Neurodegenerative disorders in humans: the role of glutathione in oxidative stress-mediated neuronal death

Oxidative stress has been implicated in both normal aging and in various neurodegenerative disorders and may be a common mechanism underlying various forms of cell death including necrosis, apoptosis, and excitotoxicity. In this review, we develop the hypothesis that oxidative stress-mediated neuronal loss may be initiated by a decline in the antioxidant molecule glutathione (GSH). GSH plays multiple roles in the nervous system including free radical scavenger, redox modulator of ionotropic receptor activity, and possible neurotransmitter. GSH depletion can enhance oxidative stress and may also increase the levels of excitotoxic molecules; both types of action can initiate cell death in distinct neuronal populations. Evidence for a role of oxidative stress and diminished GSH status is presented for Lou Gehrig's disease (ALS), Parkinson's disease, and Alzheimer's disease. Potential links to the Guamanian variant of these diseases (ALS–PD complex) are discussed. In context to the above, we provide a GSH-depletion model of neurodegenerative disorders, suggest experimental verifications of this model, and propose potential therapeutic approaches for preventing or halting these diseases.     

Amyloid beta peptide, 4-hydroxynonenal and apoptosis

Considerable evidence suggests a role for oxidative stress in the pathogenesis of neuron degeneration in several neurodegenerative disorders including Alzheimer's disease (AD). Although debated, increasing evidence suggests that oxidative stress/damage (amyloid beta peptide, iron/hydrogen peroxide) or neurotoxic by-products of lipid peroxidation (4-hydroxy-2-nonenal, acrolein) lead to cell death through apoptosis or programmed cell death in AD. This review discusses current evidence supporting the role of oxidative stress/damage mediated apoptosis in in vitro models of neurodegeneration.     

Effects of EGb 761 Ginkgo biloba extract on mitochondrial function and oxidative stress

As major sources of reactive oxygen species (ROS), mitochondrial structures are exposed to high concentrations of ROS and may therefore be particularly susceptible to oxidative damage. Mitochondrial damage could play a pivotal role in the cell death decision. A decrease in mitochondrial energy charge and redox state, loss of transmembrane potential (depolarization), mitochondrial respiratory chain impairment, and release of substances such as calcium and cytochrome c all contribute to apoptosis. These mitochondrial abnormalities may constitute a part of the spectrum of chronic oxidative stress in Alzheimer's disease. Accumulation of amyloid beta (Abeta) in form of senile plaques is also thought to play a central role in the pathogenesis of Alzheimer's disease mediated by oxidative stress. In addition, increasing evidence shows that Abeta generates free radicals in vitro, which mediate the toxicity of this peptide. In our study, PC12 cells were used to examine the protective features of EGb 761(definition see editorial) on mitochondria stressed with hydrogen peroxide and antimycin, an inhibitor of complex III. In addition, we investigated the efficacy of EGb 761 in Abeta-induced MTT reduction in PC12 cells. Moreover, we examined the effects of EGb 761 on ROS levels and ROS-induced apoptosis in lymphocytes from aged mice after in vivo administration. Here, we will report that EGb 761 was able to protect mitochondria from the attack of hydrogen peroxide, antimycin and Abeta. Furthermore, EGb 761 reduced ROS levels and ROS-induced apoptosis in lymphocytes from aged mice treated orally with EGb 761 for 2 weeks. Our data further emphasize neuroprotective properties of EGb 761, such as protection against Abeta-toxicity, and antiapoptotic properties, which are probably due to its preventive effects on mitochondria.     

