Biochemical and immunocytochemical characterization of antipeptide antibodies to a cloned GluR1 glutamate receptor subunit: cellular and subcellular distribution in the rat forebrain

Isolation and characterization culturable microbes on the surface of ‘Granny Smith’ apples treated with electrolyzed water during cold storage

Response of culturable microbes on the surface of apples treated with slightly alkaline electrolyzed water (SAIEW) is largely unexplored. Thus, the aim of this study was to characterize culturable microbes on the surface of SAIEW treated 'Granny Smith' apples using conventional and molecular approach. Results showed that SAIEW treatments and storage duration influenced culturable microbes isolated from the surface of 'Granny Smith' apples stored at 5 °C for 21 days. Enterobacterial repetitive intergenic consensus (ERIC-PCR) analysis distinctively identified 27 groups of bacteria from 56 plate isolates. Using random amplified polymorphic DNA (RAPD-PCR) typing and RAPD1283 primers, 10 distinct band patterns were identified from 30 fungal isolates. Sequencing of 16S rRNA and intergenic spacer (ITS1 and ITS4) region, identified eight bacteria and four fungi, respectively, to species level. Study showed that SAIEW treatment inhibited growth of Staphylococcus epidermidis, S. capitis, Ochrobactrum soli, and Aspergillus inuii on the surface apples during storage.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s10068-022-01148-2.

Zinc as a Neuromodulator in the Central Nervous System with a Focus on the Olfactory Bulb

The olfactory bulb (OB) is central to the sense of smell, as it is the site of the first synaptic relay involved in the processing of odor information. Odor sensations are first transduced by olfactory sensory neurons (OSNs) before being transmitted, by way of the OB, to higher olfactory centers that mediate olfactory discrimination and perception. Zinc is a common trace element, and it is highly concentrated in the synaptic vesicles of subsets of glutamatergic neurons in some brain regions including the hippocampus and OB. In addition, zinc is contained in the synaptic vesicles of some glycinergic and GABAergic neurons. Thus, zinc released from synaptic vesicles is available to modulate synaptic transmission mediated by excitatory (e.g., N-methyl-D aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)) and inhibitory (e.g., gamma-aminobutyric acid (GABA), glycine) amino acid receptors. Furthermore, extracellular zinc can alter the excitability of neurons through effects on a variety of voltage-gated ion channels. Consistent with the notion that zinc acts as a regulator of neuronal activity, we and others have shown zinc modulation (inhibition and/or potentiation) of amino acid receptors and voltage-gated ion channels expressed by OB neurons. This review summarizes the locations and release of vesicular zinc in the central nervous system (CNS), including in the OB. It also summarizes the effects of zinc on various amino acid receptors and ion channels involved in regulating synaptic transmission and neuronal excitability, with a special emphasis on the actions of zinc as a neuromodulator in the OB. An understanding of how neuroactive substances such as zinc modulate receptors and ion channels expressed by OB neurons will increase our understanding of the roles that synaptic circuits in the OB play in odor information processing and transmission.

Pharmacological Rescue of Long-Term Potentiation in Alzheimer Diseased Synapses

Long-term potentiation (LTP) is an activity-dependent and persistent increase in synaptic transmission. Currently available techniques to measure LTP are time-intensive and require highly specialized expertise and equipment, and thus are not well suited for screening of multiple candidate treatments, even in animal models. To expand and facilitate the analysis of LTP, here we use a flow cytometry-based method to track chemically induced LTP by detecting surface AMPA receptors in isolated synaptosomes: fluorescence analysis of single-synapse long-term potentiation (FASS-LTP). First, we demonstrate that FASS-LTP is simple, sensitive, and models electrically induced LTP recorded in intact circuitries. Second, we conducted FASS-LTP analysis in two well-characterized Alzheimer's disease (AD) mouse models (3xTg and Tg2576) and, importantly, in cryopreserved human AD brain samples. By profiling hundreds of synaptosomes, our data provide the first direct evidence to support the idea that synapses from AD brain are intrinsically defective in LTP. Third, we used FASS-LTP for drug evaluation in human synaptosomes. Testing a panel of modulators of cAMP and cGMP signaling pathways, FASS-LTP identified vardenafil and Bay-73-6691 (phosphodiesterase-5 and -9 inhibitors, respectively) as potent enhancers of LTP in synaptosomes from AD cases. These results indicate that our approach could provide the basis for protocols to study LTP in both healthy and diseased human brains, a previously unattainable goal.

SIGNIFICANCE STATEMENT

Learning and memory depend on the ability of synapses to strengthen in response to activity. Long-term potentiation (LTP) is a rapid and persistent increase in synaptic transmission that is thought to be affected in Alzheimer's disease (AD). However, direct evidence of LTP deficits in human AD brain has been elusive, primarily due to methodological limitations. Here, we analyze LTP in isolated synapses from AD brain using a novel approach that allows testing LTP in cryopreserved brain. Our analysis of hundreds of synapses supports the idea that AD-diseased synapses are intrinsically defective in LTP. Further, we identified pharmacological agents that rescue LTP in AD, thus opening up a new avenue for drug screening and evaluation of strategies for alleviating memory impairments.

A parallel panning scheme used for selection of a GluA4-specific Fab targeting the ligand-binding domain

A method for development of murine Fab fragments towards extracellular domains of a surface receptor is presented. The GluA4 ionotropic glutamate receptor is used as a model system. Recombinant GluA4 ectodomain comprising both the N-terminal domain (NTD) and the ligand-binding domain (LBD) in one molecule was used for immunization. A Fab-phage library was constructed and a parallel panning approach enabled selection of murine Fab fragments towards either intact ectodomain or the isolated LBD of the GluA4 receptor. One LBD-Fab (FabL9) showed exclusive selectivity for the GluA4 LBD, over a panel of LBDs from GluA2, GluK1, GluK2 and GluD2. Soluble FabL9 was produced in amounts suitable for characterization. Competitive ELISA and rat-brain immunoprecipitation experiments confirmed that the FabL9 epitope is conserved in the LBD and in the intact native receptor. By an alignment of GluA2 and GluA4, the likely binding epitope for FabL9 was predicted. This study demonstrates a simple approach for development of antibody fragments towards specific sub-domains of a large ligand-gated ion channel, and this method could be utilized for all multi-domain surface receptors where antibody domain-selectivity may be desirable. Furthermore, we present for the first time a GluA4 subtype-specific murine Fab fragment targeting the LBD of the receptor.

AMPARs and synaptic plasticity: the last 25 years

The study of synaptic plasticity and specifically LTP and LTD is one of the most active areas of research in neuroscience. In the last 25 years we have come a long way in our understanding of the mechanisms underlying synaptic plasticity. In 1988, AMPA and NMDA receptors were not even molecularly identified and we only had a simple model of the minimal requirements for the induction of plasticity. It is now clear that the modulation of the AMPA receptor function and membrane trafficking is critical for many forms of synaptic plasticity and a large number of proteins have been identified that regulate this complex process. Here we review the progress over the last two and a half decades and discuss the future challenges in the field.

