Chava B. Pocernich

Evidence of oxidative damage in Alzheimer's disease brain: central role for amyloid β-peptide

Amyloid beta-peptide (Abeta) is heavily deposited in the brains of Alzheimers disease (AD) patients. Free-radical oxidative stress, particularly of neuronal lipids, proteins and DNA, is extensive in those AD brain areas in which Abeta is abundant. Recent research suggests that these observations might be linked, and it is postulated that Abeta-induced oxidative stress leads to neurodegeneration in AD brain. Consonant with this postulate, Abeta leads to neuronal lipid peroxidation, protein oxidation and DNA oxidation by means that are inhibited by free-radical antioxidants. Here, we summarize current research on phospholipid peroxidation, as well as protein and DNA oxidation, in AD brain, and discuss the potential role of Abeta in this oxidative stress.

Nutritional approaches to combat oxidative stress in Alzheimer’s disease

Alzheimers disease (AD) brains are characterized by extensive oxidative stress. Additionally, large depositions of amyloid beta-peptide (Abeta) are observed, and many researchers opine that Abeta is central to the pathogenesis of AD. Our laboratory combined these two observations in a comprehensive model for neurodegeneration in AD brains centered around Abeta-induced oxidative stress. Given the oxidative stress in AD and its potentially important role in neurodegeneration, considerable research has been conducted on the use of antioxidants to slow or reverse the pathology and course of AD. One source of antioxidants is the diet. This review examines the literature of the effects of endogenous and exogenous, nutritionally-derived antioxidants in relation to AD. In particular, studies of glutathione and other SH-containing antioxidants, vitamins, and polyphenolic compounds and their use in AD and modulation of Abeta-induced oxidative stress and neurotoxicity are reviewed.

The glutamatergic system and Alzheimer's disease: therapeutic implications.

Alzheimer’s disease affects nearly 5 million Americans currently and, as a result of the baby boomer cohort, is predicted to affect 14 million Americans and 22 million persons totally worldwide in just a few decades. Alzheimer’s disease is present in nearly half of individuals aged 85 years.The main symptom of Alzheimer’s disease is a gradual loss of cognitive function. Glutamatergic neurotransmission, an important process in learning and memory, is severely disrupted in patients with Alzheimer’s disease. Loss of glutamatergic function in Alzheimer’s disease may be related to the increase in oxidative stress associated with the amyloid β-peptide that is found in the brains of individuals who have the disease. Therefore, therapeutic strategies directed at the glutamatergic system may hold promise. Therapies addressing oxidative stress induced by hyperactivity of glutamate receptors include supplementation with estrogen and antioxidants such as tocopherol (vitamin E) and acetylcysteine (N-acetylcysteine). Therapy for hypoactivity of glutamate receptors is aimed at inducing the NMDA receptor with glycine and cycloserine (D-cycloserine). Recently, memantine, an NMDA receptor antagonist that addresses the hyperactivity of these receptors, has been approved in some countries for use in Alzheimer’s disease.

In-vivo glutathione elevation protects against hydroxyl free radical-induced protein oxidation in rat brain

Glutathione deficiency has been associated with a number of neurodegenerative diseases including Lou Gehrigs disease, Parkinsons disease, and HIV. A crucial role for glutathione is as a free radical scavenger. Alzheimers disease (AD) brain is characterized by oxidative stress, manifested by protein oxidation, lipid oxidation, oxidized glutathione, and decreased activity of glutathione S-transferase, among others. Reasoning that elevated levels of endogenous glutathione would offer protection against free radical-induced oxidative stress, rodents were given in vivo injections of N-acetylcysteine (NAC), a known precursor of glutathione, to study the vulnerability of isolated synaptosomal membranes treated with Fe2+/H2O2, a known hydroxyl free radical producer. Protein carbonyls, a marker of protein oxidation, were measured. NAC significantly increased endogenous glutathione levels in cortical synaptosome cytosol (P < 0.01). As reported previously, protein carbonyl levels of the Fe2+/H2O2-treated synaptosomes were significantly higher compared to that of non-treated controls (P < 0.01), consistent with increased oxidative stress. In contrast, protein carbonyl levels in Fe2+/H2O2-treated synaptosomes isolated from NAC-injected animals were not significantly different from saline-injected non-treated controls, demonstrating protection against hydroxyl radical induced oxidative stress. These results are consistent with the notion that methods to increase endogenous glutathione levels in neurodegenerative diseases associated with oxidative stress, including AD, may be promising.

