Influence of GluN2 subunit identity on NMDA receptor function

EphB2 receptor tyrosine kinase-mediated excitatory synaptic functions are negatively modulated by MDGA2

MDGA2 is an excitatory synapse-specific suppressor that uses distinct extracellular mechanisms to negatively regulate various postsynaptic properties. Here, we identify EphB2, an excitatory synapse-specific receptor tyrosine kinase, as a new binding partner for MDGA2. The first three immunoglobulin domains of MDGA2 undergo cis-binding to the ligand-binding domain of EphB2, enabling MDGA2 to compete with Ephrin-B1 for binding to EphB2. Moreover, EphB2 forms complexes with MDGA2 and GluN2B-containing NMDA receptors (NMDARs) in mouse brains. MDGA2 deletion promotes formation of the EphB2/Ephrin-B1 complex but does not alter the surface expression levels and Ephrin-stimulated activation of EphB2 receptors and downstream GluN2B-containing NMDARs in cultured neurons. AlphaFold-based molecular replacement experiments reveal that MDGA2 must bind EphB2 to suppress spontaneous synaptic transmission and NMDAR-mediated, but not AMPAR-mediated, postsynaptic responses at excitatory synapses in cultured neurons. These results collectively suggest that MDGA2 is a versatile factor that suppresses distinct excitatory postsynaptic properties via different transsynaptic pathways.

Significance of NMDA receptor-targeting compounds in neuropsychological disorders: An In-depth Review

N-methyl-D-aspartate receptors (NMDARs), a subclass of glutamate-gated ion channels, play an integral role in the maintenance of synaptic plasticity and excitation-inhibition balance within the central nervous system (CNS). Any irregularities in NMDAR functions, whether hypo-activation or over-activation, can destabilize neural networks and impair CNS function. Several decades of experimental and clinical investigations have demonstrated that NMDAR dysfunction is implicated in the pathophysiology of various neurological disorders. Despite designing a long list of compounds that differentially modulate NMDARs, success in developing drugs that can selectively and effectively regulate various NMDAR subtypes while showing encouraging efficacy in clinical settings remains limited. A better understanding of the basic mechanism of NMDAR function, particularly its selective regulation in pathological conditions, could aid in designing effective drugs for the treatment of neurological conditions. Here, we reviewed the experimental and clinical investigations that studied the effects of available NMDAR modulators in various neurological disorders and weighed up the pros and cons of the use of these substances on the improvement of functional outcomes of these disorders. Despite numerous efforts to develop NMDAR modulatory drugs that did not produce the desired outcomes, NMDARs remain a significant target for advancing novel drugs to treat neurological disorders. This article reviews the complexity of NMDAR signaling dysfunction in different neurological diseases, the efforts taken to examine designed compounds targeting specific subtypes of NMDARs, including challenges accompanied by using these substances, and the potential enhancements in drug discovery for NMDAR modulatory compounds by innovative technologies.

Subunit specific altered expression and activity of casein kinase 2 in the brain tissues resected from mesial temporal lobe epilepsy with hippocampal sclerosis patients & rodent temporal lobe epilepsy model

INTRODUCTION

Mesial temporal lobe epilepsy (MTLE), is associated with dysregulated excitatory-inhibitory balance in the brain. Numerous enzymes, protein kinases, that are modulated through phosphorylation, have been linked with key processes involved in the pathogenesis of epilepsy. Therefore, in this study, we determined the subunit specific expression and activity of multi-subunit casein Kinase 2 (CK2) which influences NMDARs through phosphorylation events, in MTS patients as well as pilocarpine model of TLE.

METHODS

mRNA expression of CK2 (α, α', β) & NR2B was measured by real time PCR andprotein expression of CK2 (α, α', β), NR2B, and NR2B Ser1480 were evaluated using western blotting and immunohistochemistry in experimental models of TLE and MTS patients. CK2 α and α' activity was measured by kinase assay.

RESULTS

Significant increase in CK2α', CK2β, and NR2B mRNA expression were noted in chronic TLE rat model. Similarly, MTS patients displayed upregulated CK2α' and CK2β expressions, but NR2B mRNA remained unchanged. CK2α', CK2β, and NR2B Ser1480 protein expressions were higher in chronic TLE and MTS patients in relation to controls (p < 0.05), as was kinase activity (p < 0.05). In acute TLE rats, only NR2B protein expression was upregulated (p < 0.05).

CONCLUSION

Our research demonstrated for the first time the upregulation of CK2α' subunit and its increased kinase activityin resected brain samples from MTS patients as well as pilocarpine model of TLE. Altered expression and higher activity of CK2 α' highlights subunit specific contribution, suggesting the modulation of NMDA receptors by Casein Kinase 2 may contribute to hyperexcitability in MTLE.

Contrasting patterns of extrasynaptic NMDAR-GluN2B expression in macaque subgenual cingulate and dorsolateral prefrontal cortices

Expression of the N-methyl-D-aspartate receptor, particularly when containing the GluN2B subunit (NMDAR-GluN2B), varies across the prefrontal cortex (PFC). In humans, the subgenual cingulate cortex (SGC) contains among the highest levels of NMDAR-GluN2B expression, while the dorsolateral prefrontal cortex (dlPFC) exhibits a more moderate level of NMDAR-GluN2B expression. NMDAR-GluN2B are commonly associated with ionotropic synaptic function and plasticity and are essential to the neurotransmission underlying working memory in the macaque dlPFC in the layer III circuits, which in humans are afflicted in schizophrenia. However, NMDAR-GluN2B can also be found at extrasynaptic sites, where they may trigger distinct events, including some linked to neurodegenerative processes. The SGC is an early site of tau pathology in sporadic Alzheimer's disease (sAD), which mirrors its high NMDAR-GluN2B expression. Additionally, the SGC is hyperactive in depression, which can be treated with NMDAR antagonists. Given the clinical relevance of NMDAR in the SGC and dlPFC, the current study used immunoelectron microscopy (immunoEM) to quantitatively compare the synaptic and extrasynaptic expression patterns of NMDAR-GluN2B across excitatory and inhibitory neuron dendrites in rhesus macaque layer III SGC and dlPFC. We found a larger population of extrasynaptic NMDAR-GluN2B in dendrites of putative pyramidal neurons in SGC as compared to the dlPFC, while the dlPFC had a higher proportion of synaptic NMDAR-GluN2B. In contrast, in putative inhibitory dendrites from both areas, extrasynaptic expression of NMDAR-GluN2B was far more frequently observed over synaptic expression. These findings may provide insight into varying cortical vulnerability to alterations in excitability and neurodegenerative forces.

Preliminary pharmacokinetics and in vivo studies indicate analgesic and stress mitigation effects of a novel NMDA receptor modulator

N-methyl D-aspartate receptor (NMDAR) channel blockers produce analgesic and antidepressant effects by preferentially inhibiting the GluN2D subtype at lower doses. Given the distinct physiological role of GluN2 subunits, we hypothesized that compounds capable of simultaneously modulating GluN2A and GluN2D subtypes in opposite directions could serve as effective analgesics with minimal cognitive adverse effects. In this translational study, we investigated the in vivo effects of costa NMDAR stimulator 4 (CNS4), a recently discovered glutamate concentration-dependent NMDAR modulator. Pharmacokinetic data revealed that CNS4 reaches peak plasma and brain concentrations within 0.25 hours after intraperitoneal injection, with brain concentrations reaching values up to 8.4% of those in plasma (64.9 vs 5.47 μg/mL). Preliminary results showed that CNS4, a nonopioid compound, increased escape latency in mice during a hotplate assay by 1.74-fold compared with saline. In a fear conditioning experiment, CNS4 anecdotally reduced the electric shock sensation and significantly decreased stress-related defecation (fecal pellets: males, 21 vs 1; females, 19 vs 3). CNS4 also improved hyperarousal behavior (25 vs 4 jumps), without affecting fear memory parameters such as freezing episodes, duration, or latency. CNS4 caused no changes in locomotion across 8 of 9 parameters studied. Remarkably, approximately 50 hours after fear conditioning training, CNS4 prevented stress-induced excessive sucrose drinking behavior by more than 2-fold both in male and female mice. These findings suggest that CNS4 penetrates brain tissue and produces pharmacological effects such as those of NMDAR-targeting drugs but with a distinct mechanism, avoiding the undesirable side effects typical of traditional NMDAR blockers. Therefore, CNS4 holds potential as a novel nonopioid analgesic, warranting further investigation. SIGNIFICANCE STATEMENT: N-methyl D-aspartate (NMDA)-subtype glutamate receptors are an attractive target for chronic pain and posttraumatic stress disorder treatments because they play a critical role in forming emotional memories of stressful events. In this translational pharmacology work, we demonstrate the central analgesic and stress-mitigating characteristics of a novel glutamate concentration-biased NMDA receptor modulator, costa NMDA receptor stimulator 4.

