Role of the carboxy-terminal region of the GluR epsilon2 subunit in synaptic localization of the NMDA receptor channel

Limb-Clasping Response in NMDA Receptor Palmitoylation-Deficient Mice

Proper regulation of N-methyl-D-aspartate-type glutamate receptor (NMDA receptor) expression is responsible for excitatory synaptic functions in the mammalian brain. NMDA receptor dysfunction can cause various neuropsychiatric disorders and neurodegenerative diseases. Posttranslational protein S-palmitoylation, the covalent attachment of palmitic acid to intracellular cysteine residues via thioester bonds, occurs in the carboxyl terminus of GluN2B, which is the major regulatory NMDA receptor subunit. Mutations of three palmitoylatable cysteine residues in the membrane-proximal cluster of GluN2B to non-palmitoylatable serine (3CS) lead to the dephosphorylation of GluN2B Tyr1472 in the hippocampus and cerebral cortex, inducing a reduction in the surface expression of GluN2B-containig NMDA receptors. Furthermore, adult GluN2B 3CS homozygous mice demonstrated a definite clasping response without abnormalities in the gross brain structure, other neurological reflexes, or expression levels of synaptic proteins in the cerebrum. This behavioral disorder, observed in the GluN2B 3CS knock-in mice, indicated that complex higher brain functions are coordinated through the palmitoylation-dependent regulation of NMDA receptors in excitatory synapses.

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.

Edonerpic maleate regulates glutamate receptors through CRMP2- and Arc-mediated mechanisms in response to brain trauma

Dysfunction of ionotropic glutamate receptors (iGluRs) is a key molecular mechanism of excitotoxic neuronal injury following traumatic brain injury (TBI). Edonerpic maleate is a low molecular-weight compound that was screened as a candidate neuroprotective agent. In this study, we investigated its effects on TBI and GluRs signaling. Traumatic neuronal injury (TNI) induced by scratch followed by glutamate treatment was performed to mimic TBI in vitro. Edonerpic maleate at 1 and 10 μM exerted protective activity when it was added within 2 h following injury. The protective activities were also confirmed by the reduction of lipid peroxidation and oxidative stress. In addition, edonerpic maleate inhibited the expression of surface NR2B, total GluR1, and surface GluR1, and mitigated the intracellular Ca²⁺ responses following injury in vitro. Western blot analysis showed that edonerpic maleate reduced the cleavage of collapsing response mediator protein 2 (CRMP2), but increased the expression of postsynaptic protein Arc. By using gene overexpression and silencing technologies, CRMP2 was overexpressed and Arc was knockdown in cortical neurons. The results showed that the effect of edonerpic maleate on NMDA receptor expression was mediated by CRMP2, whereas the edonerpic maleate-induced AMPA receptor regulation was dependent on Arc activation. In in vivo TBI model, 30 mg/kg edonerpic maleate alleviated the TBI-induced brain edema, neuronal loss, and microglial activation, with no effect on locomotor function at 24 h. However, edonerpic maleate improves long-term neurological function after TBI. Furthermore, edonerpic maleate inhibited CRMP2 cleavage but increased Arc activation in vivo. In summary, our results identify edonerpic maleate as a clinically potent small compound with which to attenuate TBI-related brain damage through regulating GluRs signaling.

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.

Protein Kinase C-Mediated Phosphorylation and α2δ-1 Interdependently Regulate NMDA Receptor Trafficking and Activity

N-methyl-d-aspartate receptors (NMDARs) are important for synaptic plasticity associated with many physiological functions and neurologic disorders. Protein kinase C (PKC) activation increases the phosphorylation and activity of NMDARs, and α2δ-1 is a critical NMDAR-interacting protein and controls synaptic trafficking of NMDARs. In this study, we determined the relative roles of PKC and α2δ-1 in the control of NMDAR activity. We found that α2δ-1 coexpression significantly increased NMDAR activity in HEK293 cells transfected with GluN1/GluN2A or GluN1/GluN2B. PKC activation with phorbol 12-myristate 13-acetate (PMA) increased receptor activity only in cells coexpressing GluN1/GluN2A and α2δ-1. Remarkably, PKC inhibition with Gӧ6983 abolished α2δ-1-coexpression-induced potentiation of NMDAR activity in cells transfected with GluN1/GluN2A or GluN1/GluN2B. Treatment with PMA increased the α2δ-1-GluN1 interaction and promoted α2δ-1 and GluN1 cell surface trafficking. PMA also significantly increased NMDAR activity of spinal dorsal horn neurons and the amount of α2δ-1-bound GluN1 protein complexes in spinal cord synaptosomes in wild-type mice, but not in α2δ-1 knockout mice. Furthermore, inhibiting α2δ-1 with pregabalin or disrupting the α2δ-1-NMDAR interaction with the α2δ-1 C-terminus peptide abolished the potentiating effect of PMA on NMDAR activity. Additionally, using quantitative phosphoproteomics and mutagenesis analyses, we identified S929 on GluN2A and S1413 (S1415 in humans) on GluN2B as the phosphorylation sites responsible for NMDAR potentiation by PKC and α2δ-1. Together, our findings demonstrate the interdependence of α2δ-1 and PKC phosphorylation in regulating NMDAR trafficking and activity. The phosphorylation-dependent, dynamic α2δ-1-NMDAR interaction constitutes an important molecular mechanism of synaptic plasticity.SIGNIFICANCE STATEMENT A major challenge in studies of protein phosphorylation is to define the functional significance of each phosphorylation event and determine how various signaling pathways are coordinated in response to neuronal activity to shape synaptic plasticity. PKC phosphorylates transporters, ion channels, and G-protein-coupled receptors in signal transduction. In this study, we showed that α2δ-1 is indispensable for PKC-activation-induced surface and synaptic trafficking of NMDARs, whereas the α2δ-1-NMDAR interaction is controlled by PKC-induced phosphorylation. Our findings reveal that α2δ-1 mainly functions as a phospho-binding protein in the control of NMDAR trafficking and activity. This information provides new mechanistic insight into the reciprocal roles of PKC-mediated phosphorylation and α2δ-1 in regulating NMDARs and in the therapeutic actions of gabapentinoids.

Regulation of the NMDA receptor by its cytoplasmic domains: (How) is the tail wagging the dog?

Excitatory neurotransmission mediated by N-methyl-d-aspartate receptors (NMDARs) is critical for synapse development, function, and plasticity in the brain. NMDARs are tetra-heteromeric cation-channels that mediate synaptic transmission and plasticity. Extensive human studies show the existence of genetic variants in NMDAR subunits genes (GRIN genes) that are associated with neurodevelopmental and neuropsychiatric disorders, including autism spectrum disorders (ASD), epilepsy (EP), intellectual disability (ID), attention deficit hyperactivity disorder (ADHD), and schizophrenia (SCZ). NMDAR subunits have a unique modular architecture with four semiautonomous domains. Here we focus on the carboxyl terminal domain (CTD), also known as the intracellular C-tail, which varies in length among the glutamate receptor subunits and is the most diverse domain in terms of amino acid sequence. The CTD shows no sequence homology to any known proteins but encodes short docking motifs for intracellular binding proteins and covalent modifications. Our review will discuss the many important functions of the CTD in regulating NMDA membrane and synaptic targeting, stabilization, degradation targeting, allosteric modulation and metabotropic signaling of the receptor. This article is part of the special issue on 'Glutamate Receptors - NMDA Receptors'.

Architecture and function of NMDA receptors: an evolutionary perspective

Ionotropic glutamate receptors (iGluRs) are a major class of ligand-gated ion channels that are widespread in the living kingdom. Their critical role in excitatory neurotransmission and brain function of arthropods and vertebrates has made them a compelling subject of interest for neurophysiologists and pharmacologists. This is particularly true for NMDA receptor (NMDARs), a subclass of iGluRs that act as central drivers of synaptic plasticity in the CNS. How and when the unique properties of NMDARs arose during evolution, and how they relate to the evolution of the nervous system, remain open questions. Recent years have witnessed a boom in both genomic and structural data, such that it is now possible to analyse the evolution of iGluR genes on an unprecedented scale and within a solid molecular framework. In this review, combining insights from phylogeny, atomic structure and physiological and mechanistic data, we discuss how evolution of NMDAR motifs and sequences shaped their architecture and functionalities. We trace differences and commonalities between NMDARs and other iGluRs, emphasizing a few distinctive properties of the former regarding ligand binding and gating, permeation, allosteric modulation and intracellular signalling. Finally, we speculate on how specific molecular properties of iGuRs arose to supply new functions to the evolving structure of the nervous system, from early metazoan to present mammals.

