China experienced several large measles outbreaks in the past two decades, and a series of enhanced control measures were implemented to achieve the goal of measles elimination. Molecular epidemiologic surveillance of wild-type measles viruses (MeV) provides valuable information about the viral transmission patterns. Since 1993, virologic surveillnace has confirmed that a single endemic genotype H1 viruses have been predominantly circulating in China. A component of molecular surveillance is to monitor the genetic characteristics of the hemagglutinin (H) gene of MeV, the major target for virus neutralizing antibodies.
Analysis of the sequences of the complete H gene from 56 representative wild-type MeV strains circulating in China during 1993–2009 showed that the H gene sequences were clustered into 2 groups, cluster 1 and cluster 2. Cluster1 strains were the most frequently detected cluster and had a widespread distribution in China after 2000. The predicted amino acid sequences of the H protein were relatively conserved at most of the functionally significant amino acid positions. However, most of the genotype H1 cluster1 viruses had an amino acid substitution (Ser240Asn), which removed a predicted N-linked glycosylation site. In addition, the substitution of Pro397Leu in the hemagglutinin noose epitope (HNE) was identified in 23 of 56 strains. The evolutionary rate of the H gene of the genotype H1 viruses was estimated to be approximately 0.76×10−3 substitutions per site per year, and the ratio of dN to dS (dN/dS) was <1 indicating the absence of selective pressure.
Although H genes of the genotype H1 strains were conserved and not subjected to selective pressure, several amino acid substitutions were observed in functionally important positions. Therefore the antigenic and genetic properties of H genes of wild-type MeVs should be monitored as part of routine molecular surveillance for measles in China.
The incidence of measles in China from 1991 to 2008 was reviewed, and the nucleotide sequences from 1507 measles viruses (MeV) isolated during 1993 to 2008 were phylogenetically analyzed. The results showed that measles epidemics peaked approximately every 3 to 5 years with the range of measles cases detected between 56,850 and 140,048 per year. The Chinese MeV strains represented three genotypes; 1501 H1, 1 H2 and 5 A. Genotype H1 was the predominant genotype throughout China continuously circulating for at least 16 years. Genotype H1 sequences could be divided into two distinct clusters, H1a and H1b. A 4.2% average nucleotide divergence was found between the H1a and H1b clusters, and the nucleotide sequence and predicted amino acid homologies of H1a viruses were 92.3%–100% and 84.7%–100%, H1b were 97.1%–100% and 95.3%–100%, respectively. Viruses from both clusters were distributed throughout China with no apparent geographic restriction and multiple co-circulating lineages were present in many provinces. Cluster H1a and H1b viruses were co-circulating during 1993 to 2005, while no H1b viruses were detected after 2005 and the transmission of that cluster has presumably been interrupted. Analysis of the nucleotide and predicted amino acid changes in the N proteins of H1a and H1b viruses showed no evidence of selective pressure. This study investigated the genotype and cluster distribution of MeV in China over a 16-year period to establish a genetic baseline before MeV elimination in Western Pacific Region (WPR). Continuous and extensive MeV surveillance and the ability to quickly identify imported cases of measles will become more critical as measles elimination goals are achieved in China in the near future. This is the first report that a single endemic genotype of measles virus has been found to be continuously circulating in one country for at least 16 years.
We evaluated performance of three commercial Japanese encephalitis virus (JEV) IgM antibody capture enzyme-linked immunosorbent assay (MAC ELISA) kits with a panel of serological specimens collected during a surveillance project of acute encephalitis syndrome in India and acute meningitis and encephalitis syndrome in Bangladesh. The serum and cerebral spinal fluid specimens had been referred to the Centers for Disease Control and Prevention (CDC) for confirmatory testing. The CDC results and specimen classifications were considered the reference standard. All three commercial kits had high specificity (95–99.5%), but low sensitivities, ranging from 17–57%, with both serum and cerebrospinal fluid samples. Specific factors contributing to low sensitivity compared with the CDC ELISA could not be determined through further analysis of the limits and dilution end points of IgM detection.
