Oxidative stress and ageing in Caenorhabditis elegans

The spe-10 mutant has longer life and increased stress resistance

We investigated the life span of spe-10 mutant nematodes. We also tested resistance of spe-10 mutants to ultraviolet (UV) light, heat, and paraquat and examined the relationship between resistance to UV light and the fertility defect of these animals. The spe-10 mutation significantly increased mean life span. Additionally, the mutation significantly increased resistance to both UV light and to heat. Resistance to paraquat was not significantly different from that of wild-type, nor were any dauers formed at 27 degrees C. No significant correlation was found between the UV resistance and the fertility defect, nor was the UV resistance attributable to a hormetic effect. These results reinforce the importance of stress resistance in specifying increased life span and indirectly suggest that this fertility defect is not a direct cause of life span extension.

Nematode ageing: Putting metabolic theories to the test

The increased life span caused by certain mutations in the nematode Caenorhabditis elegans has been interpreted in terms of two metabolic theories of ageing: the oxidative damage theory and the rate of living theory. New findings support the former, but not the latter interpretation.

Mouse models of mitochondrial disease, oxidative stress, and senescence

During the course of normal respiration, reactive oxygen species are produced which are particularly detrimental to mitochondrial function. This is shown by recent studies with a mouse that lacks the mitochondrial form of superoxide dismutase (Sod2). Tissues that are heavily dependent on mitochondrial function such as the brain and heart are most severely affected in the Sod2 mutant mouse. Recent work with a mouse mutant for the heart/muscle specific isoform of the mitochondrial adenine nuclear translocator (Ant1) demonstrates a potential link between mitochondrial oxidative stress and mitochondrial DNA mutations. These mutations can be detected by Long-extension PCR, a method for detecting a wide variety of mutations of the mitochondrial genome. Such mutations have also been observed in the mitochondrial genome with senescence regardless of the mean or maximal lifespan of the organism being studied. Mutations have been detected with age in Caenorhabditis elegans, mice, chimpanzees, and humans. This implies that a causal relationship may exist between mitochondrial reactive oxygen species production, and the senescence specific occurrence of mitochondrial DNA mutations.

A cytosolic catalase is needed to extend adult lifespan in C. elegans daf-C and clk-1 mutants

The dauer larva is an alternative larval stage in Caenorhabditis elegans which allows animals to survive through periods of low food availability. Well-fed worms live for about three weeks, but dauer larvae can live for at least two months without affecting post-dauer lifespan. Mutations in daf-2 and age-1, which produce a dauer constitutive (Daf-C) phenotype, and in clk-1, which are believed to slow metabolism, markedly increase adult lifespan. Here we show that a ctl-1 mutation reduces adult lifespan in otherwise wild-type animals and eliminates the daf-c and clk-1-mediated extension of adult lifespan. ctl-1 encodes an unusual cytosolic catalase; a second gene, ctl-2, encodes a peroxisomal catalase. ctl-1 messenger RNA is increased in dauer larvae and adults with the daf-c mutations. We suggest that the ctl-1 catalase is needed during periods of starvation, as in the dauer larva, and that its misexpression in daf-c and clk-1 adults extends lifespan. Cytosolic catalase may have evolved to protect nematodes from oxidative damage produced during prolonged dormancy before reproductive maturity, or it may represent a general mechanism for permitting organisms to cope with the metabolic changes that accompany starvation.

Interorganelle signaling is a determinant of longevity in Saccharomyces cerevisiae

Replicative capacity, which is the number of times an individual cell divides, is the measure of longevity in the yeast Saccharomyces cerevisiae. In this study, a process that involves signaling from the mitochondrion to the nucleus, called retrograde regulation, is shown to determine yeast longevity, and its induction resulted in postponed senescence. Activation of retrograde regulation, by genetic and environmental means, correlated with increased replicative capacity in four different S. cerevisiae strains. Deletion of a gene required for the retrograde response, RTG2, eliminated the increased replicative capacity. RAS2, a gene previously shown to influence longevity in yeast, interacts with retrograde regulation in setting yeast longevity. The molecular mechanism of aging elucidated here parallels the results of genetic studies of aging in nematodes and fruit flies, as well as the caloric restriction paradigm in mammals, and it underscores the importance of metabolic regulation in aging, suggesting a general applicability.

Carbonylated proteins in aging and exercise: immunoblot approaches

Protein carbonyls were studied in aging and exercise by immunoblot followed by one- or two-dimensional polyacrylamide gel electrophoresis using antibodies against 2,4-dinitrophenylhydrazones. Proteins of rat kidneys exhibited significant age-related increase in the amount of carbonyl while those of the brain and liver did not. Major carbonylated proteins in the kidney included serum albumin. In nematodes in which protein carbonyls increased with age, one of the carbonylated proteins was identified as vitellogenin, an egg-yolk protein. A possible biological significance of this protein present in abundance even after egg-laying stages is discussed in terms of protection against oxidative stress. Exhaustive exercise induced significant increase in the carbonylation of selected but unidentified proteins in the lung. This oxidative stress might be caused by xanthine oxidase in this tissue and hypoxanthine derived from ATP-depleted muscles. Exercise at high altitude caused higher carbonylation of the skeletal muscle proteins, most notably a protein likely to be actin, than that at sea level but no significant difference was observed in lipid peroxidation. These studies emphasize the value of immunoblot analysis of tissue protein carbonyls in a variety of situations where oxidative stress is likely involved.

