Tissue-specific distribution of multiple mitochondrial DNA rearrangements during human aging

Mitochondrial DNA damage patterns and aging: revising the evidences for humans and mice

A significant body of work, accumulated over the years, strongly suggests that damage in mitochondrial DNA (mtDNA) contributes to aging in humans. Contradictory results, however, are reported in the literature, with some studies failing to provide support to this hypothesis. With the purpose of further understanding the aging process, several models, among which mouse models, have been frequently used. Although important affinities are recognized between humans and mice, differences on what concerns physiological properties, disease pathogenesis as well as life-history exist between the two; the extent to which such differences limit the translation, from mice to humans, of insights on the association between mtDNA damage and aging remains to be established. In this paper we revise the studies that analyze the association between patterns of mtDNA damage and aging, investigating putative alterations in mtDNA copy number as well as accumulation of deletions and of point mutations. Reports from the literature do not allow the establishment of a clear association between mtDNA copy number and age, either in humans or in mice. Further analysis, using a wide spectrum of tissues and a high number of individuals would be necessary to elucidate this pattern. Likewise humans, mice demonstrated a clear pattern of age-dependent and tissue-specific accumulation of mtDNA deletions. Deletions increase with age, and the highest amount of deletions has been observed in brain tissues both in humans and mice. On the other hand, mtDNA point mutations accumulation has been clearly associated with age in humans, but not in mice. Although further studies, using the same methodologies and targeting a larger number of samples would be mandatory to draw definitive conclusions, the revision of the available data raises concerns on the ability of mouse models to mimic the mtDNA damage patterns of humans, a fact with implications not only for the study of the aging process, but also for investigations of other processes in which mtDNA dysfunction is a hallmark, such as neurodegeneration.

Effect of aging on mitochondrial and nuclear DNA oxidative damage in the heart and brain throughout the life-span of the rat

The oxygen radical-induced DNA lesion 8-oxo,7,8-dihydro-2'-deoxyguanosine (8-oxodG) is the most commonly measured marker of oxidative DNA damage, which is currently considered a main cause of aging. However, a detailed study of the age-related variations of this marker in both mitochondrial (mtDNA) and nuclear (nDNA) DNA of post-mitotic organs throughout the life span has not been previously performed. In this investigation 8-oxodG steady-state levels were simultaneously measured in mtDNA and nDNA in the heart and brain of Sprague-Dawley rats at up to five different ages covering most of the adult life span, 4, 8, 12, 17 and 24 months of age, using exactly the same digestion of DNA to deoxynucleosides and chromatographic procedures for mtDNA and nDNA. 8-oxodG levels were maintained without changes during young and middle age in all cases, but showed statistically significant increases at the older ages studied in the majority of the kinds of DNA investigated. These age-related increases in oxidative damage occurred in brain nDNA at 17 and 24 months of age, in heart nDNA at 24 months of age, and in brain mtDNA at 24 months of age, whereas no significant age-related changes were detected in heart mtDNA. Besides, 8-oxodG levels were various fold higher in mtDNA than in nDNA, both in brain and heart, at all the ages studied. The results show that oxidative damage to DNA is higher in the mtDNA than in the nDNA of post-mitotic tissues throughout the whole life span of the rat and that and increase in mtDNA and nDNA oxidative stress occurs in most cases in old animals.

Improving upon nature's somatic mitochondrial DNA therapies

Mitochondrial DNA (mtDNA) directs key metabolic functions in eukaryotic cells. While a number of mtDNA mutations are known causes of human diseases and age-related dysfunctions, some mtDNA haplotypes are associated with extreme longevity. Despite the mutagenic mitochondrial environment naturally enhancing somatic mtDNA mutation rates, mtDNA remains grossly stable along generations of plant and animal species including man. This relative stability can be accounted for by the purging of deleterious mutations by natural selection operating on growing cells, tissues, organisms and populations, as observed in gametogenesis, embryogenesis, oncogenesis and cladogenesis. In the adult multicellular organism, however, mtDNA mutations accumulate in slowly dividing cells, and, to a much higher degree, in postmitotic cells and tissues. Dynamic mitochondrial fusion and fission, by redistributing polymorphic mtDNA molecules; mitophagy, by clearing defective mitochondria and mutated mtDNA; compensatory mutations and mtDNA repair can compensate for the accumulation of mtDNA mutations only to a certain extent, thereby creating a dysfunctional threshold. Here we hypothesize that this threshold is naturally up-regulated by both vertical and horizontal transfers of mtDNA from stem cells or cell types which retain the capacity of purging deleterious mtDNA through cell division and natural selection in the adult organism. When these natural cell and tissue mtDNA reserves are exhausted, artificial mtDNA therapy may provide for additional threshold up-regulation. Replacement of mtDNA has been already successfully accomplished in early stage embryos and stem cells in a number of species including primates. It is thus simply a matter of refinement of technique that somatic mtDNA therapy, i.e., therapy of pathological conditions based on the transfer of mtDNA to somatic eukaryotic cells and tissues, becomes a medical reality.

