Genetic, molecular and developmental analysis of the glutamine synthetase isozymes of Drosophila melanogaster

Tet controls axon guidance in early brain development through glutamatergic signaling

Mutations in ten-eleven translocation (TET) proteins are associated with human neurodevelopmental disorders. We find a function of Tet in regulating Drosophila early brain development. The Tet DNA-binding domain (TetAXXC) is required for axon guidance in the mushroom body (MB). Glutamine synthetase 2 (Gs2), a key enzyme in glutamatergic signaling, is significantly down-regulated in the TetAXXC brains. Loss of Gs2 recapitulates the TetAXXC phenotype. Surprisingly, Tet and Gs2 act in the insulin-producing cells (IPCs) to control MB axon guidance, and overexpression of Gs2 in IPCs rescues the defects of TetAXXC. Feeding TetAXXC with metabotropic glutamate receptor antagonist MPEP rescues the phenotype while glutamate enhances it. Mutants in Tet and Drosophila Fmr1, the homolog of human FMR1, have similar defects, and overexpression of Gs2 in IPCs also rescues the Fmr1 phenotype. We provide the first evidence that Tet controls the guidance of developing brain axons by modulating glutamatergic signaling.

Tet Controls Axon Guidance in Early Brain Development through Glutamatergic Signaling

Mutations in human TET proteins have been found in individuals with neurodevelopmental disorders. Here we report a new function of Tet in regulating Drosophila early brain development. We found that mutation in the Tet DNA-binding domain ( Tet AXXC ) resulted in axon guidance defects in the mushroom body (MB). Tet is required in early brain development during the outgrowth of MB β axons. Transcriptomic study shows that glutamine synthetase 2 (Gs2), a key enzyme in glutamatergic signaling, is significantly downregulated in the Tet AXXC mutant brains. CRISPR/Cas9 mutagenesis or RNAi knockdown of Gs2 recapitulates the Tet AXXC mutant phenotype. Surprisingly, Tet and Gs2 act in the insulin-producing cells (IPCs) to control MB axon guidance, and overexpression of Gs2 in these cells rescues the axon guidance defects of Tet AXXC . Treating Tet AXXC with the metabotropic glutamate receptor antagonist MPEP can rescue while treating with glutamate enhances the phenotype confirming Tet function in regulating glutamatergic signaling. Tet AXXC and the Drosophila homolog of Fragile X Messenger Ribonucleoprotein protein mutant ( Fmr1 3 ) have similar axon guidance defects and reduction in Gs2 mRNA levels. Interestingly, overexpression of Gs2 in the IPCs also rescues the Fmr1 3 phenotype, suggesting functional overlapping of the two genes. Our studies provide the first evidence that Tet can control the guidance of axons in the developing brain by modulating glutamatergic signaling and the function is mediated by its DNA-binding domain.

Global characterization of gene expression in the brain of starved immature Rhodnius prolixus

BACKGROUND

Rhodnius prolixus is a vector of Chagas disease and has become a model organism to study physiology, behavior, and pathogen interaction. The publication of its genome allowed initiating a process of comparative characterization of the gene expression profiles of diverse organs exposed to varying conditions. Brain processes control the expression of behavior and, as such, mediate immediate adjustment to a changing environment, allowing organisms to maximize their chances to survive and reproduce. The expression of fundamental behavioral processes like feeding requires fine control in triatomines because they obtain their blood meals from potential predators. Therefore, the characterization of gene expression profiles of key components modulating behavior in brain processes, like those of neuropeptide precursors and their receptors, seems fundamental. Here we study global gene expression profiles in the brain of starved R. prolixus fifth instar nymphs by means of RNA sequencing (RNA-Seq).

RESULTS

The expression of neuromodulatory genes such as those of precursors of neuropeptides, neurohormones, and their receptors; as well as the enzymes involved in the biosynthesis and processing of neuropeptides and biogenic amines were fully characterized. Other important gene targets such as neurotransmitter receptors, nuclear receptors, clock genes, sensory receptors, and takeouts genes were identified and their gene expression analyzed.

