Aging-dependent large accumulation of point mutations in the human mtDNA control region for replication

Mitochondrial Genome (mtDNA) Mutations that Generate Reactive Oxygen Species

Mitochondria are critical for the energetic demands of virtually every cellular process within nucleated eukaryotic cells. They harbour multiple copies of their own genome (mtDNA), as well as the protein-synthesing systems required for the translation of vital subunits of the oxidative phosphorylation machinery used to generate adenosine triphosphate (ATP). Molecular lesions to the mtDNA cause severe metabolic diseases and have been proposed to contribute to the progressive nature of common age-related diseases such as cancer, cardiomyopathy, diabetes, and neurodegenerative disorders. As a consequence of playing a central role in cellular energy metabolism, mitochondria produce reactive oxygen species (ROS) as a by-product of respiration. Here we review the evidence that mutations in the mtDNA exacerbate ROS production, contributing to disease.

Sharing of heteroplasmies between human liver lobes varies across the mtDNA genome

Mitochondrial DNA (mtDNA) heteroplasmy (intra-individual variation) varies among different human tissues and increases with age, suggesting that the majority of mtDNA heteroplasmies are acquired, rather than inherited. However, the extent to which heteroplasmic sites are shared across a tissue remains an open question. We therefore investigated heteroplasmy in two liver samples (one from each primary lobe) from 83 Europeans, sampled at autopsy. Minor allele frequencies (MAF) at heteroplasmic sites were significantly correlated between the two liver samples from an individual, with significantly more sharing of heteroplasmic sites in the control region than in the non-control region. We show that this increased sharing for the control region cannot be explained by recent mutations at just a few specific heteroplasmic sites or by the possible presence of 7S DNA. Moreover, we carried out simulations to show that there is significantly more sharing than would be predicted from random genetic drift from a common progenitor cell. We also observe a significant excess of non-synonymous vs. synonymous heteroplasmies in the protein-coding region, but significantly more sharing of synonymous heteroplasmies. These contrasting patterns for the control vs. the non-control region, and for non-synonymous vs. synonymous heteroplasmies, suggest that selection plays a role in heteroplasmy sharing.

Effects of Ageing on the Mitochondrial Genome in Rat Vestibular Organs

Background:

Deterioration in vestibular function occurs with ageing and is linked to age-related falls. Sensory hair cells located in the inner ear vestibular labyrinth are critical to vestibular function. Vestibular hair cells rely predominantly on oxidative phosphorylation (OXPHOS) for ener-gy production and contain numerous mitochondria. Mitochondrial DNA (mtDNA) mutations and perturbed energy production are associated with the ageing process.

Objective:

We investigated the effects of ageing on mtDNA in vestibular hair and support cells, and vestibular organ gene expression, to better understand mechanisms of age-related vestibular deficits.

Methods:

Vestibular hair and supporting cell layers were microdissected from young and old rats, and mtDNA was quantified by qPCR. Additionally, vestibular organ gene expression was analysed by microarray and gene set enrichment analyses.

Results:

In contrast to most other studies, we found no evidence of age-related mtDNA deletion mu-tations. However, we found an increase in abundance of major arc genes near the mtDNA control re-gion. There was also a marked age-related reduction in mtDNA copy number in both cell types. Ves-tibular organ gene expression, gene set enrichment analysis showed the OXPHOS pathway was down regulated in old animals.

Conclusion:

Given the importance of mtDNA to mitochondrial OXPHOS and hair cell function, our findings suggest the vestibular organs are potentially on the brink of an energy crisis in old animals

Is mitochondrial DNA profiling predictive for athletic performance?

Mitochondrial DNA encodes some proteins of the oxidative phosphorylation enzymatic complex, playing an important role in aerobic ATP production; therefore, it can contribute to the ability to respond to endurance exercise training. The accumulation of mitochondrial mutations and the migratory processes of populations have given a great contribution to the development of haplogroups with a different distribution in the world. Several studies have shown the important role of gene polymorphisms in aerobic performance. In this review, some mitochondrial haplogroups and multiple rare alleles were taken into consideration and could be linked to the athlete's physical performance of different ethnic groups.

Energetic costs of cellular and therapeutic control of stochastic mitochondrial DNA populations

The dynamics of the cellular proportion of mutant mtDNA molecules is crucial for mitochondrial diseases. Cellular populations of mitochondria are under homeostatic control, but the details of the control mechanisms involved remain elusive. Here, we use stochastic modelling to derive general results for the impact of cellular control on mtDNA populations, the cost to the cell of different mtDNA states, and the optimisation of therapeutic control of mtDNA populations. This formalism yields a wealth of biological results, including that an increasing mtDNA variance can increase the energetic cost of maintaining a tissue, that intermediate levels of heteroplasmy can be more detrimental than homoplasmy even for a dysfunctional mutant, that heteroplasmy distribution (not mean alone) is crucial for the success of gene therapies, and that long-term rather than short intense gene therapies are more likely to beneficially impact mtDNA populations.

Mitochondria's Role in Skin Ageing

Skin ageing is the result of a loss of cellular function, which can be further accelerated by external factors. Mitochondria have important roles in skin function, and mitochondrial damage has been found to accumulate with age in skin cells, but also in response to solar light and pollution. There is increasing evidence that mitochondrial dysfunction and oxidative stress are key features in all ageing tissues, including skin. This is directly linked to skin ageing phenotypes: wrinkle formation, hair greying and loss, uneven pigmentation and decreased wound healing. The loss of barrier function during skin ageing increases susceptibility to infection and affects wound healing. Therefore, an understanding of the mechanisms involved is important clinically and also for the development of antiageing skin care products.

Oxidative Stress in Neurodegenerative Diseases: From a Mitochondrial Point of View

Age is the main risk factor for a number of human diseases, including neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis, which increasing numbers of elderly individuals suffer. These pathological conditions are characterized by progressive loss of neuron cells, compromised motor or cognitive functions, and accumulation of abnormally aggregated proteins. Mitochondrial dysfunction is one of the main features of the aging process, particularly in organs requiring a high-energy source such as the heart, muscles, brain, or liver. Neurons rely almost exclusively on the mitochondria, which produce the energy required for most of the cellular processes, including synaptic plasticity and neurotransmitter synthesis. The brain is particularly vulnerable to oxidative stress and damage, because of its high oxygen consumption, low antioxidant defenses, and high content of polyunsaturated fats very prone to be oxidized. Thus, it is not surprising the importance of protecting systems, including antioxidant defenses, to maintain neuronal integrity and survival. Here, we review the role of mitochondrial oxidative stress in the aging process, with a specific focus on neurodegenerative diseases. Understanding the molecular mechanisms involving mitochondria and oxidative stress in the aging and neurodegeneration may help to identify new strategies for improving the health and extending lifespan.