CytochromecOxidase Rather than Cytochromecis a Major Determinant of Mitochondrial Respiratory Capacity in Skeletal Muscle of Aged Rats: Role of Carnitine and Lipoic Acid

The release of mitochondrial cytochrome c followed by activation of caspase cascade has been reported with aging in various tissues, whereas little is known about the caspase-independent pathway involved in mitochondrial dysfunction. To determine the functional impact of cytochrome c loss on mitochondrial respiratory capacity, we monitored NADH redox transitions and oxygen consumption in isolated skeletal muscle mitochondria of 4- and 24-month-old rats in the presence and absence of exogenous cytochrome c; and assessed the efficacy of cosupplementation of carnitine and lipoic acid on age-related alteration in mitochondrial respiration. The loss of mitochondrial cytochrome c with age was accompanied with alteration in respiratory transition, which in turn was not rescued by exogenous addition of cytochrome c to isolated mitochondria. The analysis of mitochondrial and nuclear-encoded cytochrome c oxidase subunits suggests that the decreased levels of cytochrome c oxidase may be attributed for the irresponsiveness to exogenously added cytochrome c on mitochondrial respiratory transitions, possibly through reduction of upstream electron carriers. Oral supplementation of carnitine and lipoic acid to aged rats help to maintaining the mitochondrial oxidative capacity by regulating the release of cytochrome c and improves cytochrome c oxidase transcript levels. Thus, carnitine and lipoic acid supplementation prevents the loss of cytochrome c and their associated decline in cytochrome c oxidase activity; thereby, effectively attenuating any putative decrease in cellular energy and redox status with age.

Aging in vertebrates, and the effect of caloric restriction: a mitochondrial free radical production-DNA damage mechanism?

Oxygen is toxic to aerobic animals because it is univalently reduced inside cells to oxygen free radicals. Studies dealing with the relationship between oxidative stress and aging in different vertebrate species and in caloric-restricted rodents are discussed in this review. Healthy tissues mainly produce reactive oxygen species (ROS) at mitochondria. These ROS can damage cellular lipids, proteins and, most importantly, DNA. Although antioxidants help to control this oxidative stress in cells in general, they do not decrease the rate of aging, because their concentrations are lower in long- than in short-lived animals and because increasing antioxidant levels does not increase vertebrate maximum longevity. However, long-lived homeothermic vertebrates consistently have lower rates of mitochondrial ROS production and lower levels of steady-state oxidative damage in their mitochondrial DNA than short-lived ones. Caloric-restricted rodents also show lower levels of these two key parameters than controls fed ad libitum. The decrease in mitochondrial ROS generation of the restricted animals has been recently localized at complex I and the mechanism involved is related to the degree of electronic reduction of the complex I ROS generator. Strikingly, the same site and mechanism have been found when comparing a long- with a short-lived animal species. It is suggested that a low rate of mitochondrial ROS generation extends lifespan both in long-lived and in caloric-restricted animals by determining the rate of oxidative attack and accumulation of somatic mutations in mitochondrial DNA.

Direct repeats in mitochondrial DNA and mammalian lifespan

Mitochondria have long been suspected to be among the leading determinants of aging due to their functional importance and accelerated deterioration caused by accumulation of mutations in the mitochondrial DNA. Direct repeats are known to contribute to deletion formation in mtDNA and are a powerful source of reactive oxygen species (ROS)-independent mutagenesis. To evaluate the potential importance of homology-based deletion formation, we have analyzed the association between direct repeats in the mtDNA sequence and the lifespans of 65 mammalian species. Here, we report a significant negative correlation between the mutagenic potential of direct repeats and the mammalian lifespan, which is especially evident in closely related species.