Hyperglycemia potentiates carbonyl stress-induced apoptosis in naïve PC-12 cells: relationship to cellular redox and activator protease factor-1 expression

The mechanism(s) of central nervous system complication associated with neurodegenerative disorders such as diabetes is unknown. Previous studies demonstrated that carbonyl stress induced by methylglyoxal (MG) mediates differential apoptosis of rat pheochromocytoma (PC12) cells in the na?ve or differentiated transition states. Since chronic hyperglycemia is central to diabetic complications, and poorly differentiated cells are oxidatively more vulnerable, we currently investigated the effect of glycemic status on MG-induced apoptosis in na?ve (nPC12) cells focusing on glutathione-to-glutathione disulfide (GSH/GSSG) redox signaling. nPC12 cells were exposed to 25 mM glucose acutely for 24h or chronically for 1 week. A role for glycemic fluctuation was tested in chronic high glucose-adapted cells subjected to acute reduction in glucose availability. Acute hyperglycemia potentiated MG-induced nPC12 apoptosis in accordance with cellular redox (GSH-to-Disulfide (GSSG plus protein-bound SSG)) imbalance. Chronic hyperglycemia exacerbated baseline and MG-induced apoptosis that corresponded to exaggerated loss of cytosolic and mitochondrial redox balance, impaired glucose 6-phosphate dehydrogenase (G6PD) activity, and enhanced basal expression of apoptosis protease activator factor-1 (Apaf-1). Reduced glucose availability in hyperglycemia-adapted nPC12 cells induced by acute lowering of glucose or by dehydroepiandrosterone (DHEA, G6PD inhibitor) further enhanced MG-induced apoptosis in association with greater cytosolic and mitochondrial redox and G6PD impairment and elevated basal Apaf-1 expression. These findings demonstrate that chronic hyperglycemia or acute glucose reduction from the chronic hyperglycemic state potentiates carbonyl stress, which collectively contribute to oxidative susceptibility of poorly differentiated cells such as that which occurs in brain neurons of neurodegenerative disorders like diabetes and Alzheimer's disease.     

Apoptosis: a key in neurodegenerative disorders

Apoptosis is an important process in the development of the nervous system. Typically, approximately 50% of the neurons apoptose during neurogenesis before the nervous system matures. However, recent paradigms implicate premature apoptosis and/or aberrations in the fine control of neuronal apoptosis in the pathogenesis of a variety of neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, spinal muscular atrophy, stroke, brain trauma, spinal cord injury, and diabetic neuropathy. This review will focus on the current concepts salient to understanding the apoptosis death program, the mediators and control of cellular apoptosis, and the relationship between aberrant apoptosis and genesis of neurodegenerative disorders. The discussion will also highlight current advances in methodology, such as utilization of neuronal cell lines and mutant animal models, in investigations of neuronal apoptotic death. The knowledge of apoptosis mechanisms could underpin the basis for development of novel therapeutic strategies and treatment modalities that are directed at control of the neuronal apoptotic death program.     

Calcium signaling in the ER: its role in neuronal plasticity and neurodegenerative disorders

Endoplasmic reticulum (ER) is a multifaceted organelle that regulates protein synthesis and trafficking, cellular responses to stress, and intracellular Ca2+ levels. In neurons, it is distributed between the cellular compartments that regulate plasticity and survival, which include axons, dendrites, growth cones and synaptic terminals. Intriguing communication networks between ER, mitochondria and plasma membrane are being revealed that provide mechanisms for the precise regulation of temporal and spatial aspects of Ca2+ signaling. Alterations in Ca2+ homeostasis in ER contribute to neuronal apoptosis and excitotoxicity, and are being linked to the pathogenesis of several different neurodegenerative disorders, including Alzheimer's disease and stroke.     

