Nucleic acid oxidation in Alzheimer disease

Involvement of heat shock protein 70 in the DNA protective effect from estrogen

As an endogenous cytoprotective factor, the protection of estrogen and heat shock protein-70 (Hsp70) on DNA has been documented, respectively, but the functional interaction between estrogen and Hsp70 on DNA damage repair is largely unknown. We therefore investigated the relation between estrogen and Hsp70 in terms of DNA protection in in vitro. The findings showed a significant reduction in cell survival and elevation in oxidative stress while cells were exposed to amyloid β (Aβ25-35) peptide, but preincubation of the cells with 17β-estradiol (17β-E2) ameliorated this situation. In addition, 17β-E2 alleviated oxidized DNA damage induced by Aβ and elevated the expression of Hsp70. However, the beneficial properties of 17β-E2 on reducing DNA damage were attenuated when Hsp70 gene was silenced accordingly. These results indicate that Hsp70 plays a role in DNA protection mediated by estrogen, and the DNA protection may be involved in Alzheimer's disease preventive effect from estrogen.

Effect of aging on 5-hydroxymethylcytosine in the mouse hippocampus

PURPOSE

Aging is believed to affect epigenetic marking of brain DNA with 5-methylcytosine (5mC) and possibly via the 5mC to 5-hydroxymethylcytosine (5hmC) conversion by TET (ten-eleven translocation) enzymes. We investigated the impact of aging on hippocampal DNA 5-hydroxymethylation including in the sequence of aging-susceptible 5-lipoxygenase (5-LOX) gene.

METHODS

Hippocampal samples were obtained from C57BL6 mice. Cellular 5hmC localization was determined by immunofluorescence. The global 5mC and 5hmC contents were measured with the corresponding ELISA. The 5-LOX 5hmC content was measured using a glucosyltransferase/enzymatic restriction digest assay. TET mRNA was measured using qRT-PCR.

RESULTS

Global hippocampal 5hmC content increased during aging as did the 5hmC content in the 5-LOX gene. This occurred without alterations of TET1-3 mRNAs and without changes in the content of 8-hydroxy-2-deoxy-guanosine, a marker of non-enzymatic DNA oxidation.

CONCLUSIONS

The aging-associated increase of hippocampal 5hmC content (global and 5-LOX) appears to be unrelated to oxidative stress. It may be driven by an altered activity but not by the increased expression of the three TET enzymes. Global 5hmC content was increased during aging in the absence of 5mC decrease, suggesting that 5hmC could act as an epigenetic marker and not only as an intermediary in DNA demethylation. Further research is needed to elucidate the functional implications of the impact of aging on hippocampal cytosine hydroxymethylation.

Physiologic brain activity causes DNA double-strand breaks in neurons, with exacerbation by amyloid-β

We show that a natural behavior, exploration of a novel environment, causes DNA double-strand breaks (DSBs) in neurons of young adult wild-type mice. DSBs occurred in multiple brain regions, were most abundant in the dentate gyrus, which is involved in learning and memory, and were repaired within 24 h. Increasing neuronal activity by sensory or optogenetic stimulation increased neuronal DSBs in relevant but not irrelevant networks. Mice transgenic for human amyloid precursor protein (hAPP), which simulate key aspects of Alzheimer's disease, had increased neuronal DSBs at baseline and more severe and prolonged DSBs after exploration. Interventions that suppress aberrant neuronal activity and improve learning and memory in hAPP mice normalized their levels of DSBs. Blocking extrasynaptic NMDA-type glutamate receptors prevented amyloid-β (Aβ)-induced DSBs in neuronal cultures. Thus, transient increases in neuronal DSBs occur as a result of physiological brain activity, and Aβ exacerbates DNA damage, most likely by eliciting synaptic dysfunction.

Modulation of DNA base excision repair during neuronal differentiation

Neurons are terminally differentiated cells with a high rate of metabolism and multiple biological properties distinct from their undifferentiated precursors. Previous studies showed that nucleotide excision DNA repair is downregulated in postmitotic muscle cells and neurons. Here, we characterize DNA damage susceptibility and base excision DNA repair (BER) capacity in undifferentiated and differentiated human neural cells. The results show that undifferentiated human SH-SY5Y neuroblastoma cells are less sensitive to oxidative damage than their differentiated counterparts, in part because they have robust BER capacity, which is heavily attenuated in postmitotic neurons. The reduction in BER activity in differentiated cells correlates with diminished protein levels of key long patch BER components, flap endonuclease-1, proliferating cell nuclear antigen, and ligase I. Thus, because of their higher BER capacity, proliferative neural progenitor cells are more efficient at repairing DNA damage compared with their neuronally differentiated progeny.

Alzheimer's disease-associated neurotoxic peptide amyloid-β impairs base excision repair in human neuroblastoma cells

Alzheimer's disease (AD) is the leading cause of dementia in developed countries. It is characterized by two major pathological hallmarks, one of which is the extracellular aggregation of the neurotoxic peptide amyloid-β (Aβ), which is known to generate oxidative stress. In this study, we showed that the presence of Aβ in a neuroblastoma cell line led to an increase in both nuclear and mitochondrial DNA damage. Unexpectedly, a concomitant decrease in basal level of base excision repair, a major route for repairing oxidative DNA damage, was observed at the levels of both gene expression and protein activity. Moreover, the addition of copper sulfate or hydrogen peroxide, used to mimic the oxidative stress observed in AD-affected brains, potentiates Aβ-mediated perturbation of DNA damage/repair systems in the "Aβ cell line". Taken together, these findings indicate that Aβ could act as double-edged sword by both increasing oxidative nuclear/mitochondrial damage and preventing its repair. The synergistic effects of increased ROS production, accumulated DNA damage and impaired DNA repair could participate in, and partly explain, the massive loss of neurons observed in Alzheimer's disease since both oxidative stress and DNA damage can trigger apoptosis.

