The rate of mitochondrial mutagenesis is faster in mice than humans

Phenotypic molecular features of long-lived animal species

One of the challenges facing science/biology today is uncovering the molecular bases that support and determine animal and human longevity. Nature, in offering a diversity of animal species that differ in longevity by more than 5 orders of magnitude, is the best 'experimental laboratory' to achieve this aim. Mammals, in particular, can differ by more than 200-fold in longevity. For this reason, most of the available evidence on this topic derives from comparative physiology studies. But why can human beings, for instance, reach 120 years whereas rats only last at best 4 years? How does nature change the longevity of species? Longevity is a species-specific feature resulting from an evolutionary process. Long-lived animal species, including humans, show adaptations at all levels of biological organization, from metabolites to genome, supported by signaling and regulatory networks. The structural and functional features that define a long-lived species may suggest that longevity is a programmed biological property.

Hyperphosphorylated Human Tau Accumulates at the Synapse, Localizing on Synaptic Mitochondrial Outer Membranes and Disrupting Respiration in a Mouse Model of Tauopathy

Neurogenerative disorders, such as Alzheimer’s disease (AD), represent a growing public health challenge in aging societies. Tauopathies, a subset of neurodegenerative disorders that includes AD, are characterized by accumulation of fibrillar and hyperphosphorylated forms of microtubule-associated protein tau with coincident mitochondrial abnormalities and neuronal dysfunction. Although, in vitro, tau impairs axonal transport altering mitochondrial distribution, clear in vivo mechanisms associating tau and mitochondrial dysfunction remain obscure. Herein, we investigated the effects of human tau on brain mitochondria in vivo using transgenic htau mice at ages preceding and coinciding with onset of tauopathy. Subcellular proteomics combined with bioenergetic assessment revealed pathologic forms of tau preferentially associate with synaptic over non-synaptic mitochondria coinciding with changes in bioenergetics, reminiscent of an aged synaptic mitochondrial phenotype in wild-type mice. While mitochondrial content was unaltered, mitochondrial maximal respiration was impaired in synaptosomes from htau mice. Further, mitochondria-associated tau was determined to be outer membrane-associated using the trypsin protection assay and carbonate extraction. These findings reveal non-mutant human tau accumulation at the synapse has deleterious effects on mitochondria, which likely contributes to synaptic dysfunction observed in the context of tauopathy.

Proteomic landscape of liver tissue in old male mice that are long-term treated with polysaccharides from Sargassum fusiforme

Sargassum fusiforme is a type of brown algae and well known as a longevity promoting vegetable in Northeastern Asia. The polysaccharides derived from Sargassum fusiforme (SFPs) have been suggested as an antioxidant component for anti-aging function. However, global molecular changes in vivo by SFPs have not been fully elucidated. Here, we present a proteomics study using liver tissues of aged male mice that were fed with SFPs. Of forty-nine protein spots, thirty-eight were up-regulated and eleven were down-regulated, showing significant changes in abundance by two-dimensional gel electrophoresis. These differentially expressed proteins were mainly involved in oxidation-reduction, amino acid metabolism, and energy metabolism. Forty-six proteins were integrated into a unified network, with catalase (Cat) at the center. Intriguingly, most of the proteins were speculated as mitochondrial-located proteins. Our findings suggested that SFPs modulated antioxidant enzymes to scavenge redundant free radicals, thus preventing oxidative damage. In conclusion, our study provides a proteomic view on how SFPs have beneficial effects on the aspects of antioxidant and energy metabolism during the aging process. This study facilitates the understanding of anti-aging molecular mechanisms in polysaccharides derived from Sargassum fusiforme.

Analysis of the Mitochondrial 4977 Bp Deletion in Patients with Hepatocellular Carcinoma

Mutations in the mitochondrial (mt) genome that result in mt dysfunction, have long been proposed to play important roles in the pathogenesis of hepatocellular carcinoma (HCC). Among these, the common mtDNA 4977 bp deletion is one of the most frequent mutations observed in various cancers. To understand the relationship between the mtDNA 4977 bp deletion and HCC, we performed mutational screening for the presence of this deletion in 105 HCC patients and 69 unrelated healthy subjects. After nested-polymerase chain reaction (nested-PCR) amplification, we found that there were 10 patients carrying the mtDNA 4977 bp deletion, and this deletion was absent in control subjects. Moreover, HCC patients carrying this deletion showed a marked increase in reactive oxygen species (ROS) level and mtDNA copy number when compared with the healthy controls. Taken together, our data indicated that the mtDNA 4977 bp deletion may play important role in the carcinogenesis of HCC, possibly via the alternation of mtDNA copy number and oxidative stress.

Aging synaptic mitochondria exhibit dynamic proteomic changes while maintaining bioenergetic function

Aging correlates with a progressive impairment of mitochondrial homeostasis and is an influential factor for several forms of neurodegeneration. However, the mechanisms underlying age-related alterations in synaptosomal mitochondria, a neuronal mitochondria population highly susceptible to insults and critical for brain function, remain incompletely understood. Therefore this study investigates the synaptic mitochondrial proteomic and bioenergetic alterations that occur with age. The utilization of a state of the art quantitative proteomics approach allowed for the comparison of protein expression levels in synaptic mitochondria isolated from 5 (mature), 12 (old), and 24 (aged) month old mice. During the process of aging we find that dynamic proteomic alterations occur in synaptic mitochondria. Despite direct (mitochondrial DNA deletions) and indirect (increased antioxidant protein levels) signs of mitochondrial damage in the aged mice, there was an overall maintenance of mitochondrial function. Therefore the synaptic mitochondrial proteomic changes that occur with aging correlate with preservation of synaptic mitochondrial function.

