RNA interference-mediated silencing of Sod2 in Drosophila leads to early adult-onset mortality and elevated endogenous oxidative stress

ROS leads to MnSOD upregulation through ERK2 translocation and p53 activation in selenite-induced apoptosis of NB4 cells

Following our previous finding that sodium selenite induces apoptosis in human leukemia NB4 cells, we now show that the expression of the critical antioxidant enzyme manganese superoxide dismutase (MnSOD) is remarkably elevated during this process. We further reveal that reactive oxygen species (ROS), especially superoxide radicals, play a crucial role in selenite-induced MnSOD upregulation, with extracellular regulated kinase (ERK) and p53 closely implicated. Specifically, ERK2 translocates into the nucleus driven by ROS, where it directly phosphorylates p53, leading to dissociation of p53 from its inhibitory protein mouse double minute 2 (MDM2). Active p53 directly mediates the expression of MnSOD, serving as the link between ERK2 translocation and MnSOD upregulation.

Update on the oxidative stress theory of aging: does oxidative stress play a role in aging or healthy aging?

The oxidative stress theory of aging predicts that manipulations that alter oxidative stress/damage will alter aging. The gold standard for determining whether aging is altered is life span, i.e., does altering oxidative stress/damage change life span? Mice with genetic manipulations in their antioxidant defense system designed to directly address this prediction have, with few exceptions, shown no change in life span. However, when these transgenic/knockout mice are tested using models that develop various types of age-related pathology, they show alterations in progression and/or severity of pathology as predicted by the oxidative stress theory: increased oxidative stress accelerates pathology and reduced oxidative stress retards pathology. These contradictory observations might mean that (a) oxidative stress plays a very limited, if any, role in aging but a major role in health span and/or (b) the role that oxidative stress plays in aging depends on environment. In environments with minimal stress, as expected under optimal husbandry, oxidative damage plays little role in aging. However, under chronic stress, including pathological phenotypes that diminish optimal health, oxidative stress/damage plays a major role in aging. Under these conditions, enhanced antioxidant defenses exert an "antiaging" action, leading to changes in life span, age-related pathology, and physiological function as predicted by the oxidative stress theory of aging.

Mitochondrial electron transport chain dysfunction during development does not extend lifespan in Drosophila melanogaster

Since the initial identification of reactive oxygen species (ROS) as the major factor in aging, many studies have provided evidence for the central role of mitochondria in longevity. A few years ago, an unexpected finding showed that the inactivation of the mitochondrial respiratory chain (MRC) in Caenorhabditis elegans, during the developmental stages only, extended lifespan. Activation of this mitochondrial pathway affecting aging (MIT) is associated with several phenotypic features: increased longevity, increased time of development, decreased fertility/fecundity and reduced adult size. Here, we investigated this pathway in another model organism, Drosophila melanogaster. To assess the role of mitochondrial activity in the Drosophila aging process, we partially inactivated the MRC using RNA interference (RNAi) during larval stages. Developmental perturbation of the respiratory process prolonged development, increased lethality during developmental stage, reduced both fecundity and fertility and slightly reduced individual weight. However, in contrast to the nematode, this genetic intervention either shortened or had no effect on lifespan, depending on the level of gene inactivation. Thus, the effects of MRC disruption during development on aging differ between species. We discuss the possible origins of such differences.

Açai palm fruit (Euterpe oleracea Mart.) pulp improves survival of flies on a high fat diet

Reducing oxidative damage is thought to be an effective aging intervention. Açai, a fruit indigenous to the Amazon, is rich in phytochemicals that possesses high anti-oxidant activities, and has anti-inflammatory, anti-cancer and anti-cardiovascular disease properties. However, little is known about its potential anti-aging properties especially at the organismal level. Here we evaluated the effect of açai pulp on modulating lifespan in Drosophila melanogaster. We found that açai supplementation at 2% in the food increased the lifespan of female flies fed a high fat diet compared to the non-supplemented control. We measured transcript changes induced by açai for age-related genes. Although transcript levels of most genes tested were not altered, açai increased the transcript level of l(2)efl, a small heat-shock-related protein, and two detoxification genes, GstD1 and MtnA, while decreasing the transcript level of phosphoenolpyruvate carboxykinase (Pepck), a key gene involved in gluconeogenesis. Furthermore, açai increased the lifespan of oxidative stressed females caused by sod1 RNAi. This suggests that açai improves survival of flies fed a high fat diet through activation of stress response pathways and suppression of Pepck expression. Açai has the potential to antagonize the detrimental effect of fat in the diet and alleviate oxidative stress in aging.

Cerium caused life span shortening and oxidative stress resistance in Drosophila melanogaster

To investigate the effects of the rare earth element cerium (Ce) on the life span and biomarkers of oxidative stress in the fruit fly (Drosophila melanogaster). Fruit flies were fed on media with different dose of ceric sulfate (1, 4, 16, 64, 256, 1024mg/L, corresponding to cerium concentrations of 0.45, 1.65, 6.91, 26.3, 104, and 429microg/g culture medium). Mean life span, maximum life span, and fertility were calculated. There was a significant decrease in mean life span and maximum life span with increasing doses of cerium. At some concentrations, there was a decrease in reproductive output, especially concentrations >6.91microg/g. We also measured superoxide dismutase (SOD) activity, catalase (CAT) activity, and lipid peroxidation product levels (malondialdehyde (MDA) content). Cerium caused a significant increase in MDA content and decrease in SOD and CAT activities at concentrations >6.91microg/g. These results suggest that cerium may result in oxidative toxicity to D. melanogaster.

Mitochondrial manganese superoxide dismutase mRNA expression in human chorioamniotic membranes and its association with labor, inflammation, and infection

OBJECTIVE

Human parturition is characterized by the activation of genes involved in acute inflammatory responses in the fetal membranes. Manganese superoxide dismutase (Mn SOD) is a mitochondrial enzyme that scavenges reactive oxygen species (ROS). Mn SOD is up-regulated in sites of inflammation and has an important role in the down-regulation of acute inflammatory processes. Therefore, the aim of this study was to determine the differences in Mn SOD mRNA expression in the fetal membranes in patients with term and preterm labor (PTL) as well as in acute chorioamnionitis.

STUDY DESIGN

Fetal membranes were obtained from patients in the following groups: (1) term not in labor (n = 29); (2) term in labor (n = 29); (3) spontaneous PTL with intact mebranes (n = 16); (4) PTL with histological chorioamnionitis (n = 12); (5) preterm prelabor rupture of the membranes (PPROM; n = 17); and (6) PPROM with histological chorioamnionitis (n = 21). Mn SOD mRNA expression in the membranes was determined by quantitative real-time reverse transcription-polymerase chain reaction.

RESULTS

(1) Mn SOD mRNA expression was higher in the fetal membranes of patients at term in labor than those not in labor (2.4-fold; p = 0.02); (2) the amount of Mn SOD mRNA in the fetal membranes was higher in PTL than in term labor or in PPROM (7.2-fold, p = 0.03; 3.2-fold, p = 0.03, respectively); (3) Mn SOD mRNA expression was higher when histological chorioamnionitis was present both among patients with PPROM (3.8-fold, p = 0.02) and with PTL (5.4-fold, p = 0.02) than in patients with these conditions without histological chorioamnionitis; (4) expression of Mn SOD mRNA was higher in PTL with chorioamnionitis than in PPROM with chorioamnionitis (4.3-fold, p = 0.03).

