Molecular cloning of an invertebrate glutamate receptor subunit expressed in Drosophila muscle

Insertional mutation of the Drosophila nuclear lamin Dm0 gene results in defective nuclear envelopes, clustering of nuclear pore complexes, and accumulation of annulate lamellae

Nuclear lamins are thought to play an important role in disassembly and reassembly of the nucleus during mitosis. Here, we describe a Drosophila lamin Dm0 mutant resulting from a P element insertion into the first intron of the Dm0 gene. Homozygous mutant animals showed a severe phenotype including retardation in development, reduced viability, sterility, and impaired locomotion. Immunocytochemical and ultrastructural analysis revealed that reduced lamin Dm0 expression caused an enrichment of nuclear pore complexes in cytoplasmic annulate lamellae and in nuclear envelope clusters. In several cells, particularly the densely packed somata of the central nervous system, defective nuclear envelopes were observed in addition. All aspects of the mutant phenotype were rescued upon P element-mediated germline transformation with a lamin Dm0 transgene. These data constitute the first genetic proof that lamins are essential for the structural organization of the cell nucleus.

Desensitization and resensitization kinetics of glutamate receptor channels from Drosophila larval muscle

Outside-out patches from wild-type Drosophila larval muscle were exposed to L-glutamate (glu) using a piezo-driven application system. Glu receptor channels opened and desensitized in response to rapid applications of 10 mM glu. Desensitization was fitted with an exponential function with a mean time constant of desensitization (tau d) of 15 ms in response to 10 mM glu. The tau d was concentration dependent and decreased to 6 ms (on average) with 0.7 mM glu and increased again to 12 ms (on average) in response to 0.5 mM glu. Desensitization in response to longer applications of glu was almost complete, but surprisingly, even a 1-ms pulse of 3 mM glu produced about 30% desensitization. In the presence of low glu concentrations, the response to a pulse was reduced and was about halved by preequilibration with 30 microM glu. Recovery from desensitization was not concentration dependent and was fitted with an exponential function with a mean time constant of 150 ms. During recovery the channels rarely opened. Kinetic schemes were fitted to these results, and a circular reaction scheme was found to fit the data best. An important feature of the scheme is desensitization from a lower ligated closed state. This allows substantial desensitization of synaptic receptor channels in response to quantal release of transmitter, in part without opening of the channels. Desensitization reduces the probability of the channels opening in response to a subsequent release for a period of time determined by the rate of recovery from desensitization and might serve as a form of molecular short-term memory.

Neural activity affects distribution of glutamate receptors during neuromuscular junction formation in Drosophila embryos

Changes in the distribution and density of transmitter receptors in the postsynaptic cell are required steps for functional synapse formation. We raised antibodies against Drosophila glutamate receptors (DGluR-II) and visualized the distribution of receptors during neuromuscular junction formation in embryos. In wild-type embryos, embryonic development is complete within 22 hr after egg lying (AEL) and neuromuscular junction (NMJ) formation begins at 13 hr AEL. At the time of initial synapse formation, DGluR-IIs appeared as clusters closely associated with some muscle nuclei. Subsequently, these nonjunctional clusters dispersed while DGluR-IIs accumulated at the junctional region. In a paralytic temperature-sensitive mutant, para(ts1), neural activity decreases drastically at restrictive temperatures. When neural activity was blocked throughout synaptogenesis by rearing embryos at a restrictive temperature prior to the beginning of synaptogenesis, 12 hr AEL, the dispersal of extrajunctional clusters was significantly suppressed and no accumulation of receptors at the junction was observed at 22 hr AEL. However, when neural activity was blocked later, by rearing embryos at a restrictive temperature from 13 hr AEL, DGluR-IIs did not accumulate at the NMJ, although extrajunctional clusters dispersed normally. These findings suggest that the neural activity differentially regulates dissipation of receptor clusters in the nonjunctional region and accumulation of receptors at the junctional region.

Synapse maturation and structural plasticity at Drosophila neuromuscular junctions

The Drosophila larval neuromuscular junction has recently emerged as a powerful model system to characterize the cellular and molecular events involved in the formation and flexibility of synapses. The combination of molecular, genetic, electrophysiological and anatomical approaches has revealed, for example, the functional significance of the discs-large gene product (a novel synapse-organizing protein) in the nervous system. This protein is involved in the clustering of at least one ion channel and in the structural modification of glutamatergic synapses during target muscle growth. The manipulation of the genes encoding ion channels, components of second-messenger cascades, and cell adhesion molecules is beginning to tease apart the mechanisms underlying structural synaptic plasticity.