Mitochondrial DNA content increase in response to cigarette smoking

An increase in mitochondrial DNA (mtDNA) content and decline in mitochondrial function occurs with aging and in response to DNA-damaging agents, including tobacco smoke. We did a cross-sectional study and quantified changes in mtDNA content in a population of individuals with varied smoking and alcohol exposure. Age, smoking history, ethanol intake, and other demographic data were characterized for 604 individuals participating in a screening study for smoking-related upper aerodigestive malignancy. Total DNA was extracted from exfoliated cells in saliva. DNA from a nuclear gene, beta-actin, and two mitochondrial genes, cytochrome c oxidase I and II (Cox I and Cox II), were quantified by real-time PCR. mtDNA content was correlated with age, exposure history, and other variables using multivariate regression analyses. A significant increase (P<0.001) in mtDNA content was noted in smokers (31% and 29% increase for Cox I and Cox II, respectively) and former smokers (31% and 34%) when compared with never smokers. This association persisted after adjustment for other significant factors including age, alcohol drinking, and income (P<0.001). Increased mtDNA content was positively associated with pack-years of smoking (P=0.02). Despite an average smoking cessation interval of 21 years in former smokers, tobacco cessation interval was not statistically significantly associated with mtDNA content. Smoking is associated with increased mtDNA content in a dose-dependent fashion. Mitochondrial DNA alterations in response to smoking persist for several decades after smoking cessation, consistent with long-term, smoking-related damage.

What the papers say: Where is the somatic mutation that causes aging?

It has been proposed that somatic mutations make major contributions to aging. The first paper, based on a gene knock-in mouse, supports a contributory role for mutation in mtDNA in aging, but does not support a damaged-mtDNA-producing-more-damaged-mtDNA hypothesis. The second paper indicates some GC-rich sequences in the nuclear DNA are more sensitive to oxidative damage than mtDNA. As a result, key genes involved in brain function and mitochondrial function are progressively inactivated with age. Failure in these nucleus-encoded mitochondrial genes may be a primary reason for mitochondrial failure in old age.

Down-regulation of mitochondrial cytochrome c oxidase in senescent porcine pulmonary artery endothelial cells

Cellular aging is associated with dysfunction of the mitochondrial respiration chain. Deficiency of mitochondrial cytochrome c oxidase (complex IV) plays a critical role in aging-induced mitochondrial dysfunction. We investigated whether in vitro cellular aging causes the downregulation of complex IV activity and gene expression using senescent (passage 45) and young (passage 3) pulmonary artery endothelial cells (PAEC). In senescent PAEC, the catalytic activity of complex IV decreased 84%, compared to that in young cells. Relative protein levels of complex IV subunits I and IV (complex IV S1 and S4) in senescent cells decreased 91%, compared to those in young cells. This suggests that lack of complex IV S1 and S4 in senescent cells may contribute to the deficiency of complex IV. Total steady state levels of mRNA for complex IV S1 and S4 in senescent cells were decreased to 20% and 18% of those in young cells. The relative rates of mRNA synthesis of complex IV S1 and S4 were decreased 46% and 37% in senescent cells, respectively, compared to young cells. The degradation of complex IV S1 and S4 was increased 76% and 64% in senescent cells, compared to young cells. These data indicate that mitochondrial DNA-encoded subunit I and nuclear DNA-encoded subunit IV of complex IV are downregulated through reduced synthesis and enhanced degradation of their mRNA, which may be responsible for the deficiency of complex IV in replicative senescent PAEC.

Experimental gerontological research in Australia

Recently there has been recognition in Australia of the importance of experimental gerontological research. To date, the major areas of research into the biology of ageing have been oxidative stress and mitochondrial dysfunction. Future directions for gerontological research in Australia are likely to place more emphasis on comprehensive studies that integrate basic biological research, with clinical studies and the behavioural and social domains of ageing. Priority will be given to studies incorporating biological markers that are population based and longitudinal. The growing interest in experimental gerontological research complements Australian research strengths in other domains such as disease-based research, social gerontology, population studies and health policy research.

