Expression of hMTH1 in the hippocampi of control and Alzheimer's disease

The two faces of DNA oxidation in genomic and functional mosaicism during aging in human neurons

Maintaining genomic integrity in post-mitotic neurons in the human brain is paramount because these cells must survive for an individual's entire lifespan. Due to life-long synaptic plasticity and electrochemical transmission between cells, the brain engages in an exceptionally high level of mitochondrial metabolic activity. This activity results in the generation of reactive oxygen species with 8-oxo-7,8-dihydroguanine (8-oxoG) being one of the most prevalent oxidation products in the cell. 8-oxoG is important for the maintenance and transfer of genetic information into proper gene expression: a low basal level of 8-oxoG plays an important role in epigenetic modulation of neurodevelopment and synaptic plasticity, while a dysregulated increase in 8-oxoG damages the genome leading to somatic mutations and transcription errors. The slow yet persistent accumulation of DNA damage in the background of increasing cellular 8-oxoG is associated with normal aging as well as neurological disorders such as Alzheimer's disease and Parkinson's disease. This review explores the current understanding of how 8-oxoG plays a role in brain function and genomic instability, highlighting new methods being used to advance pathological hallmarks that differentiate normal healthy aging and neurodegenerative disease.

Structural Insights into the Specificity of 8-Oxo-7,8-dihydro-2′-deoxyguanosine Bypass by Family X DNA Polymerases

8-oxo-guanine (8OG) is a common base lesion, generated by reactive oxygen species, which has been associated with human diseases such as cancer, aging-related neurodegenerative disorders and atherosclerosis. 8OG is highly mutagenic, due to its dual-coding potential it can pair both with adenine or cytidine. Therefore, it creates a challenge for DNA polymerases striving to correctly replicate and/or repair genomic or mitochondrial DNA. Numerous structural studies provide insights into the mechanistic basis of the specificity of 8OG bypass by DNA polymerases from different families. Here, we focus on how repair polymerases from Family X (Pols β, λ and µ) engage DNA substrates containing the oxidized guanine. We review structures of binary and ternary complexes for the three polymerases, which represent distinct steps in their catalytic cycles—the binding of the DNA substrate and the incoming nucleotide, followed by its insertion and extension. At each of these steps, the polymerase may favor or exclude the correct C or incorrect A, affecting the final outcome, which varies depending on the enzyme.

MUTYH Actively Contributes to Microglial Activation and Impaired Neurogenesis in the Pathogenesis of Alzheimer's Disease

Oxidative stress is a major risk factor for Alzheimer's disease (AD), which is characterized by brain atrophy, amyloid plaques, neurofibrillary tangles, and loss of neurons. 8-Oxoguanine, a major oxidatively generated nucleobase highly accumulated in the AD brain, is known to cause neurodegeneration. In mammalian cells, several enzymes play essential roles in minimizing the 8-oxoguanine accumulation in DNA. MUTYH with adenine DNA glycosylase activity excises adenine inserted opposite 8-oxoguanine in DNA. MUTYH is reported to actively contribute to the neurodegenerative process in Parkinson and Huntington diseases and some mouse models of neurodegenerative diseases by accelerating neuronal dysfunction and microgliosis under oxidative conditions; however, whether or not MUTYH is involved in AD pathogenesis remains unclear. In the present study, we examined the contribution of MUTYH to the AD pathogenesis. Using postmortem human brains, we showed that various types of MUTYH transcripts and proteins are expressed in most hippocampal neurons and glia in both non-AD and AD brains. We further introduced MUTYH deficiency into App ^{NL-G-F/NL-G-F} knock-in AD model mice, which produce humanized toxic amyloid-β without the overexpression of APP protein, and investigated the effects of MUTYH deficiency on the behavior, pathology, gene expression, and neurogenesis. MUTYH deficiency improved memory impairment in App ^{NL-G-F/NL-G-F} mice, accompanied by reduced microgliosis. Gene expression profiling strongly suggested that MUTYH is involved in the microglial response pathways under AD pathology and contributes to the phagocytic activity of disease-associated microglia. We also found that MUTYH deficiency ameliorates impaired neurogenesis in the hippocampus, thus improving memory impairment. In conclusion, we propose that MUTYH, which is expressed in the hippocampus of AD patients as well as non-AD subjects, actively contributes to memory impairment by inducing microgliosis with poor neurogenesis in the preclinical AD phase and that MUTYH is a novel therapeutic target for AD, as its deficiency is highly beneficial for ameliorating AD pathogenesis.

