Novel CNS glutamate receptor subunit genes of Drosophila melanogaster

Chromatin gene-gene loops support the cross-regulation of genes with related function

Chromatin loops between gene pairs have been observed in diverse contexts in both flies and vertebrates. Combining high-resolution Capture-C, DNA fluorescence in situ hybridization, and genetic perturbations, we dissect the functional role of three loops between genes with related function during Drosophila embryogenesis. By mutating the loop anchor (but not the gene) or the gene (but not loop anchor), we disentangle loop formation and gene expression and show that the 3D proximity of paralogous gene loci supports their co-regulation. Breaking the loop leads to either an attenuation or enhancement of expression and perturbs their relative levels of expression and cross-regulation. Although many loops appear constitutive across embryogenesis, their function can change in different developmental contexts. Taken together, our results indicate that chromatin gene-gene loops act as architectural scaffolds that can be used in different ways in different contexts to fine-tune the coordinated expression of genes with related functions and sustain their cross-regulation.

Glutamate Is a Wake-Active Neurotransmitter in Drosophila melanogaster

Introduction

In mammals, there is evidence that glutamate has a role as a wake-active neurotransmitter. So using video-based analysis of Drosophila behavior, we undertook a study to examine if glutamate, which has been previously shown to have an excitatory role in neuromuscular junctions in Drosophila, may have a conserved wake-active role in the adult brain.

Aims and Methods

Using 6- to 9-day-old female flies, we examined the effect of perturbations of the glutamatergic signaling on total wakefulness and wake bout architecture. We increased and decreased neuronal activity of glutamatergic neurons in the brains of adult flies using Upstream Activating Sequence (UAS) NaChBac and UAS EKO, respectively. We blocked neurotransmission from glutamatergic neurons in adult flies using the UAS-driven temperature-sensitive dynamin mutation shibirets. We examined the behavior of flies with loss of function mutations of individual subunits of brain-specific ionotropic glutamate receptors.

Results

Increasing the activity of glutamatergic neurons in the adult brain led to a significant increase in wakefulness compared to the control groups both in the daytime and nighttime and decreasing the activity of these same neurons reduced wakefulness in the nighttime. Blocking neurotransmitter release in glutamatergic neurons significantly reduced wake in the nighttime. The ionotropic receptor mutants had significantly less wake in the nighttime than their respective genetic background controls.

Conclusion

The results show the following: glutamate is indeed a wake-active neurotransmitter in Drosophila; there is a major time of day effect associated with loss of glutamatergic neurotransmission; and it is a major wake-active neurotransmitter in the nighttime.

Rapid feedback regulation of synaptic efficacy during high-frequency activity at the Drosophila larval neuromuscular junction

High-frequency firing of neurons depresses transmitter release at many synapses. At the glutamatergic synapse of the Drosophila larval neuromuscular junction, we find that presynaptic depression is modulated by postsynaptic ionotropic glutamate receptor (iGluR) activity. Although basal release at low frequency was insensitive to postsynaptic iGluR activity, recovery from depression elicited by high-frequency presynaptic trains decreased with partial block of native iGluRs. Moreover, recovery from depression increased with optical activation of the light-gated mammalian iGluR6 (LiGluR) expressed postsynaptically. The enhancement of recovery from depression occurred within 2 min of optical activation of LiGluR and persisted for minutes after optical deactivation. This effect depended on cAMP-dependent presynaptic recruitment of vesicles from the reserve pool. Our findings reveal a unique dimension to postsynaptic iGluR activity: fast retrograde signaling that preserves transmission efficacy during high-frequency presynaptic firing.

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.

Synchronized bilateral synaptic inputs to Drosophila melanogaster neuropeptidergic rest/arousal neurons

Neuropeptide PDF (pigment-dispersing factor)-secreting large ventrolateral neurons (lLN(v)s) in the Drosophila brain regulate daily patterns of rest and arousal. These bilateral wake-promoting neurons are light responsive and integrate information from the circadian system, sleep circuits, and light environment. To begin to dissect the synaptic circuitry of the circadian neural network, we performed simultaneous dual whole-cell patch-clamp recordings of pairs of lLN(v)s. Both ipsilateral and contralateral pairs of lLN(v)s exhibit synchronous rhythmic membrane activity with a periodicity of ∼ 5-10 s. This rhythmic lLN(v) activity is blocked by TTX, voltage-gated sodium blocker, or α-bungarotoxin, nicotinic acetylcholine receptor antagonist, indicating that action potential-dependent cholinergic synaptic connections are required for rhythmic lLN(v) activity. Since injecting current into one neuron of the pair had no effect on the membrane activity of the other neuron of the pair, this suggests that the synchrony is attributable to bilateral inputs and not coupling between the pairs of lLN(v)s. To further elucidate the nature of these synaptic inputs to lLN(v)s, we blocked or activated a variety of neurotransmitter receptors and measured effects on network activity and ionic conductances. These measurements indicate the lLN(v)s possess excitatory nicotinic ACh receptors, inhibitory ionotropic GABA(A) receptors, and inhibitory ionotropic GluCl (glutamate-gated chloride) receptors. We demonstrate that cholinergic input, but not GABAergic input, is required for synchronous membrane activity, whereas GABA can modulate firing patterns. We conclude that neuropeptidergic lLN(v)s that control rest and arousal receive synchronous synaptic inputs mediated by ACh.

