Immunoautoradiographic analysis of NMDA receptor subunits and associated postsynaptic density proteins in the brain of dyskinetic MPTP-treated common marmosets

l-3,4-dihydroxyphenylalanine methyl ester (l-DOPA)-induced dyskinesia in Parkinson's disease may result from aberrant glutamatergic stimulation of the striatum due to synaptic plasticity in the motor cortex or striatum as a consequence of adaptation of striatal output pathways. This might result from changes in NMDA receptor subunit or NMDA receptor associated postsynaptic density (PSD) scaffold protein expression. Using immunoautoradiography the expression levels of NR1 and NR2B subunits of the NMDA receptor and the postsynaptic density scaffold proteins, PSD-95, PSD-93, and neurofilament light (NFL) were examined in normal common marmosets (Callithrix jacchus) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned animals that exhibited high or low levels of l-DOPA-induced dyskinesia. Brains from MPTP-lesioned animals that were not primed for l-DOPA-induced dyskinesia were not included in this study. No alterations in the NR1 NMDA receptor subunit were observed. The NR2B NMDA receptor subunit was increased in caudal caudate nucleus and putamen, hippocampus, cingulate motor area (CMA), supplementary motor area (SMA) and dorsal primary motor cortex (dMI) of highly dyskinetic MPTP-lesioned marmosets, but not in animals with low levels of dyskinesia. PSD-93 was decreased in the globus pallidus of marmosets with high and low levels of dyskinesia and increased in the CMA, SMA and dMI of highly dyskinetic marmosets. PSD-95 was increased in the SMA of highly dyskinetic marmosets, but not in animals with low dyskinesia. NFL expression was elevated in the SMA and dorsal and ventral MI of highly dyskinetic marmosets. These results suggest that l-DOPA treatment of MPTP-lesioned marmosets can affect glutamatergic systems and indicate that altered NMDA receptor function may relate to dyskinesia.     

Receptor compartmentalization and trafficking at glutamate synapses: a developmental proposal

This article focuses on NMDA receptor subunit changes that occur in the forebrain and midbrain during development, namely the switch from predominance of NMDA receptors rich in NR2B subunits to that of NMDA receptors rich in NR2A subunits. We review the potential roles in brain plasticity of two membrane-associated guanylate kinases (MAGUKs), SAP102 and PSD95, which form a scaffold for the ion-passing glutamate receptors at the postsynaptic density, and we consider the known functional significance of these molecules in subunit switching. In addition, based on recent analyses of the synaptic location of glutamate receptors, activity-dependent changes in developing visual neurons, and extensive data on MAGUKs, we propose a model of glutamatergic synaptic differentiation. In this model, different NMDA receptor scaffolding and signaling complexes effect the trafficking and synaptic localization of NR2A-rich and NR2B-rich receptors, leading to tangential compartmentalization of these receptors and their movement between synaptic and extrasynaptic compartments.     

Gene expression of PSD95 in prefrontal cortex and hippocampus in schizophrenia

A number of studies have suggested that disturbance in glutamatergic transmission in the cerebral cortex may underlie, or contribute to the pathophysiology of schizophrenia. In this study we examined expression of the postsynaptic density protein 95 (PSD95) mRNA in the prefrontal cortex and hippocampus in postmortem material from neuroleptic-treated schizophrenics and normal controls. PSD95 is known to bind to NMDA receptor subunits and is known to be involved in synaptic plasticity. In situ hybridization analysis showed that the expression of PSD95 was significantly decreased in Brodmann area 9 of the prefrontal cortex but not in the hippocampus. These results further implicate the prefrontal cortex in the pathophysiology of schizophrenia and suggest dysfunction of NMDA receptors in the schizophrenic cortex.     

Developmental changes in the association of NMDA receptors with lipid rafts

Lipid rafts (LR) are lipid microdomains present in the cell surface membrane that are organizational platforms involved in protein trafficking and formation of cell signaling complexes. In the adult brain, NMDA receptors (NMDAR) and receptor-associated proteins such as membrane-associated guanylate kinases (PSD-95 and SAP102), are distributed between the postsynaptic density (PSD) and lipid rafts. However, the time course of the association of NMDAR with LR during neural development is not known. We therefore investigated the effect of development on the association of NMDAR with LR prepared from rat brains ranging in postnatal age from 1-35 days and compared this with their expression in PSDs. LR and PSD fractions were prepared by extraction of P2 membranes with Tx-100 followed by sucrose density gradient centrifugation. The yield of LR, as reflected by levels of protein, Thy-1, and flotillin-1 increased during postnatal development. NR2A was associated predominantly with the lipid raft fraction at all ages examined whereas NR2B underwent a gradual shift from PSDs to lipid rafts during the first 3 weeks after birth. These changes in the distribution of NR2A and NR2B were paralleled by changes in the distribution of PSD-95 and SAP102 respectively. Tyrosine-phosphorylated proteins, including NR2A and NR2B, were preferentially associated with lipid rafts in older, as compared to younger, animals. These results show that the association of NMDAR with LR is regulated developmentally.     

