Mitochondrial DNA remains intact during Drosophila aging, but the levels of mitochondrial transcripts are significantly reduced

Age-related flexibility of energetic metabolism in the honey bee Apis mellifera

The mechanisms that underpin aging are still elusive. In this study, we suggest that the ability of mitochondria to oxidize different substrates, which is known as metabolic flexibility, is involved in this process. To verify our hypothesis, we used honey bees (Apis mellifera carnica) at different ages, to assess mitochondrial oxygen consumption and enzymatic activities of key enzymes of the energetic metabolism as well as ATP5A1 content (subunit of ATP synthase) and adenylic energy charge (AEC). We also measured mRNA abundance of genes involved in mitochondrial functions and the antioxidant system. Our results demonstrated that mitochondrial respiration increased with age and favored respiration through complexes I and II of the electron transport system (ETS) while glycerol-3-phosphate (G3P) oxidation was relatively decreased. In addition, glycolytic, tricarboxylic acid cycle and ETS enzymatic activities increased, which was associated with higher ATP5A1 content and AEC. Furthermore, we detected an early decrease in the mRNA abundance of subunits of NADH ubiquinone oxidoreductase subunit B2 (NDUFB2, complex I), mitochondrial cytochrome b (CYTB, complex III) of the ETS as well as superoxide dismutase 1 and a later decrease for vitellogenin, catalase and mitochondrial cytochrome c oxidase subunit 1 (COX1, complex IV). Thus, our study suggests that the energetic metabolism is optimized with aging in honey bees, mainly through quantitative and qualitative mitochondrial changes, rather than showing signs of senescence. Moreover, aging modulated metabolic flexibility, which might reflect an underpinning mechanism that explains lifespan disparities between the different castes of worker bees.

Estrogen Replacement Therapy Induces Antioxidant and Longevity-Related Genes in Women after Medically Induced Menopause

Females live longer than males in many species, including humans, and estrogens are in part responsible for this protection against aging. We reported previously that estrogens can protect rats against oxidative stress, by inducing antioxidant and longevity-related genes. Thus, this study was aimed at confirming the ability of estrogens to upregulate antioxidant and longevity-related genes in humans. For this purpose, we selected 16 women of reproductive age (18-42 years old) undergoing a fertility treatment that includes a medically induced menopause, at the Valencian Infertility Institute. We took blood samples at each time point of the treatment (basal, induced menopause, estrogen, and estrogen plus progesterone replacement therapy). mRNA expression of antioxidant and longevity-related genes in peripheral blood mononuclear cells (PBMC) was determined by real-time reverse transcriptase-polymerase chain reaction (RT-PCR). Determination of reduced glutathione (GSH) in total blood was carried out using high-performance liquid chromatography (HPLC). As expected, we found that medically induced menopause significantly decreased sexual hormone (estrogens and progesterone) levels. It also lowered glutathione peroxidase (GPx), 16S rRNA, P21, and TERF2 mRNA expression and blood GSH levels. Estrogen replacement therapy significantly restored estrogen levels and induced mRNA expression of manganese superoxide dismutase (MnSOD), GPx, 16S rRNA, P53, P21, and TERF2 and restored blood GSH levels. Progesterone replacement therapy induced a significant increase in MnSOD, P53, sestrin 2 (SENS2), and TERF2 mRNA expression when compared to basal conditions. These findings provide evidence for estrogen beneficial effects in upregulating antioxidant and longevity-related genes in women.

Moderate Red Wine Consumption Increases the Expression of Longevity-Associated Genes in Controlled Human Populations and Extends Lifespan in Drosophila melanogaster

The beneficial effects of moderate red wine consumption on cardiovascular health are well known. The composition of red wine includes several compounds, such as the phytoestrogen resveratrol, that exert these beneficial effects, although not all the mechanisms by which they act are known. Our aim was to study the effect of red wine consumption on longevity-related genes in controlled human populations, such as cloistered nuns. We found that the expression of catalase, manganese-superoxide dismutase, Sirt1, and p53 was increased in peripheral blood mononuclear cells after 14 days of moderate red wine consumption. This increase was accompanied by an enhanced metabolic wellness: fatty acids, cholesterol, branched chain amino acids (isoleucine and leucine), ketone bodies (acetoacetate), bacterial co-metabolites (trimethylamine), and cellular antioxidants (taurine) contributed to a change in metabolic profile after moderate red wine consumption by the nuns. No serious unwanted side effects were observed. Finally, we tested the effect of moderate red wine consumption on longevity in a controlled animal population, such as D. melanogaster, and found that it increased average life span by 7%. In conclusion, moderate red wine consumption increases the expression of key longevity-related genes and improves metabolic health in humans and increases longevity in flies.

Minimal effects of spargel (PGC-1) overexpression in a Drosophila mitochondrial disease model

PGC-1α and its homologues have been proposed to act as master regulators of mitochondrial biogenesis in animals. Most relevant studies have been conducted in mammals, where interpretation is complicated by the fact that there are three partially redundant members of the gene family. In Drosophila, only a single PGC-1 homologue, spargel (srl), is present in the genome. Here, we analyzed the effects of srl overexpression on phenotype and on gene expression in tko^25t, a recessive bang-sensitive mutant with a global defect in oxidative phosphorylation, resulting from a deficiency of mitochondrial protein synthesis. In contrast to previous reports, we found that substantial overexpression of srl throughout development had only minimal effects on the tko^25t mutant phenotype. Copy number of mtDNA was unaltered and srl overexpression produced no systematic effects on a representative set of transcripts related to mitochondrial OXPHOS and other metabolic enzymes, although these were influenced by sex and genetic background. This study provides no support to the concept of Spargel as a global regulator of mitochondrial biogenesis, at least in the context of the tko^25t model.

Summary: Overexpression of spargel, the fly PGC-1 homologue proposed as a mitochondrial biogenesis regulator, has minimal effects on the phenotype of tko^25t, considered a fly model for mitochondrial disease.

Humanin, a Mitochondrial-Derived Peptide Released by Astrocytes, Prevents Synapse Loss in Hippocampal Neurons

Astroglial cells are crucial for central nervous system (CNS) homeostasis. They undergo complex morpho-functional changes during aging and in response to hormonal milieu. Ovarian hormones positively affect different astroglia parameters, including regulation of cell morphology and release of neurotrophic and neuroprotective factors. Thus, ovarian hormone loss during menopause has profound impact in astroglial pathophysilogy and has been widely associated to the process of brain aging. Humanin (HN) is a secreted mitochondrial-encoded peptide with neuroprotective effects. It is localized in several tissues with high metabolic rate and its expression decreases with age. In the brain, humanin has been found in glial cells in physiological conditions. We previously reported that surgical menopause induces hippocampal mitochondrial dysfunction that mimics an aging phenotype. However, the effect of ovarian hormone deprivation on humanin expression in this area has not been studied. Also, whether astrocytes express and release humanin and the regulation of such processes by ovarian hormones remain elusive. Although humanin has also proven to be beneficial in ameliorating cognitive impairment induced by different insults, its putative actions on structural synaptic plasticity have not been fully addressed. In a model of surgical menopause in rats, we studied hippocampal humanin expression and localization by real-time quantitative polymerase chain reaction (RT-qPCR) and double immunohistochemistry, respectively. Humanin production and release and ovarian hormone regulation of such processes were studied in cultured astrocytes by flow cytometry and ELISA, respectively. Humanin effects on glutamate-induced structural synaptic alterations were determined in primary cultures of hippocampal neurons by immunocytochemistry. Humanin expression was lower in the hippocampus of ovariectomized rats and its immunoreactivity colocalized with astroglial markers. Chronic ovariectomy also promoted the presence of less complex astrocytes in this area. Ovarian hormones increased humanin intracellular content and release by cultured astrocytes. Humanin prevented glutamate-induced dendritic atrophy and reduction in puncta number and total puncta area for pre-synaptic marker synaptophysin in cultured hippocampal neurons. In conclusion, astroglial functional and morphological alterations induced by chronic ovariectomy resemble an aging phenotype and could affect astroglial support to neuronal function by altering synaptic connectivity and functionality. Reduced astroglial-derived humanin may represent an underlying mechanism for synaptic dysfunction and cognitive decline after menopause.

