The GLUR2 subunit of AMPA receptors: Synaptic role

Mesenchymal-derived extracellular vesicles enhance microglia-mediated synapse remodeling after cortical injury in aging Rhesus monkeys

Understanding the microglial neuro-immune interactions in the primate brain is vital to developing therapeutics for cortical injury, such as stroke or traumatic brain injury. Our previous work showed that mesenchymal-derived extracellular vesicles (MSC-EVs) enhanced motor recovery in aged rhesus monkeys following injury of primary motor cortex (M1), by promoting homeostatic ramified microglia, reducing injury-related neuronal hyperexcitability, and enhancing synaptic plasticity in perilesional cortices. A focal lesion was induced via surgical ablation of pial blood vessels over lying the cortical hand representation of M1 of aged female rhesus monkeys, that received intravenous infusions of either vehicle (veh) or EVs 24 h and again 14 days post-injury. The current study used this same cohort to address how these injury- and recovery-associated changes relate to structural and molecular interactions between microglia and neuronal synapses. Using multi-labeling immunohistochemistry, high-resolution microscopy, and gene expression analysis, we quantified co-expression of synaptic markers (VGLUTs, GLURs, VGAT, GABARs), microglia markers (Iba1, P2RY12), and C1q, a complement pathway protein for microglia-mediated synapse phagocytosis, in perilesional M1 and premotor cortices (PMC). We compared this lesion cohort to age-matched non-lesion controls (ctr). Our findings revealed a lesion-related loss of excitatory synapses in perilesional areas, which was ameliorated by EV treatment. Further, we found region-dependent effects of EVs on microglia and C1q expression. In perilesional M1, EV treatment and enhanced functional recovery were associated with increased expression of C1q + hypertrophic microglia, which are thought to have a role in debris-clearance and anti-inflammatory functions. In PMC, EV treatment was associated with decreased C1q + synaptic tagging and microglia-spine contacts. Our results suggest that EV treatment may enhance synaptic plasticity via clearance of acute damage in perilesional M1, and thereby preventing chronic inflammation and excessive synaptic loss in PMC. These mechanisms may act to preserve synaptic cortical motor networks and a balanced normative M1/PMC synaptic function to support functional recovery after injury.

Mesenchymal-Derived Extracellular Vesicles Enhance Microglia-mediated Synapse Remodeling after Cortical Injury in Rhesus Monkeys

Understanding the microglial neuro-immune interactions in the primate brain is vital to developing therapeutics for cortical injury, such as stroke. Our previous work showed that mesenchymal-derived extracellular vesicles (MSC-EVs) enhanced motor recovery in aged rhesus monkeys post-injury of primary motor cortex (M1), by promoting homeostatic ramified microglia, reducing injury-related neuronal hyperexcitability, and enhancing synaptic plasticity in perilesional cortices. The current study addresses how these injury- and recovery-associated changes relate to structural and molecular interactions between microglia and neuronal synapses. Using multi-labeling immunohistochemistry, high resolution microscopy, and gene expression analysis, we quantified co-expression of synaptic markers (VGLUTs, GLURs, VGAT, GABARs), microglia markers (Iba-1, P2RY12), and C1q, a complement pathway protein for microglia-mediated synapse phagocytosis, in perilesional M1 and premotor cortices (PMC) of monkeys with intravenous infusions of either vehicle (veh) or EVs post-injury. We compared this lesion cohort to aged-matched non-lesion controls. Our findings revealed a lesion-related loss of excitatory synapses in perilesional areas, which was ameliorated by EV treatment. Further, we found region-dependent effects of EV on microglia and C1q expression. In perilesional M1, EV treatment and enhanced functional recovery were associated with increased expression of C1q + hypertrophic microglia, which are thought to have a role in debris-clearance and anti-inflammatory functions. In PMC, EV treatment was associated with decreased C1q + synaptic tagging and microglial-spine contacts. Our results provided evidence that EV treatment facilitated synaptic plasticity by enhancing clearance of acute damage in perilesional M1, and thereby preventing chronic inflammation and excessive synaptic loss in PMC. These mechanisms may act to preserve synaptic cortical motor networks and a balanced normative M1/PMC synaptic connectivity to support functional recovery after injury.

A maternal high-fat diet during pregnancy and lactation disrupts short-term memory functions via altered hippocampal glutamatergic signaling in female rat offspring

A maternal high-fat diet (HFD) provokes changes in the offspring's brain's structure, function, and development. These changes may cause neuropsychiatric disorders in the early life of offspring the basis of which may be memory impairment. In this study, the effects of maternal HFD during pregnancy and lactation on the short-term memory in adolescent and young adult offspring were evaluated. We analyzed the expression of genes encoding the glutamatergic transporters in the hippocampus to verify the association between changes in glutamatergic transporters and behavioral changes in offspring. Next, we examined whether maternal diet-induced changes in the mRNA levels of genes encoding the NMDA receptor subunits and the AMPA receptor subunits, as well as BDNF in this structure in offspring. All significant changes were validated at the protein level. We found that a maternal HFD during pregnancy and lactation disrupts short-term memory in adolescent and young adult females. The latter change is likely related to the dysregulation of hippocampal levels of GluN2B subunit of NMDA receptors and of reduced levels of BDNF. In summary, we showed that a maternal HFD during pregnancy and lactation triggered several changes within the glutamatergic system in the hippocampus of rat offspring, which may be related to producing behavioral changes in offspring.

The role of AMPA and NMDA receptors in mitragynine effects on hippocampal synaptic plasticity

Mitragynine, an indole alkaloid from the plant Mitragyna speciosa (Kratom), has been reported to modify hippocampal synaptic transmission. However, the role of glutamatergic neurotransmission modulating synaptic plasticity in mitragynine-induced synaptic changes is still unknown. Here, we determined the role of AMPA- and NMDA glutamate receptors in mitragynine-induced synaptic plasticity in the hippocampus. Male Sprague Dawley rats received either vehicle or mitragynine (10 mg/kg), with or without the AMPA receptor antagonist, NBQX (3 mg/kg), or the NMDA receptor antagonist, MK-801 (0.2 mg/kg). Field excitatory postsynaptic potentials (fEPSP) during baseline, paired-pulse facilitation (PPF) and long-term potentiation (LTP) were recorded in-vivo in the hippocampal CA1 area of anaesthetised rats. Basal synaptic transmission and LTP were significantly impaired after mitragynine, NBQX, and MK-801 alone, without an effect on PPF. Combined effects suggest a weak functional AMPA- as well as NMDA receptor antagonist action of mitragynine.

