Increases of mitochondrial mass and mitochondrial genome in association with enhanced oxidative stress in human cells harboring 4,977 BP-deleted mitochondrial DNA

Mitochondrial diseases: from molecular mechanisms to therapeutic advances

Mitochondria are essential for cellular function and viability, serving as central hubs of metabolism and signaling. They possess various metabolic and quality control mechanisms crucial for maintaining normal cellular activities. Mitochondrial genetic disorders can arise from a wide range of mutations in either mitochondrial or nuclear DNA, which encode mitochondrial proteins or other contents. These genetic defects can lead to a breakdown of mitochondrial function and metabolism, such as the collapse of oxidative phosphorylation, one of the mitochondria's most critical functions. Mitochondrial diseases, a common group of genetic disorders, are characterized by significant phenotypic and genetic heterogeneity. Clinical symptoms can manifest in various systems and organs throughout the body, with differing degrees and forms of severity. The complexity of the relationship between mitochondria and mitochondrial diseases results in an inadequate understanding of the genotype-phenotype correlation of these diseases, historically making diagnosis and treatment challenging and often leading to unsatisfactory clinical outcomes. However, recent advancements in research and technology have significantly improved our understanding and management of these conditions. Clinical translations of mitochondria-related therapies are actively progressing. This review focuses on the physiological mechanisms of mitochondria, the pathogenesis of mitochondrial diseases, and potential diagnostic and therapeutic applications. Additionally, this review discusses future perspectives on mitochondrial genetic diseases.

Evidence of compensation for mitochondrial reactive oxygen species increase in Caenorhabditis briggsae cytoplasmic-nuclear hybrids

Hybrid offspring dysfunction in cytoplasmic-nuclear hybrids (cybrids) implies that one parent's mitochondrial genome is incompatible with the nuclear genome of the other parent. In Caenorhabditis briggsae , cybrids exhibit increased mitochondrial reactive oxygen species (ROS). In this study, we measured the specific activity of markers for mitochondrial abundance (citrate synthase) and antioxidant enzyme response (catalase) in four C. briggsae cybrid lines. An increase of catalase expression but not in mitochondrial abundance was found in dysfunctional cybrids. This suggests that organisms might compensate for some genetic incompatibilities by modulating gene expression of key oxidative stress enzymes such as catalase.

Prenatal arsenic metabolite exposure is associated with increased newborn mitochondrial DNA copy number: evidence from a birth cohort study

While mitochondria are susceptible to environmental detriments, little is known about potential associations between arsenic metabolites and mitochondria DNA copy number (mtDNAcn). We attempted to examine whether maternal urinary arsenic metabolite levels in different trimesters were related to neonatal cord blood mtDNAcn. We included 819 mother-newborn pairs embedded in an in-progress birth cohort survey performed from April 2014 to October 2016 in Wuhan, China. We determined maternal urinary arsenic species concentrations in different trimesters. We determined cord blood mtDNAcn using quantitative real-time polymerase chain reaction. In covariate-adjusted models, each one-unit increment of dimethylated arsenic (DMA) and total arsenic (TAs) in the third trimester was related to 8.43% (95% CI 1.13%, 16.26%) and 12.15% (95% CI 4.35%, 20.53%) increases in mtDNAcn, respectively. The dose-response trend with statistical significance was observed across tertiles of DMA and TAs in the third trimester with mtDNAcn (DMA percent changes (%Δ) = 25.60 (95% CI 6.73, 47.82), for the highest vs the lowest tertile (P = 0.02); TAs %Δ = 40.31 (95% CI 19.25, 65.10), for the highest vs the lowest tertile (P = 0.0002)). These findings may prove the relationships between prenatal arsenic species levels and neonatal mitochondrial dysfunction.

Salivary Biomarkers for Oral Cancer Detection: Insights from Human DNA and RNA Analysis

Oral cancer is a significant global health concern, with a high mortality rate mainly due to late-stage diagnosis. Early detection plays a critical role in improving patient outcomes, highlighting the need for non-invasive and accessible screening methods. Salivary biomarkers have emerged as a promising avenue for oral cancer detection, leveraging advancements in human DNA and RNA analysis. Several DNA-based biomarkers, such as genetic mutations, chromosomal aberrations, and epigenetic alterations, have shown promise in detecting oral cancer at various stages. Likewise, RNA-based biomarkers, including microRNAs, long non-coding RNAs, and messenger RNAs, have demonstrated potential for diagnosing oral cancer and predicting treatment outcomes. The integration of high-throughput sequencing technologies, such as next-generation sequencing and transcriptomic profiling, has enabled the identification of novel biomarkers and provided deeper insights into the molecular mechanisms underlying oral cancer development and progression. Despite the promising results, challenges remain in standardizing sample collection, establishing robust biomarker panels, and validating their clinical utility. Nevertheless, salivary biomarkers hold great promise as a non-invasive, cost-effective, and accessible approach for oral cancer detection, ultimately leading to improved patient outcomes through early diagnosis and intervention. The analysis of genetic material obtained from saliva offers several advantages, including ease of collection, non-invasiveness, and the potential for repeated sampling. Furthermore, saliva reflects the physiological and pathological status of the oral cavity, making it an ideal source for biomarker discovery and validation. This article presents a comprehensive review of the current research on salivary biomarkers for oral cancer detection, focusing on insights gained from human DNA and RNA analysis.

Altered cord blood mitochondrial DNA content and prenatal exposure to arsenic metabolites in low-arsenic areas

While mitochondria are susceptible to environmental detriments, little is known about potential associations between arsenic metabolites and mitochondria DNA copy number (mtDNAcn). We attempted to examine whether arsenic metabolism in different trimesters was related to cord blood mtDNAcn alteration. We included 819 mother-newborn pairs embedded in an in-progress birth cohort survey performed from April 2014 to October 2016 in Wuhan, China. We determined maternal urinary arsenic species concentrations in different trimesters using HPLC-ICPMS. We decided on cord blood mtDNAcn using quantitative real-time polymerase chain reaction. In covariate-adjusted models, each two-fold increment of dimethylated arsenic (DMA) and total arsenic (TAs) in the 3rd trimester were related to 8.43% (95% CI: 1.13%, 16.26%) and 12.15% (95% CI:4.35%, 20.53%) increases in mtDNAcn, respectively. The dose-response trend with statistical significance was observed across tertiles of DMA and TAs in the 3rd trimester with mtDNAcn. These findings may prove the relationships between arsenic species and mitochondrial dysfunction.

