The effect of superoxide dismutase alleles on aging in Drosophila

Genetic repression of the antioxidant enzymes reduces the lifespan in Drosophila melanogaster

Aging is a biological process associated with gradual loss of function caused by cellular and molecular damages ultimately leading to mortality. Free radicals are implicated in oxidative damage which affects the longevity of organisms. Natural cellular defenses involving antioxidant enzymes delay or prevent oxidative damage and, therefore, influence the aging process and longevity has been shown in many species including Drosophila. We and others have shown that oxidative resistance is an important mechanism in the aging process in Drosophila. Therefore, we hypothesized that repressing endogenous antioxidant defenses shortens longevity in Drosophila. To study the influence of natural defense mechanisms against oxidative stress in aging, we have investigated the effect of genetic repression of the antioxidant enzymes, superoxide dismutase (SOD) and catalase (CAT), on longevity in Drosophila using transgenic RNAi flies and in vivo inhibition of the enzymes with chemical inhibitors. RNAi lines of Drosophila viz., UAS-sod1-IR and UAS-cat-IR, are driven ubiquitously using Act5C-Gal4 and Tubulin-Gal4 to achieve the suppression of SOD1 and CAT activities, respectively. We show that genetic repression of SOD1 and CAT by RNAi in transgenic flies led to drastically reduced longevity (SOD1, 77%; CAT, 83%), presenting the evidence for the role of endogenous antioxidant defenses in lifespan extension in Drosophila. Further, our study shows that the enzyme inhibitors, diethyldithiocarbamate and 3-amino-1,2,4-triazole, although lower the enzyme activities in vivo in flies, but did not affect longevity, which could be attributed to the factors such as bioavailability and metabolism of the inhibitors and adaptive mechanisms involving de novo synthesis of the enzymes. Our study of genetic repression using transgenic RNAi provides experimental evidence that extended longevity is associated with endogenous antioxidant defenses and aging is correlated with oxidative stress resistance.

Healthy aging - insights from Drosophila

Human life expectancy has nearly doubled in the past century due, in part, to social and economic development, and a wide range of new medical technologies and treatments. As the number of elderly increase it becomes of vital importance to understand what factors contribute to healthy aging. Human longevity is a complex process that is affected by both environmental and genetic factors and interactions between them. Unfortunately, it is currently difficult to identify the role of genetic components in human longevity. In contrast, model organisms such as C. elegans, Drosophila, and rodents have facilitated the search for specific genes that affect lifespan. Experimental evidence obtained from studies in model organisms suggests that mutations in a single gene may increase longevity and delay the onset of age-related symptoms including motor impairments, sexual and reproductive and immune dysfunction, cardiovascular disease, and cognitive decline. Furthermore, the high degree of conservation between diverse species in the genes and pathways that regulate longevity suggests that work in model organisms can both expand our theoretical knowledge of aging and perhaps provide new therapeutic targets for the treatment of age-related disorders.

Changed gene expression for candidate ageing genes in long-lived Bicyclus anynana butterflies

Candidate genes for the regulation of lifespan have emerged from studies that use mutants and genetically manipulated model organisms. However, it is rarely addressed whether these genes contribute to lifespan variation in populations of these species that capture natural standing genetic variation. Here, we explore expression variation in three candidate ageing genes, Indy, sod2, and catalase, in Bicyclus anynana, a butterfly with well understood ecology. We used lines established from natural populations and artificially selected for increased adult starvation resistance. They show a considerable increase in adult lifespan under both starvation and optimal food conditions. We measured adult butterflies of various ages, under a range of optimal and starvation diets, from two selected populations and one unselected control population. In all lines, Indy and catalase are up-regulated in response to starvation while this is not evident for sod2. Under starvation, Indy and catalase are up-regulated in, while this is not evident for sod2. Under optimal food conditions, Indy is down-regulated at a later age, with Indy expression showing relatively high inter-individual variation. We find differences between the selected lines and the unselected line. Under starvation conditions, expression is higher for catalase in one, and for sod2 in both selected lines. Importantly, sod2 expression is also higher in the selected populations under optimal food conditions. We conclude that sod2, but not Indy, is involved in the response to artificial selection for increased starvation resistance. The role of catalase is less clear because of the differences between the two selected lines. Moreover, sod2 appears to be a candidate gene that underpins the genetic correlation between starvation resistance and longevity. Our study indicates that some, but not all, genes identified through mutant screens in other organisms may underpin standing genetic variation for ageing-related traits in stocks of Bicyclus butterflies established from natural populations. Clearly, this needs to be investigated in other organisms as well, especially in the organisms to which mutants screens were applied. This information will narrow down the list of genes that underpin variation in lifespan and ageing in extant populations of organisms, and which may serve as candidate genes in humans.

