HSP70 induction and oxidative stress protection mediated by a subtoxic dose of NMDA in the retinoic acid-differentiated C6 glioma cell line

Mechanisms of Inhibition of Excessive Microglial Activation by Melatonin

As the innate immune cells that permanently reside in the central nervous system (CNS), microglia play an increasingly important role in maintaining brain function. Normally, microglia act as resting phenotype, which can be activated by various types of stimuli and release a variety of inflammatory mediators. Melatonin is an endogenous rhythmic hormone secreted principally by the pineal gland. Increasing evidence suggests that melatonin can detoxify reactive oxygen species (ROS) and prevent microglia from over-activation. This review summarizes the mechanisms of melatonin in inhibiting excessive activation of microglia and demonstrates the feasibility of melatonin in the treatment of diseases related to microglial over-activation.

Influence of mTOR signaling pathway on ketamine-induced injuries in the hippocampal neurons of rats

OBJECTIVE

To explore the influences of mammalian target of rapamycin (mTOR) signaling pathway on ketamine-induced apoptosis, oxidative stress and Ca²⁺ concentration in the hippocampal neurons of rats.

METHODS

The primary hippocampal neurons isolated from fetal Sprague Dawley rats were treated with ketamine (0, 50, 100 and 500 μM) for 4 days to observe its effect on mTOR signaling pathway and apoptosis of rat hippocampal neurons. Then, the hippocampal neurons were divided into C (Control), R (Rapamycin, an inhibitor of mTOR signaling pathway), K (Ketamine) and R + K (Rapamycin + Ketamine) groups to detect the apoptosis, reactive oxygen species (ROS) production, and Ca²⁺ concentration via the terminal transferase uridyl nick end labelling (TUNEL) assay, dichloro-dihydro-fluorescein diacetate (DCFH-DA) method and Fluo-3 acetoxymethyl ester (Fluo-3AM) staining, respectively. The expressions of mTOR signaling pathway and apoptosis-related proteins in hippocampal neurons were examined by qRT-PCR and Western blot.

RESULTS

Ketamine could dose-dependently promote the apoptosis of rat hippocampal neurons with upregulation of p-mTOR and its downstream regulators (p-4E-BP-1 and p-p70S6K). However, ketamine-induced apoptosis in hippocampal neurons was reversed significantly by the administration of rapamycin, as evident by the decrease in expressions of pro-apoptotic proteins (Bax and cleaved Caspase-3) and the increase in anti-apoptotic protein (Bcl-2). Meanwhile, the ROS generation and Ca²⁺ concentration was inhibited accompanied with reduced malonildialdehyde levels but elevated superoxide and glutathione peroxidase activities.

CONCLUSION

Inhibition of mTOR signaling pathway protected rat hippocampal neurons from ketamine-induced injuries via reducing apoptosis, oxidative stress, as well as Ca²⁺ concentration.

ABBREVIATIONS

mTOR: mammalian target of rapamycin; SD: Sprague-Dawley; SPF: Specific-pathogen free; ROS: reactive oxygen species; TUNEL: terminal transferase uridyl nick end labelling; DCFH-DA: Dichloro-dihydro-fluorescein diacetate; Fluo-3A: Fluo-3 acetoxymethyl ester; NMDAR: non-competitive N-methyl-D-aspartame glutamate receptor; 4E-BP1: 4E binding protein 1; p70S6K: p70 S6 Kinase; PCR: Polymerase chain reaction; MDA: malonildialdehyde; GSH-PX: glutathione peroxidase; ANOVA: One-way Analysis of Variance.

