Selective high-affinity transport of aspartate by a Drosophila homologue of the excitatory amino-acid transporters

Novel EAAT2 activators improve motor and cognitive impairment in a transgenic model of Huntington's disease

Introduction

Glutamate excitotoxicity is causal in striatal neurodegeneration underlying motor dysfunction and cognitive deficits in Huntington's disease (HD). Excitatory amino acid transporter 2 (EAAT2), the predominant glutamate transporter accounting for >90% of glutamate transport, plays a key role in preventing excitotoxicity by clearing excess glutamate from the intrasynaptic cleft. Accordingly, EAAT2 has emerged as a promising therapeutic target for prevention of neuronal excitotoxicity underlying HD and other neurodegenerative diseases.

Methods

We have previously designed novel EAAT2 positive allosteric modulator GT951, GTS467, and GTS551, with low nanomolar efficacy in glutamate uptake and favorable pharmacokinetic properties. In this study, we test the neuroprotective abilities of these novel EAAT2 activators in vivo using the robust Drosophila HD transgenic model expressing human huntingtin gene with expanded repeats (Htt128Q).

Results

All three compounds significantly restored motor function impaired under HD pathology over a wide dose range. Additionally, treatment with all three compounds significantly improved HD-associated olfactory associative learning and short-term memory defects, while GT951 and GTS551 also improved middle-term memory in low-performing group. Similarly, treatment with GT951 and GTS551 partially protected against early mortality observed in our HD model. Further, treatment with all three EAAT2 activators induced epigenetic expression of EAAT2 Drosophila homolog and several cognition-associated genes.

Conclusion

Together, these results highlight the efficacy of GT951, GTS467 and GTS551 in treating motor and cognitive impairments under HD pathology and support their development for treatment of HD.

A Medicinal Chemistry Perspective on Excitatory Amino Acid Transporter 2 Dysfunction in Neurodegenerative Diseases

The excitatory amino acid transporter 2 (EAAT2) plays a key role in the clearance and recycling of glutamate - the major excitatory neurotransmitter in the mammalian brain. EAAT2 loss/dysfunction triggers a cascade of neurodegenerative events, comprising glutamatergic excitotoxicity and neuronal death. Nevertheless, our current knowledge regarding EAAT2 in neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS) and Alzheimer's disease (AD), is restricted to post-mortem analysis of brain tissue and experimental models. Thus, detecting EAAT2 in the living human brain might be crucial to improve diagnosis/therapy for ALS and AD. This perspective article describes the role of EAAT2 in physio/pathological processes and provides a structure-activity relationship of EAAT2-binders, bringing two perspectives: therapy (activators) and diagnosis (molecular imaging tools).

Review of Venoms of Non-Polydnavirus Carrying Ichneumonoid Wasps

Parasitoids are predominantly insects that develop as larvae on or inside their host, also usually another insect, ultimately killing it after various periods of parasitism when both parasitoid larva and host are alive. The very large wasp superfamily Ichneumonoidea is composed of parasitoids of other insects and comprises a minimum of 100,000 species. The superfamily is dominated by two similarly sized families, Braconidae and Ichneumonidae, which are collectively divided into approximately 80 subfamilies. Of these, six have been shown to release DNA-containing virus-like particles, encoded within the wasp genome, classified in the virus family Polydnaviridae. Polydnaviruses infect and have profound effects on host physiology in conjunction with various venom and ovarial secretions, and have attracted an immense amount of research interest. Physiological interactions between the remaining ichneumonoids and their hosts result from adult venom gland secretions and in some cases, ovarian or larval secretions. Here we review the literature on the relatively few studies on the effects and chemistry of these ichneumonoid venoms and make suggestions for interesting future research areas. In particular, we highlight relatively or potentially easily culturable systems with features largely lacking in currently studied systems and whose study may lead to new insights into the roles of venom chemistry in host-parasitoid relationships as well as their evolution.

A circuit-dependent ROS feedback loop mediates glutamate excitotoxicity to sculpt the Drosophila motor system

Overproduction of reactive oxygen species (ROS) is known to mediate glutamate excitotoxicity in neurological diseases. However, how ROS burdens can influence neural circuit integrity remains unclear. Here, we investigate the impact of excitotoxicity induced by depletion of Drosophila Eaat1, an astrocytic glutamate transporter, on locomotor central pattern generator (CPG) activity, neuromuscular junction architecture, and motor function. We show that glutamate excitotoxicity triggers a circuit-dependent ROS feedback loop to sculpt the motor system. Excitotoxicity initially elevates ROS, thereby inactivating cholinergic interneurons and consequently changing CPG output activity to overexcite motor neurons and muscles. Remarkably, tonic motor neuron stimulation boosts muscular ROS, gradually dampening muscle contractility to feedback-enhance ROS accumulation in the CPG circuit and subsequently exacerbate circuit dysfunction. Ultimately, excess premotor excitation of motor neurons promotes ROS-activated stress signaling that alters neuromuscular junction architecture. Collectively, our results reveal that excitotoxicity-induced ROS can perturb motor system integrity through a circuit-dependent mechanism.