Absence of GluN2A in hippocampal CA1 neurons leads to altered dendritic structure and reduced frequency of miniature excitatory synaptic events

GluN2A is a NMDA receptor subunit postulated as important for learning and memory. In humans, heterozygous loss of function variants in the gene encoding it (GRIN2A) increase the risk of epilepsy, intellectual disability and schizophrenia. Haploinsufficient mouse models show electrophysiological abnormalities and thus to improve and widen understanding of the pathogenesis of GRIN2A-associated disorders in humans, this study aimed to assess the impact of Grin2a absence and haploinsufficiency on core neuronal and synaptic properties in genetically modified rats. Electrophysiological whole-cell current- and voltage-clamp recordings were made from CA1 pyramidal neurons in acute hippocampal slices from wild-type and Grin2a heterozygous (Grin2a+/- ) and homozygous (Grin2a-/- ) knock out rats aged postnatal day 27-34. While reduced levels or absence of GluN2A did not affect neuronal excitability or intrinsic membrane properties in both Grin2a+/- and Grin2a-/- rats, we found a reduced frequency of miniature excitatory post synaptic currents and a reduced density of proximal dendrites suggestive of a reduced number of excitatory synapses. Recordings from CA1 neurons in slices prepared from Grin2a+/- and Grin2a-/- rats revealed there was a reduced ratio of the current mediated by NMDA receptors compared to AMPA receptors, while in Grin2a-/- recordings, there was a slowing of the decay time-constant of the NMDA receptor-mediated excitatory postsynaptic currents. Moreover, neither summation of sub-threshold excitatory postsynaptic potentials nor summation of supra-threshold excitatory postsynaptic potentials to initiate action potential firing in CA1 pyramidal neurons indicated any dependence on GluN2A. We conclude that reduced levels of GluN2A alters the kinetics of NMDA receptor-mediated synaptic events and dendritic structure of CA1 neurons, but do not affect several other core neuronal functions. These relatively subtle changes are consistent with the largely intact neural functioning of the majority of humans carrying GRIN2A loss of function variants. Further research could explore whether the changes in synaptic properties we observed contribute to alterations in higher level circuit dynamics and computation, which may manifest as disorders of cognition and excitability in humans.

Contrasting patterns of extrasynaptic NMDAR-GluN2B expression in macaque subgenual cingulate and dorsolateral prefrontal cortices

Expression of the N-methyl-D-aspartate receptor, particularly when containing the GluN2B subunit (NMDAR-GluN2B) varies across the prefrontal cortex (PFC). In humans, the subgenual cingulate cortex (SGC) contains among the highest levels of NMDAR-GluN2B expression, while the dorsolateral prefrontal cortex (dlPFC) exhibits a more moderate level of NMDAR-GluN2B expression. NMDAR-GluN2B are commonly associated with ionotropic synaptic function and plasticity, and are essential to the neurotransmission underlying working memory in the macaque dlPFC in the layer III circuits afflicted in schizophrenia. However, NMDAR-GluN2B can also be found at extrasynaptic sites, where they may trigger distinct events, including some linked to neurodegenerative processes. The SGC is an early site of tau pathology in sporadic Alzheimer's Disease (sAD), which mirrors its high NMDAR-GluN2B expression. Additionally, the SGC is hyperactive in depression, which is treated with NMDAR antagonists. Given the clinical relevance of NMDAR in the SGC and dlPFC, the current study used immunoelectron microscopy (immunoEM) to quantitatively compare the synaptic and extrasynaptic expression patterns of NMDAR-GluN2B across excitatory and inhibitory neuron dendrites in the rhesus macaque SGC and dlPFC. We found a larger population of extrasynaptic NMDAR-GluN2B in dendritic shafts and spines of putative pyramidal neurons in SGC as compared to the dlPFC, while the dlPFC had a higher proportion of synaptic NMDAR-GluN2B. In contrast, in putative inhibitory dendrites from both areas, extrasynaptic expression of NMDAR-GluN2B was far more frequently observed over synaptic expression. These findings may provide insight into varying cortical vulnerability to alterations in excitability and to neurodegenerative forces.

Scope Statement

NMDAR are ionotropic receptors that contribute to neurotransmission and second messenger signaling events. NMDAR can induce a diverse array of neuronal events, in part due to variation in subunit composition and subcellular localization of receptor expression. Expression of the GluN2B subunit varies across the prefrontal cortex in humans. This subunit is highly expressed in the subgenual cingulate, an area associated with mood and emotion, and more moderately expressed in the dorsolateral prefrontal cortex, an area associated with cognitive processes. Extrasynaptic NMDAR, which often contain with the GluN2B subunit, have been linked to detrimental cellular events like neurodegeneration. Here, using high resolution electron microscopy in rhesus macaques, we found evidence that extrasynaptic NMDAR-GluN2B expression may be more prominent in subgenual cortex than in the dorsolateral prefrontal cortex. Conversely, synaptic NMDAR-GluN2B may be more prominent in the dorsolateral prefrontal cortex, consistent with their essential contribution to neuronal firing during working memory. These findings may help to illuminate the propensity of the subgenual cortex to tonic hyperactivity in major depression and its vulnerability to neurodegeneration in Alzheimer's disease, and may help to explain how rapid acting antidepressants exert therapeutic action across diverse neural circuits.

Evaluation of Prenatal Transportation Stress on DNA Methylation (DNAm) and Gene Expression in the Hypothalamic-Pituitary-Adrenal (HPA) Axis Tissues of Mature Brahman Cows

Background/Objectives: The experience of prenatal stress results in various physiological disorders due to an alteration of an offspring's methylome and transcriptome. The objective of this study was to determine whether PNS affects DNA methylation (DNAm) and gene expression in the stress axis tissues of mature Brahman cows. Methods: Samples were collected from the paraventricular nucleus (PVN), anterior pituitary (PIT), and adrenal cortex (AC) of 5-year-old Brahman cows that were prenatally exposed to either transportation stress (PNS, n = 6) or were not transported (Control, n = 8). The isolated DNA and RNA samples were, respectively, used for methylation and RNA-Seq analyses. A gene ontology and KEGG pathway enrichment analysis of each data set within each sample tissue was conducted with the DAVID Functional Annotation Tool. Results: The DNAm analysis revealed 3, 64, and 99 hypomethylated and 2, 93, and 90 hypermethylated CpG sites (FDR < 0.15) within the PVN, PIT, and AC, respectively. The RNA-Seq analysis revealed 6, 25, and 5 differentially expressed genes (FDR < 0.15) in the PVN, PIT, and AC, respectively, that were up-regulated in the PNS group relative to the Control group, as well as 24 genes in the PIT that were down-regulated. Based on the enrichment analysis, several developmental and cellular processes, such as maintenance of the actin cytoskeleton, cell motility, signal transduction, neurodevelopment, and synaptic function, were potentially modulated. Conclusions: The methylome and transcriptome were altered in the stress axis tissues of mature cows that had been exposed to prenatal transportation stress. These findings are relevant to understanding how prenatal experiences may affect postnatal neurological functions.

Differential responses of disease-related GRIN variants located in pore-forming M2 domain of N-methyl-D-aspartate receptor to FDA-approved inhibitors

N-methyl-D-aspartate receptors (NMDAR), ionotropic glutamate receptors, mediate a slow component of excitatory synaptic transmission in the central nervous system and play a key role in normal brain function and development. Genetic variations in GRIN genes encoding NMDAR subunits that alter the receptor's functional characteristics are associated with a wide range of neurological and neuropsychiatric conditions. Pathological GRIN variants located in the M2 re-entrant loop lining the channel pore cause significant functional changes, the most consequential alteration being a reduction in voltage-dependent Mg2+ inhibition. Voltage-dependent Mg2+ block is a unique feature of NMDAR biology whereby channel activation requires both ligand binding and postsynaptic membrane depolarization. Thus, loss of NMDAR Mg2+ block will have a profound impact on synaptic function and plasticity. Here, we choose 11 missense variants within the GRIN1, GRIN2A, and GRIN2B genes that alter residues located in the M2 loop and significantly reduce Mg2+ inhibition. Each variant was evaluated for tolerance to genetic variation using the 3-dimensional structure and assessed for functional rescue pharmacology via electrophysiological recordings. Three FDA-approved NMDAR drugs-memantine, dextromethorphan, and ketamine-were chosen based on their ability to bind near the M2 re-entrant loop, potentially rectifying dysregulated NMDAR function by supplementing the reduced voltage-dependent Mg2+ block. These results provide insight of structural determinants of FDA-approved NMDAR drugs at their binding sites in the channel pore and may further define conditions necessary for the use of such agents as potential rescue pharmacology.

How is Excitotoxicity Being Modelled in iPSC-Derived Neurons?