Deficiency of MTH1 and/or OGG1 increases the accumulation of 8-oxoguanine in the brain of the AppNL-G-F/NL-G-F knock-in mouse model of Alzheimer's disease, accompanied by accelerated microgliosis and reduced anxiety-like behavior

Oxidative stress is a major risk factor for Alzheimer's disease (AD). Among various oxidized molecules, the marked accumulation of an oxidized form of guanine, 8-oxo-7,8-dihydroguanine (8-oxoG), is observed in the AD brain. 8-oxo-2'-deoxyguanosine triphosphatase (MTH1) and 8-oxoG DNA glycosylase (OGG1) minimize the 8-oxoG accumulation in DNA, and their expression is decreased in the AD brain. MTH1 and/or OGG1 may suppress the pathogenesis of AD; however, their exact roles remain unclear. We evaluated the roles of MTH1 and OGG1 during the pathogenesis of AD using App^{NL-G-F/NL-G-F} knock-in mice (a preclinical AD model). Six-month-old female App^{NL-G-F/NL-G-F} mice with MTH1 and/or OGG1 deficiency exhibited reduced anxiety-related behavior, but their cognitive and locomotive functions were unchanged; the alteration was less evident in 12-month-old mice. MTH1 and/or OGG1 deficiency accelerated the 8-oxoG accumulation and microgliosis in the amygdala and cortex of six-month-old mice; the alteration was less evident in 12-month-old mice. Astrocytes and neurons were not influenced. We showed that MTH1 and OGG1 are essential for minimizing oxidative DNA damage in the App^{NL-G-F/NL-G-F} brain, and the effects are age-dependent. MTH1 and/or OGG1 deficiency reduced anxiety-related behavior in App^{NL-G-F/NL-G-F} mice with a significant acceleration of the 8-oxoG burden and microgliosis, especially in the cortex and amygdala.

MTH1 and OGG1 maintain a low level of 8-oxoguanine in Alzheimer's brain, and prevent the progression of Alzheimer's pathogenesis

8-Oxoguanine (8-oxoG), a major oxidative base lesion, is highly accumulated in Alzheimer’s disease (AD) brains during the pathogenic process. MTH1 hydrolyzes 8-oxo-dGTP to 8-oxo-dGMP, thereby avoiding 8-oxo-dG incorporation into DNA. 8-OxoG DNA glycosylase-1 (OGG1) excises 8-oxoG paired with cytosine in DNA, thereby minimizing 8-oxoG accumulation in DNA. Levels of MTH1 and OGG1 are significantly reduced in the brains of sporadic AD cases. To understand how 8-oxoG accumulation in the genome is involved in AD pathogenesis, we established an AD mouse model with knockout of Mth1 and Ogg1 genes in a 3xTg-AD background. MTH1 and OGG1 deficiency increased 8-oxoG accumulation in nuclear and, to a lesser extent, mitochondrial genomes, causing microglial activation and neuronal loss with impaired cognitive function at 4–5 months of age. Furthermore, minocycline, which inhibits microglial activation and reduces neuroinflammation, markedly decreased the nuclear accumulation of 8-oxoG in microglia, and inhibited microgliosis and neuronal loss. Gene expression profiling revealed that MTH1 and OGG1 efficiently suppress progression of AD by inducing various protective genes against AD pathogenesis initiated by Aß/Tau accumulation in 3xTg-AD brain. Our findings indicate that efficient suppression of 8-oxoG accumulation in brain genomes is a new approach for prevention and treatment of AD.

RNA and Oxidative Stress in Alzheimer's Disease: Focus on microRNAs

Oxidative stress (OS) is one of the major pathomechanisms of Alzheimer's disease (AD), which is closely associated with other key events in neurodegeneration such as mitochondrial dysfunction, inflammation, metal dysregulation, and protein misfolding. Oxidized RNAs are identified in brains of AD patients at the prodromal stage. Indeed, oxidized mRNA, rRNA, and tRNA lead to retarded or aberrant protein synthesis. OS interferes with not only these translational machineries but also regulatory mechanisms of noncoding RNAs, especially microRNAs (miRNAs). MiRNAs can be oxidized, which causes misrecognizing target mRNAs. Moreover, OS affects the expression of multiple miRNAs, and conversely, miRNAs regulate many genes involved in the OS response. Intriguingly, several miRNAs embedded in upstream regulators or downstream targets of OS are involved also in neurodegenerative pathways in AD. Specifically, seven upregulated miRNAs (miR-125b, miR-146a, miR-200c, miR-26b, miR-30e, miR-34a, miR-34c) and three downregulated miRNAs (miR-107, miR-210, miR-485), all of which are associated with OS, are found in vulnerable brain regions of AD at the prodromal stage. Growing evidence suggests that altered miRNAs may serve as targets for developing diagnostic or therapeutic tools for early-stage AD. Focusing on a neuroprotective transcriptional repressor, REST, and the concept of hormesis that are relevant to the OS response may provide clues to help us understand the role of the miRNA system in cellular and organismal adaptive mechanisms to OS.

8-Oxoguanine accumulation in aged female brain impairs neurogenesis in the dentate gyrus and major island of Calleja, causing sexually dimorphic phenotypes

In mammals, including humans, MTH1 with 8-oxo-dGTPase and OGG1 with 8-oxoguanine DNA glycosylase minimize 8-oxoguanine accumulation in genomic DNA. We investigated age-related alterations in behavior, 8-oxoguanine levels, and neurogenesis in the brains of Mth1/Ogg1-double knockout (TO-DKO), Ogg1-knockout, and human MTH1-transgenic (hMTH1-Tg) mice. Spontaneous locomotor activity was significantly decreased in wild-type mice with age, and females consistently exhibited higher locomotor activity than males. This decrease was significantly suppressed in female but not male TO-DKO mice and markedly enhanced in female hMTH1-Tg mice. Long-term memory retrieval was impaired in middle-aged female TO-DKO mice. 8-Oxoguanine accumulation significantly increased in nuclear DNA, particularly in the dentate gyrus (DG), subventricular zone (SVZ) and major island of Calleja (ICjM) in middle-aged female TO-DKO mice. In middle-aged female TO-DKO mice, neurogenesis was severely impaired in SVZ and DG, accompanied by ICjM and DG atrophy. Conversely, expression of hMTH1 efficiently suppressed 8-oxoguanine accumulation in both SVZ and DG with hypertrophy of ICjM. These findings indicate that newborn neurons from SVZ maintain ICjM in the adult brain, and increased accumulation of 8-oxoguanine in nuclear DNA of neural progenitors in females is caused by 8-oxo-dGTP incorporation during proliferation, causing depletion of neural progenitors, altered behavior, and cognitive function changes with age.