Rat models of caloric intake and activity: relationships to animal physiology and human health

Every rodent experiment is based on important parameters concerning the levels of caloric intake and physical activity. In many cases, these decisions are not made consciously, but are based on traditional models. For experimental models directed at the study of caloric intake and activity, the selection of parameters is usually aimed at modeling human conditions, the ultimate goal of which is to gain insight into the pathophysiology of the disease process in man. In each model, it is important to understand the influence of diet, exercise, and genetic background on physiology and the development of disease states. Along the continuum of energy intake from caloric restriction to high-fat feeding, and of energy output from total inactivity to forced exercise, a number of models are used to study different disease states. In this paper, we will evaluate the influence of the quantity and composition of diet and exercise in several animal models, and will discuss how each model can be applied to various human conditions. This review will be limited to traditional models using the rat as the experimental animal, and although it is not an exhaustive list, the models presented are those most commonly represented in the literature. We will also review the mechanisms by which each affects rat physiology, and will compare these to the analogous mechanisms in the modeled human disease state. We hope that the information presented here will help researchers make choices among the available models and will encourage discussion on the interpretation and extrapolation of results obtained from traditional and novel rodent experiments on diet, exercise, and chronic disease.

C47T polymorphism in manganese superoxide dismutase (MnSOD), antioxidant intake and survival

INTRODUCTION AND OBJECTIVE

Manganese superoxide dismutase (MnSOD), an enzyme that catalyzes superoxide radical quenching, is hypothesized to protect against premature aging. A C47T transition in the MnSOD gene may affect the enzyme's distribution to the mitochondrion, a site of high oxidative stress. We examined the association between this polymorphism and survival.

METHODS

Individuals who donated a blood sample to the CLUE I and II campaigns in 1974 and 1989, respectively, and completed a food frequency questionnaire in 1989 (N=6151) were included in the analysis. Hazard ratios (HR) and 95% confidence intervals (CI) were calculated by Cox proportional hazards models. Mortality follow-up extended from 1989 to 2002.

RESULTS

MnSOD genotype distributions were 27% CC (wildtype homozygotes), 50% CT (heterozygotes) and 23% TT (variant homozygotes). TT and CT genotypes compared to the CC genotype were not associated with all-cause or cardiovascular disease mortality. A slight, but non-statistically significant higher risk of cancer mortality was observed for the CT (HR=1.13, 95% CI: 0.86-1.49) and TT (HR=1.24, 95% CI: 0.90-1.70) genotypes compared to CC genotype (p-trend=0.19).

CONCLUSION

We did not observe an association between the C47T polymorphism in the MnSOD gene and survival. These null associations were not modified by fruit and vegetable intake, cigarette smoking status, or body mass index.

What evidence is there for the existence of individual genes with antagonistic pleiotropic effects?

Classical evolutionary theory predicts the existence of genes with antagonistic effects on longevity and various components of early-life fitness. Quantitative genetic studies have provided convincing evidence that such genes exist. However, antagonistic pleiotropic effects have rarely been attributed to individual loci. We examine several classes of longevity-assurance genes: those involved in regulation of the gonad; the insulin-like growth factor pathway; free-radical scavenging; heat shock proteins and apoptosis. We find initial evidence that antagonistic pleiotropic effects are pervasive in each of these classes of genes and in various model systems--although most studies lack explicit studies of fitness components. This is particularly true of human studies. Very little is known about the early-life fitness effects of longevity loci. Given the possible medical importance of such effects we urge their future study.