Aging in the Drosophila ovary: contrasting changes in the expression of the piRNA machinery and mitochondria but no global release of transposable elements

Background

Evolutionary theory indicates that the dynamics of aging in the soma and reproductive tissues may be distinct. This difference arises from the fact that only the germline lineage establishes future generations. In the soma, changes in the landscape of heterochromatin have been proposed to have an important role in aging. This is because redistribution of heterochromatin during aging has been linked to the derepression of transposable elements and an overall loss of somatic gene regulation. A role for changes in the chromatin landscape in the aging of reproductive tissues is less well established. Whether or not epigenetic factors, such as heterochromatin marks, are perturbed in aging reproductive tissues is of interest because, in special cases, epigenetic variation may be heritable. Using mRNA sequencing data from late-stage egg chambers in Drosophila melanogaster, we characterized the landscape of altered gene and transposable element expression in aged reproductive tissues. This allowed us to test the hypothesis that reproductive tissues may differ from somatic tissues in their response to aging.

Results

We show that age-related expression changes in late-stage egg chambers tend to occur in genes residing in heterochromatin, particularly on the largely heterochromatic 4th chromosome. However, these expression differences are seen as both decreases and increases during aging, inconsistent with a general loss of heterochromatic silencing. We also identify an increase in expression of the piRNA machinery, suggesting an age-related increased investment in the maintenance of genome stability. We further identify a strong age-related reduction in the expression of mitochondrial transcripts. However, we find no evidence for global TE derepression in reproductive tissues. Rather, the observed effects of aging on TEs are primarily strain and family specific.

Conclusions

These results identify unique responses in somatic versus reproductive tissue with regards to aging. As in somatic tissues, female reproductive tissues show reduced expression of mitochondrial genes. In contrast, the piRNA machinery shows increased expression during aging. Overall, these results also indicate that global loss of TE control observed in other studies may be unique to the soma and sensitive to genetic background and TE family.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5668-3) contains supplementary material, which is available to authorized users.

The Interplay Between Respiratory Supercomplexes and ROS in Aging

SIGNIFICANCE

The molecular mechanism of aging is still vigorously debated, although a general consensus exists that mitochondria are significantly involved in this process. However, the previously postulated role of mitochondrial-derived reactive oxygen species (ROS) as the damaging agents inducing functional loss in aging has fallen out of favor in the recent past. In this review, we critically examine the role of ROS in aging in the light of recent advances on the relationship between mitochondrial structure and function.

RECENT ADVANCES

The functional mitochondrial respiratory chain is now recognized as a reflection of the dynamic association of respiratory complexes in the form of supercomplexes (SCs). Besides providing kinetic advantage (channeling), SCs control ROS generation by the respiratory chain, thus providing a means to regulate ROS levels in the cell. Depending on their concentration, these ROS are either physiological signals essential for the life of the cell or toxic species that damage cell structure and functions.

CRITICAL ISSUES

We propose that under physiological conditions the dynamic nature of SCs reversibly controls the generation of ROS as signals involved in mitochondrial-nuclear communication. During aging, there is a progressive loss of control of ROS generation so that their production is irreversibly enhanced, inducing a vicious circle in which signaling is altered and structural damage takes place.

FUTURE DIRECTIONS

A better understanding on the forces affecting SC association would allow the manipulation of ROS generation, directing these species to their physiological signaling role.

Selective propagation of functional mitochondrial DNA during oogenesis restricts the transmission of a deleterious mitochondrial variant

Although mitochondrial DNA (mtDNA) is prone to mutation and few mtDNA repair mechanisms exist, crippling mitochondrial mutations are exceedingly rare. Recent studies have demonstrated strong purifying selection in the mouse female germline. However, the mechanisms underlying positive selection of healthy mitochondria remain to be elucidated. We visualized mtDNA replication during Drosophila melanogaster oogenesis, finding that mtDNA replication commenced before oocyte determination during the late germarium stage and was dependent on mitochondrial fitness. We isolated a temperature-sensitive lethal mtDNA allele, mt:CoI(T300I), which resulted in reduced mtDNA replication in the germarium at the restrictive temperature. Additionally, the frequency of the mt:CoI(T300I) allele in heteroplasmic flies was decreased, both during oogenesis and over multiple generations, at the restrictive temperature. Furthermore, we determined that selection against mt:CoI(T300I) overlaps with the timing of selective replication of mtDNA in the germarium. These findings establish a previously uncharacterized developmental mechanism for the selective amplification of wild-type mtDNA, which may be evolutionarily conserved to limit the transmission of deleterious mutations.

Oxidative stress is not a major contributor to somatic mitochondrial DNA mutations

The accumulation of somatic mitochondrial DNA (mtDNA) mutations is implicated in aging and common diseases of the elderly, including cancer and neurodegenerative disease. However, the mechanisms that influence the frequency of somatic mtDNA mutations are poorly understood. To develop a simple invertebrate model system to address this matter, we used the Random Mutation Capture (RMC) assay to characterize the age-dependent frequency and distribution of mtDNA mutations in the fruit fly Drosophila melanogaster. Because oxidative stress is a major suspect in the age-dependent accumulation of somatic mtDNA mutations, we also used the RMC assay to explore the influence of oxidative stress on the somatic mtDNA mutation frequency. We found that many of the features associated with mtDNA mutations in vertebrates are conserved in Drosophila, including a comparable somatic mtDNA mutation frequency (∼10(-5)), an increased frequency of mtDNA mutations with age, and a prevalence of transition mutations. Only a small fraction of the mtDNA mutations detected in young or old animals were G∶C to T∶A transversions, a signature of oxidative damage, and loss-of-function mutations in the mitochondrial superoxide dismutase, Sod2, had no detectable influence on the somatic mtDNA mutation frequency. Moreover, a loss-of-function mutation in Ogg1, which encodes a DNA repair enzyme that removes oxidatively damaged deoxyguanosine residues (8-hydroxy-2'-deoxyguanosine), did not significantly influence the somatic mtDNA mutation frequency of Sod2 mutants. Together, these findings indicate that oxidative stress is not a major cause of somatic mtDNA mutations. Our data instead suggests that somatic mtDNA mutations arise primarily from errors that occur during mtDNA replication. Further studies using Drosophila should aid in the identification of factors that influence the frequency of somatic mtDNA mutations.

