Toxicity induced by cumene hydroperoxide in PC12 cells: protective role of thiol donors

Oxidative shock and production of reactive oxygen species are known to play a major role in situations leading to neuron degeneration, but the precise mechanisms responsible for cell degeneration remain uncertain. In the present article, we have studied in PC 12 cells the effect of cumene hydroxyperoxide on both cell metabolism and morphology. We observed that relatively low concentrations of the drug (100 μM) led to a significant decrease in the cellular content of ATP and reduced glutathione as well as to a decreased mitochondrial potential. These metabolic alterations were followed by an important increase in intracellular free calcium and membrane disruption and death. In parallel, we observed profound changes in cell morphology with a shortening of cell extensions, the formation of ruffles and blebs at the cell surface, and a progressive detachment of the cells from the surface of the culture flasks. We also showed that addition of thiol donors such as N-acetylcysteine or β-mercaptoethanol, which were able to enhance cell glutathione content, almost completely protected PC 12 cells from the toxic action of cumene hydroperoxide whereas pretreatment by buthionine sulfoximine, a selective inhibitor of GSH synthesis, enhanced its action.

Reverse glial glutamate uptake triggers neuronal cell death through extrasynaptic NMDA receptor activation

Evidence have accumulated that reverse glutamate uptake plays a key role in the pathophysiology of cerebral ischemia. Here, we investigated the effects of glial glutamate transporter dysfunction on neuronal survival using the substrate inhibitor of glutamate transporters, L-trans-pyrrolidine,2-4,dicarboxylate (PDC), that partly mimics reverse glutamate uptake. On mice primary cortical co-cultures of neurons and astrocytes, PDC treatment triggered an elevation of extracellular glutamate concentration, induced neuronal calcium influx and a massive NMDA receptor (NMDAR) mediated-neuronal death without having any direct agonist activity on NMDARs. We investigated the NMDAR subpopulation activated by PDC-induced glutamate release. PDC application led to the activation of both subtypes of NMDARs but the presence of astrocytes was required to activate NMDARs located extra-synaptically. Extrasynaptic NMDAR activation was also confirmed by the loss of neuronal mitochondrial membrane potential and the inhibition of pro-survival p-ERK signalling pathway. These data suggest that reverse glial glutamate uptake may trigger neuronal death through preferential activation of extrasynaptic NMDAR-related pathways.

Neuronal viability is controlled by a functional relation between synaptic and extrasynaptic NMDA receptors

N-methyl-D-aspartate receptors (NMDARs) are critical for synaptic plasticity that underlies learning and memory. But, they have also been described as a common source of neuronal damage during stroke and neurodegenerative diseases. Several studies have suggested that cellular location of NMDARs (synaptic or extrasynaptic) is a key parameter controlling their effect on neuronal viability. The aim of the study was to understand the relation between these two pools of receptors and to determine their implication in both beneficial and/or deleterious events related to NMDAR activation. We demonstrated that selective extrasynaptic NMDAR activation, as well as NMDA bath application, does not activate extracellular signal-regulated kinase (ERK) pathways, but induces mitochondrial membrane potential breakdown and triggers cell body and dendrite damages, whereas synaptic NMDAR activation is innocuous and induces a sustained ERK activation. The functional dichotomy between these two NMDAR pools is tightly controlled by glutamate uptake systems. Finally, we demonstrated that the only clinically approved NMDAR antagonist, memantine, preferentially antagonizes extrasynaptic NMDARs. Together, these results suggest that extrasynaptic NMDAR activation contributes to excitotoxicity and that a selective targeting of the extrasynaptic NMDARs represents a promising therapeutic strategy for brain injuries.

Increased expression of transforming growth factor-beta after cerebral ischemia in the baboon: an endogenous marker of neuronal stress?

