Regulation of NR1/NR2C N-Methyl-D-aspartate (NMDA) Receptors by Phosphorylation*

Functional Evaluation of a Novel GRIN2B Missense Variant Associated with Epilepsy and Intellectual Disability

Epilepsy, a neurological condition, is widely prevalent among individuals with intellectual disability (ID). It is well established that N-methyl-D-aspartate (NMDA) receptors play an important role in both epilepsy and ID. Autosomal dominant mutations in the GRIN2B gene, which encodes the GluN2B subunit of the NMDA receptor, have been reported to be associated with epilepsy and ID. However, the underlying mechanism of this association is not well-understood. In this study, we identified a novel GRIN2B mutation (c.3272A > C, p.K1091T) in a patient with epilepsy and ID. The proband was a one year and ten months old girl. GRIN2B variant was inherited from her mother. We further investigated the functional consequences of this mutation. Our findings revealed that the p.K1091T mutation created a Casein kinase 2 phosphorylation site. Using recombinant NMDA receptors containing the GluN2B-K1091T along with GluN1 in HEK 293T cells, we observed significant defects in its interactions with postsynaptic density 95. It is accompanied by reduced delivery of the receptors to the cell membrane and a decrease in glutamate affinity. Moreover, primary neurons expressing GluN2B-K1091T also exhibited impaired surface expression of NMDA receptors, a reduction in dendritic spine number and excitatory synaptic transmission. In summary, our study reports a novel GRIN2B mutation and provides functional characteristics of this mutation in vitro, thereby contributing to the understanding of GRIN2B variants in epilepsy and ID.

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.

Dynamic postnatal development of the cellular and circuit properties of striatal D1 and D2 spiny projection neurons

KEY POINTS

Imbalances in the activity of the D1-expressing direct pathway and D2-expressing indirect pathway striatal projection neurons (SPNs) are thought to contribute to many basal ganglia disorders, including early-onset neurodevelopmental disorders such as obsessive-compulsive disorder, attention deficit hyperactivity disorder and Tourette's syndrome. This study provides the first detailed quantitative investigation of development of D1 and D2 SPNs, including their cellular properties and connectivity within neural circuits, during the first postnatal weeks. This period is highly dynamic with many properties changing, but it is possible to make three main observations: many aspects of D1 and D2 SPNs progressively mature in parallel; there are notable exceptions when they diverge; and many of the defining properties of mature striatal SPNs and circuits are already established by the first and second postnatal weeks, suggesting guidance through intrinsic developmental programmes. These findings provide an experimental framework for future studies of striatal development in both health and disease.

ABSTRACT

Many basal ganglia neurodevelopmental disorders are thought to result from imbalances in the activity of the D1-expressing direct pathway and D2-expressing indirect pathway striatal projection neurons (SPNs). Insight into these disorders is reliant on our understanding of normal D1 and D2 SPN development. Here we provide the first detailed study and quantification of the striatal cellular and circuit changes occurring for both D1 and D2 SPNs in the first postnatal weeks using in vitro whole-cell patch-clamp electrophysiology. Characterization of their intrinsic electrophysiological and morphological properties, the excitatory long-range inputs coming from cortex and thalamus, as well their local gap junction and inhibitory synaptic connections reveals this period to be highly dynamic with numerous properties changing. However it is possible to make three main observations. Firstly, many aspects of SPNs mature in parallel, including intrinsic membrane properties, increases in dendritic arbours and spine densities, general synaptic inputs and expression of specific glutamate receptors. Secondly, there are notable exceptions, including a transient stronger thalamic innervation of D2 SPNs and stronger cortical NMDA receptor-mediated inputs to D1 SPNs, both in the second postnatal week. Thirdly, many of the defining properties of mature D1 and D2 SPNs and striatal circuits are already established by the first and second postnatal weeks, including different electrophysiological properties as well as biased local inhibitory connections between SPNs, suggesting this is guided through intrinsic developmental programmes. Together these findings provide an experimental framework for future studies of D1 and D2 SPN development in health and disease.

Changes of Protein Phosphorylation Are Associated with Synaptic Functions during the Early Stage of Alzheimer's Disease

Alzheimer's disease is an irreversible neurodegenerative disorder for which we have limited knowledge of the mechanisms underlying its pathogenesis, especially the molecular events that trigger the deterioration of neuronal functions in the early stage. Protein phosphorylation and dephosphorylation are highly dynamic and reversible post-translational modifications that control protein signaling and hence neuronal functions, aberrations of which are implicated in various neurodegenerative diseases including Alzheimer's disease. We conducted a quantitative phosphoproteomic analysis in the brains of APP/PS1 mice, an Aβ-deposition transgenic mouse model, at 3 months old, the stage at which amyloid pathology just initiates. Compared to the wild-type mouse brains, we found that changes in serine phosphorylation were predominant in the APP/PS1 mouse brains, and that the occurrence of proline-directed phosphorylation was most common among the overrepresented phosphopeptides. Further analysis of the 167 phosphoproteins that were significantly up- or downregulated in APP/PS1 mouse brains revealed the enrichment of these proteins in synapse-related pathways. In particular, Western blot analysis validated the increased phosphorylation of chromogranin B, a protein enriched in large dense-core vesicles, in APP/PS1 mouse brains. These findings collectively suggest that changes in the phosphoprotein network may be associated with the deregulation of synaptic functions during the pathogenesis of Alzheimer's disease.

Potentiation of NMDA receptor-mediated synaptic transmission at the parabrachial-central amygdala synapses by CGRP in mice

The capsular part of the central amygdala (CeC) is called the “nociceptive amygdala,” as it receives nociceptive information from various pathways, including monosynaptic input from the lateral part of the parabrachial nucleus (LPB), a major target of ascending neurons in the spinal and medullary dorsal horn. LPB-CeC synaptic transmission is mediated by glutamate but the fibers from the LPB also contain calcitonin gene-related peptide (CGRP) and the CeC is rich in CGRP-binding sites. CGRP might be released in response to strong nociception and activate these CGRP receptors. Though it has been shown that CGRP affects the excitatory postsynaptic current (EPSC) amplitude at this synapse in a manner sensitive to NMDA receptor (NMDA-R) blockers, the effect of CGRP on postsynaptic NMDA-R-mediated current recorded in isolation has never been directly examined. Thus, we evaluated the effects of CGRP on NMDA-R-mediated EPSCs that were pharmacologically isolated in brain slices from naïve mice. CGRP significantly increased the amplitude of EPSCs mediated by NMDA-Rs in a manner dependent on protein kinase A activation, but not that mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, in concentration-dependent and antagonist-sensitive manners. This CGRP-induced potentiation of synaptic NMDA-R function would have a potent impact on the strengthening of the nociception-emotion link in persistent pain.