The C-terminal domains of the NMDA receptor: How intrinsically disordered tails affect signalling, plasticity and disease

NMDA receptors are part of the ionotropic glutamate receptor family, and are crucial for neurotransmission and memory. At the cellular level, the effects of activating these receptors include long-term potentiation (LTP) or depression (LTD). The NMDA receptor is a stringently gated cation channel permeable to Ca²⁺ , and it shares the molecular architecture of a tetrameric ligand-gated ion channel with the other family members. Its subunits, however, have uniquely long cytoplasmic C-terminal domains (CTDs). While the molecular gymnastics of the extracellular domains have been described in exquisite detail, much less is known about the structure and function of these CTDs. The CTDs vary dramatically in length and sequence between receptor subunits, but they all have a composition characteristic of intrinsically disordered proteins. The CTDs affect channel properties, trafficking and downstream signalling output from the receptor, and these functions are regulated by alternative splicing, protein-protein interactions, and post-translational modifications such as phosphorylation and palmitoylation. Here, we review the roles of the CTDs in synaptic plasticity with a focus on biochemical mechanisms. In total, the CTDs play a multifaceted role as a modifier of channel function, a regulator of cellular location and abundance, and signalling scaffold control the downstream signalling output.

Regulation of NMDA glutamate receptor functions by the GluN2 subunits

The N-methyl-D-aspartate receptors (NMDARs) are ionotropic glutamate receptors that mediate the flux of calcium (Ca²⁺ ) into the post-synaptic compartment. Ca²⁺ influx subsequently triggers the activation of various intracellular signalling cascades that underpin multiple forms of synaptic plasticity. Functional NMDARs are assembled as heterotetramers composed of two obligatory GluN1 subunits and two GluN2 or GluN3 subunits. Four different GluN2 subunits (GluN2A-D) are present throughout the central nervous system; however, they are differentially expressed, both developmentally and spatially, in a cell- and synapse-specific manner. Each GluN2 subunit confers NMDARs with distinct ion channel properties and intracellular trafficking pathways. Regulated membrane trafficking of NMDARs is a dynamic process that ultimately determines the number of NMDARs at synapses, and is controlled by subunit-specific interactions with various intracellular regulatory proteins. Here we review recent progress made towards understanding the molecular mechanisms that regulate the trafficking of GluN2-containing NMDARs, focusing on the roles of several key synaptic proteins that interact with NMDARs via their carboxyl termini.

An autism-associated mutation in GluN2B prevents NMDA receptor trafficking and interferes with dendrite growth

Autism spectrum disorders (ASDs) are neurodevelopmental disorders with multiple genetic associations. Analysis of de novo mutations identified GRIN2B, which encodes the GluN2B subunit of NMDA receptors, as a gene linked to ASDs with high probability. However, the mechanisms by which GRIN2B mutations contribute to ASD pathophysiology are not understood. Here, we investigated the cellular phenotypes induced by a human mutation that is predicted to truncate GluN2B within the extracellular loop. This mutation abolished NMDA-dependent Ca²⁺ influx. Mutant GluN2B co-assembled with GluN1 but was not trafficked to the cell surface or dendrites. When mutant GluN2B was expressed in developing cortical neurons, dendrites appeared underdeveloped, with shorter and fewer branches, while spine density was unaffected. Mutant dendritic arbors were often dysmorphic, displaying abnormal filopodial-like structures. Interestingly, dendrite maldevelopment appeared when mutant GluN2B was expressed on a wild-type background, reflecting the disease given that individuals are heterozygous for GRIN2B mutations. Restoring the fourth transmembrane domain and cytoplasmic tail did not rescue the phenotypes. Finally, abnormal development was not accompanied by reduced mTOR signaling. These data suggest that mutations in GluN2B contribute to ASD pathogenesis by disrupting dendrite development.

The role of CaMKII-Tiam1 complex on learning and memory

A stimulation inducing long-term potentiation (LTP) of synaptic transmission induces a persistent expansion of dendritic spines, a phenomenon known as structural LTP (sLTP). We previously proposed that the formation of a reciprocally activating kinase-effector complex (RAKEC) between CaMKII and Tiam1, an activator of the small G-protein Rac1, locks CaMKII into an active conformation, which in turn maintains the phosphorylation status of Tiam1. This makes Rac1 persistently active, specifically in the stimulated spine. To understand the significance of the CaMKII-Tiam1 RAKEC in vivo, we generated a Tiam1 mutant knock-in mouse line in which critical residues for CaMKII binding were mutated into alanines. We confirmed the central role of this interaction on sLTP by observing that KI mice showed reduced Rac1 activity, had smaller spines and a diminished sLTP as compared to their wild-type littermates. Moreover, behavioral tests showed that the novel object recognition memory of these animals was impaired. We thus propose that the CaMKII-Tiam1 interaction regulates spine morphology in vivo and is required for memory storage.

Calpains and neuronal damage in the ischemic brain: The swiss knife in synaptic injury

The excessive extracellular accumulation of glutamate in the ischemic brain leads to an overactivation of glutamate receptors with consequent excitotoxic neuronal death. Neuronal demise is largely due to a sustained activation of NMDA receptors for glutamate, with a consequent increase in the intracellular Ca(2+) concentration and activation of calcium- dependent mechanisms. Calpains are a group of Ca(2+)-dependent proteases that truncate specific proteins, and some of the cleavage products remain in the cell, although with a distinct function. Numerous studies have shown pre- and post-synaptic effects of calpains on glutamatergic and GABAergic synapses, targeting membrane- associated proteins as well as intracellular proteins. The resulting changes in the presynaptic proteome alter neurotransmitter release, while the cleavage of postsynaptic proteins affects directly or indirectly the activity of neurotransmitter receptors and downstream mechanisms. These alterations also disturb the balance between excitatory and inhibitory neurotransmission in the brain, with an impact in neuronal demise. In this review we discuss the evidence pointing to a role for calpains in the dysregulation of excitatory and inhibitory synapses in brain ischemia, at the pre- and post-synaptic levels, as well as the functional consequences. Although targeting calpain-dependent mechanisms may constitute a good therapeutic approach for stroke, specific strategies should be developed to avoid non-specific effects given the important regulatory role played by these proteases under normal physiological conditions.

Neurological dysfunctions associated with altered BACE1-dependent Neuregulin-1 signaling

Inhibition of BACE1 is being pursued as a therapeutic target to treat patients suffering from Alzheimer's disease because BACE1 is the sole β-secretase that generates β-amyloid peptide. Knowledge regarding other cellular functions of BACE1 is therefore critical for the safe use of BACE1 inhibitors in human patients. Neuregulin-1 (Nrg1) is a BACE1 substrate and BACE1 cleavage of Nrg1 is critical for signaling functions in myelination, remyelination, synaptic plasticity, normal psychiatric behaviors, and maintenance of muscle spindles. This review summarizes the most recent discoveries associated with BACE1-dependent Nrg1 signaling in these areas. This body of knowledge will help to provide guidance for preventing unwanted Nrg1-based side effects following BACE1 inhibition in humans. To initiate its signaling cascade, membrane anchored Neuregulin (Nrg), mainly type I and III β1 Nrg1 isoforms and Nrg3, requires ectodomain shedding. BACE1 is one of such indispensable sheddases to release the functional Nrg signaling fragment. The dependence of Nrg on the cleavage by BACE1 is best manifested by disrupting the critical role of Nrg in the control of axonal myelination, schizophrenic behaviors as well as the formation and maintenance of muscle spindles.