Drosophila Nrf2/Keap1 Mediated Redox Signaling Supports Synaptic Function and Longevity and Impacts on Circadian Activity

Many neurodegenerative conditions and age-related neuropathologies are associated with increased levels of reactive oxygen species (ROS). The cap "n" collar (CncC) family of transcription factors is one of the major cellular system that fights oxidative insults, becoming activated in response to oxidative stress. This transcription factor signaling is conserved from metazoans to human and has a major developmental and disease-associated relevance. An important mammalian member of the CncC family is nuclear factor erythroid 2-related factor 2 (Nrf2) which has been studied in numerous cellular systems and represents an important target for drug discovery in different diseases. CncC is negatively regulated by Kelch-like ECH associated protein 1 (Keap1) and this interaction provides the basis for a homeostatic control of cellular antioxidant defense. We have utilized the Drosophila model system to investigate the roles of CncC signaling on longevity, neuronal function and circadian rhythm. Furthermore, we assessed the effects of CncC function on larvae and adult flies following exposure to stress. Our data reveal that constitutive overexpression of CncC modifies synaptic mechanisms that positively impact on neuronal function, and suppression of CncC inhibitor, Keap1, shows beneficial phenotypes on synaptic function and longevity. Moreover, supplementation of antioxidants mimics the effects of augmenting CncC signaling. Under stress conditions, lack of CncC signaling worsens survival rates and neuronal function whilst silencing Keap1 protects against stress-induced neuronal decline. Interestingly, overexpression and RNAi-mediated downregulation of CncC have differential effects on sleep patterns possibly via interactions with redox-sensitive circadian cycles. Thus, our data illustrate the important regulatory potential of CncC signaling in neuronal function and synaptic release affecting multiple aspects within the nervous system.

Taurine Transporter dEAAT2 is Required for Auditory Transduction in Drosophila

Drosophila dEAAT2, a member of the excitatory amino-acid transporter (EAAT) family, has been described as mediating the high-affinity transport of taurine, which is a free amino-acid abundant in both insects and mammals. However, the role of taurine and its transporter in hearing is not clear. Here, we report that dEAAT2 is required for the larval startle response to sound stimuli. dEAAT2 was found to be enriched in the distal region of chordotonal neurons where sound transduction occurs. The Ca2+ imaging and electrophysiological results showed that disrupted dEAAT2 expression significantly reduced the response of chordotonal neurons to sound. More importantly, expressing dEAAT2 in the chordotonal neurons rescued these mutant phenotypes. Taken together, these findings indicate a critical role for Drosophila dEAAT2 in sound transduction by chordotonal neurons.

Sleep and Metabolism: Eaat-ing Your Way to ZZZs

A new study in fruit flies identifies a molecule, Eaat2, that regulates both sleep and metabolic rate. Surprisingly, Eaat2 acts in a specific glial subtype to modulate both processes, suggesting a cellular link in the brain between sleep and metabolism.

Focal adhesion molecules regulate astrocyte morphology and glutamate transporters to suppress seizure-like behavior

Astrocytes are important regulators of neural circuit function and behavior in the healthy and diseased nervous system. We screened for molecules in Drosophila astrocytes that modulate neuronal hyperexcitability and identified multiple components of focal adhesion complexes (FAs). Depletion of astrocytic Tensin, β-integrin, Talin, focal adhesion kinase (FAK), or matrix metalloproteinase 1 (Mmp1), resulted in enhanced behavioral recovery from genetic or pharmacologically induced seizure. Overexpression of Mmp1, predicted to activate FA signaling, led to a reciprocal enhancement of seizure severity. Blockade of FA-signaling molecules in astrocytes at basal levels of CNS excitability resulted in reduced astrocytic coverage of the synaptic neuropil and expression of the excitatory amino acid transporter EAAT1. However, induction of hyperexcitability after depletion of FA-signaling components resulted in enhanced astrocyte coverage and an approximately twofold increase in EAAT1 levels. Our work identifies FA-signaling molecules as important regulators of astrocyte outgrowth and EAAT1 expression under normal physiological conditions. Paradoxically, in the context of hyperexcitability, this pathway negatively regulates astrocytic process outgrowth and EAAT1 expression, and their blockade leading to enhanced recovery from seizure.

Ultrastructural comparison of the Drosophila larval and adult ventral abdominal neuromuscular junction

Drosophila melanogaster has recently emerged as model system for studying synaptic transmission and plasticity during adulthood, aging and neurodegeneration. However, still little is known about the basic neuronal mechanisms of synaptic function in the adult fly. Per se, adult Drosophila neuromuscular junctions should be highly suited for studying these aspects as they allow for genetic manipulations in combination with ultrastructural and electrophysiological analyses. Although different neuromuscular junctions of the adult fly have been described during the last years, a direct ultrastructural comparison with their larval counterpart is lacking. The present study was designed to close this gap by providing a detailed ultrastructural comparison of the larval and the adult neuromuscular junction of the ventrolongitudinal muscle. Assessment of several parameters revealed similarities but also major differences in the ultrastructural organisation of the two model neuromuscular junctions. While basic morphological parameters are retained from the larval into the adult stage, the analysis discovered major differences of potential functional relevance in the adult: The electron-dense membrane apposition of the presynaptic and postsynaptic membrane is shorter, the subsynaptic reticulum is less elaborated and the number of synaptic vesicles at a certain distance of the presynaptic membrane is higher.

