Importance of the intracellular domain of NR2 subunits for NMDA receptor function in vivo

Inhibition of the NMDA Currents by Probenecid in Amygdaloid Kindling Epilepsy Model

Epilepsy is characterized by a sustained depolarization and repeated discharge of neurons, attributed to overstimulation of N-methyl-D-aspartate receptors (NMDAr). Herein, we propose that probenecid (PROB), an inhibitor of the activity of some ATP binding-cassette transporters (ABC-transporters) can modify NMDAr activity and expression in amygdaloid kindled model. Some studies have suggested that NMDAr expression could be regulated by inhibiting the activity of P-glycoprotein (MDR1) and drug resistance protein-1 (MRP1). Besides, PROB was found to interact with other proteins with proven activity in the kindling model, such as TRPV2 channels, OAT1, and Panx1. Administering PROB at two doses (100 and 300 mg/kg/d) for 5 d decreased after-discharge duration and Racine behavioral scores. It also reduced the expression of NR2B and the activity of total NOS and the expression of nNOS with respect to the kindling group. In a second protocol, voltage-clamp measurements of NMDA-evoked currents were performed in CA1 hippocampal cells dissociated from control and kindled rats. PROB produced a dose-dependent reduction in NMDA-evoked currents. In neurons from kindled rats, a residual NMDA-evoked current was registered with respect to control animals, while a reduction in NMDA-evoked currents was observed in the presence of 20 mM PROB. Finally, we evaluated the expression of MRP1 and MDR1 in order to establish a relationship between the reduction of kindling parameters, the inhibition of NMDA-type currents, and the expression of these transporters. Based on our results, we conclude that at the concentrations used, PROB inhibits currents evoked by NMDA in dissociated neurons of control and kindled rats. In the kindling model, at the tested doses, PROB decreases the after-discharge duration and Racine behavioral score in the kindling model. We propose a mechanism that could be dependent on the expression of ABC-type transporters.

Characterization of Mice Carrying a Neurodevelopmental Disease-Associated GluN2B(L825V) Variant

N-Methyl-d-aspartate receptors (NMDARs), encoded by GRIN genes, are ionotropic glutamate receptors playing a critical role in synaptic transmission, plasticity, and synapse development. Genome sequence analyses have identified variants in GRIN genes in patients with neurodevelopmental disorders, but the underlying disease mechanisms are not well understood. Here, we have created and evaluated a transgenic mouse line carrying a missense variant Grin2bL825V , corresponding to a de novo GRIN2B variant encoding GluN2B(L825V) found in a patient with intellectual disability (ID) and autism spectrum disorder (ASD). We used HEK293T cells expressing recombinant receptors and primary hippocampal neurons prepared from heterozygous Grin2bL825V/+ (L825V/+) and wild-type (WT) Grin2b+/+ (+/+) male and female mice to assess the functional impact of the variant. Whole-cell NMDAR currents were reduced in neurons from L825V/+ compared with +/+ mice. The peak amplitude of NMDAR-mediated evoked excitatory postsynaptic currents (NMDAR-eEPSCs) was unchanged, but NMDAR-eEPSCs in L825V/+ neurons had faster deactivation compared with +/+ neurons and were less sensitive to a GluN2B-selective antagonist ifenprodil. Together, these results suggest a decreased functional contribution of GluN2B subunits to synaptic NMDAR currents in hippocampal neurons from L825V/+ mice. The analysis of the GluN2B(L825V) subunit surface expression and synaptic localization revealed no differences compared with WT GluN2B. Behavioral testing of mice of both sexes demonstrated hypoactivity, anxiety, and impaired sensorimotor gating in the L825V/+ strain, particularly affecting males, as well as cognitive symptoms. The heterozygous L825V/+ mouse offers a clinically relevant model of GRIN2B-related ID/ASD, and our results suggest synaptic-level functional changes that may contribute to neurodevelopmental pathology.

Disease-Associated Variants in GRIN1, GRIN2A and GRIN2B genes: Insights into NMDA Receptor Structure, Function, and Pathophysiology

N-methyl-D-aspartate receptors (NMDARs) are a subtype of ionotropic glutamate receptors critical for synaptic transmission and plasticity, and for the development of neural circuits. Rare or de-novo variants in GRIN genes encoding NMDAR subunits have been associated with neurodevelopmental disorders characterized by intellectual disability, developmental delay, autism, schizophrenia, or epilepsy. In recent years, some disease-associated variants in GRIN genes have been characterized using recombinant receptors expressed in non-neuronal cells, and a few variants have also been studied in neuronal preparations or animal models. Here we review the current literature on the functional evaluation of human disease-associated variants in GRIN1, GRIN2A and GRIN2B genes at all levels of analysis. Focusing on the impact of different patient variants at the level of receptor function, we discuss effects on receptor agonist and co-agonist affinity, channel open probability, and receptor cell surface expression. We consider how such receptor-level functional information may be used to classify variants as gain-of-function or loss-of-function, and discuss the limitations of this classification at the synaptic, cellular, or system level. Together this work by many laboratories worldwide yields valuable insights into NMDAR structure and function, and represents significant progress in the effort to understand and treat GRIN disorders. Keywords: NMDA receptor , GRIN genes, Genetic variants, Electrophysiology, Synapse, Animal models.

Anterior cingulate cortex-related functional hyperconnectivity underlies sensory hypersensitivity in Grin2b-mutant mice

Sensory abnormalities are observed in ~90% of individuals with autism spectrum disorders (ASD), but the underlying mechanisms are poorly understood. GluN2B, an NMDA receptor subunit that regulates long-term depression and circuit refinement during brain development, has been strongly implicated in ASD, but whether GRIN2B mutations lead to sensory abnormalities remains unclear. Here, we report that Grin2b-mutant mice show behavioral sensory hypersensitivity and brain hyperconnectivity associated with the anterior cingulate cortex (ACC). Grin2b-mutant mice with a patient-derived C456Y mutation (Grin2bC456Y/+) show sensory hypersensitivity to mechanical, thermal, and electrical stimuli through supraspinal mechanisms. c-fos and functional magnetic resonance imaging indicate that the ACC is hyperactive and hyperconnected with other brain regions under baseline and stimulation conditions. ACC pyramidal neurons show increased excitatory synaptic transmission. Chemogenetic inhibition of ACC pyramidal neurons normalizes ACC hyperconnectivity and sensory hypersensitivity. These results suggest that GluN2B critically regulates ASD-related cortical connectivity and sensory brain functions.

Disease-associated nonsense and frame-shift variants resulting in the truncation of the GluN2A or GluN2B C-terminal domain decrease NMDAR surface expression and reduce potentiating effects of neurosteroids

N-methyl-D-aspartate receptors (NMDARs) play a critical role in normal brain function, and variants in genes encoding NMDAR subunits have been described in individuals with various neuropsychiatric disorders. We have used whole-cell patch-clamp electrophysiology, fluorescence microscopy and in-silico modeling to explore the functional consequences of disease-associated nonsense and frame-shift variants resulting in the truncation of GluN2A or GluN2B C-terminal domain (CTD). This study characterizes variant NMDARs and shows their reduced surface expression and synaptic localization, altered agonist affinity, increased desensitization, and reduced probability of channel opening. We also show that naturally occurring and synthetic steroids pregnenolone sulfate and epipregnanolone butanoic acid, respectively, enhance NMDAR function in a way that is dependent on the length of the truncated CTD and, further, is steroid-specific, GluN2A/B subunit-specific, and GluN1 splice variant-specific. Adding to the previously described effects of disease-associated NMDAR variants on the receptor biogenesis and function, our results improve the understanding of the molecular consequences of NMDAR CTD truncations and provide an opportunity for the development of new therapeutic neurosteroid-based ligands.