Physiological Roles of Calpain 1 Associated to Multiprotein NMDA Receptor Complex

We have recently demonstrated that in resting conditions calpain 1, but not calpain 2, is specifically associated to the N-Methyl-D-Aspartate receptor (NMDAR) multiprotein complex. We are here reporting that in SKNBE neuroblastoma cells or in freshly isolated nerve terminals from adult rat hippocampus, the proteolytic activity of calpain 1 resident at the NMDAR is very low under basal conditions and greatly increases following NMDAR stimulation. Since the protease resides at the NMDAR in saturating amounts, variations in Ca2+ influx promote an increase in calpain 1 activity without affecting the amount of the protease originally associated to NMDAR. In all the conditions examined, resident calpain 1 specifically cleaves NR2B at the C-terminal region, leading to its internalization together with NR1 subunit. While in basal conditions intracellular membranes include small amounts of NMDAR containing the calpain-digested NR2B, upon NMDAR stimulation nearly all the receptor molecules are internalized. We here propose that resident calpain 1 is involved in NMDAR turnover, and following an increase in Ca2+ influx, the activated protease, by promoting the removal of NMDAR from the plasma membranes, can decrease Ca2+ entrance through this channel. Due to the absence of calpastatin in such cluster, the activity of resident calpain 1 may be under the control of HSP90, whose levels are directly related to the activation of this protease. Observations of different HSP90/calpain 1 ratios in different ultrasynaptic compartments support this conclusion.

Semi-quantitative analyses of antibodies to N-methyl-d-aspartate type glutamate receptor subunits (GluN2B & GluN1) in the clinical course of Rasmussen syndrome

OBJECTIVE

In Rasmussen syndrome (RS), in addition to the predominant involvement of cytotoxic T cells, heterogeneous autoantibodies against neural molecules are also found, but their function has not been elucidated. We examined antibodies to N-methyl-d-aspartate (NMDA) type glutamate receptor (GluR) subunits (GluN2B & GluN1) semi-quantitatively in cerebrospinal fluid (CSF) samples from RS patients, and evaluated their changes over time and their roles in immunopathogenesis.

METHODS

Autoantibodies against N-terminal and C-terminal of GluN2B and GluN1 were examined in 40 CSF samples collected from 18 RS patients 5 to 180 months after the onset of RS. Epileptic patients without infectious etiology or progressive clinical course served as disease controls (n=23). Synthesized peptides encoding the extracellular and intracellular domains of human GluN2B and GluN1 subunits were used as antigens in ELISA. We defined the cut-off for these antibodies as mean +2 standard deviations (optimal density) of the disease controls. MRI were evaluated according to the MRI staging proposed by Bien et al. (2002b, Neurology 58, 250).

RESULTS

CSF levels of antibodies against N-terminal and C-terminal of GluN2B were higher in RS patients than in disease controls (p<0.01). Likewise, CSF levels of antibodies against N-terminal and C-terminal of GluN1 were also higher in RS patients than in disease controls (p<0.01). All four antibodies tested were below cut-off levels in almost all CSF samples collected within one year from epilepsy onset. The proportions of CSF samples with these antibodies above cut-off levels were highest from 12 to 23 months after epilepsy onset, and declined after 24 months. CSF levels of these antibodies were higher when seizure occurred daily than when seizure occurred less frequently (p<0.01), and were higher at MRI stage 3 than at MRI stages 0, 2 and 4 (p<0.05), except for anti-GluN1-CT antibody at stage 2.

CONCLUSIONS

Broad epitope recognition spectrum and delayed production of autoantibodies to NMDA type GluR in CSF of RS patients suggest that the autoantibodies are produced against NMDA type GluR antigens derived from cytotoxic T cell-mediated neuronal damages. These antibodies may impact the pathophysiology of RS in the most active stage, and could be a marker for active inflammation in the clinical course of RS. Further studies including passive transfer of the antibodies to mice may reveal the pivotal roles of the antibodies in RS.

Opposing role of NMDA receptor GluN2B and GluN2D in somatosensory development and maturation

Development of correct topographical connections between peripheral receptors and central somatosensory stations requires activity-dependent synapse refinement, in which the NMDA type of glutamate receptors plays a key role. Here we compared functional roles of GluN2B (GluRε2 or NR2B) and GluN2D (GluRε4 or NR2D), two major regulatory subunits of neonatal NMDA receptors, in development of whisker-related patterning at trigeminal relay stations. Compared with control littermates, both the appearance of whisker-related patterning and the termination of the critical period, as assessed by unilateral infraorbital nerve transection, were delayed by nearly a day in the somatosensory cortex of GluN2B(+/-) mice but advanced by nearly a day in GluN2D(-/-) mice. Similar temporal shifts were found at subcortical relay stations in the thalamus and brainstem of GluN2B(+/-) and GluN2D(-/-) mice. In comparison, the magnitude of lesion-induced critical period plasticity in the somatosensory cortex, as assessed following row-C whisker removal, was normal in both mutants. Thus, GluN2B and GluN2D play counteractive roles in temporal development and maturation of somatosensory maps without affecting the magnitude of critical period plasticity. To understand the opposing action, we then examined neuronal and synaptic expressions of the two subunits along the trigeminal pathway. At each trigeminal station, GluN2B was predominant at asymmetrical synapses of non-GABAergic neurons, whereas GluN2D was selective to asymmetrical synapses of GABAergic neurons. Together, our findings suggest that GluN2B expressed at glutamatergic synapses on glutamatergic projection neurons facilitates refinement of ascending pathway synapses directly, whereas GluN2D expressed at glutamatergic synapses on GABAergic interneurons delays it indirectly.

Brain perfusion SPECT in limbic encephalitis associated with autoantibody against the glutamate receptor epsilon 2

OBJECTIVES

The aim of this study was to elucidate the single-photon emission computed tomography (SPECT) pattern at the acute stage of disease in non-herpetic limbic encephalitis (NHLE) patients associated with the N-methyl-D-aspartate-type glutamate receptor epsilon 2 (GluR ϵ2) autoantibody using Z-score imaging system (eZIS) analyses.

METHODS

Brain magnetic resonance imaging (MRI) and brain perfusion SPECT using technetium-99 ethyl cysteinate dimer ((99m)Tc-ECD) were performed in eight patients with NHLE (5 men and 3 women; mean age 48.8±22 years) within 20days after clinical onset.

RESULTS

All patients had various clinical limbic-associated symptoms and no evidence of herpes simplex infection or systemic malignancies. Two of eight patients showed abnormally hyperintense lesions on diffusion-weighted images and significant hyperperfusion in ipsilateral cerebral cortex on eZIS analysis, whereas other patients showed normal MRI findings and significant hypoperfusion in one or both sides of the limbic and paralimbic areas.

CONCLUSION

We suggest that (99m)Tc-ECD SPECT study using eZIS analyses may be helpful to detect the neuronal dysfunction, particularly in NHLE patients without abnormal MRI findings.

Surface trafficking of NMDA receptors: gathering from a partner to another

Understanding the molecular and cellular pathways by which neurons integrate signals from different neurotransmitter systems has been among the major challenges of modern neuroscience. The ionotropic glutamate NMDA receptor plays a key role in the maturation and plasticity of glutamate synapses, both in physiology and pathology. It recently appeared that the surface distribution of NMDA receptors is dynamically regulated through lateral diffusion, providing for instance a powerful way to rapidly affect the content and composition of synaptic receptors. The ability of various neuromodulators to regulate NMDA receptor signaling revealed that this receptor can also serve as a molecular integrator of the ambient neuronal environment. Although still in its infancy, we here review our current understanding of the cellular regulation of NMDA receptor surface dynamics. We specifically discuss the roles of well-known modulators, such as dopamine, and membrane interactors in these regulatory processes, exemplifying the recent evidence that the direct interaction between NMDAR and dopamine receptors regulates their surface diffusion and distribution. In addition to the well-established modulation of NMDA receptor signaling by intracellular pathways, the surface dynamics of the receptor is now emerging as the first level of regulation, opening new pathophysiological perspectives for innovative therapeutical strategies.