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.

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.

What have two decades of laboratory life-history evolution studies on Drosophila melanogaster taught us?

A series of laboratory selection experiments on Drosophila melanogaster over the past two decades has provided insights into the specifics of life-history tradeoffs in the species and greatly refined our understanding of how ecology and genetics interact in life-history evolution. Much of what has been learnt from these studies about the subtlety of the microevolutionary process also has significant implications for experimental design and inference in organismal biology beyond life-history evolution, as well as for studies of evolution in the wild. Here we review work on the ecology and evolution of life-histories in laboratory populations of D. melanogaster, emphasizing how environmental effects on life-history-related traits can influence evolutionary change. We discuss life-history tradeoffs - many unexpected - revealed by selection experiments, and also highlight recent work that underscores the importance to life-history evolution of cross-generation and cross-life-stage effects and interactions, sexual antagonism and sexual dimorphism, population dynamics, and the possible role of biological clocks in timing life-history events. Finally, we discuss some of the limitations of typical selection experiments, and how these limitations might be transcended in the future by a combination of more elaborate and realistic selection experiments, developmental evolutionary biology, and the emerging discipline of phenomics.

Patterns of DNA sequence polymorphism at Sod vicinities in Drosophila melanogaster: unraveling the footprint of a recent selective sweep

We survey DNA sequence polymorphisms at the Sod locus and four neighboring regions of Drosophila melanogaster, spanning 55,513 base pairs (bp), in 15 strains from a natural population, plus one reference laboratory strain and one strain of Drosophila simulans. Our objective is to characterize a proposed selective sweep that occurred at a locus close to Sod in D. melanogaster and to characterize the strength of the selection event, its time, and the size of the hitchhiked region. Two regions, 1819 and 6kbr3r, show a pattern of polymorphism very similar to the one of Sod, implying that they have been affected by the same evolutionary process that impacted Sod. A third fragment, 2021 seems unaffected by the event. A fourth one, 4039, on the opposite flank of Sod in relation to 2021, is only partially affected. We estimate that the length of the chromosomal segment impacted by the selective sweep is 41-54 kb, the age of the selective sweep is 2,600-22,000 years, and the selective advantage is 0.020 < s < 0.103.

Phenotypic effects of familial amyotrophic lateral sclerosis mutant Sod alleles in transgenic Drosophila

A subset of patients suffering from familial amyotrophic lateral sclerosis (FALS) exhibit point mutations in the gene encoding Cu-Zn superoxide dismutase [superoxide:superoxide oxidoreductase, EC (SOD)]. The human wild-type and five FALS Sod mutant transgenes were introduced into the fruit fly, Drosophila melanogaster, in a Cu-Zn Sod null background. Sod null flies had dramatically decreased life span, glutathione and methionine content, fertility, locomotor activity, and resistance to hyperoxic stress, compared with wild-type controls. All of these phenotypic manifestations were rescued fully by a single human wild-type allele, expressing 5-10% of wild-type SOD activity. Full recovery of wild-type life span was also observed when human mutant and wild-type alleles were placed together in the fly Sod null background. The FALS Sod mutations alone caused a recessive phenotype, usually involving low or undetectable levels of SOD activity, in which: (i) full restoration of the wild-type phenotype was observed among young adults, and (ii) older adults exhibited a sudden increase in oxidative stress, accompanied by physiological impairment of abrupt onset, and followed by premature death. Thus, the minimal SOD activity associated with the FALS Sod mutations appears to determine longevity, not by chronically increasing oxidative stress, but by limiting the time in which a viable redox environment can be maintained. However, the dominant gain of function by mutant SOD, which occurs in human patients and in the transgenic mouse model of FALS, is not observed in Drosophila.

