Distribution of the NMDA receptor NR3A subunit in the adult pig-tail macaque brain

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

A new insight into mechanisms of interferon alpha neurotoxicity: Expression of GRIN3A subunit of NMDA receptors and NMDA-evoked exocytosis

Neurological and psychiatric side effects accompany the high-dose interferon-alpha (IFNA) therapy. The primary genes responsible for these complications are mostly unknown. Our genome-wide search in mouse and rat genomes for the conservative genes containing IFN-stimulated response elements (ISRE) in their promoters revealed a new potential target gene of IFNA, Grin3α, which encodes the 3A subunit of NMDA receptor. This study aimed to explore the impact of IFNA on the expression of Grin3α and Ifnα genes and neurotransmitters endo/exocytosis in the mouse brain. We administered recombinant human IFN-alpha 2b (rhIFN-α2b) intracranially, and 24 h later, we isolated six brain regions and used the samples for RT-qPCR and western blot analysis. Synaptosomes were isolated from the cortex to analyze endo/exocytosis with acridine orange and L-[14C]glutamate. IFNA induced an increase in Grin3α mRNA and GRIN3A protein, but a decrease in Ifnα mRNA and protein. IFNA did not affect the accumulation and distribution of L-[14C]glutamate and acridine orange between synaptosomes and the extra-synaptosomal space. It caused the more significant acridine orange release activated by NMDA or glutamate than from control mice's synaptosomes. In response to IFNA, the newly discovered association between elevated Grin3α expression and NMDA- and glutamate-evoked neurotransmitters release from synaptosomes implies a new molecular mechanism of IFNA neurotoxicity.

Expression of NMDA receptor subunits in human blood lymphocytes: A peripheral biomarker in online computer game addiction

Background and aims Repeated performance of some behaviors such as playing computer games could result in addiction. The NMDA receptor is critically involved in the development of behavioral and drug addictions. It has been claimed that the expression level of neurotransmitter receptors in the brain may be reflected in peripheral blood lymphocytes (PBLs). Methods Here, using a real-time PCR method, we have investigated the mRNA expression of GluN2A, GluN2D, GluN3A, and GluN3B subunits of the NMDA receptor in PBLs of male online computer game addicts (n = 25) in comparison with normal subjects (n = 26). Results Expression levels of GluN2A, GluN2D, and GluN3B subunits were not statistically different between game addicts and the control group. However, the mRNA expression of the GluN3A subunit was downregulated in PBLs of game addicts. Discussion and conclusions Transcriptional levels of GluN2A and GluN2D subunits in online computer game addicts are similar to our previously reported data of opioid addiction and are not different from the control group. However, unlike our earlier finding of drug addiction, the mRNA expression levels of GluN3A and GluN3B subunits in PBLs of game addicts are reduced and unchanged, respectively, compared with control subjects. It seems that the downregulated state of the GluN3A subunit of NMDA receptor in online computer game addicts is a finding that deserves more studies in the future to see whether it can serve as a peripheral biomarker in addiction studies, where the researcher wants to rule out the confusing effects of abused drugs.

NMDA Receptors in the Central Nervous System

NMDA-type glutamate receptors are ligand-gated ion channels that mediate a major component of excitatory neurotransmission in the central nervous system (CNS). They are widely distributed at all stages of development and are critically involved in normal brain functions, including neuronal development and synaptic plasticity. NMDA receptors are also implicated in the pathophysiology of numerous neurological and psychiatric disorders, such as ischemic stroke, traumatic brain injury, Alzheimer's disease, epilepsy, mood disorders, and schizophrenia. For these reasons, NMDA receptors have been intensively studied in the past several decades to elucidate their physiological roles and to advance them as therapeutic targets. Seven NMDA receptor subunits exist that assemble into a diverse array of tetrameric receptor complexes, which are differently regulated, have distinct regional and developmental expression, and possess a wide range of functional and pharmacological properties. The diversity in subunit composition creates NMDA receptor subtypes with distinct physiological roles across neuronal cell types and brain regions, and enables precise tuning of synaptic transmission. Here, we will review the relationship between NMDA receptor structure and function, the diversity and significance of NMDA receptor subtypes in the CNS, as well as principles and rules by which NMDA receptors operate in the CNS under normal and pathological conditions.

