Insensitivity to glutamate neurotoxicity mediated by NMDA receptors in association with delayed mitochondrial membrane potential disruption in cultured rat cortical neurons

Spinal Astrocyte-Neuron Lactate Shuttle Contributes to the Pituitary Adenylate Cyclase-Activating Polypeptide/PAC1 Receptor-Induced Nociceptive Behaviors in Mice

We have previously shown that spinal pituitary adenylate cyclase-activating polypeptide (PACAP)/PACAP type 1 (PAC1) receptor signaling triggered long-lasting nociceptive behaviors through astroglial activation in mice. Since astrocyte-neuron lactate shuttle (ANLS) could be essential for long-term synaptic facilitation, we aimed to elucidate a possible involvement of spinal ANLS in the development of the PACAP/PAC1 receptor-induced nociceptive behaviors. A single intrathecal administration of PACAP induced short-term spontaneous aversive behaviors, followed by long-lasting mechanical allodynia in mice. These nociceptive behaviors were inhibited by 1,4-dideoxy-1,4-imino-d-arabinitol (DAB), an inhibitor of glycogenolysis, and this inhibition was reversed by simultaneous L-lactate application. In the cultured spinal astrocytes, the PACAP-evoked glycogenolysis and L-lactate secretion were inhibited by DAB. In addition, a protein kinase C (PKC) inhibitor attenuated the PACAP-induced nociceptive behaviors as well as the PACAP-evoked glycogenolysis and L-lactate secretion. Finally, an inhibitor for the monocarboxylate transporters blocked the L-lactate secretion from the spinal astrocytes and inhibited the PACAP- and spinal nerve ligation-induced nociceptive behaviors. These results suggested that spinal PAC1 receptor-PKC-ANLS signaling contributed to the PACAP-induced nociceptive behaviors. This signaling system could be involved in the peripheral nerve injury-induced pain-like behaviors.

Identification of N-methyl-D-aspartate receptor antagonists using the rat postnatal mixed cortical and hippocampal neurons

The goal of this study was to evaluate mixed cortical and hippocampal primary rat postnatal neuronal culture as in vitro tool for identification of N-methyl-D-aspartate receptor (NMDAR) antagonists and to find out, whether this model is comparable with other commonly used primary rat neuronal models differing in their origin (pure cortical vs. mixed cortical and hippocampal) and differentiation state (embryonal vs. postnatal). Induced pluripotent stem cell (iPSC) - derived human glutamatergic neurons have been included in this study as well. First, the cultures were characterized by their neuron/astrocyte composition, the mRNA expression of NR2B/NR2A NMDAR subunit ratios, and the expression of glutamate transporters (GLT1, GLAST). Then, selected endogenous steroids and synthetic neuroactive steroids that have been previously identified as negative allosteric modulators of recombinant GluN1/GluN2B NMDA receptors, were evaluated for their ability to prevent an NMDA or glutamate-induced Ca²⁺ influx (acute effect) and excitotoxicity over 24 h. Though the neuroprotective potential against excitotoxic stimuli varied among the models studied, postnatal mixed cortical and hippocampal culture proved to be a convenient and robust tool for NMDAR antagonist screening. The most widely used embryonal (E18) cultures offered higher cell yields but at the expense of a higher sensitivity to compounds' cytotoxicity. iPSC-derived neurons were not found to be superior to rat cultures for screening purposes.

Acute and Delayed Effects of Mechanical Injury on Calcium Homeostasis and Mitochondrial Potential of Primary Neuroglial Cell Culture: Potential Causal Contributions to Post-Traumatic Syndrome