MtDNA Copy Number in Oral Epithelial Cells Serves as a Potential Biomarker of Mitochondrial Damage by Neonicotinoid Exposure: A Cross-Sectional Study

As the mitochondrial DNA copy number (mtDNAcn) has been reported to be a biomarker for mtDNA damage in honeybees when exposed to sublethal neonicotinoids, the feasibility of using human mitochondria as a predictor upon neonicotinoid exposure remains elusive. This study investigated the association between the urinary neonicotinoid and the relative mtDNAcn (RmtDNAcn) of oral epithelial cells collected in a cross-sectional study with repeated measurements over 6 weeks. The molecular mechanism underlying neonicotinoid-caused mitochondrial damage was also examined by in vitro assay. Herein, the average integrated urinary neonicotinoid (IMIRPF) concentration ranged from 8.01 to 13.70 μg/L (specific gravity-adjusted) during the sampling period. Concomitantly, with an increase in the urinary IMIRPF, the RmtDNAcn significantly increased from 1.20 (low group) to 1.93 (high group), indicating potential dose-dependent mitochondrial damage. Furthermore, the linear regression analysis confirmed the significant correlation between the IMIRPF and RmtDNAcn. Results from in vitro assays demonstrated that neonicotinoid exposure led to the inhibition of the genes encoding mitochondrial oxidative phosphorylation (OXPHOS) complexes I and III (e.g., ND2, ND6, CytB, and CYC1), accompanied by increased reactive oxygen species production in SH-SY5Y cells. Conjointly, neonicotinoid exposure led to mitochondrial dysfunction and a resulting increase in the RmtDNAcn, which may serve as a plausible biomarker in humans.

Potential Roles of mtDNA Mutations in PCOS-IR: A Review

Polycystic ovary syndrome (PCOS) is the most common heterogeneous endocrine disease that affecting females in reproductive age. Insulin resistance (IR), an important molecular basis for PCOS, accounts for at least 75% of women carrying this syndrome. Although there have been many studies on PCOS-IR, the detailed mechanisms are not fully understood. As essential hub for energy generation, mitochondria are critical to insulin secretion and normal function, whereas mutations in mitochondrial DNA (mtDNA) result in mitochondrial dysfunctions contributing to the pathophysiology of PCOS-IR via the regulation of balance of oxidative stress (OS), energy deficiency, or hormone metabolism. In the current review, we summarize the clinical and molecular features of PCOS-IR and discuss molecular mechanisms related to mtDNA mutations.

Induction of Oxidative Stress in SH-SY5Y Cells by Overexpression of hTau40 and Its Mitigation by Redox-Active Nanoparticles

Abnormally phosphorylated tau protein is the principal component of neurofibrillary tangles, accumulating in the brain in many neurodegenerative diseases, including Alzheimer's disease. The aim of this study was to examine whether overexpression of tau protein leads to changes in the redox status of human neuroblastoma SH-SY5Y cells. The level of reactive oxygen species (ROS) was elevated in tau-overexpressing cells (TAU cells) as compared with cells transfected with the empty vector (EP cells). The level of glutathione was increased in TAU cells, apparently due to overproduction as an adaptation to oxidative stress. The TAU cells had elevated mitochondrial mass. They were more sensitive to 6-hydroxydopamine, delphinidin, 4-amino-TEMPO, and nitroxide-containing nanoparticles (NPs) compared to EP controls. These results indicate that overexpression of the tau protein imposes oxidative stress on the cells. The nitroxide 4-amino-TEMPO and nitroxide-containing nanoparticles (NPs) mitigated oxidative stress in TAU cells, decreasing the level of ROS. Nitroxide-containing nanoparticles lowered the level of lipid peroxidation in both TAU and EP cells, suggesting that nitroxides and NPs may mitigate tau-protein-induced oxidative stress.

The war against Alzheimer, the mitochondrion strikes back!

Alzheimer's disease (AD) is a leading neurodegenerative pathology associated with aging worldwide. It is estimated that AD prevalence will increase from 5.8 million people today to 13.8 million by 2050 in the United States alone. AD effects in the brain are well known; however, there is still a lack of knowledge about the cellular mechanisms behind the origin of AD. It is known that AD induces cellular stress affecting the energy metabolism in brain cells. During the pathophysiological advancement of AD, damaged mitochondria enter a vicious cycle, producing reactive oxygen species (ROS), harming mitochondrial DNA and proteins, leading to more ROS and cellular death. Additionally, mitochondria are interconnected with the plaques formed by amyloid-β in AD and have underlying roles in the progression of the disease and severity. For years, the biomedical field struggled to develop new therapeutic options for AD without a significant advancement. However, mitochondria are striking back existing outside cells in a new mechanism of intercellular communication. Extracellular mitochondria are exchanged from healthy to damaged cells to rescue those with a perturbed metabolism in a process that could be applied as a new therapeutic option to repair those brain cells affected by AD. In this review we highlight key aspects of mitochondria's role in CNS' physiology and neurodegenerative disorders, focusing on AD. We also suggest how mitochondria strikes back as a therapeutic target and as a potential agent to be transplanted to repair neurons affected by AD.

Peripheral blood mononuclear cell mitochondrial copy number and adenosine triphosphate inhibition test in NAFLD

BACKGROUND AND AIM

Non-alcoholic fatty liver disease (NAFLD) is associated with mitochondrial dysfunction. This study aims to develop biomarkers for assessing mitochondrial dysfunction in patients with NAFLD.

METHODS

Mitochondrion-associated transcriptome analysis was performed. Peripheral blood mononuclear cells obtained from patients with NAFLD (69) and healthy controls (19) were used to determine the mitochondrial DNA (mtDNA) copy number. A mitochondrial inhibition substrate test (ATP assay) was performed in HepG2 cells using the patient serum.

RESULTS

Hepatic mRNA transcriptome analysis showed that the gene expression related to mitochondrial functions (mitochondrial fusion, apoptotic signal, and mitochondrial envelope) increased in patients with steatohepatitis, but not in those with NAFL. Gene set enrichment analysis revealed that the upregulated expression of genes is related to the pathways of the tricarboxylic (TCA) cycle and deoxyribonucleic acid (DNA) replication in patients with steatohepatitis, but not in healthy controls. The mtDNA copy number in the peripheral blood mononuclear cells was 1.28-fold lower in patients with NAFLD than that in healthy controls (P <.0001). The mitochondrial inhibition substrate test showed that the cellular adenosine triphosphate (ATP) concentration was 1.2-fold times less in NAFLD patients than that in healthy controls (P <.0001). The mtDNA copy number and mitochondrial ATP inhibition substrate test demonstrated negative correlations with the degree of hepatic steatosis, whereas the ATP concentration showed a positive correlation with the mtDNA copy number.