Not breathing is not an option: How to deal with oxidative DNA damage

Oxidative DNA damage constitutes a major threat to genetic integrity, and has thus been implicated in the pathogenesis of a wide variety of diseases, including cancer and neurodegeneration. 7,8-dihydro-8oxo-deoxyGuanine (8-oxo-G) is one of the best characterised oxidative DNA lesions, and it can give rise to point mutations due to its miscoding potential that instructs most DNA polymerases (Pols) to preferentially insert Adenine (A) opposite 8-oxo-G instead of the correct Cytosine (C). If uncorrected, A:8-oxo-G mispairs can give rise to C:G→A:T transversion mutations. Cells have evolved a variety of pathways to mitigate the mutational potential of 8-oxo-G that include i) mechanisms to avoid incorporation of oxidized nucleotides into DNA through nucleotide pool sanitisation enzymes (by MTH1, MTH2, MTH3 and NUDT5), ii) base excision repair (BER) of 8-oxo-G in DNA (involving MUTYH, OGG1, Pol λ, and other components of the BER machinery), and iii) faithful bypass of 8-oxo-G lesions during replication (using a switch between replicative Pols and Pol λ). In the following, the fate of 8-oxo-G in mammalian cells is reviewed in detail. The differential origins of 8-oxo-G in DNA and its consequences for genetic stability will be covered. This will be followed by a thorough discussion of the different mechanisms in place to cope with 8-oxo-G with an emphasis on Pol λ-mediated correct bypass of 8-oxo-G during MUTYH-initiated BER as well as replication across 8-oxo-G. Furthermore, the multitude of mechanisms in place to regulate key proteins involved in 8-oxo-G repair will be reviewed. Novel functions of 8-oxo-G as an epigenetic-like regulator and insights into the repair of 8-oxo-G within the cellular context will be touched upon. Finally, a discussion will outline the relevance of 8-oxo-G and the proteins involved in dealing with 8-oxo-G to human diseases with a special emphasis on cancer.

Consequences of RNA oxidation on protein synthesis rate and fidelity: implications for the pathophysiology of neuropsychiatric disorders

Unlike DNA, oxidative damage to RNA has received little attention presumably due to the assumed transient nature of RNA. However, RNAs including mRNA can persist for several hours to days in certain tissues and are demonstrated to sustain greater oxidative damage than DNA. Because neuronal cells in the brain are continuously exposed to reactive oxygen species due to a high oxygen consumption rate, it is not surprising that neuronal RNA oxidation is observed as a common feature at an early stage in a series of neurodegenerative disorders. A recent study on a well-defined bacterial translation system has revealed that mRNA containing 8-oxo-guanosine (8-oxoGuo) has little effect on fidelity despite the anticipated miscoding. Indeed, 8-oxoGuo-containing mRNA leads to ribosomal stalling with a reduced rate of peptide-bond formation by 3–4 orders of magnitude and is subject to no-go decay, a ribosome-based mRNA surveillance mechanism. Another study demonstrates that transfer RNA oxidation catalyzed by cytochrome c (cyt c) leads to its depurination and cross-linking, which may facilitate cyt c release from mitochondria and subsequently induce apoptosis. Even more importantly, a discovery of oxidized microRNA has been recently reported. The oxidized microRNA causes misrecognizing the target mRNAs and subsequent down-regulation in the protein synthesis. It is noteworthy that oxidative modification to RNA not only interferes with the translational machinery but also with regulatory mechanisms of noncoding RNAs that contribute toward the biological complexity of the mammalian brain. Oxidative RNA damage might be a promising therapeutic target potentially useful for an early intervention of diverse neuropsychiatric disorders.

The Response to Oxidative DNA Damage in Neurons: Mechanisms and Disease

There is a growing body of evidence indicating that the mechanisms that control genome stability are of key importance in the development and function of the nervous system. The major threat for neurons is oxidative DNA damage, which is repaired by the base excision repair (BER) pathway. Functional mutations of enzymes that are involved in the processing of single-strand breaks (SSB) that are generated during BER have been causally associated with syndromes that present important neurological alterations and cognitive decline. In this review, the plasticity of BER during neurogenesis and the importance of an efficient BER for correct brain function will be specifically addressed paying particular attention to the brain region and neuron-selectivity in SSB repair-associated neurological syndromes and age-related neurodegenerative diseases.

