The expanding reach of the GAL4/UAS system into the behavioral neurobiology of Drosophila

Fighting Flies: Quantifying and Analyzing Drosophila Aggression

Aggression is an innate behavior that likely evolved in the framework of defending or obtaining resources. This complex social behavior is influenced by genetic, environmental, and internal factors. Drosophila melanogaster remains an effective and exciting model organism with which to unravel the mechanistic basis of aggression due to its small but sophisticated brain, an impressive array of neurogenetic tools, and robust stereotypical behavioral patterns. The investigations of many laboratories have led to the identification of external and internal state factors that promote aggression, sex differences in the patterns and outcome of aggression, and neurotransmitters that regulate aggression.

Hereditary spastic paraparesis (HSP) presenting as cerebral palsy due to ADD3 variant with mechanistic insight provided by a Drosophila γ-adducin model

INTRODUCTION

Cerebral palsy (CP) causes neurological disability in early childhood. Hypoxic-ischaemic injury plays a major role in its aetiology, nevertheless, genetic and epigenetic factors may contribute to the clinical presentation. Mutations in ADD3 (encoding γ-adducin) gene have been described in a monogenic form of spastic quadriplegic cerebral palsy (OMIM 601568).

METHODS

We studied a sixteen-year-old male with spastic diplegia. Several investigations including neurometabolic testing, brain and spine magnetic resonance imaging (MRI) and CGH-Array were normal. Further, clinical genetics assessment and Whole Exome Sequencing (WES) gave the diagnosis. We generated an animal model using Drosophila to study the effects of γ-adducin loss and gain of function.

RESULTS

WES revealed a biallelic variant in the ADD3 gene, NM_016824.5(ADD3): c.1100G>A, p.(Gly367Asp). Mutations in this gene have been described as an ultra-rare autosomal recessive which is a known form of inherited cerebral palsy. Molecular modelling suggests that this mutation leads to a loss of structural integrity of γ-adducin and is therefore expected to result in a decreased level of functional protein. Pan-neuronal over-expression or knock-down of the Drosophila ortholog of ADD3 called hts caused a reduction of life span and impaired locomotion thereby phenocopying aspects of the human disease.

CONCLUSION

Our animal experiments present a starting point to understand the biological processes underpinning the clinical phenotype and pathogenic mechanisms, to gain insights into potential future methods for treating or preventing ADD3 related spastic quadriplegic cerebral palsy.

Animal Models of Neurodegenerative Disease: Recent Advances in Fly Highlight Innovative Approaches to Drug Discovery

Neurodegenerative diseases represent a formidable challenge to global health. As advances in other areas of medicine grant healthy living into later decades of life, aging diseases such as Alzheimer's disease (AD) and other neurodegenerative disorders can diminish the quality of these additional years, owed largely to the lack of efficacious treatments and the absence of durable cures. Alzheimer's disease prevalence is predicted to more than double in the next 30 years, affecting nearly 15 million Americans, with AD-associated costs exceeding $1 billion by 2050. Delaying onset of AD and other neurodegenerative diseases is critical to improving the quality of life for patients and reducing the burden of disease on caregivers and healthcare systems. Significant progress has been made to model disease pathogenesis and identify points of therapeutic intervention. While some researchers have contributed to our understanding of the proteins and pathways that drive biological dysfunction in disease using in vitro and in vivo models, others have provided mathematical, biophysical, and computational technologies to identify potential therapeutic compounds using in silico modeling. The most exciting phase of the drug discovery process is now: by applying a target-directed approach that leverages the strengths of multiple techniques and validates lead hits using Drosophila as an animal model of disease, we are on the fast-track to identifying novel therapeutics to restore health to those impacted by neurodegenerative disease.

GABA transmission from mAL interneurons regulates aggression in Drosophila males

Aggression is dependent on the sex of the conspecific in almost all animal species. But the neuronal basis of how sex-specific chemosensory signals regulate aggression is poorly understood. Using the fruit fly model of Drosophila melanogaster, we demonstrate that activation of a group of GABAergic central brain neurons, known to respond to sex-specific pheromonal stimuli, enhances aggression in dyadic male encounters. Inactivation of this neuronal group decreases aggression and increases the reciprocal social behavior of courtship. Our results can help trace the neural circuit from pheromone processing in the sensory neurons to behavior integration in the central brain and ultimately help understand how neurons encode the behavior of aggression.

