Nuclear and mitochondrial DNA oxidation in Alzheimer's disease

The study of Alzheimer's disease neuropathology has been intimately associated with the field of oxidative stress for nearly 20 years. Indeed, increased markers of oxidative stress have been associated with this neurodegenerative condition, resulting from oxidation of lipids, proteins and nucleic acids. Increased nuclear and mitochondrial DNA oxidation are observed in Alzheimer's disease, stemming from increased reactive oxygen species attack to DNA bases and from the impairment of DNA repair mechanisms. Moreover, mitochondrial DNA is found to be more extensively oxidized than nuclear DNA. This review is intended to summarizes the most important cellular reactive oxygen species producers and how mitochondrial dysfunction, redox-active metals dyshomeostasis and NADPH oxidases contribute to increased oxidative stress in Alzheimer's disease. A summary of the antioxidant system malfunction will also be provided. Moreover, we will highlight the mechanisms of DNA oxidation and repair. Importantly, we will discuss evidence relating the DNA repair machinery and accumulated DNA oxidation with Alzheimer's disease.     

Nucleic acid oxidation in Alzheimer disease

Increasing evidence suggests that oxidative stress is intimately associated with Alzheimer disease pathophysiology. Nucleic acids (nuclear DNA, mitochondrial DNA, and RNA) are one of the several cellular macromolecules damaged by reactive oxygen species, particularly the hydroxyl radical. Because neurons are irreplaceable and survive as long as the organism does, they need elaborate defense mechanisms to ensure their longevity. In Alzheimer disease, however, an accumulation of nucleic acid oxidation is observed, indicating an increased level of oxidative stress and/or a decreased capacity to repair the nucleic acid damage. In this review, we present data supporting the notion that mitochondrial and metal abnormalities are key sources of oxidative stress in Alzheimer disease. Furthermore, we outline the mechanisms of nucleic acid oxidation and repair. Finally, evidence showing the occurrence of nucleic acid oxidation in Alzheimer disease will be discussed.     

Quinone reductase (NQO1), a sensitive redox indicator,is increased in Alzheimer's disease

Abstract

NAD(P)H:quinone oxidoreductase 1 (NQO1), a redox-regulated flavoenzyme, plays a central rolein monitoring cellular redox state. NQO1 acts to protect against oxidative stress induced by a variety of metabolic situations, including metabolism of quinones and other xenobiotics, by: (i)functioning as a two electron donor to provide a shunt that competes with the formation of reactive oxygen species; (ii) maintaining reduced coenzyme Q; and (iii) regulating the stress activated kinase pathway. In Alzheimer's disease, while there is abundant evidence for the involvement of oxidative stress, the cause or the consequences are largely unresolved. We suspected that increased NQO1 could signal a major shift in redox balance in Alzheimer's disease and, in this study, found that NQO1 is localized not only to neurofibrillary tangles but also the cytoplasm of hippocampal neurons. By marked contrast, there is very little NQO1 in the same neuronal populations in young and age-matched controls. This novel association of NQO1 further buttresses the nexus of oxidative stress, via free radicals, with selective neuronal vulnerability and also supports a fundamental abnormality in redox balance in Alzheimer's disease.     

Oxidative stress and mitochondrial dysfunction in Alzheimer's disease

Alzheimer's disease (AD) exhibits extensive oxidative stress throughout the body, being detected peripherally as well as associated with the vulnerable regions of the brain affected in disease. Abundant evidence not only demonstrates the full spectrum of oxidative damage to neuronal macromolecules, but also reveals the occurrence of oxidative events early in the course of the disease and prior to the formation of the pathology, which support an important role of oxidative stress in AD. As a disease of abnormal aging, AD demonstrates oxidative damage at levels that significantly surpass that of elderly controls, which suggests the involvement of additional factor(s). Structurally and functionally damaged mitochondria, which are more proficient at producing reactive oxygen species but less so in ATP, are also an early and prominent feature of the disease. Since mitochondria are also vulnerable to oxidative stress, it is likely that a vicious downward spiral involving the interactions between mitochondrial dysfunction and oxidative stress contributes to the initiation and/or amplification of reactive oxygen species that is critical to the pathogenesis of AD. This article is part of a Special Issue entitled: Misfolded Proteins, Mitochondrial Dysfunction and Neurodegenerative Diseases.     

