Presynaptic glutamic acid decarboxylase is required for induction of the postsynaptic receptor field at a glutamatergic synapse

Muscle cofilin alters neuromuscular junction postsynaptic development to strengthen functional neurotransmission

Cofilin, an actin-severing protein, plays key roles in muscle sarcomere addition and maintenance. Our previous work found that Drosophila cofilin (DmCFL) knockdown in muscle causes progressive deterioration of muscle structure and function and produces features seen in nemaline myopathy caused by cofilin mutations. We hypothesized that disruption of actin cytoskeleton dynamics by DmCFL knockdown would impact other aspects of muscle development, and, thus, conducted an RNA-sequencing analysis that unexpectedly revealed upregulated expression of numerous neuromuscular junction (NMJ) genes. We found that DmCFL is enriched in the muscle postsynaptic compartment and that DmCFL muscle knockdown causes F-actin disorganization in this subcellular domain prior to the sarcomere defects observed later in development. Despite NMJ gene expression changes, we found no significant changes in gross presynaptic Bruchpilot active zones or total postsynaptic glutamate receptor levels. However, DmCFL knockdown resulted in mislocalization of GluRIIA class glutamate receptors in more deteriorated muscles and strongly impaired NMJ transmission strength. These findings expand our understanding of the roles of cofilin in muscle to include NMJ structural development and suggest that NMJ defects may contribute to the pathophysiology of nemaline myopathy.

Paraneoplastic encephalitis

The first reports of encephalitis associated with cancer date to the 1960s and were characterized by clinical and pathologic involvement of limbic areas. This specific association was called limbic encephalitis (LE). The subsequent discovery of several "onconeural" antibodies (Abs), i.e., Abs targeting an antigen shared by neurons and tumor cells, supported the hypothesis of an autoimmune paraneoplastic etiology of LE and other forms of rapidly progressive encephalopathy. Over the past 20 years, similar clinical pictures with different clinical courses have been described in association with novel Abs-binding neuronal membrane proteins and proved to be pathogenic. The most well-known encephalitis in this group was described in 2007 as an association of a complex neuro-psychiatric syndrome, N-methyl-d-aspartate (NMDA) receptor-Abs, and ovarian teratoma in young women. Later on, nonparaneoplastic cases of NMDA receptor encephalitis were also described. Since then, the historical concept of LE and Ab associated encephalitis has changed. Some of these occur in fact more commonly in the absence of a malignancy (e.g., anti-LG1 Abs). Lastly, seronegative cases were also described. The term paraneoplastic encephalitis nowadays encompasses different syndromes that may be triggered by occult tumors.

The Neurotransmitters Involved in Drosophila Alcohol-Induced Behaviors

Alcohol is a widely used and abused substance with numerous negative consequences for human health and safety. Historically, alcohol's widespread, non-specific neurobiological effects have made it a challenge to study in humans. Therefore, model organisms are a critical tool for unraveling the mechanisms of alcohol action and subsequent effects on behavior. Drosophila melanogaster is genetically tractable and displays a vast behavioral repertoire, making it a particularly good candidate for examining the neurobiology of alcohol responses. In addition to being experimentally amenable, Drosophila have high face and mechanistic validity: their alcohol-related behaviors are remarkably consistent with humans and other mammalian species, and they share numerous conserved neurotransmitters and signaling pathways. Flies have a long history in alcohol research, which has been enhanced in recent years by the development of tools that allow for manipulating individual Drosophila neurotransmitters. Through advancements such as the GAL4/UAS system and CRISPR/Cas9 mutagenesis, investigation of specific neurotransmitters in small subsets of neurons has become ever more achievable. In this review, we describe recent progress in understanding the contribution of seven neurotransmitters to fly behavior, focusing on their roles in alcohol response: dopamine, octopamine, tyramine, serotonin, glutamate, GABA, and acetylcholine. We chose these small-molecule neurotransmitters due to their conservation in mammals and their importance for behavior. While neurotransmitters like dopamine and octopamine have received significant research emphasis regarding their contributions to behavior, others, like glutamate, GABA, and acetylcholine, remain relatively unexplored. Here, we summarize recent genetic and behavioral findings concerning these seven neurotransmitters and their roles in the behavioral response to alcohol, highlighting the fitness of the fly as a model for human alcohol use.

The voltage-gated potassium channel Shaker promotes sleep via thermosensitive GABA transmission

Genes and neural circuits coordinately regulate animal sleep. However, it remains elusive how these endogenous factors shape sleep upon environmental changes. Here, we demonstrate that Shaker (Sh)-expressing GABAergic neurons projecting onto dorsal fan-shaped body (dFSB) regulate temperature-adaptive sleep behaviors in Drosophila. Loss of Sh function suppressed sleep at low temperature whereas light and high temperature cooperatively gated Sh effects on sleep. Sh depletion in GABAergic neurons partially phenocopied Sh mutants. Furthermore, the ionotropic GABA receptor, Resistant to dieldrin (Rdl), in dFSB neurons acted downstream of Sh and antagonized its sleep-promoting effects. In fact, Rdl inhibited the intracellular cAMP signaling of constitutively active dopaminergic synapses onto dFSB at low temperature. High temperature silenced GABAergic synapses onto dFSB, thereby potentiating the wake-promoting dopamine transmission. We propose that temperature-dependent switching between these two synaptic transmission modalities may adaptively tune the neural property of dFSB neurons to temperature shifts and reorganize sleep architecture for animal fitness.

Ji-hyung Kim and Yoonhee Ki et al. show that low temperatures suppress sleep in Drosophila by increasing GABA transmission in Shaker-expressing GABAergic neurons projecting onto the dorsal fan-shaped body, while high temperatures potentiate dopamine-induced arousal by reducing GABA transmission. This study highlights a role for Shaker in sleep modulation via a temperature-dependent switch in GABA signaling.

A GABAergic Maf-expressing interneuron subset regulates the speed of locomotion in Drosophila

Interneurons (INs) coordinate motoneuron activity to generate appropriate patterns of muscle contractions, providing animals with the ability to adjust their body posture and to move over a range of speeds. In Drosophila larvae several IN subtypes have been morphologically described and their function well documented. However, the general lack of molecular characterization of those INs prevents the identification of evolutionary counterparts in other animals, limiting our understanding of the principles underlying neuronal circuit organization and function. Here we characterize a restricted subset of neurons in the nerve cord expressing the Maf transcription factor Traffic Jam (TJ). We found that TJ⁺ neurons are highly diverse and selective activation of these different subtypes disrupts larval body posture and induces specific locomotor behaviors. Finally, we show that a small subset of TJ⁺ GABAergic INs, singled out by the expression of a unique transcription factors code, controls larval crawling speed.

Spinal interneurons (IN) coordinate motoneuron activity to modulate locomotion behavior. Here, the authors characterize a subset of IN subtypes expressing the Maf transcription factor Traffic Jam (TJ) and report the distinct effects of their activation on body posture and locomotion in Drosophila larvae.

Nuclear Transcriptomes of the Seven Neuronal Cell Types That Constitute the Drosophila Mushroom Bodies

The insect mushroom body (MB) is a conserved brain structure that plays key roles in a diverse array of behaviors. The Drosophila melanogaster MB is the primary invertebrate model of neural circuits related to memory formation and storage, and its development, morphology, wiring, and function has been extensively studied. MBs consist of intrinsic Kenyon Cells that are divided into three major neuron classes (γ, α'/β' and α/β) and 7 cell subtypes (γd, γm, α'/β'ap, α'/β'm, α/βp, α/βs and α/βc) based on their birth order, morphology, and connectivity. These subtypes play distinct roles in memory processing, however the underlying transcriptional differences are unknown. Here, we used RNA sequencing (RNA-seq) to profile the nuclear transcriptomes of each MB neuronal cell subtypes. We identified 350 MB class- or subtype-specific genes, including the widely used α/β class marker Fas2 and the α'/β' class marker trio Immunostaining corroborates the RNA-seq measurements at the protein level for several cases. Importantly, our data provide a full accounting of the neurotransmitter receptors, transporters, neurotransmitter biosynthetic enzymes, neuropeptides, and neuropeptide receptors expressed within each of these cell types. This high-quality, cell type-level transcriptome catalog for the Drosophila MB provides a valuable resource for the fly neuroscience community.