Quantification of peripheral whole blood, cell-free plasma and exosome encapsulated mitochondrial DNA copy numbers in patients with atrial fibrillation

Mitochondrial DNA (mtDNA) copy number changes have been associated with various diseases. Several studies showed that mtDNA content in peripheral blood was associated with oxidative stress and cardiovascular disease. Atrial fibrillation (AF) is one of the severe cardiovascular diseases. We aimed to determine the mtDNA copy numbers in peripheral blood, in cell-free plasma and in exosomes of AF patients and healthy controls. Peripheral blood was drawn from 60 AF patients and 72 healthy controls. DNA was isolated from EDTA blood and plasma. Exosomes were isolated from cell-free plasma and then exosome encapsulated DNA (exoDNA) was extracted. Quantitative-real-time PCR was performed with Human Mitochondrial DNA (mtDNA) Monitoring Primer Set. Statistical analysis of the data was performed. We found statistically significant difference in mtDNA copy numbers in DNA isolated from peripheral whole blood, cell-free plasma and exosome samples of controls' (44.4 ± 18.0, 27.2 ± 30.1, 11.5 ± 8.7), and patients' group (43.4 ± 13.6, 26.2 ± 26.4, 14.5 ± 12.3). However there was no significant difference in mtDNA copy number between the two study groups either in peripheral blood, in cell-free plasma and in exosomes, and even in different sexes and ages. We found the highest copy number of mtDNA in peripheral blood, followed by plasma and exosomes. We did not find differences between patients and controls, neither age nor gender had effect on the mtDNA copy number. According to our results the mtDNA copy numbers did not differ in AF patients.

When the skin is in the center of interest: An aging issue

The skin represents the first bearer of marks of time as well as an easily accessible model for the assessment and determination of the involved molecular mechanisms. The deterioration of important skin functions due to intrinsic and extrinsic aging leads to clinical manifestations, which mirror several internal age-associated diseases, such as neurodegenerative, cardiovascular, skeletal, and endocrine/metabolic skin diseases. Current molecular data indicate that skin aging, especially intrinsic aging, mirrors age-related deficiencies in the entire human body. These data and the development of new biologic technologies highlight the importance of the skin in aging research and should enable future interdisciplinary projects on internal diseases, which could barely have been performed until recently due mainly to the lack of respective tissue.

Pyrimidine Ring-Opened Product from Oxidative DNA Damage of 5-Formyl-2'-deoxyuridine

After thymidine (dT) was treated with a Fenton-type reagent and further incubated for a long period (6 days) under physiological conditions (37 °C, pH 7.4), a new product, named dT*, was detected by HPLC in addition to the free thymine base and the known oxidative dT damage, 5-formyl-2'-deoxyuridine (f⁵dU). dT* was found to be formed from f⁵dU. The structure of dT* was determined to be 3-amino-2-carbamoyl-2-propenal-N³-2'-deoxyriboside, a pyrimidine ring-opened product from f⁵dU, on the basis of ¹H- and ¹³C NMR analyses and mass spectra. From the model compound 1-methyl-5-formyluracil, a similar ring-opened product was formed after the incubation. dT* was also detected in DNA treated with a Fenton-type reagent or γ-rays, followed by the prolonged incubation. dT* will be a new promising marker of oxidative DNA damage. The possible role of this product in oxy-radical-induced mutagenesis is discussed.

Reduced cardiomyocyte Na+ current in the age-dependent murine Pgc-1β-/- model of ventricular arrhythmia

Peroxisome proliferator-activated receptor-γ coactivator-1 deficient (Pgc-1β-/- ) murine hearts model the increased, age-dependent, ventricular arrhythmic risks attributed to clinical conditions associated with mitochondrial energetic dysfunction. These were accompanied by compromised action potential (AP) upstroke rates and impaired conduction velocities potentially producing arrhythmic substrate. We tested a hypothesis implicating compromised Na+ current in these electrophysiological phenotypes by applying loose patch-clamp techniques in intact young and aged, wild-type (WT) and Pgc-1β-/- , ventricular cardiomyocyte preparations for the first time. This allowed conservation of their in vivo extracellular and intracellular conditions. Depolarising steps elicited typical voltage-dependent activating and inactivating inward Na+ currents with peak amplitudes increasing or decreasing with their respective activating or preceding inactivating voltage steps. Two-way analysis of variance associated Pgc-1β-/- genotype with independent reductions in maximum peak ventricular Na+ currents from -36.63 ± 2.14 (n = 20) and -35.43 ± 1.96 (n = 18; young and aged WT, respectively), to -29.06 ± 1.65 (n = 23) and -27.93 ± 1.63 (n = 20; young and aged Pgc-1β-/- , respectively) pA/μm2 (p < 0.0001), without independent effects of, or interactions with age. Voltages at half-maximal current V*, and steepness factors k in plots of voltage dependences of both Na+ current activation and inactivation, and time constants for its postrepolarisation recovery from inactivation, remained indistinguishable through all experimental groups. So were the activation and rectification properties of delayed outward (K+ ) currents, demonstrated from tail currents reflecting current recoveries from respective varying or constant voltage steps. These current-voltage properties directly implicate decreases specifically in maximum available Na+ current with unchanged voltage dependences and unaltered K+ current properties, in proarrhythmic reductions in AP conduction velocity in Pgc-1β-/- ventricles.

Age-related Changes in the Global DNA Methylation Profile of Oligodendrocyte Progenitor Cells Derived from Rat Spinal Cords

Demyelination of axons plays an important role in the pathology of many spinal cord diseases and injuries. Remyelination in demyelinated lesions is primarily performed by oligodendrocyte progenitor cells (OPCs), which generate oligodendrocytes in the developing and mature central nervous system. The efficiency of remyelination decreases with age. Many reports suggest that this decline in remyelination results from impaired OPC recruitment and differentiation during aging. Of the various molecular mechanisms involved in aging, changes in epigenetic modifications have received particular attention. Global DNA methylation is a major epigenetic modification that plays important roles in cellular senescence and organismal aging. Thus, we aimed to evaluate the dynamic changes in the global DNA methylation profiles of OPCs derived from rat spinal cords during the aging process. We separated and cultured OPCs from the spinal cords of neonatal, 4-month-old, and 16-month-old rats and investigated the age-related alterations of genomic DNA methylation levels by using quantitative colorimetric analysis. To determine the potential cause of dynamic changes in global DNA methylation, we further analyzed the activity of DNA methyltransferases (DNMTs) and the expression of DNMT1, DNMT3a, DNMT3b, TET1, TET2, TET3, MBD2, and MeCP2 in the OPCs from each group. Our results showed the genomic DNA methylation level and the activity of DNMTs from OPCs derived from rat spinal cords decreased gradually during aging, and OPCs from 16-month-old rats were characterized by global hypomethylation. During OPC aging, the mRNA and protein expression levels of DNMT3a, DNMT3b, and MeCP2 were significantly elevated; those of DNMT1 were significantly down-regulated; and no significant changes were observed in those for TET1, TET2, TET3, or MBD2. Our results indicated that global DNA hypomethylation in aged OPCs is correlated with DNMT1 downregulation. Together, these data provide important evidence for partly elucidating the mechanism of age-related impaired OPC recruitment and differentiation and assist in the development of new treatments for promoting efficient remyelination.