Metabolite transport across the mammalian and insect brain diffusion barriers

The nervous system in higher vertebrates is separated from the circulation by a layer of specialized endothelial cells. It protects the sensitive neurons from harmful blood-derived substances, high and fluctuating ion concentrations, xenobiotics or even pathogens. To this end, the brain endothelial cells and their interlinking tight junctions build an efficient diffusion barrier. A structurally analogous diffusion barrier exists in insects, where glial cell layers separate the hemolymph from the neural cells. Both types of diffusion barriers, of course, also prevent influx of metabolites from the circulation. Because neuronal function consumes vast amounts of energy and necessitates influx of diverse substrates and metabolites, tightly regulated transport systems must ensure a constant metabolite supply. Here, we review the current knowledge about transport systems that carry key metabolites, amino acids, lipids and carbohydrates into the vertebrate and Drosophila brain and how this transport is regulated. Blood-brain and hemolymph-brain transport functions are conserved and we can thus use a simple, genetically accessible model system to learn more about features and dynamics of metabolite transport into the brain.

Astrocyte transport of glutamate and neuronal activity reciprocally modulate tau pathology in Drosophila

Abnormal buildup of the microtubule associated protein tau is a major pathological hallmark of Alzheimer's disease (AD) and various tauopathies. The mechanisms by which pathological tau accumulates and spreads throughout the brain remain largely unknown. Previously, we demonstrated that a restoration of the major astrocytic glutamate transporter, GLT1, ameliorated a buildup of tau pathology and rescued cognition in a mouse model of AD. We hypothesized that aberrant extracellular glutamate and abnormal neuronal excitatory activities promoted tau pathology. In the present study, we investigated genetic interactions between tau and the GLT1 homolog dEaat1 in Drosophila melanogaster. Neuronal-specific overexpression of human wildtype tau markedly shortened lifespan and impaired motor behavior. RNAi depletion of dEaat1 in astrocytes worsened these phenotypes, whereas overexpression of dEaat1 improved them. However, the synaptic neuropil appeared unaffected, and we failed to detect any major neuronal loss with tau overexpression in combination with dEaat1 depletion. To mimic glutamate-induced aberrant excitatory input in neurons, repeated depolarization of neurons via transgenic TrpA1 was applied to the adult Drosophila optic nerves, and we examined the change of tau deposits. Repeated depolarization significantly increased the accumulation of tau in these neurons. We propose that increased neuronal excitatory activity exacerbates tau-mediated neuronal toxicity and behavioral deficits.

De novo sequencing and transcriptome analysis of venom glands of endoparasitoid Aenasius arizonensis (Girault) (=Aenasius bambawalei Hayat) (Hymenoptera, Encyrtidae)

Aenasius bambawalei Hayat (Encyrtidae: Hymenoptera) has been synonymized with Aenasius arizonensis (Girault) is a small, newly discovered endoparasitoid of the cotton mealybug Phenacoccuss solenopsis Tinsley (Pseudococcidae: Hemiptera), which completes its life cycle inside the body of its host and it is a potential insect control tool. Despite the acquired knowledge regarding host-parasitoid interaction, little information is available on the factors of parasitoid origin able to modulate mealybug physiology. The components of A. arizonensis venom have not been well studied but venom from other parasitoids and wasps contain biologically active proteins that have potential applications in pest management or may be of medicinal importance. To provide an insight into the transcripts expressed in the venom gland of A. arizonensis, a transcriptomic database was developed utilizing high throughput RNA sequencing approaches to analyze the genes expressed in venom glands of this endoparasitic wasp. The resulting A. arizonensis RNA sequences were assembled de-novo with contigs then blasted against the NCBI non-redundant sequence database. Contigs which matched database sequences were mostly homologous to genes from hymenopteran parasitoids such as Nasonia vitripennis, Copidosoma floridanum, Fopius arsenus and Pteromalas puparium. Further analysis of the A. arizonensis database was then performed which focused on selected genes encoding proteins potentially involved in host developmental arrest, disrupting the host immune system, host paralysis, and transcripts that support these functions. Sequenced mRNAS predicted to encode full length ORFs of Calreticulin, Serine Protease Precursor and Arginine kinase proteins were identified and the tissue specific expression of these putative venom genes was analyzed by RT-PCR. In addition, results also demonstrate that de novo transcriptome assembly allows useful venom gene expression analysis in a species lacking a genome sequence database and may provide useful information for devising control tools for insect pests and other applications.

New insight into the role of MMP14 in metabolic balance

Membrane-anchored matrix metalloproteinase 14 (MMP14) is involved broadly in organ development through both its proteolytic and signal-transducing functions. Knockout of Mmp14 (KO) in mice results in a dramatic reduction of body size and wasting followed by premature death, the mechanism of which is poorly understood. Since the mammary gland develops after birth and is thus dependent for its functional progression on systemic and local cues, we chose it as an organ model for understanding why KO mice fail to thrive. A global analysis of the mammary glands' proteome in the wild type (WT) and KO mice provided insight into an unexpected role of MMP14 in maintaining metabolism and homeostasis. We performed mass spectrometry and quantitative proteomics to determine the protein signatures of mammary glands from 7 to 11 days old WT and KO mice and found that KO rudiments had a significantly higher level of rate-limiting enzymes involved in catabolic pathways. Glycogen and lipid levels in KO rudiments were reduced, and the circulating levels of triglycerides and glucose were lower. Analysis of the ultrastructure of mammary glands imaged by electron microscopy revealed a significant increase in autophagy signatures in KO mice. Finally, Mmp14 silenced mammary epithelial cells displayed enhanced autophagy. Applied to a systemic level, these findings indicate that MMP14 is a crucial regulator of tissue homeostasis. If operative on a systemic level, these findings could explain how Mmp14KO litter fail to thrive due to disorder in metabolism.