Cloning and functional expression of a Drosophila metabotropic glutamate receptor expressed in the embryonic CNS

The excitatory neurotransmitter glutamate plays important roles in the mammalian brain, ranging from synaptic plasticity to memory. To mediate these functions, glutamate activates two types of receptors: ligand-gated channels and metabotropic receptors coupled to G-proteins. Both families of glutamate receptors share no sequence homology and possess original structural features compared with other ligand-gated channels and G-protein-coupled receptors, respectively. Glutamate-gated receptor-channel subunits have already been characterized in invertebrates. Here we report the cloning and functional characterization of an invertebrate metabotropic glutamate receptor (DmGluRA) isolated from Drosophila melanogaster. This receptor displays 45 and 43% amino acid sequence identity with its mammalian homologs mGluR3 and mGluR2, respectively. Moreover, its pharmacology and transduction mechanisms are surprisingly similar to those of mGluR2 and mGluR3. DmGluRA is expressed in the CNS of the late embryo. These results indicate that the original structural features of both glutamate receptor types are conserved from insects to mammals and suggest that the functions of these receptors have been highly conserved during evolution.

Cloning and characterization of cDNAs encoding putative glutamate transporters from Caenorhabditis elegans and Onchocerca volvulus

We report the identification and partial characterization of cDNAs encoding for putative glutamate transporters from the free-living nematode Caenorhabditis elegans and the filarial parasite Onchocerca volvulus. Glutamate transporters can be used as reliable markers for identifying cells and neurons that synaptically release glutamate and aspartate. An amplified PCR fragment containing a highly conserved amino acid heptamer found in all vertebrate glutamate transporters was used to screen a C. elegans cDNA library. Two full-length cDNA sequences from C. elegans were deduced from the isolated cDNA clones and RT-PCR products with the splice leader. The two C. elegans cDNA sequences differ by only 97 nucleotides at the 5' end. The C. elegans glutamate transporter gene glt-1 spans at least 2.9 kb of chromosomal DNA and possesses nine exons and eight introns. Primers directed to the CeGlt cDNA were used with O. volvulus first-strand cDNA to amplify and isolate the O. volvulus cDNA homolog. The C. elegans and O. volvulus glutamate transporters are 98% identical over 492 amino acids to each other and 52 to 58% identical to the mammalian glutamate transporters. Antibodies generated against partial coding regions of the C. elegans glutamate transporter recognized a protein of approximately 66 kDa in C. elegans and O. volvulus protein extracts.

Structure and pharmacological properties of a molluscan glutamate-gated cation channel and its likely role in feeding behavior

We describe the isolation of a molluscan (Lymnaea stagnalis) full-length complementary DNA that encodes a mature polypeptide (which we have named Lym-eGluR2) with a predicted molecular weight of 105 kDa that exhibits 44-48% identity to the mammalian kainate-selective glutamate receptor GluR5, GluR6, and GluR7 subunits. Injection of in vitro-transcribed RNA from this clone into Xenopus laevis oocytes results in the robust expression of homo-oligomeric cation channels that can be gated by L-glutamate (EC50 = 1.2 +/- 0.3 micron) and several other glutamate receptor agonists; rank order of potency: glutamate >> kainate > ibotenate > AMPA. These currents can be blocked by the mammalian non-NMDA receptor antagonists 6,7-dinitroquinoxaline-2,3-dione, 6-cyano-7-nitroquinoxaline-2,3-dione, and 1-(4-chlorobenzoyl)piperazine-2,3-dicarboxylic acid. Ionic-replacement experiments have shown that the agonist-induced current is carried entirely by sodium and potassium ions. In situ hybridization has revealed that the Lym-eGluR2 transcript is present in all 11 ganglia of the Lymnaea CNS, including the 4-cluster motorneurons within the paired buccal ganglia. The pharmacological properties and deduced location of Lym-eGluR2 are entirely consistent with it being (a component of) the receptor, which has been identified previously on buccal motorneurons, that mediates the excitatory effects of glutamate released from neurons within the feeding central pattern generator.