Alzheimer's disease: Effects of β-amyloid on mitochondria

The impairment of the respiratory chain or defects in the detoxification system can decrease electron transfer efficiency, reduce ATP production, and increase reactive oxygen species (ROS) production by mitochondria. Accumulation of ROS results in oxidative stress, a hallmark of neurodegenerative diseases such as Alzheimer's disease (AD). β-amyloid has been implicated in the pathogenesis of AD, and its accumulation may lead to degeneration of neuronal or non-neuronal cells. There is evidence that β-amyloid interacts with mitochondria but little is known concerning the significance of this interaction in the physiopathology of AD. This review explores possible mechanisms of β-amyloid-induced mitochondrial toxicity.     

Oxidative stress and metabolic syndrome

Metabolic syndrome is a collection of cardiometabolic risk factors that includes obesity, insulin resistance, hypertension and dyslipidemia. Although there has been significant debate regarding the criteria and concept of the syndrome, this clustering of risk factors is unequivocally linked to an increased risk of developing type 2 diabetes and cardiovascular disease. Metabolic syndrome is often characterized by oxidative stress, a condition in which an imbalance results between the production and inactivation of reactive oxygen species. Reactive oxygen species can best be described as double-edged swords; while they play an essential role in multiple physiological systems, under conditions of oxidative stress, they contribute to cellular dysfunction. Oxidative stress is thought to play a major role in the pathogenesis of a variety of human diseases, including atherosclerosis, diabetes, hypertension, aging, Alzheimer's disease, kidney disease and cancer. The purpose of this review is to discuss the role of oxidative stress in metabolic syndrome and its major clinical manifestations (namely coronary artery disease, hypertension and diabetes). It will also highlight the effects of lifestyle modification in ameliorating oxidative stress in metabolic syndrome. Discussion will be limited to human data.     

Evaluation of the relationship among gene expressions and enzyme activities with antioxidant role and presenilin 1 expression in Alzheimer's disease

It is known that oxidative stress originating from reactive oxygen species plays a role in the pathogenesis of Alzheimer's disease. In this study, the role of antioxidant status associated with oxidative stress in Alzheimer's disease was investigated. Peripheral blood samples were obtained from 28 healthy individuals (as control) and 28 Alzheimer's patients who met the National Institute of Neurological and Communicative Diseases and Stroke/Alzheimer's Disease and Related Disorders Association criteria. Catalase, glutathione S-transferase and paraoxonase 1 enzyme activities in blood plasma and glutathione S-transferase enzyme activities in erythrocytes were determined by spectrophotometer. Catalase, glutathione S-transferase and presenilin 1 gene expressions in leukocytes were determined using qRT-PCR. Data were analysed with SPSS one-way anova , a LSD post hoc test at p < 0.05. The activity of each enzyme was significantly reduced in Alzheimer's patients compared to control. The catalase gene expression level did not change compared to the control. Glutathione S-transferase and presenilin 1 gene expression levels were increased compared to the control.     

Impairment of Glucose and Glutamate Transport and Induction of Mitochondrial Oxidative Stress and Dysfunction in Synaptosomes by Amyloid β-Peptide: Role of the Lipid Peroxidation Product 4-Hydroxynonenal

Abstract: Deposits of amyloid β-peptide (Aβ), reduced glucose uptake into brain cells, oxidative damage to cellular proteins and lipids, and excitotoxic mechanisms have all been suggested to play roles in the neurodegenerative process in Alzheimer's disease. Synapse loss is closely correlated with cognitive impairments in Alzheimer's disease, suggesting that the synapse may be the site at which degenerative mechanisms are initiated and propagated. We report that Aβ causes oxyradical-mediated impairment of glucose transport, glutamate transport, and mitochondrial function in rat neocortical synaptosomes. Aβ induced membrane lipid peroxidation in synaptosomes that occurred within 1 h of exposure; significant decreases in glucose transport occurred within 1 h of exposure to Aβ and decreased further with time. The lipid peroxidation product 4-hydroxynonenal conjugated to synaptosomal proteins and impaired glucose transport; several antioxidants prevented Aβ-induced impairment of glucose transport, indicating that lipid peroxidation was causally linked to this adverse action of Aβ. FeSO₄ (an initiator of lipid peroxidation), Aβ, and 4-hydroxynonenal each induced accumulation of mitochondrial reactive oxygen species, caused concentration-dependent decreases in 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction, and reduced cellular ATP levels significantly. Aβ also impaired glutamate transport, an effect blocked by antioxidants. These data suggest that Aβ induces membrane lipid peroxidation, which results in impairment of the function of membrane glucose and glutamate transporters, altered mitochondrial function, and a deficit in ATP levels; 4-hydroxynonenal appears to be a mediator of these actions of Aβ. These data suggest that oxidative stress occurring at synapses may contribute to the reduced glucose uptake and synaptic degeneration that occurs in Alzheimer's disease patients. They further suggest a sequence of events whereby oxidative stress promotes excitotoxic synaptic degeneration and neuronal cell death in a variety of different neurodegenerative disorders.     

