Subtype-specific regulation of recombinant NMDA receptor-channels by protein tyrosine kinases of the src family

Src dependency of the regulation of LTP by alternative splicing of GRIN1 exon 5

Alternative splicing of Grin1 exon 5 regulates induction of long-term potentiation (LTP) at Schaffer collateral-CA1 synapses: LTP in mice lacking the GluN1 exon 5-encoded N1 cassette (GluN1a mice) is significantly increased compared with that in mice compulsorily expressing this exon (GluN1b mice). The mechanism underlying this difference is unknown. Here, we report that blocking the non-receptor tyrosine kinase Src prevents induction of LTP in GluN1a mice but not in GluN1b. We find that activating Src enhances pharmacologically isolated synaptic N-methyl-d-aspartate receptor (NMDAR) currents in GluN1a mice but not in GluN1b. Moreover, we observe that Src activation increases the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor component of Schaffer collateral-evoked excitatory post-synaptic potentials in GluN1a mice, but this increase is prevented by blocking NMDARs. We conclude that at these synapses, NMDARs in GluN1a mice are subject to upregulation by Src that mediates induction of LTP, whereas NMDARs in GluN1b mice are not regulated by Src, leading to Src-resistance of LTP. Thus, we have uncovered that a key regulatory mechanism for synaptic potentiation is gated by differential splicing of exon 5 of Grin1. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

CD3ζ-Mediated Signaling Protects Retinal Ganglion Cells in Glutamate Excitotoxicity of the Retina

Excessive levels of glutamate activity could potentially damage and kill neurons. Glutamate excitotoxicity is thought to play a critical role in many CNS and retinal diseases. Accordingly, glutamate excitotoxicity has been used as a model to study neuronal diseases. Immune proteins, such as major histocompatibility complex (MHC) class I molecules and their receptors, play important roles in many neuronal diseases, while T-cell receptors (TCR) are the primary receptors of MHCI. We previously showed that a critical component of TCR, CD3ζ, is expressed by mouse retinal ganglion cells (RGCs). The mutation of CD3ζ or MHCI molecules compromises the development of RGC structure and function. In this study, we investigated whether CD3ζ-mediated molecular signaling regulates RGC death in glutamate excitotoxicity. We show that mutation of CD3ζ significantly increased RGC survival in NMDA-induced excitotoxicity. In addition, we found that several downstream molecules of TCR, including Src (proto-oncogene tyrosine-protein kinase) family kinases (SFKs) and spleen tyrosine kinase (Syk), are expressed by RGCs. Selective inhibition of an SFK member, Hck, or Syk members, Syk or Zap70, significantly increased RGC survival in NMDA-induced excitotoxicity. These results provide direct evidence to reveal the underlying molecular mechanisms that control RGC death under disease conditions.

An innate immune response to adeno-associated virus genomes decreases cortical dendritic complexity and disrupts synaptic transmission

Recombinant adeno-associated viruses (AAVs) allow rapid and efficient gene delivery to the nervous system, are widely used in neuroscience research, and are the basis of FDA-approved neuron-targeting gene therapies. Here we find that an innate immune response to the AAV genome reduces dendritic length and complexity and disrupts synaptic transmission in mouse somatosensory cortex. Dendritic loss is apparent 3 weeks after injection of experimentally relevant viral titers, is not restricted to a particular capsid serotype, transgene, promoter, or production facility, and cannot be explained by responses to surgery or transgene expression. AAV-associated dendritic loss is accompanied by a decrease in the frequency and amplitude of miniature excitatory postsynaptic currents and an increase in the proportion of GluA2-lacking, calcium-permeable AMPA receptors. The AAV genome is rich in unmethylated CpG DNA, which is recognized by the innate immunoreceptor Toll-like receptor 9 (TLR9), and acutely blocking TLR9 preserves dendritic complexity and AMPA receptor subunit composition in AAV-injected mice. These results reveal unexpected impacts of an immune response to the AAV genome on neuronal structure and function and identify approaches to improve the safety and efficacy of AAV-mediated gene delivery in the nervous system.

Homocysteine-induced sustained GluN2A NMDA receptor stimulation leads to mitochondrial ROS generation and neurotoxicity

Homocysteine, a sulfur-containing amino acid derived from methionine metabolism, is a known agonist of N-methyl-D-aspartate receptor (NMDAR) and is involved in neurotoxicity. Our previous findings showed that neuronal exposure to elevated homocysteine levels leads to sustained low-level increase in intracellular Ca2+, which is dependent on GluN2A subunit-containing NMDAR (GluN2A-NMDAR) stimulation. These studies further showed a role of ERK MAPK in homocysteine-GluN2A-NMDAR-mediated neuronal death. However, the intracellular mechanisms associated with such sustained GluN2A-NMDAR stimulation and subsequent Ca2+ influx have remained unexplored. Using live-cell imaging with Fluo3-AM and biochemical approaches, we show that homocysteine-GluN2A NMDAR-induced initial Ca2+ influx triggers sequential phosphorylation and subsequent activation of the proline rich tyrosine kinase 2 (Pyk2) and Src family kinases, which in turn phosphorylates GluN2A-Tyr1325 residue of GluN2A-NMDARs to maintain channel activity. The continuity of this cycle of events leads to sustained Ca2+ influx through GluN2A-NMDAR. Our findings also show that lack of activation of the regulatory tyrosine phosphatase STEP, which can limit Pyk2 and Src family kinase activity further contributes to the maintenance of this cycle. Additional studies using live-cell imaging of neurons expressing a redox-sensitive GFP targeted to the mitochondrial matrix show that treatment with homocysteine leads to a progressive increase in mitochondrial reactive oxygen species generation, which is dependent on GluN2A-NMDAR-mediated sustained ERK MAPK activation. This later finding demonstrates a novel role of GluN2A-NMDAR in homocysteine-induced mitochondrial ROS generation and highlights the role of ERK MAPK as the intermediary signaling pathway between GluN2A-NMDAR stimulation and mitochondrial reactive oxygen species generation.

Cellular mechanisms of contextual fear memory reconsolidation: Role of hippocampal SFKs, TrkB receptors and GluN2B-containing NMDA receptors

Memories are stored into long-term representations through a process that depends on protein synthesis. However, a consolidated memory is not static and inflexible and can be reactivated under certain circumstances, the retrieval is able to reactivate memories and destabilize them engaging a process of restabilization known as reconsolidation. Although the molecular mechanisms that mediate fear memory reconsolidation are not entirely known, so here we investigated the molecular mechanisms in the hippocampus involved in contextual fear conditioning memory (CFC) reconsolidation in male Wistar rats. We demonstrated that the blockade of Src family kinases (SFKs), GluN2B-containing NMDA receptors and TrkB receptors (TrkBR) in the CA1 region of the hippocampus immediately after the reactivation session impaired contextual fear memory reconsolidation. These impairments were blocked by the neurotrophin BDNF and the NMDAR agonist, D-Serine. Considering that the study of the link between synaptic proteins is crucial for understanding memory processes, targeting the reconsolidation process may provide new ways of disrupting maladaptive memories, such as those seen in post-traumatic stress disorder. Here we provide new insights into the cellular mechanisms involved in contextual fear memory reconsolidation, demonstrating that SFKs, GluN2B-containing NMDAR, and TrkBR are necessary for the reconsolidation process. Our findings suggest a link between BDNF and SFKs and GluN2B-containing NMDAR as well as a link between NMDAR and SFKs and TrkBR in fear memory reconsolidation. These preliminary pharmacological findings provide new evidence of the mechanisms involved in the reconsolidation of fear memory and have the potential to contribute to the development of treatments for psychiatric disorders involving maladaptive memories.

