Refocusing the Brain: New Approaches in Neuroprotection Against Ischemic Injury

A new era for neuroprotective strategies is emerging in ischemia/reperfusion. This has forced to review the studies existing to date based in neuroprotection against oxidative stress, which have undoubtedly contributed to clarify the brain endogenous mechanisms, as well as to identify possible therapeutic targets or biomarkers in stroke and other neurological diseases. The efficacy of exogenous administration of neuroprotective compounds has been shown in different studies so far. However, something must be missing to get these treatments successfully applied in the clinical environment. Here, the mechanisms involved in neuronal protection against physiological level of ROS and the main neuroprotective signaling pathways induced by excitotoxic and ischemic stimuli are reviewed. Also, the endogenous ischemic tolerance in terms of brain self-protection mechanisms against subsequent cerebral ischemia is revisited to highlight how the preconditioning has emerged as a powerful tool to understand these phenomena. A better understanding of endogenous defense against exacerbated ROS and metabolism in nervous cells will therefore aid to design pharmacological antioxidants targeted specifically against oxidative damage induced by ischemic injury, but also might be very valuable for translational medicine.

Single-Nucleotide Polymorphism 309T>G in the MDM2 Promoter Determines Functional Outcome After Stroke

Background and Purpose- The E3 ubiquitin ligase MDM2 (murine double minute 2) is the main negative regulator of the p53 protein-a key player in neuronal apoptosis after ischemia. A functional single-nucleotide polymorphism in the human MDM2 gene promoter (rs2279744) regulates MDM2 protein expression. We investigated whether the MDM2 SNP309, by controlling p53-mediated apoptosis, determines functional outcome after stroke. Methods- Primary cortical neurons were subjected to oxygen and glucose deprivation. Mice were subjected to ischemic (transient middle cerebral artery occlusion) or hemorrhagic (collagenase injection) stroke models. Protein and mRNA levels of MDM2 and p53 were measured in both neuronal and brain extracts. The interaction of MDM2 with p53 was disrupted by neuronal treatment with nutlin-3a. siRNA was used to knockdown MDM2 expression. We analyzed the link between the MDM2 SNP309 and functional outcome, measured by the modified Rankin Scale scores, in 2 independent hospital-based stroke cohorts: ischemic stroke cohort (408 patients) and intracerebral hemorrhage cohort (128 patients). Results- Experimental stroke and oxygen and glucose deprivation induced the expression of MDM2 in the brain and neurons, respectively. Moreover, oxygen and glucose deprivation promoted MDM2 binding with p53 in neurons. Disruption of the MDM2-p53 interaction with nutlin-3a, or MDM2 knockdown by siRNA, triggered p53 accumulation, which increased neuronal susceptibility to oxygen and glucose deprivation-induced apoptosis. Finally, we showed that patients harboring the G allele in the MDM2 promoter had higher MDM2 protein levels and showed better functional outcome after stroke than those harboring the T/T genotype. The T/T genotype was also associated with large infarct volume in ischemic stroke and increased lesion volume in patients with intracerebral hemorrhage. Conclusions- Our results reveal a novel role for the MDM2-p53 interaction in neuronal apoptosis after ischemia and show that the MDM2 SNP309 determines the functional outcome of patients after stroke.

Human neuroblastoma cells with MYCN amplification are selectively resistant to oxidative stress by transcriptionally up-regulating glutamate cysteine ligase

Neuroblastoma is a sympathetic nervous system tumour whose degree of malignancy, prognosis and therapy resistance has been associated with the amplification of MYCN oncogene. However, the molecular pathway responsible for such resistance is unknown. To contribute addressing this issue, in this study, we have compared the vulnerability of four human neuroblastoma cell lines differentially amplifying MYCN, namely SK-N-BE-2 and IMR-32 (MYCN-amplified cells) and SH-SY5Y and SK-N-SH (MCYN-non-amplified cells), to H(2)O(2)-mediated apoptotic death. We found that the high resistance of the MYCN-amplified neuroblastoma cells against oxidative damage can be accounted for by their greater expression of both the mRNA and protein of the catalytic subunit of glutamate-cysteine ligase (GCL(cat)), the rate-limiting step in GSH biosynthesis. Furthermore, we found that MYCN directly binds to an E-box containing GCL(cat) promoter and that over-expression of MYCN in MYCN-non-amplified cells stimulated GCL(cat) expression and provided resistance to oxidative damage; whereas knock down of MYCN in MYCN-amplified cells decreased GCL(cat) expression and sensitized them to oxidative damage. Finally, GCL(cat) knock down enhanced the vulnerability of MYCN-amplified cells to oxidative damage. These results demonstrate that regulation of GCL(cat) by MYCN accounts for the survival of neuroblastoma cells against oxidative damage, and suggest that GCL should be considered a potential therapeutic target for the treatment of MYCN-amplified neuroblastoma.

Inhibition of mitochondrial respiration by nitric oxide: its role in glucose metabolism and neuroprotection

There is an increasing body of evidence demonstrating that inhibition of cytochrome c oxidase by nitric oxide (NO) may be one more step in a signaling cascade involved in the physiologic regulation of cell functions. For example, in both astrocytes and neurons the inhibition of mitochondrial respiration by endogenously produced NO induces transient and modest decreases in cellular ATP concentrations. This mitochondrial impairment may serve as a cellular sensor of energy charges, hence modulating metabolic pathways, such as glycolysis, through AMP-activated protein kinase (AMPK) in astrocytes. In neurons, the NO derivative peroxynitrite anion triggers signaling pathways leading to glucose oxidation through the pentose-phosphate pathway to form reducing equivalents in the form of NADPH. The modulation of these metabolic pathways by nitric oxide or its derivatives may be important for understanding the mechanisms by which this free radical affects neuronal death or survival.