Candidate glutamatergic neurons in the visual system of Drosophila

The visual system of Drosophila contains approximately 60,000 neurons that are organized in parallel, retinotopically arranged columns. A large number of these neurons have been characterized in great anatomical detail. However, studies providing direct evidence for synaptic signaling and the neurotransmitter used by individual neurons are relatively sparse. Here we present a first layout of neurons in the Drosophila visual system that likely release glutamate as their major neurotransmitter. We identified 33 different types of neurons of the lamina, medulla, lobula and lobula plate. Based on the previous Golgi-staining analysis, the identified neurons are further classified into 16 major subgroups representing lamina monopolar (L), transmedullary (Tm), transmedullary Y (TmY), Y, medulla intrinsic (Mi, Mt, Pm, Dm, Mi Am), bushy T (T), translobula plate (Tlp), lobula intrinsic (Lcn, Lt, Li), lobula plate tangential (LPTCs) and lobula plate intrinsic (LPi) cell types. In addition, we found 11 cell types that were not described by the previous Golgi analysis. This classification of candidate glutamatergic neurons fosters the future neurogenetic dissection of information processing in circuits of the fly visual system.

Drosophila glial glutamate transporter Eaat1 is regulated by fringe-mediated notch signaling and is essential for larval locomotion

In the mammalian CNS, glial cells expressing excitatory amino acid transporters (EAATs) tightly regulate extracellular glutamate levels to control neurotransmission and protect neurons from excitotoxic damage. Dysregulated EAAT expression is associated with several CNS pathologies in humans, yet mechanisms of EAAT regulation and the importance of glutamate transport for CNS development and function in vivo remain incompletely understood. Drosophila is an advanced genetic model with only a single high-affinity glutamate transporter termed Eaat1. We found that Eaat1 expression in CNS glia is regulated by the glycosyltransferase Fringe, which promotes neuron-to-glia signaling through the Delta-Notch ligand-receptor pair during embryogenesis. We made Eaat1 loss-of-function mutations and found that homozygous larvae could not perform the rhythmic peristaltic contractions required for crawling. We found no evidence for excitotoxic cell death or overt defects in the development of neurons and glia, and the crawling defect could be induced by postembryonic inactivation of Eaat1. Eaat1 fully rescued locomotor activity when expressed in only a limited subpopulation of glial cells situated near potential glutamatergic synapses within the CNS neuropil. Eaat1 mutants had deficits in the frequency, amplitude, and kinetics of synaptic currents in motor neurons whose rhythmic patterns of activity may be regulated by glutamatergic neurotransmission among premotor interneurons; similar results were seen with pharmacological manipulations of glutamate transport. Our findings indicate that Eaat1 expression is promoted by Fringe-mediated neuron-glial communication during development and suggest that Eaat1 plays an essential role in regulating CNS neural circuits that control locomotion in Drosophila.

Increased vesicular glutamate transporter expression causes excitotoxic neurodegeneration

Increases in vesicular glutamate transporter (VGLUT) levels are observed after a variety of insults including hypoxic injury, stress, methamphetamine treatment, and in genetic seizure models. Such overexpression can cause an increase in the amount of glutamate released from each vesicle, but it is unknown whether this is sufficient to induce excitotoxic neurodegeneration. Here we show that overexpression of the Drosophila vesicular glutamate transporter (DVGLUT) leads to excess glutamate release, with some vesicles releasing several times the normal amount of glutamate. Increased DVGLUT expression also leads to an age-dependent loss of motor function and shortened lifespan, accompanied by a progressive neurodegeneration in the postsynaptic targets of the DVGLUT-overexpressing neurons. The early onset lethality, behavioral deficits, and neuronal pathology require overexpression of a functional DVGLUT transgene. Thus overexpression of DVGLUT is sufficient to generate excitotoxic neuropathological phenotypes and therefore reducing VGLUT levels after nervous system injury or stress may mitigate further damage.

Dynamics of glutamatergic signaling in the mushroom body of young adult Drosophila

Background

The mushroom bodies (MBs) are paired brain centers located in the insect protocerebrum involved in olfactory learning and memory and other associative functions. Processes from the Kenyon cells (KCs), their intrinsic neurons, form the bulk of the MB's calyx, pedunculus and lobes. In young adult Drosophila, the last-born KCs extend their processes in the α/β lobes as a thin core (α/β cores) that is embedded in the surrounding matrix of other mature KC processes. A high level of L-glutamate (Glu) immunoreactivity is present in the α/β cores (α/βc) of recently eclosed adult flies. In a Drosophila model of fragile X syndrome, the main cause of inherited mental retardation, treatment with metabotropic Glu receptor (mGluR) antagonists can rescue memory deficits and MB structural defects.

Results

To address the role of Glu signaling in the development and maturation of the MB, we have compared the time course of Glu immunoreactivity with the expression of various glutamatergic markers at various times, that is, 1 hour, 1 day and 10 days after adult eclosion. We observed that last-born α/βc KCs in young adult as well as developing KCs in late larva and at various pupal stages transiently express high level of Glu immunoreactivity in Drosophila. One day after eclosion, the Glu level was already markedly reduced in the α/βc neurons. Glial cell processes expressing glutamine synthetase and the Glu transporter dEAAT1 were found to surround the Glu-expressing KCs in very young adults, subsequently enwrapping the α/β lobes to become distributed equally over the entire MB neuropil. The vesicular Glu transporter DVGluT was detected by immunostaining in processes that project within the MB lobes and pedunculus, but this transporter is apparently never expressed by the KCs themselves. The NMDA receptor subunit dNR1 is widely expressed in the MB neuropil just after eclosion, but was not detected in the α/βc neurons. In contrast, we provide evidence that DmGluRA, the only Drosophila mGluR, is specifically expressed in Glu-accumulating cells of the MB α/βc immediately and for a short time after eclosion.