Endogenous ion channels expressed in human embryonic kidney (HEK-293) cells

Mammalian expression systems, particularly the human embryonic kidney (HEK-293) cells, combined with electrophysiological studies, have greatly benefited our understanding of the function, characteristic, and regulation of various ion channels. It was previously assumed that the existence of endogenous ion channels in native HEK-293 cells could be negligible. Still, more and more ion channels are gradually reported in native HEK-293 cells, which should draw our attention. In this regard, we summarize the different ion channels that are endogenously expressed in HEK-293 cells, including voltage-gated Na⁺ channels, Ca²⁺ channels, K⁺ channels, Cl^- channels, nonselective cation channels, TRP channels, acid-sensitive ion channels, and Piezo channels, which may complicate the recording of the heterogeneously expressed ion channels to a certain degree. We noted that the expression patterns and channel profiles varied with different studies, which may be due to the distinct originality of the cells, cell culture conditions, passage numbers, and different recording protocols. Therefore, a better knowledge of endogenous ion channels may help minimize potential problems in characterizing heterologously expressed ion channels. Based on this, it is recommended that HEK-293 cells from unknown sources should be examined before transfection for the characterization of their functional profile, especially when the expression level of exogenous ion channels does not overwhelm the endogenous ion channels largely, or the current amplitude is not significantly higher than the native currents.

Synaptic tau: A pathological or physiological phenomenon?

In this review, we discuss the synaptic aspects of Tau pathology occurring during Alzheimer's disease (AD) and how this may relate to memory impairment, a major hallmark of AD. Whilst the clinical diagnosis of AD patients is a loss of working memory and long-term declarative memory, the histological diagnosis is the presence of neurofibrillary tangles of hyperphosphorylated Tau and Amyloid-beta plaques. Tau pathology spreads through synaptically connected neurons to impair synaptic function preceding the formation of neurofibrillary tangles, synaptic loss, axonal retraction and cell death. Alongside synaptic pathology, recent data suggest that Tau has physiological roles in the pre- or post- synaptic compartments. Thus, we have seen a shift in the research focus from Tau as a microtubule-stabilising protein in axons, to Tau as a synaptic protein with roles in accelerating spine formation, dendritic elongation, and in synaptic plasticity coordinating memory pathways. We collate here the myriad of emerging interactions and physiological roles of synaptic Tau, and discuss the current evidence that synaptic Tau contributes to pathology in AD.

Src and Fyn regulation of NMDA receptors in health and disease

The Src family kinases (SFKs) are cytoplasmic non-receptor tyrosine kinases involved in multiple signalling pathways. In the central nervous system (CNS), SFKs are key regulators of N-methyl-d-aspartate receptor (NMDAR) function and major points of convergence for neuronal transduction pathways. Physiological upregulation of NMDAR activity by members of the SFKs, namely Src and Fyn, is crucial for induction of plasticity at Schaffer collateral-CA1 synapses of the hippocampus. Aberrant SFK regulation of NMDARs is implicated in several pathological conditions in the CNS including schizophrenia and pain hypersensitivity. Here, evidence is presented to highlight the current understanding of the intermolecular interactions of SFKs within the NMDAR macromolecular complex, the upstream regulators of SFK activity on NMDAR function and the role Src and Fyn have in synaptic plasticity and metaplasticity. The targeting of SFK protein-protein interactions is discussed as a potential therapeutic strategy to restore signalling activity underlying glutamatergic dysregulation in CNS disease pathophysiology.

Leptin modulates pancreatic β-cell membrane potential through Src kinase-mediated phosphorylation of NMDA receptors

The adipocyte-derived hormone leptin increases trafficking of KATP and Kv2.1 channels to the pancreatic β-cell surface, resulting in membrane hyperpolarization and suppression of insulin secretion. We have previously shown that this effect of leptin is mediated by the NMDA subtype of glutamate receptors (NMDARs). It does so by potentiating NMDAR activity, thus enhancing Ca2+ influx and the ensuing downstream signaling events that drive channel trafficking to the cell surface. However, the molecular mechanism by which leptin potentiates NMDARs in β-cells remains unknown. Here, we report that leptin augments NMDAR function via Src kinase-mediated phosphorylation of the GluN2A subunit. Leptin-induced membrane hyperpolarization diminished upon pharmacological inhibition of GluN2A but not GluN2B, indicating involvement of GluN2A-containing NMDARs. GluN2A harbors tyrosine residues that, when phosphorylated by Src family kinases, potentiate NMDAR activity. We found that leptin increases phosphorylation of Tyr-418 in Src, an indicator of kinase activation. Pharmacological inhibition of Src or overexpression of a kinase-dead Src mutant prevented the effect of leptin, whereas a Src kinase activator peptide mimicked it. Using mutant GluN2A overexpression, we show that Tyr-1292 and Tyr-1387 but not Tyr-1325 are responsible for the effect of leptin. Importantly, β-cells from db/db mice, a type 2 diabetes mouse model lacking functional leptin receptors, or from obese diabetic human donors failed to respond to leptin but hyperpolarized in response to NMDA. Our study reveals a signaling pathway wherein leptin modulates NMDARs via Src to regulate β-cell excitability and suggests NMDARs as a potential target to overcome leptin resistance.

Antimania-Like Effect of Panax ginseng Regulating the Glutamatergic Neurotransmission in REM-Sleep Deprivation Rats

Previous studies have shown the therapeutic properties of ginseng and ginsenosides on hyperactive and impulsive behaviors in several psychiatric diseases. Herein, we investigated the effect of Panax ginseng Meyer (PG) on hyperactive/impulsive behaviors in a manic-like animal model, sleep deprivation (SD) rats. Male rats were sleep-deprived for 48 h, and PG (200 mg/kg) was administered for 4 days, from 2 days prior to the start of SD to the end date of SD. The elevated plus maze (EPM) test showed that PG alleviated the increased frequency of entries into and spent time within open arms by SD. In order to investigate the molecular mechanism on this effect of PG, we assessed differentially expressed genes (DEGs) in the prefrontal cortex of PG-treated SD rats using RNA sequencing (RNA-seq) and performed gene-enrichment analysis for DEGs. The gene-enrichment analysis showed that PG most prominently affected the glutamatergic synapse pathway. Among the glutamatergic synapse pathway genes, particularly, PG enhanced the expressions of glutamate transporter Slc1a3 and Slc1a2 reduced in SD rats. Moreover, we found that PG could inhibit the SD-induced phosphorylation of the NR2A subunit of the NMDA receptor. These results suggested that PG might have a therapeutic effect against the manic-like behaviors, regulating the glutamatergic neurotransmission.

A common mechanism allows selective targeting of GluN2B subunit-containing N-methyl-D-aspartate receptors

N-methyl-D-aspartate receptors (NMDARs), especially GluN2B-containing NMDARs, are associated with neurodegenerative diseases like Parkinson, Alzheimer and Huntington based on their high Ca2+ conductivity. Overactivation leads to high intracellular Ca2+ concentrations and cell death rendering GluN2B-selective inhibitors as promising drug candidates. Ifenprodil represents the first highly potent prototypical, subtype-selective inhibitor of GluN2B-containing NMDARs. However, activity of ifenprodil on serotonergic, adrenergic and sigma receptors limits its therapeutic use. Structural reorganization of the ifenprodil scaffold to obtain 3-benzazepines retained inhibitory GluN2B activity but decreased the affinity at the mentioned non-NMDARs. While scaffold optimization improves the selectivity, the molecular inhibitory mechanism of these compounds is still not known. Here, we show a common inhibitory mechanism of ifenprodil and the related 3-benzazepines by mutational modifications of the receptor binding site, chemical modifications of the 3-benzazepine scaffold and subsequent in silico simulation of the inhibitory mechanism.