CONCLUSION

The increase in Mn SOD mRNA expression by fetal membranes in term labor and in histological chorioamnionitis in PTL and PPROM suggests that the fetus deploys anti-oxidant mechanisms to constrain the inflammatory processes in the chorioamniotic membranes.

Is the oxidative stress theory of aging dead?

Currently, the oxidative stress (or free radical) theory of aging is the most popular explanation of how aging occurs at the molecular level. While data from studies in invertebrates (e.g., C. elegans and Drosophila) and rodents show a correlation between increased lifespan and resistance to oxidative stress (and in some cases reduced oxidative damage to macromolecules), direct evidence showing that alterations in oxidative damage/stress play a role in aging are limited to a few studies with transgenic Drosophila that overexpress antioxidant enzymes. Over the past eight years, our laboratory has conducted an exhaustive study on the effect of under- or overexpressing a large number and wide variety of genes coding for antioxidant enzymes. In this review, we present the survival data from these studies together. Because only one (the deletion of the Sod1 gene) of the 18 genetic manipulations we studied had an effect on lifespan, our data calls into serious question the hypothesis that alterations in oxidative damage/stress play a role in the longevity of mice.

Differential contribution of the mitochondrial respiratory chain complexes to reactive oxygen species production by redox cycling agents implicated in parkinsonism

Exposure to environmental pesticides can cause significant brain damage and has been linked with an increased risk of developing neurodegenerative disorders, including Parkinson's disease. Bipyridyl herbicides, such as paraquat (PQ), diquat (DQ), and benzyl viologen (BV), are redox cycling agents known to exert cellular damage through the production of reactive oxygen species (ROS). We examined the involvement of the mitochondrial respiratory chain in ROS production by bipyridyl herbicides. In isolated rat brain mitochondria, H2O2 production occurred with the following order of potency: BV > DQ > PQ in accordance with their measured ability to redox cycle. H2O2 production was significantly attenuated in all cases by antimycin A, an inhibitor of complex III. Interestingly, at micromolar (< or = 300 microM) concentrations, PQ-induced H2O2 production was unaffected by complex I inhibition via rotenone, whereas DQ-induced H2O2 production was equally attenuated by inhibition of complex I or III. Moreover, complex I inhibition decreased BV-induced H2O2 production to a greater extent than with PQ or DQ. These data suggest that multiple sites within the respiratory chain contribute to H2O2 production by redox cycling bipyridyl herbicides. In primary midbrain cultures, H2O2 differed slightly with the following order of potency: DQ > BV > PQ. In this model, inhibition of complex III resulted in roughly equivalent inhibition of H2O2 production with all three compounds. These data identify a novel role for complex III dependence of mitochondrial ROS production by redox cycling herbicides, while emphasizing the importance of identifying mitochondrial mechanisms by which environmental agents generate oxidative stress contributing to parkinsonism.

Sod2 knockdown in the musculature has whole-organism consequences in Drosophila

Oxidative damage to cell macromolecules by reactive oxygen species is associated with numerous diseases and aging. In Drosophila, RNAi-mediated silencing of the mitochondrial antioxidant manganese superoxide dismutase (SOD2) throughout the body dramatically reduces life span, accelerates senescence of locomotor function, and enhances sensitivity to applied oxidative stress. Here, we show that Sod2 knockdown in the musculature alone is sufficient to cause the shortened life span and accelerated locomotor declines observed with knockdown of Sod2 throughout the body, indicating that Sod2 deficiency in muscle is central to these phenotypes. Knockdown of Sod2 in the muscle also increased caspase activity (a marker for apoptosis) and caused a mitochondrial pathology characterized by swollen mitochondria, decreased mitochondrial content, and reduced ATP levels. These findings indicate that Sod2 plays a crucial role in the musculature in Drosophila and that the consequences of SOD2 loss in this tissue extend to the viability of the organism as a whole.

Decreased expression and the Lys751Gln polymorphism of the XPD gene are associated with extreme longevity

Aging is associated with progressing genomic instability. The XPD gene encodes a DNA helicase involved in nucleotide excision repair and in transcription. We analyzed the common XPD polymorphisms that were previously shown to affect protein's DNA repair efficiency and to increase the risk of developing various cancers. Analysis was performed in 149 centenarians (mean age 101.1 years old) and in 413 young subjects (mean age 27.1 years old). We showed that the distribution of the Lys751Gln genotypes differed significantly between these groups (P = 0.017). In centenarians, the homozygous genotypes AA and CC were found less frequently than in young controls (29 vs. 36%, OR = 0.71, and 14 vs. 20%, OR = 0.652, respectively). The Arg156Arg and Asp312Asn were not significantly associated with extreme longevity. Analysis of the XPD mRNA level in blood mononuclear cells of people divided into three age groups (mean ages 28.7, 65.8 and 92.7 years old) showed that extreme longevity is associated with the decrease of the mean level of the specific mRNA; the differences between young or middle-aged vs. extremely old group were significant (P < 0.0001, P < 0.0001, respectively). In addition, the methylation pattern of the XPD promoter was analyzed in 30 people divided into three age groups (29.5, 65.9, and 101.4 years old). We showed that overall methylation of the XPD promoter is a rare event; however, aging is associated with the increase of methylation level upstream of the transcription start site. In summary, we showed for the first time that both the XPD polymorphic variants and the decreased level of its expression might be associated with aging.

The interaction of mitochondrial iron with manganese superoxide dismutase

Superoxide dismutase 2 (SOD2) is one of the rare mitochondrial enzymes evolved to use manganese as a cofactor over the more abundant element iron. Although mitochondrial iron does not normally bind SOD2, iron will misincorporate into Saccharomyces cerevisiae Sod2p when cells are starved for manganese or when mitochondrial iron homeostasis is disrupted by mutations in yeast grx5, ssq1, and mtm1. We report here that such changes in mitochondrial manganese and iron similarly affect cofactor selection in a heterologously expressed Escherichia coli Mn-SOD, but not a highly homologous Fe-SOD. By x-ray absorption near edge structure and extended x-ray absorption fine structure analyses of isolated mitochondria, we find that misincorporation of iron into yeast Sod2p does not correlate with significant changes in the average oxidation state or coordination chemistry of bulk mitochondrial iron. Instead, small changes in mitochondrial iron are likely to promote iron-SOD2 interactions. Iron binds Sod2p in yeast mutants blocking late stages of iron-sulfur cluster biogenesis (grx5, ssq1, and atm1), but not in mutants defective in the upstream Isu proteins that serve as scaffolds for iron-sulfur biosynthesis. In fact, we observed a requirement for the Isu proteins in iron inactivation of yeast Sod2p. Sod2p activity was restored in mtm1 and grx5 mutants by depleting cells of Isu proteins or using a dominant negative Isu1p predicted to stabilize iron binding to Isu1p. In all cases where disruptions in iron homeostasis inactivated Sod2p, we observed an increase in mitochondrial Isu proteins. These studies indicate that the Isu proteins and the iron-sulfur pathway can donate iron to Sod2p.

