Properties of Triheteromeric N-Methyl-d-Aspartate Receptors Containing Two Distinct GluN1 Isoforms

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.

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.

Selective Cell-Surface Expression of Triheteromeric NMDA Receptors

NMDA-type ionotropic glutamate receptors are critically involved in many brain functions and are implicated in a variety of brain disorders. Seven NMDA receptor subunits exist (GluN1, GluN2A-D, and GluN3A-B) that assemble into tetrameric receptor subtypes with distinct functional properties and physiological roles. The majority NMDA receptors are composed of two GluN1 and two GluN2 subunits, which can assemble into four diheteromeric receptors subtypes composed of GluN1 and one type of GluN2 subunit (e.g., GluN1/2A), and presumably also six triheteromeric receptor subtypes composed of GluN1 and two different GluN2 subunits (e.g., GluN1/2A/2B). Furthermore, the GluN1 subunit exists as eight splice variants (e.g., GluN1-1a and GluN1-1b isoforms), and two different GluN1 isoforms can co-assemble to also form triheteromeric NMDA receptors (e.g., GluN1-1a/1b/2A). Here, we describe a method to faithfully express triheteromeric NMDA receptors in heterologous expression systems by controlling the identity of two of the four subunits. This method overcomes the problem that co-expression of three different NMDA receptor subunits generates two distinct diheteromeric receptor subtypes as well as one triheteromeric receptor subtype, thereby confounding studies that require a homogenous population of triheteromeric NMDA receptors. The method has been applied to selectively express recombinant triheteromeric GluN1/2A/2B, GluN1/2A/2C, GluN1/2B/2D, GluN1-1a/GluN1-1b/2A, GluN1-1a/GluN1-1b/2B receptors with negligible co-expression of the respective diheteromeric receptor subtypes. This method therefore enables quantitative evaluation of functional and pharmacological properties of triheteromeric NMDA receptors, some of which are abundant NMDA receptor subtypes in the adult brain.

Downstream Allosteric Modulation of NMDA Receptors by 3-Benzazepine Derivatives

N-Methyl-D-aspartate receptors (NMDARs) composed of different splice variants display distinct pH sensitivities and are crucial for learning and memory, as well as for inflammatory or injury processes. Dysregulation of the NMDAR has been linked to diseases like Parkinson's, Alzheimer's, schizophrenia, and drug addiction. The development of selective receptor modulators, therefore, constitutes a promising approach for numerous therapeutical applications. Here, we identified (R)-OF-NB1 as a promising splice variant selective NMDAR antagonist. We investigated the interaction of (R)-OF-NB1 and NMDAR from a biochemical, bioinformatical, and electrophysiological perspective to characterize the downstream allosteric modulation of NMDAR by 3-benzazepine derivatives. The allosteric modulatory pathway starts at the ifenprodil binding pocket in the amino terminal domain and immobilizes the connecting α5-helix to the ligand binding domain, resulting in inhibition. In contrast, the exon 5 splice variant GluN1-1b elevates the NMDARs flexibility and promotes the open state of its ligand binding domain.

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 t-N-methyl-d-aspartate receptor: Making the case for d-Serine to be considered its inverse co-agonist

The N-methyl-d-aspartate receptor (NMDAR) is an enigmatic macromolecule that has garnered a good deal of attention on account of its involvement in the cellular processes that underlie learning and memory, following its discovery in the mid twentieth century (Baudry and Davis, 1991). Yet, despite advances in knowledge about its function, there remains much more to be uncovered regarding the receptor's biophysical properties, subunit composition, and role in CNS physiology and pathophysiology. The motivation for this review stems from the need for synthesizing new information gathered about these receptors that sheds light on their role in synaptic plasticity and their dichotomous relationship with the amino acid d-serine through which they influence the pathogenesis of neurodegenerative diseases like temporal lobe epilepsy (TLE), the most common type of adult epilepsies (Beesley et al., 2020a). This review will outline pertinent ideas relating structure and function of t-NMDARs (GluN3 subunit-containing triheteromeric NMDARs) for which d-serine might serve as an inverse co-agonist. We will explore how tracing d-serine's origins blends glutamate-receptor biology with glial biology to help provide fresh perspectives on how neurodegeneration might interlink with neuroinflammation to initiate and perpetuate the disease state. Taken together, we envisage the review to deepen our understanding of endogenous d-serine's new role in the brain while also recognizing its therapeutic potential in the treatment of TLE that is oftentimes refractory to medications.