Awakened by cellular stress: isolation and characterization of a novel population of pluripotent stem cells derived from human adipose tissue

Advances in stem cell therapy face major clinical limitations, particularly challenged by low rates of post-transplant cell survival. Hostile host factors of the engraftment microenvironment such as hypoxia, nutrition deprivation, pro-inflammatory cytokines, and reactive oxygen species can each contribute to unwanted differentiation or apoptosis. In this report, we describe the isolation and characterization of a new population of adipose tissue (AT) derived pluripotent stem cells, termed Multilineage Differentiating Stress-Enduring (Muse) Cells, which are isolated using severe cellular stress conditions, including long-term exposure to the proteolytic enzyme collagenase, serum deprivation, low temperatures and hypoxia. Under these conditions, a highly purified population of Muse-AT cells is isolated without the utilization of cell sorting methods. Muse-AT cells grow in suspension as cell spheres reminiscent of embryonic stem cell clusters. Muse-AT cells are positive for the pluripotency markers SSEA3, TR-1-60, Oct3/4, Nanog and Sox2, and can spontaneously differentiate into mesenchymal, endodermal and ectodermal cell lineages with an efficiency of 23%, 20% and 22%, respectively. When using specific differentiation media, differentiation efficiency is greatly enhanced in Muse-AT cells (82% for mesenchymal, 75% for endodermal and 78% for ectodermal). When compared to adipose stem cells (ASCs), microarray data indicate a substantial up-regulation of Sox2, Oct3/4, and Rex1. Muse-ATs also exhibit gene expression patterns associated with the down-regulation of genes involved in cell death and survival, embryonic development, DNA replication and repair, cell cycle and potential factors related to oncogenecity. Gene expression analysis indicates that Muse-ATs and ASCs are mesenchymal in origin; however, Muse-ATs also express numerous lymphocytic and hematopoietic genes, such as CCR1 and CXCL2, encoding chemokine receptors and ligands involved in stem cell homing. Being highly resistant to severe cellular stress, Muse-AT cells have the potential to make a critical impact on the field of regenerative medicine and cell-based therapy.

MSK1 regulates homeostatic and experience-dependent synaptic plasticity

The ability of neurons to modulate synaptic strength underpins synaptic plasticity, learning and memory, and adaptation to sensory experience. Despite the importance of synaptic adaptation in directing, reinforcing, and revising the behavioral response to environmental influences, the cellular and molecular mechanisms underlying synaptic adaptation are far from clear. Brain-derived neurotrophic factor (BDNF) is a prime initiator of structural and functional synaptic adaptation. However, the signaling cascade activated by BDNF to initiate these adaptive changes has not been elucidated. We have previously shown that BDNF activates mitogen- and stress-activated kinase 1 (MSK1), which regulates gene transcription via the phosphorylation of both CREB and histone H3. Using mice with a kinase-dead knock-in mutation of MSK1, we now show that MSK1 is necessary for the upregulation of synaptic strength in response to environmental enrichment in vivo. Furthermore, neurons from MSK1 kinase-dead mice failed to show scaling of synaptic transmission in response to activity deprivation in vitro, a deficit that could be rescued by reintroduction of wild-type MSK1. We also show that MSK1 forms part of a BDNF- and MAPK-dependent signaling cascade required for homeostatic synaptic scaling, which likely resides in the ability of MSK1 to regulate cell surface GluA1 expression via the induction of Arc/Arg3.1. These results demonstrate that MSK1 is an integral part of a signaling pathway that underlies the adaptive response to synaptic and environmental experience. MSK1 may thus act as a key homeostat in the activity- and experience-dependent regulation of synaptic strength.

Piperazine-2,3-dicarboxylic acid derivatives as dual antagonists of NMDA and GluK1-containing kainate receptors

Competitive N-methyl-d-aspartate receptor (NMDAR) antagonists bind to the GluN2 subunit, of which there are four types (GluN2A-D). We report that some N(1)-substituted derivatives of cis-piperazine-2,3-dicarboxylic acid display improved relative affinity for GluN2C and GluN2D versus GluN2A and GluN2B. These derivatives also display subtype selectivity among the more distantly related kainate receptor family. Compounds 18i and (-)-4 were the most potent kainate receptor antagonists, and 18i was selective for GluK1 versus GluK2, GluK3 and AMPA receptors. Modeling studies revealed structural features required for activity at GluK1 subunits and suggested that S674 was vital for antagonist activity. Consistent with this hypothesis, replacing the equivalent residue in GluK3 (alanine) with a serine imparts 18i antagonist activity. Antagonists with dual GluN2D and GluK1 antagonist activity may have beneficial effects in various neurological disorders. Consistent with this idea, antagonist 18i (30 mg/kg ip) showed antinociceptive effects in an animal model of mild nerve injury.

LTP in hippocampal neurons is associated with a CaMKII-mediated increase in GluA1 surface expression

The use of hippocampal dissociated neuronal cultures has enabled the study of molecular changes in endogenous native proteins associated with long-term potentiation. Using immunofluorescence labelling of the active (Thr286-phosphorylated) alpha-Ca(2+) /calmodulin-dependent protein kinase II (CaMKII) we found that CaMKII activity was increased by transient (3 × 1 s) depolarisation in 18- to 21-day-old cultures but not in 9- to 11-day-old cultures. The increase in Thr286 phosphorylation of CaMKII required the activation of NMDA receptors and was greatly attenuated by the CaMKII inhibitor KN-62. We compared the effects of transient depolarisation on the surface expression of GluA1 and GluA2 subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor and found a preferential recruitment of the GluA1 subunit. CaMKII inhibition prevented this NMDA receptor-dependent delivery of GluA1 to the cell surface. CaMKII activation is therefore an important factor in the activity-dependent recruitment of native GluA1 subunit-containing alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors to the cell surface of hippocampal neurons.

Mapping the ligand binding sites of kainate receptors: molecular determinants of subunit-selective binding of the antagonist [3H]UBP310

Kainate receptors (KARs) modulate synaptic transmission and plasticity, and their dysfunction has been linked to several disease states such as epilepsy and chronic pain. KARs are tetramers formed from five different subunits. GluK1-3 are low affinity kainate binding subunits, whereas GluK4/5 bind kainate with high affinity. A number of these subunits can be present in any given cell type, and different combinations of subunits confer different properties to KARs. Here we report the characterization of a new GluK1 subunit-selective radiolabeled antagonist (S)-1-(2-amino-2-carboxyethyl)-3-(2-carboxythiophene-3-yl-methyl)-5-methylpyrimidine-2,4-dione ([(3)H]UBP310) using human recombinant KARs. [(3)H]UBP310 binds to GluK1 with low nanomolar affinity (K(D) = 21 ± 7 nM) but shows no specific binding to GluK2. However, [(3)H]UBP310 also binds to GluK3 (K(D) = 0.65 ± 0.19 μM) but with ~30-fold lower affinity than that observed for GluK1. Competition [(3)H]UBP310 binding experiments on GluK1 revealed the same rank order of affinity of known GluK1-selective ligands as reported previously in functional assays. Nonconserved residues in GluK1-3 adjudged in modeling studies to be important in determining the GluK1 selectivity of UBP310 were point-mutated to switch residues between subunits. None of the mutations altered the expression or trafficking of KAR subunits. Whereas GluK1-T503A mutation diminished [(3)H]UBP310 binding, GluK2-A487T mutation rescued it. Likewise, whereas GluK1-N705S/S706N mutation decreased, GluK3-N691S mutation increased [(3)H]UBP310 binding activity. These data show that Ala487 in GluK2 and Asn691 in GluK3 are important determinants in reducing the affinity of UBP310 for these subunits. Insights from these modeling and point mutation studies will aid the development of new subunit-selective KAR antagonists.