CONCLUSION

We propose that the set of neuromodulatory-related genes highly expressed in the brain of starved R. prolixus nymphs deserves functional characterization to allow the subsequent development of tools targeting them for bug control. As the brain is a complex structure that presents functionally specialized areas, future studies should focus on characterizing gene expression profiles in target areas, e.g. mushroom bodies, to complement our current knowledge.

Structural Insight into the Contributions of the N-Terminus and Key Active-Site Residues to the Catalytic Efficiency of Glutamine Synthetase 2

Glutamine synthetase (GS) catalyzes the condensation of ammonia and glutamate, along with ATP, to form glutamine. Despite extensive studies on GSs from eukaryotes and prokaryotes, the roles of the N-terminus and other structural features in catalysis remain unclear. Here we report the decameric structure of Drosophila melanogaster GS 2 (DmGS2). The N-terminal short helices, α1 and α2, constitute a meander region, and form hydrogen bonds with residues 3–5 in the N-terminal loop, which are not present in the GSs of other species. Deletion of α1 or α1-α2 inactivates DmGS2. Notably, the Arg4 in each monomer of one pentamer forms hydrogen bonds with Glu7, and Asp8 in the adjacent monomer of the other pentamer. Replacement of Arg4 with Asp (R4D) abolishes activity. Analytical ultracentrifugation revealed that Arg4 is crucial for oligomerization. Circular dichroism spectra revealed that R4D may alter the secondary structure. We mutated key residues to identify the substrate-binding site. As Glu140 binds glutamate and Glu311 binds ammonia, mutants E140A and E311A have little activity. Conversely, mutant P214A (P contributes to ATP binding) has higher activity than wild-type DmGS2. These findings expand the understanding of the structural and functional features of the N-terminal meander region of DmGS2 and the residues important for catalytic efficiency.

Genetic and metabolomic architecture of variation in diet restriction-mediated lifespan extension in Drosophila

In most organisms, dietary restriction (DR) increases lifespan. However, several studies have found that genotypes within the same species vary widely in how they respond to DR. To explore the mechanisms underlying this variation, we exposed 178 inbred Drosophila melanogaster lines to a DR or ad libitum (AL) diet, and measured a panel of 105 metabolites under both diets. Twenty four out of 105 metabolites were associated with the magnitude of the lifespan response. These included proteinogenic amino acids and metabolites involved in α-ketoglutarate (α-KG)/glutamine metabolism. We confirm the role of α-KG/glutamine synthesis pathways in the DR response through genetic manipulations. We used covariance network analysis to investigate diet-dependent interactions between metabolites, identifying the essential amino acids threonine and arginine as “hub” metabolites in the DR response. Finally, we employ a novel metabolic and genetic bipartite network analysis to reveal multiple genes that influence DR lifespan response, some of which have not previously been implicated in DR regulation. One of these is CCHa2R, a gene that encodes a neuropeptide receptor that influences satiety response and insulin signaling. Across the lines, variation in an intronic single nucleotide variant of CCHa2R correlated with variation in levels of five metabolites, all of which in turn were correlated with DR lifespan response. Inhibition of adult CCHa2R expression extended DR lifespan of flies, confirming the role of CCHa2R in lifespan response. These results provide support for the power of combined genomic and metabolomic analysis to identify key pathways underlying variation in this complex quantitative trait.

Dietary restriction extends lifespan across most organisms in which it has been tested. However, several studies have now demonstrated that this effect can vary dramatically across different genotypes within a population. Within a population, dietary restriction might be beneficial for some, yet detrimental for others. Here, we measure the metabolome of 178 genetically characterized fly strains on fully fed and restricted diets. The fly strains vary widely in their lifespan response to dietary restriction. We then use information about each strain’s genome and metabolome (a measure of small molecules circulating in flies) to pinpoint cellular pathways that govern this variation in response. We identify a novel pathway involving the gene CCHa2R, which encodes a neuropeptide receptor that has not previously been implicated in dietary restriction or age-related signaling pathways. This study demonstrates the power of leveraging systems biology and network biology methods to understand how and why different individuals vary in their response to health and lifespan-extending interventions.