Novel Players in the Aging Synapse: Impact on Cognition

While neuronal loss has long been considered as the main contributor to age-related cognitive decline, these alterations are currently attributed to gradual synaptic dysfunction driven by calcium dyshomeostasis and alterations in ionotropic/metabotropic receptors. Given the key role of the hippocampus in encoding, storage, and retrieval of memory, the morpho- and electrophysiological alterations that occur in the major synapse of this network-the glutamatergic-deserve special attention. We guide you through the hippocampal anatomy, circuitry, and function in physiological context and focus on alterations in neuronal morphology, calcium dynamics, and plasticity induced by aging and Alzheimer's disease (AD). We provide state-of-the art knowledge on glutamatergic transmission and discuss implications of these novel players for intervention. A link between regular consumption of caffeine-an adenosine receptor blocker-to decreased risk of AD in humans is well established, while the mechanisms responsible have only now been uncovered. We review compelling evidence from humans and animal models that implicate adenosine A2A receptors (A2AR) upsurge as a crucial mediator of age-related synaptic dysfunction. The relevance of this mechanism in patients was very recently demonstrated in the form of a significant association of the A2AR-encoding gene with hippocampal volume (synaptic loss) in mild cognitive impairment and AD. Novel pathways implicate A2AR in the control of mGluR5-dependent NMDAR activation and subsequent Ca2+ dysfunction upon aging. The nature of this receptor makes it particularly suited for long-term therapies, as an alternative for regulating aberrant mGluR5/NMDAR signaling in aging and disease, without disrupting their crucial constitutive activity.

HIV gp120-induced neuroinflammation potentiates NMDA receptors to overcome basal suppression of inhibitory synapses by p38 MAPK

HIV-associated neurocognitive disorder affects about half of HIV-infected patients. HIV impairs neuronal function through indirect mechanisms mainly mediated by inflammatory cytokines and neurotoxic viral proteins, such as the envelope protein gp120. HIV gp120 elicits a neuroinflammatory response that potentiates NMDA receptor function and induces the loss of excitatory synapses. How gp120 influences neuronal inhibition remains unknown. In this study, we expressed a green fluorescent protein (GFP)-tagged recombinant antibody-like protein that binds to the post-synaptic scaffolding protein gephyrin to label inhibitory synapses in living neurons. Treatment with 600 pM gp120 for 24 h increased the number of labeled inhibitory synapses. HIV gp120 evoked the release of interleukin-1β (IL-1β) from microglia to activate IL-1 receptors on neurons. Subsequent activation of the tyrosine kinase Src and GluN2A-containing NMDA receptors increased the number of inhibitory synapses via a process that required protein synthesis. In naïve cultures, inhibition of neuronal p38 mitogen-activated protein kinase (p38 MAPK) increased the number of inhibitory synapses suggesting that p38 MAPK produces a basal suppression of inhibitory synapses that is overcome in the presence of gp120. Direct activation of a mutant form of p38 MAPK expressed in neurons mimicked basal suppression of inhibitory synapses. This study shows for the first time that gp120-induced neuroinflammation increases the number of inhibitory synapses and that this increase overcomes a basal suppression of synaptic inhibition. Increased inhibition may be an adaptive mechanism enabling neurons to counteract excess excitatory input in order to maintain network homeostasis. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

Activation of Phosphotyrosine-Mediated Signaling Pathways in the Cortex and Spinal Cord of SOD1G93A, a Mouse Model of Familial Amyotrophic Lateral Sclerosis

Degeneration of cortical and spinal motor neurons is the typical feature of amyotrophic lateral sclerosis (ALS), a progressive neurodegenerative disease for which a pathogenetic role for the Cu/Zn superoxide dismutase (SOD1) has been demonstrated. Mice overexpressing a mutated form of the SOD1 gene (SOD1^G93A) develop a syndrome that closely resembles the human disease. The SOD1 mutations confer to this enzyme a “gain-of-function,” leading to increased production of reactive oxygen species. Several oxidants induce tyrosine phosphorylation through direct stimulation of kinases and/or phosphatases. In this study, we analyzed the activities of src and fyn tyrosine kinases and of protein tyrosine phosphatases in synaptosomal fractions prepared from the motor cortex and spinal cord of transgenic mice expressing SOD1^G93A. We found that (i) protein phosphotyrosine level is increased, (ii) src and fyn activities are upregulated, and (iii) the activity of tyrosine phosphatases, including the striatal-enriched tyrosine phosphatase (STEP), is significantly decreased. Moreover, the NMDA receptor (NMDAR) subunit GluN2B tyrosine phosphorylation was upregulated in SOD1^G93A. Tyrosine phosphorylation of GluN2B subunits regulates the NMDAR function and the recruitment of downstream signaling molecules. Indeed, we found that proline-rich tyrosine kinase 2 (Pyk2) and ERK1/2 kinase are upregulated in SOD1^G93A mice. These results point out an involvement of tyrosine kinases and phosphatases in the pathogenesis of ALS.

Noonan Syndrome-Associated SHP2 Dephosphorylates GluN2B to Regulate NMDA Receptor Function

Hyperactivating mutations in the non-receptor tyrosine phosphatase SHP2 cause Noonan syndrome (NS). NS is associated with cognitive deficits, but how hyperactivation of SHP2 in NS changes neuron function is not well understood. We find that mice bearing an NS-associated SHP2 allele (NS mice) have selectively impaired Schaffer collateral-CA1 NMDA (N-methyl-D-aspartate) receptor (NMDAR)-mediated neurotransmission and that residual NMDAR-mediated currents decay faster in NS mice because of reduced contribution of GluN1:GluN2B diheteromers. Consistent with altered GluN2B function, we identify GluN2B Y1252 as an NS-associated SHP2 substrate both in vitro and in vivo. Mutation of Y1252 does not alter recombinant GluN1:GluN2B receptor kinetics. Instead, phospho-Y1252 binds the actin-regulatory adaptor protein Nck2, and this interaction is required for proper NMDAR function. These results establish SHP2 and Nck2 as NMDAR regulatory proteins and strongly suggest that NMDAR dysfunction contributes to NS cognitive deficits.

The role of non-receptor protein tyrosine kinases in the excitotoxicity induced by the overactivation of NMDA receptors

Protein tyrosine phosphorylation is one of the primary modes of regulation of N-methyl-d-aspartate (NMDA) receptors. The non-receptor tyrosine kinases are one of the two types of protein tyrosine kinases that are involved in this process. The overactivation of NMDA receptors is a primary reason for neuron death following cerebral ischemia. Many studies have illustrated the important role of non-receptor tyrosine kinases in ischemia insults. This review introduces the roles of Src, Fyn, focal adhesion kinase, and proline-rich tyrosine kinase 2 in the excitotoxicity induced by the overactivation of NMDA receptors following cerebral ischemia.

Pyk2 modulates hippocampal excitatory synapses and contributes to cognitive deficits in a Huntington's disease model

The structure and function of spines and excitatory synapses are under the dynamic control of multiple signalling networks. Although tyrosine phosphorylation is involved, its regulation and importance are not well understood. Here we study the role of Pyk2, a non-receptor calcium-dependent protein-tyrosine kinase highly expressed in the hippocampus. Hippocampal-related learning and CA1 long-term potentiation are severely impaired in Pyk2-deficient mice and are associated with alterations in NMDA receptors, PSD-95 and dendritic spines. In cultured hippocampal neurons, Pyk2 has autophosphorylation-dependent and -independent roles in determining PSD-95 enrichment and spines density. Pyk2 levels are decreased in the hippocampus of individuals with Huntington and in the R6/1 mouse model of the disease. Normalizing Pyk2 levels in the hippocampus of R6/1 mice rescues memory deficits, spines pathology and PSD-95 localization. Our results reveal a role for Pyk2 in spine structure and synaptic function, and suggest that its deficit contributes to Huntington's disease cognitive impairments.