In vitro models of traumatic brain injury (TBI) help to elucidate the pathological mechanisms responsible for cell dysfunction and death. To simulate in vitro the mechanical brain trauma, primary neuroglial cultures were scratched during different periods of network formation. Fluorescence microscopy was used to measure changes in intracellular free Ca²⁺ concentration ([Ca²⁺]_i) and mitochondrial potential (ΔΨm) a few minutes later and on days 3 and 7 after scratching. An increase in [Ca²⁺]_i and a decrease in ΔΨm were observed ~10 s after the injury in cells located no further than 150–200 µm from the scratch border. Ca²⁺ entry into cells during mechanical damage of the primary neuroglial culture occurred predominantly through the NMDA-type glutamate ionotropic channels. MK801, an inhibitor of this type of glutamate receptor, prevented an acute increase in [Ca²⁺]_i in 99% of neurons. Pathological changes in calcium homeostasis persisted in the primary neuroglial culture for one week after injury. Active cell migration in the scratch area occurred on day 11 after neurotrauma and was accompanied by a decrease in the ratio of live to dead cells in the areas adjacent to the injury. Immunohistochemical staining of glial fibrillary acidic protein and β-III tubulin showed that neuronal cells migrated to the injured area earlier than glial cells, but their repair potential was insufficient for survival. Mitochondrial Ca²⁺ overload and a drop in ΔΨm may cause delayed neuronal death and thus play a key role in the development of the post-traumatic syndrome. Preventing prolonged ΔΨm depolarization may be a promising therapeutic approach to improve neuronal survival after traumatic brain injury.

N-methyl-D-aspartate (NMDA) impairs myogenesis in C2C12 cells

INTRODUCTION

N-methyl-d-aspartate (NMDA) is expressed in sensory neurons and plays important roles in peripheral pain mechanisms. The aim of this study was to examine the effects and molecular mechanisms of NMDA on C2C12 myoblast proliferation and differentiation.

METHODS

Cytotoxicity and differentiation were examined by the MTT assay, reverse transcription-polymerase chain reaction, and immunofluorescence.

RESULTS

NMDA had no cytotoxicity (10-500 μM) and inhibited myoblastic differentiation of C2C12 cells, as assessed by F-actin immunofluorescence and levels of mRNAs encoding myogenic markers such as myogenin and myosin heavy-chain 2. It inhibited phosphorylation of mammalian target of rapamycin (mTOR) by inactivating mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinase, c-Jun N-terminal kinase, and p38. It induced reactive oxygen species production. Furthermore, NMDA-suppressed expression of F-actin was reversed by adding the antioxidant N-acetylcysteine.

CONCLUSIONS

Collectively, these results indicate that NMDA impairs myogenesis or myogenic differentiation in C2C12 cells through the mTOR/MAPK signaling pathways and may lead to skeletal muscle degeneration. Muscle Nerve 56: 510-518, 2017.

Protective upregulation of activating transcription factor-3 against glutamate neurotoxicity in neuronal cells under ischemia

This study evaluates the pathological role of the stress sensor activating transcription factor-3 (ATF3) in ischemic neurotoxicity. Upregulation of the transcript and protein for ATF3 was seen 2-10 hr after reperfusion in the ipsilateral cerebral hemisphere of mice with transient middle cerebral artery occlusion for 2 hr. Immunohistochemical analysis confirmed the expression of ATF3 by cells immunoreactive for a neuronal marker in neocortex, hippocampus, and striatum within 2 hr after reperfusion. In murine neocortical neurons previously cultured under ischemic conditions for 2 hr, transient upregulation of both Atf3 and ATF3 expression was similarly found during subsequent culture for 2-24 hr under normoxia. Lentiviral overexpression of ATF3 ameliorated the neurotoxicity of glutamate (Glu) in cultured murine neurons along with a slight but statistically significant inhibition of both Fluo-3 and rhodamine-2 fluorescence increases by N-methyl-D-aspartate. Similarly, transient upregulation was seen in Atf3 and ATF3 expression during the culture for 48 hr in neuronal Neuro2A cells previously cultured under ischemic conditions for 2 hr. Luciferase reporter analysis with ATF3 promoter together with immunoblotting revealed the possible involvement of several transcription factors responsive to extracellular and intracellular stressors in the transactivation of the Atf3 gene in Neuro2A cells. ATF3 could be upregulated to play a role in mechanisms underlying mitigation of the neurotoxicity mediated by the endogenous neurotoxin Glu at an early stage after ischemic signal inputs.