Downregulation of glutamic acid decarboxylase in Drosophila TDP-43-null brains provokes paralysis by affecting the organization of the neuromuscular synapses

Amyotrophic lateral sclerosis is a progressive neurodegenerative disease that affects the motor system, comprised of motoneurons and associated glia. Accordingly, neuronal or glial defects in TDP-43 function provoke paralysis due to the degeneration of the neuromuscular synapses in Drosophila. To identify the responsible molecules and mechanisms, we performed a genome wide proteomic analysis to determine differences in protein expression between wild-type and TDP-43-minus fly heads. The data established that mutant insects presented reduced levels of the enzyme glutamic acid decarboxylase (Gad1) and increased concentrations of extracellular glutamate. Genetic rescue of Gad1 activity in neurons or glia was sufficient to recuperate flies locomotion, synaptic organization and glutamate levels. Analogous recovery was obtained by treating TDP-43-null flies with glutamate receptor antagonists demonstrating that Gad1 promotes synapses formation and prevents excitotoxicity. Similar suppression of TDP-43 provoked the downregulation of GAD67, the Gad1 homolog protein in human neuroblastoma cell lines and analogous modifications were observed in iPSC-derived motoneurons from patients carrying mutations in TDP-43, uncovering conserved pathological mechanisms behind the disease.

Transcriptional Reorganization of Drosophila Motor Neurons and Their Muscular Junctions toward a Neuroendocrine Phenotype by the bHLH Protein Dimmed

Neuroendocrine cells store and secrete bulk amounts of neuropeptides, and display morphological and molecular characteristics distinct from neurons signaling with classical neurotransmitters. In Drosophila the transcription factor Dimmed (Dimm), is a prime organizer of neuroendocrine capacity in a majority of the peptidergic neurons. These neurons display large cell bodies and extensive axon terminations that commonly do not form regular synapses. We ask which molecular compartments of a neuron are affected by Dimm to generate these morphological features. Thus, we ectopically expressed Dimm in glutamatergic, Dimm-negative, motor neurons and analyzed their characteristics in the central nervous system and the neuromuscular junction. Ectopic Dimm results in motor neurons with enlarged cell bodies, diminished dendrites, larger axon terminations and boutons, as well as reduced expression of synaptic proteins both pre and post-synaptically. Furthermore, the neurons display diminished vesicular glutamate transporter, and signaling components known to sustain interactions between the developing axon termination and muscle, such as wingless and frizzled are down regulated. Ectopic co-expression of Dimm and the insulin receptor augments most of the above effects on the motor neurons. In summary, ectopic Dimm expression alters the glutamatergic motor neuron phenotype toward a neuroendocrine one, both pre- and post-synaptically. Thus, Dimm is a key organizer of both secretory capacity and morphological features characteristic of neuroendocrine cells, and this transcription factor affects also post-synaptic proteins.

The comprehensive connectome of a neural substrate for 'ON' motion detection in Drosophila

Analysing computations in neural circuits often uses simplified models because the actual neuronal implementation is not known. For example, a problem in vision, how the eye detects image motion, has long been analysed using Hassenstein-Reichardt (HR) detector or Barlow-Levick (BL) models. These both simulate motion detection well, but the exact neuronal circuits undertaking these tasks remain elusive. We reconstructed a comprehensive connectome of the circuits of Drosophila's motion-sensing T4 cells using a novel EM technique. We uncover complex T4 inputs and reveal that putative excitatory inputs cluster at T4's dendrite shafts, while inhibitory inputs localize to the bases. Consistent with our previous study, we reveal that Mi1 and Tm3 cells provide most synaptic contacts onto T4. We are, however, unable to reproduce the spatial offset between these cells reported previously. Our comprehensive connectome reveals complex circuits that include candidate anatomical substrates for both HR and BL types of motion detectors.

Transgenic line for the identification of cholinergic release sites in Drosophila melanogaster

The identification of neurotransmitter type used by a neuron is important for the functional dissection of neuronal circuits. In the model organism Drosophila melanogaster, several methods for discerning the neurotransmitter systems are available. Here, we expanded the toolbox for the identification of cholinergic neurons by generating a new line FRT-STOP-FRT-VAChT::HA that is a conditional tagged knock-in of the vesicular acetylcholine transporter (VAChT) gene in its endogenous locus. Importantly, in comparison to already available tools for the detection of cholinergic neurons, the FRT-STOP-FRT-VAChT::HA allele also allows for identification of the subcellular localization of the cholinergic presynaptic release sites in a cell-specific manner. We used the newly generated FRT-STOP-FRT-VAChT::HA line to characterize the Mi1 and Tm3 neurons in the fly visual system and found that VAChT is present in the axons of both cell types, suggesting that Mi1 and Tm3 neurons provide cholinergic input to the elementary motion detectors, the T4 neurons.

Summary: A new transgenic Drosophila melanogaster line for the cell-type-specific identification of cholinergic release sites expands the available methods toolbox for discerning the neurotransmitter systems in the fly nervous system.

Presynaptic neuromuscular transmission defect in the stiff person syndrome

BACKGROUND

The stiff person syndrome (SPS) is a rare disorder characterized by muscular rigidity and stiffness.

CASE PRESENTATIONS

We describe an SPS patient presenting with longstanding fatigue and electrophysiological evidence of presynaptic neuromuscular transmission defect, who responded to administration of pyridostigmine. In contrast, no electrophysiolgical evidence of neuromuscular transmission defect was demonstrated in 2 other SPS patients without fatigue symptoms.

CONCLUSIONS

Our findings suggest that glutamic acid decarboxylase (GAD) antibodies may play a role in presynaptic neuromuscular transmission defect of SPS patients with fatigue.

Rationally subdividing the fly nervous system with versatile expression reagents

The ability to image and manipulate specific cell populations in Drosophila enables the investigation of how neural circuits develop and coordinate appropriate motor behaviors. Gal4 lines give genetic access to many types of neurons, but the expression patterns of these reagents are often complex. Here, we present the generation and expression patterns of LexA lines based on the vesicular neurotransmitter transporters and Hox transcription factors. Intersections between these LexA lines and existing Gal4 collections provide a strategy for rationally subdividing complex expression patterns based on neurotransmitter or segmental identity.

The Amino Acid Transporter JhI-21 Coevolves with Glutamate Receptors, Impacts NMJ Physiology, and Influences Locomotor Activity in Drosophila Larvae

Changes in synaptic physiology underlie neuronal network plasticity and behavioral phenomena, which are adjusted during development. The Drosophila larval glutamatergic neuromuscular junction (NMJ) represents a powerful synaptic model to investigate factors impacting these processes. Amino acids such as glutamate have been shown to regulate Drosophila NMJ physiology by modulating the clustering of postsynaptic glutamate receptors and thereby regulating the strength of signal transmission from the motor neuron to the muscle cell. To identify amino acid transporters impacting glutmatergic signal transmission, we used Evolutionary Rate Covariation (ERC), a recently developed bioinformatic tool. Our screen identified ten proteins co-evolving with NMJ glutamate receptors. We selected one candidate transporter, the SLC7 (Solute Carrier) transporter family member JhI-21 (Juvenile hormone Inducible-21), which is expressed in Drosophila larval motor neurons. We show that JhI-21 suppresses postsynaptic muscle glutamate receptor abundance, and that JhI-21 expression in motor neurons regulates larval crawling behavior in a developmental stage-specific manner.

Reward signal in a recurrent circuit drives appetitive long-term memory formation

Dopamine signals reward in animal brains. A single presentation of a sugar reward to Drosophila activates distinct subsets of dopamine neurons that independently induce short- and long-term olfactory memories (STM and LTM, respectively). In this study, we show that a recurrent reward circuit underlies the formation and consolidation of LTM. This feedback circuit is composed of a single class of reward-signaling dopamine neurons (PAM-α1) projecting to a restricted region of the mushroom body (MB), and a specific MB output cell type, MBON-α1, whose dendrites arborize that same MB compartment. Both MBON-α1 and PAM-α1 neurons are required during the acquisition and consolidation of appetitive LTM. MBON-α1 additionally mediates the retrieval of LTM, which is dependent on the dopamine receptor signaling in the MB α/β neurons. Our results suggest that a reward signal transforms a nascent memory trace into a stable LTM using a feedback circuit at the cost of memory specificity.

DOI:http://dx.doi.org/10.7554/eLife.10719.001

An animal that finds particularly nutritious and palatable food will often develop a long-lasting memory—even if they experience that event only once.

One example of this is the ability of the fruit fly Drosophila to form a long-term association between a sugar reward and a specific odor that was present when they received the reward. The consumption of sugar triggers the release of a chemical called dopamine on specific compartments of a brain structure called the mushroom body. Dopamine then acts to modify the connection between cells called “Kenyon cells”, which encode specific odors, and the neurons that send signals out from the mushroom body (called MBONs). The result is the formation of a memory that links the odor with the reward.

