Turnover analysis of glutamate receptors identifies a rapidly degraded pool of the N-methyl-D-aspartate receptor subunit, NR1, in cultured cerebellar granule cells

NMDARs in Alzheimer's Disease: Between Synaptic and Extrasynaptic Membranes

N-methyl-D-aspartate receptors (NMDARs) are glutamate receptors with key roles in synaptic communication and plasticity. The activation of synaptic NMDARs initiates plasticity and stimulates cell survival. In contrast, the activation of extrasynaptic NMDARs can promote cell death underlying a potential mechanism of neurodegeneration occurring in Alzheimer's disease (AD). The distribution of synaptic versus extrasynaptic NMDARs has emerged as an important parameter contributing to neuronal dysfunction in neurodegenerative diseases including AD. Here, we review the concept of extrasynaptic NMDARs, as this population is present in numerous neuronal cell membranes but also in the membranes of various non-neuronal cells. Previous evidence regarding the membranal distribution of synaptic versus extrasynaptic NMDRs in relation to AD mice models and in the brains of AD patients will also be reviewed.

Quantitative analysis of NMDA receptor subunits proteins in mouse brain

NMDA-type glutamate receptors (NMDARs) are tetrameric channel complex composed of two subunits of GluN1, which is encoded by a single gene and diversified by alternative splicing, and two subunits from four subtypes of GluN2, leading to various combinations of subunits and channel specificities. However, there is no comprehensive quantitative analysis of GluN subunit proteins for relative comparison, and their compositional ratios at various regions and developmental stages have not been clarified. Here we prepared six chimeric subunits, by fusing an N-terminal side of the GluA1 subunit with a C-terminal side of each of two splicing isoforms of GluN1 subunit and four GluN2 subunits, with which titers of respective NMDAR subunit antibodies could be standardized using common GluA1 antibody, thus enabling quantification of relative protein levels of each NMDAR subunit by western blotting. We determined relative protein amounts of NMDAR subunits in crude, membrane (P2) and microsomal fractions prepared from the cerebral cortex, hippocampus and cerebellum in adult mice. We also examined amount changes in the three brain regions during developmental stages. Their relative amounts in the cortical crude fraction were almost parallel to those of mRNA expression, except for some subunits. Interestingly, a considerable amount of GluN2D protein existed in adult brains, although its transcription level declines after early postnatal stages. GluN1 was larger in quantity than GluN2 in the crude fraction, whereas GluN2 increased in the membrane component-enriched P2 fraction, except in the cerebellum. These data will provide the basic spatio-temporal information on the amount and composition of NMDARs.

Age-dependent NMDA receptor function is regulated by the amyloid precursor protein

N-methyl-D-aspartate receptors (NMDARs) are critical for the maturation and plasticity of glutamatergic synapses. In the hippocampus, NMDARs mainly contain GluN2A and/or GluN2B regulatory subunits. The amyloid precursor protein (APP) has emerged as a putative regulator of NMDARs, but the impact of this interaction to their function is largely unknown. By combining patch-clamp electrophysiology and molecular approaches, we unravel a dual mechanism by which APP controls GluN2B-NMDARs, depending on the life stage. We show that APP is highly abundant specifically at the postnatal postsynapse. It interacts with GluN2B-NMDARs, controlling its synaptic content and mediated currents, both in infant mice and primary neuronal cultures. Upon aging, the APP amyloidogenic-derived C-terminal fragments, rather than APP full-length, contribute to aberrant GluN2B-NMDAR currents. Accordingly, we found that the APP processing is increased upon aging, both in mice and human brain. Interfering with stability or production of the APP intracellular domain normalized the GluN2B-NMDARs currents. While the first mechanism might be essential for synaptic maturation during development, the latter could contribute to age-related synaptic impairments.

Protein quality control of N-methyl-D-aspartate receptors

N-methyl-D-aspartate receptors (NMDARs) are glutamate-gated cation channels that mediate excitatory neurotransmission and are critical for synaptic development and plasticity in the mammalian central nervous system (CNS). Functional NMDARs typically form via the heterotetrameric assembly of GluN1 and GluN2 subunits. Variants within GRIN genes are implicated in various neurodevelopmental and neuropsychiatric disorders. Due to the significance of NMDAR subunit composition for regional and developmental signaling at synapses, properly folded receptors must reach the plasma membrane for their function. This review focuses on the protein quality control of NMDARs. Specifically, we review the quality control mechanisms that ensure receptors are correctly folded and assembled within the endoplasmic reticulum (ER) and trafficked to the plasma membrane. Further, we discuss disease-associated variants that have shown disrupted NMDAR surface expression and function. Finally, we discuss potential targeted pharmacological and therapeutic approaches to ameliorate disease phenotypes by enhancing the expression and surface trafficking of subunits harboring disease-associated variants, thereby increasing their incorporation into functional receptors.

SAPAP3 regulates epileptic seizures involving GluN2A in post-synaptic densities

Aberrantly synchronized neuronal discharges in the brain lead to epilepsy, a devastating neurological disease whose pathogenesis and mechanism are unclear. SAPAP3, a cytoskeletal protein expressed at high levels in the postsynaptic density (PSD) of excitatory synapses, has been well studied in the striatum, but the role of SAPAP3 in epilepsy remains elusive. In this study, we sought to investigate the molecular, cellular, electrophysiological and behavioral consequences of SAPAP3 perturbations in the mouse hippocampus. We identified a significant increase in the SAPAP3 levels in patients with temporal lobe epilepsy (TLE) and in mouse models of epilepsy. In addition, behavioral studies showed that the downregulation of SAPAP3 by shRNA decreased the seizure severity and that the overexpression of SAPAP3 by recombinant SAPAP3 yielded the opposite effect. Moreover, SAPAP3 affected action potentials (APs), miniature excitatory postsynaptic currents (mEPSCs) and N-methyl-D-aspartate receptor (NMDAR)-mediated currents in the CA1 region, which indicated that SAPAP3 plays an important role in excitatory synaptic transmission. Additionally, the levels of the GluN2A protein, which is involved in synaptic function, were perturbed in the hippocampal PSD, and this perturbation was accompanied by ultrastructural morphological changes. These results revealed a previously unknown function of SAPAP3 in epileptogenesis and showed that SAPAP3 may represent a novel target for the treatment of epilepsy.

The Extracellular Domains of GluN Subunits Play an Essential Role in Processing NMDA Receptors in the ER

N-methyl-D-aspartate receptors (NMDARs) belong to a family of ionotropic glutamate receptors that play essential roles in excitatory neurotransmission and synaptic plasticity in the mammalian central nervous system (CNS). Functional NMDARs consist of heterotetramers comprised of GluN1, GluN2A-D, and/or GluN3A-B subunits, each of which contains four membrane domains (M1 through M4), an intracellular C-terminal domain, a large extracellular N-terminal domain composed of the amino-terminal domain and the S1 segment of the ligand-binding domain (LBD), and an extracellular loop between M3 and M4, which contains the S2 segment of the LBD. Both the number and type of NMDARs expressed at the cell surface are regulated at several levels, including their translation and posttranslational maturation in the endoplasmic reticulum (ER), intracellular trafficking via the Golgi apparatus, lateral diffusion in the plasma membrane, and internalization and degradation. This review focuses on the roles played by the extracellular regions of GluN subunits in ER processing. Specifically, we discuss the presence of ER retention signals, the integrity of the LBD, and critical N-glycosylated sites and disulfide bridges within the NMDAR subunits, each of these steps must pass quality control in the ER in order to ensure that only correctly assembled NMDARs are released from the ER for subsequent processing and trafficking to the surface. Finally, we discuss the effect of pathogenic missense mutations within the extracellular domains of GluN subunits with respect to ER processing of NMDARs.