Aggression is known to be regulated by pheromonal information in many species. But how central brain neurons processing this information modulate aggression is poorly understood. Using the fruit fly model of Drosophila melanogaster, we systematically characterize the role of a group of sexually dimorphic GABAergic central brain neurons, popularly known as mAL, in aggression regulation. The mAL neurons are known to be activated by male and female pheromones. In this report, we show that mAL activation robustly increases aggression, whereas its inactivation decreases aggression and increases intermale courtship, a behavior considered reciprocal to aggression. GABA neurotransmission from mAL is crucial for this behavior regulation. Exploiting the genetic toolkit of the fruit fly model, we also find a small group of approximately three to five GABA⁺ central brain neurons with anatomical similarities to mAL. Activation of the mAL resembling group of neurons is necessary for increasing intermale aggression. Overall, our findings demonstrate how changes in activity of GABA⁺ central brain neurons processing pheromonal information, such as mAL in Drosophila melanogaster, directly modulate the social behavior of aggression in male–male pairings.

The Effects of Chloride Flux on Drosophila Heart Rate

Approaches are sought after to regulate ionotropic and chronotropic properties of the mammalian heart. Electrodes are commonly used for rapidly exciting cardiac tissue and resetting abnormal pacing. With the advent of optogenetics and the use of tissue-specific expression of light-activated channels, cardiac cells cannot only be excited but also inhibited with ion-selective conductance. As a proof of concept for the ability to slow down cardiac pacing, anion-conducting channelrhodopsins (GtACR1/2) and the anion pump halorhodopsin (eNpHR) were expressed in hearts of larval Drosophila and activated by light. Unlike body wall muscles in most animals, the equilibrium potential for Cl⁻ is more positive as compared to the resting membrane potential in larval Drosophila. As a consequence, upon activating the two forms of GtACR1 and 2 with low light intensity the heart rate increased, likely due to depolarization and opening of voltage-gated Ca²⁺ channels. However, with very intense light activation the heart rate ceases, which may be due to Cl^– shunting to the reversal potential for chloride. Activating eNpHR hyperpolarizes body wall and cardiac muscle in larval Drosophila and rapidly decreases heart rate. The decrease in heart rate is related to light intensity. Intense light activation of eNpHR stops the heart from beating, whereas lower intensities slowed the rate. Even with upregulation of the heart rate with serotonin, the pacing of the heart was slowed with light. Thus, regulation of the heart rate in Drosophila can be accomplished by activating anion-conducting channelrhodopsins using light. These approaches are demonstrated in a genetically amenable insect model.

Optogenetic activation of short neuropeptide F (sNPF) neurons induces sleep in Drosophila melanogaster

Sleep abnormalities have widespread and costly public health consequences, yet we have only a rudimentary understanding of the events occurring at the cellular level in the brain that regulate sleep. Several key signaling molecules that regulate sleep across taxa come from the family of neuropeptide transmitters. For example, in Drosophila melanogaster, the neuropeptide Y (NPY)-related transmitter short neuropeptide F (sNPF) appears to promote sleep. In this study, we utilized optogenetic activation of neuronal populations expressing sNPF to determine the causal effects of precisely timed activity in these cells on sleep behavior. Combining sNPF-GAL4 and UAS-Chrimson transgenes allowed us to activate sNPF neurons using red light. We found that activating sNPF neurons for as little as 3 s at a time of day when most flies were awake caused a rapid transition to sleep that persisted for another 2+ hours following the stimulation. Changing the timing of red light stimulation to times of day when flies were already asleep caused the control flies to wake up (due to the pulse of light), but the flies in which sNPF neurons were activated stayed asleep through the light pulse, and then showed further increases in sleep at later points when they would have normally been waking up. Video recording of individual fly responses to short-term (0.5-20 s) activation of sNPF neurons demonstrated a clear light duration-dependent decrease in movement during the subsequent 4-min period. These results provide supportive evidence that sNPF-producing neurons promote long-lasting increases in sleep, and show for the first time that even brief periods of activation of these neurons can cause changes in behavior that persist after cessation of activation. We have also presented evidence that sNPF neuron activation produces a homeostatic sleep drive that can be dissipated at times long after the neurons were stimulated. Future studies will determine the specific roles of sub-populations of sNPF-producing neurons, and will also assess how sNPF neurons act in concert with other neuronal circuits to control sleep.

