A family of glutamate receptor genes: evidence for the formation of heteromultimeric receptors with distinct channel properties

Structure, Function, and Regulation of the Kainate Receptor

Neural communication and modulation are complex processes. Ionotropic glutamate receptors (iGluRs) significantly contribute to mediating the fast-excitatory branch of neurotransmission in the mammalian brain. Kainate receptors (KARs), a subfamily of the iGluRs, act as modulators of the neuronal circuitry by playing important roles at both the post- and presynaptic sites of specific neurons. The functional tetrameric receptors are formed by two different gene families, low agonist affinity (GluK1-GluK3) and high agonist affinity (GluK4-GluK5) subunits. These receptors garnered attention in the past three decades, and since then, much work has been done to understand their localization, interactome, physiological functions, and regulation. Cloning of the receptor subunits (GluK1-GluK5) in the early 1990s led to recombinant expression of kainate receptors in heterologous systems. This facilitated understanding of the functional differences between subunit combinations, splice variants, trafficking, and drug discovery. Structural studies of individual domains and recent full-length homomeric and heteromeric kainate receptors have revealed unique functional mechanisms, which have answered several long-standing questions in the field of kainate receptor biology. In this chapter, we review the current understanding of kainate receptors and associated disorders.

The Mechanisms Behind Rapid Antidepressant Effects of Ketamine: A Systematic Review With a Focus on Molecular Neuroplasticity

The mechanism of action underlying ketamine's rapid antidepressant effects in patients with depression, both suffering from major depressive disorder (MDD) and bipolar disorder (BD), including treatment resistant depression (TRD), remains unclear. Of the many speculated routes that ketamine may act through, restoring deficits in neuroplasticity may be the most parsimonious mechanism in both human patients and preclinical models of depression. Here, we conducted a literature search using PubMed for any reports of ketamine inducing neuroplasticity relevant to depression, to identify cellular and molecular events, relevant to neuroplasticity, immediately observed with rapid mood improvements in humans or antidepressant-like effects in animals. After screening reports using our inclusion/exclusion criteria, 139 publications with data from cell cultures, animal models, and patients with BD or MDD were included (registered on PROSPERO, ID: CRD42019123346). We found accumulating evidence to support that ketamine induces an increase in molecules involved in modulating neuroplasticity, and that these changes are paired with rapid antidepressant effects. Molecules or complexes of high interest include glutamate, AMPA receptors (AMPAR), mTOR, BDNF/TrkB, VGF, eEF2K, p70S6K, GSK-3, IGF2, Erk, and microRNAs. In summary, these studies suggest a robust relationship between improvements in mood, and ketamine-induced increases in molecular neuroplasticity, particularly regarding intracellular signaling molecules.

Astrocytic contribution to glutamate-related central respiratory chemoreception in vertebrates

Central respiratory chemoreceptors play a key role in the respiratory homeostasis by sensing CO₂ and H⁺ in brain and activating the respiratory neural network. This ability of specific brain regions to respond to acidosis and hypercapnia is based on neuronal and glial mechanisms. Several decades ago, glutamatergic transmission was proposed to be involved as a main mechanism in central chemoreception. However, a complete identification of mechanism has been elusive. At the rostral medulla, chemosensitive neurons of the retrotrapezoid nucleus (RTN) are glutamatergic and they are stimulated by ATP released by RTN astrocytes in response to hypercapnia. In addition, recent findings show that caudal medullary astrocytes in brainstem can also contribute as CO₂ and H⁺ sensors that release D-serine and glutamate, both gliotransmitters able to activate the respiratory neural network. In this review, we describe the mammalian astrocytic glutamatergic contribution to the central respiratory chemoreception trying to trace in vertebrates the emergence of several components involved in this process.

Structural biology of kainate receptors

This review summarizes structural studies on kainate receptors that explain unique functional properties of this receptor family. A large number of structures have been solved for ligand binding domain dimer assemblies, giving insight into the subtype selective pharmacology of agonists, antagonists, and allosteric modulators. Structures and biochemical studies on the amino terminal domain reveal mechanisms that play a key role in assembly of heteromeric receptors. Surprisingly, structures of full length homomeric GluK2, GluK3 and heteromeric GluK2/GluK5, receptors reveal a novel structure for the desensitized state that is strikingly different from that for AMPA receptors.

