Intracellular localization of 8-oxo-dGTPase in human cells, with special reference to the role of the enzyme in mitochondria

Mitochondrial H2O2 release does not directly cause damage to chromosomal DNA

Reactive Oxygen Species (ROS) derived from mitochondrial respiration are frequently cited as a major source of chromosomal DNA mutations that contribute to cancer development and aging. However, experimental evidence showing that ROS released by mitochondria can directly damage nuclear DNA is largely lacking. In this study, we investigated the effects of H2O2 released by mitochondria or produced at the nucleosomes using a titratable chemogenetic approach. This enabled us to precisely investigate to what extent DNA damage occurs downstream of near- and supraphysiological amounts of localized H2O2. Nuclear H2O2 gives rise to DNA damage and mutations and a subsequent p53 dependent cell cycle arrest. Mitochondrial H2O2 release shows none of these effects, even at levels that are orders of magnitude higher than what mitochondria normally produce. We conclude that H2O2 released from mitochondria is unlikely to directly damage nuclear genomic DNA, limiting its contribution to oncogenic transformation and aging.

MTH1 protects platelet mitochondria from oxidative damage and regulates platelet function and thrombosis

Human MutT Homolog 1 (MTH1) is a nucleotide pool sanitization enzyme that hydrolyzes oxidized nucleotides to prevent their mis-incorporation into DNA under oxidative stress. Expression and functional roles of MTH1 in platelets are not known. Here, we show MTH1 expression in platelets and its deficiency impairs hemostasis and arterial/venous thrombosis in vivo. MTH1 deficiency reduced platelet aggregation, phosphatidylserine exposure and calcium mobilization induced by thrombin but not by collagen-related peptide (CRP) along with decreased mitochondrial ATP production. Thrombin but not CRP induced Ca2+-dependent mitochondria reactive oxygen species generation. Mechanistically, MTH1 deficiency caused mitochondrial DNA oxidative damage and reduced the expression of cytochrome c oxidase 1. Furthermore, MTH1 exerts a similar role in human platelet function. Our study suggests that MTH1 exerts a protective function against oxidative stress in platelets and indicates that MTH1 could be a potential therapeutic target for the prevention of thrombotic diseases.

Discovery and properties of a monoclonal antibody targeting 8-oxoA, an oxidized adenine lesion in DNA and RNA

8-oxoA, a major oxidation product of adenosine, is a mispairing, mutagenic lesion that arises in DNA and RNA when ^•OH radicals or one-electron oxidants attack the C8 adenine atom or polymerases misincorporate 8-oxo(d)ATP. The danger of 8-oxoA is underscored by the existence of dedicated cellular repair machinery that explicitly excise it from DNA, the attenuation of translation induced by 8-oxoA-mRNA or damaged ribosomes, and its potency as a TLR7 agonist. Here we present the discovery, purification, and biochemical characterization of a new mouse IgGk1 monoclonal antibody (6E4) that specifically targets 8-oxoA. Utilizing an AchE-based competitive ELISA assay, we demonstrate the selectivity of 6E4 for 8-oxoA over a plethora of canonical and chemically modified nucleosides including 8-oxoG, A, m6A, 2-oxoA, and 5-hoU. We further show the ability of 6E4 to exclusively recognize 8-oxoA in nucleoside triphosphates (8-oxoATP) and DNA/RNA oligonucleotides containing a single 8-oxoA. 6E4 also binds 8-oxoA in duplex DNA/RNA antigens where the lesion is either paired correctly or base mismatched. Our findings define the 8-oxoAde nucleobase as the critical epitope and indicate mAb 6E4 is ideally suited for a broad range of immunological applications in nucleic acid detection and quality control.

Inhibition of human APE1 and MTH1 DNA repair proteins by dextran-coated γ-Fe2O3 ultrasmall superparamagnetic iron oxide nanoparticles

Aim: To quantitatively evaluate the inhibition of human DNA repair proteins APE1 and MTH1 by dextran-coated γ-Fe₂O₃ ultrasmall superparamagnetic iron oxide nanoparticles (dUSPIONs). Materials & methods: Liquid chromatography-tandem mass spectrometry with isotope-dilution was used to measure the expression levels of APE1 and MTH1 in MCL-5 cells exposed to increasing doses of dUSPIONs. The expression levels of APE1 and MTH1 were measured in cytoplasmic and nuclear fractions of cell extracts. Results: APE1 and MTH1 expression was significantly inhibited in both cell fractions at the highest dUSPION dose. The expression of MTH1 was linearly inhibited across the full dUSPION dose range in both fractions. Conclusion: These findings warrant further studies to characterize the capacity of dUSPIONs to inhibit other DNA repair proteins in vitro and in vivo.

Dynamic features of human mitochondrial DNA maintenance and transcription

Mitochondria are the primary sites for cellular energy production and are required for many essential cellular processes. Mitochondrial DNA (mtDNA) is a 16.6 kb circular DNA molecule that encodes only 13 gene products of the approximately 90 different proteins of the respiratory chain complexes and an estimated 1,200 mitochondrial proteins. MtDNA is, however, crucial for organismal development, normal function, and survival. MtDNA maintenance requires mitochondrially targeted nuclear DNA repair enzymes, a mtDNA replisome that is unique to mitochondria, and systems that control mitochondrial morphology and quality control. Here, we provide an overview of the current literature on mtDNA repair and transcription machineries and discuss how dynamic functional interactions between the components of these systems regulate mtDNA maintenance and transcription. A profound understanding of the molecular mechanisms that control mtDNA maintenance and transcription is important as loss of mtDNA integrity is implicated in normal process of aging, inflammation, and the etiology and pathogenesis of a number of diseases.

Redox Activation of Mitochondrial DAMPs and the Metabolic Consequences for Development of Autoimmunity

Significance: Reactive oxygen species (ROS) are well known to promote innate immune responses during and in the absence of microbial infections. However, excessive or prolonged exposure to ROS provokes innate immune signaling dysfunction and contributes to the pathogenesis of many autoimmune diseases. The relatively high basal expression of pattern recognition receptors (PRRs) in innate immune cells renders them prone to activation in response to minor intrinsic or extrinsic ROS misbalances in the absence of pathogens. Critical Issues: A prominent source of ROS are mitochondria, which are also major inter-organelle hubs for innate immunity activation, since most PRRs and downstream receptor molecules are directly located either at mitochondria or at mitochondria-associated membranes. Due to their ancestral bacterial origin, mitochondria can also act as quasi-intrinsic self-microbes that mimic a pathogen invasion and become a source of danger-associated molecular patterns (DAMPs) that triggers innate immunity from within. Recent Advances: The release of mitochondrial DAMPs correlates with mitochondrial metabolism changes and increased generation of ROS, which can lead to the oxidative modification of DAMPs. Recent studies suggest that ROS-modified mitochondrial DAMPs possess increased, persistent immunogenicity. Future Directions: Herein, we discuss how mitochondrial DAMP release and oxidation activates PRRs, changes cellular metabolism, and causes innate immune response dysfunction by promoting systemic inflammation, thereby contributing to the onset or progression of autoimmune diseases. The future goal is to understand what the tipping point for DAMPs is to become oxidized, and whether this is a road without return. Antioxid. Redox Signal. 36, 441-461.