Treatment resistance NMDA receptor pathway polygenic score is associated with brain glutamate in schizophrenia

Dysfunction of glutamate neurotransmission has been implicated in the pathophysiology of schizophrenia and may be particularly relevant in severe, treatment-resistant symptoms. The underlying mechanism may involve hypofunction of the NMDA receptor. We investigated whether schizophrenia-related pathway polygenic scores, composed of genetic variants within NMDA receptor encoding genes, are associated with cortical glutamate in schizophrenia. Anterior cingulate cortex (ACC) glutamate was measured in 70 participants across 4 research sites using Proton Magnetic Resonance Spectroscopy (1H-MRS). Two NMDA receptor gene sets were sourced from the Molecular Signatories Database and NMDA receptor pathway polygenic scores were constructed using PRSet. The NMDA receptor pathway polygenic scores were weighted by single nucleotide polymorphism (SNP) associations with treatment-resistant schizophrenia, and associations with ACC glutamate were tested. We then tested whether NMDA receptor pathway polygenic scores with SNPs weighted by associations with non-treatment-resistant schizophrenia were associated with ACC glutamate. A higher NMDA receptor complex pathway polygenic score was significantly associated with lower ACC glutamate (β = -0.25, 95 % CI = -0.49, -0.02, competitive p = 0.03). When SNPs were weighted by associations with non-treatment-resistant schizophrenia, there was no association between the NMDA receptor complex pathway polygenic score and ACC glutamate (β = 0.05, 95 % CI = -0.18, 0.27, competitive p = 0.79). These results provide initial evidence of an association between common genetic variation implicated in NMDA receptor function and ACC glutamate levels in schizophrenia. This association was specific to when the NMDA receptor complex pathway polygenic score was weighted by SNP associations with treatment-resistant schizophrenia.

GRIN2A (NR2A): a gene contributing to glutamatergic involvement in schizophrenia

Involvement of the glutamate system, particularly N-methyl-D-aspartate (NMDA) receptor hypofunction, has long been postulated to be part of the pathophysiology of schizophrenia. An important development is provided by recent data that strongly implicate GRIN2A, the gene encoding the NR2A (GluN2A) NMDA receptor subunit, in the aetiology of the disorder. Rare variants and common variants are both robustly associated with genetic risk for schizophrenia. Some of the rare variants are point mutations likely affecting channel function, but most are predicted to cause protein truncation and thence result, like the common variants, in reduced gene expression. We review the genomic evidence, and the findings from Grin2a mutant mice and other models which give clues as to the likely phenotypic impacts of GRIN2A genetic variation. We suggest that one consequence of NR2A dysfunction is impairment in a form of hippocampal synaptic plasticity, producing deficits in short-term habituation and thence elevated and dysregulated levels of attention, a phenotype of relevance to schizophrenia and its cognitive aspects.

Structural insights into NMDA receptor pharmacology

N-methyl-d-aspartate receptors (NMDARs) comprise a subfamily of ionotropic glutamate receptors that form heterotetrameric ligand-gated ion channels and play fundamental roles in neuronal processes such as synaptic signaling and plasticity. Given their critical roles in brain function and their therapeutic importance, enormous research efforts have been devoted to elucidating the structure and function of these receptors and developing novel therapeutics. Recent studies have resolved the structures of NMDARs in multiple functional states, and have revealed the detailed gating mechanism, which was found to be distinct from that of other ionotropic glutamate receptors. This review provides a brief overview of the recent progress in understanding the structures of NMDARs and the mechanisms underlying their function, focusing on subtype-specific, ligand-induced conformational dynamics.

mGluR5 from Primary Sensory Neurons Promotes Opioid-Induced Hyperalgesia and Tolerance by Interacting with and Potentiating Synaptic NMDA Receptors

Aberrant activation of presynaptic NMDARs in the spinal dorsal horn is integral to opioid-induced hyperalgesia and analgesic tolerance. However, the signaling mechanisms responsible for opioid-induced NMDAR hyperactivity remain poorly identified. Here, we show that repeated treatment with morphine or fentanyl reduced monomeric mGluR5 protein levels in the dorsal root ganglion (DRG) but increased levels of mGluR5 monomers and homodimers in the spinal cord in mice and rats of both sexes. Coimmunoprecipitation analysis revealed that monomeric and dimeric mGluR5 in the spinal cord, but not monomeric mGluR5 in the DRG, directly interacted with GluN1. By contrast, mGluR5 did not interact with μ-opioid receptors in the DRG or spinal cord. Repeated morphine treatment markedly increased the mGluR5-GluN1 interaction and protein levels of mGluR5 and GluN1 in spinal synaptosomes. The mGluR5 antagonist MPEP reversed morphine treatment-augmented mGluR5-GluN1 interactions, GluN1 synaptic expression, and dorsal root-evoked monosynaptic EPSCs of dorsal horn neurons. Furthermore, CRISPR-Cas9-induced conditional mGluR5 knockdown in DRG neurons normalized mGluR5 levels in spinal synaptosomes and NMDAR-mediated EPSCs of dorsal horn neurons increased by morphine treatment. Correspondingly, intrathecal injection of MPEP or conditional mGluR5 knockdown in DRG neurons not only potentiated the acute analgesic effect of morphine but also attenuated morphine treatment-induced hyperalgesia and tolerance. Together, our findings suggest that opioid treatment promotes mGluR5 trafficking from primary sensory neurons to the spinal dorsal horn. Through dimerization and direct interaction with NMDARs, presynaptic mGluR5 potentiates and/or stabilizes NMDAR synaptic expression and activity at primary afferent central terminals, thereby maintaining opioid-induced hyperalgesia and tolerance.SIGNIFICANCE STATEMENT Opioids are essential analgesics for managing severe pain caused by cancer, surgery, and tissue injury. However, these drugs paradoxically induce pain hypersensitivity and tolerance, which can cause rapid dose escalation and even overdose mortality. This study demonstrates, for the first time, that opioids promote trafficking of mGluR5, a G protein-coupled glutamate receptor, from peripheral sensory neurons to the spinal cord; there, mGluR5 proteins dimerize and physically interact with NMDARs to augment their synaptic expression and activity. Through dynamic interactions, the two distinct glutamate receptors mutually amplify and sustain nociceptive input from peripheral sensory neurons to the spinal cord. Thus, inhibiting mGluR5 activity or disrupting mGluR5-NMDAR interactions could reduce opioid-induced hyperalgesia and tolerance and potentiate opioid analgesic efficacy.

Loss of TDP-43 function underlies hippocampal and cortical synaptic deficits in TDP-43 proteinopathies

TDP-43 proteinopathy is linked to neurodegenerative diseases that feature synaptic loss in the cortex and hippocampus, although it remains unclear how TDP-43 regulates mature synapses. We report that, in adult mouse hippocampus, TDP-43 knockdown, but not overexpression, induces robust structural and functional damage to excitatory synapses, supporting a role for TDP-43 in maintaining mature synapses. Dendritic spine loss induced by TDP-43 knockdown is rescued by wild-type TDP-43, but not ALS/FTLD-associated mutants, suggesting a common TDP-43 functional deficiency in neurodegenerative diseases. Interestingly, M337V and A90V mutants also display dominant negative activities against WT TDP-43, partially explaining why M337V transgenic mice develop hippocampal degeneration similar to that in excitatory neuronal TDP-43 knockout mice, and why A90V mutation is associated with Alzheimer's disease. Further analyses reveal that a TDP-43 knockdown-induced reduction in GluN2A contributes to synaptic loss. Our results show that loss of TDP-43 function underlies hippocampal and cortical synaptic degeneration in TDP-43 proteinopathies.