Aging-specific expression of Drosophila hsp22

hsp22 is among the least abundantly expressed Drosophila heat shock (hs) genes during both development and heat stress. In contrast, hsp22 was found to be the most abundantly expressed hs gene during Drosophila aging. During aging, hsp22 RNA was induced 60-fold in the head, with somewhat lower level induction in abdomen and thorax. Induction of the other hs gene RNAs was </=3-fold, except for hsp23, which was induced approximately 5-fold in thorax. hsp22 protein was detected using rat anti-hsp22 polyclonal antisera and was induced >150-fold, with particularly abundant expression in eye tissue. Aging-specific induction of hsp22 was reproduced by hsp22:lacZ fusion reporter constructs in transgenic flies. Analysis of specific promoter mutations in transgenic flies indicated that functional heat shock response elements are required for hsp22 induction during aging. Finally, comparison of hsp22 RNA and protein expression patterns suggests that aging-specific expression of hsp22 is regulated at both the transcriptional and the posttranscriptional levels. Aging-specific induction of hsp22 is discussed with regard to current evolutionary theories of aging.

Cloning of the oxidative stress-responsive genes in Caenorhabditis elegans

Defense systems against free radicals and reactive oxygen species minimize the lethal and mutagenic consequences of these destructive agents. To investigate the genetic response to oxidative stress in a eukaryote, we cloned and characterized oxidative stress-responsive genes by comparing gene expression in the free-living nematode Caenorhabditis elegans under atmospheric conditions and high oxygen concentrations using a method of RNA arbitrarily primed polymerase chain reaction method (RAP-PCR). We identified four genes whose expression levels increase under high oxygen. These encoded 18 s, 5.8 s and 26 s rRNAs, 16 kDa heat shock proteins (hsp16-1 and 16-48) and a vacuolar ATPase G subunit. In addition, we also established that oxi-1, an oxidative stress-responsive gene we previously cloned, encodes a family of proteins related to human E6-AP ubiquitin-protein ligase. The similarity between human and nematode was 54% in one conserved amino acid region.

Molecular mechanisms of bacterial virulence elucidated using a Pseudomonas aeruginosa-Caenorhabditis elegans pathogenesis model

The human opportunistic pathogen Pseudomonas aeruginosa strain PA14 kills Caenorhabditis elegans. Using systematic mutagenesis of PA14 to identify mutants that fail to kill C. elegans and a C. elegans mutant that lacks P-glycoproteins, we identified phenazines, secreted P. aeruginosa pigments, as one of the mediators of killing. Analysis of C. elegans mutants with altered responses to oxidative stress suggests that phenazines exert their toxic effects on C. elegans through the generation of reactive oxygen species. Finally, we show that phenazines and other P. aeruginosa factors required for C. elegans killing are also required for pathogenesis in plants and mice, illustrating that this model tackles the dual challenges of identifying bacterial virulence factors as well as host responses to them.

Heat stress-induced life span extension in yeast

The yeast Saccharomyces cerevisiae has a limited life span that can be measured by the number of times individual cells divide. Several genetic manipulations have been shown to prolong the yeast life span. However, environmental effects that extend longevity have been largely ignored. We have found that mild, nonlethal heat stress extended yeast life span when it was administered transiently early in life. The increased longevity was due to a reduction in the mortality rate that persisted over many cell divisions (generations) but was not permanent. The genes RAS1 and RAS2 were necessary to observe this effect of heat stress. The RAS2 gene is consistently required for maintenance of life span when heat stress is chronic or in its extension when heat stress is transient or absent altogether. RAS1, on the other hand, appears to have a role in signaling life extension induced by transient, mild heat stress, which is distinct from its life-span-curtailing effect in the absence of stress and its lack of involvement in the response to chronic heat stress. This distinction between the RAS genes may be partially related to their different effects on growth-promoting genes and stress-responsive genes. The ras2 mutation clearly hindered resumption of growth and recovery from stress, while the ras1 mutation did not. The HSP104 gene, which is largely responsible for induced thermotolerance in yeast, was necessary for life extension induced by transient heat stress. An interaction between mitochondrial petite mutations and heat stress was found, suggesting that mitochondria may be necessary for life extension by transient heat stress. The results raise the possibility that the RAS genes and mitochondria may play a role in the epigenetic inheritance of reduced mortality rate afforded by transient, mild heat stress.

Homologs of the yeast longevity gene LAG1 in Caenorhabditis elegans and human

LAG1 is a longevity gene, the first such gene to be identified and cloned from the yeast Saccharomyces cerevisiae. A close homolog of this gene, which we call LAC1, has been found in the yeast genome. We have cloned the human homolog of LAG1 with the ultimate goal of examining its possible function in human aging. In the process, we have also cloned a homolog from the nematode worm Caenorhabditis elegans. Both of these homologs, LAG1Hs and LAG1Ce-1, functionally complemented the lethality of a lag1delta lac1delta double deletion, despite low overall sequence similarity to the yeast proteins. The proteins shared a short sequence, the Lag1 motif, and a similar transmembrane domain profile. Another, more distant human homolog, TRAM, which lacks this motif, did not complement. LAG1Hs also restored the life span of the double deletion, demonstrating that it functions in establishing the longevity phenotype in yeast. LAG1Hs mapped to 19p12, and it was expressed in only three tissues: brain, skeletal muscle, and testis. This gene possesses a trinucleotide (CTG) repeat within exon 1. This and its expression profile raise the possibility that it may be involved in neurodegenerative disease. This possibility suggests at least one way in which LAG1Hs might be involved in human aging.