CONCLUSION

The mitochondrial copy number of peripheral blood mononuclear cells and mitochondrial ATP inhibition substrate can be used as biomarkers for assessing the mitochondrial dysfunction in patients with NAFLD.

Personality traits are consistently associated with blood mitochondrial DNA copy number estimated from genome sequences in two genetic cohort studies

Background

Mitochondrial DNA copy number (mtDNAcn) in tissues and blood can be altered in conditions like diabetes and major depression and may play a role in aging and longevity. However, little is known about the association between mtDNAcn and personality traits linked to emotional states, metabolic health, and longevity. This study tests the hypothesis that blood mtDNAcn is related to personality traits and mediates the association between personality and mortality.

Methods

We assessed the big five personality domains and facets using the Revised NEO Personality Inventory (NEO-PI-R), assessed depressive symptoms with the Center for Epidemiologic Studies Depression Scale (CES-D), estimated mtDNAcn levels from whole-genome sequencing, and tracked mortality in participants from the Baltimore Longitudinal Study of Aging. Results were replicated in the SardiNIA Project.

Results

We found that mtDNAcn was negatively associated with the Neuroticism domain and its facets and positively associated with facets from the other four domains. The direction and size of the effects were replicated in the SardiNIA cohort and were robust to adjustment for potential confounders in both samples. Consistent with the Neuroticism finding, higher depressive symptoms were associated with lower mtDNAcn. Finally, mtDNAcn mediated the association between personality and mortality risk.

Conclusions

To our knowledge, this is the first study to show a replicable association between mtDNAcn and personality. Furthermore, the results support our hypothesis that mtDNAcn is a biomarker of the biological process that explains part of the association between personality and mortality.

Funding

Support for this work was provided by the Intramural Research Program of the National Institute on Aging (Z01-AG000693, Z01-AG000970, and Z01-AG000949) and the National Institute of Neurological Disorders and Stroke of the National Institutes of Health. AT was also supported by the National Institute on Aging of the National Institutes of Health Grant R01AG068093.

Blood mitochondrial DNA copy number: What are we counting?

There is growing scientific interest to develop scalable biological measures that capture mitochondrial (dys)function. Mitochondria have their own genome, the mitochondrial DNA (mtDNA). It has been proposed that the number of mtDNA copies per cell (mtDNA copy number; mtDNAcn) reflects mitochondrial health. The common availability of stored DNA material or existing DNA sequencing data, especially from blood and other easy-to-collect samples, has made its quantification a popular approach in clinical and epidemiological studies. However, the interpretation of mtDNAcn is not univocal, and either a reduction or elevation in mtDNAcn can indicate dysfunction. The major determinants of blood-derived mtDNAcn are the heterogeneous cell type composition of leukocytes and platelet abundance, which can change with time of day, aging, and with disease. Hematopoiesis is a likely driver of blood mtDNAcn. Here we discuss the rationale and available methods to quantify mtDNAcn, the influence of blood cell type variations, and consider important gaps in knowledge that need to be resolved to maximize the scientific value around the investigation of blood mtDNAcn.

Low Glucose Mediated Fluconazole Tolerance in Cryptococcus neoformans

Chronic meningoencephalitis is caused by Cryptococcus neoformans and is treated in many parts of the world with fluconazole (FLC) monotherapy, which is associated with treatment failure and poor outcome. In the host, C. neoformans propagates predominantly under low glucose growth conditions. We investigated whether low glucose, mimicked by growing in synthetic media (SM) with 0.05% glucose (SM^lowglu), affects FLC-resistance. A > 4-fold increase in FLC tolerance was observed in seven C. neoformans strains when minimum inhibitory concentration (MIC) was determined in SM^lowglu compared to MIC in SM with normal (2%) glucose (SM^nlglu). In SM^lowglu, C. neoformans cells exhibited upregulation of efflux pump genes AFR1 (8.7-fold) and AFR2 (2.5-fold), as well as decreased accumulation (2.6-fold) of Nile Red, an efflux pump substrate. Elevated intracellular ATP levels (3.2-fold and 3.4-fold), as well as decreased mitochondrial reactive oxygen species levels (12.8-fold and 17-fold), were found in the presence and absence of FLC, indicating that low glucose altered mitochondrial function. Fluorescence microscopy revealed that mitochondria of C. neoformans grown in SM^lowglu were fragmented, whereas normal glucose promoted a reticular network of mitochondria. Although mitochondrial membrane potential (MMP) was not markedly affected in SM^lowglu, it significantly decreased in the presence of FLC (12.5-fold) in SM^nlglu, but remained stable in SM^lowglu-growing C. neoformans cells. Our data demonstrate that increased FLC tolerance in low glucose-growing C. neoformans is the result of increased efflux pump activities and altered mitochondrial function, which is more preserved in SM^lowglu. This mechanism of resistance is different from FLC heteroresistance, which is associated with aneuploidy of chromosome 1 (Chr1).

Mitocytosis, a migrasome-mediated mitochondrial quality-control process

Damaged mitochondria need to be cleared to maintain the quality of the mitochondrial pool. Here, we report mitocytosis, a migrasome-mediated mitochondrial quality-control process. We found that, upon exposure to mild mitochondrial stresses, damaged mitochondria are transported into migrasomes and subsequently disposed of from migrating cells. Mechanistically, mitocytosis requires positioning of damaged mitochondria at the cell periphery, which occurs because damaged mitochondria avoid binding to inward motor proteins. Functionally, mitocytosis plays an important role in maintaining mitochondrial quality. Enhanced mitocytosis protects cells from mitochondrial stressor-induced loss of mitochondrial membrane potential (MMP) and mitochondrial respiration; conversely, blocking mitocytosis causes loss of MMP and mitochondrial respiration under normal conditions. Physiologically, we demonstrate that mitocytosis is required for maintaining MMP and viability in neutrophils in vivo. We propose that mitocytosis is an important mitochondrial quality-control process in migrating cells, which couples mitochondrial homeostasis with cell migration.

