Mechanism of NMDA Receptor Inhibition and Activation

Bi-directional allosteric pathway in NMDA receptor activation and modulation

N-methyl-D-aspartate (NMDA) receptors are ionotropic glutamate receptors involved in learning and memory. NMDA receptors primarily comprise two GluN1 and two GluN2 subunits. The GluN2 subunit dictates biophysical receptor properties, including the extent of receptor activation and desensitization. GluN2A- and GluN2D-containing receptors represent two functional extremes. To uncover the conformational basis of their functional divergence, we utilize single-molecule fluorescence resonance energy transfer to probe the extracellular domains of these receptor subtypes under resting and ligand-bound conditions. We find that the conformational profile of the GluN2 amino-terminal domain correlates with the disparate functions of GluN2A- and GluN2D-containing receptors. Changes at the pre-transmembrane segments inversely correlate with those observed at the amino-terminal domain, confirming direct allosteric communication between these domains. Additionally, binding of a positive allosteric modulator at the transmembrane domain shifts the conformational profile of the amino-terminal domain towards the active state, revealing a bidirectional allosteric pathway between extracellular and transmembrane domains.

Subtype-specific conformational landscape of NMDA receptor gating

N-methyl-D-aspartate receptors are ionotropic glutamate receptors that mediate synaptic transmission and plasticity. Variable GluN2 subunits in diheterotetrameric receptors with identical GluN1 subunits set very different functional properties. To understand this diversity, we use single-molecule fluorescence resonance energy transfer (smFRET) to measure the conformations of the ligand binding domain and modulatory amino-terminal domain of the common GluN1 subunit in receptors with different GluN2 subunits. Our results demonstrate a strong influence of the GluN2 subunits on GluN1 rearrangements, both in non-agonized and partially agonized activation intermediates, which have been elusive to structural analysis, and in the fully liganded state. Chimeric analysis reveals structural determinants that contribute to these subtype differences. Our study provides a framework for understanding the conformational landscape that supports highly divergent levels of activity, desensitization, and agonist potency in receptors with different GluN2s and could open avenues for the development of subtype-specific modulators.

Structural prediction of GluN3 NMDA receptors

N-methyl-D-aspartate (NMDA) receptors are heterotetrametric ion channels composed of two obligatory GluN1 subunits and two alternative GluN2 or GluN3 subunits, forming GluN1-N2, GluN1-N3, and GluN1-N2-N3 type of NMDA receptors. Extensive research has focused on the functional and structural properties of conventional GluN1-GluN2 NMDA receptors due to their early discovery and high expression levels. However, the knowledge of unconventional GluN1-N3 NMDA receptors remains limited. In this study, we modeled the GluN1-N3A, GluN1-N3B, and GluN1-N3A-N3B NMDA receptors using deep-learned protein-language predication algorithms AlphaFold and RoseTTAFold All-Atom. We then compared these structures with GluN1-N2 and GluN1-N3A receptor cryo-EM structures and found that GluN1-N3 receptors have distinct properties in subunit arrangement, domain swap, and domain interaction. Furthermore, we predicted the agonist- or antagonist-bound structures, highlighting the key molecular-residue interactions. Our findings shed new light on the structural and functional diversity of NMDA receptors and provide a new direction for drug development. This study uses advanced AI algorithms to model GluN1-N3 NMDA receptors, revealing unique structural properties and interactions compared to conventional GluN1-N2 receptors. By highlighting key molecular-residue interactions and predicting ligand-bound structures, our research enhances the understanding of NMDA receptor diversity and offers new insights for targeted drug development.

Molecular mechanism of ligand gating and opening of NMDA receptor

Glutamate transmission and activation of ionotropic glutamate receptors are the fundamental means by which neurons control their excitability and neuroplasticity1. The N-methyl-D-aspartate receptor (NMDAR) is unique among all ligand-gated channels, requiring two ligands-glutamate and glycine-for activation. These receptors function as heterotetrameric ion channels, with the channel opening dependent on the simultaneous binding of glycine and glutamate to the extracellular ligand-binding domains (LBDs) of the GluN1 and GluN2 subunits, respectively2,3. The exact molecular mechanism for channel gating by the two ligands has been unclear, particularly without structures representing the open channel and apo states. Here we show that the channel gate opening requires tension in the linker connecting the LBD and transmembrane domain (TMD) and rotation of the extracellular domain relative to the TMD. Using electron cryomicroscopy, we captured the structure of the GluN1-GluN2B (GluN1-2B) NMDAR in its open state bound to a positive allosteric modulator. This process rotates and bends the pore-forming helices in GluN1 and GluN2B, altering the symmetry of the TMD channel from pseudofourfold to twofold. Structures of GluN1-2B NMDAR in apo and single-liganded states showed that binding of either glycine or glutamate alone leads to distinct GluN1-2B dimer arrangements but insufficient tension in the LBD-TMD linker for channel opening. This mechanistic framework identifies a key determinant for channel gating and a potential pharmacological strategy for modulating NMDAR activity.

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.

Promising Effects of Casearins in Tumor-Bearing Mice and Antinociceptive Action against Oncologic Pain: Molecular Docking and In Vivo Findings

Safer analgesic drugs remain a hard challenge because of cardiovascular and/or gastrointestinal toxicity, mainly. So, this study evaluated in vivo the antiproliferative actions of a fraction with casearins (FC) from Casearia sylvestris leaves against human colorectal carcinomas and antihyperalgesic effects on inflammatory- or opiate-based pain relief and oncologic pain in Sarcoma 180 (S180)-bearing mice. Moreover, docking investigations evaluated the binding among Casearin X and NMDA(N-methyl-D-aspartate)-type glutamate receptors. HCT-116 colorectal carcinoma-xenografted mice were treated with FC for 15 days. Antinociceptive assays included chemically induced algesia and investigated mechanisms by pharmacological blockade. Intraplantar region S180-bearing animals received a single dose of FC and were examined for mechanical allodynia and behavior alterations. AutoDock Vina determined molecular interactions among Cas X and NMDA receptor subunits. FC reduced tumor growth at i.p. (5 and 10 mg/kg) and oral (25 mg/kg/day) doses (31.12-39.27%). FC reduced abdominal pain, as confirmed by formalin and glutamate protocols, whose antinociception activity was blocked by naloxone and L-NAME (neurogenic phase) and naloxone, atropine, and flumazenil (inflammatory phase). Meanwhile, glibenclamide potentiated the FC analgesic effects. FC increased the paw withdrawal threshold without producing changes in exploratory parameters or motor coordination. Cas X generated a more stable complex with active sites of the NMDA receptor GluN2B subunits. FC is a promising antitumor agent against colorectal carcinomas, has peripheral analgesic effects by desensitizing secondary afferent neurons, and inhibits glutamate release from presynaptic neurons and/or their action on cognate receptors. These findings emphasize the use of clerodane diterpenes against cancer-related pain conditions.