Disruption of the interaction between myosin VI and SAP97 is associated with a reduction in the number of AMPARs at hippocampal synapses

Myosin VI is an actin-based motor protein that is enriched at the postsynaptic density and appears to interact with alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate-type glutamate receptors (AMPARs) via synapse associated protein 97 (SAP97). Here, we find that a Flag epitope-tagged dominant negative construct that inhibits the interaction between SAP97 and myosin VI (Flag-myoVI-DN) causes a dramatic reduction in the number of synapses and the surface expression of AMPARs in cultured hippocampal neurons. Furthermore, we find that Flag-myoVI-DN also prevents the rapid delivery of AMPARs to synapses that can be induced by the transient activation of N-methyl-d-aspartate receptors. The Flag-myoVI-DN induced decrease in surface AMPARs is not because of reduced AMPAR subunit protein synthesis. Using whole-cell recording, we show that Flag-myoVI-DN also prevents the activity-induced increase in miniature excitatory postsynaptic current frequency that is normally associated with recruitment of AMPARs to the cell surface at synaptic sites that lack these receptors (i.e. 'silent' synapses). Together, these results indicate that myosin VI/SAP97 plays an important role in trafficking and activity-dependent recruitment of AMPARs to synapses.

Nucleus accumbens neurons exhibit synaptic scaling that is occluded by repeated dopamine pre-exposure

Synaptic scaling has been proposed as a form of plasticity that may contribute to drug addiction but it has not been previously demonstrated in the nucleus accumbens (NAc), a critical region for addiction. Here we demonstrate bidirectional synaptic scaling in postnatal rat NAc neurons that were co-cultured with prefrontal cortical neurons to restore excitatory input. Prolonged activity blockade (1-3 days) with an AMPA receptor antagonist increased cell surface (synaptic and extrasynaptic) glutamate receptor 1 (GluR1) and GluR2 but not GluR3, as well as GluR1/2 co-localization on the cell surface and total GluR1 and GluR2 protein levels. A prolonged increase in activity (bicuculline, 48 h) produced opposite effects. These results suggest that GluR1/2-containing AMPA receptors undergo synaptic scaling in NAc neurons. GluR1 and GluR2 surface expression was also increased by tetrodotoxin alone or in combination with an N-methyl-d-aspartate receptor or AMPA receptor antagonist but not by the l-type Ca(2+) channel antagonist nifedipine. A cobalt-quenching assay confirmed the immunocytochemical results indicating that synaptic scaling after activity blockade did not involve a change in abundance of GluR2-lacking AMPA receptors. Increased AMPA receptor surface expression after activity blockade required protein synthesis and was occluded by inhibition of the ubiquitin-proteasome system. Repeated dopamine (DA) treatment, which leads to upregulation of surface GluR1 and GluR2, occluded activity blockade-induced synaptic scaling. These latter results indicate an interaction between cellular mechanisms involved in synaptic scaling and adaptive mechanisms triggered by repeated DA receptor stimulation, suggesting that synaptic scaling may not function normally after exposure to DA-releasing drugs such as cocaine.

Estrogen receptor alpha and beta specific agonists regulate expression of synaptic proteins in rat hippocampus

Changes in hippocampal CA1 dendritic spine density and synaptic number across the estrous cycle in female rats correlate with increased hippocampal-dependent cognitive performance in a manner that is dependent on estrogen receptors (ERs). Two isoforms of the estrogen receptor, alpha and beta are present in the rat hippocampus and distinct effects on cognitive behavior have been described for each receptor. The present study generated a profile of synaptic proteins altered by administration of estradiol benzoate, the ERalpha selective agonist PPT (1,3,5-tris (4-hydroxyphenyl)-4-propyl-1H-pyrazole) and the ERbeta selective agonist DPN (2,3-bis (4-hydroxyphenyl) propionitrile) alone and in combination in comparison to vehicle in the CA1 region of the dorsal hippocampus. In the stratum radiatum, estradiol, DPN, and PPT increased PSD-95 and AMPA-type glutamate receptor subunit GluR1. Only DPN administration regulated expression of AMPA receptor subunits GluR2 and GluR3, increasing and decreasing levels respectively. DPN also increased GluR2 expression in the other lamina of the CA1. These results support previous reports that estradiol and isoform specific agonists differentially activate ERalpha and ERbeta to regulate protein expression. The distinct effects of DPN and PPT administration on synaptic proteins suggest that the desired therapeutic outcome of estrogen may be accomplished by using specific estrogen receptor agonists. Moreover, the effects of estradiol treatment on PSD-95 expression are consistent with a growing body of evidence that this postsynaptic protein is a key marker of estrogen action related to spine synapse formation.

Glutamate receptor dynamics in dendritic microdomains

Among diverse factors regulating excitatory synaptic transmission, the abundance of postsynaptic glutamate receptors figures prominently in molecular memory and learning-related synaptic plasticity. To allow for both long-term maintenance of synaptic transmission and acute changes in synaptic strength, the relative rates of glutamate receptor insertion and removal must be tightly regulated. Interactions with scaffolding proteins control the targeting and signaling properties of glutamate receptors within the postsynaptic membrane. In addition, extrasynaptic receptor populations control the equilibrium of receptor exchange at synapses and activate distinct signaling pathways involved in plasticity. Here, we review recent findings that have shaped our current understanding of receptor mobility between synaptic and extrasynaptic compartments at glutamatergic synapses, focusing on AMPA and NMDA receptors. We also examine the cooperative relationship between intracellular trafficking and surface diffusion of glutamate receptors that underlies the expression of learning-related synaptic plasticity.

2,3-Benzodiazepine-type AMPA receptor antagonists and their neuroprotective effects

AMPA receptors are fast ligand-gated members of glutamate receptors in neuronal and many types of non-neuronal cells. The heterotetramer complexes are assembled from four subunits (GluR1-4) in region-, development- and function-selective patterns. Each subunit contains three extracellular domains (a large amino terminal domain, an agonist-binding domain and a transducer domain), and three transmembrane segments with a loop (pore forming domain), as well as the intracellular carboxy terminal tail (traffic and conductance regulatory domain). The binding of the agonist (excitatory amino acids and their derivatives) initiates conformational realignments, which transmit to the transducer domain and membrane spanning segments to gate the channel permeable to Na+, K+ and more or less to Ca2+. Several 2,3-benzodiazepines act as non-competitive antagonists of the AMPA receptor (termed also negative allosteric modulators), which are thought to bind to the transducer domains and inhibit channel gating. Analysing their effects in vitro, it has been possible to recognize a structure-activity relationship, and to describe the critical parts of the molecules involved in their action at AMPA receptors. Blockade of AMPA receptors can protect the brain from apoptotic and necrotic cell death by preventing neuronal excitotoxicity during pathophysiological activation of glutamatergic neurons. Animal experiments provided evidence for the potential usefulness of non-competitive AMPA antagonists in the treatment of human ischemic and neurodegenerative disorders including stroke, multiple sclerosis, Parkinson's disease, periventricular leukomalacia and motoneuron disease. 2,3-benzodiazepine AMPA antagonists can protect against seizures, decrease levodopa-induced dyskinesia in animal models of Parkinson's disease demonstrating their utility for the treatment of a variety of CNS disorders.