A forward genetic screen in Drosophila implicates insulin signaling in age-related locomotor impairment

Age-related locomotor impairment (ARLI) is one of the most detrimental changes that occurs during aging. Elderly individuals with ARLI are at increased risks for falls, depression and a number of other co-morbidities. Despite its clinical significance, little is known about the genes that influence ARLI. We consequently performed a forward genetic screen to identify Drosophila strains with delayed ARLI using negative geotaxis as an index of locomotor function. One of the delayed ARLI strains recovered from the screen had a P-element insertion that decreased expression of the insulin signaling gene phosphoinositide-dependent kinase 1 (PDK1) Precise excision of the P-element insertion reverted PDK1 expression and ARLI to the same as control flies, indicating that disruption of PDK1 leads to delayed ARLI. Follow-up studies showed that additional loss of function mutations in PDK1 as well as loss of function alleles of two other insulin signaling genes, Dp110 and Akt (the genes for the catalytic subunit of phosphoinositide 3-kinase and AKT), also forestalled ARLI. Interestingly, only some of the strains with delayed ARLI had elevated resistance to paraquat, indicating that enhanced resistance to this oxidative stressor is not required for preservation of locomotor function across age. Our studies implicate insulin signaling as a key regulator of ARLI in Drosophila.

Manipulation of Sod1 expression ubiquitously, but not in the nervous system or muscle, impacts age-related parameters in Drosophila

Superoxide dismutase 1 (SOD1) is an important antioxidant previously shown to impact life span in Drosophila. We examined the consequences of manipulating Sod1 expression throughout the body or in the nervous system or musculature on life span and age-related locomotor impairment (ARLI) in Drosophila. Ubiquitous overexpression of SOD1 extended life span but did not substantially forestall ARLI, whereas ubiquitous knock-down of Sod1 shortened life span and accelerated ARLI. Interestingly, neither overexpression of Sod1 nor expression of Sod1 RNAi in the nervous system or muscle altered life span or ARLI. Our studies suggest that the control of reactive oxygen species by SOD1 in tissues other than the nervous system and musculature support life span and ARLI in Drosophila.

Overexpression of methionine-R-sulfoxide reductases has no influence on fruit fly aging

Methionine sulfoxide reductases (Msrs) are enzymes that repair oxidized methionine residues in proteins. This function implicated Msrs in antioxidant defense and the regulation of aging. There are two known Msr types in animals: MsrA specific for the reduction of methionine-S-sulfoxide, and MsrB that catalyzes the reduction of methionine-R-sulfoxide. In a previous study, overexpression of MsrA in the nervous system of Drosophila was found to extend lifespan by 70%. Overexpression of MsrA in yeast also extended lifespan, whereas MsrB overexpression did so only under calorie restriction conditions. The effect of MsrB overexpression on lifespan has not yet been characterized in animal model systems. Here, the GAL4-UAS binary system was used to drive overexpression of cytosolic Drosophila MsrB and mitochondrial mouse MsrB2 in whole body, fatbody, and the nervous system of flies. In contrast to MsrA, MsrB overexpression had no consistent effect on the lifespan of fruit flies on either corn meal or sugar yeast diets. Physical activity, fecundity, and stress resistance were also similar in MsrB-overexpressing and control flies. Thus, MsrA and MsrB, the two proteins with similar function in antioxidant protein repair, have different effects on aging in fruit flies.

Mitochondrial superoxide radicals differentially affect muscle activity and neural function

Cellular superoxide radicals (O(2)(-)) are mostly generated during mitochondrial oxygen metabolism. O(2)(-) serves as the raw material for many reactive oxygen species (ROS) members like H(2)O(2) and OH(.-) radicals following its catalysis by superoxide dismutase (SOD) enzymes and also by autocatalysis (autodismutation) reactions. Mitochondrial ROS generation could have serious implications on degenerative diseases. In model systems overproduction of mitochondrial O(2)(-) resulting from the loss of SOD2 function leads to movement disorders and drastic reduction in life span in vertebrates and invertebrates alike. With the help of a mitochondrial SOD2 loss-of-function mutant, Sod2(n283), we measured the sensitivity of muscles and neurons to ROS attack. Neural outputs from flight motor neurons and sensory neurons were unchanged in Sod2(n283) and the entire neural circuitry between the giant fiber (GF) and the dorsal longitudinal muscles (DLM) showed no overt defect due to elevated ROS. Such insensitivity of neurons to mitochondrial superoxides was further established through neuronal expression of SOD2, which failed to improve survival or locomotive ability of Sod2(n283). On the other hand, ultrastructural analysis of Sod2(n283) muscles revealed fewer mitochondria and reduced muscle ATP production. By targeting the SOD2 expression to the muscle we demonstrate that the early mortality phenotype of Sod2(n283) can be ameliorated along with signs of improved mobility. In summary, muscles appear to be more sensitive to superoxide attack relative to the neurons and such overt phenotypes observed in SOD2-deficient animals can be directly attributed to the muscle.

Effects of four oxidants, menadione, 1-chloro-2,4-dinitrobenzene, hydrogen peroxide and cumene hydroperoxide, on fission yeast Schizosaccharmoyces pombe

Several chemical agents have been used to exert oxidative stress in the study of stress response, but differences in the effects of different reagents have received little attention. To elucidate whether such differences exist, the response of Schizosaccharomyces pombe to menadione (MD), 1-chloro-2,4-dinitrobenzene (CDNB), hydrogen peroxide and cumene hydroperoxide (CHP), which are frequently used to exert oxidative stress, was investigated. Sensitivity to these reagents differed among mutants deficient in genes involved in oxidative stress resistance. N-Acetylcysteine restored resistance to MD, CHP and hydrogen peroxide but did not change sensitivity to CDNB. The induction kinetics of genes induced by oxidative stress differed for each reagent. MD, CDNB and hydrogen peroxide caused a transient induction of genes, but the peak times of induction differed among the reagents. CHP gave quite different kinetics in that the induction continued for up to 2 h. The ctt1(+) gene was not induced by CHP. GSH rapidly decreased in the cells treated with high concentrations of these reagents, but at a low concentration only CDNB decreased GSH. These results indicated that S. pombe responded differently to the oxidative stress exerted by these different reagents.

Sod2 haploinsufficiency does not accelerate aging of telomere dysfunctional mice

Telomere shortening represents a causal factor of cellular senescence. At the same time, several lines of evidence indicate a pivotal role of oxidative DNA damage for the aging process in vivo. A causal connection between the two observations was suggested by experiments showing accelerated telomere shorting under conditions of oxidative stress in cultured cells, but has never been studied in vivo. We therefore have analysed whether an increase in mitochondrial derived oxidative stress in response to heterozygous deletion of superoxide dismutase (Sod2^+/-) would exacerbate aging phenotypes in telomere dysfunctional (mTerc^-/-) mice. Heterozygous deletion of Sod2 resulted in reduced SOD2 protein levels and increased oxidative stress in aging telomere dysfunctional mice, but this did not lead to an increase in basal levels of oxidative nuclear DNA damage, an accumulation of nuclear DNA breaks, or an increased rate of telomere shortening in the mice. Moreover, heterozygous deletion of Sod2 did not accelerate the depletion of stem cells and the impairment in organ maintenance in aging mTerc^-/- mice. In agreement with these observations, Sod2 haploinsufficiency did not lead to a further reduction in lifespan of mTerc^-/- mice. Together, these results indicate that a decrease in SOD2-dependent antioxidant defence does not exacerbate aging in the context of telomere dysfunction.

