Oxidative stress and ageing in Caenorhabditis elegans

The nuclear hormone receptor DAF-12 has opposing effects on Caenorhabditis elegans lifespan and regulates genes repressed in multiple long-lived worms

The orphan nuclear hormone receptor gene daf-12 in Caenorhabditis elegans plays a key role in the regulation of development and determination of adult longevity. To understand the effects of daf-12 on aging we characterized the lifespan of loss-of-function and gain-of-function daf-12 alleles that have been identified on the basis of their effects on dauer development. We find that these mutations have opposing effects on longevity and resistance to oxidative and thermal stress which makes daf-12 the first gene with alleles that can extend or shorten lifespan. We find that the shortened lifespan of the loss-of-function mutation is due to accelerated aging in young adulthood rather than an adverse effect of the mutation on development. Microarray analysis of worms carrying the two alleles revealed a relatively small number of genes differentially expressed between the two genotypes. Comparison of the expression profiles with the profiles associated with dauer formation and long-lived daf-2 mutants revealed that while the profiles are largely different, there is significant overlap among the genes down-regulated, but not up-regulated, in all profiles. Several of these genes down-regulated in multiple long-lived worms have known effects on lifespan, and many of the genes belong to a family of poorly characterized genes that are strongly down-regulated in dauers, daf-2 mutants, and long-lived daf-12 mutants. Our results point to daf-12 modulating aging and stress responses in part through the repression of specific genes, and emphasize the role that the repression of genes that curtail maximal lifespan plays in lifespan determination.

Thioredoxin-ASK1 complex levels regulate ROS-mediated p38 MAPK pathway activity in livers of aged and long-lived Snell dwarf mice

We have proposed that the age-associated increase of reactive oxygen species (ROS) by electron transport chain (ETC) dysfunction may cause the elevated basal level of p38 MAPK stress response pathway activity. However, the mechanism by which ROS activates this pathway is not clear. Here we propose that activation of the p38 MAPK pathway by complex I (CI) generated ROS, in response to rotenone (ROT) treatment, is based on the ability of reduced Trx to bind to and inhibit ASK 1 and its release from the complex upon oxidation. This balance of free vs. bound ASK1 regulates the level of p38 MAPK pathway activity. To support this mechanism we demonstrate that the production of ROS by ROT treated AML12 hepatocyte cells dissociates the Trx-ASK1 complex, thereby increasing p38 MAPK pathway activity. This mechanism is supported by the ability of N-acetyl cysteine (NAC) to prevent dissociation of Trx-ASK1 and activation of the p38 MAPK pathway. We also demonstrated that the ratio of ASK1/Trx-ASK1 increases in aged mouse livers and that this correlates with the increased basal activity of the p38 MAPK pathway. The longevity of Snell dwarf mice has been attributed to their resistance to oxidative stress. A comparison of the levels of Trx-ASK1 in young and aged dwarfs showed a higher abundance of the complex than in their age-matched controls. These results, which are indicative of a decreased level of oxidative stress, suggest that increased ROS production in aged liver may alter the ratio of ASK1 and Trx-ASK1, thereby increasing the age-associated basal level of p38 MAPK pathway activity.

Beyond the evolutionary theory of ageing, from functional genomics to evo-gero

By the mid 1970s, the mechanisms by which ageing can evolve had a secure theoretical basis in population genetics. Here, we discuss how subsequent evolutionary work has focussed on testing and extending this theory, and on attempting to integrate it with other emerging facets of the biology of ageing, such as genetic studies of long-lived mutants and of phenotypic plasticity in ageing, such as in response to nutritional status. We also describe how functional genomic studies are providing new insights into the evolutionary forces shaping genome evolution and lifespan control. Future challenges include understanding the biochemistry of longevity and how its failure generates ageing and associated diseases, and the determination of the genetic basis of lifespan evolution and the great plasticity that it displays.

Aging-dependent and -independent modulation of associative learning behavior by insulin/insulin-like growth factor-1 signal in Caenorhabditis elegans

Mutations in the insulin/IGF-1 neuroendocrine pathway extend lifespan and affect development, metabolism, and other biological processes in Caenorhabditis elegans and in other species. In addition, they may play a role in learning and memory. Investigation of the insulin/IGF-1 pathway may provide clues for the prevention of age-related declines in cognitive functions. Here, we examined the effects of the life-extending (Age) mutations, such as the age-1 (phosphatidylinositol 3-OH kinase) and daf-2 (insulin/IGF-1 receptor) mutations, on associative learning behavior called isothermal tracking. This thermotaxis learning behavior associates paired stimuli, temperature, and food. The age-1 mutation delayed the age-related decline of isothermal tracking, resulting in a 210% extension of the period that ensures it. The effect is dramatic compared with the extension of other physiological health spans. In addition, young adults of various Age mutants (age-1, daf-2, clk-1, and eat-2) showed increased consistency of temperature-food association, which may be caused by a common feature of the mutants, such as the secondary effects of life extension (i.e., enhanced maintenance of neural mechanisms). The age-1 and daf-2 mutants but not the other Age mutants showed an increase in temperature-starvation association through a different mechanism. Increased temperature-food association of the daf-2 mutant was dependent on neuronal Ca2+-sensor ncs-1, which modulates isothermal tracking in the AIY interneuron. Interestingly, mutations in the daf-7 TGFbeta gene, which functions in parallel to the insulin/IGF-1 pathway, caused deficits in acquisition of temperature-food and temperature-starvation association. This study highlights roles of the Age mutations in modulation of certain behavioral plasticity.