Inhibitor development of MTH1 via high-throughput screening with fragment based library and MTH1 substrate binding cavity

MutT Homolog 1 (MTH1) has been proven to hydrolyze oxidized nucleotide triphosphates during DNA repair. It can prevent the incorporation of wrong nucleotides during DNA replication and mitigate cell apoptosis. In a cancer cell, abundant reactive oxygen species can lead to substantial DNA damage and DNA mutations by base-pairing mismatch. MTH1 could eliminate oxidized dNTP and prevent cancer cells from entering cell death. Therefore, inhibition of MTH1 activity is considered to be an anti-cancer therapeutic target. In this study, high-throughput screening techniques were combined with a fragment-based library containing 2,313 compounds, which were used to screen for lead compounds with MTH1 inhibitor activity. Four compounds with MTH1 inhibitor ability were selected, and compound MI0639 was found to have the highest effective inhibition. To discover the selectivity and specificity of this action, several derivatives based on the MTH1 and MI0639 complex structure were synthesized. We compared 14 complex structures of MTH1 and the various compounds in combination with enzymatic inhibition and thermodynamic analysis. Nanomolar-range IC₅₀ inhibition abilities by enzyme kinetics and K_d values by thermodynamic analysis were obtained for two compounds, named MI1020 and MI1024. Based on structural information and compound optimization, we aim to provide a strategy for the development of MTH1 inhibitors with high selectivity and specificity.

Radiolabeled 6-(2, 3-Dichlorophenyl)-N4-methylpyrimidine-2, 4-diamine (TH287): A Potential Radiotracer for Measuring and Imaging MTH1

MTH1 (MutT homolog 1) or NUDT1 (Nudix Hydrolase 1), also known as oxidized purine nucleoside triphosphatase, has potential as a biomarker for monitoring cancer progression and quantifying target engagement for relevant therapies. In this study, we validate one MTH1 inhibitor TH287 as a PET MTH1 radiotracer. TH287 was radiolabeled with tritium and the binding of [³H]TH287 to MTH1 was evaluated in live glioblastoma cells (U251MG) through saturation and competitive binding assays, together with in vitro enzymatic assays. Furthermore, TH287 was radiolabeled with carbon-11 for in vivo microPET studies. Saturation binding assays show that [³H]TH287 has a dissociation constant (K_d) of 1.97 ± 0.18 nM, B_max of 2676 ± 122 fmol/mg protein for U251MG cells, and n_H of 0.98 ± 0.02. Competitive binding assays show that TH287 (K_i: 3.04 ± 0.14 nM) has a higher affinity for MTH1 in U251MG cells compared to another well studied MTH1 inhibitor: (S)-crizotinib (K_i: 153.90 ± 20.48 nM). In vitro enzymatic assays show that TH287 has an IC₅₀ of 2.2 nM in inhibiting MTH1 hydrolase activity and a K_i of 1.3 nM from kinetics assays, these results are consistent with our radioligand binding assays. Furthermore, MicroPET imaging shows that [¹¹C]TH287 gets into the brain with rapid clearance from the brain, kidney, and heart. The results presented here indicate that radiolabeled TH287 has favorable properties to be a useful tool for measuring MTH1 in vitro and for further evaluation for in vivo PET imaging MTH1 of brain tumors and other central nervous system disorders.

Mitochondrial apurinic/apyrimidinic endonuclease 1 enhances mtDNA repair contributing to cell proliferation and mitochondrial integrity in early stages of hepatocellular carcinoma

BACKGROUND

Hepatocellular carcinoma (HCC) is the leading cause of primary liver cancers. Surveillance of individuals at specific risk of developing HCC, early diagnostic markers, and new therapeutic approaches are essential to obtain a reduction in disease-related mortality. Apurinic/apyrimidinic endonuclease 1 (APE1) expression levels and its cytoplasmic localization have been reported to correlate with a lower degree of differentiation and shorter survival rate. The aim of this study is to fully investigate, for the first time, the role of the mitochondrial form of APE1 in HCC.

METHODS

As a study model, we analyzed samples from a cohort of patients diagnosed with HCC who underwent surgical resection. Mitochondrial APE1 content, expression levels of the mitochondrial import protein Mia40, and mtDNA damage of tumor tissue and distal non-tumor liver of each patient were analyzed. In parallel, we generated a stable HeLa clone for inducible silencing of endogenous APE1 and re-expression of the recombinant shRNA resistant mitochondrially targeted APE1 form (MTS-APE1). We evaluated mtDNA damage, cell growth, and mitochondrial respiration.

RESULTS

APE1's cytoplasmic positivity in Grades 1 and 2 HCC patients showed a significantly higher expression of mitochondrial APE1, which accounted for lower levels of mtDNA damage observed in the tumor tissue with respect to the distal area. In the contrast, the cytoplasmic positivity in Grade 3 was not associated with APE1's mitochondrial accumulation even when accounting for the higher number of mtDNA lesions measured. Loss of APE1 expression negatively affected mitochondrial respiration, cell viability, and proliferation as well as levels of mtDNA damage. Remarkably, the phenotype was efficiently rescued in MTS-APE1 clone, where APE1 is present only within the mitochondrial matrix.

CONCLUSIONS

Our study confirms the prominent role of the mitochondrial form of APE1 in the early stages of HCC development and the relevance of the non-nuclear fraction of APE1 in the disease progression. We have also confirmed overexpression of Mia40 and the role of the MIA pathway in the APE1 import process. Based on our data, inhibition of the APE1 transport by blocking the MIA pathway could represent a new therapeutic approach for reducing mitochondrial metabolism by preventing the efficient repair of mtDNA.

Altered Mitochondrial Dynamics in Motor Neuron Disease: An Emerging Perspective

Mitochondria plays privotal role in diverse pathways that regulate cellular function and survival, and have emerged as a prime focus in aging and age-associated motor neuron diseases (MNDs), such as amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Accumulating evidence suggests that many amyloidogenic proteins, including MND-associated RNA/DNA-binding proteins fused in sarcoma (FUS) and TAR DNA binding protein (TDP)-43, are strongly linked to mitochondrial dysfunction. Animal model and patient studies have highlighted changes in mitochondrial structure, plasticity, replication/copy number, mitochondrial DNA instability, and altered membrane potential in several subsets of MNDs, and these observations are consistent with the evidence of increased excitotoxicity, induction of reactive oxygen species, and activation of intrinsic apoptotic pathways. Studies in MND rodent models also indicate that mitochondrial abnormalities begin prior to the clinical and pathological onset of the disease, suggesting a causal role of mitochondrial dysfunction. Our recent studies, which demonstrated the involvement of specific defects in DNA break-ligation mediated by DNA ligase 3 (LIG3) in FUS-associated ALS, raised a key question of its potential implication in mitochondrial DNA transactions because LIG3 is essential for both mitochondrial DNA replication and repair. This question, as well as how wild-type and mutant MND-associated factors affect mitochondria, remain to be elucidated. These new investigation avenues into the mechanistic role of mitochondrial dysfunction in MNDs are critical to identify therapeutic targets to alleviate mitochondrial toxicity and its consequences. In this article, we critically review recent advances in our understanding of mitochondrial dysfunction in diverse subgroups of MNDs and discuss challenges and future directions.

Mitochondria in the signaling pathways that control longevity and health span

Genetic and pharmacological intervention studies have identified evolutionarily conserved and functionally interconnected networks of cellular energy homeostasis, nutrient-sensing, and genome damage response signaling pathways, as prominent regulators of longevity and health span in various species. Mitochondria are the primary sites of ATP production and are key players in several other important cellular processes. Mitochondrial dysfunction diminishes tissue and organ functional performance and is a commonly considered feature of the aging process. Here we review the evidence that through reciprocal and multilevel functional interactions, mitochondria are implicated in the lifespan modulation function of these pathways, which altogether constitute a highly dynamic and complex system that controls the aging process. An important characteristic of these pathways is their extensive crosstalk and apparent malleability to modification by non-invasive pharmacological, dietary, and lifestyle interventions, with promising effects on lifespan and health span in animal models and potentially also in humans.