Exposure of rat hippocampal astrocytes to Ziram increases oxidative stress

Pesticides have been shown in several studies to be the leading candidates of environmental toxins and may contribute to the pathogenesis of several neurodegenerative diseases. Ziram (zinc-bis(dimethyldithiocarbamate)) is an agricultural dithiocarbamate fungicide that is used to treat a variety of plant diseases. In spite of their generally acknowledged low toxicity, dithiocarbamates are known to cause a wide range of neurobehavioral effects as well as neuropathological changes in the brain. Astrocytes play a key role in normal brain physiology and in the pathology of the nervous system. This investigation studied the effects of 1.0 µM Ziram on rat hippocampal astrocytes. The thiobarbituric acid reactive substance assay performed showed a significant increase in malondialdehyde, a product of lipid peroxidation, in the Ziram-treated cells. Biochemical analysis also revealed a significant increase in the induction of 70 kDa heat shock and heme oxygenase 1 stress proteins. In addition, an increase of glutathione peroxidase (GPx) and a significant increase in oxidized glutathione (GSSG) were observed in the Ziram-treated cells. The ratio GSH to GSSG calculated from the treated cells was also decreased. Light and transmission electron microscopy supported the biochemical findings in Ziram-treated astrocytes. This data suggest that the cytotoxic effects observed with Ziram treatments may be related to the increase of oxidative stress.

Role of Acorus calamus and alpha-asarone on hippocampal dependent memory in noise stress exposed rats

Stress is a condition or stimulus that threatens an organism's survival. Noise is an environmental stressor. It is well known that long term as well as acute exposure to noise led to oxidative stress. In the present study, it was investigated that the persistence of noise stress (100 dBA/4 h/d for 30 days) could cause memory impairment in rats and whether ethylacetate extract of AC EAAC (50 mg kg(-1) b.wt.) and alpha-Asarone (9 mg kg(-1) b.wt.). treatment can prevent or not. In order to understand the possible mechanism behind it, antioxidant status and acetylcholinesterase (AChE) activity in hippocampus was evaluated after rats were tested in Radial Eight-arm Maze (RAM). Heat shock protein 70 (hsp 70) expression in hippocampus was also evaluated to understand the intensity of stress level. Results showed that after noise stress exposure, time taken to visit all the baited arms, working and reference memory errors were increased in RAM. The superoxide dismutase, lipid peroxidation, AChE activity, hsp 70 were significantly increased with concomitant decrease in catalase, glutathione peroxidase activity and G6PD activity of non-enzymatic levels was observed in the 30 days noise stress exposed group. When rats were co-administrated with EAAC and alpha-Asarone prevents the noise stress induced alterations significantly. In Conclusion, noise stress induced oxidative stress, increased AChE activity, and over expression of hsp 70 in hippocampus region might have led to the impairment of spatial memory. EAAC and alpha-Asarone prevents this noise stress induced memory impairment.

Transcriptional regulation of PSA-NCAM expression by NMDA receptor activation in RA-differentiated C6 glioma cultures

N-Methyl-d-aspartate (NMDA) receptors exhibit a dichotomy of signaling with both toxic and plastic responses. Recent reports have shown that exposure to subtoxic concentration of NMDA results in a neuroprotective state that was measured when these neurons were subsequently challenged with toxic doses of glutamate or kainate. Control of polysialylated neural cell adhesion molecule (PSA-NCAM) expression by NMDA receptor activation has been described in several systems, suggesting a functional link between these two proteins. The perception of glial role in CNS function has changed dramatically over the past few years from simple trophic functions to that of cells with important roles in development and maintenance of CNS in cooperation with neurons. We report here the transcriptional regulation of PSA-NCAM expression by subtoxic dose of NMDA in retinoic acid differentiated C6 glioma cell cultures. C6 glioma cell cultures differentiated with retinoic acid (10microM) were exposed to NMDA (100microM) or to antagonist MK-801 (200nM) prior to treatment with NMDA and cells were harvested after 24h of treatment to study the expression of total NCAM, PSA-NCAM, nuclear factor kappaB (NF-kappaB) and activator protein-1 (AP-1) by Western blotting and dual immunocytofluorescence and expression of PST mRNA by fluorescent in situ hybridization (FISH). Significant increase in the levels of PSA-NCAM, NF-kappaB, AP-1 and PST mRNA was observed in NMDA treated cultures. Treatment of cultures with MK-801, a non-competitive NMDA receptor antagonist, prior to NMDA exposure prevented the NMDA-mediated changes indicating the involvement of NMDA receptor activation. The results elucidate the possible cellular and molecular mechanisms of regulation of PSA-NCAM expression in astroglial cultures by extracellular signals.