Quantitative proteomics of synaptic and nonsynaptic mitochondria: insights for synaptic mitochondrial vulnerability

Synaptic mitochondria are essential for maintaining calcium homeostasis and producing ATP, processes vital for neuronal integrity and synaptic transmission. Synaptic mitochondria exhibit increased oxidative damage during aging and are more vulnerable to calcium insult than nonsynaptic mitochondria. Why synaptic mitochondria are specifically more susceptible to cumulative damage remains to be determined. In this study, the generation of a super-SILAC mix that served as an appropriate internal standard for mouse brain mitochondria mass spectrometry based analysis allowed for the quantification of the proteomic differences between synaptic and nonsynaptic mitochondria isolated from 10-month-old mice. We identified a total of 2260 common proteins between synaptic and nonsynaptic mitochondria of which 1629 were annotated as mitochondrial. Quantitative proteomic analysis of the proteins common between synaptic and nonsynaptic mitochondria revealed significant differential expression of 522 proteins involved in several pathways including oxidative phosphorylation, mitochondrial fission/fusion, calcium transport, and mitochondrial DNA replication and maintenance. In comparison to nonsynaptic mitochondria, synaptic mitochondria exhibited increased age-associated mitochondrial DNA deletions and decreased bioenergetic function. These findings provide insights into synaptic mitochondrial susceptibility to damage.

Regulation of lifespan by the mitochondrial electron transport chain: reactive oxygen species-dependent and reactive oxygen species-independent mechanisms

SIGNIFICANCE

Aging is a consequence of the accumulation of cellular damage that impairs the capacity of an aging organism to adapt to stress. The Mitochondrial Free Radical Theory of Aging (MFRTA) has been one of the most influential ideas over the past 50 years. The MFRTA is supported by the accumulation of oxidative damage during aging along with comparative studies demonstrating that long-lived species or individuals produce fewer mitochondrial reactive oxygen species and have lower levels of oxidative damage.

RECENT ADVANCES

Recently, however, species that combine high oxidative damage with a longer lifespan (i.e., naked mole rats) have been described. Moreover, most of the interventions based on antioxidant supplementation do not increase longevity, as would be predicted by the MFRTA. Studies to date provide a clear understanding that mitochondrial function regulates the rate of aging, but the underlying mechanisms remain unclear.

CRITICAL ISSUES

Here, we review the reactive oxygen species (ROS)-dependent and ROS-independent mechanisms by which mitochondria can affect longevity. We discuss the role of different ROS (superoxide, hydrogen peroxide, and hydroxyl radical), both as oxidants as well as signaling molecules. We also describe how mitochondria can regulate longevity by ROS-independent mechanisms. We discuss alterations in mitochondrial DNA, accumulation of cellular waste as a consequence of glyco- and lipoxidative damage, and the regulation of DNA maintenance enzymes as mechanisms that can determine longevity without involving ROS.

FUTURE DIRECTIONS

We also show how the regulation of longevity is a complex process whereby ROS-dependent and ROS-independent mechanisms interact to determine the maximum lifespan of species and individuals.

Mitochondrial common deletion, a potential biomarker for cancer occurrence, is selected against in cancer background: a meta-analysis of 38 studies

Mitochondrial dysfunction has been long proposed to play a major role in tumorigenesis. Mitochondrial DNA (mtDNA) mutations, especially the mtDNA 4,977 bp deletion has been found in patients of various types of cancer. In order to comprehend the mtDNA 4,977 bp deletion status in various cancer types, we performed a meta-analysis composed of 33 publications, in which a total of 1613 cancer cases, 1516 adjacent normals and 638 healthy controls were included. When all studies were pooled, we found that cancerous tissue carried a lower mtDNA 4,977 bp deletion frequency than adjacent non-cancerous tissue (OR = 0.43, 95% CI = 0.20-0.92, P = 0.03 for heterogeneity test, I(2) = 91.5%) among various types of cancer. In the stratified analysis by cancer type the deletion frequency was even lower in tumor tissue than in adjacent normal tissue of breast cancer (OR = 0.19, 95% CI = 0.06-0.61, P = 0.005 for heterogeneity test, I(2)= 82.7%). Interestingly, this observation became more significant in the stratified studies with larger sample sizes (OR = 0.70, 95% CI = 0.58-0.86, P = 0.0005 for heterogeneity test, I(2) = 95.1%). Furthermore, when compared with the normal tissue from the matched healthy controls, increased deletion frequencies were observed in both adjacent non-cancerous tissue (OR = 3.02, 95% CI = 2.13-4.28, P<0.00001 for heterogeneity test, I(2)= 53.7%), and cancerous tissue (OR = 1.36, 95% CI = 1.04-1.77, P = 0.02 for heterogeneity test, I(2)= 83.5%). This meta-analysis suggests that the mtDNA 4,977 bp deletion is often found in cancerous tissue and thus has the potential to be a biomarker for cancer occurrence in the tissue, but at the same time being selected against in various types of carcinoma tissues. Larger and better-designed studies are still warranted to confirm these findings.