However, little is known about how this process differs for long-term vs. short-term memories, and how it can occur when the fly has experienced the odor and reward together on only a single occasion. To find out, Ichinose et al. combined behavioral testing of fruit flies with genetics. The results confirmed that the dopamine neurons and the MBONs that project to a single compartment of the mushroom body, called α1, are both required for the formation of long-term odor-reward memories, but not their short-term equivalents. These neurons are called PAM-α1 and MBON-α1, respectively.

Unexpectedly, anatomical data revealed that PAM-α1 dopamine neurons receive input from MBON-α1; that is, long-term memory formation involves a feedback circuit: from PAM-α1 to Kenyon cells, then to MBON-α1 and back to PAM-α1. Blocking feedback from the MBON-α1 onto the PAM-α1 neurons shortly after odor-reward training disrupted long-term memory formation. Conversely, blocking feedback at a later stage did not. This suggests that prolonged activation of PAM-α1 by MBON-α1 helps to strengthen newly established memories, converting them into memories that will last for a long time.

The discovery of a specific circuit that supports long-term, but not short-term, memory formation in fruit flies is consistent with evidence of distinct mechanisms underlying these processes in mammals. Further work is now required to determine whether feedback circuits similar to those in fruit flies also contribute to reward-based learning in other animals.

DOI:http://dx.doi.org/10.7554/eLife.10719.002

A Case of Stiff Person Syndrome: Immunomodulatory Effect of Benzodiazepines: Successful Rituximab and Tizanidine Therapy

Stiff person syndrome (SPS) is a rare autoimmune disease. Most patients have high-titer antibodies against glutamate decarboxylase (GADAb), which is without practical value in disease monitoring. Benzodiazepines are the first line drugs, but long-term use is not well characterized. This report demonstrates ineffective benzodiazepine therapy of SPS that prompts tachyphylaxis, loss of responsiveness, and finally benzodiazepine withdrawal syndrome. Convulsion and anxiety correlate with high level of creatine phosphokinase (CK). Although tonus and spasm attacks were successfully controlled by tizanidine, glutamate release inhibitor, the immune response, and autoimmune diabetes development require the plasmapheresis, mycophenolat mofetil, and rituximab therapy that results in a significant decrease of GADAb, impaired glucose tolerance (IGT), lactate dehydrogenase (LDH), and CK normalization. Unfortunately, reintroduction of benzodiazepine was a source of rapid and high increase of CK, LDH, GADAb titer (up to 1:15,000), IGT, and SPS relapse. Contrary to previous publications, we observed IGT that correlated with high anti-GAD level, but without high immunogenetic susceptibility to haplotype human leukocyte antigens-DR3, DQw2. This preliminary observation and the last finding of immunomodulatory properties of peripheral benzodiazepine receptor suggest that increased antigenic stimulation during benzodiazepine therapy and glutamatergic hyperactivity could account for convulsions observed in SPS. Benzodiazepine withdrawal prompted alternative muscle relaxant therapy (tizanidine). Muscular and brain abnormalities observed in SPS indicate that noncardiac CK level may be a useful tool in SPS therapy monitoring.

Differentially timed extracellular signals synchronize pacemaker neuron clocks

Circadian pacemaker neurons in Drosophila are regulated by two synchronizing signals that are released at opposite times of day, generating a rhythm in intracellular cyclic AMP.

Synchronized neuronal activity is vital for complex processes like behavior. Circadian pacemaker neurons offer an unusual opportunity to study synchrony as their molecular clocks oscillate in phase over an extended timeframe (24 h). To identify where, when, and how synchronizing signals are perceived, we first studied the minimal clock neural circuit in Drosophila larvae, manipulating either the four master pacemaker neurons (LN_vs) or two dorsal clock neurons (DN₁s). Unexpectedly, we found that the PDF Receptor (PdfR) is required in both LN_vs and DN₁s to maintain synchronized LN_v clocks. We also found that glutamate is a second synchronizing signal that is released from DN₁s and perceived in LN_vs via the metabotropic glutamate receptor (mGluRA). Because simultaneously reducing Pdfr and mGluRA expression in LN_vs severely dampened Timeless clock protein oscillations, we conclude that the master pacemaker LN_vs require extracellular signals to function normally. These two synchronizing signals are released at opposite times of day and drive cAMP oscillations in LN_vs. Finally we found that PdfR and mGluRA also help synchronize Timeless oscillations in adult s-LN_vs. We propose that differentially timed signals that drive cAMP oscillations and synchronize pacemaker neurons in circadian neural circuits will be conserved across species.

Circadian molecular clocks are essential for daily cycles in animal behavior and we have a good understanding of how these clocks work in individual pacemaker neurons. However, the accuracy of these individual clocks is meaningless unless they are synchronized with one another. In this study we show that synchronizing the principal pacemaker LN_v neurons in Drosophila larvae require two extracellular signals that are received at opposite times of day: namely, the neuropeptide PDF released from LN_vs themselves at dawn and glutamate released from dorsal clock neurons at dusk. LN_vs perceive both PDF and glutamate via G-protein coupled receptors that increase or decrease intracellular cAMP, respectively. The alternating phases of PDF and glutamate release generate oscillations in intracellular cyclic AMP. In addition to maintaining synchrony between LN_vs, this rhythm is also required for molecular clock oscillations in individual larval LN_vs. We show that disruption of PDF and glutamate signaling also reduces synchrony in adult LN_vs. This impairs the oscillations of clock proteins and flies have delayed onset of sleep. Our data highlight the importance of intercellular signaling in ensuring synchrony between clock neurons within the circadian network. Our findings help extend the conservation of clock properties between Drosophila and mammals beyond clock genes to include clock circuitry.

γ-glutamyl transpeptidase 1 specifically suppresses green-light avoidance via GABAA receptors in Drosophila

Drosophila larvae innately show light avoidance behavior. Compared with robust blue-light avoidance, larvae exhibit relatively weaker green-light responses. In our previous screening for genes involved in larval light avoidance, compared with control w(1118) larvae, larvae with γ-glutamyl transpeptidase 1 (Ggt-1) knockdown or Ggt-1 mutation were found to exhibit higher percentage of green-light avoidance which was mediated by Rhodopsin6 (Rh6) photoreceptors. However, their responses to blue light did not change significantly. By adjusting the expression level of Ggt-1 in different tissues, we found that Ggt-1 in malpighian tubules was both necessary and sufficient for green-light avoidance. Our results showed that glutamate levels were lower in Ggt-1 null mutants compared with controls. Feeding Ggt-1 null mutants glutamate can normalize green-light avoidance, indicating that high glutamate concentrations suppressed larval green-light avoidance. However, rather than directly, glutamate affected green-light avoidance indirectly through GABA, the level of which was also lower in Ggt-1 mutants compared with controls. Mutants in glutamate decarboxylase 1, which encodes GABA synthase, and knockdown lines of the GABAA receptor, both exhibit elevated levels of green-light avoidance. Thus, our results elucidate the neurobiological mechanisms mediating green-light avoidance, which was inhibited in wild-type larvae.

GABAergic circuit dysfunction in the Drosophila Fragile X syndrome model

Fragile X syndrome (FXS), caused by loss of FMR1 gene function, is the most common heritable cause of intellectual disability and autism spectrum disorders. The FMR1 protein (FMRP) translational regulator mediates activity-dependent control of synapses. In addition to the metabotropic glutamate receptor (mGluR) hyperexcitation FXS theory, the GABA theory postulates that hypoinhibition is causative for disease state symptoms. Here, we use the Drosophila FXS model to assay central brain GABAergic circuitry, especially within the Mushroom Body (MB) learning center. All 3 GABAA receptor (GABAAR) subunits are reportedly downregulated in dfmr1 null brains. We demonstrate parallel downregulation of glutamic acid decarboxylase (GAD), the rate-limiting GABA synthesis enzyme, although GABAergic cell numbers appear unaffected. Mosaic analysis with a repressible cell marker (MARCM) single-cell clonal studies show that dfmr1 null GABAergic neurons innervating the MB calyx display altered architectural development, with early underdevelopment followed by later overelaboration. In addition, a new class of extra-calyx terminating GABAergic neurons is shown to include MB intrinsic α/β Kenyon Cells (KCs), revealing a novel level of MB inhibitory regulation. Functionally, dfmr1 null GABAergic neurons exhibit elevated calcium signaling and altered kinetics in response to acute depolarization. To test the role of these GABAergic changes, we attempted to pharmacologically restore GABAergic signaling and assay effects on the compromised MB-dependent olfactory learning in dfmr1 mutants, but found no improvement. Our results show that GABAergic circuit structure and function are impaired in the FXS disease state, but that correction of hypoinhibition alone is not sufficient to rescue a behavioral learning impairment.

