Differential sensitivity of cultured tanycytes and astrocytes to hydrogen peroxide toxicity

Copper induces neuron-sparing, ferredoxin 1-independent astrocyte toxicity mediated by oxidative stress

Copper is an essential enzyme cofactor in oxidative metabolism, anti-oxidant defenses, and neurotransmitter synthesis. However, intracellular copper, when improperly buffered, can also lead to cell death. Given the growing interest in the use of copper in the presence of the ionophore elesclomol (CuES) for the treatment of gliomas, we investigated the effect of this compound on the surround parenchyma-namely neurons and astrocytes in vitro. Here, we show that astrocytes were highly sensitive to CuES toxicity while neurons were surprisingly resistant, a vulnerability profile that is opposite of what has been described for zinc and other toxins. Bolstering these findings, a human astrocytic cell line was similarly sensitive to CuES. Modifications of cellular metabolic pathways implicated in cuproptosis, a form of copper-regulated cell death, such as inhibition of mitochondrial respiration or knock-down of ferredoxin 1 (FDX1), did not block CuES toxicity to astrocytes. CuES toxicity was also unaffected by inhibitors of apoptosis, necrosis or ferroptosis. However, we did detect the presence of lipid peroxidation products in CuES-treated astrocytes, indicating that oxidative stress is a mediator of CuES-induced glial toxicity. Indeed, treatment with anti-oxidants mitigated CuES-induced cell death in astrocytes indicating that oxidative stress is a mediator of CuES-induced glial toxicity. Lastly, prior induction of metallothioneins 1 and 2 in astrocytes with zinc plus pyrithione was strikingly protective against CuES toxicity. As neurons express high levels of metallothioneins basally, these results may partially account for their resistance to CuES toxicity. These results demonstrate a unique toxic response to copper in glial cells which contrasts with the cell selectivity profile of zinc, another biologically relevant metal.

Protective effects of N-acetylcysteine against monosodium glutamate-induced astrocytic cell death

Monosodium glutamate (MSG) is a flavor enhancer, largely used in the food industry and it was reported to have excitotoxic effects. Higher amounts of MSG consumption have been related with increased risk of many diseases, including Chinese restaurant syndrome and metabolic syndromes in human. This study investigated the protective effects of N-acetylcysteine (NAC) on MSG-induced cytotoxicity in C6 astrocytic cells. MSG (20 mM)-induced reactive oxygen species (ROS) generation and apoptotic cell death were significantly attenuated by NAC (500 μM) pretreatment. NAC effectively inhibited the MSG-induced mitochondrial membrane potential (MMP) loss and intracellular reduced glutathione (GSH) depletion. In addition, NAC significantly attenuated MSG-induced endoplasmic reticulum (ER) stress markers, such as XBP1 splicing and CHOP, PERK, and GRP78 up-regulation. Furthermore, NAC prevented the changes of MSG-induced Bcl-2 expression level. These results suggest that NAC can protect C6 astrocytic cells against MSG-induced oxidative stress, mitochondrial dysfunction, and ER stress.

Preconditioning protects against oxidative injury involving hypoxia-inducible factor-1 and vascular endothelial growth factor in cultured astrocytes

Tolerance to brain injury involves hypoxia-inducible factor-1 (HIF-1) and its target genes as the key pathway mediating a cascade of events including cell survival, energetics, and angiogenesis. In this study, we established the treatment paradigms for an in vitro model of tolerance to oxidative injury in primary astrocytic cultures and further examined the roles for the HIF-1 signalling cascade. Isolated murine astrocytes were preconditioned with sub-toxic concentrations of HIF-1 inducers and subsequently exposed to a H(2)O(2) insult, where changes in cell viability and protein expression were determined. Preconditioning with non-damaging concentrations of desferrioxamine (DFO) and ethyl-3,4-dihydroxybenzoate (EDHB) significantly improved cellular viability after H(2)O(2) injury treatment. Time course studies revealed that DFO and EDHB treatments alone induced sequential activation of HIF-1 signal transduction where nuclear HIF-1alpha protein accumulation was detected as early as 2h, followed by downstream upregulation of intracellular and released VEGF from 4h and 8h onwards, respectively. The protective effects of DFO and EDHB preconditioning against H(2)O(2) injury were abolished by co-treatment with cycloheximide, an inhibitor of protein synthesis. Importantly, when the anti-HIF-1 compound, 3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole (YC-1) was used, the cytoprotection and VEGF accumulation produced by DFO and EDHB preconditioning were diminished. These results indicate the essential role of the HIF-1 pathway in our model of tolerance against oxidative injury in cultured astrocytes, and suggest roles for astrocytic HIF-1 expression and VEGF release which may influence the function of surrounding cells and vasculature during oxidative stress-related brain diseases.