Mitochondrial c-Fos May Increase the Vulnerability of Neuro2a Cells to Cellular Stressors

Although c-Fos expression in mitochondria is known to increase under excitatory injury via kainic acid or N-methyl-D-aspartate injection, the authentic function of c-Fos in mitochondria remains unknown. We found that c-Fos expression in the mitochondria of neuroblastoma Neuro2a cells was augmented by oxygen and glucose deprivation (OGD), which is a common in vitro model for brain ischemia. Then we demonstrated that Neuro2a cells stably expressing c-Fos exclusively in the mitochondria were more vulnerable to stressors such as OGD, rotenone (which is known to induce mitochondrial dysfunction) and hydrogen peroxide (a reactive oxygen species). Since mitochondrial dysfunction and the generation of reactive oxygen species are known to be caused by OGD, our findings indicate that mitochondrial c-Fos increases neuronal vulnerability to brain ischemia. This suggests that mitochondrial c-Fos play a potential role in inducing neuronal death on, and can therefore act as a potential drug target for brain ischemia.

Selective inhibition by ethanol of mitochondrial calcium influx mediated by uncoupling protein-2 in relation to N-methyl-D-aspartate cytotoxicity in cultured neurons

BACKGROUND

We have shown the involvement of mitochondrial uncoupling protein-2 (UCP2) in the cytotoxicity by N-methyl-D-aspartate receptor (NMDAR) through a mechanism relevant to the increased mitochondrial Ca(2+) levels in HEK293 cells with acquired NMDAR channels. Here, we evaluated pharmacological profiles of ethanol on the NMDA-induced increase in mitochondrial Ca(2+) levels in cultured murine neocortical neurons.

METHODOLOGY/PRINCIPAL FINDINGS

In neurons exposed to glutamate or NMDA, a significant increase was seen in mitochondrial Ca(2+) levels determined by Rhod-2 at concentrations of 0.1 to 100 µM. Further addition of 250 mM ethanol significantly inhibited the increase by glutamate and NMDA in Rhod-2 fluorescence, while similarly potent inhibition of the NMDA-induced increase was seen after exposure to ethanol at 50 to 250 mM in cultured neurons. Lentiviral overexpression of UCP2 significantly accelerated the increase by NMDA in Rhod-2 fluorescence in neurons, without affecting Fluo-3 fluorescence for intracellular Ca(2+) levels. In neurons overexpressing UCP2, exposure to ethanol resulted in significantly more effective inhibition of the NMDA-induced increase in mitochondrial free Ca(2+) levels than in those without UCP2 overexpression, despite a similarly efficient increase in intracellular Ca(2+) levels irrespective of UCP2 overexpression. Overexpression of UCP2 significantly increased the number of dead cells in a manner prevented by ethanol in neurons exposed to glutamate. In HEK293 cells with NMDAR containing GluN2B subunit, more efficient inhibition was similarly induced by ethanol at 50 and 250 mM on the NMDA-induced increase in mitochondrial Ca(2+) levels than in those with GluN2A subunit. Decreased protein levels of GluN2B, but not GluN2A, subunit were seen in immunoprecipitates with UCP2 from neurons with brief exposure to ethanol at concentrations over 50 mM.

CONCLUSIONS/SIGNIFICANCE

Ethanol could inhibit the interaction between UCP2 and NMDAR channels to prevent the mitochondrial Ca(2+) incorporation and cell death after NMDAR activation in neurons.

