Early exposure of cultured hippocampal neurons to excitatory amino acids protects from later excitotoxicity

NMDA Receptors: Distribution, Role, and Insights into Neuropsychiatric Disorders

BACKGROUND

N-methyl-D-aspartate receptors (NMDARs) are members of the ionotropic glutamate receptor family. These ligand-gated channels are entwined with numerous fundamental neurological functions within the central nervous system (CNS), and numerous neuropsychiatric disorders may arise from their malfunction.

METHODS

The purpose of the present review is to provide a detailed description of NMDARs by addressing their molecular structures, activation mechanisms, and physiological roles in the mammalian brain. In the second part, their role in various neuropsychiatric disorders including stroke, epilepsy, anti-NMDA encephalitis, Alzheimer's and Huntington's diseases, schizophrenia, depression, neuropathic pain, opioid-induced tolerance, and hyperalgesia will be covered.

RESULTS

Finally, through a careful exploration of the main non-competitive NMDARs antagonists (channel-blockers).

CONCLUSION

We discuss the strengths and limitations of the various molecular structures developed for diagnostic or therapeutic purposes.

Exposing immature hippocampal neurons to excitotoxins reveals distinct transcriptome and protein regulation with induction of common survival signaling pathways

Early life traumas lead to neuroprotection by preconditioning mechanisms. To determine which genes and pathways are most likely involved in specific adaptive effects, immature hippocampal cultures were exposed to a single high dose of glutamate (250 μM), NMDA (100 μM), or KA (300 μM) for 48 h (5-7 DIV) based on our prior "two hit" in vitro model of preconditioning. Transcriptome profiling and immunocytochemistry of gene candidates were performed 7 days later when cultured neurons mature (14 DIV). Many genes were up- and down- regulated involving distinct Ca²⁺-binding protein families, G-coupled proteins, various growth factors, synaptic vesicle docking factors, certain neurotransmitter receptors, heat shock, oxidative stress, and certain anti-apoptotic Bcl-2 gene members that influence neuronal survival. Immunohistochemistry showed a marked decrease in the number of Calb1 and Calm2 positive neurons following NMDA but not after glutamate exposure whereas ryanodine and Cav1.2 voltage gated channel expression was less affected. Survivors had marked increases in Calm2 immunostaining; however, high-density neural clusters observed in controls, were depleted after NMDA and partly diminished after glutamate. While NR1 mRNA expression was decreased in the microarray, specific antibodies revealed selective loss of the NR1C1 splice variant. Calm2 which can inactivate NMDA receptors by binding to C1 but not C2 regions of its NR1 subunit suggests that loss of the C1 splice variant will reduce co-regulation with Calm2 and alter NR1 trafficking, phosphorylation, and NMDA currents following early life NMDA exposure. A dramatic reduction in the density of GABAAα5 and GABAB receptor expressing neurons was observed after NMDA exposure but immunodensity measurements were unchanged as was the expression of the GABA synthesizing enzyme, GAD, suggesting that fast inhibitory neurotransmission and response to benzodiazepines and GABAB-mediated IPSPs may be preserved in matured survivors. Selective upregulation of Chat and CNRIP was detected after glutamate treatment suggesting this condition would decrease cholinergic and excitatory neurotransmission by decreasing Ach content and CB1 interacting protein function. This decrease likely contributes to memory and attention tasks deficits that follow a single early neurological insult. Diverse changes that follow overactivation of excitatory networks of immature neurons appear long-lasting or permanent and are expected to have profound effects on network function and adaptive responses to further insult.

NMDA receptor activity determines neuronal fate: location or number?