Genetics of longevity

Recent studies on the genetics of aging in the yeast Saccharomyces cerevisiae, the roundworm Caenorhabditis elegans, and the fruit fly Drosophila melanogaster have converged revealing the central role of metabolic capacity and resistance to stress in determining life span. Signal transduction has emerged from these studies as an important molecular mechanism underlying longevity. In their broad features, the results obtained in these genetic models are applicable to the dietary restriction paradigm in mammals, suggesting a general significance. It will be of interest to determine whether many of the molecular details will also pertain. The examination of centenarian populations for the frequency of certain alleles of pertinent genes may provide insights into the relevance of the conclusions of studies in invertebrates to human aging. These population genetic studies can be augmented by mechanistic studies in transgenic mice.

Extended life-span and stress resistance in the Drosophila mutant methuselah

Toward a genetic dissection of the processes involved in aging, a screen for gene mutations that extend life-span in Drosophila melanogaster was performed. The mutant line methuselah (mth) displayed approximately 35 percent increase in average life-span and enhanced resistance to various forms of stress, including starvation, high temperature, and dietary paraquat, a free-radical generator. The mth gene predicted a protein with homology to several guanosine triphosphate-binding protein-coupled seven-transmembrane domain receptors. Thus, the organism may use signal transduction pathways to modulate stress response and life-span.

Life extension and stress resistance in Caenorhabditis elegans modulated by the tkr-1 gene

The nematode Caenorhabditis elegans is widely used to study aging, development, behavior and other basic metazoan processes [1-3]. The only mutants directly identified on the basis of their extended longevity in any metazoan have been isolated in C. elegans [4,5]. All life-extension mutants (Age mutants) previously identified in C. elegans result from hypo-morphic or nullo-morphic mutations. We have identified a new class of gerontogene (a gene whose alteration causes life extension) that increases life span when overexpressed. The first gene in this class has been designated tyrosine kinase receptor-1 (tkr-1); it encodes a putative receptor tyrosine kinase. Overexpression of tkr-1 in transgenics increases longevity 40-100% (average 65%), confers increased resistance to heat and ultraviolet (UV) irradiation in transgenic nematodes, and does not alter development or fertility. Unlike previously identified gerontogenes, tkr-1 positively modulates stress resistance and longevity. These results further support the positive relationship between increased stress resistance and increased longevity seen in all previously studied longevity mutants. This transgenic system is an effective means for identifying overexpression gerontogenes.

Transgenic mice with elevated level of CuZnSOD are highly susceptible to malaria infection

Copper/zinc superoxide dismutase (CuZnSOD) catalyses the conversion of O2.- into H2O2. Constitutive overexpression of CuZnSOD in cells and animals creates an indigenous oxidative stress that predisposes them to added insults. In this study, we used transgenic CuZnSOD (Tg-CuZnSOD) mice with elevated levels of CuZnSOD to determine whether overexpression of CuZnSOD affected the susceptibility of these mice to plasmodium infection. Acute malaria is associated with oxidative stress, mediated by redox-active iron released from the infected RBC. Two independently derived Tg-CuZnSOD lines showed higher sensitivity than control mice to infection by Plasmodium berghei (P. berghei), reflected by an earlier onset and increased rate of mortality. Nevertheless, while Tg-CuZnSOD mice were more vulnerable than control mice, the levels of parasitemia were comparable in both strains. Moreover, treatment of infected red blood cells (RBC) with oxidative stress inducers, such as ascorbate or paraquat, reduced the viability of parasites equally in both transgenic and control RBC. This further confirms that increased CuZnSOD does not support plasmodia development. The data are consistent with the possibility that the combination of increased redox-active iron and elevated H2O2 in the plasmodium-infected Tg-CuZnSOD mice, led to an enhanced Fenton's reaction-mediated HO. production, and the resulting oxidative injury renders the transgenic mice more vulnerable to parasite infection.

Identification of stress-responsive genes in Caenorhabditis elegans using RT-PCR differential display

In order to identify genes that are differentially expressed as a consequence of oxidative stress due to paraquat we used the differential display technique to compare mRNA expression patterns in Caenorhabditis elegans . A C.elegans mixed stage worm population and a homogeneous larval population were treated with 100 mM paraquat, in parallel with controls. Induction of four cDNA fragments, designated L-1, M-47, M-96 and M-132, was confirmed by Northern blot analysis with RNA from stressed and unstressed worm populations. A 40-fold increase in the steady-state mRNA level in the larval population was observed for the L-1/M-47 gene, which encodes the detoxification enzyme glutathione S-transferase. A potential stress-responsive transcription factor (M-132) with C2H2-type zinc finger motifs and an N-terminal leucine zipper domain was identified. The M-96 gene encodes a novel stress-responsive protein. Since paraquat is known to generate superoxide radicals in vivo , the response of the C.elegans superoxide dismutase (SOD) genes to paraquat was also investigated in this study. The steady-state mRNA levels of the manganese-type and the copper/zinc-type SODs increased 2-fold in the larval population in response to paraquat, whereas mixed stage populations did not show any apparent increase in the levels of these SOD mRNAs.