Glutathione elevation and its protective role in acrolein-induced protein damage in synaptosomal membranes: relevance to brain lipid peroxidation in neurodegenerative disease

Oxidative stress may be a hallmark of several neurodegenerative disorders, including Alzheimers disease (AD) Huntingtons, and Parkinsons diseases as well as amyotrophic lateral sclerosis. Acrolein is a highly reactive product of lipid peroxidation that is elevated in the brains of persons with AD. This alkenal potentially can react with proteins by Michael addition to alter their structure and function. In the present study, we used electron paramagnetic resonance in conjunction with a protein-specific spin label to monitor synaptosomal membrane protein conformational alterations induced by acrolein. A dose-dependent increased conformational alteration was observed. Consistent with this finding, protein carbonyl levels from protein-bound acrolein were significantly elevated. However, pretreatment of synaptosomes with glutathione ethyl ester (GEE) significantly ameliorated both the conformational alterations and protein carbonyls induced by acrolein. Based on this success, we tested the hypothesis that elevated levels of endogenous glutathione (GSH) would offer protection against acrolein-induced oxidative stress. In-vivo elevation of GSH (215% over control, P<0.04) was produced by i.p. injection of N-acetylcysteine (NAC), a known precursor of GSH. Synaptosomes were treated with vehicle or 2 nM acrolein, the level of this alkenal found in AD brain. In contrast to synaptosomes from control animals, which had significantly increased protein carbonyl levels following addition of 2 nM acrolein, synaptosomes that were isolated from NAC-treated rodents and treated with 2 nM acrolein showed no increased carbonyl levels compared to untreated controls. These results demonstrate protection by increased in-vivo GSH levels against acrolein-induced oxidative stress at levels found in AD brain and are consistent with the notion that methods to increase endogenous GSH levels in neurodegenerative diseases associated with oxidative stress may be promising.

Elevation of Glutathione as a Therapeutic Strategy in Alzheimer Disease

Oxidative stress has been associated with the onset and progression of mild cognitive impairment (MCI) and Alzheimer disease (AD). AD and MCI brain and plasma display extensive oxidative stress as indexed by protein oxidation, lipid peroxidation, free radical formation, DNA oxidation, and decreased antioxidants. The most abundant endogenous antioxidant, glutathione, plays a significant role in combating oxidative stress. The ratio of oxidized to reduced glutathione is utilized as a measure of intensity of oxidative stress. Antioxidants have long been considered as an approach to slow down AD progression. In this review, we focus on the elevation on glutathione through N-acetyl-cysteine (NAC) and γ-glutamylcysteine ethyl ester (GCEE) as a potential therapeutic approach for Alzheimer disease. This article is part of a Special Issue entitled: Antioxidants and Antioxidant Treatment in Disease.

Acrolein inhibits NADH-linked mitochondrial enzyme activity: implications for Alzheimer's disease.

In Alzheimers disease (AD) brain increased lipid peroxidation and decreased energy utilization are found. Mitochondria membranes contain a significant amount of arachidonic and linoleic acids, precursors of lipid peroxidation products, 4-hydroxynonenal (HNE) and 2-propen-1-al (acrolein), that are extremely reactive. Both alkenals are increased in AD brain. In this study, we examined the effects of nanomolar levels of acrolein on the activities pyruvate dehydrogenase (PDH) and α-ketoglutarate dehydrogenase (KGDH), both reduced nicotinamide adenine dinucleotide (NADH)-linked mitochondrial enzymes. Acrolein decreased PDH and KGDH activities significantly in a dose-dependent manner. Using high performance liquid chromatography coupled to mass spectrometry (HPLCMS), acrolein was found to bind lipoic acid, a component in both the PDH and KGDH complexes, most likely explaining the loss of enzyme activity. Acrolein also interacted with oxidized nicotinamide adenine dinucleotide (NAD+) in such a way as to decrease the production of NADH. Acrolein, which is increased in AD brain, may be partially responsible for the dysfunction of mitochondria and loss of energy found in AD brain by inhibition of PDH and KGDH activities, potentially contributing to the neurodegeneration in this disorder.