NMDA and AMPA receptors dysregulation in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative condition characterized by cognitive dysfunction and synaptic failure. The current therapeutic approaches are mainly focused on symptomatic treatment and possess limited effectiveness in addressing the pathophysiology of AD. It is known that neurodegeneration is negatively correlated with synaptic plasticity. This negative correlation highlights glutamatergic neurotransmission via N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors and (AMPA) receptors as a critical mediator of synaptic plasticity. Despite this favorable role, extensive extracellular glutamate concentration induces excitotoxicity and neurodegeneration. NMDA receptors containing GluN2A subunits are located at synaptic sites, implicated in the protective pathways. In comparison, GluN2B containing receptors are located mainly at extrasynaptic sites and increase neuronal vulnerability. AMPA receptors are consistently endocytosed and recycled back to the membrane. An increase in the rate of endocytosis has been implicated as a part of AD pathophysiology through inducing long-term depression (LTD) and synaptic disintegration. In the present review, we focused on the mechanisms of glutamatergic system dysregulation in AD, particularly on its interaction with amyloid-beta. We concluded that assigning a specific role to an individual subtype of either NMDA receptors or AMPA receptors might be an oversimplification as they are not static receptors. Therefore, any imbalance between synaptic and extrasynaptic NMDA receptors and a reduced number of surface AMPA receptors will lead to synaptopathy.

The well responsiveness of drug-resistant focal seizures in anti-AMPA2 receptor encephalitis to perampanel treatment

Anti-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor encephalitis is an anti-neuronal surface antigen autoimmune encephalitis and is relatively rare. Our study evaluated a patient who developed anti-AMPA2 receptor encephalitis with memory deficits and refractory focal seizures as paroxysmal jerking on right face as well as dystonic seizure on right hand. On this patient, the combination treatment of levetiracetam, carbamazepine, and clonazepam, monthly periodic intravenous immunoglobin and immunosuppressive therapies for 5 months was not effective for the focal seizures, while his memory loss was slightly improved. However, adjunctive perampanel treatment led to a rapid relief of seizures. Perampanel is suggested in seizures associated with anti-AMPA receptor encephalitis by directly attenuating nerve hyperexcitability caused by glutamate and Ca2+-permeable GluA4 subunit of AMPA receptors.

AMPA Receptor Surface Expression Is Regulated by S-Nitrosylation of Thorase and Transnitrosylation of NSF

The regulation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) trafficking affects multiple brain functions, such as learning and memory. We have previously shown that Thorase plays an important role in the internalization of AMPARs from the synaptic membrane. Here, we show that N-methyl-d-aspartate receptor (NMDAR) activation leads to increased S-nitrosylation of Thorase and N-ethylmaleimide-sensitive factor (NSF). S-nitrosylation of Thorase stabilizes Thorase-AMPAR complexes and enhances the internalization of AMPAR and interaction with protein-interacting C kinase 1 (PICK1). S-nitrosylated NSF is dependent on the S-nitrosylation of Thorase via trans-nitrosylation, which modulates the surface insertion of AMPARs. In the presence of the S-nitrosylation-deficient C137L Thorase mutant, AMPAR trafficking, long-term potentiation, and long-term depression are impaired. Overall, our data suggest that both S-nitrosylation and interactions of Thorase and NSF/PICK1 are required to modulate AMPAR-mediated synaptic plasticity. This study provides critical information that elucidates the mechanism underlying Thorase and NSF-mediated trafficking of AMPAR complexes.

TSPAN5 Enriched Microdomains Provide a Platform for Dendritic Spine Maturation through Neuroligin-1 Clustering

Tetraspanins are a class of evolutionarily conserved transmembrane proteins with 33 members identified in mammals that have the ability to organize specific membrane domains, named tetraspanin-enriched microdomains (TEMs). Despite the relative abundance of different tetraspanins in the CNS, few studies have explored their role at synapses. Here, we investigate the function of TSPAN5, a member of the tetraspanin superfamily for which mRNA transcripts are found at high levels in the mouse brain. We demonstrate that TSPAN5 is localized in dendritic spines of pyramidal excitatory neurons and that TSPAN5 knockdown induces a dramatic decrease in spine number because of defects in the spine maturation process. Moreover, we show that TSPAN5 interacts with the postsynaptic adhesion molecule neuroligin-1, promoting its correct surface clustering. We propose that membrane compartmentalization by tetraspanins represents an additional mechanism for regulating excitatory synapses.

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TSPAN5 is expressed in pyramidal neurons and localizes mainly to dendritic spines

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TSPAN5 interacts with neuroligin-1 and promotes its clustering

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TSPAN5-neuroligin-1 complex is fundamental for dendritic spine maturation

In Vivo Regulation of Oligodendrocyte Precursor Cell Proliferation and Differentiation by the AMPA-Receptor Subunit GluA2

The functional role of AMPA receptor (AMPAR)-mediated synaptic signaling between neurons and oligodendrocyte precursor cells (OPCs) remains enigmatic. We modified the properties of AMPARs at axon-OPC synapses in the mouse corpus callosum in vivo during the peak of myelination by targeting the GluA2 subunit. Expression of the unedited (Ca²⁺ permeable) or the pore-dead GluA2 subunit of AMPARs triggered proliferation of OPCs and reduced their differentiation into oligodendrocytes. Expression of the cytoplasmic C-terminal (GluA2(813-862)) of the GluA2 subunit (C-tail), a modification designed to affect the interaction between GluA2 and AMPAR-binding proteins and to perturb trafficking of GluA2-containing AMPARs, decreased the differentiation of OPCs without affecting their proliferation. These findings suggest that ionotropic and non-ionotropic properties of AMPARs in OPCs, as well as specific aspects of AMPAR-mediated signaling at axon-OPC synapses in the mouse corpus callosum, are important for balancing the response of OPCs to proliferation and differentiation cues.