The impact of base excision DNA repair in age-related neurodegenerative diseases

The aging process and several age-related neurodegenerative disorders have been linked to elevated levels of DNA damage induced by ROS and deficiency in DNA repair mechanisms. DNA damage induced by ROS is a byproduct of cellular respiration and accumulation of damage over time, is a fundamental aspect of a main theory of aging. Mitochondria have a pivotal role in generating cellular oxidative stress, and mitochondrial dysfunction has been associated with several diseases. DNA base excision repair is considered the major pathway for repair of oxidized bases in DNA both in the nuclei and in mitochondria, and in neurons this mechanism is particularly important because non-diving cells have limited back-up DNA repair mechanisms. An association between elevated oxidative stress and a decrease in BER is strongly related to the aging process and has special relevance in age-related neurodegenerative diseases. Here, we review the role of DNA repair in aging, focusing on the implications of the DNA base excision repair pathways and how alterations in expression of these DNA repair proteins are related to the aging process and to age-related neurodegenerative diseases.

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.

8-Oxoguanine causes neurodegeneration during MUTYH-mediated DNA base excision repair

8-Oxoguanine (8-oxoG), a common DNA lesion caused by reactive oxygen species, is associated with carcinogenesis and neurodegeneration. Although the mechanism by which 8-oxoG causes carcinogenesis is well understood, the mechanism by which it causes neurodegeneration is unknown. Here, we report that neurodegeneration is triggered by MUTYH-mediated excision repair of 8-oxoG-paired adenine. Mutant mice lacking 8-oxo-2'-deoxyguanosine triphosphate-depleting (8-oxo-dGTP-depleting) MTH1 and/or 8-oxoG-excising OGG1 exhibited severe striatal neurodegeneration, whereas mutant mice lacking MUTYH or OGG1/MUTYH were resistant to neurodegeneration under conditions of oxidative stress. These results indicate that OGG1 and MTH1 are protective, while MUTYH promotes neurodegeneration. We observed that 8-oxoG accumulated in the mitochondrial DNA of neurons and caused calpain-dependent neuronal loss, while delayed nuclear accumulation of 8-oxoG in microglia resulted in PARP-dependent activation of apoptosis-inducing factor and exacerbated microgliosis. These results revealed that neurodegeneration is a complex process caused by 8-oxoG accumulation in the genomes of neurons and microglia. Different signaling pathways were triggered by the accumulation of single-strand breaks in each type of DNA generated during base excision repair initiated by MUTYH, suggesting that suppression of MUTYH may protect the brain under conditions of oxidative stress.

DNA damage and impairment of DNA repair in Alzheimer's disease

Deoxyribonucleic acid (DNA) damage has been implicated in ageing and neurodegenerative disorders including Alzheimer's disease (AD) for a few decades. Although it is an established finding, yet there are limited studies on DNA damage. In both nucleus and mitochondria, DNA damage is primarily free radical mediated. It has been proven that mitochondrial DNA is more vulnerable to damage compared to the nuclear DNA. A few studies summarized in this review throw light on the mechanisms of free radical mediated DNA damage and impairment of DNA repair mechanisms in AD. There is a growing need to initiate studies on DNA damage and repair and unravel the molecular underpinnings entailed in the etiopathogenesis of the disease. The outcome of such studies substantiates the corner stone streamlined to employ therapeutic strategies.

Oxidized purine nucleotides, genome instability and neurodegeneration

Oxidative DNA damage can be the consequence of endogenous metabolic processes and exogenous insults and several DNA repair enzymes provide protection against the toxic effects of oxidized DNA bases. Here we review the increasing knowledge on the relationship between an oxidized dNTPs pool and genome instability. The review also describes some important progress toward understanding the role of oxidative DNA damage and its repair in neurodegenerative diseases. In particular the hMTH1 hydrolase destroys oxidized nucleic acid precursors to prevent their harmful incorporation into DNA and RNA. Based on results obtained in our transgenic mouse overexpressing hMTH1 in the brain we discussed the mechanisms by which this hydrolase protects against neurodegeneration in Huntington disease models.

Programmed cell death triggered by nucleotide pool damage and its prevention by MutT homolog-1 (MTH1) with oxidized purine nucleoside triphosphatase

Accumulation of oxidized bases such as 8-oxoguanine in either nuclear or mitochondrial DNA triggers various cellular dysfunctions including mutagenesis, and programmed cell death or senescence. Recent studies have revealed that oxidized nucleoside triphosphates such as 8-oxo-dGTP in the nucleotide pool are the main source of oxidized bases accumulating in the DNA of cells under oxidative stress. To counteract such deleterious effects of nucleotide pool damage, mammalian cells possess MutT homolog-1 (MTH1) with oxidized purine nucleoside triphosphatase and related enzymes, thus minimizing the accumulation of oxidized bases in cellular DNA. Depletion or increased expression of the MTH1 protein have revealed its significant roles in avoiding programmed cell death or senescence as well as mutagenesis, and accumulating evidences indicate that MTH1 is involved in suppression of degenerative disorders such as neurodegeneration.