Muscle-specific overexpression of the catalytic subunit of DNA polymerase γ induces pupal lethality in Drosophila melanogaster

We show the physiological effects and molecular characterization of overexpression of the catalytic core of mitochondrial DNA (mtDNA) polymerase (pol γ-α) in muscle of Drosophila melanogaster. Muscle-specific overexpression of pol γ-α using the UAS/GAL4 (where UAS is upstream activation sequence) system produced more than 90% of lethality at the end of pupal stage at 25°C, and the survivor adult flies showed a significant reduction in life span. The survivor flies displayed a decreased mtDNA level that is accompanied by a corresponding decrease in the levels of the nucleoid-binding protein mitochondrial transcription factor A (mtTFA). Furthermore, an increase in apoptosis is detected in larvae and adults overexpressing pol γ-α. We suggest that the pupal lethality and reduced life span of survivor adult flies are both caused mainly by massive apoptosis of muscle cells induced by mtDNA depletion.

Chronic caloric restriction preserves mitochondrial function in senescence without increasing mitochondrial biogenesis

Caloric restriction (CR) mitigates many detrimental effects of aging and prolongs life span. CR has been suggested to increase mitochondrial biogenesis, thereby attenuating age-related declines in mitochondrial function, a concept that is challenged by recent studies. Here we show that lifelong CR in mice prevents age-related loss of mitochondrial oxidative capacity and efficiency, measured in isolated mitochondria and permeabilized muscle fibers. We find that these beneficial effects of CR occur without increasing mitochondrial abundance. Whole-genome expression profiling and large-scale proteomic surveys revealed expression patterns inconsistent with increased mitochondrial biogenesis, which is further supported by lower mitochondrial protein synthesis with CR. We find that CR decreases oxidant emission, increases antioxidant scavenging, and minimizes oxidative damage to DNA and protein. These results demonstrate that CR preserves mitochondrial function by protecting the integrity and function of existing cellular components rather than by increasing mitochondrial biogenesis.

Sex and life expectancy

BACKGROUND

A sexual dimorphism in human life expectancy has existed in almost every country for as long as records have been kept. Although human life expectancy has increased each year, females still live longer, on average, than males. Undoubtedly, the reasons for the sex gap in life expectancy are multifaceted, and it has been discussed from both sociological and biological perspectives. However, even if biological factors make up only a small percentage of the determinants of the sex difference in this phenomenon, parity in average life expectancy should not be anticipated.

OBJECTIVE

The aim of this review is to highlight biological mechanisms that may underlie the sexual dimorphism in life expectancy.

METHODS

Using PubMed, ISI Web of Knowledge, and Google Scholar, as well as cited and citing reference histories of articles through August 2012, English-language articles were identified, read, and synthesized into categories that could account for biological sex differences in human life expectancy.

RESULTS

The examination of biological mechanisms accounting for the female-based advantage in human life expectancy has been an active area of inquiry; however, it is still difficult to prove the relative importance of any 1 factor. Nonetheless, biological differences between the sexes do exist and include differences in genetic and physiological factors such as progressive skewing of X chromosome inactivation, telomere attrition, mitochondrial inheritance, hormonal and cellular responses to stress, immune function, and metabolic substrate handling among others. These factors may account for at least a part of the female advantage in human life expectancy.

CONCLUSIONS

Despite noted gaps in sex equality, higher body fat percentages and lower physical activity levels globally at all ages, a sex-based gap in life expectancy exists in nearly every country for which data exist. There are several biological mechanisms that may contribute to explaining why females live longer than men on average, but the complexity of the human life experience makes research examining the contribution of any single factor for the female advantage difficult. However, this information may still prove important to the development of strategies for healthy aging in both sexes.

Whole transcriptome analysis of a reversible neurodegenerative process in Drosophila reveals potential neuroprotective genes

Background

Neurodegenerative diseases are progressive and irreversible and they can be initiated by mutations in specific genes. Spalt-like genes (Sall) encode transcription factors expressed in the central nervous system. In humans, SALL mutations are associated with hereditary syndromes characterized by mental retardation, sensorineural deafness and motoneuron problems, among others. Drosophila sall mutants exhibit severe neurodegeneration of the central nervous system at embryonic stage 16, which surprisingly reverts later in development at embryonic stage 17, suggesting a potential to recover from neurodegeneration. We hypothesize that this recovery is mediated by a reorganization of the transcriptome counteracting SALL lost. To identify genes associated to neurodegeneration and neuroprotection, we used mRNA-Seq to compare the transcriptome of Drosophila sall mutant and wild type embryos from neurodegeneration and reversal stages.

Results

Neurodegeneration stage is associated with transcriptional changes in 220 genes, of which only 5% were already described as relevant for neurodegeneration. Genes related to the groups of Redox, Lifespan/Aging and Mitochondrial diseases are significantly represented at this stage. By contrast, neurodegeneration reversal stage is associated with significant changes in 480 genes, including 424 not previously associated with neuroprotection. Immune response and Salt stress are the most represented groups at this stage.

Conclusions

We identify new genes associated to neurodegeneration and neuroprotection by using an mRNA-Seq approach. The strong homology between Drosophila and human genes raises the possibility to unveil novel genes involved in neurodegeneration and neuroprotection also in humans.

The role of estrogen deficiency in skin ageing and wound healing

The links between hormonal signalling and lifespan have been well documented in a range of model organisms. For example, in C. elegans or D. melanogaster, lifespan can be modulated by ablating germline cells, or manipulating reproductive history or pregnenolone signalling. In mammalian systems, however, hormonal contribution to longevity is less well understood. With increasing age human steroid hormone profiles change substantially, particularly following menopause in women. This article reviews recent links between steroid sex hormones and ageing, with special emphasis on the skin and wound repair. Estrogen, which substantially decreases with advancing age in both males and females, protects against multiple aspects of cellular ageing in rodent models, including oxidative damage, telomere shortening and cellular senescence. Estrogen's effects are particularly pronounced in the skin where cutaneous changes post-menopause are well documented, and can be partially reversed by classical Hormone Replacement Therapy (HRT). Our research shows that while chronological ageing has clear effects on skin wound healing, falling estrogen levels are the principle mediator of these effects. Thus, both HRT and topical estrogen replacement substantially accelerate healing in elderly humans, but are associated with unwanted deleterious effects, particularly cancer promotion. In fact, much current research effort is being invested in exploring the therapeutic potential of estrogen signalling manipulation to reverse age-associated pathology in peripheral tissues. In the case of the skin the differential targeting of estrogen receptors to promote healing in aged subjects is a real therapeutic possibility.

Heat shock proteins and Drosophila aging

Since their discovery in Drosophila, the heat shock proteins (Hsps) have been shown to regulate both stress resistance and life-span. Aging is characterized by increased oxidative stress and the accumulation of abnormal (malfolded) proteins, and these stresses induce Hsp gene expression through the transcription factor HSF. In addition, a subset of Hsps is induced by oxidative stress through the JNK signaling pathway and the transcription factor Foxo. The Hsps counteract the toxicity of abnormal proteins by facilitating protein refolding and turnover, and through other mechanisms including inhibition of apoptosis. The Hsps are up-regulated in tissue-specific patterns during aging, and their expression correlates with, and sometimes predicts, life span, making them ideal biomarkers of aging. The tools available for experimentally manipulating gene function and assaying healthspan in Drosophila provides an unparalleled opportunity to further study the role of Hsps in aging.

