Surface Expression Of N-methyl-D-aspartate Receptors Research Articles

Functional Evaluation of a Novel GRIN2B Missense Variant Associated with Epilepsy and Intellectual Disability

Epilepsy, a neurological condition, is widely prevalent among individuals with intellectual disability (ID). It is well established that N-methyl-D-aspartate (NMDA) receptors play an important role in both epilepsy and ID. Autosomal dominant mutations in the GRIN2B gene, which encodes the GluN2B subunit of the NMDA receptor, have been reported to be associated with epilepsy and ID. However, the underlying mechanism of this association is not well-understood. In this study, we identified a novel GRIN2B mutation (c.3272A > C, p.K1091T) in a patient with epilepsy and ID. The proband was a one year and ten months old girl. GRIN2B variant was inherited from her mother. We further investigated the functional consequences of this mutation. Our findings revealed that the p.K1091T mutation created a Casein kinase 2 phosphorylation site. Using recombinant NMDA receptors containing the GluN2B-K1091T along with GluN1 in HEK 293T cells, we observed significant defects in its interactions with postsynaptic density 95. It is accompanied by reduced delivery of the receptors to the cell membrane and a decrease in glutamate affinity. Moreover, primary neurons expressing GluN2B-K1091T also exhibited impaired surface expression of NMDA receptors, a reduction in dendritic spine number and excitatory synaptic transmission. In summary, our study reports a novel GRIN2B mutation and provides functional characteristics of this mutation in vitro, thereby contributing to the understanding of GRIN2B variants in epilepsy and ID.

High Pressure Stress Response: Involvement of NMDA Receptor Subtypes and Molecular Markers.

Professional divers who are exposed to high pressure (HP) above 1.1 MPa suffer from high pressure neurological syndrome (HPNS), which is characterized by reversible CNS hyperexcitability and cognitive and motor deficits. HPNS remains the final major constraints on deep diving at HP. Prolonged and repetitive exposure to HP during deep sea saturation dives may result in permanent memory and motor impairment. Previous studies revealed that CNS hyperexcitability associated with HPNS is largely induced by N-methyl-D-aspartate receptors (NMDARs). NMDARs that contain the GluN2A subunit are the only ones that show a large (∼60%) current increase at He HP. NMDAR subtypes that contain other GluN2 members show minor decrease or no change of the current. Immunoprecipitation was used in order to test the hypothesis that current augmentation may result from inserting additional NMDARs into the membrane during the 20–25 min compression. The results indicated that there is no increase in surface expression of NMDARs in the oocyte membrane under HP conditions. In contrast, consistent increase in glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and β-actin was discovered. GAPDH and β-actin are cytosolic proteins which involve in various cellular control processes, increase of their expression suggests the presence of a general cellular stress response to HP. Understanding the precise hyperexcitation mechanism(s) of specific NMDAR subtypes and other possible neurotoxic processes during HP exposure could provide the key for eliminating the adverse, yet reversible, short-term effects of HPNS and hopefully the deleterious long-term ones.

Effects of estrogen and aging on synaptic morphology and distribution of phosphorylated Tyr1472 NR2B in the female rat hippocampus.

Age and estrogens may impact the mobility of N-methyl-D-aspartate receptors (NMDARs) in hippocampal synapses. Here, we used serial section immunogold electron microscopy to examine whether phosphorylated tyrosine 1472 NR2B (pY1472), which is involved in the surface expression of NMDARs, is altered in the dorsal hippocampus of young (3-4months old) and aged (∼24months old) ovariectomized rats treated with 17β-estradiol or vehicle for 2days. The number of gold particles labeling pY1472 was higher in presynaptic and postsynaptic compartments of aged rats with low estradiol (vehicle-treated) compared to other groups. In terminals, pY1472 levels were elevated in aged rats but reduced by estradiol treatment to levels seen in young rats. Conversely, the mitochondria number was lower in aged females but was restored to young levels by estradiol. In the postsynaptic density and dendritic spines, estradiol reduced pY1472 in young and aged rats. As phosphorylation at Y1472 blocks NR2B endocytosis, reduction of pY1472 by estradiol suggests another mechanism through which estrogen enhances synaptic plasticity by altering localization of NMDAR subunits within synapses.