Diapause-associated metabolic traits reiterated in long-lived daf-2 mutants in the nematode Caenorhabditis elegans

The longevity of the Caenorhabditis elegans diapausal dauer larva greatly exceeds that of the adult. Dauer formation and adult ageing are both regulated by insulin/IGF-1 signaling (IIS). Reduced IIS, e.g. by mutation of the daf-2 insulin/IGF-1 receptor gene, increases adult lifespan. This may reflect mis-expression in the adult of dauer longevity-assurance processes. Since IIS plays a central role in the regulation of metabolism, metabolic alterations shared by dauer larvae and daf-2 adults represent candidate mechanisms for lifespan determination. We have conducted a detailed comparison of transcript profile data from dauers and daf-2 mutant adults, focusing on expression of metabolic pathway genes. Our results imply up-regulation in both dauers and daf-2 mutant adults of gluconeogenesis, glyoxylate pathway activity, and trehalose biosynthesis. Down-regulation of the citric acid cycle and mitochondrial respiratory chain occurs in dauers, but not daf-2 adults. However, the F(1) ATPase inhibitor was up-regulated in both, implying enhanced homeostasis in conditions where mitochondria are stressed. Overall, the data implies increased conversion of fat to carbohydrate, and conservation of ATP stocks in daf-2 mutant adults, suggesting a state of increased energy availability. We postulate that this fuels increased somatic maintenance activity, as suggested by the disposable soma theory.

Long-term starvation and ageing induce AGE-1/PI 3-kinase-dependent translocation of DAF-16/FOXO to the cytoplasm

Background

The provision of stress resistance diverts resources from development and reproduction and must therefore be tightly regulated. In Caenorhabditis elegans, the switch to increased stress resistance to promote survival through periods of starvation is regulated by the DAF-16/FOXO transcription factor. Reduction-of-function mutations in AGE-1, the C. elegans Class IA phosphoinositide 3-kinase (PI3K), increase lifespan and stress resistance in a daf-16 dependent manner. Class IA PI3Ks downregulate FOXOs by inducing their translocation to the cytoplasm. However, the circumstances under which AGE-1 is normally activated are unclear. To address this question we used C. elegans first stage larvae (L1s), which when starved enter a developmentally-arrested diapause stage until food is encountered.

Results

We find that in L1s both starvation and daf-16 are necessary to confer resistance to oxidative stress in the form of hydrogen peroxide. Accordingly, DAF-16 is localised to cell nuclei after short-term starvation. However, after long-term starvation, DAF-16 unexpectedly translocates to the cytoplasm. This translocation requires functional age-1. H₂O₂ treatment can replicate the translocation and induce generation of the AGE-1 product PIP₃. Because feeding reduces to zero in ageing adult C. elegans, these animals may also undergo long-term starvation. Consistent with our observation in L1s, DAF-16 also translocates to the cytoplasm in old adult worms in an age-1-dependent manner.

Conclusion

DAF-16 is activated in the starved L1 diapause. The translocation of DAF-16 to the cytoplasm after long-term starvation may be a feedback mechanism that prevents excessive expenditure on stress resistance. H₂O₂ is a candidate second messenger in this feedback mechanism. The lack of this response in age-1(hx546) mutants suggests a novel mechanism by which this mutation increases longevity.

Endocrine targets for pharmacological intervention in aging in Caenorhabditis elegans

Studies in the nematode Caenorhabditis elegans have been instrumental in defining genetic pathways that are involved in modulating lifespan. Multiple processes such as endocrine signaling, nutritional sensing and mitochondrial function play a role in determining lifespan in the worm and these mechanisms appear to be conserved across species. These discoveries have identified a range of novel targets for pharmacological manipulation of lifespan and it is likely that the nematode model will now prove useful in the discovery of compounds that slow aging. This review will focus on the endocrine targets for intervention in aging and the use of C. elegans as a system for high throughput screens of compounds for their effects on aging.

Energy, quiescence and the cellular basis of animal life spans

Animals are routinely faced with harsh environmental conditions in which insufficient energy is available to grow and reproduce. Many animals adapt to this challenge by entering a dormant, or quiescent state. In some animals, such as the nematode Caenorhabditis elegans, quiescence is coincident with increased stress resistance and longevity. Here we review evidence that the rules of life span extension established in C. elegans may be generally true of most animals. That is, that the rate of animal aging correlates inversely with cellular resistance to physiological stress, particularly oxidative stress, and that stress resistance is co-regulated with the quiescence adaptation (where the latter occurs). We discuss evidence for highly conserved intracellular signalling pathways involved in energy sensing that are sensitive to aspects of mitochondrial energy transduction and can be modulated in response to energetic flux. We provide a broad overview of the current knowledge of the relationships between energy, metabolism and life span.

Dietary restriction in C. elegans: from rate-of-living effects to nutrient sensing pathways

The nematode Caenorhabditis elegans has been subjected to dietary restriction (DR) by a number of means, with varying results in terms of fecundity and lifespan. Two possible mechanisms by which DR increases lifespan are reduction of metabolic rate and reduction of insulin/IGF-1 signalling. Experimental tests have not supported either possibility. However, interaction studies suggest that DR and insulin/IGF-1 signalling may act in parallel on common regulated processes. In this review, we discuss recent developments in C. elegans DR research, including new discoveries about the biology of nutrient uptake in the gut, and the importance of invasion by the bacterial food source as a determinant of lifespan. The evidence that the effect of DR on lifespan in C. elegans is mediated by the TOR pathway is discussed. We conclude that the effect of DR on lifespan is likely to involve multiple mechanisms, which may differ according to the DR regimen used and the organism under study.

The C. elegans ortholog of mammalian Ku70, interacts with insulin-like signaling to modulate stress resistance and life span

The mammalian Ku heterodimer has important roles in DNA double strand break repair, telomere maintenance, cell cycle checkpoint-arrest, tumor suppression, and cellular stress resistance. To investigate the evolutionarily conserved functions of Ku, we knocked down expression by RNA interference (RNAi) of Ku genes in C. elegans. We found that C. elegans Ku70 (CKU-70) is required for resistance to genotoxic stress, regulates cytotoxic stress responses, and influences aging. The latter effects are dependent on an IGF-1/insulin-like signaling pathway previously shown to affect life span. Reduction of CKU-70 activity amplifies the aging phenotype of long-lived insulin receptor daf-2 mutations in a daf-16-dependent manner. These observations support the view that organismal stress resistance determines life span and Ku70 modulates these effects.