"What You Need, Baby, I Got It": Transposable Elements as Suppliers of Cis-Operating Sequences in Drosophila

Transposable elements (TEs) are constitutive components of both eukaryotic and prokaryotic genomes. The role of TEs in the evolution of genes and genomes has been widely assessed over the past years in a variety of model and non-model organisms. Drosophila is undoubtedly among the most powerful model organisms used for the purpose of studying the role of transposons and their effects on the stability and evolution of genes and genomes. Besides their most intuitive role as insertional mutagens, TEs can modify the transcriptional pattern of host genes by juxtaposing new cis-regulatory sequences. A key element of TE biology is that they carry transcriptional control elements that fine-tune the transcription of their own genes, but that can also perturb the transcriptional activity of neighboring host genes. From this perspective, the transposition-mediated modulation of gene expression is an important issue for the short-term adaptation of physiological functions to the environmental changes, and for long-term evolutionary changes. Here, we review the current literature concerning the regulatory and structural elements operating in cis provided by TEs in Drosophila. Furthermore, we highlight that, besides their influence on both TEs and host genes expression, they can affect the chromatin structure and epigenetic status as well as both the chromosome's structure and stability. It emerges that Drosophila is a good model organism to study the effect of TE-linked regulatory sequences, and it could help future studies on TE-host interactions in any complex eukaryotic genome.

Reduced Glutamine Synthetase Activity Alters the Fecundity of Female Bactrocera dorsalis (Hendel)

Glutamine synthetase (GS) is a key enzyme in glutamine synthesis and is associated with multiple physiological processes in insects, such as embryonic development, heat shock response, and fecundity regulation. However, little is known about the influence of GS on female fecundity in the oriental fruit fly, Bactrocera dorsalis. Based on the cloning of BdGSs, mitochondrial BdGSm and cytoplasmic BdGSc, we determined their expressions in the tissues of adult B. dorsalis. BdGSm was highly expressed in the fat body, while BdGSc was highly expressed in the head and midgut. Gene silencing by RNA interference against two BdGSs isoforms suppressed target gene expression at the transcriptional level, leading to a reduced ovarian size and lower egg production. The specific inhibitor L-methionine S-sulfoximine suppressed enzyme activity, but only the gene expression of BdGSm was suppressed. A similar phenotype of delayed ovarian development occurred in the inhibitor bioassay. Significantly lower expression of vitellogenin and vitellogenin receptor was observed when GS enzyme activity was suppressed. These data illustrate the effects of two GS genes on adult fecundity by regulating vitellogenin synthesis in different ways.

Metabolic enzymes in glial cells of the honeybee brain and their associations with aging, starvation and food response

The honey bee has been extensively studied as a model for neuronal circuit and memory function and more recently has emerged as an unconventional model in biogerontology. Yet, the detailed knowledge of neuronal processing in the honey bee brain contrasts with the very sparse information available on glial cells. In other systems glial cells are involved in nutritional homeostasis, detoxification, and aging. These glial functions have been linked to metabolic enzymes, such as glutamine synthetase and glycogen phosphorylase. As a step in identifying functional roles and potential differences among honey bee glial types, we examined the spatial distribution of these enzymes and asked if enzyme abundance is associated with aging and other processes essential for survival. Using immunohistochemistry and confocal laser microscopy we demonstrate that glutamine synthetase and glycogen phosphorylase are abundant in glia but appear to co-localize with different glial sub-types. The overall spatial distribution of both enzymes was not homogenous and differed markedly between different neuropiles and also within each neuropil. Using semi-quantitative Western blotting we found that rapid aging, typically observed in shortest-lived worker bees (foragers), was associated with declining enzyme levels. Further, we found enzyme abundance changes after severe starvation stress, and that glutamine synthetase is associated with food response. Together, our data indicate that aging and nutritional physiology in bees are linked to glial specific metabolic enzymes. Enzyme specific localization patterns suggest a functional differentiation among identified glial types.