Excitotoxicity linked either to environmental causes (pesticide and cyanotoxin exposure), excitatory neurotransmitter imbalance, or to intrinsic neuronal hyperexcitability, is a pathological mechanism central to neurodegeneration in amyotrophic lateral sclerosis (ALS). Investigation of excitotoxic mechanisms using in vitro and in vivo animal models has been central to understanding ALS mechanisms of disease. In particular, advances in induced pluripotent stem cell (iPSC) technologies now provide human cell-based models that are readily amenable to environmental and network-based excitotoxic manipulations. The cell-type specific differentiation of iPSC, combined with approaches to modelling excitotoxicity that include editing of disease-associated gene variants, chemogenetics, and environmental risk-associated exposures make iPSC primed to examine gene-environment interactions and disease-associated excitotoxic mechanisms. Critical to this is knowledge of which neurotransmitter receptor subunits are expressed by iPSC-derived neuronal cultures being studied, how their activity responds to antagonists and agonists of these receptors, and how to interpret data derived from multi-parameter electrophysiological recordings. This review explores how iPSC-based studies have contributed to our understanding of ALS-linked excitotoxicity and highlights novel approaches to inducing excitotoxicity in iPSC-derived neurons to further our understanding of its pathological pathways.

Quinolone bioisosteres of phenolic GluN2B-selective NMDA receptor antagonists

Cyclopenta[g]quinolones of type 4 were designed with the aim to bioisosterically replace the phenol of potent GluN2B ligands such as ifenprodil and Ro 25-6981 by the quinolone system and to restrict the conformational flexibility of the aminopropanol substructure in a cyclopentane system. The designed ligands were synthesized in an eight-step sequence starting with terephthalaldehyde (5). Key steps pf the synthesis were the intramolecular Friedel-Crafts acylation of propionic acids 10 to yield the cyclopenta[g]quinolinediones 11 and the Mannich reaction of diketone 11a followed by conjugate addition at the α,β-unsaturated ketone 12a. Although the quinolones 13a, 15a, and 16a contain an H-bond donor group (secondary lactam) as ifenprodil and Ro 25-6981, they show only moderate GluN2B affinity (Ki > 410 nM). However, the introduction of lipophilic substituents at the quinolone N-atom resulted in more than 10-fold increased GluN2B affinity of the benzyl and benzyloxymethyl derivatives cis-13c (Ko = 36 nM) and 13e (Ko = 27 nM). All compounds are selective over the phencyclidine (PCP) binding site of the N-methyl-D-aspartate (NMDA) receptor. The benzyl derivative 13c showed six- and threefold selectivity over σ1 and σ2 receptors, respectively.

GluN2A- and GluN2B-containing pre-synaptic N-methyl-d-aspartate receptors differentially regulate action potential-evoked Ca2+ influx via modulation of SK channels

N-methyl-d-aspartate receptors (NMDARs) play a pivotal role in synaptic plasticity. While the functional role of post-synaptic NMDARs is well established, pre-synaptic NMDAR (pre-NMDAR) function is largely unexplored. Different pre-NMDAR subunit populations are documented at synapses, suggesting that subunit composition influences neuronal transmission. Here, we used electrophysiological recordings at Schaffer collateral-CA1 synapses partnered with Ca2+ imaging and glutamate uncaging at boutons of CA3 pyramidal neurones to reveal two populations of pre-NMDARs that contain either the GluN2A or GluN2B subunit. Activation of the GluN2B population decreases action potential-evoked Ca2+ influx via modulation of small-conductance Ca2+-activated K+ channels, while activation of the GluN2A population does the opposite. Critically, the level of functional expression of the subunits is subject to homeostatic regulation, bidirectionally affecting short-term facilitation, thus providing a capacity for a fine adjustment of information transfer. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

NMDA Receptor Antagonists: Emerging Insights into Molecular Mechanisms and Clinical Applications in Neurological Disorders

Neurodegenerative disorders (NDs) include a range of chronic conditions characterized by progressive neuronal loss, leading to cognitive, motor, and behavioral impairments. Common examples include Alzheimer's disease (AD) and Parkinson's disease (PD). The global prevalence of NDs is on the rise, imposing significant economic and social burdens. Despite extensive research, the mechanisms underlying NDs remain incompletely understood, hampering the development of effective treatments. Excitotoxicity, particularly glutamate-mediated excitotoxicity, is a key pathological process implicated in NDs. Targeting the N-methyl-D-aspartate (NMDA) receptor, which plays a central role in excitotoxicity, holds therapeutic promise. However, challenges, such as blood-brain barrier penetration and adverse effects, such as extrapyramidal effects, have hindered the success of many NMDA receptor antagonists in clinical trials. This review explores the molecular mechanisms of NMDA receptor antagonists, emphasizing their structure, function, types, challenges, and future prospects in treating NDs. Despite extensive research on competitive and noncompetitive NMDA receptor antagonists, the quest for effective treatments still faces significant hurdles. This is partly because the same NMDA receptor that necessitates blockage under pathological conditions is also responsible for the normal physiological function of NMDA receptors. Allosteric modulation of NMDA receptors presents a potential alternative, with the GluN2B subunit emerging as a particularly attractive target due to its enrichment in presynaptic and extrasynaptic NMDA receptors, which are major contributors to excitotoxic-induced neuronal cell death. Despite their low side-effect profiles, selective GluN2B antagonists like ifenprodil and radiprodil have encountered obstacles such as poor bioavailability in clinical trials. Moreover, the selectivity of these antagonists is often relative, as they have been shown to bind to other GluN2 subunits, albeit minimally. Recent advancements in developing phenanthroic and naphthoic acid derivatives offer promise for enhanced GluN2B, GluN2A or GluN2C/GluN2D selectivity and improved pharmacodynamic properties. Additional challenges in NMDA receptor antagonist development include conflicting preclinical and clinical results, as well as the complexity of neurodegenerative disorders and poorly defined NMDA receptor subtypes. Although multifunctional agents targeting multiple degenerative processes are also being explored, clinical data are limited. Designing and developing selective GluN2B antagonists/modulators with polycyclic moieties and multitarget properties would be significant in addressing neurodegenerative disorders. However, advancements in understanding NMDA receptor structure and function, coupled with collaborative efforts in drug design, are imperative for realizing the therapeutic potential of these NMDA receptor antagonists/modulators.

Early expression of GluN2A-containing NMDA receptors in a model of fragile X syndrome

NMDA-type glutamate receptors (NMDARs) play a crucial role in synaptogenesis, circuit development, and synaptic plasticity, serving as fundamental components in cellular models of learning and memory. Their dysregulation has been implicated in several neurological disorders and synaptopathies. NMDARs are heterotetrameric complexes composed of two GluN1 and two GluN2 subunits. The composition of GluN2 subunits determines the main biophysical properties of the channel, such as calcium permeability and gating kinetics, and influences the ability of the receptor to interact with postsynaptic proteins involved in normal synaptic physiology and plasticity, including scaffolding proteins and signaling molecules. During early development, NMDARs in the forebrain contain solely the GluN2B subunit, a necessary subunit for proper synaptogenesis and synaptic plasticity. As the animal matures, the expression of the GluN2A subunit increases, leading to a partial replacement of GluN2B-containing synaptic NMDARs with GluN2A-containing receptors. The switch in the synaptic GluN2A-to-GluN2B ratio has a significant impact on the kinetics of excitatory postsynaptic currents and diminishes the synaptic plasticity capacity. In this study, we present findings indicating that GluN2A expression occurs earlier in a mouse model of fragile X syndrome (FXS). This altered timing of GluN2A expression affects various important parameters of NMDAR-mediated excitatory postsynaptic currents, including maximal current amplitude, decay time, and response to consecutive stimuli delivered in close temporal proximity. These observations suggest that the early expression of GluN2A during a critical period when synapses and circuits are developing could be an underlying factor contributing to the formation of pathological circuits in the FXS mouse model.NEW & NOTEWORTHY NMDA receptors (NMDARs) play important roles in synaptic transmission and are involved in multiple neurological disorders. During development, GluN2A in the forebrain becomes incorporated into previously GluN2B-dominated NMDARs, leading to the "GluN2A/GluN2B ratio switch." This is a crucial step for normal brain development. Here we present findings indicating that GluN2A expression occurs earlier in the fragile X mouse and this could be an underlying factor contributing to the pathology found in the fragile X model.

Distinct GABAergic modulation of timing-dependent LTP in CA1 pyramidal neurons along the longitudinal axis of the mouse hippocampus

Synaptic plasticity in the hippocampus underlies episodic memory formation, with dorsal hippocampus being instrumental for spatial memory whereas ventral hippocampus is crucial for emotional learning. Here, we studied how GABAergic inhibition regulates physiologically relevant low repeat spike timing-dependent LTP (t-LTP) at Schaffer collateral-CA1 synapses along the dorsoventral hippocampal axis. We used two t-LTP protocols relying on only 6 repeats of paired spike-firing in pre- and postsynaptic cells within 10 s that differ in postsynaptic firing patterns. GABAA receptor mechanisms played a greater role in blocking 6× 1:1 t-LTP that recruits single postsynaptic action potentials. 6× 1:4 t-LTP that depends on postsynaptic burst-firing unexpectedly required intact GABAB receptor signaling. The magnitude of both t-LTP-forms decreased along the dorsoventral axis, despite increasing excitability and basal synaptic strength in this direction. This suggests that GABAergic inhibition contributes to the distinct roles of dorsal and ventral hippocampus in memory formation.