Glutamate receptors modulate oxidative stress in neuronal cells. A mini-review

Multiple lines of evidence demonstrate that reactive oxygen species (ROS) are involved in regulation of normal cell metabolism as second messengers. Under extreme conditions, these molecules induce oxidative stress, which may stimulate (or accompany) a number of neurodegenerative processes. In the glutamatergic system, ROS levels are under control of ionotropic and metabotropic glutamate receptors, which modulate ion fluxes through the neuronal membrane. The Na+/K(+)-pump is also one of the important participants affecting stationary ROS levels through several distinct mechanisms. This review describes the involvement of the Na+/K(+)-pump in intracellular signaling mechanisms via cross-talk between the pump and glutamate receptors in cerebellum granule cells. Selective dysfunction of mGlu II receptors may also lead to abnormal protein phosphorylation (i.e., tau phosphorylation), culminating in neurodegenerative disorders (i.e., Alzheimer disease). Also, unregulated production of intracellular ROS resulting from an imbalance of ionotropic and metabotropic receptors may activate one or more protein kinases. In summary, Glu receptor dysfunction, leading to a deficit in glutamate-mediated signal transduction may represent one of the earliest stages of neurodegenerative disorders. The Na+/K(+)-pump is able to prevent over-production of intracellular ROS, thus increasing oxidative stability of neuronal cells.     

The role of ER stress-induced apoptosis in neurodegeneration

Post-mortem analyses of human brain tissue samples from patients suffering from neurodegenerative disorders have demonstrated dysfunction of the endoplasmic reticulum (ER). A common characteristic of the aforementioned disorders is the intracellular accumulation and aggregation of proteins due to genetic mutations or exogenous factors, leading to the activation of a stress mechanism known as the unfolded protein response (UPR). This mechanism aims to restore cellular homeostasis, however, if prolonged, can trigger pro-apoptotic signals, which are thought to contribute to neuronal cell death. The authors present evidence to support the role of ER stress-induced apoptosis in Alzheimer's, Parkinson's and Huntington's diseases, and further examine the interplay between ER dyshomeostasis and mitochondrial dysfunction, and the function of reactive oxygen species (ROS) and calcium ions (Ca(2+)) in the intricate relationship between the two organelles. Possible treatments for neurodegenerative diseases that are based on combating ER stress are finally presented.     

Apple juice prevents oxidative stress induced by amyloid-beta in culture

Increased oxidative stress contributes to the decline in cognitive performance during normal aging and in neurodegenerative conditions such as Alzheimer's disease. Dietary supplementation with fruits and vegetables that are high in antioxidant potential have in some cases compensated for oxidative stress. Herein, we examined whether apple juice could alleviate the neurotoxic consequences of exposure of cultured neuronal cells to amyloid-beta (Abeta), since at least a portion of the neurotoxicity of Abeta is due to oxidative stress. Apple juice concentrate (AJC; 70 degree brix) was diluted into culture medium of SH-SY-5Y human neuroblastoma cells that had been differentiated for 7 days with 5 microM retinoic acid concurrent with the addition of 20 microM Abeta. AJC prevented the increased generation of reactive oxygen species (ROS) normally induced by Abeta treatment under these conditions. AJC also prevented Abeta-induced calcium influx and apoptosis, each of which results in part due to increased ROS. These findings suggest that the antioxidant potential of apple products can prevent Abeta-induced oxidative damage.     

Role of mitochondria in neuronal cell death induced by oxidative stress; neuroprotection by Coenzyme Q10

Neuronal cells depend on mitochondrial oxidative phosphorylation for most of their energy needs and therefore are at a particular risk for oxidative stress. Mitochondria play an important role in energy production and oxidative stress-induced apoptosis. In the present study, we have demonstrated that external oxidative stress induces mitochondrial dysfunction leading to increased ROS generation and ultimately apoptotic cell death in neuronal cells. Furthermore, we have investigated the role of Coenzyme Q10 as a neuroprotective agent. Coenzyme Q10 is a component of the mitochondrial respiratory chain and a potent anti-oxidant. Our results indicate that total cellular ROS generation was inhibited by Coenzyme Q10. Further, pre-treatment with Coenzyme Q10 maintained mitochondrial membrane potential during oxidative stress and reduced the amount of mitochondrial ROS generation. Our study suggests that water-soluble Coenzyme Q10 acts by stabilizing the mitochondrial membrane when neuronal cells are subjected to oxidative stress. Therefore, Coenzyme Q10 has the potential to be used as a therapeutic intervention for neurodegenerative diseases.     