AMPA receptor contribution to methylmercury-mediated alteration of intracellular Ca2+ concentration in human induced pluripotent stem cell motor neurons

α motor neurons (MNs) are a target of the environmental neurotoxicant methylmercury (MeHg), accumulating MeHg and subsequently degenerating. In mouse spinal cord MN cultures, MeHg increased intracellular Ca²⁺ [Ca²⁺]_i; the AMPA receptor (AMPAR) antagonist CNQX delayed the increase in [Ca²⁺]_i, implicating the role of AMPARs in this response. Here we used human induced pluripotent stem cell-derived MNs (hiPSC-MNs), to characterize the role of MN AMPARs in MeHg neurotoxicity. Acute exposure to MeHg (0.1, 0.2, 0.5, 1 and 1.5 μM), fura-2 microfluorimetry, and a standard cytotoxicity assay, were used to examine MN regulation of [Ca²⁺]_i, and cytotoxicity, respectively. Contribution of Ca²⁺-permeable and impermeable AMPARs was compared using either CNQX, or the Ca²⁺-permeable AMPAR antagonist N-acetyl spermine (NAS). MeHg-induced cytotoxicity was evaluated following a 24 h delay subsequent to 1 h exposure of hiPSC-MNs. MeHg caused a characteristic biphasic increase in [Ca²⁺]_i, the onset of which was concentration-dependent; higher MeHg concentrations hastened onset of both phases. CNQX significantly delayed MeHg's effect on onset time of both phases. In contrast, NAS significantly delayed only the 2nd phase increase in fura-2 fluorescence. Exposure to MeHg for 1 h followed by a 24 h recovery period caused a concentration-dependent incidence of cell death. These results demonstrate for the first time that hiPSC-derived MNs are highly sensitive to effects of MeHg on [Ca²⁺]_i, and cytotoxicity, and that both Ca²⁺-permeable and impermeable AMPARs contribute the elevations in [Ca²⁺]_i.

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.

SAP97-mediated rescue of NMDA receptor surface distribution in a neuronal model of Huntington's disease

Huntington's disease (HD) is a genetic neurodegenerative disorder caused by an expansion of the CAG repeat tract in the HTT gene, leading to motor, cognitive, and psychiatric symptoms. At the cellular level, NMDA-type glutamate receptors are upregulated at glutamatergic extrasynaptic sites in HD, triggering cell death signaling pathways and driving HD neurodegeneration. Extrasynaptic and synaptic glutamate receptor trafficking and surface distribution are regulated by the α and β N-terminal isoforms of SAP97, a postsynaptic density protein localized at glutamatergic synapses. Here we examined the surface distribution of NMDARs and AMPARs in a cellular model of HD, and whether the manipulation of individual SAP97 isoforms can regulate receptor distribution in diseased neurons. Using dSTORM super-resolution imaging, we reveal that mutant HTT drives the elevation of extrasynaptic NMDAR clusters located 100-500 nm from the postsynaptic density. This was accompanied by a decline in synaptic NMDAR-mediated currents while surface NMDAR-mediated currents remained unchanged. These effects were induced within 3 days of mutant HTT expression in rat hippocampal neurons in vitro, and were specific for NMDARs and not observed with AMPARs. Intriguingly, upregulation of either α- or βSAP97 expression increased synaptic and/or perisynaptic NMDAR localization and prevented the shift of NMDARs to extrasynaptic sites in mutant HTT neurons. This was accompanied by the rescue of normal synaptic NMDAR-mediated currents. Taken together, our high-resolution data reveals plasticity in surface NMDAR localization driven by mutant HTT and identifies the similar but independent roles of SAP97 N-terminal isoforms in maintaining normal synaptic function in pathological states.

Structural features in the glycine-binding sites of the GluN1 and GluN3A subunits regulate the surface delivery of NMDA receptors

N-methyl-D-aspartate receptors (NMDARs) are ionotropic glutamate receptors that play an essential role in mediating excitatory neurotransmission in the mammalian central nervous system (CNS). Functional NMDARs are tetramers composed of GluN1, GluN2A-D, and/or GluN3A-B subunits, giving rise to a wide variety of NMDAR subtypes with unique functional properties. Here, we examined the surface delivery and functional properties of NMDARs containing mutations in the glycine-binding sites in GluN1 and GluN3A subunits expressed in mammalian cell lines and primary rat hippocampal neurons. We found that the structural features of the glycine-binding sites in both GluN1 and GluN3A subunits are correlated with receptor forward trafficking to the cell surface. In addition, we found that a potentially clinically relevant mutation in the glycine-binding site of the human GluN3A subunit significantly reduces surface delivery of NMDARs. Taken together, these findings provide novel insight into how NMDARs are regulated by their glycine-binding sites and may provide important information regarding the role of NMDARs in both physiological and pathophysiological processes in the mammalian CNS.

Lectins modulate the functional properties of GluN1/GluN3-containing NMDA receptors

N-methyl-d-aspartate receptors (NMDARs) play an essential role in excitatory neurotransmission within the mammalian central nervous system (CNS). NMDARs are heteromultimers containing GluN1, GluN2, and/or GluN3 subunits, thus giving rise to a wide variety of subunit combinations, each with unique functional and pharmacological properties. Importantly, GluN1/GluN3A and GluN1/GluN3B receptors form glycine-gated receptors. Here, we combined electrophysiology with rapid solution exchange in order to determine whether the presence of specific N-glycans and/or interactions with specific lectins regulates the functional properties of GluN1/GluN3A and GluN1/GluN3B receptors expressed in human embryonic kidney 293 (HEK293) cells. We found that removing putative N-glycosylation sites alters the functional properties of GluN1/GluN3B receptors, but has no effect on GluN1/GluN3A receptors. Moreover, we found that the functional properties of both GluN1/GluN3A and GluN1/GluN3B receptors are modulated by a variety of lectins, including Concanavalin A (ConA), Wheat Germ Agglutinin (WGA), and Aleuria Aurantia Lectin (AAL), and this effect is likely mediated by a reduction in GluN1 subunit-mediated desensitization. We also found that AAL has the most profound effect on GluN1/GluN3 receptors, and this effect is mediated partly by a single N-glycosylation site on the GluN3 subunit (specifically, N565 on GluN3A and N465 on GluN3B). Finally, we found that lectins mediate their effect only when applied to non-activated receptors and have no effect when applied in the continuous presence of glycine. These findings provide further evidence to distinguish GluN1/GluN3 receptors from the canonical GluN1/GluN2 receptors and offer insight into how GluN1/GluN3 receptors may be regulated in the mammalian CNS.

Altered Regulation of Striatal Neuronal N-Methyl-D-Aspartate Receptor Trafficking by Palmitoylation in Huntington Disease Mouse Model

N-methyl-D-aspartate receptors (NMDARs) play a critical role in synaptic signaling, and alterations in the synaptic/extrasynaptic NMDAR balance affect neuronal survival. Studies have shown enhanced extrasynaptic GluN2B-type NMDAR (2B-NMDAR) activity in striatal neurons in the YAC128 mouse model of Huntington disease (HD), resulting in increased cell death pathway activation contributing to striatal vulnerability to degeneration. However, the mechanism(s) of altered GluN2B trafficking remains unclear. Previous work shows that GluN2B palmitoylation on two C-terminal cysteine clusters regulates 2B-NMDAR trafficking to the surface membrane and synapses in cortical neurons. Notably, two palmitoyl acyltransferases (PATs), zDHHC17 and zDHHC13, also called huntingtin-interacting protein 14 (HIP14) and HIP14-like (HIP14L), directly interact with the huntingtin protein (Htt), and mutant Htt disrupts this interaction. Here, we investigated whether GluN2B palmitoylation is involved in enhanced extrasynaptic surface expression of 2B-NMDARs in YAC128 striatal neurons and whether this process is regulated by HIP14 or HIP14L. We found reduced GluN2B palmitoylation in YAC128 striatum, specifically on cysteine cluster II. Consistent with that finding, the palmitoylation-deficient GluN2B Cysteine cluster II mutant exhibited enhanced, extrasynaptic surface expression in striatal neurons from wild-type mice, mimicking increased extrasynaptic 2B-NMDAR observed in YAC128 cultures. We also found that HIP14L palmitoylated GluN2B cysteine cluster II. Moreover, GluN2B palmitoylation levels were reduced in striatal tissue from HIP14L-deficient mice, and siRNA-mediated HIP14L knockdown in cultured neurons enhanced striatal neuronal GluN2B surface expression and susceptibility to NMDA toxicity. Thus, altered regulation of GluN2B palmitoylation levels by the huntingtin-associated PAT HIP14L may contribute to the cell death-signaling pathways underlying HD.