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.

NMDARs mediate peripheral and central sensitization contributing to chronic orofacial pain

Peripheral and central sensitizations of the trigeminal nervous system are the main mechanisms to promote the development and maintenance of chronic orofacial pain characterized by allodynia, hyperalgesia, and ectopic pain after trigeminal nerve injury or inflammation. Although the pathomechanisms of chronic orofacial pain are complex and not well known, sufficient clinical and preclinical evidence supports the contribution of the N-methyl-D-aspartate receptors (NMDARs, a subclass of ionotropic glutamate receptors) to the trigeminal nociceptive signal processing pathway under various pathological conditions. NMDARs not only have been implicated as a potential mediator of pain-related neuroplasticity in the peripheral nervous system (PNS) but also mediate excitatory synaptic transmission and synaptic plasticity in the central nervous system (CNS). In this review, we focus on the pivotal roles and mechanisms of NMDARs in the trigeminal nervous system under orofacial neuropathic and inflammatory pain. In particular, we summarize the types, components, and distribution of NMDARs in the trigeminal nervous system. Besides, we discuss the regulatory roles of neuron-nonneuronal cell/neuron-neuron communication mediated by NMDARs in the peripheral mechanisms of chronic orofacial pain following neuropathic injury and inflammation. Furthermore, we review the functional roles and mechanisms of NMDARs in the ascending and descending circuits under orofacial neuropathic and inflammatory pain conditions, which contribute to the central sensitization. These findings are not only relevant to understanding the underlying mechanisms, but also shed new light on the targeted therapy of chronic orofacial pain.

Synaptic Dysfunction by Mutations in GRIN2B: Influence of Triheteromeric NMDA Receptors on Gain-of-Function and Loss-of-Function Mutant Classification

GRIN2B mutations are rare but often associated with patients having severe neurodevelopmental disorders with varying range of symptoms such as intellectual disability, developmental delay and epilepsy. Patient symptoms likely arise from mutations disturbing the role that the encoded NMDA receptor subunit, GluN2B, plays at neuronal connections in the developing nervous system. In this study, we investigated the cell-autonomous effects of putative gain- (GoF) and loss-of-function (LoF) missense GRIN2B mutations on excitatory synapses onto CA1 pyramidal neurons in organotypic hippocampal slices. In the absence of both native GluN2A and GluN2B subunits, functional incorporation into synaptic NMDA receptors was attenuated for GoF mutants, or almost eliminated for LoF GluN2B mutants. NMDA-receptor-mediated excitatory postsynaptic currents (NMDA-EPSCs) from synaptic GoF GluN1/2B receptors had prolonged decays consistent with their functional classification. Nonetheless, in the presence of native GluN2A, molecular replacement of native GluN2B with GoF and LoF GluN2B mutants all led to similar functional incorporation into synaptic receptors, more rapidly decaying NMDA-EPSCs and greater inhibition by TCN-201, a selective antagonist for GluN2A-containing NMDA receptors. Mechanistic insight was gained from experiments in HEK293T cells, which revealed that GluN2B GoF mutants slowed deactivation in diheteromeric GluN1/2B, but not triheteromeric GluN1/2A/2B receptors. We also show that a disease-associated missense mutation, which severely affects surface expression, causes opposing effects on NMDA-EPSC decay and charge transfer when introduced into GluN2A or GluN2B. Finally, we show that having a single null Grin2b allele has only a modest effect on NMDA-EPSC decay kinetics. Our results demonstrate that functional incorporation of GoF and LoF GluN2B mutants into synaptic receptors and the effects on EPSC decay times are highly dependent on the presence of triheteromeric GluN1/2A/2B NMDA receptors, thereby influencing the functional classification of NMDA receptor variants as GoF or LoF mutations. These findings highlight the complexity of interpreting effects of disease-causing NMDA receptor missense mutations in the context of neuronal function.

The Use of Low-Dose Methadone as Add-On to Ongoing Opioid Treatment in Palliative Cancer Care-An Underrated Treatment?