Cause or casualty: The role of mitochondrial DNA in aging and age-associated disease

The mitochondrial genome (mtDNA) represents a tiny fraction of the whole genome, comprising just 16.6 kilobases encoding 37 genes involved in oxidative phosphorylation and the mitochondrial translation machinery. Despite its small size, much interest has developed in recent years regarding the role of mtDNA as a determinant of both aging and age-associated diseases. A number of studies have presented compelling evidence for key roles of mtDNA in age-related pathology, although many are correlative rather than demonstrating cause. In this review we will evaluate the evidence supporting and opposing a role for mtDNA in age-associated functional declines and diseases. We provide an overview of mtDNA biology, damage and repair as well as the influence of mitochondrial haplogroups, epigenetics and maternal inheritance in aging and longevity.

Mitochondrial DNA Integrity: Role in Health and Disease

As the primary cellular location for respiration and energy production, mitochondria serve in a critical capacity to the cell. Yet, by virtue of this very function of respiration, mitochondria are subject to constant oxidative stress that can damage one of the unique features of this organelle, its distinct genome. Damage to mitochondrial DNA (mtDNA) and loss of mitochondrial genome integrity is increasingly understood to play a role in the development of both severe early-onset maladies and chronic age-related diseases. In this article, we review the processes by which mtDNA integrity is maintained, with an emphasis on the repair of oxidative DNA lesions, and the cellular consequences of diminished mitochondrial genome stability.

Mitochondrial DNA m.13514G>A heteroplasmy is associated with depressive symptoms in the elderly

OBJECTIVES

Mitochondrial DNA (mtDNA) heteroplasmy is a mixture of normal and mutated mtDNA molecules in a cell. High levels of heteroplasmy at several mtDNA sites in complex I lead to inherited neurological neurologic diseases and brain magnetic resonance imaging (MRI) abnormalities. Here, we test the hypothesis that mtDNA heteroplasmy at these complex I sites is associated with depressive symptoms in the elderly.

METHODS

We examined platelet mtDNA heteroplasmy for associations with depressive symptoms among 137 participants over age 70 from the community-based Health, Aging and Body Composition Study. Depressive symptoms were assessed using the 10-point version of the Center for Epidemiologic Studies Depression Scale (CES-D 10). Complete mtDNA sequencing was performed and heteroplasmy derived for 5 mtDNA sites associated with neurologic mitochondrial diseases and tested for associations with depressive symptoms.

RESULTS

Of 5 candidate complex I mtDNA mutations examined for effects on depressive symptoms, increased heteroplasmy at m.13514A>G, ND5, was significantly associated with higher CES-D score (P = .01). A statistically significant interaction between m.13514A > G heteroplasmy and sex was detected (P = .04); in sex-stratified analyses, the impact of m.13514A>G heteroplasmy was stronger in male (P = .003) than in female (P = .98) participants. Men in highest tertile of mtDNA heteroplasmy exhibited significantly higher (P = .0001) mean ± SE CES-D 10 scores, 5.37 ± 0.58, when compared with those in the middle, 2.13 ± 0.52, and lowest tertiles, 2.47 ± 0.58. No associations between the 4 other candidate sites and depressive symptoms were observed.

CONCLUSIONS

Increased mtDNA heteroplasmy at m.13514A>G is associated with depressive symptoms in older men. Heteroplasmy may represent a novel biological risk factor for depression.

The Energy Maintenance Theory of Aging: Maintaining Energy Metabolism to Allow Longevity

Fused, elongated mitochondria are more efficient in generating ATP than fragmented mitochondria. In diverse C. elegans longevity pathways, increased levels of fused mitochondria are associated with lifespan extension. Blocking mitochondrial fusion in these animals abolishes their extended longevity. The long-lived C. elegans vhl-1 mutant is an exception that does not have increased fused mitochondria, and is not dependent on fusion for longevity. Loss of mammalian VHL upregulates alternate energy generating pathways. This suggests that mitochondrial fusion facilitates longevity in C. elegans by increasing energy metabolism. In diverse animals, ATP levels broadly decreases with age. Substantial evidence supports the theory that increasing or maintaining energy metabolism promotes the survival of older animals. Increased ATP levels in older animals allow energy-intensive repair and homeostatic mechanisms such as proteostasis that act to prevent cellular aging. These observations support the emerging paradigm that maintaining energy metabolism promotes the survival of older animals.

Germline and somatic mtDNA mutations in mouse aging

The accumulation of acquired mitochondrial genome (mtDNA) mutations with aging in somatic cells has been implicated in mitochondrial dysfunction and linked to age-onset diseases in humans. Here, we asked if somatic mtDNA mutations are also associated with aging in the mouse. MtDNA integrity in multiple organs and tissues in young and old (2-34 months) wild type (wt) mice was investigated by whole genome sequencing. Remarkably, no acquired somatic mutations were detected in tested tissues. However, we identified several non-synonymous germline mtDNA variants whose heteroplasmy levels (ratio of normal to mutant mtDNA) increased significantly with aging suggesting clonal expansion of inherited mtDNA mutations. Polg mutator mice, a model for premature aging, exhibited both germline and somatic mtDNA mutations whose numbers and heteroplasmy levels increased significantly with age implicating involvement in premature aging. Our results suggest that, in contrast to humans, acquired somatic mtDNA mutations do not accompany the aging process in wt mice.

Reversing wrinkled skin and hair loss in mice by restoring mitochondrial function

Mitochondrial DNA (mtDNA) depletion is involved in mtDNA depletion syndromes, mitochondrial diseases, aging and aging-associated chronic diseases, and other human pathologies. To evaluate the consequences of depletion of mtDNA in the whole animal, we created an inducible mtDNA-depleter mouse expressing, in the polymerase domain of POLG1, a dominant-negative mutation to induce depletion of mtDNA in various tissues. These mice showed reduced mtDNA content, reduced mitochondrial gene expression, and instability of supercomplexes involved in oxidative phosphorylation (OXPHOS) resulting in reduced OXPHOS enzymatic activities. We demonstrate that ubiquitous depletion of mtDNA in mice leads to predominant and profound effects on the skin resulting in wrinkles and visual hair loss with an increased number of dysfunctional hair follicles and inflammatory responses. Development of skin wrinkle was associated with the significant epidermal hyperplasia, hyperkeratosis, increased expression of matrix metalloproteinases, and decreased expression of matrix metalloproteinase inhibitor TIMP1. We also discovered markedly increased skin inflammation that appears to be a contributing factor in skin pathology. Histopathologic analyses revealed dysfunctional hair follicles. mtDNA-depleter mice also show changes in expression of aging-associated markers including IGF1R, KLOTHO, VEGF, and MRPS5. mtDNA-repleter mice showed that, by turning off the mutant POLG1 transgene expression, mitochondrial function, as well as the skin and hair pathology, is reversed to wild-type level. To our knowledge that restoration of mitochondrial functions can reverse the skin and hair pathology is unprecedented.

Ventricular pro-arrhythmic phenotype, arrhythmic substrate, ageing and mitochondrial dysfunction in peroxisome proliferator activated receptor-γ coactivator-1β deficient (Pgc-1β-/-) murine hearts

INTRODUCTION

Ageing and age-related bioenergetic conditions including obesity, diabetes mellitus and heart failure constitute clinical ventricular arrhythmic risk factors.

