Mutations in the transmembrane domain M3 generate spontaneously open orphan glutamate δ1 receptor

GRID1/GluD1 homozygous variants linked to intellectual disability and spastic paraplegia impair mGlu1/5 receptor signaling and excitatory synapses

The ionotropic glutamate delta receptor GluD1, encoded by the GRID1 gene, is involved in synapse formation, function, and plasticity. GluD1 does not bind glutamate, but instead cerebellin and D-serine, which allow the formation of trans-synaptic bridges, and trigger transmembrane signaling. Despite wide expression in the nervous system, pathogenic GRID1 variants have not been characterized in humans so far. We report homozygous missense GRID1 variants in five individuals from two unrelated consanguineous families presenting with intellectual disability and spastic paraplegia, without (p.Thr752Met) or with (p.Arg161His) diagnosis of glaucoma, a threefold phenotypic association whose genetic bases had not been elucidated previously. Molecular modeling and electrophysiological recordings indicated that Arg161His and Thr752Met mutations alter the hinge between GluD1 cerebellin and D-serine binding domains and the function of this latter domain, respectively. Expression, trafficking, physical interaction with metabotropic glutamate receptor mGlu1, and cerebellin binding of GluD1 mutants were not conspicuously altered. Conversely, upon expression in neurons of dissociated or organotypic slice cultures, we found that both GluD1 mutants hampered metabotropic glutamate receptor mGlu1/5 signaling via Ca2+ and the ERK pathway and impaired dendrite morphology and excitatory synapse density. These results show that the clinical phenotypes are distinct entities segregating in the families as an autosomal recessive trait, and caused by pathophysiological effects of GluD1 mutants involving metabotropic glutamate receptor signaling and neuronal connectivity. Our findings unravel the importance of GluD1 receptor signaling in sensory, cognitive and motor functions of the human nervous system.

Loss of activation by GABA in vertebrate delta ionotropic glutamate receptors

Ionotropic glutamate receptors (iGluRs) mediate excitatory signals between cells by binding neurotransmitters and conducting cations across the cell membrane. In the mammalian brain, most of these signals are mediated by two types of iGluRs: AMPA and NMDA (i.e. iGluRs sensitive to 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid and N-methyl-D-aspartic acid, respectively). Delta-type iGluRs of mammals also form neurotransmitter-binding channels in the cell membrane, but in contrast, their channel is not activated by neurotransmitter binding, raising biophysical questions about iGluR activation and biological questions about the role of delta iGluRs. We therefore investigated the divergence of delta iGluRs from their iGluR cousins using molecular phylogenetics, electrophysiology, and site-directed mutagenesis. We find that delta iGluRs are found in numerous bilaterian animals (e.g., worms, starfish, and vertebrates) and are closely related to AMPA receptors, both genetically and functionally. Surprisingly, we observe that many iGluRs of the delta family are activated by the classical inhibitory neurotransmitter, γ-aminobutyric acid (GABA). Finally, we identify nine amino acid substitutions that likely gave rise to the inactivity of today's mammalian delta iGluRs, and these mutations abolish activity when engineered into active invertebrate delta iGluRs, partly by inducing receptor desensitization. These results offer biophysical insight into iGluR activity and point to a role for GABA in excitatory signaling in invertebrates.

Clinical features, functional consequences, and rescue pharmacology of missense GRID1 and GRID2 human variants