Deletion of the mitochondrial superoxide dismutase sod-2 extends lifespan in Caenorhabditis elegans

The oxidative stress theory of aging postulates that aging results from the accumulation of molecular damage caused by reactive oxygen species (ROS) generated during normal metabolism. Superoxide dismutases (SODs) counteract this process by detoxifying superoxide. It has previously been shown that elimination of either cytoplasmic or mitochondrial SOD in yeast, flies, and mice results in decreased lifespan. In this experiment, we examine the effect of eliminating each of the five individual sod genes present in Caenorhabditis elegans. In contrast to what is observed in other model organisms, none of the sod deletion mutants shows decreased lifespan compared to wild-type worms, despite a clear increase in sensitivity to paraquat- and juglone-induced oxidative stress. In fact, even mutants lacking combinations of two or three sod genes survive at least as long as wild-type worms. Examination of gene expression in these mutants reveals mild compensatory up-regulation of other sod genes. Interestingly, we find that sod-2 mutants are long-lived despite a significant increase in oxidatively damaged proteins. Testing the effect of sod-2 deletion on known pathways of lifespan extension reveals a clear interaction with genes that affect mitochondrial function: sod-2 deletion markedly increases lifespan in clk-1 worms while clearly decreasing the lifespan of isp-1 worms. Combined with the mitochondrial localization of SOD-2 and the fact that sod-2 mutant worms exhibit phenotypes that are characteristic of long-lived mitochondrial mutants-including slow development, low brood size, and slow defecation-this suggests that deletion of sod-2 extends lifespan through a similar mechanism. This conclusion is supported by our demonstration of decreased oxygen consumption in sod-2 mutant worms. Overall, we show that increased oxidative stress caused by deletion of sod genes does not result in decreased lifespan in C. elegans and that deletion of sod-2 extends worm lifespan by altering mitochondrial function.

Hypoxia rescues early mortality conferred by superoxide dismutase deficiency

Oxidative stress is widely associated with disease and aging but the underlying mechanisms are incompletely understood. Here we show that the premature mortality of Drosophila deficient in superoxide scavengers, superoxide dismutase (SOD) 1 or SOD2, is rescued by chronic hypoxia. Strikingly, switching moribund SOD2-deficient adults from normoxia into hypoxia abruptly arrests their impending premature mortality and endows the survivors with a near-normal life span. This finding challenges the notion that irreversible oxidative damage initiated by unscavenged superoxide in the mitochondrial matrix underpins the premature mortality of SOD2-deficient adults. In contrast, switching moribund SOD1-deficient flies from normoxia into hypoxia fails to alter their mortality trajectory, suggesting that the deleterious effects of unscavenged superoxide in the cytoplasm/intermembrane space compartment are cumulative and largely irreversible. We conclude that cellular responses to superoxide-initiated oxidative stress are mediated through different compartment-specific pathways. Elucidating these pathways should provide novel insights into how aerobic cells manage oxidative stress in health, aging, and disease.

Chapter 21 Paraquat-induced production of reactive oxygen species in brain mitochondria

Paraquat (PQ) is a prototypical redox cycling agent commonly used experimentally to generate reactive oxygen species and oxidative stress. Recently, PQ has also come under investigation as a potential environmental neurotoxin associated with increased risk for neurodegenerative disease developing after chronic exposure. The interactions of PQ with mitochondria remain an important aspect of its toxicity, particularly in the brain, although the underlying mechanisms are relatively uncharacterized. Here, we describe the basic measurement of PQ-induced hydrogen peroxide (H(2)O(2)) production in isolated brain mitochondria by use of two independent methods, polarography and fluorometry. The advantages of the use of these two independent methods include the capability to validate results and overcoming limitations in the use of either method exclusively. These simplified in vitro techniques for measurement of mitochondrial-generated H(2)O(2) can be easily applied to other tissues and models.

Chapter 22 The uptake and interactions of the redox cycler paraquat with mitochondria

Paraquat (1,1'-dimethyl-4,4'-bipyridinium dichloride) is widely used as a redox cycler to stimulate superoxide production in organisms, cells, and mitochondria. Paraquat is also used to induce symptoms of Parkinson's disease in experimental models of this neurodegenerative disorder. Paraquat causes extensive mitochondrial oxidative damage, and in mammalian systems, complex I of the respiratory chain has been identified as the major site of superoxide production by paraquat. Although much progress has been made at explaining how paraquat interacts with mitochondria, several aspects remain to be clarified-most notably the pathway of paraquat uptake into mitochondria. This chapter describes methods for further investigating the interaction of paraquat with mitochondria and also provides practical information for the general use of paraquat as a superoxide generator and agent of oxidative stress. The techniques covered include the detection and quantitation of the paraquat dication and the paraquat monocation radical (by electron paramagnetic resonance, spectrophotometry, and with an ion-selective electrode); assays for measuring paraquat-induced superoxide production by intact mitochondria or mitochondrial membranes (including aconitase inactivation, and coelenterazine chemiluminescence); methods for assessing paraquat uptake by mitochondria; and screens for identifying paraquat sensitivity or resistance in yeast mutants.

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.

Antioxidants cannot suppress the lethal phenotype of a Drosophila melanogaster model of Huntington’s disease

Substantial evidence suggests that antioxidants may play a major role in delaying the progress of Huntington’s disease (HD). Here we investigated the effects of superoxide dismutase (cytoplasmic Cu/ZnSOD and mitochondrial MnSOD) and supplementation with dietary antioxidants (α-tocopherol and coenzyme Q10) on survival to adulthood in a Drosophila melanogaster model of HD. Our results illustrate that neither overexpression of superoxide dismutase nor supplementation of dietary antioxidants can rescue the lethal phenotype of HD flies. We discuss these results in conjunction with other evidence that antioxidants may only avert the oxidative stress induced progression of HD.

The effects of vitamin supplementation on Drosophila life span under normoxia and under oxidative stress

Vitamin A (retinol), vitamin C (ascorbic acid), and vitamin E (alpha-tocopherol) are each thought to play an important role in the aging process. Here, we investigated the effects of these vitamins on Drosophila melanogaster life span under different oxidative stress conditions. Among the vitamins tested, alpha-tocopherol exhibited the strongest antioxidant activity, extending average and maximum life span for wild-type flies under hyperoxia and for Cu/Zn superoxide dismutase-deficient (SOD1-deficient) flies under normoxia. Retinol supplementation extended life span of SOD1-deficient flies under normoxia, and ascorbic acid supplementation extended life span of wild-type flies under normoxia. However, both retinol and ascorbic acid exhibited a strong prooxidant activity under hyperoxia and shortened life span. Furthermore, ascorbic acid supplementation enhanced the toxic effects of iron, with the iron pool significantly increased in adult whole-body extracts. Taken together, our results document antioxidant and prooxidant contributions of vitamins to D. melanogaster life-span determination under normoxia and under oxidative stress.

Transcriptional profiling of MnSOD-mediated lifespan extension in Drosophila reveals a species-general network of aging and metabolic genes

Transcriptional profiling of MnSOD-mediated life-span extension in Drosophila identifies a set of candidate biomarkers of aging, consisting primarily of carbohydrate metabolism and electron transport genes.

Background

Several interventions increase lifespan in model organisms, including reduced insulin/insulin-like growth factor-like signaling (IIS), FOXO transcription factor activation, dietary restriction, and superoxide dismutase (SOD) over-expression. One question is whether these manipulations function through different mechanisms, or whether they intersect on common processes affecting aging.