The effect of transient global ischemia on the interaction of Src and Fyn with the N-methyl-D-aspartate receptor and postsynaptic densities: possible involvement of Src homology 2 domains.

Transient ischemia increases tyrosine phosphorylation of N-methyl-D-aspartate (NMDA) receptor subunits NR2A and NR2B in the rat hippocampus. The authors investigated the effects of this increase on the ability of the receptor subunits to bind to the Src homology 2 (SH2) domains of Src and Fyn expressed as glutathione-S-transferase-SH2 fusion proteins. The NR2A and NR2B bound to each of the SH2 domains and binding was increased approximately twofold after ischemia and reperfusion. Binding was prevented by prior incubation of hippocampal homogenates with a protein tyrosine phosphatase or by a competing peptide for the Src SH2 domain. Ischemia induced a marked increase in the tyrosine phosphorylation of several proteins in the postsynaptic density (PSD), including NR2A and NR2B, but had no effect on the amounts of individual NMDA receptor subunits in the PSD. The level of Src and Fyn in PSDs, but not in other subcellular fractions, was increased after ischemia. The ischemia-induced increase in the interaction of NR2A and NR2B with the SH2 domains of Src and Fyn suggests a possible mechanism for the recruitment of signaling proteins to the PSD and may contribute to altered signal transduction in the postischemic hippocampus.     

Glutamate signaling proteins and tyrosine hydroxylase in the locus coeruleus of alcoholics

It has been postulated that alcoholism is associated with abnormalities in glutamatergic neurotransmission. This study examined the density of glutamate NMDA receptor subunits and its associated proteins in the noradrenergic locus coeruleus (LC) in deceased alcoholic subjects. Our previous research indicated that the NMDA receptor in the human LC is composed of obligatory NR1 and regulatory NR2C subunits. At synapses, NMDA receptors are stabilized through interactions with postsynaptic density protein (PSD-95). PSD-95 provides structural and functional coupling of the NMDA receptor with neuronal nitric oxide synthase (nNOS), an intracellular mediator of NMDA receptor activation. LC tissue was obtained from 10 alcohol-dependent subjects and eight psychiatrically healthy controls. Concentrations of NR1 and NR2C subunits, as well as PSD-95 and nNOS, were measured using Western blotting. In addition, we have examined tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of norepinephrine. The amount of NR1 was lower in the rostral (-30%) and middle (-41%) portions of the LC of alcoholics as compared to control subjects. No differences in the amounts of NR2C, PSD-95, nNOS and TH were detected comparing alcoholic to control subjects. Lower levels of NR1 subunit of the NMDA receptor in the LC implicates altered glutamate-norepinephrine interactions in alcoholism.     

Ontogeny of postsynaptic density proteins at glutamatergic synapses

In glutamatergic synapses, glutamate receptors (GluRs) associate with many other proteins involved in scaffolding and signal transduction. The ontogeny of these postsynaptic density (PSD) proteins involves changes in their composition during development, paralleling changes in GluR type and function. In the CA1 region of the hippocampus, at postnatal day 2 (P2), many synapses already have a distinct PSD. We used immunoblot analysis, subcellular fractionation, and quantitative immunogold electron microscopy to examine the distribution of PSD proteins during development of the hippocampus. Synapses at P2 contained substantial levels of NR1 and NR2B and most GluR-associated proteins, including SAP102, SynGAP, the chain of proteins from GluRs/SAP102 through GKAP/Shank/Homer and metabotropic glutamate receptors, and the adhesion factors, cadherin, catenin, neuroligin, and Nr-CAM. Development was marked by substantial decreases in NR2B and SAP102 and increases in NR2A, PSD-95, AMPA receptors, and CaMKII. Other components showed more moderate changes.     