Mitochondria, oxidative DNA damage, and aging

Protection from reactive oxygen species (ROS) and from mitochondrial oxidative damage is well known to be necessary to longevity. The relevance of mitochondrial DNA (mtDNA) to aging is suggested by the fact that the two most commonly measured forms of mtDNA damage, deletions and the oxidatively induced lesion 8-oxo-dG, increase with age. The rate of increase is species-specific and correlates with maximum lifespan. It is less clear that failure or inadequacies in the protection from reactive oxygen species (ROS) and from mitochondrial oxidative damage are sufficient to explain senescence. DNA containing 8-oxo-dG is repaired by mitochondria, and the high ratio of mitochondrial to nuclear levels of 8-oxo-dG previously reported are now suspected to be due to methodological difficulties. Furthermore, MnSOD -/+ mice incur higher than wild type levels of oxidative damage, but do not display an aging phenotype. Together, these findings suggest that oxidative damage to mitochondria is lower than previously thought, and that higher levels can be tolerated without physiological consequence. A great deal of work remains before it will be known whether mitochondrial oxidative damage is a "clock" which controls the rate of aging. The increased level of 8-oxo-dG seen with age in isolated mitochondria needs explanation. It could be that a subset of cells lose the ability to protect or repair mitochondria, resulting in their incurring disproportionate levels of damage. Such an uneven distribution could exceed the reserve capacity of these cells and have serious physiological consequences. Measurements of damage need to focus more on distribution, both within tissues and within cells. In addition, study must be given to the incidence and repair of other DNA lesions, and to the possibility that repair varies from species to species, tissue to tissue, and young to old.

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.

Implications of quantum metabolism and natural selection for the origin of cancer cells and tumor progression

Empirical studies give increased support for the hypothesis that the sporadic form of cancer is an age-related metabolic disease characterized by: (a) metabolic dysregulation with random abnormalities in mitochondrial DNA, and (b) metabolic alteration - the compensatory upregulation of glycolysis to offset mitochondrial impairments. This paper appeals to the theory of Quantum Metabolism and the principles of natural selection to formulate a conceptual framework for a quantitative analysis of the origin and proliferation of the disease. Quantum Metabolism, an analytical theory of energy transduction in cells inspired by the methodology of the quantum theory of solids, elucidates the molecular basis for differences in metabolic rate between normal cells, utilizing predominantly oxidative phosphorylation, and cancer cells utilizing predominantly glycolysis. The principles of natural selection account for the outcome of competition between the two classes of cells. Quantum Metabolism and the principles of natural selection give an ontogenic and evolutionary rationale for cancer proliferation and furnish a framework for effective therapeutic strategies to impede the spread of the disease.

Assurance of mitochondrial integrity and mammalian longevity by the p62-Keap1-Nrf2-Nqo1 cascade

Sqstm1/p62 functions in the non-canonical activation of nuclear factor (erythroid-derived 2)-like 2 (Nrf2). However, its physiological relevance is not certain. Here, we show that p62(-/-) mice exhibited an accelerated presentation of ageing phenotypes, and tissues from these mice created a pro-oxidative environment owing to compromised mitochondrial electron transport. Accordingly, mitochondrial function rapidly declined with age in p62(-/-) mice. In addition, p62 enhanced basal Nrf2 activity, conferring a higher steady-state expression of NAD(P)H dehydrogenase, quinone 1 (Nqo1) to maintain mitochondrial membrane potential and, thereby, restrict excess oxidant generation. Together, the p62-Nrf2-Nqo1 cascade functions to assure mammalian longevity by stabilizing mitochondrial integrity.

An evolutionary comparative scan for longevity-related oxidative stress resistance mechanisms in homeotherms

Key mechanisms relating oxidative stress to longevity from an interespecies comparative approach are reviewed. Long-lived animal species show low rates of reactive oxygen species (ROS) generation and oxidative damage at their mitochondria. Comparative physiology also shows that the specific compositional pattern of tissue macromolecules (proteins, lipids and nucleic acids) in long-lived animal species gives them an intrinsically high resistance to modification that likely contributes to their superior longevity. This is obtained in the case of lipids by decreasing the degree of fatty acid unsaturation, and in the case of proteins by lowering their methionine content. These findings are also substantiated from a phylogenomic approach. Nutritional or/and pharmacological interventions focused to modify some of these molecular traits were translated with modifications in animal longevity. It is proposed that natural selection tends to decrease the mitochondrial ROS generation and to increase the molecular resistance to the oxidative damage in long-lived species.

Stability of mitochondrial DNA against reactive oxygen species (ROS) generated in diabetes

BACKGROUND

Increased production of reactive oxygen species (ROS) in mitochondria has been proposed as the pathogenic mechanism for chronic complications of diabetes. Mitochondrial DNA (mtDNA) is more vulnerable to reactive oxygen species. However, there are few data on the mitochondrial DNA damage in diabetes and these are available only from patients with different duration of the disease and tissues not relevant to the chronic complications of diabetes. We therefore proposed to study the stability of mitochondrial DNA under controlled experimental conditions, to understand its contribution to chronic complications of diabetes.

METHODS

The mitochondrial DNA damage was evaluated by long-fragment polymerase chain reaction in human dermal fibroblasts exposed to high glucose level and hypoxia (an additional source of reactive oxygen species) or in organs from diabetic animals (db/db mice) at different ages. Reactive oxygen species production was assessed in vitro by fluorescence and in vivo by nitrosylation of the proteins. The antioxidant enzymes were assessed by enzyme activity and by quantitative real-time polymerase chain reaction while the mitochondrial repair activity (base excision repair) was determined by using abasic site-containing oligonucleotides as substrates.

RESULTS

Hyperglycaemia, when combined with hypoxia, is able to induce mitochondrial DNA damage in human dermal fibroblasts. The deleterious effect is mediated by mitochondrial reactive oxygen species, being abolished when the mitochondria electron transport is blocked. The accumulation of mitochondrial DNA damage in vivo is, however, decreased in 'old' diabetic animals (db/db) despite higher reactive oxygen species levels. This mitochondrial DNA protection might be conferred by an increased base excision repair activity.

CONCLUSION

Increased base excision repair activity in tissues affected by the chronic complications of diabetes is a potential mechanism that can overcome mitochondrial DNA damage induced by hyperglycaemia-related reactive oxygen species overproduction.