NMDA hypofunction as a convergence point for progression and symptoms of schizophrenia

Schizophrenia is a disabling mental illness that is now recognized as a neurodevelopmental disorder. It is likely that genetic risk factors interact with environmental perturbations to affect normal brain development and that this altered trajectory results in a combination of positive, negative, and cognitive symptoms. Although the exact pathophysiology of schizophrenia is unknown, the N-methyl-D-aspartate receptor (NMDAR), a major glutamate receptor subtype, has received great attention. Proper expression and regulation of NMDARs in the brain is critical for learning and memory processes as well as cortical plasticity and maturation. Evidence from both animal models and human studies implicates a dysfunction of NMDARs both in disease progression and symptoms of schizophrenia. Furthermore, mutations in many of the known genetic risk factors for schizophrenia suggest that NMDAR hypofunction is a convergence point for schizophrenia. In this review, we discuss how disrupted NMDAR function leads to altered neurodevelopment that may contribute to the progression and development of symptoms for schizophrenia, particularly cognitive deficits. We review the shared signaling pathways among the schizophrenia susceptibility genes DISC1, neuregulin1, and dysbindin, focusing on the AKT/GSK3β pathway, and how their mutations and interactions can lead to NMDAR dysfunction during development. Additionally, we explore what open questions remain and suggest where schizophrenia research needs to move in order to provide mechanistic insight into the cause of NMDAR dysfunction, as well as generate possible new avenues for therapeutic intervention.

Evolution of GluN2A/B cytoplasmic domains diversified vertebrate synaptic plasticity and behavior

Two genome duplications early in the vertebrate lineage expanded gene families, including GluN2 subunits of the NMDA receptor. Diversification between the four mammalian GluN2 proteins occurred primarily at their intracellular C-terminal domains (CTDs). To identify shared ancestral functions and diversified subunit-specific functions, we exchanged the exons encoding the GluN2A (also known as Grin2a) and GluN2B (also known as Grin2b) CTDs in two knock-in mice and analyzed the mice's biochemistry, synaptic physiology, and multiple learned and innate behaviors. The eight behaviors were genetically separated into four groups, including one group comprising three types of learning linked to conserved GluN2A/B regions. In contrast, the remaining five behaviors exhibited subunit-specific regulation. GluN2A/B CTD diversification conferred differential binding to cytoplasmic MAGUK proteins and differential forms of long-term potentiation. These data indicate that vertebrate behavior and synaptic signaling acquired increased complexity from the duplication and diversification of ancestral GluN2 genes.

Gap junction-mediated electrical transmission: regulatory mechanisms and plasticity

The term synapse applies to cellular specializations that articulate the processing of information within neural circuits by providing a mechanism for the transfer of information between two different neurons. There are two main modalities of synaptic transmission: chemical and electrical. While most efforts have been dedicated to the understanding of the properties and modifiability of chemical transmission, less is still known regarding the plastic properties of electrical synapses, whose structural correlate is the gap junction. A wealth of data indicates that, rather than passive intercellular channels, electrical synapses are more dynamic and modifiable than was generally perceived. This article will discuss the factors determining the strength of electrical transmission and review current evidence demonstrating its dynamic properties. Like their chemical counterparts, electrical synapses can also be plastic and modifiable. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.

The App-Runx1 region is critical for birth defects and electrocardiographic dysfunctions observed in a Down syndrome mouse model

Down syndrome (DS) leads to complex phenotypes and is the main genetic cause of birth defects and heart diseases. The Ts65Dn DS mouse model is trisomic for the distal part of mouse chromosome 16 and displays similar features with post-natal lethality and cardiovascular defects. In order to better understand these defects, we defined electrocardiogram (ECG) with a precordial set-up, and we found conduction defects and modifications in wave shape, amplitudes, and durations in Ts65Dn mice. By using a genetic approach consisting of crossing Ts65Dn mice with Ms5Yah mice monosomic for the App-Runx1 genetic interval, we showed that the Ts65Dn viability and ECG were improved by this reduction of gene copy number. Whole-genome expression studies confirmed gene dosage effect in Ts65Dn, Ms5Yah, and Ts65Dn/Ms5Yah hearts and showed an overall perturbation of pathways connected to post-natal lethality (Coq7, Dyrk1a, F5, Gabpa, Hmgn1, Pde10a, Morc3, Slc5a3, and Vwf) and heart function (Tfb1m, Adam19, Slc8a1/Ncx1, and Rcan1). In addition cardiac connexins (Cx40, Cx43) and sodium channel sub-units (Scn5a, Scn1b, Scn10a) were found down-regulated in Ts65Dn atria with additional down-regulation of Cx40 in Ts65Dn ventricles and were likely contributing to conduction defects. All these data pinpoint new cardiac phenotypes in the Ts65Dn, mimicking aspects of human DS features and pathways altered in the mouse model. In addition they highlight the role of the App-Runx1 interval, including Sod1 and Tiam1, in the induction of post-natal lethality and of the cardiac conduction defects in Ts65Dn. These results might lead to new therapeutic strategies to improve the care of DS people.

Pathophysiology of Huntington's disease: time-dependent alterations in synaptic and receptor function

Huntington's disease (HD) is a progressive, fatal neurological condition caused by an expansion of CAG (glutamine) repeats in the coding region of the Huntington gene. To date, there is no cure but great strides have been made to understand pathophysiological mechanisms. In particular, genetic animal models of HD have been instrumental in elucidating the progression of behavioral and physiological alterations, which had not been possible using classic neurotoxin models. Our groups have pioneered the use of transgenic HD mice to examine the excitotoxicity hypothesis of striatal neuronal dysfunction and degeneration, as well as alterations in excitation and inhibition in striatum and cerebral cortex. In this review, we focus on synaptic and receptor alterations of striatal medium-sized spiny (MSNs) and cortical pyramidal neurons in genetic HD mouse models. We demonstrate a complex series of alterations that are region-specific and time-dependent. In particular, many changes are bidirectional depending on the degree of disease progression, that is, early vs. late, and also on the region examined. Early synaptic dysfunction is manifested by dysregulated glutamate release in striatum followed by progressive disconnection between cortex and striatum. The differential effects of altered glutamate release on MSNs originating the direct and indirect pathways is also elucidated, with the unexpected finding that cells of the direct striatal pathway are involved early in the course of the disease. In addition, we review evidence for early N-methyl-D-aspartate receptor (NMDAR) dysfunction leading to enhanced sensitivity of extrasynaptic receptors and a critical role of GluN2B subunits. Some of the alterations in late HD could be compensatory mechanisms designed to cope with early synaptic and receptor dysfunctions. The main findings indicate that HD treatments need to be designed according to the stage of disease progression and should consider regional differences.

Glutamate receptor dynamics and protein interaction: lessons from the NMDA receptor

The plasticity of excitatory glutamate synapses emerged over the last decades as a core cellular mechanism for the encoding and processing of various cognitive functions. This property relies in part on the ability to dynamically adjust the content of glutamate receptors in the postsynaptic membrane. Among these receptors, NMDA receptors (NMDAR), which are composed of two obligatory GluN1 and two regulatory GluN2/3 subunits, play a key role in the induction of many forms of plasticity processes. Understanding how NMDAR subtypes are trafficked and regulated in the synapse has thus captured considerable attention. It has emerged that NMDAR synaptic content relies on an equilibrium between intracellular trafficking and rapid lateral diffusion of the receptor within the synaptic area. Here, we review our current understanding of NMDAR trafficking, mostly the ones at the surface membrane, with a specific focus on the role of interacting PDZ-containing proteins during the journey of NMDAR to and around the synaptic area. The cellular and molecular lessons obtained from examining NMDAR dynamics and regulation by interacting proteins appear to apply to other ionotropic neurotransmitter receptors, and thus shed new light on the modulation of excitatory, inhibitory, and modulatory transmission. This article is part of a Special Issue entitled 'Neuronal Function'.

mGluR5 in cortical excitatory neurons exerts both cell-autonomous and -nonautonomous influences on cortical somatosensory circuit formation

Glutamatergic neurotransmission plays important roles in sensory map formation. The absence of the group I metabotropic glutamate receptor 5 (mGluR5) leads to abnormal sensory map formation throughout the mouse somatosensory pathway. To examine the role of cortical mGluR5 expression on barrel map formation, we generated cortex-specific mGluR5 knock-out (KO) mice. Eliminating mGluR5 function solely in cortical excitatory neurons affects, not only the whisker-related organization of cortical neurons (barrels), but also the patterning of their presynaptic partners, the thalamocortical axons (TCAs). In contrast, subcortical whisker maps develop normally in cortical-mGluR5 KO mice. In the S1 cortex of cortical-mGluR5 KO, layer IV neurons are homogenously distributed and have no clear relationship to the location of TCA clusters. The altered dendritic morphology of cortical layer IV spiny stellate neurons in cortical-mGluR5 KO mice argues for a cell-autonomous role of mGluR5 in dendritic patterning. Furthermore, morphometric analysis of single TCAs in both cortical- and global-mGluR5 KO mice demonstrated that in these mice, the complexity of axonal arbors is reduced, while the area covered by TCA arbors is enlarged. Using voltage-clamp whole-cell recordings in acute thalamocortical brain slices, we found that KO of mGluR5 from cortical excitatory neurons reduced inhibitory but not excitatory inputs onto layer IV neurons. This suggests that mGluR5 signaling in cortical excitatory neurons nonautonomously modulates the functional development of GABAergic circuits. Together, our data provide strong evidence that mGluR5 signaling in cortical principal neurons exerts both cell-autonomous and -nonautonomous influences to modulate the formation of cortical sensory circuits.