Conclusions

The distribution and dynamics of glutamatergic markers indicate that newborn KCs transiently accumulate Glu at a high level in late pupal and young eclosed Drosophila, and may locally release this amino acid by a mechanism that would not involve DVGluT. At this stage, Glu can bind to intrinsic mGluRs abundant in the α/βc KCs, and to NMDA receptors in the rest of the MB neuropil, before being captured and metabolized in surrounding glial cells. This suggests that Glu acts as an autocrine or paracrine agent that contributes to the structural and functional maturation of the MB during the first hours of Drosophila adult life.

Furthering pharmacological and physiological assessment of the glutamatergic receptors at the Drosophila neuromuscular junction

Drosophila melanogaster larval neuromuscular junctions (NMJs) serve as a model for synaptic physiology. The molecular sequences of the postsynaptic glutamate receptors have been described; however, the pharmacological profile has not been fully elucidated. The postsynaptic molecular sequence suggests a novel glutamate receptor subtype. Kainate does not depolarize the muscle, but dampens evoked EPSP amplitudes. Quantal responses show a decreased amplitude and area under the voltage curve indicative of reduced postsynaptic receptor sensitivity to glutamate transmission. ATPA, a kainate receptor agonist, did not mimic kainate's action. The metabotropic glutamate receptor agonist t-ACPD had no effect. Domoic acid, a kainate/AMPA receptor agonist, blocks the postsynaptic receptors without depolarizing the muscle. However, SYM 2081, a kainate receptor agonist, did depolarize the muscle and reduce the EPSP amplitude at 1 mM but not at 0.1 mM. This supports the notion that these are generally a quisqualate subtype receptors with some oddities in the pharmacological profile. The results suggest a direct postsynaptic action of kainate due to partial antagonist action on the quisqualate receptors. There does not appear to be presynaptic auto-regulation via a kainate receptor subtype or a metabotropic auto-receptor. This study aids in furthering the pharmokinetic profiling and specificity of the receptor subtypes.

Comparative analysis of the locations of the NR1 and NR2 NMDA receptor subunits in honeybee (Apis mellifera) and fruit fly (Drosophila melanogaster, Canton-S wild-type) cerebral ganglia

The locations of the NR1 and NR2 subunits of the GABA receptor were studied in brain structures in insects--honeybees and fruit flies--using an immunohistochemical method. The specificities of the antibodies to the NR1 and NR2 subunits were confirmed by the antisense knockdown method for the NR1 subunit and western blotting. The data obtained here lead to the conclusion that the distributions of the NR1 and NR2 subunits of the NMDA receptor complex in the cerebral ganglia of the honeybee and fruit fly are similar; areas with the highest concentrations of NR1 and NR2 subunits were identified, and these were found to be different in the different insects. This is associated with the behavioral characteristics of these two insect species.

Visualizing glutamatergic cell bodies and synapses in Drosophila larval and adult CNS

Glutamate is the major excitatory neurotransmitter in the vertebrate central nervous system (CNS) and at Drosophila neuromuscular junctions (NMJs). Although glutamate is also used as a transmitter in the Drosophila CNS, there has been no systematic description of the central glutamatergic signaling system in the fly. With the recent cloning of the Drosophila vesicular glutamate transporter (DVGLUT), it is now possible to mark many, if not all, central glutamatergic neurons and synapses. Here we present the pattern of glutamatergic synapses and cell bodies in the late larval CNS and in the adult fly brain by using an anti-DVGLUT antibody. We also introduce two new tools for studying the Drosophila glutamatergic system: a dvglut promoter fragment fused to Gal4 whose expression labels glutamatergic neurons and a green fluorescent protein (GFP)-tagged DVGLUT transgene that localizes to synapses. In the larval CNS, we find synaptic DVGLUT immunoreactivity prominent in all brain lobe neuropil compartments except for the mushroom body. Likewise in the adult CNS, glutamatergic synapses are abundant throughout all major brain structures except the mushroom body. We also find that the larval ventral nerve cord neuropil is rich in glutamatergic synapses, which are primarily located near the dorsal surface of the neuropil, segregated from the ventrally positioned cholinergic processes. This description of the glutamatergic system in Drosophila highlights the prevalence of glutamatergic neurons in the CNS and presents tools for future study and manipulation of glutamatergic transmission.

Widespread brain distribution of the Drosophila metabotropic glutamate receptor

Glutamate is the predominant excitatory neurotransmitter in the vertebrate brain, whereas acetylcholine has been considered to play the same role in insects. Recent studies have, however, questioned the latter view by showing a rather general distribution of glutamate transporters. Here, we describe the expression pattern of the receptor DmGlu-A (DmGluRA), the unique homolog of vertebrate metabotropic glutamate receptors. Metabotropic glutamate receptors play important roles in the regulation of glutamatergic neurotransmission. Using a specific antibody, we report DmGluRA expression in most neuropile areas in both larvae and adults, but not in the lobes of the mushroom bodies. These observations suggest a key role for glutamate in the insect brain.