Fibroblast cell lines from young adult mice of long-lived mutant strains are resistant to multiple forms of stress

Previous studies have shown that dermal fibroblast cell lines derived from young adult mice of the long-lived Snell dwarf mutant stock are resistant, in vitro, to the cytotoxic effects of H(2)O(2), cadmium, UV light, paraquat, and heat. We show here that similar resistance profiles are seen in fibroblast cells derived from a related mutant, the Ames dwarf mouse, and that cells from growth hormone receptor-null mice are resistant to H(2)O(2), paraquat, and UV but not to cadmium. Resistance to UV light, cadmium, and H(2)O(2) are similar in cells derived from 1-wk-old Snell dwarf or normal mice, and thus the resistance of cell lines derived from young adult donors reflects developmental processes, presumably hormone dependent, that take place in the first few months of life. The resistance of cells from Snell dwarf mice to these stresses does not reflect merely antioxidant defenses: dwarf-derived cells are also resistant to the DNA-alkylating agent methyl methanesulfonate. Furthermore, inhibitor studies show that fibroblast resistance to UV light is unaffected by the antioxidants ascorbic acid and N-acetyl-L-cysteine. These data suggest that postnatal exposure to altered levels of pituitary hormones leads to development of cellular resistance to oxidative and nonoxidative stressors, which are stable through many rounds of in vitro cell division and could contribute to the remarkable disease resistance of long-lived mutant mice.

Longevity genes: from primitive organisms to humans

Recent results indicate that the longevity of both invertebrates and vertebrates can be altered through genetic manipulation and pharmacological intervention. Most of these interventions involve alterations of one or more of the following: insulin/IGF-I signaling pathway, caloric intake, stress resistance and nuclear structure. How longevity regulation relates to aging per se is less clear, but longevity increases are usually accompanied by extended periods of good health. How these results will translate to primate aging and longevity remains to be shown.

The effects of aging and oxidative stress on learning behavior in C. elegans

Oxidative stress is associated with age-related declines of biological functions. However, the nervous system is preserved during aging in Caenorhabditis elegans and, thus, it is not well explored whether aging and oxidative stress affect nervous functions. Here we report that age-related decline can be observed in a type of associative-learning behavior, referred to as isothermal tracking. We also report the effects of mutants with altered sensitivity to oxidative stress on learning behavior and motor activity in young adults. The isp-1 and clk-1 mutants are members of the Clk class of mutants and have deficits in the function of the mitochondrial respiratory chain, leading to reduced levels of oxidative stress, increased longevity, delayed rhythmic behaviors and other phenotypes. Both the Clk mutations and pretreatment with a metabolic antioxidant, alpha-lipoic acid (LA), increased the ability to show isothermal tracking and modestly reduced motor activity. Mutants with increased oxidative stress showed severely impaired learning behavior and modestly reduced motor activity. Therefore, physiological levels of oxidative stress may be too high for learning behavior but, perhaps, not for motor activity. We discuss the relevance of oxidative stress to the aging and evolution of behaviors.

Analysis of long-lived C. elegans daf-2 mutants using serial analysis of gene expression

We have identified longevity-associated genes in a long-lived Caenorhabditis elegans daf-2 (insulin/IGF receptor) mutant using serial analysis of gene expression (SAGE), a method that efficiently quantifies large numbers of mRNA transcripts by sequencing short tags. Reduction of daf-2 signaling in these mutant worms leads to a doubling in mean lifespan. We prepared C. elegans SAGE libraries from 1, 6, and 10-d-old adult daf-2 and from 1 and 6-d-old control adults. Differences in gene expression between daf-2 libraries representing different ages and between daf-2 versus control libraries identified not only single genes, but whole gene families that were differentially regulated. These gene families are part of major metabolic pathways including lipid, protein, and energy metabolism, stress response, and cell structure. Similar expression patterns of closely related family members emphasize the importance of these genes in aging-related processes. Global analysis of metabolism-associated genes showed hypometabolic features in mid-life daf-2 mutants that diminish with advanced age. Comparison of our results to recent microarray studies highlights sets of overlapping genes that are highly conserved throughout evolution and thus represent strong candidate genes that control aging and longevity.

Metabolism, physiology and stress defense in three aging Ins/IGF-1 mutants of the nematode Caenorhabditis elegans

The insulin/insulin-like growth factor-1 (Ins/IGF-1) pathway regulates the aging rate of the nematode Caenorhabditis elegans. We describe other features of the three Ins/IGF-1 mutants daf-2, age-1 and aap-1. We show that the investigated Ins/IGF-1 mutants all have a reduced body volume, reduced reproductive capacity, increased ATP concentrations and an elevated stress resistance. We also observed that heat production is lower in these mutants, although the respiration rate was similar or higher compared with wild-type individuals, suggesting a metabolic shift in these mutants.

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?

The role of oxidative damage and stress in aging

The Free Radical/Oxidative Stress Theory of Aging, which was first proposed in 1956, is currently one of the most popular explanations for how aging occurs at the biochemical/molecular level. However, most of the evidence in support of this theory is correlative, e.g., oxidative damage to various biomolecules increases with age, and caloric restriction, which increases life span and retards aging, reduces the age-related increase in oxidative damage to biomolecules. The most direct test of the Free Radical/Oxidative Stress Theory of Aging is to specifically alter the age-related increase in oxidative damage and determine how this alteration affects life span. For the first time, investigators can use genetically altered animals to test directly the role of oxidative damage in aging. In this manuscript, we critically review the past research in this area and discuss potential future research directions in testing the Free Radical/Oxidative Theory of Aging.