Glutamatergic Signaling in the Central Nervous System: Ionotropic and Metabotropic Receptors in Concert

Glutamate serves as both the mammalian brain's primary excitatory neurotransmitter and as a key neuromodulator to control synapse and circuit function over a wide range of spatial and temporal scales. This functional diversity is decoded by two receptor families: ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs). The challenges posed by the complexity and physiological importance of each of these subtypes has limited our appreciation and understanding of how these receptors work in concert. In this review, by comparing both receptor families with a focus on their crosstalk, we argue for a more holistic understanding of neural glutamate signaling.

TAK-137, an AMPA-R potentiator with little agonistic effect, has a wide therapeutic window

Activation of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPA-R) is a promising strategy to treat psychiatric and neurological diseases if issues of bell-shaped response and narrow safety margin against seizure can be overcome. Here, we show that structural interference at Ser743 in AMPA-R is a key to lower the agonistic effect of AMPA-R potentiators containing dihydropyridothiadiazine 2,2-dioxides skeleton. With this structural insight, TAK-137, 9-(4-phenoxyphenyl)-3,4-dihydropyrido[2,1-c][1,2,4]thiadiazine 2,2-dioxide, was discovered as a novel AMPA-R potentiator with a lower agonistic effect than an AMPA-R potentiator LY451646 ((R)-N-(2-(4'-cyanobiphenyl-4-yl)propyl)propane-2-sulfonamide) in rat primary neurons. TAK-137 induced brain-derived neurotrophic factor in neurons in rodents and potently improved cognition in both rats and monkeys. Compared to LY451646, TAK-137 had a wider safety margin against seizure in rats. TAK-137 enhanced neural progenitor proliferation over a broader range of doses in rodents. Thus, TAK-137 is a promising AMPA-R potentiator with potent procognitive effects and lower risks of bell-shaped response and seizure. These data may open the door for the development of AMPA-R potentiators as therapeutic drugs for psychiatric and neurological diseases.

Electron Acceptive Mass Tag for Mass Spectrometric Imaging-Guided Synergistic Targeting to Mice Brain Glutamate Receptors

Dysfunctional glutamate receptors (GluRs) have been implicated in neurological disorders and injuries. Hetero-tetrameric assemblies of different GluR subunits or splicing variants have distinct spatiotemporal expression patterns and pharmacological properties. Mass spectrometric imaging of GluRs-targeted small molecules is important for determining the regional preferences of these compounds. We report herein the development of a mass tag covalently bonded with glutamate or N-methyl-d-aspartate that functions as both an electron acceptor to generate mass spectrometric signals on irradiated (Bi₂O₃)_0.07(CoO)_0.03(ZnO)_0.9 nanoparticles with the third harmonic (355 nm) of Nd³⁺:YAG laser and as the core component to target bilobed clamshell-like structures of GluRs. In this approach, different molecules produce the same tag ion. It provides a new avenue for quantitative assessment of spatial densities of different compounds, which cannot be achieved with well-established stable isotope labeling technique due to different ionization efficiency of different compounds. Various coexisting endogenous molecules are also simultaneously detected for investigation of overall physiological changes induced by these compounds. Because semiconductors do not generate background peaks, this method eliminates interferences from organic matrix materials that are used in regular MALDI (matrix assisted laser desorption ionization). The localized ionization provides high spatial resolution that can be down to sub-micrometers.

AMPA Receptor-Dependent Glutamatergic Signaling is Present in the Carotid Chemoreceptor

Exposure to both sustained and intermittent hypoxia for as little as a day produces sustained augmentation of carotid chemoreceptor sensitivity; however, the molecular basis for this chemoreflex plasticity remains uncertain. We previously reported that NMDA receptor-dependent glutamatergic signaling in rat carotid body played a role in altered hypoxic sensitivity after exposure to cyclic intermittent hypoxia (CIH). Here we found that mRNAs of multiple AMPA and Kainate glutamate receptors were expressed in rat carotid body. The AMPA receptor subunit GluR1 showed intense immunoreactivity in the carotid body, co-localizing with tyrosine hydroxylase in type I cells. Treatment of rat carotid body-derived primary cells with AMPA activated ERK1/2 in a time-dependent manner. Exposing Sprague-Dawley rats to CIH for 8 h/day for 3 weeks significantly enhanced the expression level of GluA1 mRNA as well as GluR1 protein in the carotid body. In addition, our results showed that multiple of vesicular glutamate transporters (VGLUTs) and excitatory amino acid transporters (EAATs) were expressed in the rat carotid body, indicating that glutamate might be as a neurotransmitter stored, released and uptake in the carotid body. Finally, we found that mRNAs of GluA1, GluA2 and GluA3 as well as PSD-95-like membrane-associated granulate kinase family members, PSD-95, PSD-93, and SAP97, were expressed in human carotid body. Our data suggest AMPA receptor-dependent glutamatergic signaling is present in the carotid body and might be involved in the carotid chemoreceptor response to hypoxia.