Regulation of skeletal muscle mitochondrial function: genes to proteins

The impact of ageing on mitochondrial function and the deterministic role of mitochondria on senescence continue to be topics of vigorous debate. Many studies report that skeletal muscle mitochondrial content and function are reduced with ageing and metabolic diseases associated with insulin resistance. However, an accumulating body of literature suggests that physical inactivity typical of ageing may be a more important determinant of mitochondrial function than chronological age, per se. Reports of age-related declines in mitochondrial function have spawned a vast body of literature devoted to understanding the underlying mechanisms. These mechanisms include decreased abundance of mtDNA, reduced mRNA levels, as well as decreased synthesis and expression of mitochondrial proteins, ultimately resulting in decreased function of the whole organelle. Effective therapies to prevent, reverse or delay the onset of the aforementioned mitochondrial changes, regardless of their inevitability or precise underlying causes, require an intimate understanding of the processes that regulate mitochondrial biogenesis, which necessitates the coordinated regulation of nuclear and mitochondrial genomes. Herein we review the current thinking on regulation of mitochondrial biogenesis by transcription factors and transcriptional co-activators and the role of hormones and exercise in initiating this process. We review how exercise may help preserve mitochondrial content and functionality across the lifespan, and how physical inactivity is emerging as a major determinant of many age-associated changes at the level of the mitochondrion. We also review evidence that some mitochondrial changes with ageing are independent of exercise or physical activity and appear to be inevitable consequences of old age.

Gene expression profiling implicates OXPHOS complexes in lifespan extension of flies over-expressing a small mitochondrial chaperone, Hsp22

Aging is a complex process accompanied by a decreased capacity to tolerate and respond to various stresses. Heat shock proteins as part of cell defense mechanisms are up-regulated following stress. In Drosophila, the mitochondrial Hsp22 is preferentially up-regulated in aged flies. Its over-expression results in an extension of lifespan and an increased resistance to stress. Hsp22 has chaperone-like activity in vitro, but the mechanism(s) by which it increases lifespan in flies are unknown. Genome-wide analysis was performed on long-lived Hsp22+ and control flies to unveil transcriptional changes brought by Hsp22. Transcriptomes obtained at 45days, 90% and 50% survival were then compared between them to focus more on genes up- or down-regulated in presence of higher levels of hsp22 mRNA. Hsp22+ flies display an up-regulation of genes mainly related to mitochondrial energy production and protein biosynthesis, two functions normally down-regulated during aging. Interestingly, among the 26 genes up-regulated in Hsp22+ flies, 7 genes encode for mitochondrial proteins, 5 of which being involved in OXPHOS complexes. Other genes that could influence aging such as CG5002, dGCC185 and GstS1 also displayed a regulation linked to Hsp22 expression. The up-regulation of genes of the OXPHOS system in Hsp22+ flies suggest that mitochondrial homeostasis is at the center of Hsp22 beneficial effects on lifespan.

Stochastic drift in mitochondrial DNA point mutations: a novel perspective ex silico

The mitochondrial free radical theory of aging (mFRTA) implicates Reactive Oxygen Species (ROS)-induced mutations of mitochondrial DNA (mtDNA) as a major cause of aging. However, fifty years after its inception, several of its premises are intensely debated. Much of this uncertainty is due to the large range of values in the reported experimental data, for example on oxidative damage and mutational burden in mtDNA. This is in part due to limitations with available measurement technologies. Here we show that sample preparations in some assays necessitating high dilution of DNA (single molecule level) may introduce significant statistical variability. Adding to this complexity is the intrinsically stochastic nature of cellular processes, which manifests in cells from the same tissue harboring varying mutation load. In conjunction, these random elements make the determination of the underlying mutation dynamics extremely challenging. Our in silico stochastic study reveals the effect of coupling the experimental variability and the intrinsic stochasticity of aging process in some of the reported experimental data. We also show that the stochastic nature of a de novo point mutation generated during embryonic development is a major contributor of different mutation burdens in the individuals of mouse population. Analysis of simulation results leads to several new insights on the relevance of mutation stochasticity in the context of dividing tissues and the plausibility of ROS ”vicious cycle” hypothesis.

Aging is characterized by a systemic decline of an organism's capacity in responding to internal and external stresses, leading to increased mortality. The mitochondrial Free Radical Theory of Aging (mFRTA) attributes this decline to the accumulation of damages, in the form of mitochondrial DNA (mtDNA) mutations, caused by free radical byproducts of metabolism. However, there is still a great deal of uncertainty with this theory due to the difficulties in quantifying mtDNA mutation burden. In this modeling study, we have shown that a random drift in mtDNA point mutation during life, in combination with the experimental sampling can explain the variability seen in some of the reported experimental data. Particularly, we found that while the average mutation increases in a linear fashion, the variability in the mutation load data increases over time, and thus a low number of data replicates can often lead to a deceptive inference of the mutation burden dynamics. The model also predicted a significant contribution from the embryonic developmental phase to the accumulation of mtDNA mutation burden. Furthermore, the model revealed that the replication rate of mtDNA is a major determinant of new mutations during development and in fast-dividing tissues.

Mitochondrial function as a determinant of life span

Average human life expectancy has progressively increased over many decades largely due to improvements in nutrition, vaccination, antimicrobial agents, and effective treatment/prevention of cardiovascular disease, cancer, etc. Maximal life span, in contrast, has changed very little. Caloric restriction (CR) increases maximal life span in many species, in concert with improvements in mitochondrial function. These effects have yet to be demonstrated in humans, and the duration and level of CR required to extend life span in animals is not realistic in humans. Physical activity (voluntary exercise) continues to hold much promise for increasing healthy life expectancy in humans, but remains to show any impact to increase maximal life span. However, longevity in Caenorhabditis elegans is related to activity levels, possibly through maintenance of mitochondrial function throughout the life span. In humans, we reported a progressive decline in muscle mitochondrial DNA abundance and protein synthesis with age. Other investigators also noted age-related declines in muscle mitochondrial function, which are related to peak oxygen uptake. Long-term aerobic exercise largely prevented age-related declines in mitochondrial DNA abundance and function in humans and may increase spontaneous activity levels in mice. Notwithstanding, the impact of aerobic exercise and activity levels on maximal life span is uncertain. It is proposed that age-related declines in mitochondrial content and function not only affect physical function, but also play a major role in regulation of life span. Regular aerobic exercise and prevention of adiposity by healthy diet may increase healthy life expectancy and prolong life span through beneficial effects at the level of the mitochondrion.

Identification of genes differentially expressed by calorie restriction in the rotifer (Brachionus plicatilis)

A monogonont rotifer Brachionus plicatilis has been widely used as a model organism for physiological, ecological studies and for ecotoxicology. Because of the availability of parthenogenetic mode of reproduction as well as its versatility to be used as live food in aquaculture, the population dynamic studies using the rotifer have become more important and acquired the priority over those using other species. Although many studies have been conducted to identify environmental factors that influence rotifer populations, the molecular mechanisms involved still remain to be elucidated. In this study, gene(s) differentially expressed by calorie restriction in the rotifer was analyzed, where a calorie-restricted group was fed 3 h day(-1) and a well-fed group fed ad libitum. A subtracted cDNA library from the calorie-restricted rotifer was constructed using suppression subtractive hybridization (SSH). One hundred sixty-three expressed sequence tags (ESTs) were identified, which included 109 putative genes with a high identity to known genes in the publicly available database as well as 54 unknown ESTs. After assembling, a total of 38 different genes were obtained among 109 ESTs. Further validation of expression by semi-quantitative reverse transcription-PCR showed that 29 out of the 38 genes obtained by SSH were up regulated by calorie restriction.