Cell Senescence, Multiple Organelle Dysfunction and Atherosclerosis

Atherosclerosis is an age-related disorder associated with long-term exposure to cardiovascular risk factors. The asymptomatic progression of atherosclerotic plaques leads to major cardiovascular diseases (CVD), including acute myocardial infarctions or cerebral ischemic strokes in some cases. Senescence, a biological process associated with progressive structural and functional deterioration of cells, tissues and organs, is intricately linked to age-related diseases. Cell senescence involves coordinated modifications in cellular compartments and has been demonstrated to contribute to different stages of atheroma development. Senescence-based therapeutic strategies are currently being pursued to treat and prevent CVD in humans in the near-future. In addition, distinct experimental settings allowed researchers to unravel potential approaches to regulate anti-apoptotic pathways, facilitate excessive senescent cell clearance and eventually reverse atherogenesis to improve cardiovascular function. However, a deeper knowledge is required to fully understand cellular senescence, to clarify senescence and atherogenesis intertwining, allowing researchers to establish more effective treatments and to reduce the cardiovascular disorders' burden. Here, we present an objective review of the key senescence-related alterations of the major intracellular organelles and analyze the role of relevant cell types for senescence and atherogenesis. In this context, we provide an updated analysis of therapeutic approaches, including clinically relevant experiments using senolytic drugs to counteract atherosclerosis.

Combined Treatment of Sulfonyl Chromen-4-Ones (CHW09) and Ultraviolet-C (UVC) Enhances Proliferation Inhibition, Apoptosis, Oxidative Stress, and DNA Damage against Oral Cancer Cells

The sensitizing effect of chromone-derived compounds on UVC-induced proliferation inhibition has not been comprehensively investigated so far. The subject of this study was to examine the proliferation change of oral cancer cells while using the combined treatment of UVC (254 nm) with our previously developed sulfonyl chromen-4-ones (CHW09), namely UVC/CHW09. Cell viability, apoptosis, oxidative stress, and DNA damage for the individual and combined treatments for UVC and/or CHW09 were examined in oral cancer Ca9-22 cells. In 24 h MTS assay, UVC (30 J/m²; UVC30), or CHW09 (25 and 50 µg/mL; namely, CHW09-25 and CHW09-50) show 54%, 59%, and 45% viability. The combined treatment (UVC30/CHW09-25 and UVC30/CHW09-50) show lower cell viability (45% and 35%). Mechanistically, UVC/CHW09 induced higher apoptosis than individual treatments and untreated control, which were supported by the evidence of flow cytometry for subG1, annexin V/7-aminoactinomycin D, pancaspase and caspases 3/7 activity, and western blotting for cleaved poly(ADP-ribose) polymerase. Moreover, this cleaved PARP expression was downregulated by pancaspase inhibitor Z-VAD-FMK. UVC/CHW09 showed higher oxidative stress than individual treatments and untreated control in terms of flow cytometry for reactive oxygen species, mitochondrial membrane potential, and mitochondrial mass. Furthermore, UVC/CHW09 showed higher DNA damage than individual treatments and untreated control in terms of flow cytometry for H2A histone family member X and 8-oxo-2’-deoxyguanosine. In conclusion, combined treatment UVC/CHW09 suppresses proliferation, and promotes apoptosis, oxidative stress, and DNA damage against oral cancer cells, providing a novel application of sulfonyl chromen-4-ones in order to sensitize UVC induced proliferation inhibition for oral cancer therapy.

Prevalence of mitochondrial DNA common deletion in patients with gliomas and meningiomas: A first report from a Malaysian study group

BACKGROUND

The 4977-bp common deletion (mtDNA) is a well-established mitochondrial genome alteration that has been described in various types of human cancers. However, to date, no studies on mtDNA in brain tumors have been reported. The present study aimed to determine mtDNA prevalence in common brain tumors, specifically, low- and high-grade gliomas (LGGs and HGGs), and meningiomas in Malaysian cases. Its correlation with clinicopathological parameters was also evaluated.

METHODS

A total of 50 patients with pathologically confirmed brain tumors (13 LGGs, 20 HGGs, and 17 meningiomas) were enrolled in this study. mtDNA was detected by using polymerase chain reaction (PCR) technique and later confirmed via Sanger DNA sequencing.

RESULTS

Overall, mtDNA was observed in 16 (32%) patients and it was significantly correlated with the type of tumor group and sex, being more common in the HGG group and in male patients.

CONCLUSION

The prevalence of mtDNA in Malaysian glioma and meningioma cases has been described for the first time and it was, indeed, comparable with previously published studies. This study provides initial insights into mtDNA in brain tumor and these findings can serve as new data for the global mitochondrial DNA mutations database.

Mitochondrial DNA copy number in cervical exfoliated cells and risk of cervical cancer among HPV-positive women

Background

Although human papillomavirus (HPV) infection has been regarded as the cause of cervical cancer in over 99% of cases, only a small fraction of HPV-infected women develop this malignancy. Emerging evidence suggests that alterations of mitochondrial DNA copy number (mtCN) may contribute to carcinogenesis. However, the relationship between mtCN and cervical cancer remains undetermined.

Methods

The current study included 591 cervical cancer cases and 373 cancer-free controls, all of whom were infected with high-risk HPV. Relative mtCN in cervical cancer exfoliated cells was measured by qRT-PCR assays, and logistic regression analysis was performed to compute odds ratios (ORs) and 95% confidence intervals (CIs). Interaction between mtCN and HPV types was assessed by using the Wald test in logistic regression models.

Results

HPV16, 18, 52, and 58 were the most common types in both case and control groups. Median mtCN in cases was significantly higher than that in controls (1.63 vs. 1.23, P = 0.03). After adjustment for age and HPV types, the highest quartile of mtCN was associated with increased odds of having cervical cancer (OR = 1.77, 95% CI = 1.19, 2.62; P < 0.01), as compared to the lowest quartile. A dose-response effect of mtCN on cervical cancer was also observed (P_trend < 0.001). The interaction between mtCN and HPV types was statistically nonsignificant.

Conclusions

In women who test HPV positive, the increase of mtCN in cervical exfoliated cells is associated with cervical cancer. This suggests a potential role of mtCN in cervical carcinogenesis.

Damage in Mitochondrial DNA Associated with Parkinson's Disease

Mitochondria are the only organelles that contain their own genetic material (mtDNA). Mitochondria are involved in several key physiological functions, including ATP production, Ca²⁺ homeostasis, and metabolism of neurotransmitters. Since these organelles perform crucial processes to maintain neuronal homeostasis, mitochondrial dysfunctions can lead to various neurodegenerative diseases. Several mitochondrial proteins involved in ATP production are encoded by mtDNA. Thus, any mtDNA alteration can ultimately lead to mitochondrial dysfunction and cell death. Accumulation of mutations, deletions, and rearrangements in mtDNA has been observed in animal models and patients suffering from Parkinson's disease (PD). Also, specific inherited variations associated with mtDNA genetic groups (known as mtDNA haplogroups) are associated with lower or higher risk of developing PD. Consequently, mtDNA alterations should now be considered important hallmarks of this neurodegenerative disease. This review provides an update about the role of mtDNA alterations in the physiopathology of PD.