High-throughput genetic screen for synaptogenic factors: identification of LRP6 as critical for excitatory synapse development

Genetic screens in invertebrates have discovered many synaptogenic genes and pathways. However, similar genetic studies have not been possible in mammals. We have optimized an automated high-throughput platform that employs automated liquid handling and imaging of primary mammalian neurons. Using this platform, we have screened 3,200 shRNAs targeting 800 proteins. One of the hits identified was LRP6, a coreceptor for canonical Wnt ligands. LRP6 regulates excitatory synaptogenesis and is selectively localized to excitatory synapses. In vivo knockdown of LRP6 leads to a reduction in the number of functional synapses. Moreover, we show that the canonical Wnt ligand, Wnt8A, promotes synaptogenesis via LRP6. These results provide a proof of principle for using a high-content approach to screen for synaptogenic factors in the mammalian nervous system and identify and characterize a Wnt ligand receptor complex that is critical for the development of functional synapses in vivo.

The ADAR RNA editing enzyme controls neuronal excitability in Drosophila melanogaster

RNA editing by deamination of specific adenosine bases to inosines during pre-mRNA processing generates edited isoforms of proteins. Recoding RNA editing is more widespread in Drosophila than in vertebrates. Editing levels rise strongly at metamorphosis, and Adar^5G1 null mutant flies lack editing events in hundreds of CNS transcripts; mutant flies have reduced viability, severely defective locomotion and age-dependent neurodegeneration. On the other hand, overexpressing an adult dADAR isoform with high enzymatic activity ubiquitously during larval and pupal stages is lethal. Advantage was taken of this to screen for genetic modifiers; Adar overexpression lethality is rescued by reduced dosage of the Rdl (Resistant to dieldrin), gene encoding a subunit of inhibitory GABA receptors. Reduced dosage of the Gad1 gene encoding the GABA synthetase also rescues Adar overexpression lethality. Drosophila Adar^5G1 mutant phenotypes are ameliorated by feeding GABA modulators. We demonstrate that neuronal excitability is linked to dADAR expression levels in individual neurons; Adar-overexpressing larval motor neurons show reduced excitability whereas Adar^5G1 null mutant or targeted Adar knockdown motor neurons exhibit increased excitability. GABA inhibitory signalling is impaired in human epileptic and autistic conditions, and vertebrate ADARs may have a relevant evolutionarily conserved control over neuronal excitability.

Glutamate decarboxylase of the parasitic arthropods Ctenocephalides felis and Rhipicephalus microplus: gene identification, cloning, expression, assay development, identification of inhibitors by high throughput screening and comparison with the orthologs from Drosophila melanogaster and mouse

Glutamate decarboxylase (l-glutamate 1-carboxylyase, E.C. 4.1.1.15, GAD) is the rate-limiting enzyme for the production of γ-aminobutyric acid (GABA), the major inhibitory neurotransmitter in vertebrates and invertebrates. We report the identification, isolation and characterization of cDNAs encoding GAD from the parasitic arthropods Ctenocephalides felis (cat flea) and Rhipicephalus microplus (cattle tick). Expression of the parasite GAD genes and the corresponding Drosophila melanogaster (fruit fly) GAD1 as well as the mouse GAD(65) and GAD(67) genes in Escherichia coli as maltose binding protein fusions resulted in functional enzymes in quantities compatible with the needs of high throughput inhibitor screening (HTS). A novel continuous coupled spectrophotometric assay for GAD activity based on the detection cascade GABA transaminase/succinic semialdehyde dehydrogenase was developed, adapted to HTS, and a corresponding screen was performed with cat flea, cattle tick and fruit fly GAD. Counter-screening of the selected 38 hit substances on mouse GAD(65) and GAD(67) resulted in the identification of non-specific compounds as well as inhibitors with preferences for arthropod GAD, insect GAD, tick GAD and the two mouse GAD forms. Half of the identified hits most likely belong to known classes of GAD inhibitors, but several substances have not been described previously as GAD inhibitors and may represent lead optimization entry points for the design of arthropod-specific parasiticidal compounds.

SMN is required for sensory-motor circuit function in Drosophila

Spinal muscular atrophy (SMA) is a lethal human disease characterized by motor neuron dysfunction and muscle deterioration due to depletion of the ubiquitous survival motor neuron (SMN) protein. Drosophila SMN mutants have reduced muscle size and defective locomotion, motor rhythm, and motor neuron neurotransmission. Unexpectedly, restoration of SMN in either muscles or motor neurons did not alter these phenotypes. Instead, SMN must be expressed in proprioceptive neurons and interneurons in the motor circuit to nonautonomously correct defects in motor neurons and muscles. SMN depletion disrupts the motor system subsequent to circuit development and can be mimicked by the inhibition of motor network function. Furthermore, increasing motor circuit excitability by genetic or pharmacological inhibition of K(+) channels can correct SMN-dependent phenotypes. These results establish sensory-motor circuit dysfunction as the origin of motor system deficits in this SMA model and suggest that enhancement of motor neural network activity could ameliorate the disease.

Glycosylated synaptomatrix regulation of trans-synaptic signaling

Synapse formation is driven by precisely orchestrated intercellular communication between the presynaptic and the postsynaptic cell, involving a cascade of anterograde and retrograde signals. At the neuromuscular junction (NMJ), both neuron and muscle secrete signals into the heavily glycosylated synaptic cleft matrix sandwiched between the two synapsing cells. These signals must necessarily traverse and interact with the extracellular environment, for the ligand-receptor interactions mediating communication to occur. This complex synaptomatrix, rich in glycoproteins and proteoglycans, comprises heterogeneous, compartmentalized domains where specialized glycans modulate trans-synaptic signaling during synaptogenesis and subsequent synapse modulation. The general importance of glycans during development, homeostasis and disease is well established, but this important molecular class has received less study in the nervous system. Glycan modifications are now understood to play functional and modulatory roles as ligands and co-receptors in numerous tissues; however, roles at the synapse are relatively unexplored. We highlight here properties of synaptomatrix glycans and glycan-interacting proteins with key roles in synaptogenesis, with a particular focus on recent advances made in the Drosophila NMJ genetic system. We discuss open questions and interesting new findings driving this investigation of complex, diverse, and largely understudied glycan mechanisms at the synapse.

Cyclic adenosine monophosphate metabolism in synaptic growth, strength, and precision: neural and behavioral phenotype-specific counterbalancing effects between dnc phosphodiesterase and rut adenylyl cyclase mutations

Two classic learning mutants in Drosophila, rutabaga (rut) and dunce (dnc), are defective in cyclic adenosine monophosphate (cAMP) synthesis and degradation, respectively, exhibiting a variety of neuronal and behavioral defects. We ask how the opposing effects of these mutations on cAMP levels modify subsets of phenotypes, and whether any specific phenotypes could be ameliorated by biochemical counter balancing effects in dnc rut double mutants. Our study at larval neuromuscular junctions (NMJs) demonstrates that dnc mutations caused severe defects in nerve terminal morphology, characterized by unusually large synaptic boutons and aberrant innervation patterns. Interestingly, a counterbalancing effect led to rescue of the aberrant innervation patterns but the enlarged boutons in dnc rut double mutant remained as extreme as those in dnc. In contrast to dnc, rut mutations strongly affect synaptic transmission. Focal loose-patch recording data accumulated over 4 years suggest that synaptic currents in rut boutons were characterized by unusually large temporal dispersion and a seasonal variation in the amount of transmitter release, with diminished synaptic currents in summer months. Experiments with different rearing temperatures revealed that high temperature (29-30°C) decreased synaptic transmission in rut, but did not alter dnc and wild-type (WT). Importantly, the large temporal dispersion and abnormal temperature dependence of synaptic transmission, characteristic of rut, still persisted in dnc rut double mutants. To interpret these results in a proper perspective, we reviewed previously documented differential effects of dnc and rut mutations and their genetic interactions in double mutants on a variety of physiological and behavioral phenotypes. The cases of rescue in double mutants are associated with gradual developmental and maintenance processes whereas many behavioral and physiological manifestations on faster time scales could not be rescued. We discuss factors that could contribute to the effectiveness of counterbalancing interactions between dnc and rut mutations for phenotypic rescue.