CDNA cloning of chick brain alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors reveals conservation of structure, function and post-transcriptional processes with mammalian receptors

Several types of functional ionotropic glutamate receptor have been cloned in the recent years from the mammalian central nervous system, but till now, none from other vertebrate species. Here, we report the cloning and functional analysis of four chick brain cDNAs, coding for members of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subtype of glutamate receptors. These receptors are highly homologous to the mammalian GluR1-4 (A-D) receptors ( > 90%), and conserve their post-transcriptional modifications. The flip/flop exons are conserved not only at the amino acid level but also at the nucleotide level, and the intron of GluR4 involved in the RNA editing of the R/G site displays a rat-chick sequence conservation of 95%. Significant sequence differences are found only in the region containing the immunogenic epitope of neuroactive anti-GluR3 antibodies. Chick AMPA receptors are expressed in both the cerebrum and cerebellum. The ion channel activities of chick GluR1-4 were analyzed in Xenopus oocytes and found to be similar to those of mammalian AMPA receptors. Though their contribution to kainate binding activity in the cerebellum is minor, the profile of channel activity of the chick GluR1-4 suggests that they account for the kainatergic channel activity expressed by total chick cerebellar mRNAs.

Evolution and overview of classical transmitter molecules and their receptors

All the classical transmitter ligand molecules evolved at least 1000 million years ago. With the possible exception of the Porifera and coelenterates (Cnidaria), they occur in all the remaining phyla. All transmitters have evolved the ability to activate a range of ion channels, resulting in excitation, inhibition and biphasic or multiphasic responses. All transmitters can be synthesised in all three basic types of neurones, i.e. sensory, interneurone and motoneurone. However their relative importance as sensory, interneurone or motor transmitters varies widely between the phyla. It is likely that all neurones contain more than one type of releasable molecule, often a combination of a classical transmitter and a neuroactive peptide. Second messengers, i.e. G proteins and phospholipase C systems, appeared early in evolution and occur in all phyla that have been investigated. Although the evidence is incomplete, it is likely that all the classical transmitter receptor subtypes identified in mammals, also occur throughout the phyla. The invertebrate receptors so far cloned show some interesting homologies both between those from different invertebrate phyla and with mammalian receptors. This indicates that many of the basic receptor subtypes, including benzodiazepine subunits, evolved at an early period, probably at least 800 million years ago. Overall, the evidence stresses the similarity between the major phyla rather than their differences, supporting a common origin from primitive helminth stock.

Molecular biology and electrophysiology of glutamate-gated chloride channels of invertebrates

In this chapter we summarize the available data on a novel class of ligand-gated anion channels that are gated by the neurotransmitter glutamate. Glutamate is classically thought to be a stimulatory neurotransmitter, however, studies in invertebrates have proven that glutamate also functions as an inhibitory ligand. The bulk of studies conducted in vivo have been on insects and crustaceans, where glutamate was first postulated to act on H-receptors resulting in a hyperpolarizing response to glutamate. Recently, glutamate-gated chloride channels have been cloned from several nematodes and Drosophila. The pharmacology and electrophysiological properties of these channels have been studied by expression in Xenopus oocytes. Studies on the cloned channels demonstrate that the invertebrate glutamate-gated chloride channels are the H-receptors and represent important targets for the antiparasitic avermectins.

Recordings of glutamate-gated ion channels in outside-out patches from Drosophila larval muscle

Outside-out patches of membrane were excised from muscle fibers 6 and 7 of third-instar wild-type Drosophila larvae. Channels were observed to open in response to short pulses of L-glutamate. At a holding potential of -60 mV, the channels opened to one main conductance level of about 120 pS. The current-voltage plot for the channels was linear and reversed around 0 mV holding potential. The channels were also activated by quisqualate but not by aspartate, N-methyl-D-aspartate (NMDA), kainate, glycine, gamma-aminobutyric acid (GABA) and acetylcholine. At high glutamate concentrations (3 or 10 mM), channel activation reached a peak within 0.3 ms. The channels opened in 'bursts' flickering between open and closed states. The channels opened only for a few milliseconds after switching on the glutamate and channel activity declined after the initial surge to zero with time constants between 5 and 20 ms. During applications of low glutamate concentrations (0.2-0.5 mM) to the same patch the channels opened much less frequently and during most applications no openings were observed. The openings observed were short and 'bursts' of openings were rare. Two exponential components were identified in the open-time distribution obtained with pulsed applications of glutamate (0.5-10 mM) with time constants of about 0.1 and 2.0 ms. The kinetics of the channels seem to be similar to the kinetics of certain glutamate gated channels found on muscle of crayfish and locust.