Loss of the sustained antidepressant-like effect of (2R,6R)-hydroxynorketamine in NMDA receptor GluN2D subunit knockout mice

Ketamine, an N-methyl-d-aspartate (NMDA) receptor antagonist, has attracted attention for its acute and sustained antidepressant effects in patients with depression. Hydroxynorketamine (HNK), a metabolite of ketamine, exerts antidepressant effects without exerting ketamine's side effects and has attracted much attention in recent years. However, the detailed pharmacological mechanism of action of HNK remains unclear. We previously showed that the GluN2D NMDA receptor subunit is important for sustained antidepressant-like effects of (R)-ketamine. Therefore, we investigated whether the GluN2D subunit is involved in antidepressant-like effects of (2R,6R)-HNK and (2S,6S)-HNK. Treatment with (2R,6R)-HNK but not (2S,6S)-HNK exerted acute and sustained antidepressant-like effects in the tail-suspension test in wildtype mice. Interestingly, sustained antidepressant-like effects of (2R,6R)-HNK were abolished in GluN2D-knockout mice, whereas acute antidepressant-like effects were maintained in GluN2D-knockout mice. When expression levels of GluN2A and GluN2B subunits were evaluated, a decrease in GluN2B protein expression in the nucleus accumbens was found in stressed wildtype mice but not in stressed GluN2D-knockout mice. These results suggest that the GluN2D subunit and possibly the GluN2B subunit are involved in the sustained antidepressant-like effect of (2R,6R)-HNK.

Pathological Interplay between Inflammation and Mitochondria Aggravates Glutamate Toxicity

Mitochondrial dysfunction and glutamate toxicity are associated with neural disorders, including brain trauma. A review of the literature suggests that toxic and transmission actions of neuronal glutamate are spatially and functionally separated. The transmission pathway utilizes synaptic GluN2A receptors, rapidly released pool of glutamate, evoked release of glutamate mediated by Synaptotagmin 1 and the amount of extracellular glutamate regulated by astrocytes. The toxic pathway utilizes extrasynaptic GluN2B receptors and a cytoplasmic pool of glutamate, which results from the spontaneous release of glutamate mediated by Synaptotagmin 7 and the neuronal 2-oxoglutarate dehydrogenase complex (OGDHC), a tricarboxylic acid (TCA) cycle enzyme. Additionally, the inhibition of OGDHC observed upon neuro-inflammation is due to an excessive release of reactive oxygen/nitrogen species by immune cells. The loss of OGDHC inhibits uptake of glutamate by mitochondria, thus facilitating its extracellular accumulation and stimulating toxic glutamate pathway without affecting transmission. High levels of extracellular glutamate lead to dysregulation of intracellular redox homeostasis and cause ferroptosis, excitotoxicity, and mitochondrial dysfunction. The latter affects the transmission pathway demanding high-energy supply and leading to cell death. Mitochondria aggravate glutamate toxicity due to impairments in the TCA cycle and become a victim of glutamate toxicity, which disrupts oxidative phosphorylation. Thus, therapies targeting the TCA cycle in neurological disorders may be more efficient than attempting to preserve mitochondrial oxidative phosphorylation.

Single-locus and Haplotype Associations of GRIN2B Gene with Autism Spectrum Disorders and the Demographic and Clinical Characteristics of Patients in Guilan, Iran

Autism spectrum disorders (ASDs) are described as generalized developmental disorders, with an average age of onset of 36 months. Genetic and environmental factors may contribute to this multifactorial disorder. The present study aimed to investigate the association of three GRIN2B polymorphisms, including rs1019385, rs1024893, and rs3764028, with ASDs. Based on the results, there was a significant difference regarding the genotype frequency of rs3764028 polymorphism between the control and case (ASD) groups (P = 0.027). According to the recessive model, this variant was associated with ASDs (P = 0.23). None of the eight haplotype models with frequencies above 0.5 showed significant differences between the case and control groups in terms of allelic frequency. The present results showed that the rs376028 variant was directly related to the phenotypic symptoms of ASDs.

Esketamine induces apoptosis of nasopharyngeal carcinoma cells through the PERK/CHOP pathway

Nasopharyngeal carcinoma, a malignant tumor prevalent in southeast Asia and north Africa, still lacks effective treatment. Esketamine, an N-methyl-D-aspartatic acid (NMDA) receptor (NMDAR) antagonist, is widely used in clinical anesthesia. Emerging evidence suggests that esketamine plays an important role in inhibiting tumor cell activity. However, the underlying mechanisms of esketamine on nasopharyngeal carcinoma remain unknown. In this study, we found that esketamine inhibited the proliferation and migration of nasopharyngeal carcinoma cells. Mechanically, transcriptome sequencing and subsequent verification experiments revealed that esketamine promoted the apoptosis of nasopharyngeal carcinoma cells through endoplasmic reticulum stress PERK/ATF4/CHOP signaling pathway mediated by NMDAR. Additionally, when combined with esketamine, the inhibitory effect of cisplatin on the proliferation of nasopharyngeal carcinoma cells was significantly enhanced. These findings provide new insights into future anti-nasopharyngeal carcinoma clinical strategies via targeting the NMDAR/PERK/CHOP axis alone or in combination with cisplatin.

The neuroprotective potential of phytochemicals in traumatic brain injury: mechanistic insights and pharmacological implications

Traumatic brain injury (TBI) leads to brain damage, comprising both immediate primary damage and a subsequent cascade of secondary injury mechanisms. The primary injury results in localized brain damage, while the secondary damage initiates inflammatory responses, followed by the disruption of the blood-brain barrier, infiltration of peripheral blood cells, brain edema, and the release of various immune mediators, including chemotactic factors and interleukins. TBI disrupts molecular signaling, cell structures, and functions. In addition to physical tissue damage, such as axonal injuries, contusions, and haemorrhages, TBI interferes with brain functioning, impacting cognition, decision-making, memory, attention, and speech capabilities. Despite a deep understanding of the pathophysiology of TBI, an intensive effort to evaluate the underlying mechanisms with effective therapeutic interventions is imperative to manage the repercussions of TBI. Studies have commenced to explore the potential of employing natural compounds as therapeutic interventions for TBI. These compounds are characterized by their low toxicity and limited interactions with conventional drugs. Moreover, many natural compounds demonstrate the capacity to target various aspects of the secondary injury process. While our understanding of the pathophysiology of TBI, there is an urgent need for effective therapeutic interventions to mitigate its consequences. Here, we aimed to summarize the mechanism of action and the role of phytochemicals against TBI progression. This review discusses the therapeutic implications of various phytonutrients and addresses primary and secondary consequences of TBI. In addition, we highlighted the roles of emerging phytochemicals as promising candidates for therapeutic intervention of TBI. The review highlights the neuroprotective roles of phytochemicals against TBI and the mechanistic approach. Furthermore, our efforts focused on the underlying mechanisms, providing a better understanding of the therapeutic potential of phytochemicals in TBI therapeutics.

Boltzmann's Theorem Revisited: Inaccurate Time-to-Action Clocks in Affective Disorders

Timely goal-oriented behavior is essential for survival and is shaped by experience. In this paper, a multileveled approach was employed, ranging from the polymorphic level through thermodynamic molecular, cellular, intracellular, extracellular, non-neuronal organelles and electrophysiological waves, attesting for signal variability. By adopting Boltzmann's theorem as a thermodynamic conceptualization of brain work, we found deviations from excitation-inhibition balance and wave decoupling, leading to wider signal variability in affective disorders compared to healthy individuals. Recent evidence shows that the overriding on-off design of clock genes paces the accuracy of the multilevel parallel sequencing clocks and that the accuracy of the time-to-action is more crucial for healthy behavioral reactions than their rapidity or delays. In affective disorders, the multilevel clocks run free and lack accuracy of responsivity to environmentally triggered time-to-action as the clock genes are not able to rescue mitochondria organelles from oxidative stress to produce environmentally-triggered energy that is required for the accurate time-to-action and maintenance of the thermodynamic equilibrium. This maintenance, in turn, is dependent on clock gene transcription of electron transporters, leading to higher signal variability and less signal accuracy in affective disorders. From a Boltzmannian thermodynamic and energy-production perspective, the option of reversibility to a healthier time-toaction, reducing entropy is implied. We employed logic gates to show deviations from healthy levelwise communication and the reversed conditions through compensations implying the role of nonneural cells and the extracellular matrix in return to excitation-inhibition balance and accuracy in the time-to-action signaling.