Glutamate and its metabotropic receptor in Drosophila clock neuron circuits

Identification of the neurotransmitters in clock neurons is critical for understanding the circuitry of the neuronal network that controls the daily behavioral rhythms in Drosophila. Except for the neuropeptide pigment-dispersing factor, no neurotransmitters have been clearly identified in the Drosophila clock neurons. Here we show that glutamate and its metabotropic receptor, DmGluRA, are components of the clock circuitry and modulate the rhythmic behavior pattern of Drosophila. The dorsal clock neurons, DN1s in the larval brain and some DN1s and DN3s in the adult brain, were immunolabeled with antibodies against Drosophila vesicular glutamate transporter (DvGluT), suggesting that they are glutamatergic. Because the DN1s may communicate with the primary pacemaker neurons, s-LN(v)s, we tested glutamate responses of dissociated larval s-LN(v)s by means of calcium imaging. Application of glutamate dose dependently decreased intracellular calcium in the s-LN(v)s. Pharmacology of the response suggests the presence of DmGluRA on the s-LN(v)s. Antibodies against DmGluRA labeled dissociated s-LN(v)s and the LN(v) dendrites in the intact larval and adult brain. The role of metabotropic glutamate signaling was tested in behavior assays in transgenic larvae and flies with altered DmGluRA expression in the LN(v)s and other clock neurons. Larval photophobic behavior was enhanced in DmGluRA mutants. For adults, we could induce altered activity patterns in the dark phase under LD conditions and increase the period during constant darkness by knockdown of DmGluRA expression in LN(v)s. Our results suggest that a glutamate signal from some of the DNs modulates the rhythmic behavior pattern via DmGluRA on the LN(v)s in Drosophila.

Eph receptor and ephrin signaling in developing and adult brain of the honeybee (Apis mellifera)

Roles for Eph receptor tyrosine kinase and ephrin signaling in vertebrate brain development are well established. Their involvement in the modulation of mammalian synaptic structure and physiology is also emerging. However, less is known of their effects on brain development and their function in adult invertebrate nervous systems. Here, we report on the characterization of Eph receptor and ephrin orthologs in the honeybee, Apis mellifera (Am), and their role in learning and memory. In situ hybridization for mRNA expression showed a uniform distribution of expression of both genes across the developing pupal and adult brain. However, in situ labeling with Fc fusion proteins indicated that the AmEphR and Amephrin proteins were differentially localized to cell body regions in the mushroom bodies and the developing neuropiles of the antennal and optic lobes. In adults, AmEphR protein was localized to regions of synaptic contacts in optic lobes, in the glomeruli of antennal lobes, and in the medial lobe of the mushroom body. The latter two regions are involved in olfactory learning and memory in the honeybee. Injections of EphR-Fc and ephrin-Fc proteins into the brains of adult bees, 1 h before olfactory conditioning of the proboscis extension reflex, significantly reduced memory 24 h later. Experimental amnesia in the group injected with ephrin-Fc was apparent 1 h post-training. Experimental amnesia was also induced by post-training injections with ephrin-Fc suggesting a role in recall. This is the first demonstration that Eph molecules function to regulate the formation of memory in insects.

Characterization of a metabotropic glutamate receptor in the honeybee (Apis mellifera): implications for memory formation

G-protein-coupled metabotropic glutamate receptors (GPC mGluRs) are important constituents of glutamatergic synapses where they contribute to synaptic plasticity and development. Here we characterised a member of this family in the honeybee. We show that the honeybee genome encodes a genuine mGluR (AmGluRA) that is expressed at low to medium levels in both pupal and adult brains. Analysis of honeybee protein sequence places it within the type 3 GPCR family, which includes mGlu receptors, GABA-B receptors, calcium-sensing receptors, and pheromone receptors. Phylogenetic comparisons combined with pharmacological evaluation in HEK 293 cells transiently expressing AmGluRA show that the honeybee protein belongs to the group II mGluRs. With respect to learning and memory AmGluRA appears to be required for memory formation. Both agonists and antagonists selective against the group II mGluRs impair long-term (24 h) associative olfactory memory formation when applied 1 h before training, but have no effect when injected post-training or pre-testing. Our results strengthen the notion that glutamate is a key neurotransmitter in memory processes in the honeybee.

Drosophila melanogaster neurobiology, neuropharmacology, and how the fly can inform central nervous system drug discovery

Central nervous system (CNS) drug discovery in the post-genomic era is rapidly evolving. Older empirical methods are giving way to newer technologies that include bioinformatics, structural biology, genetics, and modern computational approaches. In the search for new medical therapies, and in particular treatments for disorders of the central nervous system, there has been increasing recognition that identification of a single biological target is unlikely to be a recipe for success; a broad perspective is required. Systems biology is one such approach, and has been increasingly recognized as a very important area of research, as it places specific molecular targets within a context of overall biochemical action. Understanding the complex interactions between the components within a given biological system that lead to modifications in output, such as changes in behavior or development, may be important avenues of discovery to identify new therapies. One avenue to drug discovery that holds tremendous potential is the use of model genetic organisms such as the fruit fly, Drosophila melanogaster. The similarity between mode of drug action, behavior, and gene response in D. melanogaster and mammalian systems, combined with the power of genetics, have recently made the fly a very attractive system to study fundamental neuropharmacological processes relevant to human diseases. The promise that the use of model organisms such as the fly offers is speed, high throughput, and dramatically reduced overall costs that together should result in an enhanced rate of discovery.