The unexplored role of alkali and alkaline earth elements (ALAEs) on the structure, processing, and biological effects of bioactive glasses

Bioactive glass has been employed in several medical applications since its inception in 1969. The compositions of these materials have been investigated extensively with emphasis on glass network formers, therapeutic transition metals, and glass network modifiers. Through these experiments, several commercial and experimental compositions have been developed with varying chemical durability, induced physiological responses, and hydroxyapatite forming abilities. In many of these studies, the concentrations of each alkali and alkaline earth element have been altered to monitor changes in structure and biological response. This review aims to discuss the impact of each alkali and alkaline earth element on the structure, processing, and biological effects of bioactive glass. We explore critical questions regarding these elements from both a glass science and biological perspective. Should elements with little biological impact be included? Are alkali free bioactive glasses more promising for greater biological responses? Does this mixed alkali effect show increased degradation rates and should it be employed for optimized dissolution? Each of these questions along with others are evaluated comprehensively and discussed in the final section where guidance for compositional design is provided.

Bi-directional allosteric pathway in NMDA receptor activation and modulation

N-methyl-D-aspartate (NMDA) receptors are ionotropic glutamate receptors involved in learning and memory. NMDA receptors primarily comprise two GluN1 and two GluN2 subunits. The GluN2 subunit dictates biophysical receptor properties, including the extent of receptor activation and desensitization. GluN2A- and GluN2D-containing receptors represent two functional extremes. To uncover the conformational basis of their functional divergence, we utilized single-molecule fluorescence resonance energy transfer to probe the extracellular domains of these receptor subtypes under resting and ligand-bound conditions. We find that the conformational profile of the GluN2 amino-terminal domain correlates with the disparate functions of GluN2A- and GluN2D-containing receptors. Changes at the pre-transmembrane segments inversely correlate with those observed at the amino-terminal domain, confirming direct allosteric communication between these domains. Additionally, binding of a positive allosteric modulator at the transmembrane domain shifts the conformational profile of the amino-terminal domain towards the active state, revealing a bidirectional allosteric pathway between extracellular and transmembrane domains.

The role of NMDARs in the anesthetic and antidepressant effects of ketamine

BACKGROUND

As a phencyclidine (PCP) analog, ketamine can generate rapid-onset and substantial anesthetic effects. Contrary to traditional anesthetics, ketamine is a dissociative anesthetic and can induce loss of consciousness in patients. Recently, the subanaesthetic dose of ketamine was found to produce rapid-onset and lasting antidepressant effects.

AIM

However, how different concentrations of ketamine can induce diverse actions remains unclear. Furthermore, the molecular mechanisms underlying the NMDAR-mediated anesthetic and antidepressant effects of ketamine are not fully understood.

METHOD

In this review, we have introduced ketamine and its metabolism, summarized recent advances in the molecular mechanisms underlying NMDAR inhibition in the anesthetic and antidepressant effects of ketamine, explored the possible functions of NMDAR subunits in the effects of ketamine, and discussed the future directions of ketamine-based anesthetic and antidepressant drugs.

RESULT

Both the anesthetic and antidepressant effects of ketamine were thought to be mediated by N-methyl-D-aspartate receptor (NMDAR) inhibition.

CONCLUSION

The roles of NMDARs have been extensively studied in the anaesthetic effects of ketamine. However, the roles of NMDARs in antidepressant effects of ketamine are complicated and controversial.

Mechanistic and structural studies reveal NRAP-1-dependent coincident activation of NMDARs

N-methyl-D-aspartate (NMDA)-type ionotropic glutamate receptors have essential roles in neurotransmission and synaptic plasticity. Previously, we identified an evolutionarily conserved protein, NRAP-1, that is required for NMDA receptor (NMDAR) function in C. elegans. Here, we demonstrate that NRAP-1 was sufficient to gate NMDARs and greatly enhanced glutamate-mediated NMDAR gating, thus conferring coincident activation properties to the NMDAR. Intriguingly, vertebrate NMDARs-and chimeric NMDARs where the amino-terminal domain (ATD) of C. elegans NMDARs was replaced by the ATD from vertebrate receptors-were spontaneously active when ectopically expressed in C. elegans neurons. Thus, the ATD is a primary determinant of NRAP-1- and glutamate-mediated gating of NMDARs. We determined the crystal structure of NRAP-1 at 1.9-Å resolution, which revealed two distinct domains positioned around a central low-density lipoprotein receptor class A domain. The NRAP-1 structure, combined with chimeric and mutational analyses, suggests a model where the three NRAP-1 domains work cooperatively to modify the gating of NMDARs.

Neuroprotective Effects of Cannabispirenone A against NMDA-Induced Excitotoxicity in Differentiated N2a Cells

The endocannabinoid system is found throughout the central nervous system, and its cannabinoids receptor 1 is critical in preventing neurotoxicity caused by N-methyl-D-aspartate receptor activation (NMDARs). The activity of NMDARs places demands on endogenous cannabinoids to regulate their calcium currents. Endocannabinoids keep NMDAR activity within safe limits, protecting neural cells from excitotoxicity. Cannabinoids are remembered to deliver this outcome by repressing presynaptic glutamate discharge or obstructing postsynaptic NMDAR-managed flagging pathways. The endocannabinoid system must exert a negative influence proportional to the strength of NMDAR signaling for such control to be effective. The goal of this paper is to draw the attention towards the neuroprotective mechanism of constituents of Cannabis sativa against NMDA-induced excitotoxic result. Phytochemical investigation of the cannabis flowers led to the isolation of nine secondary metabolites. A spiro-compound, Cannabispirenone A, which on treatment of the cells prior to NMDA exposure significantly increases cell survival while decreasing ROS production, lipid peroxidation, and intracellular calcium. Our findings showed that this compound showed neuroprotection against NMDA-induced excitotoxic insult, has antioxidative properties, and increased cannabinoid receptor 1 expression, which may be involved in the signaling pathway for this neuroprotection.

Conformational basis of subtype-specific allosteric control of NMDA receptor gating

N-methyl-D-aspartate receptors are ionotropic glutamate receptors that are integral to synaptic transmission and plasticity. Variable GluN2 subunits in diheterotetrameric receptors with identical GluN1 subunits set very different functional properties, which support their individual physiological roles in the nervous system. To understand the conformational basis of this diversity, we assessed the conformation of the common GluN1 subunit in receptors with different GluN2 subunits using single-molecule fluorescence resonance energy transfer (smFRET). We established smFRET sensors in the ligand binding domain and modulatory amino-terminal domain to study an apo-like state and partially liganded activation intermediates, which have been elusive to structural analysis. Our results demonstrate a strong, subtype-specific influence of apo and glutamate-bound GluN2 subunits on GluN1 rearrangements, suggesting a conformational basis for the highly divergent levels of receptor activity, desensitization and agonist potency. Chimeric analysis reveals structural determinants that contribute to the subtype differences. Our study provides a framework for understanding GluN2-dependent functional properties and could open new avenues for subtype-specific modulation.

Expression and Purification of Mammalian NMDA Receptor Protein for Functional Characterization

N-methyl-D-aspartate (NMDA) receptors are critical for brain function and serve as drug targets for the treatment of neurological and psychiatric disorders. They typically form the tetrameric assembly of GluN1-GluN2 (2A to 2D) subtypes, with their diverse three-dimensional conformations linked with the physiologically relevant function in vivo. Purified proteins of tetrameric assembled NMDA receptors have broad applications in the structural elucidation, hybridoma technology for antibody production, and high-throughput drug screening. However, obtaining sufficient quantity and monodisperse NMDA receptor protein is still technically challenging. Here, we summarize a paradigm for the expression and purification of diverse NMDA receptor subtypes, with detailed descriptions on screening constructs by fluorescence size-exclusion chromatography (FSEC), generation of recombinant baculovirus, expression in the eukaryotic expression system, protein purification by affinity chromatography and size-exclusion chromatography (SEC), biochemical and functional validation assays.