The antidepressant tianeptine reverts synaptic AMPA receptor defects caused by deficiency of CDKL5

Mutations in the X-linked cyclin-dependent kinase-like 5 (CDKL5) gene cause a complex neurological disorder, characterized by infantile seizures, impairment of cognitive and motor skills and autistic features. Loss of Cdkl5 in mice affects dendritic spine maturation and dynamics but the underlying molecular mechanisms are still far from fully understood. Here we show that Cdkl5 deficiency in primary hippocampal neurons leads to deranged expression of the alpha-amino-3-hydroxy-5-methyl-4-iso-xazole propionic acid receptors (AMPA-R). In particular, a dramatic reduction of expression of the GluA2 subunit occurs concomitantly with its hyper-phosphorylation on Serine 880 and increased ubiquitination. Consequently, Cdkl5 silencing skews the composition of membrane-inserted AMPA-Rs towards the GluA2-lacking calcium-permeable form. Such derangement is likely to contribute, at least in part, to the altered synaptic functions and cognitive impairment linked to loss of Cdkl5. Importantly, we find that tianeptine, a cognitive enhancer and antidepressant drug, known to recruit and stabilise AMPA-Rs at the synaptic sites, can normalise the expression of membrane inserted AMPA-Rs as well as the number of PSD-95 clusters, suggesting its therapeutic potential for patients with mutations in CDKL5.

Neurointegrity and neurophysiology: astrocyte, glutamate, and carbon monoxide interactions

Astrocyte contributions to brain function and prevention of neuropathologies are as extensive as that of neurons. Astroglial regulation of glutamate, a primary neurotransmitter, is through uptake, release through vesicular and non-vesicular pathways, and catabolism to intermediates. Homeostasis by astrocytes is considered to be of primary importance in determining normal central nervous system health and central nervous system physiology - glutamate is central to dynamic physiologic changes and central nervous system stability. Gasotransmitters may affect diverse glutamate interactions positively or negatively. The effect of carbon monoxide, an intrinsic central nervous system gasotransmitter, in the complex astrocyte homeostasis of glutamate may offer insights to normal brain development, protection, and its use as a neuromodulator and neurotherapeutic. In this article, we will review the effects of carbon monoxide on astrocyte homeostasis of glutamate.

MEF2A regulates mGluR-dependent AMPA receptor trafficking independently of Arc/Arg3.1

Differential trafficking of AMPA receptors (AMPARs) to and from the postsynaptic membrane is a key determinant of the strength of excitatory neurotransmission, and is thought to underlie learning and memory. The transcription factor MEF2 is a negative regulator of memory in vivo, in part by regulating trafficking of the AMPAR subunit GluA2, but the molecular mechanisms behind this have not been established. Here we show, via knockdown of endogenous MEF2A in primary neuronal culture, that MEF2A is specifically required for Group I metabotropic glutamate receptor (mGluR)-mediated GluA2 internalisation, but does not regulate AMPAR expression or trafficking under basal conditions. Furthermore, this process occurs independently of changes in expression of Arc/Arg3.1, a previously characterised MEF2 transcriptional target and mediator of mGluR-dependent long-term depression. These data demonstrate a novel MEF2A-dependent mechanism for the regulation of activity-dependent AMPAR trafficking.

Maternal fluoride exposure during gestation and lactation decreased learning and memory ability, and glutamate receptor mRNA expressions of mouse pups

Previous investigations demonstrated that high fluoride (F) exposure may adversely affect the neurodevelopment and learning and memory ability. However, whether maternal F exposure during gestation and lactation can influence the learning, memory ability, and glutamate receptor expressions of offspring has not yet been elucidated. Hence, in the present study, maternal mice were exposed to F (25, 50, or 100 mg/L sodium fluoride (NaF) in drinking water) during gestation and lactation. Results showed that exposure to 100 mg/L NaF significantly enhanced the number of total arm entries and working memory errors of offspring in the radial arm maze test compared to the control group. However, no difference was observed in open-field behaviors. For the subtypes of glutamate receptors in hippocampus, expression of GluR2 mRNA was significantly reduced by 25, 50, and 100 mg/L NaF. Besides, F exposure also suppressed the expression of NR2A, NR2B, and mGluR2 mRNA levels in a dose-dependent manner, where NR2A was significantly suppressed by 50 mg/L NaF and NR2B and mGluR2 by 100 mg/L NaF. However, no significant changes were observed in GluR1 and mGluR5 mRNA expression levels. Collectively, these findings suggested that F can pass through the cord blood and breast milk and may have deleterious impact on learning and memory of the mouse pups, which was mediated by reduced mRNA expression of glutamate receptor subunits.

Disrupting GluA2-GAPDH Interaction Affects Axon and Dendrite Development

GluA2-containing AMPA receptors (AMPARs) play a critical role in various aspects of neurodevelopment. However, the molecular mechanisms underlying these processes are largely unknown. We report here that the interaction between GluA2 and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) is necessary for neuron and cortical development. Using an interfering peptide (GluA2-G-Gpep) that specifically disrupts this interaction, we found that primary neuron cultures with peptide treatment displayed growth cone development deficits, impairment of axon formation, less dendritic arborization and lower spine protrusion density. Consistently, in vivo data with mouse brains from pregnant dams injected with GluA2-G-Gpep daily during embryonic day 8 to 19 revealed a reduction of cortical tract axon integrity and neuronal density in post-natal day 1 offspring. Disruption of GluA2-GAPDH interaction also impairs the GluA2-Plexin A4 interaction and reduces p53 acetylation in mice, both of which are possible mechanisms leading to the observed neurodevelopmental abnormalities. Furthermore, electrophysiological experiments indicate altered long-term potentiation (LTP) in hippocampal slices of offspring mice. Our results provide novel evidence that AMPARs, specifically the GluA2 subunit via its interaction with GAPDH, play a critical role in cortical neurodevelopment.