Antioxidant function of a novel selenoprotein in Drosophila melanogaster

BACKGROUND

Insects appear to have diverged from both higher and lower organisms in their defense mechanisms against oxidative damage. They do not encode glutathione peroxidases or glutathione reductases, and their thioredoxin reductases exhibit distinct properties from those of higher and lower species. Nonetheless, appropriate balance of anti-oxidants and pro-oxidants, and protection from damaging reactive oxygen species are clearly crucial in insects for viability, normal functioning of signalling pathways and morphogenesis, and have been implicated in studies on longevity in flies and other organisms.

RESULTS

Two novel selenoproteins, dselH and dselK, were recently identified in Drosophila melanogaster. We have used RNAi in D. melanogaster embryos and in Schneider S2 cells to inhibit expression of these proteins. We report that inhibition of either dselH or dselK expression significantly reduces viability in embryos. We further show that dselH silencing decreases total anti-oxidant capacity in embryos and Schneider cells, and increases lipid peroxidation in cells. Conversely, transient expression of dselH in the cell line decreases lipid peroxidation, and reverses the toxic effects of a glutathione-depleting drug. The latter correlates with sparing of glutathione levels.

CONCLUSIONS

These studies suggest that the well-known role of selenoproteins in vertebrate anti-oxidant defenses also extends to include invertebrates.

Ticking fast or ticking slow, through Shc must you go?

Circumstantial evidence places the p66 isoform of the adapter protein Shc in a position to mediate the accelerated aging phenotype displayed by mice expressing shortened forms of the tumor suppressor protein p53. We present a model in which p66(shc) may be responsible for integrating signals from the p53 pathway with signals from the insulin-like growth factor-1/Daf pathway in mammals. A full understanding of how interactions between p53 and p66(shc) affect longevity will require the production of animals with mutations in the genes encoding both proteins.

Shared transcriptional signature in Caenorhabditis elegans Dauer larvae and long-lived daf-2 mutants implicates detoxification system in longevity assurance

In the nematode Caenorhabditis elegans, formation of the long-lived dauer larva and adult aging are both controlled by insulin/insulin-like growth factor-1 signaling. Potentially, increased adult life span in daf-2 insulin/insulin-like growth factor-1 receptor mutants results from mis-expression in the adult of a dauer larva longevity program. By using oligonucleotide microarray analysis, we identified a dauer transcriptional signature in daf-2 mutant adults. By means of a nonbiased statistical approach, we identified gene classes whose expression is altered similarly in dauers and daf-2 mutants, which represent potential determinants of life span. These include known determinants of longevity; the small heat shock protein/alpha-crystallins are up-regulated in both milieus. The cytochrome P450, short-chain dehydrogenase/reductase, UDP-glucuronosyltransferase, and glutathione S-transferase (in daf-2 mutants) gene classes were also up-regulated. These four gene classes act together in metabolism and excretion of toxic endobiotic and xenobiotic metabolites. This suggests that diverse toxic lipophilic and electrophilic metabolites, disposed of by phase 1 and phase 2 drug metabolism, may be the major determinants of the molecular damage that causes aging. In addition, we observed down-regulation of genes linked to nutrient uptake, including nhx-2 and pep-2. These work together in the uptake of dipeptides in the intestine, implying dietary restriction in daf-2 mutants. Some gene groups up-regulated in dauers and/or daf-2 were enriched for certain promoter elements as follows: the daf-16-binding element, the heat shock-response element, the heat shock-associated sequence, or the hif-1-response element. By contrast, the daf-16-associated element was enriched in genes down-regulated in dauers and daf-2 mutants. Thus, particular promoter elements appear longevity-associated or aging associated.

Deletion of the Intestinal Peptide Transporter Affects Insulin and TOR Signaling in Caenorhabditis elegans

The mammalian intestinal peptide transporter PEPT1 mediates the uptake of di- and tripeptides from the gut lumen into intestinal epithelial cells and acts in parallel with amino acid transporters. Here we address the importance of the PEPT1 orthologue PEP-2 for the assimilation of dietary protein and for overall protein nutrition in Caenorhabditis elegans. pep-2 is expressed specifically along the apical membrane of the intestinal cells, and in pep-2 deletion mutant animals, uptake of intact peptides from the gut lumen is abolished. The consequences are a severely retarded development, reduced progeny and body size, and increased stress tolerance. We show here that pep-2 cross-talks with both the C. elegans target of rapamycin (TOR) and the DAF-2/insulin-signaling pathways. The pep-2 mutant enhances the developmental and longevity phenotypes of daf-2, resulting, among other effects, in a pronounced increase in adult life span. Moreover, all aspects of a weak let-363/TOR RNA interference phenotype are intensified by pep-2 deletion, indicating that pep-2 function upstream of TOR-mediated nutrient sensing. Our findings provide evidence for a predominant role of the intestinal peptide transporter for the delivery of bulk quantities of amino acids for growth and development, which consequently affects signaling pathways that regulate metabolism and aging.

Altered oxidative stress response of the long-lived Snell dwarf mouse

Several single gene mutations in mice that increase the murine life span have been identified, including the Pit-1 mutation which results in the Snell dwarf (Pit1(dw/dw)), however, the biological mechanism of this life-span extension is still unclear. Based on studies that show oxidative stress plays an important role in the aging process, we hypothesized that the increased longevity seen in Snell dwarf mice may result from a resistance to oxidative stress. We report that Snell dwarf mice respond to oxidative stress induced by 3-NPA differently than their wild type littermates. This altered response results in diminished activation of the MEK-ERK kinase cascade and virtually no phosphorylation of c-Jun at Ser63 in dwarf mice after 3-NPA treatment, despite a robust phosphorylation of Ser63 in wild type mice. We propose that this altered management of oxidative stress in dwarf mice is partially responsible for the increased longevity in Snell dwarf mice.