Role of mitochondrial oxidative stress to explain the different longevity between genders. Protective effect of estrogens

Females live longer than males. Work from our laboratory has shown that this may be due to the up-regulation of longevity-associated genes by estrogens. Estrogens bind to the estrogen receptors and subsequently activate the mitogen activated protein kinase and nuclear factor kappa B signalling pathways, resulting in an up-regulation of antioxidant enzymes. Estrogen administration, however, has serious undesirable effects and of course, cannot be administered to males because of its powerful feminizing effects. Thus, we tested the effect of genistein, a phytoestrogen of high nutritional importance whose structure is similar to estradiol, on the regulation of the expression of antioxidant, longevity-related genes and consequently on oxidant levels in mammary gland tumour cells in culture. Phytoestrogens mimic the protective effect of oestradiol using the same signalling pathway. The critical importance of up-regulating antioxidant genes, by hormonal and dietary manipulations, to increase longevity is discussed.

Part of the Series: From Dietary Antioxidants to Regulators in Cellular Signalling and Gene ExpressionRole of reactive oxygen species and (phyto)oestrogens in the modulation of adaptive response to stress

There is increasing evidence that reactive oxygen species (ROS) are not only toxic but play an important role in cellular signalling and in the regulation of gene expression. We, here, discuss two examples of improved adaptive response to an altered cellular redox state. First, differences in longevity between males and females may be explained by a higher expression of antioxidant enzymes in females resulting in a lower yield of mitochondrial ROS. Oestrogens are made responsible for these phenomena. Oestradiol induces glutathione peroxidase-1 and MnSOD by processes requiring the cell surface oestrogen receptor (ER) and the activation of pathways usually involved in oxidative stress response. Second, oxygen radicals produced during moderate exercise as performed during training up-regulate the expression of antioxidant enzymes in muscle cells. An increased level of these enzymes might prevent oxidative damage during exhaustive exercise and should, therefore, not be prevented by antioxidants. The relevance of these findings is discussed in the context with observations made in transgenic animals overexpressing MnSOD or catalase.

Mitochondria and ageing in Drosophila

Studies in different organisms have revealed that ageing is a complex process involving a tight regulation of gene expression. Among other features, ageing organisms generally display an increased oxidative stress and a decreased mitochondrial function. The increase in oxidative stress can be attributable to reactive oxygen species, which are mainly produced by mitochondria as a by-product of energy metabolism. Consistent with these data, mitochondria have been suggested to play a significant role in lifespan determination. The fruitfly Drosophila melanogaster is a well-suited organism to study ageing as it is relatively short-lived, mainly composed of post-mitotic cells, has sequenced nuclear and mitochondrial genomes, and multiple genetic tools are available. It has been used in genome-wide studies to unveil the molecular signature of ageing, in different feeding and dietary restriction protocols and in overexpression and down-regulation studies to examine the effect of specific compounds or genes/proteins on lifespan. Here we review the various features linking mitochondria and ageing in Drosophila melanogaster.

Age-related decrease in expression of mitochondrial DNA encoded subunits of cytochrome c oxidase in Drosophila melanogaster

A key feature of the aging process is that the mitochondrial respiratory capacity declines and, the production of reactive oxygen species increases in the later part of life span. In previous studies, cytochrome c oxidase (CcO), the terminal component of the mitochondrial electron transport chain, was found to be the only oxidoreductase exhibiting an age-related decrease in activity in Drosophila melanogaster. The present study tested the hypothesis that decreases in the abundance of catalytic subunits of CcO, encoded in mitochondrial DNA, could underlie the age-associated loss of enzyme activity. Protein amounts of subunits I, II and III, which form the catalytic core of CcO, were determined by immunoblot analysis in 15-, 25-, 35-, 47- and 60-day-old flies. Subunits II and III decreased with age by up to 43% and 75%, respectively, whereas the decrease in subunit I was only 15%. The results pinpoint specific changes in a component of the mitochondrial electron transport chain, which could underlie the age-related decrease in mitochondrial respiratory activity and an increase in oxidant production. Apparently, the stoichiometry of CcO holoprotein is dynamically altered during the aging process in D. melanogaster.

Modulation of longevity-associated genes by estrogens or phytoestrogens

Females live longer than males. We have shown that the higher levels of estrogens in females protect them against aging, by up-regulating the expression of antioxidant, longevity-related genes, such as that of selenium-dependent glutathione peroxidase (GPx) and Mn-superoxide dismutase (Mn-SOD). Both estradiol and genistein (the most abundant phytoestrogen in soybeans) share chemical properties which confer antioxidant features to these compounds. However, the low concentration of estrogens and phytoestrogens make it unlikely that they exhibit significant antioxidant capacity in the organism. Physiological concentrations of estrogens and nutritionally relevant concentrations of genistein activate the MAP kinase pathway. These, in turn, activate the nuclear factor kappa B (NF-kappa B) signaling pathway. Activation of NF-kappa B by estrogens subsequently activates the expression of Mn-SOD and GPx, but genistein is only capable of activating Mn-SOD expression. This could be due to the fact that genistein binds preferably to estrogen receptor beta. The antioxidant protection is reflected in the lower peroxide levels found in cells treated with estrogens or phytoestrogens when compared with controls. The challenge for the future is to find molecules that have the beneficial effects of estradiol, but without its feminizing effects. Phytoestrogens or phytoestrogen-related molecules may be good candidates to meet this challenge.

Over-expression of the catalytic core of mitochondrial DNA (mtDNA) polymerase in the nervous system of Drosophila melanogaster reduces median life span by inducing mtDNA depletion

DNA polymerase gamma (pol gamma) is the sole DNA polymerase devoted to mitochondrial DNA (mtDNA) replication. We have characterized the molecular and physiological effects of over-expression of the catalytic subunit of pol gamma, pol gamma-alpha, in the nervous system of Drosophila melanogaster using the upstream activation sequence (UAS)/yeast transcriptional activator by binding to UAS (GAL4) system. Tissue-specific over-expression of pol gamma-alpha was confirmed by immunoblot analysis, whereas the very low levels of endogenous protein are undetectable in UAS or GAL4 control lines. The transgenic flies over-expressing pol gamma-alpha in the nervous system showed a moderate increase in pupal lethality, and a significant decrease in the median life span of adult flies. Moreover, these flies displayed a decrease in the rate of synthesis of mtDNA, which is accompanied by a significant mtDNA depletion, and a corresponding decrease in the levels of mitochondrial transcription factor A (mtTFA). Biochemical analysis showed an oxidative phosphorylation (OXPHOS) defect in transgenic flies, which were more susceptible to oxidative stress. Although we did not detect apoptosis in the nervous system of adult transgenic flies, brains of larvae over-expressing pol gamma-alpha showed evidence of increased cell death that correlates with the observed phenotypes. Our data establish an animal model that mimics some of the features of human mtDNA depletion syndromes.