Immunolocalization of AMPA receptor subunits within the enteric nervous system of the mouse colon and the effect of their activation on spontaneous colonic contractions

BACKGROUND

The appropriate expression of specific neurotransmitter receptors within the cellular networks that compose the enteric nervous system (ENS) is central to the regulation of gastrointestinal (GI) functions. While the ENS expression patterns of the neurotransmitter glutamate have been well documented, the localization of its receptors on ENS neurons remains to be fully characterized. We investigated the expression patterns of glutamate receptor AMPA subunits within ENS neurons of the mouse colon and the consequences of their pharmacological activation on spontaneous colonic contractility.

METHODS

RT-PCR was used to detect individual AMPA receptor (GluR 1-4) subunit expression at the mRNA level in mouse colon tissue. Immunohistochemistry and confocal microscopy was used to localize the expression of the GluR1 and 4 subunits in colon tissue. Brain tissue was used as a positive control. Organ bath preparations were used to determine the effect of AMPA receptors activation on the force and frequency of colonic longitudinal smooth muscle spontaneous contractions.

KEY RESULTS

GluR1, 3, 4 mRNA was detected in the mouse colon. Immunoreactivity for GluR1 and 4 subunits was detected on the somatic and dendritic surfaces of subpopulations of neurochemically defined ENS neurons. The pharmacological activation of AMPA receptors increased the force but not frequency of spontaneous colonic contractions.

CONCLUSIONS & INFERENCES

Molecularly distinct AMPA receptor subtypes are differentially expressed within the neural networks of the mouse colon and have a direct role in motility. These data provide the rationale for the development of AMPA-selective ligands for the therapeutic delivery to the GIT in motility disorders.

Neuroplasticity and Repair in Rodent Neurotoxic Models of Spinal Motoneuron Disease

Retrogradely transported toxins are widely used to set up protocols for selective lesioning of the nervous system. These methods could be collectively named "molecular neurosurgery" because they are able to destroy specific types of neurons by using targeted neurotoxins. Lectins such as ricin, volkensin, or modeccin and neuropeptide- or antibody-conjugated saporin represent the most effective toxins used for neuronal lesioning. Some of these specific neurotoxins could be used to induce selective depletion of spinal motoneurons. In this review, we extensively describe two rodent models of motoneuron degeneration induced by volkensin or cholera toxin-B saporin. In particular, we focus on the possible experimental use of these models to mimic neurodegenerative diseases, to dissect the molecular mechanisms of neuroplastic changes underlying the spontaneous functional recovery after motoneuron death, and finally to test different strategies of neural repair. The potential clinical applications of these approaches are also discussed.

Excitatory synaptic function and plasticity is persistently altered in ventral tegmental area dopamine neurons after prenatal ethanol exposure

Prenatal ethanol exposure (PE) is one of the developmental factors leading to increased addiction propensity (risk). However, the neuronal mechanisms underlying this effect remain unknown. We examined whether increased excitatory synaptic transmission in ventral tegmental area (VTA) dopamine (DA) neurons, which is associated with drug addiction, was impacted by PE. Pregnant rats were exposed to ethanol (0 or 6 g/kg/day) via intragastric intubation from gestational day 8-20. Amphetamine self-administration, whole-cell recordings, and electron microscopy were performed in male offspring between 2 and 12-week-old. The results showed enhanced amphetamine self-administration in PE animals. In PE animals, we observed a persistent augmentation in calcium-permeable AMPA receptor (CP-AMPAR) expression, indicated by increased rectification and reduced decay time of AMPAR-mediated excitatory postsynaptic currents (AMPAR-EPSCs), enhanced depression of AMPAR-EPSCs by NASPM (a selective CP-AMPAR antagonist), and increased GluA3 subunits in VTA DA neuron dendrites. Increased CP-AMPAR expression in PE animals led to enhanced excitatory synaptic strength and the induction of CP-AMPAR-dependent long-term potentiation (LTP), an anti-Hebbian form of LTP. These observations suggest that, in PE animals, increased excitatory synaptic strength in VTA DA neurons might be susceptible to further strengthening even in the absence of impulse flow. The PE-induced persistent increase in CP-AMPAR expression, the resulting enhancement in excitatory synaptic strength, and CP-AMPAR-dependent LTP are similar to effects observed after repeated exposure to drugs of abuse, conditions known to increase addiction risk. Therefore, these mechanisms could be important neuronal substrates underlying PE-induced enhancement in amphetamine self-administration and increased addiction risk in individuals with fetal alcohol spectrum disorders.

Trigeminal Medullary Dorsal Horn Neurons Activated by Nasal Stimulation Coexpress AMPA, NMDA, and NK1 Receptors

Afferent information initiating the cardiorespiratory responses during nasal stimulation projects from the nasal passages to neurons within the trigeminal medullary dorsal horn (MDH) via the anterior ethmoidal nerve (AEN). Central AEN terminals are thought to release glutamate to activate the MDH neurons. This study was designed to determine which neurotransmitter receptors (AMPA, kainate, or NMDA glutamate receptor subtypes or the Substance P receptor NK1) are expressed by these activated MDH neurons. Fos was used as a neuronal marker of activated neurons, and immunohistochemistry combined with epifluorescent microscopy was used to determine which neurotransmitter receptor subunits were coexpressed by activated MDH neurons. Results indicate that, during nasal stimulation with ammonia vapors in urethane-anesthetized Sprague-Dawley rats, activated neurons within the superficial MDH coexpress the AMPA glutamate receptor subunits GluA1 (95.8%) and GluA2/3 (88.2%), the NMDA glutamate receptor subunits GluN1 (89.1%) and GluN2A (41.4%), and NK1 receptors (64.0%). It is therefore likely that during nasal stimulation the central terminals of the AEN release glutamate and substance P that then produces activation of these MDH neurons. The involvement of AMPA and NMDA receptors may mediate fast and slow neurotransmission, respectively, while NK1 receptor involvement may indicate activation of a nociceptive pathway.

AMPAR trafficking in synapse maturation and plasticity

Glutamate ionotropic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (AMPARs) mediate most fast excitatory synaptic transmission in the central nervous system. The content and composition of AMPARs in postsynaptic membranes (which determine synaptic strength) are dependent on the regulated trafficking of AMPAR subunits in and out of the membranes. AMPAR trafficking is a key mechanism that drives nascent synapse development, and is the main determinant of both Hebbian and homeostatic plasticity in mature synapses. Hebbian plasticity seems to be the biological substrate of at least some forms of learning and memory; while homeostatic plasticity (also known as synaptic scaling) keeps neuronal circuits stable by maintaining changes within a physiological range. In this review, we examine recent findings that provide further understanding of the role of AMPAR trafficking in synapse maturation, Hebbian plasticity, and homeostatic plasticity.

