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

Bisphenol-A induces neurodegeneration through disturbance of intracellular calcium homeostasis in human embryonic stem cells-derived cortical neurons

Bisphenol-A (BPA) is a representative exogenous endocrine disruptor, which is extensively composed in plastic products. Due to the capability of passing through the blood-brain barrier, evidence has linked BPA exposure with multiple neuropsychological dysfunctions, neurobehavioral disorders and neurodegenerative diseases. However, the underlying mechanism by which BPA induces neurodegeneration still remains unclear. Our study used human embryonic stem cells-derived human cortical neurons (hCNs) as a cellular model to investigate the adverse neurotoxic effects of BPA. hCNs were treated with 0, 0.1, 1 and 10 μM BPA for 14 days. Impacts of BPA exposure on cell morphology, cell viability and neural marker (MAP2) were measured for evaluating the neurodegeneration. The intracellular calcium homeostasis, reactive oxygen species (ROS) generation and organelle functions were also taken into consideration. Results revealed that chronic exposure of BPA damaged the neural morphology, induced neuronal apoptosis and decreased MAP2 expression at the level of both transcription and translation. The intracellular calcium levels were elevated in hCNs after BPA exposure through NMDARs-nNOS-PSD-95 mediating. Meanwhile, BPA led to oxidative stress by raising the ROS generation and attenuating the antioxidant defense in hCNs. Furthermore, BPA triggered ER stress and increased cytochrome c release by impairing the mitochondrial function. Ultimately, BPA triggered the cell apoptosis by regulating Bcl-2 family and caspase-dependent signaling pathway. Taken together, BPA exerted neurotoxic effects on hCNs by eliciting apoptosis, which might due to the intracellular calcium homeostasis perturbation and cell organellar dysfunction.

Assay of Calcium Transients and Synapses in Rat Hippocampal Neurons by Kinetic Image Cytometry and High-Content Analysis: An In Vitro Model System for Postchemotherapy Cognitive Impairment

Postchemotherapy cognitive impairment (PCCI) is commonly exhibited by cancer patients treated with a variety of chemotherapeutic agents, including the endocrine disruptor tamoxifen (TAM). The etiology of PCCI is poorly understood. Our goal was to develop high-throughput assay methods to test the effects of chemicals on neuronal function applicable to PCCI. Rat hippocampal neurons (RHNs) were plated in 96- or 384-well dishes and exposed to test compounds (forskolin [FSK], 17β-estradiol [ES]), TAM or fulvestrant [FUL], aka ICI 182,780) for 6-14 days. Kinetic Image Cytometry™ (KIC™) methods were developed to quantify spontaneously occurring intracellular calcium transients representing the activity of the neurons, and high-content analysis (HCA) methods were developed to quantify the expression, colocalization, and puncta formed by synaptic proteins (postsynaptic density protein-95 [PSD-95] and presynaptic protein Synapsin-1 [Syn-1]). As quantified by KIC, FSK increased the occurrence and synchronization of the calcium transients indicating stimulatory effects on RHN activity, whereas TAM had inhibitory effects. As quantified by HCA, FSK also increased PSD-95 puncta and PSD-95:Syn-1 colocalization, whereas ES increased the puncta of both PSD-95 and Syn-1 with little effect on colocalization. The estrogen receptor antagonist FUL also increased PSD-95 puncta. In contrast, TAM reduced Syn-1 and PSD-95:Syn-1 colocalization, consistent with its inhibitory effects on the calcium transients. Thus TAM reduced activity and synapse formation by the RHNs, which may relate to the ability of this agent to cause PCCI. The results illustrate that KIC and HCA can be used to quantify neurotoxic and neuroprotective effects of chemicals in RHNs to investigate mechanisms and potential therapeutics for PCCI.