Drosophila astrocytes cover specific territories of the CNS neuropil and are instructed to differentiate by Prospero, a key effector of Notch

Astrocytes are crucial in the formation, fine-tuning, function and plasticity of neural circuits in the central nervous system. However, important questions remain about the mechanisms instructing astrocyte cell fate. We have studied astrogenesis in the ventral nerve cord of Drosophila larvae, where astrocytes exhibit remarkable morphological and molecular similarities to those in mammals. We reveal the births of larval astrocytes from a multipotent glial lineage, their allocation to reproducible positions, and their deployment of ramified arbors to cover specific neuropil territories to form a stereotyped astroglial map. Finally, we unraveled a molecular pathway for astrocyte differentiation in which the Ets protein Pointed and the Notch signaling pathway are required for astrogenesis; however, only Notch is sufficient to direct non-astrocytic progenitors toward astrocytic fate. We found that Prospero is a key effector of Notch in this process. Our data identify an instructive astrogenic program that acts as a binary switch to distinguish astrocytes from other glial cells.

A Distinct Perisynaptic Glial Cell Type Forms Tripartite Neuromuscular Synapses in the Drosophila Adult

Previous studies of Drosophila flight muscle neuromuscular synapses have revealed their tripartite architecture and established an attractive experimental model for genetic analysis of glial function in synaptic transmission. Here we extend these findings by defining a new Drosophila glial cell type, designated peripheral perisynaptic glia (PPG), which resides in the periphery and interacts specifically with fine motor axon branches forming neuromuscular synapses. Identification and specific labeling of PPG was achieved through cell type-specific RNAi-mediated knockdown (KD) of a glial marker, Glutamine Synthetase 2 (GS2). In addition, comparison among different Drosophila neuromuscular synapse models from adult and larval developmental stages indicated the presence of tripartite synapses on several different muscle types in the adult. In contrast, PPG appear to be absent from larval body wall neuromuscular synapses, which do not exhibit a tripartite architecture but rather are imbedded in the muscle plasma membrane. Evolutionary conservation of tripartite synapse architecture and peripheral perisynaptic glia in vertebrates and Drosophila suggests ancient and conserved roles for glia-synapse interactions in synaptic transmission.

The Drosophila blood-brain barrier: development and function of a glial endothelium

The efficacy of neuronal function requires a well-balanced extracellular ion homeostasis and a steady supply with nutrients and metabolites. Therefore, all organisms equipped with a complex nervous system developed a so-called blood-brain barrier, protecting it from an uncontrolled entry of solutes, metabolites or pathogens. In higher vertebrates, this diffusion barrier is established by polarized endothelial cells that form extensive tight junctions, whereas in lower vertebrates and invertebrates the blood-brain barrier is exclusively formed by glial cells. Here, we review the development and function of the glial blood-brain barrier of Drosophila melanogaster. In the Drosophila nervous system, at least seven morphologically distinct glial cell classes can be distinguished. Two of these glial classes form the blood-brain barrier. Perineurial glial cells participate in nutrient uptake and establish a first diffusion barrier. The subperineurial glial (SPG) cells form septate junctions, which block paracellular diffusion and thus seal the nervous system from the hemolymph. We summarize the molecular basis of septate junction formation and address the different transport systems expressed by the blood-brain barrier forming glial cells.

Drosophila melanogaster as a genetic model system to study neurotransmitter transporters

The model genetic organism Drosophila melanogaster, commonly known as the fruit fly, uses many of the same neurotransmitters as mammals and very similar mechanisms of neurotransmitter storage, release and recycling. This system offers a variety of powerful molecular-genetic methods for the study of transporters, many of which would be difficult in mammalian models. We review here progress made using Drosophila to understand the function and regulation of neurotransmitter transporters and discuss future directions for its use.

Detecting the immune system response of a 500 year-old Inca mummy

Disease detection in historical samples currently relies on DNA extraction and amplification, or immunoassays. These techniques only establish pathogen presence rather than active disease. We report the first use of shotgun proteomics to detect the protein expression profile of buccal swabs and cloth samples from two 500-year-old Andean mummies. The profile of one of the mummies is consistent with immune system response to severe pulmonary bacterial infection at the time of death. Presence of a probably pathogenic Mycobacterium sp. in one buccal swab was confirmed by DNA amplification, sequencing, and phylogenetic analyses. Our study provides positive evidence of active pathogenic infection in an ancient sample for the first time. The protocol introduced here is less susceptible to contamination than DNA-based or immunoassay-based studies. In scarce forensic samples, shotgun proteomics narrows the range of pathogens to detect using DNA assays, reducing cost. This analytical technique can be broadly applied for detecting infection in ancient samples to answer questions on the historical ecology of specific pathogens, as well as in medico-legal cases when active pathogenic infection is suspected.