MATERIALS AND METHODS

Pro-arrhythmic properties in electrocardiographic and intracellular recordings were compared in young and aged, peroxisome proliferator-activated receptor-γ coactivator-1β knockout (Pgc-1β-/-) and wild type (WT), Langendorff-perfused murine hearts, during regular and programmed stimulation (PES), comparing results by two-way ANOVA.

RESULTS AND DISCUSSION

Young and aged Pgc-1β-/- showed higher frequencies and durations of arrhythmic episodes through wider PES coupling-interval ranges than WT. Both young and old, regularly-paced, Pgc-1β-/- hearts showed slowed maximum action potential (AP) upstrokes, (dV/dt)max (∼157 vs. 120-130 V s-1), prolonged AP latencies (by ∼20%) and shortened refractory periods (∼58 vs. 51 ms) but similar AP durations (∼50 ms at 90% recovery) compared to WT. However, Pgc-1β-/- genotype and age each influenced extrasystolic AP latencies during PES. Young and aged WT ventricles displayed distinct, but Pgc-1β-/- ventricles displayed similar dependences of AP latency upon (dV/dt)max resembling aged WT. They also independently increased myocardial fibrosis. AP wavelengths combining activation and recovery terms paralleled contrasting arrhythmic incidences in Pgc-1β-/- and WT hearts. Mitochondrial dysfunction thus causes pro-arrhythmic Pgc-1β-/- phenotypes by altering AP conduction through reducing (dV/dt)max and causing age-dependent fibrotic change.

Mechanisms of Mitochondrial DNA Repair in Mammals

Accumulation of mutations in mitochondrial DNA leads to the development of severe, currently untreatable diseases. The contribution of these mutations to aging and progress of neurodegenerative diseases is actively studied. Elucidation of DNA repair mechanisms in mitochondria is necessary for both developing approaches to the therapy of diseases caused by mitochondrial mutations and understanding specific features of mitochondrial genome functioning. Mitochondrial DNA repair systems have become a subject of extensive studies only in the last decade due to development of molecular biology methods. DNA repair systems of mammalian mitochondria appear to be more diverse and effective than it had been thought earlier. Even now, one may speak about the existence of mitochondrial mechanisms for the repair of single- and double-stranded DNA lesions. Homologous recombination also takes place in mammalian mitochondria, although its functional significance and molecular mechanisms remain obscure. In this review, I describe DNA repair systems in mammalian mitochondria, such as base excision repair (BER) and microhomology-mediated end joining (MMEJ) and discuss a possibility of existence of mitochondrial DNA repair mechanisms otherwise typical for the nuclear DNA, e.g., nucleotide excision repair (NER), mismatch repair (MMR), homologous recombination, and classical non-homologous end joining (NHEJ). I also present data on the mechanisms for coordination of the nuclear and mitochondrial DNA repair systems that have been actively studied recently.

Homologous recombination-mediated repair of DNA double-strand breaks operates in mammalian mitochondria

Mitochondrial DNA is frequently exposed to oxidative damage, as compared to nuclear DNA. Previously, we have shown that while microhomology-mediated end joining can account for DNA deletions in mitochondria, classical nonhomologous DNA end joining, the predominant double-strand break (DSB) repair pathway in nucleus, is undetectable. In the present study, we investigated the presence of homologous recombination (HR) in mitochondria to maintain its genomic integrity. Biochemical studies revealed that HR-mediated repair of DSBs is more efficient in the mitochondria of testes as compared to that of brain, kidney and spleen. Interestingly, a significant increase in the efficiency of HR was observed when a DSB was introduced. Analyses of the clones suggest that most of the recombinants were generated through reciprocal exchange, while ~ 30% of recombinants were due to gene conversion in testicular extracts. Colocalization and immunoblotting studies showed the presence of RAD51 and MRN complex proteins in the mitochondria and immunodepletion of MRE11, RAD51 or NIBRIN suppressed the HR-mediated repair. Thus, our results reveal importance of homologous recombination in the maintenance of mitochondrial genome stability.

Deep sequencing of the mitochondrial genome reveals common heteroplasmic sites in NADH dehydrogenase genes

Increasing evidence implicates mitochondrial dysfunction in aging and age-related conditions. But little is known about the molecular basis for this connection. A possible cause may be mutations in the mitochondrial DNA (mtDNA), which are often heteroplasmic-the joint presence of different alleles at a single locus in the same individual. However, the involvement of mtDNA heteroplasmy in aging and age-related conditions has not been investigated thoroughly. We deep-sequenced the complete mtDNA genomes of 356 Framingham Heart Study participants (52% women, mean age 43, mean coverage 4570-fold), identified 2880 unique mutations and comprehensively annotated them by MITOMAP and PolyPhen-2. We discovered 11 heteroplasmic "hot" spots [NADH dehydrogenase (ND) subunit 1, 4, 5 and 6 genes, n = 7; cytochrome c oxidase I (COI), n = 2; 16S rRNA, n = 1; D-loop, n = 1] for which the alternative-to-reference allele ratios significantly increased with advancing age (Bonferroni correction p < 0.001). Four of these heteroplasmic mutations in ND and COI genes were predicted to be deleterious nonsynonymous mutations which may have direct impact on ATP production. We confirmed previous findings that healthy individuals carry many low-frequency heteroplasmy mutations with potentially deleterious effects. We hypothesize that the effect of a single deleterious heteroplasmy may be minimal due to a low mutant-to-wildtype allele ratio, whereas the aggregate effects of many deleterious mutations may cause changes in mitochondrial function and contribute to age-related diseases. The identification of age-related mtDNA mutations is an important step to understand the genetic architecture of age-related diseases and may uncover novel therapeutic targets for such diseases.

Associations of mitochondrial polymorphisms with sporadic colorectal adenoma

Somatic mutations in mitochondrial DNA have been reported in colorectal adenomatous polyps (adenomas), the precursors to most colorectal cancers. However, there are no reports of associations of germline variation in mitochondrial DNA with adenoma risk. We investigated associations of germline polymorphisms in the displacement loop (D-loop) and non-D-loop region of the mitochondrial genome with incident, sporadic colorectal adenoma in three pooled colonoscopy-based case-control studies (n = 327 adenoma cases, 420 controls) that used identical methods for case and risk factor ascertainment. We sequenced a 1124 bp fragment to identify all genetic variation in the mitochondrial D-loop region, and used the Sequenom platform to genotype 64 tagSNPs in the non-D-loop region. We used multivariable unconditional logistic regression to estimate associations of the polymorphisms with adenoma. The odds ratios (OR) for associations of four polymorphisms in the HV1 region (mt16294, mt16296, mt16278, mt16069) with adenoma were 2.30, 2.63, 3.34, and 0.56, respectively; all 95% confidence intervals (CI) excluded 1.0, however, after correction for multiple comparisons, none of the findings remained statistically significant. Similar results were found for six polymorphisms in the non-D-loop region. In the HV1 region poly C tract, relative to those with 5 repeats, the ORs for those with fewer or more repeats were, respectively, 2.29 (95%CI 1.07-4.89) and 0.63 (95%CI 0.36-1.08), but repeat numbers in the HV2 region were not associated with adenoma. These findings suggest that mitochondrial D-loop HV1 region polymorphisms may be associated with colorectal adenoma risk and support further investigation.