Genetic polymorphism at two linked loci, Sod and Est-6, in Drosophila melanogaster

We have examined the patterns of polymorphism at two linked loci, Sod and Est-6, separated by nearly 1000 kb on the left arm of chromosome 3 of Drosophila melanogaster. The evidence suggests that natural selection has been involved in shaping the polymorphisms. At the Sod locus, a fairly strong (s>0.01) selective sweep, started >or=2600 years ago, increased the frequency of a rare haplotype, F(A), to about 50% frequency in populations of Europe, Asia, and the Americas. More recently, an F(A) allele mutated to an S allele, which has increased to frequencies 5-15% in populations of Europe, Asia and North America. All S alleles are identical (or very nearly) in sequence and differ by one nucleotide substitution (which accounts for the F-->S electrophoretic difference) from F(A) alleles. At the Est-6 locus, the evidence indicates both directional and balancing selection impacting separately the promoter and the coding regions of the gene, with linkage disequilibrium occurring within each region. Some linkage disequilibrium also exists between the two genes.

Molecular evolution of the Est-6 gene in Drosophila melanogaster: contrasting patterns of DNA variability in adjacent functional regions

We have investigated nucleotide polymorphism at the esterase 6 gene (Est-6) gene, including the complete coding region (1686 bp), as well as the 5'-flanking (1183 bp) and 3'-flanking (193 bp) regions of the gene, in 30 strains of Drosophila melanogaster and in one strain of Drosophila simulans. The level of silent variation is similar in the coding and in the 3'-flanking region, but smaller in the 5'-flanking region. Strong linkage disequilibrium occurs within each region; and also, although less pronounced, between the 5'-flanking region and the rest of the gene, including the 3'-flanking region. We suggest that the pattern of nucleotide polymorphism of Est-6 may be shaped by: (1) directional and balancing selection acting on the promoter and the coding region; and (2) interactions between the two regions that involve variable degrees of hitchhiking. The patterns of linkage disequilibrium, as well as the statistics Z(nS) (Genetics 146 (1997)1197) and B and Q (Genet. Res. 74 (1999) 65), may be interpreted as there being multiple targets of selection within the gene. The previously reported Est-6 allozyme latitudinal clines may be accounted for by the interaction between selective processes in the promoter and coding regions.

High-quality life extension by the enzyme peptide methionine sulfoxide reductase

Cumulative oxidative damages to cell constituents are considered to contribute to aging and age-related diseases. The enzyme peptide methionine sulfoxide reductase A (MSRA) catalyzes the repair of oxidized methionine in proteins by reducing methionine sulfoxide back to methionine. However, whether MSRA plays a role in the aging process is poorly understood. Here we report that overexpression of the msrA gene predominantly in the nervous system markedly extends the lifespan of the fruit fly Drosophila. The MSRA transgenic animals are more resistant to paraquat-induced oxidative stress, and the onset of senescence-induced decline in the general activity level and reproductive capacity is delayed markedly. The results suggest that oxidative damage is an important determinant of lifespan, and MSRA may be important in increasing the lifespan in other organisms including humans.

Doxycycline-induced expression of sense and inverted-repeat constructs modulates phosphogluconate mutase (Pgm) gene expression in adult Drosophila melanogaster

BACKGROUND

A tetracycline-regulated (conditional) system for RNA interference (RNAi) would have many practical applications. Such a strategy was developed using RNAi of the gene for phosphogluconate mutase (Pgm). Pgm is a candidate lifespan regulator: PgmS allele frequency is increased by selection for increased lifespan, whereas PgmM and PgmF allele frequencies are decreased.

RESULTS

The Pgm alleles were cloned and sequenced and were found to differ by amino-acid substitutions consistent with the relative electrophoretic mobilities of the proteins. The 'tet-on' doxycycline-regulated promoter system was used to overexpress PgmS in a wild-type (PgmM) background. Enzyme activity increases of two- to five-fold were observed in five independent transgenic lines. Tet-on was also used to drive expression of an inverted-repeat fragment of Pgm coding region. The inverted-repeat transcript was expected to form a dsRNA hairpin, induce RNAi, and thereby reduce endogenous Pgm gene expression at the RNA level. Endogenous Pgm RNA levels in adult flies were found to be reduced or eliminated by doxycycline treatment in five independent inverted-repeat transgenic lines. Our results show that doxycycline-regulated expression of inverted-repeat constructs can cause a conditional reduction in specific gene expression. The effect of sense and inverted-repeat construct expression on lifespan was assayed in multiple transgenic lines. Under the conditions tested, altered Pgm gene expression had no detectable effect on adult Drosophila lifespan.