Base-excision repair deficiency alone or combined with increased oxidative stress does not increase mtDNA point mutations in mice

Mitochondrial DNA (mtDNA) mutations become more prevalent with age and are postulated to contribute to the ageing process. Point mutations of mtDNA have been suggested to originate from two main sources, i.e. replicative errors and oxidative damage, but the contribution of each of these processes is much discussed. To elucidate the origin of mtDNA mutations, we measured point mutation load in mice with deficient mitochondrial base-excision repair (BER) caused by knockout alleles preventing mitochondrial import of the DNA repair glycosylases OGG1 and MUTYH (Ogg1 dMTS, Mutyh dMTS). Surprisingly, we detected no increase in the mtDNA mutation load in old Ogg1 dMTS mice. As DNA repair is especially important in the germ line, we bred the BER deficient mice for five consecutive generations but found no increase in the mtDNA mutation load in these maternal lineages. To increase reactive oxygen species (ROS) levels and oxidative damage, we bred the Ogg1 dMTS mice with tissue specific Sod2 knockout mice. Although increased superoxide levels caused a plethora of changes in mitochondrial function, we did not detect any changes in the mutation load of mtDNA or mtRNA. Our results show that the importance of oxidative damage as a contributor of mtDNA mutations should be re-evaluated.

Mouse Nudt13 is a Mitochondrial Nudix Hydrolase with NAD(P)H Pyrophosphohydrolase Activity

The mammalian NUDT13 protein possesses a sequence motif characteristic of the NADH pyrophosphohydrolase subfamily of Nudix hydrolases. Due to the persistent insolubility of the recombinant product expressed in Escherichia coli, active mouse Nudt13 was expressed in insect cells from a baculovirus vector as a histidine-tagged recombinant protein. In vitro, it efficiently hydrolysed NADH to NMNH and AMP and NADPH to NMNH and 2',5'-ADP and had a marked preference for the reduced pyridine nucleotides. Much lower activity was obtained with other nucleotide substrates tested. K m and k cat values for NADH were 0.34 mM and 7 s-1 respectively. Expression of Nudt13 as an N-terminal fusion to green fluorescent protein revealed that it was targeted exclusively to mitochondria by the N-terminal targeting peptide, suggesting that Nudt13 may act to regulate the concentration of mitochondrial reduced pyridine nucleotide cofactors and the NAD(P)+/NAD(P)H ratio in this organelle and elsewhere. Future studies of the enzymology of pyridine nucleotide metabolism in relation to energy homeostasis, redox control, free radical production and cellular integrity should consider the possible regulatory role of Nudt13.

MTH1 as a nucleotide pool sanitizing enzyme: Friend or foe?

8-Oxo-7,8-dihydroguanine (GO) can originate as 8-oxo-7,8-dihydro-2'-deoxyguanosine 5'-triphosphate (8-oxo-dGTP), an oxidized form of dGTP in the nucleotide pool, or by direct oxidation of guanine base in DNA. Accumulation of GO in cellular genomes can result in mutagenesis or programmed cell death, and is thus minimized by the actions of MutT homolog-1 (MTH1) with 8-oxo-dGTPase, OGG1 with GO DNA glycosylase and MutY homolog (MUTYH) with adenine DNA glycosylase. Studies on Mth1/Ogg1/Mutyh-triple knockout mice demonstrated that the defense systems efficiently minimize GO accumulation in cellular genomes, and thus maintain low incidences of spontaneous mutagenesis and tumorigenesis. Mth1/Ogg1-double knockout mice increased GO accumulation in the genome, but exhibited little susceptibility to spontaneous tumorigenesis, thus revealing that accumulation of GO in cellular genomes induces MUTYH-dependent cell death. Cancer cells are exposed to high oxidative stress levels and accumulate a high level of 8-oxo-dGTP in their nucleotide pools; cancer cells consequently express increased levels of MTH1 to eliminate 8-oxo-dGTP, indicating that increased expression of MTH1 in cancer cells may be detrimental for cancer patients. Mth1/Ogg1-double knockout mice are highly vulnerable to neurodegeneration under oxidative conditions, while transgenic expression of human MTH1 efficiently prevents neurodegeneration by avoiding GO accumulation in mitochondrial genomes of neurons and/or nuclear genomes of microglia, indicating that increased expression of MTH1 may be beneficial for neuronal tissues.

Association of MTH1 expression with the tumor malignant potential and poor prognosis in patients with resected lung cancer

OBJECTIVES

The oxidized purine nucleoside triphosphatase, mutT homolog 1 (MTH1), physiologically sanitizes 8-oxo-dGTP in the nucleotide pool. Previous studies indicated that MTH1 is associated with tumor proliferation and invasion in non-small cell lung cancer (NSCLC) cell lines; however, the role of MTH1 in patients with NSCLC remains unclear.

MATERIALS AND METHODS

Two patient cohorts that underwent surgery for NSCLC in our institution were investigated retrospectively. In one cohort consisting of 197 patients, the associations between MTH1 expression and clinicopathological factors or prognosis were analyzed. In another cohort consisting of 41 patients, the relationship between MTH1 expression in the tumors and serum oxidative stress levels (evaluated by the diacron-reactive oxygen metabolites [d-ROMs] test) or antioxidant capacity in the patients (evaluated by the biological antioxidant potential (BAP) test) was analyzed. A total of 238 patients were assessed for MTH1 protein levels using immunohistochemistry.

RESULTS

Among the 197 patients in the former cohort, 111 (56.3%) exhibited high MTH1 expression, while 86 (43.7%) exhibited low MTH1 expression. Male sex, smoking habit of ≥20 pack-years, squamous cell carcinoma, pathological stage ≥ II, tumor diameter ≥30mm, lymph node metastases, pleural invasion, lymphatic permeation and vascular infiltration were significantly associated with high MTH1 expression (p<0.05). The high MTH1 expression group had a significantly worse prognosis than that of the low MTH1 expression group (5-year overall survival: 81.6% vs. 92.3%, p=0.0011; 5-year disease-free survival: 55.0% vs. 83.7%, p=0.0002). d-ROMs and BAP test values were significantly higher in the high than in the low MTH1 expression group (p<0.05).

CONCLUSION

This study showed that MTH1 protein expression was closely related to factors associated with a high malignant potential and poor patient survival. MTH1 may be a novel therapeutic target for NSCLC.

Mammalian Mitochondria and Aging: An Update

Mitochondria were first postulated to contribute to aging more than 40 years ago. During the following decades, multiple lines of evidence in model organisms and humans showed that impaired mitochondrial function can contribute to age-associated disease phenotypes and aging. However, in contrast to the original theory favoring oxidative damage as a cause for mtDNA mutations, there are now strong data arguing that most mammalian mtDNA mutations originate as replication errors made by the mtDNA polymerase. Currently, a substantial amount of mitochondrial research is focused on finding ways to either remove or counteract the effects of mtDNA mutations with the hope of extending the human health- and lifespan. This review summarizes the current knowledge regarding the formation of mtDNA mutations and their impact on mitochondrial function. We also critically discuss proposed pathways interlinked with mammalian mtDNA mutations and suggest future research strategies to elucidate the role of mtDNA mutations in aging.

Structural and Kinetic Studies of the Human Nudix Hydrolase MTH1 Reveal the Mechanism for Its Broad Substrate Specificity

The human MutT homolog 1 (hMTH1, human NUDT1) hydrolyzes oxidatively damaged nucleoside triphosphates and is the main enzyme responsible for nucleotide sanitization. hMTH1 recently has received attention as an anticancer target because hMTH1 blockade leads to accumulation of oxidized nucleotides in the cell, resulting in mutations and death of cancer cells. Unlike Escherichia coli MutT, which shows high substrate specificity for 8-oxoguanine nucleotides, hMTH1 has broad substrate specificity for oxidized nucleotides, including 8-oxo-dGTP and 2-oxo-dATP. However, the reason for this broad substrate specificity remains unclear. Here, we determined crystal structures of hMTH1 in complex with 8-oxo-dGTP or 2-oxo-dATP at neutral pH. These structures based on high quality data showed that the base moieties of two substrates are located on the similar but not the same position in the substrate binding pocket and adopt a different hydrogen-bonding pattern, and both triphosphate moieties bind to the hMTH1 Nudix motif (i.e. the hydrolase motif) similarly and align for the hydrolysis reaction. We also performed kinetic assays on the substrate-binding Asp-120 mutants (D120N and D120A), and determined their crystal structures in complex with the substrates. Analyses of bond lengths with high-resolution X-ray data and the relationship between the structure and enzymatic activity revealed that hMTH1 recognizes the different oxidized nucleotides via an exchange of the protonation state at two neighboring aspartate residues (Asp-119 and Asp-120) in its substrate binding pocket. To our knowledge, this mechanism of broad substrate recognition by enzymes has not been reported previously and may have relevance for anticancer drug development strategies targeting hMTH1.