Complex functional phenotypes of NMDA receptor disease variants

NMDA receptors have essential roles in the physiology of central excitatory synapses and their dysfunction causes severe neuropsychiatric symptoms. Recently, a series of genetic variants have been identified in patients, however, functional information about these variants is sparse and their role in pathogenesis insufficiently known. Here we investigate the mechanism by which two GluN2A variants may be pathogenic. We use molecular dynamics simulation and single-molecule electrophysiology to examine the contribution of GluN2A subunit-residues, P552 and F652, and their pathogenic substitutions, P552R and F652V, affect receptor functions. We found that P552 and F652 interact during the receptors' normal activity cycle; the interaction stabilizes receptors in open conformations and is required for a normal electrical response. Engineering shorter side-chains at these positions (P552A and/or F652V) caused a loss of interaction energy and produced receptors with severe gating, conductance, and permeability deficits. In contrast, the P552R side chain resulted in stronger interaction and produced a distinct, yet still drastically abnormal electrical response. These results identify the dynamic contact between P552 and F652 as a critical step in the NMDA receptor activation, and show that both increased and reduced communication through this interaction cause dysfunction. Results show that subtle differences in NMDA receptor primary structure can generate complex phenotypic alterations whose binary classification is too simplistic to serve as a therapeutic guide.

Protein quality control of N-methyl-D-aspartate receptors

N-methyl-D-aspartate receptors (NMDARs) are glutamate-gated cation channels that mediate excitatory neurotransmission and are critical for synaptic development and plasticity in the mammalian central nervous system (CNS). Functional NMDARs typically form via the heterotetrameric assembly of GluN1 and GluN2 subunits. Variants within GRIN genes are implicated in various neurodevelopmental and neuropsychiatric disorders. Due to the significance of NMDAR subunit composition for regional and developmental signaling at synapses, properly folded receptors must reach the plasma membrane for their function. This review focuses on the protein quality control of NMDARs. Specifically, we review the quality control mechanisms that ensure receptors are correctly folded and assembled within the endoplasmic reticulum (ER) and trafficked to the plasma membrane. Further, we discuss disease-associated variants that have shown disrupted NMDAR surface expression and function. Finally, we discuss potential targeted pharmacological and therapeutic approaches to ameliorate disease phenotypes by enhancing the expression and surface trafficking of subunits harboring disease-associated variants, thereby increasing their incorporation into functional receptors.

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.

Developmental regulation and lateralization of N-methyl-d-aspartate (NMDA) receptors in the rat hippocampus

N-methyl-d-aspartate receptors (NMDARs) are expressed abundantly in the brain and play a crucial role in the regulation of central nervous system (CNS) development, learning, and memory. During early neuronal development, NMDARs modulate neurogenesis, neuronal differentiation and migration, and synaptogenesis. The present study aimed to examine the developmental expression of NMDARs subunits, NR1 and NR2B, in the developing hippocampus of neonatal rats during the first two postnatal weeks. Fifty-four male offspring were randomly divided into three age groups, postnatal days (P) 0, 7, and 14. Real-time-PCR, western blotting, and immunohistochemistry (IHC) analyses were employed to examine and compare the hippocampal expression of the NMDA receptor subunits. The highest mRNA expression of NR1 and NR2B subunits was observed at P7, regardless of its laterality. The mRNA expression of both subunits in the right hippocampus was significantly higher than that of the left one at P0 and P7. Similarly, the highest protein level expression of NR1 and NR2B subunits was also observed at P7 in both sides hippocampi. Although the protein expression of NR1 was significantly higher on the right side in all studied days, the NR2B was significantly higher in the right hippocampus only at P7. The analysis of optical density (OD) has shown a marked increase in the distribution pattern of the NR1 and NR2B subunits at P7 in all hippocampal subregions. In conclusion, there is a marked right-left asymmetry in the expression of NR1 and NR2B subunits in the developing rat hippocampus, which might be considered as a probable mechanism for the lateral differences in the structure and function of the hippocampus in rats.

Protein Kinase C-Mediated Phosphorylation and α2δ-1 Interdependently Regulate NMDA Receptor Trafficking and Activity

N-methyl-d-aspartate receptors (NMDARs) are important for synaptic plasticity associated with many physiological functions and neurologic disorders. Protein kinase C (PKC) activation increases the phosphorylation and activity of NMDARs, and α2δ-1 is a critical NMDAR-interacting protein and controls synaptic trafficking of NMDARs. In this study, we determined the relative roles of PKC and α2δ-1 in the control of NMDAR activity. We found that α2δ-1 coexpression significantly increased NMDAR activity in HEK293 cells transfected with GluN1/GluN2A or GluN1/GluN2B. PKC activation with phorbol 12-myristate 13-acetate (PMA) increased receptor activity only in cells coexpressing GluN1/GluN2A and α2δ-1. Remarkably, PKC inhibition with Gӧ6983 abolished α2δ-1-coexpression-induced potentiation of NMDAR activity in cells transfected with GluN1/GluN2A or GluN1/GluN2B. Treatment with PMA increased the α2δ-1-GluN1 interaction and promoted α2δ-1 and GluN1 cell surface trafficking. PMA also significantly increased NMDAR activity of spinal dorsal horn neurons and the amount of α2δ-1-bound GluN1 protein complexes in spinal cord synaptosomes in wild-type mice, but not in α2δ-1 knockout mice. Furthermore, inhibiting α2δ-1 with pregabalin or disrupting the α2δ-1-NMDAR interaction with the α2δ-1 C-terminus peptide abolished the potentiating effect of PMA on NMDAR activity. Additionally, using quantitative phosphoproteomics and mutagenesis analyses, we identified S929 on GluN2A and S1413 (S1415 in humans) on GluN2B as the phosphorylation sites responsible for NMDAR potentiation by PKC and α2δ-1. Together, our findings demonstrate the interdependence of α2δ-1 and PKC phosphorylation in regulating NMDAR trafficking and activity. The phosphorylation-dependent, dynamic α2δ-1-NMDAR interaction constitutes an important molecular mechanism of synaptic plasticity.SIGNIFICANCE STATEMENT A major challenge in studies of protein phosphorylation is to define the functional significance of each phosphorylation event and determine how various signaling pathways are coordinated in response to neuronal activity to shape synaptic plasticity. PKC phosphorylates transporters, ion channels, and G-protein-coupled receptors in signal transduction. In this study, we showed that α2δ-1 is indispensable for PKC-activation-induced surface and synaptic trafficking of NMDARs, whereas the α2δ-1-NMDAR interaction is controlled by PKC-induced phosphorylation. Our findings reveal that α2δ-1 mainly functions as a phospho-binding protein in the control of NMDAR trafficking and activity. This information provides new mechanistic insight into the reciprocal roles of PKC-mediated phosphorylation and α2δ-1 in regulating NMDARs and in the therapeutic actions of gabapentinoids.

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.

Regulation of the NMDA receptor by its cytoplasmic domains: (How) is the tail wagging the dog?

Excitatory neurotransmission mediated by N-methyl-d-aspartate receptors (NMDARs) is critical for synapse development, function, and plasticity in the brain. NMDARs are tetra-heteromeric cation-channels that mediate synaptic transmission and plasticity. Extensive human studies show the existence of genetic variants in NMDAR subunits genes (GRIN genes) that are associated with neurodevelopmental and neuropsychiatric disorders, including autism spectrum disorders (ASD), epilepsy (EP), intellectual disability (ID), attention deficit hyperactivity disorder (ADHD), and schizophrenia (SCZ). NMDAR subunits have a unique modular architecture with four semiautonomous domains. Here we focus on the carboxyl terminal domain (CTD), also known as the intracellular C-tail, which varies in length among the glutamate receptor subunits and is the most diverse domain in terms of amino acid sequence. The CTD shows no sequence homology to any known proteins but encodes short docking motifs for intracellular binding proteins and covalent modifications. Our review will discuss the many important functions of the CTD in regulating NMDA membrane and synaptic targeting, stabilization, degradation targeting, allosteric modulation and metabotropic signaling of the receptor. This article is part of the special issue on 'Glutamate Receptors - NMDA Receptors'.