HBT1, a Novel AMPA Receptor Potentiator with Lower Agonistic Effect, Avoided Bell-Shaped Response in In Vitro BDNF Production

α-Amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptor (AMPA-R) potentiators with brain-derived neurotrophic factor (BDNF)-induction potential could be promising as therapeutic drugs for neuropsychiatric and neurologic disorders. However, AMPA-R potentiators such as LY451646 have risks of narrow bell-shaped responses in pharmacological effects, including in vivo BDNF induction. Interestingly, LY451646 and LY451395, other AMPA-R potentiators, showed agonistic effects and exhibited bell-shaped responses in the BDNF production in primary neurons. We hypothesized that the agonistic property is related to the bell-shaped response and endeavored to discover novel AMPA-R potentiators with lower agonistic effects. LY451395 showed an agonistic effect in primary neurons, but not in a cell line expressing AMPA-Rs, in Ca²⁺ influx assays; thus, we used a Ca²⁺ influx assay in primary neurons and, from a chemical library, discovered two AMPA-R potentiators with lower agonistic effects: 2-(((5-methyl-3-(trifluoromethyl)-1H-pyrazol-1-yl)acetyl)amino)-4,5,6,7-tetrahydro-1-benzothiophene-3-carboxamide (HBT1) and (3S)-1-(4-tert-butylphenyl)-N-((1R)-2-(dimethylamino)-1-phenylethyl)-3-isobutyl-2-oxopyrrolidine-3-carboxamide (OXP1). In a patch-clamp study using primary neurons, HBT1 showed little agonistic effect, whereas both LY451395 and OXP1 showed remarkable agonistic effects. HBT1, but not OXP1, did not show remarkable bell-shaped response in BDNF production in primary neurons. HBT1 bound to the ligand-binding domain (LBD) of AMPA-R in a glutamate-dependent manner. The mode of HBT1 and LY451395 binding to a pocket in the LBD of AMPA-R differed: HBT1, but not LY451395, formed hydrogen bonds with S518 in the LBD. OXP1 may bind to a cryptic binding pocket on AMPA-R. Lower agonistic profile of HBT1 may associate with its lower risks of bell-shaped responses in BDNF production in primary neurons.

Hippocampal GABAergic Inhibitory Interneurons

In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10-15% of the total neuronal population; however, their remarkable anatomical and physiological diversity allows them to regulate virtually all aspects of cellular and circuit function. Here we provide an overview of the current state of the field of interneuron research, focusing largely on the hippocampus. We discuss recent advances related to the various cell types, including their development and maturation, expression of subtype-specific voltage- and ligand-gated channels, and their roles in network oscillations. We also discuss recent technological advances and approaches that have permitted high-resolution, subtype-specific examination of their roles in numerous neural circuit disorders and the emerging therapeutic strategies to ameliorate such pathophysiological conditions. The ultimate goal of this review is not only to provide a touchstone for the current state of the field, but to help pave the way for future research by highlighting where gaps in our knowledge exist and how a complete appreciation of their roles will aid in future therapeutic strategies.

Diverse roles for ionotropic glutamate receptors on inhibitory interneurons in developing and adult brain

Glutamate receptor-mediated recruitment of GABAergic inhibitory interneurons is a critical determinant of network processing. Early studies observed that many, but not all, interneuron glutamatergic synapses contain AMPA receptors that are GluA2-subunit lacking and Ca(2+) permeable, making them distinct from AMPA receptors at most principal cell synapses. Subsequent studies demonstrated considerable alignment of synaptic AMPA and NMDA receptor subunit composition within specific subtypes of interneurons, suggesting that both receptor expression profiles are developmentally and functionally linked. Indeed glutamate receptor expression profiles are largely predicted by the embryonic origins of cortical interneurons within the medial and caudal ganglionic eminences of the developing telencephalon. Distinct complements of AMPA and NMDA receptors within different interneuron subpopulations contribute to the differential recruitment of functionally divergent interneuron subtypes by common afferent inputs for appropriate feed-forward and feedback inhibitory drive and network entrainment. In contrast, the lesser-studied kainate receptors, which are often present at both pre- and postsynaptic sites, appear to follow an independent developmental expression profile. Loss of specific ionotropic glutamate receptor (iGluR) subunits during interneuron development has dramatic consequences for both cellular and network function, often precipitating circuit inhibition-excitation imbalances and in some cases lethality. Here we briefly review recent findings highlighting the roles of iGluRs in interneuron development.