Prognostic impact of MutT homolog-1 expression on esophageal squamous cell carcinoma

MutT homolog‐1 (MTH1) is a pyrophosphatase that acts on oxidized nucleotides and hydrolyzes 8‐oxo‐2’‐deoxyguanosine triphosphate in deoxynucleoside triphosphate pool to prevent its incorporation into nuclear and mitochondrial DNA, result in reduce cytotoxicity in tumor cells. MTH1 is overexpressed in various cancers and is considered as a therapeutic target. Environmental factors such as cigarette smoking and alcohol consumption are critical risk factors for the development and progression of esophageal squamous cell carcinoma (ESCC), suggesting that oxidative stress contributes to the pathogenesis of ESCC. We examined the expression of MTH1 and the accumulation of 8‐oxo‐2’‐deoxyguanosine (8‐oxo‐dG) in 84 patients with ESCC who underwent curative resection without neoadjuvant therapy. MTH1 mRNA level was quantified by performing quantitative reverse transcription‐PCR. Immunohistochemical analysis of paraffin‐embedded cancer tissues was performed to determine MTH1 protein expression and 8‐oxo‐dG accumulation. MTH1 mRNA expression was higher in cancerous tissues than in the corresponding normal epithelium (P < 0.0001). Immunohistochemical analysis showed that high MTH1 expression was significantly associated with deeper tumor invasion and venous invasion, advanced cancer stage, and poor overall survival (P = 0.0021) and disease‐specific survival (P = 0.0013) compared with low MTH1 expression. Furthermore, high MTH1 expression was an independent predictor of poor disease‐specific survival (P = 0.0121). In contrast, 8‐oxo‐dG accumulation was not associated with any clinicopathological factor and poor prognosis. These results suggest that MTH1 overexpression is a predictor of ESCC progression and poor prognosis and that MTH1 can serve as a therapeutic target for treating patients with ESCC.

8-Oxoguanine accumulation in mitochondrial DNA causes mitochondrial dysfunction and impairs neuritogenesis in cultured adult mouse cortical neurons under oxidative conditions

Oxidative stress and mitochondrial dysfunction are implicated in aging-related neurodegenerative disorders. 8-Oxoguanine (8-oxoG), a common oxidised base lesion, is often highly accumulated in brains from patients with neurodegenerative disorders. MTH1 hydrolyses 8-oxo-2'-deoxyguanosine triphosphate (8-oxo-dGTP) to 8-oxo-dGMP and pyrophosphate in nucleotide pools, while OGG1 excises 8-oxoG paired with cytosine in DNA, thereby minimising the accumulation of 8-oxoG in DNA. Mth1/Ogg1-double knockout (TO-DKO) mice are highly susceptible to neurodegeneration under oxidative conditions and show increased accumulation of 8-oxoG in mitochondrial DNA (mtDNA) in neurons, suggesting that 8-oxoG accumulation in mtDNA causes mitochondrial dysfunction. Here, we evaluated the contribution of MTH1 and OGG1 to the prevention of mitochondrial dysfunction during neuritogenesis in vitro. We isolated cortical neurons from adult wild-type and TO-DKO mice and maintained them with or without antioxidants for 2 to 5 days and then examined neuritogenesis. In the presence of antioxidants, both TO-DKO and wild-type neurons exhibited efficient neurite extension and arborisation. However, in the absence of antioxidants, the accumulation of 8-oxoG in mtDNA of TO-DKO neurons was increased resulting in mitochondrial dysfunction. Cells also exhibited poor neurite outgrowth with decreased complexity of neuritic arborisation, indicating that MTH1 and OGG1 are essential for neuritogenesis under oxidative conditions.

Human REV3 DNA Polymerase Zeta Localizes to Mitochondria and Protects the Mitochondrial Genome

To date, mitochondrial DNA polymerase γ (POLG) is the only polymerase known to be present in mammalian mitochondria. A dogma in the mitochondria field is that there is no other polymerase present in the mitochondria of mammalian cells. Here we demonstrate localization of REV3 DNA polymerase in the mammalian mitochondria. We demonstrate localization of REV3 in the mitochondria of mammalian tissue as well as cell lines. REV3 associates with POLG and mitochondrial DNA and protects the mitochondrial genome from DNA damage. Inactivation of Rev3 leads to reduced mitochondrial membrane potential, reduced OXPHOS activity, and increased glucose consumption. Conversely, inhibition of the OXPHOS increases expression of Rev3. Rev3 expression is increased in human primary breast tumors and breast cancer cell lines. Inactivation of Rev3 decreases cell migration and invasion, and localization of Rev3 in mitochondria increases survival and the invasive potential of cancer cells. Taken together, we demonstrate that REV3 functions in mammalian mitochondria and that mitochondrial REV3 is associated with the tumorigenic potential of cells.

Eosinophil extracellular trap cell death-derived DNA traps: Their presence in secretions and functional attributes

BACKGROUND

Activated human eosinophils, as well as neutrophils, can release extracellular chromatin to form DNA traps through cytolytic extracellular trap cell death (ETosis). Although formations of neutrophil DNA traps are recognized in patients with various inflammatory conditions, neither the presence of ETosis-derived eosinophil DNA traps in human allergic diseases nor the characteristics of these DNA traps have been studied.

OBJECTIVE

We investigated the presence of ETosis-derived DNA traps in eosinophil-rich sinus and ear secretions and the functional attributes of ETosis DNA traps.

METHODS

Eosinophil-rich secretions obtained from patients with eosinophilic chronic rhinosinusitis and eosinophilic otitis media were studied microscopically. In vitro studies of ETosis and DNA trap formation used blood-derived eosinophils and neutrophils, and studies of the binding capacities of DNA traps used labeled bacteria and fluorescent microbeads. Stabilities of DNA traps were evaluated by using fluorescence microscopy.

RESULTS

Abundant nuclear histone H1-bearing DNA traps formed in vivo in the eosinophilic secretions and contributed to their increased viscosity. In vitro, after brief shear flow, eosinophil ETosis-elicited DNA traps assembled to form stable aggregates. Eosinophil DNA traps entrapped bacteria and fungi and, through hydrophobic interactions, microbeads. In comparison with neutrophil-derived DNA traps, eosinophil DNA traps ultrastructurally exhibited thicker fibers with globular structures and were less susceptible to leukocyte-derived proteolytic degradation, likely because of the lesser protease activities of eosinophils.

CONCLUSIONS

In human allergic diseases local cytolysis of eosinophils not only releases free eosinophil granules but also generates nuclear-derived DNA traps that are major extracellular structural components within eosinophil-rich secretions.

Different faces of mitochondrial DNA mutators

A number of studies have shown that ageing is associated with increased amounts of mtDNA deletions and/or point mutations in a variety of species as diverse as Caenorhabditis elegans, Drosophila melanogaster, mice, rats, dogs, primates and humans. This detected vulnerability of mtDNA has led to the suggestion that the accumulation of somatic mtDNA mutations might arise from increased oxidative damage and could play an important role in the ageing process by producing cells with a decreased oxidative capacity. However, the vast majority of DNA polymorphisms and disease-causing base-substitution mutations and age-associated mutations that have been detected in human mtDNA are transition mutations. They are likely arising from the slight infidelity of the mitochondrial DNA polymerase. Indeed, transition mutations are also the predominant type of mutation found in mtDNA mutator mice, a model for premature ageing caused by increased mutation load due to the error prone mitochondrial DNA synthesis. These particular misincorporation events could also be exacerbated by dNTP pool imbalances. The role of different repair, replication and maintenance mechanisms that contribute to mtDNA integrity and mutagenesis will be discussed in details in this article. This article is part of a Special Issue entitled: Mitochondrial Dysfunction in Aging.