Cell-Based Enzyme-Linked Immunosorbent Assay (Cell-ELISA) Analysis of Native and Recombinant Glutamate Receptors

Glutamate receptors (GluRs) located primarily on the membranes of neurons and glial cells are responsible for excitatory synaptic transmission in the central nervous system. The transport of GluRs to the cell surface is a highly regulated dynamic process that determines neuronal excitability and synaptic responses. The molecular and cellular mechanisms of GluR trafficking are often studied in cell cultures. These studies require sensitive techniques that allow the measurement of total and surface-expressed GluRs in cell populations. The cell-based enzyme-linked immunosorbent assay (cell-ELISA) combines steps of direct immunochemical labelling of cell cultures and ELISA. It can be used for quantitative comparisons of surface-expressed and total protein contents of various cell cultures. While several cell-ELISA protocols are available for different cell types, in this chapter we describe the procedure that we have applied for the investigation of quantitative changes in the cell surface expression of recombinant ionotropic glutamate receptors (iGluRs) in adherent human embryonic kidney 293 (HEK293) cells and endogenous iGluR proteins in primary neuronal cultures.

Pulse-Chase Labeling of Protein Antigens with [35S]Methionine

Pulse-chase labeling of antigens with [³⁵S]methionine is used to determine the relative half-life of a protein. In this protocol, intracellular unlabeled methionine levels are reduced by starvation of cells for 0.5-1 h, and then the cells are briefly labeled with [³⁵S]methionine to create the pulse of newly synthesized proteins. Upon completion of cell labeling, the addition of Chasing medium containing an excess of unlabeled methionine is used to create the chase, reducing the likelihood that any remaining [³⁵S]methionine will be incorporated into newly synthesized proteins. Labeling and chasing reactions of adherent cells can be directly performed in cell culture dishes in an incubator, whereas suspension cells are labeled and chased in a polypropylene tube kept in a water bath set at 37°C. At intervals after the pulse, aliquots of chased labeled cells are collected and pelleted with the option of immediately preparing cell lysates or freezing and storing the cell pellets at -80°C. Upon cell lysis and antigen purification by immunoprecipitation, SDS-PAGE-resolved proteins can be fixed on the gel and enhanced with fluorography or can be transferred to a nitrocellulose or polyvinylidene fluoride (PVDF) membrane followed by autoradiography or exposure in a phosphorimager. Membrane blotting has the advantage of allowing for detection of the target of interest by probing with an antigen-specific antibody to confirm that equal amounts of steady-state levels of the target protein were immunoprecipitated at each interval.

Reciprocal control of excitatory synapse numbers by Wnt and Wnt inhibitor PRR7 secreted on exosomes

Secreted Wnts play crucial roles in synaptogenesis and synapse maintenance, but endogenous factors promoting synapse elimination in central neurons remain unknown. Here we show that proline-rich 7 (PRR7) induces specific removal of excitatory synapses and acts as a Wnt inhibitor. Remarkably, transmembrane protein PRR7 is activity-dependently released by neurons via exosomes. Exosomal PRR7 is uptaken by neurons through membrane fusion and eliminates excitatory synapses in neighboring neurons. Conversely, PRR7 knockdown in sparse neurons greatly increases excitatory synapse numbers in all surrounding neurons. These non-cell autonomous effects of PRR7 are effectively negated by augmentation or blockade of Wnt signaling. PRR7 exerts its effect by blocking the exosomal secretion of Wnts, activation of GSK3β, and promoting proteasomal degradation of PSD proteins. These data uncover a proximity-dependent, reciprocal mechanism for the regulation of excitatory synapse numbers in local neurons and demonstrate the significance of exosomes in inter-neuronal signaling in the vertebrate brain.

Wnts are important for synapse formation and maintenance. Here, the authors show that proline-rich 7 (PRR7) is a Wnt inhibitor that is secreted via exosomes to regulate excitatory synapse numbers.

N-Glycosylation Regulates the Trafficking and Surface Mobility of GluN3A-Containing NMDA Receptors

N-methyl-D-aspartate receptors (NMDARs) play critical roles in both excitatory neurotransmission and synaptic plasticity. NMDARs containing the nonconventional GluN3A subunit have different functional properties compared to receptors comprised of GluN1/GluN2 subunits. Previous studies showed that GluN1/GluN2 receptors are regulated by N-glycosylation; however, limited information is available regarding the role of N-glycosylation in GluN3A-containing NMDARs. Using a combination of microscopy, biochemistry, and electrophysiology in mammalian cell lines and rat hippocampal neurons, we found that two asparagine residues (N203 and N368) in the GluN1 subunit and three asparagine residues (N145, N264 and N275) in the GluN3A subunit are required for surface delivery of GluN3A-containing NMDARs. Furthermore, deglycosylation and lectin-based analysis revealed that GluN3A subunits contain extensively modified N-glycan structures, including hybrid/complex forms of N-glycans. We also found (either using a panel of inhibitors or by studying human fibroblasts derived from patients with a congenital disorder of glycosylation) that N-glycan remodeling is not required for the surface delivery of GluN3A-containing NMDARs. Finally, we found that the surface mobility of GluN3A-containing NMDARs in hippocampal neurons is increased following incubation with 1-deoxymannojirimycin (DMM, an inhibitor of the formation of the hybrid/complex forms of N-glycans) and decreased in the presence of specific lectins. These findings provide new insight regarding the mechanisms by which neurons can regulate NMDAR trafficking and function.

The non-coding RNA BC1 regulates experience-dependent structural plasticity and learning

The brain cytoplasmic (BC1) RNA is a non-coding RNA (ncRNA) involved in neuronal translational control. Absence of BC1 is associated with altered glutamatergic transmission and maladaptive behavior. Here, we show that pyramidal neurons in the barrel cortex of BC1 knock out (KO) mice display larger excitatory postsynaptic currents and increased spontaneous activity in vivo. Furthermore, BC1 KO mice have enlarged spine heads and postsynaptic densities and increased synaptic levels of glutamate receptors and PSD-95. Of note, BC1 KO mice show aberrant structural plasticity in response to whisker deprivation, impaired texture novel object recognition and altered social behavior. Thus, our study highlights a role for BC1 RNA in experience-dependent plasticity and learning in the mammalian adult neocortex, and provides insight into the function of brain ncRNAs regulating synaptic transmission, plasticity and behavior, with potential relevance in the context of intellectual disabilities and psychiatric disorders.

Brain cytoplasmic (BC1) RNA is a non-coding RNA that has been implicated in translational regulation, seizure, and anxiety. Here, the authors show that in the cortex, BC1 RNA is required for sensory deprivation-induced structural plasticity of dendritic spines, as well as for correct sensory learning and social behaviors.

NR2B Expression in Rat DRG Is Differentially Regulated Following Peripheral Nerve Injuries That Lead to Transient or Sustained Stimuli-Evoked Hypersensitivity

Following injury, primary sensory neurons undergo changes that drive central sensitization and contribute to the maintenance of persistent hypersensitivity. NR2B expression in the dorsal root ganglia (DRG) has not been previously examined in neuropathic pain models. Here, we investigated if changes in NR2B expression within the DRG are associated with hypersensitivities that result from peripheral nerve injuries. This was done by comparing the NR2B expression in the DRG derived from two modalities of the spared nerve injury (SNI) model, since each variant produces different neuropathic pain phenotypes. Using the electronic von Frey to stimulate the spared and non-spared regions of the hindpaws, we demonstrated that sural-SNI animals develop sustained neuropathic pain in both regions while the tibial-SNI animals recover. NR2B expression was measured at Day 23 and Day 86 post-injury. At Day 23 and 86 post-injury, sural-SNI animals display strong hypersensitivity, whereas tibial-SNI animals display 50 and 100% recovery from post-injury-induced hypersensitivity, respectively. In tibial-SNI at Day 86, but not at Day 23 the perinuclear region of the neuronal somata displayed an increase in NR2B protein. This retention of NR2B protein within the perinuclear region, which will render them non-functional, correlates with the recovery observed in tibial-SNI. In sural-SNI at Day 86, DRG displayed an increase in NR2B mRNA which correlates with the development of sustained hypersensitivity in this model. The increase in NR2B mRNA was not associated with an increase in NR2B protein within the neuronal somata. The latter may result from a decrease in kinesin Kif17, since Kif17 mediates NR2B transport to the soma’s plasma membrane. In both SNIs, microglia/macrophages showed a transient increase in NR2B protein detected at Day 23 but not at Day 86, which correlates with the initial post-injury induced hypersensitivity in both SNIs. In tibial-SNI at Day 86, but not at Day 23, satellite glia cells (SGCs) displayed an increase in NR2B protein. This study is the first to characterize of cell-specific changes in NR2B expression within the DRG following peripheral nerve injury. We discuss how the observed NR2B changes in DRG can contribute to the different neuropathic pain phenotypes displayed by each SNI variant.