Endogenous oxidative stress: relationship to aging, longevity and caloric restriction

Available studies are consistent with the possibility that oxygen radicals endogenously produced by mitochondria are causally involved in the determination of the rate of aging in homeothermic vertebrates. Oxidative damage to tissue macromolecules seems to increase during aging. The rate of mitochondrial oxygen radical generation of post-mitotic tissues is negatively correlated with animal longevity. In agreement with this, long-lived animals show lower levels of oxidative damage in their mitochondrial DNA (mtDNA) than short-lived ones, whereas this does not occur in nuclear DNA (nDNA). Caloric restriction, which decreases the rate of aging, also decreases mitochondrial oxygen radical generation and oxidative damage to mitochondrial DNA. This decrease in free radical generation occurs in complex I and is due to a decrease in the degree of electronic reduction of the complex I free radical generator, similarly to what has been described in various cases in long-lived animals. These results suggest that similar mechanisms have been used to extend longevity through decreases in oxidative stress in caloric restriction and during the evolution of species with different longevities.

Human aging and global function of coenzyme Q10

In this paper, we review two parts of our recent work on human skeletal muscle. The first part mainly describes changes occurring during aging, whereas the second part discusses the functions of coenzyme Q10 (CoQ10), particularly in relation to the aging process. During the lifetime of an individual, mtDNA undergoes a variety of mutation events and rearrangements. These mutations and their consequent bioenergenic decline, together with nuclear DNA damage, contribute to the reduced function of cells and organs, especially in postmitotic tissues. In skeletal muscle, this functional decline can be observed by means of changes with age in fiber type profile and the reduction in the number and size of the muscle fibers. In addition to the functions of coenzyme Q10 as an electron carrier in the respiratory chain and as an antioxidant, CoQ10 has been shown to regulate global gene expression in skeletal muscle. We hypothesize that this regulation is achieved via superoxide formation with H2O2 as a second messenger to the nucleus.

Genes, telomeres and mammalian ageing

Although there appear to be several influences, which contribute to the ageing of mammals, the role of DNA appears to be pivotal. There is increasing evidence that oxidative damage is an important factor in producing mutations in genes, shortening telomeres and damaging mitochondrial DNA. Accumulation of mutations in genomic DNA could result in the gradual decline in cellular function, which is exhibited in a variety of tissues. The random nature of these mutations, could also offer an explanation for differences in the degree and time of onset of age-related changes, exhibited by different individuals. Shortening of telomeres, caused by oxidative damage or the end-replication problem, could result in the accumulation of post-mitotic cells in-vivo during ageing. This might impair certain aspects of physiology, such as wound healing. Mutation of mitochondrial DNA may also be important in causing loss of cells in post-mitotic tissues such as muscle or brain. In addition changes in the redox state during the life of an animal may alter transcription factor activities, leading to consistent changes in the gene expression profiles of mammalian tissues. The latter could explain consistent age-related changes that have been observed in cell structure and physiology. Although all of these mechanisms may make a contribution to ageing, it is likely that it is the interplay between them that produces the most prominent effects.

Preferential amplification is minimised in long-PCR systems

The advent of long PCR (XL-PCR) has proven to be a major advance in PCR technology and is currently being utilised to investigate numerous biological systems. The analysis of mixed DNA populations is a particularly useful application for XL-PCR. For example, XL-PCR has been used to investigate the occurrence of heterogeneous mitochondrial DNA (mtDNA) rearrangement mutations. With XL-PCR it became possible to amplify the entire length of the mtDNA chromosome and detect any mtDNA deletion or insertion mutations based on a measurable change in overall sequence length. In the present communication, XL-PCR and conventional short-length PCR were used to amplify mitochondrial DNA sequences from several human vastus lateralis skeletal muscle samples. The experiments demonstrated that there was minimal preferential amplification of shorter DNA sequences with XL-PCR and was significantly less than the preferential amplification of shorter sequences observed with conventional PCR. Also, XL-PCR amplification of the complete mtDNA sequence from control DNA containing a single mtDNA template (leucocyte extracts) showed that the generation of PCR artefacts was not a predisposed failing of the system but was dependant on the standard rules that govern the set up and optimisation of any PCR reaction. In optimised systems, XL-PCR artefacts were not generated and a single PCR product was always recovered.