Therapeutic potential of N-methyl-D-aspartate receptor modulators in psychiatry

N-methyl-D-aspartate (NMDA) receptors mediate a slow component of excitatory synaptic transmission, are widely distributed throughout the central nervous system, and regulate synaptic plasticity. NMDA receptor modulators have long been considered as potential treatments for psychiatric disorders including depression and schizophrenia, neurodevelopmental disorders such as Rett Syndrome, and neurodegenerative conditions such as Alzheimer's disease. New interest in NMDA receptors as therapeutic targets has been spurred by the findings that certain inhibitors of NMDA receptors produce surprisingly rapid and robust antidepressant activity by a novel mechanism, the induction of changes in the brain that well outlast the presence of drug in the body. These findings are driving research into an entirely new paradigm for using NMDA receptor antagonists in a host of related conditions. At the same time positive allosteric modulators of NMDA receptors are being pursued for enhancing synaptic function in diseases that feature NMDA receptor hypofunction. While there is great promise, developing the therapeutic potential of NMDA receptor modulators must also navigate the potential significant risks posed by the use of such agents. We review here the emerging pharmacology of agents that target different NMDA receptor subtypes, offering new avenues for capturing the therapeutic potential of targeting this important receptor class.

Modeling and Simulation of the NMDA Receptor at Coarse-Grained and Atomistic Levels

N-Methyl-D-aspartate (NMDA) receptors are glutamate-gated excitatory channels that play essential roles in brain functions. While high-resolution structures were solved for an allosterically inhibited form of functional NMDA receptor, other key functional states (particularly the active open-channel state) have not yet been resolved at atomic resolutions. To decrypt the molecular mechanism of the NMDA receptor activation, structural modeling and simulation are instrumental in providing detailed information about the dynamics and energetics of the receptor in various functional states. In this chapter, we describe coarse-grained modeling of the NMDA receptor using an elastic network model and related modeling/analysis tools (e.g., normal mode analysis, flexibility and hotspot analysis, cryo-EM flexible fitting, and transition pathway modeling) based on available structures. Additionally, we show how to build an atomistic model of the active-state receptor with targeted molecular dynamics (MD) simulation and explore its energetics and dynamics with conventional MD simulation. Taken together, these modeling and simulation can offer rich structural and dynamic information which will guide experimental studies of the activation of this key receptor.

Diabetic Encephalopathy: Role of Oxidative and Nitrosative Factors in Type 2 Diabetes

Diabetes mellitus is a set of complex metabolic disorders characterized by chronic hyperglycaemic condition due to defective insulin secretion (Type 1) and action (Type 2), which leads to serious micro and macro-vascular damage, inflammation, oxidative and nitrosative stress and a deranged energy homeostasis due to imbalance in the glucose and lipid metabolism. Moreover, patient with diabetes mellitus often showed the nervous system disorders known as diabetic encephalopathy. The precise pathological mechanism of diabetic encephalopathy by which it effects the central nervous system directly or indirectly causing the cognitive and motor impairment, is not completely understood. However, it has been speculated that like other extracerebellar tissues, oxidative and nitrosative stress may play significant role in the pathogenesis of diabetic encephalopathy. Therefore, the present review aimed to explain the possible association of the oxidative and nitrosative stress caused by the chronic hyperglycaemic condition with the central nervous system complications of the type 2 diabetes mellitus induced diabetic encephalopathy.

Nitric oxide modulates NMDA receptor through a negative feedback mechanism and regulates the dynamical behavior of neuronal postsynaptic components

Nitric oxide (NO) is known to be an important regulator of neurological processes in the central nervous system which acts directly on the presynaptic neuron and enhances the release of neurotransmitters like glutamate into the synaptic cleft. Calcium influx activates a cascade of biochemical reactions to influence the production of nitric oxide in the postsynaptic neuron. This has been modeled in the present work as a system of ordinary differential equations, to explore the dynamics of the interacting components and predict the dynamical behavior of the postsynaptic neuron. It has been hypothesized that nitric oxide modulates the NMDA receptor via a feedback mechanism and regulates the dynamic behavior of postsynaptic components. Results obtained by numerical analyses indicate that the biochemical system is stimulus-dependent and shows oscillations of calcium and other components within a limited range of concentration. Some of the parameters such as stimulus strength, extracellular calcium concentration, and rate of nitric oxide feedback are crucial for the dynamics of the components in the postsynaptic neuron.

Structural insights into gating mechanism and allosteric regulation of NMDA receptors

N-methyl-d-aspartate receptors (NMDARs) belong to the ionotropic glutamate receptors (iGluRs) superfamily and act as coincidence detectors that are crucial to neuronal development and synaptic plasticity. They typically assemble as heterotetramers of two obligatory GluN1 subunits and two alternative GluN2 (from 2A to 2D) and/or GluN3 (3A and 3B) subunits. These alternative subunits mainly determine the diverse biophysical and pharmacological properties of different NMDAR subtypes. Over the past decade, the unprecedented advances in structure elucidation of these tetrameric NMDARs have provided atomic insights into channel gating, allosteric modulation and the action of therapeutic drugs. A wealth of structural and functional information would accelerate the artificial intelligence-based drug design to exploit more NMDAR subtype-specific molecules for the treatment of neurological and psychiatric disorders.

d-Amino acids in biological systems

Investigations on the occurrence and biochemical roles of free D-amino acids and D-amino acid-containing peptides and proteins in living systems have increased in frequency and significance. Their occurrence and roles may vary substantially with progression from microbiotic to evermore advanced macrobiotic systems. We now understand many of the biosynthetic and regulatory pathways, which are outlined herein. Important uses for D-amino acids in plants, invertebrates, and vertebrates are reviewed. Given its importance, a separate section on the occurrence and role of D-amino acids in human disease is presented.

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.

The structural basis of divalent cation block in a tetrameric prokaryotic sodium channel

Divalent cation block is observed in various tetrameric ion channels. For blocking, a divalent cation is thought to bind in the ion pathway of the channel, but such block has not yet been directly observed. So, the behaviour of these blocking divalent cations remains still uncertain. Here, we elucidated the mechanism of the divalent cation block by reproducing the blocking effect into NavAb, a well-studied tetrameric sodium channel. Our crystal structures of NavAb mutants show that the mutations increasing the hydrophilicity of the inner vestibule of the pore domain enable a divalent cation to stack on the ion pathway. Furthermore, non-equilibrium molecular dynamics simulation showed that the stacking calcium ion repel sodium ion at the bottom of the selectivity filter. These results suggest the primary process of the divalent cation block mechanism in tetrameric cation channels.