Glutamatergic stimulation induces GluN2B translation by the nitric oxide-Heme-Regulated eIF2α kinase in cortical neurons

The activation of N-Methyl D-Aspartate Receptor (NMDAR) by glutamate is crucial in the nervous system function, particularly in memory and learning. NMDAR is composed by two GluN1 and two GluN2 subunits. GluN2B has been reported to participate in the prevalent NMDAR subtype at synapses, the GluN1/2A/2B. Here we studied the regulation of GluN2B expression in cortical neurons finding that glutamate up-regulates GluN2B translation through the action of nitric oxide (NO), which induces the phosphorylation of the eukaryotic translation initiation factor 2 α (eIF2α). It is a process mediated by the NO-heme-regulated eIF2α kinase (HRI), as the effect was avoided when a specific HRI inhibitor or a HRI small interfering RNA (siHRI) were used. We found that the expressed GluN2B co-localizes with PSD-95 at the postsynaptic ending, which strengthen the physiological relevance of the proposed mechanism. Moreover the receptors bearing GluN2B subunits upon NO stimulation are functional as high Ca2+ entry was measured and increases the co-localization between GluN2B and GluN1 subunits. In addition, the injection of the specific HRI inhibitor in mice produces a decrease in memory retrieval as tested by the Novel Object Recognition performance. Summarizing our data suggests that glutamatergic stimulation induces HRI activation by NO to trigger GluN2B expression and this process would be relevant to maintain postsynaptic activity in cortical neurons.

Biochemical and electrophysiological characterization of N-glycans on NMDA receptor subunits

In mammals, excitatory synapses contain two major types of ionotropic glutamate receptors: α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and N-methyl-d-aspartate receptors (NMDARs). Both receptor types are comprised of several subunits that are post-translationally modified by N-glycosylation. However, the precise N-glycans that are attached to these receptor types are largely unknown. Here, we used biochemistry to confirm that native NMDARs are extensively N-glycosylated; moreover, we found that the NMDAR GluN2B subunit differs from GluN1 subunits with respect to endoglycosidase H sensitivity. Next, we used a complete panel of lectins to determine the glycan composition of NMDARs in both cerebellar tissue and cultured cerebellar granule cells. Our experiments identified 23 lectins that pulled down both the GluN1 and GluN2B NMDAR subunits. We then performed an electrophysiological analysis using representative lectins and found that pre-incubating cerebellar granule cells with the AAL, WGA, or ConA alters the receptor's biophysical properties; this lectin-mediated effect was eliminated when the cells were deglycosylated with peptide-N-glycosidase F. Similar lectin-mediated effects were observed using HEK293 cells that express recombinant GluN1/GluN2B receptors. Finally, using mutant recombinant GluN subunits expressed in HEK293 cells, we found that 11 out of 12 predicted N-glycosylation sites in GluN1 and 7 out of 7 N-glycosylation sites in GluN2B are occupied by N-glycans. These data provide new insight into the role that N-glycosylation plays in regulating the function of NMDA receptors in the central nervous system. All animal experiments were performed in accordance with relevant institutional ethics guidelines and regulations with respect to protecting animal welfare. We examined the N-glycan composition of NMDA receptors (NMDARs) using deglycosylating enzymes, lectin-based biochemistry, and electrophysiology. Our results revealed that cerebellar NMDARs associate with 23 different lectins that have unique specificities for glycan structures. Furthermore, we found that 11 out of 12 predicted N-glycosylation sites in GluN1 and 7 out of 7 N-glycosylation sites in GluN2B are occupied by N-glycans. These data shed light on the glycan composition of NMDARs, revealing potential targets for the development of novel therapeutic approaches.

VCP and ATL1 regulate endoplasmic reticulum and protein synthesis for dendritic spine formation

Imbalanced protein homeostasis, such as excessive protein synthesis and protein aggregation, is a pathogenic hallmark of a range of neurological disorders. Here, using expression of mutant proteins, a knockdown approach and disease mutation knockin mice, we show that VCP (valosin-containing protein), together with its cofactor P47 and the endoplasmic reticulum (ER) morphology regulator ATL1 (Atlastin-1), regulates tubular ER formation and influences the efficiency of protein synthesis to control dendritic spine formation in neurons. Strengthening the significance of protein synthesis in dendritic spinogenesis, the translation blocker cyclohexamide and the mTOR inhibitor rapamycin reduce dendritic spine density, while a leucine supplement that increases protein synthesis ameliorates the dendritic spine defects caused by Vcp and Atl1 deficiencies. Because VCP and ATL1 are the causative genes of several neurodegenerative and neurodevelopmental disorders, we suggest that impaired ER formation and inefficient protein synthesis are significant in the pathogenesis of multiple neurological disorders.

Protein homeostasis is crucial for maintaining a variety of cellular functions. Here the authors show that valosin-containing protein and its cofactors regulate tubular ER formation and protein synthesis efficiency, thereby control dendritic spine formation in neurons.

The involvement of N-methyl-D-aspartate receptor (NMDAR) subunit NR1 in the pathophysiology of schizophrenia

Schizophrenia is a severe mental illness that afflicts nearly 1% of the world population. Although the exact pathophysiology of schizophrenia is unknown, the N-methyl-d-aspartate receptor (NMDAR), a major glutamate receptor subtype, has received great attention. The NR1 subunit is often considered indispensable for functional NMDAR assemblies, abnormal modulation of which is found in patients with schizophrenia. In this review, we discuss how disrupted function of NR1 subunits in NMDAR leads to the progression and development of symptoms of schizophrenia-like behaviors in a variety of genetically modified mouse models. We also discuss some of the susceptible genes and shared signaling pathways among the schizophrenia, and how their mutations lead to NR1 subunits hypofunction. Finally, we suggest that the subunit-selective modulators of NR1 subunits in NMDA receptors may be promising tools for the therapy of schizophrenia.

Vesicular Location and Transport of S100A8 and S100A9 Proteins in Monocytoid Cells

We show here, by using surface biotinylation, followed by Western blotting or surface plasmon resonance analysis, that very low levels of S100A8 and/or S100A9 can be detected on the surface of THP-1 cells or freshly isolated human monocytes. This was supported by immune-electron microscopy where we observed membrane-associated expression of the proteins restricted to small patches. By using confocal microscopy we could determine that S100A8 and S100A9 protein in THP-1 cells or freshly isolated human monocytes was mostly present in vesicular structures. This finding was confirmed using immune-electron microscopy. Subcellular fractionation and confocal microscopy showed that these vesicular structures are mainly early endosomes and endolysosomes. Our subsequent studies showed that accumulation of S100A8 and S100A9 in the endolysosomal compartment is associated with induction of their release from the cells. Furthermore, an inhibitor of lysosomal activity could modulate the release of S100A8 and S100A9 in the extracellular milieu. Our current results suggest that the S100A8 and S100A9 proteins are primarily associated with certain kinds of cytosolic vesicles and may be secreted via an endolysosomal pathway.