Mechanism of oxidative DNA damage repair and relevance to human pathology

Since DNA is prone to oxidative attack cells have evolved multiple protective strategies to prevent the deleterious effects of DNA oxidation. Base excision repair is the major mechanism for repair of DNA base damage by reactive oxygen species but recent evidence indicate that nucleotide excision repair proteins, that are mutated in human syndromes, are involved too. The mechanisms of repair dealing with the direct oxidation of DNA will be reviewed taking as prototype the oxidized base 7,8-dihydro-8-hydroxyguanine. The function of the individual repair components as inferred from model mice indicate that the ablation of two gene functions is mostly required to lead to accumulation of oxidative DNA damage, mutagenesis and cancer development. The recent identification of human diseases associated with mutations in oxidative damage repair show that defects in this pathway may lead to increased cancer but their major causative role seems to be in neurological diseases.     

Reactive oxygen species in spermatozoa: methods for monitoring and significance for the origins of genetic disease and infertility

Human spermatozoa generate low levels of reactive oxygen species in order to stimulate key events, such as tyrosine phosphorylation, associated with sperm capacitation. However, if the generation of these potentially pernicious oxygen metabolites becomes elevated for any reason, spermatozoa possess a limited capacity to protect themselves from oxidative stress. As a consequence, exposure of human spermatozoa to intrinsically- or extrinsically- generated reactive oxygen intermediates can result in a state of oxidative stress characterized by peroxidative damage to the sperm plasma membrane and DNA damage to the mitochondrial and nuclear genomes. Oxidative stress in the male germ line is associated with poor fertilization rates, impaired embryonic development, high levels of abortion and increased morbidity in the offspring, including childhood cancer. In this review, we consider the possible origins of oxidative damage to human spermatozoa and reflect on the important contribution such stress might make to the origins of genetic disease in our species.     

DNA damage in telomeres and mitochondria during cellular senescence: is there a connection?

Cellular senescence is the ultimate and irreversible loss of replicative capacity occurring in primary somatic cell culture. It is triggered as a stereotypic response to unrepaired nuclear DNA damage or to uncapped telomeres. In addition to a direct role of nuclear DNA double-strand breaks as inducer of a DNA damage response, two more subtle types of DNA damage induced by physiological levels of reactive oxygen species (ROS) can have a significant impact on cellular senescence: Firstly, it has been established that telomere shortening, which is the major contributor to telomere uncapping, is stress dependent and largely caused by a telomere-specific DNA single-strand break repair inefficiency. Secondly, mitochondrial DNA (mtDNA) damage is closely interrelated with mitochondrial ROS production, and this might also play a causal role for cellular senescence. Improvement of mitochondrial function results in less telomeric damage and slower telomere shortening, while telomere-dependent growth arrest is associated with increased mitochondrial dysfunction. Moreover, telomerase, the enzyme complex that is known to re-elongate shortened telomeres, also appears to have functions independent of telomeres that protect against oxidative stress. Together, these data suggest a self-amplifying cycle between mitochondrial and telomeric DNA damage during cellular senescence.     