Effect of gender on mitochondrial toxicity of Alzheimer's Abeta peptide

The aim of this article is to review the role of mitochondria in the pathogenesis of Alzheimer's disease. Additionally, the effect of gender on the incidence of Alzheimer's disease and the pathophysiological mechanisms involved will be discussed. Mitochondria, in the presence of Alzheimer's amyloid-beta peptide, increase the formation of reactive oxygen species which act both as damaging agents and also as signaling molecules. These radicals, in fact, unleash a mechanism involving the liberation of cytochrome c that leads to neuronal apoptosis. Notably, young females appear protected against the mitochondrial toxicity of amyloid-beta, likely due to the upregulation of antioxidant enzymes which occur in females. Estrogens are responsible for this effect. Overall, the findings support the notion that amyloid-beta causes intracellular toxicity via the increased production of oxidant species. Reactive oxygen species generated by mitochondria act as a signal to start the mitochondrial apoptotic pathway. There is a possibility of prevention, and indirect evidence shows that estrogenic compounds (either endogenous estradiol or phytoestrogens such as genistein) may increase the expression of antioxidant enzymes, leading to a lowering of oxidative stress and thus protection against intracellular toxicity of amyloid-beta peptide. These ideas open up the possibility of using phytoestrogens to prevent the onset of Alzheimer's disease. More studies are required to determine whether estrogens and/or phytoestrogens fulfill these expectations.

Aging perturbs 26S proteasome assembly in Drosophila melanogaster

Aging is associated with loss of quality control in protein turnover. The ubiquitin-proteasome pathway is critical to this quality control process as it degrades mutated and damaged proteins. We identified a unique aging-dependent mechanism that contributes to proteasome dysfunction in Drosophila melanogaster. Our studies are the first to show that the major proteasome form in old (43-47 days old) female and male flies is the weakly active 20S core particle, while in younger (1-32 days old) flies highly active 26S proteasomes are preponderant. Old (43-47 days) flies of both genders also exhibit a decline (approximately 50%) in ATP levels, which is relevant to 26S proteasomes, as their assembly is ATP-dependent. The steep declines in 26S proteasome and ATP levels were observed at an age (43-47 days) when the flies exhibited a marked drop in locomotor performance, attesting that these are "old age" events. Remarkably, treatment with a proteasome inhibitor increases ubiquitinated protein levels and shortens the life span of old but not young flies. In conclusion, our data reveal a previously unknown mechanism that perturbs proteasome activity in "old-age" female and male Drosophila most likely depriving them of the ability to effectively cope with proteotoxic damages caused by environmental and/or genetic factors.

Aging impact on biochemical activities and gene expression of Drosophila melanogaster mitochondria

The consequences of aging are characterized by a decline in the main cellular functions, including those of the mitochondria. Although these consequences have been much studied, efforts have often focused solely on a few parameters used to assess the "state" of mitochondrial function during aging. We performed comparative measurements of several parameters in young (a few days) and old (8 and 12 weeks) adult male Drosophila melanogaster: respiratory complex activities, mitochondrial respiration, ATP synthesis, lipid composition of the inner membrane, concentrations of respiratory complex subunits, expression of genes (nuclear and mitochondrial) coding for mitochondrial proteins. Our results show that, in the mitochondria of "old" flies, the activities of three respiratory complexes (I, III, IV) are greatly diminished, ATP synthesis is decreased, and the lipid composition of the inner membrane (fatty acids, cardiolipin) is modified. However, the respiration rate and subunit concentrations measured by Western blot are unaffected. Although cellular mitochondrial DNA (mtDNA) content remains constant, there is a decrease in concentrations of nuclear and mitochondrial transcripts apparently coordinated. The expression of nuclear genes encoding the transcription factors TFAM, TFB1, TFB2, and DmTTF, which are essential for the maintenance and expression of mtDNA are also decreased. The decrease in nuclear and mitochondrial transcript concentrations may be one of the principal effects of aging on mitochondria, and could explain observed decreases in mitochondrial efficiency.

Ceasing of muscle function with aging: is it the consequence of intrinsic muscle degeneration or a secondary effect of neuronal impairments?

Aging is associated with a significant decline in neuromuscular function leading to an eventual loss of independence and mobility of senescent people. Age-related sarcopenia, characterised by a reduction in muscle mass and strength, is considered one of the most striking features of aging at the level of the skeletal muscle. Morphological alterations in skeletal muscle can be considered as one of the consequences responsible for muscle weakness in the aged population. Beyond 60 years of age, human muscle undergoes a process of continuous denervation and reinnervation, due to an accelerating loss of motor units. It appears evident that phenotypic alterations in muscle depend on the motor drives provided by the nervous system. Because the peripheral nerves, the neuromuscular junction and motor neurons exhibit degenerative features during advanced age, sarcopenia does not seem to intrinsically develop, but is rather a secondary effect of impaired neuronal function. It is therefore recommended that elderly subjects undergo an exercise program that is aimed towards the improvement of coordinative skills and of muscle strength.

Gene expression patterns associated with queen honey bee longevity

The oxidative stress theory of aging proposes that accumulation of oxidative damage is the main proximate cause of aging and that lifespan is determined by the rate at which this damage occurs. Two predictions from this theory are that long-lived organisms produce fewer ROS or have increased antioxidant production. Based in these predictions, molecular mechanisms to promote longevity could include either changes in the regulation of mitochondrial genes that affect ROS production or elevated expression of antioxidant genes. We explored these possibilities in the honey bee, a good model for the study of aging because it has a caste system in which the same genome produces both a long-lived queen and a short-lived worker. We measured mRNA levels for genes encoding eight of the most prominent antioxidant enzymes and five mitochondrial proteins involved in respiration. The expression of antioxidant genes generally decreased with age in queens, but not in workers. Expression of most mitochondrial genes, in particular CytC, was higher in young queens, but these genes showed a faster age-related decline relative to workers. One exception to this trend was COX-I in thorax. This resulted in higher COX-I/CytC ratios in old queens compared to old workers, which suggests caste-specific differences in mitochondrial function that might be related to the caste-specific differences in longevity. Queen honey bee longevity appears to have evolved via mechanisms other than increased antioxidant gene expression.

Superoxide dismutase evolution and life span regulation

Superoxide is among the most abundant reactive oxygen species (ROS) produced by the mitochondria, and is involved in cellular signaling pathways. Superoxide and other ROS can damage cellular macromolecules and levels of oxidative damage products are positively correlated with aging. Superoxide dismutase (SOD) enzymes catalyze the breakdown of superoxide into hydrogen peroxide and water and are therefore central regulators of ROS levels. Genetic and transgenic manipulation of SOD activities in model systems such as S. cereviseae, mouse and Drosophila are consistent with a central role for SOD enzymes in regulating oxidative stress resistance. Over-expression of SOD in S. cereviseae and Drosophila can reduce oxidative damage and extend life span, but the mechanism(s) are not yet clear. A phylogenetic analysis of publicly available SOD protein sequences suggests several additional conserved gene families. For example, in addition to the well-characterized soluble Cu/Zn enzyme (Sod) and mitochondrial manganese-containing form (Sod2), Drosophila melanogaster is found to contain a putative copper chaperone (CCS), an extracellular Cu/Zn enzyme (Sod3), and an extracellular protein distantly related to the Cu/Zn forms (Sodq). C. elegans and blue crab are unusual in having two Mn-containing SODs, and A. gambiae contains an unusual internally repeated SOD. The most parsimonius conclusion from the analysis of the extracellular SODs is that they evolved independently multiple times by addition of a signal peptide to cytoplasmic SOD.