Regulated internalization of NMDA receptors drives PKD1-mediated suppression of the activity of residual cell-surface NMDA receptors

Background

Constitutive and regulated internalization of cell surface proteins has been extensively investigated. The regulated internalization has been characterized as a principal mechanism for removing cell-surface receptors from the plasma membrane, and signaling to downstream targets of receptors. However, so far it is still not known whether the functional properties of remaining (non-internalized) receptor/channels may be regulated by internalization of the same class of receptor/channels. The N-methyl-D-aspartate receptor (NMDAR) is a principal subtype of glutamate-gated ion channel and plays key roles in neuronal plasticity and memory functions. NMDARs are well-known to undergo two types of regulated internalization – homologous and heterologous, which can be induced by high NMDA/glycine and DHPG, respectively. In the present work, we investigated effects of regulated NMDAR internalization on the activity of residual cell-surface NMDARs and neuronal functions.

Results

In electrophysiological experiments we discovered that the regulated internalization of NMDARs not only reduced the number of cell surface NMDARs but also caused an inhibition of the activity of remaining (non-internalized) surface NMDARs. In biochemical experiments we identified that this functional inhibition of remaining surface NMDARs was mediated by increased serine phosphorylation of surface NMDARs, resulting from the activation of protein kinase D1 (PKD1). Knockdown of PKD1 did not affect NMDAR internalization but prevented the phosphorylation and inhibition of remaining surface NMDARs and NMDAR-mediated synaptic functions.

Conclusion

These data demonstrate a novel concept that regulated internalization of cell surface NMDARs not only reduces the number of NMDARs on the cell surface but also causes an inhibition of the activity of remaining surface NMDARs through intracellular signaling pathway(s). Furthermore, modulating the activity of remaining surface receptors may be an effective approach for treating receptor internalization-induced changes in neuronal functions of the CNS.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-015-0167-1) contains supplementary material, which is available to authorized users.

The Regulation of Synaptic Protein Turnover

Emerging evidence indicates that protein synthesis and degradation are necessary for the remodeling of synapses. These two processes govern cellular protein turnover, are tightly regulated, and are modulated by neuronal activity in time and space. The anisotropic anatomy of the neurons presents a challenge for the study of protein turnover, but the understanding of protein turnover in neurons and its modulation in response to activity can help us to unravel the fine-tuned changes that occur at synapses in response to activity. Here we review the key experimental evidence demonstrating the role of protein synthesis and degradation in synaptic plasticity, as well as the turnover rates of specific neuronal proteins.

GluN2B-Containing NMDA Receptors Regulate AMPA Receptor Traffic through Anchoring of the Synaptic Proteasome

NMDA receptors play a central role in shaping the strength of synaptic connections throughout development and in mediating synaptic plasticity mechanisms that underlie some forms of learning and memory formation in the CNS. In the hippocampus and the neocortex, GluN1 is combined primarily with GluN2A and GluN2B, which are differentially expressed during development and confer distinct molecular and physiological properties to NMDA receptors. The contribution of each subunit to the synaptic traffic of NMDA receptors and therefore to their role during development and in synaptic plasticity is still controversial. We report a critical role for the GluN2B subunit in regulating NMDA receptor synaptic targeting. In the absence of GluN2B, the synaptic levels of AMPA receptors are increased and accompanied by decreased constitutive endocytosis of GluA1-AMPA receptor. We used quantitative proteomic analysis to identify changes in the composition of postsynaptic densities from GluN2B(-/-) mouse primary neuronal cultures and found altered levels of several ubiquitin proteasome system components, in particular decreased levels of proteasome subunits. Enhancing the proteasome activity with a novel proteasome activator restored the synaptic levels of AMPA receptors in GluN2B(-/-) neurons and their endocytosis, revealing that GluN2B-mediated anchoring of the synaptic proteasome is responsible for fine tuning AMPA receptor synaptic levels under basal conditions.

Loss of F-box only protein 2 (Fbxo2) disrupts levels and localization of select NMDA receptor subunits, and promotes aberrant synaptic connectivity

NMDA receptors (NMDARs) play an essential role in some forms of synaptic plasticity, learning, and memory. Therefore, these receptors are highly regulated with respect to their localization, activation, and abundance both within and on the surface of mammalian neurons. Fundamental questions remain, however, regarding how this complex regulation is achieved. Using cell-based models and F-box Only Protein 2 (Fbxo2) knock-out mice, we found that the ubiquitin ligase substrate adaptor protein Fbxo2, previously reported to facilitate the degradation of the NMDAR subunit GluN1 in vitro, also functions to regulate GluN1 and GluN2A subunit levels in the adult mouse brain. In contrast, GluN2B subunit levels are not affected by the loss of Fbxo2. The loss of Fbxo2 results in greater surface localization of GluN1 and GluN2A, together with increases in the synaptic markers PSD-95 and Vglut1. These synaptic changes do not manifest as neurophysiological differences or alterations in dendritic spine density in Fbxo2 knock-out mice, but result instead in increased axo-dendritic shaft synapses. Together, these findings suggest that Fbxo2 controls the abundance and localization of specific NMDAR subunits in the brain and may influence synapse formation and maintenance.

Two N-glycosylation Sites in the GluN1 Subunit Are Essential for Releasing N-methyl-d-aspartate (NMDA) Receptors from the Endoplasmic Reticulum

NMDA receptors (NMDARs) comprise a subclass of neurotransmitter receptors whose surface expression is regulated at multiple levels, including processing in the endoplasmic reticulum (ER), intracellular trafficking via the Golgi apparatus, internalization, recycling, and degradation. With respect to early processing, NMDARs are regulated by the availability of GluN subunits within the ER, the presence of ER retention and export signals, and posttranslational modifications, including phosphorylation and palmitoylation. However, the role of N-glycosylation, one of the most common posttranslational modifications, in regulating NMDAR processing has not been studied in detail. Using biochemistry, confocal and electron microscopy, and electrophysiology in conjunction with a lentivirus-based molecular replacement strategy, we found that NMDARs are released from the ER only when two asparagine residues in the GluN1 subunit (Asn-203 and Asn-368) are N-glycosylated. Although the GluN2A and GluN2B subunits are also N-glycosylated, their N-glycosylation sites do not appear to be essential for surface delivery of NMDARs. Furthermore, we found that removing N-glycans from native NMDARs altered the receptor affinity for glutamate. Our results suggest a novel mechanism by which neurons ensure that postsynaptic membranes contain sufficient numbers of functional NMDARs.

Female protection from slow-pressor effects of angiotensin II involves prevention of ROS production independent of NMDA receptor trafficking in hypothalamic neurons expressing angiotensin 1A receptors

Renin–angiotensin system overactivity, upregulation of postsynaptic NMDA receptor function, and increased reactive oxygen species (ROS) production in the hypothalamic paraventricular nucleus (PVN) are hallmarks of angiotensin II (AngII)-induced hypertension, which is far more common in young males than in young females. We hypothesize that the sex differences in hypertension are related to differential AngII-induced changes in postsynaptic trafficking of the essential NMDA receptor GluN1 subunit and ROS production in PVN cells expressing angiotensin Type 1a receptor (AT1aR). We tested this hypothesis using slow-pressor (14-day) infusion of AngII (600 ng/kg/min) in mice, which elicits hypertension in males but not in young females. Two-month-old male and female transgenic mice expressing enhanced green fluorescent protein (EGFP) in AT1aR-containing cells were used. In males, but not in females, AngII increased blood pressure and ROS production in AT1aR–EGFP PVN cells at baseline and following NMDA treatment. Electron microscopy showed that AngII increased cytoplasmic and total GluN1–silver-intensified immunogold (SIG) densities and induced a trend toward an increase in near plasmalemmal GluN1–SIG density in AT1aR–EGFP dendrites of males and females. Moreover, AngII decreased dendritic area and diameter in males, but increased dendritic area of small (<1 µm) dendrites and decreased diameter of large (>1 µm) dendrites in females. Fluorescence microscopy revealed that AT1aR and estrogen receptor β do not colocalize, suggesting that if estrogen is involved, its effect is indirect. These data suggest that the sexual dimorphism in AngII-induced hypertension is associated with sex differences in ROS production in AT1aR-containing PVN cells but not with postsynaptic NMDA receptor trafficking.