Protein-protein interactions at the NMDA receptor complex: From synaptic retention to synaptonuclear protein messengers

N-methyl-d-aspartate receptors (NMDARs) are glutamate-gated ion channels that support essential functions throughout the brain. NMDARs are tetramers composed of the GluN1 subunit in complex with GluN2- and GluN3-type regulatory subunits, resulting in the formation of various receptor subtypes throughout the central nervous system (CNS), characterised by different kinetics, biophysical and pharmacological properties, and the abilities to interact with specific partners at dendritic spines. NMDARs are expressed at high levels, are widely distributed throughout the brain, and are involved in several physiological and pathological conditions. Here, we will focus on the GluN2A- and GluN2B-containing NMDARs found at excitatory synapses and their interactions with plasticity-relevant proteins, such as the postsynaptic density family of membrane-associated guanylate kinases (PSD-MAGUKs), Ca²⁺/calmodulin-dependent kinase II (CaMKII) and synaptonuclear protein messengers. The dynamic interactions between NMDAR subunits and various proteins regulating synaptic receptor retention and synaptonuclear signalling mediated by protein messengers suggest that the NMDAR serves as a key molecular player that coordinates synaptic activity and cell-wide events that require gene transcription. Importantly, protein-protein interactions at the NMDAR complex can also contribute to synaptic dysfunction in several brain disorders. Therefore, the modulation of the molecular composition of the NMDAR complex might represent a novel pharmacological approach for the treatment of certain disease states.

Voltage-independent GluN2A-type NMDA receptor Ca2+ signaling promotes audiogenic seizures, attentional and cognitive deficits in mice

The NMDA receptor-mediated Ca2+ signaling during simultaneous pre- and postsynaptic activity is critically involved in synaptic plasticity and thus has a key role in the nervous system. In GRIN2-variant patients alterations of this coincidence detection provoked complex clinical phenotypes, ranging from reduced muscle strength to epileptic seizures and intellectual disability. By using our gene-targeted mouse line (Grin2aN615S), we show that voltage-independent glutamate-gated signaling of GluN2A-containing NMDA receptors is associated with NMDAR-dependent audiogenic seizures due to hyperexcitable midbrain circuits. In contrast, the NMDAR antagonist MK-801-induced c-Fos expression is reduced in the hippocampus. Likewise, the synchronization of theta- and gamma oscillatory activity is lowered during exploration, demonstrating reduced hippocampal activity. This is associated with exploratory hyperactivity and aberrantly increased and dysregulated levels of attention that can interfere with associative learning, in particular when relevant cues and reward outcomes are disconnected in space and time. Together, our findings provide (i) experimental evidence that the inherent voltage-dependent Ca2+ signaling of NMDA receptors is essential for maintaining appropriate responses to sensory stimuli and (ii) a mechanistic explanation for the neurological manifestations seen in the NMDAR-related human disorders with GRIN2 variant-meidiated intellectual disability and focal epilepsy.

D-serine reduces memory impairment and neuronal damage induced by chronic lead exposure

Although exogenous D-serine has been applied as a neural regulatory intervention in many studies, the role played by D-serine in hippocampal injuries caused by lead exposure remains poorly understood. Rat models of chronic lead exposure were established through the administration of 0.05% lead acetate for 8 weeks. Simultaneously, rats were administered 30 or 60 mg/kg D-serine, intraperitoneally, twice a day. Our results showed that D-serine treatment shortened the escape latency from the Morris water maze, increased the number of times that mice crossed the original platform location, and alleviated the pathological damage experienced by hippocampal neurons in response to lead exposure. Although D-serine administration did not increase the expression levels of the N-methyl-D-aspartate receptor subtype 2B (NR2B) in the hippocampi of lead-exposed rats, 60 mg/kg D-serine treatment restored the expression levels of NR2A, which are reduced by lead exposure. These findings suggested that D-serine can alleviate learning and memory impairments induced by lead exposure and that the underlying mechanism is associated with the increased expression of NR2A in the hippocampus. This study was approved by the Animal Ethics Committee of North China University of Science and Technology, China (approval No. LX2018155) on December 21, 2018.

Acute blockade of NR2C/D subunit-containing N-methyl-D-aspartate receptors modifies sleep and neural oscillations in mice

N-methyl-d-aspartate receptors (NMDARs) play an important role in excitatory neurotransmission and have been associated with psychiatric conditions including schizophrenia and major depressive disorder. NMDARs are composed of two NR1 and two NR2 subunits. The type of NR2 subunit determines electrophysiological and pharmacological properties of the receptor. As the precise role of NR2C/D subunit-containing NMDARs is poorly understood in vivo, we have performed behavioural, quantitative electroencephalographic (qEEG) and polysomnographic analysis following acute pharmacological blockade of these receptor subtypes in adult male CD1 mice. We found that NR2C/D blockade impaired motor coordination and decreased the amount of gross movement. Moreover, EEG power in multiple frequency bands including theta and sigma were found to decrease significantly together with a decrease of theta oscillation frequency. Changes of these qEEG measures were accompanied by a decrease in time spent in slow-wave and rapid eye movement sleep, but an increase of time spent in quiet wakefulness. Furthermore, there was a significant decrease of sleep spindle oscillation density. These findings highlight the importance of NR2C/D-containing NMDARs and take a step towards establishing a link between electrophysiological correlates of psychiatric disorders and underlying synaptic dysfunctions.

Architecture and function of NMDA receptors: an evolutionary perspective

Ionotropic glutamate receptors (iGluRs) are a major class of ligand-gated ion channels that are widespread in the living kingdom. Their critical role in excitatory neurotransmission and brain function of arthropods and vertebrates has made them a compelling subject of interest for neurophysiologists and pharmacologists. This is particularly true for NMDA receptor (NMDARs), a subclass of iGluRs that act as central drivers of synaptic plasticity in the CNS. How and when the unique properties of NMDARs arose during evolution, and how they relate to the evolution of the nervous system, remain open questions. Recent years have witnessed a boom in both genomic and structural data, such that it is now possible to analyse the evolution of iGluR genes on an unprecedented scale and within a solid molecular framework. In this review, combining insights from phylogeny, atomic structure and physiological and mechanistic data, we discuss how evolution of NMDAR motifs and sequences shaped their architecture and functionalities. We trace differences and commonalities between NMDARs and other iGluRs, emphasizing a few distinctive properties of the former regarding ligand binding and gating, permeation, allosteric modulation and intracellular signalling. Finally, we speculate on how specific molecular properties of iGuRs arose to supply new functions to the evolving structure of the nervous system, from early metazoan to present mammals.

Alternative Anesthesia of Neonatal Mice for Global rAAV Delivery in the Brain With Non-detectable Behavioral Interference in Adults

Viral-transduced gene expression is the current standard for cell-type-specific labeling and cell tacking in experimental neuroscience. To achieve widespread gene expression, a viral delivery method to neonatal rodents was introduced more than two decades ago. Most of those neonatal viral vector injection-based gene transduction methods in mice used deep hypothermia for anesthesia, which was reported to be associated with behavioral impairments. To explore other options for neonatal viral applications, we applied a combination of Medetomidine, Midazolam, and Fentanyl (MMF), each of which can be antagonized by a specific antagonist. Later in their adulthood, we found that adult mice, that received the MMF-induced anesthesia, combined with virus-injected into the brain at postnatal day 2, showed similar performance in all behavioral tasks tested, including tasks for motor coordination, anxiety-related tasks, and spatial memory when compared to adult naïve littermates. This demonstrates that MMF anesthesia could be safely applied to mice for neonatal viral transduction at P2.