The aim of this review is to summarize the current knowledge of low-dose methadone treatment in palliative cancer care. In Sweden, methadone is quite common in specialized palliative care, where almost a tenth of patients are prescribed this drug. Negative attitudes towards methadone do not seem to prevent it from being used for pain management, and by starting with low doses and then increasing slowly and gradually, methadone can apparently be introduced safely. It is still uncertain whether methadone has a better analgesic effect than other opioids. However, for pain relief in cancer patients with severe and complex cancer-related pain, NMDA receptor inhibition with methadone may, in selected cases, be an attractive alternative, especially in the form of low-dose supplements to other ongoing opioids. Due to long half-life and complex metabolism, the use of methadone requires an experienced physician and solid follow-up. Continuous administration of opioids, including low-dose methadone, has been proven effective and safe in reducing pain in dying patients without increasing the risk of confusion, regardless of age.

Common synaptic phenotypes arising from diverse mutations in the human NMDA receptor subunit GluN2A

Dominant mutations in the human gene GRIN2A, encoding NMDA receptor (NMDAR) subunit GluN2A, make a significant and growing contribution to the catalogue of published single-gene epilepsies. Understanding the disease mechanism in these epilepsy patients is complicated by the surprising diversity of effects that the mutations have on NMDARs. Here we have examined the cell-autonomous effect of five GluN2A mutations, 3 loss-of-function and 2 gain-of-function, on evoked NMDAR-mediated synaptic currents (NMDA-EPSCs) in CA1 pyramidal neurons in cultured hippocampal slices. Despite the mutants differing in their functional incorporation at synapses, prolonged NMDA-EPSC current decays (with only marginal changes in charge transfer) were a common effect for both gain- and loss-of-function mutants. Modelling NMDA-EPSCs with mutant properties in a CA1 neuron revealed that the effect of GRIN2A mutations can lead to abnormal temporal integration and spine calcium dynamics during trains of concerted synaptic activity. Investigations beyond establishing the molecular defects of GluN2A mutants are much needed to understand their impact on synaptic transmission.

The cell-autonomous effect of five severe loss- or gain-of-function GluN2A (NMDA receptor) mutations is assessed on evoked NMDAR-mediated synaptic currents in CA1 pyramidal neurons in cultured mouse hippocampal slices. Data and modelling suggest that mutant-like NMDA-EPSCs can lead to abnormal temporal summation and spine calcium dynamics.

Allosteric antagonist action at triheteromeric NMDA receptors

NMDA receptors are one subtype of glutamate receptor that play fundamental roles in synaptic physiology and synaptic plasticity in the nervous system, in addition to being implicated in several neurological disorders. It is now established that many NMDA receptors in the nervous system are triheteromeric, composed of two glycine-binding GluN1 subunits and two different glutamate binding GluN2 subunits. The pharmacology of NMDA receptor has become well established since the pioneering work of Watkins and Evans almost half a century ago and has seen a resurgence of interest in the past decade as new subtype-selective allosteric modulators have been discovered. In this article, features specific to allosteric antagonist action at triheteromeric NMDA receptors are reviewed with a focus on understanding the mechanism of action of drugs acting at triheteromeric GluN1/GluN2B/GluN2D receptors. These receptors are of importance in the basal ganglia and in interneurons of the hippocampus and implications for understanding the action of allosteric antagonists at synaptic triheteromeric receptors are considered.

The σ1 Receptor and the HINT1 Protein Control α2δ1 Binding to Glutamate NMDA Receptors: Implications in Neuropathic Pain

Nerve injury produces neuropathic pain through the binding of α2δ1 proteins to glutamate N-methyl-D-aspartate receptors (NMDARs). Notably, mice with a targeted deletion of the sigma 1 receptor (σ1R) gene do not develop neuropathy, whereas mice lacking the histidine triad nucleotide-binding protein 1 (Hint1) gene exhibit exacerbated allodynia. σ1R antagonists more effectively diminish neuropathic pain of spinal origin when administered by intracerebroventricular injection than systemically. Thus, in mice subjected to unilateral sciatic nerve chronic constriction injury (CCI), we studied the participation of σ1Rs and HINT1 proteins in the formation of α2δ1-NMDAR complexes within the supraspinal periaqueductal gray (PAG). We found that δ1 peptides required σ1Rs in order to interact with the NMDAR NR1 variant that contains the cytosolic C1 segment. σ1R antagonists or low calcium levels provoke the dissociation of σ1R-NR1 C1 dimers, while they barely affect the integrity of δ1-σ1R-NR1 C1 trimers. However, HINT1 does remove δ1 peptides from the trimer, thereby facilitating the subsequent dissociation of σ1Rs from NMDARs. In σ1R-/- mice, CCI does not promote the formation of NMDAR-α2δ1 complexes and allodynia does not develop. The levels of α2δ1-σ1R-NMDAR complexes increase in HINT1-/- mice and after inducing CCI, degradation of α2δ1 proteins is observed. Notably, σ1R antagonists but not gabapentinoids alleviate neuropathic pain in these mice. During severe neuropathy, the metabolism of α2δ1 proteins may account for the failure of many patients to respond to gabapentinoids. Therefore, σ1Rs promote and HINT1 proteins hinder the formation α2δ1-NMDAR complexes in the PAG, and hence, the appearance of mechanical allodynia depends on the interplay between these proteins.