An Essential Role of Fyn in the Modulation of Metabotropic Glutamate Receptor 1 in Neurons

Fyn is a member of the Src family of nonreceptor tyrosine kinases and is broadly expressed in the CNS. As a synapse-enriched kinase, Fyn interacts with and phosphorylates local substrates to regulate synaptic transmission and plasticity, although our knowledge of specific targets of Fyn at synaptic sites remains incomplete and the accurate role of Fyn in regulating synaptic proteins is poorly understood. In this study, we initiated an effort to explore the interaction of Fyn with a metabotropic glutamate receptor (mGluR). We found that recombinant Fyn directly binds to mGluR1a at a consensus binding motif located in the intracellular C-terminus (CT) of mGluR1a in vitro. Similarly, endogenous Fyn interacts with mGluR1a in adult rat cerebellar neurons in vivo. Active Fyn phosphorylates mGluR1a at a conserved tyrosine residue in the CT region. In cerebellar neurons and transfected HEK293T cells, the Fyn-mediated tyrosine phosphorylation of mGluR1a is constitutively active and acts to facilitate the surface expression of mGluR1a and to potentiate the mGluR1a postreceptor signaling. These results support mGluR1a to be a novel substrate of Fyn. Fyn, by binding to and phosphorylating mGluR1a, potentiates surface expression and signaling of the receptors.

PDI regulates seizure activity via NMDA receptor redox in rats

Redox modulation of cysteine residues is one of the post-translational modifications of N-methyl-D-aspartate receptor (NMDAR). Protein disulfide isomerases (PDI), an endoplasmic reticulum (ER) chaperone, plays a crucial role in catalyzing disulfide bond formation, reduction, and isomerization. In the present study, we found that PDI bound to NMDAR in the normal hippocampus, and that this binding was increased in chronic epileptic rats. In vitro thiol reductase assay revealed that PDI increased the amount of thiols on full-length recombinant NR1 protein. PDI siRNA, 5-5'-dithio-bis(2-nitrobenzoic acid) (DTNB), bacitracin and PDI antibody reduced seizure susceptibility in response to pilocarpine. In addition, PDI knockdown effectively ameliorated spontaneous seizure activity in chronic epileptic rats. Anticonvulsive effects of PDI siRNA were correlated to the reduction of the amount of free- and nitrosothiols on NMDAR, accompanied by the inhibition of PDI activity. However, PDI knockdown did not lead to alteration in basal neurotransmission or ER stress under physiological condition. These findings provide mechanistic insight into sulfhydration of disulfide bonds on NMDAR by PDI, and suggest that PDI may represent a target of potential therapeutics for epilepsy, which avoids a possible side effect on physiological receptor functionality.

Protease activated receptor 1 (PAR1) enhances Src-mediated tyrosine phosphorylation of NMDA receptor in intracerebral hemorrhage (ICH)

It has been demonstrated that Src could modulate NMDA receptor, and PAR1 could also affect NMDAR signaling. However, whether PAR1 could regulate NMDAR through Src under ICH has not yet been investigated. In this study, we demonstrated the role of Src-PSD95-GluN2A signaling cascades in rat ICH model and in vitro thrombin challenged model. Using the PAR1 agonist SFLLR, antagonist RLLFS and Src inhibitor PP2, electrophysiological analysis showed that PAR1 regulated NMDA-induced whole-cell currents (INMDA) though Src in primary cultured neurons. Both in vivo and in vitro results showed the elevated phosphorylation of tyrosine in Src and GluN2A and enhanced interaction of the Src-PSD95-GluN2A under model conditions. Treatment with the PAR1 antagonist RLLFS, AS-PSD95 (Antisense oligonucleotide against PSD95) and Src inhibitor PP2 inhibited the interaction among Src-PSD95-GluN2A, and p-Src, p-GluN2A. Co-application of SFLLR and AS-PSD95, PP2, or MK801 (NMDAR inhibitor) abolished the effect of SF. In conclusion, our results demonstrated that activated thrombin receptor PAR1 induced Src activation, enhanced the interaction among Src-PSD95-GluN2A signaling modules, and up-regulated GluN2A phosphorylation after ICH injury. Elucidation of such signaling cascades would possibly provide novel targets for ICH treatment.

Role of Fyn-mediated NMDA receptor function in prediabetic neuropathy in mice

Diabetic neuropathy is a common complication of diabetes. This study evaluated the role of Fyn kinase and N-methyl-d-aspartate receptors (NMDARs) in the spinal cord in diabetic neuropathy using an animal model of high-fat diet-induced prediabetes. We found that prediabetic wild-type mice exhibited tactile allodynia and thermal hypoalgesia after a 16-wk high-fat diet, relative to normal diet-fed wild-type mice. Furthermore, prediabetic wild-type mice exhibited increased tactile allodynia and thermal hypoalgesia at 24 wk relative to 16 wk. Such phenomena were correlated with increased expression and activation of NR2B subunit of NMDARs, as well as Fyn-NR2B interaction in the spinal cord. Fyn(-/-) mice developed prediabetes after 16-wk high-fat diet treatment and exhibited thermal hypoalgesia, without showing tactile allodynia or altered expression and activation of NR2B subunit, relative to normal diet-fed Fyn(-/-) mice. Finally, intrathecal administrations of Ro 25-6981 (selective NR2B subunit-containing NMDAR antagonist) dose-dependently alleviated tactile allodynia, but not thermal hypoalgesia, at 16 and 24 wk in prediabetic wild-type mice. Our results suggested that Fyn-mediated NR2B signaling plays a critical role in regulation of prediabetic neuropathy and that the increased expression/function of NR2B subunit-containing NMDARs may contribute to the progression of neuropathy in type 2 diabetes.

Simvastatin enhances NMDA receptor GluN2B expression and phosphorylation of GluN2B and GluN2A through increased histone acetylation and Src signaling in hippocampal CA1 neurons

Simvastatin (SV) can improve cognitive deficits in Alzheimer's disease patients and mice. Herein, we report that the administration of SV (20 mg/kg) for 5 days in mice (SV-mice) or the treatment of slices with SV (10 μM) for 4 h (SV-slices) could increase the density of NMDA-evoked inward currents (INMDA) in hippocampal CA1 pyramidal cells, which were blocked by farnesol (FOH) that converts farnesyl pyrophosphate (FPP), but not geranylgeraniol (GGOH) that increases geranylgeranylpyrophosphate (GGPP). Sensitivity of INMDA to ifenprodil in SV-mice or SV-slices was significantly increased. The levels of hippocampal GluN2B and GluN2A or Src phosphorylation in SV-mice or SV-slices were higher than controls, which were sensitive to FOH. The Src inhibitor PP2 could inhibit the SV-enhanced phosphorylation of GluN2B and GluN2A and SV-augmented INMDA, but PI3K inhibitor LY294002 did not. The levels of GluN2B mRNA and protein were elevated in SV-mice, which was abolished by FOH, but not by GGOH or PP2. Furthermore, the histone H3K9 and H3K27 acetylation of GluN2B promoter was increased in SV-mice, which was suppressed by FOH rather than GGOH or PP2. In control mice and slices, the reduction of FPP by farnesyl transferase inhibitor could increase the levels of GluN2B expression, the histone H3K9 and H3K27 acetylation and enhance the phosphorylation of GluN2B, GluN2A and Src. The findings indicate that the administration of SV can enhance GluN2B expression and GluN2B and GluN2A phosphorylation leading to augmentation of NMDAR activity through reducing FPP to increase histone acetylation of GluN2B and Src signaling.