Near-infrared spectroscopy detects age-related differences in skeletal muscle oxidative function: promising implications for geroscience

Age is the greatest risk factor for chronic disease and is associated with a marked decline in functional capacity and quality of life. A key factor contributing to loss of function in older adults is the decline in skeletal muscle function. While the exact mechanism(s) remains incompletely understood, age-related mitochondrial dysfunction is thought to play a major role. To explore this question further, we studied 15 independently living seniors (age: 72 ± 5 years; m/f: 4/11; BMI: 27.6 ± 5.9) and 17 young volunteers (age: 25 ± 4 years; m/f: 8/9; BMI: 24.0 ± 3.3). Skeletal muscle oxidative function was measured in forearm muscle from the recovery kinetics of muscle oxygen consumption using near-infrared spectroscopy (NIRS). Muscle oxygen consumption was calculated as the slope of change in hemoglobin saturation during a series of rapid, supra-systolic arterial cuff occlusions following a brief bout of exercise. Aging was associated with a significant prolongation of the time constant of oxidative recovery following exercise (51.8 ± 5.4 sec vs. 37.1 ± 2.1 sec, P = 0.04, old vs. young, respectively). This finding suggests an overall reduction in mitochondrial function with age in nonlocomotor skeletal muscle. That these data were obtained using NIRS holds great promise in gerontology for quantitative assessment of skeletal muscle oxidative function at the bed side or clinic.

Mitochondrial mutations drive prostate cancer aggression

Nuclear mutations are well known to drive tumor incidence, aggression and response to therapy. By contrast, the frequency and roles of mutations in the maternally inherited mitochondrial genome are poorly understood. Here we sequence the mitochondrial genomes of 384 localized prostate cancer patients, and identify a median of one mitochondrial single-nucleotide variant (mtSNV) per patient. Some of these mtSNVs occur in recurrent mutational hotspots and associate with aggressive disease. Younger patients have fewer mtSNVs than those who diagnosed at an older age. We demonstrate strong links between mitochondrial and nuclear mutational profiles, with co-occurrence between specific mutations. For example, certain control region mtSNVs co-occur with gain of the MYC oncogene, and these mutations are jointly associated with patient survival. These data demonstrate frequent mitochondrial mutation in prostate cancer, and suggest interplay between nuclear and mitochondrial mutational profiles in prostate cancer.

In prostate cancer, the role of mutations in the maternally-inherited mitochondrial genome are not well known. Here, the authors demonstrate frequent, age-dependent mitochondrial mutation in prostate cancer. Strong links between mitochondrial and nuclear mutational profiles are associated with clinical aggressivity.

RAP80 binds p32 to preserve the functional integrity of mitochondria

RAP80, a member of the BRCA1-A complex, is a well-known crucial regulator of cell cycle checkpoint and DNA damage repair in the nucleus. However, it is still unclear whether Rap80 localizes to another region outside the nucleus and plays different roles with its partners. Here, we found mitochondrial p32 as a novel binding partner of RAP80 by using yeast two-hybrid screening. RAP80 directly binds the internal region of p32 through its arginine rich C-terminal domain. Based on the interaction, we showed that a subset of RAP80 localizes to mitochondria where p32 exists. Loss of function study revealed that RAP80 deficiency reduces the protein level of p32 and p32 dependent mitochondrial translating proteins such as Rieske and COX1. As a result, mitochondrial membrane potential and oxygen consumption are reduced in RAP80 knockdown cells, indicating mitochondrial dysfunction. Our study identifies a novel interaction between RAP80 and p32, which is important for preserving intact mitochondrial function.

Mitochondrial dysfunction on sinoatrial node and pulmonary vein electrophysiological activities

Atrial fibrillation (AF) is associated with mitochondrial dysfunction. Sinoatrial node (SAN) dysfunction increases arrhythmogenesis of pulmonary veins (PVs), which is the most important trigger of AF; however, it is not clear whether mitochondrial dysfunction differentially regulates electrical activity of SANs and PVs. In the present study, conventional microelectrodes were used to record the action potentials (APs) in isolated rabbit PVs, SANs, left atrium (LA) and right atrium (RA) before and after application of trifluorocarbonylcyanide phenylhydrazone (FCCP; a mitochondrial uncoupling agent) at 10, 100 and 300 nM. FCCP application at 100 and 300 nM decreased spontaneous rates in PVs and in SANs at 10, 100 and 300 nM. FCCP shortened the 20, 50 and 90% AP durations in the LA, and shortened only the 20% AP duration in the RA. FCCP caused a greater rate reduction in SANs than in PVs; however, in the presence of coenzyme-Q₁₀ (10 µM), FCCP reduced the beating rate in PVs and SANs to a similar extent. In SAN-PV preparations with intact electrical connections, FCCP (100 nM) application shifted the SAN-PV electrical conduction into PV-SAN conduction in 5 (62.5%) of 8 preparations. In conclusion, mitochondrial dysfunction modulates PV and SAN electrical activities, which may contribute to atrial arrhythmogenesis.

miR-24-mediated knockdown of H2AX damages mitochondria and the insulin signaling pathway

Mitochondrial deficits or altered expressions of microRNAs are associated with the pathogenesis of various diseases, and microRNA-operated control of mitochondrial activity has been reported. Using a retrovirus-mediated short-hairpin RNA (shRNA) system, we observed that miR-24-mediated H2AX knockdown (H2AX-KD) impaired both mitochondria and the insulin signaling pathway. The overexpression of miR-24 decreased mitochondrial H2AX and disrupted mitochondrial function, as indicated by the ATP content, membrane potential and oxygen consumption. Similar mitochondrial damage was observed in shH2AX-mediated specific H2AX-KD cells. The H2AX-KD reduced the expression levels of mitochondrial transcription factor A (TFAM) and mitochondrial DNA-dependent transcripts. H2AX-KD mitochondria were swollen, and their cristae were destroyed. H2AX-KD also blocked the import of precursor proteins into mitochondria and the insulin-stimulated phosphorylation of IRS-1 (Y632) and Akt (S473 and T308). The rescue of H2AX, but not the nuclear form of ΔC24-H2AX, restored all features of miR-24- or shH2AX-mediated impairment of mitochondria. Hepatic miR-24 levels were significantly increased in db/db and ob/ob mice. A strong feedback loop may be present among miR-24, H2AX, mitochondria and the insulin signaling pathway. Our findings suggest that H2AX-targeting miR-24 may be a novel negative regulator of mitochondrial function and is implicated in the pathogenesis of insulin resistance.

Influence of omega-3 fatty acids on skeletal muscle protein metabolism and mitochondrial bioenergetics in older adults

Omega-3 polyunsaturated fatty acids (n3-PUFA) are recognized for their anti-inflammatory effects and may be beneficial in the context of sarcopenia. We determined the influence of n3-PUFA on muscle mitochondrial physiology and protein metabolism in older adults. Twelve young (18-35 years) and older (65-85 years) men and women were studied at baseline. Older adults were studied again following n3-PUFA supplementation (3.9g/day, 16 weeks). Muscle biopsies were used to evaluate respiratory capacity (high resolution respirometry) and oxidant emissions (spectrofluorometry) in isolated mitochondria. Maximal respiration was significantly lower in older compared to young. n3-PUFA did not change respiration, but significantly reduced oxidant emissions. Participants performed a single bout of resistance exercise, followed by biopsies at 15 and 18 hours post exercise. Several genes involved in muscle protein turnover were significantly altered in older adults at baseline and following exercise, yet muscle protein synthesis was similar between age groups under both conditions. Following n3-PUFA supplementation, mixed muscle, mitochondrial, and sarcoplasmic protein synthesis rates were increased in older adults before exercise. n3-PUFA increased post-exercise mitochondrial and myofibrillar protein synthesis in older adults. These results demonstrate that n3-PUFA reduce mitochondrial oxidant emissions, increase postabsorptive muscle protein synthesis, and enhance anabolic responses to exercise in older adults.