Physiological requirement for the glutamate transporter dEAAT1 at the adult Drosophila neuromuscular junction

L-glutamate is the major excitatory neurotransmitter in the mammalian brain. Specific proteins, the Na+/K+-dependent high affinity excitatory amino acid transporters (EAATs), are involved in the extracellular clearance and recycling of this amino acid. Type I synapses of the Drosophila neuromuscular junction (NMJ) similarly use L-glutamate as an excitatory transmitter. However, the localization and function of the only high-affinity glutamate reuptake transporter in Drosophila, dEAAT1, at the NMJ was unknown. Using a specific antibody and transgenic strains, we observed that dEAAT1 is present at the adult, but surprisingly not at embryonic and larval NMJ, suggesting a physiological maturation of the junction during metamorphosis. We found that dEAAT1 is not localized in motor neurons but in glial extensions that closely follow motor axons to the adult NMJ. Inactivation of the dEAAT1 gene by RNA interference generated viable adult flies that were able to walk but were flight-defective. Electrophysiological recordings of the thoracic dorso-lateral NMJ were performed in adult dEAAT1-deficient flies. The lack of dEAAT1 prolonged the duration of the individual responses to motor nerve stimulation and this effect was progressively increased during physiological trains of stimulations. Therefore, glutamate reuptake by glial cells is required to ensure normal activity of the Drosophila NMJ, but only in adult flies.

Focal and temporal release of glutamate in the mushroom bodies improves olfactory memory in Apis mellifera

In contrast to vertebrates, the role of the neurotransmitter glutamate in learning and memory in insects has hardly been investigated. The reason is that a pharmacological characterization of insect glutamate receptors is still missing; furthermore, it is difficult to locally restrict pharmacological interventions. In this study, we overcome these problems by using locally and temporally defined photo-uncaging of glutamate to study its role in olfactory learning and memory formation in the honeybee, Apis mellifera. Uncaging glutamate in the mushroom bodies immediately after a weak training protocol induced a higher memory rate 2 d after training, mimicking the effect of a strong training protocol. Glutamate release before training does not facilitate memory formation, suggesting that glutamate mediates processes triggered by training and required for memory formation. Uncaging glutamate in the antennal lobes shows no effect on memory formation. These results provide the first direct evidence for a temporally and locally restricted function of glutamate in memory formation in honeybees and insects.

Identification and localisation of the NR1 sub-unit homologue of the NMDA glutamate receptor in the honeybee brain

The NR1 sub-unit homologue of the NMDA glutamate receptor was characterised in the honeybee. Sequence analysis suggests that the honeybee NMDA receptor may act as a coincidence detector molecule similar to its counterpart in the mammalian nervous system. The localisation of the expression sites at the mRNA and the protein levels indicates that the receptor is expressed throughout the brain, in neurons and in glial cells.

Increased synaptic microtubules and altered synapse development in Drosophila sec8 mutants

Background

Sec8 is highly expressed in mammalian nervous systems and has been proposed to play a role in several aspects of neural development and function, including neurite outgrowth, calcium-dependent neurotransmitter secretion, trafficking of ionotropic glutamate receptors and regulation of neuronal microtubule assembly. However, these models have never been tested in vivo. Nervous system development and function have not been described after mutation of sec8 in any organism.

Results

We identified lethal sec8 mutants in an unbiased forward genetic screen for mutations causing defects in development of glutamatergic Drosophila neuromuscular junctions (NMJs). The Drosophila NMJ is genetically malleable and accessible throughout development to electrophysiology and immunocytochemistry, making it ideal for examination of the sec8 mutant synaptic phenotype. We developed antibodies to Drosophila Sec8 and showed that Sec8 is abundant at the NMJ. In our sec8 null mutants, in which the sec8 gene is specifically deleted, Sec8 immunoreactivity at the NMJ is eliminated but immunoblots reveal substantial maternal contribution in the rest of the animal. Contrary to the hypothesis that Sec8 is required for neurite outgrowth or synaptic terminal growth, immunocytochemical examination revealed that sec8 mutant NMJs developed more branches and presynaptic terminals during larval development, compared to controls. Synaptic electrophysiology showed no evidence that Sec8 is required for basal neurotransmission, though glutamate receptor trafficking was mildly disrupted in sec8 mutants. The most dramatic NMJ phenotype in sec8 mutants was an increase in synaptic microtubule density, which was approximately doubled compared to controls.

Conclusion

Sec8 is abundant in the Drosophila NMJ. Sec8 is required in vivo for regulation of synaptic microtubule formation, and (probably secondarily) regulation of synaptic growth and glutamate receptor trafficking. We did not find any evidence that Sec8 is required for basal neurotransmission.

Decreasing glutamate buffering capacity triggers oxidative stress and neuropil degeneration in the Drosophila brain

L-glutamate is both the major brain excitatory neurotransmitter and a potent neurotoxin in mammals. Glutamate excitotoxicity is partly responsible for cerebral traumas evoked by ischemia and has been implicated in several neurodegenerative diseases including amyotrophic lateral sclerosis (ALS). In contrast, very little is known about the function or potential toxicity of glutamate in the insect brain. Here, we show that decreasing glutamate buffering capacity is neurotoxic in Drosophila. We found that the only Drosophila high-affinity glutamate transporter, dEAAT1, is selectively addressed to glial extensions that project ubiquitously through the neuropil close to synaptic areas. Inactivation of dEAAT1 by RNA interference led to characteristic behavior deficits that were significantly rescued by expression of the human glutamate transporter hEAAT2 or the administration in food of riluzole, an anti-excitotoxic agent used in the clinic for human ALS patients. Signs of oxidative stress included hypersensitivity to the free radical generator paraquat and rescue by the antioxidant melatonin. Inactivation of dEAAT1 also resulted in shortened lifespan and marked brain neuropil degeneration characterized by widespread microvacuolization and swollen mitochondria. This suggests that the dEAAT1-deficient fly provides a powerful genetic model system for molecular analysis of glutamate-mediated neurodegeneration.