Drosophila TAP/p32 is a core histone chaperone that cooperates with NAP-1, NLP, and nucleophosmin in sperm chromatin remodeling during fertilization

Nuclear DNA in the male gamete of sexually reproducing animals is organized as sperm chromatin compacted primarily by sperm-specific protamines. Fertilization leads to sperm chromatin remodeling, during which protamines are expelled and replaced by histones. Despite our increased understanding of the factors that mediate nucleosome assembly in the nascent male pronucleus, the machinery for protamine removal remains largely unknown. Here we identify four Drosophila protamine chaperones that mediate the dissociation of protamine-DNA complexes: NAP-1, NLP, and nucleophosmin are previously characterized histone chaperones, and TAP/p32 has no known function in chromatin metabolism. We show that TAP/p32 is required for the removal of Drosophila protamine B in vitro, whereas NAP-1, NLP, and Nph share roles in the removal of protamine A. Embryos from P32-null females show defective formation of the male pronucleus in vivo. TAP/p32, similar to NAP-1, NLP, and Nph, facilitates nucleosome assembly in vitro and is therefore a histone chaperone. Furthermore, mutants of P32, Nlp, and Nph exhibit synthetic-lethal genetic interactions. In summary, we identified factors mediating protamine removal from DNA and reconstituted in a defined system the process of sperm chromatin remodeling that exchanges protamines for histones to form the nucleosome-based chromatin characteristic of somatic cells.

Cellular levels of 8-oxoguanine in either DNA or the nucleotide pool play pivotal roles in carcinogenesis and survival of cancer cells

8-Oxoguanine, a major oxidized base lesion formed by reactive oxygen species, causes G to T transversion mutations or leads to cell death in mammals if it accumulates in DNA. 8-Oxoguanine can originate as 8-oxo-dGTP, formed in the nucleotide pool, or by direct oxidation of the DNA guanine base. MTH1, also known as NUDT1, with 8-oxo-dGTP hydrolyzing activity, 8-oxoguanine DNA glycosylase (OGG1) an 8-oxoG DNA glycosylase, and MutY homolog (MUTYH) with adenine DNA glycosylase activity, minimize the accumulation of 8-oxoG in DNA; deficiencies in these enzymes increase spontaneous and induced tumorigenesis susceptibility. However, different tissue types have different tumorigenesis susceptibilities. These can be reversed by combined deficiencies in the defense systems, because cell death induced by accumulation of 8-oxoG in DNA is dependent on MUTYH, which can be suppressed by MTH1 and OGG1. In cancer cells encountering high oxidative stress levels, a high level of 8-oxo-dGTP accumulates in the nucleotide pool, and cells therefore express increased levels of MTH1 in order to eliminate 8-oxo-dGTP. Suppression of MTH1 may be an efficient strategy for killing cancer cells; however, because MTH1 and OGG1 protect normal tissues from oxidative-stress-induced cell death, it is important that MTH1 inhibition does not increase the risk of healthy tissue degeneration.

Formation and repair of oxidative damage in the mitochondrial DNA

The mitochondrial DNA (mtDNA) encodes for only 13 polypeptides, components of 4 of the 5 oxidative phosphorylation complexes. But despite this apparently small numeric contribution, all 13 subunits are essential for the proper functioning of the oxidative phosphorylation circuit. Thus, accumulation of lesions, mutations and deletions/insertions in the mtDNA could have severe functional consequences, including mitochondrial diseases, aging and age-related diseases. The DNA is a chemically unstable molecule, which can be easily oxidized, alkylated, deaminated and suffer other types of chemical modifications, throughout evolution the organisms that survived were those who developed efficient DNA repair processes. In the last two decades, it has become clear that mitochondria have DNA repair pathways, which operate, at least for some types of lesions, as efficiently as the nuclear DNA repair pathways. The mtDNA is localized in a particularly oxidizing environment, making it prone to accumulate oxidatively generated DNA modifications (ODMs). In this article, we: i) review the major types of ODMs formed in mtDNA and the known repair pathways that remove them; ii) discuss the possible involvement of other repair pathways, just recently characterized in mitochondria, in the repair of these modifications; and iii) address the role of DNA repair in mitochondrial function and a possible cross-talk with other pathways that may potentially participate in mitochondrial genomic stability, such as mitochondrial dynamics and nuclear-mitochondrial signaling. Oxidative stress and ODMs have been increasingly implicated in disease and aging, and thus we discuss how variations in DNA repair efficiency may contribute to the etiology of such conditions or even modulate their clinical outcomes.

The maintenance of mitochondrial DNA integrity--critical analysis and update

DNA molecules in mitochondria, just like those in the nucleus of eukaryotic cells, are constantly damaged by noxious agents. Eukaryotic cells have developed efficient mechanisms to deal with this assault. The process of DNA repair in mitochondria, initially believed nonexistent, has now evolved into a mature area of research. In recent years, it has become increasingly appreciated that mitochondria possess many of the same DNA repair pathways that the nucleus does. Moreover, a unique pathway that is enabled by high redundancy of the mitochondrial DNA and allows for the disposal of damaged DNA molecules operates in this organelle. In this review, we attempt to present a unified view of our current understanding of the process of DNA repair in mitochondria with an emphasis on issues that appear controversial.

Ribonucleotide reductase association with mammalian liver mitochondria

Deoxyribonucleoside triphosphate pools in mammalian mitochondria are highly asymmetric, and this asymmetry probably contributes to the elevated mutation rate for the mitochondrial genome as compared with the nuclear genome. To understand this asymmetry, we must identify pathways for synthesis and accumulation of dNTPs within mitochondria. We have identified ribonucleotide reductase activity specifically associated with mammalian tissue mitochondria. Examination of immunoprecipitated proteins by mass spectrometry revealed R1, the large ribonucleotide reductase subunit, in purified mitochondria. Significant enzymatic and immunological activity was seen in rat liver mitochondrial nucleoids, isolated as described by Wang and Bogenhagen (Wang, Y., and Bogenhagen, D. F. (2006) J. Biol. Chem. 281, 25791-25802). Moreover, incubation of respiring rat liver mitochondria with [(14)C]cytidine diphosphate leads to accumulation of radiolabeled deoxycytidine and thymidine nucleotides within the mitochondria. Comparable results were seen with [(14)C]guanosine diphosphate. Ribonucleotide reduction within the mitochondrion, as well as outside the organelle, needs to be considered as a possibly significant contributor to mitochondrial dNTP pools.

hMTH1 expression protects mitochondria from Huntington's disease-like impairment

Huntington disease (HD) is a neurodegenerative disease caused by expansion of CAG repeats in the huntingtin (Htt) gene. The expression of hMTH1, the human hydrolase that degrades oxidized purine nucleoside triphosphates, grants protection in a chemical HD mouse model in which HD-like features are induced by the mitochondrial toxin 3-nitropropionic acid (3-NP). To further examine the relationship between oxidized dNTPs and HD-like neurodegeneration, we studied the effects of hMTH1 expression in a genetic cellular model for HD, such as striatal cells expressing mutant htt (Hdh^Q111). hMTH1 expression protected these cells from 3-NP and H₂O₂-induced killing, by counteracting the mutant htt-dependent increased vulnerability and accumulation of nuclear and mitochondrial DNA 8-hydroxyguanine levels. hMTH1 expression reverted the decreased mitochondrial membrane potential characteristic of Hdh^Q111 cells and delayed the increase in mitochondrial reactive oxygen species associated with 3-NP treatment. Further indications of hMTH1-mediated mitochondrial protection are the partial reversion of 3-NP-induced alterations in mitochondrial morphology and the modulation of DRP1 and MFN1 proteins, which control fusion/fission rates of mitochondria. Finally, in line with the in vitro findings, upon 3-NP in vivo treatment, 8-hydroxyguanine levels in mitochondrial DNA from heart, muscle and brain are significantly lower in transgenic hMTH1-expressing mice than in wild-type animals.