Mitochondria, Cybrids, Aging, and Alzheimer's Disease

Mitochondrial and bioenergetic function change with advancing age and may drive aging phenotypes. Mitochondrial and bioenergetic changes are also documented in various age-related neurodegenerative diseases, including Alzheimer's disease (AD). In some instances AD mitochondrial and bioenergetic changes are reminiscent of those observed with advancing age but are greater in magnitude. Mitochondrial and bioenergetic dysfunction could, therefore, link neurodegeneration to brain aging. Interestingly, mitochondrial defects in AD patients are not brain-limited, and mitochondrial function can be linked to classic AD histologic changes including amyloid precursor protein processing to beta amyloid. Also, transferring mitochondria from AD subjects to cell lines depleted of endogenous mitochondrial DNA (mtDNA) creates cytoplasmic hybrid (cybrid) cell lines that recapitulate specific biochemical, molecular, and histologic AD features. Such findings have led to the formulation of a "mitochondrial cascade hypothesis" that places mitochondrial dysfunction at the apex of the AD pathology pyramid. Data pertinent to this premise are reviewed.

Role of the mitochondrial DNA replication machinery in mitochondrial DNA mutagenesis, aging and age-related diseases

As regulators of bioenergetics in the cell and the primary source of endogenous reactive oxygen species (ROS), dysfunctional mitochondria have been implicated for decades in the process of aging and age-related diseases. Mitochondrial DNA (mtDNA) is replicated and repaired by nuclear-encoded mtDNA polymerase γ (Pol γ) and several other associated proteins, which compose the mtDNA replication machinery. Here, we review evidence that errors caused by this replication machinery and failure to repair these mtDNA errors results in mtDNA mutations. Clonal expansion of mtDNA mutations results in mitochondrial dysfunction, such as decreased electron transport chain (ETC) enzyme activity and impaired cellular respiration. We address the literature that mitochondrial dysfunction, in conjunction with altered mitochondrial dynamics, is a major driving force behind aging and age-related diseases. Additionally, interventions to improve mitochondrial function and attenuate the symptoms of aging are examined.

Mouse models of ageing and their relevance to disease

Ageing is a process that gradually increases the organism's vulnerability to death. It affects different biological pathways, and the underlying cellular mechanisms are complex. In view of the growing disease burden of ageing populations, increasing efforts are being invested in understanding the pathways and mechanisms of ageing. We review some mouse models commonly used in studies on ageing, highlight the advantages and disadvantages of the different strategies, and discuss their relevance to disease susceptibility. In addition to addressing the genetics and phenotypic analysis of mice, we discuss examples of models of delayed or accelerated ageing and their modulation by caloric restriction.

Biomarkers of skin aging

Aging is a complex process not only influenced by inherited but also by several environmental factors. It is characterized by a progressive loss of function in multiple tissues, which leads to an increased probability of death. On the other hand, several morphological and histological changes are registered in aged skin that is mostly dependent on the cumulative exposure in environmental aging promoters, such as ultraviolet radiation. Understanding of individual pathogenesis and introduction of preventive measurements require objective assessment, i.e., the administration of biomarkers. Because of the complexity of skin aging, the exact definition of biomarkers is a major research challenge. In this article, we summarize the basic knowledge involving skin aging and its biomarkers.

Mitochondrial compromise in 3-year old patas monkeys exposed in utero to human-equivalent antiretroviral therapies

Antiretroviral (ARV) drug therapy, given during pregnancy for prevention of mother-to-child transmission of human immunodeficiency virus 1 (HIV-1), induces fetal mitochondrial dysfunction in some children. However, the persistence/reversibility of that dysfunction is unclear. Here we have followed Erythrocebus patas (patas) monkey offspring for up to 3 years of age (similar in development to a 15-year old human) after exposure of the dams to human-equivalent in utero ARV exposure protocols. Pregnant patas dams (3-5/exposure group) were given ARV drug combinations that included zidovudine (AZT)/lamivudine (3TC)/abacavir (ABC), or AZT/3TC/nevirapine (NVP), for the last 10 weeks (50%) of gestation. Infants kept for 1 and 3 years also received drug for the first 6 weeks of life. In offpsring at birth, 1 and 3 years of age mitochondrial morphology, examined by electron microscopy (EM), was compromised compared to the unexposed controls. Mitochondrial DNA (mtDNA), measured by hybrid capture chemiluminescence assay (HCCA) was depleted in hearts of patas exposed to AZT/3TC/NVP at all ages (P < 0.05), but not in those exposed to AZT/3TC/ABC at any age. Compared to unexposed controls, mitochondrial reserve capacity oxygen consumption rate (OCR by Seahorse) in cultured bone marrow mesenchymal fibroblasts from 3-year-old patas offspring was ∼50% reduced in AZT/3TC/ABC-exposed patas (P < 0.01), but not in AZT/3TC/NVP-exposed patas. Overall the data show that 3-year-old patas sustain persistent mitochondrial dysfunction as a result of perinatal ARV drug exposure. Environ. Mol. Mutagen. 57:526-534, 2016. © 2016 Wiley Periodicals, Inc.

Mitochondrial remodeling: Rearranging, recycling, and reprogramming

Mitochondria are highly dynamic and responsive organelles that respond to environmental cues with fission and fusion. They undergo mitophagy and biogenesis, and are subject to extensive post-translational modifications. Calcium plays an important role in regulating mitochondrial functions. Mitochondria play a central role in metabolism of glucose, fatty acids, and amino acids, and generate ATP with effects on redox poise, oxidative stress, pH, and other metabolites including acetyl-CoA and NAD(+) which in turn have effects on chromatin remodeling. The complex interplay of mitochondria, cytosolic factors, and the nucleus ensure a well-coordinated response to environmental stresses.