The metabolism of histamine in the Drosophila optic lobe involves an ommatidial pathway: β-alanine recycles through the retina

Flies recycle the photoreceptor neurotransmitter histamine by conjugating it to β-alanine to form β-alanyl-histamine (carcinine). The conjugation is regulated by Ebony, while Tan hydrolyses carcinine, releasing histamine and β-alanine. In Drosophila, β-alanine synthesis occurs either from uracil or from the decarboxylation of aspartate but detailed roles for the enzymes responsible remain unclear. Immunohistochemically detected β-alanine is present throughout the fly's entire brain, and is enhanced in the retina especially in the pseudocone, pigment and photoreceptor cells of the ommatidia. HPLC determinations reveal 10.7 ng of β-alanine in the wild-type head, roughly five times more than histamine. When wild-type flies drink uracil their head β-alanine increases more than after drinking l-aspartic acid, indicating the effectiveness of the uracil pathway. Mutants of black, which lack aspartate decarboxylase, cannot synthesize β-alanine from l-aspartate but can still synthesize it efficiently from uracil. Our findings demonstrate a novel function for pigment cells, which not only screen ommatidia from stray light but also store and transport β-alanine and carcinine. This role is consistent with a β-alanine-dependent histamine recycling pathway occurring not only in the photoreceptor terminals in the lamina neuropile, where carcinine occurs in marginal glia, but vertically via a long pathway that involves the retina. The lamina's marginal glia are also a hub involved in the storage and/or disposal of carcinine and β-alanine.

l(2)01810 is a novel type of glutamate transporter that is responsible for megamitochondrial formation

l(2)01810 causes glutamine-dependent megamitochondrial formation when it is overexpressed in Drosophila cells. In the present study, we elucidated the function of l(2)01810 during megamitochondrial formation. The overexpression of l(2)01810 and the inhibition of glutamine synthesis showed that l(2)01810 is involved in the accumulation of glutamate. l(2)01810 was predicted to contain transmembrane domains and was found to be localized to the plasma membrane. By using (14)C-labelled glutamate, l(2)01810 was confirmed to uptake glutamate into Drosophila cells with high affinity (K(m)=69.4 μM). Also, l(2)01810 uptakes glutamate in a Na(+)-independent manner. Interestingly, however, this uptake was not inhibited by cystine, which is a competitive inhibitor of Na(+)-independent glutamate transporters, but by aspartate. A signal peptide consisting of 34 amino acid residues targeting to endoplasmic reticulum was predicted at the N-terminus of l(2)01810 and this signal peptide is essential for the protein's localization to the plasma membrane. In addition, l(2)01810 has a conserved functional domain of a vesicular-type glutamate transporter, and Arg(146) in this domain was found to play a key role in glutamate transport and megamitochondrial formation. These results indicate that l(2)01810 is a novel type of glutamate transporter and that glutamate uptake is a rate-limiting step for megamitochondrial formation.

A tripartite synapse model in Drosophila

Tripartite (three-part) synapses are defined by physical and functional interactions of glia with pre- and post-synaptic elements. Although tripartite synapses are thought to be of widespread importance in neurological health and disease, we are only beginning to develop an understanding of glial contributions to synaptic function. In contrast to studies of neuronal mechanisms, a significant limitation has been the lack of an invertebrate genetic model system in which conserved mechanisms of tripartite synapse function may be examined through large-scale application of forward genetics and genome-wide genetic tools. Here we report a Drosophila tripartite synapse model which exhibits morphological and functional properties similar to those of mammalian synapses, including glial regulation of extracellular glutamate, synaptically-induced glial calcium transients and glial coupling of synapses with tracheal structures mediating gas exchange. In combination with classical and cell-type specific genetic approaches in Drosophila, this model is expected to provide new insights into the molecular and cellular mechanisms of tripartite synapse function.

Venom proteins from endoparasitoid wasps and their role in host-parasite interactions

Endoparasitoids introduce a variety of factors into their host during oviposition to ensure successful parasitism. These include ovarian and venom fluids that may be accompanied by viruses and virus-like particles. An overwhelming number of venom components are enzymes with similarities to insect metabolic enzymes, suggesting their recruitment for expression in venom glands with modified functions. Other components include protease inhibitors, paralytic factors, and constituents that facilitate/enhance entry and expression of genes from symbiotic viruses or virus-like particles. In addition, the venom gland may itself support replication/production of some viruses or virus-like entities. Overlapping functions and structural similarities of some venom, ovarian, and virus-encoded proteins suggest coevolution of molecules recruited by endoparasitoids to maintain their fitness relative to their host.

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.

Glial solute carrier transporters in Drosophila and mice

Glia regulate brain physiology primarily by regulating the movement and concentration of substances in the extracellular fluid. Therefore, one approach to understanding the role of glia in brain physiology is to study what happens when glial transporters are removed or modified. The largest and most highly conserved class of transporter is solute carrier (SLC) proteins. SLC proteins are highly expressed in brain, and many are found in glia. The function of many SLC proteins in the brain--particularly in glia--is very poorly understood. SLC proteins can be relatively easily knocked out or modified in genetic model organisms to better understand glial function. Drosophila are popular genetic model organisms that offer a nice balance between genetic malleability and brain complexity. They are ideal for such an endeavor. This article lists and discusses SLC transporter family members that are expressed in both mouse and Drosophila glia, in an effort to provide a foundation for studies of glial SLC transporters using Drosophila as a model.