Cytoplasmic glutamine synthetase gene expression regulates larval development in Bactrocera dorsalis (Hendel)

In insects, glutamine synthetase (GS), a key enzyme in the synthesis of glutamine, has been reported to be associated with embryonic development, heat shock response, and fecundity regulation. However, little is known about the influence of GS on postembryonic development. In this study, we demonstrate that blocking the activity of GS in the oriental fruit fly (Bactrocera dorsalis) with use of a GS-specific inhibitor (L-methionine S-sulfoximine), led to a significant delay in larval development, pupal weight loss, and inhibition of pupation. We further identify cloned and characterized two GS genes (BdGS-c and BdGS-m) from B. dorsalis. The two GS genes identified in B. dorsalis were predicted to be located in the cytosol (BdGS-c) and mitochondria (BdGS-m), and homology analysis indicated that both genes were similar to homologs from other Dipterans, such as Drosophila melanogaster and Aedes aegypti. BdGS-c was highly expressed in the larval stages, suggesting that cytosolic GS plays a predominant role in larval development. Furthermore, RNA interference experiments against BdGS-c, to specifically decrease the expression of cytosolic GS, resulted in delay in larval development as well as pupal weight loss. This study presents the prominent role played by BdGS-c in regulating larval development and suggests that the observed effect could have been modulated through ecdysteroid synthesis, agreeing with the reduced expression of the halloween gene spook. Also, the direct effects of BdGS-c silencing on B. dorsalis, such as larval lethality, delayed pupation, and late emergence, can be further exploited as novel insecticide target in the context of pest management.

Mitochondrial glutamate carriers from Drosophila melanogaster: biochemical, evolutionary and modeling studies

The mitochondrial carriers are members of a family of transport proteins that mediate solute transport across the inner mitochondrial membrane. Two isoforms of the glutamate carriers, GC1 and GC2 (encoded by the SLC25A22 and SLC25A18 genes, respectively), have been identified in humans. Two independent mutations in SLC25A22 are associated with severe epileptic encephalopathy. In the present study we show that two genes (CG18347 and CG12201) phylogenetically related to the human GC encoding genes are present in the D. melanogaster genome. We have functionally characterized the proteins encoded by CG18347 and CG12201, designated as DmGC1p and DmGC2p respectively, by overexpression in Escherichia coli and reconstitution into liposomes. Their transport properties demonstrate that DmGC1p and DmGC2p both catalyze the transport of glutamate across the inner mitochondrial membrane. Computational approaches have been used in order to highlight residues of DmGC1p and DmGC2p involved in substrate binding. Furthermore, gene expression analysis during development and in various adult tissues reveals that CG18347 is ubiquitously expressed in all examined D. melanogaster tissues, while the expression of CG12201 is strongly testis-biased. Finally, we identified mitochondrial glutamate carrier orthologs in 49 eukaryotic species in order to attempt the reconstruction of the evolutionary history of the glutamate carrier function. Comparison of the exon/intron structure and other key features of the analyzed orthologs suggests that eukaryotic glutamate carrier genes descend from an intron-rich ancestral gene already present in the common ancestor of lineages that diverged as early as bilateria and radiata.

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.