It is widely believed that the proper activation of N-methyl-D-aspartate (NMDA) receptors (NMDARs) promotes neuronal survival, whereas an excessive activation of NMDARs leads to neuronal damage. NMDARs are found at both synaptic and extrasynaptic sites. One current prevailing theory proposes the dichotomy of NMDAR activity. The role of the two population receptors is mutual antagonism. The activation of synaptic NMDARs, such as synaptic activity at physiological levels, promotes neuronal survival. However, the activation of extrasynaptic NMDARs occurring during stroke, brain injury, and chronic neurological diseases contributes to neuronal death. Thus, the location of NMDARs determines the neuronal fate. However, the theory is greatly challenged. Several studies suggested that synaptic NMDARs are involved in neuronal death. Recently, our work further showed that the coactivation of synaptic and extrasynaptic NMDARs contributes to neuronal death under neuronal insults. Therefore, we propose that the magnitude and duration of NMDAR activation determines the neuronal fate. More interestingly, there appears to be some subtle differences in the affinity between synaptic and extrasynaptic NMDARs, shedding light on the development of selective drugs to block extrasynaptic NMDARs.

De novo expression of parvalbumin in ependymal cells in response to brain injury promotes ependymal remodeling and wound repair

The calcium-binding protein parvalbumin (PV) hallmarks subpopulations of interneurons in the murine brain. We serendipitously observed the de novo expression of PV in ependymal cells of the lateral ventricle wall following in vivo lesioning and brain slicing for the preparation of organotypic hippocampal slice cultures (OHSCs). In OHSCs, de novo PV-expression begins shortly after the onset of culturing, and the number of ependymal cells implicated in this process increases with time. PV-immunopositive ependymal cells aggregate and form compact cell clusters, which are characterized by lumen-formation and beating cilia. Scratches inflicted on such clusters with a sharp knife are rapidly closed. Exposure of OHSCs to NF-КB-inhibitors and to antioxidants reduces PV-expression in ependymal cells, thereby implicating injury-induced inflammation in this process. Indeed, in vivo stab injury enhances PV-expression in ependymal cells adjacent to the lesion, whereas neuraminidase denudation is without effect. PV-knock-out mice manifest an impaired wound-healing response to in vivo injury, and a reduced scratch-wound reparation capacity in OHSCs. Whole-transcriptome analysis of ependymal-cell clusters in OHSCs revealed down-regulation of genes involved in cytoskeletal rearrangement, cell motility and cell adhesion in PV-knock out mice as compared with wild-type mice. Our data indicate that the injury-triggered up-regulation of PV-expression is mediated by inflammatory cytokines, and promotes the motility and adhesion of ependymal cells, thereby contributing to leakage closure by the re-establishment of a continuous ependymal layer.

Apoptotic and neurotoxic actions of 4-para-nonylphenol are accompanied by activation of retinoid X receptor and impairment of classical estrogen receptor signaling

4-para-Nonylphenol (NP) is a non-ionic surfactant that has widespread and uncontrolled distribution in the environment. Little is known, however, about its actions on neuronal cells during critical developmental periods. This study aimed to investigate the mechanisms underlying the apoptotic and toxic actions of NP on mouse embryonic neuronal cells and the possible interactions of NP with estrogen receptor (ER)- and retinoid X receptor (RXR)-mediated intracellular signaling. Treatment of mouse hippocampal neuronal cell cultures with NP (5 and 10μM) induced apoptotic and neurotoxic effects. The 2 and 7 day-old mouse hippocampal cultures were vulnerable to 5 and 10μM NP, whereas 12 day-old cultures responded only to the highest concentration of NP, thus suggesting an age-dependent action of the chemical on neuronal cells. The use of specific inhibitors did not support the involvement of calpains in NP-induced apoptosis, but indicated caspase-8- and caspase-9-dependent effects of NP. Specific ER antagonists MPP and PHTPP potentiated the NP-induced loss of mitochondrial membrane potential and increase in lactate dehydrogenase (LDH) release whereas, ER agonists PPT and DPN inhibited these effects. RXR antagonist HX531 diminished the NP-evoked loss of mitochondrial membrane potential, the activity of caspase-3 and LDH release. In addition, exposure to NP inhibited ERα- and ERβ-specific immunofluorescence but stimulated RXR-specific immunolabeling in mouse hippocampal cells. In conclusion, our study demonstrated that the apoptotic and toxic actions of NP on neuronal cells in early development is accompanied by an impairment of ER- and stimulation of RXR-mediated signaling pathways. Taking into account NP-induced alterations in mRNA expression levels of particular types of RXRs, we suggest that NP affected mainly RXRα and RXRβ, but not RXRγ signaling.