GRID1 and GRID2 encode the enigmatic GluD1 and GluD2 proteins, which form tetrameric receptors that play important roles in synapse organization and development of the central nervous system. Variation in these genes has been implicated in neurodevelopmental phenotypes. We evaluated GRID1 and GRID2 human variants from the literature, ClinVar, and clinical laboratories and found that many of these variants reside in intolerant domains, including the amino terminal domain of both GRID1 and GRID2. Other conserved regions, such as the M3 transmembrane domain, show different intolerance between GRID1 and GRID2. We introduced these variants into GluD1 and GluD2 cDNA and performed electrophysiological and biochemical assays to investigate the mechanisms of dysfunction of GRID1/2 variants. One variant in the GRID1 distal amino terminal domain resides at a position predicted to interact with Cbln2/Cbln4, and the variant disrupts complex formation between GluD1 and Cbln2, which could perturb its role in synapse organization. We also discovered that, like the lurcher mutation (GluD2-A654T), other rare variants in the GRID2 M3 domain create constitutively active receptors that share similar pathogenic phenotypes. We also found that the SCHEMA schizophrenia M3 variant GluD1-A650T produced constitutively active receptors. We tested a variety of compounds for their ability to inhibit constitutive currents of GluD receptor variants and found that pentamidine potently inhibited GluD2-T649A constitutive channels (IC50 50 nM). These results identify regions of intolerance to variation in the GRID genes, illustrate the functional consequences of GRID1 and GRID2 variants, and suggest how these receptors function normally and in disease.

GluD1 binds GABA and controls inhibitory plasticity

Fast synaptic neurotransmission in the vertebrate central nervous system relies primarily on ionotropic glutamate receptors (iGluRs), which drive neuronal excitation, and type A γ-aminobutyric acid receptors (GABAARs), which are responsible for neuronal inhibition. However, the GluD1 receptor, an iGluR family member, is present at both excitatory and inhibitory synapses. Whether and how GluD1 activation may affect inhibitory neurotransmission is unknown. In this work, by using a combination of biochemical, structural, and functional analyses, we demonstrate that GluD1 binds GABA, a previously unknown feature of iGluRs. GluD1 activation produces long-lasting enhancement of GABAergic synaptic currents in the adult mouse hippocampus through a non-ionotropic mechanism that is dependent on trans-synaptic anchoring. The identification of GluD1 as a GABA receptor that controls inhibitory synaptic plasticity challenges the classical dichotomy between glutamatergic and GABAergic receptors.

Glutamate delta-1 receptor regulates oligodendrocyte progenitor cell differentiation and myelination in normal and demyelinating conditions

In this study, we investigated the role of glutamate delta 1 receptor (GluD1) in oligodendrocyte progenitor cell (OPC)-mediated myelination during basal (development) and pathophysiological (cuprizone-induced demyelination) conditions. Initially, we sought to determine the expression pattern of GluD1 in OPCs and found a significant colocalization of GluD1 puncta with neuron-glial antigen 2 (NG2, OPC marker) in the motor cortex and dorsal striatum. Importantly, we found that the ablation of GluD1 led to an increase in the number of myelin-associated glycoprotein (MAG+) cells in the corpus callosum and motor cortex at P40 without affecting the number of NG2+ OPCs, suggesting that GluD1 loss selectively facilitates OPC differentiation rather than proliferation. Further, deletion of GluD1 enhanced myelination in the corpus callosum and motor cortex, as indicated by increased myelin basic protein (MBP) staining at P40, suggesting that GluD1 may play an essential role in the developmental regulation of myelination during the critical window period. In contrast, in cuprizone-induced demyelination, we observed reduced MBP staining in the corpus callosum of GluD1 KO mice. Furthermore, cuprizone-fed GluD1 KO mice showed more robust motor deficits. Collectively, our results demonstrate that GluD1 plays a critical role in OPC regulation and myelination in normal and demyelinating conditions.

Differences between the GluD1 and GluD2 receptors revealed by GluD1 X-ray crystallography, binding studies and molecular dynamics