Targeting NMDA receptor in Alzheimer's disease: identifying novel inhibitors using computational approaches

The glutamate-gated ion channels known as N-methyl-d-aspartate receptors (NMDARs) are important for both normal and pathological brain function. Subunit-selective antagonists have high therapeutic promise since many pathological conditions involve NMDAR over activation, although few clinical successes have been reported. Allosteric inhibitors of GluN2B-containing receptors are among the most potential NMDAR targeting drugs. Since the discovery of ifenprodil, a variety of GluN2B-selective compounds have been discovered, each with remarkably unique structural motifs. These results expand the allosteric and pharmacolog-ical spectrum of NMDARs and provide a new structural basis for the development of next-generation GluN2B antagonists that have therapeutic potential in brain diseases. Small molecule therapeutic inhibitors targeting NMDA have recently been developed to target CNS disorders such as Alzheimer's disease. In the current study, a cheminformatics method was used to discover potential antagonists and to identify the structural requirements for Gly/NMDA antagonism. In this case we have created a useful pharmacophore model with solid statistical values. Through pharmacophore mapping, the verified model was used to filter out virtual matches from the ZINC database. Assessing receptor-ligand binding mechanisms and affinities used molecular docking. To find the best hits, the GlideScore and the interaction of molecules with important amino acids were considered essential features. We found some molecular inhibitors, namely, ZINC13729211, ZINC07430424, ZINC08614951, ZINC60927204, ZINC12447511, and ZINC18889258 with high binding affinity using computational methods. The molecules in our studies showed characteristics such as good stability, hydrogen bonding and higher binding affinities in the solvation-based assessment method than ifenprodil with acceptable ADMET profile. Moreover, these six leads have been proposed as potential new perspectives for exploring potent Gly/NMDA receptor antagonists. In addition, it can be tested in the laboratory for potential therapeutic strategies for both in vitro and in vivo research.

GluD receptors are functional ion channels

In this article, we review contemporary evidence that GluD receptors are functional ion channels whose depolarizing currents contribute to their biological functions, akin to all other members of the ionotropic glutamate receptor (iGluR) family.

Allosteric modulation of GluN1/GluN3 NMDA receptors by GluN1-selective competitive antagonists

NMDA-type ionotropic glutamate receptors are critical for normal brain function and are implicated in central nervous system disorders. Structure and function of NMDA receptors composed of GluN1 and GluN3 subunits are less understood compared to those composed of GluN1 and GluN2 subunits. GluN1/3 receptors display unusual activation properties in which binding of glycine to GluN1 elicits strong desensitization, while glycine binding to GluN3 alone is sufficient for activation. Here, we explore mechanisms by which GluN1-selective competitive antagonists, CGP-78608 and L-689,560, potentiate GluN1/3A and GluN1/3B receptors by preventing glycine binding to GluN1. We show that both CGP-78608 and L-689,560 prevent desensitization of GluN1/3 receptors, but CGP-78608-bound receptors display higher glycine potency and efficacy at GluN3 subunits compared to L-689,560-bound receptors. Furthermore, we demonstrate that L-689,560 is a potent antagonist of GluN1FA+TL/3A receptors, which are mutated to abolish glycine binding to GluN1, and that this inhibition is mediated by a non-competitive mechanism involving binding to the mutated GluN1 agonist binding domain (ABD) to negatively modulate glycine potency at GluN3A. Molecular dynamics simulations reveal that CGP-78608 and L-689,560 binding or mutations in the GluN1 glycine binding site promote distinct conformations of the GluN1 ABD, suggesting that the GluN1 ABD conformation influences agonist potency and efficacy at GluN3 subunits. These results uncover the mechanism that enables activation of native GluN1/3A receptors by application of glycine in the presence of CGP-78608, but not L-689,560, and demonstrate strong intra-subunit allosteric interactions in GluN1/3 receptors that may be relevant to neuronal signaling in brain function and disease.

Excitatory GluN1/GluN3A glycine receptors (eGlyRs) in brain signaling

GluN3A is a glycine-binding subunit belonging to the NMDA receptor (NMDAR) family that can assemble with GluN1 subunits to form unconventional NMDARs insensitive to glutamate and activated by glycine only. The existence of such excitatory glycine receptors (eGlyRs) in the central nervous system (CNS) has long remained elusive. Recently, eGlyRs have been identified in specific brain regions, where they represent a novel neuronal signaling modality by which extracellular glycine tunes neuronal excitability, circuit function, and behavior. In this review, we summarize the emerging knowledge regarding these underappreciated receptors. The existence of eGlyRs reshapes current understanding of NMDAR diversity and of glycinergic signaling, previously thought to be primarily inhibitory. Given that GluN3A expression is concentrated in brain regions regulating emotional responses, eGlyRs are potential new targets of therapeutic interest in neuropsychiatry.

Binding Affinity and Mechanisms of Potential Antidepressants Targeting Human NMDA Receptors

Depression, a mental disorder that plagues the world, is a burden on many families. There is a great need for new, fast-acting antidepressants to be developed. N-methyl-D-aspartic acid (NMDA) is an ionotropic glutamate receptor that plays an important role in learning and memory processes and its TMD region is considered as a potential target to treat depression. However, due to the unclear binding sites and pathways, the mechanism of drug binding lacks basic explanation, which brings great complexity to the development of new drugs. In this study, we investigated the binding affinity and mechanisms of an FDA-approved antidepressant (S-ketamine) and seven potential antidepressants (R-ketamine, memantine, lanicemine, dextromethorphan, Ro 25-6981, ifenprodil, and traxoprodil) targeting the NMDA receptor by ligand-protein docking and molecular dynamics simulations. The results indicated that Ro 25-6981 has the strongest binding affinity to the TMD region of the NMDA receptor among the eight selected drugs, suggesting its potential effective inhibitory effect. We also calculated the critical binding-site residues at the active site and found that residues Leu124 and Met63 contributed the most to the binding energy by decomposing the free energy contributions on a per-residue basis. We further compared S-ketamine and its chiral molecule, R-ketamine, and found that R-ketamine had a stronger binding capacity to the NMDA receptor. This study provides a computational reference for the treatment of depression targeting NMDA receptors, and the proposed results will provide potential strategies for further antidepressant development and is a useful resource for the future discovery of fast-acting antidepressant candidates.

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.

Functional crosstalk of the glycine transporter GlyT1 and NMDA receptors

NMDA-type glutamate receptors (NMDARs) constitute one of the main glutamate (Glu) targets in the central nervous system and are involved in synaptic plasticity, which is the molecular substrate of learning and memory. Hypofunction of NMDARs has been associated with schizophrenia, while overstimulation causes neuronal death in neurodegenerative diseases or in stroke. The function of NMDARs requires coincidental binding of Glu along with other cellular signals such as neuronal depolarization, and the presence of other endogenous ligands that modulate their activity by allosterism. Among these allosteric modulators are zinc, protons and Gly, which is an obligatory co-agonist. These characteristics differentiate NMDARs from other receptors, and their structural bases have begun to be established in recent years. In this review we focus on the crosstalk between Glu and glycine (Gly), whose concentration in the NMDAR microenvironment is maintained by various Gly transporters that remove or release it into the medium in a regulated manner. The GlyT1 transporter is particularly involved in this task, and has become a target of great interest for the treatment of schizophrenia since its inhibition leads to an increase in synaptic Gly levels that enhances the activity of NMDARs. However, the only drug that has completed phase III clinical trials did not yield the expected results. Notwithstanding, there are additional drugs that continue to be investigated, and it is hoped that knowledge gained from the recently published 3D structure of GlyT1 may allow the rational design of more effective new drugs.