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.

The Negative Allosteric Modulator EU1794-4 Reduces Single-Channel Conductance and Ca2+ Permeability of GluN1/GluN2A N-Methyl-d-Aspartate Receptors

NMDA receptors are ligand-gated ion channels that mediate a slow, Ca²⁺-permeable component of excitatory synaptic currents. These receptors are involved in several important brain functions, including learning and memory, and have also been implicated in neuropathological conditions and acute central nervous system injury, which has driven therapeutic interest in their modulation. The EU1794 series of positive and negative allosteric modulators of NMDA receptors has structural determinants of action near the preM1 helix that is involved in channel gating. Here, we describe the effects of the negative allosteric modulator EU1794-4 on GluN1/GluN2A channels studied in excised outside-out patches. Coapplication of EU1794-4 with a maximally effective concentration of glutamate and glycine increases the fraction of time the channel is open by nearly 1.5-fold, yet reduces single-channel conductance by increasing access of the channel to several subconductance levels, which has the net overall effect of reducing the macroscopic current. The lack of voltage-dependence of negative modulation suggests this is unrelated to a channel block mechanism. As seen with other NMDA receptor modulators that reduce channel conductance, EU1794-4 also reduces the Ca²⁺ permeability relative to monovalent cations of GluN1/GluN2A receptors. We conclude that EU1794-4 is a prototype for a new class of NMDA receptor negative allosteric modulators that reduce both the overall current that flows after receptor activation and the flux of Ca²⁺ ion relative to monovalent cations. SIGNIFICANCE STATEMENT: NMDA receptors are implicated in many neurological conditions but are challenging to target given their ubiquitous expression. Several newly identified properties of the negative allosteric modulator EU1794-4, including reducing Ca²⁺ flux through NMDA receptors and attenuating channel conductance, explain why this modulator reduces but does not eliminate NMDA receptor function. A modulator with these properties could have therapeutic advantages for indications in which attenuation of NMDA receptor function is beneficial, such as neurodegenerative disease and acute injury.

N-Methyl-D-Aspartate Receptors in Hematopoietic Cells: What Have We Learned?

The N-methyl-D-aspartate receptor (NMDAR) provides a pathway for glutamate-mediated inter-cellular communication, best known for its role in the brain but with multiple examples of functionality in non-neuronal cells. Data previously published by others and us provided ex vivo evidence that NMDARs regulate platelet and red blood cell (RBC) production. Here, we summarize what is known about these hematopoietic roles of the NMDAR. Types of NMDAR subunits expressed in megakaryocytes (platelet precursors) and erythroid cells are more commonly found in the developing rather than adult brain, suggesting trophic functions. Nevertheless, similar to their neuronal counterparts, hematopoietic NMDARs function as ion channels, and are permeable to calcium ions (Ca2+). Inhibitors that block open NMDAR (memantine and MK-801) interfere with megakaryocytic maturation and proplatelet formation in primary culture. The effect on proplatelet formation appears to involve Ca2+ influx-dependent regulation of the cytoskeletal remodeling. In contrast to normal megakaryocytes, NMDAR effects in leukemic Meg-01 cells are diverted away from differentiation to increase proliferation. NMDAR hypofunction triggers differentiation of Meg-01 cells with the bias toward erythropoiesis. The underlying mechanism involves changes in the intracellular Ca2+ homeostasis, cell stress pathways, and hematopoietic transcription factors that upon NMDAR inhibition shift from the predominance of megakaryocytic toward erythroid regulators. This ability of NMDAR to balance both megakaryocytic and erythroid cell fates suggests receptor involvement at the level of a bipotential megakaryocyte-erythroid progenitor. In human erythroid precursors and circulating RBCs, NMDAR regulates intracellular Ca2+ homeostasis. NMDAR activity supports survival of early proerythroblasts, and in mature RBCs NMDARs impact cellular hydration state, hemoglobin oxygen affinity, and nitric oxide synthase activity. Overexcitation of NMDAR in mature RBCs leads to Ca2+ overload, K+ loss, RBC dehydration, and oxidative stress, which may contribute to the pathogenesis of sickle cell disease. In summary, there is growing evidence that glutamate-NMDAR signaling regulates megakaryocytic and erythroid cells at different stages of maturation, with some intriguing differences emerging in NMDAR expression and function between normal and diseased cells. NMDAR signaling may provide new therapeutic opportunities in hematological disease, but in vivo applicability needs to be confirmed.