Role of the mitochondrial stress response in human cancer progression

IMPACT STATEMENT

Dysregulated mitochondria often occurred in cancers. Mitochondrial dysfunction might contribute to cancer progression. We reviewed several mitochondrial stresses in cancers. Mitochondrial stress responses might contribute to cancer progression. Several mitochondrion-derived molecules (ROS, Ca2+, oncometabolites, exported mtDNA, mitochondrial double-stranded RNA, humanin, and MOTS-c), integrated stress response, and mitochondrial unfolded protein response act as retrograde signaling pathways and might be critical in the development and progression of cancer. Targeting these mitochondrial stress responses may be an important strategy for cancer treatment.

A comprehensive overview of mitochondrial DNA 4977-bp deletion in cancer studies

Mitochondria are cellular machines essential for energy production. The biogenesis of mitochondria is a highly complex and it depends on the coordination of the nuclear and mitochondrial genome. Mitochondrial DNA (mtDNA) mutations and deletions are suspected to be associated with carcinogenesis. The most described mtDNA deletion in various human cancers is called the 4977-bp common deletion (mDNA⁴⁹⁷⁷) and it has been explored since two decades. In spite of that, its implication in carcinogenesis still unknown and its predictive and prognostic impact remains controversial. This review article provides an overview of some of the cellular and molecular mechanisms underlying mDNA⁴⁹⁷⁷ formation and a detailed summary about mDNA⁴⁹⁷⁷ reported in various types of cancers. The current knowledges of mDNA⁴⁹⁷⁷ as a prognostic and predictive marker are also discussed.

Neonatal exposure to environmental pollutants and placental mitochondrial DNA content: A multi-pollutant approach

BACKGROUND

Placental mitochondrial DNA (mtDNA) content can be indicative of oxidative damage to the placenta during fetal development and is responsive to external stressors. In utero exposure to environmental pollutants that may influence placental mtDNA needs further exploration.

OBJECTIVES

We evaluated if placental mtDNA content is altered by environmental pollution in newborns and identified pollutants independently associated to alterations in placental mtDNA content.

METHODS

mtDNA content was measured in placental tissue of 233 newborns. Four perfluoroalkyl compounds and nine organochlorine compounds were quantified in cord blood plasma samples and six toxic metals in whole cord blood. We first applied a LASSO (least absolute shrinkage and selection operator) penalized regression model to identify independent associations between environmental pollutants and placental mtDNA content, without penalization of several covariates. Then adjusted estimates were obtained using an ordinary least squares (OLS) regression model evaluating the pollutants' association with placental mtDNA content, adjusted for several covariates.

RESULTS

Based on LASSO penalized regression, oxychlordane, p,p'-dichlorodiphenyldichloroethylene, β-hexachlorocyclohexane, perfluorononanoic acid, arsenic, cadmium and thallium were identified to be independently associated with placental mtDNA content. The OLS model showed a higher placental mtDNA content of 2.71% (95% CI: 0.3 to 5.2%; p=0.03) and 1.41% (0.1 to 2.8%, p=0.04) for a 25% concentration increase of respectively cord blood β-hexachlorocyclohexane and arsenic. For a 25% concentration increase of cord blood thallium, a 4.88% lower placental mtDNA content (95% CI: -9.1 to -0.5%, p=0.03) was observed.

CONCLUSION

In a multi-pollutant approach, low fetal exposure levels of environmental organic and inorganic pollutants might compromise placental mitochondrial function as exemplified in this study by alterations in mtDNA content.

Generating Rho-0 Cells Using Mesenchymal Stem Cell Lines

Introduction

The generation of Rho-0 cells requires the use of an immortalization process, or tumor cell selection, followed by culture in the presence of ethidium bromide (EtBr), incurring the drawbacks its use entails. The purpose of this work was to generate Rho-0 cells using human mesenchymal stem cells (hMSCs) with reagents having the ability to remove mitochondrial DNA (mtDNA) more safely than by using EtBr.

Methodology

Two immortalized hMSC lines (3a6 and KP) were used; 143B.TK-Rho-0 cells were used as reference control. For generation of Rho-0 hMSCs, cells were cultured in medium supplemented with each tested reagent. Total DNA was isolated and mtDNA content was measured by real-time polymerase chain reaction (PCR). Phenotypic characterization and gene expression assays were performed to determine whether 3a6 Rho-0 hMSCs maintain the same stem properties as untreated 3a6 hMSCs. To evaluate whether 3a6 Rho-0 hMSCs had a phenotype similar to that of 143B.TK-Rho-0 cells, in terms of reactive oxygen species (ROS) production, apoptotic levels and mitochondrial membrane potential (Δψm) were measured by flow cytometry and mitochondrial respiration was evaluated using a SeaHorse XFp Extracellular Flux Analyzer. The differentiation capacity of 3a6 and 3a6 Rho-0 hMSCs was evaluated using real-time PCR, comparing the relative expression of genes involved in osteogenesis, adipogenesis and chondrogenesis.

Results

The results showed the capacity of the 3a6 cell line to deplete its mtDNA and to survive in culture with uridine. Of all tested drugs, Stavudine (dt4) was the most effective in producing 3a6-Rho cells. The data indicate that hMSC Rho-0 cells continue to express the characteristic MSC cell surface receptor pattern. Phenotypic characterization showed that 3a6 Rho-0 cells resembled 143B.TK-Rho-0 cells, indicating that hMSC Rho-0 cells are Rho-0 cells. While the adipogenic capability was higher in 3a6 Rho-0 cells than in 3a6 cells, the osteogenic and chondrogenic capacities were lower.

Conclusion

Among the drugs and conditions tested, the use of d4t was the best option for producing Rho-0 cells from hMSCs. Rho-0 cells are useful for studying the role of mitochondria in hMSC differentiation.

