC gene expression by RNA sequencing (RNA-seq) and found that responsive genes fall into diverse signaling classes. One factor, matrix metalloproteinase-1 (MMP-1), is induced in Drosophila ensheathing glia responding to severed axons. Interestingly, glial induction of MMP-1 requires the highly conserved engulfment receptor Draper, as well as AP-1 and STAT92E. In MMP-1 depleted flies, glia do not properly infiltrate neuropil regions after axotomy and, as a consequence, fail to clear degenerating axonal debris. This work identifies Draper-dependent activation of MMP-1 as a novel cascade required for proper glial clearance of severed axons.

Rhythmic Behavior Is Controlled by the SRm160 Splicing Factor in Drosophila melanogaster

Circadian clocks organize the metabolism, physiology, and behavior of organisms throughout the day-night cycle by controlling daily rhythms in gene expression at the transcriptional and post-transcriptional levels. While many transcription factors underlying circadian oscillations are known, the splicing factors that modulate these rhythms remain largely unexplored. A genome-wide assessment of the alterations of gene expression in a null mutant of the alternative splicing regulator SR-related matrix protein of 160 kDa (SRm160) revealed the extent to which alternative splicing impacts on behavior-related genes. We show that SRm160 affects gene expression in pacemaker neurons of the Drosophila brain to ensure proper oscillations of the molecular clock. A reduced level of SRm160 in adult pacemaker neurons impairs circadian rhythms in locomotor behavior, and this phenotype is caused, at least in part, by a marked reduction in period (per) levels. Moreover, rhythmic accumulation of the neuropeptide PIGMENT DISPERSING FACTOR in the dorsal projections of these neurons is abolished after SRm160 depletion. The lack of rhythmicity in SRm160-downregulated flies is reversed by a fully spliced per construct, but not by an extra copy of the endogenous locus, showing that SRm160 positively regulates per levels in a splicing-dependent manner. Our findings highlight the significant effect of alternative splicing on the nervous system and particularly on brain function in an in vivo model.

A Screening of UNF Targets Identifies Rnb, a Novel Regulator of Drosophila Circadian Rhythms

Behavioral circadian rhythms are controlled by multioscillator networks comprising functionally different subgroups of clock neurons. Studies have demonstrated that molecular clocks in the fruit fly Drosophila melanogaster are regulated differently in clock neuron subclasses to support their specific functions (Lee et al., 2016; Top et al., 2016). The nuclear receptor unfulfilled (unf) represents a regulatory node that provides the small ventral lateral neurons (s-LNvs) unique characteristics as the master pacemaker (Beuchle et al., 2012). We previously showed that UNF interacts with the s-LNv molecular clocks by regulating transcription of the core clock gene period (per) (Jaumouillé et al., 2015). To gain more insight into the mechanisms by which UNF contributes to the functioning of the circadian master pacemaker, we identified UNF target genes using chromatin immunoprecipitation. Our data demonstrate that a previously uncharacterized gene CG7837, which we termed R and B (Rnb), acts downstream of UNF to regulate the function of the s-LNvs as the master circadian pacemaker. Mutations and LNv-targeted adult-restricted knockdown of Rnb impair locomotor rhythms. RNB localizes to the nucleus, and its loss-of-function blunts the molecular rhythms and output rhythms of the s-LNvs, particularly the circadian rhythms in PDF accumulation and axonal arbor remodeling. These results establish a second pathway by which UNF interacts with the molecular clocks in the s-LNvs and highlight the mechanistic differences in the molecular clockwork within the pacemaker circuit.SIGNIFICANCE STATEMENT Circadian behavior is generated by a pacemaker circuit comprising diverse classes of pacemaker neurons, each of which contains a molecular clock. In addition to the anatomical and functional diversity, recent studies have shown the mechanistic differences in the molecular clockwork among the pacemaker neurons in Drosophila Here, we identified the molecular characteristics distinguishing the s-LNvs, the master pacemaker of the locomotor rhythms, from other clock neuron subtypes. We demonstrated that a newly identified gene Rnb is an s-LNv-specific regulator of the molecular clock and essential for the generation of circadian locomotor behavior. Our results provide additional evidence to the emerging view that the differential regulation of the molecular clocks underlies the functional differences among the pacemaker neuron subgroups.