Blockade of AMPA-receptors attenuates 4-aminopyridine seizures, decreases the activation of inhibitory neurons but is ineffective against seizure-related astrocytic swelling

The neurotransmitter glutamate plays a pivotal role in the development of the neuropathological sequelae following acute seizures. Our previous data proved the efficacy of the NMDA-receptor antagonists on the symptoms, survival and neuronal activation in the 4-aminopyridine- (4-AP) induced seizures. In this study, we examined the effects of two different doses of a non-competitive, selective, allosteric AMPA-receptor antagonist, GYKI 52466. GYKI 52466 was effective in prolonging the latency to generalised seizures and reduction of seizure mortality. However, the effects on neuronal c-fos expression and astrocyte swelling were complex. The 25mg/kg dose of GYKI 52466 was effective in reducing the c-fos immunoreactivity (IR) in the hippocampus only. In the neocortex the overall c-fos-IR cell counts were increased significantly. Investigation of the neocortical parvalbumin-containing interneuron population proved that GYKI 52466 decreased c-fos expression. The 50mg/kg dose of GYKI 52466 significantly reduced the c-fos-IR in the neo- and allocortex, not only in principal neurons, but also in the parvalbumin-positive interneurons. The GYKI 52466-pretreatment did not prevent the astrocyte swelling in the investigated cortical areas; thus we conclude that the AMPA-receptors have little if any involvement in the in the mediation of neuropathological alterations in acute convulsions.

mGluR5 is involved in dendrite differentiation and excitatory synaptic transmission in NTERA2 human embryonic carcinoma cell-derived neurons

The pluripotent human embryonic carcinoma cell line NTERA2 readily differentiates into neurons when exposed to retinoic acid in vitro. These neurons show characteristic morphology with long processes and they express neuronal markers TUJ-1 and NeuN. NTERA2-derived neurons can regulate Ca2+ signalling through ionotropic glutamate (iGluR) and muscarinic receptors (mAChRs). Little is known, however, about the role of metabotropic glutamate receptors (mGluRs) in these neurons. Here we show that NTERA2-derived neurons express functional mGluR5, which is involved in Ca2+ signalling. Blocking mGluR5 activity at early stages of differentiation leads to fewer dendrites and a reduction in miniature excitatory postsynaptic currents (mEPSCs). Furthermore, cells cultured in the presence of the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) show reduced N-methyl-D-aspartate (NMDA) receptor-mediated Ca2+ mobilisation but increased alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor Ca2+ permeability. During normal neuronal development, the edited GluR2 renders AMPARs Ca2+ impermeable. The increased Ca2+ permeability of AMPARs in MPEP-treated neurons is due to the reduced expression of GluR2 subunit protein. Thus, mGluR5 activity at early stages of differentiation is likely to play a role in the development of multipotent cell-derived neurons.

Tyrosine phosphatases regulate AMPA receptor trafficking during metabotropic glutamate receptor-mediated long-term depression

Two forms of long-term depression (LTD), triggered by activation of NMDA receptors (NMDARs) and metabotropic glutamate receptors (mGluRs), respectively, can be induced at CA1 synapses in the hippocampus. Compared with NMDAR-LTD, relatively little is known about mGluR-LTD. Here, we show that protein tyrosine phosphatase (PTP) inhibitors, orthovanadate and phenylarsine oxide, selectively block mGluR-LTD induced by application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG-LTD), because NMDAR-LTD is unaffected by these inhibitors. Furthermore, DHPG-LTD measured using whole-cell recording is similarly blocked by either bath-applied or patch-loaded PTP inhibitors. These inhibitors also block the changes in paired-pulse facilitation and coefficient of variation that are associated with the expression of DHPG-LTD. DHPG treatment of hippocampal slices was associated with a decrease in the level of tyrosine phosphorylation of GluR2 AMPA receptor (AMPAR) subunits, an effect blocked by orthovanadate. Finally, in dissociated hippocampal neurons, orthovanadate blocked the ability of DHPG to reduce the number of AMPA receptor clusters on the surface of dendrites. Again, the effects of PTP blockade were selective, because NMDA-induced decreases in surface AMPAR clusters was unaffected by orthovanadate. Together, these data suggest that activation of postsynaptic PTP results in tyrosine dephosphorylation of AMPARs and their removal from the synapse.

Changes in glutamate receptor subunit 4 expression in the deafferented olfactory bulb of zebrafish

The distribution of ionotropic glutamate receptor subunit 4 (iGluR4) was examined in both normal and deafferented olfactory bulbs of adult zebrafish, Danio rerio. With the exception of the olfactory nerve layer, there was extensive labeling with antibodies to iGluR4 in the olfactory bulbs, specifically in juxtaglomerular cell bodies and their processes. These results are consistent with previous work, which has suggested differential distribution of glutamate receptors in the vertebrate olfactory system. Analysis of bulbs following olfactory organ removal revealed a significant loss of iGluR4 immunoreactivity by 24 h post-deafferentation. At 48 h after denervation, iGluR4 labeling had returned to normal levels and was retained through 3 weeks post-surgery. Thus, afferent input plays a role in reduced labeling of this protein immediately following injury, but return of immunoreactivity can occur even without sensory innervation.

Activation of metabotropic glutamate receptors does not alter the phosphorylation state of GluR1 AMPA receptor subunit at serine 845 in perirhinal cortical neurons

Activation of group I and group II metabotropic glutamate receptors (mGluRs) is thought to be required for long-term depression (LTD) of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor-mediated synaptic transmission in the perirhinal cortex. However, little is known about how activation of mGluRs leads to this form of synaptic plasticity. AMPA receptor phosphorylation has been implicated in several forms of modulation of synaptic transmission. In the CA1 area of the hippocampus, N-methyl-d-aspartate (NMDA) receptor-dependent LTD is associated with the reduced phosphorylation of the GluR1 AMPA receptor subunit at serine 845 (GluR1-S845). Immunoblot analysis of perirhinal cortical neurons using GluR1 and GluR1-S845 phosphorylation state specific antibodies showed that stimulation of adenylyl cyclase (AC) with forskolin (FSK) dramatically increased PKA-mediated phosphorylation of GluR1-S845. However, selective or simultaneous application of mGluR5 agonist (S)-3,5-dihydroxyphenylglycine (CHPG) and mGluR2/3 agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) did not produce detectable changes in GluR1-S845 phosphorylation. These results indicate that in the perirhinal cortex mGluR activation does not alter the phosphorylation state of GluR1-S845. Therefore, it is likely that the process involved in the modification of AMPA receptors in mGluR activation dependent LTD in the perirhinal cortex is mechanistically distinct from NMDA receptor-mediated LTD described in hippocampal neurons.

A Comparison of GluR-A-Deficient and Wild-Type Mice on a Test Battery Assessing Sensorimotor, Affective, and Cognitive Behaviors

Previous studies have demonstrated a spatial working memory deficit in glutamate receptor (GluR)-A (GluR1) AMPA receptor subunit knockout mice. The present study evaluated male and female wild-type and GluR-A-/- mice on a test battery that assessed sensorimotor, affective, and cognitive behaviors. Results revealed a behavioral phenotype more extensive than previously described. GluR-A-/- mice were hyperactive, displayed a subtle lack of motor coordination, and were generally more anxious than wild-type controls. In addition, they showed a deficit in spontaneous alternation, consistent with previous reports of a role for GluR-A-dependent plasticity in hippocampus-dependent, spatial working memory. Although changes in motor coordination or anxiety cannot explain the dissociations already reported within the spatial memory domain, it is clear that they could significantly affect interpretation of results obtained in other kinds of behavioral tasks.