Evolutionary and Expression Analysis Provides Evidence for the Plant Glutamate-like Receptors Family is Involved in Woody Growth-related Function

Glutamate-like receptors (GLRs) is a highly conserved family of ligand-gated ion channels, which have been associated with various physiological and developmental processes. Here, we investigated the evolutionary pattern of GLRs in plants. We observed that tandem duplications occupied the largest proportion of the plant GLR gene family expansion. Based on a phylogenetic tree, we suggested a new subfamily, GLR4, which is widespread in angiosperm but absence on Brassicales. Meanwhile, because GLR1 and GLR2 subfamilies were potential sister clades, we combined them into the GLR1&2 subfamily. A comparative analysis of plant GLR subfamilies revealed that selective forces shaped the GLR1&2 repertoires in the stems of eudicotyledons with distinct functional preferences. Moreover, GLR1&2 formed a species-specific highwoody-expanded subfamily, with preferential expression in the cambial-enriched and shoot apical meristem fractions of the highwood species. Together, these findings lay the foundation for evolutionary analysis of plant GLRs over the entire plant timescale and identified unique targets for manipulating the woody-growth behaviours of plant GLRs.

REVIEW ■ : How Glutamate Receptors Are Built

Glutamate was shown to excite central neurons almost 40 years ago, but it was not until the mid-1980s that it was widely accepted as a neurotransmitter in the mammalian CNS. In the past decade, the ability to make high-resolution electrophysiological recordings from CNS neurons and the application of molecular biology techniques to the study of glutamate receptors has begun to elucidate the relationship between the structure of these receptors and their functional characteristics. Somewhat surprisingly, these investigations have shown that the ionotropic glutamate receptors make up a novel family of ligand-gated ion channels. Recent work has revealed the protein domains involved in ion permeation and ligand binding, and has begun to identify structural elements involved in channel gating, especially receptor desensitization. Additional se quence motifs have been found that are important for the synaptic localization of glutamate-receptor sub units. Although the subunit composition and stoichiometry of native receptors is still partially unresolved, work over the past decade has shown that the glutamate receptor family exhibits an unexpectedly rich diversity and that the regulation of the structure and function of these receptors is both complex and highly dynamic. NEUROSCIENTIST 5:311-323, 1999

Changes in synaptic plasticity and expression of glutamate receptor subunits in the CA1 and CA3 areas of the hippocampus after transient global ischemia

Excess glutamate release from the presynaptic membrane has been thought to be the major cause of ischemic neuronal death. Although both CA1 and CA3 pyramidal neurons receive presynaptic glutamate input, transient cerebral ischemia induces CA1 neurons to die while CA3 neurons remain relatively intact. This suggests that changes in the properties of pyramidal cells may be the main cause related to ischemic neuronal death. Our previous studies have shown that the densities of dendritic spines and asymmetric synapses in the CA1 area are increased at 12h and 24h after ischemia. In the present study, we investigated changes in synaptic structures in the CA3 area and compared the expression of glutamate receptors in the CA1 and CA3 hippocampal regions of rats after ischemia. Our results demonstrated that the NR2B/NR2A ratio became larger after ischemia although the expression of both the NR2B subunit (activation of apoptotic pathway) and NR2A subunit (activation of survival pathway) decreased in the CA1 area from 6h to 48h after reperfusion. Furthermore, expression of the GluR2 subunit (calcium impermeable) of the AMPA receptor class significantly decreased while the GluR1 subunit (calcium permeable) remained unchanged at the same examined reperfusion times, which subsequently caused an increase in the GluR1/GluR2 ratio. Despite these notable differences in subunit expression, there were no obvious changes in the density of synapses or expression of NMDAR and AMPAR subunits in the CA3 area after ischemia. These results suggest that delayed CA1 neuronal death may be related to the dramatic fluctuation in the synaptic structure and relative upregulation of NR2B and GluR1 subunits induced by transient global ischemia.