Probing oxidative stress: Small molecule fluorescent sensors of metal ions, reactive oxygen species, and thiols

Oxidative stress is a common feature shared by many diseases, including neurodegenerative diseases. Factors that contribute to cellular oxidative stress include elevated levels of reactive oxygen species, diminished availability of detoxifying thiols, and the misregulation of metal ions (both redox-active iron and copper as well as non-redox active calcium and zinc). Deciphering how each of these components interacts to contribute to oxidative stress presents an interesting challenge. Fluorescent sensors can be powerful tools for detecting specific analytes within a complicated cellular environment. Reviewed here are several classes of small molecule fluorescent sensors designed to detect several molecular participants of oxidative stress. We focus our review on describing the design, function and application of probes to detect metal cations, reactive oxygen species, and intracellular thiol-containing compounds. In addition, we highlight the intricacies and complications that are often faced in sensor design and implementation.

Altered transcription factor trafficking in oxidatively-stressed neuronal cells

Age-related neurodegenerative diseases are associated with alterations in gene expression in affected neurons. One of the mechanisms that could account for this is altered subcellular localization of transcription factors, which has been observed in human post-mortem brains of each of the major neurodegenerative diseases, including Parkinson's disease (PD). The specific mechanisms are yet to be elucidated; however a potential mechanism involves alterations in nuclear transport. In this study, we examined the nucleocytoplasmic trafficking of select transcription factors in response to a PD-relevant oxidative injury, 6-hydroxydopamine (6OHDA). Utilizing a well-established model of ligand-regulated nucleocytoplasmic shuttling, the glucocorticoid receptor, we found that 6OHDA selectively impaired nuclear import through an oxidative mechanism without affecting nuclear export or nuclear retention. Interestingly, impaired nuclear import was selective as Nrf2 (nuclear factor E2-related factor 2) nuclear localization remained intact in 6OHDA-treated cells. Thus, oxidative stress specifically impacts the subcellular localization of some but not all transcription factors, which is consistent with observations in post-mortem PD brains. Our data further implicate a role for altered microtubule dependent trafficking in the differential effects of 6OHDA on transcription factor import. Oxidative disruption of microtubule-dependent nuclear transport may contribute to selective declines in transcriptional responses of aging or diseased dopaminergic cells.

A Cell-Permeable Fluorescent Prochelator Responds to Hydrogen Peroxide and Metal Ions by Decreasing Fluorescence

Described here is the development of two boronic ester-based fluorescent prochelators, FloB (2-(6-hydroxy-3-oxo-3H-xanthen-9-yl)-4(5)-[2-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-yl)-benzylidene-hydrazinocarbonyl]-benzoic acid) and FloB-SI (2-(6-hydroxy-3-oxo-3Hxanthen-9-yl)-4(5)-[2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyloxy)-benzylidene-hydrazinocarbonyl]-benzoic acid) that show a fluorescence response to a variety of transition metal ions only after reaction with H(2)O(2). Both prochelators' boronic ester masks are oxidized by H(2)O(2) to reveal a fluorescein-tagged metal chelator, FloS (4(5)-(2-hydroxy-benzylidenehydrazinocarbonyl)-2-(6-hydroxy-3-oxo-3H-xanthen-9-yl)-benzoic acid). Chelation of Fe(3+) or Cu(2+) elicits a 70% decrease in the emission signal of FloS, while Zn(2+), Ni(2+), and Co(2+) produce a more modest fluorescence decrease. The conversion of FloB to FloS proceeds in organic solvents, but hydrolytic decomposition of its hydrazone backbone is observed in aqueous solution. However, FloB-SI oxidizes cleanly with H(2)O(2) within 1 h in aqueous solutions to produce FloS. Fluorescence microscopy studies in HeLa cells with FloB-SI show that the sensor's fluorescence intensity remains unchanged until incubation with exogenous H(2)O(2), which results in a decreased fluorescent signal. Incubation with a competitive chelator restores the emission response, thus suggesting that FloB-SI can effectively report on a H(2)O(2)-induced increase in intracellular labilized metal.

RNA oxidation adducts 8-OHG and 8-OHA change with Aβ42 levels in late-stage Alzheimer's disease

While research supports amyloid-β (Aβ) as the etiologic agent of Alzheimer's disease (AD), the mechanism of action remains unclear. Evidence indicates that adducts of RNA caused by oxidation also represent an early phenomenon in AD. It is currently unknown what type of influence these two observations have on each other, if any. We quantified five RNA adducts by gas chromatography/mass spectroscopy across five brain regions from AD cases and age-matched controls. We then used a reductive directed analysis to compare the RNA adducts to common indices of AD neuropathology and various pools of Aβ. Using data from four disease-affected brain regions (Brodmann's Area 9, hippocampus, inferior parietal lobule, and the superior and middle temporal gyri), we found that the RNA adduct 8-hydroxyguanine (8-OHG) decreased, while 8-hydroxyadenine (8-OHA) increased in AD. The cerebellum, which is generally spared in AD, did not show disease related changes, and no RNA adducts correlated with the number of plaques or tangles. Multiple regression analysis revealed that SDS-soluble Aβ₄₂ was the best predictor of changes in 8-OHG, while formic acid-soluble Aβ₄₂ was the best predictor of changes in 8-OHA. This study indicates that although there is a connection between AD related neuropathology and RNA oxidation, this relationship is not straightforward.