Mitochondrial DNA damage and animal longevity: insights from comparative studies

Chemical reactions in living cells are under strict enzyme control and conform to a tightly regulated metabolic program. However, uncontrolled and potentially deleterious endogenous reactions occur, even under physiological conditions. Aging, in this chemical context, could be viewed as an entropic process, the result of chemical side reactions that chronically and cumulatively degrade the function of biological systems. Mitochondria are a main source of reactive oxygen species (ROS) and chemical sidereactions in healthy aerobic tissues and are the only known extranuclear cellular organelles in animal cells that contain their own DNA (mtDNA). ROS can modify mtDNA directly at the sugar-phosphate backbone or at the bases, producing many different oxidatively modified purines and pyrimidines, as well as single and double strand breaks and DNA mutations. In this scenario, natural selection tends to decrease the mitochondrial ROS generation, the oxidative damage to mtDNA, and the mitochondrial mutation rate in long-lived species, in agreement with the mitochondrial oxidative stress theory of aging.

Chronically elevated glucose compromises myocardial mitochondrial DNA integrity by alteration of mitochondrial topoisomerase function

Mitochondrial dysfunction has a significant role in the development and complications of diabetic cardiomyopathy. Mitochondrial dysfunction and mitochondrial DNA (mtDNA) mutations are also associated with different types of cancer and neurodegenerative diseases. The goal of this study was to determine if chronically elevated glucose increase in mtDNA damage contributed to mitochondrial dysfunction and identify the underlying basis for mtDNA damage. H9c2 myotubes (a cardiac-derived cell line) were studied in the presence of 5.5, 16.5, or 33.0 mM glucose for up to 13 days. Tests of mitochondria function (Complex I and IV activity and ATP generation) were all significantly depressed by elevated media glucose. Intramitochondrial superoxide and intracellular superoxide levels were transiently increased during the experimental period. AnnexinV binding (a marker of apoptosis) was significantly increased after 7 and 13 days of high glucose. Thirteen days of elevated glucose significantly increased mtDNA damage globally and across the region encoding for the three subunits of cytochrome oxidase. Using mitochondria isolated from cells chronically exposed to elevated glucose, we observed significant increases in topoisomerase-linked DNA cleavage. Mitochondria-dependent DNA cleavage was significantly exacerbated by H(2)O(2) and that immunoprecipitation of mitochondrial extracts with a mtTOP1 antibody significantly decreased DNA cleavage, indicating that at least part of this activity could be attributed to mtTOP1. We conclude that even mild increases in glucose presentation compromised mitochondrial function as a result of a decline in mtDNA integrity. Separate from a direct impact of oxidative stress on mtDNA, ROS-induced alteration of mitochondrial topoisomerase activity exacerbated and propagated increases in mtDNA damage. These findings are significant in that the activation/inhibition state of the mitochondrial topoisomerases will have important consequences for mitochondrial DNA integrity and the well being of the myocardium.

Derivation of a reference dose and drinking water equivalent level for 1,2,3-trichloropropane

In some US potable water supplies, 1,2,3-trichloropropane (TCP) has been present at ranges of non-detect to less than 100 ppb, resulting from past uses. In subchronic oral studies, TCP produced toxicity in kidneys, liver, and other tissues. TCP administered by corn oil gavage in chronic studies produced tumors at multiple sites in rats and mice; however, interpretation of these studies was impeded by substantial premature mortality. Drinking water equivalent levels (DWELs) were estimated for a lifetime of consumption by applying biologically-based safety/risk assessment approaches, including Monte Carlo techniques, and with consideration of kinetics and modes of action, to possibly replace default assumptions. Internationally recognized Frameworks for human relevance of animal data were employed to interpret the findings. Calculated were a reference dose (=39 microg/kg d) for non-cancer and Cancer Values (CV) (=10-14 microg/kg d) based on non-linear dose-response relationships for mutagenicity as a precursor of cancer. Lifetime Average Daily Intakes (LADI) are 3130 and 790-1120 microg/person-d for non-cancer and cancer, respectively. DWELs, estimated by applying a relative source contribution (RSC) of 50% to the LADIs, are 780 and 200-280 microg/L for non-cancer and cancer, respectively. These DWELs may inform establishment of formal/informal guidelines and standards to protect public health.

Functional outcome is impaired following traumatic brain injury in aging Nogo-A/B-deficient mice

Increasing age is associated with a poor prognosis following traumatic brain injury (TBI). CNS axons may recover poorly following TBI due to expression of myelin-derived inhibitors to axonal outgrowth such as Nogo-A. To study the role of Nogo-A/B in the pathophysiological response of the elderly to TBI, 1-year-old mice deficient in Nogo-A/B (Nogo-A/B homozygous(-/-) mice), Nogo-A/B heterozygous(-/+) mice, and age-matched wild-type (WT) littermate controls were subjected to a controlled cortical impact (CCI) TBI. Sham-injured WT mice (7 months old) and 12 month old naïve Nogo-A/B(-/-) and Nogo-A/B(-/+) served as controls. Neurological motor function was evaluated up to 3 weeks, and cognitive function, hemispheric tissue loss, myelin staining and hippocampal beta-amyloid (A beta) immunohistochemistry were evaluated at 4 weeks post-injury. In WT littermates, TBI significantly impaired learning ability at 4 weeks and neurological motor function up to 2 weeks post-injury and caused a significant loss of hemispheric tissue. Following TBI, Nogo-A/B(-/-) mice showed significantly less recovery from neurological motor and cognitive deficits compared to brain-injured WT mice. Naïve Nogo-A/B(-/-) and Nogo-A/B(-/+) mice quickly learned the MWM task in contrast to brain-injured Nogo-A/B(-/-) mice who failed to learn the MWM task at 4 weeks post-injury. Hemispheric tissue loss and cortical lesion volume were similar among the brain-injured genotypes. Neither TBI nor the absence of NogoA/B caused an increased A beta expression. Myelin staining showed a reduced area and density in the corpus callosum in brain-injured Nogo-A/B(-/-) animals compared to their littermate controls. These novel and unexpected behavioral results demonstrate that the absence of Nogo-A/B may negatively influence outcome, possibly related to hypomyelination, following TBI in mice and suggest a complex role for this myelin-associated axonal growth inhibitor following TBI.