Ionotropic glutamate receptors are ligand-gated ion channels essential for fast excitatory neurotransmission in the brain. In contrast to most other members of the iGluR family, the subfamily of delta receptors, GluD1 and GluD2, does not bind glutamate but glycine/D-serine. GluD1 is widely expressed in the brain and the inner ear, where it is required for high-frequency hearing. Furthermore, it has been associated with schizophrenia, autism and depression. X-ray structures of the ligand-binding domain (LBD) of GluD2 have been published; however, no high-resolution structure is available for the ligand-binding domain of GluD1 (GluD1-LBD). Here, we report the X-ray crystal structure of the GluD1-LBD in its apo form at 2.57 Å resolution. Using isothermal titration calorimetry, we show that D-serine binds to the GluD1-LBD in an exothermic manner with a K_d of 160 μm, which is approximately five-fold greater than at GluD2. Furthermore, we identify Glu822 as a critical determinant of receptor activation in GluD1 A654T. In contrast to studies on the GluD2 lurcher mutant A654T, we did not observe any effect of 1 mm D-serine on the spontaneous currents at mouse GluD1 A654T by electrophysiological recordings of Xenopus laevis oocytes as previously also reported by others. These results point towards differences in the structure and dynamics between GluD1 and GluD2. Molecular dynamics simulations were employed to address this observation, suggesting that the apo structure of GluD1 is less flexible than the apo structure of GluD2 and that Pro725 in GluD1 may affect the interlobe closure of the ligand-binding domain of GluD1.

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.

Potentiation of neuronal activity by tonic GluD1 current in brain slices

Ion channel function of native delta glutamate receptors (GluD_R ) is incompletely understood. Previously, we and others have shown that activation of Gαq protein-coupled receptors (GqPCR) produces a slow inward current carried by GluD1_R . GluD1_R also carries a tonic cation current of unknown cause. Here, using voltage-clamp electrophysiological recordings from adult mouse brain slices containing the dorsal raphe nucleus, we find no role of ongoing G-protein-coupled receptor activity in generating or sustaining tonic GluD1_R currents. Neither augmentation nor disruption of G protein activity affects tonic GluD1_R currents, suggesting that ongoing G-protein-coupled receptor activity does not give rise to tonic GluD1_R currents. Further, the tonic GluD1_R current is unaffected by the addition of external glycine or D-serine, which influences GluD2_R current at millimolar concentrations. Instead, GqPCR-stimulated and tonic GluD1_R currents are regulated by physiological levels of external calcium. In current-clamp recordings, block of GluD1_R channels hyperpolarizes the membrane by ~7 mV at subthreshold potentials, reducing excitability. Thus, GluD1_R carries a G-protein-independent tonic current that contributes to subthreshold neuronal excitation in the dorsal raphe nucleus.

Dysfunction of Glutamate Delta-1 Receptor-Cerebellin 1 Trans-Synaptic Signaling in the Central Amygdala in Chronic Pain

Chronic pain is a debilitating condition involving neuronal dysfunction, but the synaptic mechanisms underlying the persistence of pain are still poorly understood. We found that the synaptic organizer glutamate delta 1 receptor (GluD1) is expressed postsynaptically at parabrachio-central laterocapsular amygdala (PB-CeLC) glutamatergic synapses at axo-somatic and punctate locations on protein kinase C δ -positive (PKCδ+) neurons. Deletion of GluD1 impairs excitatory neurotransmission at the PB-CeLC synapses. In inflammatory and neuropathic pain models, GluD1 and its partner cerebellin 1 (Cbln1) are downregulated while AMPA receptor is upregulated. A single infusion of recombinant Cbln1 into the central amygdala led to sustained mitigation of behavioral pain parameters and normalized hyperexcitability of central amygdala neurons. Cbln2 was ineffective under these conditions and the effect of Cbln1 was antagonized by GluD1 ligand D-serine. The behavioral effect of Cbln1 was GluD1-dependent and showed lateralization to the right central amygdala. Selective ablation of GluD1 from the central amygdala or injection of Cbln1 into the central amygdala in normal animals led to changes in averse and fear-learning behaviors. Thus, GluD1-Cbln1 signaling in the central amygdala is a teaching signal for aversive behavior but its sustained dysregulation underlies persistence of pain. Significance statement: Chronic pain is a debilitating condition which involves synaptic dysfunction, but the underlying mechanisms are not fully understood. Our studies identify a novel mechanism involving structural synaptic changes in the amygdala caused by impaired GluD1-Cbln1 signaling in inflammatory and neuropathic pain behaviors. We also identify a novel means to mitigate pain in these conditions using protein therapeutics.

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.