The C-terminal domains of the NMDA receptor: How intrinsically disordered tails affect signalling, plasticity and disease

NMDA receptors are part of the ionotropic glutamate receptor family, and are crucial for neurotransmission and memory. At the cellular level, the effects of activating these receptors include long-term potentiation (LTP) or depression (LTD). The NMDA receptor is a stringently gated cation channel permeable to Ca²⁺ , and it shares the molecular architecture of a tetrameric ligand-gated ion channel with the other family members. Its subunits, however, have uniquely long cytoplasmic C-terminal domains (CTDs). While the molecular gymnastics of the extracellular domains have been described in exquisite detail, much less is known about the structure and function of these CTDs. The CTDs vary dramatically in length and sequence between receptor subunits, but they all have a composition characteristic of intrinsically disordered proteins. The CTDs affect channel properties, trafficking and downstream signalling output from the receptor, and these functions are regulated by alternative splicing, protein-protein interactions, and post-translational modifications such as phosphorylation and palmitoylation. Here, we review the roles of the CTDs in synaptic plasticity with a focus on biochemical mechanisms. In total, the CTDs play a multifaceted role as a modifier of channel function, a regulator of cellular location and abundance, and signalling scaffold control the downstream signalling output.

Early correction of synaptic long-term depression improves abnormal anxiety-like behavior in adult GluN2B-C456Y-mutant mice

Extensive evidence links Glutamate receptor, ionotropic, NMDA2B (GRIN2B), encoding the GluN2B/NR2B subunit of N-methyl-D-aspartate receptors (NMDARs), with various neurodevelopmental disorders, including autism spectrum disorders (ASDs), but the underlying mechanisms remain unclear. In addition, it remains unknown whether mutations in GluN2B, which starts to be expressed early in development, induces early pathophysiology that can be corrected by early treatments for long-lasting effects. We generated and characterized Grin2b-mutant mice that carry a heterozygous, ASD-risk C456Y mutation (Grin2b+/C456Y). In Grin2b+/C456Y mice, GluN2B protein levels were strongly reduced in association with decreased hippocampal NMDAR currents and NMDAR-dependent long-term depression (LTD) but unaltered long-term potentiation, indicative of mutation-induced protein degradation and LTD sensitivity. Behaviorally, Grin2b+/C456Y mice showed normal social interaction but exhibited abnormal anxiolytic-like behavior. Importantly, early, but not late, treatment of young Grin2b+/C456Y mice with the NMDAR agonist D-cycloserine rescued NMDAR currents and LTD in juvenile mice and improved anxiolytic-like behavior in adult mice. Therefore, GluN2B-C456Y haploinsufficiency decreases GluN2B protein levels, NMDAR-dependent LTD, and anxiety-like behavior, and early activation of NMDAR function has long-lasting effects on adult mouse behavior.

A Model to Study NMDA Receptors in Early Nervous System Development

N-methyl-D-aspartate receptors (NMDARs) are glutamate-gated ion channels that play critical roles in neuronal development and nervous system function. Here, we developed a model to study NMDARs in early development in zebrafish, by generating CRISPR-mediated lesions in the NMDAR genes, grin1a and grin1b, which encode the obligatory GluN1 subunits. While receptors containing grin1a or grin1b show high Ca²⁺ permeability, like their mammalian counterpart, grin1a is expressed earlier and more broadly in development than grin1b Both grin1a ^-/- and grin1b ^-/- zebrafish are viable. Unlike in rodents, where the grin1 knockout is embryonic lethal, grin1 double-mutant fish (grin1a ^-/- ; grin1b ^-/-), which lack all NMDAR-mediated synaptic transmission, survive until ∼10 d dpf (days post fertilization), providing a unique opportunity to explore NMDAR function during development and in generating behaviors. Many behavioral defects in the grin1 double-mutant larvae, including abnormal evoked responses to light and acoustic stimuli, prey-capture deficits, and a failure to habituate to acoustic stimuli, are replicated by short-term treatment with the NMDAR antagonist MK-801, suggesting that they arise from acute effects of compromised NMDAR-mediated transmission. Other defects, however, such as periods of hyperactivity and alterations in place preference, are not phenocopied by MK-801, suggesting a developmental origin. Together, we have developed a unique model to study NMDARs in the developing vertebrate nervous system.SIGNIFICANCE STATEMENT Rapid communication between cells in the nervous system depends on ion channels that are directly activated by chemical neurotransmitters. One such ligand-gated ion channel, the NMDAR, impacts nearly all forms of nervous system function. It has been challenging, however, to study the prolonged absence of NMDARs in vertebrates, and hence their role in nervous system development, due to experimental limitations. Here, we demonstrate that zebrafish lacking all NMDAR transmission are viable through early development and are capable of a wide range of stereotypic behaviors. As such, this zebrafish model provides a unique opportunity to study the role of NMDAR in the development of the early vertebrate nervous system.

Regulation of NMDA glutamate receptor functions by the GluN2 subunits

The N-methyl-D-aspartate receptors (NMDARs) are ionotropic glutamate receptors that mediate the flux of calcium (Ca²⁺ ) into the post-synaptic compartment. Ca²⁺ influx subsequently triggers the activation of various intracellular signalling cascades that underpin multiple forms of synaptic plasticity. Functional NMDARs are assembled as heterotetramers composed of two obligatory GluN1 subunits and two GluN2 or GluN3 subunits. Four different GluN2 subunits (GluN2A-D) are present throughout the central nervous system; however, they are differentially expressed, both developmentally and spatially, in a cell- and synapse-specific manner. Each GluN2 subunit confers NMDARs with distinct ion channel properties and intracellular trafficking pathways. Regulated membrane trafficking of NMDARs is a dynamic process that ultimately determines the number of NMDARs at synapses, and is controlled by subunit-specific interactions with various intracellular regulatory proteins. Here we review recent progress made towards understanding the molecular mechanisms that regulate the trafficking of GluN2-containing NMDARs, focusing on the roles of several key synaptic proteins that interact with NMDARs via their carboxyl termini.

Knock-in Mice Expressing an Ethanol-Resistant GluN2A NMDA Receptor Subunit Show Altered Responses to Ethanol

BACKGROUND

N-methyl-D-aspartate receptors (NMDARs) are glutamate-activated, heterotetrameric ligand-gated ion channels critically important in virtually all aspects of glutamatergic signaling. Ethanol (EtOH) inhibition of NMDARs is thought to mediate specific actions of EtOH during acute and chronic exposure. Studies from our laboratory, and others, identified EtOH-sensitive sites within specific transmembrane (TM) domains involved in channel gating as well as those in subdomains of extracellular and intracellular regions of GluN1 and GluN2 subunits that affect channel function. In this study, we characterize for the first time the physiological and behavioral effects of EtOH on knock-in mice expressing a GluN2A subunit that shows reduced sensitivity to EtOH.

METHODS

A battery of tests evaluating locomotion, anxiety, sedation, motor coordination, and voluntary alcohol intake were performed in wild-type mice and those expressing the GluN2A A825W knock-in mutation. Whole-cell patch-clamp electrophysiological recordings were used to confirm reduced EtOH sensitivity of NMDAR-mediated currents in 2 separate brain regions (mPFC and the cerebellum) where the GluN2A subunit is known to contribute to NMDAR-mediated responses.

RESULTS

Male and female mice homozygous for the GluN2A(A825W) knock-in mutation showed reduced EtOH inhibition of NMDAR-mediated synaptic currents in mPFC and cerebellar neurons as compared to their wild-type counterparts. GluN2A(A825W) male but not female mice were less sensitive to the sedative and motor-incoordinating effects of EtOH and showed a rightward shift in locomotor-stimulating effects of EtOH. There was no effect of the mutation on EtOH-induced anxiolysis or voluntary EtOH consumption in either male or female mice.

CONCLUSIONS

These findings show that expression of EtOH-resistant GluN2A NMDARs results in selective and sex-specific changes in the behavioral sensitivity to EtOH.