Stress differentially alters mu opioid receptor density and trafficking in parvalbumin-containing interneurons in the female and male rat hippocampus

Stress differentially affects hippocampal-dependent learning relevant to addiction and morphology in male and female rats. Mu opioid receptors (MORs), which are located in parvalbumin (PARV)-containing GABAergic interneurons and are trafficked in response to changes in the hormonal environment, play a critical role in promoting principal cell excitability and long-term potentiation. Here, we compared the effects of acute and chronic immobilization stress (AIS and CIS) on MOR trafficking in PARV-containing neurons in the hilus of the dentate gyrus in female and male rats using dual label immunoelectron microscopy. Following AIS, the density of MOR silver-intensified gold particles (SIGs) in the cytoplasm of PARV-labeled dendrites was significantly reduced in females (estrus stage). Conversely, AIS significantly increased the proportion of cytoplasmic MOR SIGs in PARV-labeled dendrites in male rats. CIS significantly reduced the number of PARV-labeled neurons in the dentate hilus of males but not females. However, MOR/PARV-labeled dendrites and terminals were significantly smaller in CIS females, but not males, compared with controls. Following CIS, the density of cytoplasmic MOR SIGs increased in PARV-labeled dendrites and terminals in females. Moreover, the proportion of near-plasmalemmal MOR SIGs relative to total decreased in large PARV-labeled dendrites in females. After CIS, no changes in the density or trafficking of MOR SIGs were seen in PARV-labeled dendrites or terminals in males. These data show that AIS and CIS differentially affect available MOR pools in PARV-containing interneurons in female and male rats. Furthermore, they suggest that CIS could affect principal cell excitability in a manner that maintains learning processes in females but not males.

Noggin and Sonic hedgehog are involved in compensatory changes within the motoneuron-depleted mouse spinal cord

Sonic hedgehog and Noggin are morphogenetic factors involved in neural induction and ventralization of the neural tube, but recent findings suggest that they could participate in regeneration and functional recovery after injury. Here, in order to verify if these mechanisms could occur in the spinal cord and involve synaptic plasticity, we measured the expression levels of Sonic hedgehog, Noggin, Choline Acetyltransferase, Synapsin-I and Glutamate receptor subunits (GluR1, GluR2, GluR4), in a motoneuron-depleted mouse spinal cord lesion model obtained by intramuscular injection of Cholera toxin-B saporin. The lesion caused differential expression changes of the analyzed proteins. Moreover, motor performance was found correlated with Sonic hedgehog and Noggin expression in lesioned animals. The results also suggest that Sonic hedgehog could collaborate in modulating synaptic plasticity. Together, these findings confirm that the injured mammalian spinal cord has intrinsic potential for repair and that some proteins classically involved in development, such as Sonic hedgehog and Noggin could have important roles in regeneration and functional restoration, by mechanisms including synaptic plasticity.

Rapid progression and brain atrophy in anti-AMPA receptor encephalitis

Anti-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor encephalitis is an anti-neuronal surface antigen autoimmune encephalitis that is rarely reported. Our study evaluated the first known patient who developed anti-AMPA receptor encephalitis during pregnancy. Initial brain MRI revealed bilateral limbic encephalitis. However, rapid brain atrophy on MRI with extensive hypometabolism of cerebral cortices, caudate nuclei and brain stem hypoperfusion on (18)F-FDG PET developed when clinically progressed. IgG index of serial CSF studies reflected the clinical improvements after plasmapheresis and plasma exchange. The clinical spectrum of anti-AMPA receptor encephalitis may be expanded from limited limbic involvement to extended central nervous system.

Expression, subunit composition, and function of AMPA-type glutamate receptors are changed in activated microglia; possible contribution of GluA2 (GluR-B)-deficiency under pathological conditions

Microglia express AMPA (α-amino-hydroxy-5-methyl-isoxazole-4-propionate)-type of glutamate (Glu) receptors (AMPAR), which are highly Ca(2+) impermeable due to the expression of GluA2. However, the functional importance of AMPAR in microglia remains to be investigated, especially under pathological conditions. As low expression of GluA2 was reported in some neurodegenerative diseases, GluA2(-/-) mice were used to show the functional change of microglial AMPARs in response to Glu or kainate (KA). Here we found that Glu-induced currents in the presence of 100 μM cyclothiazide, an inhibitor of AMPAR desensitization, showed time-dependent decrease after activation of microglia with lipopolysaccharide (LPS) in GluA2(+/+) microglia, but not in GluA2(-/-) microglia. Upon activation of microglia, expression level of GluA2 subunits significantly increased, while expression of GluA1, A3 and A4 subunits on membrane surface significantly decreased. These results suggest that nearly homomeric GluA2 subunits were the main reason for low conductance of AMPAR in activated microglia. Increased expression of GluA2 in microglia was also detected partially in brain slices from LPS-injected mice. Cultured microglia from GluA2(-/-) mice showed higher Ca(2+) -permeability, consequently inducing significant increase in the release of proinflammatory cytokine, such as TNF-α. The conditioning medium from KA-treated GluA2(-/-) microglia had more neurotoxic effect on wild type cultured neurons than that from KA-treated GluA2(+/+) microglia. These results suggest that membrane translocation of GluA2-containing AMPARs in activated microglia has functional importance and thus, dysfunction or decreased expression of GluA2 may accelerate Glu neurotoxicity via excess release of proinflammatory cytokines from microglia.