Chemical analysis of neurotransmitter candidates in clonal cell lines from Drosophila central nervous system, II: Neuropeptides and amino acids

In a previous study, we have found acetylcholine and/or L-DOPA in 10 colonial clones from one cell line of Drosophila larval central nervous system (CNS). In this study, to characterize clonal neuronal phenotypes further, we have examined three neuropeptides and 19 amino acids using HPLC system. Substance P and proctolin were found in seven and eight out of ten clones, respectively. On the other hand, somatostatin was expressed in all ten clones. GABA and taurine were not detected in any clones. Glutamate, which is an excitatory transmitter in Drosophila, was found in all the clones, although its content was different seven times among them. Glycine, which is not known as a transmitter in Drosophila, was found to be unevenly expressed among them. Therefore, the conspicuous expression of peptides or amino acids in some clones suggests that the substances have a special role in Drosophila CNS.

Drosophila glutamate receptor RNA expression in embryonic and larval muscle fibers

Glutamate is the excitatory transmitter at neuromuscular synapses in Drosophila, and electrophysiological studies indicate that the receptors for glutamate are concentrated in muscle fibers at synaptic sites. Acetylcholine is the excitatory transmitter at vertebrate neuromuscular synapses, and previous studies have shown that accumulation of acetylcholine receptors (AChRs) at synaptic sites is controlled both by transcriptional and post-translational mechanisms. The transcriptional pathway culminates in selective expression of AChR subunit genes in nuclei near the synaptic site, causing AChR mRNA to accumulate in the synaptic region of the muscle fiber. We used a cDNA encoding a subunit of the Drosophila muscle glutamate receptor (DGluR-II) to determine the temporal and spatial expression pattern of the DGluR-II gene during embryogenesis and in larval muscle. We show that DGluR-II mRNA is first expressed at stage 12 of embryogenesis and that expression is detected in developing dorsal, lateral, and ventral somatic muscles within the next 2 hr. By stage 16 DGluR-II mRNA is expressed in all somatic muscles and in pharyngeal muscles. In third instar larvae DGluR-II mRNA is expressed in all body-wall muscle fibers. DGluR-II mRNA, however, is expressed throughout the larval muscle fibers and is not concentrated within muscle fibers at neuromuscular synapses. These results indicate that although the DGluR-II gene is expressed in somatic muscle cells it is not selectively expressed in nuclei near the synaptic site.

Unraveling the modular design of glutamate-gated ion channels

Glutamate receptors that function as ligand-gated ion channels are essential components of cell-cell communication in the nervous system. Despite a wealth of information concerning these receptors, details of their structure are just beginning to emerge. We propose that glutamate receptors comprise four modules: two modules that are related to bacterial periplasmic-binding proteins, one module that is related to the pore-forming region of K+ channels, and one regulatory module of unknown origin. A K(+)-channel-like domain inserted into a crucial region of a periplasmic-binding protein-like domain suggests a mechanism for transduction of binding energy to channel opening. This modular design also suggests an evolutionary link between a ligand-gated ion-channel family and voltage-gated ion channels.

Identification of neuron-specific ivermectin binding sites in Drosophila melanogaster and Schistocerca americana

High affinity avermectin binding sites have been identified and partially characterized in membranes from two insect species, Drosophila melanogaster and the locus Schistocerca americana. There is a 10-fold increase in the density of ivermectin binding sites associated with membranes isolated from Drosophila heads (a neuronally enriched tissue source) compared to the bodies (Bmax values were 3.5 and 0.22 pmol/mg, respectively) with only a small difference in the apparent dissociation constant (Kd values of 0.20 and 0.34 nM for heads and bodies, respectively). Membranes prepared from metathoracic ganglia of the locust, Schistocerca americana, were highly enriched in high affinity avermectin binding sites (Kd = 0.2 nM and Bmax = 42 pmol/mg). Using an [125I]arylazido-avermectin analog as a photoaffinity probe, a 45 kDa protein was identified in both the Drosophila head and body tissue preparations. A 45 kDa protein was also specifically labeled with [125I]azido-avermectin in the locust neuronal membranes.

Immunoaffinity purification of avermectin-binding proteins from the free-living nematode Caenorhabditis elegans and the fruitfly Drosophila melanogaster

Avermectin-binding proteins from the free-living nematode worm Caenorhabditis elegans and from the fruitfly Drosophila melanogaster were purified to homogeneity via a three-step procedure. The binding proteins were covalently labelled using a radioactive photoaffinity probe and then partially purified on a Sephacryl S-300 gel-filtration column. The radiolabelled binding proteins were then purified by immunoaffinity chromatography using a monoclonal antibody to avermectin covalently attached to Protein A-Sepharose beads. Three affinity-labelled Drosophila proteins with molecular masses between 45 and 50 kDa were isolated in this way and then separated from each other by electroelution. This three-step protocol provides a rapid technique for receptor purification which may be of use in the purification of other binding proteins.