Ex vivo protective effects of nicotinamide and 3‐aminobenzamide on rat synaptosomes treated with Aβ(1–42)

Alzheimer's disease (AD) is the most common form of dementia and is characterized by the presence of senile plaques and neurofibrillary tangles, along with synaptic loss. The underlying mechanisms of AD are not clarified yet, but oxidative stress and mitochondrial dysfunction are important factors. Overactivation of poly(adenosine diphosphate ribose) polymerase‐1 (PARP‐1) enzyme has been known to cause neuroinflammation and cell death in neurodegenerative processes. The aim of the present study was to investigate the protective effects of the PARP‐1 inhibitors, 3‐aminobenzamide (3‐AB) and nicotinamide (NA), against amyloid β peptide (1–42) (Aβ(1–42))‐induced oxidative damage and mitochondrial reduction capacity on isolated synaptosomes. Rats were injected intraperitoneally with 3‐AB (30–100 mg kg^?1), NA (100–500 mg kg^?1) or with saline for 7 days. Synaptosomes were incubated with 10–30 μM Aβ(1–42) or saline for 6 h at 37 °C. Ex vivo Aβ(1–42) treatment significantly induced oxidative stress and mitochondrial dysfunction in synaptosomes of the saline group, while synaptosomes of 3‐AB and NA groups showed significant decreases in lipid peroxidation, reactive oxygen species production and protein oxidation. Moreover, both NA and 3‐AB were able to improve the mitochondrial reduction capacity against Aβ(1–42). These data suggest that NA and 3‐AB may have protective effects in neurodegenerative processes because of the reduced levels of oxidative stress and the improvement of mitochondrial function. Copyright © 2014 John Wiley & Sons, Ltd.     

Mitochondrial Telomerase Protects Cancer Cells from Nuclear DNA Damage and Apoptosis

Most cancer cells express high levels of telomerase and proliferate indefinitely. In addition to its telomere maintenance function, telomerase also has a pro-survival function resulting in an increased resistance against DNA damage and decreased apoptosis induction. However, the molecular mechanisms for this protective function remain elusive and it is unclear whether it is connected to telomere maintenance or is rather a non-telomeric function of the telomerase protein, TERT. It was shown recently that the protein subunit of telomerase can shuttle from the nucleus to the mitochondria upon oxidative stress where it protects mitochondrial function and decreases intracellular oxidative stress. Here we show that endogenous telomerase (TERT protein) shuttles from the nucleus into mitochondria upon oxidative stress in cancer cells and analyzed the nuclear exclusion patterns of endogenous telomerase after treatment with hydrogen peroxide in different cell lines. Cell populations excluded TERT from the nucleus upon oxidative stress in a heterogeneous fashion. We found a significant correlation between nuclear localization of telomerase and high DNA damage, while cells which excluded telomerase from the nucleus displayed no or very low DNA damage. We modeled nuclear and mitochondrial telomerase using organelle specific localization vectors and confirmed that mitochondrial localization of telomerase protects the nucleus from inflicted DNA damage and apoptosis while, in contrast, nuclear localization of telomerase correlated with higher amounts of DNA damage and apoptosis. It is known that nuclear DNA damage can be caused by mitochondrially generated reactive oxygen species (ROS). We demonstrate here that mitochondrial localization of telomerase specifically prevents nuclear DNA damage by decreasing levels of mitochondrial ROS. We suggest that this decrease of oxidative stress might be a possible cause for high stress resistance of cancer cells and could be especially important for cancer stem cells.     

Measurement of o- and m-tyrosine as markers of oxidative damage in motor neuron disease

Abstract

Oxidative damage is thought to play an important role in many disease states. Damage caused by reactive oxygen species has been linked to degenerative conditions such as cancer, atherosclerosis, inflammatory disease, lens tissue disease, Alzheimer's disease and motor neurone disease, as well as to the aging process itself. Damage could arise through increased levels of reactive species or through alterations in levels of free radical scavenger molecules such as superoxide dismutase.     

The role of DNA repair in brain related disease pathology

Oxidative DNA damage is implicated in brain aging, neurodegeneration and neurological diseases. Damage can be created by normal cellular metabolism, which accumulates with age, or by acute cellular stress conditions which create bursts of oxidative damage. Brain cells have a particularly high basal level of metabolic activity and use distinct oxidative damage repair mechanisms to remove oxidative damage from DNA and dNTP pools. Accumulation of this damage in the background of a functional DNA repair response is associated with normal aging, but defective repair in brain cells can contribute to neurological dysfunction. Emerging research strongly associates three common neurodegenerative conditions, Alzheimer's, Parkinson's and stroke, with defects in the ability to repair chronic or acute oxidative damage in neurons. This review explores the current knowledge of the role of oxidative damage repair in preserving brain function and highlights the emerging models and methods being used to advance our knowledge of the pathology of neurodegenerative disease.     