Human age estimation from blood using mRNA, DNA methylation, DNA rearrangement, and telomere length

Establishing the age of unknown persons, or persons with unknown age, can provide important leads in police investigations, disaster victim identification, fraud cases, and in other legal affairs. Previous methods mostly relied on morphological features available from teeth or skeletal parts. The development of molecular methods for age estimation allowing to use human specimens that possess no morphological age information, such as bloodstains, is extremely valuable as this type of samples is commonly found at crime scenes. Recently, we introduced a DNA-based approach for human age estimation from blood based on the quantification of T-cell specific DNA rearrangements (sjTRECs), which achieves accurate assignment of blood DNA samples to one of four 20-year-interval age categories. Aiming at improving the accuracy of molecular age estimation from blood, we investigated different types of biomarkers. We started out by systematic genome-wide surveys for new age-informative mRNA and DNA methylation markers in blood from the same young and old individuals using microarray technologies. The obtained candidate markers were validated in independent samples covering a wide age range using alternative technologies together with previously proposed DNA methylation, sjTREC, and telomere length markers. Cross-validated multiple regression analysis was applied for estimating and validating the age predictive power of various sets of biomarkers within and across different marker types. We found that DNA methylation markers outperformed mRNA, sjTREC, and telomere length in age predictive power. The best performing model included 8 DNA methylation markers derived from 3 CpG islands reaching a high level of accuracy (cross-validated R(2)=0.88, SE±6.97 years, mean absolute deviation 5.07 years). However, our data also suggest that mRNA markers can provide independent age information: a model using a combined set of 5 DNA methylation markers and one mRNA marker could provide similarly high accuracy (cross-validated R(2)=0.86, SE±7.62 years, mean absolute deviation 4.60 years). Overall, our study provides new and confirms previously suggested molecular biomarkers for age estimation from blood. Moreover, our comparative study design revealed that DNA methylation markers are superior for this purpose over other types of molecular biomarkers tested. While the new and some previous findings are highly promising, before molecular age estimation can eventually meet forensic practice, the proposed biomarkers should be tested further in larger sets of blood samples from both healthy and unhealthy individuals, and markers and genotyping methods shall be validated to meet forensic standards.

Nuclear DNA damage signalling to mitochondria in ageing

Mitochondrial dysfunction is a hallmark of ageing, and mitochondrial maintenance may lead to increased healthspan. Emerging evidence suggests a crucial role for signalling from the nucleus to mitochondria (NM signalling) in regulating mitochondrial function and ageing. An important initiator of NM signalling is nuclear DNA damage, which accumulates with age and may contribute to the development of age-associated diseases. DNA damage-dependent NM signalling constitutes a network that includes nuclear sirtuins and controls genomic stability and mitochondrial integrity. Pharmacological modulation of NM signalling is a promising novel approach for the prevention and treatment of age-associated diseases.

Age-Related Accumulation of Somatic Mitochondrial DNA Mutations in Adult-Derived Human iPSCs

The genetic integrity of iPSCs is an important consideration for therapeutic application. In this study, we examine the accumulation of somatic mitochondrial genome (mtDNA) mutations in skin fibroblasts, blood, and iPSCs derived from young and elderly subjects (24-72 years). We found that pooled skin and blood mtDNA contained low heteroplasmic point mutations, but a panel of ten individual iPSC lines from each tissue or clonally expanded fibroblasts carried an elevated load of heteroplasmic or homoplasmic mutations, suggesting that somatic mutations randomly arise within individual cells but are not detectable in whole tissues. The frequency of mtDNA defects in iPSCs increased with age, and many mutations were non-synonymous or resided in RNA coding genes and thus can lead to respiratory defects. Our results highlight a need to monitor mtDNA mutations in iPSCs, especially those generated from older patients, and to examine the metabolic status of iPSCs destined for clinical applications.

Scattering pulse of label free fine structure cells to determine the size scale of scattering structures

Scattering pulse is sensitive to the morphology and components of each single label-free cell. The most direct detection result, label free cell's scattering pulse is studied in this paper as a novel trait to recognize large malignant cells from small normal cells. A set of intrinsic scattering pulse calculation method is figured out, which combines both hydraulic focusing theory and small particle's scattering principle. Based on the scattering detection angle ranges of widely used flow cytometry, the scattering pulses formed by cell scattering energy in forward scattering angle 2°-5° and side scattering angle 80°-110° are discussed. Combining the analysis of cell's illuminating light energy, the peak, area, and full width at half maximum (FWHM) of label free cells' scattering pulses for fine structure cells with diameter 1-20 μm are studied to extract the interrelations of scattering pulse's features and cell's morphology. The theoretical and experimental results show that cell's diameter and FWHM of its scattering pulse agree with approximate linear distribution; the peak and area of scattering pulse do not always increase with cell's diameter becoming larger, but when cell's diameter is less than about 16 μm the monotone increasing relation of scattering pulse peak or area with cell's diameter can be obtained. This relationship between the features of scattering pulse and cell's size is potentially a useful but very simple criterion to distinguishing malignant and normal cells by their sizes and morphologies in label free cells clinical examinations.

Age-Related and Heteroplasmy-Related Variation in Human mtDNA Copy Number

The mitochondrial (mt) genome is present in many copies in human cells, and intra-individual variation in mtDNA sequences is known as heteroplasmy. Recent studies found that heteroplasmies are highly tissue-specific, site-specific, and allele-specific, however the functional implications have not been explored. This study investigates variation in mtDNA copy numbers (mtCN) in 12 different tissues obtained at autopsy from 152 individuals (ranging in age from 3 days to 96 years). Three different methods to estimate mtCN were compared: shotgun sequencing (in 4 tissues), capture-enriched sequencing (in 12 tissues) and droplet digital PCR (ddPCR, in 2 tissues). The highest precision in mtCN estimation was achieved using shotgun sequencing data. However, capture-enrichment data provide reliable estimates of relative (albeit not absolute) mtCNs. Comparisons of mtCN from different tissues of the same individual revealed that mtCNs in different tissues are, with few exceptions, uncorrelated. Hence, each tissue of an individual seems to regulate mtCN in a tissue-related rather than an individual-dependent manner. Skeletal muscle (SM) samples showed an age-related decrease in mtCN that was especially pronounced in males, while there was an age-related increase in mtCN for liver (LIV) samples. MtCN in SM samples was significantly negatively correlated with both the total number of heteroplasmic sites and with minor allele frequency (MAF) at two heteroplasmic sites, 408 and 16327. Heteroplasmies at both sites are highly specific for SM, accumulate with aging and are part of functional elements that regulate mtDNA replication. These data support the hypothesis that selection acting on these heteroplasmic sites is reducing mtCN in SM of older individuals.

Microhomology-mediated end joining is the principal mediator of double-strand break repair during mitochondrial DNA lesions

Repair of double-strand breaks in mammalian mitochondria depends on microhomology-mediated end joining (MMEJ). Classical NHEJ is not detectable in mitochondria. DNA ligase III, but not ligase IV or ligase I, is involved in mitochondrial MMEJ. The protein machinery involved in miitochondrial MMEJ includes CtIP, FEN1, ligase III, MRE11, and PARP1.

Mitochondrial DNA (mtDNA) deletions are associated with various mitochondrial disorders. The deletions identified in humans are flanked by short, directly repeated mitochondrial DNA sequences; however, the mechanism of such DNA rearrangements has yet to be elucidated. In contrast to nuclear DNA (nDNA), mtDNA is more exposed to oxidative damage, which may result in double-strand breaks (DSBs). Although DSB repair in nDNA is well studied, repair mechanisms in mitochondria are not characterized. In the present study, we investigate the mechanisms of DSB repair in mitochondria using in vitro and ex vivo assays. Whereas classical NHEJ (C-NHEJ) is undetectable, microhomology-mediated alternative NHEJ efficiently repairs DSBs in mitochondria. Of interest, robust microhomology-mediated end joining (MMEJ) was observed with DNA substrates bearing 5-, 8-, 10-, 13-, 16-, 19-, and 22-nt microhomology. Furthermore, MMEJ efficiency was enhanced with an increase in the length of homology. Western blotting, immunoprecipitation, and protein inhibition assays suggest the involvement of CtIP, FEN1, MRE11, and PARP1 in mitochondrial MMEJ. Knockdown studies, in conjunction with other experiments, demonstrated that DNA ligase III, but not ligase IV or ligase I, is primarily responsible for the final sealing of DSBs during mitochondrial MMEJ. These observations highlight the central role of MMEJ in maintenance of mammalian mitochondrial genome integrity and is likely relevant for deletions observed in many human mitochondrial disorders.