Product and mechanistic analysis of the reactivity of a C6-pyrimidine radical in RNA

Nucleobase radicals are the major reactive intermediates produced when hydroxyl radical reacts with nucleic acids. 5,6-Dihydrouridin-6-yl radical (1) was independently generated from a ketone precursor via Norrish Type I photocleavage in a dinucleotide, single-stranded, and double-stranded RNA. This radical is a model of the major hydroxyl radical adduct of uridine. Tandem lesions resulting from addition of the peroxyl radical derived from 1 to the 5'-adjacent nucleotide are observed by ESI-MS. Radical 1 produces direct strand breaks at the 5'-adjacent nucleotide and at the initial site of generation. The preference for cleavage at these two positions depends upon the secondary structure of the RNA and whether O(2) is present or not. Varying the identity of the 5'-adjacent nucleotide has little effect on strand scission. In general, strand scission is significantly more efficient under anaerobic conditions than when O(2) is present. Strand scission is more than twice as efficient in double-stranded RNA than in a single-stranded oligonucleotide under anaerobic conditions. Internucleotidyl strand scission occurs via β-fragmentation following C2'-hydrogen atom abstraction by 1. The subsequently formed olefin cation radical ultimately yields products containing 3'-phosphate or 3'-deoxy-2'-ketouridine termini. These end groups are proposed to result from competing deprotonation pathways. The dependence of strand scission efficiency from 1 on secondary structure under anaerobic conditions suggests that this reactivity may be useful for extracting additional RNA structural information from hydroxyl radical reactions.

The aging stress response

Aging is the outcome of a balance between damage and repair. The rate of aging and the appearance of age-related pathology are modulated by stress response and repair pathways that gradually decline, including the proteostasis and DNA damage repair networks and mitochondrial respiratory metabolism. Highly conserved insulin/IGF-1, TOR, and sirtuin signaling pathways in turn control these critical cellular responses. The coordinated action of these signaling pathways maintains cellular and organismal homeostasis in the face of external perturbations, such as changes in nutrient availability, temperature, and oxygen level, as well as internal perturbations, such as protein misfolding and DNA damage. Studies in model organisms suggest that changes in signaling can augment these critical stress response systems, increasing life span and reducing age-related pathology. The systems biology of stress response signaling thus provides a new approach to the understanding and potential treatment of age-related diseases.

Evidence of severe mitochondrial oxidative stress and a protective effect of low oxygen in mouse models of inherited photoreceptor degeneration

The role of oxidative stress within photoreceptors (PRs) in inherited photoreceptor degeneration (IPD) is unclear. We investigated this question using four IPD mouse models (Pde6b(rd1/rd1), Pde6b(atrd1/atrd1), Rho(-/-) and Prph2(rds/rds)) and compared the abundance of reduced glutathione (GSH) and the activity of mitochondrial NADH:ubiquinone oxidoreductase (complex I), which is oxidative stress sensitive, as indirect measures of redox status, in the retinas of wild type and IPD mice. All four IPD mutants had significantly reduced retinal complex I activities (14-29% of wild type) and two showed reduced GSH, at a stage prior to the occurrence of significant cell death, whereas mitochondrial citrate synthase, which is oxidative stress insensitive, was unchanged. We orally administered the mitochondrially targeted anti oxidant MitoQ in order to reduce oxidative stress but without any improvement in retinal complex I activity, GSH or rates of PR degeneration. One possible source of oxidative stress in IPDs is oxygen toxicity in the outer retina due to reduced consumption by PR mitochondria. We therefore asked whether a reduction in the ambient O(2) concentration might improve PR survival in Pde6b(rd1/rd1) retinal explants either directly, by reducing reactive oxygen species formation, or indirectly by a neuroprotective mechanism. Pde6b(rd1/rd1) retinal explants cultured in 6% O(2) showed 31% less PR death than normoxic explants. We conclude that (i) mitochondrial oxidative stress is a significant early feature of IPDs; (ii) the ineffectiveness of MitoQ may indicate its inability to reduce some mediators of oxidative stress, such as hydrogen peroxide; and (iii) elucidation of the mechanisms by which hypoxia protects mutant PRs may identify novel neuroprotective pathways in the retina.

Mitochondria and antioxidant targeted therapeutic strategies for Alzheimer's disease

Oxidative stress and mitochondrial dysfunction are important features present in Alzheimer's disease (AD). They appear early and contribute to disease progression, both in human postmortem AD brains as well as in transgenic AD mouse brains. For this reason, targeting oxidative stress and mitochondria in AD may lead to the development of promising therapeutic strategies. Several exogenous antioxidant compounds have been tested and found beneficial in transgenic AD mice, such as vitamins and spices. However, their efficacy was much more modest in human trials. More recently, new strategies have been elaborated to promote endogenous antioxidant systems. Different pathways involved in oxidative stress response have been identified. Compounds able to upregulate these pathways are being generated and tested in animal models of AD and in human patients. Upregulation of antioxidant gene expression was beneficial in mice, giving hope for future avenues in the treatment of AD and other neurodegenerative disorders.