The vesicle protein, synaptotagmin I, is the Ca2+ sensor that triggers fast, synchronous release of neurotransmitter. Specifically, Ca2+ binding by the C2B domain of synaptotagmin is required at intact synapses. Yet the mechanism whereby Ca2+ binding results in vesicle fusion remains controversial. Ca2+-dependent interactions between synaptotagmin and SNARE complexes and/or anionic membranes are possible effector interactions. However, no effector-interaction mutations to date impact synaptic transmission as severely as mutation of the C2B Ca2+-binding motif suggesting that these interactions are facilitatory rather than essential. Here we use Drosophila to show the functional role of a highly-conserved, hydrophobic residue located at the tip of each of synaptotagmin’s two Ca2+-binding pockets. Mutation of this residue in the C2A domain (F286) resulted in a ~50% decrease in evoked transmitter release at an intact synapse, again indicative of a facilitatory role. Mutation of this hydrophobic residue in the C2B domain (I420), on the other hand, blocked all locomotion, was embryonic lethal even in syt I heterozygotes, and resulted in less evoked transmitter release than that in sytnull mutants, which is more severe than the phenotype of C2B Ca2+-binding mutants. Thus mutation of a single, C2B hydrophobic residue required for Ca2+-dependent penetration of anionic membranes, results in the most severe disruption of synaptotagmin function in vivo to date. Our results provide direct support for the hypothesis that plasma membrane penetration, specifically by synaptotagmin’s C2B domain, is the critical effector interaction for coupling Ca2+ binding with vesicle fusion.
synaptic vesicle release; exocytosis; Drosophila; synaptic transmission; MEPP; EPP (6)
Drosophila is a premier genetic model for the study of both embryonic development and functional neuroscience. Traditionally, these fields are quite isolated from each other, with largely independent histories and scientific communities. However, the interface between these usually disparate fields is the developmental programs underlying acquisition of functional electrical signaling properties and differentiation of functional chemical synapses during the final phases of neural circuit formation. This interface is a critically important area for investigation. In Drosophila, these phases of functional development occur during a period of <8 hours (at 25°C) during the last third of embryogenesis. This late developmental period was long considered intractable to investigation owing to the deposition of a tough, impermeable epidermal cuticle. A breakthrough advance was the application of water-polymerizing surgical glue that can be locally applied to the cuticle to enable controlled dissection of late-stage embryos. With a dorsal longitudinal incision, the embryo can be laid flat, exposing the ventral nerve cord and body wall musculature to experimental investigation. Whole-cell patch-clamp techniques can then be employed to record from individually-identifiable neurons and somatic muscles. These recording configurations have been used to track the appearance and maturation of ionic currents and action potential propagation in both neurons and muscles. Genetic mutants affecting these electrical properties have been characterized to reveal the molecular composition of ion channels and associated signaling complexes, and to begin exploration of the molecular mechanisms of functional differentiation. A particular focus has been the assembly of synaptic connections, both in the central nervous system and periphery. The glutamatergic neuromuscular junction (NMJ) is most accessible to a combination of optical imaging and electrophysiological recording. A glass suction electrode is used to stimulate the peripheral nerve, with excitatory junction current (EJC) recordings made in the voltage-clamped muscle. This recording configuration has been used to chart the functional differentiation of the synapse, and track the appearance and maturation of presynaptic glutamate release properties. In addition, postsynaptic properties can be assayed independently via iontophoretic or pressure application of glutamate directly to the muscle surface, to measure the appearance and maturation of the glutamate receptor fields. Thus, both pre- and postsynaptic elements can be monitored separately or in combination during embryonic synaptogenesis. This system has been heavily used to isolate and characterize genetic mutants that impair embryonic synapse formation, and thus reveal the molecular mechanisms governing the specification and differentiation of synapse connections and functional synaptic signaling properties.