Why females live longer than males? Importance of the upregulation of longevity-associated genes by oestrogenic compounds

Females live longer than males in many mammalian species, including humans. Mitochondria from females produce approximately half the amount of H(2)O(2) than males. We have found that females behave as double transgenics overexpressing both superoxide dismutase and glutathione peroxidase. This is due to oestrogens that act by binding to the estrogen receptors and subsequently activating the mitogen activated protein (MAP) kinase and nuclear factor kappa B (NF-kappaB) signalling pathways. Phytoestrogens mimic the protective effect of oestradiol using the same signalling pathway. The critical importance of upregulating antioxidant genes, by hormonal and dietary manipulations, in order to increase longevity is discussed.

Evolutionary and mechanistic theories of aging

Senescence (aging) is defined as a decline in performance and fitness with advancing age. Senescence is a nearly universal feature of multicellular organisms, and understanding why it occurs is a long-standing problem in biology. Here we present a concise review of both evolutionary and mechanistic theories of aging. We describe the development of the general evolutionary theory, along with the mutation accumulation, antagonistic pleiotropy, and disposable soma versions of the evolutionary model. The review of the mechanistic theories focuses on the oxidative stress resistance, cellular signaling, and dietary control mechanisms of life span extension. We close with a discussion of how an approach that makes use of both evolutionary and molecular analyses can address a critical question: Which of the mechanisms that can cause variation in aging actually do cause variation in natural populations?

Mitochondrial DNA and aging

Among the numerous theories that explain the process of aging, the mitochondrial theory of aging has received the most attention. This theory states that electrons leaking from the ETC (electron transfer chain) reduce molecular oxygen to form O2•− (superoxide anion radicals). O2•−, through both enzymic and non-enzymic reactions, can cause the generation of other ROS (reactive oxygen species). The ensuing state of oxidative stress results in damage to ETC components and mtDNA (mitochondrial DNA), thus increasing further the production of ROS. Ultimately, this ‘vicious cycle’ leads to a physiological decline in function, or aging. This review focuses on recent developments in aging research related to the role played by mtDNA. Both supportive and contradictory evidence is discussed.

Expression of cytochrome c oxidase subunit 1 gene in the brain at an early stage in the termination of pupal diapause in the sweet potato hornworm, Agrius convolvuli

Suppression-subtractive hybridization was used to isolate cDNAs specifically expressed in the brain at the termination of pupal diapause in Agrius convolvuli. One of the isolated clones shows similarity to the cytochrome c oxidase subunit 1 (COX1) gene. The full-length cDNA was obtained from brain mRNA by rapid amplification of cDNA ends (RACE). The insert is 1.65 kb in length and has an open reading frame of 1.46 kb which encodes a putative protein of 486 amino acid residues. RT-PCR reveals that the mRNA increases dramatically at an early stage of diapause termination. Activity of cytochrome c oxidase in the brain also increases at the same time. The up-regulation of this gene suggests that expression of the COX1 gene and ATP synthesis are initiated in the brain in association with diapause termination.

Drosophila melanogaster and the future of 'evo-devo' biology in space. Challenges and problems in the path of an eventual colonization project outside the earth

Space exploration, especially its future phase involving the International Space Station (ISS) makes possible the study of the effects on living systems of long-term expositions to such a strange environment. This phase is being initiated when Biological Sciences are crossing a no-return line into a new territory where the connection between phenotype and genotype may be finally made. We briefly review the paradoxical results obtained in Space experiments performed during the last third of the XX Century. They reveal that simple unicellular systems sense the absence of gravity changing their cytoskeletal organization and the signal transduction pathways, while animal development proceeds unaltered in these conditions, in spite of the fact that these processes are heavily involved in embryogenesis. Longer-term experiments possible in the ISS may solve this apparent contradiction. On the other hand, the current constraints on the scientific use of the ISS makes necessary the development of new hardware and the modification of current techniques to start taking advantage of this extraordinary technological facility. We discuss our advances in this direction using one of the current key biological model systems, Drosophila melanogaster. In addition, the future phase of Space exploration, possibly leading to the exploration and, may be, the colonization of another planet, will provide the means of performing interesting evolutionary experiments, studying how the terrestrial biological systems will change in their long-term adaptation to new, very different environments. In this way, Biological Research in Space may contribute to the advancement of the new Biology, in particular to the branch known as "Evo-Devo". On the other hand, as much as the Space Adventure will continue involving human beings as the main actors in the play, long-term multi-generation experiments using a fast reproducing species, such as Drosophila melanogaster, capable of producing more than 300 generations in 15 years, the useful life foreseen for ISS, will be important. Among other useful information, they will help in detecting the possible changes that a biological species may undergo in such an environment, preventing the uncontrolled occurrence of irreversible deleterious effects with catastrophic consequences on the living beings participating in this endeavour.

Mitochondrial theory of aging: importance to explain why females live longer than males

Females live longer than males in many species, including humans. This can be explained on the basis of the mitochondrial theory of aging. Mitochondria from females produce significantly less hydrogen peroxide than those from males and have higher levels of mitochondrial reduced glutathione, manganese superoxide dismutase, and glutathione peroxidase than males. Oxidative damage to mitochondrial DNA is also fourfold higher in males than in females. These differences may be explained by estrogens. Ovariectomy abolishes the gender differences between males and females and estrogen replacement rescues the ovariectomy effect. The challenge for the future is to find molecules that have the beneficial effects of estradiol, but without its feminizing effects. Phytoestrogens or phytoestrogen-related molecules may be good candidates to meet this challenge.

Reduced amount of mitochondrial DNA in aged human muscle

Muscle concentrations of mRNAs encoded by mitochondrial DNA (mtDNA) decline with aging. To determine whether this can be explained by diminished mtDNA levels, we measured the relative concentrations of mtDNA and a representative mtDNA transcript [encoding cytochrome-c oxidase, subunit 2 (COX-2)] in muscle of young (21-27 yr) and older subjects (65-75 yr). The amount of COX-2 mRNA (relative to 28S rRNA) was 22% lower (P = 0.04) in older muscle, and the amount of mtDNA (relative to nuclear DNA) was 38% lower (P = 0.0002). The average level of mitochondrial transcription factor A (Tfam), a protein essential for mtDNA replication, was similar in younger and older muscle. Tfam mRNA, nuclear respiratory factor-1 mRNA, and several mRNAs encoding proteins required for mtDNA replication were expressed at similar levels in younger and older muscle. The mtDNA concentrations were only weakly related to age-adjusted aerobic fitness (maximal oxygen consumption) and self-reported physical activity levels. We conclude that the lower concentration of mitochondrial mRNAs in older muscle can be explained by a reduced concentration of mtDNA.