Long-term imipramine treatment increases N-methyl-d-aspartate receptor activity and expression via epigenetic mechanisms

Imipramine, a major antidepressant, is known to inhibit reuptake of serotonin and norepinephrine, which contributes to recovery from major depressive disorder. It has recently been reported that acute imipramine treatment inhibits N-methyl-d-aspartate (NMDA) receptor activity. However, the mechanisms underlying long-term effects of imipramine have not been identified. We tested these distinct effects in mouse cortical neurons and found that acute (30s) imipramine treatment decreased Ca(2+) influx through NMDA receptors, whereas long-term treatment (48h) increased Ca(2+) influx via the same receptors. Furthermore, long-term treatment increased NMDA receptor 2B (NR2B) subunit expression via epigenetic changes, including increased acetylation of histones H3K9 and H3K27 in the NR2B promoter and decreased activity of histone deacetylase 3 (HDAC3) and HDAC4. These results suggest that the long-term effects of imipramine on NMDA receptors are quite different from its acute effects. Furthermore, increased NR2B expression via epigenetic alterations might be a part of the mechanism responsible for this long-term effect.

SAP97 blocks the RXR ER retention signal of NMDA receptor subunit GluN1-3 through its SH3 domain

SAP97 is directly involved in exporting NMDA receptors with a specific subunit composition from the endoplasmic reticulum (ER). Characterization of the interactions between SAP97 and an NMDA receptor splice variant, GluN1-3, and of the effects on forward trafficking revealed that an ER-level interaction blocked the RXR ER-retention motif in the GluN1-3 cytoplasmic C-terminus in the context of both reporter molecules and full-length receptors. Binding of SAP97 to the PDZ-binding domain of GluN1-3 was required, but the blockade of ER-retention was mediated by the SH3-GuK domains coupled with the action of the N-terminus of SAP97. While other domains of SAP97 were involved in forward trafficking of GluN1-3 out of the ER, the SH3 domain was necessary and sufficient to block the ER retention. This is the first direct evidence for the masking of ER-retention signals by PDZ domain-containing proteins, and provides detailed underlying mechanistic requirements. Such a mechanism could be central to modulating the ER exit of receptors into local, non-conventional or conventional, secretory pathways in neurons.

ER to synapse trafficking of NMDA receptors

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. There are three distinct subtypes of ionotropic glutamate receptors (GluRs) that have been identified including 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid receptors (AMPARs), N-methyl-D-aspartate receptors (NMDARs) and kainate receptors. The most common GluRs in mature synapses are AMPARs that mediate the fast excitatory neurotransmission and NMDARs that mediate the slow excitatory neurotransmission. There have been large numbers of recent reports studying how a single neuron regulates synaptic numbers and types of AMPARs and NMDARs. Our current research is centered primarily on NMDARs and, therefore, we will focus in this review on recent knowledge of molecular mechanisms occurring (1) early in the biosynthetic pathway of NMDARs, (2) in the transport of NMDARs after their release from the endoplasmic reticulum (ER); and (3) at the plasma membrane including excitatory synapses. Because a growing body of evidence also indicates that abnormalities in NMDAR functioning are associated with a number of human psychiatric and neurological diseases, this review together with other chapters in this issue may help to enhance research and to gain further knowledge of normal synaptic physiology as well as of the etiology of many human brain diseases.

Kalirin-9 and Kalirin-12 Play Essential Roles in Dendritic Outgrowth and Branching

Proteins derived from the Kalrn gene, encoding 2 Rho guanine nucleotide exchange factor (GEF) domains, affect dendritic and axonal morphogenesis. The roles of endogenous Kalirin-9 (Kal9) and Kalirin-12 (Kal12), the Kalrn isoforms expressed before synaptogenesis, have not been studied in neurite growth and maturation during early development. The Caenorhabditis elegans and Drosophila melanogaster orthologues of Kalrn encode proteins equivalent to Kal9 but, lacking a kinase domain, neither organism expresses a protein equivalent to Kal12. Both in vivo and in vitro analyses of cortical neurons from total Kalrn knockout mice, lacking all major Kalirin isoforms, revealed a simplified dendritic arbor and reduced neurite length. Using isoform-specific shRNAs to reduce Kal9 or Kal12 expression in hippocampal cultures resulted in stunted dendritic outgrowth and branching in vitro, without affecting axonal polarity. Exposing hippocampal cultures to inhibitors of the first GEF domain of Kalirin (ITX3, Z62954982) blunted neurite outgrowth and branching, confirming its essential role, without altering the morphology of neurons not expressing Kalrn. In addition, exogenous expression of the active kinase domain unique to Kal12 increased neurite number and length, whereas that of the inactive kinase domain decreased neurite growth. Our results demonstrate that both endogenous Kal9 and endogenous Kal12 contribute to dendritic maturation in early development.

NMDA receptors are upregulated and trafficked to the plasma membrane after sigma-1 receptor activation in the rat hippocampus

Sigma-1 receptors (σ-1Rs) are endoplasmic reticulum resident chaperone proteins implicated in many physiological and pathological processes in the CNS. A striking feature of σ-1Rs is their ability to interact and modulate a large number of voltage- and ligand-gated ion channels at the plasma membrane. We have reported previously that agonists for σ-1Rs potentiate NMDA receptor (NMDAR) currents, although the mechanism by which this occurs is still unclear. In this study, we show that in vivo administration of the selective σ-1R agonists (+)-SKF 10,047 [2S-(2α,6α,11R*]-1,2,3,4,5,6-hexahydro-6,11-dimethyl-3-(2-propenyl)-2,6-methano-3-benzazocin-8-ol hydrochloride (N-allylnormetazocine) hydrochloride], PRE-084 (2-morpholin-4-ylethyl 1-phenylcyclohexane-1-carboxylate hydrochloride), and (+)-pentazocine increases the expression of GluN2A and GluN2B subunits, as well as postsynaptic density protein 95 in the rat hippocampus. We also demonstrate that σ-1R activation leads to an increased interaction between GluN2 subunits and σ-1Rs and mediates trafficking of NMDARs to the cell surface. These results suggest that σ-1R may play an important role in NMDAR-mediated functions, such as learning and memory. It also opens new avenues for additional studies into a multitude of pathological conditions in which NMDARs are involved, including schizophrenia, dementia, and stroke.

The protective effect of heat acclimation from hypoxic damage in the brain involves changes in the expression of glutamate receptors

Long-term heat acclimation (34 °C, 30d) alters the physiological responses and the metabolic state of organisms. It also improves ability to cope with hypoxic stress via a cross-tolerance mechanism. Within the brain, the hippocampal and frontal cortex neurons are the most sensitive to hypoxia and cell death is mainly caused by calcium influx via glutamate-gated ion channels, specifically NMDA and AMPA receptors. GluN1 subunit levels of NMDA-R correspond to NMDA-R levels. GluN2B/GluN2A subunit ratio is a qualitative index of channel activity; a higher ratio implies lower calcium permeability. The GluA2 subunit of AMPA-R controls channel permeability by inhibiting calcium penetration. Here, in rats model we (i)used behavioral-assessment tests to evaluate heat acclimation mediated hypoxic (15' 4.5 ± 0.5% O2) neuroprotection, (ii) measured protein and transcript levels of NMDA-R and AMPA-R subunits before and after hypoxia in the hippocampus and the frontal cortex, to evaluate the role of Ca(2+) in neuro-protection/cross-tolerance. Behavioral tests confirmed hypoxic tolerance in long-term (30d) but not in short-term (2d) heat acclimated rats. Hypoxic tolerance in the long-term acclimated phenotype was accompanied by a significant decrease in basal NMDA receptor GluN1 protein and an increase in its mRNA. The long-term acclimated rats also showed post ischemic increases in the GluN2B/GluN2A subunit ratio and GluA2 subunit of the AMPA receptor, supporting the hypothesis that reduced calcium permeability contributes to heat acclimation mediated hypoxia cross-tolerance. Abrupt post ischemic change in GluN2B/GluN2A subunit ratio with no change in NMDA-R subunits transcript levels implies that post-translational processes are inseparable acclimatory cross-tolerance mechanism.