[Calorie restriction, oxidative stress and longevity]

Studies on the relationship between oxidative stress and ageing in different vertebrate species and in calorie-restricted animals are reviewed. Endogenous antioxidants inversely correlate with maximum longevity in animal species and experiments modifying levels of these antioxidants can increase survival and mean life span but not maximum life span (MLSP). The available evidence shows that long-living vertebrates consistently have low rates of mitochondrial free radical generation, as well as a low grade of fatty acid unsaturation on cellular membranes, which are two crucial factors determining their ageing rate. Oxidative damage to mitochondrial DNA is also lower in long-living vertebrates than in short-living vertebrates. Calorie restriction, the best described experimental strategy that consistently increases mean and maximum life span, also decreases mitochondrial reactive oxygen species (ROS) generation and oxidative damage to mitochondrial DNA. Recent data indicate that the decrease in mitochondrial ROS generation is due to protein restriction rather than to calorie restriction, and more specifically to dietary methionine restriction. Greater longevity would be partly achieved by a low rate of endogenous oxidative damage generation, but also by a macromolecular composition highly resistant to oxidative modification, as is the case for lipids and proteins.

Molecular mechanisms of life- and health-span extension: role of calorie restriction and exercise intervention

The aging process results in a gradual and progressive structural deterioration of biomolecular and cellular compartments and is associated with many pathological conditions, including cardiovascular disease, stroke, Alzheimer's disease, osteoporosis, sarcopenia, and liver dysfunction. Concomitantly, each of these conditions is associated with progressive functional decline, loss of independence, and ultimately disability. Because disabled individuals require care in outpatient or home care settings, and in light of the social, emotional, and fiscal burden associated with caring for an ever-increasing elderly population, research in geriatric medicine has recently focused on the biological mechanisms that are involved in the progression towards functional decline and disability to better design treatment and intervention strategies. Although not completely understood, the mechanisms underlying the aging process may partly involve inflammatory processes, oxidative damage, mitochondrial dysfunction, and apoptotic tissue degeneration. These hypotheses are based on epidemiological evidence and data from animal models of aging, as well as interventional studies. Findings from these studies have identified possible strategies to decrease the incidence of age-related diseases and delay the aging process. For example, lifelong exercise is known to extend mean life-span, whereas calorie restriction (CR) increases both mean and maximum life-span in a variety of species. Optimal application of these intervention strategies in the elderly may positively affect health-related outcomes and possibly longevity. Therefore, the scope of this article is to (i) provide an interpretation of various theories of aging from a "health-span" perspective; (ii) describe interventional testing in animals (CR and exercise); and (iii) provide a translational interpretation of these data.

The role of mitochondrial DNA mutations in aging and sarcopenia: implications for the mitochondrial vicious cycle theory of aging

Aging is associated with a progressive loss of skeletal muscle mass and strength and the mechanisms mediating these effects likely involve mitochondrial DNA (mtDNA) mutations, mitochondrial dysfunction and the activation of mitochondrial-mediated apoptosis. Because the mitochondrial genome is densely packed and close to the main generator of reactive oxygen species (ROS) in the cell, the electron transport chain (ETC), an important role for mtDNA mutations in aging has been proposed. Point mutations and deletions in mtDNA accumulate with age in a wide variety of tissues in mammals, including humans, and often coincide with significant tissue dysfunction. Here, we examine the evidence supporting a causative role for mtDNA mutations in aging and sarcopenia. We review experimental outcomes showing that mtDNA mutations, leading to mitochondrial dysfunction and possibly apoptosis, are causal to the process of sarcopenia. Moreover, we critically discuss and dispute an important part of the mitochondrial 'vicious cycle' theory of aging which proposes that accumulation of mtDNA mutations may lead to an enhanced mitochondrial ROS production and ever increasing oxidative stress which ultimately leads to tissue deterioration and aging. Potential mechanism(s) by which mtDNA mutations may mediate their pathological consequences in skeletal muscle are also discussed.

Highly resistant macromolecular components and low rate of generation of endogenous damage: two key traits of longevity

Key characteristics relating oxidative damage to aging and longevity are reviewed. Available information indicates that the specific composition of tissue macromolecules (proteins, lipids and mitochondrial DNA) in long-lived animal species gives them an intrinsically high resistance to modification that likely contributes to the superior longevity of these species. This is obtained in the case of lipids by decreasing fatty acid unsaturation, and in the proteins by lowering their methionine content. Long-lived animals also show low rates of reactive oxygen species (ROS) generation and oxidative damage at their mitochondria. On the other hand, dietary restriction decreases mitochondrial ROS production and oxidative damage to mitochondrial DNA and proteins. These changes are due to the decreased intake of dietary proteins (not of lipids or carbohydrates) of the dietary restricted animals. In turn, these effects of protein restriction seem to be specifically due to the lowered methionine intake of the protein and dietary restricted animals. It is emphasized that both a low rate of generation of endogenous damage and an intrinsically high resistance to modification of tissue macromolecules are key traits of animal longevity.