A juvenile form of postsynaptic hippocampal long-term potentiation in mice deficient for the AMPA receptor subunit GluR-A

In adult mice, long-term potentiation (LTP) of synaptic transmission at CA3-to-CA1 synapses induced by tetanic stimulation requires L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors containing GluR-A subunits. Here, we report a GluR-A-independent form of LTP, which is comparable in size to LTP in wild-type mice at postnatal day 14 (P14) but diminishes between P14 and P42 in brain slices of GluR-A-deficient mice. The GluR-A-independent form of LTP is sensitive to D(-)-2-amino-5-phosphonopentanoic acid (D-AP5), but lacks short-term potentiation (STP) and can also be observed in the pairing induction protocol. As judged by unaltered paired-pulse facilitation, this LTP form is postsynaptically expressed despite depleted extrasynaptic AMPA receptor pools with reduced levels of GluR-B, which accumulates in somata and synapses of CA1 pyramidal neurons in GluR-A-deficient mice. Our results show that in the developing hippocampus synaptic plasticity can be expressed by AMPA receptors lacking the GluR-A subunit.

Neuroadaptive processes in GABAergic and glutamatergic systems in benzodiazepine dependence

Knowledge of the neural mechanisms underlying the development of benzodiazepine (BZ) dependence remains incomplete. The gamma-aminobutyric acid (GABA(A)) receptor, being the main locus of BZ action, has been the main focus to date in studies performed to elucidate the neuroadaptive processes underlying BZ tolerance and withdrawal in preclinical studies. Despite this intensive effort, however, no clear consensus has been reached on the exact contribution of neuroadaptive processes at the level of the GABA(A) receptor to the development of BZ tolerance and withdrawal. It is likely that changes at the level of this receptor are inadequate in themselves as an explanation of these neuroadaptive processes and that neuroadaptations in other receptor systems are important in the development of BZ dependence. In particular, it has been hypothesised that as part of compensatory mechanisms to diazepam-induced chronic enhancement of GABAergic inhibition, excitatory mechanisms (including the glutamatergic system) become more sensitive [Behav. Pharmacol. 6 (1995) 425], conceivably contributing to BZ tolerance development and/or expression of withdrawal symptoms on cessation of treatment, including increased anxiety and seizure activity. Glutamate is a key candidate for changes in excitatory transmission mechanisms and BZ dependence, (1) since there are defined neuroanatomical relationships between glutamatergic and GABAergic neurons in the CNS and (2) because of the pivotal role of glutamatergic neurotransmission in mediating many forms of synaptic plasticity in the CNS, such as long-term potentiation and kindling events. Thus, it is highly possible that glutamatergic processes are also involved in the neuroadaptive processes in drug dependence, which can conceivably be considered as a form of synaptic plasticity. This review provides an overview of studies investigating changes in the GABAergic and glutamatergic systems in the brain associated with BZ dependence, with particular attention to the possible differential involvement of N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors in these processes.

AMPA receptor trafficking and long-term potentiation

Activity-dependent changes in synaptic function are believed to underlie the formation of memories. A prominent example is long-term potentiation (LTP), whose mechanisms have been the subject of considerable scrutiny over the past few decades. I review studies from our laboratory that support a critical role for AMPA receptor trafficking in LTP and experience-dependent plasticity.

Functional metabotropic glutamate receptors are expressed in oligodendrocyte progenitor cells

We investigated the expression of metabotropic glutamate receptor (mGluR) isoforms in CG-4 rodent oligodendroglial progenitor cells (OPC) and rat brain oligodendrocytes. Our RT-PCR analysis detected mRNAs for mGluR3 and mGluR5 isoforms in OPCs. Although neurons express both mGluR5a and mGluR5b splice variants, only mGluR5a was identified in OPCs. Antibodies to mGluR2/3 and mGluR5 detected the corresponding receptor proteins in immunoblots of OPC membrane fractions. Furthermore, immunocytochemical analysis identified mGluR5 in oligodendrocyte marker O4-positive OPCs. The expression of mGluR5 was also demonstrated in oligodendrocyte marker (O4 and O1) positive cells in white matter of postnatal 4- and 7-day-old rat brain sections using immunofluorescent double labelling and confocal microscopy. The mGluR5 receptor function was assessed in CG-4 OPCs with fura-2 microfluorometry. Application of the mGluR1/5 specific agonist (S)-3,5-dihydroxyphenylglycine (DHPG) induced calcium oscillations, which were inhibited by the selective mGluR5 antagonist 2-methyl-6-(phenylethynyl) pyridine hydrochloride (MPEP). The DHPG induced calcium oscillations required Ca2+ release from intracellular stores. In OPCs the group II mGluR agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) decreased forskolin-stimulated cAMP synthesis, indicating the presence of functional mGluR3. The newly identified mGluR3 and mGluR5a may be involved in the differentiation of oligodendrocytes, myelination and the development of white matter damage.

Non-competitive NMDA receptor antagonists moderate seizure-induced c-fos expression in the rat cerebral cortex

We examined the effects of non-competitive NMDA glutamate receptor antagonists on seizures elicited by 4-aminopyridine (4-AP), and in particular, on the expression of the transcription factor c-fos induced by these seizures. Induction of c-fos mRNA due to 4-AP-elicited seizures was ascertained by reverse transcription polymerase chain reaction in samples of the neocortex. Adult rats were pretreated with the NMDA receptor antagonists amantadine (40 mg/kg), ketamine (3mg/kg), dizocilpine (MK-801; 1mg/kg) or dextrometorphan (40 mg/kg); 4-AP (5mg/kg) was then injected i.p. Controls were treated with either antagonist only or with 4-AP only. Pretreatment with the antagonists (with the exception of amantadine) increased the latency of behavioural seizures, but not all of the antagonists caused symptomatic seizure protection. In the brains which were processed for Fos immunohistochemistry, quantitative evaluation of immunostained cells was performed in the neocortex and hippocampus. Treatment with either antagonist did not induce by itself c-fos expression, with the exception of amantadine, which caused slight Fos induction in the neocortex. Pretreatment with all the antagonists resulted in decrease of seizure-induced Fos immunoreactivity with respect to non-pretreated animals. Decrease of immunostained cells was significant in the neocortex, in the granule cell layer and hilus of the dentate gyrus, in hippocampal areas CA1 and CA2. MK-801, ketamine and dextrometorphan decreased significantly Fos immunoreactivity also in area CA3. The decrease of Fos immunostaining was not directly correlated with a suppression of behavioural seizures. The results support an important role of NMDA receptors in c-fos gene induction in acute 4-AP seizures.