NMDAR PAMs: Multiple Chemotypes for Multiple Binding Sites

The N-methyl-D-aspartate receptor (NMDAR) is a member of the ionotropic glutamate receptor (iGluR) family that plays a crucial role in brain signalling and development. NMDARs are nonselective cation channels that are involved with the propagation of excitatory neurotransmission signals with important effects on synaptic plasticity. NMDARs are functionally and structurally complex receptors, they exist as a family of subtypes each with its own unique pharmacological properties. Their implication in a variety of neurological and psychiatric conditions means they have been a focus of research for many decades. Disruption of NMDAR-related signalling is known to adversely affect higherorder cognitive functions (e.g. learning and memory) and the search for molecules that can recover (or even enhance) receptor output is a current strategy for CNS drug discovery. A number of positive allosteric modulators (PAMs) that specifically attempt to overcome NMDAR hypofunction have been discovered. They include various chemotypes that have been found to bind to several different binding sites within the receptor. The heterogeneity of chemotype, binding site and NMDAR subtype provide a broad landscape of ongoing opportunities to uncover new features of NMDAR pharmacology. Research on NMDARs continues to provide novel mechanistic insights into receptor activation and this review will provide a high-level overview of the research area and discuss the various chemical classes of PAMs discovered so far.

Linking NMDA Receptor Synaptic Retention to Synaptic Plasticity and Cognition

NMDA receptor (NMDAR) subunit composition plays a pivotal role in synaptic plasticity at excitatory synapses. Still, the mechanisms responsible for the synaptic retention of NMDARs following induction of plasticity need to be fully elucidated. Rabphilin3A (Rph3A) is involved in the stabilization of NMDARs at synapses through the formation of a complex with GluN2A and PSD-95. Here we used different protocols to induce synaptic plasticity in the presence or absence of agents modulating Rph3A function. The use of Forskolin/Rolipram/Picrotoxin cocktail to induce chemical LTP led to synaptic accumulation of Rph3A and formation of synaptic GluN2A/Rph3A complex. Notably, Rph3A silencing or use of peptides interfering with the GluN2A/Rph3A complex blocked LTP induction. Moreover, in vivo disruption of GluN2A/Rph3A complex led to a profound alteration of spatial memory. Overall, our results demonstrate a molecular mechanism needed for NMDAR stabilization at synapses after plasticity induction and to trigger downstream signaling events necessary for cognitive behavior.

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LTP induces trafficking of Rph3A at synapses and formation of GluN2A/Rph3A complex

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Disruption of Rph3A/GluN2A complex leads to LTP impairment

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Rph3A/GluN2A complex is needed for modifications of dendritic spines induced by LTP

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Disruption of Rph3A/GluN2A complex leads to spatial memory impairment

Disruption of NMDAR Function Prevents Normal Experience-Dependent Homeostatic Synaptic Plasticity in Mouse Primary Visual Cortex

Homeostatic regulation of synaptic strength allows for maintenance of neural activity within a dynamic range for proper circuit function. There are largely two distinct modes of synaptic plasticity that allow for homeostatic adaptation of cortical circuits: synaptic scaling and sliding threshold (BCM theory). Previous findings suggest that the induction of synaptic scaling is not prevented by blocking NMDARs, whereas the sliding threshold model posits that the synaptic modification threshold of LTP and LTD readjusts with activity and thus the outcome of synaptic plasticity is NMDAR dependent. Although synaptic scaling and sliding threshold have been considered two distinct mechanisms, there are indications from recent studies that these two modes of homeostatic plasticity may interact or that they may operate under two distinct activity regimes. Here, we report using both sexes of mouse that acute genetic knock-out of the obligatory subunit of NMDAR or acute pharmacological block of NMDAR prevents experience-dependent homeostatic regulation of AMPAR-mediated miniature EPSCs in layer 2/3 of visual cortex. This was not due to gross changes in postsynaptic neuronal activity with inhibiting NMDAR function as determine by c-Fos expression and two-photon Ca²⁺ imaging in awake mice. Our results suggest that experience-dependent homeostatic regulation of intact cortical circuits is mediated by NMDAR-dependent plasticity mechanisms, which supports a sliding threshold model of homeostatic adaptation.SIGNIFICANCE STATEMENT Prolonged changes in sensory experience lead to homeostatic adaptation of excitatory synaptic strength in sensory cortices. Both sliding threshold and synaptic scaling models can account for the observed homeostatic synaptic plasticity. Here we report that visual experience-dependent homeostatic plasticity of excitatory synapses observed in superficial layers of visual cortex is dependent on NMDAR function. In particular, both strengthening of synapses induced by visual deprivation and the subsequent weakening by reinstatement of visual experience were prevented in the absence of functional NMDARs. Our results suggest that sensory experience-dependent homeostatic adaptation depends on NMDARs, which supports the sliding threshold model of plasticity and input-specific homeostatic control observed in vivo.

NMDAR Hypofunction Animal Models of Schizophrenia

The N-methyl-d-aspartate receptor (NMDAR) hypofunction hypothesis has been proposed to help understand the etiology and pathophysiology of schizophrenia. This hypothesis was based on early observations that NMDAR antagonists could induce a full range of symptoms of schizophrenia in normal human subjects. Accumulating evidence in humans and animal studies points to NMDAR hypofunctionality as a convergence point for various symptoms of schizophrenia. Here we review animal models of NMDAR hypofunction generated by pharmacological and genetic approaches, and how they relate to the pathophysiology of schizophrenia. In addition, we discuss the limitations of animal models of NMDAR hypofunction and their potential utility for therapeutic applications.

Postsynaptic SNARE Proteins: Role in Synaptic Transmission and Plasticity

Soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) proteins mediate membrane fusion events in eukaryotic cells. Traditionally recognized as major players in regulating presynaptic neurotransmitter release, accumulative evidence over recent years has identified several SNARE proteins implicated in important postsynaptic processes such as neurotransmitter receptor trafficking and synaptic plasticity. Here we analyze the emerging data revealing this novel functional dimension for SNAREs with a focus on the molecular specialization of vesicular recycling and fusion in dendrites compared to those at axon terminals and its impact in synaptic transmission and plasticity.

GPR40 modulates epileptic seizure and NMDA receptor function

Epilepsy is a common neurological disease, and approximately 30% of patients do not respond adequately to antiepileptic drug treatment. Recent studies suggest that G protein-coupled receptor 40 (GPR40) is expressed in the central nervous system and is involved in the regulation of neurological function. However, the impact of GPR40 on epileptic seizures remains unclear. In this study, we first reported that GPR40 expression was increased in epileptic brains. In the kainic acid-induced epilepsy model, GPR40 activation after status epilepticus alleviated epileptic activity, whereas GPR40 inhibition showed the opposite effect. In the pentylenetetrazole-induced kindling model, susceptibility to epilepsy was reduced with GPR40 activation and increased with GPR40 inhibition. Whole-cell patch-clamp recordings demonstrated that GPR40 affected N-methyl-d-aspartate (NMDA) receptor-mediated synaptic transmission. Moreover, GPR40 regulated NR2A and NR2B expression on the surface of neurons. In addition, endocytosis of NMDA receptors and binding of GPR40 with NR2A and NR2B can be regulated by GPR40. Together, our findings indicate that GPR40 modulates epileptic seizures, providing a novel antiepileptic target.

Inhibition of GluN2A NMDA receptors ameliorates synaptic plasticity deficits in the Fmr1-/y mouse model

KEY POINTS

Fragile X syndrome (FXS) is a genetic condition that is the most common form of inherited intellectual impairment and causes a range of neurodevelopmental complications including learning disabilities and intellectual disability and shares many characteristics with autism spectrum disorder (ASD). In the FXS mouse model, Fmr1^-/y , impaired synaptic plasticity was restored by pharmacologically inhibiting GluN2A-containing NMDA receptors but not GluN2B-containing receptors. Similar results were obtained by crossing Fmr1^-/y with GluN2A knock-out (Grin2A^-/- ) mice. These results suggest that dampening the elevated levels of GluN2A-containing NMDA receptors in Fmr1^-/y mice has the potential to restore hyperexcitability of the neural circuitry to (a more) normal-like level of brain activity.