There has been an increasing interest in recent years in the evaluation of the neuronal and glial responses to ischemic insult. Some cytokines, including transforming growth factor-beta (TGF-beta), that are overexpressed after experimental stroke in rodents are thought to be implicated in the neuronal processes that lead to necrosis. Thus, such cytokines could predict tissue fate after stroke in humans, although data are currently sparse for gyrencephalic species. The current study addressed the expression pattern of TGF-beta1 in a nonhuman primate model of middle cerebral artery occlusion. Focal permanent ischemia was induced for 1 or 7 days in 6 baboons and the following investigations were undertaken: cerebral oxygen metabolism (CMRO2) positron emission tomography studies, magnetic resonance imaging, postmortem histology, and reverse transcription-polymerase chain reaction. The aim of the current study was to correlate the expression of TGF-beta1 to the underlying metabolic and histologic state of the threatened cerebral parenchyma. The authors evidenced increased TGF-beta1 mRNA levels (up to 25-fold) in those regions displaying a moderate (20% to 49%) reduction in CMRO2. The current findings suggest that the greatly enhanced expression of TGF-beta1 in the penumbral zones that surround tissue destined to infarction may represent a robust index of potentially salvageable brain. The current investigation, in the nonhuman primate, strengthens the authors' hypothesis, derived from rodent models, that TGF-beta1 may be involved in the physiopathology of human stroke.

Neuroprotection mediated by glial cell line-derived neurotrophic factor: involvement of a reduction of NMDA-induced calcium influx by the mitogen-activated protein kinase pathway

The glial cell line-derived neurotrophic factor (GDNF) is first characterized for its trophic activity on dopaminergic neurons. Recent data suggested that GDNF could modulate the neuronal death induced by ischemia. The purpose of this study was to characterize the influence of GDNF on cultured cortical neurons subjected to two paradigms of injury (necrosis and apoptosis) that have been identified during cerebral ischemia and to determine the molecular mechanisms involved. First, we demonstrated that both neurons and astrocytes express the mRNA and the protein for GDNF and its receptor complex (GFRalpha-1 and c-Ret). Next, we showed that the application of recombinant human GDNF to cortical neurons and astrocytes induces the activation of the MAP kinase (MAPK) pathway, as visualized by an increase in the phosphorylated forms of extracellular signal-regulated kinases (ERKs). Thereafter, we demonstrated that GDNF fails to prevent apoptotic neuronal death but selectively attenuates slowly triggered NMDA-induced excitotoxic neuronal death via a direct effect on cortical neurons. To further characterize the neuroprotective mechanisms of GDNF against NMDA-mediated neuronal death, we showed that a pretreatment with GDNF reduces NMDA-induced calcium influx. This effect likely results from a reduction of NMDA receptor activity rather than an enhanced buffering or extrusion capacity for calcium. Finally, we also demonstrated that an ERKs activation pathway is necessary for GDNF-mediated reduction of the NMDA-induced calcium response. Together, these results describe a novel mechanism by which the activation of MAPK induced by GDNF modulates NMDA receptor activity, a mechanism that could be responsible for the neuroprotective effect of GDNF in acute brain injury.

Complement anaphylatoxin C3a is selectively protective against NMDA-induced neuronal cell death

The anaphylatoxin C3a is a potent inflammatory polypeptide released at sites of complement activation. To test whether C3a might alter neuronal outcome following an ischemic insult, we determined the effects of purified human C3a on murine primary cortical cell cultures exposed to apoptotic or excitotoxic paradigms. C3a prevented neither serum deprivation-induced apoptotic neuronal death, nor AMPA/kainate-mediated excitotoxicity. However, in mixed cultures of neurons and astrocytes, C3a dose-dependently protected neurons against NMDA toxicity (47% neuroprotection using 100 nM C3a, p < 0.01, n = 12). The neuroprotective effect of C3a was observable only in the presence of astrocytes. These observations suggest that C3a is involved in excitotoxicity-mediated neuronal death through astrocyte stimulation and extend its role beyond immune functions.