Sigma receptors [σRs]: biology in normal and diseased states

This review compares the biological and physiological function of Sigma receptors [σRs] and their potential therapeutic roles. Sigma receptors are widespread in the central nervous system and across multiple peripheral tissues. σRs consist of sigma receptor one (σ₁R) and sigma receptor two (σ₂R) and are expressed in numerous regions of the brain. The sigma receptor was originally proposed as a subtype of opioid receptors and was suggested to contribute to the delusions and psychoses induced by benzomorphans such as SKF-10047 and pentazocine. Later studies confirmed that σRs are non-opioid receptors (not an µ opioid receptor) and play a more diverse role in intracellular signaling, apoptosis and metabolic regulation. σ₁Rs are intracellular receptors acting as chaperone proteins that modulate Ca²⁺ signaling through the IP₃ receptor. They dynamically translocate inside cells, hence are transmembrane proteins. The σ₁R receptor, at the mitochondrial-associated endoplasmic reticulum membrane, is responsible for mitochondrial metabolic regulation and promotes mitochondrial energy depletion and apoptosis. Studies have demonstrated that they play a role as a modulator of ion channels (K⁺ channels; N-methyl-d-aspartate receptors [NMDAR]; inositol 1,3,5 triphosphate receptors) and regulate lipid transport and metabolism, neuritogenesis, cellular differentiation and myelination in the brain. σ₁R modulation of Ca²⁺ release, modulation of cardiac myocyte contractility and may have links to G-proteins. It has been proposed that σ₁Rs are intracellular signal transduction amplifiers. This review of the literature examines the mechanism of action of the σRs, their interaction with neurotransmitters, pharmacology, location and adverse effects mediated through them.

Polymorphisms in GRIA1 gene are a risk factor for asparaginase hypersensitivity during the treatment of childhood acute lymphoblastic leukemia

l-asparaginase is an effective antineoplastic agent used in chemotherapy of acute lymphoblastic leukemia. The drug effect may be compromised by an elicited immune response, resulting in the production of anti-asparaginase antibodies causing an anaphylactic reaction or silent inactivation of the enzyme. To elucidate possible genetic predisposition for inter-individual differences in asparaginase hypersensitivity, we studied single nucleotide polymorphisms (SNPs) in the GRIA1 gene in 146 pediatric patients treated with l-asparaginase. Allergic reaction to l-asparaginase occurred in 49.3% of patients. We observed a statistically significant association between SNPs in the GRIA1 gene and the occurrence of asparaginase allergy: rs4958351 with p = 0.003, rs4958676 with p = 0.005, rs6889909 with p = 0.005, rs6890057 with p = 0.005 and rs10070447 with p = 0.006. We found a statistically significant correlation between asparaginase allergy and event-free survival (p-value 0.005).

Retour aux sources: defining the structural basis of glutamate receptor activation

Ionotropic glutamate receptors (iGluRs) are the major excitatory neurotransmitter receptor in the vertebrate CNS and, as a result, their activation properties lie at the heart of much of the neuronal network activity observed in the developing and adult brain. iGluRs have also been implicated in many nervous system disorders associated with postnatal development (e.g. autism, schizophrenia), cerebral insult (e.g. stroke, epilepsy), and disorders of the ageing brain (e.g. Alzheimer's disease, Parkinsonism). In view of this, an emphasis has been placed on understanding how iGluRs activate and desensitize in functional and structural terms. Early structural models of iGluRs suggested that the strength of the agonist response was primarily governed by the degree of closure induced in the ligand-binding domain (LBD). However, recent studies have suggested a more nuanced role for the LBD with current evidence identifying the iGluR LBD interface as a "hotspot" regulating agonist behaviour. Such ideas remain to be consolidated with recently solved structures of full-length iGluRs to account for the global changes that underlie channel activation and desensitization.

FOXO/TXNIP pathway is involved in the suppression of hepatocellular carcinoma growth by glutamate antagonist MK-801

Background

Accumulating evidence has suggested the importance of glutamate signaling in cancer growth, yet the signaling pathway has not been fully elucidated. N-methyl-D-aspartic acid (NMDA) receptor activates intracellular signaling pathways such as the extracellular-signal-regulated kinase (ERK) and forkhead box, class O (FOXO). Suppression of lung carcinoma growth by NMDA receptor antagonists via the ERK pathway has been reported. However, series of evidences suggested the importance of FOXO pathways for the regulation of normal and cancer cell growth. In the liver, FOXO1 play important roles for the cell proliferation such as hepatic stellate cells as well as liver metabolism. Our aim was to investigate the involvement of the FOXO pathway and the target genes in the growth inhibitory effects of NMDA receptor antagonist MK-801 in human hepatocellular carcinoma.

Methods

Expression of NMDAR1 in cancer cell lines from different tissues was examined by Western blot. NMDA receptor subunits in HepG2, HuH-7, and HLF were examined by reverse transcriptase polymerase chain reaction (RT-PCR), and growth inhibition by MK-801 and NBQX was determined using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The effects of MK-801 on the cell cycle were examined by flow cytometry and Western blot analysis. Expression of thioredoxin-interacting protein (TXNIP) and p27 was determined by real-time PCR and Western blotting. Activation of the FOXO pathway and TXNIP induction were examined by Western blotting, fluorescence microscopy, Chromatin immunoprecipitation (ChIP) assay, and reporter gene assay. The effects of TXNIP on growth inhibition were examined using the gene silencing technique.