The functional organisation of glia in the adult brain of Drosophila and other insects

This review annotates and categorises the glia of adult Drosophila and other model insects and analyses the developmental origins of these in the Drosophila optic lobe. The functions of glia in the adult vary depending upon their sub-type and location in the brain. The task of annotating glia is essentially complete only for the glia of the fly's lamina, which comprise: two types of surface glia-the pseudocartridge and fenestrated glia; two types of cortex glia-the distal and proximal satellite glia; and two types of neuropile glia-the epithelial and marginal glia. We advocate that the term subretinal glia, as used to refer to both pseudocartridge and fenestrated glia, be abandoned. Other neuropiles contain similar glial subtypes, but other than the antennal lobes these have not been described in detail. Surface glia form the blood brain barrier, regulating the flow of substances into and out of the nervous system, both for the brain as a whole and the optic neuropiles in particular. Cortex glia provide a second level of barrier, wrapping axon fascicles and isolating neuronal cell bodies both from neighbouring brain regions and from their underlying neuropiles. Neuropile glia can be generated in the adult and a subtype, ensheathing glia, are responsible for cleaning up cellular debris during Wallerian degeneration. Both the neuropile ensheathing and astrocyte-like glia may be involved in clearing neurotransmitters from the extracellular space, thus modifying the levels of histamine, glutamate and possibly dopamine at the synapse to ultimately affect behaviour.

Effect of ambient extracellular glutamate on Drosophila glutamate receptor trafficking and function

Measurements suggest that the hemolymph glutamate concentrations in Drosophila are relatively high. This raises the possibility that extracellular glutamate could be an important regulator of glutamatergic transmission in vivo. Using voltage clamp electrophysiology, we found that synaptic currents in D. melanogaster larval neuromuscular junctions are reduced by extracellular glutamate (EC50: approximately 0.4 mM), such that only 10-30% of receptors were functionally available in 1 mM extracellular glutamate. The kinetics of synaptic currents were also slowed in a dose-dependent fashion (EC50: approximately 1 mM), consistent with the idea that extracellular glutamate preferentially removes the fastest-desensitizing receptors from the functional pool. Prolonged exposure (several hours) to extracellular glutamate also triggers loss of glutamate receptor immunoreactivity from neuromuscular junctions. To determine whether this receptor loss requires that glutamate bind directly to the lost receptors, we examined glutamate-dependent loss of receptor immunoreactivity in larvae with glutamate receptor ligand binding mutations. Our results suggest that glutamate-dependent receptor loss requires binding of glutamate directly to the lost receptors. To determine whether lost receptor protein is degraded or merely redistributed, we used immunoblots. Results suggest that glutamate receptor protein is redistributed, but not degraded, after prolonged exposure to high extracellular glutamate.

Comparisons of mass spectrometry compatible surfactants for global analysis of the mammalian brain proteome

Methods for the global analysis of protein expression offer an approach to study the molecular basis of disease. Studies of protein expression in tissue, such as brain, are complicated by the need for efficient and unbiased digestion of proteins that permit identification of peptides by shotgun proteomic methods. In particular, identification and characterization of less abundant membrane proteins has been of great interest for studies of brain physiology, but often proteins of interest are of low abundance or exist in multiple isoforms. Parsing protein isoforms as a function of disease will be essential. In this study, we develop a digestion scheme using detergents compatible with mass spectrometry that improves membrane protein identification from brain tissue. We show the modified procedure yields close to 5,000 protein identifications from 1.8 mg of rat brain homogenate with an average of 25% protein sequence coverage. This procedure achieves a remarkable reduction in the amount of starting material required to observe a broad spectrum of membrane proteins. Among the proteins identified from a mammalian brain homogenate, 1897 (35%) proteins are annotated by Gene Ontology as membrane proteins, and 1225 (22.6%) proteins are predicted to contain at least one transmembrane domain. Membrane proteins identified included neurotransmitter receptors and ion channels implicated in important physiological functions and disease.

A screen for neurotransmitter transporters expressed in the visual system of Drosophila melanogaster identifies three novel genes

The fly eye provides an attractive substrate for genetic studies, and critical transport activities for synaptic transmission and pigment biogenesis in the insect visual system remain unknown. We therefore screened for transporters in Drosophila melanogaster that are down-regulated by genetically ablating the eye. Using a large panel of transporter specific probes on Northern blots, we identified three transcripts that are down-regulated in flies lacking eye tissue. Two of these, CG13794 and CG13795, are part of a previously unknown subfamily of putative solute carriers within the neurotransmitter transporter family. The third, CG4476, is a member of a related subfamily that includes characterized nutrient transporters expressed in the insect gut. Using imprecise excision of a nearby transposable P element, we have generated a series of deletions in the CG4476 gene. In fast phototaxis assays, CG4476 mutants show a decreased behavioral response to light, and the most severe mutant behaves as if it were blind. These data suggest an unforeseen role for the "nutrient amino acid transporter" subfamily in the nervous system, and suggest new models to study transport function using the fly eye.

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.

EAAC1 is expressed in rat and human prostate epithelial cells; functions as a high-affinity L-aspartate transporter; and is regulated by prolactin and testosterone

Background

Prostate epithelial cells accumulate a high level of aspartate that is utilized as a substrate for their unique function of production and secretion of enormously high levels of citrate. In most mammalian cells aspartate is synthesized; and, therefore is a non-essential amino acid. In contrast, in citrate-producing prostate cells, aspartate is an essential amino acid that must be derived from circulation. The prostate intracellular/extracellular conditions present a 40:1 concentration gradient. Therefore, these cells must possess a plasma membrane-associated aspartate uptake transport process to achieve their functional activity. In earlier kinetic studies we identified the existence of a unique Na+-dependent high-affinity L-aspartate transport process in rat prostate secretory epithelial cells. The present report is concerned with the identification of this putative L-aspartate transporter in rat and human prostate cells.