Phosphorylation of Tyrosine 1070 at the GluN2B Subunit Is Regulated by Synaptic Activity and Critical for Surface Expression of N-Methyl-D-aspartate (NMDA) Receptors

The number and subunit composition of synaptic N-methyl-d-aspartate receptors (NMDARs) play critical roles in synaptic plasticity, learning, and memory and are implicated in neurological disorders. Tyrosine phosphorylation provides a powerful means of regulating NMDAR function, but the underling mechanism remains elusive. In this study we identified a tyrosine site on the GluN2B subunit, Tyr-1070, which was phosphorylated by a proto-oncogene tyrosine-protein (Fyn) kinase and critical for the surface expression of GluN2B-containing NMDARs. The phosphorylation of GluN2B at Tyr-1070 was required for binding of Fyn kinase to GluN2B, which up-regulated the phosphorylation of GluN2B at Tyr-1472. Moreover, our results revealed that the phosphorylation change of GluN2B at Tyr-1070 accompanied the Tyr-1472 phosphorylation and Fyn associated with GluN2B in synaptic plasticity induced by both chemical and contextual fear learning. Taken together, our findings provide a new mechanism for regulating the surface expression of NMDARs with implications for synaptic plasticity.

Fyn in Neurodevelopment and Ischemic Brain Injury

The Src family kinases (SFKs) are nonreceptor protein tyrosine kinases that are implicated in many normal and pathological processes in the nervous system. The SFKs Fyn, Src, Yes, Lyn, and Lck are expressed in the brain. This review will focus on Fyn, as Fyn mutant mice have striking phenotypes in the brain and Fyn has been shown to be involved in ischemic brain injury in adult rodents and, with our work, in neonatal animals. An understanding of Fyn's role in neurodevelopment and disease will allow researchers to target pathological pathways while preserving protective ones.

Girdin phosphorylation is crucial for synaptic plasticity and memory: a potential role in the interaction of BDNF/TrkB/Akt signaling with NMDA receptor

Synaptic plasticity in hippocampal neurons has been thought to represent a variety of memories. Although accumulating evidence indicates a crucial role of BDNF/TrkB/Akt signaling in the synaptic plasticity of the hippocampus, the mechanism by which Akt, a serine/threonine kinase, controls activity-dependent neuronal plasticity remains unclear. Girdin (also known as APE, GIV, and HkRP1), an actin-binding protein involved both in the remodeling of the actin cytoskeleton and in cell migration, has been identified as a substrate of Akt. Previous studies have demonstrated that deficit of neuronal migration in the hippocampus of Girdin-deficient (Girdin(-/-)) mice is independent on serine phosphorylation of Girdin at S1416 (Girdin S1416) by Akt. In the present study, we focused on the role of Girdin S1416 phosphorylation in BDNF/TrkB/Akt signaling associated with synaptic plasticity. We found that Girdin in the hippocampus was phosphorylated at S1416 in an activity-dependent manner. Phosphorylation-deficient knock-in mice (Girdin(SA/SA) mice), in which S1416 is replaced with alanine, exhibited shrinkage of spines, deficit of hippocampal long-term potentiation, and memory impairment. These phenotypes of Girdin(SA/SA) mice resembled those of Girdin(+/-) mice, which have 50% loss of Girdin expression. Furthermore, Girdin interacted with Src kinase and NR2B subunit of NMDA receptor, leading to phosphorylation of the NR2B subunit and NMDA receptor activation. Our findings suggest that Girdin has two different functions in the hippocampus: Akt-independent neuronal migration and Akt-dependent NR2B phosphorylation through the interaction with Src, which is associated with synaptic plasticity in the hippocampus underlying memory formation.

Modulation of the NMDA Receptor Through Secreted Soluble Factors

Synaptic activity is a critical determinant in the formation and development of excitatory synapses in the central nervous system (CNS). The excitatory current is produced and regulated by several ionotropic receptors, including those that respond to glutamate. These channels are in turn regulated through several secreted factors that function as synaptic organizers. Specifically, Wnt, brain-derived neurotrophic factor (BDNF), fibroblast growth factor (FGF), and transforming growth factor (TGF) particularly regulate the N-methyl-D-aspartate receptor (NMDAR) glutamatergic channel. These factors likely regulate early embryonic development and directly control key proteins in the function of important glutamatergic channels. Here, we review the secreted molecules that participate in synaptic organization and discuss the cell signaling behind of this fine regulation. Additionally, we discuss how these factors are dysregulated in some neuropathologies associated with glutamatergic synaptic transmission in the CNS.

The prostaglandin EP1 receptor potentiates kainate receptor activation via a protein kinase C pathway and exacerbates status epilepticus

Prostaglandin E2 (PGE2) regulates membrane excitability, synaptic transmission, plasticity, and neuronal survival. The consequences of PGE2 release following seizures has been the subject of much study. Here we demonstrate that the prostaglandin E2 receptor 1 (EP1, or Ptger1) modulates native kainate receptors, a family of ionotropic glutamate receptors widely expressed throughout the central nervous system. Global ablation of the EP1 gene in mice (EP1-KO) had no effect on seizure threshold after kainate injection but reduced the likelihood to enter status epilepticus. EP1-KO mice that did experience typical status epilepticus had reduced hippocampal neurodegeneration and a blunted inflammatory response. Further studies with native prostanoid and kainate receptors in cultured cortical neurons, as well as with recombinant prostanoid and kainate receptors expressed in Xenopus oocytes, demonstrated that EP1 receptor activation potentiates heteromeric but not homomeric kainate receptors via a second messenger cascade involving phospholipase C, calcium and protein kinase C. Three critical GluK5 C-terminal serines underlie the potentiation of the GluK2/GluK5 receptor by EP1 activation. Taken together, these results indicate that EP1 receptor activation during seizures, through a protein kinase C pathway, increases the probability of kainic acid induced status epilepticus, and independently promotes hippocampal neurodegeneration and a broad inflammatory response.

HIV-1 protein Tat produces biphasic changes in NMDA-evoked increases in intracellular Ca2+ concentration via activation of Src kinase and nitric oxide signaling pathways

HIV-associated neurocognitive disorders afflict about half of HIV-infected patients. HIV-infected cells shed viral proteins, such as the transactivator of transcription (Tat), which can cause neurotoxicity by over activation of NMDA receptors. Here, we show that Tat causes a time-dependent, biphasic change in NMDA-evoked increases in intracellular Ca(2+) concentration ([Ca(2+)]i). NMDA-evoked responses were potentiated following 2-h exposure to Tat (50 ng/mL). Tat-induced potentiation of NMDA-evoked increases in [Ca(2+)]i peaked by 8 h and then adapted by gradually reversing to baseline by 24 h and eventually dropping below control by 48 h. Tat-induced potentiation of NMDA-evoked responses was blocked by inhibition of lipoprotein receptor-related protein (LRP) or Src tyrosine kinase. Potentiation was unaffected by inhibition of nitric oxide synthase (NOS). However, NOS activity was required for adaptation. Adaptation was also prevented by inhibition of soluble guanylate cyclase (sGC) and cyclic guanosine monophosphate-dependent protein kinase G (PKG). Together, these findings indicate that Tat potentiates NMDA-evoked increases in [Ca(2+)]i via LRP-dependent activation of Src and that this potentiation adapts via activation of the NOS/sGC/PKG pathway. Adaptation may protect neurons from excessive Ca(2+) influx and could reveal targets for the treatment of HIV-associated neurocognitive disorders. HIV-associated neurocognitive disorders (HAND) afflict about half of HIV-infected patients. HIV-infected cells shed viral proteins, such as the transactivator of transcription (Tat), which can cause neurotoxicity by over activation of NMDA receptors (NMDARs). We show that HIV-1 Tat evoked biphasic changes in NMDA-evoked [Ca(2+) ]i responses. Initially, Tat potentiated NMDA-evoked responses following LRP-mediated activation of Src kinase. Subsequently, Tat-induced NMDAR potentiation adapted by activation of a NOS/sGC/PKG pathway that attenuated NMDA-evoked increases in [Ca(2+)]i . Adaptation may be a novel neuroprotective mechanism to prevent excessive Ca(2+) influx. Solid and dashed arrows represent direct and potentially indirect connections, respectively.