The flux of free radical attack through mitochondrial DNA is related to aging rate

Aging is a progressive and universal process originated endogenously which manifests best in post-mitotic cells. Available data indicate that the relation between oxidative stress and aging is due to the presence of low rates of mitochondrial free radical production and low degrees of fatty acid unsaturation of cellular membranes in the post-mitotic tissues of long-lived animals in relation to those of short-lived ones. Recent research shows that long-lived animals also have lower steady-state levels of oxidative damage in the mitochondrial DNA (mtDNA) of post-mitotic cells than short-lived species. This study shows that the flux of free radical attack to mtDNA is higher in short- than in long-lived animals, and proposes that this is a main determinant of the rate of accumulation of mtDNA mutations, and thus the rate of aging. This implies that aging has been slowed evolutionarily by mechanisms that decrease the generation of endogenous damage rather than try to intercept damaging agents, or to repair the damage already inflicted. The first kind of mechanisms are more efficient and less energetically expensive. Free radicals of mitochondrial origin, oxidative damage to DNA, evolution of aging rate, and possibilities and consequences of their future modification are also discussed.

Tissue mitochondrial DNA changes. A stochastic system

Several lines of evidence support the view that the bioenergetic function of the mitochondria in postmitotic tissue deteriorates during normal aging. Skeletal muscle is one such tissue that undergoes age-related fiber loss and atrophy and an age-associated rise in the number of cytochrome c oxidase (COX) deficient fibers. With such metabolic pressure placed on skeletal muscle it would be an obvious advantage to supplement the cellular requirement for energy by up-regulating glycolysis, and alternative pathway for energy synthesis. Analysis of rat skeletal muscle utilizing antibodies directed against key enzymes involved in glycolysis has provided evidence of an age-associated increase in the enzymes involved in glycolysis. Fructose-6-phosphate kinase, aldolase, glyceraldehyde-3-phosphate dehydrogenase, and pyruvate kinase protein levels appeared to increase in the soleus, gracilis, and quadriceps muscle from aged rats. The increase in the level of these proteins appeared to correlate to a corresponding decrease in the amount of cytochrome c oxidase protein measured in the same tissue. Together these results are interpreted to represent a general upregulation of glycolysis that occurs in response to the age-associated decrease in mitochondrial energy capacity. Mitochondrial DNA (mtDNA) damage and mutations may accumulate with advancing age until they reach a threshold level were they impinge on the bioenergy capacity of the cell or tissue. Evidence indicates that mtDNA from the skeletal muscle of both aged rats and humans not only undergoes changes at the nucleotide sequence level (mutations and DNA damage), but also undergoes modifications at the tertiary level to generate unique age-related conformational mtDNA species. One particular age-related conformational form was only detected in aged rat tissues with high demands on respiration, specifically in heart, kidney, soleus muscle, and, to a lesser extent, the quadriceps muscle. The age-related form was not detected in gracilis muscle which is predominantly dependent upon glycolysis with regard to its energy requirements. Finally, a comprehensive hypothesis is presented that features the stochastic nature of the mitochondrial system. The basis of the hypothesis is that a dynamic relationship exists between endogenous mutagen production, DNA repair, mtDNA turnover, and nuclear control of mtDNA copy number and that age-associated changes in the dynamics of this relationship lead to a loss of functional full-length mtDNA that eventually leads to bioenergy decline.

Differential effects of antiretroviral nucleoside analogs on mitochondrial function in HepG2 cells