Results

The studies show for the first time that EAAC1 is expressed in normal rat prostate epithelial cells, in normal and hyperplastic human prostate glands, and in human malignant prostate cell lines. EAAC1 expression and high-affinity L-aspartate transport are correspondingly down-regulated by EAAC1 siRNA knock down. Exposure of prostate cells to physiological levels of prolactin or testosterone results in an up-regulation of EAAC1 expression and a corresponding increase in the high-affinity transport of L-aspartate into the cells.

Conclusion

This study shows that EAAC1 functions as the high-affinity L-aspartate transporter that is responsible for the uptake and accumulation of aspartate in prostate cells. In other cells (predominantly excitable tissue cells), EAAC1 has been reported to function as a glutamate transporter rather than as an aspartate transporter. The regulation of EAAC1 expression and L-aspartate transport by testosterone and prolactin is consistent with their regulation of citrate production in prostate cells. The identification of EAAC1 as the high-affinity L-aspartate transporter now permits studies to elucidate the mechanism of hormonal regulation of EAAC1 gene expression, and to investigate the mechanism by which the cellular environment effects the functioning of EAAC1 as an aspartate transporter or as a glutamate transporter.

Advances and prospects on biosynthesis, structures and functions of venom proteins from parasitic wasps

Molecular and biochemical properties of parasitoid Hymenoptera's venom proteins are currently receiving an increasing interest. In this review, we will highlight the progress that has been made over the past 10 years in fundamental research on this field. Main knowledge acquired on the structural features of parasitoid venom peptides, proteins and enzymes will be summarized and discussed and several examples showing the diversity of their biological functions will be given with respect to future prospects and applications.

High Affinity Transport of Taurine by the Drosophila Aspartate Transporter dEAAT2

Excitatory amino acid transporters (EAATs) are structurally related plasma membrane proteins known to mediate the Na(+)/K(+)-dependent uptake of the amino acids l-glutamate and dl-aspartate. In the nervous system, these proteins contribute to the clearance of glutamate from the synaptic cleft and maintain excitatory amino acid concentrations below excitotoxic levels. Two homologues exist in Drosophila melanogaster, dEAAT1 and dEAAT2, which are specifically expressed in the nervous tissue. We previously reported that dEAAT2 shows unique substrate discrimination as it mediates high affinity transport of aspartate but not glutamate. We now show that dEAAT2 can also transport the amino acid taurine with high affinity, a property that is not shared by two other transporters of the same family, Drosophila dEAAT1 and human hEAAT2. Taurine transport by dEAAT2 was efficiently blocked by an EAAT antagonist but not by inhibitors of the structurally unrelated mammalian taurine transporters. Taurine and aspartate are transported with similar K(m) and relative efficacy and behave as mutually competitive inhibitors. dEAAT2 can mediate either net uptake or the heteroexchange of its two substrates, both being dependent on the presence of Na(+) ions in the external medium. Interestingly, heteroexchange only occurs in one preferred substrate orientation, i.e. with taurine transported inwards and aspartate outwards, suggesting a mechanism of transinhibition of aspartate uptake by intracellular taurine. Therefore, dEAAT2 is actually an aspartate/taurine transporter. Further studies of this protein are expected to shed light on the role of taurine as a candidate neuromodulator and cell survival factor in the Drosophila nervous system.

Asobara, braconid parasitoids of Drosophila larvae: unusual strategies to avoid encapsulation without VLPs

Ichneumonoidae parasitoids have been well described for their regulatory effects on host physiology which are usually associated with the activity of polydnaviruses (PDVs) or viruslike-particles (VLPs) injected by the female wasps at oviposition. Among them, parasitoids of the braconid families display specific characteristics like the required activity of secretions from the maternal venom glands or of teratocytes from embryological origin. However, none of these features were observed in two braconid species of the Asobara genus parasitizing Drosophila hosts. In the absence of PDVs and VLPs, the two species A. tabida and A. citri seem to have developed unique strategies to avoid immunity defenses and to succeed in their Drosophila larval hosts. The aim of this study is to report on the complex relationships of braconid parasitoids with their hosts and to present some of the insights from studying Drosophila parasitoids.