Structural biology of ionotropic glutamate delta receptors and their crosstalk with metabotropic glutamate receptors

Enigmatic orphan glutamate delta receptors (GluD) are one of the four classes of the ionotropic glutamate receptors (iGluRs) that play key roles in synaptic transmission and plasticity. While members of other iGluR families viz AMPA, NMDA, and kainate receptors are gated by glutamate, the GluD receptors neither bind glutamate nor evoke ligand-induced currents upon binding of glycine and D-serine. Thus, the GluD receptors were considered to function as structural proteins that facilitate the formation, maturation, and maintenance of synapses in the hippocampus and cerebellum. Recent work has revealed that GluD receptors have extensive crosstalk with metabotropic glutamate receptors (mGlus) and are also gated by their activation. The latest development of a novel optopharamcological tool and the cryoEM structures of GluD receptors would help define the molecular and chemical basis of the GluD receptor's role in synaptic physiology. This article is part of the special Issue on "Glutamate Receptors - Orphan iGluRs".

Structure of the Arabidopsis thaliana glutamate receptor-like channel GLR3.4

Glutamate receptor-like channels (GLRs) play vital roles in various physiological processes in plants, such as wound response, stomatal aperture control, seed germination, root development, innate immune response, pollen tube growth, and morphogenesis. Despite the importance of GLRs, knowledge about their molecular organization is limited. Here we use X-ray crystallography and single-particle cryo-EM to solve structures of the Arabidopsis thaliana GLR3.4. Our structures reveal the tetrameric assembly of GLR3.4 subunits into a three-layer domain architecture, reminiscent of animal ionotropic glutamate receptors (iGluRs). However, the non-swapped arrangement between layers of GLR3.4 domains, binding of glutathione through S-glutathionylation of cysteine C205 inside the amino-terminal domain clamshell, unique symmetry, inter-domain interfaces, and ligand specificity distinguish GLR3.4 from representatives of the iGluR family and suggest distinct features of the GLR gating mechanism. Our work elaborates on the principles of GLR architecture and symmetry and provides a molecular template for deciphering GLR-dependent signaling mechanisms in plants.

Glycine agonism in ionotropic glutamate receptors

Ionotropic glutamate receptors (iGluRs) are ligand-gated ion channels that mediate the majority of excitatory neurotransmission in the vertebrate CNS. Classified as AMPA, kainate, delta and NMDA receptors, iGluRs are central drivers of synaptic plasticity widely considered as a major cellular substrate of learning and memory. Surprisingly however, five out of the eighteen vertebrate iGluR subunits do not bind glutamate but glycine, a neurotransmitter known to mediate inhibitory neurotransmission through its action on pentameric glycine receptors (GlyRs). This is the case of GluN1, GluN3A, GluN3B, GluD1 and GluD2 subunits, all also binding the D amino acid d-serine endogenously present in many brain regions. Glycine and d-serine action and affinities broadly differ between glycinergic iGluR subtypes. On 'conventional' GluN1/GluN2 NMDA receptors, glycine (or d-serine) acts in concert with glutamate as a mandatory co-agonist to set the level of receptor activity. It also regulates the receptor's trafficking and expression independently of glutamate. On 'unconventional' GluN1/GluN3 NMDARs, glycine acts as the sole agonist directly triggering opening of excitatory glycinergic channels recently shown to be physiologically relevant. On GluD receptors, d-serine on its own mediates non-ionotropic signaling involved in excitatory and inhibitory synaptogenesis, further reinforcing the concept of glutamate-insensitive iGluRs. Here we present an overview of our current knowledge on glycine and d-serine agonism in iGluRs emphasizing aspects related to molecular mechanisms, cellular function and pharmacological profile. The growing appreciation of the critical influence of glycine and d-serine on iGluR biology reshapes our understanding of iGluR signaling diversity and complexity, with important implications in neuropharmacology.