AAV-mediated overexpression of neuroserpin in the hippocampus decreases PSD-95 expression but does not affect hippocampal-dependent learning and memory

Neuroserpin is a serine protease inhibitor, or serpin, that is expressed in the nervous system and inhibits the protease tissue plasminogen activator (tPA). Neuroserpin has been suggested to play a role in learning and memory but direct evidence for such a role is lacking. Here we have used an adeno-associated virus (AAV) vector expression system to investigate the effect of neuroserpin on hippocampal-dependent learning and memory in the young adult rat. A FLAG-tagged neuroserpin construct was initially characterized by in vitro transcription/translation and transfection into HEK293 cells and shown to interact with tPA and be targeted to the secretory pathway. Targeted injection of a chimeric AAV1/2 vector expressing FLAG-neuroserpin resulted in localized overexpression in the dorsal hippocampus. Neuroserpin overexpression led to the appearance of an unstable neuroserpin:tPA complex in zymographic assays consistent with interaction with endogenous tPA in vivo. Rats overexpressing neuroserpin also showed a significant decrease in the levels of postsynaptic density protein 95, a major postsynaptic scaffolding protein. Three weeks after injection, a range of behavioural tests was performed to measure spatial and associative learning and memory, as well as innate and acquired fear. These tests provided no evidence of a role for neuroserpin in hippocampal-dependent learning and memory. In summary this study does not support a role for neuroserpin in hippocampal-dependent learning and memory in young adult rats but does suggest an involvement of neuroserpin in hippocampal synaptic plasticity.

F-box only protein 2 (Fbxo2) regulates amyloid precursor protein levels and processing

The amyloid precursor protein (APP) is an integral membrane glycoprotein whose cleavage products, particularly amyloid-β, accumulate in Alzheimer disease (AD). APP is present at synapses and is thought to play a role in both the formation and plasticity of these critical neuronal structures. Despite the central role suggested for APP in AD pathogenesis, the mechanisms regulating APP in neurons and its processing into cleavage products remain incompletely understood. F-box only protein 2 (Fbxo2), a neuron-enriched ubiquitin ligase substrate adaptor that preferentially binds high-mannose glycans on glycoproteins, was previously implicated in APP processing by facilitating the degradation of the APP-cleaving β-secretase, β-site APP-cleaving enzyme. Here, we sought to determine whether Fbxo2 plays a similar role for other glycoproteins in the amyloid processing pathway. We present in vitro and in vivo evidence that APP is itself a substrate for Fbxo2. APP levels were decreased in the presence of Fbxo2 in non-neuronal cells, and increased in both cultured hippocampal neurons and brain tissue from Fbxo2 knock-out mice. The processing of APP into its cleavage products was also increased in hippocampi and cultured hippocampal neurons lacking Fbxo2. In hippocampal slices, this increase in cleavage products was accompanied by a significant reduction in APP at the cell surface. Taken together, these results suggest that Fbxo2 regulates APP levels and processing in the brain and may play a role in modulating AD pathogenesis.

Wnts in action: from synapse formation to synaptic maintenance

A proper balance between synapse assembly and disassembly is crucial for the formation of functional neuronal circuits and synaptic plasticity in the adult brain. During development, synaptogenesis generates a vast excess of synapses, which are subsequently eliminated. Importantly, aberrant synaptic disassembly during development underpins many neurological disorders. Wnt secreted proteins are robust synaptogenic factors that regulate synapse assembly and function in the developing and mature brain. Recent studies show that Wnt blockade with the antagonist Dickkopf-1 (Dkk1) induces the rapid disassembly of synapses in mature neurons. Importantly, Dkk1 mediates synaptic loss induced by Amyloid-ß, a key pathogenic molecule in Alzheimer's disease (AD). These findings provide new insights into the potential contribution of dysfunctional Wnt signaling to synaptic loss observed in neurodegenerative diseases. In this review, we discuss the role of Wnt signaling in vertebrate synaptic assembly, function and maintenance, and consider how dysfunction of Wnt signaling could contribute to synaptic disassembly in neurodegenerative diseases such as AD.

A eukaryotic specific transmembrane segment is required for tetramerization in AMPA receptors

Most fast excitatory synaptic transmission in the nervous system is mediated by glutamate acting through ionotropic glutamate receptors (iGluRs). iGluRs (AMPA, kainate, and NMDA receptor subtypes) are tetrameric assemblies, formed as a dimer of dimers. Still, the mechanism underlying tetramerization--the necessary step for the formation of functional receptors that can be inserted into the plasma membrane--is unknown. All eukaryotic compared to prokaryotic iGluR subunits have an additional transmembrane segment, the M4 segment, which positions the physiologically critical C-terminal domain on the cytoplasmic side of the membrane. AMPA receptor (AMPAR) subunits lacking M4 do not express on the plasma membrane. Here, we show that these constructs are retained in the endoplasmic reticulum, the major cellular compartment mediating protein oligomerization. Using approaches to assay the native oligomeric state of AMPAR subunits, we find that subunits lacking M4 or containing single amino acid substitutions along an "interacting" face of the M4 helix that block surface expression no longer tetramerize in either homomeric or heteromeric assemblies. In contrast, subunit dimerization appears to be largely intact. These experiments define the M4 segment as a unique functional unit in AMPARs that is required for the critical dimer-to-tetramer transition.

Neuroprotective effect of ceftriaxone on the penumbra in a rat venous ischemia model

OBJECTIVE

Glutamate transporter-1 (GLT-1) maintains low concentrations of extracellular glutamate by removing glutamate from the extracellular space. It is controversial, however, whether upregulation of GLT-1 is neuroprotective under all ischemic/hypoxic conditions. Recently, a neuroprotective effect of preconditioning with a β-lactam antibiotic ceftriaxone (CTX) that increases expression of GLT-1 has been reported in animal models of focal ischemia. On the other hand, it is said that CTX does not play a neuroprotective role in an in vitro study. Thus, we examined the effect of CTX on ischemic injury in a rat model of two-vein occlusion (2VO). This model mimics venous ischemia during, e.g. tumor surgery, a clinical situation that is best suitable for pretreatment with CTX.

METHODS

CTX (100mg/kg, 200mg/kg per day) or vehicle (0.9% NaCl) was intraperitoneally injected into Wistar rats for 5days before venous ischemia (n=57). Then, animals were prepared for occlusion of two adjacent cortical veins (2VO) by photothrombosis with rose bengal that was followed by KCl-induced cortical spreading depression (CSD). Infarct volume was evaluated with hematoxylin and eosin (H&E) staining 2days after venous occlusion. [(3)H]MK-801, [(3)H]AMPA and [(3)H]Muscimol ligand binding were examined autoradiographically in additional two groups without 2VO (n=5/group). Animals were injected either with NaCl (vehicle) or CTX 200mg/kg for 5days in order to evaluate whether NMDA, AMPA and GABAA ligand binding densities were affected.

RESULTS

CTX pretreatment reduced infarct volume compared to vehicle pretreatment (p<0.05). The effect of CTX pretreatment was attenuated by administration of the GLT-1 inhibitor, dihydrokainate (DHK) 30min before 2VO. CTX had no effect on the number of spontaneous spreading depressions after 2VO. Analysis of quantitative receptor autoradiography showed no statistically significant difference between rats after administration with CTX compared to control rats.

CONCLUSIONS

Pretreatment with CTX has neuroprotective potential without effect on NMDA, AMPA and GABAA receptor density and spontaneous spreading depression. This effect can be abolished by GLT-1 inhibition, indicating that upregulation of GLT-1 is an important mechanism for neuroprotective action in penumbra-like conditions, e.g. if neurosurgeons plan to occlude cerebral veins during tumor surgery.