Surface expression of NMDA receptor changes during memory consolidation in the crab Neohelice granulata.

The aim of the present study was to analyze the surface expression of the NMDA-like receptors during the consolidation of contextual learning in the crab Neohelice granulata. Memory storage is based on alterations in the strength of synaptic connections between neurons. The glutamatergic synapses undergo various forms of N-methyl-D aspartate receptor (NMDAR)-dependent changes in strength, a process that affects the abundance of other receptors at the synapse and underlies some forms of learning and memory. Here we propose a direct regulation of the NMDAR. Changes in NMDAR's functionality might be induced by the modification of the subunit's expression or cellular trafficking. This trafficking does not only include NMDAR's movement between synaptic and extra-synaptic localizations but also the cycling between intracellular compartments and the plasma membrane, a process called surface expression. Consolidation of contextual learning affects the surface expression of the receptor without affecting its general expression. The surface expression of the GluN1 subunit of the NMDAR is down-regulated immediately after training, up-regulated 3 h after training and returns to naïve and control levels 24 h after training. The changes in NMDAR surface expression observed in the central brain are not seen in the thoracic ganglion. A similar increment in surface expression of GluN1 in the central brain is observed 3 h after administration of the competitive GABAA receptor antagonist, bicuculline. These consolidation changes are part of a plasticity event that first, during the down-regulation, stabilizes the trace and later, at 3-h post-training, changes the threshold for synapse activation.

Altered NMDA receptor function in primary cultures of hippocampal neurons from mice lacking the Homer2 gene.

N-Methyl-D-Aspartate (NMDA) receptors are inhibited during acute exposure to ethanol and are involved in changes in neuronal plasticity following repeated ethanol exposure. The postsynaptic scaffolding protein Homer2 can regulate the cell surface expression of NMDA receptors in vivo, and mice with a null mutation of the Homer2 gene exhibit an alcohol-avoiding and -intolerant phenotype that is accompanied by a lack of ethanol-induced glutamate sensitization. Thus, Homer2 deletion may perturb the function or acute ethanol sensitivity of the NMDA receptor. In this study, the function and ethanol sensitivity of glutamate receptors in cultured hippocampal neurons from wild-type (WT) and Homer2 knock-out (KO) mice were examined at 7 and 14 days in vitro (DIV) using standard whole-cell voltage-clamp electrophysiology. As compared with wild-type controls, NMDA receptor current density was reduced in cultured hippocampal neurons from Homer2 KO mice at 14 DIV, but not at 7 DIV. There were no genotype-dependent changes in whole-cell capacitance or in currents evoked by kainic acid. The GluN2B-selective antagonist ifenprodil inhibited NMDA-evoked currents to a similar extent in both wild-type and Homer2 KO neurons and inhibition was greater at 7 versus 14 DIV. NMDA receptor currents from both WT and KO mice were inhibited by ethanol (10-100 mM) and the degree of inhibition did not differ as a function of genotype. In conclusion, NMDA receptor function, but not ethanol sensitivity, is reduced in hippocampal neurons lacking the Homer2 gene.

Phosphorylation of Tyrosine 1070 at the GluN2B Subunit Is Regulated by Synaptic Activity and Critical for Surface Expression of N-Methyl-d-aspartate (NMDA) Receptors

The number and subunit composition of synaptic N-methyl-d-aspartate receptors (NMDARs) play critical roles in synaptic plasticity, learning, and memory and are implicated in neurological disorders. Tyrosine phosphorylation provides a powerful means of regulating NMDAR function, but the underling mechanism remains elusive. In this study we identified a tyrosine site on the GluN2B subunit, Tyr-1070, which was phosphorylated by a proto-oncogene tyrosine-protein (Fyn) kinase and critical for the surface expression of GluN2B-containing NMDARs. The phosphorylation of GluN2B at Tyr-1070 was required for binding of Fyn kinase to GluN2B, which up-regulated the phosphorylation of GluN2B at Tyr-1472. Moreover, our results revealed that the phosphorylation change of GluN2B at Tyr-1070 accompanied the Tyr-1472 phosphorylation and Fyn associated with GluN2B in synaptic plasticity induced by both chemical and contextual fear learning. Taken together, our findings provide a new mechanism for regulating the surface expression of NMDARs with implications for synaptic plasticity.