Distribution of metabotropic receptors of serotonin, dopamine, GABA, glutamate, and short neuropeptide F in the central complex of Drosophila

The central complex is a prominent set of midline neuropils in the insect brain, known to be a higher locomotor control center that integrates visual inputs and modulates motor outputs. It is composed of four major neuropil structures, the ellipsoid body (EB), fan-shaped body (FB), noduli (NO), and protocerebral bridge (PB). In Drosophila different types of central complex neurons have been shown to express multiple neuropeptides and neurotransmitters; however, the distribution of corresponding receptors is not known. Here, we have mapped metabotropic, G-protein-coupled receptors (GPCRs) of several neurotransmitters to neurons of the central complex. By combining immunocytochemistry with GAL4 driven green fluorescent protein, we examined the distribution patterns of six different GPCRs: two serotonin receptor subtypes (5-HT(1B) and 5-HT(7)), a dopamine receptor (DopR), the metabotropic GABA(B) receptor (GABA(B)R), the metabotropic glutamate receptor (DmGluR(A)) and a short neuropeptide F receptor (sNPFR1). Five of the six GPCRs were mapped to different neurons in the EB (sNPFR1 was not seen). Different layers of the FB express DopR, GABA(B)R, DmGluR(A,) and sNPFR1, whereas only GABA(B)R and DmGluR(A) were localized to the PB. Finally, strong expression of DopR and DmGluR(A) was detected in the NO. In most cases the distribution patterns of the GPCRs matched the expression of markers for their respective ligands. In some nonmatching regions it is likely that other types of dopamine and serotonin receptors or ionotropic GABA and glutamate receptors are expressed. Our data suggest that chemical signaling and signal modulation are diverse and highly complex in the different compartments and circuits of the Drosophila central complex. The information provided here, on receptor distribution, will be very useful for future analysis of functional circuits in the central complex, based on targeted interference with receptor expression.

Extracellular protons reduce quantal content and prolong synaptic currents at the Drosophila larval neuromuscular junction

Fluctuations in extracellular pH occur in the nervous system in response to a number of physiological and pathological processes, such as ischemia, hypercapnea, and high-frequency activity. Using the Drosophila larval neuromuscular junction, the author has examined acute effects of low and high pH on excitability and synaptic transmission. Acidification rapidly and reversibly reduces the size of electrically evoked excitatory junctional currents (EJCs) in a concentration-dependent manner, with transmission nearly abolished at pH 5.0. Conversely, raising pH to 7.8 increases EJC amplitude significantly. Further elevation to pH 8.5 causes an initial increase in amplitude, followed by profound, long-lasting depression of the synapse. Amplitudes of spontaneous miniature EJCs (mEJCs) are modestly, but significantly reduced at pH 5.0. It is therefore the number of quanta released per action potential, rather than the size of individual quanta, that is most strongly affected. Decay times of both EJCs and mEJCs are dramatically lengthened at low pH, suggesting that glutamate remains in the synaptic cleft for much longer than normal. Presynaptic excitability is also reduced, as indicated by increased latency between nerve shock and EJC onset. The response to low pH was not altered by mutations in genes encoding Transient Receptor Potential, Mucolipin subfamily (TRPML) and Slowpoke ion channels, which had previously been implicated as possible targets of extracellular protons. The author concludes that extracellular protons have strong effects on the release of glutamate and the time course of synaptic currents. These phenotypes can be exploited to study the mechanisms of acid-mediated changes in neuronal function, and to pursue the way in which pH modulates synaptic function in normal and pathophysiological conditions.

Pre and postsynaptic roles for Drosophila CASK

CASK ('calcium/calmodulin-dependent serine protein kinase'), also known in Drosophila as 'Caki' or 'Camguk/CMG', and in C. elegans as 'Lin-2', is thought to play an important role in cell-cell junction formation and at synapses in particular. To understand the role of CASK in synapse formation and function, we functionally and morphologically analyzed Drosophila embryonic and larval glutamatergic neuromuscular junctions (NMJs) after pan-cellular and tissue-specific manipulation of CASK expression. Our results show that Drosophila CASK is associated with both pre and postsynaptic membranes. Loss of presynaptic CASK led to less evoked synaptic transmission, fewer spontaneous synaptic events, and reduced synaptic vesicle cycling. These changes were accompanied by a reduction in the number of synapses but no change in overall NMJ size. Loss of postsynaptic CASK, on the other hand, caused reduced spontaneous synaptic current amplitudes and smaller glutamate-gated currents. These changes were accompanied by loss of postsynaptic glutamate receptors, but the receptor loss was subtype-specific: Only receptors containing GluRIIA subunits were lost in CASK mutants. Receptors containing GluRIIB were unaffected.

Mushroom body efferent neurons responsible for aversive olfactory memory retrieval in Drosophila

Aversive olfactory memory is formed in the mushroom bodies in Drosophila melanogaster. Memory retrieval requires mushroom body output, but the manner in which a memory trace in the mushroom body drives conditioned avoidance of a learned odor remains unknown. To identify neurons that are involved in olfactory memory retrieval, we performed an anatomical and functional screen of defined sets of mushroom body output neurons. We found that MB-V2 neurons were essential for retrieval of both short- and long-lasting memory, but not for memory formation or memory consolidation. MB-V2 neurons are cholinergic efferent neurons that project from the mushroom body vertical lobes to the middle superiormedial protocerebrum and the lateral horn. Notably, the odor response of MB-V2 neurons was modified after conditioning. As the lateral horn has been implicated in innate responses to repellent odorants, we propose that MB-V2 neurons recruit the olfactory pathway involved in innate odor avoidance during memory retrieval.

Insulin signaling, lifespan and stress resistance are modulated by metabotropic GABA receptors on insulin producing cells in the brain of Drosophila

Insulin-like peptides (ILPs) regulate growth, reproduction, metabolic homeostasis, life span and stress resistance in worms, flies and mammals. A set of insulin producing cells (IPCs) in the Drosophila brain that express three ILPs (DILP2, 3 and 5) have been the main focus of interest in hormonal DILP signaling. Little is, however, known about factors that regulate DILP production and release by these IPCs. Here we show that the IPCs express the metabotropic GABA_B receptor (GBR), but not the ionotropic GABA_A receptor subunit RDL. Diminishing the GBR expression on these cells by targeted RNA interference abbreviates life span, decreases metabolic stress resistance and alters carbohydrate and lipid metabolism at stress, but not growth in Drosophila. A direct effect of diminishing GBR on IPCs is an increase in DILP immunofluorescence in these cells, an effect that is accentuated at starvation. Knockdown of irk3, possibly part of a G protein-activated inwardly rectifying K⁺ channel that may link to GBRs, phenocopies GBR knockdown in starvation experiments. Our experiments suggest that the GBR is involved in inhibitory control of DILP production and release in adult flies at metabolic stress and that this receptor mediates a GABA signal from brain interneurons that may convey nutritional signals. This is the first demonstration of a neurotransmitter that inhibits insulin signaling in its regulation of metabolism, stress and life span in an invertebrate brain.

Mutation of the Drosophila vesicular GABA transporter disrupts visual figure detection

The role of gamma amino butyric acid (GABA) release and inhibitory neurotransmission in regulating most behaviors remains unclear. The vesicular GABA transporter (VGAT) is required for the storage of GABA in synaptic vesicles and provides a potentially useful probe for inhibitory circuits. However, specific pharmacologic agents for VGAT are not available, and VGAT knockout mice are embryonically lethal, thus precluding behavioral studies. We have identified the Drosophila ortholog of the vesicular GABA transporter gene (which we refer to as dVGAT), immunocytologically mapped dVGAT protein expression in the larva and adult and characterized a dVGAT(minos) mutant allele. dVGAT is embryonically lethal and we do not detect residual dVGAT expression, suggesting that it is either a strong hypomorph or a null. To investigate the function of VGAT and GABA signaling in adult visual flight behavior, we have selectively rescued the dVGAT mutant during development. We show that reduced GABA release does not compromise the active optomotor control of wide-field pattern motion. Conversely, reduced dVGAT expression disrupts normal object tracking and figure-ground discrimination. These results demonstrate that visual behaviors are segregated by the level of GABA signaling in flies, and more generally establish dVGAT as a model to study the contribution of GABA release to other complex behaviors.