Mitochondrial DNA damage and its consequences for mitochondrial gene expression

How mitochondria process DNA damage and whether a change in the steady-state level of mitochondrial DNA damage (mtDNA) contributes to mitochondrial dysfunction are questions that fuel burgeoning areas of research into aging and disease pathogenesis. Over the past decade, researchers have identified and measured various forms of endogenous and environmental mtDNA damage and have elucidated mtDNA repair pathways. Interestingly, mitochondria do not appear to contain the full range of DNA repair mechanisms that operate in the nucleus, although mtDNA contains types of damage that are targets of each nuclear DNA repair pathway. The reduced repair capacity may, in part, explain the high mutation frequency of the mitochondrial chromosome. Since mtDNA replication is dependent on transcription, mtDNA damage may alter mitochondrial gene expression at three levels: by causing DNA polymerase γ nucleotide incorporation errors leading to mutations, by interfering with the priming of mtDNA replication by the mitochondrial RNA polymerase, or by inducing transcriptional mutagenesis or premature transcript termination. This review summarizes our current knowledge of mtDNA damage, its repair, and its effects on mtDNA integrity and gene expression. This article is part of a special issue entitled: Mitochondrial Gene Expression.

Functional regulation of FEN1 nuclease and its link to cancer

Flap endonuclease-1 (FEN1) is a member of the Rad2 structure-specific nuclease family. FEN1 possesses FEN, 5′-exonuclease and gap-endonuclease activities. The multiple nuclease activities of FEN1 allow it to participate in numerous DNA metabolic pathways, including Okazaki fragment maturation, stalled replication fork rescue, telomere maintenance, long-patch base excision repair and apoptotic DNA fragmentation. Here, we summarize the distinct roles of the different nuclease activities of FEN1 in these pathways. Recent biochemical and genetic studies indicate that FEN1 interacts with more than 30 proteins and undergoes post-translational modifications. We discuss how FEN1 is regulated via these mechanisms. Moreover, FEN1 interacts with five distinct groups of DNA metabolic proteins, allowing the nuclease to be recruited to a specific DNA metabolic complex, such as the DNA replication machinery for RNA primer removal or the DNA degradosome for apoptotic DNA fragmentation. Some FEN1 interaction partners also stimulate FEN1 nuclease activities to further ensure efficient action in processing of different DNA structures. Post-translational modifications, on the other hand, may be critical to regulate protein–protein interactions and cellular localizations of FEN1. Lastly, we also review the biological significance of FEN1 as a tumor suppressor, with an emphasis on studies of human mutations and mouse models.

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.

Mitochondrial import of PKCepsilon is mediated by HSP90: a role in cardioprotection from ischaemia and reperfusion injury

Aims

Protein kinase C epsilon (PKCε) is critical for cardiac protection from ischaemia and reperfusion (IR) injury. PKCε substrates that mediate cytoprotection reside in the mitochondria. However, the mechanism enabling mitochondrial translocation and import of PKCε to enable phosphorylation of these substrates is not known. Heat shock protein 90 (HSP90) is a cytoprotective protein chaperone that participates in mitochondrial import of a number of proteins. Here, we investigated the role of HSP90 in mitochondrial import of PKCε.

Methods and results

Using an ex vivo perfused rat heart model of IR, we found that PKCε translocates from the cytosol to the mitochondrial fraction following IR. Immunogold electron microscopy and mitochondrial fractionation demonstrated that following IR, mitochondrial PKCε is localized within the mitochondria, on the inner mitochondrial membrane. Pharmacological inhibition of HSP90 prevented IR-induced interaction between PKCε and the translocase of the outer membrane (Tom20), reduced mitochondrial import of PKCε, and increased necrotic cell death by ∼70%. Using a rational approach, we designed a 7-amino acid peptide activator of PKCε, derived from an HSP90 homologous sequence located in the C2 domain of PKCε (termed ψεHSP90). Treatment with this peptide (conjugated to the cell permeating TAT protein-derived peptide, TAT_47–57) increased PKCε–HSP90 protein–protein interaction, enhanced mitochondrial translocation of PKCε, increased phosphorylation and activity of an intra-mitochondrial PKCε substrate, aldehyde dehydrogenase 2, and reduced cardiac injury in ex vivo and in vivo models of myocardial infarction.

Conclusion

Our results suggest that HSP90-mediated mitochondrial import of PKCε plays an important role in the protection of the myocardium from IR injury.

DNA repair in mammalian mitochondria: Much more than we thought?

For many years, the repair of most damage in mitochondrial DNA (mtDNA) was thought limited to short-patch base excision repair (SP-BER), which replaces a single nucleotide by the sequential action of DNA glycosylases, an apurinic/apyrimidinic (AP) endonuclease, the mitochondrial DNA polymerase gamma, an abasic lyase activity, and mitochondrial DNA ligase. However, the likely array of lesions inflicted on mtDNA by oxygen radicals and the possibility of replication errors and disruptions indicated that such a restricted repair repertoire would be inadequate. Recent studies have considerably expanded our knowledge of mtDNA repair to include long-patch base excision repair (LP-BER), mismatch repair, and homologous recombination and nonhomologous end-joining. In addition, elimination of mutagenic 8-oxodeoxyguanosine triphosphate (8-oxodGTP) helps prevent cell death due to the accumulation of this oxidation product in mtDNA. Although it was suspected for many years that irreparably damaged mtDNA might be targeted for degradation, only recently was clear evidence provided for this hypothesis. Therefore, multiple DNA repair pathways and controlled degradation of mtDNA function together to maintain the integrity of mitochondrial genome.

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.

Cell Cycle Arrest Induced by Hydrogen Peroxide Is Associated with Modulation of Oxidative Stress Related Genes in Breast Cancer Cells

Depending on the amounts present, reactive oxygen species can exert either beneficial or deleterious effect to cells. In the present study, we observed a decrease in cell viability concomitant with an increase of malondialdehyde concentration in hydrogen peroxide (H2O2)-treated MCF-7 breast cancer cells. There was also a concurrent G1/S phase cell cycle arrest with increased apoptosis in H2O2-treated cells. Analysis of 84 oxidative stress related genes showed that five genes were significantly and differentially regulated, namely, Cytoglobin (CYGB), Forkhead box M1 (FOXM1), NADPH oxidase ( NOX5), Nudix (nucleoside diphosphate linked moiety X)-type motif 1 (NUDT1) and Selenoprotein P1 (SEPP1) genes with H2O2 treatment. It would seem that oxidative stress induces cell cycle arrest in the breast cancer by modulation of these genes. Manipulation of these genes, in particular FOXM1, a proliferation-specific gene associated with human malignancies, could stifle cancer progression and enhance the therapeutic efficacy of drugs which exert their effects by oxidative stress.