Isolation of long-lived mutants in Caenorhabditis elegans using selection for resistance to juglone

The accumulation of molecular damage from attack by reactive oxygen species is one cause of aging. Therefore, some mutant organisms showing increased resistance to reactive oxygen species should live longer. We show that selection for Caenorhabditis elegans mutants that are resistant to juglone, a reactive oxygen species-generating compound, leads to the identification of long-lived mutants. Indeed, four of six resistant mutants isolated were also long-lived. This study illustrates once more the strong relationship between oxidative stress and the aging processes.

Superoxide dismutase mimetics elevate superoxide dismutase activity in vivo but do not retard aging in the nematode Caenorhabditis elegans

According to the oxidative damage theory a primary cause of aging is the accrual of molecular damage from reactive oxygen species (ROS), particularly superoxide and its derivatives. This predicts that treatments that reduce ROS levels should retard aging. Using the nematode Caenorhabditis elegans, we tested the effects on stress resistance and life span of treatment with EUK-8 and EUK-134, synthetic mimetics of the antioxidant enzyme superoxide dismutase (SOD), which neutralises superoxide. Treatment with SOD mimetics elevated in vivo SOD activity levels, particularly in mitochondria, where up to 5-fold increases in SOD activity were recorded. Treatment with exogenous SOD mimetics did not affect endogenous protein SOD levels. Where life span was reduced by the superoxide generators paraquat and plumbagin, EUK-8 treatment increased life span in a dose-dependent fashion. Yet in the absence of a superoxide generator, treatment with EUK-8 or EUK-134 did not increase life span, even at doses that were optimal for protection against pro-oxidants. Thus, an elevation of SOD activity levels sufficient to increase life span when it is limited by superoxide generators does not retard aging in the absence of superoxide generators. This suggests that C. elegans life span is not normally limited by levels of superoxide and its derivatives.

Understanding aging: revealing order out of chaos

Aging is often described as an extremely complex process affecting all of the vital parameters of an individual. In this article, we review how understanding of aging evolved from the first analyses of population survival to the identification of the molecular mechanisms regulating life span. Abundant evidence implicates mitochondria in aging and we focus on the three main components of the mitochondrial theory of aging: (1) increased reactive oxygen species (ROS) production, (2) mitochondrial DNA (mtDNA) damage accumulation, and (3) progressive respiratory chain dysfunction. Experimental evidence shows a relationship between respiratory chain dysfunction, ROS damage, and aging in most of the model organisms. However, involvement of the mtDNA mutations in the aging process is still debated. We recently created a mutant mouse strain with increased levels of somatic mtDNA mutations causing a progressive respiratory chain deficiency and premature aging. These mice demonstrate the fundamental importance of the accumulation of mtDNA alterations in aging. We present here an integrative model where aging is provoked by a single primary event leading to a variety of effects and secondary causes.

S.W.A.T.--SOD weapons and tactics

The accumulation of damage caused by reactive oxygen species (ROS) is held to be one of the underlying causes of age-related decline and has been shown to be involved in a number of pathological states. Inherent defense mechanisms have evolved to limit this damage by reducing the levels of ROS, which are produced mainly by the mitochondria in aerobic organisms. One such defense is superoxide dismutase 1 (SOD1). It is well established that oxidative stress results in increased transcription and translation of the SOD1 gene, but it is now known that an additional level of posttranslational control exists. A recent paper describes the presence of an inactive pool of SOD1 whose activation is wholly reliant on the presence of superoxide or oxygen and a specific copper-containing chaperone. This mechanism highlights the importance of rapid responses in the fight against oxidative stress.

Germ-line versus somatic cells. I. Stereological study of differentiating embryonic tissues of Tetrodontophora bielanensis (Hexapoda, Collembola)

We examined five different somatic tissues and compared them with germ-line cells to verify the "disposable soma" theory. Two embryonic stages and second-stage juveniles of Tetrodontophora bielanensis Waga, 1842 (Hexapoda, Collembola) were studied. Our results show that changes in relative volume of mitochondria during differentiation of cells correlate well with transformations of cell morphology. During morphological transformation of differentiating somatic cells, the relative volume of mitochondria in their cytoplasm is high, whereas in the differentiated tissues, this parameter is much lower. Surprisingly, the highest value of relative volume density of mitochondria is found in the cytoplasm of germ-line cells. If we accept that this parameter indicates the cell metabolism rate, then our results should be taken as supporting the "disposable soma" theory. It is also conceivable that the higher volume of mitochondria in the germ-line cells have nothing to do with energy production but, for instance, with the production or function of nuage material (germ-cell determinant) in the germ-line cells. These two possibilities are discussed.

Comparative genomics of gene expression in the parasitic and free-living nematodes Strongyloides stercoralis and Caenorhabditis elegans

Although developmental timing of gene expression is used to infer potential gene function, studies have yet to correlate this information between species. We analyzed 10,921 ESTs in 3311 clusters from first- and infective third-stage larva (L1, L3i) of the parasitic nematode Strongyloides stercoralis and compared the results to Caenorhabditis elegans, a species that has an L3i-like dauer stage. In the comparison of S. stercoralis clusters with stage-specific expression to C. elegans homologs expressed in either dauer or nondauer stages, matches between S. stercoralis L1 and C. elegans nondauer-expressed genes dominated, suggesting conservation in the repertoire of genes expressed during growth in nutrient-rich conditions. For example, S. stercoralis collagen transcripts were abundant in L1 but not L3i, a pattern consistent with C. elegans collagens. Although a greater proportion of S. stercoralis L3i than L1 genes have homologs among the C. elegans dauer-specific transcripts, we did not uncover evidence of a robust conserved L3i/dauer 'expression signature.' Strikingly, in comparisons of S. stercoralis clusters to C. elegans homologs with RNAi knockouts, those with significant L1-specific expression were more than twice as likely as L3i-specific clusters to match genes with phenotypes. We also provide functional classifications of S. stercoralis clusters.