Cysteine residues 87 and 320 in the amino terminal domain of NMDA receptor GluN2A govern its homodimerization but do not influence GluN2A/GluN1 heteromeric assembly

N-Methyl-D-aspartate receptors (NMDARs) play a central role in various physiological and pathological processes in the central nervous system. And they are commonly composed of four subunits, two GluN1 subunits and two GluN2 or GluN3 subunits. The different subunit compositions make NMDARs a heterogeneous population with distinct electrophysiological and pharmacological properties and thus with different abilities to conduct neuronal activities. The subunit composition, assembly process, and final structure of assembled NMDARs have been studied for years but no consensus has been achieved. In this study, we investigated the role of the amino terminal domain (ATD) of GluN2A in regulating NMDAR assembly. The ATD of GluN2A was first expressed in heterogeneous cells and the homodimer formation was investigated by fluorescent resonance energy transfer and non-reducing SDSPAGE electrophoresis. Each of the three cysteine residues located in the ATD was mutated into alanine, and the homodimerization of the ATD or GluN2A, as well as the heteromeric assembly of NMDARs was assessed by non-reducing SDSPAGE electrophoresis, co-immunoprecipitation and immunocytochemistry. We found that two cysteine residues, C87 and C320, in the ATD of the GluN2A subunit were required for the formation of disulfide bonds and GluN2A ATD homodimers. Furthermore, the disruption of GluN2A ATD domain dimerization had no influence on the assembly and surface expression of NMDARs. These results suggest that the two ATD domains of GluN2A are structurally adjacent in fully-assembled NMDARs. However, unlike GluN1, the homomerization of the ATD domain of GluN2A is not required for the assembly of NMDARs, implying that GluN2A and GluN1 play unequal roles in NMDAR assembly.

The tyrosine phosphatase STEP: implications in schizophrenia and the molecular mechanism underlying antipsychotic medications

Glutamatergic signaling through N-methyl-D-aspartate receptors (NMDARs) is required for synaptic plasticity. Disruptions in glutamatergic signaling are proposed to contribute to the behavioral and cognitive deficits observed in schizophrenia (SZ). One possible source of compromised glutamatergic function in SZ is decreased surface expression of GluN2B-containing NMDARs. STEP61 is a brain-enriched protein tyrosine phosphatase that dephosphorylates a regulatory tyrosine on GluN2B, thereby promoting its internalization. Here, we report that STEP61 levels are significantly higher in the postmortem anterior cingulate cortex and dorsolateral prefrontal cortex of SZ patients, as well as in mice treated with the psychotomimetics MK-801 and phencyclidine (PCP). Accumulation of STEP61 after MK-801 treatment is due to a disruption in the ubiquitin proteasome system that normally degrades STEP61. STEP knockout mice are less sensitive to both the locomotor and cognitive effects of acute and chronic administration of PCP, supporting the functional relevance of increased STEP61 levels in SZ. In addition, chronic treatment of mice with both typical and atypical antipsychotic medications results in a protein kinase A-mediated phosphorylation and inactivation of STEP61 and, consequently, increased surface expression of GluN1/GluN2B receptors. Taken together, our findings suggest that STEP61 accumulation may contribute to the pathophysiology of SZ. Moreover, we show a mechanistic link between neuroleptic treatment, STEP61 inactivation and increased surface expression of NMDARs, consistent with the glutamate hypothesis of SZ.