Mitochondria from females exhibit higher antioxidant gene expression and lower oxidative damage than males

We have investigated the differential mitochondrial oxidative stress between males and females to understand the molecular mechanisms enabling females to live longer than males. Mitochondria are a major source of free radicals in cells. Those from female rats generate half the amount of peroxides than those of males. This does not occur in ovariectomized animals. Estrogen replacement therapy prevents the effect of ovariectomy. Mitochondria from females have higher levels of reduced glutathione than those from males. Those from ovariectomized rats have similar levels to males, and estrogen therapy prevents the fall in glutathione levels that occurs in ovariectomized animals. Oxidative damage to mitochondrial DNA in males is 4-fold higher than that in females. This is due to higher expression and activities of Mn-superoxide dismutase and of glutathione peroxidase in females, which behave as double transgenics overexpressing superoxide dismutase and glutathione peroxidase, conferring protection against free-radical-mediated damage in aging. Moreover, 16S rRNA expression, which decreases significantly with aging, is four times higher in mitochondria from females than in those from males of the same chronological age. The facts reported here provide molecular evidence to explain the different life span in males and females.

Mitochondrial DNA repair and aging

The mitochondrial electron transport chain plays an important role in energy production in aerobic organisms and is also a significant source of reactive oxygen species that damage DNA, RNA and proteins in the cell. Oxidative damage to the mitochondrial DNA is implicated in various degenerative diseases, cancer and aging. The importance of mitochondrial ROS in age-related degenerative diseases is further strengthened by studies using animal models, Caenorhabditis elegans, Drosophila and yeast. Research in the last several years shows that mitochondrial DNA is more susceptible to various carcinogens and ROS when compared to nuclear DNA. DNA damage in mammalian mitochondria is repaired by base excision repair (BER). Studies have shown that mitochondria contain all the enzymes required for BER. Mitochondrial DNA damage, if not repaired, leads to disruption of electron transport chain and production of more ROS. This vicious cycle of ROS production and mtDNA damage ultimately leads to energy depletion in the cell and apoptosis.

Oxidative stress, mitochondrial DNA mutation, and impairment of antioxidant enzymes in aging

Mitochondria do not only produce less ATP, but they also increase the production of reactive oxygen species (ROS) as by-products of aerobic metabolism in the aging tissues of the human and animals. It is now generally accepted that aging-associated respiratory function decline can result in enhanced production of ROS in mitochondria. Moreover, the activities of free radical-scavenging enzymes are altered in the aging process. The concurrent age-related changes of these two systems result in the elevation of oxidative stress in aging tissues. Within a certain concentration range, ROS may induce stress response of the cells by altering expression of respiratory genes to uphold the energy metabolism to rescue the cell. However, beyond the threshold, ROS may cause a wide spectrum of oxidative damage to various cellular components to result in cell death or elicit apoptosis by induction of mitochondrial membrane permeability transition and release of apoptogenic factors such as cytochrome c. Moreover, oxidative damage and large-scale deletion and duplication of mitochondrial DNA (mtDNA) have been found to increase with age in various tissues of the human. Mitochondria act like a biosensor of oxidative stress and they enable cell to undergo changes in aging and age-related diseases. On the other hand, it has recently been demonstrated that impairment in mitochondrial respiration and oxidative phosphorylation elicits an increase in oxidative stress and causes a host of mtDNA rearrangements and deletions. Here, we review work done in the past few years to support our view that oxidative stress and oxidative damage are a result of concurrent accumulation of mtDNA mutations and defective antioxidant enzymes in human aging.

Flow cytometry of isolated mitochondria during development and under some pathological conditions

Mitochondria play an essential role in the generation of the energy needed for eukaryotic cell life and in the release of molecules involved in initiation of cell death. Here we review the changes in isolated mitochondrial fluorescent populations as distinguished by flow cytometry during postnatal development and their regulation by thyroid hormones and catecholamines. The use of flow cytometry in the study of mitochondrial changes occurring under hypothyroidism, alcohol abuse and aging is also reviewed.

Mitochondrial DNA mutations, oxidative stress, and aging

Advances in understanding of mitochondrial physiology and genetics in relation to pathology have exploded in the last decade. Paralleling this increase has been an active debate about the role of mitochondrial oxidative stress with regard to mitochondrial DNA mutations, aging, and disease. We discuss in a historical context the rapid progress in our understanding of the role of mitochondrial DNA mutations in disease, mitochondrial oxidative stress in aging, and the potential interplay between these two phenomena.

Gene therapy by mitochondrial transfer

Mitochondrial DNA (mtDNA) is highly susceptible to mutation. Novel approaches such as those involving cytoplast fusion and mitochondrial microinjection are essential for gene therapy of diseases caused by these mutations, due to the non-Mendelian genetics of these diseases. In this fusion method, mtDNA in the cytoplast is transferred into mutant cells via the formation of cybrids; once inside the cell the mtDNA complement the defect correctly and safely. The genes in cloned animals are composed of nuclear DNA (nDNA) of a mature tissue and mtDNA from an oocyte. Recent advances in transmitochondrial mice depends on the microinjection of mitochondria into the oocyte. Here we present data on in vitro gene therapy using human mtDNA, cybrid formation and microinjection.

Doxorubicin-induced persistent oxidative stress to cardiac myocytes

We recently reported a cardioselective and cumulative oxidation of cardiac mitochondrial DNA (mtDNA) following subchronic administration of doxorubicin to rats. The mtDNA adducts persist for up to 5 weeks after cessation of doxorubicin treatment. Since the evidence suggests that this persistence of mtDNA adducts cannot be attributed to a lack of repair and replication, we investigated whether it might reflect a long-lasting stimulation of free radical-mediated adduct formation. Male Sprague-Dawley rats received weekly s.c. injections of either doxorubicin (2 mg/kg) or an equivalent volume of saline. Cardiac myocytes isolated from rats following 6 weekly injections of doxorubicin expressed a much higher rate of reactive oxygen species (ROS) formation compared to saline controls. This higher rate of ROS formation persisted for 5 weeks following the last injection. Associated with this was a persistent depression of GSH in heart tissue, while protein-thiol content was not markedly altered. These data suggest that the accumulation and persistence of oxidized mtDNA may be due, not to the stability of the adducts, but to some as yet undefined toxic lesion that causes long-lasting stimulation of ROS generation by doxorubicin. This persistent generation of ROS may contribute to the cumulative and irreversible cardiotoxicity observed clinically with the drug.

High-abundance mRNAs in human muscle: comparison between young and old

To gain a better understanding of the potential role of altered gene expression in the diminished muscle function in old age, we performed a broad search for transcripts expressed at quantitatively different levels in younger (21-24 yr) and older (66-77 yr) human vastus lateralis muscle by serial analysis of gene expression (SAGE). Because SAGE was based on RNA pooled from muscle of several different subjects, relative concentrations of selected mRNAs also were determined in individual muscle samples by quantitative RT-PCR. There were 702 SAGE tags detected at least 10 times in one or both mRNA pools, and the detection frequency was different (at P < 0.01) between young and older muscle for 89 of these. The ratio of myosin heavy chain 2a mRNA to myosin heavy chain 1 mRNA was reduced in older muscle. The mRNAs encoding several mitochondrial proteins involved in electron transport (including several subunits of cytochrome-c oxidase and NADH dehydrogenase) and subunits of ATP synthase were approximately 30% less abundant in older muscle. Several mRNAs encoding enzymes involved in glucose metabolism also were less abundant in older muscle. Analysis of individual samples revealed that the differences suggested by SAGE were not artifacts of atypical gene expression in one or a few individuals. These data suggest that some of the phenotypic changes in senescent muscle may be related to altered gene transcription.