Intercellular signaling pathway among Endothelia, astrocytes and neurons in excitatory neuronal damage

Neurons interact closely with astrocytes via glutamate; this neuron-glia circuit may play a pivotal role in synaptic transmission. On the other hand, astrocytes contact vascular endothelial cells with their end-feet. It is becoming obvious that non-neuronal cells play a critical role in regulating the neuronal activity in the brain. We find that kainic acid (KA) administration induces the expression of microsomal prostaglandin E synthase-1 (mPGES-1) in venous endothelial cells and the prostaglandin E2 (PGE2) receptor prostaglandin E receptor (EP)-3 on astrocytes. Endothelial mPGES-1 exacerbates KA-induced neuronal damage in in vivo experiments. In in vitro experiments, mPGES-1 produces PGE2, which enhances astrocytic Ca2+ levels via the EP3 receptor and increases Ca2+-dependent glutamate release, thus aggravating neuronal injury. This novel endothelium-astrocyte-neuron signaling pathway may be crucial for driving neuronal damage after repetitive seizures and could be a new therapeutic target for epilepsy and other brain disorders.

Delayed mitochondrial membrane potential disruption by ATP in cultured rat hippocampal neurons exposed to N-methyl-D-aspartate

Necrotic damage leads to a massive leakage from injured cells of different intracellular constituents such as glutamate (Glu) and ATP, which are believed to play a role in the neuronal survival in the brain. In this study, we evaluated pharmacological properties of ATP, which is shown to be an endogenous inhibitor of N-methyl-D-aspartate (NMDA) receptors, on the neurotoxicity relevant to mitochondrial membrane potential disruption in cultured rat hippocampal neurons. Exposure to Glu or NMDA significantly inhibited cellular viability determined 24 and 48 h later, while simultaneous addition of 1 mM ATP significantly ameliorated the decreased viability in neurons exposed to Glu and NMDA, but not in those exposed to other cytotoxins. Both Glu and NMDA markedly increased intracellular free Ca(2+) levels in a manner sensitive to blockade by the exposure to ATP, but not by that to adenosine. Exposure to ATP significantly delayed the rate of mitochondrial membrane potential disruption induced by Glu and NMDA. These results suggest that extracellular ATP would play a role as an endogenous antagonist endowed to protect rat hippocampal neurons from the excitotoxicity mediated by NMDA receptors in association with the delayed mitochondrial membrane potential disruption after the liberation from adjacent cells under necrotic death.

Role of mitochondrial activation in PACAP dependent neurite outgrowth

Pituitary adenylate cyclase-activating polypeptide (PACAP) increases neurite outgrowth, although signaling via its receptor PACAP-specific receptor (PAC1R) has not been fully characterized. Because mitochondria also play an important role in neurite outgrowth, we examined whether mitochondria contribute to PACAP-mediated neurite outgrowth. When mouse primary hippocampal neurons and Neuro2a cells were exposed to PACAP, neurite outgrowth and the mitochondrial membrane potential increased in both cell types. These results were reproduced using the PAC1R-specific agonist maxadilan and the adenylate cyclase activator forskolin, whereas the protein kinase A inhibitor H89 and mitochondrial uncoupling agent carbonyl cyanide m-chlorophenyl hydrazone (CCCP) inhibited these effects. Expression levels of peroxisome proliferator-activated receptor γ coactivator 1α (Pgc1α), a master regulator of mitochondrial activation, and its downstream effectors, such as cytochrome C and cytochrome C oxidase subunit 4, increased in response to PACAP. Knocking down Pgc1α expression using small interfering RNA or treatment with CCCP significantly attenuated neurite outgrowth and reduced the mitochondrial membrane potential in PACAP-treated cells. These data suggest that mitochondrial activation plays a key role in PACAP-induced neurite outgrowth via a signaling pathway that includes PAC1R, PKA, and Pgc1α.