Distinct structure and gating mechanism in diverse NMDA receptors with GluN2C and GluN2D subunits

N-methyl-D-aspartate (NMDA) receptors are heterotetramers comprising two GluN1 and two alternate GluN2 (N2A-N2D) subunits. Here we report full-length cryo-EM structures of the human N1-N2D di-heterotetramer (di-receptor), rat N1-N2C di-receptor and N1-N2A-N2C tri-heterotetramer (tri-receptor) at a best resolution of 3.0 Å. The bilobate N-terminal domain (NTD) in N2D intrinsically adopts a closed conformation, leading to a compact NTD tetramer in the N1-N2D receptor. Additionally, crosslinking the ligand-binding domain (LBD) of two N1 protomers significantly elevated the channel open probability (Po) in N1-N2D di-receptors. Surprisingly, the N1-N2C di-receptor adopted both symmetric (minor) and asymmetric (major) conformations, the latter further locked by an allosteric potentiator, PYD-106, binding to a pocket between the NTD and LBD in only one N2C protomer. Finally, the N2A and N2C subunits in the N1-N2A-N2C tri-receptor display a conformation close to one protomer in the N1-N2A and N1-N2C di-receptors, respectively. These findings provide a comprehensive structural understanding of diverse function in major NMDA receptor subtypes.

NMDARs regulate the excitatory-inhibitory balance within neural circuits

Excitatory-inhibitory (E/I) balance is essential for normal neural development, behavior and cognition. E/I imbalance leads to a variety of neurological disorders, such as autism and schizophrenia. NMDA receptors (NMDARs) regulate AMPAR-mediated excitatory and GABAAR-mediated inhibitory synaptic transmission, suggesting that NMDARs play an important role in the establishment and maintenance of the E/I balance. In this review, we briefly introduced NMDARs, AMPARs and GABAARs, summarized the current studies on E/I balance mediated by NMDARs, and discussed the current advances in NMDAR-mediated AMPAR and GABAAR development. Specifically, we analyzed the role of NMDAR subunits in the establishment and maintenance of E/I balance, which may provide new therapeutic strategies for the recovery of E/I imbalance in neurological disorders.

The Weakened Interaction Between HECTD4 and GluN2B in Ischemic Stroke Promotes Calcium Overload and Brain Injury Through a Mechanism Involving the Decrease of GluN2B and MALT1 Ubiquitination

Glutamate receptor ionotropic NMDA 2B (GluN2B) plays an essential role in calcium overload during excitotoxicity. Reverse-phase nano-liquid chromatography-tandem mass spectrometry has revealed an interaction between GluN2B and HECT domain E3 ubiquitin protein ligase 4 (HECTD4), an E3 ubiquitin ligase highly expressed in the brain. As a potential substrate for HECTD4, mucosa-associated lymphoid tissue lymphoma translocation protein 1 (MALT1) acts as a scaffold with hydrolysis activity. This study explores the relationship between HECTD4, GluN2B, and MALT1, focusing on their role in brain injury in ischemic stroke. Rats were subjected to 2 h-ischemia followed by 24-h reperfusion to establish an ischemic stroke model. We observed the downregulation of HECTD4 and the upregulation of MALT1. Additionally, an increased GluN2B phosphorylation was concomitant with weakened interactions between HECTD4 and GluN2B, followed by decreased striatal-enriched protein phosphatase (STEP61). Knockdown of HECTD4 exacerbated hypoxia- or NMDA-induced injury in nerve cells coincident with a decrease in GluN2B and MALT1 ubiquitination, and an increase in GluN2B phosphorylation as well as an increase in intracellular calcium level, which were counteracted by MALT1 siRNA. Blockage of MALT1 with its inhibitor or siRNA reduced STEP61 degradation, accompanied by a decrease in GluN2B phosphorylation, intracellular calcium concentration, and brain cell injury, which were reversed by overexpression of MALT1. Based on these observations, we conclude that the downregulation of HECTD4 in ischemic stroke rat brain accounts for calcium overload and brain injury due to activating GluN2B directly and indirectly through a mechanism involving the reduced ubiquitination of GluN2B and MALT1, respectively.

The kynurenine pathway implicated in patient delirium: possible indications for indoleamine 2,3 dioxygenase inhibitors

Tryptophan (Trp) metabolism plays a central role in sleep, mood, and immune system regulation. The kynurenine pathway (KP), which is regulated by the enzymes tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3 dioxygenase (IDO), which catalyze the conversion of Trp to kynurenine (Kyn), facilitates immune regulation and influences neurocognition. Notably, Kyn metabolites bind the N-methyl-d-aspartate receptor (NMDAR), essential for memory encoding, and in turn, cognition. Aberrant NMDAR activity through agonist binding influences excitability and cell death. In this issue of the JCI, Watne and authors demonstrate that KP pathway end products were elevated in the serum and the cerebrospinal fluid (CSF) of subjects with delirium. This observation provides insight regarding the basis of a variety of commonly observed clinical conditions including sundowning, abnormal sleep-wake cycles in hospitalized patients, neurodegenerative cognitive impairment, radiation-induced cognitive impairment, neurocognitive symptomatology related to COVID-19, and clinical outcomes observed in patients with CNS tumors, such as gliomas.

Datumetine Preferentially Upregulates N-methyl-D-aspartate Receptor Signalling Pathways in Different Brain Regions of Mice

Introduction

We previously reported that datumetine possesses binding affinity with N-methyl-D-aspartate receptor (NMDAR) and that 14-day exposure to datumetine altered NMDAR signaling by mimicking glutamate toxicity. Here, we investigated the potential neuroprotective effect of a single shot of a low dose of datumetine administration in BALB/c mice.

Methods

30 male adult BALB/c mice were used for the study. The mice were randomly divided into three groups of ten mice each with an intraperitoneal injection of 0.1 mL of 10% DMSO for the Vehicle group, Datumetine group were administered 0.1 mg/kg body weight (bw) of datumetine and MK-801+Datumetine group were administered 0.5 mg/kg bw of MK-801 (to block NMDAR) followed by 0.1 mg/kg bw of datumetine after 30 minutes. 24 hours after administration, mice were euthanized in an isoflurane chamber followed by perfusion with 1X PBS. Brains were excised and stored at -20°C till further processing. Mice designated for IHC were further perfused with 4% PFA and brain excised and stored in 4% PFA till further processing. NMDAR signalling molecules expression was evaluated in frozen brain samples and the fixed brain samples were stained for neuron, vGlut and NMDAR subtypes.

Results

Relative to vehicle (Veh), datumetine downregulate calcium calmodulin kinase II alpha (CamKIIα) expression in the hippocampus and prefrontal cortex (PFC) but not in the cerebellum, cyclic AMP response element binding protein (CREB) was also upregulated only in the PFC but phosphorylated CREB (pCREB) was also upregulated in three brain regions observed, while brain-derived neurotrophic factor (BDNF) was only upregulated in hippocampus and PFC of Datumetine relative to vehicle (Veh). On the other hand, dizocilpine (MK-801) reversed some of the effects of datumetine in the observed brain regions. No major histological alterations were observed in the different brain regions immunohistochemically.

Conclusion

We conclude that a low dose of datumetine moderately enhances NMDAR activity. This showed the neuroprotective potentials of low datumetine exposure.