An emerging map of glutamate delta 1 receptors in the forebrain

Glutamate delta 1 (GluD1) and glutamate delta 2 (GluD2) form the delta family of ionotropic glutamate receptors; these proteins plays widespread roles in synaptic architecture, motor behavior, and cognitive function. Though the role of GluD2 at cerebellar parallel fiber-Purkinje cell synapses is well established, attention now turns to the function of GluD receptors in the forebrain. GluD1 regulates synaptic assembly and modulation in multiple higher brain regions, acting as a postsynaptic cell adhesion molecule with effects on both excitatory and inhibitory transmission. Furthermore, variations and mutations in the GRID1 gene, which codes for GluD1, and in genes which code for proteins functionally linked to GluD1, are associated with mental disorders including autism, schizophrenia, bipolar disorder, and major depression. Cerebellin (Cbln) family proteins, the primary binding partners of delta receptors, are secreted C1q-like proteins which also bind presynaptic neurexins (NRXNs), forming a tripartite synaptic bridge. Published research explores this bridge's function in regions including the striatum, hippocampus, cortex, and cerebellum. In this review, we summarize region- and circuit-specific functions and expression patterns for GluD1 and its related proteins, and their implications for behavior and disease.

Structural biology and thermodynamics of GluD receptors

Glutamate delta (GluD) receptors are a functionally enigmatic subfamily of ionotropic glutamate receptors. Despite sharing similar sequences and structures with AMPA, NMDA, and kainate receptors, GluD receptors do not bind glutamate nor function as ligand-gated ion channels. Binding d-serine and engaging in transsynaptic protein-protein interactions, GluD receptors are thought to undergo complex conformational rearrangements for non-ionotropic signaling that regulates synaptic plasticity. Recent structural, biochemical, and computational studies have elucidated multiple conformational and thermodynamic factors governing the unique properties of GluD receptors. Here, we review advances in biophysical insights into GluD receptors and discuss the structural thermodynamic relationships that underpin their neurobiological functions.

Delta glutamate receptor conductance drives excitation of mouse dorsal raphe neurons

The dorsal raphe nucleus is the predominant source of central serotonin, where neuronal activity regulates complex emotional behaviors. Action potential firing of serotonin dorsal raphe neurons is driven via α1-adrenergic receptors (α1-AR) activation. Despite this crucial role, the ion channels responsible for α1-AR-mediated depolarization are unknown. Here, we show in mouse brain slices that α1-AR-mediated excitatory synaptic transmission is mediated by the ionotropic glutamate receptor homolog cation channel, delta glutamate receptor 1 (GluD1). GluD1R-channels are constitutively active under basal conditions carrying tonic inward current and synaptic activation of α1-ARs augments tonic GluD1R-channel current. Further, loss of dorsal raphe GluD1R-channels produces an anxiogenic phenotype. Thus, GluD1R-channels are responsible for α1-AR-dependent induction of persistent pacemaker-type firing of dorsal raphe neurons and regulate dorsal raphe-related behavior. Given the widespread distribution of these channels, ion channel function of GluD1R as a regulator of neuronal excitability is proposed to be widespread in the nervous system.

Striatal glutamate delta-1 receptor regulates behavioral flexibility and thalamostriatal connectivity

Impaired behavioral flexibility and repetitive behavior is a common phenotype in autism and other neuropsychiatric disorders, but the underlying synaptic mechanisms are poorly understood. The trans-synaptic glutamate delta (GluD)-Cerebellin 1-Neurexin complex, critical for synapse formation/maintenance, represents a vulnerable axis for neuropsychiatric diseases. We have previously found that GluD1 deletion results in reversal learning deficit and repetitive behavior. In this study, we show that selective ablation of GluD1 from the dorsal striatum impairs behavioral flexibility in a water T-maze task. We further found that striatal GluD1 is preferentially found in dendritic shafts, and more frequently associated with thalamic than cortical glutamatergic terminals suggesting localization to projections from the thalamic parafascicular nucleus (Pf). Conditional deletion of GluD1 from the striatum led to a selective loss of thalamic, but not cortical, terminals, and reduced glutamatergic neurotransmission. Optogenetic studies demonstrated functional changes at thalamostriatal synapses from the Pf, but no effect on the corticostriatal system, upon ablation of GluD1 in the dorsal striatum. These studies suggest a novel molecular mechanism by which genetic variations associated with neuropsychiatric disorders may impair behavioral flexibility, and reveal a unique principle by which GluD1 subunit regulates forebrain circuits.