Stimulation and clustering of cytochrome b5 reductase in caveolin-rich lipid microdomains is an early event in oxidative stress-mediated apoptosis of cerebellar granule neurons

The apoptosis of cerebellar granule neurons (CGN) induced by low potassium in the extracellular medium is a model of neuronal apoptosis where an overshot of reactive oxygen species (ROS) triggers the neuronal death. In this work, using dihydroethidium and L-012 as specific dyes for superoxide anion detection we show that this ROS overshot can be accounted by an increased release of superoxide anion to the extracellular medium. The amplitude and time course of the increase of superoxide anion observed early during apoptosis correlated with the increase of the content of soluble cytochrome b(5), a substrate of the NADH-dependent oxidase activity of the cytochrome b(5) reductase associated with lipid rafts in CGN. Western blotting and immunofluorescence microscopy approaches, including fluorescence energy transfer, pointed out an enhanced clustering of cytochrome b(5) reductase within caveolins-rich lipid rafts microdomains. Protein/protein docking analysis suggests that cytochrome b(5) reductase can form complexes with caveolins 1α, 1β and 2, playing electrostatic interactions a major role in this association. In conclusion, our results indicate that overstimulation of cytochrome b(5) reductase associated with lipid rafts can account for the overshot of plasma membrane-focalized superoxide anion production that triggers the entry of CGN in the irreversible phase of apoptosis. This article is part of a Special Issue entitled: Proteomics: The clinical link.

Homeostatic NMDA receptor down-regulation via brain derived neurotrophic factor and nitric oxide-dependent signalling in cortical but not in hippocampal neurons

Nitric oxide (NO) has been proposed to down-regulate NMDA receptors (NMDA-Rs) in a homeostatic manner. However, NMDA-R-dependent NO synthesis also can cause excitotoxic cell death. Using bicuculline-stimulated hippocampal and cortical cell cultures, we have addressed the role of the brain-derived neurotrophic factor-NO pathway in NMDA-R down-regulation. This pathway protected cortical cells from NMDA-induced death and led to NMDA-R inhibition. In contrast, no evidence was gained for the presence of this protective pathway in hippocampal neurons, in which NMDA-induced NO synthesis was confirmed to be toxic. Therefore, opposing effects of NO depended on the activation of different signalling pathways. The pathophysiological relevance of this observation was investigated in synaptosomes and post-synaptic densities isolated from rat hippocampi and cerebral cortices following kainic acid-induced status epilepticus. In cortical, but not in hippocampal synaptosomes, brain-derived neurotrophic factor induced NO synthesis and inhibited NMDA-R currents present in isolated post-synaptic densities. In conclusion, we identified a NO-dependent homeostatic response in the rat cerebral cortex induced by elevated activity. A low performance of this pathway in brain areas including the hippocampus may be related to their selective vulnerability in pathologies such as temporal lobe epilepsy.

A possible pivotal role of mitochondrial free calcium in neurotoxicity mediated by N-methyl-d-aspartate receptors in cultured rat hippocampal neurons

We have previously shown that mitochondrial membrane potential disruption is involved in mechanisms underlying differential vulnerabilities to the excitotoxicity mediated by N-methyl-d-aspartate (NMDA) receptors between primary cultured neurons prepared from rat cortex and hippocampus. To further elucidate the role of mitochondria in the excitotoxicity after activation of NMDA receptors, neurons were loaded with the fluorescent dye calcein diffusible in the cytoplasm and organelles for determination of the activity of mitochondrial permeability transition pore (mPTP) responsible for the leakage of different mitochondrial molecules. The addition of CoCl(2) similarly quenched the intracellular fluorescence except mitochondria in both cultured neurons, while further addition of NMDA led to a leakage of the dye into the cytoplasm in hippocampal neurons only. An mPTP inhibitor prevented the NMDA-induced loss of viability in hippocampal neurons, while an activator of mPTP induced a similarly potent loss of viability in cortical and hippocampal neurons. Although NMDA was more effective in increasing rhodamine-2 fluorescence as a mitochondrial calcium indicator in hippocampal than cortical neurons, a mitochondrial calcium uniporter inhibitor significantly prevented the NMDA-induced loss of viability in hippocampal neurons. Expression of mRNA was significantly higher for the putative uniporter uncoupling protein-2 in hippocampal than cortical neurons. These results suggest that mitochondrial calcium uniporter would be at least in part responsible for the NMDA neurotoxicity through a mechanism relevant to promotion of mPTP orchestration in hippocampal neurons.