Characterizing Functional Contributions of Specific GluN2 Subunits to Individual Postsynaptic NMDAR Responses Using Biophysical Parameters

The NMDAR is a heterotetramer composed of two GluN1 subunits and two GluN2 and/or GluN3 subunits, with the GluN2 subunits exhibiting significant diversity in their structure and function. Recent studies have highlighted the importance of characterizing the specific roles of each GluN2 subunit across central nervous system regions and developmental stages, as well as their unique contributions to NMDAR-mediated signaling and plasticity. Understanding the distinct functions of GluN2 subunits is critical for the development of targeted therapeutic strategies for NMDAR-related disorders. However, measuring the functional contribution of individual GluN2 subtypes in ex vivo slices is challenging. Conventionally, pharmacological or genetic approaches are used, but, in many cases, this is not possible or is restricted to population-level NMDAR responses. Here, we describe a technique for using biophysical properties of miniature synaptic NMDAR responses as a proxy to measure the functional contribution of specific GluN2-NMDAR subunits to individual synapses within a neuron.

Understanding the role of the NMDA receptor subunit, GluN2D, in mediating NMDA receptor antagonist-induced behavioral disruptions in male and female mice

Noncompetitive NMDA receptor (NMDAR) antagonists like phencyclidine (PCP) and ketamine cause psychosis-like symptoms in healthy humans, exacerbate schizophrenia symptoms in people with the disorder, and disrupt a range of schizophrenia-relevant behaviors in rodents, including hyperlocomotion. This is negated in mice lacking the GluN2D subunit of the NMDAR, suggesting the GluN2D subunit mediates the hyperlocomotor effects of these drugs. However, the role of GluN2D in mediating other schizophrenia-relevant NMDAR antagonist-induced behavioral disturbances, and in both sexes, is unclear. This study aimed to investigate the role of the GluN2D subunit in mediating schizophrenia-relevant behaviors induced by a range of NMDA receptor antagonists. Using both male and female GluN2D knockout (KO) mice, we examined the effects of the NMDAR antagonist's PCP, the S-ketamine enantiomer (S-ket), and the ketamine metabolite R-norketamine (R-norket) on locomotor activity, anxiety-related behavior, and recognition and short-term spatial memory. GluN2D-KO mice showed a blunted locomotor response to R-norket, S-ket, and PCP, a phenotype present in both sexes. GluN2D-KO mice of both sexes showed an anxious phenotype and S-ket, R-norket, and PCP showed anxiolytic effects that were dependent on sex and genotype. S-ket disrupted spatial recognition memory in females and novel object recognition memory in both sexes, independent of genotype. This datum identifies a role for the GluN2D subunit in sex-specific effects of NMDAR antagonists and on the differential effects of the R- and S-ket enantiomers.

Salicylate- and Noise-induced Tinnitus. Different Mechanisms Producing the same Result? An Experimental Model

PURPOSE

Tinnitus, the generation of phantom sounds, can be the result of noise exposure, however, understanding of its underlying mechanisms is limited. Purpose of the study was is to determine whether different concentrations of salicylate can cause tinnitus of different intensity.

METHODS

For the purposes of this study 50 male Wistar rats were used. The animals were divided into 5 groups (10 rats in each group). The animals that did not receive any substance were allocated to the control group (Group A). The second group (Group B) of rats received salicylate (Sigma Aldrich) intraperitoneally for 7 days (300 mg/Kg/day). The 3rd group (Group C) received salicylate intraperitoneally for 7 days, but at twice the concentration of the animals in the second group (600 mg/kg/d). The 4th group (Group D) simultaneously received salicylate (300 mg/Kg/day) and pure Memantine (Sigma Aldrich, 10 mg/kg/d) intraperitoneally for 7 days. The 5th group (Group E) did not receive any substance but was exposed for 168 consecutive hours (7 days) to sound to induce tinnitus. Cochlear activity was evaluated with the use of Distortion Product Otoacoustic Emissions (DPOAEs). At the end of the experimental period, the animals were sacrificed, and the right cochlea was removed and prepared for further histological and immunohistochemical studies.

RESULTS

The DPOAEs of animals treated either with salicylate as monotherapy or salicylate combined with memantine were indistinguishable from the noise floor, did not differ significantly compared to the animals of the control group or those expose to constant noise. The cochlear structures of Group E remained anatomically and functionally unaffected from the exposure to constant noise. Memantine does not seem to offer substantial protection to the cochlear structures, according to histological examination and hearing tests, however, the rats receiving it exhibited better results in behavioral tests.

CONCLUSIONS

The administration of memantine does not contribute significantly to the reduction of tinnitus.

GluN2A and GluN2B N-Methyl-D-Aspartate Receptor (NMDARs) Subunits: Their Roles and Therapeutic Antagonists in Neurological Diseases

N-methyl-D-aspartate receptors (NMDARs) are ion channels that respond to the neurotransmitter glutamate, playing a crucial role in the permeability of calcium ions and excitatory neurotransmission in the central nervous system (CNS). Composed of various subunits, NMDARs are predominantly formed by two obligatory GluN1 subunits (with eight splice variants) along with regulatory subunits GluN2 (GluN2A-2D) and GluN3 (GluN3A-B). They are widely distributed throughout the CNS and are involved in essential functions such as synaptic transmission, learning, memory, plasticity, and excitotoxicity. The presence of GluN2A and GluN2B subunits is particularly important for cognitive processes and has been strongly implicated in neurodegenerative diseases like Parkinson's disease and Alzheimer's disease. Understanding the roles of GluN2A and GluN2B NMDARs in neuropathologies provides valuable insights into the underlying causes and complexities of major nervous system disorders. This knowledge is vital for the development of selective antagonists targeting GluN2A and GluN2B subunits using pharmacological and molecular methods. Such antagonists represent a promising class of NMDA receptor inhibitors that have the potential to be developed into neuroprotective drugs with optimal therapeutic profiles.

Negative allosteric modulators of NMDA receptors with GluN2B subunit: synthesis of β-aminoalcohols by epoxide opening and subsequent rearrangement

In order to obtain novel antagonists of GluN2B subunit containing NMDA receptors, aryloxiranes were opened with benzylpiperidines. Phenyloxiranes 6 and (indazolyl)oxirane 15 were opened regioselectively at the position bearing the aryl moiety. Reaction of the resulting β-aminoalcohols 7 and 16 with carboxylic acids under Mitsunobu conditions (DIAD, PPh3) led to rearrangement and after ester hydrolysis to the regioisomeric β-aminoalcohols 9 and 18. This strategy allows the synthesis of amino-ifenprodil 12 as well using phthalimide in the Mitsunobu reaction. Unexpectedly, the isomeric (indazolyl)oxirane 21 reacted with benzylpiperidines to afford both regioisomeric β-aminoalcohols 22 and 23. In radioligand receptor binding studies, the indazolyl derivative 18a, which can be regarded as indazole bioisostere of ifenprodil, showed high GluN2B affinity (Ki = 31 nM). Replacement of the benzylic OH moiety of ifenprodil by the NH2 moiety in amino-ifenprodil 12 also resulted in low nanomolar GluN2B affinity (Ki = 72 nM). In TEVC experiments, 18a inhibited the ion flux to the same extent as ifenprodil proving that the phenol of ifenprodil can be replaced bioisosterically by an indazole ring maintaining affinity and inhibitory activity. Whereas 10-fold selectivity was found for the ifenprodil binding site over σ1 receptors, only low preference for the GluN2B receptor over σ2 receptors was detected. The log D7.4 value of 18a (log D7.4 = 2.08) indicates promising bioavailability.

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.

Indazole as a Phenol Bioisostere: Structure-Affinity Relationships of GluN2B-Selective NMDA Receptor Antagonists

Negative allosteric modulation of GluN2B subunit-containing NMDA receptors prevents overstimulation, resulting in neuroprotective effects. Since the phenol of prominent negative allosteric modulators is prone to rapid glucuronidation, its bioisosteric replacement by an indazole was envisaged. The key step in the synthesis was a Sonogashira reaction of non-protected iodoindazoles with propargylpiperidine derivatives. Modification of the alkynyl moiety allowed the introduction of several functional groups. The synthesized indazoles showed very high GluN2B affinity but limited selectivity over σ receptors. Molecular dynamics simulations revealed the same molecular interactions with the ifenprodil binding site as the analogous phenols. In two-electrode voltage-clamp experiments, enantiomeric 3-(4-benzylpiperidin-1-yl)-1-(1H-indazol-5-yl)propan-1-ols (S)-10a and (R)-10a displayed higher inhibitory activity than ifenprodil. In contrast to phenolic GluN2B antagonists, the indazoles were not conjugated with glucuronic acid. It can be concluded that the phenol of potent GluN2B antagonists can be replaced bioisosterically by an indazole, retaining the high GluN2B affinity and activity but inhibiting glucuronidation.