The free radical theory of aging matures

The free radical theory of aging, conceived in 1956, has turned 40 and is rapidly attracting the interest of the mainstream of biological research. From its origins in radiation biology, through a decade or so of dormancy and two decades of steady phenomenological research, it has attracted an increasing number of scientists from an expanding circle of fields. During the past decade, several lines of evidence have convinced a number of scientists that oxidants play an important role in aging. (For the sake of simplicity, we use the term oxidant to refer to all "reactive oxygen species," including O2-., H2O2, and .OH, even though the former often acts as a reductant and produces oxidants indirectly.) The pace and scope of research in the last few years have been particularly impressive and diverse. The only disadvantage of the current intellectual ferment is the difficulty in digesting the literature. Therefore, we have systematically reviewed the status of the free radical theory, by categorizing the literature in terms of the various types of experiments that have been performed. These include phenomenological measurements of age-associated oxidative stress, interspecies comparisons, dietary restriction, the manipulation of metabolic activity and oxygen tension, treatment with dietary and pharmacological antioxidants, in vitro senescence, classical and population genetics, molecular genetics, transgenic organisms, the study of human diseases of aging, epidemiological studies, and the ongoing elucidation of the role of active oxygen in biology.

Thioredoxin peroxidases from Brugia malayi

Parasite-derived antioxidant proteins have been implicated in playing an important role in protection against the oxygen radicals that are generated during aerobic metabolism and in defense against host immune cell attack. Here we report that filarial nematodes include the thioredoxin peroxidase/thiol-specific antioxidant (TPx/TSA) family of antioxidant proteins as part of their complex defense against radical-mediated damage. At the protein level, the TPx/TSA from Brugia malayi (Bm-TPx-1) was approximately 50% identical and approximately 60% similar to TPx/TSAs from mammals, amphibians and yeast. Bm-TPx-1 was also approximately 60% identical to putative TPx proteins from a related filarial nematode, Onchocerca volvulus, and from the free-living nematode Caenorhabditis elegans. That B. malayi may express multiple forms of molecules with TPx/TSA activity was indicated by the identification of a B. malayi gene encoding a second, distinct member of the TPx/TSA family (Bm-tpx-2). Bm-tpx-1 was found to be transcribed in all stages of the parasite present in the mammalian host and the 25 kDa translation product was present in all of the developmental stages studied. The results of immunohistochemical, immunofluorescent and immunoprecipitation studies showed Bm-TPx-1 to be localized in the cells of the hypodermis/lateral chord in adult parasites and not to be present at the surface or in excretory/secretory products. The distribution in the parasite suggests that Bm-TPx-1 may play its major role in countering radicals produced within cells. A recombinant form of Bm-TPx-1 was biologically active and capable of protecting DNA from oxygen radical-mediated damage. Thioredoxin peroxidases may prove to be a critical component in the parasite's defense against injury caused by oxygen radicals derived from endogenous and exogenous sources.

An insulin-like signaling pathway affects both longevity and reproduction in Caenorhabditis elegans

Mutations in daf-2 and age-1 cause a dramatic increase in longevity as well as developmental arrest at the dauer diapause stage in Caenorhabditis elegans. daf-2 and age-1 encode components of an insulin-like signaling pathway. Both daf-2 and age-1 act at a similar point in the genetic epistasis pathway for dauer arrest and longevity and regulate the activity of the daf-16 gene. Mutations in daf-16 cause a dauer-defective phenotype and are epistatic to the diapause arrest and life span extension phenotypes of daf-2 and age-1 mutants. Here we show that mutations in this pathway also affect fertility and embryonic development. Weak daf-2 alleles, and maternally rescued age-1 alleles that cause life span extension but do not arrest at the dauer stage, also reduce fertility and viability. We find that age-1(hx546) has reduced both maternal and zygotic age-1 activity. daf-16 mutations suppress all of the daf-2 and age-1 phenotypes, including dauer arrest, life span extension, reduced fertility, and viability defects. These data show that insulin signaling, mediated by DAF-2 through the AGE-1 phosphatidylinositol-3-OH kinase, regulates reproduction and embryonic development, as well as dauer diapause and life span, and that DAF-16 transduces these signals. The regulation of fertility, life span, and metabolism by an insulin-like signaling pathway is similar to the endocrine regulation of metabolism and fertility by mammalian insulin signaling.

Molecular genetics of life span in C. elegans: how much does it teach us?

Several loci have been identified in the nematode worm Caenorhabditis elegans that, when mutated, can increase life span. Three of these genes, age-1, daf-2 and clk-1, have now been cloned. Mutations in these three genes are highly pleiotropic and affect many aspects of worm development and behaviour, age-1 and daf-2 act in the same genetic pathway and have similar effects on the worm, age-1 encodes a homologue of the p110 subunit of phosphatidylinositol 3-kinase and daf-2 encodes an insulin receptor family member, clk-1 encodes a protein of unknown biochemical function similar to the yeast metabolic regulator Cat5p/Coq7p. The implications of these findings for our understanding of organismal ageing are discussed.

Cloning, expression, and characterization of two manganese superoxide dismutases from Caenorhabditis elegans

Two genes encoding manganese superoxide dismutase (sod-2 and sod-3) have been identified in the nematode Caenorhabditis elegans. Each gene is composed of five exons, and intron positions are identical; however, intron sizes and sequences are not the same. The predicted protein sequences are 86.3% homologous (91.8% conservative), and the cDNAs are only 75.2% homologous. Both deduced protein sequences contain the expected N-terminal mitochondrial transit peptides. Reverse transcriptase polymerase chain reaction analysis shows that both genes are expressed under normal growth conditions and that their RNA transcripts are trans-spliced to the SL-1 leader sequence. The latter result together with Northern blot analysis indicate that both genes have mono-cistronic transcripts. The sod-3 gene was mapped to chromosome X, and the location of sod-2 was confirmed to be chromosome I. Polymerase chain reaction was used to amplify the cDNA regions encoding the predicted mature manganese superoxide dismutase proteins and each was cloned and expressed to high levels in Escherichia coli cells deficient in cytosolic superoxide dismutases. Both proteins were shown to be active in E. coli, providing similar protection against methyl viologen-induced oxidative stress. The expressed enzymes, which were not inhibited by hydrogen peroxide or cyanide, are dimeric, show quite different electrophoretic mobilities and isoelectric points, but exhibit comparable specific activities.