Drosophila is a premier genetic model for the study of both embryonic development and functional neuroscience. Traditionally, these fields are quite isolated from each other, with largely independent histories and scientific communities. However, the interface between these usually disparate fields is the developmental programs underlying acquisition of functional electrical signaling properties and differentiation of functional chemical synapses during the final phases of neural circuit formation. This interface is a critically important area for investigation. In Drosophila, these phases of functional development occur during a period of <8 hours (at 25°C) during the last third of embryogenesis. This late developmental period was long considered intractable to investigation owing to the deposition of a tough, impermeable epidermal cuticle. A breakthrough advance was the application of water-polymerizing surgical glue that can be locally applied to the cuticle to enable controlled dissection of late-stage embryos. With a dorsal longitudinal incision, the embryo can be laid flat, exposing the ventral nerve cord and body wall musculature to experimental investigation. This system has been heavily used to isolate and characterize genetic mutants that impair embryonic synapse formation, and thus reveal the molecular mechanisms governing the specification and differentiation of synapse connections and functional synaptic signaling properties.
Most intercellular glutamate signaling in the nervous system occurs at synapses. Some intercellular glutamate signaling occurs outside synapses, however, and even outside the nervous system where high ambient extracellular glutamate might be expected to preclude the effectiveness of glutamate as an intercellular signal. Here, I briefly review the types of intercellular glutamate signaling in the nervous system and beyond, with emphasis on the diversity of signaling mechanisms and fundamental unanswered questions.
Glutamate; synaptic; paracrine; receptor
Neurexin is a synaptic cell adhesion protein critical for synapse formation and function. Mutations in neurexin and neurexin-interacting proteins have been implicated in several neurological diseases. Previous studies have described Drosophila neurexin mutant phenotypes in third instar larvae and adults. However, the expression and function of Drosophila neurexin early in synapse development, when neurexin function is thought to be most important, has not been described.
We use a variety of techniques, including immunohistochemistry, electron microscopy, in situ hybridization, and electrophysiology, to characterize neurexin expression and phenotypes in embryonic Drosophila neuromuscular junctions (NMJs). Our results surprisingly suggest that neurexin in embryos is present both pre and postsynaptically. Presynaptic neurexin promotes presynaptic active zone formation and neurotransmitter release, but along with postsynaptic neurexin, also suppresses formation of ectopic glutamate receptor clusters. Interestingly, we find that loss of neurexin only affects receptors containing the subunit GluRIIA.
Our study extends previous results and provides important detail regarding the role of neurexin in Drosophila glutamate receptor abundance. The possibility that neurexin is present postsynaptically raises new hypotheses regarding neurexin function in synapses, and our results provide new insights into the role of neurexin in synapse development.
Better understanding of transmission patterns will enhance control and elimination programs.
To determine the origin of the virus associated with a measles outbreak in Menglian County, Yunnan Province, People’s Republic of China, in 2009, we conducted genetic analyses. Phylogenetic analyses based on nucleoprotein (N) and hemagglutinin (H) gene sequences showed that these Menglian viruses were not closely related to sequences of any World Health Organization (WHO) reference strains representing the 23 currently recognized genotypes. The minimum nucleotide divergence between the Menglian viruses and the most closely related reference strain, genotype D7, was 3.3% for the N gene and 3.0% for the H gene. A search of the databases of GenBank, WHO, and the Health Protection Agency Measles Nucleotide Surveillance showed that the Menglian viruses, together with the 2 older non-Menglian viruses, could be members of a new proposed measles genotype, d11. The new genotype designation will allow for better description of measles transmission patterns, especially in the Southeast Asian and Western Pacific regions.