Regulation of SAP102 Synaptic Targeting by Phosphorylation

Synapse-associated protein 102 (SAP102) is a scaffolding protein highly expressed early in development and plays a critical role in mediating glutamate receptor trafficking during synaptogenesis. Mutations in human SAP102 have been reported to cause intellectual disability, which is thought to be due to mislocalization of the mutant protein. However, little is known about the regulation of SAP102 synaptic targeting. Here, we investigate the role of phosphorylation of SAP102 in regulating its synaptic targeting. Previous studies have shown that synaptic targeting of SAP102 is regulated by C-terminal splicing. We now identify a phosphorylation site, serine 632, within the C-terminal alternatively spliced region, which is phosphorylated by casein kinase II (CK2). We show that Ser632 on SAP102 is phosphorylated in vitro, in heterologous cells, and in neurons. Moreover, we demonstrate that synaptic enrichment of SAP102 is increased by Ser632 phosphorylation. Consistently, elevation of synaptic activity that suppresses Ser632 phosphorylation reduces synaptic enrichment of SAP102. Furthermore, the mobility of SAP102 is decreased by Ser632 phosphorylation. Therefore, not only SAP102 synaptic targeting but also its mobility is regulated by Ser632 phosphorylation. These data provide evidence for a novel mechanism in regulating SAP102 function and glutamate receptor trafficking.

Behavioral and physiological characterization of PKC-dependent phosphorylation in the Grin2a∆PKC mouse

Activity-dependent plasticity in NMDA receptor-containing synapses can be regulated by phosphorylation of serines and tyrosines in the C-terminal domain of the receptor subunits by various kinases. We have previously identified S1291/S1312 as important sites for PKC phosphorylation; while Y1292/Y1312 are the sites indirectly phosphorylated by PKC via Src kinase. In the oocyte expression system, mutation of those Serine sites to Alanine (that cannot be phosphorylated) in the GluN2A subunit, resulted in a decreased PKC stimulated current enhancement through the receptors compared to wild-type NMDA receptors. To investigate the behavioral and physiological significance of those PKC-mediated phosphorylation sites in vivo, the Grin2a∆PKC mouse expressing GluN2A with four mutated amino acids: S1291A, S1312A, Y1292F and Y1387F was generated using homologous recombination. The Grin2a∆PKC mice exhibit reduced anxiety in the open field test, light dark emergence test, and elevated plus maze. The mutant mice show reduced alternation in a Y maze spontaneous alternation task and a in a non-reinforced T maze alternation task. Interestingly, when the mutant mice were exposed to novel environments, there was no increase in context-induced Fos levels in hippocampal CA1 and CA3 compared to home-cage Fos levels, while the Fos increased in the WT mice in CA1, CA3 and DG. When the SC-CA1 synapses in slices from mutant mice were stimulated using a theta-burst protocol, there was no impairment in LTP. Overall, these results suggest that at least one of those PKC-mediated phosphorylation sites regulates NMDAR-mediated signaling that modulates anxiety.

Altered thalamocortical development in the SAP102 knockout model of intellectual disability

Genetic mutations known to cause intellectual disabilities (IDs) are concentrated in specific sets of genes including both those encoding synaptic proteins and those expressed during early development. We have characterized the effect of genetic deletion of Dlg3, an ID-related gene encoding the synaptic NMDA-receptor interacting protein synapse-associated protein 102 (SAP102), on development of the mouse somatosensory cortex. SAP102 is the main representative of the PSD-95 family of postsynaptic MAGUK proteins during early development and is proposed to play a role in stabilizing receptors at immature synapses. Genetic deletion of SAP102 caused a reduction in the total number of thalamocortical (TC) axons innervating the somatosensory cortex, but did not affect the segregation of barrels. On a synaptic level SAP102 knockout mice display a transient speeding of NMDA receptor kinetics during the critical period for TC plasticity, despite no reduction in GluN2B-mediated component of synaptic transmission. These data indicated an interesting dissociation between receptor kinetics and NMDA subunit expression. Following the critical period NMDA receptor function was unaffected by loss of SAP102 but there was a reduction in the divergence of TC connectivity. These data suggest that changes in synaptic function early in development caused by mutations in SAP102 result in changes in network connectivity later in life.

Subunit-specific regulation of N-methyl-D-aspartate (NMDA) receptor trafficking by SAP102 protein splice variants

Synapse-associated protein 102 (SAP102) is a scaffolding protein abundantly expressed early in development that mediates glutamate receptor trafficking during synaptogenesis. Mutations in human SAP102 have been reported to cause intellectual disability, which is consistent with its important role during early postnatal development. SAP102 contains PDZ, SH3, and guanylate kinase (GK)-like domains, which mediate specific protein-protein interactions. SAP102 binds directly to N-methyl-D-aspartate receptors (NMDARs), anchors receptors at synapses, and facilitates transduction of NMDAR signals. Proper localization of SAP102 at the postsynaptic density is essential to these functions. However, how SAP102 is targeted to synapses is unclear. In the current study we find that synaptic localization of SAP102 is regulated by alternative splicing. The SAP102 splice variant that possesses a C-terminal insert (I2) between the SH3 and GK domains is highly enriched at dendritic spines. We also show that there is an intramolecular interaction between the SH3 and GK domains in SAP102 but that the I2 splicing does not influence SH3-GK interaction. Previously, we have shown that SAP102 expression promotes spine lengthening. We now find that the spine lengthening effect is independent of the C-terminal alternative splicing of SAP102. In addition, expression of I2-containing SAP102 isoforms is regulated developmentally. Knockdown of endogenous I2-containing SAP102 isoforms differentially affect NMDAR surface expression in a subunit-specific manner. These data shed new light on the role of SAP102 in the regulation of NMDAR trafficking.

Distinct regions within the GluN2C subunit regulate the surface delivery of NMDA receptors

N-methyl-D-aspartate (NMDA) receptors mediate fast excitatory synaptic transmission in the mammalian central nervous system. The activation of NMDA receptors plays a key role in brain development, synaptic plasticity, and memory formation, and is a major contributor to many neuropsychiatric disorders. Here, we investigated the mechanisms that underlie the trafficking of GluN1/GluN2C receptors. Using an approach combining molecular biology, microscopy, and electrophysiology in mammalian cell lines and cultured cerebellar granule cells, we found that the surface delivery of GluN2C-containing receptors is reduced compared to GluN2A- and GluN2B-containing receptors. Furthermore, we identified three distinct regions within the N-terminus, M3 transmembrane domain, and C-terminus of GluN2C subunits that are required for proper intracellular processing and surface delivery of NMDA receptors. These results shed new light on the regulation of NMDA receptor trafficking, and these findings can be exploited to develop new strategies for treating some forms of neuropsychiatric disorders.