Chronic Opioid Potentiates Presynaptic but Impairs Postsynaptic N-Methyl-d-aspartic Acid Receptor Activity in Spinal Cords: IMPLICATIONS FOR OPIOID HYPERALGESIA AND TOLERANCE

Chronic Opioid Potentiates Presynaptic but Impairs Postsynaptic N-Methyl-d-aspartic Acid Receptor Activity in Spinal Cords: IMPLICATIONS FOR OPIOID HYPERALGESIA AND TOLERANCE

NMDA receptor/amyloid precursor protein interactions: A comparison between wild-type and amyloid precursor protein mutations associated with familial Alzheimer's disease

Two recent reports showed that amyloid precursor protein (APP) may contribute to postsynaptic mechanisms via the regulation of the surface trafficking of excitatory N-methyl-d-aspartate (NMDA) receptors. Here we have investigated the interactions and surface trafficking of NR1-1a/NR2A and NR1-1a/NR2B NMDA receptor subtypes with three APP mutations linked to familial Alzheimer's disease, APP695Indiana, APP695London and APP695Swedish. Flag-tagged mutated APP695s were generated and shown to be expressed at equivalent levels to wild-type APP695 in mammalian cells. Each APP mutant co-precipitated with NR1-1a/NR2A and NR1-1a/NR2B receptors following co-expression in mammalian cells. Further, as found for wild-type APP695, each enhanced NMDA receptor surface expression with no concomitant increase in total NR1-1a, NR2A or NR2B subunit expression. Thus these three familial APP mutations behave as wild-type APP695 with respect to their association with assembled NMDA receptors and their APP695-enhanced receptor cell surface trafficking.

AMPA reduces surface expression of NR1 through regulation of GSK3β

Emerging evidence has suggested that alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) protects neurons from glutamate-induced neurotoxicity. In this study, we examined the effect of AMPA on the cell surface expression of the N-methyl-D-aspartate (NMDA) receptor, a central player in glutamate-induced neurotoxicity, using rat cortical neurons. AMPA (10 microM, 24 h) attenuated the expression of cell surface NR1, an NMDA receptor subunit, and also inhibited glutamate-induced increases in intracellular Ca2+. SB216763, an inhibitor of glycogen synthase kinase 3beta (GSK3beta), had effects similar to those of AMPA. We have earlier shown that AMPA treatment attenuated GSK3beta activity. Our data suggest that AMPA reduces the cell surface expression of NMDA receptors through the regulation of GSK3beta and, consequently, reduces the amount of intracellular Ca2+.

The Integrity of the Glycine Co-agonist Binding Site of N-Methyl-d-aspartate Receptors Is a Functional Quality Control Checkpoint for Cell Surface Delivery

N-Methyl-D-aspartate receptors are a subclass of ligand-gated, heteromeric glutamatergic neurotransmitter receptors whose cell surface expression is regulated by quality control mechanisms. Functional quality control checkpoints are known to contribute to cell surface trafficking of non-N-methyl-D-aspartate glutamate receptors. Here we investigated if similar mechanisms operate for the surface delivery of NMDA receptors. Point mutations in the glycine binding domain of the NR1-1a subunit were generated: D732A, a mutation that results in an approximately 3 x 10(4) decrease in glycine binding affinity; D732E, a conservative change; and D723A, a residue in the same NR1-1a domain that has no effect on glycine binding affinity. Each NR1-1a subunit was co-expressed with NR2A in mammalian cells. Immunoblotting and immunoprecipitations showed that all mutants were expressed to similar levels as wild-type NR1-1a and associated with NR2A. Cell surface expression measured by an enzyme-linked immunosorbent assay found that whereas NR1-1a (D732E)/NR2A and NR1-1a (D723A)/NR2A trafficked as efficiently as NR1-1a/NR2A, there was a 90% decrease in surface expression for NR1-1a (D732A)/NR2A. This was confirmed by confocal microscopy imaging and cell surface biotinylation. Further imaging showed that NR1-1a (D732A) and co-transfected NR2A co-localized with an endoplasmic reticulum marker. Dichlorokynurenic acid, a competitive glycine site antagonist, partially rescued surface expression. Mutation of the NR1-1a ER retention motif showed that the ligand binding checkpoint is an early event preceding endoplasmic reticulum sorting mechanisms. These findings demonstrate that integrity of the glycine co-agonist binding site is a functional checkpoint requisite for efficient cell surface trafficking of assembled NMDA receptors.