Identification and In Vivo Characterisation of Cardioactive Peptides in Drosophila melanogaster

Neuropeptides and peptide hormones serve as critical regulators of numerous biological processes, including development, growth, reproduction, physiology, and behaviour. In mammals, peptidergic regulatory systems are complex and often involve multiple peptides that act at different levels and relay to different receptors. To improve the mechanistic understanding of such complex systems, invertebrate models in which evolutionarily conserved peptides and receptors regulate similar biological processes but in a less complex manner have emerged as highly valuable. Drosophila melanogaster represents a favoured model for the characterisation of novel peptidergic signalling events and for evaluating the relevance of those events in vivo. In the present study, we analysed a set of neuropeptides and peptide hormones for their ability to modulate cardiac function in semi-intact larval Drosophila melanogaster. We identified numerous peptides that significantly affected heart parameters such as heart rate, systolic and diastolic interval, rhythmicity, and contractility. Thus, peptidergic regulation of the Drosophila heart is not restricted to chronotropic adaptation but also includes inotropic modulation. By specifically interfering with the expression of corresponding peptides in transgenic animals, we assessed the in vivo relevance of the respective peptidergic regulation. Based on the functional conservation of certain peptides throughout the animal kingdom, the identified cardiomodulatory activities may be relevant not only to proper heart function in Drosophila, but also to corresponding processes in vertebrates, including humans.

Isolation of Aggressive Behavior Mutants in Drosophila Using a Screen for Wing Damage

Aggression is a complex social behavior that is widespread in nature. To date, only a limited number of genes that affect aggression have been identified, in large part because the complexity of the phenotype makes screening difficult and time-consuming regardless of the species that is studied. We discovered that aggressive group-housed Drosophila melanogaster males inflict damage on each other's wings, and show that wing damage negatively affects their ability to fly and mate. Using this wing-damage phenotype, we screened males from ∼1400 chemically mutagenized strains and found ∼40 mutant strains with substantial wing damage. Five of these mutants also had increased aggressive behavior. To identify the causal mutation in one of our top aggressive strains, we used whole-genome sequencing and genomic duplication rescue strategies. We identified a novel mutation in the voltage-gated potassium channel Shaker (Sh) and show that a nearby previously identified Sh mutation also results in increased aggression. This simple screen can be used to dissect the molecular mechanisms underlying aggression.

Synthetic regulatory RNAs selectively suppress the progression of bladder cancer

The traditional treatment for cancer is lack of specificity and efficacy. Modular synthetic regulatory RNAs, such as inhibitive RNA (iRNA) and active RNA (aRNA), may overcome these limitations. Here, we synthesize a new iRNA to bind the upstream activating sequence (UAS) of a minimal promoter that drives expression of artificial miRNAs (amiRNAs) targeting MYC, which represses the binding interaction between UAS and GAL4 fusion protein (GAL4-VP64) in GAL4/UAS system. The aRNA driven by a tumor-specific mutant human telomerase reverse transcriptase (hTERT) promoter is created to interact with iRNA to expose UAS again in bladder cancer. Without the aRNA, mRNA and protein levels of MYC, cell growth, cell apoptosis and cell migration were no significance in two bladder cancer cell lines, T24 and 5637, and human foreskin fibroblast (HFF) cells. The aRNA significantly inhibited the expression of MYC in mRNA and protein levels, as well as the proliferation and migration of the cancer cells, but not in HFF cells. These results indicated that regulatory RNAs selectively controlled the expression of amiRNAs and ultimately suppress the progression of bladder cancer cells without affecting normal cells. Synthetic regulatory RNAs might be a selective therapeutic approach for bladder cancer.