Immunogold electron microscopic evidence of differential regulation of GluN1, GluN2A, and GluN2B, NMDA-type glutamate receptor subunits in rat hippocampal CA1 synapses during benzodiazepine withdrawal

Benzodiazepine withdrawal-anxiety is associated with enhanced α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR)-mediated glutamatergic transmission in rat hippocampal CA1 synapses due to enhanced synaptic insertion and phosphorylation of GluA1 homomers. Interestingly, attenuation of withdrawal-anxiety is associated with a reduction in N-methyl-D-aspartate receptor (NMDAR)-mediated currents and subunit expression, secondary to AMPA receptor potentiation. Therefore, in this study ultrastructural evidence for possible reductions in NMDAR GluN1, GluN2A, and GluN2B subunits was sought at CA1 stratum radiatum synapses in proximal dendrites using postembedding immunogold labeling of tissues from rats withdrawn for 2 days from 1-week daily oral administration of the benzodiazepine, flurazepam (FZP). GluN1-immunogold density and the percentage of immunopositive synapses were significantly decreased in tissues from FZP-withdrawn rats. Similar decreases were observed for GluN2B subunits; however, the relative lateral distribution of GluN2B-immunolabeling within the postsynaptic density did not change after BZ withdrawal. In contrast to the GluN2B subunit, the percentage of synapses labeled with the GluN2A subunit antibody and the density of immunogold labeling for this subunit was unchanged. The spatial localization of immunogold particles associated with each NMDAR subunit was consistent with a predominantly postsynaptic localization. The data therefore provide direct evidence for reduced synaptic GluN1/GluN2B receptors and preservation of GluN1/GluN2A receptors in the CA1 stratum radiatum region during BZ withdrawal. Based on collective findings in this benzodiazepine withdrawal-anxiety model, we propose a functional model illustrating the changes in glutamate receptor populations at excitatory synapses during benzodiazepine withdrawal.

The kinesin superfamily protein KIF17 is regulated by the same transcription factor (NRF-1) as its cargo NR2B in neurons

The kinesin superfamily of motor proteins is known to be ATP-dependent transporters of various types of cargoes. In neurons, KIF17 is found to transport vesicles containing the N-methyl-D-aspartate receptor NR2B subunit from the cell body specifically to the dendrites. These subunits are intimately associated with glutamatergic neurotransmission as well as with learning and memory. Glutamatergic synapses are highly energy-dependent, and recently we found that the same transcription factor, nuclear respiratory factor 1 (NRF-1), co-regulates energy metabolism (via its regulation of cytochrome c oxidase and other mitochondrial enzymes) and neurochemicals of glutamatergic transmission (NR1, NR2B, GluR2, and nNOS). The present study tested our hypothesis that NRF-1 also transcriptionally regulates KIF17. By means of in silico analysis, electrophoretic mobility shift and supershift assays, in vivo chromatin immunoprecipitation assays, promoter mutations, and real-time quantitative PCR, we found that NRF-1 (but not NRF-2) functionally regulates Kif17, but not Kif1a, gene. NRF-1 binding sites on Kif17 gene are highly conserved among mice, rats, and humans. Silencing of NRF-1 with small interference RNA blocked the up-regulation of Kif17 mRNA and proteins (and of Grin1 and Grin2b) induced by KCl-mediated depolarization, whereas over-expressing NRF-1 rescued these transcripts and proteins from being suppressed by TTX. Thus, NRF-1 co-regulates oxidative enzymes that generate energy and neurochemicals that consume energy related to glutamatergic neurotransmission, such as KIF17, NR1, and NR2B, thereby ensuring that energy production matches energy utilization at the molecular and cellular levels.

Dynamic and specific interaction between synaptic NR2-NMDA receptor and PDZ proteins

The relative content of NR2 subunits in the NMDA receptor confers specific signaling properties and plasticity to synapses. However, the mechanisms that dynamically govern the retention of synaptic NMDARs, in particular 2A-NMDARs, remain poorly understood. Here, we investigate the dynamic interaction between NR2 C termini and proteins containing PSD-95/Discs-large/ZO-1 homology (PDZ) scaffold proteins at the single molecule level by using high-resolution imaging. We report that a biomimetic divalent competing ligand, mimicking the last 15 amino acids of NR2A C terminus, specifically and efficiently disrupts the interaction between 2A-NMDARs, but not 2B-NMDARs, and PDZ proteins on the time scale of minutes. Furthermore, displacing 2A-NMDARs out of synapses lead to a compensatory increase in synaptic NR2B-NMDARs, providing functional evidence that the anchoring mechanism of 2A- or 2B-NMDARs is different. These data reveal an unexpected role of the NR2 subunit divalent arrangement in providing specific anchoring within synapses, highlighting the need to study such dynamic interactions in native conditions.

De novo truncating mutation in Kinesin 17 associated with schizophrenia

BACKGROUND

Schizophrenia (SCZ) is one of the most disabling psychiatric disorders. It is thought to be due to a complex interplay between polygenic and various environmental risk factors, although recent reports on genomic copy number variations suggest that a fraction of the cases could result from variably penetrant de novo variants. The gene encoding the synaptic motor protein kinesin 17 (KIF17) involved in glutamatergic synapse is a candidate gene for SCZ.

METHODS

As part of our Synapse to Disease project, we resequenced KIF17 in a cohort of individuals with sporadic SCZ (188 subjects). Additional populations included autism spectrum disorder (142 subjects), nonsyndromic mental retardation (95 subjects), and control subjects (568 subjects). Functional validation of the human mutation was done in developing zebrafish.

RESULTS

Here we report the identification of a de novo nonsense truncating mutation in one patient with SCZ, in kinesin 17, a synaptic motor protein. No de novo or truncating KIF17 mutations were found in the additional samples. We further validated the pathogenic nature of this mutation by knocking down its expression in zebrafish embryos, which resulted in a developmental defect.

CONCLUSIONS

Together our findings suggest that disruption of KIF17, although rare, could result in a schizophrenia phenotype and emphasize the possible involvement of rare de novo mutations in this disorder.

Variability of distribution of Ca(2+)/calmodulin-dependent kinase II at mixed synapses on the mauthner cell: colocalization and association with connexin 35

In contrast to chemical transmission, few proteins have been shown associated with gap junction-mediated electrical synapses. Mixed (electrical and glutamatergic) synaptic terminals on the teleost Mauthner cell known as "Club endings" constitute because of their unusual large size and presence of connexin 35 (Cx35), an ortholog of the widespread mammalian Cx36, a valuable model for the study of electrical transmission. Remarkably, both components of their mixed synaptic response undergo activity-dependent potentiation. Changes in electrical transmission result from interactions with colocalized glutamatergic synapses, the activity of which leads to the activation of Ca(2+)/calmodulin-dependent kinase II (CaMKII), required for the induction of changes in both forms of transmission. However, the distribution of this kinase and potential localization to electrical synapses remains undetermined. Taking advantage of the unparalleled experimental accessibility of Club endings, we explored the presence and intraterminal distribution of CaMKII within these terminals. Here we show that (1) unlike other proteins, both CaMKII labeling and distribution were highly variable between contiguous contacts, and (2) CaMKII was not restricted to the periphery of the terminals, in which glutamatergic synapses are located, but also was present at the center in which gap junctions predominate. Accordingly, double immunolabeling indicated that Cx35 and CaMKII were colocalized, and biochemical analysis showed that these proteins associate. Because CaMKII characteristically undergoes activity-dependent translocation, the observed variability of labeling likely reflects physiological differences between electrical synapses of contiguous Club endings, which remarkably coexist with differing degrees of conductance. Together, our results indicate that CaMKII should be considered a component of electrical synapses, although its association is nonobligatory and likely driven by activity.