Lack of peroxisomal catalase causes a progeric phenotype in Caenorhabditis elegans

Studies using the nematode Caenorhabditis elegans as a model system to investigate the aging process have implicated the insulin/insulin-like growth factor-I signaling pathway in the regulation of organismal longevity through its action on a subset of target genes. These targets can be classified into genes that shorten or extend life-span upon their induction. Genes that shorten life-span include a variety of stress response genes, among them genes encoding catalases; however, no evidence directly implicates catalases in the aging process of nematodes or other organisms. Using genetic mutants, we show that lack of peroxisomal catalase CTL-2 causes a progeric phenotype in C. elegans. Lack of peroxisomal catalase also affects the developmental program of C. elegans, since Deltactl-2 mutants exhibit decreased egg laying capacity. In contrast, lack of cytosolic catalase CTL-1 has no effect on either nematode aging or egg laying capacity. The Deltactl-2 mutation also shortens the maximum life-span of the long lived Deltaclk-1 mutant and accelerates the onset of its egg laying period. The more rapid aging of Deltactl-2 worms is apparently not due to increased carbonylation of the major C. elegans proteins, although altered peroxisome morphology in the Deltactl-2 mutant suggests that changes in peroxisomal function, including increased production of reactive oxygen species, underlie the progeric phenotype of the Deltactl-2 mutant. Our findings support an important role for peroxisomal catalase in both the development and aging of C. elegans and suggest the utility of the Deltactl-2 mutant as a convenient model for the study of aging and the human diseases acatalasemia and hypocatalasemia.

Tissue-specific activities of C. elegans DAF-16 in the regulation of lifespan

In C. elegans, the transcription factor DAF-16 promotes longevity in response to reduced insulin/IGF-1 signaling or germline ablation. In this study, we have asked how different tissues interact to specify the lifespan of the animal. We find that several tissues act as signaling centers. In particular, DAF-16 activity in the intestine, which is also the animal's adipose tissue, completely restores the longevity of daf-16(-) germline-deficient animals, and increases the lifespans of daf-16(-) insulin/IGF-1-pathway mutants substantially. Our findings indicate that DAF-16 may control two types of downstream signals: DAF-16 activity in signaling cells upregulates DAF-16 in specific responding tissues, possibly via regulation of insulin-like peptides, and also evokes DAF-16-independent responses. We suggest that this network of tissue interactions and feedback regulation allows the tissues to equilibrate and fine-tune their expression of downstream genes, which, in turn, coordinates their rates of aging within the animal.

Life extension in the dwarf mouse

Ames dwarf mice and Snell dwarf mice lack growth hormone (GH), prolactin (PRL), and thyroid-stimulating hormone (TSH), live much longer than their normal siblings, and exhibit many symptoms of delayed aging. "Laron dwarf mice," produced by targeted disruption of the GH receptor/GH-binding protein gene (GHR-KO mice), are GH resistant and also live much longer than normal animals from the same line. Isolated GH deficiency in "little" mice is similarly associated with increased life span, provided that obesity is prevented by reducing fat content in the diet. Long-lived dwarf mice share many phenotypic characteristics with genetically normal (wild-type) animals subjected to prolonged caloric restriction (CR) but are not CR mimetics. We propose that mechanisms linking GH deficiency and GH resistance with delayed aging include reduced hepatic synthesis of insulin-like growth factor 1 (IGF-1), reduced secretion of insulin, increased hepatic sensitivity to insulin actions, reduced plasma glucose, reduced generation of reactive oxygen species, improved antioxidant defenses, increased resistance to oxidative stress, and reduced oxidative damage. The possible role of hypothyroidism, reduced body temperature, reduced adult body size, delayed puberty, and reduced fecundity in producing the long-lived phenotype of dwarf mice remains to be evaluated. An important role of IGF-1 and insulin in the control of mammalian longevity is consistent with the well-documented actions of homologous signaling pathways in invertebrates.

Reduced free-radical production and extreme longevity in the little brown bat (Myotis lucifugus) versus two non-flying mammals

The extended longevity of bats, despite their high metabolic rate, may provide insight to patterns and mechanisms of aging. Here I test predictions of the free radical or oxidative stress theory of aging as an explanation for differences in lifespan between the little brown bat, Myotis lucifugus (maximum lifespan potential MLSP=34 years), the short-tailed shrew, Blarina brevicauda (MLSP=2 years), and the white-footed mouse, Peromyscus leucopus (MLSP=8 years) by comparing whole-organism oxygen consumption, hydrogen peroxide production, and superoxide dismutase activity in heart, kidney, and brain tissue. Mitochondria from M. lucifugus produced half to one-third the amount of hydrogen peroxide per unit of oxygen consumed compared to mitochondria from B. brevicauda and P. leucopus, respectively. Superoxide dismutase (SOD) activity did not differ among the three species. These results are similar to those found for birds, which like bats have high metabolic rates and extended longevities, and provide support for the free radical theory of aging as an at least partial explanation for the extreme longevity of bats.

Oxidative stress in Caenorhabditis elegans: protective effects of superoxide dismutase/catalase mimetics

The lifespan of Caenorhabditis elegans can be extended by the administration of synthetic superoxide dismutase/ catalase mimetics (SCMs) without any effects on development or fertility. Here we demonstrate that the mimetics, Euk-134 and Euk-8, confer resistance to the oxidative stress-inducing agent, paraquat and to thermal stress. The protective effects of the compounds are apparent with treatments either during development or during adulthood and are independent of an insulin/IGF-I-like signalling pathway also known to affect thermal and oxidative stress resistance. Worms exposed to the compounds do not induce a cellular stress response and no detrimental effects are observed.