Results

NMDA receptor subunits were expressed in all cell lines examined, and MK-801, but not NBQX, inhibited cell growth of hepatocellular carcinomas. Cell cycle analysis showed that MK-801 induced G1 cell cycle arrest by down-regulating cyclin D1 and up-regulating p27. MK-801 dephosphorylated Thr24 in FOXO1 and induced its nuclear translocation, thus increasing transcription of TXNIP, a tumor suppressor gene. Knock-down of TXNIP ameliorated the growth inhibitory effects of MK-801.

Conclusions

Our results indicate that functional NMDA receptors are expressed in hepatocellular carcinomas and that the FOXO pathway is involved in the growth inhibitory effects of MK-801. This mechanism could be common in hepatocellular carcinomas examined, but other mechanisms such as ERK pathway could exist in other cancer cells as reported in lung carcinoma cells. Altered expression levels of FOXO target genes including cyclin D1 and p27 may contribute to the inhibition of G1/S cell cycle transition. Induction of the tumor suppressor gene TXNIP plays an important role in the growth inhibition by MK-801. Our report provides new evidence that FOXO-TXNIP pathway play a role in the inhibition of the hepatocellular carcinoma growth by MK-801.

Evolution of synapse complexity and diversity

Proteomic studies of the composition of mammalian synapses have revealed a high degree of complexity. The postsynaptic and presynaptic terminals are molecular systems with highly organized protein networks producing emergent physiological and behavioral properties. The major classes of synapse proteins and their respective functions in intercellular communication and adaptive responses evolved in prokaryotes and eukaryotes prior to the origins of neurons in metazoa. In eukaryotes, the organization of individual proteins into multiprotein complexes comprising scaffold proteins, receptors, and signaling enzymes formed the precursor to the core adaptive machinery of the metazoan postsynaptic terminal. Multiplicative increases in the complexity of this protosynapse machinery secondary to genome duplications drove synaptic, neuronal, and behavioral novelty in vertebrates. Natural selection has constrained diversification in mammalian postsynaptic mechanisms and the repertoire of adaptive and innate behaviors. The evolution and organization of synapse proteomes underlie the origins and complexity of nervous systems and behavior.

Influence of GRIA1, GRIA2 and GRIA4 polymorphisms on diagnosis and response to treatment in patients with major depressive disorder

The present study is aimed to exploring whether some single nucleotide polymorphisms (SNPs) within GRIA1, GRIA2 and GRIA4 could be associated with major depressive disorder (MDD) and whether they could predict clinical outcomes in Korean in-patients, respectively, treated with antidepressants. One hundred forty-five (145) patients with MDD and 170 healthy controls were genotyped for 17 SNPs within GRIA1, GRIA2 and GRIA4. Baseline and final clinical measures, including the Montgomery-Asberg Depression Rating Scale (MADRS) for patients with MDD, were recorded. No association was observed between alleles, genotypes and haplotypes under investigation and clinical and demographical variables. As a secondary finding, a marginal association was observed between rs4302506 and rs4403097 alleles within GRIA2 and age of onset in patients with MDD. Our findings provide evidence for a possible association between rs4302506 and rs4403097 SNPs and age of onset in patients with MDD. However, taking into account that the several limitations of our study including the moderately small sample size of our study, our findings should be considered with caution and further research is needed to draw more definitive conclusions.

Possible modulation of process extension by N-methyl-D-aspartate receptor expressed in osteocytic MLO-Y4 cells

In contrast to osteoblasts, little attention has been paid to expression profiles of different glutamatergic signaling machineries in osteocytes, which are the most abundant cells with a possible role as a mechanical sensor in bone. Here, we show that N-methyl-D-aspartate receptor (NMDAR) is expressed by osteocytic cells in five-weeks-old mouse tibiae in vivo as well as by osteocytic MLO-Y4 cells in vitro. Sustained exposure to the NMDAR antagonist dizocilpine significantly increased the number of cells with processes in cultured MLO-Y4 cells. These results suggest that NMDAR would be expressed by osteocytes with an unidentified role in the process extension.