Methamphetamine toxicity is attenuated in mice that overexpress human manganese superoxide dismutase.

We have investigated methamphetamine (MA) toxicity in transgenic mice that overexpress the human form of mitochondrial manganese superoxide dismutase (MnSOD). Our results reveal a significant reduction in the long-term depletion of striatal dopamine and protein oxidation following repeated administration of MA in transgenic vs. non-transgenic littermates. These findings support the notion that ROS contribute to MA-induced brain damage and suggest that mitochondria may play an important role in this form of neurodegeneration.

Striatal Damage and Oxidative Stress Induced by the Mitochondrial Toxin Malonate Are Reduced in Clorgyline-Treated Rats and MAO-A Deficient Mice

Intrastriatal administration of the succinate dehydrogenase (SDH) inhibitor malonate produces neuronal injury by a “secondary excitotoxic” mechanism involving the generation of reactive oxygen species (ROS). Recent evidence indicates dopamine may contribute to malonate-induced striatal neurodegeneration; infusion of malonate causes a pronounced increase in extracellular dopamine and dopamine deafferentation attenuates malonate toxicity. Inhibition of the catabolic enzyme monoamine oxidase (MAO) also attenuates striatal lesions induced by malonate. In addition to forming 3,4-dihydroxyphenylacetic acid, metabolism of dopamine by MAO generates H2O2, suggesting that dopamine metabolism may be a source of ROS in malonate toxicity. There are two isoforms of MAO, MAO-A and MAO-B. In this study, we have investigated the role of each isozyme in malonate-induced striatal injury using both pharmacological and genetic approaches. In rats treated with either of the specific MAO-A or -B inhibitors, clorgyline or deprenyl, respectively, malonate lesion volumes were reduced by 30% compared to controls. In knock-out mice lacking the MAO-A isoform, malonate-induced lesions were reduced by 50% and protein carbonyls, an index ROS formation, were reduced by 11%, compared to wild-type animals. In contrast, mice deficient in MAO-B showed highly variable susceptibility to malonate toxicity precluding us from determining the precise role of MAO-B in this form of brain damage. These findings indicate that normal levels of MAO-A participate in expression of malonate toxicity by a mechanism involving oxidative stress.

Effects of apolipoprotein E on the human immunodeficiency virus protein Tat in neuronal cultures and synaptosomes

Human immunodeficiency virus type 1 (HIV‐1)‐associated dementia is observed in 20–30% of patients with acquired immunodeficiency syndrome (AIDS). The ϵ4 allele of the apolipoprotein E (APOE) gene currently is thought to play a role as a risk factor for the development of HIV dementia. The HIV protein Tat is neurotoxic and binds to the same receptor as apoE, the low‐density lipoprotein receptor‐related protein (LRP). In this study, we investigated the role apoE plays in Tat toxicity. Synaptosomes from wild‐type mice treated with Tat had increased reactive oxygen species (ROS), increased lipid and protein oxidation, and decreased mitochondrial membrane potential. Synaptosomes from APOE‐knockout mice also had increased ROS, increased protein oxidation, and decreased mitochondrial membrane potential, but to a significantly lesser degree. Treatment of synaptosomes with heparinase and Tat increased Tat‐induced oxidative stress, consistent with the notion of Tat requiring interaction with neuronal membranes to induce oxidative damage. Human lipidated apoE3 greatly protected neurons from Tat‐induced toxicity, whereas human lipidated apoE4 showed no protection. We demonstrated that human apoE3 has antioxidant properties against Tat‐induced toxicity. Taken together, the data suggest that murine apoE and human apoE4 act similarly and do not protect the cell from Tat‐induced toxicity. This would allow excess Tat to remain outside the cell and interact with synaptosomal membranes, leading to oxidative stress and neurotoxicity, which could contribute to dementia associated with HIV. We show that the antioxidant properties of apoE3 greatly outweigh the competition for clearance in deterring Tat‐induced oxidative stress.