An automated high-throughput assay for survival of the nematode Caenorhabditis elegans

Many genetic or environmental manipulations that extend life span in the nematode Caenorhabditis elegans (C. elegans) also enhance survival following acute stresses such as oxidative damage and thermal stress. This coupling of stress response and aging mechanisms has proved a useful tool in identifying new genes that affect the aging process without the need for performing lengthy life span analyses. Therefore, it is likely that this approach may also be applied to the identification of pharmacological agents that extend life span through enhanced resistance to oxygen radicals or other stressors. To facilitate high-throughput drug screens in the nematode, we have developed a microtitre plate survival assay that uses uptake of the fluorescent dye SYTOX green as a marker of nematode death. An increase in throughput compared with the conventional survival assay was achieved by combining automated worm-handling technology with automated real-time fluorescence detection. We have validated this assay by examining survival during acute heat stress and protection against oxidative stress with the superoxide dismutase/catalase mimetic Euk-134. We propose that this novel method of survival analysis will accelerate the discovery of new pharmacological interventions in aging and oxidative stress.

Protein oxidation in aging: endoplasmic reticulum as a target

Oxidatively modified proteins have been shown to correlate with the age of an organism or its tissues. An increase in tissue-susceptibility to experimentally induced protein oxidation not only depends on tissue type and age, but also on the maximum lifespan potential of the species. A general, although tissue dependent, decline in anti-oxidative defenses during aging may very well be responsible for this difference in vulnerability. In addition, the level of protein modifications also depends on the nature and the subcellular localization of the proteins involved. Damage to the endoplasmic reticulum (ER), and its subsequent impaired functionality may be involved in the process of aging. This is suggested by; (1) an upregulation of ER stress-response chaperones, (2) a preferential oxidation of ER-resident proteins and, (3) a disturbance of calcium homeostasis. Therefore, this review will focus on the putative involvement of the oxidized endoplasmic reticulum in the process of aging.

Multiplex stress resistance in cells from long-lived dwarf mice

Mutations that extend nematode longevity by interference with IGF-I/insulin sensing pathways also lead to resistance to multiple forms of stress. Here, we report that skin-derived fibroblasts from Snell dwarf mice, already known to show increased longevity and delayed aspects of aging, are resistant to multiple forms of cellular stress, including UV light, heat, paraquat, H2O2, and the toxic metal cadmium. The findings suggest that increases in cellular resistance to stress may mediate extended longevity in mammals.

Does anti-aging equal anti-microbial?

Aging is the dominant risk factor for human disease in developed countries. Could it be that a wide variety of disease states all have their origins in a common mechanism? Major signaling pathways that determine the rate of aging, such as the insulin/insulin-like growth factor 1 (IGF-1) pathway, might give clues to the nature of this major disease risk factor. It has now been shown that insulin/IGF-1 signaling influences Caenorhabditis elegans resistance to bacteria in such a way that long-lived worms are stress-resistant and slow to succumb to infection. Perhaps enhanced innate immunity is a feature of genetically determined longevity.

Decline of nuclear and mitochondrial oxidative base excision repair activity in late passage human diploid fibroblasts

There are numerous studies documenting the increase of oxidative DNA damage in the nuclei and mitochondria of senescing cells as well as in tissues of aging animals. Here, we show that in IMR 90 human diploid fibroblasts, DNA repair activity is robust in both nuclear and mitochondrial extracts, however, the levels of activity differed against the three substrates tested. In extracts, cleavage of the 8-oxoguanine substrate, and to a lesser extent the dihydrouracil-containing substrate, occurred in a concerted reaction between the DNA glycosylases and the second enzyme in the reaction, hAPE. Cleavage of both the furan and the dihydrouracil-containing substrates was unchanged when nuclear extracts from early and late passage cells were compared. However, cleavage of the 8-oxoguanine substrate was substantially reduced in the nuclear extracts from late passage cells and significantly reduced transcription from the hOGG1 gene was observed. When mitochondrial extracts were examined, activity on all three substrates was significantly reduced, with the reduction in hAPE activity being the most marked. The reduction in cleavage of the furan substrate was not simply due to inactive mitochondrial AP endonuclease but a substantially reduced amount of hAPE protein; transcription from the hAPE gene was also reduced. Confocal microscopic analysis confirmed that hAPE was present in the mitochondria of early passage cells but greatly reduced in the mitochondria of late passage cells. Cytoplasmic extracts from late passage fibroblasts also showed reduced activity with all three substrates suggesting that the residual hAPE, and activities that recognized dihydrouracil, were preferentially targeted to the nuclei. Taken together the data support the concept that the increase in oxidative damage in the mitochondrial DNA of senescing cells and tissues from aging animals is due to reduced base excision repair activity.

A stress-responsive glutathione S-transferase confers resistance to oxidative stress in Caenorhabditis elegans

Previous studies demonstrated that the Caenorhabditis elegans GST-p24 is upregulated at the steady state mRNA level in response to oxidative stress. A transcriptional upregulation was confirmed in the current study by analyzing Ce-GST-p24 promoter-reporter constructs in transgenic C. elegans strains CL2166 and CL3166. The transgenic strain BL1, which overexpresses the Ce-GST-p24 enzyme (as a GFP fusion protein controlled by its own promoter), was generated to investigate the function of this enzyme in vivo. Stress experiments with BL1 demonstrated an increased resistance to intracellularly induced oxidative stress, as compared to wild type. The consequences of a decrease in the Ce-GST-p24 enzyme concentration were examined by RNAi-treatment of BL1 C. elegans to silence both the endogene and the transgene Ce-GST-p24 and by the analysis of the K08F4.7 homozygous deletion mutant. In both cases, the reduced Ce-GST-p24 enzyme level resulted in a significant decrease in the stress resistance of the nematodes. These results clearly demonstrate a direct correlation between the concentration of Ce-GST-p24 and the resistance to oxidative stress. We have demonstrated for the first time that manipulation of the expression of a single GST can modulate the organismal response to oxidative stress. The enzymatic activity of this detoxification enzyme was examined with various substrates, giving emphasis to lipid peroxidation products. The Ce-GST-p24 was also localized in BL1 C. elegans by confocal laser-scanning microscopy, revealing a wide-spread distribution profile.