Measles virus; new genotype; phylogenetic analysis; viruses; research
Molecular characterization of wild-type measles viruses in China during 1995-2004 demonstrated that genotype H1 was endemic and widely distributed throughout the country. H1-associated cases and outbreaks caused a resurgence of measles beginning in 2005. A total of 210,094 measles cases and 101 deaths were reported by National Notifiable Diseases Reporting System (NNDRS) and Chinese Measles Laboratory Network (LabNet) from 2006 to 2007, and the incidences of measles were 6.8/100,000 population and 7.2/100,000 population in 2006 and 2007, respectively. Five hundred and sixty-five wild-type measles viruses were isolated from 24 of 31 provinces in mainland China during 2006 and 2007, and all of the wild type virus isolates belonged to cluster 1 of genotype H1. These results indicated that H1-cluster 1 viruses were the predominant viruses circulating in China from 2006 to 2007. This study contributes to previous efforts to generate critical baseline data about circulating wild-type measles viruses in China that will allow molecular epidemiologic studies to help measure the progress made toward China's goal of measles elimination by 2012.
Japanese encephalitis (JE) is the most important form of viral encephalitis in Asia. Surveillance for the disease in many countries has been limited. To improve collection of accurate surveillance data in order to increase understanding of the full impact of JE and monitor control programs, World Health Organization (WHO) Recommended Standards for JE Surveillance have been developed. To aid acceptance of the Standards, we describe the process of development, provide the supporting evidence, and explain the rationale for the recommendations made in the document.
A JE Core Working Group was formed in 2002 and worked on development of JE surveillance standards. A series of questions on specific topics was initially developed. A literature review was undertaken and the findings were discussed and documented. The group then prepared a draft document, with emphasis placed on the feasibility of implementation in Asian countries. A field test version of the Standards was published by WHO in January 2006. Feedback was then sought from countries that piloted the Standards and from public health professionals in forums and individual meetings to modify the Standards accordingly.
After revisions, a final version of the JE surveillance standards was published in August 2008. The supporting information is presented here together with explanations of the rationale and levels of evidence for specific recommendations.
Provision of the supporting evidence and rationale should help to facilitate successful implementation of the JE surveillance standards in JE-endemic countries which will in turn enable better understanding of disease burden and the impact of control programs.
The efficacy of synaptic transmission depends, to a large extent, on postsynaptic receptor abundance. The molecular mechanisms controlling receptor abundance are poorly understood. We tested whether abundance of postsynaptic glutamate receptors (GluRs) in Drosophila neuromuscular junctions is controlled by microRNAs, and provide evidence that it is. We show here that postsynaptic knockdown of dicer-1, the endoribonuclease necessary for microRNA synthesis, leads to large increases in postsynaptic GluR subunit messenger RNA and protein. Specifically, we measured increases in GluRIIA and GluRIIB but not GluRIIC. Further, knockout of MiR-284, a microRNA predicted to bind to GluRIIA and GluRIIB but not GluRIIC, increases expression of GluRIIA and GluRIIB but not GluRIIC proportional to the number of predicted binding sites in each transcript. Most of the de-repressed GluR protein, however, does not appear to be incorporated into functional receptors, and only minor changes in synaptic strength are observed, which suggests that microRNAs primarily regulate Drosophila receptor subunit composition rather than overall receptor abundance or synaptic strength.
Measles morbidity and mortality decreased significantly after measles vaccine was introduced into China in 1965. From 1995 to 2004, average annual measles incidence decreased to 5.6 cases per 100,000 population following the establishment of a national two-dose regimen. Molecular characterization of wild-type measles viruses demonstrated that genotype H1 was endemic and widely distributed throughout the country in China during 1995-2004. A total of 124,865 cases and 55 deaths were reported from the National Notifiable Diseases Reporting System (NNDRS) in 2005, which represented a 69.05% increase compared with 2004. Over 16,000 serum samples obtained from 914 measles outbreaks and the measles IgM positive rate was 81%. 213 wild-type measles viruses were isolated from 18 of 31 provinces in China during 2005, and all of the isolates belonged to genotype H1. The ranges of the nucleotide sequence and predicted amino acid sequence homologies of the 213 genotype H1 strains were 93.4%-100% and 90.0%-100%, respectively. H1-associated cases and outbreaks caused the measles resurgence in China in 2005. H1 genotype has the most inner variation within genotype, it could be divided into 2 clusters, and cluster 1 viruses were predominant in China throughout 2005.