Oxidative stress, mitochondrial dysfunction and cellular stress response in Friedreich's ataxia

There is significant evidence that the pathogenesis of several neurodegenerative diseases, including Parkinson's disease, Alzheimer's disease, Friedreich's ataxia (FRDA), multiple sclerosis and amyotrophic lateral sclerosis, may involve the generation of reactive oxygen species (ROS) and/or reactive nitrogen species (RNS) associated with mitochondrial dysfunction. The mitochondrial genome may play an essential role in the pathogenesis of these diseases, and evidence for mitochondria being a site of damage in neurodegenerative disorders is based in part on observed decreases in the respiratory chain complex activities in Parkinson's, Alzheimer's, and Huntington's disease. Such defects in respiratory complex activities, possibly associated with oxidant/antioxidant imbalance, are thought to underlie defects in energy metabolism and induce cellular degeneration. The precise sequence of events in FRDA pathogenesis is uncertain. The impaired intramitochondrial metabolism with increased free iron levels and a defective mitochondrial respiratory chain, associated with increased free radical generation and oxidative damage, may be considered possible mechanisms that compromise cell viability. Recent evidence suggests that frataxin might detoxify ROS via activation of glutathione peroxidase and elevation of thiols, and in addition, that decreased expression of frataxin protein is associated with FRDA. Many approaches have been undertaken to understand FRDA, but the heterogeneity of the etiologic factors makes it difficult to define the clinically most important factor determining the onset and progression of the disease. However, increasing evidence indicates that factors such as oxidative stress and disturbed protein metabolism and their interaction in a vicious cycle are central to FRDA pathogenesis. Brains of FRDA patients undergo many changes, such as disruption of protein synthesis and degradation, classically associated with the heat shock response, which is one form of stress response. Heat shock proteins are proteins serving as molecular chaperones involved in the protection of cells from various forms of stress. In the central nervous system, heat shock protein (HSP) synthesis is induced not only after hyperthermia, but also following alterations in the intracellular redox environment. The major neurodegenerative diseases, Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), multiple sclerosis (MS), Huntington's disease (HD) and FRDA are all associated with the presence of abnormal proteins. Among the various HSPs, HSP32, also known as heme oxygenase I (HO-1), has received considerable attention, as it has been recently demonstrated that HO-1 induction, by generating the vasoactive molecule carbon monoxide and the potent antioxidant bilirubin, could represent a protective system potentially active against brain oxidative injury. Given the broad cytoprotective properties of the heat shock response there is now strong interest in discovering and developing pharmacological agents capable of inducing the heat shock response. This may open up new perspectives in medicine, as molecules inducing this defense mechanism appear to be possible candidates for novel cytoprotective strategies. In particular, manipulation of endogenous cellular defense mechanisms, such as the heat shock response, through nutritional antioxidants, pharmacological compounds or gene transduction, may represent an innovative approach to therapeutic intervention in diseases causing tissue damage, such as neurodegeneration.     

Colostrinin: an oxidative stress modulator for prevention and treatment of age-related disorders

Colostrum-derived proline-rich polypeptide, also known as Colostrinin (CLN), has been shown to have a stabilizing effect on cognitive function in Alzheimer's disease patients. This complex action of CLN could be related to prevention of amyloid-beta peptide aggregation, as shown in in vitro studies, and its impact on delicate cassettes of signaling pathways common to cellular redox regulation, proliferation and differentiation. Studies on cultured cells showed that CLN modulates intracellular levels of reactive oxygen species (ROS), via regulation of glutathione metabolism, activity of antioxidant enzymes and mitochondria function. Due to an improvement in senescence-associated mitochondrial dysfunction and a decrease in ROS generation, CLN decelerates the aging processes of both cultured cells and experimental animals. When given orally to mice, CLN increased the lifespan and improved various motor and sensory activities. Although the molecular basis by which CLN exerts its diverse effects are still under investigation, the regulatory effect on the cellular redox state via maintenance of mitochondrial function and modification of ROS-induced cell signaling seem to be of great importance. In this article, we examine experimental data pertinent to the mechanism of action, including a review of CLN's utility in the maintenance of physiological processes in which oxidative stress has an etiological role.     