Role of NMDA Receptor-Mediated Glutamatergic Signaling in Chronic and Acute Neuropathologies

N-Methyl-D-aspartate receptors (NMDARs) have two opposing roles in the brain. On the one hand, NMDARs control critical events in the formation and development of synaptic organization and synaptic plasticity. On the other hand, the overactivation of NMDARs can promote neuronal death in neuropathological conditions. Ca(2+) influx acts as a primary modulator after NMDAR channel activation. An imbalance in Ca(2+) homeostasis is associated with several neurological diseases including schizophrenia, Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. These chronic conditions have a lengthy progression depending on internal and external factors. External factors such as acute episodes of brain damage are associated with an earlier onset of several of these chronic mental conditions. Here, we will review some of the current evidence of how traumatic brain injury can hasten the onset of several neurological conditions, focusing on the role of NMDAR distribution and the functional consequences in calcium homeostasis associated with synaptic dysfunction and neuronal death present in this group of chronic diseases.

Kangxian capsules: Effects on convulsive injuries, N-methyl-d-aspartate (NMDA) receptor subunit expression, and free Ca(2+) concentration in a rat hippocampal neuron epileptic discharge model

PURPOSE

To investigate the effects and mechanisms of Kangxian (KX) capsules on hippocampal neuron convulsive injuries.

METHODS

An epileptic discharge model was prepared with hippocampal neurons and divided into groups that were subjected to control, Mg-free, MK801, or anti-epilepsy (KX) interventions for 6 or 24h. The N-methyl-d-aspartate (NMDA) receptor channel current was recorded with a whole-cell patch-clamp technique, and the decay tau was determined from the receptor channel attenuation. The NMDA receptor subunits (NR1, NR2A, and NR2B) were detected by immunoblot assays, and intracellular free Ca(2+) was detected by laser confocal microscopy.

RESULTS

The discharge times (6h: 100.66±36.51min, 24h: 134.42±86.43min) and tau values (6h: 934.0±564.9s, 24h: 846.6±488.0) of the Mg-free group were significantly increased (P<0.05) compared to the control group. All of the groups had similar levels of NR1 expression. NR2A and NR2B expression was significantly decreased in the Mg-free group and significantly increased most in the MK801 group, which was followed by the KX group (P<0.01). The free Ca(2+) concentrations in the control group were lower than those in the MK-801 and KX groups, the concentrations of which were significantly lower than those in the Mg-free group and which decreased with time.

CONCLUSION

Kangxian capsules played its antiepileptic and neuroprotective roles via multiple targets and the underlying mechanisms included acceleration of the attenuation time course of NMDA receptor channels, alterations in the expression of NMDA receptor subunits, and reductions in the concentration of intraneuronal Ca(2+).

Potential interactions of calcium-sensitive reagents with zinc ion in different cultured cells

Background

Several chemicals have been widely used to evaluate the involvement of free Ca²⁺ in mechanisms underlying a variety of biological responses for decades. Here, we report high reactivity to zinc of well-known Ca²⁺-sensitive reagents in diverse cultured cells.

Methodology/Principal Findings

In rat astrocytic C6 glioma cells loaded with the fluorescent Ca²⁺ dye Fluo-3, the addition of ZnCl₂ gradually increased the fluorescence intensity in a manner sensitive to the Ca²⁺ chelator EGTA irrespective of added CaCl₂. The addition of the Ca²⁺ ionophore A23187 drastically increased Fluo-3 fluorescence in the absence of ZnCl₂, while the addition of the Zn²⁺ ionophore pyrithione rapidly and additionally increased the fluorescence in the presence of ZnCl₂, but not in its absence. In cells loaded with the zinc dye FluoZin-3 along with Fluo-3, a similarly gradual increase was seen in the fluorescence of Fluo-3, but not of FluoZin-3, in the presence of both CaCl₂ and ZnCl₂. Further addition of pyrithione drastically increased the fluorescence intensity of both dyes, while the addition of the Zn²⁺ chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethane-1,2-diamine (TPEN) rapidly and drastically decreased FluoZin-3 fluorescence. In cells loaded with FluoZin-3 alone, the addition of ZnCl₂ induced a gradual increase in the fluorescence in a fashion independent of added CaCl₂ but sensitive to EGTA. Significant inhibition was found in the vitality to reduce 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide in a manner sensitive to TPEN, EDTA and BAPTA in C6 glioma cells exposed to ZnCl₂, with pyrithione accelerating the inhibition. Similar inhibition occurred in an EGTA-sensitive fashion after brief exposure to ZnCl₂ in pluripotent P19 cells, neuronal Neuro2A cells and microglial BV2 cells, which all expressed mRNA for particular zinc transporters.