Results

A doxycycline-regulated system was used to over-express manganese-SOD (MnSOD) in adult Drosophila, yielding increases in mean and maximal lifespan of 20%. Increased lifespan resulted from lowered initial mortality rate and required MnSOD over-expression in the adult. Transcriptional profiling indicated that the expression of specific genes was altered by MnSOD in a manner opposite to their pattern during normal aging, revealing a set of candidate biomarkers of aging enriched for carbohydrate metabolism and electron transport genes and suggesting a true delay in physiological aging, rather than a novel phenotype. Strikingly, cross-dataset comparisons indicated that the pattern of gene expression caused by MnSOD was similar to that observed in long-lived Caenorhabditis elegans insulin-like signaling mutants and to the xenobiotic stress response, thus exposing potential conserved longevity promoting genes and implicating detoxification in Drosophila longevity.

Conclusion

The data suggest that MnSOD up-regulation and a retrograde signal of reactive oxygen species from the mitochondria normally function as an intermediate step in the extension of lifespan caused by reduced insulin-like signaling in various species. The results implicate a species-conserved net of coordinated genes that affect the rate of senescence by modulating energetic efficiency, purine biosynthesis, apoptotic pathways, endocrine signals, and the detoxification and excretion of metabolites.

Overexpression of frataxin in the mitochondria increases resistance to oxidative stress and extends lifespan in Drosophila

In Friedreich's ataxia, reduction of the mitochondria protein frataxin results in the accumulation of iron and reactive oxygen species, which leads to oxidative damage, neurodegeneration and a diminished lifespan. Recent studies propose that frataxin might play a role in the antioxidative process. Here we show that overexpression of Drosophila frataxin in the mitochondria of female transgenic animals increases antioxidant capability, resistance to oxidative stress insults, and longevity. This suggests that Drosophila frataxin may function to protect the mitochondria from oxidative stresses and the ensuing cellular damage.

Complex I is the major site of mitochondrial superoxide production by paraquat

Paraquat (1,1'-dimethyl-4,4'-bipyridinium dichloride) is widely used as a redox cycler to stimulate superoxide production in organisms, cells, and mitochondria. This superoxide production causes extensive mitochondrial oxidative damage, however, there is considerable uncertainty over the mitochondrial sites of paraquat reduction and superoxide formation. Here we show that in yeast and mammalian mitochondria, superoxide production by paraquat occurs in the mitochondrial matrix, as inferred from manganese superoxide dismutase-sensitive mitochondrial DNA damage, as well as from superoxide assays in isolated mitochondria, which were unaffected by exogenous superoxide dismutase. This paraquat-induced superoxide production in the mitochondrial matrix required a membrane potential that was essential for paraquat uptake into mitochondria. This uptake was of the paraquat dication, not the radical monocation, and was carrier-mediated. Experiments with disrupted mitochondria showed that once in the matrix paraquat was principally reduced by complex I (mammals) or by NADPH dehydrogenases (yeast) to form the paraquat radical cation that then reacted with oxygen to form superoxide. Together this membrane potential-dependent uptake across the mitochondrial inner membrane and the subsequent rapid reduction to the paraquat radical cation explain the toxicity of paraquat to mitochondria.

Hydrogen peroxide scavenging rescues frataxin deficiency in a Drosophila model of Friedreich's ataxia

Friedreich's ataxia (FRDA) is a neurodegenerative disorder arising from a deficit of the mitochondrial iron chaperone, frataxin. Evidence primarily from yeast and mammalian cells is consistent with the hypothesis that a toxic hydroxyl radical generated from hydrogen peroxide (H2O2) via iron-catalyzed Fenton chemistry at least partially underlies the pathology associated with this disease. However, no whole-organism studies have been presented that directly test this hypothesis. We recently developed a Drosophila model that recapitulates the principal hallmarks of FRDA [Anderson PR, Kirby K, Hilliker A, Phillips JP (2005) Hum Mol Genet 14:3397-3405]. Using the Drosophila FRDA model, we now report that ectopic expression of enzymes that scavenge H2O2 suppresses the deleterious phenotypes associated with frataxin deficiency. In contrast, genetic augmentation with enzymes that scavenge superoxide is without effect. Augmentation of endogenous catalase restores the activity of the reactive oxygen species (ROS)-sensitive mitochondrial enzyme, aconitase and enhances resistance to H2O2 exposure, both of which are diminished by frataxin deficiency. Collectively, these data argue that H2O2 is an important pathogenic substrate underlying the phenotypes arising from frataxin deficiency in Drosophila and that interventions that reduce this specific ROS can effectively ameliorate these phenotypes. The therapeutic implications of these findings are clear and we believe warrant immediate clinical investigation.

Targeting mitochondria

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are closely linked to degenerative diseases such as Alzheimer's disease, Parkinson's, neuronal death including ischemic and hemorrhagic stroke, acute and chronic degenerative cardiac myocyte death, and cancer. As a byproduct of oxidative phosphorylation, a steady stream of reactive species emerge from our cellular energy plants, the mitochondria. ROS and RNS potentially cause damage to all cellular components. Structure alteration, biomolecule fragmentation, and oxidation of side chains are trade-offs of cellular energy production. ROS and RNS escape results in the activation of cytosolic stress pathways, DNA damage, and the upregulation of JNK, p38, and p53. Incomplete scavenging of ROS and RNS particularly affects the mitochondrial lipid cardiolipin (CL), triggers the release of mitochondrial cytochrome c, and activates the intrinsic death pathway. Due to the active redox environment and the excess of NADH and ATP at the inner mitochondrial membrane, a broad range of agents including electron acceptors, electron donors, and hydride acceptors can be used to influence the biochemical pathways. The key to therapeutic value is to enrich selective redox modulators at the target sites. Our approach is based on conjugating nitroxides to segments of natural products with relatively high affinity for mitochondrial membranes. For example, a modified gramicidin S segment was successfully used for this purpose and proven to be effective in preventing superoxide production in cells and CL oxidation in mitochondria and in protecting cells against a range of pro-apoptotic triggers such as actinomycin D, radiation, and staurosporine. More importantly, these mitochondria-targeted nitroxide/gramicidin conjugates were able to protect against apoptosis in vivo by preventing CL oxidation induced by intestinal hemorrhagic shock. Optimization of nitroxide carriers could lead to a new generation of effective antiapoptotic agents acting at an early mitochondrial stage. Alternative chemistry-based approaches to targeting mitochondria include the use of proteins and peptides, as well as the attachment of payloads to lipophilic cationic compounds, sulfonylureas, anthracyclines, and other agents with proven or hypothetical affinities for mitochondria. Manganese superoxide dismutase (MnSOD), SS tetrapeptides with 2',6'-dimethyltyrosine (Dmt) residues, rhodamine, triphenylphosphonium salts, nonopioid analgesics, adriamycin, and diverse electron-rich aromatics and stilbenes were used to influence mitochondrial biochemistry and the biology of aging. Some general structural principles for effective therapeutic agents are now emerging. Among these are the presence of basic or positively charged functional groups, hydrophobic substructures, and, most promising for future selective strategies, classes of compounds that are actively shuttled into mitochondria, bind to mitochondria-specific proteins, or show preferential affinity to mitochondria-specific lipids.