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

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

Regionally specific distribution of the binding of anti-glutamine synthetase and anti-S100 antibodies and of Datura stramonium lectin in glial domains of the optic lobe of the giant prawn

We previously characterized some crustacean glial cells by markers such as 2',3'-cyclic nucleotide 3'-phosphodiesterase and glial fibrillary acidic protein. Here we use antibodies against glutamine synthetase full-length molecule (anti-GS/FL), a GS C-terminal peptide (anti-GS/20aa-C), and brain S100 (anti-S100), as well as the binding of the insect glia and rat astrocytic marker Datura stramonium lectin (DSL), in the optic lobe of the prawn Macrobrachium rosenbergii. All markers label the lamina ganglionaris cartridge region (lighter: anti-GS/FL; heavier: DSL). In addition, anti-GS/FL labels superficial somata of external and internal medullas and internal chiasm cells. Both anti-GS/20aa-C and anti-S100 label heavily the glial sheaths of the lamina ganglionaris. In addition, anti-S100 binds to the perineurial glia of medullary parenchymal vessels. Western blot analyses show that both anti-GS/FL and anti-GS/20aa-C bind mostly to a band of 50-55 kDa, compatible with a long isoform of vertebrate GS, and accessorily to a possible dimer and, in the case of anti-GS/20aa-C, to an ill-defined band of intermediate mass. Binding of anti-S100 is selective for a single band of about 68 kDa but shows no protein in the weight range of the canonical S100 protein superfamily. DSL reveals two bands of about 75 and about 120 kDa, thus within the range of maximal recognition for rat astrocytes. Our results suggest that phenotype protein markers of the optic lobe glia share antigenic determinants with S100 and (a long form of) GS and that, similarly to vertebrate and insect glia, crustacean glia protein and N-glycan residue markers display regional heterogeneity.

Roles of glia in the Drosophila nervous system

Glial cells have diverse functions that are necessary for the proper development and function of complex nervous systems. Various insects, primarily the fruit fly Drosophila melanogaster and the moth Manduca sexta, have provided useful models of glial function during development. The present review will outline evidence of glial contributions to embryonic, visual, olfactory and wing development. We will also outline evidence for non-developmental functions of insect glia including blood-brain-barrier formation, homeostatic functions and potential contributions to synaptic function. Where relevant, we will also point out similarities between the functions of insect glia and their vertebrate counterparts.

Ammonia metabolism in Aedes aegypti

We investigated the mechanisms by which Aedes aegypti mosquitoes are able to metabolize ammonia. When females were given access to solutions containing NH(4)Cl or to a blood meal, hemolymph glutamine and proline concentrations increased markedly, indicating that ammonium/ammonia can be removed from the body through the synthesis of these two amino acids. The importance of glutamine synthetase was shown when an inhibitor of the enzyme was added to the meal causing the glutamine concentration in hemolymph to decrease significantly, while the proline concentration increased dramatically. Unexpectedly, we found an important role for glutamate synthase. When mosquitoes were fed azaserine, an inhibitor of glutamate synthase, the glutamine concentration increased and the proline concentration decreased significantly. This confirms the presence of glutamate synthase in mosquitoes and suggests that this enzyme contributes to the production of glutamate for proline synthesis. Several key enzymes related to ammonium/ammonia metabolism showed activity in homogenates of mosquito fat body and midgut. The mosquito genes encoding glutamate dehydrogenase, glutamine synthetase, glutamate synthase, pyrroline-5-carboxylate synthase were cloned and sequenced. The mRNA expression patterns of these genes were examined by a real-time RT-PCR in fat body and midgut. The results show that female mosquitoes have evolved efficient mechanisms to detoxify large loads of ammonium/ammonia.

Expression of Four Glutamine Synthetase Genes in the Early Stages of Development of Rainbow Trout (Oncorhynchus mykiss) in Relationship to Nitrogen Excretion