Neurexin in embryonic Drosophila neuromuscular junctions

Background

Neurexin is a synaptic cell adhesion protein critical for synapse formation and function. Mutations in neurexin and neurexin-interacting proteins have been implicated in several neurological diseases. Previous studies have described Drosophila neurexin mutant phenotypes in third instar larvae and adults. However, the expression and function of Drosophila neurexin early in synapse development, when neurexin function is thought to be most important, has not been described.

Methodology/Principal Findings

We use a variety of techniques, including immunohistochemistry, electron microscopy, in situ hybridization, and electrophysiology, to characterize neurexin expression and phenotypes in embryonic Drosophila neuromuscular junctions (NMJs). Our results surprisingly suggest that neurexin in embryos is present both pre and postsynaptically. Presynaptic neurexin promotes presynaptic active zone formation and neurotransmitter release, but along with postsynaptic neurexin, also suppresses formation of ectopic glutamate receptor clusters. Interestingly, we find that loss of neurexin only affects receptors containing the subunit GluRIIA.

Conclusions/Significance

Our study extends previous results and provides important detail regarding the role of neurexin in Drosophila glutamate receptor abundance. The possibility that neurexin is present postsynaptically raises new hypotheses regarding neurexin function in synapses, and our results provide new insights into the role of neurexin in synapse development.

Sialyltransferase regulates nervous system function in Drosophila

In vertebrates, sialylated glycans participate in a wide range of biological processes and affect the development and function of the nervous system. While the complexity of glycosylation and the functional redundancy among sialyltransferases provide obstacles for revealing biological roles of sialylation in mammals, Drosophila possesses a sole vertebrate-type sialyltransferase, Drosophila sialyltransferase (DSiaT), with significant homology to its mammalian counterparts, suggesting that Drosophila could be a suitable model to investigate the function of sialylation. To explore this possibility and investigate the role of sialylation in Drosophila, we inactivated DSiaT in vivo by gene targeting and analyzed phenotypes of DSiaT mutants using a combination of behavioral, immunolabeling, electrophysiological, and pharmacological approaches. Our experiments demonstrated that DSiaT expression is restricted to a subset of CNS neurons throughout development. We found that DSiaT mutations result in significantly decreased life span, locomotor abnormalities, temperature-sensitive paralysis, and defects of neuromuscular junctions. Our results indicate that DSiaT regulates neuronal excitability and affects the function of a voltage-gated sodium channel. Finally, we showed that sialyltransferase activity is required for DSiaT function in vivo, which suggests that DSiaT mutant phenotypes result from a defect in sialylation of N-glycans. This work provided the first evidence that sialylation has an important biological function in protostomes, while also revealing a novel, nervous system-specific function of alpha2,6-sialylation. Thus, our data shed light on one of the most ancient functions of sialic acids in metazoan organisms and suggest a possibility that this function is evolutionarily conserved between flies and mammals.

Analysis of GABAergic and non-GABAergic neuron activity in the optic lobes of the forager and re-orienting worker honeybee (Apis mellifera L.)

Background

European honeybee (Apis mellifera L.) foragers have a highly developed visual system that is used for navigation. To clarify the neural basis underlying the highly sophisticated visual ability of foragers, we investigated the neural activity pattern of the optic lobes (OLs) in pollen-foragers and re-orienting bees, using the immediate early gene kakusei as a neural activity marker.

Methodology/Principal Findings

We performed double-in situ hybridization of kakusei and Amgad, the honeybee homolog of the GABA synthesizing enzyme GAD, to assess inhibitory neural activity. kakusei-related activity in GABAergic and non-GABAergic neurons was strongly upregulated in the OLs of the foragers and re-orienting bees, suggesting that both types of neurons are involved in visual information processing. GABAergic neuron activity was significantly higher than non-GABAergic neuron activity in a part of the OLs of only the forager, suggesting that unique information processing occurs in the OLs of foragers. In contrast, GABAergic neuron activity in the antennal lobe was significantly lower than that of GABAergic neurons in the OLs in the forager and re-orienting bees, suggesting that kakusei-related visual activity is dominant in the brains of these bees.

Conclusions/Significance

The present study provides the first evidence that GABAergic neurons are highly active in the OL neurons of free-moving honeybees and essential clue to reveal neural basis of the sophisticated visual ability that is equipped in the small and simple brain.

Maturation of active zone assembly by Drosophila Bruchpilot

Synaptic vesicles fuse at active zone (AZ) membranes where Ca²⁺ channels are clustered and that are typically decorated by electron-dense projections. Recently, mutants of the Drosophilamelanogaster ERC/CAST family protein Bruchpilot (BRP) were shown to lack dense projections (T-bars) and to suffer from Ca²⁺ channel–clustering defects. In this study, we used high resolution light microscopy, electron microscopy, and intravital imaging to analyze the function of BRP in AZ assembly. Consistent with truncated BRP variants forming shortened T-bars, we identify BRP as a direct T-bar component at the AZ center with its N terminus closer to the AZ membrane than its C terminus. In contrast, Drosophila Liprin-α, another AZ-organizing protein, precedes BRP during the assembly of newly forming AZs by several hours and surrounds the AZ center in few discrete punctae. BRP seems responsible for effectively clustering Ca²⁺ channels beneath the T-bar density late in a protracted AZ formation process, potentially through a direct molecular interaction with intracellular Ca²⁺ channel domains.

Electrophysiological recording in the Drosophila embryo

Drosophila is a premier genetic model for the study of both embryonic development and functional neuroscience. Traditionally, these fields are quite isolated from each other, with largely independent histories and scientific communities. However, the interface between these usually disparate fields is the developmental programs underlying acquisition of functional electrical signaling properties and differentiation of functional chemical synapses during the final phases of neural circuit formation. This interface is a critically important area for investigation. In Drosophila, these phases of functional development occur during a period of <8 hours (at 25 degrees C) during the last third of embryogenesis. This late developmental period was long considered intractable to investigation owing to the deposition of a tough, impermeable epidermal cuticle. A breakthrough advance was the application of water-polymerizing surgical glue that can be locally applied to the cuticle to enable controlled dissection of late-stage embryos. With a dorsal longitudinal incision, the embryo can be laid flat, exposing the ventral nerve cord and body wall musculature to experimental investigation. Whole-cell patch-clamp techniques can then be employed to record from individually-identifiable neurons and somatic muscles. These recording configurations have been used to track the appearance and maturation of ionic currents and action potential propagation in both neurons and muscles. Genetic mutants affecting these electrical properties have been characterized to reveal the molecular composition of ion channels and associated signaling complexes, and to begin exploration of the molecular mechanisms of functional differentiation. A particular focus has been the assembly of synaptic connections, both in the central nervous system and periphery. The glutamatergic neuromuscular junction (NMJ) is most accessible to a combination of optical imaging and electrophysiological recording. A glass suction electrode is used to stimulate the peripheral nerve, with excitatory junction current (EJC) recordings made in the voltage-clamped muscle. This recording configuration has been used to chart the functional differentiation of the synapse, and track the appearance and maturation of presynaptic glutamate release properties. In addition, postsynaptic properties can be assayed independently via iontophoretic or pressure application of glutamate directly to the muscle surface, to measure the appearance and maturation of the glutamate receptor fields. Thus, both pre- and postsynaptic elements can be monitored separately or in combination during embryonic synaptogenesis. This system has been heavily used to isolate and characterize genetic mutants that impair embryonic synapse formation, and thus reveal the molecular mechanisms governing the specification and differentiation of synapse connections and functional synaptic signaling properties.

Harvesting and preparing Drosophila embryos for electrophysiological recording and other procedures

Drosophila is a premier genetic model for the study of both embryonic development and functional neuroscience. Traditionally, these fields are quite isolated from each other, with largely independent histories and scientific communities. However, the interface between these usually disparate fields is the developmental programs underlying acquisition of functional electrical signaling properties and differentiation of functional chemical synapses during the final phases of neural circuit formation. This interface is a critically important area for investigation. In Drosophila, these phases of functional development occur during a period of <8 hours (at 25°C) during the last third of embryogenesis. This late developmental period was long considered intractable to investigation owing to the deposition of a tough, impermeable epidermal cuticle. A breakthrough advance was the application of water-polymerizing surgical glue that can be locally applied to the cuticle to enable controlled dissection of late-stage embryos. With a dorsal longitudinal incision, the embryo can be laid flat, exposing the ventral nerve cord and body wall musculature to experimental investigation. This system has been heavily used to isolate and characterize genetic mutants that impair embryonic synapse formation, and thus reveal the molecular mechanisms governing the specification and differentiation of synapse connections and functional synaptic signaling properties.