Many neuroscientists assume that ambient extracellular glutamate concentrations in the nervous system are biologically negligible under nonpathological conditions. This assumption is false. Hundreds of studies over several decades suggest that ambient extracellular glutamate levels in the intact mammalian brain are ~0.5 to ~5 μM. This has important implications. Glutamate receptors are desensitized by glutamate concentrations significantly lower than needed for receptor activation; 0.5 to 5 μM of glutamate is high enough to cause constitutive desensitization of most glutamate receptors. Therefore, most glutamate receptors in vivo may be constitutively desensitized, and ambient extracellular glutamate and receptor desensitization may be potent but generally unrecognized regulators of synaptic transmission. Unfortunately, the mechanisms regulating ambient extracellular glutamate and glutamate receptor desensitization remain poorly understood and understudied.
Glutamate; Synaptic; Synapse; Glutamate receptor; Ambient extracellular glutamate
One of the most widely used transgenic animal models in biology is Drosophila melanogaster, the fruit fly. Chemical information from this exceedingly small organism is usually accomplished by studying populations to attain sample volumes suitable for standard analysis methods. This paper describes a direct sampling technique capable of obtaining 50–300 nL of hemolymph from individual Drosophila larvae. Hemolymph sampling performed under mineral oil and in air at 30 s intervals up to 120 s after piercing larvae revealed that the effect of evaporation on amino acid concentrations is insignificant when the sample was collected within 60 s. Qualitative and quantitative amino acid analyses of obtained hemolymph were carried out in two optimized buffer conditions by capillary electrophoresis with laser-induced fluorescence detection after derivatizing with fluorescamine. Thirteen amino acids were identified from individual hemolymph samples of both wild-type (WT) control and the genderblind (gb) mutant larvae. The levels of glutamine, glutamate, and taurine in the gb hemolymph were significantly lower at 35%, 38%, and 57% of WT levels, respectively. The developed technique that samples only the hemolymph fluid is efficient and enables accurate organism-level chemical information while minimizing errors associated with possible sample contaminations, estimations, and effects of evaporation compared to the traditional hemolymph-sampling techniques.
Neural function is dependent upon the proper formation and development of synapses. We show here that Wnt5 regulates the growth of the Drosophila neuromuscular junction (NMJ) by signaling through the Derailed receptor. Mutations in both wnt5 and drl result in a significant reduction in the number of synaptic boutons. Cell-type specific rescue experiments show that wnt5 functions in the presynaptic motor neuron while drl likely functions in the postsynaptic muscle cell. Epistatic analyses indicate that drl acts downstream of wnt5 to promote synaptic growth. Structure–function analyses of the Drl protein indicate that normal synaptic growth requires the extracellular Wnt inhibitory factor domain and the intracellular domain, which includes an atypical kinase. Our findings reveal a novel signaling mechanism that regulates morphology of the Drosophila NMJ.
Wnt; Drl; Ryk; NMJ; synapse; boutons
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: ~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: ~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.
Drosophila melanogaster; Glutamate; Glutamate receptor; Hemolymph; Synaptic
AMPA receptors are responsible for fast excitatory transmission in the CNS and the trafficking of these receptors has been implicated in LTP and learning and memory. These receptors reside in the postsynaptic density, a network of proteins that links the receptors to downstream signaling components and to the neuronal cytoskeleton. To determine whether the fruit fly, Drosophila melanogaster, possesses a similar array of proteins as are found at the mammalian PSD, we identified Drosophila homologs of 95.8% of mammalian PSD proteins. We investigated, for the first time, the role of one of these PSD proteins, Pod1 in GluR cluster formation at the Drosophila neuromuscular junction and found that mutations in pod1 resulted in a specific loss of A-type receptors at the synapse.