Cryo-EM structures of the ionotropic glutamate receptor GluD1 reveal a non-swapped architecture

Ionotropic orphan delta receptors (GluD) are not gated by glutamate or any other endogenous ligand but are grouped with ionotropic glutamate receptors based on sequence similarity. GluD1 receptors play critical roles in synaptogenesis, synapse maintenance and have been implicated in neuronal disorders including schizophrenia, cognitive deficits, and cerebral ataxia. Here we report cryo-electron microscopy structures of the rat GluD1 receptor complexed with calcium and the ligand 7-chlorokynurenic acid, elucidating molecular architecture and principles of receptor assembly. The structures reveal a non-swapped architecture at the extracellular amino-terminal (ATD) and ligand-binding domain (LBD) interface. This is in contrast to other families of ionotropic glutamate receptors (iGluRs) where the dimer partners between the ATD and LBD layers are swapped. Our results demonstrate that principles of architecture and symmetry are not conserved between delta receptors and other iGluRs and provide a molecular blueprint for understanding the functions of the “orphan” class of iGluRs.

Genome wide association and gene enrichment analysis reveal membrane anchoring and structural proteins associated with meat quality in beef

BACKGROUND

Meat quality related phenotypes are difficult and expensive to measure and predict but are ideal candidates for genomic selection if genetic markers that account for a worthwhile proportion of the phenotypic variation can be identified. The objectives of this study were: 1) to perform genome wide association analyses for Warner-Bratzler Shear Force (WBSF), marbling, cooking loss, tenderness, juiciness, connective tissue and flavor; 2) to determine enriched pathways present in each genome wide association analysis; and 3) to identify potential candidate genes with multiple quantitative trait loci (QTL) associated with meat quality.

RESULTS

The WBSF, marbling and cooking loss traits were measured in longissimus dorsi muscle from 672 steers. Out of these, 495 animals were used to measure tenderness, juiciness, connective tissue and flavor by a sensory panel. All animals were genotyped for 221,077 markers and included in a genome wide association analysis. A total number of 68 genomic regions covering 52 genes were identified using the whole genome association approach; 48% of these genes encode transmembrane proteins or membrane associated molecules. Two enrichment analysis were performed: a tissue restricted gene enrichment applying a correlation analysis between raw associated single nucleotide polymorphisms (SNPs) by trait, and a functional classification analysis performed using the DAVID Bioinformatic Resources 6.8 server. The tissue restricted gene enrichment approach identified eleven pathways including "Endoplasmic reticulum membrane" that influenced multiple traits simultaneously. The DAVID functional classification analysis uncovered eleven clusters related to transmembrane or structural proteins. A gene network was constructed where the number of raw associated uncorrelated SNPs for each gene across all traits was used as a weight. A multiple SNP association analysis was performed for the top five most connected genes in the gene-trait network. The gene network identified the EVC2, ANXA10 and PKHD1 genes as potentially harboring multiple QTLs. Polymorphisms identified in structural proteins can modulate two different processes with direct effect on meat quality: in vivo myocyte cytoskeletal organization and postmortem proteolysis.

CONCLUSION

The main result from the present analysis is the uncovering of several candidate genes associated with meat quality that have structural function in the skeletal muscle.

Glutamate delta-1 receptor regulates cocaine-induced plasticity in the nucleus accumbens