Molecular mechanisms underlying the N-methyl-d-aspartate receptor antagonists: Highlighting their potential for transdiagnostic therapeutics

The so-called "psychedelic renaissance" has stimulated expanded interest in several classes of drugs that appear to possess transdiagnostic effects in the treatment of mental health disorders, specifically. N-methyl-d-aspartate receptor (NMDAR) antagonists are one such class with diverse therapeutic potential. NMDARs mediate excitatory postsynaptic signalling in the central nervous system (CNS) and are integral to normal neurobiological processes including neuronal development, synaptic transmission, and plasticity, and thus involved in learning and memory. However, NMDAR hyper-function is also implicated in acute CNS trauma, neuropsychiatric and neurodegenerative disorders, as well as chronic pain. The complex structure of NMDARs permits several locations for therapeutic inhibition, making these receptors a potential target for multiple drugs which modulate them in different ways. NMDAR antagonists, which may be competitive, non-competitive, or uncompetitive, either block glutamate from binding the receptor or modulate the response to glutamate binding. Despite longstanding concerns about side effects of NMDAR antagonists, recent research suggests that, when appropriately used, these agents have favourable safety profiles. Furthermore, their fast-acting mechanism of action, resulting in rapid effects compared to other therapeutic agents, makes them a promising class of drugs that may yield effective therapeutics for multiple CNS disorders.

Structural insights into assembly and function of GluN1-2C, GluN1-2A-2C, and GluN1-2D NMDARs

Neurotransmission mediated by diverse subtypes of N-methyl-D-aspartate receptors (NMDARs) is fundamental for basic brain functions and development as well as neuropsychiatric diseases and disorders. NMDARs are glycine- and glutamate-gated ion channels that exist as heterotetramers composed of obligatory GluN1 and GluN2(A-D) and/or GluN3(A-B). The GluN2C and GluN2D subunits form ion channels with distinct properties and spatio-temporal expression patterns. Here, we provide the structures of the agonist-bound human GluN1-2C NMDAR in the presence and absence of the GluN2C-selective positive allosteric potentiator (PAM), PYD-106, the agonist-bound GluN1-2A-2C tri-heteromeric NMDAR, and agonist-bound GluN1-2D NMDARs by single-particle electron cryomicroscopy. Our analysis shows unique inter-subunit and domain arrangements of the GluN2C NMDARs, which contribute to functional regulation and formation of the PAM binding pocket and is distinct from GluN2D NMDARs. Our findings here provide the fundamental blueprint to study GluN2C- and GluN2D-containing NMDARs, which are uniquely involved in neuropsychiatric disorders.

An Insight into Animal Glutamate Receptors Homolog of Arabidopsis thaliana and Their Potential Applications-A Review

Most excitatory impulses received by neurons are mediated by ionotropic glutamate receptors (iGluRs). These receptors are located at the apex and play an important role in memory, neuronal development, and synaptic plasticity. These receptors are ligand-dependent ion channels that allow a wide range of cations to pass through. Glutamate, a neurotransmitter, activates three central ionotropic receptors: N-methyl-D-aspartic acid (NMDA), -amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), and kainic acid (KA). According to the available research, excessive glutamate release causes neuronal cell death and promotes neurodegenerative disorders. Arabidopsis thaliana contains 20 glutamate receptor genes (AtGluR) comparable to the human ionotropic glutamate (iGluRs) receptor. Many studies have proved that AtGL-rec genes are involved in a number of plant growth and physiological activities, such as in the germination of seeds, roots, abiotic and biotic stress, and cell signaling, which clarify the place of these genes in plant biology. In spite of these, the iGluRs, Arabidopsis glutamate receptors (AtGluR), is associated with the ligand binding activity, which confirms the evolutionary relationship between animal and plant glutamate receptors. Along with the above activities, the impact of mammalian agonists and antagonists on Arabidopsis suggests a correlation between plant and animal glutamate receptors. In addition, these glutamate receptors (plant/animal) are being utilized for the early detection of neurogenerative diseases using the fluorescence resonance energy transfer (FRET) approach. However, a number of scientific laboratories and institutes are consistently working on glutamate receptors with different aspects. Currently, we are also focusing on Arabidopsis glutamate receptors. The current review is focused on updating knowledge on AtGluR genes, their evolution, functions, and expression, and as well as in comparison with iGluRs. Furthermore, a high throughput approach based on FRET nanosensors developed for understanding neurotransmitter signaling in animals and plants via glutamate receptors has been discussed. The updated information will aid in the future comprehension of the complex molecular dynamics of glutamate receptors and the exploration of new facts in plant/animal biology.

Naringenin Attenuates Cognitive Impairment in a Rat Model of Vascular Dementia by Inhibiting Hippocampal Oxidative Stress and Inflammatory Response and Promoting N-Methyl-D-Aspartate Receptor Signaling Pathway

Vascular dementia (VaD) is the second most common form of dementia globally, yet there are no efficient treatments. Naringenin, a natural flavonoid, exerts antioxidative, anti-inflammatory, and neuroprotective properties; however, its potential effect on VaD remain unclear. Herein, the purpose of present study was to elucidate whether naringenin attenuates cognitive dysfunction in VaD via inhibiting hippocampal oxidative stress and inflammatory response, and promoting N-methyl-D-aspartate receptors (NMDARs) signaling pathway. A rat model of VaD was established by permanent bilateral common carotid artery occlusion [2-vessel occlusion (2VO)]. Behavioral performance analyses results revealed that administration of naringenin improves cognitive impairment in rats with VaD according to the new object recognition test and the Morris water maze test. In addition, naringenin attenuated hippocampal oxidative stress by reducing reactive oxygen species generation, decreasing malondialdehyde content and recombinant reactive oxygen species modulator 1 (Romo-1) expression, and increasing superoxide dismutase and glutathione peroxidase activities in the hippocampus of VaD rats. Moreover, naringenin decreased the proinflammatory cytokines (IL-1β, IL-6, and TNF-α) levels and increased the anti-inflammatory cytokines (IL-10 and IL-4) levels in the hippocampus of 2VO surgery-treated rats, attenuating hippocampal inflammatory response during VaD. Furthermore, naringenin promoted synaptophysin (SYP), postsynaptic density protein 95 (PSD95), N-methyl-Daspartic acid receptor 1 (NR1) and N-methyl-D-aspartate receptor subunit 2B (NR2B) expressions levels in hippocampus of VaD rats. Collectively, these findings indicated that naringenin mitigates cognitive impairment in VaD rats partly via inhibiting hippocampal oxidative stress and inflammatory response and restoring NMDARs signaling pathway.

Biphenyl scaffold for the design of NMDA-receptor negative modulators: molecular modeling, synthesis, and biological activity

NMDA (N-methyl-d-aspartate) receptor antagonists are promising tools for the treatment of a wide variety of central nervous system impairments including major depressive disorder. We present here the activity optimization process of a biphenyl-based NMDA negative allosteric modulator (NAM) guided by free energy calculations, which led to a 100 times activity improvement (IC50 = 50 nM) compared to a hit compound identified in virtual screening. Preliminary calculation results suggest a low affinity for the human ether-a-go-go-related gene ion channel (hERG), a high affinity for which was earlier one of the main obstacles for the development of first-generation NMDA-receptor negative allosteric modulators. The docking study and the molecular dynamics calculations suggest a completely different binding mode (ifenprodil-like) compared to another biaryl-based NMDA NAM EVT-101.