The proteolytic activity of tissue-plasminogen activator enhances NMDA receptor-mediated signaling

Tissue-plasminogen activator (t-PA) is now available for the treatment of thrombo-embolic stroke but adverse effects have been reported in some patients, particularly hemorrhaging. In contrast, the results of animal studies have indicated that t-PA could increase neuronal damage after focal cerebral ischemia. Here we report for the first time that t-PA potentiates signaling mediated by glutamatergic receptors by modifying the properties of the N-methyl-D-aspartate (NMDA) receptor. When depolarized, cortical neurons release bio-active t-PA that interacts with and cleaves the NR1 subunit of the NMDA receptor. Moreover, the treatment with recombinant t-PA leads to a 37% increase in NMDA-stimulated fura-2 fluorescence, which may reflect an increased NMDA-receptor function. These results were confirmed in vivo by the intrastriatal injection of recombinant-PA, which potentiated the excitotoxic lesions induced by NMDA. These data provide insight into the regulation of NMDA-receptor-mediated signaling and could initiate therapeutic strategies to improve the efficacy of t-PA treatment in man.

Ischemia-induced interleukin-6 as a potential endogenous neuroprotective cytokine against NMDA receptor-mediated excitotoxicity in the brain

In the brain, the expression of the pleiotropic cytokine interleukin-6 (IL-6) is enhanced in various chronic or acute central nervous system disorders. However, the significance of IL-6 production in such neuropathologic states remains controversial. The present study investigated the role of IL-6 after cerebral ischemia. First, the authors showed that focal cerebral ischemia in rats early up-regulated the expression of IL-6 mRNA, without affecting the transcription of its receptors (IL-6Ralpha and gp130). Similarly, the striatal injection of N-methyl-D-aspartate (NMDA) in rats, a paradigm of excitotoxic injury, activated the expression of IL-6 mRNA. The involvement of glutamatergic receptor activation was further investigated by incubating cortical neurons with NMDA or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA). NMDA and ionomycin (a calcium ionophore) up-regulated IL-6 mRNA, suggesting that neurons may produce IL-6 in response to the calcium influx mediated through NMDA receptors. The potential role of IL-6 during ischemic/excitotoxic insults was then studied by testing the effect of IL-6 against apoptotic or excitotoxic challenges in cortical cultures. IL-6 did not prevent serum deprivation- or staurosporine-induced apoptotic neuronal death, or AMPA/kainate-mediated excitotoxicity. However, in both mixed and pure neuronal cultures, IL-6 dose-dependently protected neurons against NMDA toxicity. This effect was blocked by a competitive inhibitor of IL-6. Overall, the results suggest that the up-regulation of IL-6 induced by cerebral ischemia could represent an endogenous neuroprotective mechanism against NMDA receptor-mediated injury.

A transforming growth factor-beta antagonist unmasks the neuroprotective role of this endogenous cytokine in excitotoxic and ischemic brain injury

Various studies describe increased concentrations of transforming growth factor-beta (TGF-beta) in brain tissue after acute brain injury. However, the role of endogenously produced TGF-beta after brain damage to the CNS remains to be clearly established. Here, the authors examine the influence of TGF-beta produced after an episode of cerebral ischemia by injecting a soluble TGF-beta type II receptor fused with the Fc region of a human immunoglobulin (TbetaRIIs-Fc). First, this molecular construct was characterized as a selective antagonist of TGF-beta. Then, the authors tested its ability to reverse the effect of TGF-beta1 on excitotoxic cell death in murine cortical cell cultures. The addition of 1 microg/mL of TbetaRIIs-Fc to the exposure medium antagonized the neuroprotective activity of TGF-beta1 in N-methyl-D-aspartate (NMDA)-induced excitotoxic cell death. These results are consistent with the hypothesis that TGF-beta1 exerts a negative modulatory action on NMDA receptor-mediated excitotoxicity. To determine the role of TGF-beta1 produced in response to brain damage, the authors used a model of an excitotoxic lesion induced by the intrastriatal injection of 75 nmol of NMDA in the presence of 1.5 microg of TbetaRIIs-Fc. The intrastriatal injection of NMDA was demonstrated to induce an early upregulation of the expression of TGF-beta1 mRNA. Furthermore, when added to the excitotoxin, TbetaRIIs-Fc increased (by 2.2-fold, P < 0.05) the lesion size. These observations were strengthened by the fact that an intracortical injection of TbetaRIIs-Fc in rats subjected to a 30-minute reversible cerebral focal ischemia aggravated the volume of infarction. In the group injected with the TGF-beta1 antagonist, a 3.5-fold increase was measured in the infarction size (43.3 +/- 9.5 versus 152.8 +/- 46.3 mm3; P < 0.05). In conclusion, by antagonizing the influence of TGF-beta in brain tissue subjected to excitotoxic or ischemic lesion, the authors markedly exacerbated the resulting extent of necrosis. These results suggest that, in response to such insults, brain tissue responds by the synthesis of a neuroprotective cytokine, TGF-beta1, which is involved in the limitation of the extent of the injury. The pharmacologic potentiation of this endogenous defensive mechanism might represent an alternative and novel strategy for the therapy of hypoxic-ischemic cerebral injury.