Microglial Priming in Bilirubin-Induced Neurotoxicity

Neuroinflammation is a major contributor to bilirubin-induced neurotoxicity, which results in severe neurological deficits. Microglia are the primary immune cells in the brain, with M1 microglia promoting inflammatory injury and M2 microglia inhibiting neuroinflammation. Controlling microglial inflammation could be a promising therapeutic strategy for reducing bilirubin-induced neurotoxicity. Primary microglial cultures were prepared from 1-3-day-old rats. In the early stages of bilirubin treatment, pro-/anti-inflammatory (M1/M2) microglia mixed polarization was observed. In the late stages, bilirubin persistence induced dominant proinflammatory microglia, forming an inflammatory microenvironment and inducing iNOS expression as well as the release of tumor necrosis factor (TNF)-α, interleukin (IL)-6, and IL-1β. Simultaneously, nuclear factor-kappa B (NF-κB) was activated and translocated into the nucleus, upregulating inflammatory target genes. As well known, neuroinflammation can have an effect on N-methyl-D-aspartate receptor (NMDAR) expression or function, which is linked to cognition. Treatment with bilirubin-treated microglia-conditioned medium did affect the expression of IL-1β, NMDA receptor subunit 2A (NR2A), and NMDA receptor subunit 2B (NR2B) in neurons. However, VX-765 effectively reduces the levels of proinflammatory cytokines TNF-α, IL-6, and IL-1β, as well as the expressions of CD86, and increases the expressions of anti-inflammatory related Arg-1. A timely reduction in proinflammatory microglia could protect against bilirubin-induced neurotoxicity.

GRIN2A and GRIN2B and Their Related Phenotypes

Glutamate is the most relevant excitatory neurotransmitter of the central nervous system; it binds with several receptors, including N-methyl-D-aspartate receptors (NMDARs), a subtype of ionotropic glutamate receptor that displays voltage-dependent block by Mg2+ and a high permeability to Ca2+. GRIN2A and GRIN2B genes encode the GluN2A and GluN2B subunits of the NMDARs, which play important roles in synaptogenesis, synaptic transmission, and synaptic plasticity, as well as contributing to neuronal loss and dysfunction in several neurological disorders. Recently, individuals with a range of childhood-onset drug-resistant epilepsies, such as Landau–Kleffner or Lennox–Gastaut syndrome, intellectual disability (ID), and other neurodevelopmental abnormalities have been found to carry mutations in GRIN2A and GRIN2B, with high variable expressivity in phenotype. The first one is found mainly in epilepsy-aphasia syndromes, while the second one mainly in autism, schizophrenia, and ID, such as autism spectrum disorders. Brain magnetic resonance imaging alterations are found in some patients, even if without a clear clinical correlation. At the same time, increasing data on genotype–phenotype correlation have been found, but this is still not fully demonstrated. There are no specific therapies for the treatment of correlated NMDARs epilepsy, although some evidence with memantine, an antagonist of glutamate receptor, is reported in the literature in selected cases with mutation determining a gain of function.

Pharmacological Potential of 3-Benzazepines in NMDAR-Linked Pathophysiological Processes

The number of N-Methyl-D-aspartate receptor (NMDAR) linked neurodegenerative diseases such as Alzheimer's disease and dementia is constantly increasing. This is partly due to demographic change and presents new challenges to societies. To date, there are no effective treatment options. Current medications are nonselective and can lead to unwanted side effects in patients. A promising therapeutic approach is the targeted inhibition of NMDARs in the brain. NMDARs containing different subunits and splice variants display different physiological properties and play a crucial role in learning and memory, as well as in inflammatory or injury processes. They become overactivated during the course of the disease, leading to nerve cell death. Until now, there has been a lack of understanding of the general functions of the receptor and the mechanism of inhibition, which need to be understood in order to develop inhibitors. Ideal compounds should be highly targeted and even splice-variant-selective. However, a potent and splice-variant-selective NMDAR-targeting drug has yet to be developed. Recently developed 3-benzazepines are promising inhibitors for further drug development. The NMDAR splice variants GluN1-1b-4b carry a 21-amino-acid-long, flexible exon 5. Exon 5 lowers the NMDAR's sensitivity to allosteric modulators by probably acting as an NMDAR modulator itself. The role of exon 5 in NMDAR modulation is still poorly understood. In this review, we summarize the structure and pharmacological relevance of tetrahydro-3-benzazepines.

Opioids and autism spectrum disorder: liaisons dangereuses?

A recent laboratory study in the Journal examined the effects of repeated exposures of neonatal mice to fentanyl on autism-like behaviour via opioid receptor-mediated DNA hypermethylation of the Grin2B gene, which encodes the GluN2B subunit of the NMDA receptor. These experiments provide mechanisms and biological plausibility but do not directly demonstrate that opioid exposure in early life induces autism spectrum disorder in humans. Experimental modelling of human neuropsychiatric disorders is extremely challenging since most subjective psychiatric symptoms used to establish diagnosis in humans cannot be convincingly ascertained in laboratory rodents. While some human epidemiological data show associations between repeated exposures to opioids during early life, it remains undetermined whether opioid exposure is an independent risk factor for developing autism spectrum disorder in the young.

A novel NMDA receptor test model based on hiPSC-derived neural cells

N-Methyl-D-aspartate receptors (NMDARs) are central for learning and information processing in the brain. Dysfunction of NMDARs can play a key role in the pathogenesis of neurodegeneration and drug addiction. The development of selective NMDAR modulators represents a promising strategy to target these diseases. Among such modulating compounds are ifenprodil and its 3-benzazepine derivatives. Classically, the effects of these NMDAR modulators have been tested by techniques like two-electrode voltage clamp (TEVC), patch clamp, or fluorescence-based assays. However, testing their functional effects in complex human systems requires more advanced approaches. Here, we established a human induced pluripotent stem cell-derived (hiPSC-derived) neural cell system and proved its eligibility as a test system for investigating NMDAR modulators and pharmaceutical effects on human neurons.

In vitro ADME characterization of a very potent 3-acylamino-2-aminopropionic acid-derived GluN2C-NMDA receptor agonist and its ester prodrugs

The GluN2C subunit exists predominantly, but not exclusively in NMDA receptors within the cerebellum. Antagonists such as UBP1700 and positive allosteric modulators including PYD-106 and 3-acylamino-2-aminopropionic acid derivatives such as UA3-10 ((R)-2-amino-3-{[5-(2-bromophenyl)thiophen-2-yl]carboxamido}propionic acid) represent promising tool compounds to investigate the role of GluN2C-containing NMDA receptors in the signal transduction in the brain. However, due to its high polarity the bioavailability and CNS penetration of the amino acid UA3-10 are expected to be rather low. Herein, three ester prodrugs 12a-c of the NMDA receptor glycine site agonist UA3-10 were prepared and pharmacokinetically characterized. The esters 12a-c showed higher lipophilicity (higher logD _7.4 values) than the acid UA3-10 but almost the same binding at human serum albumin. The acid UA3-10 was rather stable upon incubation with mouse liver microsomes and NADPH, but the esters 12a-c were fast hydrolyzed to afford the acid UA3-10. Incubation with pig liver esterase and mouse serum led to rapid hydrolysis of the esters 12a-c. The isopropyl ester 12c showed a promising logD _7.4 value of 3.57 and the highest stability in the presence of pig liver esterase and mouse serum. These results demonstrate that ester prodrugs of UA3-10 can potentially afford improved bioavailability and CNS penetration.

Chemical, pharmacodynamic and pharmacokinetic characterization of the GluN2B receptor antagonist 3-(4-phenylbutyl)-2,3,4,5-tetrahydro-1H-3-benzazepine-1,7-diol - starting point for PET tracer development

GluN2B-NMDA receptors play a key role in several neurological and neurodegenerative disorders. In order to develop novel negative allosteric GluN2B-NMDA receptor modulators, the concept of conformational restriction was pursued, i.e. the flexible aminoethanol substructure of ifenprodil was embedded into a more rigid tetrahydro-3-benzazepine system. The resulting tetrahydro-3-benzazepine-1,7-diol (±)-2 (WMS-1410) showed promising receptor affinity in receptor binding studies (K _i = 84 nM) as well as pharmacological activity in two-electrode-voltage-clamp experiments (IC ₅₀ = 116 nM) and in cytoprotective assays (IC ₅₀ = 18.5 nM). The interactions of (R)-2 with the ifenprodil binding site of GluN2B-NMDA receptors were analyzed on the molecular level and the "foot-in-the-door" mechanism was developed. Due to promising pharmacokinetic parameters (logD_7.4 = 1.68, plasma protein binding of 76-77%, sufficient metabolic stability) F-substituted analogs were prepared and evaluated as tracers for positron emission tomography (PET). Both fluorine-18-labeled PET tracers [¹⁸F]11 and [¹⁸F]15 showed high brain uptake, specific accumulation in regions known for high GluN2B-NMDA receptor expression, but no interactions with σ ₁ receptors. Radiometabolites were not observed in the brain. Both PET tracers might be suitable for application in humans.