Up-regulation of 8-oxo-dGTPase activity of MTH1 protein in the brain, testes and kidneys of mice exposed to (137)Cs gamma radiation

Abstract Mammalian MTH1 protein is an antimutagenic (2'-deoxy)ribonucleoside 5'-triphosphate pyrophosphohydrolase that prevents the incorporation of oxidatively modified nucleotides into nucleic acids. It decomposes most specifically the miscoding products of oxidative damage to purine nucleic acid precursors (e.g. 8-oxo-dGTP, 2-oxo-dATP, 2-oxo-ATP, 8-oxo-GTP) that may cause point mutations or transcription errors when incorporated into DNA and RNA, respectively. The increased expression of MTH1 mRNA and MTH1 protein was previously proposed as a molecular marker of oxidative stress. Therefore, we hypothesized that increased 8-oxo-dGTPase activity of MTH1 protein in mouse organs could serve as a dose-dependent marker of exposure to ionizing radiation, which is known to induce oxidative stress. To test our hypothesis, we measured 8-oxo-dGTPase activity in six organs of male BL6 mice after exposure to 0, 10, 25 and 50 cGy and 1 Gy of (137)Cs gamma radiation given as a single whole-body dose (1 Gy/min). The mice were killed 4, 8 and 24 h after irradiation. A statistically significant induction of 8-oxo-dGTPase was found in brains, testes and kidneys but not in lungs, hearts or livers. Brains, which demonstrated the highest (4.3-fold) increase of 8-oxo-dGTPase activity, were shown to express approximately 50% higher levels of MTH1 protein. However, due to the lack of a simple positive correlation between the dose and the observed 8-oxo-dGTPase activity in brain, testes and kidneys, we conclude that measurements of 8-oxo-dGTPase activity in these organs may serve as a rough indicator rather than a quantifiable marker of radiation-induced oxidative stress.

Age-related alterations in the expression of MTH2 in the hippocampus of the SAMP8 mouse with learning and memory deterioration

MutT-related proteins degrade 8-oxo-7,8-dihydrodeoxyguanosine triphosphate (8-oxo-dGTP), a mutagenic substrate for DNA synthesis in the nucleotide pool, thereby preventing DNA replication errors. MTH2 (Mut T homolog 2), which belongs to this family of proteins, possesses 8-oxo-7,8-dihydro-2'-deoxyguanosine triphosphatase (8-oxo-dGTPase) activity and appears to function in the protection of the genetic material from the untoward effects of endogenous oxygen radicals. To examine the roles of MTH2 in the aging process, we used the senescence-accelerated prone mouse 8 (SAMP8), which exhibits early aging syndromes and declining abilities of learning and memory. Immunohistochemical and western blot analysis revealed that the level of MTH2 protein in the hippocampus of the SAMP8 mouse progressively decreases beginning from four months after birth, whereas no such change was observed in the control senescence-accelerated resistant mouse 1 (SAMR1). Under these conditions, 8-oxoguanine accumulates in the nuclear DNA in the CA1 and CA3 subregions of the hippocampus of SAMP8 in an age-dependent manner. In SAMR1 mice, accumulation of 8-oxoguanine in the DNA was not observed. These results suggest that the MTH2 deficiency might be one of the causative factors for accelerated aging.

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.

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.

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.

Oxidative DNA damage in mild cognitive impairment and late-stage Alzheimer's disease

Increasing evidence supports a role for oxidative DNA damage in aging and several neurodegenerative diseases including Alzheimer's disease (AD). Attack of DNA by reactive oxygen species (ROS), particularly hydroxyl radicals, can lead to strand breaks, DNA–DNA and DNA–protein cross-linking, and formation of at least 20 modified bases adducts. In addition, α,β-unsaturated aldehydic by-products of lipid peroxidation including 4-hydroxynonenal and acrolein can interact with DNA bases leading to the formation of bulky exocyclic adducts. Modification of DNA bases by direct interaction with ROS or aldehydes can lead to mutations and altered protein synthesis. Several studies of DNA base adducts in late-stage AD (LAD) brain show elevations of 8-hydroxyguanine (8-OHG), 8-hydroxyadenine (8-OHA), 5-hydroxycytosine (5-OHC), and 5-hydroxyuracil, a chemical degradation product of cytosine, in both nuclear and mitochondrial DNA (mtDNA) isolated from vulnerable regions of LAD brain compared to age-matched normal control subjects. Previous studies also show elevations of acrolein/guanine adducts in the hippocampus of LAD subjects compared to age-matched controls. In addition, studies of base excision repair show a decline in repair of 8-OHG in vulnerable regions of LAD brain. Our recent studies show elevated 8-OHG, 8-OHA, and 5,6-diamino-5-formamidopyrimidine in both nuclear and mtDNA isolated from vulnerable brain regions in amnestic mild cognitive impairment, the earliest clinical manifestation of AD, suggesting that oxidative DNA damage is an early event in AD and is not merely a secondary phenomenon.

The dentate gyrus in Alzheimer's disease

As part of the hippocampus, the dentate gyrus is considered to play a crucial role in associative memory. The reviewed data suggest that the dentate gyrus withstands the formation of plaques, tangles and neuronal death until late stages of Alzheimer's disease (AD). However, changes related to a disconnecting process, and more subtle intrinsic alterations, may contribute to disturbances in memory and learning observed in early stages of AD.