Regulation of glutamate receptor subunit availability by microRNAs

The efficacy of synaptic transmission depends, to a large extent, on postsynaptic receptor abundance. The molecular mechanisms controlling receptor abundance are poorly understood. We tested whether abundance of postsynaptic glutamate receptors (GluRs) in Drosophila neuromuscular junctions is controlled by microRNAs, and provide evidence that it is. We show here that postsynaptic knockdown of dicer-1, the endoribonuclease necessary for microRNA synthesis, leads to large increases in postsynaptic GluR subunit messenger RNA and protein. Specifically, we measured increases in GluRIIA and GluRIIB but not GluRIIC. Further, knockout of MiR-284, a microRNA predicted to bind to GluRIIA and GluRIIB but not GluRIIC, increases expression of GluRIIA and GluRIIB but not GluRIIC proportional to the number of predicted binding sites in each transcript. Most of the de-repressed GluR protein, however, does not appear to be incorporated into functional receptors, and only minor changes in synaptic strength are observed, which suggests that microRNAs primarily regulate Drosophila receptor subunit composition rather than overall receptor abundance or synaptic strength.

Presynaptic secretion of mind-the-gap organizes the synaptic extracellular matrix-integrin interface and postsynaptic environments

Mind-the-Gap (MTG) is required during synaptogenesis of the Drosophila glutamatergic neuromuscular junction (NMJ) to organize the postsynaptic domain. Here, we generate MTG::GFP transgenic animals to demonstrate MTG is synaptically targeted, secreted, and localized to punctate domains in the synaptic extracellular matrix (ECM). Drosophila NMJs form specialized ECM carbohydrate domains, with carbohydrate moieties and integrin ECM receptors occupying overlapping territories. Presynaptically secreted MTG recruits and reorganizes secreted carbohydrates, and acts to recruit synaptic integrins and ECM glycans. Transgenic MTG::GFP expression rescues hatching, movement, and synaptogenic defects in embryonic-lethal mtg null mutants. Targeted neuronal MTG expression rescues mutant synaptogenesis defects, and increases rescue of adult viability, supporting an essential neuronal function. These results indicate that presynaptically secreted MTG regulates the ECM-integrin interface, and drives an inductive mechanism for the functional differentiation of the postsynaptic domain of glutamatergic synapses. We suggest that MTG pioneers a novel protein family involved in ECM-dependent synaptic differentiation.

Proteome response to the panneural expression of human wild-type alpha-synuclein: a Drosophila model of Parkinson's disease

The alpha-synuclein protein is associated with several neurodegenarative diseases, including Parkinson's disease (PD). In humans, only mutated forms of alpha-synuclein are linked to PD; however, panneural expression of human wild-type (WT) alpha-synuclein induces Parkinson's like-symptoms in Drosophila. Here, we report a quantitative proteomic analysis of WT alpha-synuclein transgenic flies with age-matched controls at the presymptomatic stage utilizing a global isotopic labeling strategy combined with multidimensional liquid chromatographies and tandem mass spectrometry. The analysis includes two biological replicates, in which samples are isotopically labeled in forward and reverse directions. In total, 229 proteins were quantified from assignments of at least two peptide sequences. Of these, 188 (82%) proteins were detected in both forward and reverse labeling measurements. Twelve proteins were found to be differentially expressed in response to the expression of human WT alpha-synuclein; down-regulations of larval serum protein 2 and fat body protein 1 levels were confirmed by Western blot analysis. Gene Ontology analysis indicates that the dysregulated proteins are primarily associated with cellular metabolism and signaling, suggesting potential contributions of perturbed metabolic and signaling pathways to PD. An increased level of the iron (III)-binding protein, ferritin, typically found in the brains of PD patients, is also observed in presymptomatic WT alpha-synuclein expressing animals. The observed alterations in both pathology-associated and novel proteins may shed light on the pathological roles of alpha-synuclein that may lead to the development of diagnostic strategies at the presymptomatic stage.

Molecular cloning, partial genomic structure and functional characterization of succinic semialdehyde dehydrogenase genes from the parasitic insects Lucilia cuprina and Ctenocephalides felis

The enzyme succinic semialdehyde dehydrogenase (SSADH; EC1.2.1.24) is a component of the gamma-aminobutyric acid degradation pathway in mammals and is essential for development and function of the nervous system. Here we report the identification, cDNA cloning and functional expression of SSADH from the parasitic insects Lucilia cuprina and Ctenocephalides felis. The recombinant proteins possess potent NAD+-dependent SSADH activity, while their catalytic efficiency for other aldehyde substrates is lower. A genomic copy of the L. cuprina SSADH gene contains two introns, while a genomic gene version of C. felis is devoid of introns. In contrast to the single copy SSADH genes in Drosophila melanogaster and mammals, in L. cuprina and C. felis, multiple SSADH gene copies are present in the genome.

Activity-dependent site-specific changes of glutamate receptor composition in vivo

The subunit composition of postsynaptic non-NMDA-type glutamate receptors (GluRs) determines the function and trafficking of the receptor. Changes in GluR composition have been implicated in the homeostasis of neuronal excitability and synaptic plasticity underlying learning. Here, we imaged GluRs in vivo during the formation of new postsynaptic densities (PSDs) at Drosophila neuromuscular junctions coexpressing GluRIIA and GluRIIB subunits. GluR composition was independently regulated at directly neighboring PSDs on a submicron scale. Immature PSDs typically had large amounts of GluRIIA and small amounts of GluRIIB. During subsequent PSD maturation, however, the GluRIIA/GluRIIB composition changed and became more balanced. Reducing presynaptic glutamate release increased GluRIIA, but decreased GluRIIB incorporation. Moreover, the maturation of GluR composition correlated in a site-specific manner with the level of Bruchpilot, an active zone protein that is essential for mature glutamate release. Thus, we show that an activity-dependent, site-specific control of GluR composition can contribute to match pre- and postsynaptic assembly.

Glutamate, GABA and acetylcholine signaling components in the lamina of the Drosophila visual system

Synaptic connections of neurons in the Drosophila lamina, the most peripheral synaptic region of the visual system, have been comprehensively described. Although the lamina has been used extensively as a model for the development and plasticity of synaptic connections, the neurotransmitters in these circuits are still poorly known. Thus, to unravel possible neurotransmitter circuits in the lamina of Drosophila we combined Gal4 driven green fluorescent protein in specific lamina neurons with antisera to γ-aminobutyric acid (GABA), glutamic acid decarboxylase, a GABA_B type of receptor, L-glutamate, a vesicular glutamate transporter (vGluT), ionotropic and metabotropic glutamate receptors, choline acetyltransferase and a vesicular acetylcholine transporter. We suggest that acetylcholine may be used as a neurotransmitter in both L4 monopolar neurons and a previously unreported type of wide-field tangential neuron (Cha-Tan). GABA is the likely transmitter of centrifugal neurons C2 and C3 and GABA_B receptor immunoreactivity is seen on these neurons as well as the Cha-Tan neurons. Based on an rdl-Gal4 line, the ionotropic GABA_A receptor subunit RDL may be expressed by L4 neurons and a type of tangential neuron (rdl-Tan). Strong vGluT immunoreactivity was detected in α-processes of amacrine neurons and possibly in the large monopolar neurons L1 and L2. These neurons also express glutamate-like immunoreactivity. However, antisera to ionotropic and metabotropic glutamate receptors did not produce distinct immunosignals in the lamina. In summary, this paper describes novel features of two distinct types of tangential neurons in the Drosophila lamina and assigns putative neurotransmitters and some receptors to a few identified neuron types.

Genome-wide expression analysis of a spinal muscular atrophy model: towards discovery of new drug targets

Spinal Muscular Atrophy is a recessive genetic disease and affects lower motor neurones and muscle tissue. A single gene is disrupted in SMA: SMN1 activity is abolished but a second copy of the gene (SMN2) provides limited activity. While the SMN protein has been shown to function in the assembly of RNA-protein complexes, it is unclear how the overall reduction in SMN activity specifically results in the neuromuscular phenotypes. Similar to humans, reduced smn activity in the fly causes earliest phenotypes in neuromuscular tissues. To uncover the effects of reduced SMN activity, we have studied gene expression in control and diseased fly tissues using whole genome micro-arrays. A number of gene expression changes are recovered and independently validated. Identified genes show trends in their predicted function: several are consistent with the function of SMN, in addition some uncover novel pathways. This and subsequent genetic analysis in the fly indicates some of the identified genes could be taken for further studies as potential drug targets for SMA and other neuromuscular disorders.