Endothelial microsomal prostaglandin E synthase-1 facilitates neurotoxicity by elevating astrocytic Ca2+ levels

Recurrent seizures may cause neuronal damage in the hippocampus. As neurons form intimate interactions with astrocytes via glutamate, this neuron-glia circuit may play a pivotal role in neuronal excitotoxicity following such seizures. On the other hand, astrocytes contact vascular endothelia with their endfeet. Recently, we found kainic acid (KA) administration induced microsomal prostaglandin E synthase-1 (mPGES-1) and prostaglandin E(2) (PGE(2)) receptor EP3 in venous endothelia and on astrocytes, respectively. In addition, mice deficient in mPGES-1 exhibited an improvement in KA-induced neuronal loss, suggesting that endothelial PGE(2) might modulate neuronal damage via astrocytes. In this study, we therefore investigated whether the functional associations between endothelia and astrocytes via endothelial mPGES-1 lead to neuronal injury using primary cultures of hippocampal slices. We first confirmed the delayed induction of endothelial mPGES-1 in the wild-type (WT) slices after KA-treatment. Next, we examined the effects of endothelial mPGES-1 on Ca(2+) levels in astrocytes, subsequent glutamate release and neuronal injury using cultured slices prepared from WT and mPGES-1 knockout mice. Moreover, we investigated which EP receptor on astrocytes was activated by PGE(2). We found that endothelial mPGES-1 produced PGE(2) that enhanced astrocytic Ca(2+) levels via EP3 receptors and increased Ca(2+)-dependent glutamate release, aggravating neuronal injury. This novel endothelium-astrocyte-neuron signaling pathway may be crucial for neuronal damage after repetitive seizures, and hence could be a new target for drug development.

Induced tolerance to glutamate neurotoxicity through down-regulation of NR2 subunits of N-methyl-D-aspartate receptors in cultured rat striatal neurons

We have previously shown differential vulnerabilities to glutamate (Glu) excitotoxicity mediated by the N-methyl-D-aspartate (NMDA) receptor (NMDAR) between rat cortical and rat hippocampal neurons in culture. In this study, we evaluated the possible induced tolerance to NMDA neurotoxicity in cultured rat striatal neurons with prior sustained activation of NMDAR. Brief exposure to Glu or NMDA for 1 hr led to a significant decrease in cellular vitality determined 24 hr later in cultured rat striatal neurons, whereas no marked loss was seen in cellular survival after exposure to Glu or NMDA in striatal neurons previously cultured with Glu or NMDA. Sustained culture with Glu or NMDA invariably led to a significant decrease in protein levels of NR2, but not NR1, subunits without affecting their mRNA levels. Similar induced tolerance was seen to the excitotoxicity of NMDA in hippocampal neurons in a manner sensitive to an NMDAR antagonist. Prior culture with NMDA induced less effective alterations in both intracellular free Ca(2+) levels and mitochondrial membrane potentials after the addition of NMDA in striatal neurons. However, calpain inhibitor-I significantly prevented the decreased NR2B and NR2C protein levels in striatal neurons cultured with NMDA. These results suggest that prior tonic activation of NMDAR would induce tolerance to the excitotoxicity mediated by NMDAR through a mechanism related to calpain-induced down-regulation of particular NR2 subunits in rat striatal neurons.

Preferential inhibition by antidiarrheic 2-methoxy-4-methylphenol of Ca(2+) influx across acquired N-methyl-D-aspartate receptor channels composed of NR1/NR2B subunit assembly