Pro-oxidant diet enhances β/γ secretase-mediated APP processing in APP/PS1 transgenic mice

The etiology of Alzheimer's disease (AD) is complex with oxidative stress being a possible contributory factor to pathogenesis and disease progression. TASTPM transgenic mice expressing familial AD-associated amyloid precursor protein (APPswe) and presenilin transgenes (PS1M146V) show increased brain amyloid beta (Aβ) levels and Aβ plaques from 3 months. We tested if enhancing oxidative stress through diet would accelerate Aβ-related pathology. TASTPM were fed a pro-oxidant diet for 3 months resulting in increased brain levels of protein carbonyls, increased Nrf2, and elevated concentrations of glutathione (GSH). The diet increased both amyloid precursor protein (APP) and Aβ in the cortex of TASTPM but did not alter Aβ plaque load, presenilin 1, or β-secretase (BACE1) expression. TASTPM cortical neurons were cultured under similar pro-oxidant conditions resulting in increased levels of APP and Aβ likely as a result of enhanced β/γ secretase processing of APP. Thus, pro-oxidant conditions increase APP levels and enhance BACE1-mediated APP processing and in doing so might contribute to pathogenesis in AD.

Direct strand scission from a nucleobase radical in RNA

RNA oxidation is important in the etiology of disease and as a tool for studying the structure and folding kinetics of this biopolymer. Nucleobase radicals are the major family of reactive intermediates produced in RNA exposed to diffusible species such as hydroxyl radical. The nucleobase radicals are believed to produce direct strand breaks by abstracting hydrogen atoms from their own and neighboring ribose rings. By independently generating the formal C5 hydrogen atom addition product of uridine in RNA, we provide the first chemical characterization of the pathway for direct strand scission from an RNA nucleobase radical. The process is more efficient under anaerobic conditions. The preference for strand scission in double-stranded RNA over single-stranded RNA suggests that this chemistry may be useful for analyzing the secondary structure of RNA in hydroxyl radical cleavage experiments if they are carried out under anaerobic conditions.

Intersection between metabolic dysfunction, high fat diet consumption, and brain aging

Deleterious neurochemical, structural, and behavioral alterations are a seemingly unavoidable aspect of brain aging. However, the basis for these alterations, as well as the basis for the tremendous variability in regards to the degree to which these aspects are altered in aging individuals, remains to be elucidated. An increasing number of individuals regularly consume a diet high in fat, with high-fat diet consumption known to be sufficient to promote metabolic dysfunction, although the links between high-fat diet consumption and aging are only now beginning to be elucidated. In this review we discuss the potential role for age-related metabolic disturbances serving as an important basis for deleterious perturbations in the aging brain. These data not only have important implications for understanding the basis of brain aging, but also may be important to the development of therapeutic interventions which promote successful brain aging.

Tau Ser262 phosphorylation is critical for Abeta42-induced tau toxicity in a transgenic Drosophila model of Alzheimer's disease

The amyloid-beta 42 (Abeta42) peptide has been suggested to promote tau phosphorylation and toxicity in Alzheimer's disease (AD) pathogenesis; however, the underlying mechanisms are not fully understood. Using transgenic Drosophila expressing both human Abeta42 and tau, we show here that tau phosphorylation at Ser262 plays a critical role in Abeta42-induced tau toxicity. Co-expression of Abeta42 increased tau phosphorylation at AD-related sites including Ser262, and enhanced tau-induced neurodegeneration. In contrast, formation of either sarkosyl-insoluble tau or paired helical filaments was not induced by Abeta42. Co-expression of Abeta42 and tau carrying the non-phosphorylatable Ser262Ala mutation did not cause neurodegeneration, suggesting that the Ser262 phosphorylation site is required for the pathogenic interaction between Abeta42 and tau. We have recently reported that the DNA damage-activated Checkpoint kinase 2 (Chk2) phosphorylates tau at Ser262 and enhances tau toxicity in a transgenic Drosophila model. We detected that expression of Chk2, as well as a number of genes involved in DNA repair pathways, was increased in the Abeta42 fly brains. The induction of a DNA repair response is protective against Abeta42 toxicity, since blocking the function of the tumor suppressor p53, a key transcription factor for the induction of DNA repair genes, in neurons exacerbated Abeta42-induced neuronal dysfunction. Our results demonstrate that tau phosphorylation at Ser262 is crucial for Abeta42-induced tau toxicity in vivo, and suggest a new model of AD progression in which activation of DNA repair pathways is protective against Abeta42 toxicity but may trigger tau phosphorylation and toxicity in AD pathogenesis.

Role of mitochondrial-mediated signaling pathways in Alzheimer disease and hypoxia

Development of effective treatments for Alzheimer's disease is complicated by the poor understanding of its pathophysiology. Recent work suggests mitochondria may play a primary role in neurodegeneration, due to alterations in mitochondria turnover and that the brain is specifically susceptible, due to high energy demand. Mitochondria are the major source of cellular energy through oxidative phosphorylation and regulate intracellular calcium levels and survival pathways. Hypoxia has been implicated in several neurodegenerative diseases including Alzheimer's disease. During hypoxic events, mitochondrial complex III produces high levels of reactive oxygen species (ROS). These ROS seem to have a primary role in the regulation of the transcription factor hypoxia inducible factor 1alpha that triggers death effectors. Here we discuss the role of mitochondria in AD putting focus on the activation of hypoxia-mediated mitochondrial pathways, which could eventually lead to cell degeneration and death.