NMDA receptor GluN2B (GluR epsilon 2/NR2B) subunit is crucial for channel function, postsynaptic macromolecular organization, and actin cytoskeleton at hippocampal CA3 synapses

GluN2B (GluRepsilon2/NR2B) subunit is involved in synapse development, synaptic plasticity, and cognitive function. However, its roles in synaptic expression and function of NMDA receptors (NMDARs) in the brain remain mostly unknown because of the neonatal lethality of global knock-out mice. To address this, we generated conditional knock-out mice, in which GluN2B was ablated exclusively in hippocampal CA3 pyramidal cells. By immunohistochemistry, GluN2B disappeared and GluN1 (GluRzeta1/NR1) was moderately reduced, whereas GluN2A (GluRepsilon1/NR2A) and postsynaptic density-95 (PSD-95) were unaltered in the mutant CA3. This was consistent with protein contents in the CA3 crude fraction: 9.6% of control level for GluN2B, 47.7% for GluN1, 90.6% for GluN2A, and 98.0% for PSD-95. Despite the remaining NMDARs, NMDAR-mediated currents and long-term potentiation were virtually lost at various CA3 synapses. Then, we compared synaptic NMDARs by postembedding immunogold electron microscopy and immunoblot using the PSD fraction. In the mutant CA3, GluN1 was severely reduced in both immunogold (20.6-23.6%) and immunoblot (24.6%), whereas GluN2A and PSD-95 were unchanged in immunogold but markedly reduced in the PSD fraction (51.4 and 36.5%, respectively), indicating increased detergent solubility of PSD molecules. No such increased solubility was observed for GluN2B in the CA3 of GluN2A-knock-out mice. Furthermore, significant decreases were found in the ratio of filamentous to globular actin (49.5%) and in the density of dendritic spines (76.2%). These findings suggest that GluN2B is critically involved in NMDAR channel function, organization of postsynaptic macromolecular complexes, formation or maintenance of dendritic spines, and regulation of the actin cytoskeleton.

Interaction between connexin35 and zonula occludens-1 and its potential role in the regulation of electrical synapses

Although regulation of chemical transmission is known to involve the interaction of receptors with scaffold proteins, little is known about the existence of protein-protein interactions in regulating gap junction-mediated electrical synapses. The scaffold protein zonula-occludens-1 (ZO-1), a member of the MAGUK family of proteins, was reported to interact with several connexins (Cxs). We show here that ZO-1 extensively colocalizes with Cx35 at identifiable "mixed" (electrical and chemical) contacts on goldfish Mauthner cells, a model synapse for the study of vertebrate electrical transmission where it is possible to correlate physiological properties with molecular composition. Further, our analysis indicates that these proteins directly interact at goldfish electrical synapses. In contrast to Cx43, which interacts with ZO-1 via the PDZ2 domain, Cx35 interacts with ZO-1 via the PDZ1 domain, and this association is of lower affinity. The properties of the ZO-1/Cx35 association suggest the existence of a more dynamic relation between these two proteins, possibly including a role of ZO-1 in regulating gap junctional conductance at these highly modifiable electrical synapses. The interaction of ZO-1 with conserved regions of the C termini of Cx35/Cx36 orthologs may have a common function at electrical synapses of mammals and other vertebrates.

Surface trafficking of N-methyl-D-aspartate receptors: physiological and pathological perspectives

The N-methyl-D-aspartate receptor (NMDAR) plays a crucial role in shaping the strength of synaptic connections. Over the last decades, extensive studies have defined the cellular and molecular mechanisms by which synaptic NMDARs control the maturation and plasticity of synaptic transmission, and how altered synaptic NMDAR signaling is implicated in neurodegenerative and psychiatric disorders. It is now clear that activation of synaptic or extrasynaptic NMDARs produces different signaling cascades and thus neuronal functions. Our current understanding of NMDAR surface distribution and trafficking is only emerging. Exchange of NMDARs between synaptic and extrasynaptic areas through surface diffusion is a highly dynamic and regulated process. The aim of this review is to describe the identified mechanisms that regulate surface NMDAR behaviors and discuss the impact of this new trafficking pathway on the well-established NMDAR-dependent physiological and pathophysiological processes.

Glutamate receptor dynamics in dendritic microdomains

Among diverse factors regulating excitatory synaptic transmission, the abundance of postsynaptic glutamate receptors figures prominently in molecular memory and learning-related synaptic plasticity. To allow for both long-term maintenance of synaptic transmission and acute changes in synaptic strength, the relative rates of glutamate receptor insertion and removal must be tightly regulated. Interactions with scaffolding proteins control the targeting and signaling properties of glutamate receptors within the postsynaptic membrane. In addition, extrasynaptic receptor populations control the equilibrium of receptor exchange at synapses and activate distinct signaling pathways involved in plasticity. Here, we review recent findings that have shaped our current understanding of receptor mobility between synaptic and extrasynaptic compartments at glutamatergic synapses, focusing on AMPA and NMDA receptors. We also examine the cooperative relationship between intracellular trafficking and surface diffusion of glutamate receptors that underlies the expression of learning-related synaptic plasticity.

Post-synaptic density-93 mediates tyrosine-phosphorylation of the N-methyl-D-aspartate receptors

Src family protein kinases (SFKs) -mediated tyrosine-phosphorylation regulates N-methyl-D-aspartate (NMDA) receptor synaptic function. Some members of the membrane-associated guanylate kinase (MAGUK) family of proteins bind to both SFKs and NMDA receptors, but it is unclear whether the MAGUK family of proteins is required for SFKs-mediated tyrosine-phosphorylation of the NMDA receptors. Here, we showed by co-immunoprecipitation that post-synaptic density (PSD) -93, a member of the MAGUK family of proteins, interacts with the NMDA receptor subunits NR2A and NR2B as well as with Fyn, a member of the SFKs, in mouse cerebral cortex. Using a biochemical fractionation approach to isolate subcellular compartments revealed that the expression of Fyn, but not of other members of the SFKs (Lyn, Src, and Yes), was significantly decreased in synaptosomal membrane fractions derived from the cerebral cortex of PSD-93 knockout mice. Interestingly, we found that PSD-93 disruption causes reduction of tyrosine-phosphorylated NR2A and NR2B in the same fraction. Moreover, PSD-93 deletion markedly blocked the SFKs-mediated increase in tyrosine-phosphorylated NR2A and NR2B through the protein kinase C pathway after induction with 4-phorbol 12-myristate 13-acetate in cultured cortical neurons. Our findings indicate that PSD-93 appears to mediate tyrosine-phosphorylation of the NMDA receptors and synaptic localization of Fyn.

Excitotoxicity and focal cerebral ischemia induce truncation of the NR2A and NR2B subunits of the NMDA receptor and cleavage of the scaffolding protein PSD-95

The N-methyl-D-aspartate receptor (NMDAR) is central to physiological and pathological functioning of neurons. Although promising results are beginning to be obtained in the treatment of dementias, clinical trials with NMDAR antagonists for stroke, trauma and neurodegenerative disorders, such as Hungtinton's disease, have failed before. In order to design effective therapies to prevent excitotoxic neuronal death, it is critical to characterize the consequences of excessive NMDAR activation on its expression and function. Previous data have reported partial downregulation of the NR1 and NR2B receptor subunits in response to excitotoxicity and cerebral ischemia. However, the effect of NMDAR overactivation on NR2A, a subunit fundamental to synaptic transmission and neuronal survival, is still elusive. In this study, we report the rapid and extensive proteolytic processing of NR2A, together with the scaffolding protein postsynaptic density-95 (PSD-95), induced by excitotoxic stimulation of cortical neurons in vitro and by transient focal cerebral ischemia. Processing of the C terminus of NR2A is irreversibly induced by brief agonist exposure of NR2B-containing receptors, and requires calcium influx and the activity of calpain, also responsible for PSD-95 cleavage. The outcome is a truncated NR2A subunit that is stable and capable to interact with NR1 at the surface of neurons, but lacking the structural domains required for association with scaffolding, downstream signaling and cytoskeletal proteins. Therefore, a rapid and significant uncoupling of synaptic NMDARs from downstream survival pathways is expected to occur during ischemia. This novel mechanism induced by excitotoxicity helps to explain the failure of most therapies based on NMDAR antagonists.