Phospho-regulation of synaptic and extrasynaptic N-methyl-d-aspartate receptors in adult hippocampal slices

Recent evidence demonstrates that N-methyl-d-aspartate receptor (NMDAR) trafficking contributes to synaptic plasticity in the hippocampus. Phosphorylation of tyrosine residues, especially NR2B tyrosine 1472, appears to be a mechanism by which NMDAR endocytosis is prevented, suggesting that the tyrosine phosphorylation and surface expression of NMDARs are positively correlated. Previous work from our laboratory and others has confirmed that modulation of tyrosine phosphatase and kinase activity alters the surface expression of NMDARs. However, the changes in NMDAR surface expression described in those studies were in terms of total surface membrane versus intracellular receptors. Within the plasma membrane of glutamatergic synapses, distinct populations of NMDARs exist. Namely, receptors at the surface can be differentiated into synaptic and extrasynaptic pools based on their association with the post-synaptic density (PSD) and availability to glutamate. In the present study, we utilized a subcellular fractionation approach coupled with detergent extraction to prepare synaptic and extrasynaptic NMDARs from adult rat hippocampal slices. Using this method, we examined how tyrosine phosphatase and Src-family tyrosine kinase (SFK) inhibitors modulate the phosphorylation and localization of these different pools of NMDARs. We found that both synaptic and extrasynaptic NMDARs were modulated by tyrosine phosphatase and SFK inhibitors; however subunit- and residue-specific effects were observed. Specifically, phosphorylation of NR2B tyrosine 1472 was associated with enrichment of synaptic NMDARs, whereas phosphorylation of NR2B tyrosine 1336 was associated with enrichment of extrasynaptic NMDARs. Using electrophysiological methods, we also reveal that the biochemical modifications produced by these inhibitors were associated with corresponding changes in NMDAR function.

Aging and Surface Expression of Hippocampal NMDA Receptors

Aging is known to alter many physiological processes within the brain including synaptic responses, long-term potentiation, learning, and memory. Aging has also been shown to alter the expression and distribution of N-methyl-d-aspartate (NMDA) receptors in many different brain regions, including the hippocampus. Additionally, we have recently reported that young adult rats show an activity-dependent increase in the surface expression of NMDA receptors. We have extended these observations in the present study in aged animals and have found that aged Fischer 344 rats fail to show activity-dependent changes in the surface distribution of NMDA receptors. In conjunction with this observation we have also noted that aged rats show an expression deficit in the C2 splice variant of the NR1 subunit. This subunit is preferentially shifted to the surface following stimulation in young adult animals. As the NMDA receptor is thought to play an important role in neuronal signaling, these observations suggest possible new areas of dysfunction in this receptor that might underlie age-related deficits in neuronal physiology.

Surface expression of AMPA receptors in hippocampal neurons is regulated by an NSF-dependent mechanism.

Here, we show that disruption of N-ethylmaleimide-sensitive fusion protein- (NSF-) GluR2 interaction by infusion into cultured hippocampal neurons of a blocking peptide (pep2m) caused a rapid decrease in the frequency but no change in the amplitude of AMPA receptor-mediated miniature excitatory postsynaptic currents (mEPSCs). N-methyl-D-aspartate (NMDA) receptor-mediated mEPSCs were not changed. Viral expression of pep2m reduced the surface expression of alpha-amino-3-hydroxy-5-methyl-isoxazolepropionate (AMPA) receptors, whereas NMDA receptor surface expression in the same living cells was unchanged. In permeabilized neurons, the total amount of GluR2 immunoreactivity was unchanged, and a punctate distribution of GluR2 was observed throughout the dendritic tree. These data suggest that the NSF-GluR2 interaction is required for the surface expression of GluR2-containing AMPA receptors and that disruption of the interaction leads to the functional elimination of AMPA receptors at synapses.