Prominent corticosteroid disturbance in experimental prion disease

Prion diseases comprise a group of neurodegenerative disorders that invariably lead to death in affected individuals. The most prominent event in these diseases is a rapid and pronounced neuronal loss, although the cause and the precise mechanisms of neuronal cell death have not been identified so far. Recently, it has been suggested that corticosteroids might play a role in the pathogenesis of neurodegenerative disorders in general, as the regulation of these hormones was found to be disturbed in Alzheimer's and Parkinson's disease, as well as in a transgenic mouse model of Alzheimer's disease. To evaluate the possible corticosteroid disturbances in prion diseases, we determined the concentration of corticosterone metabolites in the faeces of scrapie-inoculated mice during the course of the clinical disease. We observed markedly elevated concentrations of the metabolites during the last 5 weeks of the disease, as well as a severe disturbance of circadian periodicity of corticosterone excretion as much as 2 weeks before this elevation. A simultaneous downregulation of cerebral neuronal glucocorticoid receptors was not detectable by immunohistochemistry, indicating that increased corticosteroids can elicit their effects in mouse scrapie freely. The dysregulation of corticosteroid excretion might act as a further cofactor in the pathogenesis of scrapie, for example by preconditioning nerve cells to disease-immanent neurotoxic stimuli, such as oxidative stress, and to apoptosis.     

Homocysteine elicits a DNA damage response in neurons that promotes apoptosis and hypersensitivity to excitotoxicity.

Elevated plasma levels of the sulfur-containing amino acid homocysteine increase the risk for atherosclerosis, stroke, and possibly Alzheimer's disease, but the underlying mechanisms are unknown. We now report that homocysteine induces apoptosis in rat hippocampal neurons. DNA strand breaks and associated activation of poly-ADP-ribose polymerase (PARP) and NAD depletion occur rapidly after exposure to homocysteine and precede mitochondrial dysfunction, oxidative stress, and caspase activation. The PARP inhibitor 3-aminobenzamide (3AB) protects neurons against homocysteine-induced NAD depletion, loss of mitochondrial transmembrane potential, and cell death, demonstrating a requirement for PARP activation and/or NAD depletion in homocysteine-induced apoptosis. Caspase inhibition accelerates the loss of mitochondrial potential and shifts the mode of cell death to necrosis; inhibition of PARP with 3AB attenuates this effect of caspase inhibition. Homocysteine markedly increases the vulnerability of hippocampal neurons to excitotoxic and oxidative injury in cell culture and in vivo, suggesting a mechanism by which homocysteine may contribute to the pathogenesis of neurodegenerative disorders.     

Oxidative stress signalling in Alzheimer's disease

Multiple lines of evidence demonstrate that oxidative stress is an early event in Alzheimer's disease (AD), occurring prior to cytopathology, and therefore may play a key pathogenic role in the disease. Indeed, that oxidative mechanisms are involved in the cell loss and other neuropathology associated with AD is evidenced by the large number of metabolic signs of oxidative stress as well as by markers of oxidative damage. However, what is intriguing is that oxidative damage decreases with disease progression, such that levels of markers of rapidly formed oxidative damage, which are initially elevated, decrease as the disease progresses to advanced AD. This finding, along with the compensatory upregulation of antioxidant enzymes found in vulnerable neurons in AD, indicates that reactive oxygen species (ROS) not only cause damage to cellular structures but also provoke cellular responses. Mammalian cells respond to extracellular stimuli by transmitting intracellular instructions by signal transduction cascades to coordinate appropriate responses. Therefore, not surprisingly stress-activated protein kinase (SAPK) pathways, pathways that are activated by oxidative stress, are extensively activated during AD. In this paper, we review the evidence of oxidative stress and compensatory responses that occur in AD with a particular focus on the roles and mechanism of activation of SAPK pathways.