Effects of haloperidol and risperidone on the expression of heat shock protein 70 in MK-801-treated rat C6 glioma cells

Non-competitive N-methyl-D-aspartate (NMDA) receptor antagonists such as dizocilpine (MK-801) produce schizophrenia-like psychosis in humans and induce the expression of heat shock protein 70 (HSP70) in rats. The present study examines the effects of antipsychotic drugs, haloperidol and risperidone, on the expression of HSP70 produced by MK-801 in rat C6 glioma cells. After pretreating with haloperidol and risperidone for 1 h, 6 h, 24 h and 72 h, respectively, C6 glioma cells were cultivated again in MK-801 for 6 h, and then, the extent of HSP70 expression was measured by immunoblotting using anti-HSP70 monoclonal antibody. The expression of HSP70 induced by MK-801 significantly decreased as the duration of haloperidol pretreatment was extended (p=0.002). Risperidone also increasingly attenuated the expression of HSP70 produced by MK-801 as the duration of pretreatment grew longer (p=0.003). The present findings show that haloperidol and risperidone decrease the HSP70 expression in MK-801-treated rat C6 glioma cells. These results suggest that HSP70 and NMDA receptors may play a significant role in the pathophysiology of schizophrenia.

Ischemic tolerance following low dose NMDA involves modulation of cellular stress proteins

Mild NMDA receptor activation is correlated with neuroprotection in models of cerebral ischemia. Neuroprotection with NMDA manifests as a form of ischemic tolerance and involves the induction of cellular stress systems sensitive to disturbances in cellular calcium homeostasis. Unilateral micro-injection of 10, 160 and 320 microM NMDA into the prefrontal cortex of a rat 30 min prior to permanent occlusion of the middle cerebral artery (MCAO) significantly reduced the area of infarct observed after 4 h of ischemia. The highest dose of NMDA (320 microM) prevented the propagation of ischemic damage through a direct toxicity on neuronal tissue adjacent to the injection site as demonstrated in thionin-stained sections. As a result, the degree of ischemia-induced damage was similar to that measured in rats pretreated with the low dose of NMDA (10 microM). Expression of heat shock protein (HSP) 70 and glucose-regulated protein (GRP) 94 in cortical samples taken from the region of infarct following MCAO was significantly reduced in rats pretreated with 10 microM NMDA compared to saline-injected control rats and rats pretreated with higher doses of NMDA. Furthermore, 10 microM NMDA did not appear to influence expression of m-calpain or GRP78, however, higher doses of NMDA did significantly induce expression of both proteins as assessed by Western blotting. In summary, our data demonstrate an in vivo rodent model of ischemic tolerance in which 30 min of neuronal preconditioning with 10 microM NMDA confers protection against a 4 h period of MCAO-induced ischemia. This effect may involve modulation of cellular stress signals, in particular HSP70 and GRP94.

Expression of heat shock transcription factors and heat shock protein 72 in rat retina after intravitreal injection of low dose N-methyl-D-aspartate

The heat shock response is a genetically well-ordered process for cell to generate heat shock protein (HSP). Various stressors can trigger the response through heat shock transcriptional factor (HSF) regulation. Recent studies demonstrated that preconditioning of N-methyl-d-aspartate (NMDA) at non-lethal levels has neuroprotective effects, but the exact mechanisms are unclear. We hypothesize that the protective mechanisms of NMDA preconditioning could involve HSP expression. To understand the regulatory mechanisms of HSP under stress, we examined the expression of Hsp72, HSF1 and HSF2 in the adult rat retina after intravitreal injection of NMDA. Retinal ganglion cell (RGC) counting with retrograde labeling showed that 8 nmol, but not 0.8 nmol, of intravitreal NMDA reduced RGC survival. Western blotting and immunohistochemistry showed that non-lethal (0.8 nmol) doses of NMDA induced a time-dependent expression of HSF1 and HSF2, and that the expression of HSF1 and HSF2 in the RGC layer peaked between 9 and 18 h after injection. Parallel to the increased HSF expression, immunohistochemistry and in situ hybridization demonstrated that Hsp72 mRNA and protein expression increased 9 and 12 h after non-lethal NMDA injection, respectively. Our findings suggest that the expression of HSF1 and HSF2 is associated with the Hsp72-related stress response.

