Scott J. Myers

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.

Transcriptional repression by REST: recruitment of Sin3A and histonedeacetylase to neuronal genes

Many genes whose expression is restricted to neurons in the brain contain a silencer element (RE1/NRSE) that limits transcription in nonneuronal cells by binding the transcription factor REST (also named NRSF or XBR). Although two independent domains of REST are known to confer repression, the mechanisms of transcriptional repression by REST remain obscure. We provide multiple lines of evidence that the N-terminal domain of REST represses transcription of the GluR2 and type II sodium-channel genes by recruiting the corepressor Sin3A and histone deacetylase (HDAC) to the promoter region in nonneuronal cells. These results identify a general mechanism for controlling the neuronal expression pattern of a specific set of genes via the RE1 silencer element.

Human GRIN2B variants in neurodevelopmental disorders.

The development of whole exome/genome sequencing technologies has given rise to an unprecedented volume of data linking patient genomic variability to brain disorder phenotypes. A surprising number of variants have been found in the N-methyl-D-aspartate receptor (NMDAR) gene family, with the GRIN2B gene encoding the GluN2B subunit being implicated in many cases of neurodevelopmental disorders, which are psychiatric conditions originating in childhood and include language, motor, and learning disorders, autism spectrum disorder (ASD), attention deficit hyperactivity disorder (ADHD), developmental delay, epilepsy, and schizophrenia. The GRIN2B gene plays a crucial role in normal neuronal development and is important for learning and memory. Mutations in human GRIN2B were distributed throughout the entire gene in a number of patients with various neuropsychiatric and developmental disorders. Studies that provide functional analysis of variants are still lacking, however current analysis of de novo variants that segregate with disease cases such as intellectual disability, developmental delay, ASD or epileptic encephalopathies reveal altered NMDAR function. Here, we summarize the current reports of disease-associated variants in GRIN2B from patients with multiple neurodevelopmental disorders, and discuss implications, highlighting the importance of functional analysis and precision medicine therapies.

Context-Dependent GluN2B-Selective Inhibitors of NMDA Receptor Function Are Neuroprotective with Minimal Side Effects

Stroke remains a significant problem despite decades of work on neuroprotective strategies. NMDA receptor (NMDAR) antagonists are neuroprotective in preclinical models, but have been clinically unsuccessful, in part due to side effects. Here we describe a prototypical GluN2B-selective antagonist with an IC50 value that is 10-fold more potent at acidic pH 6.9 associated with ischemic tissue compared to pH 7.6, a value close to the pH in healthy brain tissue. This should maximize neuroprotection in ischemic tissue while minimizing on-target side effects associated with NMDAR blockade in noninjured brain regions. We have determined the mechanism underlying pH-dependent inhibition and demonstrate the utility of this approach in vivo. We also identify dicarboxylate dimers as a novel proton sensor in proteins. These results provide insight into the molecular basis of pH-dependent neuroprotective NMDAR block, which could be beneficial in a wide range of neurological insults associated with tissue acidification.

Synthesis, structural activity-relationships, and biological evaluation of novel amide-based allosteric binding site antagonists in NR1A/NR2B N-methyl-D-aspartate receptors

The synthesis and structure-activity relationship analysis of a novel class of amide-based biaryl NR2B-selective NMDA receptor antagonists are presented. Some of the studied compounds are potent, selective, non-competitive, and voltage-independent antagonists of NR2B-containing NMDA receptors. Like the founding member of this class of antagonists (ifenprodil), several interesting compounds of the series bind to the amino terminal domain of the NR2B subunit to inhibit function. Analogue potency is modulated by linker length, flexibility, and hydrogen bonding opportunities. However, unlike previously described classes of NR2B-selective NMDA antagonists that exhibit off-target activity at a variety of monoamine receptors, the compounds described herein show much diminished effects against the hERG channel and alpha(1)-adrenergic receptors. Selections of the compounds discussed have acceptable half-lives in vivo and are predicted to permeate the blood-brain barrier. These data together suggest that masking charged atoms on the linker region of NR2B-selective antagonists can decrease undesirable side effects while still maintaining on-target potency.

Inhibition of Glutamate Receptor 2 Translation by a Polymorphic Repeat Sequence in the 5′-Untranslated Leaders

Previous studies have identified multiple transcription initiation sites for the glutamate receptor 2 (GluR2) gene, resulting in a heterogeneous population of GluR2 transcripts in vivo that differ in the length of their 5′-untranslated leaders (5′-UTR). We designed a series of monocistronic and dicistronic GluR2 cDNA constructs that model the natural in vivo transcripts and investigated their translation efficiencies in rabbit reticulocyte lysates, Xenopus oocytes, and primary cultured neurons. Transcripts containing long 5′ leaders (429 and 481 bases) were translated poorly compared with those with shorter leaders (341 or fewer bases). None of the five initiation codons in the 5′-UTR or the leader length per se were responsible for translation regulation. Rather, control of translation was mediated by a sequence containing a 34–42 nucleotide imperfect GU repeat predicted to form secondary structure in vivo. This translation suppression domain is included in some but not all rat and human GluR2 transcripts in vivo, depending on the site of transcription initiation. Rat cortex GluR2 transcripts that lack the translation suppression sequence were preferentially associated with polyribosomes. Furthermore, the GU-repeat cluster was found to be polymorphic in humans, raising the possibility that expansion or contraction of the GU-repeat cluster in certain populations might modify the level of GluR2 protein expression in neurons.