The role of ferredoxin-NADP+ reductase in the concerted cell defense against oxidative damage -- studies using Escherichia coli mutants and cloned plant genes

Ferredoxin-NADP+ reductases (FNR) participate in cellular defense against oxidative damage. Escherichia coli mutants deficient in FNR are abnormally sensitive to methyl viologen and hydrogen peroxide. Tolerance to these oxidants was regained by expression of plant FNR, superoxide dismutase, or catalase genes in the mutant cells. FNR contribution to the concerted defense against viologen toxicity under redox-cycling conditions was similar to that of the two major E. coli superoxide dismutases together, as judged by the phenotypes displayed by relevant mutant strains. However, FNR expression in sodA sodB strains failed to increase their tolerance to viologens, indicating that the FNR target is not the superoxide radical. Sensitivity of FNR-deficient cells to oxidants is related to extensive DNA damage. Incubation of the mutant bacteria with iron chelators or hydroxyl radical scavengers provided significant protection against viologens or peroxide, suggesting that oxidative injury in FNR-deficient cells was mediated by intracellular iron through the formation of hydroxyl radicals in situ. The NADP(H)-dependent activities of the reductase were necessary and sufficient for detoxification, without participation of either ferredoxin or flavodoxin in the process. Possible mechanisms by which FNR may exert its protective role are discussed.

Multi-organ characterization of mitochondrial genomic rearrangements in ad libitum and caloric restricted mice show striking somatic mitochondrial DNA rearrangements with age

Mitochondrial DNA (mtDNA) rearrangements have been shown to accumulate with age in the post-mitotic tissues of a variety of animals and have been hypothesized to result in the age-related decline of mitochondrial bioenergetics leading to tissue and organ failure. Caloric restriction in rodents has been shown to extend life span supporting an association between bioenergetics and senescence. In the present study, we use full length mtDNA amplification by long-extension polymerase chain reaction (LX-PCR) to demonstrate that mice accumulate a wide variety of mtDNA rearrangements with age in post mitotic tissues. Similarly, using an alternative PCR strategy, we have found that 2-4 kb minicircles containing the origin of heavy-strand replication accumulate with age in heart but not brain. Analysis of mtDNA structure and conformation by Southern blots of unrestricted DNA resolved by field inversion gel electrophoresis have revealed that the brain mtDNAs of young animals contain the traditional linear, nicked, and supercoiled mtDNAs while old animals accumulate substantial levels of a slower migrating species we designate age-specific mtDNAs. In old caloric restricted animals, a wide variety of rearranged mtDNAs can be detected by LX-PCR in post mitotic tissues, but Southern blots of unrestricted DNA reveals a marked reduction in the levels of the age- specific mtDNA species. These observations confirm that mtDNA mutations accumulate with age in mice and suggest that caloric restriction impedes this progress.

Properties of an oxygen-sensitive mutant mev-3 of the nematode Caenorhabditis elegans

A spontaneous mutant of mev-3 of the nematode Caenorhabditis elegans was isolated on the basis of its resistance to methyl viologen, which generates superoxide radicals. Contrary to expectation, the mutant proved hypersensitive to oxygen. Oxygen retarded development and reduced fecundity in a concentration-dependent fashion in mev-3 but not in wild-type. In addition, the mean life span of mev-3 (but not wild-type) was progressively shortened when animals were incubated under 70% oxygen.

Invertebrate gerontology: the age mutations of Caenorhabditis elegans

Ageing is a complex phenomenon which remains a major challenge to modern biology. Although the evolutionary biology of ageing is well understood, the mechanisms that limit lifespan are unknown. The isolation and analysis of single-gene mutations which extend lifespan (Age mutations) is likely to reveal processes which influence ageing. Caenorhabditis elegans is the only metazoan in which Age mutations have been identified. The Age mutations not only prolong life, but also confer a complex array of other phenotypes. Some of these phenotypes provide clues to the evolutionary origins of these genes while others allude to mechanisms of lifespan-extension. Many of the Age genes interact and share a second common phenotype, that of stress resistance. Rather than invertebrate ageing being determined by a 'clock mechanism', a picture is emerging of ageing as a non-adaptive process determined, in part, by resistance to intrinsic stress mediated by stress-response genes.

A phosphatidylinositol-3-OH kinase family member regulating longevity and diapause in Caenorhabditis elegans

A pheromone-induced neurosecretory pathway in Caenorhabditis elegans triggers developmental arrest and an increase in longevity at the dauer diapause stage. The gene age-1 is required for non-dauer development and normal senescence. age-1 encodes a homologue of mammalian phosphatidylinositol-3-OH kinase (PI(3)K) catalytic subunits. Lack of both maternal and zygotic age-1 activity causes dauer formation, whereas animals with maternal but not zygotic age-1 activity develop as non-dauers that live more than twice as long as normal. These data suggest that phosphatidylinositol signalling mediated by AGE-1 protein controls lifespan and the dauer diapause decision.