Diversity in NMDA receptor composition: many regulators, many consequences

N-methyl-D-aspartate receptors (NMDARs) are a subtype of ionotropic glutamate receptor, which play a central role in learning, memory, and synaptic development. NMDARs are assembled as tetramers composed of two GluN1 subunits and two GluN2 or GluN3 subunits. Although NMDARs are widely expressed throughout the central nervous system, their number, localization, and subunit composition are strictly regulated and differ in a cell- and synapse-specific manner. The brain area, developmental stage, and level of synaptic activity are some of the factors that regulate NMDARs. Molecular mechanisms that control subunit-specific NMDAR function include developmental regulation of subunit transcription/translation, differential trafficking through the secretory pathway, posttranscriptional modifications such as phosphorylation, and protein-protein interactions. The GluN2A and GluN2B subunits are highly expressed in cortex and hippocampus and confer many of the distinct properties on endogenous NMDARs. Importantly, the synaptic NMDAR subunit composition changes from predominantly GluN2B-containing to GluN2A-containing NMDARs during synaptic maturation and in response to activity and experience. Some of the molecular mechanisms underlying this GluN2 subunit switch have been recently identified. In addition, the balance between synaptic and extrasynaptic NMDARs is altered in several neuronal disorders. Here, the authors summarize the recent advances in the identification of NMDAR subunit-specific regulatory mechanisms.

Anti-NMDA receptor encephalitis antibody binding is dependent on amino acid identity of a small region within the GluN1 amino terminal domain

Anti-NMDA receptor (NMDAR) encephalitis is a newly identified autoimmune disorder that targets NMDARs, causing severe neurological symptoms including hallucinations, psychosis, and seizures, and may result in death (Dalmau et al., 2008). However, the exact epitope to which these antibodies bind is unknown. A clearly defined antigenic region could provide more precise testing, allow for comparison of immunogenicity between patients to explore potential clinically relevant variations, elucidate the functional effects of antibodies, and make patients' antibodies a more effective tool with which to study NMDAR function. Here, we use human CSF to explore the antigenic region of the NMDAR. We created a series of mutants within the amino terminal domain of GluN1 that change patient antibody binding in transfected cells in stereotyped ways. These mutants demonstrate that the N368/G369 region of GluN1 is crucial for the creation of immunoreactivity. Mass spectrometry experiments show that N368 is glycosylated in transfected cells and rat brain regions; however, this glycosylation is not directly required for epitope formation. Mutations of residues N368/G369 change the closed time of the receptor in single channel recordings; more frequent channel openings correlates with the degree of antibody staining, and acute antibody exposure prolongs open time of the receptor. The staining pattern of mutant receptors is similar across subgroups of patients, indicating consistent immunogenicity, although we have identified one region that has a variable role in epitope formation. These findings provide tools for detailed comparison of antibodies across patients and suggest an interaction between antibody binding and channel function.

Differential stimulus-dependent synaptic recruitment of CaMKIIα by intracellular determinants of GluN2B

The calcium-calmodulin activated kinase CaMKII mediates many forms of learning and memory. Activity-regulated translocation of CaMKII to synapses is important for its functions in synaptic plasticity. Here, we tested the role of the NMDA receptor subunit GluN2B in recruiting CaMKIIα to synapses with different paradigms: global bath stimulation of NMDA receptors, a chemical long term potentiation (cLTP) protocol that selectively activates synaptic NMDA receptors, or local stimulation of NMDA receptors at a contiguous set of ~10-30 synapses that triggers a propagating synaptic accumulation of CaMKII. Global or cLTP-induced synaptic accumulation of CaMKIIα occurred in wild-type but not sister GluN2B -/- cultured mouse hippocampal neurons. Expression of YFP-GluN2B, but not a similar level of YFP-GluN2A, rescued global and cLTP-induced CaMKIIα translocation. Using chimeric constructs, the pore-forming extracellular and membrane domains of GluN2A combined with the cytoplasmic tail of GluN2B were sufficient to rescue CaMKIIα translocation, whereas the reverse chimera was ineffective. Furthermore, the dual point mutation R1300Q,S1303D in GluN2B that blocks interaction of this high affinity site with CaMKII abolished rescue. Thus, CaMKII binding to GluN2B is required for global and cLTP-induced synaptic accumulation of CaMKIIα. However, surprisingly, locally induced propagating synaptic accumulation of CaMKIIα occurred normally in GluN2B -/- neurons, indistinguishably from wild-type. Thus, synaptic trapping of CaMKII during locally induced propagating translocation occurs by different mechanisms and molecular partners compared with global stimulation and cLTP paradigms. These findings underscore the complex regulatory properties and molecular interactions of CaMKIIα, a key player in synaptic plasticity.

Phencyclidine treatment increases NR2A and NR2B N-methyl-D-aspartate receptor subunit expression in rats

Administration of noncompetitive N-methyl-D-aspartate receptor (NMDAR) antagonist phencyclidine to rats on postnatal days 7, 9, and 11 induces apoptosis in prefrontal cortex and hippocampus. In adulthood, these animals display cognitive impairment of working memory, reversal learning and attention that are similar to clinical observations in schizophrenia. In this study, expression of different NMDAR subunits, the postsynaptic mGlu5 receptor and the connecting NMDAR-mGluR5 intracellular postsynaptic density proteins have been measured in adult rats after treatment with phencyclidine on postnatal days 7, 9, and 11. We found that these animals exhibited elevated expression in medial prefrontal cortex of the NR2A and NR2B NMDA receptor subunits in adulthood. These results indicate how behavioral changes in a developmental model for cognitive dysfunction involve changes to specific molecular subsets of the cortical glutamate system.

Glutamate binding to the GluN2B subunit controls surface trafficking of N-methyl-D-aspartate (NMDA) receptors

Trafficking of NMDA receptors to the surface of neurons and to synapses is critical for proper brain function and activity-dependent plasticity. Recent evidence suggests that surface trafficking of other ionotropic glutamate receptors requires ligand binding for exit from the endoplasmic reticulum. Here, we show that glutamate binding to GluN2 is required for trafficking of NMDA receptors to the cell surface. We expressed a panel of GluN2B ligand binding mutants in heterologous cells with GluN1 or in rat cultured neurons and found that surface expression correlates with glutamate efficacy. Such a correlation was found even in the presence of dominant negative dynamin to inhibit endocytosis and surface expression correlated with Golgi localization, indicating differences in forward trafficking. Co-expression of wild type GluN2B did not enhance surface expression of the mutants, suggesting that glutamate must bind to both GluN2 subunits in a tetramer and that surface expression is limited by the least avid of the two glutamate binding sites. Surface trafficking of a constitutively closed cleft GluN2B was indistinguishable from that of wild type, suggesting that glutamate concentrations are typically not limiting for forward trafficking. YFP-GluN2B expressed in hippocampal neurons from GluN2B(-/-) mice rescued synaptic accumulation at similar levels to wild type. Under these conditions, surface synaptic accumulation of YFP-GluN2B mutants also correlated with apparent glutamate affinity. Altogether, these results indicate that glutamate controls forward trafficking of NMDA receptors to the cell surface and to synapses and raise the intriguing idea that NMDA receptors may be functional at intracellular sites.

Phosphorylated CaMKII post-synaptic binding to NR2B subunits in the anterior cingulate cortex mediates visceral pain in visceral hypersensitive rats

The NR2B subunit of NMDA receptor in the anterior cingulate cortex (ACC) is up-regulated in viscerally hypersensitive (VH) rats induced by colonic anaphylaxis. It plays a critical role in modulation of ACC sensitization and visceral pain responses. Given the key role of calcium/calmodulin-dependent protein kinase II (CaMKII) in synaptic plasticity and behavior learning and memory, we hypothesize that phosphorylation of CaMKII binding to NR2B mediates visceral pain in VH states. We performed in vivo electroporation of CaMKII siRNA produced inhibition of colorectal distension-induced visceromotor response in the VH rats. The NR2B, CaMKII and P-CaMKII-Thr²⁸⁶ protein levels were increased in 180%, 220% and 304% fold in the post-synaptic density (PSD) fraction in VH rats separately. Western blotting following co-immunoprecipitation showed that P-CaMKII-Thr²⁸⁶ bound to NR2B in the PSD, which was increased to 267% of control in VH rats. Administration of CaMKII antagonist Antennapedia-CaMKIINtide suppressed visceromotor response in VH rats in parallel with decrease of NR2B levels and reduction of the NR2B-P-CaMKII-Thr²⁸⁶ protein complex in PSD. In conclusion, CaMKII is a critical signaling molecule in the ACC glutamatergic synaptic transmission and phosphorylation of CaMKII at Thr286, which binds to NR2B subunit at post-synaptic site, modulates visceral pain in viscerally hypersensitive state.