Neuroligin-1 loss is associated with reduced tenacity of excitatory synapses

Neuroligins (Nlgns) are postsynaptic, integral membrane cell adhesion molecules that play important roles in the formation, validation, and maturation of synapses in the mammalian central nervous system. Given their prominent roles in the life cycle of synapses, it might be expected that the loss of neuroligin family members would affect the stability of synaptic organization, and ultimately, affect the tenacity and persistence of individual synaptic junctions. Here we examined whether and to what extent the loss of Nlgn-1 affects the dynamics of several key synaptic molecules and the constancy of their contents at individual synapses over time. Fluorescently tagged versions of the postsynaptic scaffold molecule PSD-95, the AMPA-type glutamate receptor subunit GluA2 and the presynaptic vesicle molecule SV2A were expressed in primary cortical cultures from Nlgn-1 KO mice and wild-type (WT) littermates, and live imaging was used to follow the constancy of their contents at individual synapses over periods of 8-12 hours. We found that the loss of Nlgn-1 was associated with larger fluctuations in the synaptic contents of these molecules and a poorer preservation of their contents at individual synapses. Furthermore, rates of synaptic turnover were somewhat greater in neurons from Nlgn-1 knockout mice. Finally, the increased GluA2 redistribution rates observed in neurons from Nlgn-1 knockout mice were negated by suppressing spontaneous network activity. These findings suggest that the loss of Nlgn-1 is associated with some use-dependent destabilization of excitatory synapse organization, and indicate that in the absence of Nlgn-1, the tenacity of excitatory synapses might be somewhat impaired.

Kidins220/ARMS as a functional mediator of multiple receptor signalling pathways

An increasing body of evidence suggests that several membrane receptors--in addition to activating distinct signalling cascades--also engage in substantial crosstalk with each other, thereby adjusting their signalling outcome as a function of specific input information. However, little is known about the molecular mechanisms that control their coordination and integration of downstream signalling. A protein that is likely to have a role in this process is kinase-D-interacting substrate of 220 kDa [Kidins220, also known as ankyrin repeat-rich membrane spanning (ARMS), hereafter referred to as Kidins220/ARMS]. Kidins220/ARMS is a conserved membrane protein that is preferentially expressed in the nervous system and interacts with the microtubule and actin cytoskeleton. It interacts with neurotrophin, ephrin, vascular endothelial growth factor (VEGF) and glutamate receptors, and is a common downstream target of several trophic stimuli. Kidins220/ARMS is required for neuronal differentiation and survival, and its expression levels modulate synaptic plasticity. Kidins220/ARMS knockout mice show developmental defects mainly in the nervous and cardiovascular systems, suggesting a crucial role for this protein in modulating the cross talk between different signalling pathways. In this Commentary, we summarise existing knowledge regarding the physiological functions of Kidins220/ARMS, and highlight some interesting directions for future studies on the role of this protein in health and disease.

Involvement of brain-derived neurotrophic factor and sonic hedgehog in the spinal cord plasticity after neurotoxic partial removal of lumbar motoneurons

Adult mammals could spontaneously achieve a partial sensory-motor recovery after spinal cord injury, by mechanisms including synaptic plasticity. We previously showed that this recovery is associated to the expression of synapsin-I, and that sonic hedgehog and Notch-1 could be also involved in plasticity. The role of brain-derived neurotrophic factor and glutamate receptors in regulating synaptic efficacy has been explored in the last decade but, although these mechanisms are now well-defined in the brain, the molecular mechanisms underlying the so called "spinal learning" are still less clear. Here, we measured the expression levels of choline acetyltransferase, synapsin-I, sonic hedgehog, Notch-1, glutamate receptor subunits (GluR1, GluR2, GluR4, NMDAR1) and brain-derived neurotrophic factor, in a motoneuron-depleted mouse spinal lesion model obtained by intramuscular injection of cholera toxin-B saporin. The lesion caused the down-regulation of the majority of analysed proteins. Moreover, we found that in lesioned but not in control spinal tissue, synapsin-I expression is associated to that of both brain-derived neurotrophic factor and sonic hedgehog, whereas GluR2 expression is linked to that of Shh. These results suggest that brain-derived neurotrophic factor and sonic hedgehog could collaborate in modulating synaptic plasticity after the removal of motoneurons, by a mechanism involving both pre- and post-synaptic processes. Interestingly, the involvement of sonic hedgehog showed here is novel, and offers new routes to address spinal cord plasticity and repair.

High frequency stimulation alters motor maps, impairs skilled reaching performance and is accompanied by an upregulation of specific GABA, glutamate and NMDA receptor subunits

High frequency stimulation (HFS) has the potential to interfere with learning and memory. HFS and motor skill training both lead to potentiation of the stimulated network and alter motor map expression. However, the extent to which HFS can interfere with the learning and performance of a skilled motor task and the resulting effect on the representation of movement has not been examined. Moreover, the molecular mechanisms associated with HFS and skilled motor training on the motor cortex are not known. We hypothesized that HFS would impair performance on a skilled reaching task, and would be associated with alterations in motor map expression and protein levels compared to non-stimulated and untrained controls. Long Evans Hooded rats were chronically implanted with stimulating and recording electrodes in the corpus callosum and frontal neocortex, respectively. High frequency theta burst stimulation or sham stimulation was applied once daily for 20 sessions. The rats were divided into five groups: control, HFS and assessed at 1 week post stimulation, HFS and assessed 3 weeks post stimulation, reach trained, and HFS and reach trained. A subset of rats from each group was assessed with either intracortical microstimulation (ICMS) to examine motor map expression or Western blot techniques to determine protein expression of several excitatory and inhibitory receptor subunits. Firstly, we found that HFS resulted in larger and reorganized motor maps, and lower movement thresholds compared to controls. This was associated with an up-regulation of the GABA(A)α1 and NR1 receptor subunits 3 weeks after the last stimulation session only. Stimulation affected skilled reaching performance in a subset of all stimulated rats. Rats that were poor performers had larger rostral forelimb areas, higher proximal and lower distal movement thresholds compared to rats that were good performers after stimulation. Reach training alone was associated with an up-regulation of GABA(A)α1, α2, GluR2, NR1 and NR2A compared to controls. HFS and reach-trained rats showed an up-regulation of GABA(A)α2 compared to stimulated rats that were not reach-trained. Therefore, we have shown that HFS induces significant plasticity in the motor cortex, and has the potential to disrupt performance on a skilled motor task.