Oxidative stress in the progression of Alzheimer disease in the frontal cortex

We investigated oxidative stress in human postmortem frontal cortexfrom individuals characterized as mild cognitive impairment (n= 8), mild/moderate Alzheimer disease (n = 4), and late-stage Alzheimer disease (n = 9). Samples from subjects with no cognitive impairment (n = 10) that were age- and postmortem interval-matched with these cases were used as controls. The short postmortem intervalbrain samples were processed for postmitochondrial supernatant, nonsynaptic mitochondria, and synaptosome fractions. Samples were analyzed for several antioxidants (glutathione, glutathione peroxidase, glutathione reductase, glutathione-S-transferase, glucose-6-phosphate dehydrogenase, superoxide dismutase, catalase) and the oxidative marker, thiobarbituric acid reactive substances. The tissue was also analyzed for possible changes in protein damage using neurochemical markers for protein carbonyls, 3-nitrotyrosine, 4-hydroxynonenal, andacrolein. All 3 neuropil fractions (postmitochondrial supernatant, mitochondrial, and synaptosomal) demonstrated significant disease-dependent increases in oxidative markers. The highest changes were observed in the synaptosomal fraction. Both mitochondrial and synaptosomal fractions had significant declines in antioxidants (glutathione, glutathione peroxidase, glutathione-S-transferase, and superoxide dismutase). Levels of oxidative markers significantly correlated with Mini-Mental Status Examination scores. Oxidative stress was more localized to the synapses, with levels increasing in a disease-dependent fashion. These correlations implicate an involvement of oxidative stress in Alzheimer disease-related synaptic loss.

A DNA damage-activated checkpoint kinase phosphorylates tau and enhances tau-induced neurodegeneration

Hyperphosphorylation of the microtubule associated protein tau is detected in the brains of individuals with a range of neurodegenerative diseases including Alzheimer's disease (AD). An imbalance in phosphorylation and/or dephosphorylation of tau at disease-related sites has been suggested to initiate the abnormal metabolism and toxicity of tau in disease pathogenesis. However, the mechanisms underlying abnormal phosphorylation of tau in AD are not fully understood. Here, we show that the DNA damage-activated Checkpoint kinase 2 (Chk2) is a novel tau kinase and enhances tau toxicity in a transgenic Drosophila model. Overexpression of Drosophila Chk2 increases tau phosphorylation at Ser262 and enhances tau-induced neurodegeneration in transgenic flies expressing human tau. The non-phosphorylatable Ser262Ala mutation abolishes Chk2-induced enhancement of tau toxicity, suggesting that the Ser262 phosphorylation site is involved in the enhancement of tau toxicity by Chk2. In vitro kinase assays revealed that human Chk2 and a closely related checkpoint kinase 1 (Chk1) directly phosphorylate human tau at Ser262. We also demonstrate that Drosophila Chk2 does not modulate the activity of the fly homolog of microtubule affinity regulating kinase, which has been shown to be a physiological tau Ser262 kinase. Since accumulation of DNA damage has been detected in the brains of AD patients, our results suggest that the DNA damage-activated kinases Chk1 and Chk2 may be involved in tau phosphorylation and toxicity in the pathogenesis of AD.

Intrusion of a DNA repair protein in the RNome world: is this the beginning of a new era?

Apurinic/apyrimidinic endonuclease 1 (APE1), an essential protein in mammals, is known to be involved in base excision DNA repair, acting as the major abasic endonuclease; the protein also functions as a redox coactivator of several transcription factors that regulate gene expression. Recent findings highlight a novel role for APE1 in RNA metabolism. The new findings are as follows: (i) APE1 interacts with rRNA and ribosome processing protein NPM1 within the nucleolus; (ii) APE1 interacts with proteins involved in ribosome assembly (i.e., RLA0, RSSA) and RNA maturation (i.e., PRP19, MEP50) within the cytoplasm; (iii) APE1 cleaves abasic RNA; and (iv) APE1 cleaves a specific coding region of c-myc mRNA in vitro and influences c-myc mRNA level and half-life in cells. Such findings on the role of APE1 in the posttranscriptional control of gene expression could explain its ability to influence diverse biological processes and its relocalization to cytoplasmic compartments in some tissues and tumors. In addition, we propose that APE1 serves as a "cleansing" factor for oxidatively damaged abasic RNA, establishing a novel connection between DNA and RNA surveillance mechanisms. In this review, we introduce questions and speculations concerning the role of APE1 in RNA metabolism and discuss the implications of these findings in a broader evolutionary context.

Mitochondria: a therapeutic target in neurodegeneration

Mitochondrial dysfunction has long been associated with neurodegenerative disease. Therefore, mitochondrial protective agents represent a unique direction for the development of drug candidates that can modify the pathogenesis of neurodegeneration. This review discusses evidence showing that mitochondrial dysfunction has a central role in the pathogenesis of Alzheimer's, Parkinson's and Huntington's diseases and amyotrophic lateral sclerosis. We also debate the potential therapeutic efficacy of metabolic antioxidants, mitochondria-directed antioxidants and Szeto-Schiller (SS) peptides. Since these compounds preferentially target mitochondria, a major source of oxidative damage, they are promising therapeutic candidates for neurodegenerative diseases. Furthermore, we will briefly discuss the novel action of the antihistamine drug Dimebon on mitochondria.