Case-control association study of GRIA1, GRIA2 and GRIA4 polymorphisms in bipolar disorder

OBJECTIVE

The present study aimed to investigate whether some single nucleotide polymorphisms (SNPs) within GRIA1, GRIA2 and GRIA4 could be associated with bipolar disorder (BD) and they could predict clinical outcomes in in-patients with BD treated with mood stabilizers.

METHODS

One hundred and thirty-two (132) patients with BD and 170 healthy controls were genotyped for 17 SNPs within GRIA1, GRIA2 and GRIA4. Baseline and final clinical measures including Young Mania Rating Scale for patients with BD were recorded. Statistical significance was set at the 0.005 level in order to reduce the likelihood of false positive results.

RESULTS

We failed to show an evidence for a possible association of GRIA1, GRIA2 and GRIA4 with BD patients, in terms of influences on diagnosis and treatment outcomes, although this was the first study to explore the influence of such genes for bipolar disorder.

CONCLUSION

Our results suggest that 17 SNPs within GRIA1, GRIA2 and GRIA4 may not be associated with the development and treatment outcomes in BD. However, taking into account that the several limitations of our study including the moderately small sample size of our study, our findings should be considered with caution and further research is needed to draw more definitive conclusions.

Glutamate receptor homologs in plants: functions and evolutionary origins

The plant glutamate-like receptor homologs (GLRs) are homologs of mammalian ionotropic glutamate receptors (iGluRs) which were discovered more than 10 years ago, and are hypothesized to be potential amino acid sensors in plants. Although initial progress on this gene family has been hampered by gene redundancy and technical issues such as gene toxicity; genetic, pharmacological, and electrophysiological approaches are starting to uncover the functions of this protein family. In parallel, there has been tremendous progress in elucidating the structure of animal glutamate receptors (iGluRs), which in turn will help understanding of the molecular mechanisms of plant GLR functions. In this review, we will summarize recent progress on the plant GLRs. Emerging evidence implicates plant GLRs in various biological processes in and beyond N sensing, and implies that there is some overlap in the signaling mechanisms of amino acids between plants and animals. Phylogenetic analysis using iGluRs from metazoans, plants, and bacteria showed that the plant GLRs are no more closely related to metazoan iGluRs as they are to bacterial iGluRs, indicating the separation of plant, other eukaryotic, and bacterial GLRs might have happened as early on as the last universal common ancestor. Structural similarities and differences with animal iGluRs, and the implication thereof, are also discussed.

Multiple signaling pathways involved in stimulation of osteoblast differentiation by N-methyl-D-aspartate receptors activation in vitro

AIM

Glutamate receptors are expressed in osteoblastic cells. The present study was undertaken to investigate the mechanisms underlying the stimulation of osteoblast differentiation by N-methyl-D-aspartate (NMDA) receptor activation in vitro.

METHODS

Primary culture of osteoblasts was prepared from SD rats. Microarray was used to detect the changes of gene expression. The effect of NMDA receptor agonist or antagonist on individual gene was examined using RT-PCR. The activity of alkaloid phosphotase (ALP) was assessed using a commercial ALP staining kit.

RESULTS

Microarray analyses revealed that 10 genes were up-regulated by NMDA (0.5 mmol/L) and down-regulated by MK801 (100 μmol/L), while 13 genes down-regulated by NMDA (0.5 mmol/L) and up-regulated by MK801 (100 μmol/L). Pretreatment of osteoblasts with the specific PKC inhibitor Calphostin C (0.05 μmol/L), the PKA inhibitor H-89 (20 nmol/L), or the PI3K inhibitor wortmannin (100 nmol/L) blocked the ALP activity increase caused by NMDA (0.5 mmol/L). Furthermore, NMDA (0.5 mmol/L) rapidly increased PI3K phosphorylation, which could be blocked by pretreatment of wortmannin (100 nmol/L).

CONCLUSION

The results suggest that activation of NMDA receptors stimulates osteoblasts differentiation through PKA, PKC, and PI3K signaling pathways, which is a new role for glutamate in regulating bone remodeling.

Possible involvement of glutamatergic signaling machineries in pathophysiology of rheumatoid arthritis

The prevailing view is that L-glutamate (Glu) functions as an excitatory amino acid neurotransmitter through a number of molecular machineries required for the neurocrine signaling at synapses in the brain. These include Glu receptors for signal input, Glu transporters for signal termination, and vesicular Glu transporters for signal output through exocytotic release. Although relatively little attention has been paid to the functional expression of these molecules required for glutamatergic signaling in peripheral tissues, recent molecular biological analyses including ours give rise to a novel function for Glu as an extracellular signal mediator in the autocrine and/or paracrine system in several peripheral and non-neuronal tissues, including bone and cartilage. In particular, a drastic increase is demonstrated in the endogenous levels of both Glu and aspartate in the synovial fluid with intimate relevance to increased edema and sensitization to thermal hyperalgesia in experimental arthritis models. However, to date, there is only limited information about the physiological and pathological significance of glutamatergic signaling machineries expressed by articular synovial tissues. In this review, we have outlined the role of Glu in synovial fibroblasts in addition to the possible involvement of glutamatergic signaling machineries in the pathogenesis of joint diseases such as rheumatoid arthritis.