Genetic Analysis of Mosquito Detection of Humans

Mosquitoes detect the presence of humans by integrating chemosensory, thermal, and visual cues. Among these, odors are crucial for mosquito host detection. Insects have evolved a diverse repertoire of receptors to detect their plant and animal hosts. Genetic analysis of these receptors in Drosophila has set the stage for similar studies in mosquitoes. The diversity of the cues involved in mosquito host-seeking has made designing behavioral control strategies a challenge. The sensory receptors that are most important for mosquito detection of humans can now be determined using genome editing. Here, we will review our current understanding of the salient cues that attract mosquitoes, their receptors, and suggest ways forward for novel olfaction-based vector control strategies.

Optogenetic stimulation of Drosophila heart rate at different temperatures and Ca2+ concentrations

Optogenetics is a revolutionary technique that enables noninvasive activation of electrically excitable cells. In mammals, heart rate has traditionally been modulated with pharmacological agents or direct stimulation of cardiac tissue with electrodes. However, implanted wires have been known to cause physical damage and damage from electrical currents. Here, we describe a proof of concept to optically drive cardiac function in a model organism, Drosophila melanogaster. We expressed the light sensitive channelrhodopsin protein ChR2.XXL in larval Drosophila hearts and examined light-induced activation of cardiac tissue. After demonstrating optical stimulation of larval heart rate, the approach was tested at low temperature and low calcium levels to simulate mammalian heart transplant conditions. Optical activation of ChR2.XXL substantially increased heart rate in all conditions. We have developed a system that can be instrumental in characterizing the physiology of optogenetically controlled cardiac function with an intact heart.

Modulatory Action by the Serotonergic System: Behavior and Neurophysiology in Drosophila melanogaster

Serotonin modulates various physiological processes and behaviors. This study investigates the role of 5-HT in locomotion and feeding behaviors as well as in modulation of sensory-motor circuits. The 5-HT biosynthesis was dysregulated by feeding Drosophila larvae 5-HT, a 5-HT precursor, or an inhibitor of tryptophan hydroxylase during early stages of development. The effects of feeding fluoxetine, a selective serotonin reuptake inhibitor, during early second instars were also examined. 5-HT receptor subtypes were manipulated using RNA interference mediated knockdown and 5-HT receptor insertional mutations. Moreover, synaptic transmission at 5-HT neurons was blocked or enhanced in both larvae and adult flies. The results demonstrate that disruption of components within the 5-HT system significantly impairs locomotion and feeding behaviors in larvae. Acute activation of 5-HT neurons disrupts normal locomotion activity in adult flies. To determine which 5-HT receptor subtype modulates the evoked sensory-motor activity, pharmacological agents were used. In addition, the activity of 5-HT neurons was enhanced by expressing and activating TrpA1 channels or channelrhodopsin-2 while recording the evoked excitatory postsynaptic potentials (EPSPs) in muscle fibers. 5-HT2 receptor activation mediates a modulatory role in a sensory-motor circuit, and the activation of 5-HT neurons can suppress the neural circuit activity, while fluoxetine can significantly decrease the sensory-motor activity.

Activity-dependent FMRP requirements in development of the neural circuitry of learning and memory

The activity-dependent refinement of neural circuit connectivity during critical periods of brain development is essential for optimized behavioral performance. We hypothesize that this mechanism is defective in fragile X syndrome (FXS), the leading heritable cause of intellectual disability and autism spectrum disorders. Here, we use optogenetic tools in the Drosophila FXS disease model to test activity-dependent dendritogenesis in two extrinsic neurons of the mushroom body (MB) learning and memory brain center: (1) the input projection neuron (PN) innervating Kenyon cells (KCs) in the MB calyx microglomeruli and (2) the output MVP2 neuron innervated by KCs in the MB peduncle. Both input and output neuron classes exhibit distinctive activity-dependent critical period dendritic remodeling. MVP2 arbors expand in Drosophila mutants null for fragile X mental retardation 1 (dfmr1), as well as following channelrhodopsin-driven depolarization during critical period development, but are reduced by halorhodopsin-driven hyperpolarization. Optogenetic manipulation of PNs causes the opposite outcome--reduced dendritic arbors following channelrhodopsin depolarization and expanded arbors following halorhodopsin hyperpolarization during development. Importantly, activity-dependent dendritogenesis in both neuron classes absolutely requires dfmr1 during one developmental window. These results show that dfmr1 acts in a neuron type-specific activity-dependent manner for sculpting dendritic arbors during early-use, critical period development of learning and memory circuitry in the Drosophila brain.