Cocaine exposure induces plasticity of glutamatergic synapses of medium spiny neurons (MSNs) in the nucleus accumbens (NAc), which has been proposed to contribute to its addictive behavior. The mechanisms underlying cocaine-induced plasticity are not fully understood. The orphan glutamate delta-1 (GluD1) receptor is a member of the ionotropic glutamate receptor family but does not function as a typical ligand-gated ion channel. Instead it serves a synaptogenic function by interacting with presynaptic Neurexin protein. Recent neuroanatomical studies have demonstrated enriched expression of GluD1 in the NAc but its role in reward behavior, MSN function, and drug-induced plasticity remains unknown. Using a combination of constitutive and conditional GluD1 KO models, we evaluated the effect of GluD1 ablation on cocaine-conditioned place preference (CPP) and cocaine-induced structural and functional plasticity. GluD1 KO mice showed higher cocaine CPP. Selective ablation of GluD1 from striatal neurons but not cortico-limbic excitatory neurons reproduced higher CPP. Higher cocaine preference in GluD1 KO correlated with an increase in spine density, greater maturation of dendritic spines, and basally upregulated spine-regulating active cofilin. GluD1 loss did not affect basal excitatory neurotransmission or plasticity but masked the generation of cocaine-induced silent synapses. Finally, loss of GluD1 increased the GluN2B subunit contribution to NMDA receptor currents in MSNs and a partial agonist of GluN2B-containing NMDA receptors normalized the higher active cofilin and cocaine preference in GluD1 KO mice. Together, these findings demonstrate a critical role of GluD1 in controlling susceptibility to cocaine preference and cocaine-induced plasticity by modulating NMDA receptor subunit contribution.

GluD1, linked to schizophrenia, controls the burst firing of dopamine neurons

Human mutations of the GRID1 gene encoding the orphan delta1 glutamate receptor-channel (GluD1) are associated with schizophrenia but the explicit role of GluD1 in brain circuits is unknown. Based on the known function of its paralog GluD2 in cerebellum, we searched for a role of GluD1 in slow glutamatergic transmission mediated by metabotropic receptor mGlu1 in midbrain dopamine neurons, whose dysfunction is a hallmark of schizophrenia. We found that an mGlu1 agonist elicits a slow depolarizing current in HEK cells co-expressing mGlu1 and GluD1, but not in cells expressing mGlu1 or GluD1 alone. This current is abolished by additional co-expression of a dominant-negative GluD1 dead pore mutant. We then characterized mGlu1-dependent currents in dopamine neurons from midbrain slices. Both the agonist-evoked and the slow postsynaptic currents are abolished by expression of the dominant-negative GluD1 mutant, pointing to the involvement of native GluD1 channels in these currents. Likewise, both mGlu1-dependent currents are suppressed in GRID1 knockout mice, which reportedly display endophenotypes relevant for schizophrenia. It is known that mGlu1 activation triggers the transition from tonic to burst firing of dopamine neurons, which signals salient stimuli and encodes reward prediction. In vivo recordings of dopamine neurons showed that their spontaneous burst firing is abolished in GRID1 knockout mice or upon targeted expression of the dominant-negative GluD1 mutant in wild-type mice. Our results de-orphanize GluD1, unravel its key role in slow glutamatergic transmission and provide insights into how GRID1 gene alterations can lead to dopaminergic dysfunctions in schizophrenia.

Glutamate Delta-1 Receptor Regulates Metabotropic Glutamate Receptor 5 Signaling in the Hippocampus

The delta family of ionotropic glutamate receptors consists of glutamate delta-1 (GluD1) and glutamate delta-2 receptors. We have previously shown that GluD1 knockout mice exhibit features of developmental delay, including impaired spine pruning and switch in the N-methyl-D-aspartate receptor subunit, which are relevant to autism and other neurodevelopmental disorders. Here, we identified a novel role of GluD1 in regulating metabotropic glutamate receptor 5 (mGlu5) signaling in the hippocampus. Immunohistochemical analysis demonstrated colocalization of mGlu5 with GluD1 punctas in the hippocampus. Additionally, GluD1 protein coimmunoprecipitated with mGlu5 in the hippocampal membrane fraction, as well as when overexpressed in human embryonic kidney 293 cells, demonstrating that GluD1 and mGlu5 may cooperate in a signaling complex. The interaction of mGlu5 with scaffold protein effector Homer, which regulates mechanistic target of rapamycin (mTOR) signaling, was abnormal both under basal conditions and in response to mGlu1/5 agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) in GluD1 knockout mice. The basal levels of phosphorylated mTOR and protein kinase B, the signaling proteins downstream of mGlu5 activation, were higher in GluD1 knockout mice, and no further increase was induced by DHPG. We also observed higher basal protein translation and an absence of DHPG-induced increase in GluD1 knockout mice. In accordance with a role of mGlu5-mediated mTOR signaling in synaptic plasticity, DHPG-induced internalization of surface α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunits was impaired in the GluD1 knockout mice. These results demonstrate that GluD1 interacts with mGlu5, and loss of GluD1 impairs normal mGlu5 signaling potentially by dysregulating coupling to its effector. These studies identify a novel role of the enigmatic GluD1 subunit in hippocampal function.