Numerical changes in the mitochondrial DNA displacement loop in lung lesions induced by N-nitrosobis(2-hydroxypropyl)amine in rats

Mutations in the mitochondrial DNA (mtDNA) displacement loop (D-loop) region were investigated to clarify the possible molecular mechanisms underlying the development of lung tumors induced by N-nitrosobis(2-hydroxypropyl)amine (BHP) in rats. Male Wistar rats, 6 weeks of age, were given 2000 ppm BHP in their drinking water for 12 weeks and then maintained without further treatment until sacrifice at 25 weeks. Genomic DNA was extracted from paraffin-embedded tissues and polymerase chain reaction (PCR)-single strand conformation polymorphism (SSCP) analysis, followed by nucleotide sequencing, was performed. Eleven out of 24 hyperplasias (45.6%), 8 out of 16 adenomas (50.0%), and 14 out of 21 adenocarcinomas (66.7%) showed numerical changes, in a polymeric C-tract at positions 16,086-16,092 of the mtDNA D-loop, with a one base insertion of cytosine increasing the length of the C-tract, from the seven nucleotides observed in normal lung tissues from non-BHP treated rats, to eight. These changes were all homoplasmic and no changes were found in lung lesions when the length of the C-tract in the normal lung tissues adjacent to the lesions was seven. These results suggest that alterations in the mtDNA D-loop may occur in an early phase of lung carcinogenesis induced in rats by BHP.

The role of mitochondrial DNA mutations in mammalian aging

Mitochondrial DNA (mtDNA) accumulates both base-substitution mutations and deletions with aging in several tissues in mammals. Here, we examine the evidence supporting a causative role for mtDNA mutations in mammalian aging. We describe and compare human diseases and mouse models associated with mitochondrial genome instability. We also discuss potential mechanisms for the generation of these mutations and the means by which they may mediate their pathological consequences. Strategies for slowing the accumulation and attenuating the effects of mtDNA mutations are discussed.

Mitochondrial DNA Mutations and Aging

Mitochondria have been hypothesized to play a role in both aging and neurodegenerative diseases, such as Parkinson's disease (PD) and Alzheimer's disease. Many studies have shown the accumulation of mitochondrial DNA (mtDNA) mutations in post-mitotic tissues and more recent data have shown this also to be a feature of aging mitotic tissues. Much of this data has been correlative, until recently with the development of polymerase gamma deficient mice which accumulate high levels of mtDNA mutations and show a premature aging phenotype, that a more causative role has been proposed. This article focuses on recent developments in aging research into the role that mtDNA mutations play in the aging process.

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.

Mitochondrial DNA deletions inhibit proteasomal activity and stimulate an autophagic transcript

Deletions within the mitochondrial DNA (mtDNA) cause Kearns Sayre syndrome (KSS) and chronic progressive external opthalmoplegia (CPEO). The clinical signs of KSS include muscle weakness, heart block, pigmentary retinopathy, ataxia, deafness, short stature, and dementia. The identical deletions occur and rise exponentially as humans age, particularly in substantia nigra. Deletions at >30% concentration cause deficits in basic bioenergetic parameters, including membrane potential and ATP synthesis, but it is poorly understood how these alterations cause the pathologies observed in patients. To better understand the consequences of mtDNA deletions, we microarrayed six cell types containing mtDNA deletions from KSS and CPEO patients. There was a prominent inhibition of transcripts encoding ubiquitin-mediated proteasome activity, and a prominent induction of transcripts involved in the AMP kinase pathway, macroautophagy, and amino acid degradation. In mutant cells, we confirmed a decrease in proteasome biochemical activity, significantly lower concentration of several amino acids, and induction of an autophagic transcript. An interpretation consistent with the data is that mtDNA deletions increase protein damage, inhibit the ubiquitin-proteasome system, decrease amino acid salvage, and activate autophagy. This provides a novel pathophysiological mechanism for these diseases, and suggests potential therapeutic strategies.

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 mutations and apoptosis in mammalian aging

Mutations in mitochondrial DNA (mtDNA) accumulate during aging, but their significance to longevity and age-associated disease has been uncertain. Recently, in support of the hypothesis that mtDNA integrity is important, we have shown that age-associated diseases arise more rapidly in mice where mtDNA mutations and increased levels of apoptosis occur at higher rates than normal due to expression of an error-prone mtDNA polymerase. Further studies in this model may provide deeper insights into the relationship between mitochondria, aging, and susceptibility to age-associated diseases, such as cancer.

Mitochondrial DNA and ageing

The accumulation of mitochondrial DNA mutations has been proposed as a potential mechanism in the physiological processes of ageing and age-related disease. Although mitochondria have long been anticipated as a perpetrator of ageing, there was little experimental evidence to link these changes directly with the cellular pathology of ageing. Recently, considerable progress in understanding basic mitochondrial genetics and in identifying acquired mtDNA mutations in ageing has been made. Furthermore, the creation of mtDNA-mutator mice has provided the first direct evidence that accelerating the mtDNA mutation rate can result in premature ageing, consistent with the view that loss of mitochondrial function is a major causal factor in ageing. This review will, therefore, focus on recent developments in ageing research related to the role played by mtDNA.

Is the mitochondrial free radical theory of aging intact?