Life and death: metabolic rate, membrane composition, and life span of animals

Maximum life span differences among animal species exceed life span variation achieved by experimental manipulation by orders of magnitude. The differences in the characteristic maximum life span of species was initially proposed to be due to variation in mass-specific rate of metabolism. This is called the rate-of-living theory of aging and lies at the base of the oxidative-stress theory of aging, currently the most generally accepted explanation of aging. However, the rate-of-living theory of aging while helpful is not completely adequate in explaining the maximum life span. Recently, it has been discovered that the fatty acid composition of cell membranes varies systematically between species, and this underlies the variation in their metabolic rate. When combined with the fact that 1) the products of lipid peroxidation are powerful reactive molecular species, and 2) that fatty acids differ dramatically in their susceptibility to peroxidation, membrane fatty acid composition provides a mechanistic explanation of the variation in maximum life span among animal species. When the connection between metabolic rate and life span was first proposed a century ago, it was not known that membrane composition varies between species. Many of the exceptions to the rate-of-living theory appear explicable when the particular membrane fatty acid composition is considered for each case. Here we review the links between metabolic rate and maximum life span of mammals and birds as well as the linking role of membrane fatty acid composition in determining the maximum life span. The more limited information for ectothermic animals and treatments that extend life span (e.g., caloric restriction) are also reviewed.

Reduced mitochondrial SOD displays mortality characteristics reminiscent of natural aging

Manganese superoxide dismutase (MnSOD or SOD2) is a key mitochondrial enzymatic antioxidant. Arguably the most striking phenotype associated with complete loss of SOD2 in flies and mice is shortened life span. To further explore the role of SOD2 in protecting animals from aging and age-associated pathology, we generated a unique collection of Drosophila mutants that progressively reduce SOD2 expression and function. Mitochondrial aconitase activity was substantially reduced in the Sod2 mutants, suggesting that SOD2 normally ensures the functional capacity of mitochondria. Flies with severe reductions in SOD2 expression exhibited accelerated senescence of olfactory behavior as well as precocious neurodegeneration and DNA strand breakage in neurons. Furthermore, life span was progressively shortened and age-dependent mortality was increased in conjunction with reduced SOD2 expression, while initial mortality and developmental viability were unaffected. Interestingly, life span and age-dependent mortality varied exponentially with SOD2 activity, indicating that there might normally be a surplus of this enzyme for protecting animals from premature death. Our data support a model in which disruption of the protective effects of SOD2 on mitochondria manifests as profound changes in behavioral and demographic aging as well as exacerbated age-related pathology in the nervous system.

Dietary restriction alters demographic but not behavioral aging in Drosophila

Dietary restriction extends lifespan substantially in numerous species including Drosophila. However, it is unclear whether dietary restriction in flies impacts age-related functional declines in conjunction with its effects on lifespan. Here, we address this issue by assessing the effect of dietary restriction on lifespan and behavioral senescence in two wild-type strains, in our standard white laboratory stock, and in short-lived flies with reduced expression of superoxide dismutase 2. As expected, dietary restriction extended lifespan in all of these strains. The effect of dietary restriction on lifespan varied with genetic background, ranging from 40 to 90% extension of median lifespan in the seven strains tested. Interestingly, despite its robust positive effects on lifespan, dietary restriction had no substantive effects on senescence of behavior in any of the strains in our studies. Our results suggest that dietary restriction does not have a global impact on aging in Drosophila and support the hypothesis that lifespan and behavioral senescence are not driven by identical mechanisms.

Iron load and redox stress in skeletal muscle of aged rats

Loss of skeletal muscle mass (sarcopenia) is a major contributor to disability in old age. We used two-dimensional gel electrophoresis and mass spectrometry to screen for changes in proteins, and cDNA profiling to assess transcriptional regulations in the gastrocnemius muscle of adult (4 months) and aged (30 months) male Sprague-Dawley rats. Thirty-five proteins were differentially expressed in aged muscle. Proteins and mRNA transcripts involved in redox homeostasis and iron load were increased, representing novel components that were previously not associated with sarcopenia. Tissue iron levels were elevated in senescence, paralleling an increase in transferrin. Proteins involved in redox homeostasis showed a complex pattern of changes with increased SOD1 and decreased SOD2. These results suggest that an elevated iron load is a significant component of sarcopenia with the potential to be exploited clinically, and that mitochondria of aged striated muscle may be more vulnerable to radicals produced in cell respiration.

Trends in oxidative aging theories

The early observations on the rate-of-living theory by Max Rubner and the report by Gershman that oxygen free radicals exist in vivo culminated in the seminal proposal in the 1950s by Denham Harman that reactive oxygen species are a cause of aging (free radical theory of aging). The goal of this review is to analyze recent findings relevant in evaluating Harman's theory using experimental results as grouped by model organisms (i.e., invertebrate models and mice). In this regard, we have focused primarily on recent work involving genetic manipulations. Because the free radical theory of aging is not the only theorem proposed to explain the mechanism(s) involved in aging at the molecular level, we also discuss how this theory is related to other areas of research in biogerontology, specifically, telomere/cell senescence, genomic instability, and the mitochondrial hypothesis of aging. We also discuss where we think the free radical theory is headed. It is now possible to give at least a partial answer to the question whether oxidative stress determines life span as Harman posed so long ago. Based on studies to date, we argue that a tentative case for oxidative stress as a life-span determinant can be made in Drosophila melanogaster. Studies in mice argue for a role of oxidative stress in age-related disease, especially cancer; however, with regard to aging per se, the data either do not support or remain inconclusive on whether oxidative stress determines life span.

Deletions encompassing the manganese superoxide dismutase gene in the Drosophila melanogaster genome

Two deletions, Df(2R)Sod2-11 and Df(2R)Sod2-332, are recovered that encompass the manganese superoxide dismutase (MnSOD) gene or a null mutant referred to as SOD2n283 in Drosophila. Molecular analysis has revealed that the Df(2R)Sod2-332 deletion completely uncovered both MnSOD and its adjacent gene, Arp53D, whereas Df(2R)Sod2-11 was missing the promoter region of MnSOD gene. As a consequence of reduced MnSOD expression, these deletion heterozygotes are now sensitive to oxidative stress. Complementation analysis with some recently recovered deletions in the 53C/D region has established that other essential loci exist in this interval, and second, that Arp53D function is not essential for the survival of the organism. These deletions will be instrumental in the recovery of missense substitutions in the MnSOD peptide and their influence on oxidative stress resistance.

Causative role of oxidative stress in a Drosophila model of Friedreich ataxia

Friedreich ataxia (FA), the most common form of hereditary ataxia, is caused by a deficit in the mitochondrial protein frataxin. While several hypotheses have been suggested, frataxin function is not well understood. Oxidative stress has been suggested to play a role in the pathophysiology of FA, but this view has been recently questioned, and its link to frataxin is unclear. Here, we report the use of RNA interference (RNAi) to suppress the Drosophila frataxin gene (fh) expression. This model system parallels the situation in FA patients, namely a moderate systemic reduction of frataxin levels compatible with normal embryonic development. Under these conditions, fh-RNAi flies showed a shortened life span, reduced climbing abilities, and enhanced sensitivity to oxidative stress. Under hyperoxia, fh-RNAi flies also showed a dramatic reduction of aconitase activity that seriously impairs the mitochondrial respiration while the activities of succinate dehydrogenase, respiratory complex I and II, and indirectly complex III and IV are normal. Remarkably, frataxin overexpression also induced the oxidative-mediated inactivation of mitochondrial aconitase. This work demonstrates, for the first time, the essential function of frataxin in protecting aconitase from oxidative stress-dependent inactivation in a multicellular organism. Moreover our data support an important role of oxidative stress in the progression of FA and suggest a tissue-dependent sensitivity to frataxin imbalance. We propose that in FA, the oxidative mediated inactivation of aconitase, which occurs normally during the aging process, is enhanced due to the lack of frataxin.