Studies on genomic DNA stability in aluminium-maltolate treated aged new zealand rabbit: relevance to the alzheimers animal model

Background

Alzheimers disease (AD) is a devastative neurodegenerative disorder. Lack of substantial animal model that can unravel molecular underpinnings has been a major lacuna which limited the understanding of the etiology of the disease in turn limiting the employment of potential therapeutic strategies to combat the disease for a few decades. Our studies for the first time provided substantial animal model and tattered the etiology of the disease at a molecular level.

Methods

In this study DNA was isolated from Hippocampus (H), Midbrain (M) and Frontal Cortex (Fc) of control and aluminium maltolate (Al-M) treated aged New Zealand rabbit brain. DNA damage has been studied using Agarose gel electrophoresis, Ethidium Bromide (EtBr) binding and Melting temperature techniques.

Results

Al-M treated aged New Zealand rabbit's H and M showed higher DNA damage compared to corresponding controls, where as Fc showed mild DNA damage compared to corresponding controls.

Conclusions

This study tangibly provides substantial molecular level understanding of the disease in turn providing an adequate platform to streamline potential therapeutic strategies.

Keywords

Alzheimer’s disease; Aluminium maltolate; Animal model; DNA damage

Biomarkers of oxidative and nitrosative damage in Alzheimer's disease and mild cognitive impairment

Alzheimer's disease (AD) is the most common type of dementia in the elderly. Products of oxidative and nitrosative stress (OS and NS, respectively) accumulate with aging, which is the main risk factor for AD. This provides the basis for the involvement of OS and NS in AD pathogenesis. OS and NS occur in biological systems due to the dysregulation of the redox balance, caused by a deficiency of antioxidants and/or the overproduction of free radicals. Free radical attack against lipids, proteins, sugars and nucleic acids leads to the formation of bioproducts whose detection in fluids and tissues represents the currently available method for assessing oxidative/nitrosative damage. Post-mortem and in-vivo studies have demonstrated an accumulation of products of free radical damage in the central nervous system and in the peripheral tissues of subjects with AD or mild cognitive impairment (MCI). In addition to their individual role, biomarkers for OS and NS in AD are associated with altered bioenergetics and amyloid-beta (Abeta) metabolism. In this review we discuss the main results obtained in the field of biomarkers of oxidative/nitrosative stress in AD and MCI in humans, in addition to their potential role as a tool for diagnosis, prognosis and treatment efficacy in AD.

Photochemical generation and reactivity of the 5,6-dihydrouridin-6-yl radical

Nucleobase radicals are the major family of reactive intermediates formed when nucleic acids are exposed to hydroxyl radical, which is produced by gamma-radiolysis and Fe.EDTA. Significant advances have been made in understanding the role of nucleobase radicals in oxidative DNA damage by independently generating these species from photochemical precursors. However, this approach has been used much less frequently to study RNA molecules. Norrish type I photocleavage of the tert-butyl ketone (2b) enabled studying the reactivity of 5'-benzoyl-5,6-dihydrouridin-6-yl (1b). High mass balances were observed under aerobic or anaerobic conditions, and O(2) did not affect the photochemical conversion of the ketone (2b) to 1b. Competition studies with O(2) indicate that the radical abstracts hydrogen atoms from beta-mercaptoethanol with a bimolecular rate constant = 2.6 +/- 0.5 x 10(6) M(-1)s(-1). The major product formed in the presence of O(2) was 5'-benzoyl-6-hydroxy-5,6-dihydrouridine (6). In contrast, 5-benzoyl-ribonolactone (7), a hypothetical product resulting from C1'-hydrogen atom abstraction by the peroxyl radical, could not be detected. Overall, tert-butyl ketone 2b is a clean source of 5'-benzoyl-5,6-dihydrouridin-6-yl (1b) and should prove useful for studying the reactivity of the respective radical in RNA.

RNA oxidation in Alzheimer disease and related neurodegenerative disorders

RNA oxidation and its biological effects are less well studied compared to DNA oxidation. However, RNA may be more susceptible to oxidative insults than DNA, for RNA is largely single-stranded and its bases are not protected by hydrogen bonding and less protected by specific proteins. Also, cellular RNA locates in the vicinity of mitochondria, the primary source of reactive oxygen species. Oxidative modification can occur not only in protein-coding RNAs, but also in non-coding RNAs that have been recently revealed to contribute towards the complexity of the mammalian brain. Damage to coding and non-coding RNAs will cause errors in proteins and disturbances in the regulation of gene expression. While less lethal than mutations in the genome and not inheritable, such sublethal damage to cells might be associated with underlying mechanisms of degeneration, especially age-associated neurodegeneration that is commonly found in the elderly population. Indeed, oxidative RNA damage has been described recently in most of the common neurodegenerative disorders including Alzheimer disease, Parkinson disease, dementia with Lewy bodies and amyotrophic lateral sclerosis. Of particular interest, the accumulating evidence obtained from studies on either human samples or experimental models coincidentally suggests that oxidative RNA damage is a feature in vulnerable neurons at early-stage of these neurodegenerative disorders, indicating that RNA oxidation actively contributes to the onset or the development of the disorders. Further investigations aimed at understanding of the processing mechanisms related to oxidative RNA damage and its consequences may provide significant insights into the pathogenesis of neurodegenerative disorders and lead to better therapeutic strategies.

