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.

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

The GluN2C subunit exists predominantly, but not exclusively in NMDA receptors within the cerebellum. Antagonists such as UBP1700 and positive allosteric modulators including PYD-106 and 3-acylamino-2-aminopropionic acid derivatives such as UA3-10 ((R)-2-amino-3-{[5-(2-bromophenyl)thiophen-2-yl]carboxamido}propionic acid) represent promising tool compounds to investigate the role of GluN2C-containing NMDA receptors in the signal transduction in the brain. However, due to its high polarity the bioavailability and CNS penetration of the amino acid UA3-10 are expected to be rather low. Herein, three ester prodrugs 12a-c of the NMDA receptor glycine site agonist UA3-10 were prepared and pharmacokinetically characterized. The esters 12a-c showed higher lipophilicity (higher logD 7.4 values) than the acid UA3-10 but almost the same binding at human serum albumin. The acid UA3-10 was rather stable upon incubation with mouse liver microsomes and NADPH, but the esters 12a-c were fast hydrolyzed to afford the acid UA3-10. Incubation with pig liver esterase and mouse serum led to rapid hydrolysis of the esters 12a-c. The isopropyl ester 12c showed a promising logD 7.4 value of 3.57 and the highest stability in the presence of pig liver esterase and mouse serum. These results demonstrate that ester prodrugs of UA3-10 can potentially afford improved bioavailability and CNS penetration.

Structural basis of subunit selectivity for competitive NMDA receptor antagonists with preference for GluN2A over GluN2B subunits.

NMDA-type glutamate receptors are ligand-gated ion channels that contribute to excitatory neurotransmission in the central nervous system (CNS). Most NMDA receptors comprise two glycine-binding GluN1 and two glutamate-binding GluN2 subunits (GluN2A-D). We describe highly potent (S)-5-[(R)-2-amino-2-carboxyethyl]-4,5-dihydro-1H-pyrazole-3-carboxylic acid (ACEPC) competitive GluN2 antagonists, of which ST3 has a binding affinity of 52 nM at GluN1/2A and 782 nM at GluN1/2B receptors. This 15-fold preference of ST3 for GluN1/2A over GluN1/2B is improved compared with NVP-AAM077, a widely used GluN2A-selective antagonist, which we show has 11-fold preference for GluN1/2A over GluN1/2B. Crystal structures of the GluN1/2A agonist binding domain (ABD) heterodimer with bound ACEPC antagonists reveal a binding mode in which the ligands occupy a cavity that extends toward the subunit interface between GluN1 and GluN2A ABDs. Mutational analyses show that the GluN2A preference of ST3 is primarily mediated by four nonconserved residues that are not directly contacting the ligand, but positioned within 12 Å of the glutamate binding site. Two of these residues influence the cavity occupied by ST3 in a manner that results in favorable binding to GluN2A, but occludes binding to GluN2B. Thus, we reveal opportunities for the design of subunit-selective competitive NMDA receptor antagonists by identifying a cavity for ligand binding in which variations exist between GluN2A and GluN2B subunits. This structural insight suggests that subunit selectivity of glutamate-site antagonists can be mediated by mechanisms in addition to direct contributions of contact residues to binding affinity.

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.

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.

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

N-Methyl-d-aspartate (NMDA)-type glutamate receptors mediate excitatory synaptic transmission in the central nervous system and play critical roles in many neuronal processes. The physiologic roles of NMDA receptors are shaped by their functional properties, which are highly dependent on subunit composition. Most NMDA receptors are assembled from two GluN1 and two GluN2 subunits, but diversity in subunit composition is made possible by eight GluN1 splice variants (i.e., isoforms) and four distinct GluN2 subunits (GluN2A-D). We demonstrate using Förster resonance energy transfer and fluorescence lifetime imaging that GluN1-1a and GluN1-1b isoforms, which include or lack residues encoded by exon 5, form triheteromeric GluN1-1a/GluN1-1b/GluN2A (1a/1b/2A) and GluN1-1a/GluN1-1b/GluN2B (1a/1b/2B) receptors. We describe the selective expression of NMDA receptors containing two different GluN1 isoforms, and show that triheteromeric 1a/1b/2A and 1a/1b/2B receptors exhibit intermediate deactivation kinetics and pharmacological properties compared with the respective diheteromeric GluN1-1a/GluN1-1a/GluN2 and GluN1-1b/GluN1-1b/GluN2 receptors. These results highlight the intriguing possibility that neurons can finely tune NMDA receptor signaling by shifting the ratio of expressed GluN1-1a and GluN1-1b isoforms. Furthermore, we evaluate the contribution of channel pore residues to magnesium block and calcium permeability. These data point to the asymmetric contribution of pore residues in GluN1 and GluN2 to magnesium block, and reveal that a single copy of pore residues from GluN3 subunits strongly attenuates magnesium block and calcium permeability of NMDA receptors. Thus, the selective expression of NMDA receptors containing two distinct GluN1 isoforms provides new opportunities to study functional properties relevant to neuronal receptors.