Ionotropic glutamate receptors: Which ones, when, and where in the mammalian neocortex

A multitude of 18 iGluR receptor subunits, many of which are diversified by splicing and RNA editing, localize to >20 excitatory and inhibitory neocortical neuron types defined by physiology, morphology, and transcriptome in addition to various types of glial, endothelial, and blood cells. Here we have compiled the published expression of iGluR subunits in the areas and cell types of developing and adult cortex of rat, mouse, carnivore, bovine, monkey, and human as determined with antibody- and mRNA-based techniques. iGluRs are differentially expressed in the cortical areas and in the species, and all have a unique developmental pattern. Differences are quantitative rather than a mere absence/presence of expression. iGluR are too ubiquitously expressed and of limited use as markers for areas or layers. A focus has been the iGluR profile of cortical interneuron types. For instance, GluK1 and GluN3A are enriched in, but not specific for, interneurons; moreover, the interneurons expressing these subunits belong to different types. Adressing the types is still a major hurdle because type-specific markers are lacking, and the frequently used neuropeptide/CaBP signatures are subject to regulation by age and activity and vary as well between species and areas. RNA-seq reveals almost all subunits in the two morphofunctionally characterized interneuron types of adult cortical layer I, suggesting a fairly broad expression at the RNA level. It remains to be determined whether all proteins are synthesized, to which pre- or postsynaptic subdomains in a given neuron type they localize, and whether all are involved in synaptic transmission. J. Comp. Neurol. 525:976-1033, 2017. © 2016 Wiley Periodicals, Inc.

GluN3A: an NMDA receptor subunit with exquisite properties and functions

N-methyl-D-aspartate receptors (NMDAR) are pivotal for synaptic plasticity and memory formation. Conventional NMDAR consist of heterotetrameric structures composed of GluN1 and GluN2 subunits. A third subunit, GluN3, can also assemble with NMDAR subunits giving a remarkable modification of their heteromeric structure, forming a "nonconventional" NMDAR. As a consequence, the stoichiometry and kinetic properties of the receptors are dramatically changed. Among the GluN3 family, the GluN3A subunit has been the focus of a large amount of studies during recent years. These studies reveal that GluN3A is transiently expressed during development and could play a role in the fine tuning of neuronal networks as well as associated diseases. Moreover, GluN3A distribution outside the postsynaptic densities, including perisynaptic astrocytes, places it at a strategic position to play an important role in the interactions between neurons and glial cells. This review highlights GluN3A properties and addresses its role in neurophysiology and associated pathologies.

Regulatory roles of the NMDA receptor GluN3A subunit in locomotion, pain perception and cognitive functions in adult mice

Since the discovery of the glutamate NMDA receptor subunit 3A (GluN3A), the functional role of this unique inhibitory subunit has been largely obscure. GluN3A expression is high in the neonatal brain but declines to a low level in the adult brain; it is thus commonly believed that GluN3A does not have a major functional impact in adulthood. Using wild-type (WT) and GluN3A knockout (KO) mice, we show here that deletion of GluN3A affected multiple behavioural functions in adult animals. GluN3A KO mice showed impaired locomotor activity on a variety of motor function tests, and increased sensitivity to acute and sub-acute inflammatory pain. GluN3A KO mice also showed enhanced recognition and spatial learning and memory functions. Hippocampal slices from juvenile and adult GluN3A KO mice showed greater long-term potentiation (LTP) compared with WT slices. GluN3A deletion resulted in increased expression of Ca(2+)/calmodulin-dependent kinase II (CaMKII) in the forebrain, and the phosphorylated CaMKII level upon LTP induction was significantly higher in the GluN3A KO hippocampus compared with WT controls. CaMKII inhibition abrogated the enhanced LTP in GluN3A KO slices. These data reveal for the first time that the presence of GluN3A may have profound impacts on several functional/behavioural activities in adult animals, and could be a therapeutic target for neurological disorders associated with NMDA receptor functions.

Influence of the NR3A subunit on NMDA receptor functions

Various combinations of subunits assemble to form the NMDA-type glutamate receptor (NMDAR), generating diversity in its functions. Here we review roles of the unique NMDAR subunit, NR3A, which acts in a dominant-negative manner to suppress receptor activity. NR3A-containing NMDARs display striking regional and temporal expression specificity, and, unlike most other NMDAR subtypes, they have a low conductance, are only modestly permeable to Ca(2+), and pass current at hyperpolarized potentials in the presence of magnesium. While glutamate activates triheteromeric NMDARs composed of NR1/NR2/NR3A subunits, glycine is sufficient to activate diheteromeric NR1/NR3A-containing receptors. NR3A dysfunction may contribute to neurological disorders involving NMDARs, and the subunit offers an attractive therapeutic target given its distinct pharmacological and structural properties.