Direct and indirect transcriptional targets of DAF-16

Several genes involved in the determination of life span have been identified by mutation in the free-living soil nematode Caenorhabditis elegans. One of the key pathways studied in the context of life span is the DAF-2 pathway. The daf-2 gene is homologous to the insulin and insulin-like growth factor 1 receptor families. A downstream gene, daf-16, encodes a protein that is homologous to the forkhead transcription factor. A study by McElwee, Bubb, and Thomas, published in the current issue of Aging Cell, used genome-scale gene expression analysis to search for genes that are differentially expressed between long-lived daf-2(e1370) and short-lived daf-16(m27);daf-2(e1370) animals. In doing so, they identified candidate direct and indirect targets of DAF-16. In this Perspective, I discuss the results of this study.

Transcriptional outputs of the Caenorhabditis elegans forkhead protein DAF-16

In Caenorhabditis elegans, the forkhead protein DAF-16 transduces insulin-like signals that regulate larval development and adult lifespan. To identify DAF-16-dependent transcriptional alterations that occur in a long-lived C. elegans strain, we used cDNA microarrays and genomic analysis to identify putative direct and indirect DAF-16 transcriptional target genes. Our analysis suggests that DAF-16 action regulates a wide range of physiological responses by altering the expression of genes involved in metabolism, energy generation and cellular stress responses. Furthermore, we observed a large overlap between DAF-16-dependent transcription and genes normally expressed in the long-lived dauer larval stage. Finally, we examined the in vivo role of 35 of these target genes by RNA-mediated interference and identified one gene encoding a putative protease that is necessary for the daf-2 Age phenotype.

Lifespan extension in C. elegans by a molecular chaperone dependent upon insulin-like signals

Insulin-like signalling is a key determinate of lifespan in diverse species including mammals but the mechanism by which this pathway influences the rate of aging is unknown. In the roundworm Caenorhabditis elegans, mutations in the insulin-like signalling pathway extend adult lifespan and are associated with up-regulation of stress response genes including those for heat shock proteins (HSPs). We tested the hypothesis that the C. elegans insulin-like signalling pathway determines longevity through modulating HSP levels. We introduced extra copies of the gene encoding HSP-16 and this conferred stress resistance and longevity both in a wildtype and a long-lived mutant strain. The DAF-16 transcription factor is essential for maximal hsp-16 expression and for lifespan extension conferred by hsp-16. This demonstrates that lifespan is determined in part by insulin-like regulation of molecular chaperones.

Longevity and heat stress regulation in Caenorhabditis elegans

Aging is the most complex phenotype for a multicellular organism. This process is now being under severe investigation. Here I will review the different processes known to affect longevity in the nematode Caenorhabditis elegans and their relationship with thermotolerance. All the longevity mutants that have been tested so far show an increase in stress resistance. In particular, long-lived mutants affected in the IGF/insulin pathway and those affected in the germ-line formation are both thermotolerant and long-lived. The mechanisms that activate the stress resistance are now been understood including the DAF-16 fork head transcription factor transport to the nucleus and the activation of genes involved in the defense to stress. The high correlation between stress resistance and longevity suggests that the same molecular activities that defend the cell from stress can defend the cell from the damage caused by aging.

No reduction of metabolic rate in food restricted Caenorhabditis elegans

Dietary restriction (DR) is the most consistent means of extending life span throughout the animal kingdom. Multiple mechanisms by which DR may act have been proposed but none are clearly predominant. We asked whether metabolic rate and stress resistance is altered in Caenorhabditis elegans in response to DR. DR was imposed in two complementary ways: by growing wild-type worms in liquid medium supplemented with reduced concentrations of bacteria and by using eat-2 mutants, which have a feeding defect. Metabolic rate was not reduced when we fed wild-type worms reduced food and was up-regulated in the eat-2 mutants in liquid culture, as assessed by oxygen consumption rate and heat production. The specific activity levels of the antioxidant enzymes superoxide dismutase (SOD) and catalase showed small increases when we reduced food in wild-type worms, but restricted worms acquired no elevated protection against paraquat and hydrogen peroxide. eat-2 mutants showed elevated specific activities of SOD and catalase relative to wild type in liquid culture. These results indicate that the effects imparted by DR and the eat-2 mutation are not identical, and they contradict, at least in C. elegans, the widespread belief that CR acts by lowering the rate of metabolism.

Axenic growth up-regulates mass-specific metabolic rate, stress resistance, and extends life span in Caenorhabditis elegans

Culture in axenic medium causes two-fold increases in the length of development and adult life span in Caenorhabditis elegans. We asked whether axenic medium imposes dietary restriction (ADR), and causes changes in metabolic activity and stress resistance. Eat mutants, which have a reduced food intake, were studied in parallel with wild-type worms to assess potential synergistic actions of axenic culture and food restriction. We found that axenic culture enhances metabolic activity as assessed by mass-specific oxygen consumption rate and heat production. Axenic culture also caused higher activities of the antioxidant enzymes superoxide dismutase and catalase, and led to increased resistance to high temperature, which was further exacerbated by mutation in eat-2. These results show that axenic medium up-regulates a variety of somatic maintenance functions including oxidative and thermal stress resistance and that food restriction due to axenic growth and to mutation in eat-2 are very similar but not identical.

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.