Enantiomeric Propanolamines as selective N-Methyl-D-aspartate 2B Receptor Antagonists

Enantiomeric propanolamines have been identified as a new class of NR2B-selective NMDA receptor antagonists. The most effective agents are biaryl structures, synthesized in six steps with overall yields ranging from 11-64%. The compounds are potent and selective inhibitors of NR2B-containing recombinant NMDA receptors with IC 50 values between 30-100 nM. Potency is strongly controlled by substitution on both rings and the centrally located amine nitrogen. SAR analysis suggests that well-balanced polarity and chain-length factors provide the greatest inhibitory potency. Structural comparisons based on 3D shape analysis and electrostatic complementarity support this conclusion. The antagonists are neuroprotective in both in vitro and in vivo models of ischemic cell death. In addition, some compounds exhibit anticonvulsant properties. Unlike earlier generation NMDA receptor antagonists and some NR2B-selective antagonists, the present series of propanolamines does not cause increased locomotion in rodents. Thus, the NR2B-selective antagonists exhibit a range of therapeutically interesting properties.

Activity of the rat GluR4 promoter in transfected cortical neurons and glia

AMPA (alpha‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionate) receptors are assembled from four subunits, GluR1‐4. Although GluR4 is widely expressed in brain its abundance is less than GluR1‐3. We have isolated ∼5 kb of the rat GluR4 promoter region and analyzed its capacity to drive expression of a luciferase reporter gene in transfected rat cortical neurons and glia, and C6 glioma cells. Multiple transcriptional start sites were identified in a GC‐rich region lacking TATA‐boxes between − 1090 and − 1011 bp from ATG. In transfected mixed cortical cultures, luciferase expression driven by GluR4 promoter segments were found predominantly in TuJ1‐positive neurons, indicating neuronal preference of GluR4. The GluR4 promoter fragments were 6–12‐fold more active in neurons than glia, compared with a 30‐fold neuronal selectivity of GluR2. Deletion of the GluR4 transcriptional initiation region decreased luciferase activity in neurons, but increased activity in C6 cells, suggesting that regulatory elements governing neuronal expression reside in this region. An intron within the 5′‐untranslated region and Sp1, IK2 and E‐box sites are conserved in the rat, mouse and human GluR4 promoters. The relative activity of GluR4 and GluR2 promoters in transfected cells correlates with their expression in brain, and in both promoters regulatory elements for neuronal expression reside near the initiation sites.

A Novel Negative Allosteric Modulator Selective for GluN2C/2D-Containing NMDA Receptors Inhibits Synaptic Transmission in Hippocampal Interneurons

N-Methyl-d-aspartate receptors (NMDARs) are ionotropic glutamate receptors that mediate excitatory synaptic transmission and have been implicated in numerous neurological disorders. NMDARs typically comprise two GluN1 and two GluN2 subunits. The four GluN2 subtypes (GluN2A-GluN2D) have distinct functional properties and gene expression patterns, which contribute to diverse functional roles for NMDARs in the brain. Here, we present a series of GluN2C/2D-selective negative allosteric modulators built around a N-aryl benzamide (NAB) core. The prototypical compound, NAB-14, is >800-fold selective for recombinant GluN2C/GluN2D over GluN2A/GluN2B in Xenopus oocytes and has an IC50 value of 580 nM at recombinant GluN2D-containing receptors expressed in mammalian cells. NAB-14 inhibits triheteromeric (GluN1/GluN2A/GluN2C) NMDARs with modestly reduced potency and efficacy compared to diheteromeric (GluN1/GluN2C/GluN2C) receptors. Site-directed mutagenesis suggests that structural determinants for NAB-14 inhibition reside in the GluN2D M1 transmembrane helix. NAB-14 inhibits GluN2D-mediated synaptic currents in rat subthalamic neurons and mouse hippocampal interneurons, but has no effect on synaptic transmission in hippocampal pyramidal neurons, which do not express GluN2C or GluN2D. This series possesses some druglike physical properties and modest brain permeability in rat and mouse. Altogether, this work identifies a new series of negative allosteric modulators that are valuable tools for studying GluN2C- and GluN2D-containing NMDAR function in brain circuits, and suggests that the series has the potential to be developed into therapies for selectively modulating brain circuits involving the GluN2C and GluN2D subunits.

A novel missense mutation in GRIN2A causes a nonepileptic neurodevelopmental disorder: A Novel Grin2A-Associated Phenotype

Background: Mutations in the GRIN2A gene, which encodes the GluN2A (glutamate [NMDA] receptor subunit epsilon‐1) subunit of the N‐methyl‐d‐aspartate receptor, have been identified in patients with epilepsy‐aphasia spectrum disorders, idiopathic focal epilepsies with centrotemporal spikes, and epileptic encephalopathies with severe developmental delay. However, thus far, mutations in this gene have not been associated with a nonepileptic neurodevelopmental disorder with dystonia.