Transmembrane topology of two kainate receptor subunits revealed by N-glycosylation

Glutamate receptors are the primary excitatory neurotransmitter receptors in vertebrate brain and are of critical importance to a wide variety of neurological processes. Recent reports suggest that ionotropic glutamate receptors may have a unique transmembrane topology not shared by other ligand-gated ion channels. We report here the cloning of cDNAs from goldfish brain encoding two homologous kainate receptors with protein molecular masses of 41 kDa. Using a cell-free translation/translocation system, we show that (i) a portion of these receptors previously thought to be a large intracellular loop is actually located extracellularly and (ii) the putative second transmembrane region of the receptor thought to line the ion channel may not be a true membrane-spanning domain. An alternative model for the transmembrane topology of kainate receptors is proposed that could potentially serve as a framework for future detailed study of the structure of this important class of neurotransmitter receptors.

Insect calcium channels. Molecular cloning of an alpha 1-subunit from housefly (Musca domestica) muscle

The complete amino acid sequence of an invertebrate calcium channel alpha 1-subunit from housefly (Musca domestica) larvae (designated Mdl alpha 1) has been deduced by cDNA cloning and sequence analysis. Mdl alpha 1 shares higher percent sequence identity with 1,4-dihydropyridine (DHP)-sensitive L-type than with DHP-insensitive calcium channels. As shown by whole mount in situ hybridization and immunostaining Mdl alpha 1 is predominantly expressed in the larval body wall musculature.

Homologies and disparities of glutamate receptors: a critical analysis

Based on analysis of aligned amino acid sequences the following statements are made: (i) There is evolutionary homology between the N-terminal extracellular region of ionotropic Glutamate receptors/Kainate Binding Proteins and a family of procaryote amino acid binding proteins. (ii) Homology of the N-terminal extracellular domain of the metabotropic glutamate receptors with a family of receptors with a guanylate cyclase intracellular domain appears to be valid. (iii) There is no evidence for homology between the N-terminal extracellular domain of the nicotinic Acetylcholine, GABA, Glycine and 5HT3 receptors and that of the ionotropic Glutamate receptors/Kainate Binding proteins. (iv) The proposal of homology for the N-terminal extracellular domain of metabotropic Glutamate receptors and that of ionotropic Glutamate receptors does not appear to hold.

Neuromuscular development in Drosophila: insights from single neurons and single genes

How a neuron finds its synaptic target is one of the key questions of developmental neurobiology. It is a problem that must, at least in part, be explained in molecular terms. In light of this, several groups have recently examined the synapses that are made between individual motoneurons and muscle fibers in the embryos and larvae of the fruit fly Drosophila melanogaster. The work combines the traditions of small system neurobiology, which is founded on the analysis of singly identified cells, with those of developmental genetics. An important insight emerging from the work is that many of the familiar features of vertebrate synaptogenesis occur in Drosophila, where a rich array of genetic and molecular methods may be readily applied.

Glutamate receptors of Drosophila melanogaster. Primary structure of a putative NMDA receptor protein expressed in the head of the adult fly

The NMDA subtype of ionotropic glutamate receptors has been implicated in the activity-dependent modification of synaptic efficacy in the mammalian brain. Here we describe a cDNA isolated from Drosophila melanogaster which encodes a putative invertebrate NMDA receptor protein (DNMDAR-I). The deduced amino acid sequence of DNMDAR-I displays 46% amino acid identity to the rat NMDAR1 polypeptide and shows significant homology (16-23%) to other vertebrate and invertebrate glutamate receptor proteins. The DNMDAR-I gene maps to position 83AB of chromosome 3R and is highly expressed in the head of adult flies. Our data indicate that the NMDA subtype of glutamate receptors evolved early during phylogeny and suggest the existence of activity-dependent synaptic plasticity in the insect brain.