Regulation of long-term depression and climbing fiber territory by glutamate receptor delta2 at parallel fiber synapses through its C-terminal domain in cerebellar Purkinje cells

Glutamate receptor (GluR) delta2 selectively expressed in cerebellar Purkinje cells (PCs) plays key roles in long-term depression (LTD) induction at parallel fiber (PF)-PC synapses, motor learning, the matching and connection of PF-PC synapses in developing and adult cerebella, the elimination of multiple climbing fibers (CFs) during development, and the regulation of CF territory on PCs. However, it remains unsolved how GluRdelta2 regulates cerebellar synaptic plasticity, PF-PC synapse formation, and CF wiring. One possible signaling mechanism through GluRdelta2 is signaling by protein-protein interactions. The C-terminal region of GluRdelta2 contains at least three domains for protein-protein interactions. The PDZ (postsynaptic density-95/Discs large/zona occludens 1)-binding domain at the C terminal, named as the T site, interacts with several postsynaptic density proteins. Here, we generated GluRdelta2DeltaT mice carrying mutant GluRdelta2 lacking the T site. There were no significant differences in the amount of receptor proteins at synapses, histological features, and the fine structures of PF-PC synapses between wild-type and GluRdelta2DeltaT mice. However, LTD induction at PF-PC synapses and improvement in the accelerating rotarod test were impaired in GluRdelta2DeltaT mice. Furthermore, CF territory expanded distally and ectopic innervation of CFs occurred at distal dendrites in GluRdelta2DeltaT mice, but the elimination of surplus CF innervation at proximal dendrites appeared to proceed normally. These results suggest that the C-terminal T site of GluRdelta2 is essential for LTD induction and the regulation of CF territory but is dispensable for PF-PC synapse formation and the elimination of surplus CFs at proximal dendrites during development.

Hepatocyte growth factor improves synaptic localization of the NMDA receptor and intracellular signaling after excitotoxic injury in cultured hippocampal neurons

To examine the effects of HGF on synaptic densities under excitotoxic conditions, we investigated changes in the number of puncta detected by double immunostaining with NMDA receptor subunits and presynaptic markers in cultured hippocampal neurons. Exposure of hippocampal neurons to excitotoxic NMDA (100 muM) decreased the synaptic localization of NMDA receptor subunit NR2B, whereas synaptic NR1 and NR2A clusters were not altered. Colocalization of PSD-95, a scaffolding protein of the receptor, with the presynaptic protein synapsin I was also decreased after excitotoxicity. Treatment with HGF attenuated these decreases in number. The decrease in the levels of surface NR2B subunits following the addition of the excitotoxic NMDA was also attenuated by the HGF treatment. The decrease in CREB phosphorylation in response to depolarization-evoked NMDA receptor activation was prevented by the HGF treatment. These results suggest that HGF not only prevented neuronal cell death but also attenuated the decrease in synaptic localization of NMDA receptor subunits and prevented intracellular signaling through the NMDA receptor.

alpha-Isoform of calcium-calmodulin-dependent protein kinase II and postsynaptic density protein 95 differentially regulate synaptic expression of NR2A- and NR2B-containing N-methyl-d-aspartate receptors in hippocampus

N-methyl-d-aspartate receptors (NMDARs) are critical determinants of bidirectional synaptic plasticity, however, studies of NMDAR function have been based primarily on pharmacological and electrophysiological manipulations, and it is still debated whether there are subunit-selective forms of long-term potentiation (LTP) and long-term depression (LTD). Here we provide ultrastructural analyses of axospinous synapses in cornu ammonis field 1 of hippocampus (CA1) stratum radiatum of transgenic mice with mutations to two key underlying postsynaptic density (PSD) proteins, postsynaptic density protein 95 (PSD-95) and the alpha-isoform of calcium-calmodulin-dependent protein kinase II (alphaCaMKII). Distribution profiles of synaptic proteins in these mice reveal very different patterns of subunit-specific NMDAR localization, which may be related to the divergent phenotypes of the two mutants. In PSD-95, Dlg, ZO-1/Dlg-homologous region (PDZ) 3-truncated mutant mice in which LTD could not be induced but LTP was found to be enhanced, we found a subtle, yet preferential displacement of synaptic N-methyl-d-aspartate receptor subunit 2B (NR2B) subunits in lateral regions of the synapse without affecting changes in the localization of N-methyl-d-aspartate receptor subunit 2A (NR2A) subunits. In persistent inhibitory alphaCaMKII Thr305 substituted with Asp in alpha-isoform of calcium-calmodulin kinase II (T305D) mutant mice with severely impaired LTP but stable LTD expression, we found a selective reduction of NR2A subunits at both the synapse and throughout the cytoplasm of the spine without any effect on the NR2B subunit. In an experiment of mutual exclusivity, neither PSD-95 nor alphaCaMKII localization was found to be affected by mutations to the corresponding PSD protein suggesting that they are functionally independent of the other in the regulation of NR2A- and NR2B-containing NMDARs preceding synaptic activity. Consequently, there may exist at least two distinct PSD-95 and alphaCaMKII-specific NMDAR complexes involved in mediating LTP and LTD through opposing signal transduction pathways in synapses of the hippocampus. The contrasting phenotypes of the PSD-95 and alphaCaMKII mutant mice further establish the prospect of an independent and, possibly, competing mechanism for the regulation of NMDAR-dependent bidirectional synaptic plasticity.

Takusan: a large gene family that regulates synaptic activity

We have characterized a rodent-specific gene family designated alpha-takusan (meaning "many" in Japanese). We initially identified a member of the family whose expression is upregulated in mice lacking the NMDAR subunit NR3A. We then isolated cDNAs encoding 46 alpha-takusan variants from mouse brains. Most variants share an approximately 130 aa long sequence, which contains the previously identified domain of unknown function 622 (DUF622) and is predicted to form coiled-coil structures. Single-cell PCR analyses indicate that one neuron can express multiple alpha-takusan variants and particular variants may predominate in certain cell types. Forced expression in cultured hippocampal neurons of two variants, alpha1 or alpha2, which bind either directly or indirectly to PSD-95, leads to an increase in PSD-95 clustering, dendritic spine density, GluR1 surface expression, and AMPAR activity. Conversely, treating cultured neurons with RNAi targeting alpha-takusan variants resulted in the opposite phenotype. Hence, alpha-takusan represents a large gene family that regulates synaptic activity.

NMDA receptor trafficking in synaptic plasticity and neuropsychiatric disorders

The number and subunit composition of synaptic N-methyl-D-aspartate receptors (NMDARs) are not static, but change in a cell- and synapse-specific manner during development and in response to neuronal activity and sensory experience. Neuronal activity drives not only NMDAR synaptic targeting and incorporation, but also receptor retrieval, differential sorting into the endosomal-lysosomal pathway and lateral diffusion between synaptic and extrasynaptic sites. An emerging concept is that activity-dependent, bidirectional regulation of NMDAR trafficking provides a dynamic and potentially powerful mechanism for the regulation of synaptic efficacy and remodelling, which, if dysregulated, can contribute to neuropsychiatric disorders such as cocaine addiction, Alzheimer's disease and schizophrenia.