Rapid development and a long life: an association expected under a stress theory of aging

Life span and development time are considered in the context of the abiotic stresses to which free-living organisms are normally exposed. Under these circumstances, long life span depends upon metabolically efficient stress-resistance genes, which tend to be heterozygous. Similarly, rapid development time tends to be a feature of heterozygous stress-resistant individuals. Therefore, individuals who have high inherited stress resistance should develop fastest and live longest; in addition, they should show high homeostasis in the fact of the energy costs of stress. In this way, the stress theory of aging can incorporate the developmental stage, based upon oxidative stress as an important major direct challenge.

Genetic analysis of the roles of daf-28 and age-1 in regulating Caenorhabditis elegans dauer formation

Based on environmental cues, the nervous system of Caenorhabditis elegans regulates formation of the dauer larva, an alternative larval form specialized for long-term survival under harsh conditions. Mutations that cause constitutive or defective dauer formation (Daf-c or Daf-d) have been identified and the genes ordered in a branched pathway. Most Daf-c mutations also affect recovery from the dauer stage. The semi-dominant mutation daf-28(sa191) is Daf-c but has no apparent effect on dauer recovery. We use this unique aspect of daf-28(sa191) to characterize the effects of several Daf-d and synthetic Daf-c mutations on dauer recovery. We present double mutant analysis that indicates that daf-28(sa191) acts at a novel point downstream in the genetic pathway for dauer formation. We also show that daf-28(sa191) causes a modest increase (12-13%) in life span. The phenotypes and genetic interactions of daf-28(sa191) are most similar to those of daf-2 and daf-23 mutations, which also cause a dramatic increase in life span. We present mapping and complementation data that suggest that daf-23 is the same gene as age-1, identified previously by mutations that extend life span. We find that age-1 alleles are also Daf-c at 27 degrees.

A genetic pathway conferring life extension and resistance to UV stress in Caenorhabditis elegans

A variety of mechanisms have been proposed to explain the extension of adult life span (Age) seen in several mutants in Caenorhabditis elegans (age-1: an altered aging rate; daf-2 and daf-23: activation of a dauer-specific longevity program; spe-26: reduced fertility; clk-1: an altered biological clock). Using an assay for ultraviolet (UV) resistance in young adult hermaphrodites (survival after UV irradiation), we observed that all these Age mutants show increased resistance to UV. Moreover, mutations in daf-16 suppressed the UV resistance as well as the increased longevity of all the Age mutants. In contrast to the multiple mechanisms initially proposed, these results suggest that a single, daf-16-dependent pathway, specifies both extended life span and increased UV resistance. The mutations in daf-16 did not alter the reduced fertility of spe-26 and interestingly a daf-16 mutant is more fertile than wild type. We propose that life span and some aspects of stress resistance are jointly negatively regulated by a set of gerontogenes (genes whose alteration causes life extension) in C. elegans.

The limit to human longevity: an approach through a stress theory of ageing

Survival to old age is enhanced by high vitality and resilience associated with substantial physiological and morphological homeostasis. This is underlain by genes for stress resistance, which confer high metabolic efficiency and hence adaptation to the energy costs of the stresses to which free-living populations are exposed. Under the stress theory of ageing, selection for genes for stress resistance is primary, and achieved life-span is secondary. In some human populations of the modern era, selection for stress resistance is less intense than in earlier times, because of adequate nutrition and reduced exposure to environmental stresses. Such relaxed selection should permit the accumulation of deleterious mutants that are likely to be stress sensitive. Accordingly, increased maximum life-span in future human populations would appear difficult to achieve.

Determination of life-span in Caenorhabditis elegans by four clock genes

The nematode worm Caenorhabditis elegans is a model system for the study of the genetic basis of aging. Maternal-effect mutations in four genes--clk-1, clk-2, clk-3, and gro-1--interact genetically to determine both the duration of development and life-span. Analysis of the phenotypes of these mutants suggests the existence of a general physiological clock in the worm. Mutations in certain genes involved in dauer formation (an alternative larval stage induced by adverse conditions in which development is arrested) can also extend life-span, but the life extension of Clock mutants appears to be independent of these genes. The daf-2(e1370) clk-1(e2519) worms, which carry life-span-extending mutations from two different pathways, live nearly five times as long as wild-type worms.

Genetic analysis of ageing: role of oxidative damage and environmental stresses

Evolutionary theory predicts substantial interspecific and intraspecific differences in the proximal mechanisms of ageing. Our goal here is to seek evidence for common ('public') mechanisms among diverse organisms amenable to genetic analysis. Oxidative damage is a candidate for such a public mechanism of ageing. Long-lived strains are relatively resistant to different environmental stresses. The extent to which these stresses produce oxidative damage remains to be established.

Mapping quantitative trait loci affecting life history traits in the nematode Caenorhabditis elegans

We have identified chromosomal regions containing quantitative trait loci (QTLs) specifying life history traits in recombinant-inbred strains of the nematode Caenorhabditis elegans. This approach also allows us to examine epistatic interactions between loci and pleiotropic effects on different traits at specific loci. QTLs for mean life span were identified on chromosomes II (near stP101), IV (stP5) and the X (stP61), and QTLs for fertility were identified on II (maPI), III (stP19) and IV (stP51). The QTLs for mean life span accounted for 90% of the genetic component of variance. The loci for mean fertility accounted for 88% of the genetic component of variance. Additional QTLs for temperature-sensitive fertility [II (stP36) and V (stP6)] and internal hatching [IV (stP5)] were also mapped in these crosses. We found evidence for epistatic effects on mean life span between maP1 and bP1 (V), and for epistatic effects on mean fertility between stP36 and stP6, between stP98 (II) and stP192 (V), between maP1 and stP127 (III), between maP1 and stP103 (X), and between stP5 and stP6. Negatively correlated, pleiotropic effects on mean life span and internal hatching were found linked to stP5.