Cystine-glutamate transporters import cystine into cells for glutathione synthesis and protection from oxidative stress, but also export significant amounts of glutamate. Increasing evidence suggests that ‘ambient extracellular glutamate’ secreted by cystine-glutamate transporters in the nervous system modulates glutamatergic synapse strength and behavior. To date, the only cystine-glutamate transporter mutants examined behaviorally are Drosophila genderblind mutants. These animals contain loss-of-function mutations in the ‘genderblind’ gene, which encodes an xCT subunit essential for cystine-glutamate transporter function. Genderblind was named based on a mutant courtship phenotype: male genderblind mutants are attracted to normally aversive male pheromones and thus court and attempt to copulate with both male and female partners equally. However, genderblind protein is expressed in many parts of the fly brain and thus might be expected to also regulate other behaviors, including behaviors not related to male courtship or chemosensation. Here, we show that genderblind mutants display faster recovery and increased negative geotaxis after strong mechanical stimuli (e.g., they climb faster and farther after vial banging). This phenotype is displayed by both males and females, consistent with strong genderblind expression in both sexes.
Drosophila; cystine-glutamate transporters; glutamate; behavior
Mate choice is an evolutionarily critical decision that requires the detection of multiple sex-specific signals followed by central integration of these signals to direct appropriate behavior. The mechanisms controlling mate choice remain poorly understood. Here, we show that the glial amino-acid transporter genderblind controls whether Drosophila melanogaster males will attempt to mate with other males. Genderblind (gb) mutant males showed no alteration in heterosexual courtship or copulation, but were attracted to normally unappealing male species-specific chemosensory cues. As a result, genderblind mutant males courted and attempted to copulate with other Drosophila males. This homosexual behavior could be induced within hours using inducible RNAi, suggesting that genderblind controls nervous system function rather than its development. Consistent with this, and indicating that glial genderblind regulates ambient extracellular glutamate to suppress glutamatergic synapse strength in vivo, homosexual behavior could be turned on and off by altering glutamatergic transmission pharmacologically and/or genetically.
Quantal size is the postsynaptic response to the release of a single synaptic vesicle and is determined in part by the amount of transmitter within that vesicle. At glutamatergic synapses, the vesicular glutamate transporter (VGLUT) fills vesicles with glutamate. While elevated VGLUT expression increases quantal size, the minimum number of transporters required to fill a vesicle is unknown. In Drosophila DVGLUT mutants, reduced transporter levels lead to a dose-dependent reduction in the frequency of spontaneous quantal release with no change in quantal size. Quantal frequency is not limited by vesicle number or impaired exocytosis. This suggests that a single functional unit of transporter is both necessary and sufficient to fill a vesicle to completion and that vesicles without DVGLUT are empty. Consistent with the presence of empty vesicles, at dvglut mutant synapses synaptic vesicles are smaller, suggesting that vesicle filling and/or transporter level is an important determinant of vesicle size.
A molecular understanding of synaptogenesis is a critical step toward the goal of understanding how brains “wire themselves up,” and then “rewire” during development and experience. Recent genomic and molecular advances have made it possible to study synaptogenesis on a genomic scale. Here, we describe the results of a screen for genes involved in formation and development of the glutamatergic Drosophila neuromuscular junction (NMJ). We screened 2185 P-element transposon mutants representing insertions in ≈16% of the entire Drosophila genome. We first identified recessive lethal mutants, based on the hypothesis that mutations causing severe disruptions in synaptogenesis are likely to be lethal. Two hundred twenty (10%) of all insertions were homozygous lethal. Two hundred five (93%) of these lethal mutants developed at least through late embryogenesis and formed neuromusculature. We examined embryonic/larval NMJs in 202 of these homozygous mutants using immunocytochemistry and confocal microscopy. We identified and classified 88 mutants with altered NMJ morphology. Insertion loci in these mutants encode several different types of proteins, including ATP- and GTPases, cytoskeletal regulators, cell adhesion molecules, kinases, phosphatases, RNA regulators, regulators of protein formation, transcription factors, and transporters. Thirteen percent of insertions are in genes that encode proteins of novel or unknown function. Complementation tests and RT-PCR assays suggest that approximately 51% of the insertion lines carry background mutations. Our results reveal that synaptogenesis requires the coordinated action of many different types of proteins—perhaps as much as 44% of the entire genome—and that transposon mutageneses carry important caveats that must be respected when interpreting results generated using this method.