Extrasynaptic NMDA Receptor in Excitotoxicity: Function Revisited

It is generally accepted that proper activation of N-methyl-d-aspartate receptors (NMDARs) promotes neuronal survival and supports neuroplasticity, and excessive NMDAR activation leads to pathological outcomes and neurodegeneration. As NMDARs are found at both synaptic and extrasynaptic sites, there is significant interest in determining how NMDARs at different subcellular locations differentially regulate physiological as well as pathological functions. Better understanding of this issue may support the development of therapeutic strategies to attenuate neuronal death or promote normal brain function. Although the current prevailing theory emphasizes the major role of extrasynaptic NMDARs in neurodegeneration, there is growing evidence indicating the involvement of synaptic receptors. It is also evident that physiological functions of the brain also involve extrasynaptic NMDARs. Our recent studies demonstrate that the degree of cell death following neuronal insults depends on the magnitude and duration of synaptic and extrasynaptic receptor co-activation. These new results underscore the importance of revisiting the function of extrasynaptic NMDARs in cell fate. Furthermore, the development of antagonists that preferentially inhibit synaptic or extrasynaptic receptors may better clarify the role of NMDARs in neurodegeneration.

Early-life seizures in predisposing neuronal preconditioning: a critical review

Although seizures are known to be harmful, recent evidence indicates that they can also lead to adaptations that protect neurons from further insult. For example, a history of two episodes of status epilepticus during a critical period of early development can prolong the time period of resistance to hippocampal injury during the postnatal period. Neonatal seizures may lead to this neuroprotection via a preconditioning mechanism that could be attributed to attenuation of Ca(2+) currents, reduction of inflammation, and induction of survival signaling pathways. Understanding mechanisms underlying neuroprotective preconditioning may elucidate new therapeutic targets and improve outcomes and quality of life for pediatric epilepsy patients. This review will detail the specific cellular and molecular findings involved in neuronal preconditioning predisposed by early-life seizures.

Adaptation and sensitization to proteotoxic stress

Although severe stress can elicit toxicity, mild stress often elicits adaptations. Here we review the literature on stress-induced adaptations versus stress sensitization in models of neurodegenerative diseases. We also describe our recent findings that chronic proteotoxic stress can elicit adaptations if the dose is low but that high-dose proteotoxic stress sensitizes cells to subsequent challenges. In these experiments, long-term, low-dose proteasome inhibition elicited protection in a superoxide dismutase-dependent manner. In contrast, acute, high-dose proteotoxic stress sensitized cells to subsequent proteotoxic challenges by eliciting catastrophic loss of glutathione. However, even in the latter model of synergistic toxicity, several defensive proteins were upregulated by severe proteotoxicity. This led us to wonder whether high-dose proteotoxic stress can elicit protection against subsequent challenges in astrocytes, a cell type well known for their resilience. In support of this new hypothesis, we found that the astrocytes that survived severe proteotoxicity became harder to kill. The adaptive mechanism was glutathione dependent. If these findings can be generalized to the human brain, similar endogenous adaptations may help explain why neurodegenerative diseases are so delayed in appearance and so slow to progress. In contrast, sensitization to severe stress may explain why defenses eventually collapse in vulnerable neurons.

Involvement of the GluN2A and GluN2B subunits in synaptic and extrasynaptic N-methyl-D-aspartate receptor function and neuronal excitotoxicity