Expanded polyglutamine peptides disrupt EGF receptor signaling and glutamate transporter expression in Drosophila

Huntington's disease (HD) is a late onset heritable neurodegenerative disorder caused by expansion of a polyglutamine (polyQ) sequence in the protein huntingtin (Htt). Transgenic models in mice have suggested that the motor and cognitive deficits associated to this disease are triggered by extended neuronal and possibly glial dysfunction, whereas neuronal death occurs late and selectively. Here, we provide in vivo evidence that expanded polyQ peptides antagonize epidermal growth factor receptor (EGFR) signaling in Drosophila glia. We targeted the expression of the polyQ-containing domain of Htt or an extended polyQ peptide alone in a subset of Drosophila glial cells, where the only fly glutamate transporter, dEAAT1, is detected. This resulted in formation of nuclear inclusions, progressive decrease in dEAAT1 transcription and shortened adult lifespan, but no significant glial cell death. We observed that brain expression of dEAAT1 is normally sustained by the EGFR-Ras-extracellular signal-regulated kinase (ERK) signaling pathway, suggesting that polyQ could act by antagonizing this pathway. We found that the presence of polyQ peptides indeed abolished dEAAT1 upregulation by constitutively active EGFR and potently inhibited EGFR-mediated ERK activation in fly glial cells. Long polyQ also limited the effect of activated EGFR on Drosophila eye development. Our results further indicate that the polyQ acts at an upstream step in the pathway, situated between EGFR and ERK activation. This suggests that disruption of EGFR signaling and ensuing glial cell dysfunction could play a direct role in the pathogenesis of HD and other polyQ diseases in humans.

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.

Identification of an aspartylglucosaminidase-like protein in the venom of the parasitic wasp Asobara tabida (Hymenoptera: Braconidae)

This study was designed to identify one of the main components of venomous secretions of the endoparasitic wasp Asobara tabida. By using electrophoretic methods, partial amino acid sequencing and immunostaining, we demonstrated the presence of an aspartylglucosaminidase (AGA)-like protein in the venom of this insect. The enzyme had a polymeric conformation and was formed of 30 and 18 kDa subunits. The relative positions of several amino acids involved in substrate binding and catalytic activity of known AGA-proteins, which are usually lysosomal enzymes, were conserved in the NH(2)-terminal ends of these subunits. Antibodies raised against human AGA recognized the two subunits of the protein and a 44 kDa protein, suggesting the presence of a precursor molecule of the enzyme in the venom. However, no reliable measurement of the AGA activity could be performed on the venom extracts, which could be explained by the fact the enzyme would be stored in the reservoir of the venom apparatus under an inactive form. These results constitute the first description of an AGA-like protein in an insect venom and are discussed with respect to the knowledge acquired on lysosomal and venom enzymes.

Functional characterization of a glutamate/aspartate transporter from the mosquito Aedes aegypti

Glutamate elicits a variety of effects in insects, including inhibitory and excitatory signals at both neuromuscular junctions and brain. Insect glutamatergic neurotransmission has been studied in great depth especially from the standpoint of the receptor-mediated effects, but the molecular mechanisms involved in the termination of the numerous glutamatergic signals have only recently begun to receive attention. In vertebrates, glutamatergic signals are terminated by Na(+)/K(+)-dependent high-affinity excitatory amino acid transporters (EAAT), which have been cloned and characterized extensively. Cloning and characterization of a few insect homologues have followed, but functional information for these homologues is still limited. Here we report a study conducted on a cloned mosquito EAAT homologue isolated from the vector of the dengue virus, Aedes aegypti. The deduced amino acid sequence of the protein, AeaEAAT, exhibits 40-50% identity with mammalian EAATs, and 45-50% identity to other insect EAATs characterized thus far. It transports L-glutamate as well as L- and D-aspartate with high affinity in the micromolar range, and demonstrates a substrate-elicited anion conductance when heterologously expressed in Xenopus laevis oocytes, as found with mammalian homologues. Analysis of the spatial distribution of the protein demonstrates high expression levels in the adult thorax, which is mostly observed in the thoracic ganglia. Together, the work presented here provides a thorough examination of the role played by glutamate transport in Ae. aegypti.

Cellular distribution of a high-affinity glutamate transporter in the nervous system of the cabbage looperTrichoplusia ni

Glutamate functions as a neurotransmitter in the central nervous system(CNS) and neuromuscular junctions in insects. High-affinity glutamate transporters are responsible for keeping the resting levels of excitatory amino acids below the synaptic activation threshold by removing them from the extracellular fluid, thereby preventing them from reaching toxic levels. Peptides representing the N- and C-terminal regions of a glutamate transporter cloned from the cabbage looper caterpillar (Trichoplusia ni) were synthesized and used to generate polyclonal antibodies. The antibodies produced immunohistochemical staining in both muscular and nervous system T. ni tissues. Neuromuscular junctions in the skeletal muscles produced the most intense labelling, but no visceral muscle or sensory nerves were labelled. In the CNS, the neuropile of the ganglia, but not the connectives, gave a diffuse staining. Electron microscopical examination of ganglia and neuromuscular junctions showed that the plasma membrane of glial cells, but not that of neurons was labelled, in agreement with the notion that most of the glutamate uptake sites in this insect are in glial cells.

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.

Developmental regulation of glutamate receptor field size by nonvesicular glutamate release

We hypothesized that presynaptic glutamate regulates postsynaptic ionotropic glutamate receptor number during synaptogenesis. To test this idea, we genetically manipulated presynaptic glutamate levels at the glutamatergic Drosophila neuromuscular junction (NMJ), then microscopically and electrophysiologically measured postsynaptic glutamate receptor field size and function. Our data show that presynaptic glutamate is a strong negative regulator of postsynaptic receptor field size and function during development. Glutamate-triggered receptor downregulation was not affected by block of synaptic vesicle fusion, demonstrating that receptors are regulated by nonvesicular glutamate release. Our results reveal an elegant mechanism for receptor field regulation during synaptogenesis and reveal a nonpathological role for nonvesicular glutamate release at the synapse.

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.