CONCLUSIONS

A system for conditional RNAi in Drosophila adults shows promise for assay of gene functions during aging. Our results indicate that Pgm does not have a simple strong effect on longevity.

Antioxidant status and stress resistance in long- and short-lived lines of Drosophila melanogaster

The purpose of this study was to understand the nature of the biochemical and physiological variations between genetically different lines of Drosophila melanogaster. Selection for early or delayed reproduction has given rise to lines with substantial and heritable differences in longevity. The hypotheses tested were that either: (i) a compensatory slowing of metabolism, (ii) increased antioxidative enzyme activities, or (iii) elevated resistance to stressful conditions underlie these differences in longevity. The metabolic rate, metabolic potential (i.e. total amount of oxygen consumed during average lifespan) and speed of walking were all greater in long-lived than in short-lived flies, but there was no enhancement of antioxidant defenses. In fact, catalase activity was significantly lower in the long-lived flies. Long life was largely maintained under heat stress and starvation conditions, and was maintained to a lesser extent upon exposure to paraquat, a superoxide radical generator. In contrast, the 'short-lived' flies had a longer lifespan under cold stress and hyperoxia, also an inducer of radical generation. These results contradict the first two hypotheses and suggest that alleles underlying either long or short life are linked with enhanced resistance to specific kinds of stress, which may account for the preservation of these alleles in the parental population.

Variation in the reversibility of evolution

How reversible is adaptive evolution? Studies of microbes give mixed answers to this question. Reverse evolution has been little studied in sexual populations, even though the population genetics of sexual populations may be quite different. In the present study, 25 diverged replicated populations of Drosophila melanogaster are returned to a common ancestral environment for 50 generations. Here we show that reverse evolution back to the ancestral state occurs, but is not universal, instead depending on previous evolutionary history and the character studied. Hybrid populations showed no greater tendency to undergo successful reverse evolution, suggesting that insufficient genetic variation was not the factor limiting reverse evolution. Adaptive reverse evolution is a contingent process which occurs with only 50 generations of sexual reproduction.

Laboratory selection experiments using Drosophila: what do they really tell us?

Laboratory selection experiments using Drosophila, and other organisms, are widely used in experimental biology. In particular, such experiments on D. melanogaster life history and stress-related traits have been instrumental in developing the emerging field of experimental evolution. However, similar selection experiments often produce inconsistent correlated responses to selection. Unfortunately, selection experiments are vulnerable to artifacts that are difficult to control. In spite of these problems, selection experiments are a valuable research tool and can contribute to our understanding of evolution in natural populations.

Molecular evolution of two linked genes, Est-6 and Sod, in Drosophila melanogaster

We have obtained 15 sequences of Est-6 from a natural population of Drosophila melanogaster to test whether linkage disequilibrium exists between Est-6 and the closely linked Sod, and whether natural selection may be involved. An early experiment with allozymes had shown linkage disequilibrium between these two loci, while none was detected between other gene pairs. The Sod sequences for the same 15 haplotypes were obtained previously. The two genes exhibit similar levels of nucleotide polymorphism, but the patterns are different. In Est-6, there are nine amino acid replacement polymorphisms, one of which accounts for the S-F allozyme polymorphism. In Sod, there is only one replacement polymorphism, which corresponds to the S-F allozyme polymorphism. The transversion/transition ratio is more than five times larger in Sod than in Est-6. At the nucleotide level, the S and F alleles of Est-6 make up two allele families that are quite different from each other, while there is relatively little variation within each of them. There are also two families of alleles in Sod, one consisting of a subset of F alleles, and the other consisting of another subset of F alleles, designed F(A), plus all the S alleles. The Sod F(A) and S alleles are completely or nearly identical in nucleotide sequence, except for the replacement mutation that accounts for the allozyme difference. The two allele families have independent evolutionary histories in the two genes. There are traces of statistically significant linkage disequilibrium between the two genes that, we suggest, may have arisen as a consequence of selection favoring one particular sequence at each locus.