Gene expression of NMDA receptor subunits in the cerebellum of elderly patients with schizophrenia

To determine if NMDA receptor alterations are present in the cerebellum in schizophrenia, we measured NMDA receptor binding and gene expression of the NMDA receptor subunits in a post-mortem study of elderly patients with schizophrenia and non-affected subjects. Furthermore, we assessed influence of genetic variation in the candidate gene neuregulin-1 (NRG1) on the expression of the NMDA receptor in an exploratory study. Post-mortem samples from the cerebellar cortex of ten schizophrenic patients were compared with nine normal subjects. We investigated NMDA receptor binding by receptor autoradiography and gene expression of the NMDA receptor subunits NR1, NR2A, NR2B, NR2C and NR2D by in situ hybridization. For the genetic study, we genotyped the NRG1 polymorphism rs35753505 (SNP8NRG221533). Additionally, we treated rats with the antipsychotics haloperidol or clozapine and assessed cerebellar NMDA receptor binding and gene expression of subunits to examine the effects of antipsychotic treatment. Gene expression of the NR2D subunit was increased in the right cerebellum of schizophrenic patients compared to controls. Individuals carrying at least one C allele of rs35753505 (SNP8NRG221533) showed decreased expression of the NR2C subunit in the right cerebellum, compared to individuals homozygous for the T allele. Correlation with medication parameters and the animal model revealed no treatment effects. In conclusion, increased NR2D expression results in a hyperexcitable NMDA receptor suggesting an adaptive effect due to receptor hypofunction. The decreased NR2C expression in NRG1 risk variant may cause a deficit in NMDA receptor function. This supports the hypothesis of an abnormal glutamatergic neurotransmission in the right cerebellum in the pathophysiology of schizophrenia.     

Synaptic localization of NMDA receptor subunits in the rat retina

The distribution and synaptic clustering of N-methyl-D-aspartate (NMDA) receptors were studied in the rat retina by using subunit specific antisera. A punctate immunofluorescence was observed in the inner plexiform layer (IPL) for all subunits tested, and electron microscopy confirmed that the immunoreactive puncta represent labeling of receptors clustered at postsynaptic sites. Double labeling of sections revealed that NMDA receptor clusters within the IPL are composed of different subunit combinations: NR1/NR2A, NR1/NR2B, and in a small number of synapses NR1/NR2A/NR2B. The majority of NMDA receptor clusters were colocalized with the postsynaptic density proteins PSD-95, PSD-93, and SAP 102. Double labeling of the NMDA receptor subunit specific antisera with protein kinase C (PKC), a marker of rod bipolar cells, revealed very little colocalization at the rod bipolar cell axon terminal. This suggests that NMDA receptors are important in mediating neurotransmission within the cone bipolar cell pathways of the IPL. The postsynaptic neurons are a subset of amacrine cells and most ganglion cells. Usually only one of the two postsynaptic processes at the bipolar cell ribbon synapses expressed NMDA receptors. In the outer plexiform layer (OPL), punctate immunofluoresence was observed for the NR1C2; subunit, which was shown by electron microscopy to be localized presynaptically within both rod and cone photoreceptor terminals.     

Ionotropic glutamate receptors and expression of N-methyl-D-aspartate receptor subunits in subregions of human hippocampus: effects of schizophrenia

OBJECTIVE: Multiple quantifiable biologic abnormalities have been localized to the hippocampus in schizophrenia. Alterations in glutamate-mediated transmission at N-methyl-D-aspartic acid (NMDA)-sensitive receptors in hippocampus have been implicated in the pathophysiology of the illness. The authors tested the hypothesis that glutamatergic transmission within and efferent from hippocampus is altered in schizophrenia. METHOD: The authors analyzed postmortem hippocampal tissue from individuals with schizophrenia and from healthy individuals. The tissue samples had been collected by two brain tissue banks, one in Maryland and the other in Melbourne, Australia. lonotropic receptor binding for the NMDA, kainate, and (3)H-amino-3-hydroxy-5-methylisoxazol-4-propionate (AMPA) receptors was quantified by using usual radioligand techniques. In situ hybridization autoradiography was used to quantify mRNA for the NMDA receptor subunits NR1, NR2A, and NR2B. RESULTS: Ligand binding to the ionotropic glutamate receptors (NMDA, kainate, and AMPA) did not differ significantly overall or in any subregion between the schizophrenia tissue and the healthy comparison tissue. The only exception was AMPA receptor binding in hippocampal subregion CA2, which was slightly but significantly less in schizophrenia. However, the level of mRNA for the NMDA receptor subunits NR1 and NR2B was significantly different between groups; in several hippocampal subregions, the level of NR1 mRNA was lower and the level of NR2B mRNA higher in schizophrenia. CONCLUSIONS: Because the NR1 subunit of the NMDA receptor is critical to full receptor activity, a reduction of NR1 in hippocampus in schizophrenia suggests a functional impairment in glutamatergic transmission at the NMDA receptor, resulting in reduced glutamatergic transmission within and possibly efferent from the hippocampus in schizophrenia. This defect could underlie a hypoglutamatergic state in regions of limbic cortex, consistent with published results from other lines of research in schizophrenia.     