Negative allosteric modulation of GluN1/GluN3 NMDA receptors

NMDA receptors are ligand-gated ion channels that mediate excitatory neurotransmission. Most native NMDA receptors are tetrameric assemblies of two glycine-binding GluN1 and two glutamate-binding GluN2 subunits. Co-assembly of the glycine-binding GluN1 with glycine-binding GluN3 subunits (GluN3A-B) creates glycine activated receptors that possess strikingly different functional and pharmacological properties compared to GluN1/GluN2 NMDA receptors. The role of GluN1/GluN3 receptors in neuronal function remains unknown, in part due to lack of pharmacological tools with which to explore their physiological roles. We have identified the negative allosteric modulator EU1180-438, which is selective for GluN1/GluN3 receptors over GluN1/GluN2 NMDA receptors, AMPA, and kainate receptors. EU1180-438 is also inactive at GABA, glycine, and P2X receptors, but displays inhibition of some nicotinic acetylcholine receptors. Furthermore, we demonstrate that EU1180-438 produces robust inhibition of glycine-activated current responses mediated by native GluN1/GluN3A receptors in hippocampal CA1 pyramidal neurons. EU1180-438 is a non-competitive antagonist with activity that is independent of membrane potential (i.e. voltage-independent), glycine concentration, and extracellular pH. Non-stationary fluctuation analysis of neuronal current responses provided an estimated weighted mean unitary conductance of 6.1 pS for GluN1/GluN3A channels, and showed that EU1180-438 has no effect on conductance. Site-directed mutagenesis suggests that structural determinants of EU1180-438 activity reside near a short pre-M1 helix that lies parallel to the plane of the membrane below the agonist binding domain. These findings demonstrate that structural differences between GluN3 and other glutamate receptor subunits can be exploited to generate subunit-selective ligands with utility in exploring the roles GluN3 in neuronal function.

PTC-174, a positive allosteric modulator of NMDA receptors containing GluN2C or GluN2D subunits

NMDA receptors are ionotropic glutamate receptors that mediate excitatory neurotransmission. The diverse functions of these receptors are tuned by deploying different combinations of GluN1 and GluN2 subunits (GluN2A-D) to form either diheteromeric NMDA receptors, which contain two GluN1 and two identical GluN2 subunits, or triheteromeric NMDA receptors, which contain two GluN1 and two distinct GluN2 subunits. Here, we characterize PTC-174, a novel positive allosteric modulator (PAM) of receptors containing GluN2C or GluN2D subunits. PTC-174 potentiates maximal current amplitudes by 1.8-fold for diheteromeric GluN1/2B receptors and by > 10-fold for GluN1/2C and GluN1/2D receptors. PTC-174 also potentiates responses from triheteromeric GluN1/2B/2D and GluN1/2A/2C receptors by 4.5-fold and 1.7-fold, respectively. By contrast, PTC-174 produces partial inhibition of responses from diheteromeric GluN1/2A and triheteromeric GluN1/2A/2B receptors. PTC-174 increases potencies of co-agonists glutamate and glycine by 2- to 5-fold at GluN1/2C and GluN1/2D receptors, and NMDA receptor activation facilitates allosteric modulation by PTC-174. At native NMDA receptors in GluN2D-expressing subthalamic nucleus neurons, PTC-174 increases the amplitude of responses to NMDA application and slows the decay of excitatory postsynaptic currents (EPSCs) evoked by internal capsule stimulation. Furthermore, PTC-174 increases the amplitude and slows the decay of EPSCs in hippocampal interneurons, but has not effect on the amplitudes of NMDA receptor-mediated EPSCs in hippocampal CA1 pyramidal neurons. Thus, PTC-174 provides a useful new pharmacological tool to investigate the molecular pharmacology and physiology of GluN2C- and GluN2D-containing NMDA receptors.