Combined exposure of lead and high-fat diet enhanced cognitive decline via interacting with CREB-BDNF signaling in male rats

The health risks to populations induced by lead (Pb) and high-fat diets (HFD) have become a global public health problem. Pb and HFD often co-exist and are co-occurring risk factors for cognitive impairment. This study investigates effect of combined Pb and HFD on cognitive function, and explores the underlying mechanisms in terms of regulatory components of synaptic plasticity and insulin signaling pathway. We showed that the co-exposure of Pb and HFD further increased blood Pb levels, caused body weight loss and dyslipidemia. The results from Morris water maze (MWM) test and Nissl staining disclosed that Pb and HFD each contributed to cognitive deficits and neuronal damage and combined exposure enhanced this toxic injury. Pb and HFD decreased the levels of synapsin-1, GAP-43 and PSD-95 protein related to synaptic properties and SIRT1, NMDARs, phosphorylated CREB and BDNF related to synaptic plasticity regulatory, and these decreases was greater when combined exposure. Additionally, we revealed that Pb and HFD promoted IRS-1 phosphorylation and subsequently reduced downstream PI3K-Akt kinases phosphorylation in hippocampus and cortex of rats, and this process was aggravated when co-exposure. Collectively, our data suggested that combined exposure of Pb and HFD enhanced cognitive deficits, pointing to additive effects in rats than the individual stress effects related to multiple signaling pathways with CREB-BDNF signaling as the hub. This study emphasizes the need to evaluate the effects of mixed exposures on brain function in realistic environment and to better inform prevention of neurological disorders via modulating central pathway, such as CREB/BDNF signaling.

Expression optimization, purification, and biophysical characterization of a GluN2D-containing NMDA receptor

N-methyl-d-aspartate (NMDA) receptors are hetero-tetrameric ion channels typically consisting of two GluN1 and two GluN2 subunits. A GluN2D subunit containing NMDA receptor dysfunction has been implicated in several neurological diseases, including schizophrenia; however, the lack of a purified GluN2D containing NMDA receptor has been a hurdle for structural and biophysical studies. Here, we present expression and purification strategies to generate human GluN2D containing NMDA receptor, confirm its hetero-tetrameric form using fluorescence size exclusion chromatography (FSEC) and evaluated its suitability for structural studies. The purification methodology outlined here will help in the development of GluN2D specific channel modulators and enable structure activity relationship (SAR) studies.

Structural insights into binding of therapeutic channel blockers in NMDA receptors

Excitatory signaling mediated by N-methyl-D-aspartate receptor (NMDAR) is critical for brain development and function, as well as for neurological diseases and disorders. Channel blockers of NMDARs are of medical interest owing to their potential for treating depression, Alzheimer's disease, and epilepsy. However, precise mechanisms underlying binding and channel blockade have remained limited owing to challenges in obtaining high-resolution structures at the binding site within the transmembrane domains. Here, we monitor the binding of three clinically important channel blockers: phencyclidine, ketamine, and memantine in GluN1-2B NMDARs at local resolutions of 2.5-3.5 Å around the binding site using single-particle electron cryo-microscopy, molecular dynamics simulations, and electrophysiology. The channel blockers form different extents of interactions with the pore-lining residues, which control mostly off-speeds but not on-speeds. Our comparative analyses of the three unique NMDAR channel blockers provide a blueprint for developing therapeutic compounds with minimal side effects.

Nobiletin as a Neuroprotectant against NMDA Receptors: An In Silico Approach

Excitotoxicity is a type of neurodegenerative disorder. It caused by excessive glutamate receptor activation, which leads to neuronal malfunction and fatality. The N-methyl-D-aspartate (NMDA) receptors are found in glutamatergic neurons, and their excessive activation is primarily responsible for excitotoxicity. They are activated by both glutamate binding and postsynaptic depolarization, facilitating Ca²⁺ entry upon activation. Therefore, they are now widely acknowledged as being essential targets for excitotoxicity issues. Molecular docking and molecular dynamics (MD) simulation analyses have demonstrated that nobiletin efficiently targets the binding pocket of the NMDA receptor protein and exhibits stable dynamic behavior at the binding site. In this study, five potential neuroprotectants, nobiletin, silibinin, ononin, ginkgolide B, and epigallocatechin gallate (EGCG), were screened against the glutamate NMDA receptors in humans via computational methods. An in silico ADMET study was also performed, to predict the pharmacokinetics and toxicity profile for the expression of good drug-like behavior and a non-toxic nature. It was revealed that nobiletin fulfills the criteria for all of the drug-likeness rules (Veber, Lipinski, Ghose, Muegge, and Egan) and has neither PAINS nor structural alerts (Brenks). In conclusion, nobiletin demonstrated a possible promising neuroprotectant activities compared to other selected phytochemicals. Further, it can be evaluated in the laboratory for promising therapeutic approaches for in vitro and in vivo studies.

Randomised trial of intravenous thiamine and/or magnesium sulphate administration on erythrocyte transketolase activity, lactate concentrations and alcohol withdrawal scores

Alcohol withdrawal syndrome (AWS) occurs in 2% of patients admitted to U.K. hospitals. Routine treatment includes thiamine and benzodiazepines. Laboratory studies indicate that thiamine requires magnesium for optimal activity, however this has not translated into clinical practice. Patients experiencing AWS were randomized to three groups: (group 1) thiamine, (group 2) thiamine plus MgSO₄ or (group 3) MgSO₄. Pre- and 2-h post-treatment blood samples were taken. AWS severity was recorded using the Glasgow Modified Alcohol Withdrawal Score (GMAWS). The primary outcome measure was 15% change in erythrocyte transketolase activity (ETKA) in group 3. Secondary outcome measures were change in plasma lactate concentrations and time to GMAWS = 0. 127 patients were recruited, 115 patients were included in the intention-to-treat analysis. Pre-treatment, the majority of patients had normal or high erythrocyte thiamine diphosphate (TDP) concentrations (≥ 275–675/> 675 ng/gHb respectively) (99%), low serum magnesium concentrations (< 0.75 mmol/L) (59%), and high plasma lactate concentrations (> 2 mmol/L) (67%). Basal ETKA did not change significantly in groups 1, 2 or 3. Magnesium deficient patients (< 0.75 mmol/L) demonstrated less correlation between pre-treatment basal ETKA and TDP concentrations than normomagnesemic patients (R² = 0.053 and R² = 0.236). Median plasma lactate concentrations normalized (≤ 2.0 mmol/L) across all three groups (p < 0.001 for all groups), but not among magnesium deficient patients in group 1 (n = 22). The median time to achieve GMAWS = 0 for groups 1, 2 and 3 was 10, 5.5 and 6 h respectively (p < 0.001). No significant difference was found between groups for the primary endpoint of change in ETKA. Co-administration of thiamine and magnesium resulted in more consistent normalization of plasma lactate concentrations and reduced duration to achieve initial resolution of AWS symptoms.

ClinicalTrials.gov: NCT03466528.

Development and characterization of functional antibodies targeting NMDA receptors

N-methyl-D-aspartate receptors (NMDARs) are critically involved in basic brain functions and neurodegeneration as well as tumor invasiveness. Targeting specific subtypes of NMDARs with distinct activities has been considered an effective therapeutic strategy for neurological disorders and diseases. However, complete elimination of off-target effects of small chemical compounds has been challenging and thus, there is a need to explore alternative strategies for targeting NMDAR subtypes. Here we report identification of a functional antibody that specifically targets the GluN1-GluN2B NMDAR subtype and allosterically down-regulates ion channel activity as assessed by electrophysiology. Through biochemical analysis, x-ray crystallography, single-particle electron cryomicroscopy, and molecular dynamics simulations, we show that this inhibitory antibody recognizes the amino terminal domain of the GluN2B subunit and increases the population of the non-active conformational state. The current study demonstrates that antibodies may serve as specific reagents to regulate NMDAR functions for basic research and therapeutic objectives.