RNA under attack: cellular handling of RNA damage

Damage to RNA from ultraviolet light, oxidation, chlorination, nitration, and akylation can include chemical modifications to nucleobases as well as RNA-RNA and RNA-protein crosslinking. In vitro studies have described a range of possible damage products, some of which are supported as physiologically relevant by in vivo observations in normal growth, stress conditions, or disease states. Damage to both messenger RNA and noncoding RNA may have functional consequences, and work has begun to elucidate the role of RNA turnover pathways and specific damage recognition pathways in clearing cells of these damaged RNAs.

Epigenetics, oxidative stress, and Alzheimer disease

Alzheimer disease (AD) is a progressive neurodegenerative disorder whose clinical manifestations appear in old age. The sporadic nature of 90% of AD cases, the differential susceptibility to and course of the illness, as well as the late age onset of the disease suggest that epigenetic and environmental components play a role in the etiology of late-onset AD. Animal exposure studies demonstrated that AD may begin early in life and may involve an interplay between the environment, epigenetics, and oxidative stress. Early life exposure of rodents and primates to the xenobiotic metal lead (Pb) enhanced the expression of genes associated with AD, repressed the expression of others, and increased the burden of oxidative DNA damage in the aged brain. Epigenetic mechanisms that control gene expression and promote the accumulation of oxidative DNA damage are mediated through alterations in the methylation or oxidation of CpG dinucleotides. We found that environmental influences occurring during brain development inhibit DNA-methyltransferases, thus hypomethylating promoters of genes associated with AD such as the beta-amyloid precursor protein (APP). This early life imprint was sustained and triggered later in life to increase the levels of APP and amyloid-beta (Abeta). Increased Abeta levels promoted the production of reactive oxygen species, which damage DNA and accelerate neurodegenerative events. Whereas AD-associated genes were overexpressed late in life, others were repressed, suggesting that these early life perturbations result in hypomethylation as well as hypermethylation of genes. The hypermethylated genes are rendered susceptible to Abeta-enhanced oxidative DNA damage because methylcytosines restrict repair of adjacent hydroxyguanosines. Although the conditions leading to early life hypo- or hypermethylation of specific genes are not known, these changes can have an impact on gene expression and imprint susceptibility to oxidative DNA damage in the aged brain.

Reduction of oxidative stress, amyloid deposition, and memory deficit by manganese superoxide dismutase overexpression in a transgenic mouse model of Alzheimer's disease

In Alzheimer's disease (AD), oxidative stress is present early and contributes to disease pathogenesis. We previously reported that in Tg19959 transgenic AD mice, partial deficiency of the mitochondrial antioxidant enzyme manganese superoxide dismutase (MnSOD) exacerbated amyloid pathology. We therefore asked whether MnSOD overexpression would prove beneficial against AD pathogenesis, by studying the offspring of Tg19959 mice crossed with MnSOD-overexpressing mice. At 4 mo of age, there was a 2- to 3-fold increase in MnSOD protein levels in Tg19959-MnSOD mice compared to Tg19959 littermates. Tg19959-MnSOD mice also had a 50% increase in catalase protein levels, a 50% decrease in levels of oxidized protein, and a 33% reduction in cortical plaque burden compared to Tg19959 littermates. Spatial memory was impaired and synaptophysin levels were decreased in Tg19959 mice compared to wild-type littermates, but memory and synaptophysin levels were restored to wild-type levels in Tg19959-MnSOD littermates. These benefits occurred without changes in sodium dodecyl sulfate-soluble or formic acid-soluble Abeta pools or Abeta oligomers in Tg19959-MnSOD mice compared to Tg19959 littermates. These data demonstrate that facilitation of the mitochondrial antioxidant response improves resistance to Abeta, slows plaque formation or increases plaque degradation, and markedly attenuates the phenotype in a transgenic AD mouse model.

Triterpenoid CDDO-methylamide improves memory and decreases amyloid plaques in a transgenic mouse model of Alzheimer's disease

Oxidative stress is one of the earliest events in the pathogenesis of Alzheimer's disease (AD) and can markedly exacerbate amyloid pathology. Modulation of antioxidant and anti-inflammatory pathways represents an important approach for AD therapy. Synthetic triterpenoids have been found to facilitate antioxidant response and reduce inflammation in several models. We investigated the effect of the triterpenoid, 2-Cyano-3,12-Dioxooleana-1,9-Dien-28-Oic acid-MethylAmide (CDDO-MA) in Tg19959 mice, which carry the human amyloid precursor protein with two mutations. These mice develop memory impairments and amyloid plaques as early as 2-3 months of age. CDDO-MA was provided with chow (800 mg/kg) from 1 to 4 months of age. CDDO-MA significantly improved spatial memory retention and reduced plaque burden, Abeta42 levels, microgliosis, and oxidative stress in Tg19959 mice.

APE1/Ref-1 interacts with NPM1 within nucleoli and plays a role in the rRNA quality control process

APE1/Ref-1 (hereafter, APE1), a DNA repair enzyme and a transcriptional coactivator, is a vital protein in mammals. Its role in controlling cell growth and the molecular mechanisms that fine-tune its different cellular functions are still not known. By an unbiased proteomic approach, we have identified and characterized several novel APE1 partners which, unexpectedly, include a number of proteins involved in ribosome biogenesis and RNA processing. In particular, a novel interaction between nucleophosmin (NPM1) and APE1 was characterized. We observed that the 33 N-terminal residues of APE1 are required for stable interaction with the NPM1 oligomerization domain. As a consequence of the interaction with NPM1 and RNA, APE1 is localized within the nucleolus and this localization depends on cell cycle and active rRNA transcription. NPM1 stimulates APE1 endonuclease activity on abasic double-stranded DNA (dsDNA) but decreases APE1 endonuclease activity on abasic single-stranded RNA (ssRNA) by masking the N-terminal region of APE1 required for stable RNA binding. In APE1-knocked-down cells, pre-rRNA synthesis and rRNA processing were not affected but inability to remove 8-hydroxyguanine-containing rRNA upon oxidative stress, impaired translation, lower intracellular protein content, and decreased cell growth rate were found. Our data demonstrate that APE1 affects cell growth by directly acting on RNA quality control mechanisms, thus affecting gene expression through posttranscriptional mechanisms.