Essential role of GluD1 in dendritic spine development and GluN2B to GluN2A NMDAR subunit switch in the cortex and hippocampus reveals ability of GluN2B inhibition in correcting hyperconnectivity

The glutamate delta-1 (GluD1) receptor is highly expressed in the forebrain. We have previously shown that loss of GluD1 leads to social and cognitive deficits in mice, however, its role in synaptic development and neurotransmission remains poorly understood. Here we report that GluD1 is enriched in the medial prefrontal cortex (mPFC) and GluD1 knockout mice exhibit a higher dendritic spine number, greater excitatory neurotransmission as well as higher number of synapses in mPFC. In addition abnormalities in the LIMK1-cofilin signaling, which regulates spine dynamics, and a lower ratio of GluN2A/GluN2B expression was observed in the mPFC in GluD1 knockout mice. Analysis of the GluD1 knockout CA1 hippocampus similarly indicated the presence of higher spine number and synapses and altered LIMK1-cofilin signaling. We found that systemic administration of an N-methyl-d-aspartate (NMDA) receptor partial agonist d-cycloserine (DCS) at a high-dose, but not at a low-dose, and a GluN2B-selective inhibitor Ro-25-6981 partially normalized the abnormalities in LIMK1-cofilin signaling and reduced excess spine number in mPFC and hippocampus. The molecular effects of high-dose DCS and GluN2B inhibitor correlated with their ability to reduce the higher stereotyped behavior and depression-like behavior in GluD1 knockout mice. Together these findings demonstrate a critical requirement for GluD1 in normal spine development in the cortex and hippocampus. Moreover, these results identify inhibition of GluN2B-containing receptors as a mechanism for reducing excess dendritic spines and stereotyped behavior which may have therapeutic value in certain neurodevelopmental disorders such as autism.

Schizophrenia and Depression Co-Morbidity: What We have Learned from Animal Models

Patients with schizophrenia are at an increased risk for the development of depression. Overlap in the symptoms and genetic risk factors between the two disorders suggests a common etiological mechanism may underlie the presentation of comorbid depression in schizophrenia. Understanding these shared mechanisms will be important in informing the development of new treatments. Rodent models are powerful tools for understanding gene function as it relates to behavior. Examining rodent models relevant to both schizophrenia and depression reveals a number of common mechanisms. Current models which demonstrate endophenotypes of both schizophrenia and depression are reviewed here, including models of CUB and SUSHI multiple domains 1, PDZ and LIM domain 5, glutamate Delta 1 receptor, diabetic db/db mice, neuropeptide Y, disrupted in schizophrenia 1, and its interacting partners, reelin, maternal immune activation, and social isolation. Neurotransmission, brain connectivity, the immune system, the environment, and metabolism emerge as potential common mechanisms linking these models and potentially explaining comorbid depression in schizophrenia.

Type 1 metabotropic glutamate receptors (mGlu1) trigger the gating of GluD2 delta glutamate receptors

The orphan GluD2 receptor belongs to the ionotropic glutamate receptor family but does not bind glutamate. Ligand-gated GluD2 currents have never been evidenced, and whether GluD2 operates as an ion channel has been a long-standing question. Here, we show that GluD2 gating is triggered by type 1 metabotropic glutamate receptors, both in a heterologous expression system and in Purkinje cells. Thus, GluD2 is not only an adhesion molecule at synapses but also works as a channel. This gating mechanism reveals new properties of glutamate receptors that emerge from their interaction and opens unexpected perspectives regarding synaptic transmission and plasticity.