The incorporation of ammonia into glutamine, catalyzed by glutamine synthetase, is thought to be important in the detoxification of ammonia in animals. During early fish development, ammonia is continuously formed as yolk proteins and amino acids are catabolized. We followed the changes in ammonia and urea-nitrogen content, ammonia and urea-nitrogen excretion, glutamine synthetase activity, and mRNA expression of four genes coding for glutamine synthetase (Onmy-GS01-GS04) over 3-80 days post fertilization and in adult liver and skeletal muscle of the rainbow trout (Oncorhynchus mykiss). Both ammonia and urea-nitrogen accumulate before hatching, although the rate of ammonia excretion is considerably higher relative to urea-nitrogen excretion. All four genes were expressed during early development, but only Onmy-GS01 and -GS02 were expressed at appreciable levels in adult liver, and expression was very low in muscle tissue. The high level of expression of Onmy-GS01 and -GS03 prior to hatching corresponded to a linear increase in glutamine synthetase activity. We propose that the induction of glutamine synthetase genes early in development and the subsequent formation of the active protein are preparatory for the increased capacity of the embryo to convert the toxic nitrogen end product, ammonia, into glutamine, which may then be utilized in the ornithine-urea cycle or other pathways.

Molecular characterization of the glutamine synthetase gene in the Pacific oyster Crassostrea gigas: expression study in response to xenobiotic exposure and developmental stage

In this study, we characterized the full-length cDNA and genomic sequence of the gene encoding cytosolic glutamine synthetase (CgGSII) in the Pacific oyster, Crassostrea gigas. A phylogenetic analysis of GS sequences showed that CgGS clustered with the invertebrate group as expected. We analyzed the expression of mRNA CgGSII using RT-PCR to follow the expression of this gene in gills and digestive gland of oysters exposed, under experimental conditions, to hypoxia and to several contaminants (hydrocarbons and two pesticide treatments, glyphosate and a mixture of atrazine, diuron and isoproturon). We also investigated the expression of CgGSII in different developmental stages of C. gigas. Our results show that CgGSII expression was highly regulated in xenobiotic-exposed oysters compared to the control for all the treatments. Likewise, CgGSII expression was highly regulated according to the developmental stage of C. gigas. Finally, use of CgGSII as a possible marker to monitor xenobiotic exposure in disturbed ecosystems is discussed.

Three regulatory regions of the Aedes aegypti glutamine synthetase gene differentially regulate expression: identification of a crucial regulator in the first exon

Aedes aegyptiglutamine synthetase (GS) is expressed constitutively at various developmental stages and its relative mRNA abundance increases in the midgut following blood feeding in support of the biosynthesis of chitin, a component of the peritrophic matrix. To understand the regulation of GS expression better, GS-luciferase reporter fusion genes were constructed and analysed in transiently transfected C6/36 cells. These studies have identified three GS regions: GS-A, -B and -C (C1, C2) that are required for efficient transcription. The crucial regulatory DNA sequence is located within 140 nucleotides of the GS-C region in the first exon. GS-B region between -209 and +4 contains a negative modulator that represses transcription of the GS-C promoter, but the 5'-GS-A region, between -476 and -282, can negate the transcription inhibition of GS-B and promote GS transcription of the GS-C promoter. Electrophoretic mobility shift assays showed that nuclear proteins for GS-A, GS-B and GS-C1 are present in the C6/36 cells, and therefore that GS-A, GS-B and GS-C1 indeed possess regulatory function. By contrast, nuclear proteins isolated from both cultured cells and midgut tissues bound to GS-C2, suggesting that GS-C2 plays an important role in GS transcription and that GS-C2 is regulated by several different and redundant transcription factors to achieve constitutive expression in a wide variety of tissues.

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.

Aedes aegypti glutamine synthetase: expression and gene structure

The peritrophic matrix (PM) is the first natural barrier a mosquito-borne parasite faces when ingested with a blood meal; consequently, understanding the biology of PM formation could provide novel transmission control strategies. Because the PM is composed of chitin (a molecule of repeating units of N-acetyl glucosamine), glycoproteins and glucose, characterizing the regulation of enzymes involved in chitin production should provide information concerning factors that influence PM formation. We previously have shown that glutamine synthetase (GS) provides the glutamine needed in the initial steps of chitin biosynthesis in the yellow fever mosquito, Aedes aegypti. In the present study we show that GS is encoded by a single 4.5 kb gene, designated mGS, containing three exons and two introns. Multiple transcripts are generated from mGS presumably by differential splicing of the introns. Sequences of two cDNAs encoding GS are identical at the protein level, but differ in their 5'-untranslated regions. GS message is constitutively expressed in all developmental stages and in most tissues, with an increase in GS transcription observed in midgut and fat body tissues of female mosquitoes following a blood meal. Transcripts are localized to the apical side of the mosquito midgut epithelium and data suggest that mGS transcription is regulated by an Oct-1 transcription factor.