MTH1, an oxidized purine nucleoside triphosphatase, suppresses the accumulation of oxidative damage of nucleic acids in the hippocampal microglia during kainate-induced excitotoxicity

Enhanced oxidative stress has been implicated in the excitotoxicity of the CNS, and 8-oxo-7,8-dihydro-guanine (8-oxoG), a major type of oxidative damage in nucleic acids, was reported to be accumulated in the rat hippocampus after kainate administration. We herein showed that the 8-oxoG levels in mitochondrial DNA and cellular RNA increased significantly in the CA3 subregion of the mouse hippocampus 6-12 h after kainate administration but returned to basal levels within a few days. Laser-scanning confocal microscopy revealed the 8-oxoG accumulation in mitochondrial DNA to be remarkable in CA3 microglia, whereas that in nuclear DNA or cellular RNA was also detected in the CA3 pyramidal cells and astrocytes. 8-oxoG accumulation in cellular DNA or RNA should be suppressed by MutT homolog 1 (MTH1) with 8-oxo-dGTPase (8-oxo-7,8-dihydro-2'-deoxyguanosine triphosphatase) activity and 8-oxoG-DNA glycosylase 1 (OGG1) with 8-oxoG DNA glycosylase activity. We thus examined the expression level of MTH1 and OGG1 in the mouse hippocampus after kainate administration. The Mth1 mRNA level decreased soon after kainate administration and then quickly recovered beyond the basal level, and a continuously increased MTH1 protein level was observed, whereas the Ogg1 mRNA level remained constant. MTH1-null and wild-type mice exhibited a similar degree of CA3 neuron loss after kainate administration; however, the 8-oxoG levels that accumulated in mitochondrial DNA and cellular RNA in the CA3 microglia significantly increased in the MTH1-null mice in comparison with wild-type mice, thus demonstrating that MTH1 efficiently suppresses the accumulation of 8-oxoG in both cellular DNA and RNA in the hippocampus, especially in microglia, caused by excitotoxicity.

MTH1, an oxidized purine nucleoside triphosphatase, prevents the cytotoxicity and neurotoxicity of oxidized purine nucleotides

In human and rodent cells, MTH1, an oxidized purine nucleoside triphosphatase, efficiently hydrolyzes oxidized dGTP, GTP, dATP and ATP such as 2'-deoxy-8-oxoguanosine triphosphate (8-oxo-dGTP) and 2'-deoxy-2-hydroxyadenosine triphosphate (2-OH-dATP) in nucleotide pools, thus avoiding their incorporation into DNA or RNA. MTH1 is expressed in postmitotic neurons as well as in proliferative tissues, and it is localized both in the mitochondria and nucleus, thus suggesting that MTH1 plays an important role in the prevention of the mutagenicity and cytotoxicity of such oxidized purines as 8-oxoG which are known to accumulate in the cellular genome. Our recent studies with MTH1-deficient mice or cells revealed that MTH1 efficiently minimizes accumulation of 8-oxoG in both nuclear and mitochondrial DNA in the mouse brain as well as in cultured cells, thus contributing to the protection of the brain from oxidative stress.

The GT to GC single nucleotide polymorphism at the beginning of an alternative exon 2C of human MTH1 gene confers an amino terminal extension that functions as a mitochondrial targeting signal

Human MTH1 protein hydrolyzes oxidized purine nucleotides 8-oxo-2'-deoxyguanosine triphosphate (8-oxo-dGTP), 2-OH-dATP or their ribo-forms to their monophosphates, thus minimizing replicational and transcriptional errors both in the nuclei and mitochondria. MTH1 suppresses mitochondrial dysfunction and cell death caused by H(2)O(2). Furthermore, MTH1 suppresses the transient increase in 8-oxoguanine in mitochondrial DNA in the dopaminergic nerve terminals in mouse striatum after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine administration, and it protects the nerve terminals. We previously reported that a novel MTH1 allele with a single nucleotide polymorphism (SNP) in its exon 2c segment encodes the fourth MTH1 isoform, namely, MTH1a (p26), in addition to the three known isoforms, MTH1b (p22), c (p21), and d (p18). Another SNP located in exon 4 of the MTH1 gene, which is closely linked to the SNP in exon 2c, substitutes the Val83 residue in MTH1d with Met83. We herein show that all MTH1 isoforms efficiently hydrolyzed 2-OH-dATP and 8-oxo-dGTP. The amino terminal region of MTH1a functioned as a mitochondrial targeting signal when it was expressed in the HeLa cells as a fusion protein with enhanced green fluorescent protein. The cellular fractionation revealed that MTH1a(Met83) was localized in the mitochondria to the same extent as was MTH1d(Val83). However, the mitochondrial translocation of MTH1d(Met83) was less efficient than that of MTH1d(Val83).

MTH1, an oxidized purine nucleoside triphosphatase, protects the dopamine neurons from oxidative damage in nucleic acids caused by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

We previously reported that 8-oxoguanine (8-oxoG) accumulates in the cytoplasm of dopamine neurons in the substantia nigra of patients with Parkinson's disease and the expression of MTH1 carrying an oxidized purine nucleoside triphosphatase activity increases in these neurons, thus suggesting that oxidative damage in nucleic acids is involved in dopamine neuron loss. In the present study, we found that levels of 8-oxoG in cellular DNA and RNA increased in the mouse nigrostriatal system during the tyrosine hydroxylase (TH)-positive dopamine neuron loss induced by the administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). MTH1-null mice exhibited a greater accumulation of 8-oxoG in mitochondrial DNA accompanied by a more significant decrease in TH and dopamine transporter immunoreactivities in the striatum after MPTP administration, than in wild-type mice. We thus demonstrated that MTH1 protects the dopamine neurons from oxidative damage in the nucleic acids, especially in the mitochondrial DNA of striatal nerve terminals of dopamine neurons.