Regulation of glucose metabolism by nitrosative stress in neural cells

Following brain inflammatory stimuli, astrocytes actively synthesize nitric oxide and peroxynitrite. These nitrogen-derived species trigger a repertoire of biochemical effects, including alteration of mitochondrial function and redox status both in astrocytes and neighboring neurons. Furthermore, under such nitrosative stress astrocytes show remarkable resistance in spite of having their mitochondria impaired, whereas the neighboring neurons show vulnerability. In this review, we discuss recent evidence strongly suggesting that nitrogen-derived species modulate key regulatory steps of glucose metabolism. These involve up-regulation of high-affinity glucose transporter, stimulation of glycolysis at 6-phosphofructo-1-kinase, and activation of pentose-phosphate pathway at glucose-6-phosphate dehydrogenase. We conclude that the orchestrated stimulation of glucose-metabolising pathways by nitric oxide would be a transient attempt of certain neural cells to compensate for the impaired energy status and oxidised glutathione and thus emerge from an otherwise neuropathological outcome.

Tetrahydrobiopterin deficiency increases neuronal vulnerability to hypoxia

Tetrahydrobiopterin (BH4) is an essential co-factor for nitric oxide synthases (NOS). The aim of the present work was to study whether BH4 deficiency affects the vulnerability of neurones in primary culture to hypoxia. Intracellular BH4 levels were depleted by pre-incubating neurones with 5 mm 2,4-diamino-6-hydroxypyrimidine (DAHP) for 18 h, after which cells were exposed for 1 h to normoxic or hypoxic conditions. Our results showed that whereas neurones were resistant to hypoxia-induced cellular damage, BH4 deficiency in neurones led to oxidative stress, mitochondrial depolarization, ATP depletion and necrosis after 1 h of hypoxia. Indeed, hypoxia specifically inhibited mitochondrial complex IV activity in BH4-deficient neurones. All these effects were counteracted when neuronal BH4 levels were restored by incubating cells with exogenous BH4 during the hypoxic period. Moreover, hypoxia-induced damage in BH4-deficient neurones was prevented when Nomega-nitro-l-arginine monomethyl ester (NAME), haemoglobin or superoxide dismutase plus catalase were present during the hypoxic period, suggesting that peroxynitrite might be involved in the process. In fact, BH4 deficiency elicited neuronal NO dysfunction, resulting in an increase in peroxynitrite generation by cells, as shown by the enhancement in tyrosine nitration; this was prevented by supplements of BH4, NAME, haemoglobin or superoxide dismutase plus catalase during hypoxia. Our results suggest that BH4 deficiency converts neuronal NOS into an efficient peroxynitrite synthase, which is responsible for the increase in neuronal vulnerability to hypoxia-induced mitochondrial damage and necrosis.

Oxygen and glucose deprivation induces mitochondrial dysfunction and oxidative stress in neurones but not in astrocytes in primary culture

In order to investigate the potential neuroprotective role played by glucose metabolism during brain oxygen deprivation, the susceptibility of cultured neurones and astrocytes to 1 h of oxygen deprivation (hypoxia) or oxygen and glucose deprivation (OGD) was examined. OGD, but not hypoxia, promotes dihydrorhodamine 123 and glutathione oxidation in neurones but not in astrocytes reflecting free radical generation in the former cells. A specific loss of mitochondrial complex-I activity, mitochondrial membrane potential collapse, ATP depletion and necrosis occurred in the OGD neurones, but not in the OGD astrocytes. Furthermore, superoxide anion but not nitric oxide formation was responsible for these effects. OGD decreased neuronal but not astrocytic NADPH concentrations; this was not observed in hypoxia and was independent of superoxide or nitric oxide formation. These results suggest that glucose metabolism would supply NADPH, through the pentose-phosphate pathway, aimed at preventing oxidative stress, mitochondrial damage and neurotoxicity during oxygen deprivation to neural cells.

D-Glucose prevents glutathione oxidation and mitochondrial damage after glutamate receptor stimulation in rat cortical primary neurons

The possible neuroprotective effect of D-glucose against glutamate-mediated neurotoxicity was studied in rat cortical neurons in primary culture. Brief (5-min) exposure of neurons to glutamate (100 microM) increased delayed (24-h) necrosis and apoptosis by 3- and 1.8-fold, respectively. Glutamate-mediated neurotoxicity was accompanied by a D-(-)-2-amino-5-phosphonopentanoate (100 microM) and N(omega)-nitro-L-arginine methyl ester (1 mM)-inhibitable, time-dependent ATP depletion (55% at 24 h), confirming the involvement of NMDA receptor stimulation followed by nitric oxide synthesis in this process. Furthermore, the presence of D-glucose (20 mM), but not its inactive enantiomer, L-glucose, fully prevented glutamate-mediated delayed ATP depletion, necrosis, and apoptosis. Succinate- cytochrome c reductase activity, but not the activities of NADH-coenzyme Q(1) reductase or cytochrome c oxidase, was inhibited by 32% by glutamate treatment, an effect that was abolished by incubation with D-glucose. Lactate accumulation in the culture medium was unmodified by any of these treatments, ruling out the possible involvement of the glycolysis pathway in either glutamate neurotoxicity or D-glucose neuroprotection. In contrast, D-glucose, but not L-glucose, abolished glutamate-mediated glutathione oxidation and NADPH depletion. Our results suggest that NADPH production from D-glucose accounts for glutathione regeneration and protection from mitochondrial dysfunction. This supports the notion that the activity of the pentose phosphate pathway may be an important factor in protecting neurons against glutamate neurotoxicity.