Identification of AICP as a GluN2C-Selective N-Methyl-d-Aspartate Receptor Superagonist at the GluN1 Glycine Site.

N-methyl-d-aspartate (NMDA)-type ionotropic glutamate receptors mediate excitatory neurotransmission in the central nervous system and are critically involved in brain function. NMDA receptors are also implicated in psychiatric and neurological disorders and have received considerable attention as therapeutic targets. In this regard, administration of d-cycloserine (DCS), which is a glycine site NMDA receptor agonist, can enhance extinction of conditioned fear responses. The intriguing behavioral effects of DCS have been linked to its unique pharmacological profile among NMDA receptor subtypes (GluN1/2A-D), in which DCS is a superagonist at GluN2C-containing receptors compared with glycine and a partial agonist at GluN2B-containing receptors. Here, we identify (R)-2-amino-3-(4-(2-ethylphenyl)-1H-indole-2-carboxamido)propanoic acid (AICP) as a glycine site agonist with unique GluN2-dependent differences in agonist efficacy at recombinant NMDA receptor subtypes. AICP is a full agonist at GluN1/2A (100% response compared with glycine), a partial agonist at GluN1/2B and GluN1/2D (10% and 27%, respectively), and a highly efficacious superagonist at GluN1/2C receptors (353%). Furthermore, AICP potencies are enhanced compared with DCS with EC50 values in the low nanomolar range (1.7 nM at GluN1/2C). We show that GluN1/2C superagonism of AICP and DCS is mediated by overlapping but distinct mechanisms and that AICP selectively enhances responses from recombinant GluN1/2C receptors in the presence of physiological glycine concentrations. This functional selectivity of AICP for GluN2C-containing NMDA receptors is more pronounced compared with DCS, suggesting that AICP can be a useful tool compound for uncovering the roles of GluN2C subunits in neuronal circuit function and in the development of new therapeutic strategies.

Pharmacology and Structural Analysis of Ligand Binding to the Orthosteric Site of Glutamate-Like GluD2 Receptors.

The GluD2 receptor is a fundamental component of postsynaptic sites in Purkinje neurons, and is required for normal cerebellar function. GluD2 and the closely related GluD1 are classified as members of the ionotropic glutamate receptor (iGluR) superfamily on the basis of sequence similarity, but do not bind l-glutamate. The amino acid neurotransmitter D-Ser is a GluD2 receptor ligand, and endogenous D-Ser signaling through GluD2 has recently been shown to regulate endocytosis of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type iGluRs during synaptic plasticity in the cerebellum, such as long-term depression. Here, we investigate the pharmacology of the orthosteric binding site in GluD2 by examining the activity of analogs of D-Ser and GluN1 glycine site competitive antagonists at GluD2 receptors containing the lurcher mutation (GluD2(LC)), which promotes spontaneous channel activation. We identify several compounds that modulate GluD2(LC), including a halogenated alanine analog as well as the kynurenic acid analog 7-chloro-4-oxo-1H-quinoline-2-carboxylic acid (7-chlorokynurenic acid; 7-CKA). By correlating thermodynamic and structural data for 7-CKA binding to the isolated GluD2 ligand binding domain (GluD2-LBD), we find that binding 7-CKA to GluD2-LBD differs from D-Ser by inducing an intermediate cleft closure of the clamshell-shaped LBD. The GluD2 ligands identified here can potentially serve as a starting point for development of GluD2-selective ligands useful as tools in studies of the signaling role of the GluD2 receptor in the brain.