The present state of the mitochondrial free radical theory of aging is reviewed. Available studies do not support the hypothesis that antioxidants control the rate of aging because: (a) they correlate inversely with maximum longevity in vertebrates, and (b) increasing their concentration by different methods does not increase maximum lifespan. On the other hand, comparative studies consistently show that long-lived mammals and birds have low rates of mitochondrial reactive oxygen species (ROS) production and low levels of oxidative damage in their mitochondrial DNA. Furthermore, caloric restriction, which extends longevity, also decreases mitochondrial ROS production at complex I and lowers mtDNA oxidative damage. Recent data show that these changes can also be obtained with protein restriction without strong caloric restriction. Another trait of long-lived mammals and birds is the possession of low degrees of unsaturation in their cellular membranes. This is mainly due to minimizing the presence of highly unsaturated fatty acids such as 22:6n-3 and emphasizing the presence of less unsaturated fatty acids such as 18:2n-6 in long-lived animals, without changing the total amount of polyunsaturated fatty acids. This leads to lower levels of lipid peroxidation and lipoxidation-derived protein modification in long-lived species. Taken together, available information is consistent with the predictions of the mitochondrial free radical theory of aging, although definitive proof and many mechanistic details are still lacking.

Mitochondrial oxidative stress, aging and caloric restriction: the protein and methionine connection

Caloric restriction (CR) decreases aging rate and mitochondrial ROS (MitROS) production and oxidative stress in rat postmitotic tissues. Low levels of these parameters are also typical traits of long-lived mammals and birds. However, it is not known what dietary components are responsible for these changes during CR. It was recently observed that 40% protein restriction without strong CR also decreases MitROS generation and oxidative stress. This is interesting because protein restriction also increases maximum longevity (although to a lower extent than CR) and is a much more practicable intervention for humans than CR. Moreover, it was recently found that 80% methionine restriction substituting it for l-glutamate in the diet also decreases MitROS generation in rat liver. Thus, methionine restriction seems to be responsible for the decrease in ROS production observed in caloric restriction. This is interesting because it is known that exactly that procedure of methionine restriction also increases maximum longevity. Moreover, recent data show that methionine levels in tissue proteins negatively correlate with maximum longevity in mammals and birds. All these suggest that lowering of methionine levels is involved in the control of mitochondrial oxidative stress and vertebrate longevity by at least two different mechanisms: decreasing the sensitivity of proteins to oxidative damage, and lowering of the rate of ROS generation at mitochondria.

Mitochondrial DNA Mutations, Apoptosis, and the Misfolded Protein Response

Studies of transgenic mice with accelerated accumulation of mtDNA mutations specifically in the heart lead us to propose that apoptotic signaling and cell death is central to the pathogenesis of mtDNA mutations in aging. It is the cellular response to that apoptotic signaling and the organ?s compensatory response to a loss of cells that specify the phenotype of an accumulation of mtDNA mutations. In the heart, cardiomyocytes induce a vigorous anti-apoptotic, pro-survival response to counteract mitochondrial apoptotic signaling. The heart up-regulates contractility of remaining myocytes in order to maintain cardiac output. We hypothesize that mutant mitochondrial proteins originate apoptotic signaling by interacting with proteins already in place in the mitochondrial outer membrane that regulate apoptosis, for example the pro-apoptotic protein Bak. Since it is unlikely that all mutant mitochondrial proteins have the necessary structure and localization within the inner membrane to activate Bak appropriately, only a small fraction of an age-associated burden of mtDNA mutations may be pathogenic. In this model, reactive oxygen species generated by mitochondrial respiration drive the formation of mtDNA mutations, but are not the primary mechanism for their pathogenicity.

Detection of deletions flanked by short direct repeats in mitochondrial DNA of aging Drosophila

Cumulative damage due to reactive oxygen species (ROS) in mitochondria, especially in mitochondrial DNA (mtDNA), would result in a decrease in mitochondrial respiratory function and contributes to the age-related decline in the physiological functioning of organisms. Previously, we reported the tissue-specific accumulation of deleted mtDNA with age in Drosophila melanogaster. In the present study, to understand the mechanism by which mtDNA deletion is generated with age, nucleotide sequences of deleted mtDNA were determined. Consequently, 33 different sequences each containing a deletion were obtained from flies that were more than 55-day-old. Most of the deletions were found to be flanked by short direct repeats. The present results, together with those from other animals, suggest that there is a common mechanism generating mtDNA deletions through direct repeats.

Specific point mutations may not accumulate with aging in the mouse mitochondrial DNA control region

Increasing evidence suggests that mitochondrial function declines during aging in various tissues and in a wide range of organisms. This correlates with an age-dependent large accumulation of specific point mutations in the mtDNA control region that was reported recently in human fibroblast and skeletal muscle. However, evaluations of aging-related mtDNA mutations in other model animal systems. In this study, we analyzed mtDNA control regions of brain, skeletal muscle, heart, and other tissues from aged mice, in search of specific point mutations. A 948-bp fragment covering the entire mtDNA control region from various tissues of mice at the age of 25-26 months was sequenced. The sequence analysis was accomplished with a newly developed program Mutation Quantifier, which was able to accurately detect mutations with frequencies as low as 3%. Probably due to the relative shorter life-span, unlike what has been reported in human mtDNA, our results indicated there might be no significant accumulation of specific mutations in mouse mtDNA control region during aging.

The role of oxidative stress in carcinogenesis

Chemical carcinogenesis follows a multistep process involving both mutation and increased cell proliferation. Oxidative stress can occur through overproduction of reactive oxygen and nitrogen species through either endogenous or exogenous insults. Important to carcinogenesis, the unregulated or prolonged production of cellular oxidants has been linked to mutation (induced by oxidant-induced DNA damage), as well as modification of gene expression. In particular, signal transduction pathways, including AP-1 and NFkappaB, are known to be activated by reactive oxygen species, and they lead to the transcription of genes involved in cell growth regulatory pathways. This review examines the evidence of cellular oxidants' involvement in the carcinogenesis process, and focuses on the mechanisms for production, cellular damage produced, and the role of signaling cascades by reactive oxygen species.