Enhanced ethanol inhibition of recombinant N-methyl-D-aspartate receptors by magnesium: role of NR3A subunits

BACKGROUND

The effects of ethanol on brain function are thought to be partly because of altered activity of ion channels that regulate synaptic activity. Results from previous studies from this lab and others have shown that ethanol inhibits the function of the N-methyl-D-aspartate (NMDA) receptors, a calcium-permeable ion channel activated by the neurotransmitter glutamate. Factors that influence the acute sensitivity of NMDA receptors to ethanol may be critical in determining how neurons and neuronal networks respond to the presence of ethanol. In this study, we have examined the effect of physiologically relevant concentrations of magnesium on the ethanol sensitivity of recombinant NMDA receptors and how ethanol inhibition under these conditions is influenced by the NR3A subunit.

METHODS

Recombinant cDNAs encoding NMDA receptor subunits were expressed in human embryonic kidney 293 cells. Whole-cell patch-clamp electrophysiology was used to measure currents induced by rapid application of glutamate in the absence and presence of ethanol.

RESULTS

In magnesium-free recording solution, ethanol inhibited glutamate-mediated currents in cells transfected with NMDA receptor subunits. The magnitude of ethanol inhibition was significantly enhanced when recordings were carried out in media containing 1 mM magnesium. This effect was reversible and required magnesium-sensitive receptors. Magnesium did not enhance ethanol inhibition of glycine-activated NR1/NR3A/NR3B receptors. However, NR3A co-expression prevented the enhancement of ethanol's inhibitory effect on receptors composed of NR2A but not NR2B subunits.

CONCLUSIONS

These results suggest that under physiological conditions, NR3A may be an important regulator of the acute ethanol sensitivity of brain NMDA receptors.

Developmental regulation of the NMDA receptor subunits, NR3A and NR1, in human prefrontal cortex

Subunit composition of N-methyl-D-aspartate-type glutamate receptors (NMDARs) dictates their function, yet the ontogenic profiles of human NMDAR subunits from gestation to adulthood have not been determined. We examined NMDAR mRNA and protein development in human dorsolateral prefrontal cortex (DLPFC), an area in which NMDARs are critical for higher cognitive processing and NMDAR hypofunction is hypothesized in schizophrenia. Using quantitative reverse transcriptase-polymerase chain reaction and western blotting, we found NR1 expression begins low prenatally, peaks in adolescence, yet remains high throughout life, suggesting lifelong importance of NMDAR function. In contrast, NR3A levels are low during gestation, surge soon after birth, and decline progressively through adolescence and into adulthood. Because NR3A subunits uniquely attenuate NMDAR-mediated currents, limit calcium influx, and suppress dendritic spine formation, high levels during early childhood may be important for regulating neuroprotection and activity-dependent sculpting of synapses. We also examined whether subunit changes underlie reduced NMDAR activity in schizophrenia. Our results reveal normal NR1 and NR3A protein levels in DLPFC from schizophrenic patients, indicating that NMDAR hypofunction is unlikely to be maintained by gross changes in NR3A-containing NMDARs or overall NMDAR numbers. These data provide insights into NMDAR functions in the developing CNS and will contribute to designing pharmacotherapies for neurological disorders.

Analysis of NR3A receptor subunits in human native NMDA receptors

NR3A, representing the third class of NMDA receptor subunits, was first studied in rats, demonstrating ubiquitous expression in the developing central nervous system (CNS), but in the adult mainly expressed in spinal cord and some forebrain nuclei. Subsequent studies showed that rodent and non-human primate NR3A expression differs. We have studied the distribution of NR3A in the human CNS and show a widespread distribution of NR3A protein in adult human brain. NR3A mRNA and protein were found in all regions of the cerebral cortex, and also in the subcortical forebrain, midbrain and hindbrain. Only very low levels of NR3A mRNA and protein could be detected in homogenized adult human spinal cord, and in situ hybridization showed that expression was limited to ventral motoneurons. We found that NR3A is associated with NR1, NR2A and NR2B in adult human CNS, suggesting the existence of native NR1-NR2A/B-NR3A assemblies in adult human CNS. While NR1 and NR2A could only be efficiently solubilized by deoxycholate, NR3A was extracted by all detergents, suggesting that a large fraction is weakly anchored to cell membranes and other proteins. Using size exclusion chromatography we found that just as for NR1, a large fraction of NR3A exists as monomers and dimers, suggesting that these two glycine binding subunits behave similarly with regard to receptor assembly and trafficking.