Recent progress in understanding subtype specific regulation of NMDA receptors by G Protein Coupled Receptors (GPCRs)

G Protein Coupled Receptors (GPCRs) are the largest family of receptors whose ligands constitute nearly a third of prescription drugs in the market. They are widely involved in diverse physiological functions including learning and memory. NMDA receptors (NMDARs), which belong to the ionotropic glutamate receptor family, are likewise ubiquitously expressed in the central nervous system (CNS) and play a pivotal role in learning and memory. Despite its critical contribution to physiological and pathophysiological processes, few pharmacological interventions aimed directly at regulating NMDAR function have been developed to date. However, it is well established that NMDAR function is precisely regulated by cellular signalling cascades recruited downstream of G protein coupled receptor (GPCR) stimulation. Accordingly, the downstream regulation of NMDARs likely represents an important determinant of outcome following treatment with neuropsychiatric agents that target selected GPCRs. Importantly, the functional consequence of such regulation on NMDAR function varies, based not only on the identity of the GPCR, but also on the cell type in which relevant receptors are expressed. Indeed, the mechanisms responsible for regulating NMDARs by GPCRs involve numerous intracellular signalling molecules and regulatory proteins that vary from one cell type to another. In the present article, we highlight recent findings from studies that have uncovered novel mechanisms by which selected GPCRs regulate NMDAR function and consequently NMDAR-dependent plasticity.

Neuregulin-1 signalling and antipsychotic treatment: potential therapeutic targets in a schizophrenia candidate signalling pathway

Identifying the signalling pathways underlying the pathophysiology of schizophrenia is an essential step in the rational development of new antipsychotic drugs for this devastating disease. Evidence from genetic, transgenic and post-mortem studies have strongly supported neuregulin-1 (NRG1)-ErbB4 signalling as a schizophrenia susceptibility pathway. NRG1-ErbB4 signalling plays crucial roles in regulating neurodevelopment and neurotransmission, with implications for the pathophysiology of schizophrenia. Post-mortem studies have demonstrated altered NRG1-ErbB4 signalling in the brain of schizophrenia patients. Antipsychotic drugs have different effects on NRG1-ErbB4 signalling depending on treatment duration. Abnormal behaviours relevant to certain features of schizophrenia are displayed in NRG1/ErbB4 knockout mice or those with NRG1/ErbB4 over-expression, some of these abnormalities can be improved by antipsychotic treatment. NRG1-ErbB4 signalling has extensive interactions with the GABAergic, glutamatergic and dopaminergic neurotransmission systems that are involved in the pathophysiology of schizophrenia. These interactions provide a number of targets for the development of new antipsychotic drugs. Furthermore, the key interaction points between NRG1-ErbB4 signalling and other schizophrenia susceptibility genes may also potentially provide specific targets for new antipsychotic drugs. In general, identification of these targets in NRG1-ErbB4 signalling and interacting pathways will provide unique opportunities for the development of new generation antipsychotics with specific efficacy and fewer side effects.

SAP102 mediates synaptic clearance of NMDA receptors

Membrane-associated guanylate kinases (MAGUKs) are the major family of scaffolding proteins at the postsynaptic density. The PSD-MAGUK subfamily, which includes PSD-95, PSD-93, SAP97, and SAP102, is well accepted to be primarily involved in the synaptic anchoring of numerous proteins, including N-methyl-D-aspartate receptors (NMDARs). Notably, the synaptic targeting of NMDARs depends on the binding of the PDZ ligand on the GluN2B subunit to MAGUK PDZ domains, as disruption of this interaction dramatically decreases NMDAR surface and synaptic expression. We recently reported a secondary interaction between SAP102 and GluN2B, in addition to the PDZ interaction. Here, we identify two critical residues on GluN2B responsible for the non-PDZ binding to SAP102. Strikingly, either mutation of these critical residues or knockdown of endogenous SAP102 can rescue the defective surface expression and synaptic localization of PDZ binding-deficient GluN2B. These data reveal an unexpected, nonscaffolding role for SAP102 in the synaptic clearance of GluN2B-containing NMDARs.

Transcriptome analysis of amoeboid and ramified microglia isolated from the corpus callosum of rat brain

Background

Microglia, the resident immune cells of the central nervous system (CNS), have two distinct phenotypes in the developing brain: amoeboid form, known to be amoeboid microglial cells (AMC) and ramified form, known to be ramified microglial cells (RMC). The AMC are characterized by being proliferative, phagocytic and migratory whereas the RMC are quiescent and exhibit a slow turnover rate. The AMC transform into RMC with advancing age, and this transformation is indicative of the gradual shift in the microglial functions. Both AMC and RMC respond to CNS inflammation, and they become hypertrophic when activated by trauma, infection or neurodegenerative stimuli. The molecular mechanisms and functional significance of morphological transformation of microglia during normal development and in disease conditions is not clear. It is hypothesized that AMC and RMC are functionally regulated by a specific set of genes encoding various signaling molecules and transcription factors.

Results

To address this, we carried out cDNA microarray analysis using lectin-labeled AMC and RMC isolated from frozen tissue sections of the corpus callosum of 5-day and 4-week old rat brain respectively, by laser capture microdissection. The global gene expression profiles of both microglial phenotypes were compared and the differentially expressed genes in AMC and RMC were clustered based on their functional annotations. This genome wide comparative analysis identified genes that are specific to AMC and RMC.

Conclusions

The novel and specific molecules identified from the trancriptome explains the quiescent state functioning of microglia in its two distinct morphological states.

NO-dependent protective effect of VEGF against excitotoxicity on layer VI of the developing cerebral cortex