GRIN2B-related neurodevelopmental disorder: current understanding of pathophysiological mechanisms

The GRIN2B-related neurodevelopmental disorder is a rare disease caused by mutations in the GRIN2B gene, which encodes the GluN2B subunit of NMDA receptors. Most individuals with GRIN2B-related neurodevelopmental disorder present with intellectual disability and developmental delay. Motor impairments, autism spectrum disorder, and epilepsy are also common. A large number of pathogenic de novo mutations have been identified in GRIN2B. However, it is not yet known how these variants lead to the clinical symptoms of the disease. Recent research has begun to address this issue. Here, we describe key experimental approaches that have been used to better understand the pathophysiology of this disease. We discuss the impact of several distinct pathogenic GRIN2B variants on NMDA receptor properties. We then critically review pivotal studies examining the synaptic and neurodevelopmental phenotypes observed when disease-associated GluN2B variants are expressed in neurons. These data provide compelling evidence that various GluN2B mutants interfere with neuronal differentiation, dendrite morphogenesis, synaptogenesis, and synaptic plasticity. Finally, we identify important open questions and considerations for future studies aimed at understanding this complex disease. Together, the existing data provide insight into the pathophysiological mechanisms that underlie GRIN2B-related neurodevelopmental disorder and emphasize the importance of comparing the effects of individual, disease-associated variants. Understanding the molecular, cellular and circuit phenotypes produced by a wide range of GRIN2B variants should lead to the identification of core neurodevelopmental phenotypes that characterize the disease and lead to its symptoms. This information could help guide the development and application of effective therapeutic strategies for treating individuals with GRIN2B-related neurodevelopmental disorder.

Progresses in GluN2A-containing NMDA Receptors and their Selective Regulators

NMDA receptors play an important physiological role in regulating synaptic plasticity, learning and memory. GluN2A subunits are the most abundant functional subunits of NMDA receptors expressed in mature brain, and their dysfunction is related to various neurological diseases. According to subunit composition, GluN2A-containing NMDA receptors can be divided into two types: diheteromeric and triheteromeric receptors. In this review, the expression, functional and pharmacological properties of different kinds of GluN2A-containing NMDA receptors as well as selective GluN2A regulators were described to further understand this type of NMDA receptors.

From Mouse to Man: N-Methyl-d-Aspartic Acid Receptor Activation as a Promising Pharmacotherapeutic Strategy for Autism Spectrum Disorders

The BALB/c mouse displays hypersensitivity to behavioral effects of MK-801 (dizocilpine), a noncompetitive N-methyl-d-aspartic acid (NMDA) receptor "open-channel" blocker, and shows both no preference for an enclosed stimulus mouse over an inanimate object and reduced social interaction with a freely behaving stimulus mouse. NMDA receptor agonist interventions improved measures of social preference and social interaction of the BALB/c mouse model of autism spectrum disorder (ASD). A "proof of principle/proof of concept" translational 10-week clinical trial with 8-week of active medication administration was conducted comparing 20 DSM-IV-TR-diagnosed older adolescent/young adult patients with ASD randomized to once-weekly pulsed administration (50 mg/d) versus daily administration of d-cycloserine (50 mg/d). The results showed that d-cycloserine, a partial glycine agonist, was well tolerated, the 2 dosing strategies did not differ, and improvement was noted on the "lethargy/social withdrawal" and "stereotypic behavior" subscales of the Aberrant Behavior Checklist. NMDA receptor activation contributes to the regulation of mTOR signaling, a pathologic point of convergence in several monogenic syndromic forms of ASD. Furthermore, both NMDA receptor hypofunction and imbalance between NMDA receptor activation mediated by GluN2B and GluN2A-containing NMDA receptors occur as "downstream" consequences of several genetically unrelated abnormalities associated with ASD. NMDA receptor-subtype selective "positive allosteric modulators (PAMs)" are particularly appealing medication candidates for future translational trials.

Different Forms of Disorder in NMDA-Sensitive Glutamate Receptor Cytoplasmic Domains Are Associated with Differences in Condensate Formation

The N-methyl-D-aspartate (NMDA)-sensitive glutamate receptor (NMDAR) helps assemble downstream signaling pathways through protein interactions within the postsynaptic density (PSD), which are mediated by its intracellular C-terminal domain (CTD). The most abundant NMDAR subunits in the brain are GluN2A and GluN2B, which are associated with a developmental switch in NMDAR composition. Previously, we used single molecule fluorescence resonance energy transfer (smFRET) to show that the GluN2B CTD contained an intrinsically disordered region with slow, hop-like conformational dynamics. The CTD from GluN2B also undergoes liquid-liquid phase separation (LLPS) with synaptic proteins. Here, we extend these observations to the GluN2A CTD. Sequence analysis showed that both subunits contain a form of intrinsic disorder classified as weak polyampholytes. However, only GluN2B contained matched patterning of arginine and aromatic residues, which are linked to LLPS. To examine the conformational distribution, we used discrete molecular dynamics (DMD), which revealed that GluN2A favors extended disordered states containing secondary structures while GluN2B favors disordered globular states. In contrast to GluN2B, smFRET measurements found that GluN2A lacked slow conformational dynamics. Thus, simulation and experiments found differences in the form of disorder. To understand how this affects protein interactions, we compared the ability of these two NMDAR isoforms to undergo LLPS. We found that GluN2B readily formed condensates with PSD-95 and SynGAP, while GluN2A failed to support LLPS and instead showed a propensity for colloidal aggregation. That GluN2A fails to support this same condensate formation suggests a developmental switch in LLPS propensity.

GluN2A-Selective NMDA Receptor Antagonists: Mimicking the U-Shaped Bioactive Conformation of TCN-201 by a [2.2]Paracyclophane System

Under physiological conditions, N-Methyl-D-Aspartate (NMDA) receptors play a crucial role for synaptic plasticity, long-term potentiation and long-term depression. However, overactivation of NMDA receptors can result in excitotoxicity, which is associated with various neurological and neurodegenerative diseases. The physiological properties of NMDA receptors are strongly dependent on the GluN2 subunit incorporated into the heterotetrameric NMDA receptor. Therefore, subtype selective NMDA receptor modulators are of high interest. Since prototypical GluN2A-NMDA receptor antagonists TCN-201 and its MPX-analogs adopt a U-shaped conformation within the binding pocket, paracyclophanes were designed containing the phenyl rings in an already parallel orientation. Docking studies of the designed paracyclophanes show a similar binding pose as TCN-201. [2.2]Paracyclophanes with a benzoate or benzamide side chain were prepared in four-step synthesis, respectively, starting with a radical bromination in benzylic 1-position of [2.2]paracyclophane. In two-electrode voltage clamp experiments using Xenopus laevis oocytes transfected with cRNAs for the GluN1-4a and GluN2A subunits, the esters and amides (conc. 10 μM) did not show considerable inhibition of ion flux. It can be concluded that the GluN2A-NMDA receptor does not accept ligands with a paracyclophane scaffold functionalized in benzylic 1-position, although docking studies had revealed promising binding poses for benzoic acid esters and benzamides.

Dose-dependent suppression of hippocampal contextual memory formation, place cells, and spatial engrams by the NMDAR antagonist (R)-CPP

We recently reported that the competitive NMDAR antagonist (R,S)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) does not suppress NMDAR-mediated field EPSPs (fEPSPNMDA) or long-term potentiation (LTP) in vitro at concentrations that block contextual conditioning in vivo. Here we tested one possible explanation for the mismatch - that the hippocampus is relatively resistant to CPP compared to other brain structures engaged in contextual fear conditioning. Using the context pre-exposure facilitation effect (CPFE) paradigm to separate the hippocampal and extra-hippocampal components of contextual learning, we found that the active enantiomer (R)-CPP suppressed the hippocampal component with an IC50 of 3.1 mg/kg, a dose that produces brain concentrations below those required to block fEPSPNMDA or LTP. Moreover, using in-vivo calcium imaging of place cells and spatial engrams to directly assess hippocampal spatial coding, we found that (R)-CPP dose-dependently reduced the development of place cells and interfered with the formation of stable spatial engrams when it was administered prior to exposing mice to a novel context. Both effects occurred at doses that interfered with freezing to context in CPFE experiments. We conclude that (R)-CPP blocks memory formation by interfering with hippocampal function, but that it does so by modulating NMDARs at sites that are not engaged in vitro in the same manner that they are in vivo - perhaps through interneuron circuits that do not contribute to fEPSPs and are not required to elicit LTP using standard induction protocols in vitro, but are essential for successful mnemonic function in vivo.