Asbestos-induced pulmonary toxicity: role of DNA damage and apoptosis

Asbestos causes asbestosis and various malignancies by mechanisms that are not clearly defined. Here, we review the accumulating evidence showing that asbestos is directly genotoxic by inducing DNA strand breaks (DNA-SB) and apoptosis in relevant lung target cells. Although the exact mechanisms by which asbestos causes DNA damage and apoptosis are not firmly established, some of the implicated mechanisms include the generation of iron-derived reactive oxygen species (ROS) as well as reactive nitrogen species (RNS), alteration in the mitochondrial function, and activation of the death receptor pathway. We focus on the accumulating evidence implicating ROS. DNA repair mechanisms have a key role in limiting the extent of DNA damage. Recent studies show that asbestos activates DNA repair enzymes such as apurinic/apyrimidinic endonuclease (APE) and poly (ADP-ribose) polymerase (PARP). Asbestos-induced neoplastic transformation may result in the setting where DNA damage overwhelms DNA repair in the face of a persistent proliferative signal. Strategies aimed at limiting asbestos-induced oxidative stress may reduce DNA damage and, as such, prevent malignant transformation.     

Overexpression of mitochondrial methionine sulfoxide reductase B2 protects leukemia cells from oxidative stress-induced cell death and protein damage

According to the mitochondrial theory of aging, mitochondrial dysfunction increases intracellular reactive oxidative species production, leading to the oxidation of macromolecules and ultimately to cell death. In this study, we investigated the role of the mitochondrial methionine sulfoxide reductase B2 in the protection against oxidative stress. We report, for the first time, that overexpression of methionine sulfoxide reductase B2 in mitochondria of acute T-lymphoblastic leukemia MOLT-4 cell line, in which methionine sulfoxide reductase A is missing, markedly protects against hydrogen peroxide-induced oxidative stress by scavenging reactive oxygen species. The addition of hydrogen peroxide provoked a time-gradual increase of intracellular reactive oxygen species, leading to a loss in mitochondrial membrane potential and to protein carbonyl accumulation, whereas in methionine sulfoxide reductase B2-overexpressing cells, intracellular reactive oxygen species and protein oxidation remained low with the mitochondrial membrane potential highly maintained. Moreover, in these cells, delayed apoptosis was shown by a decrease in the cleavage of the apoptotic marker poly(ADP-ribose) polymerase-1 and by the lower percentage of Annexin-V-positive cells in the late and early apoptotic stages. We also provide evidence for the protective mechanism of methionine sulfoxide reductase B2 against protein oxidative damages. Our results emphasize that upon oxidative stress, the overexpression of methionine sulfoxide reductase B2 leads to the preservation of mitochondrial integrity by decreasing the intracellular reactive oxygen species build-up through its scavenging role, hence contributing to cell survival and protein maintenance.     

DNA repair in plant mitochondria – a complete base excision repair pathway in potato tuber mitochondria

Mitochondria are one of the major sites of reactive oxygen species (ROS) production in the plant cell. ROS can damage DNA, and this damage is in many organisms mainly repaired by the base excision repair (BER) pathway. We know very little about DNA repair in plants especially in the mitochondria. Combining proteomics, bioinformatics, western blot and enzyme assays, we here demonstrate that the complete BER pathway is found in mitochondria isolated from potato (Solanum tuberosum) tubers. The enzyme activities of three DNA glycosylases and an apurinic/apyrimidinic (AP) endonuclease (APE) were characterized with respect to Mg²⁺ dependence and, in the case of the APE, temperature sensitivity. Evidence for the presence of the DNA polymerase and the DNA ligase, which complete the repair pathway by replacing the excised base and closing the gap, was also obtained. We tested the effect of oxidative stress on the mitochondrial BER pathway by incubating potato tubers under hypoxia. Protein carbonylation increased significantly in hypoxic tuber mitochondria indicative of increased oxidative stress. The activity of two BER enzymes increased significantly in response to this oxidative stress consistent with the role of the BER pathway in the repair of oxidative damage to mitochondrial DNA.