The defense mechanisms in mammalian cells against oxidative damage in nucleic acids and their involvement in the suppression of mutagenesis and cell death

To counteract oxidative damage in nucleic acids, mammalian cells are equipped with several defense mechanisms. We herein review that MTH1, MUTYH and OGG1 play important roles in mammalian cells avoiding an accumulation of oxidative DNA damage, both in the nuclear and mitochondrial genomes, thereby suppressing carcinogenesis and cell death. MTH1 efficiently hydrolyzes oxidized purine nucleoside triphosphates, such as 8-oxo-dGTP, 8-oxo-dATP and 2-hydroxy (OH)-dATP, to the monophosphates, thus avoiding the incorporation of such oxidized nucleotides into the nuclear and mitochondrial genomes. OGG1 excises 8-oxoG in DNA as a DNA glycosylase and thus minimizes the accumulation of 8-oxoG in the cellular genomes. MUTYH excises adenine opposite 8-oxoG, and thus suppresses 8-oxoG-induced mutagenesis. MUTYH also possesses a 2-OH-A DNA glycosylase activity for excising 2-OH-A incorporated into the cellular genomes. Increased susceptibilities to spontaneous carcinogenesis of the liver, lung or intestine were observed in MTH1-, OGG1- and MUTYH-null mice, respectively. The increased occurrence of lung tumors in OGG1-null mice was abolished by the concomitant disruption of the Mth1 gene, indicating that an increased accumulation of 8-oxoG and/or 2-OH-A might cause cell death. Furthermore, these defense mechanisms also likely play an important role in neuroprotection.

Defense mechanism to oxidative DNA damage in glial cells

Astrocytosis is a sequential morphological change of astrocytic reaction to tissue damage, and is associated with regulation of antioxidant defense mechanisms to reduce oxidative damage. The repair enzymes to oxidative DNA damage, oxidized purine-nucleoside triphosphatase (hMTH1) and a mitochondrial type of 8-oxoguanine DNA glycosylase (hOGG1-2a) in brain tumors and neurons of Alzheimer's disease, were previously reported. In the present study, glial expression of these repair enzymes under such pathological conditions as cerebrovascular diseases and metastatic brain tumors, were investigated. Furthermore, an in-vitro experiment using a glioma cell-line under oxidative stress was performed to verify the immunohistochemical results of post-mortem materials. As a result, hOGG1-2a immunoreactivities in reactive astrocytes were more intense than those to hMTH1. Oligodendrocytes of acute or subacute stage of brain infarction were strongly immunoreactive to both repair enzymes. In-vitro study revealed that, hOGG1-2a is constitutively expressed in both untreated glioma cells and the glioma cells under oxidative stress. However, although no immunoreactivity to hMTH1 was found in the control cells, accumulation of hMTH1 was rapidly induced by oxidative stress. These results indicate that the two repair enzymes to oxidative DNA damage are differentially regulated in glial cells, and that there is a difference in the expression of the repair enzymes between reactive astrocytes and oligodendrocytes.

Structure of human MTH1, a Nudix family hydrolase that selectively degrades oxidized purine nucleoside triphosphates

Oxygen radicals generated through normal cellular respiration processes can cause mutations in genomic and mitochondrial DNA. Human MTH1 hydrolyzes oxidized purine nucleoside triphosphates, such as 8-oxo-dGTP and 2-hydroxy-dATP, to monophosphates, thereby preventing the misincorporation of these oxidized nucleotides during replication. Here we present the solution structure of MTH1 solved by multidimensional heteronuclear NMR spectroscopy. The protein adopts a fold similar to that of Escherichia coli MutT, despite the low sequence similarity between these proteins outside the conserved Nudix motif. The substrate-binding pocket of MTH1, deduced from chemical shift perturbation experiments, is located at essentially the same position as in MutT; however, a pocket-forming helix is largely displaced in MTH1 (approximately 9 A) such that the shape of the pocket differs between the two proteins. Detailed analysis of the pocket-forming residues enabled us to identify Asn33 as one of the key residues in MTH1 for discriminating the oxidized form of purine, and mutation of this residue modifies the substrate specificity. We also show that MTH1 catalyzes hydrolysis of 8-oxo-dGTP through nucleophilic substitution of water at the beta-phosphate.