Postsynaptic Proteins at Excitatory Synapses in the Brain—Relationship with Depressive Disorders

Depressive disorders (DDs) are an increasingly common health problem that affects all age groups. DDs pathogenesis is multifactorial. However, it was proven that stress is one of the most important environmental factors contributing to the development of these conditions. In recent years, there has been growing interest in the role of the glutamatergic system in the context of pharmacotherapy of DDs. Thus, it has become increasingly important to explore the functioning of excitatory synapses in pathogenesis and pharmacological treatment of psychiatric disorders (including DDs). This knowledge may lead to the description of new mechanisms of depression and indicate new potential targets for the pharmacotherapy of illness. An excitatory synapse is a highly complex and very dynamic structure, containing a vast number of proteins. This review aimed to discuss in detail the role of the key postsynaptic proteins (e.g., NMDAR, AMPAR, mGluR5, PSD-95, Homer, NOS etc.) in the excitatory synapse and to systematize the knowledge about changes that occur in the clinical course of depression and after antidepressant treatment. In addition, a discussion on the potential use of ligands and/or modulators of postsynaptic proteins at the excitatory synapse has been presented.

NMDA Receptor C-Terminal Domain Signalling in Development, Maturity, and Disease

The NMDA receptor is a Ca²⁺-permeant glutamate receptor which plays key roles in health and disease. Canonical NMDARs contain two GluN2 subunits, of which 2A and 2B are predominant in the forebrain. Moreover, the relative contribution of 2A vs. 2B is controlled both developmentally and in an activity-dependent manner. The GluN2 subtype influences the biophysical properties of the receptor through difference in their N-terminal extracellular domain and transmembrane regions, but they also have large cytoplasmic Carboxyl (C)-terminal domains (CTDs) which have diverged substantially during evolution. While the CTD identity does not influence NMDAR subunit specific channel properties, it determines the nature of CTD-associated signalling molecules and has been implicated in mediating the control of subunit composition (2A vs. 2B) at the synapse. Historically, much of the research into the differential function of GluN2 CTDs has been conducted in vitro by over-expressing mutant subunits, but more recently, the generation of knock-in (KI) mouse models have allowed CTD function to be probed in vivo and in ex vivo systems without heterologous expression of GluN2 mutants. In some instances, findings involving KI mice have been in disagreement with models that were proposed based on earlier approaches. This review will examine the current research with the aim of addressing these controversies and how methodology may contribute to differences between studies. We will also discuss the outstanding questions regarding the role of GluN2 CTD sequences in regulating NMDAR subunit composition, as well as their relevance to neurodegenerative disease and neurodevelopmental disorders.

Calcium and activity-dependent signaling in the developing cerebral cortex

Calcium influx can be stimulated by various intra- and extracellular signals to set coordinated gene expression programs into motion. As such, the precise regulation of intracellular calcium represents a nexus between environmental cues and intrinsic genetic programs. Mounting genetic evidence points to a role for the deregulation of intracellular calcium signaling in neuropsychiatric disorders of developmental origin. These findings have prompted renewed enthusiasm for understanding the roles of calcium during normal and dysfunctional prenatal development. In this Review, we describe the fundamental mechanisms through which calcium is spatiotemporally regulated and directs early neurodevelopmental events. We also discuss unanswered questions about intracellular calcium regulation during the emergence of neurodevelopmental disease, and provide evidence that disruption of cell-specific calcium homeostasis and/or redeployment of developmental calcium signaling mechanisms may contribute to adult neurological disorders. We propose that understanding the normal developmental events that build the nervous system will rely on gaining insights into cell type-specific calcium signaling mechanisms. Such an understanding will enable therapeutic strategies targeting calcium-dependent mechanisms to mitigate disease.

SPG15 protein deficits are at the crossroads between lysosomal abnormalities, altered lipid metabolism and synaptic dysfunction

Hereditary spastic paraplegia type 15 (HSP15) is a neurodegenerative condition caused by the inability to produce SPG15 protein, which leads to lysosomal swelling. However, the link between lysosomal aberrations and neuronal death is poorly explored. To uncover the functional consequences of lysosomal aberrations in disease pathogenesis, we analyze human dermal fibroblasts from HSP15 patients as well as primary cortical neurons derived from an SPG15 knockout (KO) mouse model. We find that SPG15 protein loss induces defective anterograde transport, impaired neurite outgrowth, axonal swelling and reduced autophagic flux in association with the onset of lysosomal abnormalities. Additionally, we observe lipid accumulation within the lysosomal compartment, suggesting that distortions in cellular lipid homeostasis are intertwined with lysosomal alterations. We further demonstrate that SPG15 KO neurons exhibit synaptic dysfunction, accompanied by augmented vulnerability to glutamate-induced excitotoxicity. Overall, our study establishes an intimate link between lysosomal aberrations, lipid metabolism and electrophysiological impairments, suggesting that lysosomal defects are at the core of multiple neurodegenerative disease processes in HSP15.

Understanding global changes of the mouse brain proteome after vaginal infection with HSV-2 using a label-free shotgun approach

Herpes simplex virus type 2 (HSV-2) is a common human pathogen that establishes lifelong latency in neurons of the nervous system. The number of severe central nervous system infections caused by the virus has increased recently. However, the pathogenesis of HSV-2 infection in the nervous system is not fully understood. Here, we demonstrated global proteomic changes in the brain tissue in BALB/c mice vaginally infected with HSV-2. Data are available via ProteomeXchange with identifier PXD034186. A total of 249 differentially expressed proteins were identified in infected brain tissue. The GO and KEGG enrichment analysis of these proteins indicated that they were mainly involved in the regulation of synapse formation and synaptic excitability. In addition, genes affecting autophagy, the development of other neurodegenerative diseases, and signaling pathways relevant to other neurologic diseases were identified. Additional experiments, comparing the brain tissue of asymptomatic and symptomatic mice showed a differential expression of proteins involved in synapse formation and synaptic transmission. Others were involved in autophagy, addiction, and signaling pathways of other neurologic diseases. These results suggest that changes in synaptic structure and function, as well as autophagy, may be related to the development of neurologic abnormalities that follow HSV-2 infection. We also identified a protein GluN2A encoded by Grin2a was continuously expressed at high levels after infection. We propose that GluN2A may be a key molecule in the pathogenesis of HSV-2-induced neurologic diseases.

Phenol-benzoxazolone bioisosteres of GluN2B-NMDA receptor antagonists: Unexpected rearrangement during reductive alkylation with phenylcyclohexanone

Negative allosteric modulators of N-methyl- d-aspartate receptors containing the GluN2B subunit represent promising drug candidates for the treatment of various neurological disorders including stroke, epilepsy, and Parkinson's disease. To increase the bioavailability and GluN2B affinity, the phenol of the potent benzazepine-based inhibitor, WMS-1410 (3), was replaced bioisosterically by a benzoxazolone moiety and the phenylbutyl side chain was conformationally restricted in a phenylcyclohexyl substituent. A four-step, one-pot procedure transformed the oxazolo-benzazepine 7 into the phenylcyclohexyl derivative 11. The same protocol was applied to the methylated analog 12, which unexpectedly led to ring-contracted oxazolo-isoquinolines 18. This rearrangement was explained by the additional methyl moiety in the 8-position inhibiting the formation of the planar intermediate iminium ion with phenylcyclohexanone. The allyl protective group of 11 and 18 was removed with RhCl₃ and HCl to obtain the tricyclic compounds 5 and 19 without substituent at the oxazolone ring. The structures of the rearranged products 18 and 19 were elucidated by X-ray crystal structure analysis. The oxazolo-isoquinoline trans-18 with allyl moiety (K_i  = 89 nM) and the oxazolo-benzazepine 5 without substituent at the oxazolone ring (K_i  = 114 nM) showed GluN2B affinity in the same range as the lead compound 3. In two-electrode voltage clamp measurements, 5 displayed only weak inhibitory activity.

Role of N-Methyl-D-Aspartate Receptor NR2B Subunit in Inflammatory Arthritis-Induced Chronic Pain and Peripheral Sensitized Neuropathic Pain: A Systematic Review

Arthritis is a common clinical disease that affects millions of people in the world. The most common types of arthritis are osteoarthritis and rheumatoid arthritis. Inflammatory arthritis (IA), a chronic painful disease, is characterized by synovitis and cartilage destruction in the early stages. Pathologically, IA causes inflammatory changes in the joints and eventually leads to joint destruction. Pain is associated with inflammation and abnormal regulation of the nervous system pathways involved in pain promotion and inhibition. In addition, the occurrence of pain is associated with depression and anxiety. We found that there are many factors affecting pain, in addition to inflammatory factors, glutamate receptor may be the possible cause of long-term chronic pain caused by IA. N-methyl-d-aspartate receptor subunit 2B (NR2B) has been reported to involved in IA and nervous system diseases, especially peripheral neuropathic pain. In this review, we summarized the mechanisms of the NR2B subunit of the N-methyl-D-aspartate (NMDA) receptor in peripheral nerve sensitization during IA and chronic pain.