AMPA receptor trafficking and synaptic plasticity

Activity-dependent changes in synaptic function are believed to underlie the formation of memories. Two prominent examples are long-term potentiation (LTP) and long-term depression (LTD), whose mechanisms have been the subject of considerable scrutiny over the past few decades. Here we review the growing literature that supports a critical role for AMPA receptor trafficking in LTP and LTD, focusing on the roles proposed for specific AMPA receptor subunits and their interacting proteins. While much work remains to understand the molecular basis for synaptic plasticity, recent results on AMPA receptor trafficking provide a clear conceptual framework for future studies.

Peptide dendrimers: applications and synthesis

Peptide dendrimers are radial or wedge-like branched macromolecules consisting of a peptidyl branching core and/or covalently attached surface functional units. The multimeric nature of these constructs, the unambiguous composition and ease of production make this type of dendrimer well suited to various biotechnological and biochemical applications. Applications include use as biomedical diagnostic reagents, protein mimetics, anticancer and antiviral agents, vaccines and drug and gene delivery vehicles. This review focuses on the different types of peptide dendrimers currently in use and the synthetic methods commonly employed to generate peptide dendrimers ranging from stepwise solid-phase synthesis to chemoselective and orthogonal ligation.

Developmental changes in ionotropic glutamate receptors: lessons from hippocampal synapses

Glutamatergic synapses are the primary source of excitatory transmission in the central nervous system (CNS), and their formation is critical in the establishment of neuronal connections. The refinement of these connections occurs during development and also it is postulated during learning and memory. Recent progress in understanding the molecular components of synaptic junctions, together with advances in imaging techniques, has started to offer new insights into the development of excitatory synapses. Studies performed on low-density primary neuronal cultures have enabled dissection of the temporal sequence of events, which have lead to the differentiation of pre- and postsynaptic components. A central feature of the development of excitatory synapses is the accumulation of glutamatergic receptors (GluRs) at the postsynaptic site. These receptors need to be localized and fixed opposite nerve terminals that release glutamate. But for this to occur, neurons require intracellular anchoring molecules, as well as mechanisms that ensure the efficient turnover and transport of receptor proteins. This review focuses on some of the developmental changes observed in the subcellular distribution and molecular organization of AMPA and NMDA type ionotropic GluRs (iGluRs), which mediate the majority of fast excitatory neurotransmission in the CNS.

Transient synaptic activation of NMDA receptors leads to the insertion of native AMPA receptors at hippocampal neuronal plasma membranes

The molecular mechanisms underlying long-term potentiation (LTP) of excitatory synaptic transmission in the hippocampus are not well understood. Transient depolarisation of cultured postnatal hippocampal neurones (3x1 s exposure to 90 mM K+) induces a form of LTP that is manifest primarily as an increase in mEPSC frequency. Site-directed antibodies that recognise an extracellular region of all AMPA receptor (AMPAR) subunits (GluR1-4) were used for the immunolabelling of living neurones. These antibodies were raised in two species to enable sequential immunofluorescent labelling of individual living neurones before and after the induction of LTP. High K+ treatment resulted in the appearance of new AMPAR clusters at sites on the neuronal surface that previously lacked detectable AMPARs. The appearance of new AMPAR clusters was NMDA receptor (NMDAR)-dependent since it was antagonised by the application of NMDAR antagonists. Our data indicate that the transient synaptic activation of NMDARs can lead to the insertion of native AMPARs at sites on the neuronal membrane that initially lacks AMPARs.

Water maze and radial maze learning and the density of binding sites of glutamate, GABA, and serotonin receptors in the hippocampus of inbred mouse strains

Correlations between the densities of ionotropic glutamate, GABA(A), and serotonin binding sites in the hippocampus of seven inbred mouse strains and strain-specific learning capacities in two types of maze were studied. Binding site densities were measured with quantitative receptor autoradiography. Learning capacities were determined in a water maze task as well as in spatial and nonspatial versions of an eight-arm radial maze. The densities of most binding sites differed significantly between the strains in the subfields of Ammon's horn (CA1 and CA3) and the dentate gyrus, except for serotonin binding sites in CA1. By comparing the different strains, significant receptor-behavioral correlations between the densities of the GABA(A) receptors and the activity-dependent behavior in the water maze as well as the spatial learning in the radial maze were found. The densities of D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxalone propionate (AMPA) and kainate receptors correlated positively with learning capacity in the spatial eight-arm radial maze. We conclude that hereditary variations mainly in AMPA, kainate, and GABA(A) receptor densities are involved in behavioral variations in spatial and nonspatial learning tasks.

Developmental changes in synaptic AMPA and NMDA receptor distribution and AMPA receptor subunit composition in living hippocampal neurons

AMPA and NMDA receptors mediate most excitatory synaptic transmission in the CNS. We have developed antibodies that recognize all AMPA or all NMDA receptor variants on the surface of living neurons. AMPA receptor variants were identified with a polyclonal antibody recognizing the conserved extracellular loop region of all four AMPA receptor subunits (GluR1-4, both flip and flop), whereas NMDA receptors were immunolabeled with a polyclonal antibody that binds to an extracellular N-terminal epitope of the NR1 subunit, common to all splice variants. In non-fixed brain sections these antibodies gave labeling patterns similar to autoradiographic distributions with particularly high levels in the hippocampus. Using these antibodies, in conjunction with GluR2-specific and synaptophysin antibodies, we have directly localized and quantified surface-expressed native AMPA and NMDA receptors on cultured living hippocampal neurons during development. Using a quantitative cell ELISA, a dramatic increase was observed in the surface expression of AMPA receptors, but not NMDA receptors, between 3 and 10 d in culture. Immunocytochemical analysis of hippocampal neurons between 3 and 20 d in vitro shows no change in the proportion of synapses expressing NMDA receptors (approximately 60%) but a dramatic increase (approximately 50%) in the proportion of them that also express AMPA receptors. Furthermore, over this period the proportion of AMPA receptor-positive synapses expressing the GluR2 subunit increased from approximately 67 to approximately 96%. These changes will dramatically alter the functional properties of hippocampal synapses.

Afferents from the auditory thalamus synapse on inhibitory interneurons in the lateral nucleus of the amygdala

Physiological studies suggest that afferents to the lateral nucleus of the amygdala (LA) from the auditory thalamus initiate feedforward inhibition [Li et al. (1996b)]. This model of neural processing requires that thalamic afferents synapse directly onto inhibitory interneurons. To determine whether such synaptic contacts occur, we combined anterograde tract tracing with interneuron immunocytochemistry. The anterograde tracer biotinylated dextran amine (BDA) was injected into the auditory thalamus. Inhibitory interneurons in the LA were identified using antibodies directed against gamma aminobutyric acid (GABA) or one of the calcium binding proteins (CBPs), parvalbumin (PARV), calbindin (CALB), or calretinin (CALR), since CBPs identify distinct populations of GABAergic cells within the amygdala. The distribution of GABAergic and CBP interneurons in each subregion of the LA was examined by light microscopy and the relationships between thalamo-amygdala terminals and interneurons were examined by confocal and electron microscopy. Immunoreactive cells were distributed in all three subdivisions of LA, except for CALR-ir neurons, which were sparse in the dorsal subregion and were found mainly in the ventromedial and ventrolateral subregions. Confocal microscopy revealed some thalamo-amygdala terminals in close proximity to LA interneurons, while electron microscopy showed that thalamo-amygdala terminals made direct synaptic contacts onto distal dendritic processes of inhibitory neurons. These data provide morphological evidence that thalamic afferents synapse directly onto inhibitory interneurons in LA, and are consistent with the possibility that inputs from the auditory thalamus initiate feedforward inhibition in LA. This architecture could play an important role in the suppression of background neural noise, thereby enhancing the response of LA cells to incoming auditory stimuli.