Stizolobic acid on frog spinal cord; possible species dependent activation of excitatory amino acid receptors

1. We examined the effects of stizolobic acid, an amino acid isolated from a plant, Stizolobium hassjoo, on the binding of [3H]glutamic acid and [3H]kainic acid to synaptosomes from frog spinal cords and on the depolarization at the ventral roots of frog spinal cords. 2. Stizolobic acid inhibited the binding of [3H]kainic acid more potently than that of [3H]glutamic acid. 3. Among stizolobic acid derivatives, 3-Br-stizolobic acid was the most potent inhibitor of the binding of [3H]kainic acid, but the inhibitory potency was 100 times weaker than that of kainic acid. 4. Stizolobic acid and its derivatives could cause depolarization of the ventral root of frog spinal cord in a dose dependent manner, and 3-Br-stizolobic acid was a more potent inducer of depolarization than kainic acid, but the dose dependency of 3-Br-stizolobic acid was a little different from that of kainic acid. 5. The results suggest that stizolobic acid and its analogues act as a kainic acid agonist in frog spinal cord. 6. The present results and others indicate that stizolobic acid may interact with the different types of excitatory amino acid receptors dependent on the species of animals.

Blockade of ionotropic quisqualate receptor desensitization in rat hippocampal neurons by wheatgerm agglutinin and other lectins

Previous experiments with wheatgerm agglutinin, an inhibitor of ionotropic quisqualate receptor desensitization, suggest that desensitization regulates quisqualate receptor-mediated synaptic transmission and excitotoxicity. Using whole-cell recordings from cultured postnatal rat hippocampal neurons, we have examined the wheatgerm agglutinin effect in further detail and compared it to other lectins. Wheatgerm agglutinin and other lectins belonging to the glucose/mannose, N-acetylglucosamine, and sialic acid classes inhibited desensitization. However, wheatgerm agglutinin was the most effective and had the most rapid onset of action. The inhibition was dose-dependent, and it was reduced and reversed by N-acetylglucosamine and N-acetylneuraminic acid. Treating neurons with neuraminidase, which cleaves sialic acid residues from the surface of cells, also diminished the effect. These results suggest that wheatgerm agglutinin reversibly inhibits ionotropic quisqualate receptor desensitization by interacting with carbohydrate residues on or near the quisqualate receptor complex. Future studies using the lectins may help to clarify the functional role of carbohydrate chains on the ionotropic quisqualate receptor.

Molecular biology of excitatory amino acid receptors: subtypes and subunits

Glutamate receptors coupled to ion channels have been named according to their selective agonist: N-methyl-D-Aspartate (NMDA), kainate, quisqualate and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA). The pharmacology of the NMDA receptor is clearly different from that of the kainate, quisqualate and AMPA receptors, thus differentiating two types: NMDA and non-NMDA receptors. Molecular cloning and expression of non-NMDA receptor subunits have now established that the different neuronal responses to kainate, quisqualate and AMPA are mediated by at least two subtypes of ligand-gated channels: one responding to the three ligands, the other responding to kainate and quisqualate but not to AMPA.

Channels formed by M2 peptides of a putative glutamate receptor subunit of locust

A cDNA encoding part of a polypeptide (Loc1) that exhibits similarity to the corresponding portion of the rat GluR1 subunit has been identified by screening an amplified locust cDNA library. This polypeptide is deduced to be missing about 200 amino acids of the amino-terminus and about 100 amino acids of the carboxy-terminus. cDNAs encoding two other glutamate receptor-like polypeptides (Loc2 and Loc3), which both exhibit good sequence homology with Loc1, have also been identified. So far, there is no evidence for 'flip' and 'flop' variants of Loc1, 2 and 3. A 27-mer peptide including the M1 sequence of Loc1 and a 25-mer peptide including the M2 sequence of this putative glutamate receptor subunit have been synthesised and incorporated into artificial bilayers. Channel openings, of minimum conductance 20 pS, were seen more frequently with the M2 peptide. These studies are designed to lead to the isolation of full-length cDNAs for Loc1, 2 and 3 and to the electrophysiological characterisation of their ion transport properties.

Molecular analysis of Drosophila glutamate receptors

Insects and other invertebrates use L-glutamate as a neurotransmitter in the central nervous system and at the neuromuscular junction. In contrast to the well-studied effects of L-glutamate on invertebrate muscle cells, relatively little is known about the physiological role of glutamate receptors (GluRs) in the invertebrate central nervous system. We have applied a molecular cloning approach to elucidate the molecular structure of neuronal and muscle-specific Drosophila glutamate receptor subunits (DGluRs). Several domains conserved between rat GluR subunits and DGluRs indicate regions of high functional significance. Drosophila genetics may now be used as a valuable experimental tool to gain further insight into the role of DGluRs in development, synaptic plasticity and control of gene expression.