A role for oxidized DNA precursors in Huntington's disease-like striatal neurodegeneration

Several human neurodegenerative disorders are characterized by the accumulation of 8-oxo-7,8-dihydroguanine (8-oxodG) in the DNA of affected neurons. This can occur either through direct oxidation of DNA guanine or via incorporation of the oxidized nucleotide during replication. Hydrolases that degrade oxidized purine nucleoside triphosphates normally minimize this incorporation. hMTH1 is the major human hydrolase. It degrades both 8-oxodGTP and 8-oxoGTP to the corresponding monophosphates. To investigate whether the incorporation of oxidized nucleic acid precursors contributes to neurodegeneration, we constructed a transgenic mouse in which the human hMTH1 8-oxodGTPase is expressed. hMTH1 expression protected embryonic fibroblasts and mouse tissues against the effects of oxidants. Wild-type mice exposed to 3-nitropropionic acid develop neuropathological and behavioural symptoms that resemble those of Huntington's disease. hMTH1 transgene expression conferred a dramatic protection against these Huntington's disease–like symptoms, including weight loss, dystonia and gait abnormalities, striatal degeneration, and death. In a complementary approach, an in vitro genetic model for Huntington's disease was also used. hMTH1 expression protected progenitor striatal cells containing an expanded CAG repeat of the huntingtin gene from toxicity associated with expression of the mutant huntingtin. The findings implicate oxidized nucleic acid precursors in the neuropathological features of Huntington's disease and identify the utilization of oxidized nucleoside triphosphates by striatal cells as a significant contributor to the pathogenesis of this disorder.

The oxidized purine 8-oxo-7,8-dihydroguanine (8-oxodG) is often found at high levels in DNA of post-mortem brains of patients afflicted by Huntington's disease (HD) and other neurodegenerative conditions. It has been unclear, however, whether this damage accumulation is a consequence or a cause of neurodegeneration. We have addressed this problem in a transgenic mouse model in which the human hMTH1 8-oxodGTPase, which hydrolyzes oxidized nucleoside triphosphates to prevent the incorporation of oxidized bases into nucleic acids, was expressed in several organs. We report that high-level hMTH1 expression in the transgenic mouse brain provides robust protection against neurodegeneration as well as HD-like behavioural and neuropathological symptoms in a chemical model of HD (the mitochondrial toxin 3-nitropropionic acid). In a complementary approach, we used a genetic model for HD based on the use of progenitor striatal cells containing an expanded CAG repeat of the huntingtin gene. Also, in this case, hMTH1 provided strong protection against cell death associated with expression of the mutant huntingtin protein. The new findings we report shed important light on the connection between oxidative DNA damage and human neurodegenerative disorders. Our findings go some way towards forging a causative link between neurodegeneration and oxidative DNA/RNA damage and identify the oxidized purine nucleoside triphosphates as important contributors.

Molecular characterization of organelle-type Nudix hydrolases in Arabidopsis

Nudix (for nucleoside diphosphates linked to some moiety X) hydrolases act to hydrolyze ribonucleoside and deoxyribonucleoside triphosphates, nucleotide sugars, coenzymes, or dinucleoside polyphosphates. Arabidopsis (Arabidopsis thaliana) contains 27 genes encoding Nudix hydrolase homologues (AtNUDX1 to -27) with a predicted distribution in the cytosol, mitochondria, and chloroplasts. Previously, cytosolic Nudix hydrolases (AtNUDX1 to -11 and -25) were characterized. Here, we conducted a characterization of organelle-type AtNUDX proteins (AtNUDX12 to -24, -26, and -27). AtNUDX14 showed pyrophosphohydrolase activity toward both ADP-ribose and ADP-glucose, although its K(m) value was approximately 100-fold lower for ADP-ribose (13.0+/-0.7 microm) than for ADP-glucose (1,235+/-65 microm). AtNUDX15 hydrolyzed not only reduced coenzyme A (118.7+/-3.4 microm) but also a wide range of its derivatives. AtNUDX19 showed pyrophosphohydrolase activity toward both NADH (335.3+/-5.4 microm) and NADPH (36.9+/-3.5 microm). AtNUDX23 had flavin adenine dinucleotide pyrophosphohydrolase activity (9.1+/-0.9 microm). Both AtNUDX26 and AtNUDX27 hydrolyzed diadenosine polyphosphates (n=4-5). A confocal microscopic analysis using a green fluorescent protein fusion protein showed that AtNUDX15 is distributed in mitochondria and AtNUDX14 -19, -23, -26, and -27 are distributed in chloroplasts. These AtNUDX mRNAs were detected ubiquitously in various Arabidopsis tissues. The T-DNA insertion mutants of AtNUDX13, -14, -15, -19, -20, -21, -25, -26, and -27 did not exhibit any phenotypical differences under normal growth conditions. These results suggest that Nudix hydrolases in Arabidopsis control a variety of metabolites and are pertinent to a wide range of physiological processes.

Molecular mechanisms of the combination of retinoid and interferon-gamma for inducing differentiation and increasing apoptosis in human glioblastoma T98G and U87MG cells

Glioblastoma is the deadliest brain tumor that remains incurable. We examined efficacy of combination of retinoid and interferon-gamma (IFN-gamma) in human glioblastoma T98G and U87MG cells. We conjectured that retinoid could induce differentiation with down regulation of telomerase activity to increase sensitivity to IFN-gamma for apoptosis in glioblastoma cells. Indeed, treatment of cells with 1 muM all-trans retinoic acid (ATRA) or 1 muM 13-cis retinoic acid (13-CRA) for 7 days induced astrocytic differentiation with upregulation of glial fibrillary acidic protein (GFAP) and down regulation of telomerase activity. Wright staining and ApopTag assay showed, respectively, morphological and biochemical features of apoptosis in glioblastoma cells following exposure to 200 units/ml IFN-gamma for 48 h. Induction of differentiation was associated with decreases in levels of nuclear factor kappa B (NFkappaB), inducible nitric oxide synthase (iNOS), and production of nitric oxide (NO) so as to increase sensitivity to IFN-gamma for apoptosis. Notably, IFN-gamma induced signal transducer and activator of transcription-1 (STAT-1) to bind to gamma-activated sequence (GAS) of the target gene. Also, IFN-gamma activated caspase-8 and cleaved Bid to truncated Bid (tBid) for translocation to mitochondria. Fura-2 assay showed increases in intracellular free [Ca2+] and activation of calpain in apoptotic cells. Besides, increases in Bax:Bcl-2 ratio and mitochondrial release of cytochrome c and Smac into the cytosol activated caspase-9 and caspase-3 for apoptosis. Taken together, our results showed that retinoid induced astrocytic differentiation with down regulation of telomerase activity and enhanced sensitivity to IFN-gamma for increasing apoptosis in human glioblastoma cells.

Mitochondrial DNA repair in aging and disease

Mitochondria are organelles which, according to the endosymbiosis theory, evolved from purpurbacteria approximately 1.5 billion years ago. One of the unique features of mitochondria is that they have their own genome. Mitochondria replicate and transcribe their DNA semiautonomously. Like nuclear DNA, mitochondrial DNA (mtDNA) is constantly exposed to DNA damaging agents. Regarding the repair of mtDNA, the prevailing concept for many years was that mtDNA molecules suffering an excess of damage would simply be degraded to be replaced by newly generated successors copied from undamaged genomes. However, evidence now clearly shows that mitochondria contain the machinery to repair the damage to their genomes caused by certain endogenous or exogenous damaging agents. The link between mtDNA damage and repair to aging, neurodegeneration, and carcinogenesis-associated processes is the subject of this review.

Trace amounts of 8-oxo-dGTP in mitochondrial dNTP pools reduce DNA polymerase gamma replication fidelity

Replication of the mitochondrial genome by DNA polymerase γ requires dNTP precursors that are subject to oxidation by reactive oxygen species generated by the mitochondrial respiratory chain. One such oxidation product is 8-oxo-dGTP, which can compete with dTTP for incorporation opposite template adenine to yield A-T to C-G transversions. Recent reports indicate that the ratio of undamaged dGTP to dTTP in mitochondrial dNTP pools from rodent tissues varies from ∼1:1 to >100:1. Within this wide range, we report here the proportion of 8-oxo-dGTP in the dNTP pool that would be needed to reduce the replication fidelity of human DNA polymerase γ. When various in vivo mitochondrial dNTP pools reported previously were used here in reactions performed in vitro, 8-oxo-dGTP was readily incorporated opposite template A and the resulting 8-oxo-G-A mismatch was not proofread efficiently by the intrinsic 3′ exonuclease activity of pol γ. At the dNTP ratios reported in rodent tissues, whether highly imbalanced or relatively balanced, the amount of 8-oxo-dGTP needed to reduce fidelity was <1% of dGTP. Moreover, direct measurements reveal that 8-oxo-dGTP is present at such concentrations in the mitochondrial dNTP pools of several rat tissues. The results suggest that oxidized dNTP precursors may contribute to mitochondrial mutagenesis in vivo, which could contribute to mitochondrial dysfunction and disease.