C6 glioma cells differentiated by retinoic acid overexpress the glutamate transporter excitatory amino acid carrier 1 (EAAC1)

The transport of excitatory amino acids (EAA) in CNS is performed by a family of high affinity, sodium dependent carriers. One of these transporters, excitatory amino acid carrier 1 (EAAC1), is known to be regulated by several mechanisms that modify carrier abundance on the plasma membrane. Much less is known on EAAC1 regulation at the level of gene expression. Here we report that, in C6 rat glioma cells, a line recently described to contain neural stem-like cells, EAAC1 is markedly induced by all trans-retinoic acid (ATRA), a well known differentiating agent. Consistently, ATRA stimulates EAA transport, with the maximal effect observed at concentrations>or=1 microM. After 4 days of treatment with 10 microM ATRA, the transport Vmax is fivefold enhanced, Slc1a1 mRNA is increased by 400% compared with control, EAAC1 carrier is sixfold overexpressed and the C6 culture is greatly enriched of cells with bipolar morphology strongly positive for EAAC1 immunoreactivity. Compared with untreated cells, ATRA-treated C6 cells express less Slc1a3 mRNA, for the transporter GLAST, but significantly higher levels of Slc1a2 mRNA, for the transporter GLT-1, although no expression of either protein is detected with Western blot in both untreated and ATRA-treated cells. Consistently, the inhibition pattern of aspartate transport and its stimulation by phorbol esters are indicative of a transport process due to EAAC1 operation. Under the conditions adopted, ATRA treatment causes the induction of proteolipid protein, an oligodendrocytic marker. These results indicate that, in C6 cells, ATRA stimulates the expression of EAAC1, possibly as a step toward oligodendrocytic differentiation, and constitute the first demonstration of the induction of this transporter by a differentiating agent.

Induction of hsp70, alterations in oxidative stress markers and apoptosis against dichlorvos exposure in transgenic Drosophila melanogaster: Modulation by reactive oxygen species

We examined a hypothesis that reactive oxygen species (ROS) generated by organophosphate compound dichlorvos modulates Hsp70 expression and anti-oxidant defense enzymes and acts as a signaling molecule for apoptosis in the exposed organism. Dichlorvos (0.015-15.0 ppb) without or with inhibitors of Hsp70, superoxide dismutase (SOD) and catalase (CAT) were fed to the third instar larvae of Drosophila melanogaster transgenic for hsp70 (hsp70-lacZ) Bg(9) to examine Hsp70 expression, oxidative stress and apoptotic markers. A concentration- and time-dependent significant increase in ROS generation accompanied by a significant upregulation of Hsp70 preceded changes in antioxidant defense enzyme activities and contents of glutathione, malondialdehyde and protein carbonyl in the treated organisms. An inhibitory effect on SOD and CAT activities significantly upregulated ROS generation and Hsp70 expression in the exposed organism while inhibition of Hsp70 significantly affected oxidative stress markers induced by the test chemical. A comparison made among ROS generation, Hsp70 expression and apoptotic markers showed that ROS generation is positively correlated with Hsp70 expression and apoptotic cell death end points indicating involvement of ROS in the overall adversity caused by the test chemical to the organism. The study suggests that (a) Hsp70 and anti-oxidant enzymes work together for cellular defense against xenobiotic hazard in D. melanogaster and (b) free radicals may modulate Hsp70 expression and apoptosis in the exposed organism.