In industrialized countries, cerebral palsy affects 2.5‰ of preterm and term infants. At a neurochemical level, the massive release of glutamate constitutes a major process leading to excitotoxicity and neonatal brain lesions. Previous studies, conducted in the laboratory, revealed that, in (δ/δ)VEGF(A) transgenic mice, glutamate-induced brain lesions are exacerbated suggesting that VEGF(A) could play a protective action against excitotoxicity. Using a model of cultured cortical brain slices, the aim of the study was to characterize the central effects of VEGF against glutamate-induced excitotoxicity in neonates. Exposure of brain slices to glutamate induced a strong increase of necrotic cell death in the deep cortical layer VI and a decrease of apoptotic death in superficial layers II-IV. When administered alone, a 6-h treatment with VEGF(A) had no effect on both apoptotic and necrotic deaths. In contrast, VEGF(A) abolished the glutamate-induced necrosis observed in layer VI. While MEK and PI3-K inhibitors had no effect on the protective action of VEGF(A), L-NAME, a pan inhibitor of NOS, abrogated the effect of VEGF(A) and exacerbated the excitotoxic action of glutamate. Calcimetry experiments performed on brain slices revealed that VEGF(A) reduced the massive calcium influx induced by glutamate in layer VI and this effect was blocked by L-NAME. Neuroprotective effect of VEGF(A) was also blocked by LNIO and NPLA, two inhibitors of constitutive NOS, while AGH, an iNOS inhibitor, had no effect. Nitrite measurements, electron paramagnetic resonance spectroscopy and immunohistochemistry indicated that glutamate was a potent inducer of NO production via activation of nNOS in the cortical layer VI. In vivo administration of nNOS siRNA promoted excitotoxicity and mimicked the effects of L-NAME, LNIO and NPLA. A short-term glutamate treatment increased nNOS Ser1412 phosphorylation, while a long-term exposure inhibited nNOS/NR2B protein-protein interactions. Altogether, these findings indicate that, in deep cortical layers of mice neonates, glutamate stimulates nNOS activity. Contrasting with mature brain, NO production induced by high concentrations of glutamate is neuroprotective and is required for the anti-necrotic effect of VEGF(A).

Reciprocal signalling between NR2 subunits of the NMDA receptor and neuregulin1 and their role in schizophrenia

Schizophrenia is a debilitating neurodevelopmental psychiatric disorder. Both the N-methyl-D-aspartate receptor (NMDAR) and neuregulin1 (NRG1) are key molecules involved in normal brain development that have been linked to schizophrenia pathology and aetiology. The NR2 proteins are critical structural and functional subunits of the NMDAR and are developmentally and spatially regulated. Altered NR2 gene and protein expression has been found in human post-mortem schizophrenia brain tissue together with changes in NRG1 and its receptor ErbB4. The NR2 subunits and ErbB4 share a common anchoring domain on the postsynaptic density and therefore a disruption to either of these molecules may influence the functioning of the other. It has been shown that NRG1 signalling can affect NMDAR levels and function, particularly phosphorylation of the NR2 subunits. However little is known about the possible effects of NMDAR dysfunction on NRG1 signalling, which is important with regards to schizophrenia aetiology as numerous risk factors for the disorder can alter NMDAR functioning during early brain development. This review focuses on the role of the NMDA receptor subunits and NRG1 signalling in schizophrenia and proposes a mechanism by which a disruption to the NMDAR, particularly via altering the balance of NR2 subunits during early development, could influence NRG1 signalling.

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'.

Discovery of protein phosphorylation motifs through exploratory data analysis

Background

The need for efficient algorithms to uncover biologically relevant phosphorylation motifs has become very important with rapid expansion of the proteomic sequence database along with a plethora of new information on phosphorylation sites. Here we present a novel unsupervised method, called Motif Finder (in short, F-Motif) for identification of phosphorylation motifs. F-Motif uses clustering of sequence information represented by numerical features that exploit the statistical information hidden in some foreground data. Furthermore, these identified motifs are then filtered to find “actual” motifs with statistically significant motif scores.

Results and Discussion

We have applied F-Motif to several new and existing data sets and compared its performance with two well known state-of-the-art methods. In almost all cases F-Motif could identify all statistically significant motifs extracted by the state-of-the-art methods. More importantly, in addition to this, F-Motif uncovers several novel motifs. We have demonstrated using clues from the literature that most of these new motifs discovered by F-Motif are indeed novel. We have also found some interesting phenomena. For example, for CK2 kinase, the conserved sites appear only on the right side of S. However, for CDK kinase, the adjacent site on the right of S is conserved with residue P. In addition, three different encoding methods, including a novel position contrast matrix (PCM) and the simplest binary coding, are used and the ability of F-motif to discover motifs remains quite robust with respect to encoding schemes.

Conclusions

An iterative algorithm proposed here uses exploratory data analysis to discover motifs from phosphorylated data. The effectiveness of F-Motif has been demonstrated using several real data sets as well as using a synthetic data set. The method is quite general in nature and can be used to find other types of motifs also. We have also provided a server for F-Motif at http://f-motif.classcloud.org/, http://bio.classcloud.org/f-motif/ or http://ymu.classcloud.org/f-motif/.

Behavioral analysis of NR2C knockout mouse reveals deficit in acquisition of conditioned fear and working memory

N-methyl-D-aspartate (NMDA) receptors play an important role in excitatory neurotransmission and mediate synaptic plasticity associated with learning and memory. NMDA receptors are composed of two NR1 and two NR2 subunits and the identity of the NR2 subunit confers unique electrophysiologic and pharmacologic properties to the receptor. The precise role of NR2C-containing receptors in vivo is poorly understood. We have performed a battery of behavioral tests on NR2C knockout/nβ-galactosidase knock-in mice and found no difference in spontaneous activity, basal anxiety, forced-swim immobility, novel object recognition, pain sensitivity and reference memory in comparison to wildtype counterparts. However, NR2C knockout mice were found to exhibit deficits in fear acquisition and working memory compared to wildtype mice. Deficit in fear acquisition correlated with lack of fear conditioning-induced plasticity at the thalamo-amygdala synapse. These findings suggest a unique role of NR2C-containing receptors in associative and executive learning representing a novel therapeutic target for deficits in cognition.

Post-translational modification biology of glutamate receptors and drug addiction

Post-translational covalent modifications of glutamate receptors remain a hot topic. Early studies have established that this family of receptors, including almost all ionotropic and metabotropic glutamate receptor subtypes, undergoes active phosphorylation at serine, threonine, or tyrosine residues in their intracellular domains. Recent evidence identifies several glutamate receptor subtypes to be direct substrates for palmitoylation at cysteine residues. Other modifications such as ubiquitination and sumoylation at lysine residues also occur to certain glutamate receptors. These modifications are dynamic and reversible in nature and are regulatable by changing synaptic inputs. The regulated modifications significantly impact the receptor in many ways, including interrelated changes in biochemistry (synthesis, subunit assembling, and protein–protein interactions), subcellular redistribution (trafficking, endocytosis, synaptic delivery, and clustering), and physiology, usually associated with changes in synaptic plasticity. Glutamate receptors are enriched in the striatum and cooperate closely with dopamine to regulate striatal signaling. Emerging evidence shows that modification processes of striatal glutamate receptors are sensitive to addictive drugs, such as psychostimulants (cocaine and amphetamine). Altered modifications are believed to be directly linked to enduring receptor/synaptic plasticity and drug-seeking. This review summarizes several major types of modifications of glutamate receptors and analyzes the role of these modifications in striatal signaling and in the pathogenesis of psychostimulant addiction.