Two distinct pathways of cell death triggered by oxidative damage to nuclear and mitochondrial DNAs

Oxidative base lesions, such as 8-oxoguanine (8-oxoG), accumulate in nuclear and mitochondrial DNAs under oxidative stress, resulting in cell death. However, it is not known which form of DNA is involved, whether nuclear or mitochondrial, nor is it known how the death order is executed. We established cells which selectively accumulate 8-oxoG in either type of DNA by expression of a nuclear or mitochondrial form of human 8-oxoG DNA glycosylase in OGG1-null mouse cells. The accumulation of 8-oxoG in nuclear DNA caused poly-ADP-ribose polymerase (PARP)-dependent nuclear translocation of apoptosis-inducing factor, whereas that in mitochondrial DNA caused mitochondrial dysfunction and Ca2+ release, thereby activating calpain. Both cell deaths were triggered by single-strand breaks (SSBs) that had accumulated in the respective DNAs, and were suppressed by knockdown of adenine DNA glycosylase encoded by MutY homolog, thus indicating that excision of adenine opposite 8-oxoG lead to the accumulation of SSBs in each type of DNA. SSBs in nuclear DNA activated PARP, whereas those in mitochondrial DNA caused their depletion, thereby initiating the two distinct pathways of cell death.

A novel compound, maltolyl p-coumarate, attenuates cognitive deficits and shows neuroprotective effects in vitro and in vivo dementia models

To develop a novel and effective drug that could enhance cognitive function and neuroprotection, we newly synthesized maltolyl p-coumarate by the esterification of maltol and p-coumaric acid. In the present study, we investigated whether maltolyl p-coumarate could improve cognitive decline in scopolamine-injected rats and in amyloid beta peptide(1-42)-infused rats. Maltolyl p-coumarate was found to attenuate cognitive deficits in both rat models using passive avoidance test and to reduce apoptotic cell death observed in the hippocampus of the amyloid beta peptide(1-42)-infused rats. We also examined the neuroprotective effects of maltolyl p-coumarate in vitro using SH-SY5Y cells. Cells were pretreated with maltolyl p-coumarate, before exposed to amyloid beta peptide(1-42), glutamate or H2O2. We found that maltolyl p-coumarate significantly decreased apoptotic cell death and reduced reactive oxygen species, cytochrome c release, and caspase 3 activation. Taking these in vitro and in vivo results together, our study suggests that maltolyl p-coumarate is a potentially effective candidate against Alzheimer's disease that is characterized by wide spread neuronal death and progressive decline of cognitive function.

AtNUDX1, an 8-oxo-7,8-dihydro-2'-deoxyguanosine 5'-triphosphate pyrophosphohydrolase, is responsible for eliminating oxidized nucleotides in Arabidopsis

Cellular DNA, RNA and their precursor nucleotides are at high risk of being oxidized by reactive oxygen species. An oxidized base, 8-oxo-7,8-dihydro-2'-(deoxy)guanosine, can pair with both adenine and cytosine, and thus would cause both replicational and translational errors. Previously, we have reported that an Arabidopsis Nudix hydrolase, AtNUDX1, acts to hydrolyze an oxidized deoxyribonucleotide, 8-oxo-7,8-dihydro-2'-deoxyguanosine 5'-triphosphate (8-oxo-dGTP). Here we showed that 8-oxo-dGTP pyrophosphohydrolase activity is not exhibited by any other Arabidopsis Nudix hydrolase. AtNUDX1 acted on an oxidized ribonucleotide, 8-oxo-GTP, with high affinity (K(m) 28.1 microM). In a transcriptional mutational analysis using the lacZ reporter gene, the phenotypic suppression of the lacZ amber mutation in a mutT-deficient Escherichia coli strain caused by the misincorporation of 8-oxo-GTP into the mRNA was significantly diminished by expression of AtNUDX1. These findings suggest that AtNUDX1 prevents transcriptional errors in vivo. A confocal microscopic analysis using a green fluorescent protein (GFP) fusion protein demonstrated that AtNUDX1 is distributed in the cytosol, where the main pool of nucleotides in the cells exists. The level of 8-oxo-guanosine in genomic DNA was significantly increased in knockout nudx1 plants compared with wild-type plants under normal and oxidative stress (3 microM paraquat) conditions. The results obtained here indicate that AtNUDX1 functions in cellular defense against oxidative DNA and RNA damage through the sanitization of their precursor pools in the cytosol in Arabidopsis cells.

MyD88-5 links mitochondria, microtubules, and JNK3 in neurons and regulates neuronal survival

The innate immune system relies on evolutionally conserved Toll-like receptors (TLRs) to recognize diverse microbial molecular structures. Most TLRs depend on a family of adaptor proteins termed MyD88s to transduce their signals. Critical roles of MyD88-1–4 in host defense were demonstrated by defective immune responses in knockout mice. In contrast, the sites of expression and functions of vertebrate MyD88-5 have remained elusive. We show that MyD88-5 is distinct from other MyD88s in that MyD88-5 is preferentially expressed in neurons, colocalizes in part with mitochondria and JNK3, and regulates neuronal death. We prepared MyD88-5/GFP transgenic mice via a bacterial artificial chromosome to preserve its endogenous expression pattern. MyD88-5/GFP was detected chiefly in the brain, where it associated with punctate structures within neurons and copurified in part with mitochondria. In vitro, MyD88-5 coimmunoprecipitated with JNK3 and recruited JNK3 from cytosol to mitochondria. Hippocampal neurons from MyD88-5–deficient mice were protected from death after deprivation of oxygen and glucose. In contrast, MyD88-5–null macrophages behaved like wild-type cells in their response to microbial products. Thus, MyD88-5 appears unique among MyD88s in functioning to mediate stress-induced neuronal toxicity.

The DNA polymerase gamma Y955C disease variant associated with PEO and parkinsonism mediates the incorporation and translesion synthesis opposite 7,8-dihydro-8-oxo-2'-deoxyguanosine

Mitochondrial DNA is replicated and repaired by DNA polymerase gamma (pol gamma), encoded by the POLG gene. The Y955C substitution in POLG leads to autosomal dominant progressive external ophthalmoplegia (PEO) with other severe phenotypes. PEO patients with this mutation can further develop parkinsonism or premature ovarian failure. Mouse and yeast models with this mutation show enhanced amounts of oxidative lesions and increased mtDNA damage. In DNA pol gamma, Tyr955 plays a critical role in catalysis and high fidelity DNA synthesis. 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxo-dG) is one of the most common oxidative lesions in DNA and can promote transversion mutations. Mitochondria are thought to be a major source of endogenous reactive oxygen species that can react with dG to form 8-oxo-dG as one of the more common products. DNA polymerases can mitigate mutagenesis by 8-oxo-dG through allosteric interactions from amino acid side chains, which limit the anti-conformation of the 8-oxo-dG template base during translesion DNA synthesis. Here, we show that the Y955C pol gamma displays relaxed discrimination when either incorporating 8-oxo-dGTP or translesion synthesis opposite 8-oxo-dG. Molecular modeling and biochemical analysis suggest that this residue, Tyr955, in conjunction with Phe961 helps attenuate the anti-conformation in human pol gamma for error free bypass of 8-oxo-dG and substitution to Cys allows the mutagenic syn conformation. Collectively, these results offer a biochemical link between the observed oxidative stress in model systems and parkinsonism in patients, suggesting that patients harboring the Y955C POLG mutation may undergo enhanced oxidative stress and DNA mutagenesis.