Increased thalamocortical synaptic response and decreased layer IV innervation in GAP-43 knockout mice

The growth-associated protein, GAP-43, is an axonally localized neuronal protein with high expression in the developing brain and in regenerating neurites. Mice that lack GAP-43 (GAP-43 -/-) fail to form a whisker-related barrel map. In this study, we use GAP-43 -/- mice to examine GAP-43 synaptic function in the context of thalamocortical synapse development and cortical barrel map formation. Examination of thalamocortical synaptic currents in an acute brain slice preparation and in autaptic thalamic neurons reveals that GAP-43 -/- synapses have larger alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate receptor (AMPAR)-mediated currents than controls despite similar AMPAR function and normal probability of vesicular release. Interestingly, GAP-43 -/- synapses are less sensitive to blockade by a competitive glutamate receptor antagonist, suggesting higher levels of neurotransmitter in the cleft during synaptic transmission. Field excitatory postsynaptic potentials (EPSPs) from GAP-43 -/- thalamocortical synapses reveal a reduced fiber response, and anatomical analysis shows reduced thalamic innervation of barrel cortex in GAP-43 -/- mice. Despite this fact synaptic responses in the field EPSPs are similar in GAP-43 -/- mice and wild-type littermate controls, and the ratio of AMPAR-mediated to N-methyl-d-aspartate receptor (NMDAR)-mediated currents (AMPAR:NMDAR ratio) is larger than normal. This suggests that GAP-43 -/- mice form fewer thalamocortical synapses in layer IV because of decreased anatomical innervation of the cortex, but the remaining contacts are individually stronger possibly due to increased neurotransmitter concentration in the synaptic cleft. Together, these results indicate that in addition to its well known role in axonal pathfinding GAP-43 plays a functional role in regulating neurotransmitter release.

Excitotoxicity in vitro by NR2A- and NR2B-containing NMDA receptors

Excitotoxicity, exacerbating acute brain damage from brain trauma or stroke, is mediated in part by excessive Ca(2+)-influx from prolonged NMDA receptor activation. However, the contribution to excitotoxicity by each of the main NMDAR subtypes in glutamatergic forebrain neurons, the NR2A- and NR2B-types, has remained enigmatic. Here, we investigated this issue by use of pharmacological and genetic tools in cultured cortical neurons. In wild-type neurons the contribution of the NMDA receptor subtypes to excitotoxicity changed with the age of the cultures. The blockade of NR2B-containing NMDA receptors prevented NMDA-mediated toxicity in young cultures after 14days in vitro (DIV14), but both subtypes triggered excitotoxicity in older (DIV21) cultures. Notably, blocking either of the two subtypes failed to prevent NMDA-elicited cell death, indicating that the remaining subtype triggers cell demise. Intriguingly, a neuroprotective aspect of the NR2A subtype became apparent at submaximal NMDA concentration only at DIV21. The NR2A subtype mediated NMDA toxicity as well as partial protection only if it carried a functional C-terminal domain. Upon deletion of this domain in the NR2A subtype, excitotoxicity was mediated entirely via the NR2B subtype, both at DIV14 and DIV21. Our findings predict that successful therapeutic intervention in stroke based on currently available NMDA receptor subtype-selective blockers is unlikely.

Activity dependent localization of synaptic NMDA receptors in spinal neurons

In cultured spinal neurons, NMDA receptors are absent from excitatory synapses under basal conditions, but can be made to appear at excitatory synapses following blockade of excitatory synaptic activity. The activity dependent synaptic localization of NMDA receptors is critically dependent on both the gradual, global accumulation of the NR2A and NR2B subunits and on a rapid, surface redistribution phase that is primed by the accumulation of NR2A and NR2B and inhibited by synaptic activity. Global changes in NR2A and NR2B accumulation and heterogeneous increases in synaptic NMDA receptor localization can also result from inhibitors of proteasomal processing, from manipulations of proteasomal subunit composition and from media conditioned by neurons undergoing synaptic scaling. While proteasomal processing is a mechanism shared with AMPA receptor scaling in cultured spinal neurons, diffusible factors, heterogeneity, and a rapid surface redistribution phase appear to be unique to activity dependent synaptic NMDA receptor localization.

Brain-derived neurotrophic factor rapidly increases NMDA receptor channel activity through Fyn-mediated phosphorylation

Brain-derived neurotrophic factor (BDNF) is a potent modulator of hippocampal synaptic plasticity. Previously, we found that one of the targets of BDNF modulation is NR2B-containing NMDA receptors. Furthermore, exposure to the trophin rapidly increases NMDA receptor activity and enhances tyrosine phosphorylation of NR2B in cortical and hippocampal postsynaptic densities (PSDs), potentially linking receptor phosphorylation to synaptic plasticity. To define the specific NR2B residue(s) regulated by BDNF, we focused on tyrosine 1472, phosphorylation of which increases after LTP. BDNF rapidly increased phosphorylation in cortical PSDs. The tyrosine kinase Fyn is critical since BDNF-dependent phosphorylation was abolished in Fyn knockout mice. Single-channel patch clamp recordings showed that Fyn is required for the increase in NMDA receptor activity elicited by BDNF. Collectively, our results suggest that BDNF enhances phosphorylation of NR2B tyrosine 1472 through activation of Fyn, leading to alteration of NMDA receptor activity and increased synaptic transmission.

NR2B tyrosine phosphorylation modulates fear learning as well as amygdaloid synaptic plasticity

Phosphorylation of neural proteins in response to a diverse array of external stimuli is one of the main mechanisms underlying dynamic changes in neural circuitry. The NR2B subunit of the NMDA receptor is tyrosine-phosphorylated in the brain, with Tyr-1472 its major phosphorylation site. Here, we generate mice with a knockin mutation of the Tyr-1472 site to phenylalanine (Y1472F) and show that Tyr-1472 phosphorylation is essential for fear learning and amygdaloid synaptic plasticity. The knockin mice show impaired fear-related learning and reduced amygdaloid long-term potentiation. NMDA receptor-mediated CaMKII signaling is impaired in YF/YF mice. Electron microscopic analyses reveal that the Y1472F mutant of the NR2B subunit shows improper localization at synapses in the amygdala. We thus identify Tyr-1472 phosphorylation as a key mediator of fear learning and amygdaloid synaptic plasticity.

Endocytosis and synaptic removal of NR3A-containing NMDA receptors by PACSIN1/syndapin1

A key step in glutamatergic synapse maturation is the replacement of developmentally expressed N-methyl-D-aspartate receptors (NMDARs) with mature forms that differ in subunit composition, electrophysiological properties and propensity to elicit synaptic plasticity. However, the mechanisms underlying the removal and replacement of synaptic NMDARs are poorly understood. Here we demonstrate that NMDARs containing the developmentally regulated NR3A subunit undergo rapid endocytosis from the dendritic plasma membrane in cultured rat hippocampal neurons. This endocytic removal is regulated by PACSIN1/syndapin1, which directly and selectively binds the carboxy-terminal domain of NR3A through its NPF motifs and assembles a complex of proteins including dynamin and clathrin. Endocytosis of NR3A by PACSIN1 is activity dependent, and disruption of PACSIN1 function causes NR3A accumulation at synaptic sites. Our results reveal a new activity-dependent mechanism involved in the regulation of NMDAR expression at synapses during development, and identify a brain-specific endocytic adaptor that confers spatiotemporal and subunit specificity to NMDAR endocytosis.

Postsynaptic receptor mechanisms underlying developmental speeding of synaptic transmission

As animals mature the decay of postsynaptic currents become faster at a variety of synapses. This change is thought to contribute to a refinement of motor co-ordination and to an increase in the precision of sensory perception and cognition. At cholinergic neuromuscular synapses and glycinergic and GABAergic inhibitory synapses, the developmental speeding of synaptic currents depends upon switches of receptor subunits and an ensuing acceleration in the kinetics of channel gating. At glutamatergic excitatory synapses, speeding in the decay time of NMDA receptor (NMDAR)-mediated excitatory postsynaptic currents (NMDA-EPSCs) is also dependent on developmental switches in NMDAR subunits. However, developmental speeding in the kinetics of AMPA receptor (AMPAR)-mediated EPSCs (AMPA-EPSCs) is caused by multiple factors. The decay time of AMPA-EPSCs can be shaped by the kinetics of channel gating or desensitization of AMPA receptors, depending upon the speed of transmitter clearance from the synaptic cleft. During postnatal development AMPAR channel gating and desensitization as well as the transmitter clearance speed up in kinetics. Given that the developmental speeding of synaptic currents play critical roles in the maturation of sensory and motor functions, any defect in this mechanism may seriously affect neuronal function.