NMDA receptor-dependent regulation of dendritic spine morphology by SAP102 splice variants

Membrane-associated guanylate kinases (MAGUKs) are major components of the postsynaptic density and play important roles in synaptic organization and plasticity. Most excitatory synapses are located on dendritic spines, which are dynamic structures that undergo morphological changes during synapse formation and plasticity. Synapse-associated protein 102 (SAP102) is a MAGUK that is highly expressed early in development and mediates receptor trafficking during synaptogenesis. Mutations in human SAP102 cause mental retardation, which is often accompanied with abnormalities in dendritic spines. However, little is known about the role of SAP102 in regulating synapse formation or spine morphology. We now find that SAP102 contains a novel NMDA receptor binding site in the N-terminal domain, which is specific for the NR2B subunit. The interaction between SAP102 and NR2B is PDZ (postsynaptic density-95/Discs large/zona occludens-1) domain independent and is regulated by alternative splicing of SAP102. We show that SAP102 that possesses an N-terminal insert is developmentally regulated at both mRNA and protein levels. In addition, expression of SAP102 increases synapse formation. Furthermore, the alternative splicing of SAP102 regulates dendritic spine morphology. SAP102 containing the N-terminal insert promotes lengthening of dendritic spines and preferentially promotes the formation of synapses at long spines, whereas a short hairpin RNA knockdown of the same SAP102 splice variant causes spine shrinkage. Finally, blocking NMDA receptor activity prevents the spine lengthening induced by the N-terminal splice variant of SAP102. Thus, our data provide the first evidence that SAP102 links NMDA receptor activation to alterations in spine morphology.

Mitotic inhibition of GRASP65 organelle tethering involves Polo-like kinase 1 (PLK1) phosphorylation proximate to an internal PDZ ligand

GRASP65 links cis-Golgi cisternae via a homotypic, N-terminal PDZ interaction, and its mitotic phosphorylation disrupts this activity. Neither the identity of the PDZ ligand involved in the GRASP65 self-interaction nor the mechanism by which phosphorylation inhibits its interaction is known. Phospho-mimetic mutation of known cyclin-dependent kinase 1/cyclin B sites, all of which are in the C-terminal "regulatory domain" of the molecule, failed to block organelle tethering. However, we identified a site phosphorylated by Polo-like kinase 1 (PLK1) in the GRASP65 N-terminal domain for which mutation to aspartic acid blocked tethering and alanine substitution prevented mitotic Golgi unlinking. Further, using interaction assays, we discovered an internal PDZ ligand adjacent to the PLK phosphorylation site that was required for tethering. These results reveal the mechanism of phosphoinhibition as direct inhibition by PLK1 of the PDZ ligand underlying the GRASP65 self-interaction.

Casein kinase 2 regulates the NR2 subunit composition of synaptic NMDA receptors

N-methyl-D-aspartate (NMDA) receptors (NMDARs) play a central role in development, synaptic plasticity, and neurological disease. NMDAR subunit composition defines their biophysical properties and downstream signaling. Casein kinase 2 (CK2) phosphorylates the NR2B subunit within its PDZ-binding domain; however, the consequences for NMDAR localization and function are unclear. Here we show that CK2 phosphorylation of NR2B regulates synaptic NR2B and NR2A in response to activity. We find that CK2 phosphorylates NR2B, but not NR2A, to drive NR2B-endocytosis and remove NR2B from synapses resulting in an increase in synaptic NR2A expression. During development there is an activity-dependent switch from NR2B to NR2A at cortical synapses. We observe an increase in CK2 expression and NR2B phosphorylation over this same critical period and show that the acute activity-dependent switch in NR2 subunit composition at developing hippocampal synapses requires CK2 activity. Thus, CK2 plays a central role in determining the NR2 subunit content of synaptic NMDARs.

Changes in expression of NMDA-NR1 receptor subunits in the rostral ventromedial medulla modulate pain behaviors

NMDA receptors have an important role in pain facilitation in rostral ventromedial medulla (RVM) and the NR1 subunit is essential for its function. Studies suggest that the NMDA receptors in RVM are critical to modulate both cutaneous and muscle hypersensitivity induced by repeated intramuscular acid injections. We propose that increased expression of the NR1 subunit in the RVM is critical for the full development of hypersensitivity. To test this we used recombinant lentiviruses to over-express the NR1 subunit in the RVM and measured nociceptive sensitivity to cutaneous and muscle stimuli. We also downregulated the expression of NR1 in the RVM and measured the hyperalgesia produced by repeated-acid injections. Increasing the expression of NR1 in the RVM reduces cutaneous and muscle withdrawal threshold, and decreasing the expression of NR1 in the RVM increases the muscle withdrawal threshold and prevents the development of hyperalgesia in an animal model of muscle pain. These results suggest that the NR1 subunits in the RVM are critical for modulating NMDA receptor function, which in turn sets the 'tone' of the nervous system's response to noxious stimuli and tissue injury.

Glutamate receptor ion channels: structure, regulation, and function

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

PDZ domains and their binding partners: structure, specificity, and modification

PDZ domains are abundant protein interaction modules that often recognize short amino acid motifs at the C-termini of target proteins. They regulate multiple biological processes such as transport, ion channel signaling, and other signal transduction systems. This review discusses the structural characterization of PDZ domains and the use of recently emerging technologies such as proteomic arrays and peptide libraries to study the binding properties of PDZ-mediated interactions. Regulatory mechanisms responsible for PDZ-mediated interactions, such as phosphorylation in the PDZ ligands or PDZ domains, are also discussed. A better understanding of PDZ protein-protein interaction networks and regulatory mechanisms will improve our knowledge of many cellular and biological processes.

Facilitation of synaptic transmission and pain responses by CGRP in the amygdala of normal rats

Calcitonin gene-related peptide (CGRP) plays an important role in peripheral and central sensitization. CGRP also is a key molecule in the spino-parabrachial-amygdaloid pain pathway. Blockade of CGRP1 receptors in the spinal cord or in the amygdala has antinociceptive effects in different pain models. Here we studied the electrophysiological mechanisms of behavioral effects of CGRP in the amygdala in normal animals without tissue injury.Whole-cell patch-clamp recordings of neurons in the latero-capsular division of the central nucleus of the amygdala (CeLC) in rat brain slices showed that CGRP (100 nM) increased excitatory postsynaptic currents (EPSCs) at the parabrachio-amygdaloid (PB-CeLC) synapse, the exclusive source of CGRP in the amygdala. Consistent with a postsynaptic mechanism of action, CGRP increased amplitude, but not frequency, of miniature EPSCs and did not affect paired-pulse facilitation. CGRP also increased neuronal excitability. CGRP-induced synaptic facilitation was reversed by an NMDA receptor antagonist (AP5, 50 microM) or a PKA inhibitor (KT5720, 1 microM), but not by a PKC inhibitor (GF109203X, 1 microM). Stereotaxic administration of CGRP (10 microM, concentration in microdialysis probe) into the CeLC by microdialysis in awake rats increased audible and ultrasonic vocalizations and decreased hindlimb withdrawal thresholds. Behavioral effects of CGRP were largely blocked by KT5720 (100 microM) but not by GF109203X (100 microM).The results show that CGRP in the amygdala exacerbates nocifensive and affective behavioral responses in normal animals through PKA- and NMDA receptor-dependent postsynaptic facilitation. Thus, increased CGRP levels in the amygdala might trigger pain in the absence of tissue injury.