Transforming growth factor-beta1 as a regulator of the serpins/t-PA axis in cerebral ischemia

The tissue type plasminogen activator (t-PA) is a serine protease that is involved in neuronal plasticity and cell death induced by excitotoxins and ischemia in the brain. t-PA activity in the central nervous system is regulated through the activation of serine protease inhibitors (serpins) such as the plasminogen activator inhibitor (PAI-1), the protease nexin-1 (PN-1), and neuroserpin (NSP). Recently we demonstrated in vitro that PAI-1 produced by astrocytes mediates the neuroprotective effect of the transforming growth factor-beta1 (TGF-beta1) in NMDA-induced neuronal cell death. To investigate whether serpins may be involved in neuronal cell death after cerebral ischemia, we determined, by using semiquantitative RT-PCR and in situ hybridization, that focal cerebral ischemia in mice induced a dramatic overexpression of PAI-1 without any effect on PN-1, NSP, or t-PA. Then we showed that although the expression of PAI-1 is restricted to astrocytes, PN-1, NSP, and t-PA are expressed in both neurons and astrocytes. Moreover, by using semiquantitative RT-PCR and Western blotting, we observed that only the expression of PAI-1 was modulated by TGF-beta1 treatment via a TGF-beta-inducible element contained in the PAI-1 promoter (CAGA box). Finally, we compared the specificity of TGF-beta1 action with other members of the TGF-beta family by using luciferase reporter genes. These data show that TGF-beta and activin were able to induce the overexpression of PAI-1 in astrocytes, but that bone morphogenetic proteins, glial cell line-derived neutrophic factor, and neurturin did not. These results provide new insights into the regulation of the serpins/t-PA axis and the mechanism by which TGF-beta may be neuroprotective.

Up-regulation of a serine protease inhibitor in astrocytes mediates the neuroprotective activity of transforming growth factor beta1

Serine proteases play a key role in the fundamental biology of the central nervous system (CNS), and recent data suggest their involvement in the pathophysiology of neurodegenerative diseases. Little is known about the physiological regulation of these proteases in the CNS. Among the multiple growth factors present in the brain, transforming growth factor beta1 (TGF-beta1) has been described as an injury-related growth factor. However, its beneficial or deleterious role remains unclear. In the present study, we investigated the influence of TGF-beta1 in apoptosis and necrosis, two mechanisms involved in ischemic neuronal death. We show that TGF-beta1 exerts a neuroprotective role restricted to necrosis induced by N-methyl-D-aspartate. This effect is observable only in the obligatory presence of TGF-beta1-responsive astrocytes. We demonstrate that this neuroprotective activity is mediated through an up-regulation of a serine protease inhibitor (PAI-1) in astrocytes. These results underline the involvement of serine proteases and extracellular matrix components such as the PAI-1/t-PA axis in the excitotoxic cascade. Moreover, regardless of the underlying mechanisms of t-PA involvement in excitotoxic injury, our observations might warn against the use of tissular plasminogen activator as an alternative therapy for the treatment of hypoxic-ischemic injury in the brain.