Glutamate receptor inhibitors as potential insecticides

Philanthotoxin (PhTX) is a neurotoxic constituent of the paralytic venom of the digger wasp, Philanthus triangulum. PhTX inhibits glutamate receptors of insect muscles mostly as a channel blocker, thereby producing muscle paralysis. Since glutamate receptor blockers may be of value as selective insect control agents, numerous derivatives of PhTX were synthesized and tested for their potencies as inhibitors of insect skeletal muscle glutamate receptors. Structure-activity relationship studies revealed that shortening the polyamine chain length reduced potency, and quaternarization of the nitrogen destroyed it. The potency was increased by a bulky anchoring group with moderate hydrophobicity at the end of the polyamine chain. The conversion of the tryosyl moiety to 3,5-diiodo-tyrosyl also increased potency and so did lengthening the butyryl chain from 4 to 10 carbons. Not only did PhTXs inhibit different subtypes of glutamate receptors, including the mammalian N-methyl-D-aspartate receptor, but also nicotinic receptors of insects and vertebrates. Because of this low selectively, and the hydrophilicity of the derivatives tested, which interferes with their penetration to the target receptor, these compounds cannot be used as insecticides. Nevertheless, the insect skeletal muscle glutamate receptor is a viable target for selective insecticides and major changes in PhTX structure may possibly produce derivatives that can be potential insecticides.

Molluscan ligand-gated ion-channel receptors

In this chapter we introduce the reader to the structures of the different types of ligand-gated ion-channel receptor, and the numerous receptor subtypes that have recently been revealed to exist, in both invertebrate and vertebrate species, by the application of molecular biological methods. We then review some of the data in support of the existence, in molluscs, of such receptor/channel complexes for gamma-aminobutyric acid, glutamate and acetylcholine. Finally, recent results from our laboratory on the cloning and expression of complementary DNAs, from the pond-snail Lymnaea stagnalis, that encode GABA(A) and glutamate receptor subunits will be described.

Glutamate receptors of Drosophila melanogaster: cloning of a kainate-selective subunit expressed in the central nervous system

We report the isolation and functional characterization of cDNAs encoding a Drosophila kainate-selective glutamate receptor. The deduced mature 964-residue protein (DGluR-I) is 108,482 Da and exhibits significant homology to mammalian glutamate receptor subunits. Injection of DGluR-I cRNA into Xenopus oocytes generated kainate-operated ion channels which were blocked by the selective non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione and philanthotoxin. DGluR-I transcripts are differentially expressed during Drosophila development and, in late embryogenesis, accumulate in the central nervous system.

Invertebrate GABA and glutamate receptors: molecular biology reveals predictable structures but some unusual pharmacologies

Determination of the sequences of invertebrate gamma-aminobutyric acid (GABA)-gated and glutamate-gated receptor/ion channels, through the application of recombinant DNA methods, is not just an academic exercise to effect evolutionary comparisons with the sequences of the corresponding vertebrate receptors. The isolation of DNA clones would provide the tools to investigate the exact locations and functional properties of these neurotransmitter receptors within simple nervous systems. In addition, since GABA receptors, at least, have been suggested to be the targets of certain pesticides, the availability of invertebrate receptor cDNAs might provide the agrochemical industry with the basis for 'high-throughput' screening methods for novel pesticidal compounds. Recently, the isolation of molluscan and Drosophila GABA receptor and glutamate receptor cDNAs, and the pharmacological properties of a GABA receptor expressed from one of these clones, have been reported. These studies should stimulate further research into the electrophysiology and pharmacology of native invertebrate ion channel proteins.

Receptor classes and the transmitter-gated ion channels

Transmitter-gated channels, which can be selective for cations or for anions, form an important class among the membrane receptors responsible for signal transduction. Thirteen principal types of these channels can now be recognized and most of these are available for analysis in recombinant form. It is instructive to contrast their characteristic structural features with those of the two other primary classes of the signal-transducing receptors of membranes.