Convergence of dopamine and glutamate signaling onto striatal ERK activation in response to drugs of abuse

Despite their distinct targets, all addictive drugs commonly abused by humans evoke increases in dopamine (DA) concentration within the striatum. The main DA Guanine nucleotide binding protein couple receptors (GPCRs) expressed by medium-sized spiny neurons of the striatum are the D1R and D2R, which are positively and negatively coupled to cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) signaling, respectively. These two DA GPCRs are largely segregated into distinct neuronal populations, where they are co-expressed with glutamate receptors in dendritic spines. Direct and indirect interactions between DA GPCRs and glutamate receptors are the molecular basis by which DA modulates glutamate transmission and controls striatal plasticity and behavior induced by drugs of abuse. A major downstream target of striatal D1R is the extracellular signal-regulated kinase (ERK) kinase pathway. ERK activation by drugs of abuse behaves as a key integrator of D1R and glutamate NMDAR signaling. Once activated, ERK can trigger chromatin remodeling and induce gene expression that permits long-term cellular alterations and drug-induced morphological and behavioral changes. Besides the classical cAMP/PKA pathway, downstream of D1R, recent evidence implicates a cAMP-independent crosstalk mechanism by which the D1R potentiates NMDAR-mediated calcium influx and ERK activation. The mounting evidence of reciprocal modulation of DA and glutamate receptors adds further intricacy to striatal synaptic signaling and is liable to prove relevant for addictive drug-induced signaling, plasticity, and behavior. Herein, we review the evidence that built our understanding of the consequences of this synergistic signaling for the actions of drugs of abuse.

Bidirectional modulatory effect of 17β-estradiol on NMDA receptors via ERα and ERβ in the dentate gyrus of juvenile male rats

The neurosteroid 17β-estradiol (E2) is synthesized by aromatase in both male and female hippocampi and is known to modulate hippocampal synaptic functions. However, as some contradictory findings regarding the modulatory effects of E2 have been reported in the literature, its physiological role and mechanism of action in the hippocampus remain controversial. Our recent study showed that a low E2 dose (1 nM) increased the amplitude of NMDA receptor-mediated EPSCs (NMDAR-EPSCs) and lowered the threshold for the induction of NMDA receptor-dependent long-term potentiation (NMDAR-LTP), while a high E2 dose (7 nM) exerted opposite effects in the dentate gyrus of juvenile male rat hippocampal slices. The present study is a follow-up that explores the underlying mechanism of this bidirectional effect of E2. We found that the ERα agonist PPT reproduced the actions of the low E2 dose on NMDAR-EPSCs and NMDAR-LTP, while the ERβ agonist DPN reproduced the actions of the high E2 dose. Moreover, PPT, but not DPN, restored the decrease in NMDAR-EPSCs induced by the aromatase inhibitor letrozole, suggesting that E2 synthesized constitutively in the hippocampus enhances NMDA receptor function via ERα. The PPT-induced enhancement in NMDAR-EPSCs was mediated by Src family kinase, but was not caused by NR2B modulation. These findings demonstrate that E2 exerts condition-dependent bidirectional effects on NMDA receptor-mediated transmission and, thus, synaptic plasticity via ERα and ERβ in the dentate gyrus of juvenile male rats.

Dephosphorylation of GluN2B C-terminal tyrosine residues does not contribute to acute ethanol inhibition of recombinant NMDA receptors

N-methyl-d-aspartate (NMDA) receptors are ion channels activated by the neurotransmitter glutamate and are highly expressed by neurons. These receptors are critical for excitatory synaptic signaling and inhibition of NMDA receptors leads to impaired cognition and learning. Ethanol inhibits NMDA currents at concentrations associated with intoxication and this action may underlie some of the behavioral effects of ethanol. Although numerous sites and mechanisms of action have been tested, the manner in which ethanol inhibits NMDA receptors remains unclear. Recent findings in the literature suggest that ethanol, via facilitation of tyrosine phosphatase activity, may dephosphorylate key tyrosine residues in the C-terminus of GluN2B subunits resulting in diminished channel function. To directly test this hypothesis, we engineered GluN2B mutants that contained phenylalanine in place of tyrosine at three different sites and transiently expressed them with the GluN1 subunit in human embryonic kidney (HEK) cells. Whole-cell patch clamp electrophysiology was used to record glutamate-activated currents in the absence and presence of ethanol (10-600 mM). All mutants were functional and did not differ from one another with respect to current amplitude, steady-state to peak ratio, or magnesium block. Analysis of ethanol dose-response curves showed no significant difference in IC50 values between wild-type receptors and Y1252F, Y1336F, Y1472F or triple Y-F mutants. These findings suggest that dephosphorylation of C-terminal tyrosine residues does not account for ethanol inhibition of GluN2B receptors.

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

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

Enhanced NMDA receptor tyrosine phosphorylation and increased brain injury following neonatal hypoxia-ischemia in mice with neuronal Fyn overexpression

The Src family kinases (SFKs) Src and Fyn are implicated in hypoxic-ischemic (HI) injury in the developing brain. However, it is unclear how these particular SFKs contribute to brain injury. Using neuron-specific Fyn overexpressing (OE) mice, we investigated the role of neuronal Fyn in neonatal brain HI. Wild type (WT) and Fyn OE mice were subjected to HI using the Vannucci model at postnatal day 7. Brains were scored five days later for evaluation of damage using cresyl violet and iron staining. Western blotting with postsynaptic density (PSD)-associated synaptic membrane proteins and co-immunoprecipitation with cortical lysates were performed at various time points after HI to determine NMDA receptor tyrosine phosphorylation and Fyn kinase activity. Fyn OE mice had significantly higher mortality and brain injury compared to their WT littermates. Neuronal Fyn overexpression led to sustained NR2A and NR2B tyrosine phosphorylation and enhanced NR2B phosphorylation at tyrosine (Y) 1472 and Y1252 in synaptic membranes. These early changes correlated with higher calpain activity 24h after HI in Fyn OE mice relative to WT animals. Our findings suggest a role for Fyn kinase in neuronal death after neonatal HI, possibly via up-regulation of NMDA receptor tyrosine phosphorylation.