Oxidative stress mediates tau-induced neurodegeneration in Drosophila

Markers of oxidative damage have been detected in brain tissue from patients with Alzheimer disease (AD) and other neurodegenerative disorders. These findings implicate oxidative injury in the neurodegenerative process, but whether oxidative stress is a cause or a consequence of neurotoxicity remains unclear. We used a Drosophila model of human tauopathies to investigate the role of oxidative stress in neurodegeneration. Genetic and pharmacological manipulation of antioxidant defense mechanisms significantly modified neurodegeneration in our model, suggesting that oxidative stress plays a causal role in neurotoxicity. We demonstrate that the JNK signaling pathway is activated in our model, which is in agreement with previous findings in AD tissue. Furthermore, we show that the extent of JNK activation correlates with the degree of tau-induced neurodegeneration. Finally, our findings suggest that oxidative stress acts not to promote tau phosphorylation, but to enhance tau-induced cell cycle activation. In summary, our study identifies oxidative stress as a causal factor in tau-induced neurodegeneration in Drosophila.

Genetics of proliferative aging

Human lifespan is limited by aging of both mitotic and post-mitotic cells. These two forms of aging may occur by distinct or overlapping mechanisms. Telomere erosion has been shown to limit the proliferative lifespan of human somatic cells. Other vertebrates, such as mice, possess robust telomerase activity in most cell types and their somatic cells display finite replicative lifespans as a consequence of other forms of macromolecular damage. Genetic analysis in humans, mice and yeast has provided clues regarding pathways that may affect a cell's replicative lifespan. In addition, analysis of the means by which germ cells maintain their effervescent character may provide a deeper understanding of how replicative aging occurs in somatic cells.

Activation of superoxide dismutases: putting the metal to the pedal

Superoxide dismutases (SOD) are important anti-oxidant enzymes that guard against superoxide toxicity. Various SOD enzymes have been characterized that employ either a copper, manganese, iron or nickel co-factor to carry out the disproportionation of superoxide. This review focuses on the copper and manganese forms, with particular emphasis on how the metal is inserted in vivo into the active site of SOD. Copper and manganese SODs diverge greatly in sequence and also in the metal insertion process. The intracellular copper SODs of eukaryotes (SOD1) can obtain copper post-translationally, by way of interactions with the CCS copper chaperone. CCS also oxidizes an intrasubunit disulfide in SOD1. Adventitious oxidation of the disulfide can lead to gross misfolding of immature forms of SOD1, particularly with SOD1 mutants linked to amyotrophic lateral sclerosis. In the case of mitochondrial MnSOD of eukaryotes (SOD2), metal insertion cannot occur post-translationally, but requires new synthesis and mitochondrial import of the SOD2 polypeptide. SOD2 can also bind iron in vivo, but is inactive with iron. Such metal ion mis-incorporation with SOD2 can become prevalent upon disruption of mitochondrial metal homeostasis. Accurate and regulated metallation of copper and manganese SOD molecules is vital to cell survival in an oxygenated environment.

Absence of CuZn superoxide dismutase leads to elevated oxidative stress and acceleration of age-dependent skeletal muscle atrophy

We describe a novel phenotype in mice lacking the major antioxidant enzyme, CuZn-superoxide dismutase (Sod1(-/-) mice), namely a dramatic acceleration of age-related loss of skeletal muscle mass. Sod1(-/-) mice are 17 to 20% smaller and have a significantly lower muscle mass than wild-type mice as early as 3 to 4 months of age. Muscle mass in the Sod1(-/-) mice is further reduced with age and by 20 months, the hind-limb muscle mass in Sod1(-/-) mice is nearly 50% lower than in age-matched wild-type mice. Skeletal muscle tissue from young Sod1(-/-) mice has elevated oxidative damage to proteins, lipids, and DNA compared to muscle from young wild-type mice. The reduction in muscle mass and elevated oxidative damage are accompanied by a 40% decrease in voluntary wheel running by 6 months of age and decreased performance on the Rota-rod test at 13 months of age, but are not associated with a decline in overall spontaneous activity. In some of the old Sod1(-/-) mice, the loss in muscle mass is also associated with the presence of tremors and gait disturbances. Thus, the absence of CuZnSOD imposes elevated oxidative stress, loss of muscle mass, and physiological consequences that resemble an acceleration of normal age-related sarcopenia.

Selenium Supplementation Restores the Antioxidative Capacity and Prevents Cell Damage in Bone Marrow Stromal Cells In Vitro

Bone marrow stromal cells (BMSCs) and other cell populations derived from mesenchymal precursors are developed for cell-based therapeutic strategies and undergo cellular stress during ex vivo procedures. Reactive oxygen species (ROS) of cellular and environmental origin are involved in redox signaling, cumulative cell damage, senescence, and tumor development. Selenium-dependent (glutathione peroxidases [GPxs] and thioredoxin reductases [TrxRs]) and selenium-independent (superoxide dismutases [SODs] and catalase [CAT]) enzyme systems regulate cellular ROS steady state levels. SODs process superoxide anion to hydrogen peroxide, which is subsequently neutralized by GPx and CAT; TrxR neutralizes other ROS, such as peroxinitrite. Primary BMSCs and telomerase-immortalized human mesenchymal stem cells (hMSC-TERT) express GPx1-3, TrxR1, TrxR2, SOD1, SOD2, and CAT. We show here that in standard cell cultures (5%-10% fetal calf serum, 5-10 nM selenite), the activity of antioxidative selenoenzymes is impaired in hMSC-TERT and BMSCs. Under these conditions, the superoxide anion processing enzyme SOD1 is not sufficiently stimulated by an ROS load. Resulting oxidative stress favors generation of micronuclei in BMSCs. Supplementation of selenite (100 nM) restores basal GPx and TrxR activity, rescues basal and ROS-stimulated SOD1 mRNA expression and activity, and reduces ROS accumulation in hMSC-TERT and micronuclei generation in BMSCs. In conclusion, BMSCs in routine cell culture have low antioxidative capacity and are subjected to oxidative stress, as indicated by the generation of micronuclei. Selenite supplementation of BMSC cultures appears to be an important countermeasure to restore their antioxidative capacity and to reduce cell damage in the context of tissue engineering and transplantation procedures.

The effects of exogenous antioxidants on lifespan and oxidative stress resistance in Drosophila melanogaster

We used the fruit fly Drosophila melanogaster to test the effects of feeding the superoxide dismutase (SOD) mimetic drugs Euk-8 and -134 and the mitochondria-targeted mitoquinone (MitoQ) on lifespan and oxidative stress resistance of wild type and SOD-deficient flies. Our results reaffirm the findings by other workers that exogenous antioxidant can rescue pathology associated with compromised defences to oxidative stress, but fail to extend the lifespan of normal, wild type animals. All three drugs showed a dose-dependent increase in toxicity in wild type flies, an effect that was exacerbated in the presence of the redox-cycling drug paraquat. However, important findings from this study were that in SOD-deficient flies, where the antioxidant drugs increased lifespan, the effects were sex-specific and, for either sex, the effects were also variable depending on (1) the stage of development from which the drugs were given, and (2) the magnitude of the dose. These findings place significant constraints on the role of oxidative stress in normal ageing.