Laboratory selection for the comparative physiologist

An increasingly popular experimental approach in comparative physiology is to study the evolution of physiological traits in the laboratory, using microbial, invertebrate and vertebrate models. Because selective conditions are well-defined, selected populations can be replicated and unselected control populations are available for direct comparison, strong conclusions regarding the adaptive value of an evolved response can be drawn. These studies have shown that physiological systems evolve rapidly in the laboratory, but not always as one would expect from comparative studies of different species. Laboratory environments are often not as simple as one thinks, so that the evolution of behavioral differences or selection acting on different life stages can lead to unanticipated results. In some cases, unexpected responses to laboratory selection may suggest new insights into physiological mechanisms, which might not be available using other experimental approaches. I outline here recent results (including success stories and caveats for the unwary investigator) and potential directions for selection experiments in comparative physiology.

CuZn-SOD promoter-driven expression in the Drosophila central nervous system

The aim of this study was to ascertain the status of CuZn superoxide dismutase (CuZn-SOD) expression in the central nervous system of Drosophila melanogaster. Immunoblot analysis of dissected tissue extracts revealed low levels of the CuZn-SOD protein in adult brains relative to other adult and larval tissues. To explore further this observation, three different reporter constructs containing different elements of the CuZn-SOD promoter domain were used for the generation of transgenic flies. A high level of reporter gene expression occurred during the second wave of neurogenesis (third instar and early pupal stages) in scattered, proliferating neuroblasts (NBs) and in proliferation centers of the optic lobe. In mature, postmitotic neurons, this expression was lower relative to other tissues. In adult flies, at all ages examined, there was little if any detectable reporter gene expression in cells of the central nervous system. These results suggest that one of the key components of the antioxidant defenses, CuZn-SOD, is quite low in postmitotic neural tissue, rendering it particularly susceptible to oxidative damage during aging.

Motorneurons, reactive oxygen, and life span in Drosophila

Aging and life span are widely recognized, but poorly understood, aspects of basic biology. Fortunately, genetic approaches to understanding the mechanisms governing these processes are beginning to bear fruit. One line of investigation has established that incompletely reduced forms of oxygen, arising as by-products of respiration and cellular catabolism, play an important, and perhaps universal, role in aging and life span determination. An important refinement of this model of aging, suggested by recent experiments in our laboratory, is that the critical nexus of the relationship between reactive oxygen species and life span is highly localized and, in fact, may reside principally in the motorneuron. Here we analyze the strengths and weaknesses of the reactive oxygen species/motorneuron model of aging by comparing the studies on which it is based, which used the approach of targeted transgene expression in Drosophila, with studies from other laboratories using different genetic approaches, principally mutation and selection. The results encourage the view that an understanding of the mechanisms that underlie this widely recognized aspect of basic biology is within our grasp.

Genetics of aging in Drosophila

The genetics of aging in Drosophila are reviewed under the separate headings of population genetics, physiological genetics, and molecular genetics. However, connections between these sub-fields are brought forward for discussion.

Activity of superoxide dismutase and catalase in the bean weevil (Acanthoscelides obtectus) selected for postponed senescence

Relationship of superoxide dismutase and catalase activities and aging were tested using bean weevil lines selected for postponed senescence. The beetles of different age (young and old) and mating status (virgin and mated) from the extended longevity lines were compared with their counterparts derived from the short-lived lines for activities of SOD and catalase. The old beetles from the long-lived lines had statistically significant higher activity of SOD than their controls. Although we did not find a significant effect of catalase on longevity, beetles originating from both types of lines exhibited an increased catalase activity during mating processes. In addition, we did observe an increased activity of catalase in one-day-old beetles of the short-lived lines relative to the same-aged individuals of the long-lived lines.