Activation of group I metabotropic glutamate receptors potentiates heteromeric kainate receptors

Kainate receptors (KARs), a family of ionotropic glutamate receptors, are widely expressed in the central nervous system and are critically involved in synaptic transmission. KAR activation is influenced by metabotropic glutamate receptor (mGlu) signaling, but the underlying mechanisms are not understood. We undertook studies to examine how mGlu modulation affects activation of KARs. Confocal immunohistochemistry of rat hippocampus and cultured rat cortex revealed colocalization of the high-affinity KAR subunits with group I mGlu receptors. In hippocampal and cortical cultures, the calcium signal caused by activation of native KARs was potentiated by activation of group I mGlu receptors. In Xenopus laevis oocytes, activation of group I mGlu receptors potentiated heteromeric but not homomeric KAR-mediated currents, with no change in agonist potency. The potentiation of heteromeric KARs by mGlu1 activation was attenuated by GDPβS, blocked by an inhibitor of phospholipase C or the calcium chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), prolonged by the phosphatase inhibitor okadaic acid, but unaffected by the tyrosine kinase inhibitor lavendustin A. Protein kinase C (PKC) inhibition reduced the potentiation by mGlu1 of GluK2/GluK5, and conversely, direct activation of PKC by phorbol 12-myristate,13-acetate potentiated GluK2/GluK5. Using site-directed mutagenesis, we identified three serines (Ser833, Ser836, and Ser840) within the membrane proximal region of the GluK5 C-terminal domain that, in combination, are required for mGlu1-mediated potentiation of KARs. Together, these data suggest that phosphorylation of key residues in the C-terminal domain changes the overall charge of this domain, resulting in potentiated agonist responses.

Effects of metabotropic glutamate mGlu5 receptor antagonist on tyrosine phosphorylation of NMDA receptor subunits and cell death in the hippocampus after brain ischemia in rats

Tyrosine phosphorylation of the N-methyl-D-aspartate (NMDA) receptor appears to be associated with the regulation of the receptor's ion channel. This study focused on the effect of a metabotropic glutamate mGlu5 receptor antagonist on tyrosine phosphorylation of NMDA receptor subunits and cell death in the hippocampal CA1 region after transient global ischemia and sought to explore their mechanisms. Pretreatment with the mGlu5 receptor antagonist reduced cell death in the hippocampal CA1 region on day 3 after the transient ischemia. Transient ischemia increased the tyrosine phosphorylation of NMDA receptor subunits, which are a major target of Src family tyrosine kinases. Therefore, we investigated the effect of the antagonist on tyrosine phosphorylation of the NMDA receptor subunits after transient ischemia. Tyrosine phosphorylation of the NR2A subunit, but not that of the NR2B one, was inhibited by the mGlu5 receptor antagonist. The administration of the antagonist also attenuated the increase in the amount of active form of Src after the reperfusion. We further demonstrated that the administration of a Src-family kinase inhibitor prevented cell death in the hippocampal CA1 region and attenuated the increase in the tyrosine phosphorylation of the NMDA receptor subunits after the reperfusion. These findings suggest that mGlu5 receptor in the hippocampal CA1 region after transient ischemia is involved in the activation of Src and subsequent tyrosine phosphorylation of NMDA receptor subunits, which actions may contribute to alterations of properties of the NMDA receptor and may be related to pathogenic events leading to neuronal cell death.

Chronic zinc exposure decreases the surface expression of NR2A-containing NMDA receptors in cultured hippocampal neurons

BACKGROUND

Zinc distributes widely in the central nervous system, especially in the hippocampus, amygdala and cortex. The dynamic balance of zinc is critical for neuronal functions. Zinc modulates the activity of N-methyl-D-aspartate receptors (NMDARs) through the direct inhibition and various intracellular signaling pathways. Abnormal NMDAR activities have been implicated in the aetiology of many brain diseases. Sustained zinc accumulation in the extracellular fluid is known to link to pathological conditions. However, the mechanism linking this chronic zinc exposure and NMDAR dysfunction is poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

We reported that chronic zinc exposure reduced the numbers of NR1 and NR2A clusters in cultured hippocampal pyramidal neurons. Whole-cell and synaptic NR2A-mediated currents also decreased. By contrast, zinc did not affect NR2B, suggesting that chronic zinc exposure specifically influences NR2A-containg NMDARs. Surface biotinylation indicated that zinc exposure attenuated the membrane expression of NR1 and NR2A, which might arise from to the dissociation of the NR2A-PSD-95-Src complex.

CONCLUSIONS

Chronic zinc exposure perturbs the interaction of NR2A to PSD-95 and causes the disorder of NMDARs in hippocampal neurons, suggesting a novel action of zinc distinct from its acute effects on NMDAR activity.

Targeting N-methyl-D-aspartate receptors for treatment of neuropathic pain

Neuropathic pain remains a major clinical problem and a therapeutic challenge because existing analgesics are often ineffective and can cause serious side effects. Increased N-methyl-d-aspartate receptor (NMDAR) activity contributes to central sensitization in certain types of neuropathic pain. NMDAR antagonists can reduce hyperalgesia and allodynia in animal models of neuropathic pain induced by nerve injury and diabetic neuropathy. Clinically used NMDAR antagonists, such as ketamine and dextromethorphan, are generally effective in patients with neuropathic pain, such as complex regional pain syndrome and painful diabetic neuropathy. However, patients with postherpetic neuralgia respond poorly to NMDAR antagonists. Recent studies on identifying NMDAR-interacting proteins and molecular mechanisms of increased NMDAR activity in neuropathic pain could facilitate the development of new drugs to attenuate abnormal NMDAR activity with minimal impairment of the physiological function of NMDARs. Combining NMDAR antagonists with other analgesics could also lead to better management of neuropathic pain without causing serious side effects.

Fyn kinases play a critical role in neuronal apoptosis induced by oxygen and glucose deprivation or amyloid-β peptide treatment

AIMS

Src family protein tyrosine kinases (SrcPTKs) have been implicated in the pathogenesis of brain ischemia and Alzheimer's disease (AD). In this study, we investigated whether Src and Fyn kinases, two major members of SrcPTKs in the brain, have distinct roles in the oxygen and glucose deprivation (OGD) and amyloid-β peptide (Aβ)-induced neuronal apoptosis.

METHODS AND RESULTS

The DAPI staining and caspase-3 activation analysis showed that small interfering RNAs (siRNAs) knockdown of Src or Fyn attenuated SH-SY5Y cells apoptosis after OGD and Aβ treatment. Fyn knockdown had a more potent neuroprotective effect than Src knockdown, suggesting a principal pathological significance of Fyn in brain ischemia and AD. Previously, we reported that brain ischemia promotes the phosphorylation of postsynaptic density protein 95 (PSD-95) at tyrosine 523 (Y523), which is associated with postsynaptic mechanisms of excitotoxicity. Here, immunoblot analysis indicated that not only OGD but also Aβ incubation increased the PSD-95Y523 phosphorylation. Src knockdown, especially Fyn knockdown, significantly inhibited such phosphorylation.

CONCLUSION

Fyn mediates PSD-95Y523 phosphorylation, which may be responsible for the excitotoxic signal cascades and neuronal apoptosis in brain ischemia and Aβ neurotoxicity. Fyn is a potential therapeutic target for the treatment of ischemic stroke and AD.

Paradox of schizophrenia genetics: is a paradigm shift occurring?

BACKGROUND

Genetic research of schizophrenia (SCZ) based on the nuclear genome model (NGM) has been one of the most active areas in psychiatry for the past two decades. Although this effort is ongoing, the current situation of the molecular genetics of SCZ seems disappointing or rather perplexing. Furthermore, a prominent discrepancy between persistence of the disease at a relatively high prevalence and a low reproductive fitness of patients creates a paradox. Heterozygote advantage works to sustain the frequency of a putative susceptibility gene in the mitochondrial genome model (MGM) but not in the NGM.

METHODS

We deduced a criterion that every nuclear susceptibility gene for SCZ should fulfill for the persistence of the disease under general assumptions of the multifactorial threshold model. SCZ-associated variants listed in the top 45 in the SZGene Database (the version of the 23rd December, 2011) were selected, and the distribution of the genes that could meet or do not meet the criterion was surveyed.