GluN2A and GluN2B are the major subunits of functional NMDA receptors (NMDAR). Previous studies have suggested that GluN2A and GluN2B may differentially mediate NMDAR function at synaptic and extrasynaptic locations and play opposing roles in excitotoxicity, such as neurodegeneration triggered by ischemic stroke and brain injury. By using pharmacological and molecular approaches to suppress or enhance the function of GluN2A and GluN2B in cultured cortical neurons, we examined NMDAR-mediated, bidirectional regulation of prosurvival signaling (i.e. the cAMP response element-binding protein (CREB)-Bdnf cascade) and cell death. Inhibition of GluN2A or GluN2B attenuated the up-regulation of prosurvival signaling triggered by the activation of either synaptic or extrasynaptic NMDAR. Inhibition of GluN2A or GluN2B also attenuated the down-regulation of prosurvival signaling triggered by the coactivation of synaptic and extrasynaptic receptors. The effects of GluN2B on CREB-Bdnf signaling were larger than those of GluN2A. Consistently, compared with suppression of GluN2A, suppression of GluN2B resulted in more reduction of NMDA- and oxygen glucose deprivation-induced excitotoxicity as well as NMDAR-mediated elevation of intracellular calcium. Moreover, excitotoxicity and down-regulation of CREB were exaggerated in neurons overexpressing GluN2A or GluN2B. Together, we found that GluN2A and GluN2B are involved in the function of both synaptic and extrasynaptic NMDAR, demonstrating that they play similar rather than opposing roles in NMDAR-mediated bidirectional regulation of prosurvival signaling and neuronal death.

Transcriptome profiling of hippocampal CA1 after early-life seizure-induced preconditioning may elucidate new genetic therapies for epilepsy

Injury of the CA1 subregion induced by a single injection of kainic acid (1 × KA) in juvenile animals (P20) is attenuated in animals with two prior sustained neonatal seizures on P6 and P9. To identify gene candidates involved in the spatially protective effects produced by early-life conditioning seizures we profiled and compared the transcriptomes of CA1 subregions from control, 1 × KA- and 3 × KA-treated animals. More genes were regulated following 3 × KA (9.6%) than after 1 × KA (7.1%). Following 1 × KA, genes supporting oxidative stress, growth, development, inflammation and neurotransmission were upregulated (e.g. Cacng1, Nadsyn1, Kcng1, Aven, S100a4, GFAP, Vim, Hrsp12 and Grik1). After 3 × KA, protective genes were differentially over-expressed [e.g. Cat, Gpx7, Gad1, Hspa12A, Foxn1, adenosine A1 receptor, Ca(2+) adaptor and homeostasis proteins, Cacnb4, Atp2b2, anti-apoptotic Bcl-2 gene members, intracellular trafficking protein, Grasp and suppressor of cytokine signaling (Socs3)]. Distinct anti-inflammatory interleukins (ILs) not observed in adult tissues [e.g. IL-6 transducer, IL-23 and IL-33 or their receptors (IL-F2 )] were also over-expressed. Several transcripts were validated by real-time polymerase chain reaction (QPCR) and immunohistochemistry. QPCR showed that casp 6 was increased after 1 × KA but reduced after 3 × KA; the pro-inflammatory gene Cox1 was either upregulated or unchanged after 1 × KA but reduced by ~70% after 3 × KA. Enhanced GFAP immunostaining following 1 × KA was selectively attenuated in the CA1 subregion after 3 × KA. The observed differential transcriptional responses may contribute to early-life seizure-induced pre-conditioning and neuroprotection by reducing glutamate receptor-mediated Ca(2+) permeability of the hippocampus and redirecting inflammatory and apoptotic pathways. These changes could lead to new genetic therapies for epilepsy.

NMDA receptor-mediated excitotoxicity depends on the coactivation of synaptic and extrasynaptic receptors