Reciprocal signalling between NR2 subunits of the NMDA receptor and neuregulin1 and their role in schizophrenia

Schizophrenia is a debilitating neurodevelopmental psychiatric disorder. Both the N-methyl-D-aspartate receptor (NMDAR) and neuregulin1 (NRG1) are key molecules involved in normal brain development that have been linked to schizophrenia pathology and aetiology. The NR2 proteins are critical structural and functional subunits of the NMDAR and are developmentally and spatially regulated. Altered NR2 gene and protein expression has been found in human post-mortem schizophrenia brain tissue together with changes in NRG1 and its receptor ErbB4. The NR2 subunits and ErbB4 share a common anchoring domain on the postsynaptic density and therefore a disruption to either of these molecules may influence the functioning of the other. It has been shown that NRG1 signalling can affect NMDAR levels and function, particularly phosphorylation of the NR2 subunits. However little is known about the possible effects of NMDAR dysfunction on NRG1 signalling, which is important with regards to schizophrenia aetiology as numerous risk factors for the disorder can alter NMDAR functioning during early brain development. This review focuses on the role of the NMDA receptor subunits and NRG1 signalling in schizophrenia and proposes a mechanism by which a disruption to the NMDAR, particularly via altering the balance of NR2 subunits during early development, could influence NRG1 signalling.     

Novel polymorphism in the gene region encoding the carboxyl-terminal intracellular domain of the NMDA receptor 2B subunit: analysis of association with schizophrenia

OBJECTIVE: N-methyl-D-aspartate (NMDA) receptor antagonists are known to produce a syndrome resembling schizophrenia, probably due to their blockade of NMDA receptors. The NMDA receptor 2B (NR2B) subunit has been identified as one of the major proteins in the postsynaptic density at glutamatergic synapses, suggesting that the carboxyl-terminal domain of the NR2B subunit may play a significant role in intracellular signal transduction. METHOD: The authors screened for genetic variations in the region of the NR2B subunit gene encoding the carboxyl-terminal intracellular domain in patients with schizophrenia and studied the association between schizophrenia and a novel polymorphism of the NR2B subunit gene. RESULTS: One silent mutation (2664C/T) was identified. No significant differences in the frequencies of 2664C/T genotypes and alleles were found between patients with schizophrenia and healthy comparison subjects. CONCLUSIONS: The findings provided no evidence of an association between schizophrenia and the 2664C/T polymorphism of the NR2B subunit gene.     

Reduced levels of NR2A and NR2B subunits of NMDA receptor and PSD-95 in the prefrontal cortex in major depression

Recent neuroimaging and postmortem studies have demonstrated abnormalities in glutamatergic transmission in major depression. Glutamate NMDA (N-methyl-d-aspartate) receptors are one of the major mediators of excitatory neurotransmission in the central nervous system. At synaptic sites, NMDA receptors are linked with postsynaptic density protein-95 (PSD-95) that plays a key role in mediating trafficking, clustering, and downstream signaling events, following receptor activation. In this study, we examined the expression of NMDA receptor subunits NR1, NR2A, and NR2B as well as PSD-95 in the anterior prefrontal cortex (PFC) using Western blot method. Cortical samples were obtained from age, gender and postmortem interval matched depressed and psychiatrically healthy controls. The results revealed that there was a reduced expression of the NMDA receptor subunits NR2A (-54%) and NR2B (-48%), and PSD-95 protein level (-40%) in the PFC of depressed subjects relative to controls, with no change in the NR1 subunit. The alterations in NMDA receptor subunits, especially the NR2A and NR2B, as well as PSD-95 suggest an abnormality in the NMDA receptor signaling in the PFC in major depression. Our findings in conjunction with recent clinical, cellular, and neuroimaging studies further implicate the involvement of glutamate neurotransmission in the pathophysiology of depression. This study provides additional evidence that NMDA receptor complex is a target for discovery of novel antidepressants.