FLP recombinase-mediated induction of Cu/Zn-superoxide dismutase transgene expression can extend the life span of adult Drosophila melanogaster flies

Yeast FLP recombinase was used in a binary transgenic system ("FLP-OUT") to allow induced overexpression of catalase and/or Cu/Zn-superoxide dismutase (Cu/ZnSOD) in adult Drosophila melanogaster. Expression of FLP recombinase was driven by the heat-inducible hsp70 promoter. Once expressed, FLP catalyzed the rearrangement and activation of a target construct in which expression of catalase or Cu/ZnSOD cDNAs was driven by the constitutive actin5C promoter. In this way a brief heat pulse (120 or 180 min, total) of young adult flies activated transgene expression for the rest of the life span. FLP-OUT allows the effects of induced transgene expression to be analyzed in control (no heat pulse) and experimental (heat pulse) populations with identical genetic backgrounds. Under the conditions used, the heat pulse itself always had neutral or slightly negative effects on the life span. Catalase overexpression significantly increased resistance to hydrogen peroxide but had neutral or slightly negative effects on the mean life span. Cu/ZnSOD overexpression extended the mean life span up to 48%. Simultaneous overexpression of catalase with Cu/ZnSOD had no added benefit, presumably due to a preexisting excess of catalase. The data suggest that oxidative damage is one rate-limiting factor for the life span of adult Drosophila. Finally, experimental manipulation of the genetic background demonstrated that the life span is affected by epistatic interactions between the transgene and allele(s) at other loci.

Bacterial senescence: stasis results in increased and differential oxidation of cytoplasmic proteins leading to developmental induction of the heat shock regulon

Aging, or senescence, is the progressive deterioration of every bodily function over time. A fundamental question that applies to all life forms, including growth-arrested bacteria, is why growing older by necessity causes organisms to grow more fragile. In this work, we demonstrate that the levels of oxidized proteins is correlated to the age of a stationary-phase Escherichia coli culture; both disulfide bridge formation of a cytoplasmic leader-less alkaline phosphatase and protein carbonyl levels increase during stasis. The stasis-induced increase in protein oxidation is enhanced in cells lacking the global regulators OxyR and sigmas. Some proteins were found to be specifically susceptible to stasis-induced oxidation; notably several TCA cycle enzymes, glutamine synthetase, glutamate synthase, pyruvate kinase, DnaK, and H-NS. Evidence that oxidation of target proteins during stasis serves as the signal for stationary-phase, developmental, induction of the heat shock regulon is presented by demonstrating that this induction is mitigated by overproducing the superoxide dismutase SodA. In addition, cells lacking cytoplasmic superoxide dismutase activity exhibit superinduction of heat shock proteins. The possibility that oxidative sensitivity of TCA cycle enzymes serves as a feedback mechanism down-regulating toxic respiration is discussed.

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.

DNA variation at the Sod locus of Drosophila melanogaster: an unfolding story of natural selection

Patterns of variation at the Sod locus of Drosophila melanogaster suggest that the protein polymorphism at this locus has very recently arisen. In addition, it appears that a previously rare DNA variant has been recently and rapidly driven to intermediate frequency. From the size of the region (>20 kb) that has been swept along with this rare variant, and patterns of linkage disequilibrium in the region, it is inferred that strength of selection was large (s > 0.01) and that the sweep occurred more than 25,000 generations ago. In addition, there are striking similarities to patterns of variation observed at the Est6 and Est-P loci, which are located approximately 1,000 kb from Sod.

Life history evolution in guppies (Poecilia reticulata): guppies as a model for studying the evolutionary biology of aging

Natural populations of guppies can be found with different communities of predators. We have contrasted the early life history of guppies from high and low predation localities. Life history theory predicts that such differences in mortality pattern will select for evolutionary changes in the guppy's life history. Specifically, guppies in high-predation localities are predicted to mature at an earlier age and devote more of their resources to reproduction. We have demonstrated the predicted differences in life history patterns with experiments and observations on guppies from each type of locality. We have also selected for the predicted changes in the life history by manipulating mortality patterns in natural populations. Theories for the evolution of senescence predict that these same mortality patterns will also select for changes in the rate of aging. Specifically, guppies from high-predation localities should have higher rates of aging than their counterparts from low-predation localities. Experiments that select for changes in the early life history should also select for changes in the rate of aging. The existing work on guppies, therefore, presents the opportunity to use them as a new experimental system for studying the evolutionary biology of aging. Finally, I present preliminary results from a pilot study of aging in guppies. This study differs from the earlier work by Comfort because these fish have been reproductively active since they attained maturity; Comfort's fish were maintained as virgins throughout their lives. This study makes two important points. First, age-specific changes in reproductive performance represent as important an index of aging as mortality rates. Second, the rate of aging may be far more rapid in reproductively active individuals.