Gene-specific cell labeling using MiMIC transposons

Binary expression systems such as GAL4/UAS, LexA/LexAop and QF/QUAS have greatly enhanced the power of Drosophila as a model organism by allowing spatio-temporal manipulation of gene function as well as cell and neural circuit function. Tissue-specific expression of these heterologous transcription factors relies on random transposon integration near enhancers or promoters that drive the binary transcription factor embedded in the transposon. Alternatively, gene-specific promoter elements are directly fused to the binary factor within the transposon followed by random or site-specific integration. However, such insertions do not consistently recapitulate endogenous expression. We used Minos-Mediated Integration Cassette (MiMIC) transposons to convert host loci into reliable gene-specific binary effectors. MiMIC transposons allow recombinase-mediated cassette exchange to modify the transposon content. We developed novel exchange cassettes to convert coding intronic MiMIC insertions into gene-specific binary factor protein-traps. In addition, we expanded the set of binary factor exchange cassettes available for non-coding intronic MiMIC insertions. We show that binary factor conversions of different insertions in the same locus have indistinguishable expression patterns, suggesting that they reliably reflect endogenous gene expression. We show the efficacy and broad applicability of these new tools by dissecting the cellular expression patterns of the Drosophila serotonin receptor gene family.

Impaired activity-dependent neural circuit assembly and refinement in autism spectrum disorder genetic models

Early-use activity during circuit-specific critical periods refines brain circuitry by the coupled processes of eliminating inappropriate synapses and strengthening maintained synapses. We theorize these activity-dependent (A-D) developmental processes are specifically impaired in autism spectrum disorders (ASDs). ASD genetic models in both mouse and Drosophila have pioneered our insights into normal A-D neural circuit assembly and consolidation, and how these developmental mechanisms go awry in specific genetic conditions. The monogenic fragile X syndrome (FXS), a common cause of heritable ASD and intellectual disability, has been particularly well linked to defects in A-D critical period processes. The fragile X mental retardation protein (FMRP) is positively activity-regulated in expression and function, in turn regulates excitability and activity in a negative feedback loop, and appears to be required for the A-D remodeling of synaptic connectivity during early-use critical periods. The Drosophila FXS model has been shown to functionally conserve the roles of human FMRP in synaptogenesis, and has been centrally important in generating our current mechanistic understanding of the FXS disease state. Recent advances in Drosophila optogenetics, transgenic calcium reporters, highly-targeted transgenic drivers for individually-identified neurons, and a vastly improved connectome of the brain are now being combined to provide unparalleled opportunities to both manipulate and monitor A-D processes during critical period brain development in defined neural circuits. The field is now poised to exploit this new Drosophila transgenic toolbox for the systematic dissection of A-D mechanisms in normal versus ASD brain development, particularly utilizing the well-established Drosophila FXS disease model.

Cellular dissection of the spinal cord motor column by BAC transgenesis and gene trapping in zebrafish

Bacterial artificial chromosome (BAC) transgenesis and gene/enhancer trapping are effective approaches for identification of genetically defined neuronal populations in the central nervous system (CNS). Here, we applied these techniques to zebrafish (Danio rerio) in order to obtain insights into the cellular architecture of the axial motor column in vertebrates. First, by using the BAC for the Mnx class homeodomain protein gene mnr2b/mnx2b, we established the mnGFF7 transgenic line expressing the Gal4FF transcriptional activator in a large part of the motor column. Single cell labeling of Gal4FF-expressing cells in the mnGFF7 line enabled a detailed investigation of the morphological characteristics of individual spinal motoneurons, as well as the overall organization of the motor column in a spinal segment. Secondly, from a large-scale gene trap screen, we identified transgenic lines that marked discrete subpopulations of spinal motoneurons with Gal4FF. Molecular characterization of these lines led to the identification of the ADAMTS3 gene, which encodes an evolutionarily conserved ADAMTS family of peptidases and is dynamically expressed in the ventral spinal cord. The transgenic fish established here, along with the identified gene, should facilitate an understanding of the cellular and molecular architecture of the spinal cord motor column and its connection to muscles in vertebrates.