Antioxidants have a rapid and long-lasting effect on neuritic abnormalities in APP:PS1 mice

Senile plaques are a major pathological hallmark of Alzheimer's disease (AD). Compelling evidence suggests that senile plaques lead to structural alterations of neuronal processes and that local toxicity may be mediated by increased oxidative stress. Anti-oxidant therapy can alleviate the neuronal abnormalities in APP mice, but the time-course of this beneficial effect is unknown. We used multiphoton microscopy to assess in vivo the characteristics of antioxidant treatment on senile plaques and neurites in AD model mice (APPswe/PS1dE9). We observed that α-phenyl-N-tert-butyl nitrone (PBN), Ginkgo biloba extract (EGb 761) and Trolox had no effect on the size of existing senile plaques. However, all anti-oxidants had a straightening effect on curved neurites. This effect was detected as soon as 4 days after commencing the treatment, and was maintained after 1 month of daily treatment, with no further increase in the effect. The straightening of neurites persisted 15 days after stopping the treatment. These data indicate that neuronal plasticity is fast and still active in adult animals, and suggest that amelioration of the neuritic distortions associated with senile plaques with antioxidants is both rapid and long lasting.

Diet-induced metabolic disturbances as modulators of brain homeostasis

A number of metabolic disturbances occur in response to the consumption of a high fat western diet. Such metabolic disturbances can include the progressive development of hyperglycemia, hyperinsulemia, obesity, metabolic syndrome, and diabetes. Cumulatively, diet-induced disturbances in metabolism are known to promote increased morbidity and negatively impact life expectancy through a variety of mechanisms. While the impact of metabolic disturbances on the hepatic, endocrine, and cardiovascular systems is well established there remains a noticeable void in understanding the basis by which the central nervous system (CNS) becomes altered in response to diet-induced metabolic dysfunction. In particular, it remains to be fully elucidated which established features of diet-induced pathogenesis (observed in non-CNS tissues) are recapitulated in the brain, and identification as to whether the observed changes in the brain are a direct or indirect effect of peripheral metabolic disturbances. This review will focus on each of these key issues and identify some critical experimental questions which remain to be elucidated experimentally, as well as provide an outline of our current understanding for how diet-induced alterations in metabolism may impact the brain during aging and age-related diseases of the nervous system.

Protein binding has a large effect on radical mediated DNA damage

Oxidative DNA damage is important in aging and a variety of diseases. Significant advances have been made in our understanding of the chemistry of radical mediated DNA damage. These studies have been carried out on DNA in the absence of proteins. However, in cells DNA is typically bound by proteins such as in chromatin and transiently by proteins that regulate biochemical processes. How and whether protein binding affects DNA radical reactivity is not well understood. The effect of the DNA binding protein Hbb on the reactivity of the 5-(2'-deoxyuridinyl)methyl radical (1) and 5-(2'-deoxycytidinyl)methyl radical (2) was studied. Hbb bends DNA and disrupts base stacking at the sites of kinking. The reactivity of 1 and 2 are significantly affected when they are generated at the kinking site in the presence of Hbb. The increased conformational mobility of the radicals results in significantly higher yields of DNA interstrand cross-links. These studies provide the first specific data on how protein binding affects the reactivity of a DNA radical and bring us closer to understanding oxidative DNA damage in cells.

Sublethal RNA oxidation as a mechanism for neurodegenerative disease

Although cellular RNA is subjected to the same oxidative insults as DNA and other cellular macromolecules, oxidative damage to RNA has not been a major focus in investigations of the biological consequences of free radical damage. In fact, because it is largely single-stranded and its bases lack the protection of hydrogen bonding and binding by specific proteins, RNA may be more susceptible to oxidative insults than is DNA. Oxidative damage to protein-coding RNA or non-coding RNA will, in turn, potentially cause errors in proteins and/or dysregulation of gene expression. While less lethal than mutations in the genome, such sublethal insults to cells might be associated with underlying mechanisms of several chronic diseases, including neurodegenerative disease. Recently, oxidative RNA damage has been described in several neurodegenerative diseases including Alzheimer disease, Parkinson disease, dementia with Lewy bodies, and prion diseases. Of particular interest, oxidative RNA damage can be demonstrated in vulnerable neurons early in disease, suggesting that RNA oxidation may actively contribute to the onset of the disease. An increasing body of evidence suggests that, mechanistically speaking, the detrimental effects of oxidative RNA damage to protein synthesis are attenuated, at least in part, by the existence of protective mechanisms that prevent the incorporation of the damaged ribonucleotides into the translational machinery. Further investigations aimed at understanding the processing mechanisms related to oxidative RNA damage and its consequences may provide significant insights into the pathogenesis of neurodegenerative and other degenerative diseases and lead to better therapeutic strategies.