In our previous studies, particular phenolic ingredients, such as 2-methoxy-4-methylphenol (2M4MP), of the antidiarrheic drug wood creosote significantly prevented cell death by both hydrogen peroxide and glutamate in cultured rat hippocampal neurons. In this study, we further evaluated the pharmacological properties of 2M4MP on Ca(2+) influx across native and acquired N-methyl-D-aspartate (NMDA) receptor (NMDAR) channels. The addition of 2M4MP significantly prevented the loss of cellular viability and the increase in intracellular free Ca(2+) levels as determined by Fluo-3 in cultured rat hippocampal neurons briefly exposed to NMDA. Brief exposure to NMDA also led to a marked increase in mitochondrial free Ca(2+) levels determined by Rhod-2, in addition to intracellular free Ca(2+) levels, in HEK293 cells expressing either NR1/NR2A or NR1/NR2B subunit channels. The further addition of the general NMDAR channel blocker dizocilpine similarly inhibited the increase of intracellular Ca(2+) levels by NMDA in both types of acquired NMDAR channels, whereas the NR2B subunit selective antagonist ifenprodil drastically inhibited the increase by NMDA in HEK293 cells expressing NR1/NR2B, but not NR1/NR2A, subunits. Similarly, 2M4MP significantly and selectively inhibited the NMDA-induced influx of Ca(2+) across acquired NR1/NR2B channels in a concentration-dependent manner. Moreover, prior daily oral administration of 2M4MP significantly reduced the infarct volume in the ipsilateral cerebral hemisphere in rats with middle cerebral artery occlusion 1 day after reperfusion. These results suggest that 2M4MP may protect neurons from excitotoxicity through preferential inhibition of Ca(2+) influx across NMDAR channels composed of NR1/NR2B subunits.

Mapping the high-affinity binding domain of 5-substituted benzimidazoles to the proximal N-terminus of the GluN2B subunit of the NMDA receptor

BACKGROUND AND PURPOSE

N-methyl-D-aspartate (NMDA) receptors represent an attractive drug target for the treatment of neurological and neurodegenerative disorders associated with glutamate-induced excitotoxicity. The aim of this study was to map the binding domain of high affinity 5-substituted benzimidazole derivatives [N-{2-[(4-benzylpiperidin-1-yl)methyl]benzimidazol-5-yl}methanesulphonamide (XK1) and N-[2-(4-phenoxybenzyl)benzimidazol-5-yl]methanesulphonamide (XK2)] on the GluN2B subunit of the NMDA receptor.

EXPERIMENTAL APPROACH

The pharmacological antagonistic profiles of XK1 and XK2 were assessed using in vitro rat primary cerebrocortical neurones and two-electrode voltage clamp on Xenopus oocytes expressing heterologous GluN1/GluN2B receptors. Direct ligand binding was determined using the recombinant amino-terminal domain (ATD) of GluN2B.

KEY RESULTS

XK1 and XK2 effectively protected against NMDA-induced excitotoxicity in rat primary cortical neurones. Low concentrations of XK1 (10 nM) and XK2 (1 nM) significantly reversed neuronal death. Both compounds failed to inhibit currents measured from oocytes heterologously expressing GluN1-1a subunit co-assembled with the ATD-deleted GluN2B subunit. XK1 and XK2 showed specific binding to recombinant protein of GluN2B ATD with low nanomolar affinities. Several residues in the recombinant ATD of GluN2B were identified to be critical for conferring XK1 and XK2 sensitivity. The inhibitory effects of XK1 and XK2 were pH-sensitive, being increased at acidic pH.

CONCLUSIONS AND IMPLICATIONS

These results demonstrate that XK1 and XK2 are effective neuroprotective agents in vitro and indicate that 5-substituted benzimidazole derivatives inhibit GluN1/GluN2B receptors via direct binding to the ATD of the GluN2B subunit. These compounds represent valuable alternatives to the classical antagonist ifenprodil as pharmacological tools for studying GluN2B-containing NMDA receptors.