Accumulation of deleted mitochondrial DNA in aging Drosophila melanogaster

Cumulative damage in mitochondria by reactive oxygen species is thought to result in a decrease in mitochondrial respiratory function and to contribute to the age-related decline in the physiological function of organisms. The mitochondrial genome is also subjected to damage with age through deletions. The accumulation of deleted mitochondrial DNA (mtDNA) has been observed in various animals, but still remains unclear in insects. We examined the accumulation of deleted mtDNA in D. melanogaster at various ages from larvae to 65-day-old adults. When DNA extracted from whole bodies was examined by PCR and Southern hybridization, the age-related accumulation of deletions was not clear. However, when the accumulation of deleted mtDNA with age was examined separately in three parts of the body (head, thorax and abdomen), deleted mtDNA signals were detected more frequently in the thorax and the accumulation was age-dependent. Three of the deleted mtDNA were cloned, and the breakpoints of the deletions were identified. These results strongly suggest that deleted mtDNA accumulates in Drosophila with age in a tissue-specific manner.

Is DNA cut out for a long life?

Much attention has been focused on the DNA repair hypothesis of aging. Studies in mammals that seek to test the validity of this model are complicated by both the functional redundancy and the essential nature of genes involved in the repair process. Compared to mammals, the study of DNA repair and aging in yeast has considerably fewer complicating factors. In this Perspective, I discuss results presented in this month's issue of Aging Cell that address whether the types of DNA damage repaired by the base excision repair pathway cause aging in yeast.

Mitochondrial mutagenesis in the brain in forensic and pathological research

Accumulation of alterations to the mitochondrial DNA (mtDNA) would be expected to significantly impair the bioenergetic function of mitochondria in the affected host cells. Many of these changes have been associated with several specific diseases and the process of aging. These mutations may be the result of mitochondrial oxidative stress, which is increased with age of individuals and specific degenerative diseases. Our aim with this review is to summarize the recent literature on the occurrence of mtDNA alterations and its possible relation to age-depending degenerative processes with special regards to the brain. Additionally, we show how these alterations could be used in fields of pathology and forensic medicine.

Mitochondrial DNA alterations in cancer

A number of studies have demonstrated the presence of mitochondrial DNA (mtDNA) mutations in cancer cells. In this article, we review mitochondrial genomic aberrations reported in solid tumors of the breast, colon, stomach, liver, kidney, bladder, head/neck, and lung. The tantalizing association of tumors with mtDNA mutations implicates these mutations in the process of carcinogenesis. Alterations in expression of mtDNA transcripts in a variety of cancer types are also reviewed. In solid tumors, elevated expression of mtDNA-genes coding for subunits of the mitochondrial electron respiratory chain may reflect mitochondrial adaptation to perturbations in cellular energy requirements. The role of mtDNA mutations and altered expression of mitochondrial genes in carcinogenesis is discussed. Mitochondrial DNA mutations can initiate a cascade of events leading to a continuous increase in the production of reactive oxygen species (persistent oxidative stress), a condition that probably favors tumor development.

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.

Membrane fatty acid unsaturation, protection against oxidative stress, and maximum life span: a homeoviscous-longevity adaptation?

Aging is a progressive and universal process originating endogenously that manifests during postmaturational life. Available comparative evidence supporting the mitochondrial free radical theory of aging consistently indicates that two basic molecular traits are associated with the rate of aging and thus with the maximum life span: the presence of low rates of mitochondrial oxygen radical production and low degrees of fatty acid unsaturation of cellular membranes in postmitotic tissues of long-lived homeothermic vertebrates in relation to those of short-lived ones. Recent research shows that steady-state levels of free radical-derived damage to mitochondrial DNA (mtDNA) and, in some cases, to proteins are lower in long- than in short-lived animals. Thus, nonenzymatic oxidative modification of tissue macromolecules is related to the rate of aging. The low degree of fatty acid unsaturation in biomembranes of long-lived animals may confer advantage by decreasing their sensitivity to lipid peroxidation. Furthermore, this may prevent lipoxidation-derived damage to other macromolecules. Taking into account the fatty acid distribution pattern, the origin of the low degree of membrane unsaturation in long-lived species seems to be the presence of species-specific desaturation pathways that determine membrane composition while an appropriate environment for membrane function is maintained. Mechanisms that prevent or decrease the generation of endogenous damage during the evolution of long-lived animals seem to be more important than trying to intercept those damaging agents or repairing the damage already inflicted. Here, the physiological meaning of these findings and the effects of experimental manipulations such as dietary stress, caloric restriction, and endocrine control in relation to aging and longevity are discussed.

Age-related changes in the mitochondrial depolarization induced by oxidative injury in human peripheral blood leukocytes

Aging is associated with impaired immunity and reduced host defenses. Mitochondrial bioenergetic dysfunctions and reduced antioxidative ability of immunocompetent cells may contribute to this phenomenon. In this study, 60 healthy volunteers of different age groups donated their blood after overnight fasting. Leukocytes were subjected to oxidative injuries by exposure to t-butylhydroperoxide, and were labeled with fluorochromes for measuring mitochondria transmembrane potential (delta psi m), membrane peroxidation and mitochondrial oxidant formation. delta psi m declined after t-butylhydroperoxide exposure, and the change was more prominent in leukocytes from older individuals. Cyclosporin A partly restored delta psi m, implying the contributing role of mitochondrial permeability transition pores. The mitochondrial depolarization was accompanied by increased oxidant formation and oxidation of pyridine nucleotides, which were more prominent in older subjects. The results support the view that the bioenergetic functions of mitochondria are more susceptible to oxidative injury in aged individuals. The decreased ability of leukocytes to resist oxidative stress may contribute to immunosenescence in humans.