H2O2-induced changes in astrocytic cultures from control and rapidly aging strains of mouse

The study compared the difference between the H2O2 treatment on astrocytic cultures from a rapidly aging strain of mouse (SAMP8) and its sister control (R1). A mild but statistically significant difference was observed in the numbers of dead cell between R1 and SAMP8 after H2O2 treatment. Cellular changes were equivalent in both strains after injury, including loss of cilia and side projections. Low total dose of H2O2 treatment (e.g., 400 microM for only 1 hour) caused increased cellular synthesis,while high total dose of H2O2 treatment (e.g., 200 microM for 4 hours) downregulated in intracellular synthesis and caused coagulation of microtubules.

Oxidative stress on the astrocytes in culture derived from a senescence accelerated mouse strain

Astrocytes are one of the predominant glial cell types in the adult central nervous system functioning as both supportive and metabolic cells for the brain. Our objective in this experiment is to study the direct effects of hydrogen peroxide induced oxidative stress on astrocytes in culture. These astrocytes were derived from both an aged mouse strain (P8) and a matched control strain (R1). The astrocytes for both the P8 and R1 strains were treated with increasing concentrations of hydrogen peroxide. Our results showed that the oxidative stress had a similar effect in both strains of astrocytes; decreases in 3-(4,5-dimethylthiazol-2-yl)-2,2-diphenyltetrazolium bromide (MTT) and glial fibrillary acidic protein (GFAP) levels, and increases in terminal deoxynucleotidyltransferase-mediated dUTP nick end-labeling (TUNEL) staining, lactate dehydrogenase (LDH) staining, and superoxide dismutase (SOD), caspase-3 and B-cell lymphoma 2-associated protein X (bax) levels. At a hydrogen peroxide concentration of 400 microM , the differences of the above parameters between P8 cultures and R1 cultures were statistically significant (p<0.05). This strongly suggested that astrocytes derived from P8 and R1 strains reacted to oxidative stress with similar mechanisms and consequences. However, the mechanisms were not able to compensate for the oxidative stress in the P8 strain at a hydrogen peroxide concentration of 400 microM. The inability of the P8 astrocytes to counteract the oxidative stress might lead to inadequate protection from neuronal loss possibly resulting in significantly more astrocytic death. Our results suggested that the changes of astrocytes in peroxide detoxification may play a role in aging of the central nervous system, and further aging studies should examine the oxidative status of the samples.

Glutamate-mediated glial injury: Mechanisms and clinical importance

Primary and/or secondary glial cell death can cause and/or aggravate human diseases of the central nervous system (CNS). Like neurons, glial cells are vulnerable to glutamate insults. Astrocytes, microglia, and oligodendrocytes express a wide variety of glutamate receptors and transporters that mediate many of the deleterious effects of glutamate. Astrocytes are responsible for most glutamate uptake in synaptic and nonsynaptic areas and consequently, are the major regulators of glutamate homeostasis. Microglia in turn may secrete cytokines, which can impair glutamate uptake and reduce the expression of glutamate transporters. Finally, oligodendrocytes, the myelinating cells of the CNS, are very sensitive to excessive glutamate signaling, which can lead to the apoptosis or necrosis of these cells. This review aims at summarizing the mechanisms leading to glial cell death as a consequence of alterations in glutamate signaling, and their clinical relevance. A thorough understanding of these events will undoubtedly lead to better therapeutic strategies to treat CNS diseases affecting glia and in particular, those that involve damage to white matter tracts.