Selective targeting individual subtypes of N-methyl-D-aspartate receptors (NMDARs) is a desirable therapeutic strategy for neurological disorders. Here, the authors report identification of a functional antibody that specifically targets and allosterically down-regulates ion channel activity of the GluN1—GluN2B NMDAR subtype.

Physiopathological Relevance of D-Serine in the Mammalian Cochlea

NMDA receptors (NMDARs) populate the complex between inner hair cell (IHC) and spiral ganglion neurons (SGNs) in the developing and mature cochlea. However, in the mature cochlea, activation of NMDARs is thought to mainly occur under pathological conditions such as excitotoxicity. Ototoxic drugs such as aspirin enable cochlear arachidonic-acid-sensitive NMDAR responses, and induced chronic tinnitus was blocked by local application of NMDAR antagonists into the cochlear fluids. We largely ignore if other modulators are also engaged. In the brain, D-serine is the primary physiological co-agonist of synaptic NMDARs. Whether D-serine plays a role in the cochlea had remained unexplored. We now reveal the presence of D-serine and its metabolic enzymes prior to, and at hearing onset, in the sensory and non-neuronal cells of the cochlea of several vertebrate species. In vivo intracochlear perfusion of D-serine in guinea pigs reduces sound-evoked activity of auditory nerve fibers without affecting the receptor potentials, suggesting that D-serine acts specifically on the postsynaptic auditory neurons without altering the functional state of IHC or of the stria vascularis. Indeed, we demonstrate in vitro that agonist-induced activation of NMDARs produces robust calcium responses in rat SGN somata only in the presence of D-serine, but not of glycine. Surprisingly, genetic deletion in mice of serine racemase (SR), the enzyme that catalyzes D-serine, does not affect hearing function, but offers protection against noise-induced permanent hearing loss as measured 3 months after exposure. However, the mechanisms of activation of NMDA receptors in newborn rats may be different from those in adult guinea pigs. Taken together, these results demonstrate for the first time that the neuro-messenger D-serine has a pivotal role in the cochlea by promoting the activation of silent cochlear NMDAR in pathological situations. Thus, D-serine and its signaling pathway may represent a new druggable target for treating sensorineural hearing disorders (i.e., hearing loss, tinnitus).

Protein functional dynamics from the rigorous global analysis of DEER data: Conditions, components, and conformations

The potential of spin labeling to reveal the dynamic dimension of macromolecules has been recognized since the dawn of the methodology in the 1960s. However, it was the development of pulsed electron paramagnetic resonance spectroscopy to detect dipolar coupling between spin labels and the availability of turnkey instrumentation in the 21st century that realized the full promise of spin labeling. Double electron-electron resonance (DEER) spectroscopy has seen widespread applications to channels, transporters, and receptors. In these studies, distance distributions between pairs of spin labels obtained under different biochemical conditions report the conformational states of macromolecules, illuminating the key movements underlying biological function. These experimental studies have spurred the development of methods for the rigorous analysis of DEER spectroscopic data along with methods for integrating these distributions into structural models. In this tutorial, we describe a model-based approach to obtaining a minimum set of components of the distance distribution that correspond to functionally relevant protein conformations with a set of fractional amplitudes that define the equilibrium between these conformations. Importantly, we review and elaborate on the error analysis reflecting the uncertainty in the various parameters, a critical step in rigorous structural interpretation of the spectroscopic data.

A glutamate concentration-biased allosteric modulator potentiates NMDA-induced ion influx in neurons

Precisely controlled synaptic glutamate concentration is essential for the normal function of the N-methyl D-aspartate (NMDA) receptors. Atypical fluctuations in synaptic glutamate homeostasis lead to aberrant NMDA receptor activity that results in the pathogenesis of neurological and psychiatric disorders. Therefore, glutamate concentration-dependent NMDA receptor modulators would be clinically useful agents with fewer on-target adverse effects. In the present study, we have characterized a novel compound (CNS4) that potentiates NMDA receptor currents based on glutamate concentration. This compound alters glutamate potency and exhibits no voltage-dependent effect. Patch-clamp electrophysiology recordings confirmed agonist concentration-dependent changes in maximum inducible currents. Dynamic Ca2+ and Na+ imaging assays using rat brain cortical, striatal and cerebellar neurons revealed CNS4 potentiated ion influx through native NMDA receptor activity. Overall, CNS4 is novel in chemical structure, mechanism of action and agonist concentration-biased allosteric modulatory effect. This compound or its future analogs will serve as useful candidates to develop drug-like compounds for the treatment of treatment-resistant schizophrenia and major depression disorders associated with hypoglutamatergic neurotransmission.

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.

Allosteric coupling of sub-millisecond clamshell motions in ionotropic glutamate receptor ligand-binding domains

Ionotropic glutamate receptors (iGluRs) mediate signal transmission in the brain and are important drug targets. Structural studies show snapshots of iGluRs, which provide a mechanistic understanding of gating, yet the rapid motions driving the receptor machinery are largely elusive. Here we detect kinetics of conformational change of isolated clamshell-shaped ligand-binding domains (LBDs) from the three major iGluR sub-types, which initiate gating upon binding of agonists. We design fluorescence probes to measure domain motions through nanosecond fluorescence correlation spectroscopy. We observe a broad kinetic spectrum of LBD dynamics that underlie activation of iGluRs. Microsecond clamshell motions slow upon dimerization and freeze upon binding of full and partial agonists. We uncover allosteric coupling within NMDA LBD hetero-dimers, where binding of L-glutamate to the GluN2A LBD stalls clamshell motions of the glycine-binding GluN1 LBD. Our results reveal rapid LBD dynamics across iGluRs and suggest a mechanism of negative allosteric cooperativity in NMDA receptors.

Rajab et al. study the dynamics of closure of ligand binding domains (LBD) of the three major ionotropic glutamate receptor subtypes. They find pronounced sub-millisecond fluctuations in the apo state of LBDs from all three sub-types and reveal a pathway of allosteric communication in LBD dynamics across the dimerization interface

N-acetylaspartate release by glutaminolytic ovarian cancer cells sustains protumoral macrophages

Glutaminolysis is known to correlate with ovarian cancer aggressiveness and invasion. However, how this affects the tumor microenvironment is elusive. Here, we show that ovarian cancer cells become addicted to extracellular glutamine when silenced for glutamine synthetase (GS), similar to naturally occurring GS-low, glutaminolysis-high ovarian cancer cells. Glutamine addiction elicits a crosstalk mechanism whereby cancer cells release N-acetylaspartate (NAA) which, through the inhibition of the NMDA receptor, and synergistically with IL-10, enforces GS expression in macrophages. In turn, GS-high macrophages acquire M2-like, tumorigenic features. Supporting this in␣vitro model, in silico data and the analysis of ascitic fluid isolated from ovarian cancer patients prove that an M2-like macrophage phenotype, IL-10 release, and NAA levels positively correlate with disease stage. Our study uncovers the unprecedented role of glutamine metabolism in modulating macrophage polarization in highly invasive ovarian cancer and highlights the anti-inflammatory, protumoral function of NAA.