Dissecting Long-Term Glucose Metabolism Identifies New Susceptibility Period for Metabolic Dysfunction in Aged Mice

Metabolic disorders, like diabetes and obesity, are pathogenic outcomes of imbalance in glucose metabolism. Nutrient excess and mitochondrial imbalance are implicated in dysfunctional glucose metabolism with age. We used conplastic mouse strains with defined mitochondrial DNA (mtDNA) mutations on a common nuclear genomic background, and administered a high-fat diet up to 18 months of age. The conplastic mouse strain B6-mt^FVB, with a mutation in the mt-Atp8 gene, conferred β-cell dysfunction and impaired glucose tolerance after high-fat diet. To our surprise, despite of this functional deficit, blood glucose levels adapted to perturbations with age. Blood glucose levels were particularly sensitive to perturbations at the early age of 3 to 6 months. Overall the dynamics consisted of a peak between 3–6 months followed by adaptation by 12 months of age. With the help of mathematical modeling we delineate how body weight, insulin and leptin regulate this non-linear blood glucose dynamics. The model predicted a second rise in glucose between 15 and 21 months, which could be experimentally confirmed as a secondary peak. We therefore hypothesize that these two peaks correspond to two sensitive periods of life, where perturbations to the basal metabolism can mark the system for vulnerability to pathologies at later age. Further mathematical modeling may perspectively allow the design of targeted periods for therapeutic interventions and could predict effects on weight loss and insulin levels under conditions of pre-diabetic obesity.

Melatonin: A Potential Anti-Oxidant Therapeutic Agent for Mitochondrial Dysfunctions and Related Disorders

Mitochondria play a central role in cellular physiology. Besides their classic function of energy metabolism, mitochondria are involved in multiple cell functions, including energy distribution through the cell, energy/heat modulation, regulation of reactive oxygen species (ROS), calcium homeostasis, and control of apoptosis. Simultaneously, mitochondria are the main producer and target of ROS with the result that multiple mitochondrial diseases are related to ROS-induced mitochondrial injuries. Increased free radical generation, enhanced mitochondrial inducible nitric oxide synthase (iNOS) activity, enhanced nitric oxide (NO) production, decreased respiratory complex activity, impaired electron transport system, and opening of mitochondrial permeability transition pores have all been suggested as factors responsible for impaired mitochondrial function. Because of these, neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), and aging, are caused by ROS-induced mitochondrial dysfunctions. Melatonin, the major hormone of the pineal gland, also acts as an anti-oxidant and as a regulator of mitochondrial bioenergetic function. Melatonin is selectively taken up by mitochondrial membranes, a function not shared by other anti-oxidants, and thus has emerged as a major potential therapeutic tool for treating neurodegenerative disorders. Multiple in vitro and in vivo experiments have shown the protective role of melatonin for preventing oxidative stress-induced mitochondrial dysfunction seen in experimental models of PD, AD, and HD. With these functions in mind, this article reviews the protective role of melatonin with mechanistic insights against mitochondrial diseases and suggests new avenues for safe and effective treatment modalities against these devastating neurodegenerative diseases. Future insights are also discussed.

The Role of Mitochondrial DNA in Mediating Alveolar Epithelial Cell Apoptosis and Pulmonary Fibrosis

Convincing evidence has emerged demonstrating that impairment of mitochondrial function is critically important in regulating alveolar epithelial cell (AEC) programmed cell death (apoptosis) that may contribute to aging-related lung diseases, such as idiopathic pulmonary fibrosis (IPF) and asbestosis (pulmonary fibrosis following asbestos exposure). The mammalian mitochondrial DNA (mtDNA) encodes for 13 proteins, including several essential for oxidative phosphorylation. We review the evidence implicating that oxidative stress-induced mtDNA damage promotes AEC apoptosis and pulmonary fibrosis. We focus on the emerging role for AEC mtDNA damage repair by 8-oxoguanine DNA glycosylase (OGG1) and mitochondrial aconitase (ACO-2) in maintaining mtDNA integrity which is important in preventing AEC apoptosis and asbestos-induced pulmonary fibrosis in a murine model. We then review recent studies linking the sirtuin (SIRT) family members, especially SIRT3, to mitochondrial integrity and mtDNA damage repair and aging. We present a conceptual model of how SIRTs modulate reactive oxygen species (ROS)-driven mitochondrial metabolism that may be important for their tumor suppressor function. The emerging insights into the pathobiology underlying AEC mtDNA damage and apoptosis is suggesting novel therapeutic targets that may prove useful for the management of age-related diseases, including pulmonary fibrosis and lung cancer.

Mechanisms linking mtDNA damage and aging

In the past century, considerable efforts were made to understand the role of mitochondrial DNA (mtDNA) mutations and of oxidative stress in aging. The classic mitochondrial free radical theory of aging, in which mtDNA mutations cause genotoxic oxidative stress, which in turn creates more mutations, has been a central hypothesis in the field for decades. In the past few years, however, new elements have discredited this original theory. The major sources of mitochondrial DNA mutations seem to be replication errors and failure of the repair mechanisms, and the accumulation of these mutations as observed in aged organisms seems to occur by clonal expansion and not to be caused by a reactive oxygen species-dependent vicious cycle. New hypotheses of how age-associated mitochondrial dysfunction may lead to aging are based on the role of reactive oxygen species as signaling molecules and on their role in mediating stress responses to age-dependent damage. Here, we review the changes that mtDNA undergoes during aging and the past and most recent hypotheses linking these changes to the tissue failure observed in aging.

Role of diffuse low-level heteroplasmy of mitochondrial DNA in Alzheimer's disease neurodegeneration

Alzheimer’s disease (AD) is the most common form of dementia in the elderly. The vast majority of cases are not linked to a known genetic defect and the molecular mechanisms underlying AD pathogenesis are still elusive. Evidence suggests that mitochondrial dysfunction is a prominent feature of the disease, and that mitochondrial DNA (mtDNA) alterations may represent a possible starting point of the pathophysiological cascade. Although specific mtDNA alterations have been reported in AD patients both in brain and peripheral tissues, such as D-loop mutations, 4977-bp deletion and poly-C tract D310 cytosine insertion, a generalized subtle allelic shift has also been demonstrated. This shift is significant for a few nucleotide positions (nps), but it is also detectable for most nps, although at a lower level. As single allelic substitutions can unlikely be determinant, it is proposed that the combination of all of them could lead to a less efficient oxidative phosphorylation, thus influencing AD development and course.