Somatic mutagenesis and antimutagenesis in aging research

Somatic mutagenesis and antimutagenesis are reviewed from the point of view of a gerontologist. Aging can be defined as a set of phenotypes that have escaped the force of natural selection. In mammalian species, aberrations in proliferative homeostasis, including a variety of cancers, are conspicuous examples. The author argues that there is a strong coupling between intrinsic biological aging and the biology of neoplasia. Genomic instability is likely to be a dominant mechanism underlying such coupling. Ongoing experiments from his laboratory and those of collaborators are briefly reviewed. The approaches include: (1) the characterization of a striking progeroid mutation of man, the Werner syndrome, which exhibits a deletor mutator phenotype; (2) the comparative analysis of intragenic and chromosomal mutations in mammalian species of contrasting life span potentials; (3) attempts to synthesize antimutator strains of mice. An emerging generalization is that there is a significant degree of species specificity in the patterns of somatic mutation in aging mammals.

Rate of aerobic metabolism and superoxide production rate potential in the nematode Caenorhabditis elegans

We have monitored oxygen consumption as a measure of the rate of aerobic metabolism during the lifetime of Caenorhabditis elegans. We have also developed a chemiluminescent technique which measures exogenous NADPH-stimulated superoxide anion production by freeze-thawed worms. In this assay light production depends on the combined activities of all of the enzymes involved in superoxide production, both directly and indirectly, thus reflecting their activity levels immediately prior to freeze fixation. We have designated this parameter the superoxide production rate potential. The superoxide production rate potential is controlled by the longevity determining gene age-1 and varies in a life cycle-dependent fashion. The metabolic rate generally follows these fluctuations, but additionally shows specific alterations as a response to environmental factors. Metabolic rate and superoxide production rate potential increase by 1.3- and 3-fold, respectively, in reproducing adults. This increase is not due to the contribution of embryonating eggs, however. Culture conditions have a large effect on metabolic rate, but not on the superoxide production rate potential. The energetic cost of movement, measured as consumed oxygen, is low relative to the costs of maintenance and reproduction. Identical superoxide production rate potentials are scored in paralyzed and motile worms, as would be expected.

Genetics of aging: current animal models

Studies are summarized for three organisms-Caenorhabditis elegans, Mus musculus, and Drosophila melanogaster-utilizing three distinct approaches to the identification of longevity-determining genes: the analysis of mutations that affect life span, the use of transgenic animals to assess the effects of specific gene expression on longevity, and selective breeding to identify naturally occurring allelic variations between strains that have differential effects on life span. Correlative studies of age-dependent changes in physiology, or in cellular and molecular constituents, generally cannot discern cause from effect. In contrast, analyses of genetic influences on longevity can permit underlying mechanisms to be reliably inferred; because genotype remains essentially constant throughout life, longevity comparisons of animals differing only in genetic constitution must reflect the effects of genes on long-term survival. Understanding the genetic regulation of life span may thus lead to methods of intervention in age-associated deterioration and disease.

Defining genes that govern longevity in Caenorhabditis elegans

We previously identified five regions on the chromosomal map of Caenorhabditis elegans, containing genes that help specify life span in this species, by comparing the genotypes of young and long-lived progeny from a cross between strains Bristol-N2 and Bergerac-BO [Ebert et al. (1993): Genetics 135:1003-1010]. Analyses of additional crosses, and of putative polymorphisms for the implicated genes, are necessary to clarify the roles of naturally occurring polymorphic alleles in determining longevity. We therefore carried out a second multigenerational cross, between strains Bristol-N2 and DH424 (both nonmutators at 20 degrees C), to create a different heterogeneous recombinant-inbred population. We again found strong evidence implicating multiple genes, which differ between the parental strains, in the determination of life span. Increased variance of survival, for F2 and homozygous F25 worms relative to F1 hybrids, is consistent with such alleles assorting randomly in the cross progeny. Moreover, chromosome mapping data corroborate the polygenic nature of this quantitative trait. Genotypes of young and very long-lived adult worms from a synchronous F15 population were determined by polymerase chain reaction, to identify the parental strain of origin for each of 10 polymorphic loci. Two regions, on chromosomes II and IV, each contain at least one gene with allelic differences in associated longevity. A recombinant-inbred Bergerac-BO x Bristol-N2 population, derived from the earlier cross between those strains, was exposed to an acute toxic level of hydrogen peroxide. Genotyping of H2O2-resistant worms implicated at least one of the five chromosomal regions previously identified in the same cross progeny as harboring a longevity-determining gene. Superoxide dismutase and catalase levels, determined for the three parental strains as they aged, confirm the existence of polymorphisms in the corresponding genes (or their regulatory mechanisms) inferred from the chromosome-II mapping data, and are consistent with the hypothesis that increased longevity is conferred by high levels of these enzymes late in life.