Characterization of the two distinct subtypes of metabotropic glutamate receptors from honeybee, Apis mellifera

L-Glutamate is a major neurotransmitter at the excitatory synapses in the vertebrate brain. It is also the excitatory neurotransmitter at neuromuscular junctions in insects, however its functions in their brains remain to be established. We identified and characterized two different subtypes (AmGluRA and AmGluRB) of metabotropic glutamate receptors (mGluRs) from an eusocial insect, honeybee. Both AmGluRA and AmGluRB form homodimers independently on disulfide bonds, and bind [3H]glutamate with K(D) values of 156.7 and 80.7 nM, respectively. AmGluRB is specifically expressed in the brain, while AmGluRA is expressed in the brain and other body parts, suggesting that AmGluRA is also present at the neuromuscular junctions. Both mGluRs are expressed in the mushroom bodies and the brain regions of honeybees, where motor neurons are clustered. Their expression in the brain apparently overlaps, suggesting that they may interact with each other to modulate the glutamatergic neurotransmission.

Differential localization of glutamate receptor subunits at the Drosophila neuromuscular junction

The subunit composition of postsynaptic neurotransmitter receptors is a key determinant of synaptic physiology. Two glutamate receptor subunits, Drosophila glutamate receptor IIA (DGluRIIA) and DGluRIIB, are expressed at the Drosophila neuromuscular junction and are redundant for viability, yet differ in their physiological properties. We now identify a third glutamate receptor subunit at the Drosophila neuromuscular junction, DGluRIII, which is essential for viability. DGluRIII is required for the synaptic localization of DGluRIIA and DGluRIIB and for synaptic transmission. Either DGluRIIA or DGluRIIB, but not both, is required for the synaptic localization of DGluRIII. DGluRIIA and DGluRIIB compete with each other for access to DGluRIII and subsequent localization to the synapse. These results are consistent with a model of a multimeric receptor in which DGluRIII is an essential component. At single postsynaptic cells that receive innervation from multiple motoneurons, DGluRIII is abundant at all synapses. However, DGluRIIA and DGluRIIB are differentially localized at the postsynaptic density opposite distinct motoneurons. Hence, innervating motoneurons may regulate the subunit composition of their receptor fields within a shared postsynaptic cell. The capacity of presynaptic inputs to shape the subunit composition of postsynaptic receptors could be an important mechanism for synapse-specific regulation of synaptic function and plasticity.

Effects of NMDA receptor antagonists on olfactory learning and memory in the honeybee (Apis mellifera)

In contrast to vertebrates the involvement of glutamate and N-methyl-D-aspartate (NMDA) receptors in brain functions in insects is both poorly understood and somewhat controversial. Here, we have examined the behavioural effects of two noncompetitive NMDA receptor antagonists, memantine (low affinity) and MK-801 (high affinity), on learning and memory in honeybees (Apis mellifera) using the olfactory conditioning of the proboscis extension reflex (PER). We induced memory deficit by injecting harnessed individuals with a glutamate transporter inhibitor, L-trans-2,4-PDC (L-trans-2,4-pyrrolidine dicarboxylate), that impairs long-term (24 h), but not short-term (1 h), memory in honeybees. We show that L-trans-2,4-PDC-induced amnesia is 'rescued' by memantine injected either before training, or before testing, suggesting that memantine restores memory recall rather than memory formation or storage. When injected alone memantine has a mild facilitating effect on memory. The effects of MK-801 are similar to those of L-trans-2,4-PDC. Both pretraining and pretesting injections lead to an impairment of long-term (24 h) memory, but have no effect on short-term (1 h) memory of an olfactory task. The implications of our results for memory processes in the honeybee are discussed.

Fast synaptic currents in Drosophila mushroom body Kenyon cells are mediated by alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors and picrotoxin-sensitive GABA receptors

The mushroom bodies, bilaterally symmetric regions in the insect brain, play a critical role in olfactory associative learning. Genetic studies in Drosophila suggest that plasticity underlying acquisition and storage of memory occurs at synapses on the dendrites of mushroom body Kenyon cells (Dubnau et al., 2001). Additional exploration of the mechanisms governing synaptic plasticity contributing to these aspects of olfactory associative learning requires identification of the receptors that mediate fast synaptic transmission in Kenyon cells. To this end, we developed a culture system that supports the formation of excitatory and inhibitory synaptic connections between neurons harvested from the central brain region of late-stage Drosophila pupae. Mushroom body Kenyon cells are identified as small-diameter, green fluorescent protein-positive (GFP+) neurons in cultures from OK107-GAL4;UAS-GFP pupae. In GFP+ Kenyon cells, fast EPSCs are mediated by alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors (nAChRs). The miniature EPSCs have rapid rise and decay kinetics and a broad, positively skewed amplitude distribution. Fast IPSCs are mediated by picrotoxin-sensitive chloride conducting GABA receptors. The miniature IPSCs also have a rapid rate of rise and decay and a broad amplitude distribution. The vast majority of spontaneous synaptic currents in the cultured Kenyon cells are mediated byalpha-bungarotoxin-sensitive nAChRs or picrotoxin-sensitive GABA receptors. Therefore, these receptors are also likely to mediate synaptic transmission in Kenyon cells in vivo and to contribute to plasticity during olfactory associative learning.