Biological significance of the defense mechanisms against oxidative damage in nucleic acids caused by reactive oxygen species: from mitochondria to nuclei

In mammalian cells, more than one genome in a single cell has to be maintained throughout the entire life of the cell, namely, one in the nucleus and the other in the mitochondria. The genomes and their precursor nucleotides are highly exposed to reactive oxygen species, which are inevitably generated as a result of the respiratory function in mitochondria. To counteract such oxidative damage in nucleic acids, cells are equipped with several defense mechanisms. Modified nucleotides in the nucleotide pools are hydrolyzed, thus avoiding their incorporation into DNA or RNA. Damaged bases in DNA with relatively small chemical alterations are mainly repaired by the base excision repair (BER) system, which is initiated by the excision of damaged bases by specific DNA glycosylases. MTH1 protein hydrolyzes oxidized purine nucleoside triphosphates, such as 8-oxo-dGTP, 8-oxo-dATP, and 2-hydroxy (OH)-dATP to the monophosphates, and MTH1 are located in the cytoplasm, mitochondria, and nucleus. We observed an increased susceptibility to spontaneous carcinogenesis in Mth1-deficient mice and an alteration of MTH1 expression along with the accumulation of 8-oxo-dG in patients with various neurodegenerative diseases. Enzymes for the BER pathway, namely, 8-oxoG DNA glycosylase (OGG1), 2-OH-A/adenine DNA glycosylase (MUTYH), and AP endonuclease (APEX2) are also located both in the mitochondria and in the nuclei, and the expression of mitochondrial OGG1 is altered in patients with various neurodegenerative diseases. We also observed increased susceptibilities to spontaneous carcinogenesis in OGG1 and MUTYH-deficient mice. The increased occurrence of lung tumor in OGG1-deficient mice was completely abolished by the concomitant disruption of the Mth1 gene.

An oxidized purine nucleoside triphosphatase, MTH1, suppresses cell death caused by oxidative stress

MTH1 hydrolyzes oxidized purine nucleoside triphosphates such as 8-oxo-2'-deoxyguanosine 5'-triphosphate (8-oxo-dGTP) and 2-hydroxy-2'-deoxyadenosine 5'-triphosphate (2-OH-dATP) and thus protects cells from damage caused by their misincorporation into DNA. In the present study, we established MTH1-null mouse embryo fibroblasts that were highly susceptible to cell dysfunction and death caused by exposure to H2O2, with morphological features of pyknosis and electron-dense deposits accumulated in mitochondria. The cell death observed was independent of both poly(ADP-ribose) polymerase and caspases. A high performance liquid chromatography tandem mass spectrometry analysis and immunofluorescence microscopy revealed a continuous accumulation of 8-oxo-guanine both in nuclear and mitochondrial DNA after exposure to H2O2. All of the H2O2-induced alterations observed in MTH1-null mouse embryo fibroblasts were effectively suppressed by the expression of wild type human MTH1 (hMTH1), whereas they were only partially suppressed by the expression of mutant hMTH1 defective in either 8-oxo-dGTPase or 2-OH-dATPase activity. Human MTH1 thus protects cells from H2O2-induced cell dysfunction and death by hydrolyzing oxidized purine nucleotides including 8-oxo-dGTP and 2-OH-dATP, and these alterations may be partly attributed to a mitochondrial dysfunction.

Translocation of glutamate transporter subtype excitatory amino acid carrier 1 protein in kainic acid-induced rat epilepsy

Glutamate excitotoxicity has been implicated in the pathophysiology of epilepsy. Systemic injection of kainic acid (KA) in the rat produces an animal model of human temporal lobe epilepsy. We examined the temporal expression of the sodium-dependent neuronal glutamate transporter, excitatory amino acid carrier 1 (EAAC1), in KA-induced rat epilepsy. As an early alteration, perinuclear deposits of EAAC1 protein were found mainly in the large pyramidal neurons at the hippocampus, neocortex, piriform cortex, and amygdala with the reduction of neuropil staining 6 hours after KA injection. Immunoelectron microscopic study revealed that the perinuclear EAAC1 immunoreactivity corresponded to the translocation to the Golgi complex. At this time point, EAAC1 mRNA was down-regulated. The intracellular aggregation of EAAC1 primarily disappeared by 24 hours. In vitro studies indicated that internalization of EAAC1 from the plasma membrane to the intracellular compartment by KA treatment was associated with the reduction of electrogenic transporter currents. Our results suggest that the transient EAAC1 internalization participates in the modulation of the transporter function preventing excessive glutamate uptake to pyramidal neurons during the early stage of epilepsy.

A molecular basis for the selective recognition of 2-hydroxy-dATP and 8-oxo-dGTP by human MTH1

MTH1 hydrolyzes oxidized purine nucleoside triphosphates such as 8-oxo-dGTP, 8-oxo-dATP, 2-hydroxy-dATP, and 2-hydroxy rATP to monophosphates, and thus avoids errors caused by their misincorporation during DNA replication or transcription, which may result in carcinogenesis or neurodegeneration. This substrate specificity for oxidized purine nucleoside triphosphates was investigated by mutation analyses based on the sequence comparison with the Escherichia coli homolog, MutT, which hydrolyzes only 8-oxo-dGTP and 8-oxo-rGTP but not oxidized forms of dATP or ATP. Neither a replacement of the phosphohydrolase module of MTH1 with that of MutT nor deletions of the C-terminal region of MTH1, which is unique for MTH1, altered the substrate specificity of MTH1. In contrast, the substitution of residues at position Trp-117 and Asp-119 of MTH1, which showed apparent chemical shift perturbations with 8-oxo-dGDP in NMR analyses but are not conserved in MutT, affected the substrate specificity. Trp-117 is essential for MTH1 to recognize both 8-oxo-dGTP and 2-hydroxy-dATP, whereas Asp-119 is only essential for recognizing 2-hydroxy-dATP, thus suggesting that origins of the substrate-binding pockets for MTH1 and MutT are different.