Conclusions/Significance

Taken together, comprehensive analysis is absolutely required for the demonstration of a variety of physiological and pathological responses mediated by Ca²⁺ in diverse cells enriched of Zn²⁺.

Role of sigma-1 receptors in neurodegenerative diseases

Neurodegenerative diseases with distinct genetic etiologies and pathological phenotypes appear to share common mechanisms of neuronal cellular dysfunction, including excitotoxicity, calcium dysregulation, oxidative damage, ER stress and mitochondrial dysfunction. Glial cells, including microglia and astrocytes, play an increasingly recognized role in both the promotion and prevention of neurodegeneration. Sigma receptors, particularly the sigma-1 receptor subtype, which are expressed in both neurons and glia of multiple regions within the central nervous system, are a unique class of intracellular proteins that can modulate many biological mechanisms associated with neurodegeneration. These receptors therefore represent compelling putative targets for pharmacologically treating neurodegenerative disorders. In this review, we provide an overview of the biological mechanisms frequently associated with neurodegeneration, and discuss how sigma-1 receptors may alter these mechanisms to preserve or restore neuronal function. In addition, we speculate on their therapeutic potential in the treatment of various neurodegenerative disorders.

Neuroprotective properties of the marine carotenoid astaxanthin and omega-3 fatty acids, and perspectives for the natural combination of both in krill oil

The consumption of marine fishes and general seafood has long been recommended by several medical authorities as a long-term nutritional intervention to preserve mental health, hinder neurodegenerative processes, and sustain cognitive capacities in humans. Most of the neurological benefits provided by frequent seafood consumption comes from adequate uptake of omega-3 and omega-6 polyunsaturated fatty acids, n-3/n-6 PUFAs, and antioxidants. Optimal n-3/n-6 PUFAs ratios allow efficient inflammatory responses that prevent the initiation and progression of many neurological disorders. Moreover, interesting in vivo and clinical studies with the marine antioxidant carotenoid astaxanthin (present in salmon, shrimp, and lobster) have shown promising results against free radical-promoted neurodegenerative processes and cognition loss. This review presents the state-of-the-art applications of n-3/n-6 PUFAs and astaxanthin as nutraceuticals against neurodegenerative diseases associated with exacerbated oxidative stress in CNS. The fundamental “neurohormesis” principle is discussed throughout this paper. Finally, new perspectives for the application of a natural combination of the aforementioned anti-inflammatory and antioxidant agents (found in krill oil) are also presented herewith.

NMDA reduces Tau phosphorylation in rat hippocampal slices by targeting NR2A receptors, GSK3β, and PKC activities

The molecular mechanisms that regulate Tau phosphorylation are complex and currently incompletely understood. In the present study, pharmacological inhibitors were deployed to investigate potential processes by which the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors modulates Tau phosphorylation in rat hippocampal slices. Our results demonstrated that Tau phosphorylation at Ser199-202 residues was decreased in NMDA-treated hippocampal slices, an effect that was not reproduced at Ser262 and Ser404 epitopes. NMDA-induced reduction of Tau phosphorylation at Ser199-202 was further promoted when NR2A-containing receptors were pharmacologically isolated and were completely abrogated by the NR2A receptor antagonist NVP-AAM077. Compared with nontreated slices, we observed that NMDA receptor activation was reflected in high Ser9 and low Tyr216 phosphorylation of glycogen synthase kinase-3 beta (GSK3β), suggesting that NMDA receptor activation might diminish Tau phosphorylation via a pathway involving GSK3β inhibition. Accordingly, we found that GSK3β inactivation by a protein kinase C- (PKC-) dependent mechanism is involved in the NMDA-induced reduction of Tau phosphorylation at Ser199-202 epitopes. Taken together, these data indicate that NR2A receptor activation may be important in limiting Tau phosphorylation by a PKC/GSK3β pathway and strengthen the idea that these receptors might act as an important molecular device counteracting neuronal cell death mechanisms in various pathological conditions.

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.