Age-dependent pre- and postsynaptic distribution of AMPA receptors at synapses in CA3 stratum radiatum of hippocampal slice cultures compared with intact brain

Organotypic slice cultures of rat hippocampus are widely used as experimental preparations for the study of synaptic plasticity, but their degree of correspondence with intact brain is not fully known. Here, using postembedding immunogold labelling, we describe the ultrastructural distribution of AMPA-type glutamate receptors (GluR1-4) in CA3 stratum radiatum of organotypic hippocampal slice cultures at 10 days to 11 weeks in vitro and compare the labelling with intact brain of corresponding age. In both types of preparation, the 11-week-old samples contained the highest proportion of AMPA receptor-like immunoreactive synapses. The incidence of labelled synapses, however, was higher in vivo (49%) than in vitro (24%). The intensity of labelling (number of gold particles per labelled synapse) also increased with age and was also higher in vivo than in vitro. In both organotypic cultures and intact brain, labelling was frequently found at presynaptic sites, often attached to vesicular structures. The specificity of these findings was supported both by light microscopic immunolabelling of GluR2/3 subunits and by electron microscopic double labelling of different epitopes of the GluR2 subunit. The vesicular localization of AMPA receptors was supported by Western blot analysis of subcellular fractions. Morphological evidence of presynaptic excitatory innervation of glutamatergic neurons supports a functional role for presynaptically located AMPA receptors. Our results therefore suggest that AMPA receptors occur in both pre- and postsynaptic profiles and that the distribution of AMPA receptors in cultured brain slices is fundamentally similar to intact brain, but that synaptic maturation may be retarded in vitro.

AMPA and NMDA receptors: similarities and differences in their synaptic distribution

Recent technical developments, including antigen-retrieval and electron microscopic immunogold methods, are making it possible to determine some of the basic principles governing the subcellular distribution of ionotropic glutamate receptors. Distinct AMPA and NMDA receptor subtypes are selectively targeted to functionally different synapses of a single cell, resulting in an input-selective fine-tuning and regulation of the postsynaptic responses. The amount, density and variability of AMPA receptors at a given glutamatergic synapse is governed by both pre- and postsynaptic factors, resulting in functionally distinct glutamatergic connections that display characteristic patterns of receptor expression.

Target-specific expression of pre- and postsynaptic mechanisms

Target-specific expression of pre- and postsynaptic mechanisms of synaptic transmission has been shown in a variety of central neurons by a number of laboratories. These data have demonstrated that synaptic transmission between single axons diverging onto distinct target neurons can behave independently, differentially influencing activity in the target neuron. Similarly, single neurons are capable of manufacturing molecularly distinct ligand-gated receptors and targeting them to synapses innervated by distinct converging afferent projections. A picture is emerging consistent with a role for both pre- and postsynaptic mechanisms in influencing the target-specific nature of transmission at numerous diverse synapses throughout the mammalian CNS. This target specificity adds another level of complexity in unravelling the roles played by individual neurons within a computational network. To begin to understand the coordinated activity of large ensembles of neurons it is becoming clear that the nature of transmission between individual pre- and postsynaptic elements within a circuit must first be understood for each and every neural element involved.

Interactions between AMPA receptors and intracellular proteins

alpha-Amino-3-hydroxy-5-methylisoxazolepropionate (AMPA) receptors mediate most fast excitatory synaptic transmission in the mammalian CNS. They play a central role in synapse stabilisation and plasticity and their prolonged activation is potently neurotoxic. Developmental and activity-dependent changes in the functional synaptic expression of these receptors are subject to tight cellular regulation. The molecular and cellular mechanisms which control the postsynaptic insertion and arrangement of individual AMPA receptor variants are therefore the subject of intense investigation and in the last two years there has been significant progress towards elucidating some of the processes involved. Much of the new information has come from the application of the yeast two-hybrid assay, which has led to the discovery of a hitherto unexpected complexity of proteins which selectively interact with individual AMPA receptor subunits. These proteins have been implicated in the regulation of AMPA receptor post-translational modification, targeting and trafficking, surface expression and anchoring. The aim of this article is to present an overview of the major interacting proteins described so far and to place these in the context of how they may participate in the well ordered series of events controlling the cell biology of AMPA receptors.

Organization of ionotropic glutamate receptors at dendrodendritic synapses in the rat olfactory bulb

Dendrodendritic synapses between mitral (or tufted) and granule cells of the olfactory bulb play a major role in the processes of odor discrimination and olfactory learning. Release of glutamate at these synapses activates postsynaptic receptors on the dendritic spines of granule cells, as well as presynaptic NMDA receptors in the mitral cell membrane. However, immunocytochemical studies have failed to demonstrate the presence of ionotropic glutamate receptors in granule cell dendrites. By using a postembedding immunogold procedure, we describe here the precise organization of neurotransmitter receptors at dendrodendritic synapses. We show that there is a selective localization of glutamate and GABA receptors at asymmetric and symmetric synaptic junctions, respectively. In addition, we demonstrate that NMDA and AMPA receptors are clustered at postsynaptic specializations on granule cell spines and that they are extensively colocalized. Conversely, glutamate receptors do not appear to be concentrated in clusters on mitral cell dendrites, suggesting that the presynaptic effects of glutamate are mediated by a small complement of extrasynaptic receptors. By analyzing the subsynaptic distribution of the NR1 and GluR2/3 subunits, we show that they are distributed along the entire extent of the postsynaptic specialization, indicating that both NMDA and AMPA receptors are available for dendrodendritic signaling between mitral and granule cells. These results indicate that the principles recently found to underlie the organization of glutamate receptors at axospinous synapses also apply to dendrodendritic synapses.

Expression of 15 glutamate receptor subunits and various splice variants in tissue slices and single neurons of brainstem nuclei and potential functional implications

Brainstem nuclei serve a diverse array of functions in many of which ionotropic glutamate receptors are known to be involved. However, little detailed information is available on the expression of different glutamate receptor subunits in specific nuclei. We used RT-PCR in mice to analyze the glutamate receptor subunit composition of the pre-Bötzinger complex, the hypoglossal nucleus, the nucleus of the solitary tract, and the inferior olive. Analyzing 15 receptor subunits and five variants, we found all four alpha-amino-3-hydroxy-5-methyl-4-propionic acid (AMPA) and six NMDA receptor (NR) subunits as well as three of five kainate (KA) receptors (GluR5, GluR6, and KA1) to be expressed in all nuclei. However, some distinct differences were observed: The inferior olive preferentially expresses flop variants of AMPA receptors, GluR7 is more abundant in the pre-Bötzinger complex than in the other nuclei, and NR2C is most prominent in the nucleus of the solitary tract. In single hypoglossal motoneurons and interneurons of the pre-Bötzinger complex investigation of GluR2 editing revealed strong expression of the GluR2-R editing variant, suggesting low Ca2+ permeability of AMPA receptors. Thus, Ca2+-permeable AMPA receptors are unlikely to be the cause for the reported selective vulnerability of hypoglossal motoneurons during excitotoxic events.