Numerous studies have reported effects of antiviral nucleoside analogs on mitochondrial function, but they have not correlated well with the observed toxic side effects. By comparing the effects of the five Food and Drug Administration-approved anti-human immunodeficiency virus nucleoside analogs, zidovudine (3'-azido-3'-deoxythymidine) (AZT), 2',3'-dideoxycytidine (ddC), 2', 3'-dideoxyinosine (ddI), 2',3'-didehydro-2',3'-deoxythymidine (d4T), and beta-L-2',3'-dideoxy-3'-thiacytidine (3TC), as well as the metabolite of AZT, 3'-amino-3'-deoxythymidine (AMT), on mitochondrial function in a human hepatoma cell line, this issue has been reexamined. Evidence for a number of mitochondrial defects with AZT, ddC, and ddI was found, but only AZT induced a marked rise in lactic acid levels. Only in mitochondria isolated from AZT (50 microM)-treated cells was significant inhibition of cytochrome c oxidase and citrate synthase found. Our investigations also demonstrated that AZT, d4T, and 3TC did not affect the synthesis of the 11 polypeptides encoded by mitochondrial DNA, while ddC caused 70% reduction of total polypeptide content and ddI reduced by 43% the total content of 8 polypeptides (including NADH dehydrogenase subunits 1, 2, 4, and 5, cytochrome c oxidase subunits I to III, and cytochrome b). We hypothesize that in hepatocytes the reserve capacity for mitochondrial respiration is such that inhibition of respiratory enzymes is unlikely to become critical. In contrast, the combined inhibition of the citric acid cycle and electron transport greatly enhances the dependence of the cell on glycolysis and may explain why apparent mitochondrial dysfunction is more prevalent with AZT treatment.

Oxidative damage to mitochondrial DNA is inversely related to maximum life span in the heart and brain of mammals

DNA damage is considered of paramount importance in aging. Among causes of this damage, free radical attack, particularly from mitochondrial origin, is receiving special attention. If oxidative damage to DNA is involved in aging, long-lived animals (which age slowly) should show lower levels of markers of this kind of damage than short-lived ones. However, this possibility has not heretofore been investigated. In this study, steady-state levels of 8-oxo-7, 8-dihydro-2'-deoxyguanosine (8-oxodG) referred to deoxyguanosine (dG) were measured by high performance liquid chromatography (HPLC) in the mitochondrial (mtDNA) and nuclear (nDNA) DNA from the heart of eight and the brain of six mammalian species ranging in maximum life span (MLSP) from 3.5 to 46 years. Exactly the same digestion of DNA to deoxynucleosides and HPLC protocols was used for mtDNA and nDNA. Significantly higher (three- to ninefold) 8-oxodG/dG values were found in mtDNA than in nDNA in all the species studied in both tissues. 8-oxodG/dG in nDNA did not correlate with MLSP across species either in the heart (r=-0.68; P<0.06) or brain (r = 0.53; P<0.27). However, 8-oxodG/dG in mtDNA was inversely correlated with MLSP both in heart (r=-0.92; P<0.001) and brain (r=-0.88; P<0.016) tissues following the power function y = a(.)x(b), where y is 8-oxodG/dG and x is the MLSP. This agrees with the consistent observation that mitochondrial free radical generation is also lower in long-lived than in short-lived species. The results obtained agree with the notion that oxygen radicals of mitochondrial origin oxidatively damage mtDNA in a way related to the aging rate of each species.-Barja, G., Herrero, A. Oxidative damage to mitochondrial DNA is inversely related to maximum life span in the heart and brain of mammals.

8-oxo-deoxyguanosine levels in heart and brain mitochondrial and nuclear DNA of two mammals and three birds in relation to their different rates of aging

Previous studies found that the rate of mitochondrial oxygen radical generation is lower in long-lived birds than in short-lived mammals. In the present study, the oxidative DNA damage marker 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) in heart and brain mitochondrial (mtDNA) and nuclear DNA (nDNA) was compared between mammals and birds of approximately similar body size and metabolic rates; rats (maximum life span, MLSP = 4 years) vs pigeons (MLSP = 35 years), and mice (MLSP = 3.5 years) vs parakeets (MLSP = 21 years) or canaries (MLSP = 24 years). Lower steady-state 8-oxodG values were observed in all cases in the heart mtDNA in birds than in mammals. 8-oxodG levels were also lower in brain mtDNA in pigeons than in rats, in brain nDNA in canaries than in mice, and in heart nDNA in parakeets compared with mice. The rest of the comparisons did not show significant differences between species. These results taken together indicate that oxidative damage to DNA tends to be lower in birds (highly long-lived species) than in short-lived mammals, specially in the case of mtDNA. This is consistent with the low rate of mitochondrial oxygen radical generation observed in all long-lived species investigated up to date, birds or mammals, including the bird species studied here. The results also show that 8-oxodG steady-state levels are much higher in mtDNA than in nDNA in all the tissues (heart and brain) and species (birds and mammals) studied.