NMDAR-mediated EPSCs are maintained and accelerate in time course during maturation of mouse and rat auditory brainstem in vitro

NMDA receptors (NMDARs) mediate a slow EPSC at excitatory glutamatergic synapses throughout the brain. In many areas the magnitude of the NMDAR-mediated EPSC declines with development and is associated with changes in subunit composition, but the mature channel composition is often unknown. We have employed the calyx of Held terminal with its target, the principal neuron of the medial nucleus of the trapezoid body (MNTB), to examine the NMDAR-mediated EPSC during synapse maturation from P10 to P40. Our data show that the calyx has reached a mature state by around P18. The NMDAR-mediated EPSC amplitude (and dominant decay ) fell from around 5 nA (: 40-50 ms) at P10/11 to 0.3-0.5 nA (: 10-15 ms) by P18. The mature NMDAR-EPSC showed no sensitivity to ifenprodil, indicating lack of NR2B subunits, and no block by submicromolar concentrations of zinc, consistent with NR1-1b subunit expression. Additionally, from P11 to P18 there was a reduction in voltage-dependent block and the apparent dissociation constant for [Mg(2+)](o) (K(o)) changed from 7.5 to 14 mm. Quantitative PCR showed that the relative expression of NR2A and NR2C increased, while immunohistochemistry confirmed the presence of NR2A, NR2B and NR2C protein. Although the mature NMDAR-EPSC is small, it is well coupled to NO signalling, as indicated by DAR-4M imaging. We conclude that native mature NMDAR channels at the calyx of Held have a fast time course and reduced block by [Mg(2+)](o), consistent with dominance of NR2C subunits and functional exclusion of NR2B subunits. The pharmacology suggests a single channel type and we postulate that the mature NMDARs consist of heterotrimers of NR1-1b-NR2A-NR2C.

Improvement of two-way active avoidance memory requires protein kinase a activation and brain-derived neurotrophic factor expression in the dorsal hippocampus

Previous studies have shown that two-way active avoidance (TWAA) memory processing involves a functional interaction between the pontine wave (P wave) generator and the CA3 region of the dorsal hippocampus (DH-CA3). The present experiments examined whether the interaction between P wave generator activity and the DH-CA3 involves the intracellular protein kinase A (PKA) signaling system. In the first series of experiments, rats were subjected to a session of TWAA training followed immediately by bilateral microinjection of either the PKA activation inhibitor (KT-5720) or vehicle control into the DH-CA3 and tested for TWAA memory 24 h later. The results indicated that immediate KT-5720 infusion impaired improvement of TWAA performance. Additional experiments showed that KT-5720 infusion also blocked TWAA training-induced BDNF expression in the DH-CA3. Together, these findings suggest that the PKA activation and BDNF expression in the DH-CA3 is essential for the improvement of TWAA memory.

Growth factor-dependent trafficking of cerebellar NMDA receptors via protein kinase B/Akt phosphorylation of NR2C

NMDA receptor subunit composition varies throughout the brain, providing molecular diversity in NMDA receptor function. The NR2 subunits (NR2A-D) in large part dictate the distinct functional properties of NMDA receptors and differentially regulate receptor trafficking. Although the NR2C subunit is highly enriched in cerebellar granule cells and plays a unique role in cerebellar function, little is known about NR2C-specific regulation of NMDA receptors. Here, we demonstrate that PKB/Akt directly phosphorylates NR2C on serine 1096 (S1096). In addition, we identify 14-3-3epsilon as an NR2C interactor, whose binding is dependent on S1096 phosphorylation. Both growth factor stimulation and NMDA receptor activity lead to a robust increase in both phosphorylation of NR2C on S1096 and surface expression of cerebellar NMDA receptors. Finally, we find that NR2C expression, unlike NR2A and NR2B, supports neuronal survival. Thus, our data provide a direct mechanistic link between growth factor stimulation and regulation of cerebellar NMDA receptors.

NR2B receptor blockade inhibits pain-related sensitization of amygdala neurons

Pain-related sensitization and synaptic plasticity in the central nucleus of the amygdala (CeA) depend on the endogenous activation of NMDA receptors and phosphorylation of the NR1 subunit through a PKA-dependent mechanism. Functional NMDA receptors are heteromeric assemblies of NR1 with NR2A-D or NR3A, B subunits. NMDA receptors composed of NR1 and NR2B subunits have been implicated in neuroplasticity and are present in the CeA. Here we used a selective NR2B antagonist (Ro-256981) to determine the contribution of NR2B-containing NMDA receptors to pain-related sensitization of CeA neurons. Extracellular single-unit recordings were made from CeA neurons in anesthetized adult male rats before and during the development of an acute arthritis. Arthritis was induced in one knee joint by intraarticular injections of kaolin and carrageenan. Brief (15 s) mechanical stimuli of innocuous (100-500 g/30 mm2) and noxious (1000-2000 g/30 mm2) intensity were applied to the knee and other parts of the body. In agreement with our previous studies, all CeA neurons developed increased background and evoked activity after arthritis induction. Ro-256981 (1, 10 and 100 muM; 15 min each) was administered into the CeA by microdialysis 5-6 h postinduction of arthritis. Ro-256981 concentration-dependently decreased evoked responses, but not background activity. This pattern of effect is different from that of an NMDA receptor antagonist (AP5) in our previous studies. AP5 (100 microM - 5 mM) inhibited background activity and evoked responses. The differential effects of AP5 and Ro-256981 may suggest that NMDA receptors containing the NR2B subunit are important but not sole contributors to pain-related changes of CeA neurons.