Opposite roles of NMDA receptors in relapsing and primary progressive multiple sclerosis

Synaptic transmission and plasticity mediated by NMDA receptors (NMDARs) could modulate the severity of multiple sclerosis (MS). Here the role of NMDARs in MS was first explored in 691 subjects carrying specific allelic variants of the NR1 subunit gene or of the NR2B subunit gene of this glutamate receptor. The analysis was replicated for significant SNPs in an independent sample of 1548 MS subjects. The C allele of rs4880213 was found to be associated with reduced NMDAR-mediated cortical excitability, and with increased probability of having more disability than the CT/TT MS subjects. MS severity was higher in the CC group among relapsing-remitting MS (RR-MS) patients, while primary progressive MS (PP-MS) subjects homozygous for the T allele had more pronounced clinical worsening. Mean time to first relapse, but not to an active MRI scan, was lower in the CC group of RR-MS patients, and the number of subjects with two or more clinical relapses in the first two years of the disease was higher in CC compared to CT/TT group. Furthermore, the percentage of relapses associated with residual disability was lower in subjects carrying the T allele. Lesion load at the MRI was conversely unaffected by the C or T allele of this SNP in RR-MS patients. Axonal and neuronal degeneration at the optical coherence tomography was more severe in the TT group of PP-MS patients, while reduced retinal nerve fiber thickness had less consequences on visual acuity in RR-MS patients bearing the T allele. Finally, the T allele was associated with preserved cognitive abilities at the Rao's brief repeatable neuropsychological battery in RR-MS. Signaling through glutamate NMDARs enhances both compensatory synaptic plasticity and excitotoxic neurodegeneration, impacting in opposite ways on RR-MS and PP-MS pathophysiological mechanisms.

Abnormal NMDA receptor function exacerbates experimental autoimmune encephalomyelitis

BACKGROUND AND PURPOSE

Glutamate transmission is dysregulated in both multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE), the animal model of MS. A characteristic of EAE is increased glutamate transmission associated with up-regulation of AMPA receptors. However, little is known about the role of NMDA receptors in the synaptic modifications induced by EAE.

EXPERIMENTAL APPROACH

The contribution of NMDA receptors to the alterations of glutamate transmission and disease severity in EAE mice was assessed by means of neurophysiological, morphological, Western blot, metabolic and clinical score assessments.

KEY RESULTS

In our EAE mice, there was an NMDA receptor-dependent increase of glutamate release, associated with marked activation of the astroglia. Presynaptic NMDA receptors became overactive during EAE, increasing synaptic glutamate release by a mechanism dependent on voltage-gated sodium channels. By means of NAD(P)H autofluorescence analysis, we also found that EAE has a glutamate and NMDA receptor-dependent dysfunction of mitochondrial activity, which is known to contribute to the neurodegenerative damage of MS and EAE. Furthermore, pharmacological blockade of NMDA receptors in vivo ameliorated both synaptic transmission defects and of the clinical disease course of EAE mice, while EAE induced in mice with a genetically enhanced NMDA receptor signalling had opposite effects.

CONCLUSIONS AND IMPLICATIONS

Our data, showing both sensitization of NMDA receptors and their involvement in the progression of the EAE disease, supggest that pharmacological impairment of NMDA receptor signalling would be a component of a neuroprotection strategy in MS.

Role of the N-methyl-d-aspartate receptors complex in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is an adult onset neurodegenerative disease pathologically characterized by the massive loss of motor neurons in the spinal cord, brain stem and cerebral cortex. There is a consensus in the field that ALS is a multifactorial pathology and a number of possible mechanisms have been suggested. Among the proposed hypothesis, glutamate toxicity has been one of the most investigated. Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor mediated cell death and impairment of the glutamate-transport system have been suggested to play a central role in the glutamate-mediated motor neuron degeneration. In this context, the role played by the N-methyl-d-aspartate (NMDA) receptor has received considerable less attention notwithstanding its high Ca(2+) permeability, expression in motor neurons and its importance in excitotoxicity. This review overviews the critical role of NMDA-mediated toxicity in ALS, with a particular emphasis on the endogenous modulators of the NMDAR.