RESULTS

19 SCZ-associated variants that do not meet the criterion are located outside the regions where the SCZ-associated variants that could meet the criterion are located. Since a SCZ-associated variant that does not meet the criterion cannot be a susceptibility gene, but instead must be a protective gene, it should be linked to a susceptibility gene in the NGM, which is contrary to these results. On the other hand, every protective gene on any chromosome can be associated with SCZ in the MGM. Based on the MGM we propose a new hypothesis that assumes brain-specific antioxidant defenses in which trans-synaptic activations of dopamine- and N-methyl-d-aspartate-receptors are involved. Most of the ten predictions of this hypothesis seem to accord with the major epidemiological facts and the results of association studies to date.

CONCLUSION

The central paradox of SCZ genetics and the results of association studies to date argue against the NGM, and in its place the MGM is emerging as a viable option to account for genomic and pathophysiological research findings involving SCZ.

Influence of the intracellular GluN2 C-terminal domain on NMDA receptor function

Excitatory neurotransmission mediated by N-methyl-d-aspartate receptors (NMDARs) is fundamental to learning and memory and, when impaired, causes certain neurological disorders. NMDARs are heterotetrameric complexes composed of two GluN1 [NR1] and two GluN2(A-D) [NR2(A-D)] subunits. The GluN2 subunit is responsible for subunit-specific channel activity and gating kinetics including activation (rise time), peak open probability (peak Po) and deactivation (decay time). The peak Po of recombinant NMDARs was recently described to be controlled by the extracellular GluN2 N-terminal domain (NTD). The cytoplasmic GluN2 C-terminal domain (CTD) could also be involved, because the Po of synaptic NMDARs is reduced in mice expressing C-terminally truncated GluN2 subunits. Here, we examined the role of the GluN2 cytoplasmic tail for NMDAR channel activity and gating in HEK-293 cells. C-terminal truncation of GluN2A, GluN2B or GluN2C did not change the subunit-specific rise time but accelerated the decay time of glutamate-activated currents. Furthermore, the peak Po was reduced by about 50% for GluN2A and GluN2B but not for GluN2C. These results indicated that the CTD of GluN2 has a modulating role in NMDAR gating even in the absence of interacting synaptic proteins. Reduction of peak Po and deactivation kinetics following GluN2 C-terminal truncation were reversed by re-introducing a CTD from a different GluN2 subunit. Thus, the CTDs of GluN2 subunits behave as constitutive structural elements required for normal functioning of NMDARs but are not involved in determining the subunit-specific gating properties of NMDARs.

Striatal-enriched protein tyrosine phosphatase in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia among the elderly, affecting millions of people worldwide and representing a substantial economic burden. AD is a progressive disease associated with memory loss and impaired cognitive function. The neuropathology is characterized by cortical accumulation of amyloid plaques and neurofibrillary tangles (NFTs). Amyloid plaques are small, aggregated peptides called beta amyloid (Aβ) and NFTs are aggregates of hyperphosphorylated Tau protein. Because Aβ disrupts multiple intracellular signaling pathways, resulting in some of the clinical symptoms of AD, understanding the underlying molecular mechanisms has implications for the diagnosis and treatment of AD. Recent studies have demonstrated that Aβ regulates striatal-enriched protein tyrosine phosphatase (STEP) (PTPN5). Aβ accumulation is associated with increases in STEP levels and activity that in turn disrupts glutamate receptor trafficking to and from the neuronal membrane. These findings indicate that modulating STEP levels or inhibiting its activity may have beneficial effects for patients with AD, making it an important target for drug discovery. This article reviews the biology of STEP and its role in AD as well as the potential clinical applications.

Molecular pharmacology of human NMDA receptors

N-methyl-d-aspartate (NMDA) receptors are ionotropic glutamate receptors that mediate excitatory neurotransmission. NMDA receptors are also important drug targets that are implicated in a number of pathophysiological conditions. To facilitate the transition from lead compounds in pre-clinical animal models to drug candidates for human use, it is important to establish whether NMDA receptor ligands have similar properties at rodent and human NMDA receptors. Here, we compare amino acid sequences for human and rat NMDA receptor subunits and discuss inter-species variation in the context of our current knowledge of the relationship between NMDA receptor structure and function. We summarize studies on the biophysical properties of human NMDA receptors and compare these properties to those of rat orthologs. Finally, we provide a comprehensive pharmacological characterization that allows side-by-side comparison of agonists, un-competitive antagonists, GluN2B-selective non-competitive antagonists, and GluN2C/D-selective modulators at recombinant human and rat NMDA receptors. The evaluation of biophysical properties and pharmacological probes acting at different sites on the receptor suggest that the binding sites and conformational changes leading to channel gating in response to agonist binding are highly conserved between human and rat NMDA receptors. In summary, the results of this study suggest that no major detectable differences exist in the pharmacological and functional properties of human and rat NMDA receptors.

Dysregulated Src upregulation of NMDA receptor activity: a common link in chronic pain and schizophrenia

Upregulation of N-methyl-D-aspartate (NMDA) receptor function by the nonreceptor protein tyrosine kinase Src has been implicated in physiological plasticity at glutamatergic synapses. Here, we highlight recent findings suggesting that aberrant Src upregulation of NMDA receptors may also be key in pathophysiological conditions. Within the nociceptive processing network in the dorsal horn of the spinal cord, pathologically increased Src upregulation of NMDA receptors is critical for pain hypersensitivity in models of chronic inflammatory and neuropathic pain. On the other hand, in the hippocampus and prefrontal cortex, the physiological upregulation of NMDA receptors by Src is blocked by neuregulin 1-ErbB4 signaling, a pathway that is genetically implicated in the positive symptoms of schizophrenia. Thus, either over-upregulation or under-upregulation of NMDA receptors by Src may lead to pathological conditions in the central nervous system. Therefore, normalizing Src upregulation of NMDA receptors may be a novel therapeutic approach for central nervous system disorders, without the deleterious consequences of directly blocking NMDA receptors.

Depolarization induces NR2A tyrosine phosphorylation and neuronal apoptosis

BACKGROUND

Cytosol Ca2+ overload plays a vital role in ischemic neuronal damage, which is largely contributed by the Ca2+ influx through L-type voltage-gated calcium channels (L-VGCCs) and N-methyl-D-aspartate (NMDA) type glutamate receptors. In this article, L-VGCCs were activated by depolarization to investigate the cross-talk between NMDA receptors and L-VGCCs.

METHODS

Depolarization was induced by 20 minutes incubation of 75 mM KCl in cultured rat cortical neuron. Apoptosis-like neuronal death was detected by DAPI staining. Tyrosine phosphorylation of NMDA receptor subunit 2A (NR2A), interactions of Src and NR2A were detected by immunoblot and immunoprecipitation.

RESULTS

Depolarization induced cortical neuron apoptosis-like cell death after 24 hours of restoration. The apoptosis was partially inhibited by 5 mM EGTA, 100 μM Cd2+, 10 μM nimodipine, 100 μM genistein, 20 μM MK-801, 2 μM PP2 and combined treatment of nimodipine and MK-801. NR2A tyrosine phosphorylation increased after depolarization, and the increase was inhibited by the drugs listed above. Moreover, non-receptor tyrosine kinase Src bound with NR2A after depolarization and restoration. The binding was also inhibited by the drugs listed above.

CONCLUSIONS

The results indicated that depolarization-induced neuronal death might be due to extracellular Ca2+ influx through L-VGCCs and subsequently Src activationmediated NR2A tyrosine phosphorylation.