The biochemistry, ultrastructure, and subunit assembly mechanism of AMPA receptors

The AMPA-type ionotropic glutamate receptors (AMPA-Rs) are tetrameric ligand-gated ion channels that play crucial roles in synaptic transmission and plasticity. Our knowledge about the ultrastructure and subunit assembly mechanisms of intact AMPA-Rs was very limited. However, the new studies using single particle EM and X-ray crystallography are revealing important insights. For example, the tetrameric crystal structure of the GluA2cryst construct provided the atomic view of the intact receptor. In addition, the single particle EM structures of the subunit assembly intermediates revealed the conformational requirement for the dimer-to-tetramer transition during the maturation of AMPA-Rs. These new data in the field provide new models and interpretations. In the brain, the native AMPA-R complexes contain auxiliary subunits that influence subunit assembly, gating, and trafficking of the AMPA-Rs. Understanding the mechanisms of the auxiliary subunits will become increasingly important to precisely describe the function of AMPA-Rs in the brain. The AMPA-R proteomics studies continuously reveal a previously unexpected degree of molecular heterogeneity of the complex. Because the AMPA-Rs are important drug targets for treating various neurological and psychiatric diseases, it is likely that these new native complexes will require detailed mechanistic analysis in the future. The current ultrastructural data on the receptors and the receptor-expressing stable cell lines that were developed during the course of these studies are useful resources for high throughput drug screening and further drug designing. Moreover, we are getting closer to understanding the precise mechanisms of AMPA-R-mediated synaptic plasticity.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

Cloning and characterization of glutamate receptor subunit 4 (GLUA4) and its alternatively spliced isoforms in turtle brain

Ionotropic glutamate receptors sensitive to α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), GluAs, play an important role in neural development, synaptic plasticity, and neurodegeneration. Previous studies using an in vitro model of eyeblink classical conditioning in pond turtles suggested that acquisition of conditioning is associated with synaptic delivery of AMPA receptors containing GluA4 subunits. However, sequences of the GluA4 subunit, expression profile, and its alternatively spliced isoforms in turtle brain have not been previously determined. The sequence and domain structure of turtle GluA4 (tGluA4) and its splice variants was characterized. We found ten isoforms of tGluA4 including several previously unidentified truncated variants. Analysis of the nucleotide sequences of tGluA4 flip/flop, tGluA4c flip/flop, and tGluA4s showed they are highly similar to known isoforms of the GluA4 subunit identified in chick. Examination of the relative abundance of mRNA expression for the tGluA4 variants showed that the flip and flop versions of tGluA4 and tGluA4c, and a novel truncated variant, tGluA4trc1, which is also expressed as protein, are major forms in the adult turtle brain. Identification of these alternatively spliced isoforms of tGluA4 will provide a unique opportunity to assess their role in synaptic plasticity through the application of short interfering RNAs.

Contribution of the global subunit structure and stargazin on the maturation of AMPA receptors

Subunit assembly governs regulation of AMPA receptor (AMPA-R) synaptic delivery and determines biophysical parameters of the ion channel. However, little is known about the molecular pathways of this process. Here, we present single-particle EM three-dimensional structures of dimeric biosynthetic intermediates of the GluA2 subunit of AMPA-Rs. Consistent with the structures of intact tetramers, the N-terminal domains of the biosynthetic intermediates form dimers. Transmembrane domains also dimerize despite the two ligand-binding domains (LBDs) being separated. A significant difference was detected between the dimeric structures of the wild type and the L504Y mutant, a point mutation that blocks receptor trafficking and desensitization. In contrast to the wild type, whose LBD is separated, the LBD of the L504Y mutant was detected as a single density. Our results provide direct structural evidence that separation of the LBD within the intact dimeric subunits is critical for efficient tetramerization in the endoplasmic reticulum and further trafficking of AMPA-Rs. The contribution of stargazin on the subunit assembly of AMPA-R was examined. Our data suggest that stargazin affects AMPA-R trafficking at a later stage of receptor maturation.