Oxidative damage and age-related functional declines

Most organisms experience progressive declines in physiological function as they age. Since this senescence of function is thought to underlie the decrease in quality of life in addition to the increase in susceptibility to disease and death associated with aging, identifying the mechanisms involved would be highly beneficial. One of the leading mechanistic theories for aging is the oxidative damage hypothesis. A number of studies in a variety of species support a strong link between oxidative damage and life span determination. The role of oxidative damage in functional senescence has also been investigated, albeit not as comprehensively. Here, we review these investigations. Several studies show that the age-related loss of a number of functions is associated with an accrual of oxidative damage in the tissues mediating those functions. Additionally, treatments that increase the accumulation of oxidative damage with age frequently exacerbate functional losses. Moreover, treatments that reduce the accumulation of oxidative damage often attenuate or delay the loss of function associated with aging. These data provide the foundation for a link between oxidative damage and functional senescence, thereby supporting the oxidative damage hypothesis of aging within the context of age-related functional decline.

The effect of dietary restriction on mitochondrial protein density and flight muscle mitochondrial morphology in Drosophila

Dietary restriction (DR) extends life span in diverse organisms and may do so by attenuating production of mitochondrial reactive oxygen species (ROS). However, measurements of ROS production from isolated mitochondria of organisms subjected to DR have produced inconsistent results. In the fruit fly Drosophila, DR does not reduce production of ROS from isolated mitochondria. In this study, we used Drosophila to test whether DR lowered mitochondrial density. We assessed mitochondrial densities of flies on DR and Control diets using (a) the activities of mitochondrial enzymes and (b) electron microscopy. Both methods showed no overall effect of DR on mitochondrial density; however, mitochondrial enzyme activities and morphology differed significantly between DR and Control flies. We concluded that life-span extension by DR in Drosophila is not mediated through a reduction in mitochondrial density. If DR in Drosophila extends life span by reducing ROS production, then it does so through mechanisms that operate only in vivo.

RNAi-mediated suppression of the mitochondrial iron chaperone, frataxin, in Drosophila

The mitochondrial iron chaperone, frataxin, plays a critical role in cellular iron homeostasis and the synthesis and regeneration of Fe-S centers. Genetic insufficiency for frataxin is associated with Friedreich's Ataxia in humans and confers loss of function of Fe-containing proteins including components of the respiratory chain and mitochondrial and cytosolic aconitases. Here, we report the use of RNA-interference (RNAi) to suppress frataxin in the multicellular eukaryote, Drosophila. Phenotypically, suppression of the Drosophila frataxin homologue (dfh) confers distinct phenotypes in larvae and adults, leading to giant long-lived larvae and to conditional short-lived adults. Deficiency of the DFH protein results in diminished activities of numerous heme- and iron-sulfur-containing enzymes, loss of intracellular iron homeostasis and increased susceptibility to iron toxicity. In parallel with the differential larval and adult phenotypes, our results indicate that dfh silencing differentially dysregulates ferritin expression in adults but not in larvae. Moreover, silencing of dfh in the peripheral nervous system, a specific focus of Friedreich's pathology, permits normal larval development but imposes a marked reduction in adult lifespan. In contrast, dfh silencing in motorneurons has no deleterious effect in either larvae or adults. Finally, overexpression of Sod1, Sod2 or Cat does not suppress the failure of DFH-deficient animals to successfully complete eclosion, suggesting a minimal role of oxidative stress in this phenotype. The robust developmental, biochemical and tissue-specific phenotypes conferred by DFH deficiency in Drosophila provide a platform for identifying genetic, nutritional and environmental factors, which ameliorate the symptoms arising from frataxin deficiency.

Genetic approaches to study aging in Drosophila melanogaster

The process of aging can be described as a progressive decline in an organism's function that invariably results in death. This decline results from the activities of intrinsic genetic factors within an organism. The relative contributions of the biological and environmental components to senescence are hard to measure, however different strategies have been devised in Drosophila melanogaster to isolate and identify genetic influences on aging. These strategies include selective breeding, quantitative trait loci (QTL) mapping and single gene mutant analysis. Selective breeding effectively demonstrated a genetic, heritable component to aging while QTL mapping located regions within the Drosophila genome carrying loci that influence the aging process. Within the past decade, single gene mutant analysis has facilitated the identification of specific genes whose activities play a determinative role in Drosophila aging. This review will focus on the application of selective breeding, QTL mapping and single gene mutant analysis used in Drosophila to study aging as well as the results obtained through these strategies to date.

Drosophila melanogaster Prat, a purine de novo synthesis gene, has a pleiotropic maternal-effect phenotype

In Drosophila melanogaster, two genes, Prat and Prat2, encode the enzyme, amidophosphoribosyltransferase, that performs the first and limiting step in purine de novo synthesis. Only Prat mRNA is present in the female germline and 0- to 2-hr embryos prior to the onset of zygotic transcription. We studied the maternal-effect phenotype caused by Prat loss-of-function mutations, allowing us to examine the effects of decreased purine de novo synthesis during oogenesis and the early stages of embryonic development. In addition to the purine syndrome previously characterized, we found that Prat mutant adult females have a significantly shorter life span and are conditionally semisterile. The semisterility is associated with a pleiotropic phenotype, including egg chamber defects and later effects on embryonic and larval viability. Embryos show mitotic synchrony and/or nuclear content defects at the syncytial blastoderm stages and segmentation defects at later stages. The semisterility is partially rescued by providing Prat mutant females with an RNA-enriched diet as a source of purines. Our results suggest that purine de novo synthesis is a limiting factor during the processes of cellular or nuclear proliferation that take place during egg chamber and embryonic development.

Manganese activation of superoxide dismutase 2 in the mitochondria of Saccharomyces cerevisiae

Manganese-dependent superoxide dismutase 2 (SOD2) in the mitochondria plays a key role in protection against oxidative stress. Here we probed the pathway by which SOD2 acquires its manganese catalytic cofactor. We found that a mitochondrial localization is essential. A cytosolic version of Saccharomyces cerevisiae Sod2p is largely apo for manganese and is only efficiently activated when cells accumulate toxic levels of manganese. Furthermore, Candida albicans naturally produces a cytosolic manganese SOD (Ca SOD3), yet when expressed in the cytosol of S. cerevisiae, a large fraction of Ca SOD3 also remained manganese-deficient. The cytosol of S. cerevisae cannot readily support activation of Mn-SOD molecules. By monitoring the kinetics for metalation of S. cerevisiae Sod2p in vivo, we found that prefolded Sod2p in the mitochondria cannot be activated by manganese. Manganese insertion is only possible with a newly synthesized polypeptide. Furthermore, Sod2p synthesis appears closely coupled to Sod2p import. By reversibly blocking mitochondrial import in vivo, we noted that newly synthesized Sod2p can enter mitochondria but not a Sod2p polypeptide that was allowed to accumulate in the cytosol. We propose a model in which the insertion of manganese into eukaryotic SOD2 molecules is driven by the protein unfolding process associated with mitochondrial import.