Factors contributing to the plasticity of the extended longevity phenotypes of Drosophila

A number of laboratories have constructed independently derived long-lived strains of Drosophila, each of which have similar but not identical patterns of variability in their adult longevity. Given the observed plasticity of longevity within each of these strains, it would be useful to review the operational and environmental factors that give rise to this phenotypic plasticity and ascertain whether they are common or strain specific. Our review of the more extensively analyzed strains suggests that the allelic composition of the initial genomes and the selection/transgene strategy employed yield extended longevity strains with superficially similar phenotypes but which are probably each the result of different proximal genetic mechanisms. This then offers a plausible explanation for the differential effects of various environmental factors on each strain's particular pattern of phenotypic plasticity. It also illustrates that the species has the potential to employ any one of a number of different proximal mechanisms, each of which give rise to a similar longevity phenotype.

Aging mechanisms in fruit files

Genetic analysis of Drosophila has provided evidence in support of two proposed evolutionary genetic mechanisms of aging: mutation accumulation and antagonistic pleiotropy. Both mechanisms result from the lack of natural selection acting on old organisms. Analyses of large numbers of files have revealed that mortality rates do not continue to rise with age as previously thought, but plateau at advanced ages. This phenomenon has implications both for models and for definitions of aging, and may be explained by the evolutionary theories. The physiological processes and genes most relevant to aging are being identified using Drosophila lines selected in the laboratory for postponed senescence. Oxidative stress and insufficient metabolic reserves/capacity may be particularly important factors in limiting the fruitfly lifespan. Genes which exhibit aging-related changes in expression are now being identified. Transgenic files are being used to analyze the mechanisms of such aging-related gene expression, and to test the effects of specific genes on aging and aging-related deterioration.

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.

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.

Comparative biochemical and stress analysis of genetically selected Drosophila strains with different longevities

We have performed a comparative analysis of the effects of age of reproduction on the biochemical (protein, lipid, and glycogen content) and stress resistance (ability to survive starvation, desiccation, and exogenous paraquat) parameters on 10 sister lines of five different Drosophila strains. Four pairs of these sister lines were selected under different regimens for either early or delayed reproduction; the fifth pair was maintained in a nonselected state and served as the baseline strain to which all others were compared. It is generally accepted that the early regimens give rise to short-lived phenotypes, whereas the delayed regimens give rise to long-lived phenotypes. Our results suggest that a mechanism involving lipid and starvation resistance is not operative in our long-lived strains. In addition, a mechanism involving glycogen content and desiccation resistance is only weakly supported. Finally, there is strong support for a mechanism that gives rise to enhanced paraquat resistance and therefore may involve regulatory changes in the pattern of ADS gene expression. In addition, the 15-day early age of reproduction regimen (M type) shows qualitatively similar responses to that of the late age at reproduction regimen (L type). These results suggest that correlations between biochemical traits and longevity must be interpreted with caution. We discuss possible reasons for these results, including the possibility of multiple mechanisms, each leading to a different extended longevity phenotype.

Superoxide dismutase, aging, and degenerative disease

Over 15 years of research on correlations between superoxide dismutase (SOD) activity and aging or life span have failed to provide a consistent picture of the role of SOD in aging. While genetic manipulations that increase CuZn-SOD activity have only a slight, if any, effect on maximum life span in several species, they do increase resistance to oxidative stress. However, increasing both CuZn-SOD and catalase does significantly increase maximum life span. Decreased SOD expression in a variety of species increases their vulnerability to oxidative stress, and in the case of genetically altered CuZn-SOD, leads to premature death of motor neurons in humans. Little is known about the regulation of expression of SOD and other antioxidant defense enzymes in eukaryotes. The research summarized below collectively suggest that SOD plays an important role in longevity and degenerative disease, but much remains to be learned before manipulation of SOD expression can be considered for effective intervention in either process.