N-methyl-𝒟-aspartate receptors (NMDAR) overactivation is linked to neurodegeneration. The current prevailing theory suggests that synaptic and extrasynaptic NMDAR (syn- and ex-NMDAR) impose counteracting effects on cell fate, and neuronal cell death is mainly mediated by the activation of ex-NMDAR. However, several lines of evidence implicate the limitation of this theory. Here, we demonstrate that activation of NMDAR bi-directionally regulated cell fate through stimulating pro-survival or pro-death signaling. While low-dose NMDA preferentially activated syn-NMDAR and stimulated the extracellular signal-regulated kinase ½–cAMP responsive element-binding protein–brain-derived neurotrophic factor pro-survival signaling, higher doses progressively activated increasing amount of ex-NMDAR along with syn-NMDAR and triggered cell death program. Interestingly, the activation of syn- or ex-NMDAR alone did not cause measurable cell death. Consistently, activation of syn- or ex-NMDAR alone stimulated pro-survival but not pro-death signaling. Next, we found that memantine, which was previously identified as an ex-NMDAR blocker, inhibited intracellular signaling mediated by syn- or ex-NMDAR. Simultaneous blockade of syn- and ex-NMDAR by memantine dose-dependently attenuated NMDAR-mediated death. Moreover, long- but not short-term treatment with high-dose NMDA or oxygen–glucose deprivation triggered cell death and suppressed pro-survival signaling. These data implicate that activation of syn- or ex-NMDAR alone is not neurotoxic. The degree of excitotoxicity depends on the magnitude and duration of syn- and ex-NMDAR coactivation. Finally, genome-wide examination demonstrated that the activation of syn- and ex-NMDAR lead to significant overlapping rather than counteracting transcriptional responses.

Changes in glutamate homeostasis cause retinal degeneration in Royal College of Surgeons rats

The aim of the present study was to investigate glutamate homeostasis in retinal degeneration-induced changes and the potential mechanisms of glutamate-mediated excitotoxicity in a rat model. The expression of vesicular glutamate transporter-1 (VGLUT-1) and protein kinase Cα (PKCα) in wild-type and Royal College of Surgeons (RCS) rat retinas, at postnatal Day 15 (P15), P30, P60 and P90, were detected using quantitative real-time polymerase chain reaction and immunohistochemistry. The levels of glutamine synthetase (GS) and L-glutamate/L-aspartate transporter (GLAST) were evaluated by western blotting. Compared with wild-type rats, outer nuclear layer thickness was significantly thinner and VGLUT-1 expression was upregulated in a time-dependent pattern in RCS rats. The ratio of VGLUT-1 to PKCα in RCS rats peaked at P60 (p<0.01) and subsequently decreased by P90 (p<0.01), while it remained constant in wild-type rats. The expression of GS increased gradually from P30 to P90 in RCS rats (p<0.01), while it remained constant in wild-type rats at various time-points. No significant difference in GLAST expression was found between RCS and wild-type rats at all stages of retinal degeneration. Our results confirm the occurrence of glutamate-mediated excitotoxicity to RCS rat retinas and provide an experimental foundation for safeguarding the remnant visual function in retinal degenerative disorders.

Impaired striatal GABA transmission in experimental autoimmune encephalomyelitis

Synaptic dysfunction triggers neuronal damage in experimental autoimmune encephalomyelitis (EAE), a model of multiple sclerosis (MS). While excessive glutamate signaling has been reported in the striatum of EAE, it is still uncertain whether GABA synapses are altered. Electrophysiological recordings showed a reduction of spontaneous GABAergic synaptic currents (sIPSCs) recorded from striatal projection neurons of mice with MOG((35-55))-induced EAE. GABAergic sIPSC deficits started in the acute phase of the disease (20-25days post immunization, dpi), and were exacerbated at later time-points (35, 50, 70 and 90dpi). Of note, in slices they were independent of microglial activation and of release of TNF-α. Indeed, sIPSC inhibition likely involved synaptic inputs arising from GABAergic interneurons, because EAE preferentially reduced sIPSCs of high amplitude, and was associated with a selective loss of striatal parvalbumin (PV)-positive GABAergic interneurons, which contact striatal projection neurons in their somatic region, giving rise to more efficient synaptic inhibition. Furthermore, we found also that the chronic persistence of pro-inflammatory cytokines were able, per se, to produce profound alterations of electrophysiological network properties, that were reverted by GABA administration. The results of the present investigation indicate defective GABA transmission in MS models depending from alteration of PV cells number and, in part, deriving from the effects of a chronic inflammation, and suggest that pharmacological agents potentiating GABA signaling might be considered to limit neuronal damage in MS patients.