Functional and pharmacological properties of triheteromeric GluN1/2B/2D NMDA receptors

KEY POINTS

Triheteromeric NMDA receptors contain two GluN1 and two distinct GluN2 subunits and mediate excitatory neurotransmission in the CNS. Triheteromeric GluN1/2B/2D receptors have functional properties intermediate to those of diheteromeric GluN1/2B and GluN1/2D receptors. GluN1/2B/2D receptors are more sensitive to channel blockade by ketamine and memantine compared to GluN1/2B receptors in the presence of physiological Mg²⁺ . GluN2B-selective antagonists produce robust inhibition of GluN1/2B/2D receptors, and the GluN2B-selective positive allosteric modulator spermine enhances responses from GluN1/2B/2D but not GluN1/2A/2B receptors. These insights into the properties of triheteromeric GluN1/2B/2D receptors are necessary to appreciate their physiological roles in neural circuit function and the actions of therapeutic agents targeting NMDA receptors.

ABSTRACT

Triheteromeric NMDA-type glutamate receptors that contain two GluN1 and two different GluN2 subunits contribute to excitatory neurotransmission in the adult CNS. In the present study, we report properties of the triheteromeric GluN1/2B/2D NMDA receptor subtype that is expressed in distinct neuronal populations throughout the CNS. We show that neither GluN2B, nor GluN2D dominate the functional properties of GluN1/2B/2D receptors because agonist potencies, open probability and the glutamate deactivation time course of GluN1/2B/2D receptors are intermediate to those of diheteromeric GluN1/2B and GluN1/2D receptors. Furthermore, channel blockade of GluN1/2B/2D by extracellular Mg²⁺ is intermediate compared to GluN1/2B and GluN1/2D, although GluN1/2B/2D is more sensitive to blockade by ketamine and memantine compared to GluN1/2B in the presence of physiological Mg²⁺ . Subunit-selective allosteric modulators have distinct activity at GluN1/2B/2D receptors, including GluN2B-selective antagonists, ifenprodil, EVT-101 and CP-101-606, which inhibit with similar potencies but with different efficacies at GluN1/2B/2D (∼65% inhibition) compared to GluN1/2B (∼95% inhibition). Furthermore, the GluN2B-selective positive allosteric modulator spermine enhances responses from GluN1/2B/2D but not GluN1/2A/2B receptors. We show that these key features of allosteric modulation of recombinant GluN1/2B/2D receptors are also observed for NMDA receptors in hippocampal interneurons but not CA1 pyramidal cells, which is consistent with the expression of GluN1/2B/2D receptors in interneurons and GluN1/2A/2B receptors in pyramidal cells. Altogether, we uncover previously unknown functional and pharmacological properties of triheteromeric GluN1/2B/2D receptors that can facilitate advances in our understanding of their physiological roles in neural circuit function and therapeutic drug actions.

De novo GRIN variants in NMDA receptor M2 channel pore-forming loop are associated with neurological diseases

N-methyl-D-aspartate receptors (NMDARs) mediate slow excitatory postsynaptic transmission in the central nervous system, thereby exerting a critical role in neuronal development and brain function. Rare genetic variants in the GRIN genes encoding NMDAR subunits segregated with neurological disorders. Here, we summarize the clinical presentations for 18 patients harboring 12 de novo missense variants in GRIN1, GRIN2A, and GRIN2B that alter residues in the M2 re-entrant loop, a region that lines the pore and is intolerant to missense variation. These de novo variants were identified in children with a set of neurological and neuropsychiatric conditions. Evaluation of the receptor cell surface expression, pharmacological properties, and biophysical characteristics show that these variants can have modest changes in agonist potency, proton inhibition, and surface expression. However, voltage-dependent magnesium inhibition is significantly reduced in all variants. The NMDARs hosting a single copy of a mutant subunit showed a dominant reduction in magnesium inhibition for some variants. These variant NMDARs also show reduced calcium permeability and single-channel conductance, as well as altered open probability. The data suggest that M2 missense variants increase NMDAR charge transfer in addition to varied and complex influences on NMDAR functional properties, which may underlie the patients' phenotypes.