A versatile method for cell-specific profiling of translated mRNAs in Drosophila

In Drosophila melanogaster few methods exist to perform rapid cell-type or tissue-specific expression profiling. A translating ribosome affinity purification (TRAP) method to profile actively translated mRNAs has been developed for use in a number of multicellular organisms although it has only been implemented to examine limited sets of cell- or tissue-types in these organisms. We have adapted the TRAP method for use in the versatile GAL4/UAS system of Drosophila allowing profiling of almost any tissue/cell-type with a single genetic cross. We created transgenic strains expressing a GFP-tagged ribosomal protein, RpL10A, under the control of the UAS promoter to perform cell-type specific translatome profiling. The GFP::RpL10A fusion protein incorporates efficiently into ribosomes and polysomes. Polysome affinity purification strongly enriches mRNAs from expected genes in the targeted tissues with sufficient sensitivity to analyze expression in small cell populations. This method can be used to determine the unique translatome profiles in different cell-types under varied physiological, pharmacological and pathological conditions.

Of toasters and molecular ticker tapes

Experiments in systems neuroscience can be seen as consisting of three steps: (1) selecting the signals we are interested in, (2) probing the system with carefully chosen stimuli, and (3) getting data out of the brain. Here I discuss how emerging techniques in molecular biology are starting to improve these three steps. To estimate its future impact on experimental neuroscience, I will stress the analogy of ongoing progress with that of microprocessor production techniques. These techniques have allowed computers to simplify countless problems; because they are easier to use than mechanical timers, they are even built into toasters. Molecular biology may advance even faster than computer speeds and has made immense progress in understanding and designing molecules. These advancements may in turn produce impressive improvements to each of the three steps, ultimately shifting the bottleneck from obtaining data to interpreting it.

Modeling human neurodegenerative diseases in transgenic systems

Transgenic systems are widely used to study the cellular and molecular basis of human neurodegenerative diseases. A wide variety of model organisms have been utilized, including bacteria (Escherichia coli), plants (Arabidopsis thaliana), nematodes (Caenorhabditis elegans), arthropods (Drosophila melanogaster), fish (zebrafish, Danio rerio), rodents (mouse, Mus musculus and rat, Rattus norvegicus) as well as non-human primates (rhesus monkey, Macaca mulatta). These transgenic systems have enormous value for understanding the pathophysiological basis of these disorders and have, in some cases, been instrumental in the development of therapeutic approaches to treat these conditions. In this review, we discuss the most commonly used model organisms and the methodologies available for the preparation of transgenic organisms. Moreover, we provide selected examples of the use of these technologies for the preparation of transgenic animal models of neurodegenerative diseases, including Alzheimer's disease (AD), frontotemporal lobar degeneration (FTLD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD) and Parkinson's disease (PD) and discuss the application of these technologies to AD as an example of how transgenic modeling has affected the study of human neurodegenerative diseases.

The genetics of calcium signaling in Drosophila melanogaster

BACKGROUND

Genetic screens for behavioral and physiological defects in Drosophila melanogaster, helped identify several components of calcium signaling of which some, like the Trps, were novel. For genes initially identified in vertebrates, reverse genetic methods have allowed functional studies at the cellular and systemic levels.

SCOPE OF REVIEW

The aim of this review is to explain how various genetic methods available in Drosophila have been used to place different arms of Ca2+ signaling in the context of organismal development, physiology and behavior.

MAJOR CONCLUSION

Mutants generated in genes encoding a range of Ca2+ transport systems, binding proteins and enzymes affect multiple aspects of neuronal and muscle physiology. Some also affect the maintenance of ionic balance and excretion from malpighian tubules and innate immune responses in macrophages. Aspects of neuronal physiology affected include synaptic growth and plasticity, sensory transduction, flight circuit development and function. Genetic interaction screens have shown that mechanisms of maintaining Ca2+ homeostasis in Drosophila are cell specific and require a synergistic interplay between different intracellular and plasma membrane Ca2+ signaling molecules.

GENERAL SIGNIFICANCE

Insights gained through genetic studies of conserved Ca2+ signaling pathways have helped understand multiple aspects of fly physiology. The similarities between mutant phenotypes of Ca2+ signaling genes in Drosophila with certain human disease conditions, especially where homologous genes are causative factors, are likely to aid in the discovery of underlying disease mechanisms and help develop novel therapeutic strategies. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signalling.