The essential role of AMPA receptor GluR2 subunit RNA editing in the normal and diseased brain

α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are comprised of different combinations of GluA1–GluA4 (also known asGluR1–GluR4 and GluR-A to GluR-D) subunits. The GluA2 subunit is subject to RNA editing by the ADAR2 enzyme, which converts a codon for glutamine (Gln; Q), present in the GluA2 gene, to a codon for arginine (Arg; R) found in the mRNA. AMPA receptors are calcium (Ca²⁺)-permeable if they contain the unedited GluA2(Q) subunit or if they lack the GluA2 subunit. While most AMPA receptors in the brain contain the edited GluA2(R) subunit and are therefore Ca²⁺-impermeable, recent evidence suggests that Ca²⁺-permeable AMPA receptors are important in synaptic plasticity, learning, and disease. Strong evidence supports the notion that Ca²⁺-permeable AMPA receptors are usually GluA2-lacking AMPA receptors, with little evidence to date for a significant role of unedited GluA2 in normal brain function. However, recent detailed studies suggest that Ca²⁺-permeable AMPA receptors containing unedited GluA2 do in fact occur in neurons and can contribute to excitotoxic cell loss, even where it was previously thought that there was no unedited GluA2.This review provides an update on the role of GluA2 RNA editing in the healthy and diseased brain and summarizes recent insights into the mechanisms that control this process. We suggest that further studies of the role of unedited GluA2 in normal brain function and disease are warranted, and that GluA2 editing should be considered as a possible contributing factor when Ca²⁺-permeable AMPA receptors are observed.

Effect of mild hypothermia on glutamate receptor expression after status epilepticus

Hypothermia has been shown to have neuroprotective effects in various models of neurological damage. However, its therapeutic effect on pediatric status epilepticus (SE) is still unknown. We conducted a study to investigate whether hypothermia can have an adjuvant effect on pilocarpine-induced status epilepticus in immature rats when combined with diazepam treatment. Pilocarpine-induced status epilepticus was maintained for either 30 min or 60 min, which was followed by injection with diazepam (10mg/kg body weight) and/or treatment with mild hypothermia (core temperature to 33°C). We found that the spike-wave amplitude and frequency after SE during treatment with diazepam and hypothermia was significantly lower than treatment with diazepam alone. Mild hypothermia significantly reduced the number of cells undergoing necrosis and apoptosis. In addition, α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) receptor subunit GluR1 was shown to be up-regulated by SE, while GluR2 was shown to be down-regulated. However, after combination therapy with diazepam and mild hypothermia for 8h, the expression of GluR1 was decreased and GluR2 was increased relative to the levels of diazepam alone treated juveniles. We also found that the expression of mGluR-1a was also decreased relative to diazepam alone. These findings suggest that mild hypothermia might further protect against pilocarpine-induced status epilepticus in immature rats by regulating glutamate receptor expression. This study was conducted using a pediatric model of SE so as to gain a better understanding of the role of hypothermia in the developing brain.

Neuralized1 activates CPEB3: a function for nonproteolytic ubiquitin in synaptic plasticity and memory storage

The cytoplasmic polyadenylation element-binding protein 3 (CPEB3), a regulator of local protein synthesis, is the mouse homolog of ApCPEB, a functional prion protein in Aplysia. Here, we provide evidence that CPEB3 is activated by Neuralized1, an E3 ubiquitin ligase. In hippocampal cultures, CPEB3 activated by Neuralized1-mediated ubiquitination leads both to the growth of new dendritic spines and to an increase of the GluA1 and GluA2 subunits of AMPA receptors, two CPEB3 targets essential for synaptic plasticity. Conditional overexpression of Neuralized1 similarly increases GluA1 and GluA2 and the number of spines and functional synapses in the hippocampus and is reflected in enhanced hippocampal-dependent memory and synaptic plasticity. By contrast, inhibition of Neuralized1 reduces GluA1 and GluA2 levels and impairs hippocampal-dependent memory and synaptic plasticity. These results suggest a model whereby Neuralized1-dependent ubiquitination facilitates hippocampal plasticity and hippocampal-dependent memory storage by modulating the activity of CPEB3 and CPEB3-dependent protein synthesis and synapse formation.

Trafficking of AMPA receptors at plasma membranes of hippocampal neurons

The number of AMPA receptors at synapses depends on receptor cycling. Because receptors diffuse rapidly in plasma membranes, their exocytosis and endocytosis need not occur near synapses. Here, pre-embedding immunogold electron microscopy is applied to dissociated rat hippocampal cultures to provide sensitive, high-resolution snapshots of the distribution of surface AMPA receptors in spines, dendrites, and cell bodies that will be informative about trafficking of AMPA receptors. The density of the label for GluR2 varies, but is consistent throughout cell body and dendrites in each individual neuron, except at postsynaptic densities (PSDs), where it is typically higher. Glutamate receptor 2 (GluR2) labels at PSDs significantly increase after synaptic activation by glycine treatment and increase further upon depolarization by high K(+). Islands of densely packed labels have consistent size and density but vary in frequency under different experimental conditions. These patches of label, which occur on plasma membranes of cell bodies and dendrites but not near PSDs, are taken to be the aftermath of exocytosis of AMPA receptors. A subpopulation of clathrin-coated pits in cell bodies and dendrites label for GluR2, and the number and amount of label in individual pits increase after NMDA treatment. Coated pits near synapses typically lack GluR2 label under basal conditions, but ∼40% of peri-PSD pits label for GluR2 after NMDA treatment. Thus, exocytosis and endocytosis of AMPA receptors occur mainly at extrasynaptic locations on cell bodies and dendrites. Receptors are not preferentially exocytosed near PSDs, but may be removed via endocytosis at peri-PSD locations after activation of NMDA receptors.