Direct isolation of longevity mutants in the nematode Caenorhabditis elegans

We have isolated several new EMS-induced, long-lived mutants of Caenorhabditis elegans, using a novel screen that eliminates the need for replica plating. Three new alleles of age-1 (z10, z12, and z25) were identified by failure to complement age-1 (hx546) for life span extension; these alleles had life spans ranging from 18.9 to 25.9 days at 25 degrees C, with an average 46% increase in life span. After backcrossing, alleles were examined in a wild-type background for resistance to several environmental stresses: heat (35 degrees C), ultraviolet (UV) light (20 J/m2), and hydrogen peroxide (H2O2) (0.5 M). Two replicates of the test of thermotolerance were completed on each strain, giving mean survivals of 842 min (hx546), 810 min (z10), 862 min (z12), and 860 min (z25), compared to 562 min for wild type. All the age-1 alleles were significantly tolerant, compared with wild type (P < 0.001). Two replicates for UV resistance were also completed with mean survivals of 103, 118, 108, and 89 hr, respectively, compared to 72 hr for wild type. One test of hydrogen peroxide resistance has shown that z12 and N2 had a mean survival of 41 hr, while the other age-1 alleles had mean survival of 54 hr (z10), and 62 hr (z25); H2O2 resistance is the only environmental stress that differentiates among the age-1 alleles.

Inherited stress resistance and longevity: a stress theory of ageing

Ageing is considered in the context of the abiotic stresses to which free-living organisms are normally exposed. Assuming that the primary target of selection of stress is at the level of energy carriers, trade-offs under the rate-of-living theory of ageing predict increased longevity from selection for stress resistance. Changes in longevity then become incidental to selection for stress resistance. I therefore suggest the reformulation of the rate-of-living theory to become a stress theory of ageing. This directly incorporates the characteristics of habitats in nature. Under this theory, the primary trait inherited is resistance to stress. Consequently, at extreme ages those with inherited resistance to abiotic stress should dominate. Furthermore, the reduction in homeostasis manifested by deteriorating ability to adapt to abiotic stress as ageing proceeds, should be slowest in those surviving longest.

Thermotolerance and extended life-span conferred by single-gene mutations and induced by thermal stress

We have discovered that three longevity mutants of the nematode Caenorhabditis elegans also exhibit increased intrinsic thermotolerance (Itt) as young adults. Mutation of the age-1 gene causes not only 65% longer life expectancy but also Itt. The Itt phenotype cosegregates with age-1. Long-lived spe-26 and daf-2 mutants also exhibit Itt. We investigated the relationship between increased thermotolerance and increased life-span by developing conditions for environmental induction of thermotolerance. Such pretreatments at sublethal temperatures induce significant increases in thermotolerance and small but statistically highly significant increases in life expectancy, consistent with a causal connection between these two traits. Thus, when an animal's resistance to stress is increased, by either genetic or environmental manipulation, we also observe an increase in life expectancy. These results suggest that ability to respond to stress limits the life expectancy of C. elegans and might do so in other metazoa as well.

Increased frequency of deletions in the mitochondrial genome with age of Caenorhabditis elegans

We have developed a long-extension-PCR strategy which amplifies approximately half of the mitochondrial genome (6.3 kb) of Caenorhabditis elegans using an individual worm as target. We analyzed three strains over their life span to assess the number of detectable deletions in the mitochondrial genome. Two of these strains are wild-type for life span while the third is mutant in the age-1 gene, approximately doubling its maximum life span. At the mean life span in wild-type strains, there was a significant difference between the frequency of deletions detected in the mitochondrial genome compared with the mean number of deletions in young animals. In addition, deletions in the mitochondrial genome occur at a significantly lower rate in age-1 mutants as compared with wild type. We cloned and identified the breakpoints of two deletions and found that one of the deletions had a direct repeat of 8 bp at the breakpoint. This is the largest single study (over 900 individual animals) characterizing the frequency of deletions in the mitochondrial genome as a function of age yet carried out.

Mutants of Saccharomyces cerevisiae sensitive to oxidative and osmotic stress

Although oxidative stress is involved in many human diseases, little is known of its molecular basis in eukaryotes. In a genetic approach, S. cerevisiae was used to identify elements involved in oxidative stress. By using hydrogen peroxide as an agent for oxidative stress, 34 mutants were identified. All mutants were recessive and fell into 16 complementation groups (pos1 to pos16 for peroxide sensitivity). They corresponded to single mutations as shown by a 2:2 segregation pattern. Enzymes reportedly involved in oxidative stress, such as glucose-6-phosphate dehydrogenase, glutathione reductase, superoxide dismutase, as well as glutathione concentrations, were investigated in wild-type and mutant-cells. One complementation group lacked glucose-6-phosphate dehydrogenase and was shown to be allelic to the glucose-6-phosphate dehydrogenase structural gene ZWF1/MET19. In other mutants all enzymes supposedly involved in oxidative-stress resistance were still present. However, several mutants showed strongly elevated levels of glutathione reductase, gluconate-6-phosphate dehydrogenase and glucose-6-phosphate dehydrogenase. One complementation group, pos9, was highly sensitive to oxidative stress and revealed the same growth phenotype as the previously described yap1/par1 mutant coding for the yeast homologue of mammalian transcriptional activator protein, c-Jun, of the proto-oncogenic AP-1 complex. However, unlike par1 mutants, which showed diminished activities of oxidative-stress enzymes and glutathion level, the pos9 mutants did not reveal any such changes. In contrast to other recombinants between pos mutations and par1, the sensitivity did not further increase in par1 pos9 recombinants, which may indicate that both mutations belong to the same regulating circuit.(ABSTRACT TRUNCATED AT 250 WORDS)