Use of proteoliposomes to generate phage antibodies against native AMPA receptor

To isolate antibodies against ionotropic glutamate receptors (GluRs), we prepared a phage antibody library from mice immunized with proteoliposomes containing purified alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), a selective GluRD receptor. Specific binders were selected by repeated rounds of affinity panning against immobilized GluRD liposomes. Using this approach, we obtained a panel of high-affinity antibody fragments that immunoprecipitated both recombinant and native GluRD receptors, but not GluR6, a kainate receptor subunit with a 40% sequence similarity. The antibody fragments showed subunit selectivity, some being strictly specific for GluRD, whereas others also recognized the GluRB and GluRC but not GluRA subunits. Further experiments indicated that the epitopes recognized were conformational in nature and reside in the N-terminal extracellular 400-residue X domain of GluRD. Our results suggest that proteoliposomes, in combination with phage display technology, provide an effective tool for the generation of high-affinity conformation-sensitive monoclonal antibodies against predetermined membrane proteins.

Mouse brain and muscle tissues constitutively express high levels of Homer proteins

In order to characterize expression of Homers in mouse brain and peripheral tissues we have developed a coupled reverse transcription (RT)-PCR/restriction digestion approach. This has allowed us to determine the molecular composition and relative levels of the constitutive expression of the Homer-1, -2 and -3 mRNAs across mouse tissues. We report here that mammalian brain constitutively expresses high levels of the Homer-1, -2 and -3 mRNAs. Expression of the Homer-1 mRNAs reaches 66% of the brain total Homer mRNAs expression, followed by Homer-3 mRNA (22%) and Homer-2 mRNAs (12%). Quantitative RT-PCR analysis and the Western blotting using pan-Homer antibody revealed that mouse heart, skeletal muscle and diaphragm constitutively express high levels of the Homer proteins and their mRNAs. We have shown that the molecular profile of expression of Homer-1, -2 and -3 mRNAs in muscle containing tissues resembles that obtained for mammalian brain.

Immunogold localization of AMPA and NMDA receptors in somatic sensory cortex of albino rat

We performed an electron microscopic study of S-1 cortex by using postembedding immunogold histochemistry to examine the subcellular distribution of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors (assessed with an antibody recognizing the glutamate receptor 2 and 3 [GluR2 and GluR3] subunits) and to compare this distribution with that of N-methyl-D-aspartate (NMDA) receptors (assessed with an antibody for the NR1 subunit). Both receptors were concentrated at active zones of asymmetric synapses, often directly apposed to presynaptic dense bodies. GluR2/3 showed a bias for long active zones, whereas short active zones expressed GluR2/3 at substantially lower levels; in contrast, labeling for NR1 was independent of synaptic size. Particle counts suggested that synaptic labeling was Poisson distributed and implied that the majority of synapses express both receptors. Quantitative analysis indicates that approximately one-half of synapses express high levels of GluR2/3 and that the remainder express GluR2/3 at a much lower level. Approximately three-fourths of synapses express NR1 at a uniform level; the remainder, which may lack NR1 completely, include synapses with especially large active zones. The present results suggest that the smallest active zones may play a special role in synaptic plasticity.

Association of AMPA receptors with a subset of glutamate receptor-interacting protein in vivo

The NMDA and AMPA classes of ionotropic glutamate receptors are concentrated at postsynaptic sites in excitatory synapses. NMDA receptors interact via their NR2 subunits with PSD-95/SAP90 family proteins, whereas AMPA receptors bind via their GluR2/3 subunits to glutamate receptor-interacting protein (GRIP), AMPA receptor-binding protein (ABP), and protein interacting with C kinase 1 (PICK1). We report here a novel cDNA (termed ABP-L/GRIP2) that is virtually identical to ABP except for additional GRIP-like sequences at the N-terminal and C-terminal ends. Like GRIP (which we now term GRIP1), ABP-L/GRIP2 contains a seventh PDZ domain at its C terminus. Using antibodies that recognize both these proteins, we examined the subcellular localization of GRIP1 and ABP-L/GRIP2 (collectively termed GRIP) and their biochemical association with AMPA receptors. Immunogold electron microscopy revealed the presence of GRIP at excitatory synapses and also at nonsynaptic membranes and within intracellular compartments. The association of native GRIP and AMPA receptors was confirmed biochemically by coimmunoprecipitation from rat brain extracts. A majority of detergent-extractable GluR2/3 was complexed with GRIP in the brain. However, only approximately half of GRIP was associated with AMPA receptors. Unexpectedly, immunocytochemistry of cultured hippocampal neurons and rat brain at the light microscopic level showed enrichment of GRIP in GABAergic neurons and in GABAergic nerve terminals. Thus GRIP is associated with inhibitory as well as excitatory synapses. Collectively, these findings support a role for GRIP in the synaptic anchoring of AMPA receptors but also suggest that GRIP has additional functions unrelated to the binding of AMPA receptors.

A novel two-site enzyme immunoassay reveals the regional distributions of and developmental changes in GluR1 and NMDAR1 protein contents in the rat brain

Glutamate receptors, including the alpha-amino-3-hydroxy-4-methylisoxazole-4-propionic acid (AMPA) and NMDA receptors, play an important role in neural development and synaptic plasticity in the brain. To date, it has been difficult to correlate accurately individual biochemical phenomena with quantitative and qualitative changes in receptors occurring in specific neurons or synapses. In the present study, we established a two-site enzyme immunoassay for two key subunits of the AMPA and NMDA receptors. Its sensitivities were extremely high, 30 pg for GluR1 and 15 pg for the NMDAR1 receptor containing the C2 exon [NMDAR1(C2)], which enabled us to measure their contents in a few milligrams of hippocampal tissue. Regional and developmental variations in receptor protein levels were much more marked than those reported for mRNA: The absolute GluR1 protein content was highest in the rat hippocampus, whereas the NMDAR1(C2) content was high in all the forebrain regions examined. GluR1 protein levels increased most markedly during the second and third weeks of postnatal life, whereas NMDAR1(C2) content increased during the first postnatal week. In the adult rat brain, the ratio of GluR1 protein to NMDAR1 protein was markedly lower in neocortical regions (approximately 2%) and the highest in cerebellum (22%). Therefore, this two-site enzyme immunoassay is a specific and unique method that enables us to measure absolute tissue contents of the glutamate receptors and will lead to further important discoveries on the biochemical alterations of these receptors.

Importance of AMPA receptors for hippocampal synaptic plasticity but not for spatial learning

Gene-targeted mice lacking the L-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunit GluR-A exhibited normal development, life expectancy, and fine structure of neuronal dendrites and synapses. In hippocampal CA1 pyramidal neurons, GluR-A-/- mice showed a reduction in functional AMPA receptors, with the remaining receptors preferentially targeted to synapses. Thus, the CA1 soma-patch currents were strongly reduced, but glutamatergic synaptic currents were unaltered; and evoked dendritic and spinous Ca2+ transients, Ca2+-dependent gene activation, and hippocampal field potentials were as in the wild type. In adult GluR-A-/- mice, associative long-term potentiation (LTP) was absent in CA3 to CA1 synapses, but spatial learning in the water maze was not impaired. The results suggest that CA1 hippocampal LTP is controlled by the number or subunit composition of AMPA receptors and show a dichotomy between LTP in CA1 and acquisition of spatial memory.