Intrathecal clonidine suppresses phosphorylation of the N-methyl-D-aspartate receptor NR1 subunit in spinal dorsal horn neurons of rats with neuropathic pain

BACKGROUND

Intrathecal (IT) administration of the alpha-2 adrenoceptor agonist, clonidine, produces significant analgesic effects. Although several mechanisms underlying clonidine-induced analgesia have been proposed, the possible interaction with N-methyl-D-aspartate (NMDA) receptors as a major antinociceptive mechanism has not been addressed. We designed the present study to determine whether clonidine or other analgesics can affect spinal NMDA receptor activation in rats with chronic constriction injury (CCI)-induced neuropathy.

METHODS

Rats underwent unilateral CCI, and received IT clonidine (1, 5, 20 microg/rat), [D-Ala2, NMe-Phe4, Gly-ol5]-enkephalin (DAMGO, mu opioid receptor agonist, 1 microg/rat), gabapentin (anticonvulsant, 100 microg/rat) or vehicle 2 wks later. After drug injection, we measured the pain response to thermal or mechanical stimuli and used immunohistochemistry to evaluate spinal cord phosphorylated NMDA-receptor subunit 1 (pNR1) expression.

RESULTS

Two weeks after CCI surgery, rats displayed significant mechanical allodynia and thermal hyperalgesia, and the spinal cord dorsal horn showed a significant increase in the number of pNR1 immunoreactive neurons. IT injection of clonidine (20 microg/rat), DAMGO and gabapentin potently reduced mechanical allodynia and thermal hyperalgesia. Importantly, IT clonidine, but not IT DAMGO or gabapentin, dose-dependently reduced CCI-induced pNR1 expression in all lamina of the spinal cord dorsal horn by 30 min after injection. In addition, IT injection of the alpha-2 adrenoceptor antagonist, idazoxan (40 microg/rat) 10 min before clonidine injection completely reversed clonidine's antihyperalgesic and antiallodynic effects, as well as clonidine's suppressive effect on CCI-induced NR1 phosphorylation in the spinal cord dorsal horn.

CONCLUSIONS

Our data indicate that IT clonidine's antihyperalgesic/antiallodynic effect on neuropathic pain is associated with a significant reduction in spinal NMDA receptor phosphorylation and suggests a potentially novel mechanism of clonidine's action.

Activation of recombinant NR1/NR2C NMDA receptors

The N-methyl-d-aspartate (NMDA) subtype of ionotropic glutamate receptors comprises both NR1 and NR2 subunits, and plays numerous roles in both physiological and pathophysiological processes in the central nervous system (CNS). NR2C-containing NMDA receptors are most abundant in cerebellum, thalamus and olfactory bulb, and are also expressed in oligodendrocytes and hippocampal interneurons. We have used patch clamp recording to explore the activation properties of recombinant NR1/NR2C receptors expressed in HEK293 cells. NR1/NR2C receptors activated by a maximally effective concentration of glutamate and glycine had two main conductance levels of 45 pS and 28 pS when the extracellular Ca(2+) concentration was 0.5 mm and the holding potential was -80 mV. The occurrence of the lower subconductance state was reduced in the absence of extracellular Ca(2+). The distribution of closed durations recorded from patches with a high probability of containing only one active channel were best fitted by five exponential functions; the apparent open duration histogram could be fitted by two exponential functions (n = 10 patches). The apparent mean open time of NR1/NR2C receptors was brief (0.52 +/- 0.04 ms), suggesting that the stability of the open state of the NR1/NR2C receptors is lower than other NR2-containing receptors. NR1/NR2C open probability was exceptionally low, being 0.011 +/- 0.002 in patches containing a single active receptor (n = 8). Fast agonist concentration jumps were performed on outside out patches with multiple NR1/NR2C channels, which activated with a 10-90% rise time of 3.9 +/- 0.4 ms, faster than other NR2-containing receptors. The deactivation time constant after a brief (5-8 ms) application of a maximally effective concentration of agonists was 319 +/- 34 ms. The majority of the patches also showed a modest level of desensitization that could be described by either a single or a double exponential time course with the fastest time constant between 15 and 47 ms. Conceptual models of activation were fitted using the maximum interval likelihood (MIL) method to the sequence of open and closed durations recorded from outside-out patches that contained one active NR1/NR2C channel. NR1/NR2C receptor properties including modest desensitization and low open probability could be described by gating schemes similar to those previously proposed for other NMDA receptor subunit combinations.

PKA and ERK, but not PKC, in the amygdala contribute to pain-related synaptic plasticity and behavior

The laterocapsular division of the central nucleus of the amygdala (CeLC) has emerged as an important site of pain-related plasticity and pain modulation. Glutamate and neuropeptide receptors in the CeLC contribute to synaptic and behavioral changes in the arthritis pain model, but the intracellular signaling pathways remain to be determined. This study addressed the role of PKA, PKC, and ERK in the CeLC. Adult male Sprague-Dawley rats were used in all experiments. Whole-cell patch-clamp recordings of CeLC neurons were made in brain slices from normal rats and from rats with a kaolin/carrageenan-induced monoarthritis in the knee (6 h postinduction). Membrane-permeable inhibitors of PKA (KT5720, 1 μM; cAMPS-Rp, 10 μM) and ERK (U0126, 1 μM) activation inhibited synaptic plasticity in slices from arthritic rats but had no effect on normal transmission in control slices. A PKC inhibitor (GF109203x, 1 μM) and an inactive structural analogue of U0126 (U0124, 1 μM) had no effect. The NMDA receptor-mediated synaptic component was inhibited by KT5720 or U0126; their combined application had additive effects. U0126 did not inhibit synaptic facilitation by forskolin-induced PKA-activation. Administration of KT5720 (100 μM, concentration in microdialysis probe) or U0126 (100 μM) into the CeLC, but not striatum (placement control), inhibited audible and ultrasonic vocalizations and spinal reflexes of arthritic rats but had no effect in normal animals. GF109203x (100 μM) and U0124 (100 μM) did not affect pain behavior. The data suggest that in the amygdala PKA and ERK, but not PKC, contribute to pain-related synaptic facilitation and behavior by increasing NMDA receptor function through independent signaling pathways.