Isoform sorting and the creation of intracellular compartments

The generation of isoforms via gene duplication and alternative splicing has been a valuable evolutionary tool for the creation of biological diversity. In addition to the formation of molecules with related but different functional characteristics, it is now apparent that isoforms can be segregated into different intracellular sites within the same cell. Sorting has been observed in a wide range of genes, including those encoding structural molecules, receptors, channels, enzymes, and signaling molecules. This results in the creation of intracellular compartments that (a) can be independently controlled and (b) have different functional properties. The sorting mechanisms are likely to operate at the level of both proteins and mRNAs. Isoform sorting may be an important consequence of the evolution of isoforms and is likely to have contributed to the diversity of functional properties within groups of isoforms.

Biochemical analysis of a blood meal-induced Aedes aegypti glutamine synthetase gene

Glutamine synthetase (GS) in the mosquito, Aedes aegypti, is induced in the midgut following a blood meal. Mosquito GS message is detected as soon as 1 h post-blood feeding and remains stable for 18 h. Using a PCR product encoding mosquito GS, a lambda gt10 adult female mosquito cDNA library was screened. A cDNA clone, pCl5A2, encoding the full translation product of mosquito GS was isolated and sequence analyses performed. Mosquito GS cDNA is 2.5 kb in length and its putative translation product shares all the conserved regions characteristic of the GS gene family, including the presumed ATP biding site. Glutamine synthetase activity in the mosquito midgut is highest at 18 h post-blood feeding. Activity can be detected over a broad pH range, from 6.0 to 7.5. Unlike other cellular GS enzymes, mosquito GS is not active in the presence of ATP. Very low dosages (0.05 mM) of L-methionine S-sulfoximine are sufficient to partially inhibit mosquito GS activity. Inhibition of GS disrupts the normal formation of the midgut peritrophic matrix, suggesting that GS enzyme might be involved in the initial pathway of chitin synthesis. The unique expression pattern and inducible nature of the mosquito GS gene make it an interesting candidate for studying promoter function. Additionally, the blood meal activation of the GS gene makes this a potentially valuable tool in mosquito transformation studies.

Regulation of the spatiotemporal pattern of expression of the glutamine synthetase gene

Glutamine synthetase, the enzyme that catalyzes the ATP-dependent conversion of glutamate and ammonia into glutamine, is expressed in a tissue-specific and developmentally controlled manner. The first part of this review focuses on its spatiotemporal pattern of expression, the factors that regulate its levels under (patho)physiological conditions, and its role in glutamine, glutamate, and ammonia metabolism in mammals. Glutamine synthetase protein stability is more than 10-fold reduced by its product glutamine and by covalent modifications. During late fetal development, translational efficiency increases more than 10-fold. Glutamine synthetase mRNA stability is negatively affected by cAMP, whereas glucocorticoids, growth hormone, insulin (all positive), and cAMP (negative) regulate its rate of transcription. The signal transduction pathways by which these factors may regulate the expression of glutamine synthetase are briefly discussed. The second part of the review focuses on the evolution, structure, and transcriptional regulation of the glutamine synthetase gene in rat and chicken. Two enhancers (at -6.5 and -2.5 kb) were identified in the upstream region and two enhancers (between +156 and +857 bp) in the first intron of the rat glutamine synthetase gene. In addition, sequence analysis suggests a regulatory role for regions in the 3' untranslated region of the gene. The immediate-upstream region of the chicken glutamine synthetase gene is responsible for its cell-specific expression, whereas the glucocorticoid-induced developmental appearance in the neural retina is governed by its far-upstream region.