Structural determinants and mechanism of action of a GluN2C-selective NMDA receptor positive allosteric modulator.

NMDA receptors are tetrameric complexes of GluN1, GluN2A-D, and GluN3A-B subunits and are involved in normal brain function and neurologic disorders. We identified a novel class of stereoselective pyrrolidinone (PYD) positive allosteric modulators for GluN2C-containing NMDA receptors, exemplified by methyl 4-(3-acetyl-4-hydroxy-1-[2-(2-methyl-1H-indol-3-yl)ethyl]-5-oxo-2,5-dihydro-1H-pyrrol-2-yl)benzoate. Here we explore the site and mechanism of action of a prototypical analog, PYD-106, which at 30 µM does not alter responses of NMDA receptors containing GluN2A, GluN2B, and GluN2D and has no effect on AMPA [α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid] and kainate receptors. Coapplication of 50 µM PYD-106 with a maximally effective concentration of glutamate and glycine increases the response of GluN1/GluN2C NMDA receptors in HEK-293 cells to 221% of that obtained in the absence of PYD (taken as 100%). Evaluation of the concentration dependence of this enhancement revealed an EC50 value for PYD of 13 µM. PYD-106 increased opening frequency and open time of single channel currents activated by maximally effective concentrations of agonist but only had modest effects on glutamate and glycine EC50. PYD-106 selectively enhanced the responses of diheteromeric GluN1/GluN2C receptors but not triheteromeric GluN1/GluN2A/GluN2C receptors. Inclusion of residues encoded by GluN1-exon 5 attenuated the effects of PYD. Three GluN2C residues (Arg194, Ser470, Lys470), at which mutagenesis virtually eliminated PYD function, line a cavity at the interface of the ligand binding and the amino terminal domains in a homology model of GluN1/GluN2C built from crystallographic data on GluN1/GluN2B. We propose that this domain interface constitutes a new allosteric modulatory site on the NMDA receptor.

Crystal structure and pharmacological characterization of a novel N-methyl-D-aspartate (NMDA) receptor antagonist at the GluN1 glycine binding site.

NMDA receptors are ligand-gated ion channels that mediate excitatory neurotransmission in the brain. They are tetrameric complexes composed of glycine-binding GluN1 and GluN3 subunits together with glutamate-binding GluN2 subunits. Subunit-selective antagonists that discriminate between the glycine sites of GluN1 and GluN3 subunits would be valuable pharmacological tools for studies on the function and physiological roles of NMDA receptor subtypes. In a virtual screening for antagonists that exploit differences in the orthosteric binding site of GluN1 and GluN3 subunits, we identified a novel glycine site antagonist, 1-thioxo-1,2-dihydro-[1,2,4]triazolo[4,3-a]quinoxalin-4(5H)-one (TK40). Here, we show by Schild analysis that TK40 is a potent competitive antagonist with Kb values of 21-63 nM at the GluN1 glycine-binding site of the four recombinant GluN1/N2A-D receptors. In addition, TK40 displayed >100-fold selectivity for GluN1/N2 NMDA receptors over GluN3A- and GluN3B-containing NMDA receptors and no appreciable effects at AMPA receptors. Binding experiments on rat brain membranes and the purified GluN1 ligand-binding domain using glycine site GluN1 radioligands further confirmed the competitive interaction and high potency. To delineate the binding mechanism, we have solved the crystal structure of the GluN1 ligand-binding domain in complex with TK40 and show that TK40 binds to the orthosteric binding site of the GluN1 subunit with a binding mode that was also predicted by virtual screening. Furthermore, the structure reveals that the imino acetamido group of TK40 acts as an α-amino acid bioisostere, which could be of importance in bioisosteric replacement strategies for future ligand design.

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.

Subunit-selective allosteric inhibition of glycine binding to NMDA receptors.