γ-Aminobutyric acid (GABA) signaling components inDrosophila: Immunocytochemical localization of GABABreceptors in relation to the GABAAreceptor subunit RDL and a vesicular GABA transporter

gamma-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in insects and is widely distributed in the central nervous system (CNS). GABA acts on ion channel receptors (GABA(A)R) for fast inhibitory transmission and on G-protein-coupled ones (GABA(B)R) for slow and modulatory action. We used immunocytochemistry to map GABA(B)R sites in the Drosophila CNS and compared the distribution with that of the GABA(A)R subunit RDL. To identify GABAergic synapses, we raised an antiserum to the vesicular GABA transporter (vGAT). For general GABA distribution, we utilized an antiserum to glutamic acid decarboxylase (GAD1) and a gad1-GAL4 to drive green fluorescent protein. GABA(B)R-immunoreactive (IR) punctates were seen in specific patterns in all major neuropils of the brain. Most abundant labeling was seen in the mushroom body calyces, ellipsoid body, optic lobe neuropils, and antennal lobes. The RDL distribution is very similar to that of GABA(B)R-IR punctates. However, the mushroom body lobes displayed RDL-IR but not GABA(B)R-IR material, and there were subtle differences in other areas. The vGAT antiserum labeled punctates in the same areas as the GABA(B)R and appeared to display presynaptic sites of GABAergic neurons. Various GAL4 drivers were used to analyze the relation between GABA(B)R distribution and identified neurons in adults and larvae. Our findings suggest that slow GABA transmission is very widespread in the Drosophila CNS and that fast RDL-mediated transmission generally occurs at the same sites.

Presynaptic establishment of the synaptic cleft extracellular matrix is required for post-synaptic differentiation

Formation and regulation of excitatory glutamatergic synapses is essential for shaping neural circuits throughout development. In a Drosophila genetic screen for synaptogenesis mutants, we identified mind the gap (mtg), which encodes a secreted, extracellular N-glycosaminoglycan-binding protein. MTG is expressed neuronally and detected in the synaptic cleft, and is required to form the specialized transsynaptic matrix that links the presynaptic active zone with the post-synaptic glutamate receptor (GluR) domain. Null mtg embryonic mutant synapses exhibit greatly reduced GluR function, and a corresponding loss of localized GluR domains. All known post-synaptic signaling/scaffold proteins functioning upstream of GluR localization are also grossly reduced or mislocalized in mtg mutants, including the dPix-dPak-Dock cascade and the Dlg/PSD-95 scaffold. Ubiquitous or neuronally targeted mtg RNA interference (RNAi) similarly reduce post-synaptic assembly, whereas post-synaptically targeted RNAi has no effect, indicating that presynaptic MTG induces and maintains the post-synaptic pathways driving GluR domain formation. These findings suggest that MTG is secreted from the presynaptic terminal to shape the extracellular synaptic cleft domain, and that the cleft domain functions to mediate transsynaptic signals required for post-synaptic development.

Glutamatergic functions of primary afferent neurons with special emphasis on vagal afferents

Glutamate has been identified as the main transmitter of primary afferent neurons. This was established based on biochemical, electrophysiological, and immunohistochemical data from studies on glutamatergic receptors and their agonists/antagonists. The availability of specific antibodies directed against glutamate and, more recently, vesicular glutamate transporters corroborated this and led to significant new discoveries. In particular, peripheral endings of various classes of afferents contain vesicular glutamate transporters, suggesting vesicular storage in and exocytotic release of glutamate from peripheral afferent endings. This suggests that autocrine mechanisms regulate sensory transduction processes. However, glutamate release from peripheral sensory terminals could also enable afferent neurons to influence various cells associated with them. This may be particularly relevant for vagal intraganglionic laminar endings, which could represent glutamatergic sensor-effector components of intramural reflex arcs in the gastrointestinal tract. Thus, morphological analysis of the relationships of putative glutamatergic primary afferents with associated tissues may direct forthcoming studies on their functions.

Excitable properties of adult skeletal muscle fibres from the honeybeeApis mellifera

In the hive, a wide range of honeybees tasks such as cell cleaning,nursing, thermogenesis, flight, foraging and inter-individual communication(waggle dance, antennal contact and trophallaxy) depend on proper muscle activity. However, whereas extensive electrophysiological studies have been undertaken over the past ten years to characterize ionic currents underlying the physiological neuronal activity in honeybee, ionic currents underlying skeletal muscle fibre activity in this insect remain, so far, unexplored. Here, we show that, in contrast to many other insect species, action potentials in muscle fibres isolated from adult honeybee metathoracic tibia,are not graded but actual all-or-none responses. Action potentials are blocked by Cd2+ and La3+ but not by tetrodotoxin (TTX) in current-clamp mode of the patch-clamp technique, and as assessed under voltage-clamp, both Ca2+ and K+ currents are involved in shaping action potentials in single muscle fibres. The activation threshold potential for the voltage-dependent Ca2+ current is close to–40 mV, its mean maximal amplitude is –8.5±1.9 A/F and the mean apparent reversal potential is near +40 mV. In honeybees, GABA does not activate any ionic membrane currents in muscle fibres from the tibia, but L-glutamate, an excitatory neurotransmitter at the neuromuscular synapse induces fast activation of an inward current when the membrane potential is voltage clamped close to its resting value. Instead of undergoing desensitization as is the case in many other preparations, a component of this glutamate-activated current has a sustained component, the reversal potential of which is close to 0 mV, as demonstrated with voltage ramps. Future investigations will allow extensive pharmacological characterization of membrane ionic currents and excitation–contraction coupling in skeletal muscle from honeybee, a useful insect that became a model to study many physiological phenomena and which plays a major role in plant pollination and in stability of environmental vegetal biodiversity.

Nonvesicular release of glutamate by glial xCT transporters suppresses glutamate receptor clustering in vivo

We hypothesized that cystine/glutamate transporters (xCTs) might be critical regulators of ambient extracellular glutamate levels in the nervous system and that misregulation of this glutamate pool might have important neurophysiological and/or behavioral consequences. To test this idea, we identified and functionally characterized a novel Drosophila xCT gene, which we subsequently named "genderblind" (gb). Genderblind is expressed in a previously overlooked subset of peripheral and central glia. Genetic elimination of gb causes a 50% reduction in extracellular glutamate concentration, demonstrating that xCT transporters are important regulators of extracellular glutamate. Consistent with previous studies showing that extracellular glutamate regulates postsynaptic glutamate receptor clustering, gb mutants show a large (200-300%) increase in the number of postsynaptic glutamate receptors. This increase in postsynaptic receptor abundance is not accompanied by other obvious synaptic changes and is completely rescued when synapses are cultured in wild-type levels of glutamate. Additional in situ pharmacology suggests that glutamate-mediated suppression of glutamate receptor clustering depends on receptor desensitization. Together, our results suggest that (1) xCT transporters are critical for regulation of ambient extracellular glutamate in vivo; (2) ambient extracellular glutamate maintains some receptors constitutively desensitized in vivo; and (3) constitutive desensitization of ionotropic glutamate receptors suppresses their ability to cluster at synapses.

Glutamatergic synapses of Drosophila neuromuscular junctions: a high-resolution model for the analysis of experience-dependent potentiation

The glutamatergic synapses of developing neuromuscular junctions (NMJ) of Drosophila larvae are readily accessible, morphologically simple, and physiologically well-characterized. They therefore have a long and highly successful tradition as a model system for the discovery of genetic and molecular mechanisms of target recognition, synaptogenesis, NMJ development, and synaptic plasticity. However, since the development and the activity-dependent refinement of NMJs are concurrent processes, they cannot easily be separated by the widely applied genetic manipulations that mostly have chronic effects. Recent studies have therefore begun systematically to incorporate larval foraging behavior into the physiological and genetic analysis of NMJ function in order to analyze potential experience-dependent changes of glutamatergic transmission. These studies have revealed that recent crawling experience is a potent modulator of glutamatergic transmission at NMJs, because high crawling activities result after an initial lag-phase in several subsequent phases of experience-dependent synaptic potentiation. Depending on the time window of occurrence, four distinct phases of experience-dependent potentiation have been defined. These phases of potentiation can be followed from their initial induction (phase-I) up to the morphological consolidation (phase-III/IV) of previously established functional changes (phase-II). This therefore establishes, for the first time, a temporal hierarchy of mechanisms involved in the use-dependent modification of glutamatergic synapses.