ABSTRACT

NMDA receptors (NMDARs) play important roles in synaptic plasticity at central excitatory synapses, and dysregulation of their function may lead to severe disorders such Fragile X syndrome (FXS). FXS is caused by transcriptional silencing of the FMR1 gene followed by lack of the encoding protein. Here we examined the effects of pharmacological and genetic manipulation of hippocampal NMDAR functions in long-term potentiation (LTP) and depression (LTD). We found impaired NMDAR-dependent LTP in the Fmr1-deficient mice, which could be fully restored when GluN2A-containing NMDARs was pharmacological inhibited. Interestingly, similar LTP effects were observed when the GluN2A gene (Grin2a) was deleted in Fmr1^-/y mice (Fmr1^-/y /Grin2a^-/- double knockout). In addition, GluN2A inhibition improved elevated mGluR5-dependent LTD to normal level in the Fmr1^-/y mouse. These findings suggest that GluN2A is a promising target in FXS research that could help us better understand the disorder.

Separate functional properties of NMDARs regulate distinct aspects of spatial cognition

N-methyl-d-aspartate receptors (NMDARs) at excitatory synapses are central to activity-dependent synaptic plasticity and learning and memory. NMDARs act as ionotropic and metabotropic receptors by elevating postsynaptic calcium concentrations and by direct intracellular protein signaling. In the forebrain, these properties are controlled largely by the auxiliary GluN2 subunits, GluN2A and GluN2B. While calcium conductance through NMDAR channels and intracellular protein signaling make separate contributions to synaptic plasticity, it is not known if these properties individually influence learning and memory. To address this issue, we created chimeric GluN2 subunits containing the amino-terminal domain and transmembrane domains from GluN2A or GluN2B fused to the carboxy-terminal domain of GluN2B (termed ABc) or GluN2A ATD (termed BAc), respectively, and expressed these mutated GluN2 subunits in transgenic mice. Expression was confirmed at the mRNA level and protein subunit translation and translocation into dendrites were observed in forebrain neurons. In the spatial version of the Morris water maze, BAc mice displayed signs of a learning deficit. In contrast, ABc animals performed similarly to wild-types during training, but showed a more direct approach to the goal location during a long-term memory test. There was no effect of ABc or BAc expression in a nonspatial water escape task. Since background expression is predominantly GluN2A in mature animals, the results suggest that spatial learning is more sensitive to manipulations of the amino-terminal domain and transmembrane domains (calcium conductance) and long-term memory is regulated more by the carboxy-terminal domain (intracellular protein signaling).

Rare loss of function mutations in N-methyl-D-aspartate glutamate receptors and their contributions to schizophrenia susceptibility

In schizophrenia (SCZ) and autism spectrum disorder (ASD), the dysregulation of glutamate transmission through N-methyl-D-aspartate receptors (NMDARs) has been implicated as a potential etiological mechanism. Previous studies have accumulated evidence supporting NMDAR-encoding genes' role in etiology of SCZ and ASD. We performed a screening study for exonic regions of GRIN1, GRIN2A, GRIN2C, GRIN2D, GRIN3A, and GRIN3B, which encode NMDAR subunits, in 562 participates (370 SCZ and 192 ASD). Forty rare variants were identified including 38 missense, 1 frameshift mutation in GRIN2C and 1 splice site mutation in GRIN2D. We conducted in silico analysis for all variants and detected seven missense variants with deleterious prediction. De novo analysis was conducted if pedigree samples were available. The splice site mutation in GRIN2D is predicted to result in intron retention by minigene assay. Furthermore, the frameshift mutation in GRIN2C and splice site mutation in GRIN2D were genotyped in an independent sample set comprising 1877 SCZ cases, 382 ASD cases, and 2040 controls. Both of them were revealed to be singleton. Our study gives evidence in support of the view that ultra-rare variants with loss of function (frameshift, nonsense or splice site) in NMDARs genes may contribute to possible risk of SCZ.

Integrated Bayesian analysis of rare exonic variants to identify risk genes for schizophrenia and neurodevelopmental disorders

BACKGROUND

Integrating rare variation from trio family and case-control studies has successfully implicated specific genes contributing to risk of neurodevelopmental disorders (NDDs) including autism spectrum disorders (ASD), intellectual disability (ID), developmental disorders (DDs), and epilepsy (EPI). For schizophrenia (SCZ), however, while sets of genes have been implicated through the study of rare variation, only two risk genes have been identified.

METHODS

We used hierarchical Bayesian modeling of rare-variant genetic architecture to estimate mean effect sizes and risk-gene proportions, analyzing the largest available collection of whole exome sequence data for SCZ (1,077 trios, 6,699 cases, and 13,028 controls), and data for four NDDs (ASD, ID, DD, and EPI; total 10,792 trios, and 4,058 cases and controls).

RESULTS

For SCZ, we estimate there are 1,551 risk genes. There are more risk genes and they have weaker effects than for NDDs. We provide power analyses to predict the number of risk-gene discoveries as more data become available. We confirm and augment prior risk gene and gene set enrichment results for SCZ and NDDs. In particular, we detected 98 new DD risk genes at FDR < 0.05. Correlations of risk-gene posterior probabilities are high across four NDDs (ρ>0.55), but low between SCZ and the NDDs (ρ<0.3). An in-depth analysis of 288 NDD genes shows there is highly significant protein-protein interaction (PPI) network connectivity, and functionally distinct PPI subnetworks based on pathway enrichment, single-cell RNA-seq cell types, and multi-region developmental brain RNA-seq.

CONCLUSIONS

We have extended a pipeline used in ASD studies and applied it to infer rare genetic parameters for SCZ and four NDDs ( https://github.com/hoangtn/extTADA ). We find many new DD risk genes, supported by gene set enrichment and PPI network connectivity analyses. We find greater similarity among NDDs than between NDDs and SCZ. NDD gene subnetworks are implicated in postnatally expressed presynaptic and postsynaptic genes, and for transcriptional and post-transcriptional gene regulation in prenatal neural progenitor and stem cells.

Hypoxia-ischemia modifies postsynaptic GluN2B-containing NMDA receptor complexes in the neonatal mouse brain

The N-methyl-d-aspartate-type glutamate receptor (NMDAR)-associated multiprotein complexes are indispensable for synaptic plasticity and cognitive functions. While purification and proteomic analyses of these signaling complexes have been performed in adult rodent and human brain, much less is known about the protein composition of NMDAR complexes in the developing brain and their modifications by neonatal hypoxic-ischemic (HI) brain injury. In this study, the postsynaptic density proteins were prepared from postnatal day 9 naïve, sham-operated and HI-injured mouse cortex. The GluN2B-containing NMDAR complexes were purified by immunoprecipitation with a mouse GluN2B antibody and subjected to mass spectrometry analysis for determination of the GluN2B binding partners. A total of 71 proteins of different functional categories were identified from the naïve animals as native GluN2B-interacting partners in the developing mouse brain. Neonatal HI reshaped the postsynaptic GluN2B interactome by recruiting new proteins, including multiple kinases, into the complexes; and modifying the existing associations within 1h of reperfusion. The early responses of postsynaptic NMDAR complexes and their related signaling networks may contribute to molecular processes leading to cell survival or death, brain damage and/or neurological disorders in term infants with neonatal encephalopathy.

Molecular and cellular dissection of NMDA receptor subtypes as antidepressant targets

A growing body of evidence supports the idea that drugs targeting the glutamate system may represent a valuable therapeutic alternative in major depressive disorders (MDD). The rapid and prolonged mood elevating effect of the NMDA receptor (NMDAR) antagonist ketamine has been studied intensely. However, its clinical use is hampered by deleterious side-effects, such as psychosis. Therefore, a better understanding of the mechanisms of the psychotropic effects after NMDAR blockade is necessary to develop glutamatergic antidepressants with improved therapeutic profile. Here we review recent experimental data that addressed molecular/cellular determinants of the antidepressant effect mediated by inactivating NMDAR subtypes. We refer to results obtained both in pharmacological and genetic animal models, ranging from global to conditional NMDAR manipulation. Our main focus is on the contribution of different NMDAR subtypes to the psychoactive effects induced by NMDAR ablation/blockade. We review data analyzing the effect of NMDAR subtype deletions limited to specific neuronal populations/brain areas in the regulation of mood. Altogether, these studies suggest effective and putative specific NMDAR drug targets for MDD treatment.