Topiramate protects against glutamate- and kainate-induced neurotoxicity in primary neuronal-astroglial cultures

Potential neuroprotective effects of the antiepileptic drug (AED) topiramate (TPM) were evaluated using primary neuronal-astroglial cultures or astroglial-enriched cultures from newborn rats exposed to excitotoxic concentrations of glutamate (Glu) or kainate. Neurons expressed functional Glu receptors of the NMDA and AMPA/kainate types as evaluated by immunocytochemistry and Ca(2+) imaging. When Glu (10 mM) was added to 9-10-day cultures incubated with the fluorescent dye calcein/AM for 5h, there was a marked cell loss in both culture types, but was more pronounced in the neuronal-astroglial cultures. When TPM (5-10 microM) was included in the medium together with Glu, the amount of surviving cells was significantly higher in the neuronal-astroglial cultures, but not in the astroglial-enriched cultures. Immuno-labeling of the cultures revealed an enhanced survival of MAP positive neuronal cells when TPM was included in the Glu containing medium. As TPM has a proven negative modulatory effect on kainate activated receptors, neuronal-astroglial cultures were further exposed to excitotoxic concentrations of kainate (100 microM) and analyzed immunohistochemically. Significantly more MAP positive neurons survived in the TPM containing medium and showed a morphology similar to untreated cells. Valproate and phenytoin were used as reference AEDs. In conclusion, our results demonstrate a protective effect of TPM upon neuronal cells in primary culture, exposed to excitotoxic levels of Glu or kainate.

Excitotoxicity in glial cells

Excitotoxicity results from prolonged activation of glutamate receptors expressed by cells in the central nervous system (CNS). This cell death mechanism was first discovered in retinal ganglion cells and subsequently in brain neurons. In addition, it has been recently observed that CNS glial cells can also undergo excitotoxicity. Among them, oligodendrocytes are highly vulnerable to glutamate signals and alterations in glutamate homeostasis may contribute to demyelinating disorders. We review here the available information on excitotoxity in CNS glial cells and its putative relevance to glio-pathologies.

Glutamate stimulates ascorbate transport by astrocytes

The concentrations of glutamate and ascorbate in brain extracellular fluid increase following seizure activity, trauma and ischemia. Extracellular ascorbate concentration also rises following intracerebral glutamate injection. We hypothesized that glutamate triggers the release of ascorbate from astrocytes. We observed in primary cultures of rat cerebral astrocytes that glutamate increased ascorbate efflux significantly within 30 min. The half-maximal effective concentration of glutamate was 180+/-30 microM. Glutamate-stimulated efflux of ascorbate was attenuated by hypertonic media. 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid inhibited both Na(+)-dependent glutamate uptake and ascorbate efflux. Two other inhibitors of volume-sensitive organic anion channels (1, 9-dideoxyforskolin and 5-nitro-2-(3-phenylpropylamino) benzoic acid) did not slow glutamate uptake but prevented stimulation of ascorbate efflux. Glutamate also stimulated the uptake of ascorbate by ascorbate-depleted astrocytes. In contrast, glutamate uptake was not affected by intracellular ascorbate, thus ruling out a putative glutamate-ascorbate heteroexchange mechanism. These results are consistent with activation by glutamate of ascorbate-permeant channels in astrocytes.

Tanycytes transplanted into the adult rat spinal cord support the regeneration of lesioned axons

During past years a number of therapeutic strategies have been developed in order to stimulate axonal regeneration after traumatic injuries of the spinal cord. Recently, encouraging data have been obtained by grafting specific glial cells such as Schwann cells or olfactory ensheathing glial cells, known to support the regeneration of peripheral or central axons, respectively. In a recent series of studies, we have shown that tanycytes, a particular glial cell type present in the mediobasal hypothalamus, were able to support the regeneration of a variety of axons innervating this region. The aim of the present study was to determine whether tanycytes could also support the regeneration of lesioned spinal axons. Cultured hypothalamic tanycytes and cortical astrocytes were prelabeled with Fast blue (FB) and grafted into the thoracic spinal cord of adult rats. Three weeks after the transplantation, the animals were fixed and spinal cord sections treated for multiple fluorescence detection of the FB-labeled transplanted cells on the one hand and of various glial and neuronal markers on the other hand. We show here that in all the spinal cords examined, transplanted tanycytes or astrocytes formed large spherical clusters of about 0.5 mm in diameter, located in the mediolateral spinal cord layer. The immunodetection of glial markers showed that transplanted astrocytes exhibited intense immunostaining for both glial fibrillary acidic protein (GFAP) and vimentin (VIM), whereas transplanted tanycytes were intensely immunostained for VIM, but GFAP negative. The immunodetection of axonal markers showed that contrasting with astrocyte transplants, tanycyte transplants were invaded by numerous axonal fibers. These data indicate that tanycyte transplants may represent a useful therapeutic tool for the reparation of the lesioned spinal axons.