Differential interaction of NMDA receptor subtypes with the post-synaptic density-95 family of membrane associated guanylate kinase proteins

NMDA receptors are a subclass of ionotropic glutamate receptors. They are trafficked and/or clustered at synapses by the post-synaptic density (PSD)-95 membrane associated guanylate kinase (MAGUK) family of scaffolding proteins that associate with NMDA receptor NR2 subunits via their C-terminal glutamate serine (aspartate/glutamate) valine motifs. We have carried out a systematic study investigating in a heterologous expression system, the association of the four major NMDA receptor subtypes with the PSD-95 family of MAGUK proteins, chapsyn-110, PSD-95, synapse associated protein (SAP) 97 and SAP102. We report that although each PSD-95 MAGUK was shown to co-immunoprecipitate with NR1/NR2A, NR1/NR2B, NR1/NR2C and NR1/NR2D receptor subtypes, they elicited differential effects with regard to the enhancement of total NR2 subunit expression which then results in an increased cell surface expression of NMDA receptor subtypes. PSD-95 and chapsyn-110 enhanced NR2A and NR2B total expression which resulted in increased NR1/NR2A and NR1/NR2B receptor cell surface expression whereas SAP97 and SAP102 had no effect on total or cell surface expression of these subtypes. PSD-95, chapsyn-110, SAP97 and SAP102 had no effect on either total NR2C and NR2D subunit expression or cell surface NR1/NR2C and NR1/NR2D expression. A comparison of PSD-95alpha, PSD-95beta and PSD-95alpha(C3S,C5S) showed that PSD-95-enhanced cell surface expression of NR1/NR2A receptors was dependent upon the PSD-95 N-terminal C3,C5 cysteines. These observations support differential interaction of NMDA receptor subtypes with different PSD-95 MAGUK scaffolding proteins. This has implications for the stabilisation, turnover and compartmentalisation of NMDA receptor subtypes in neurones during development and in the mature brain.

CREB modulates the functional output of nucleus accumbens neurons: a critical role of N-methyl-D-aspartate glutamate receptor (NMDAR) receptors

Nucleus accumbens (NAc) medium spiny neurons cycle between two states, a functionally inactive downstate and a functionally active upstate. Here, we show that activation of the transcription factor cAMP-response element-binding protein (CREB), a common molecular response to several drugs of abuse, increases both duration of the upstate and action potential firing during the upstate. This effect of CREB is mediated by enhanced N-methyl-d-aspartate glutamate receptor (NMDAR) function: increased CREB activity increases both NMDAR-mediated synaptic currents and surface level of NMDARs, while inhibition of NMDARs abolishes the effect of CREB on upstate duration. Furthermore, mimicking the effect of CREB by pharmacological enhancement of NMDAR function in the NAc in vivo suppressed novelty- and cocaine-elicited locomotor activity. These findings suggest that by enhancing NMDAR-mediated synaptic transmission, CREB activation promotes the proportion of time NAc neurons spend in the upstate. This effect, along with the CREB enhancement of NAc membrane excitability (Dong, Y., Green, T., Saal, D., Marie, H., Neve, R., Nestler, E. J., and Malenka, R. C. (2006) Nat. Neurosci. 9, 475-477), may counteract drug-induced maladaptations in the NAc and thus ameliorate the addictive state.

Rapid Bidirectional Switching of Synaptic NMDA Receptors

Synaptic NMDA-type glutamate receptors (NMDARs) play important roles in synaptic plasticity, brain development, and pathology. In the last few years, the view of NMDARs as relatively fixed components of the postsynaptic density has changed. A number of studies have now shown that both the number of receptors and their subunit compositions can be altered. During development, the synaptic NMDARs subunit composition changes, switching from predominance of NR2B-containing to NR2A-containing receptors, but little is known about the mechanisms involved in this developmental process. Here, we report that, depending on the pattern of NMDAR activation, the subunit composition of synaptic NMDARs is under extremely rapid, bidirectional control at neonatal synapses. This switching, which is at least as rapid as that seen with AMPARs, will have immediate and dramatic consequences on the integrative capacity of the synapse.

Regulation of NMDA receptors by phosphorylation

N-methyl-D-aspartate (NMDA) receptors are critical for neuronal development and synaptic plasticity. The molecular mechanisms underlying the synaptic localization and functional regulation of NMDA receptors have been the subject of extensive studies. In particular, phosphorylation has emerged as a fundamental mechanism that regulates NMDA receptor trafficking and can alter the channel properties of NMDA receptors. Here we summarize recent advances in the characterization of NMDA receptor phosphorylation, emphasizing subunit-specific phosphorylation, which differentially controls the trafficking and surface expression of NMDA receptors.

Phosphorylation of glutamate receptors: a potential mechanism for the regulation of receptor function and psychostimulant action

Ionotropic glutamate receptors, N-methyl-d-aspartate receptors (NMDARs) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors (AMPARs), are densely distributed in the mammalian brain and actively regulate a variety of cellular activities. Expression and function of these receptors are also under a tight regulation by many molecular mechanisms. Protein phosphorylation represents one of the important mechanisms for the posttranslational modulation of these receptors. Constitutive and regulatory phosphorylation occurs at distinct sites (serine, threonine, or tyrosine) on the intracellular C-terminal domain of almost all subunits capable of assembling a functional channel. Several key protein kinases, such as protein kinase A, protein kinase C, Ca(2+)/calmodulin-dependent protein kinases, and tyrosine kinases are involved in the site-specific catalyzation and regulation of NMDAR and AMPAR phosphorylation. Through the phosphorylation mechanism, these protein kinases as well as protein phosphatases control biochemical properties (biosynthesis, delivery, and subunit assembling), subcellular distribution, and interactions of these receptors with various synaptic proteins, which ultimately modify the efficacy and strength of excitatory synapses containing NMDARs and AMPARs and many forms of synaptic plasticity. Emerging evidence shows that psychostimulants (cocaine and amphetamine) are among effective agents that profoundly alter the phosphorylation status of both receptors in striatal neurons in vivo. Thus, psychostimulants may modulate NMDAR and AMPAR function through the phosphorylation mechanism to shape the excitatory synaptic plasticity related to additive properties of drugs of abuse.

Characterisation of the human NMDA receptor subunit NR3A glycine binding site

In this study, we characterise the binding site of the human N-methyl-d-aspartate (NMDA) receptor subunit NR3A. Saturation radioligand binding of the NMDA receptor agonists [(3)H]-glycine and [(3)H]-glutamate showed that only glycine binds to human NR3A (hNR3A) with high affinity (K(d)=535nM (277-793nM)). Eight amino acids, which correspond to amino acids that are critical for ligand binding to other NMDA receptor subunits, situated within the S1S2 predicted ligand binding domain of hNR3A were mutated, which resulted in complete or near complete loss of [(3)H]-glycine binding to hNR3A. The NMDA NR1 glycine site agonist d-serine and partial agonist HA-966 (3-amino-1-hydroxypyrrolid-2-one), similarly to glycine displaced [(3)H]-glycine monophasically, suggesting a single common binding site. However, neither the partial agonist d-cycloserine nor the antagonist 7-chlorokynurenic acid displaced [(3)H]-glycine. Using homology modelling, a model of the NR3A binding pocket was generated which we suggest can be used to identify candidate agonists and antagonists. Our data show that glycine is a ligand, and most probably the endogenous ligand, for native NR3A at a binding site with unique pharmacological characteristics.