Acute stress induces contrasting changes in AMPA receptor subunit phosphorylation within the prefrontal cortex, amygdala and hippocampus

Exposure to stress causes differential neural modifications in various limbic regions, namely the prefrontal cortex, hippocampus and amygdala. We investigated whether α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) phosphorylation is involved with these stress effects. Using an acute inescapable stress protocol with rats, we found opposite effects on AMPA receptor phosphorylation in the medial prefrontal cortex (mPFC) and dorsal hippocampus (DH) compared to the amygdala and ventral hippocampus (VH). After stress, the phosphorylation of Ser831-GluA1 was markedly decreased in the mPFC and DH, whereas the phosphorylation of Ser845-GluA1 was increased in the amygdala and VH. Stress also modulated the GluA2 subunit with a decrease in the phosphorylation of both Tyr876-GluA2 and Ser880-GluA2 residues in the amygdala, and an increase in the phosphorylation of Ser880-GluA2 in the mPFC. These results demonstrate that exposure to acute stress causes subunit-specific and region-specific changes in glutamatergic transmission, which likely lead to the reduced synaptic efficacy in the mPFC and DH and augmented activity in the amygdala and VH. In addition, these findings suggest that modifications of glutamate receptor phosphorylation could mediate the disruptive effects of stress on cognition. They also provide a means to reconcile the contrasting effects that stress has on synaptic plasticity in these regions. Taken together, the results provide support for a brain region-oriented approach to therapeutics.

Ultrastructural relationship between the AMPA-GluR2 receptor subunit and the mu-opioid receptor in the mouse central nucleus of the amygdala

Activation of GluR2-expressing non-calcium-permeable AMPA-type glutamate receptors in the central nucleus of the amygdala (CeA) may play an important role in integrating emotion and memory with goal-directed behaviors involved in opioid addiction. The location of non-calcium-permeable AMPA receptors within distinct neuronal compartments (i.e., soma, dendrite, or axon) is an important functional feature of these proteins; however, their ultrastructural location and subcellular relationship with mu-opioid receptors (μOR) in the CeA are unknown. Immunocytochemical electron microscopy was used to characterize the ultrastructural distribution of GluR2 and its association with μOR in the mouse CeA. A single-labeling analysis of GluR2 distribution employing immunoperoxidase or immunogold markers revealed that this protein was frequently affiliated with intracellular vesicular organelles, as well as the plasma membrane of CeA neuronal profiles. Among all GluR2-labeled neuronal structures, over 85% were dendrites or somata. Unlabeled axon terminals frequently formed asymmetric excitatory-type synaptic junctions with GluR2-labeled dendritic profiles. Dual-labeling immunocytochemical analysis showed that GluR2 and μOR were co-localized in neuronal compartments. Among all dual-labeled structures, approximately 80% were dendritic. Synaptic inputs to these dual-labeled dendrites were frequently from unlabeled axon terminals forming asymmetric excitatory-type synapses. The presence of GluR2 in dendritic profiles receiving asymmetric synapses suggests that activation of the non-calcium-permeable AMPA receptor plays a role in the postsynaptic modulation of excitatory signaling involving CeA neuronal circuits that coordinate sensory, affective, and behavioral processes involved in drug addiction. Given the critical role of non-calcium-permeable AMPA receptor function in neural and behavioral adaptability, their dendritic association with μOR in CeA dendrites provides a neuronal substrate for opioid-mediated plasticity.

AMPAR-mediated synaptic transmission in the CA1 hippocampal region of neonatal rats: unexpected resistance to repeated ethanol exposure

Alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate glutamatergic receptors (AMPAR) mediate most of the fast excitatory synaptic transmission in mature neurons. In contrast, a number of developing synapses do not express AMPARs; these are gradually acquired in an activity-driven manner during the first week of life in rats, which is equivalent to the third trimester of human pregnancy. Neuronal stimulation has been shown to drive high conductance Ca(2+)-permeable AMPARs into the synapse, strengthening glutamatergic synaptic transmission. Alterations in this process could induce premature stabilization or inappropriate elimination of newly formed synapses and contribute to the hippocampal abnormalities associated with fetal alcohol spectrum disorder. Previous studies from our laboratory performed with hippocampal slices from neonatal rats showed that acute ethanol exposure exerts potent stimulant effects on CA1 and CA3 neuronal networks. However, the impact of these in vitro actions of acute ethanol exposure is unknown. Here, we tested the hypothesis that repeated in vivo exposure to ethanol strengthens AMPAR-mediated neurotransmission in the CA1 region by means of an increase in synaptic expression of Ca(2+)-permeable AMPARs. We exposed rats to ethanol vapor (serum ethanol concentration approximately 40 mM) or air for 4h/day from postnatal day (P) 2-6. In brain slices prepared at P4-6, we found no significant effect of ethanol exposure on input-output curves for AMPAR-mediated field excitatory postsynaptic potentials (fEPSPs), the contribution of Ca(2+)-permeable AMPARs to these fEPSPs, or the acute effect of ethanol on fEPSP amplitude. These results suggest that homeostatic plasticity mechanisms act to maintain glutamatergic synaptic strength and ethanol sensitivity in response to repeated developmental ethanol exposure.

HNK-1 glyco-epitope regulates the stability of the glutamate receptor subunit GluR2 on the neuronal cell surface

HNK-1 (human natural killer-1) glyco-epitope, a sulfated glucuronic acid attached to N-acetyllactosamine on the nonreducing termini of glycans, is highly expressed in the nervous system. Our previous report showed that mice lacking a glucuronyltransferase (GlcAT-P), a key enzyme for biosynthesis of the HNK-1 epitope, showed reduced long term potentiation at hippocampal CA1 synapses. In this study, we identified an alpha-amino-3-hydroxy-5-methylisoxazole propionate (AMPA)-type glutamate receptor subunit, GluR2, which directly contributes to excitatory synaptic transmission and synaptic plasticity, as a novel HNK-1 carrier molecule. We demonstrated that the HNK-1 epitope is specifically expressed on the N-linked glycan(s) on GluR2 among the glutamate receptors tested, and the glycan structure, including HNK-1 on GluR2, was determined using liquid chromatography-tandem mass spectrometry. As for the function of HNK-1 on GluR2, we found that the GluR2 not carrying HNK-1 was dramatically endocytosed and expressed less on the cell surface compared with GluR2 carrying HNK-1 in both cultured hippocampal neurons and heterologous cells. These results suggest that HNK-1 stabilizes GluR2 on neuronal surface membranes and regulates the number of surface AMPA receptors. Moreover, we showed that the expression of the HNK-1 epitope enhanced the interaction between GluR2 and N-cadherin, which has important roles in AMPA receptor trafficking. Our findings suggest that the HNK-1 epitope on GluR2 regulates cell surface stability of GluR2 by modulating the interaction with N-cadherin.