Loss of Protein Arginine Methyltransferase 8 Alters Synapse Composition and Function, Resulting in Behavioral Defects

Diverse molecular mechanisms regulate synaptic composition and function in the mammalian nervous system. The multifunctional protein arginine methyltransferase 8 (PRMT8) possesses both methyltransferase and phospholipase activities. Here we examine the role of this neuron-specific protein in hippocampal plasticity and cognitive function. PRMT8 protein localizes to synaptic sites, and conditional whole-brain Prmt8 deletion results in altered levels of multiple synaptic proteins in the hippocampus, using both male and female mice. Interestingly, these altered protein levels are due to post-transcriptional mechanisms as the corresponding mRNA levels are unaffected. Strikingly, electrophysiological recordings from hippocampal slices of mice lacking PRMT8 reveal multiple defects in excitatory synaptic function and plasticity. Furthermore, behavioral analyses show that PRMT8 conditional knock-out mice exhibit impaired hippocampal-dependent fear learning. Together, these findings establish PRMT8 as an important component of the molecular machinery required for hippocampal neuronal function.SIGNIFICANCE STATEMENT Numerous molecular processes are critically required for normal brain function. Here we use mice lacking protein arginine methyltransferase 8 (PRMT8) in the brain to examine how loss of this protein affects the structure and function of neurons in the hippocampus. We find that PRMT8 localizes to the sites of communication between neurons. Hippocampal neurons from mice lacking PRMT8 have no detectable structural differences compared with controls; however, multiple aspects of their function are altered. Consistently, we find that mice lacking PRMT8 also exhibit reduced hippocampus-dependent memory. Together, our findings establish important roles for PRMT8 in regulating neuron function and cognition in the mammalian brain.

Hierarchical organization and genetically separable subfamilies of PSD95 postsynaptic supercomplexes

PSD95 is an abundant postsynaptic scaffold protein in glutamatergic synapses that assembles into supercomplexes composed of over 80 proteins including neurotransmitter receptors, ion channels and adhesion proteins. How these diverse constituents are organized into PSD95 supercomplexes in vivo is poorly understood. Here, we dissected the supercomplexes in mice combining endogenous gene‐tagging, targeted mutations and quantitative biochemical assays. Generating compound heterozygous mice with two different gene‐tags, one on each Psd95 allele, showed that each ~1.5 MDa PSD95‐containing supercomplex contains on average two PSD95 molecules. Gene‐tagging the endogenous GluN1 and PSD95 with identical Flag tags revealed N‐methyl D‐aspartic acid receptors (NMDARs) containing supercomplexes that represent only 3% of the total population of PSD95 supercomplexes, suggesting there are many other subtypes. To determine whether this extended population of different PSD95 supercomplexes use genetically defined mechanisms to specify their assembly, we tested the effect of five targeted mouse mutations on the assembly of known PSD95 interactors, Kir2.3, Arc, IQsec2/BRAG1 and Adam22. Unexpectedly, some mutations were highly selective, whereas others caused widespread disruption, indicating that PSD95 interacting proteins are organized hierarchically into distinct subfamilies of ~1.5 MDa supercomplexes, including a subpopulation of Kir2.3‐NMDAR ion channel‐channel supercomplexes. Kir2.3‐NMDAR ion channel‐channel supercomplexes were found to be anatomically restricted to particular brain regions. These data provide new insight into the mechanisms that govern the constituents of postsynaptic supercomplexes and the diversity of synapse types.

Read the Editorial Highlight for this article onpage 500.

Cover Image for this issue: doi.10.1111/jnc.13811.

Genetic Mutation of GluN2B Protects Brain Cells Against Stroke Damages

Immediately following ischemia, glutamate accumulates in the extracellular space and results in extensive stimulation of its receptors including N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors. A large amount of Ca²⁺ influx directly through the receptor-gated ion channels which leads to Ca²⁺ overload and triggers several downstream lethal reactions. As a result, cell dies via apoptosis or necrosis, or both. Death-associated protein kinase 1 (DAPK1) physically and functionally interacts with the NMDA receptor GluN2B subunit at extra-synaptic sites and this interaction acts as a central mediator for stroke damage. The goal of this study is to explore an effective strategy in the treatment of stroke with a molecular genetic manipulation to interrupt DAPK1-GluN2B interaction. We generated a mutant strain of mice with the conditional deletion of GluN2B C-terminal tail consisting of amino acids 886-1269 in the forebrain excitatory neurons (the GluN2B mutant mice) and tested the protective effects of this mutation in stroke damages. GluN2B mutation effectively disrupted the DAPK1-GluN2B interaction and inhibited extra-synaptic NMDA receptor currents without affecting synaptic NMDA receptor channel activity in the central neurons. GluN2B mutation protected against stroke damages both in vitro and in vivo and hence improved behavioral performance. Disruption of the DAPK1-GluN2B interaction is therapeutically effective against stroke damages.

Chronic early postnatal scream sound stress induces learning deficits and NMDA receptor changes in the hippocampus of adult mice

Chronic scream sounds during adulthood affect spatial learning and memory, both of which are sexually dimorphic. The long-term effects of chronic early postnatal scream sound stress (SSS) during postnatal days 1-21 (P1-P21) on spatial learning and memory in adult mice as well as whether or not these effects are sexually dimorphic are unknown. Therefore, the present study examines the performance of adult male and female mice in the Morris water maze following exposure to chronic early postnatal SSS. Hippocampal NR2A and NR2B levels as well as NR2A/NR2B subunit ratios were tested using immunohistochemistry. In the Morris water maze, stress males showed greater impairment in spatial learning and memory than background males; by contrast, stress and background females performed equally well. NR2B levels in CA1 and CA3 were upregulated, whereas NR2A/NR2B ratios were downregulated in stressed males, but not in females. These data suggest that chronic early postnatal SSS influences spatial learning and memory ability, levels of hippocampal NR2B, and NR2A/NR2B ratios in adult males. Moreover, chronic early stress-induced alterations exert long-lasting effects and appear to affect performance in a sex-specific manner.

Behavioral and physiological characterization of PKC-dependent phosphorylation in the Grin2a∆PKC mouse

Activity-dependent plasticity in NMDA receptor-containing synapses can be regulated by phosphorylation of serines and tyrosines in the C-terminal domain of the receptor subunits by various kinases. We have previously identified S1291/S1312 as important sites for PKC phosphorylation; while Y1292/Y1312 are the sites indirectly phosphorylated by PKC via Src kinase. In the oocyte expression system, mutation of those Serine sites to Alanine (that cannot be phosphorylated) in the GluN2A subunit, resulted in a decreased PKC stimulated current enhancement through the receptors compared to wild-type NMDA receptors. To investigate the behavioral and physiological significance of those PKC-mediated phosphorylation sites in vivo, the Grin2a∆PKC mouse expressing GluN2A with four mutated amino acids: S1291A, S1312A, Y1292F and Y1387F was generated using homologous recombination. The Grin2a∆PKC mice exhibit reduced anxiety in the open field test, light dark emergence test, and elevated plus maze. The mutant mice show reduced alternation in a Y maze spontaneous alternation task and a in a non-reinforced T maze alternation task. Interestingly, when the mutant mice were exposed to novel environments, there was no increase in context-induced Fos levels in hippocampal CA1 and CA3 compared to home-cage Fos levels, while the Fos increased in the WT mice in CA1, CA3 and DG. When the SC-CA1 synapses in slices from mutant mice were stimulated using a theta-burst protocol, there was no impairment in LTP. Overall, these results suggest that at least one of those PKC-mediated phosphorylation sites regulates NMDAR-mediated signaling that modulates anxiety.