NMDA receptors are ligand-gated ion channels that mediate excitatory neurotransmission in the brain and are involved in numerous neuropathological conditions. NMDA receptors are activated upon simultaneous binding of coagonists glycine and glutamate to the GluN1 and GluN2 subunits, respectively. Subunit-selective modulation of NMDA receptor function by ligand binding to modulatory sites distinct from the agonist binding sites could allow pharmacological intervention with therapeutically beneficial mechanisms. Here, we show the mechanism of action for 3-chloro-4-fluoro-N-[(4-[(2-(phenylcarbonyl)hydrazino)carbonyl]phenyl)methyl]-benzenesulfonamide (TCN-201), a new GluN1/GluN2A-selective NMDA receptor antagonist whose inhibition can be surmounted by glycine. Electrophysiological recordings from chimeric and mutant rat NMDA receptors suggest that TCN-201 binds to a novel allosteric site located at the dimer interface between the GluN1 and GluN2 agonist binding domains. Furthermore, we demonstrate that occupancy of this site by TCN-201 inhibits NMDA receptor function by reducing glycine potency. TCN-201 is therefore a negative allosteric modulator of glycine binding.

Structural determinants of agonist efficacy at the glutamate binding site of N-methyl-D-aspartate receptors.

N-methyl-d-aspartate (NMDA) receptors are ligand-gated ion channels assembled from GluN1 and GluN2 subunits. We used a series of N-hydroxypyrazole-5-glycine (NHP5G) partial agonists at the GluN2 glutamate binding site as tools to study activation of GluN1/GluN2A and GluN1/GluN2D NMDA receptor subtypes. Using two-electrode voltage-clamp electrophysiology, fast-application patch-clamp, and single-channel recordings, we show that propyl- and ethyl-substituted NHP5G agonists have a broad range of agonist efficacies relative to the full agonist glutamate (<1-72%). Crystal structures of the agonist binding domains (ABDs) of GluN2A and GluN2D do not reveal any differences in the overall domain conformation induced by binding of the full agonist glutamate or the partial agonist propyl-NHP5G, which is strikingly different from ABD structures of 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propanoate (AMPA) and kainate receptors bound to full and partial agonists. Subsequent evaluation of relative NHP5G agonist efficacy at GluN2A-GluN2D chimeric subunits implicates the amino-terminal domain (ATD) as a strong determinant of agonist efficacy, suggesting that interdomain interactions between the ABD and the ATD may be a central element in controlling the manner by which agonist binding leads to channel opening. We propose that variation in the overall receptor conformation, which is strongly influenced by the nature of interdomain interactions in resting and active states, mediates differences in agonist efficacy and partial agonism at the GluN2 subunits.

Glutamate receptor ion channels: structure, regulation, and function.

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

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.

Structural and mechanistic determinants of a novel site for noncompetitive inhibition of GluN2D-containing NMDA receptors.

NMDA receptors are ionotropic glutamate receptors that mediate excitatory synaptic transmission and have been implicated in several neurological diseases. We have evaluated the mechanism of action of a class of novel subunit-selective NMDA receptor antagonists, typified by (E)-4-(6-methoxy-2-(3-nitrostyryl)-4-oxoquinazolin-3(4H)-yl)-benzoic acid (QNZ46). We found that QNZ46 inhibits NMDA receptor function in a noncompetitive and voltage-independent manner by an unconventional mechanism that requires binding of glutamate to the GluN2 subunit, but not glycine binding to the GluN1 subunit. This dependency of antagonist association on glutamate binding to GluN2 renders these compounds nominally use-dependent, since inhibition will rely on synaptic release of glutamate. Evaluation of the structural determinants responsible for the subunit-selectivity of QNZ46 revealed that these compounds act at a new site that has not previously been described. Residues residing in the part of the agonist binding domain immediately adjacent to the transmembrane helices appear to control selectivity of QNZ46 for GluN2C- and GluN2D-containing receptors. These residues are well-positioned to sense glutamate binding to GluN2 and thus to mediate glutamate-dependent actions. This new class of noncompetitive antagonists could provide an opportunity for the development of pharmacological tools and therapeutic agents that target NMDA receptors at a new site and modulate function by a novel mechanism.

GluN1 splice variant control of GluN1/GluN2D NMDA receptors.