Terminal glial differentiation involves regulated expression of the excitatory amino acid transporters in the Drosophila embryonic CNS

The Drosophila excitatory amino acid transporters dEAAT1 and dEAAT2 are nervous-specific transmembrane proteins that mediate the high affinity uptake of L-glutamate or aspartate into cells. Here, we demonstrate by colocalization studies that both genes are expressed in discrete and partially overlapping subsets of differentiated glia and not in neurons in the embryonic central nervous system (CNS). We show that expression of these transporters is disrupted in mutant embryos deficient for the glial fate genes glial cells missing (gcm) and reversed polarity (repo). Conversely, ectopic expression of gcm in neuroblasts, which forces all nerve cells to adopt a glial fate, induces an ubiquitous expression of both EAAT genes in the nervous system. We also detected the dEAAT transcripts in the midline glia in late embryos and dEAAT2 in a few peripheral neurons in head sensory organs. Our results show that glia play a major role in excitatory amino acid transport in the Drosophila CNS and that regulated expression of the dEAAT genes contributes to generate the functional diversity of glial cells during embryonic development.

Ionotropic glutamate receptors mediate juvenile hormone synthesis in the cockroach, Diploptera punctata

By monitoring changes in the cytosolic [Ca2+](i) and rates of juvenile hormone (JH) synthesis in response to L-glutamate agonists and antagonists, we identified and characterized glutamate receptor subtypes in corpus allatum (CA) cells of the cockroach, Diploptera punctata. During the first ovarian cycle, corpora allata exhibited a cycle of changes in sensitivity to L-glutamate correlated to cyclic changes in rates of JH synthesis. When exposed to 60 microM L-glutamate in vitro, the active corpora allata of day-4 mated females produced 60% more JH, while inactive corpora allata at other ages showed 10-20% stimulatory response. Pharmacological characterization using various L-glutamate receptor agonists and antagonists indicated that several ionotropic subtypes of L-glutamate receptors were present in the CA. The CA showed an increase in rates of JH synthesis in response to NMDA, kainate, and quisqualate, but not to AMPA in both L-15 medium and minimum incubation medium. In contrast, applications of the metabotropic receptor-specific agonist trans-ACPD failed to elicit a change in the cytosolic [Ca2+](i) and JH production. An elevation of cytosolic calcium concentration, followed by 20-30% rise in JH production, was observed when active CA cells were exposed to 10-40 microM kainate. Kainate had no stimulatory effect on JH synthesis in calcium-free medium. The kainate-induced JH synthesis was blocked by 20 microM CNQX but was not affected by 20 microM NBQX. Kainate-stimulated JH production was not suppressed by MK-801 (a specific blocker of NMDA-receptor channel), nor was NMDA-stimulated JH production affected by CNQX (a specific antagonist of kainate receptor). These data suggest that active CA cells are stimulated to synthesize more JH by a glutamate-induced calcium rise via NMDA-, kainate- and/or quisqualate-sensitive subtypes of ionotropic L-glutamate receptors. The metabotropic-subtype and ionotropic AMPA-subtype L-glutamate receptors are unlikely to be present on active CA cells.

Genome-wide transcriptional orchestration of circadian rhythms in Drosophila

Circadian rhythms govern the behavior, physiology, and metabolism of living organisms. Recent studies have revealed the role of several genes in the clock mechanism both in Drosophila and in mammals. To study how gene expression is globally regulated by the clock mechanism, we used a high density oligonucleotide probe array (GeneChip) to profile gene expression patterns in Drosophila under light-dark and constant dark conditions. We found 712 genes showing a daily fluctuation in mRNA levels under light-dark conditions, and among these the expression of 115 genes was still cycling in constant darkness, i.e. under free-running conditions. Unexpectedly the expression of a large number of genes cycled exclusively under constant darkness. We found that cycling in most of these genes was lost in the arrhythmic Clock (Clk) mutant under light-dark conditions. Expression of periodically regulated genes is coordinated locally on chromosomes where small clusters of genes are regulated jointly. Our findings reveal that many genes involved in diverse functions are under circadian control and reveal the complexity of circadian gene expression in Drosophila.