Laser controlled singlet oxygen generation in mitochondria to promote mitochondrial DNA replication in vitro

Reports have shown that a certain level of reactive oxygen species (ROS) can promote mitochondrial DNA (mtDNA) replication. However, it is unclear whether it is the mitochondrial ROS that stimulate mtDNA replication and this requires further investigation. Here we employed a photodynamic system to achieve controlled mitochondrial singlet oxygen (¹O₂) generation. HeLa cells incubated with 5-aminolevulinic acid (ALA) were exposed to laser irradiation to induce ¹O₂ generation within mitochondria. Increased mtDNA copy number was detected after low doses of 630 nm laser light in ALA-treated cells. The stimulated mtDNA replication was directly linked to mitochondrial ¹O₂ generation, as verified using specific ROS scavengers. The stimulated mtDNA replication was regulated by mitochondrial transcription factor A (TFAM) and mtDNA polymerase γ. MtDNA control region modifications were induced by ¹O₂ generation in mitochondria. A marked increase in 8-Oxoguanine (8-oxoG) level was detected in ALA-treated cells after irradiation. HeLa cell growth stimulation and G1-S cell cycle transition were also observed after laser irradiation in ALA-treated cells. These cellular responses could be due to a second wave of ROS generation detected in mitochondria. In summary, we describe a controllable method of inducing mtDNA replication in vitro.

Increased Histone Deacetylase Activity Involved in the Suppressed Invasion of Cancer Cells Survived from ALA-Mediated Photodynamic Treatment

Previously, we have found that cancer cells survived from 5-Aminolevulinic acid-mediated photodynamic therapy (ALA-PDT) have abnormal mitochondrial function and suppressed cellular invasiveness. Here we report that both the mRNA expression level and enzymatic activity of histone deacetylase (HDAC) were elevated in the PDT-derived variants with dysfunctional mitochondria. The activated HDAC deacetylated histone H3 and further resulted in the reduced migration and invasion, which correlated with the reduced expression of the invasion-related genes, matrix metalloproteinase 9 (MMP9), paternally expressed gene 1 (PEG1), and miR-355, the intronic miRNA. Using chromatin immunoprecipitation, we further demonstrate the reduced amount of acetylated histone H3 on the promoter regions of MMP9 and PEG1, supporting the down-regulation of these two genes in PDT-derived variants. These results indicate that HDAC activation induced by mitochondrial dysfunction could modulate the cellular invasiveness and its related gene expression. This argument was further verified in the 51-10 cybrid cells with the 4977 bp mtDNA deletion and A375 ρ⁰ cells with depleted mitochondria. These results indicate that mitochondrial dysfunction might suppress tumor invasion through modulating histone acetylation.

Prevalence of the 4977-bp and 4408-bp mitochondrial DNA deletions in mesenteric arteries from patients with colorectal cancer

Mitochondrial DNA (mtDNA) deletions are found in many diseased tissues and lead to impairment of mitochondrial functions. In this study, we found wide presence of the common 4977-bp and a novel 4408-bp deletion in the mtDNA of mesenteric arteries from patients with colorectal cancer. These two deletions were also detected in samples from healthy individuals. The content of mtDNA with the 4977-bp deletion was significantly lower in healthy controls than cancer-associated samples, and there was no significant difference for the 4408-bp deletion between the two groups. These results suggest that mtDNA in blood vessels around cancer cells may be strongly affected by oxidative stress and tend to accumulate more large-scale variations.

Identification and biochemical characterization of the novel mutation m.8839G>C in the mitochondrial ATP6 gene associated with NARP syndrome

Mutations in the ATP6 gene are reported to be associated with Leber hereditary optic neuropathy, bilateral striatal necrosis, coronary atherosclerosis risk and neuropathy, ataxia and retinitis pigmentosa (NARP)/maternally inherited Leigh syndromes. Here, we present a patient with NARP syndrome, in whom a previously undescribed mutation was detected in the ATP6 gene: m.8839G>C. Several observations support the concept that m.8839G>C is pathogenically involved in the clinical phenotype of this patient: (1) the mutation was heteroplasmic in muscle; (2) mutation load was higher in the symptomatic patient than in the asymptomatic carriers; (3) cybrids carrying this mutation presented lower cell proliferation, increased mitochondrial DNA (mtDNA) copy number, increased steady-state OxPhos protein levels and decreased mitochondrial membrane potential with respect to isogenic wild-type cybrids; (4) this change was not observed in 2959 human mtDNAs from different mitochondrial haplogroups; (5) the affected amino acid was conserved in all the ATP6 sequences analyzed; and (6) using in silico prediction, the mutation was classified as 'probably damaging'. However, measurement of ATP synthesis showed no differences between wild-type and mutated cybrids. Thus, we suggest that m.8839G>C may lower the efficiency between proton translocation within F0 and F1 rotation, required for ATP synthesis. Further experiments are needed to fully characterize the molecular mechanisms involved in m.8839G>C pathogenicity.

Din7 and Mhr1 expression levels regulate double-strand-break-induced replication and recombination of mtDNA at ori5 in yeast

The Ntg1 and Mhr1 proteins initiate rolling-circle mitochondrial (mt) DNA replication to achieve homoplasmy, and they also induce homologous recombination to maintain mitochondrial genome integrity. Although replication and recombination profoundly influence mitochondrial inheritance, the regulatory mechanisms that determine the choice between these pathways remain unknown. In Saccharomyces cerevisiae, double-strand breaks (DSBs) introduced by Ntg1 at the mitochondrial replication origin ori5 induce homologous DNA pairing by Mhr1, and reactive oxygen species (ROS) enhance production of DSBs. Here, we show that a mitochondrial nuclease encoded by the nuclear gene DIN7 (DNA damage inducible gene) has 5′-exodeoxyribonuclease activity. Using a small ρ⁻ mtDNA bearing ori5 (hypersuppressive; HS) as a model mtDNA, we revealed that DIN7 is required for ROS-enhanced mtDNA replication and recombination that are both induced at ori5. Din7 overproduction enhanced Mhr1-dependent mtDNA replication and increased the number of residual DSBs at ori5 in HS-ρ⁻ cells and increased deletion mutagenesis at the ori5 region in ρ⁺ cells. However, simultaneous overproduction of Mhr1 suppressed all of these phenotypes and enhanced homologous recombination. Our results suggest that after homologous pairing, the relative activity levels of Din7 and Mhr1 modulate the preference for replication versus homologous recombination to repair DSBs at ori5.