NMDA receptors are ionotropic glutamate receptors that mediate a slow, Ca2+-permeable component of excitatory synaptic transmission in the central nervous system. Recombinant GluN1-1a/GluN2D receptors are characterized by low channel open probability and prolonged deactivation time course following the removal of agonist. Here, we show that the deactivation time course, agonist potency, and single channel properties of GluN2D-containing NMDA receptors are modulated by alternative RNA splicing of GluN1. Our results demonstrate that GluN1 exon 5, which encodes a 21-amino-acid insert in the amino-terminal domain, is a key determinant of GluN1/GluN2D receptor function. GluN1-1b/GluN2D receptors, which contain the residues encoded by exon 5, deactivate with a dual exponential time course described by a τFAST of 410 ms and a τSLOW of 1100 ms. This time course is 3-fold more rapid than that for exon 5-lacking GluN1-1a/GluN2D, which deactivates with a τFAST of 1100 ms and a τSLOW of 3400 ms. Exon 5-containing NMDA receptors also have a two-fold higher open probability (0.037) than exon 5-lacking receptors (0.017). Furthermore, inclusion of exon 5-encoded residues within the GluN1-1b subunit decreases the potency for the endogenous agonist l-glutamate. Evaluation of receptor kinetics for NMDA receptors containing mutated GluN1-1b subunits and wild-type GluN2D identified residue Lys211 in GluN1-1b as a key determinant of exon 5 control of the deactivation time course and glutamate potency. Evaluation of a kinetic model of GluN1/GluN2D gating suggests that residues encoded by exon 5 influence several rate-limiting steps. These data demonstrate that the GluN1 subunit is a key determinant of the kinetic and pharmacological properties of GluN2D-containing NMDA receptors.

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.

Discovery of (R)-2-amino-3-triazolpropanoic acid derivatives as NMDA receptor glycine site agonists with GluN2 subunit-specific activity.

N-Methyl-d-aspartate (NMDA) receptors play critical roles in central nervous system function and are involved in variety of brain disorders. We previously developed a series of (R)-3-(5-furanyl)carboxamido-2-aminopropanoic acid glycine site agonists with pronounced variation in activity among NMDA receptor GluN1/2A-D subtypes. Here, a series of (R)-2-amino-3-triazolpropanoic acid analogues with a novel chemical scaffold is designed and their pharmacological properties are evaluated at NMDA receptor subtypes. We found that the triazole can function as a bioisostere for amide to produce glycine site agonists with variation in activity among NMDA receptor subtypes. Compounds 13g and 13i are full and partial agonists, respectively, at GluN1/2C and GluN1/2D with 3- to 7-fold preference in agonist potency for GluN1/2C-D over GluN1/2A-B subtypes. The agonist binding mode of these triazole analogues and the mechanisms by which the triazole ring can serve as a bioisostere for amide were further explored using molecular dynamics simulations. Thus, the novel (R)-2-amino-3-triazolpropanoic acid derivatives reveal insights to agonist binding at the GluN1 subunit of NMDA receptors and provide new opportunities for the design of glycine site agonists.

Derivatives of (R)-3-(5-Furanyl)carboxamido-2-aminopropanoic Acid as Potent NMDA Receptor Glycine Site Agonists with GluN2 Subunit-Specific Activity.

NMDA receptors mediate glutamatergic neurotransmission and are therapeutic targets due to their involvement in a variety of psychiatric and neurological disorders. Here, we describe the design and synthesis of a series of (R)-3-(5-furanyl)carboxamido-2-aminopropanoic acid analogues 8a-s as agonists at the glycine (Gly) binding site in the GluN1 subunit, but not GluN3 subunits, of NMDA receptors. These novel analogues display highly variable potencies and agonist efficacies among the NMDA receptor subtypes (GluN1/2A-D) in a manner dependent on the GluN2 subunit. Notably, compound 8p is identified as a potent partial agonist at GluN1/2C (EC50 = 0.074 µM) with an agonist efficacy of 28% relative to activation by Gly and virtually no agonist activity at GluN1/2A, GluN1/2B, and GluN1/2D. Thus, these novel agonists can modulate the activity of specific NMDA receptor subtypes by replacing the full endogenous agonists Gly or d-serine (d-Ser), thereby providing new opportunities in the development of novel therapeutic agents.