Possible protection by notoginsenoside R1 against glutamate neurotoxicity mediated by N-methyl-D-aspartate receptors composed of an NR1/NR2B subunit assembly

Notoginsenoside R1 (NTR1) is the main active ingredient in Panax notoginseng, a herbal medicine widely used in Asia for years. The purpose of this study was to investigate pharmacological properties of NTR1 on neurotoxicity of glutamate (Glu) in primary cultured mouse cortical neurons along with its possible mechanism of action. We found that NTR1 significantly protected neurons from the loss of cellular viability caused by brief exposure to 10 microM Glu for 1 hr in a dose-dependent manner at concentrations from 0.1 to 10 microM, without affecting the viability alone. NTR1 significantly inhibited the increased number of cells positive to propidium iodide (PI) staining, increase of intracellular free Ca(2+) ions, overproduction of intracellular reactive oxygen species, and depolarization of mitochondrial membrane potential in cultured neurons exposed to Glu, in addition to blocking decreased Bcl-2 and increased Bax expression levels. We further evaluated the target site at which NTR1 protects neurons from Glu toxicity by using the acquired expression strategy of N-methyl-D-aspartate (NMDA) receptor subunits in human embryonic kidney 293 cells. We found that 10 microM NTR1 protected NR1/NR2B subunit expressing cells from cell death by 100 microM NMDA, but not cells expressing NR1/NR2A subunits, when determined by PI staining. These results suggest that NTR1 may preferentially protect neurons from Glu excitotoxicity mediated by NMDA receptor composed of an NR1/NR2B subunit assembly in the brain.

GABAergic striatal neurons exhibit caspase-independent, mitochondrially mediated programmed cell death

GABAergic striatal neurons are compromised in basal ganglia pathologies and we analysed how insult nature determined their patterns of injury and recruitment of the intrinsic mitochondrial pathway during programmed cell death (PCD). Stressors affecting targets implicated in striatal neurodegeneration [3-morpholinylsydnoneimine (SIN-1), 3-nitropropionic acid (3-NP), NMDA, 3,5-dihydroxyphenylglycine (DHPG), and staurosporine (STS)] were compared in cultured GABAergic neurons from murine striatum by analyzing the progression of injury and its correlation with mitochondrial involvement, the redistribution of intermembrane space (IMS) proteins, and patterns of protease activation. Stressors produced PCD exhibiting slow-onset kinetics with time-dependent annexin-V labeling and eventual DNA fragmentation. IMS proteins including cytochrome c were differentially distributed, although stressors except STS produced early redistribution of apoptosis-inducing factor and Omi, suggestive of early recruitment of both caspase-dependent and caspase-independent signaling. In general, Bax mobilization to mitochondria appeared to promote IMS protein redistribution. Caspase 3 activation was prominent after STS, whereas NMDA and SIN-1 produced mainly calpain activation, and 3-NP and DHPG elicited a mixed profile of protease activation. PCD and redistribution of IMS proteins in striatal GABAergic neurons were canonical and insult-dependent, reflecting differential interplay between the caspase cascade and alternate cell death pathways.

A critical importance of polyamine site in NMDA receptors for neurite outgrowth and fasciculation at early stages of P19 neuronal differentiation

We have investigated the role of N-methyl-d-aspartate receptors (NMDARs) and gamma-aminobutyric acid receptors type A (GABA(A)Rs) at an early stage of P19 neuronal differentiation. The subunit expression was profiled in 24-hour intervals with RT-PCR and functionality of the receptors was verified via fluo-3 imaging of Ca(2+) dynamics in the immature P19 neurons showing that both NMDA and GABA excite neuronal bodies, but only polyamine-site sensitive NMDAR stimulation leads to enhanced Ca(2+) signaling in the growth cones. Inhibition of NR1/NR2B NMDARs by 1 muM ifenprodil severely impaired P19 neurite extension and fasciculation, and this negative effect was fully reversible by polyamine addition. In contrast, GABA(A)R antagonism by a high dose of 200 microM bicuculline had no observable effect on P19 neuronal differentiation and fasciculation. Except for the differential NMDAR and GABA(A)R profiles of Ca(2+) signaling within the immature P19 neurons, we have also shown that inhibition of NR1/NR2B NMDARs strongly decreased mRNA level of NCAM-180, which has been previously implicated as a regulator of neuronal growth cone protrusion and neurite extension. Our data thus suggest a critical role of NR1/NR2B NMDARs during the process of neuritogenesis and fasciculation of P19 neurons via differential control of local growth cone Ca(2+) surges and NCAM-180 signaling.