Human Rad51 promotes mitochondrial DNA synthesis under conditions of increased replication stress

Homologous recombination is essential for productive DNA replication particularly under stress conditions. We previously demonstrated a stress-induced recruitment of Rad51 to mitochondria and a critical need for its activity in the maintenance of mitochondrial DNA (mtDNA) copy number. Using the human osteosarcoma cell line U20S, we show in the present study that recruitment of Rad51 to mitochondria under stress conditions requires ongoing mtDNA replication. Additionally, Rad51 levels in mitochondria increase in cells recovering from mtDNA depletion. Our findings highlight an important new role for Rad51 in supporting mtDNA replication, and further promote the idea that recombination is indispensable for sustaining DNA synthesis under conditions of replication stress.

p66Shc aging protein in control of fibroblasts cell fate

Reactive oxygen species (ROS) are wieldy accepted as one of the main factors of the aging process. These highly reactive compounds modify nucleic acids, proteins and lipids and affect the functionality of mitochondria in the first case and ultimately of the cell. Any agent or genetic modification that affects ROS production and detoxification can be expected to influence longevity. On the other hand, genetic manipulations leading to increased longevity can be expected to involve cellular changes that affect ROS metabolism. The 66-kDa isoform of the growth factor adaptor Shc (p66Shc) has been recognized as a relevant factor to the oxygen radical theory of aging. The most recent data indicate that p66Shc protein regulates life span in mammals and its phosphorylation on serine 36 is important for the initiation of cell death upon oxidative stress. Moreover, there is strong evidence that apart from aging, p66Shc may be implicated in many oxidative stress-associated pathologies, such as diabetes, mitochondrial and neurodegenerative disorders and tumorigenesis. This article summarizes recent knowledge about the role of p66Shc in aging and senescence and how this protein can influence ROS production and detoxification, focusing on studies performed on skin and skin fibroblasts.

A microfluidic system for fast detection of mitochondrial DNA deletion

This study reports an integrated microfluidic system capable of automatic extraction and analysis of mitochondrial DNA (mtDNA). Mitochondria are the energy production and metabolism centres of human and animal cells, which supply most of the energy for maintaining physiological functions and play an important role in the process of cell death. Because it lacks an effective repair system, mtDNA suffers much higher oxidative damage and usually harbours more mutations than nuclear DNA. Alterations of mtDNA have been reported to be strongly associated with mitochondrial dysfunction, mitochondria-related diseases, aging, and many important human diseases such as diabetes and cancers. Thus, an effective tool for automatic detection of mtDNA deletion is in great need. This study, therefore, proposed a microfluidic system integrating three enabling modules to perform the entire protocol for the detection of mtDNA deletion. Crucial processes which included mtDNA extraction, nucleic acid amplification, separation and detection of the target genes were automatically performed. When compared with traditional assays, the developed microfluidic system consumed fewer samples and reagents, achieved a higher mtDNA extraction rate, and could automate all the processes within a shorter period of time (150 minutes). It may provide a powerful tool for the analysis of mitochondria mutations in the near future.

Intracellular glycation of nuclear DNA, mitochondrial DNA, and cytosolic proteins during senescence-like growth arrest

To investigate the accumulation of intracellular advanced glycation end products (AGEs), a method was established for the simultaneous analysis of glycation products of cytosolic proteins, nuclear DNA, and mitochondrial DNA (mtDNA). Nuclear DNA, mtDNA, and cytosolic proteins were simultaneously isolated from one cell lysate by differential centrifugation and combined mechanical and chemical cell disruption methods. The major DNA-AGE N(2)-carboxyethyl-2'-deoxyguanosine (CEdG) was quantified in nuclear DNA and mtDNA by ELISA, whereas the protein-AGEs N(ɛ)-(carboxymethyl)lysine (CML) and N(ɛ)-(carboxyethyl)lysine (CEL) were determined by western blot. The method was used to analyze NIH3T3 fibroblasts. In untreated cells, CEdG levels of mtDNA (14.84 ± 3.07 pg CEdG/μg mtDNA) were significantly higher compared with nuclear DNA (4.40 ± 0.64 pg CEdG/μg DNA; p < 0.001). Then, fibroblasts were analyzed after 7 days of senescence-like growth arrest. In senescent fibroblasts, the CEdG content of nuclear DNA significantly increased by 25%. However, the CEdG level of mtDNA significantly decreased to 52%; in parallel, an increase in mitochondrial mass and mtDNA was observed. Senescence did not lead to general accumulation of protein-AGEs, but two protein bands at 32 and 34 kDa showed a significant increase in the CML/CEL modification rate (208%, p < 0.001; 196%, p = 0.0016) in senescent fibroblasts compared with control cells.

Increase of reduced nicotinamide adenine dinucleotide fluorescence lifetime precedes mitochondrial dysfunction in staurosporine-induced apoptosis of HeLa cells

In vivo noninvasive detection of apoptosis represents a new tool that may yield a more definite diagnosis, a more accurate prognosis, and help improve therapies for human diseases. The intrinsic fluorescence of reduced nicotinamide adenine dinucleotide (NADH) may be a potential optical biomarker for the apoptosis detection because NADH is involved in the respiration for the mitochondrial membrane potential (ΔΨ) formation and adenosine-5'-triphosphate (ATP) synthesis, and the depletion of ΔΨ and ATP level is the hallmark of apoptosis. We have previously observed the NADH fluorescence lifetime change is associated with staurosporine (STS)-induced mitochondria-mediated apoptosis. However, its relationship with mitochondrial functions such as ΔΨ, ATP, and oxygen consumption rate is not clear. In this study, we investigated this relationship. Our results indicate that the NADH fluorescence lifetime increased when ΔΨ and ATP levels were equal to or higher than their values of controls and decreased before the depletion of ΔΨ and ATP, and the oxygen consumption rate did not change. These findings suggest that the increased NADH fluorescence lifetime in STS-induced cell death occurred before the depletion of ΔΨ and ATP and activation of caspase 3, and was not simply caused by cellular metabolic change. Furthermore, the NADH fluorescence lifetime change is associated with the pace of apoptosis.

Fluorescence-based detection and quantification of features of cellular senescence

Cellular senescence is a spontaneous organismal defense mechanism against tumor progression which is raised upon the activation of oncoproteins or other cellular environmental stresses that must be circumvented for tumorigenesis to occur. It involves growth-arrest state of normal cells after a number of active divisions. There are multiple experimental routes that can drive cells into a state of senescence. Normal somatic cells and cancer cells enter a state of senescence upon overexpression of oncogenic Ras or Raf protein or by imposing certain kinds of stress such as cellular tumor suppressor function. Both flow cytometry and confocal imaging analysis techniques are very useful in quantitative analysis of cellular senescence phenomenon. They allow quantitative estimates of multiple different phenotypes expressed in multiple cell populations simultaneously. Here we review the various types of fluorescence methodologies including confocal imaging and flow cytometry that are frequently utilized to study a variety of senescence. First, we discuss key cell biological changes occurring during senescence and review the current understanding on the mechanisms of these changes with the goal of improving existing protocols and further developing new ones. Next, we list specific senescence phenotypes associated with each cellular trait along with the principles of their assay methods and the significance of the assay outcomes. We conclude by selecting appropriate references that demonstrate a typical example of each method.