Both Drosophila matrix metalloproteinases have released and membrane-tethered forms but have different substrates

Matrix metalloproteinases (MMPs) are extracellular proteases that can cleave extracellular matrix and alter signaling pathways. They have been implicated in many disease states, but it has been difficult to understand the contribution of individual MMPs, as there are over 20 MMPs in vertebrates. The vertebrate MMPs have overlapping substrates, they exhibit genetic redundancy and compensation, and pharmacological inhibitors are non-specific. In contrast, there are only two MMP genes in Drosophila, DmMmp1 and DmMmp2, which makes Drosophila an attractive system to analyze the basis of MMP specificity. Previously, Drosophila MMPs have been categorized by their pericellular localization, as Mmp1 appeared to be secreted and Mmp2 appeared to be membrane-anchored, suggesting that protein localization was the critical distinction in this small MMP family. We report here that products of both genes are found at the cell surface and released into media. Additionally, we show that products of both genes contain GPI-anchors, and unexpectedly, that GPI-anchored MMPs promote cell adhesion when they are rendered inactive. Finally, by using new reagents and assays, we show that the two MMPs cleave different substrates, suggesting that this is the important distinction within this smallest MMP family.

miR-124 Regulates the Phase of Drosophila Circadian Locomotor Behavior

Animals use circadian rhythms to anticipate daily environmental changes. Circadian clocks have a profound effect on behavior. In Drosophila, for example, brain pacemaker neurons dictate that flies are mostly active at dawn and dusk. miRNAs are small, regulatory RNAs (≈22 nt) that play important roles in posttranscriptional regulation. Here, we identify miR-124 as an important regulator of Drosophila circadian locomotor rhythms. Under constant darkness, flies lacking miR-124 (miR-124(KO)) have a dramatically advanced circadian behavior phase. However, whereas a phase defect is usually caused by a change in the period of the circadian pacemaker, this is not the case in miR-124(KO) flies. Moreover, the phase of the circadian pacemaker in the clock neurons that control rhythmic locomotion is not altered either. Therefore, miR-124 modulates the output of circadian clock neurons rather than controlling their molecular pacemaker. Circadian phase is also advanced under temperature cycles, but a light/dark cycle partially corrects the defects in miR-124(KO) flies. Indeed, miR-124(KO) shows a normal evening phase under the latter conditions, but morning behavioral activity is suppressed. In summary, miR-124 controls diurnal activity and determines the phase of circadian locomotor behavior without affecting circadian pacemaker function. It thus provides a potent entry point to elucidate the mechanisms by which the phase of circadian behavior is determined.

SIGNIFICANCE STATEMENT

In animals, molecular circadian clocks control the timing of behavioral activities to optimize them with the day/night cycle. This is critical for their fitness and survival. The mechanisms by which the phase of circadian behaviors is determined downstream of the molecular pacemakers are not yet well understood. Recent studies indicate that miRNAs are important regulators of circadian outputs. We found that miR-124 shapes diurnal behavioral activity and has a striking impact on the phase of circadian locomotor behavior. Surprisingly, the period and phase of the neural circadian pacemakers driving locomotor rhythms are unaffected. Therefore, miR-124 is a critical modulator of the circadian output pathways that control circadian behavioral rhythms.

Communication between circadian clusters: The key to a plastic network

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

Synchrony and desynchrony in circadian clocks: impacts on learning and memory

Circadian clocks evolved under conditions of environmental variation, primarily alternating light dark cycles, to enable organisms to anticipate daily environmental events and coordinate metabolic, physiological, and behavioral activities. However, modern lifestyle and advances in technology have increased the percentage of individuals working in phases misaligned with natural circadian activity rhythms. Endogenous circadian oscillators modulate alertness, the acquisition of learning, memory formation, and the recall of memory with examples of circadian modulation of memory observed across phyla from invertebrates to humans. Cognitive performance and memory are significantly diminished when occurring out of phase with natural circadian rhythms. Disruptions in circadian regulation can lead to impairment in the formation of memories and manifestation of other cognitive deficits. This review explores the types of interactions through which the circadian clock modulates cognition, highlights recent progress in identifying mechanistic interactions between the circadian system and the processes involved in memory formation, and outlines methods used to remediate circadian perturbations and reinforce circadian adaptation.