Insect NMDA receptors mediate juvenile hormone biosynthesis

In vertebrates, the N-methyl-D-aspartate subtype of glutamate receptors (NMDAR) appears to play a role in neuronal development, synaptic plasticity, memory formation, and pituitary activity. However, functional NMDAR have not yet been characterized in insects. We have now demonstrated immunohistochemically glutamatergic nerve terminals in the corpora allata of an adult female cockroach, Diploptera punctata. Cockroach corpus allatum (CA) cells, exposed to NMDA in vitro, exhibited elevated cytosolic [Ca(2+)], but not in culture medium nominally free of calcium or containing NMDAR-specific channel blockers: MK-801 and Mg(2+). Sensitivity of cockroach corpora allata to NMDA changed cyclically during the ovarian cycle. Highly active glands of 4-day-old mated females, exposed to 3 microM NMDA, produced 70% more juvenile hormone (JH) in vitro, but the relatively inactive glands of 8-day-old mated females showed little response to the agonist. The stimulatory effect of NMDA was eliminated by augmenting the culture medium with MK-801, conantokin, or high Mg(2+). Having obtained substantive evidence of functioning NMDAR in insect corpora allata, we used reverse transcription PCR to demonstrate two mRNA transcripts, DNMDAR1 and DNMDAR2, in the ring gland and brain of last-instar Drosophila melanogaster. Immunohistochemical labeling, using mouse monoclonal antibody against rat NMDAR1, showed that only one of the three types of endocrine cells in the ring gland, CA cells, expressed rat NMDAR1-like immunoreactive protein. This antibody also labeled two brain neurons in the lateral protocerebrum, one neuron per brain hemisphere. Finally, we used the same primers for DNMDAR1 to demonstrate a fragment of putative NMDA receptor in the corpora allata of Diploptera punctata. Our results suggest that the NMDAR has a role in regulating JH synthesis and that ionotropic-subtype glutamate receptors became specialized early in animal evolution.

Taurine-, aspartate- and glutamate-like immunoreactivity identifies chemically distinct subdivisions of Kenyon cells in the cockroach mushroom body

The lobes of the mushroom bodies of the cockroach Periplaneta americana consist of longitudinal modules called laminae. These comprise repeating arrangements of Kenyon cell axons, which like their dendrites and perikarya have an affinity to one of three antisera: to taurine, aspartate, or glutamate. Taurine-immunopositive laminae alternate with immunonegative ones. Aspartate-immunopositive Kenyon cell axons are distributed across the lobes. However, smaller leaf-like ensembles of axons that reveal particularly high affinities to anti-aspartate are embedded within taurine-positive laminae and occur in the immunonegative laminae between them. Together, these arrangements reveal a complex architecture of repeating subunits whose different levels of immunoreactivity correspond to broader immunoreactive layers identified by sera against the neuromodulator FMRFamide. Throughout development and in the adult, the most posterior lamina is glutamate immunopositive. Its axons arise from the most recently born Kenyon cells that in the adult retain their juvenile character, sending a dense system of collaterals to the front of the lobes. Glutamate-positive processes intersect aspartate- and taurine-immunopositive laminae and are disposed such that they might play important roles in synaptogenesis or synapse modification. Glutamate immunoreactivity is not seen in older, mature axons, indicating that Kenyon cells show plasticity of neurotransmitter phenotype during development. Aspartate may be a universal transmitter substance throughout the lobes. High levels of taurine immunoreactivity occur in broad laminae containing the high concentrations of synaptic vesicles.

Distribution of metabotropic glutamate receptor DmGlu-A in Drosophila melanogaster central nervous system

L-glutamate is the excitatory neurotransmitter at neuromuscular junctions in insects. It may also be involved in neurotransmission within the central nervous system (CNS), but its function therein remains elusive. The roles of glutamatergic synapses in the Drosophila melanogaster CNS were investigated, with focus on the study of DmGluRA, a G-protein-coupled glutamate receptor. In a first attempt to determine the function of this receptor, we describe its distribution in the larval and adult Drosophila CNS, using a polyclonal antibody raised against the C-terminal sequence of the protein. DmGluRA is expressed in a reproducible pattern both in the larva and in the adult. In particular, DmGluRA can be found in the antennal lobes, the optic lobes, the central complex, and the median bundle in the adult CNS. However, DmGluRA-containing neurons represented only a small fraction of all CNS neurons. DmGluRA immunoreactivity was not detected at the larval neuromuscular junction nor in the body wall muscles. The correlations between DmGluRA distribution and previously described glutamate-like immunoreactivity patterns, as well as the implications of these observations concerning the possible functions of DmGluRA in the Drosophila CNS, are discussed.

Blockade of the central generator of locomotor rhythm by noncompetitive NMDA receptor antagonists in Drosophila larvae

The noncompetitive antagonists of the vertebrate N-methyl-D-aspartate (NMDA) receptor dizocilpine (MK 801) and phencyclidine (PCP), delivered in food, were found to induce a marked and reversible inhibition of locomotor activity in Drosophila melanogaster larvae. To determine the site of action of these antagonists, we used an in vitro preparation of the Drosophila third-instar larva, preserving the central nervous system and segmental nerves with their connections to muscle fibers of the body wall. Intracellular recordings were made from ventral muscle fibers 6 and 7 in the abdominal segments. In most larvae, long-lasting (>1 h) spontaneous rhythmic motor activities were recorded in the absence of pharmacological activation. After sectioning of the connections between the brain and abdominal ganglia, the rhythm disappeared, but it could be partially restored by perfusing the muscarinic agonist oxotremorine, indicating that the activity was generated in the ventral nerve cord. MK 801 and PCP rapidly and efficiently inhibited the locomotor rhythm in a dose-dependent manner, the rhythm being totally blocked in 2 min with doses over 0.1 mg/mL. In contrast, more hydrophilic competitive NMDA antagonists had no effect on the motor rhythm in this preparation. MK 801 did not affect neuromuscular glutamatergic transmission at similar doses, as demonstrated by monitoring the responses elicited by electrical stimulation of the motor nerve or pressure applied glutamate. The presence of oxotremorine did not prevent the blocking effect of MK 801. These results show that MK 801 and PCP specifically inhibit centrally generated rhythmic activity in Drosophila, and suggest a possible role for NMDA-like receptors in locomotor rhythm control in the insect CNS.