Properties of vertebrate glutamate receptors: calcium mobilization and desensitization

Glutamate is now recognized as a major excitatory neurotransmitter in the vertebrate CNS, participating in a number of physiological and pathological processes. The importance of glutamate in the mobilization of intracellular Ca2+ as well as the relationship between excitatory and toxic properties has made it important to understand factors that regulate the responsivity of glutamate receptors. In recent years considerable insight has been gained about regulatory sites on NMDA receptors, with the recognition that these receptors are modulated by multiple endogenous and exogenous agents. Less is known about the regulation of responses mediated by AMPA, kainate, ACPD or APB receptors. Desensitization represents a potentially powerful means by which glutamate responses may be regulated. Indeed, two agents closely linked to the physiology of NMDA receptors, glycine and Ca2+, appear to modulate different types of desensitization. In the case of glycine, alteration of a rapid form of desensitization may be important in the role of this amino acid as a necessary cofactor for NMDA receptor activation. Additionally, changes in the affinity of the receptor complex for glycine may underlie the use-dependent decline in NMDA responses under certain conditions. Likewise, Ca2+ is a crucial player in the synaptic and toxic effects mediated by NMDA receptors, and is involved in a slower form of desensitization, in effect helping to regulate its own influx into neurons. The site and mechanism of the Ca2+ regulatory effects remain uncertain with evidence supporting both intracellular and ion channel sites of action. A clear role for Ca(2+)-dependent desensitization in the function of NMDA receptors under physiological conditions has not yet been demonstrated. AMPA receptor desensitization has been an area of intense investigation in recent years. The rapidity and degree of this process, coupled with its apparent rapid recovery, has suggested that desensitization is a key mechanism for the short-term regulation of responses mediated by these receptors. Furthermore, rapid desensitization appears to be one factor determining the time course and efficacy of fast excitatory synaptic transmission mediated by AMPA receptors, highlighting the physiological relevance of the process. The molecular mechanisms underlying desensitization remain uncertain. Traditionally, desensitization, like inactivation of voltage-gated channels, has been thought to represent a conformational change in the ion channel complex (Ochoa et al., 1989). However, it is unknown to what extent desensitization, in particular rapid AMPA receptor desensitization, has mechanistic features in common with inactivation. In voltage-gated channels, conformational changes in the channel protein restrict ion flow through the channel (Stuhmer, 1991).(ABSTRACT TRUNCATED AT 400 WORDS)

Mutations in a putative agonist binding region of the AMPA-selective glutamate receptor channel

The region preceding putative transmembrane segment M1 of the glutamate receptor (GluR) channel is well conserved among subunits and has been proposed to constitute a part of the agonist binding site. The functional significance of this region was examined by introducing point mutations into charged residues of the alpha 1 subunit of the mouse alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-selective GluR channel. The dose-response relationships of the mutant receptors were studied after expression in Xenopus oocytes by injection of the mutant alpha 1 subunit-specific mRNA together with the wild-type alpha 2-subunit-specific mRNA. Variable changes in the EC50 values for different agonists were found for the replacement of glutamic acid 398 by lysine and for the replacement of lysine 445 by glutamic acid. These residues may be involved in selective interaction of the GluR channel with agonists.

A novel kainate receptor in the insect nervous system

The pharmacology of excitatory amino acid receptors on an identified neurone in the insect central nervous system was investigated using single electrode recording techniques. Application of kainate, domoate and quisqualate elicited large, slow depolarizations with a rise time of approximately 4 min and recovery time of 23 min. Concentration-response curves were constructed giving an order of potency, domoate greater than quisqualate greater than kainate, and fitted curves demonstrated quisqualate to be a possible partial agonist, compared to domoate and kainate. Agonist responses were insensitive to the antagonists CNQX and picrotoxin and the vertebrate receptor-subtype selective agonists AMPA and trans-ACPD did not elicit any response, suggesting a novel type of excitatory amino acid receptor present on neurones in the insect central nervous system.

Cloning of the genes for excitatory amino acid receptors

Glutamate is the major excitatory neurotransmitter in the mammalian brain, with receptors on every neuron in the central nervous system; it has major roles in fast synaptic transmission and in the establishment of certain forms of memory. More than 20 years ago Olney and his colleagues described the 'Excitotoxic Hypothesis' which postulates that, in addition to its normal function in the healthy brain, glutamate can kill neurons by prolonged, receptor-mediated depolarization resulting in irreversible disturbances in ion homeostasis. Therefore, glutamate is a two-edged sword; in certain undefined, adverse conditions it undergoes a transition from neurotransmitter to neurotoxin. Its toxicity has been implicated in the death of neurons in ischemia, epilepsy, and the neurodegenerative disorders such as Alzheimer's, Huntington's, and Parkinson's disease. Recent advances in the molecular cloning of the genes for the glutamate family of receptors has revealed a plethora of receptor subtypes and an unexpected level of complexity in the mechanisms of receptor expression and function.