Regionalized pathology correlates with augmentation of mtDNA copy numbers in a patient with myoclonic epilepsy with ragged-red fibers (MERRF-syndrome)

Human patients with myoclonic epilepsy with ragged-red fibers (MERRF) suffer from regionalized pathology caused by a mutation in the mitochondrial DNA (m.8344A→G). In MERRF-syndrome brain and skeletal muscles are predominantly affected, despite mtDNA being present in any tissue. In the past such tissue-specificity could not be explained by varying mtDNA mutation loads. In search for a region-specific pathology in human individuals we determined the mtDNA/nDNA ratios along with the mutation loads in 43 different post mortem tissue samples of a 16-year-old female MERRF patient and in four previously healthy victims of motor vehicle accidents. In brain and muscle we further determined the quantity of mitochondrial proteins (COX subunits II and IV), transcription factors (NRF1 and TFAM), and VDAC1 (Porin) as a marker for the mitochondrial mass. In the patient the mutation loads varied merely between 89-100%. However, mtDNA copy numbers were increased 3-7 fold in predominantly affected brain areas (e.g. hippocampus, cortex and putamen) and in skeletal muscle. Similar increases were absent in unaffected tissues (e.g. heart, lung, kidney, liver, and gastrointestinal organs). Such mtDNA copy number increase was not paralleled by an augmentation of mitochondrial mass in some investigated tissues, predominantly in the most affected tissue regions of the brain. We thus conclude that "futile" stimulation of mtDNA replication per se or a secondary failure to increase the mitochondrial mass may contribute to the regionalized pathology seen in MERRF-syndrome.

Sterol regulatory element-binding protein (SREBP)-1-mediated lipogenesis is involved in cell senescence

Increased cell mass is one of the characteristics of senescent cells, but this event has not been clearly defined. When subcellular organellar mass was estimated with organelle-specific fluorescence dyes, we observed that most membranous organelles progressively increase in mass during cell senescence. This increase was accompanied by an increase in membrane lipids and augmented expression of lipogenic enzymes, such as fatty acid synthase (FAS), ATP citrate lyase, and acetyl-CoA carboxylase. The mature form of sterol regulatory element-binding protein (SREBP)-1 was also elevated. Increased expression of these lipogenic effectors was further observed in the liver tissues of aging Fischer 344 rats. Ectopic expression of mature form of SREBP-1 in both Chang cells and primary young human diploid fibroblasts was enough to induce senescence. Blocking lipogenesis with FAS inhibitors (cerulenin and C75) and via siRNA-mediated silencing of SREBP-1 and ATP citrate lyase significantly attenuated H(2)O(2)-induced senescence. Finally, old human diploid fibroblasts were effectively reversed to young-like cells by challenging with FAS inhibitors. Our results suggest that enhanced lipogenesis is not only a common event, but also critically involved in senescence via SREBP-1 induction, thereby contributing to the increase in organelle mass (as a part of cell mass), a novel indicator of senescence.

Role of reactive oxygen species-elicited apoptosis in the pathophysiology of mitochondrial and neurodegenerative diseases associated with mitochondrial DNA mutations

A wide spectrum of pathogenic mutations of mitochondrial DNA (mtDNA) has been demonstrated to cause mitochondrial dysfunction and overproduction of reactive oxygen species (ROS), in relation to mitochondrial and neurodegenerative diseases. Our previous studies have shown that large-scale deletions of mtDNA not only serve as an indicator of oxidative damage, but also result in greater susceptibility of human cells to apoptosis triggered by UV irradiation and other apoptotic stimuli. In this review, we focus on the involvement of mtDNA-mutation-associated oxidative stress and susceptibility to apoptosis in the pathophysiology of mitochondrial and neurodegenerative diseases. Different lines of research have provided concordant data to suggest that the mtDNA-mutation-elicited energy insufficiency and enhanced oxidative stress and damage lead to cell dysfunction, and increase the susceptibility of affected cells to apoptosis in patients with these diseases. Moreover, accumulating experimental evidence has shown that antioxidant therapy is a good strategy for decreasing intracellular ROS and alleviating oxidative-stress-induced apoptosis in cells of patients that harbor pathogenic mtDNA mutations.

A comparative analysis of the cell biology of senescence and aging

Various intracellular organelles, such as lysosomes, mitochondria, nuclei, and cytoskeletons, change during replicative senescence, but the utility of these changes as general markers of senescence and their significance with respect to functional alterations have not been comprehensively reviewed. Furthermore, the relevance of these alterations to cellular and functional changes in aging animals is poorly understood. In this paper, we review the studies that report these senescence-associated changes in various aging cells and their underlying mechanisms. Changes associated with lysosomes and mitochondria are found not only in cells undergoing replicative or induced senescence but also in postmitotic cells isolated from aged organisms. In contrast, other changes occur mainly in cells undergoing in vitro senescence. Comparison of age-related changes and their underlying mechanisms in in vitro senescent cells and aged postmitotic cells would reveal the relevance of replicative senescence to the physiological processes occurring in postmitotic cells as individuals age.

Reactive oxygen species regulate DNA copy number in isolated yeast mitochondria by triggering recombination-mediated replication

Mitochondrial DNA (mtDNA) encodes proteins that are essential for cellular ATP production. Reactive oxygen species (ROS) are respiratory byproducts that damage mtDNA and other cellular components. In Saccharomyces cerevisiae, the oxidized base excision-repair enzyme Ntg1 introduces a double-stranded break (DSB) at the mtDNA replication origin ori5; this DSB initiates the rolling-circle mtDNA replication mediated by the homologous DNA pairing protein Mhr1. Thus, ROS may play a role in the regulation of mtDNA copy number. Here, we show that the treatment of isolated mitochondria with low concentrations of hydrogen peroxide increased mtDNA copy number in an Ntg1- and Mhr1-dependent manner. This treatment elevated the DSB levels at ori5 of hypersuppressive [rho^–] mtDNA only if Ntg1 was active. In vitro Ntg1-treatment of hypersuppressive [rho^–] mtDNA extracted from hydrogen peroxide-treated mitochondria revealed increased oxidative modifications at ori5 loci. We also observed that purified Ntg1 created breaks in single-stranded DNA harboring oxidized bases, and that ori5 loci have single-stranded character. Furthermore, chronic low levels of hydrogen peroxide increased in vivo mtDNA copy number. We therefore propose that ROS act as a regulator of mtDNA copy number, acting through the Mhr1-dependent initiation of rolling-circle replication promoted by Ntg1-induced DSB in the single-stranded regions at ori5.