Structural biology of glutamate receptor ion channels: towards an understanding of mechanism

Ionotropic glutamate receptors (iGluRs) are tetrameric ion channels that mediate signal transmission at neuronal synapses, where they contribute centrally to the postsynaptic plasticity that underlies learning and memory. Receptor activation by l-glutamate triggers complex allosteric cascades that are transmitted through the layered and highly flexible receptor assembly culminating in opening a cation-selective pore. This process is shaped by the arrangement of the four core subunits as well as the presence of various auxiliary subunits, and is subject to regulation by an array of small molecule modulators targeting a number of sites throughout the complex. Here, we discuss recent structures of iGluR homomers and heteromers illuminating the organization and subunit arrangement of the core tetramer, co-assembled with auxiliary subunits and in complex with allosteric modulators.

Functional assessment of triheteromeric NMDA receptors containing a human variant associated with epilepsy

KEY POINTS

NMDA receptors are neurotransmitter-gated ion channels that are critically involved in brain cell communication Variations in genes encoding NMDA receptor subunits have been found in a range of neurodevelopmental disorders. We investigated a de novo genetic variant found in patients with epileptic encephalopathy that changes a residue located in the ion channel pore of the GluN2A NMDA receptor subunit. We found that this variant (GluN2AN615K ) impairs physiologically important receptor properties: it markedly reduces Mg2+ blockade and channel conductance, even for receptors in which one GluN2AN615K is co-assembled with one wild-type GluN2A subunit. Our findings are consistent with the GluN2AN615K mutation being the primary cause of the severe neurodevelopmental disorder in carriers.

ABSTRACT

NMDA receptors are ionotropic calcium-permeable glutamate receptors with a voltage-dependence mediated by blockade by Mg2+ . Their activation is important in signal transduction, as well as synapse formation and maintenance. Two unrelated individuals with epileptic encephalopathy carry a de novo variant in the gene encoding the GluN2A NMDA receptor subunit: a N615K missense variant in the M2 pore helix (GRIN2AC1845A ). We hypothesized that this variant underlies the neurodevelopmental disorders in carriers and explored its functional consequences by electrophysiological analysis in heterologous systems. We focused on GluN2AN615K co-expressed with wild-type GluN2 subunits in physiologically relevant triheteromeric NMDA receptors containing two GluN1 and two distinct GluN2 subunits, whereas previous studies have investigated the impact of the variant in diheteromeric NMDA receptors with two GluN1 and two identical GluN2 subunits. We found that GluN2AN615K -containing triheteromers showed markedly reduced Mg2+ blockade, with a value intermediate between GluN2AN615K diheteromers and wild-type NMDA receptors. Single-channel conductance was reduced by four-fold in GluN2AN615K diheteromers, again with an intermediate value in GluN2AN615K -containing triheteromers. Glutamate deactivation rates were unaffected. Furthermore, we expressed GluN2AN615K in cultured primary mouse cortical neurons, observing a decrease in Mg2+ blockade and reduction in current density, confirming that the variant continues to have significant functional impact in neuronal systems. Our results demonstrate that the GluN2AN615K variant has substantial effects on NMDA receptor properties fundamental to the roles of the receptor in synaptic plasticity, even when expressed alongside wild-type subunits. This work strengthens the evidence indicating that the GluN2AN615K variant underlies the disabling neurodevelopmental phenotype in carriers.

Structure, function, and allosteric modulation of NMDA receptors

Hansen et al. review recent structural data that have provided insight into the function and allosteric modulation of NMDA receptors.

NMDA-type glutamate receptors are ligand-gated ion channels that mediate a Ca²⁺-permeable component of excitatory neurotransmission in the central nervous system (CNS). They are expressed throughout the CNS and play key physiological roles in synaptic function, such as synaptic plasticity, learning, and memory. NMDA receptors are also implicated in the pathophysiology of several CNS disorders and more recently have been identified as a locus for disease-associated genomic variation. NMDA receptors exist as a diverse array of subtypes formed by variation in assembly of seven subunits (GluN1, GluN2A-D, and GluN3A-B) into tetrameric receptor complexes. These NMDA receptor subtypes show unique structural features that account for their distinct functional and pharmacological properties allowing precise tuning of their physiological roles. Here, we review the relationship between NMDA receptor structure and function with an emphasis on emerging atomic resolution structures, which begin to explain unique features of this receptor.