Drosophila; synapse development; axon guidance; axon branching; neuromuscular junction
Glutamatergic Drosophila neuromuscular junctions contain two spatially, biophysically, and pharmacologically distinct subtypes of postsynaptic glutamate receptor (GluR). These receptor subtypes appear to be molecularly identical except that A receptors contain the subunit GluRIIA (but not GluRIIB), and B receptors contain the subunit GluRIIB (but not GluRIIA). A- and B-type receptors are coexpressed in the same cells, in which they form homotypic clusters. During development, A- and B-type receptors can be differentially regulated. The mechanisms that allow differential segregation and regulation of A- and B-type receptors are unknown. Presumably, A-and B-type receptors are differentially anchored to the membrane cytoskeleton, but essentially nothing is known about how Drosophila glutamate receptors are localized or anchored. We identified coracle, a homolog of mammalian brain 4.1 proteins, in yeast two-hybrid and genetic screens for proteins that interact with and localize Drosophila glutamate receptors. Coracle interacts with the C terminus of GluRIIA but not GluRIIB. To test whether coracle is required for glutamate receptor localization, we immunocytochemically and electrophysiologically examined receptors in coracle mutants. In coracle mutants, synaptic A-type receptors are lost, but there is no detectable change in B-type receptor function or localization. Pharmacological disruption of postsynaptic actin phenocopies the coracle mutants, suggesting that A-type receptors are anchored to the actin cytoskeleton via coracle, whereas B-type receptors are anchored at the synapse by another (yet unknown) mechanism.
actin; Drosophila; postsynaptic; genetics; glutamate receptor; 4.1
A Drosophila forward genetic screen for mutants with defective synaptic development identified bad reception (brec). Homozygous brec mutants are embryonic lethal, paralyzed, and show no detectable synaptic transmission at the glutamatergic neuromuscular junction (NMJ). Genetic mapping, complementation tests, and genomic sequencing show that brec mutations disrupt a previously uncharacterized ionotropic glutamate receptor subunit, named here “GluRIID.” GluRIID is expressed in the postsynaptic domain of the NMJ, as well as widely throughout the synaptic neuropil of the CNS. In the NMJ of null brec mutants, all known glutamate receptor subunits are undetectable by immunocytochemistry, and all functional glutamate receptors are eliminated. Thus, we conclude that GluRIID is essential for the assembly and/or stabilization of glutamate receptors in the NMJ. In null brec mutant embryos, the frequency of periodic excitatory currents in motor neurons is significantly reduced, demonstrating that CNS motor pattern activity is regulated by GluRIID. Although synaptic development and molecular differentiation appear otherwise unperturbed in null mutants, viable hypomorphic brec mutants display dramatically undergrown NMJs by the end of larval development, suggesting that GluRIID-dependent central pattern activity regulates peripheral synaptic growth. These studies reveal GluRIID as a newly identified glutamate receptor subunit that is essential for glutamate receptor assembly/stabilization in the peripheral NMJ and required for properly patterned motor output in the CNS.
synapse; postsynaptic; synaptogenesis; glutamatergic; glutamate receptor; subunit; embryo; pattern generator
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.
glia; glutamate; glutamate receptor; neurotransmission; synapse; synaptic plasticity