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

Expression of the pro-apoptotic gene gadd153/chop is elevated in liver with aging and sensitizes cells to oxidant injury

Aging is generally accompanied by reduced tolerance to oxidative stress and altered responsiveness to proliferative signals. We have shown that hepatocytes derived from aged rats (24-26 months) exhibit greater sensitivity to H(2)O(2) treatment and reduced proliferation following epidermal growth factor (EGF) treatment than cells of young adult rats (5-6 months). Here we examined the effects of aging and calorie restriction (CR) on expression of the oxidative stress-inducible and pro-apoptotic gene gadd153 (chop) in these hepatocytes, and we investigated its influence on sensitivity to oxidants. We show that aging was associated with elevated expression of gadd153, both basally and in response to H(2)O(2) treatment. CR, which attenuates age-associated declines in stress tolerance, prevented the age-related increase in gadd153 expression. EGF treatment also resulted in gadd153 induction in old cells. This effect was absent in young cells and in old cells of CR rats. gadd153 induction by EGF was reactive oxygen species-dependent and correlated with heightened sensitivity to subsequent H(2)O(2) treatment, suggesting that elevated Gadd153 contributes to the greater sensitivity of EGF-pretreated old cells to oxidative stress. Additional support for this hypothesis was provided by experiments with Rat1 fibroblasts in which conditional expression of Gadd153 conferred increased sensitivity to H(2)O(2). We propose a model whereby the diminished ability of old hepatocytes to overcome an EGF-triggered reactive oxygen species load leads to induction of the proapoptotic gene gadd153, which, in turn, sensitizes the cells to oxidant injury. Our findings point to gadd153 expression levels as an important factor in liver aging.

Clines and adaptive evolution in the methuselah gene region in Drosophila melanogaster

In an effort to characterize further the patterns of selection and adaptive evolution at the methuselah locus in Drosophila species, we extended an analysis of geographical variation to include single nucleotide polymorphisms (SNPs) in adjacent genes on either side of the mth locus, and examined the molecular variation in a neighbouring methuselah paralogue (mth2). An analysis of 13 SNPs spanning a region of nearly 19 kilobases surrounding the mth locus demonstrated that a clinal pattern associated with the most common mth haplotype does not extend to adjacent gene loci, providing compelling evidence that the clinal pattern results from selection on as yet unidentified sites associated with the functional mth locus. mth2 exhibited a significant pattern of adaptive divergence among D. melanogaster, D. simulans and D. yakuba similar to that seen at mth. However, Ka : Ks ratios indicate a difference in levels of functional constraint at the two methuselah, loci with mth2 exhibiting a five- to six-fold reduction in levels of amino acid divergence relative to mth.

Age-related impairment of the transcriptional responses to oxidative stress in the mouse heart

To investigate the transcriptional response to oxidative stress in the heart and how it changes with age, we examined the cardiac gene expression profiles of young (5-mo-old), middle-aged (15-mo-old), and old (25-mo-old) C57BL/6 mice treated with a single intraperitoneal injection of paraquat (50 mg/kg). Mice were killed at 0, 1, 3, 5, and 7 h after paraquat treatment, and the gene expression profile was obtained with high-density oligonucleotide microarrays. Of 9,977 genes represented on the microarray, 249 transcripts in the young mice, 298 transcripts in the middle-aged mice, and 256 transcripts in the old mice displayed a significant change in mRNA levels (ANOVA, P < 0.01). Among these, a total of 55 transcripts were determined to be paraquat responsive for all age groups. Genes commonly induced in all age groups include those associated with stress, inflammatory, immune, and growth factor responses. Interestingly, only young mice displayed a significant increase in expression of all three isoforms of GADD45, a DNA damage-responsive gene. Additionally, the number of immediate early response genes (IEGs) found to be induced by paraquat was considerably higher in the younger animals. These results demonstrate that, at the transcriptional level, there is an age-related impairment of specific inducible pathways in the response to oxidative stress in the mouse heart.

Testing an 'aging gene' in long-lived drosophila strains: increased longevity depends on sex and genetic background

Molecular advances of the past decade have led to the discovery of a myriad of 'aging genes' (methuselah, Indy, InR, Chico, superoxide dismutase) that extend Drosophila lifespan by up to 85%. Despite this life extension, these mutants are no longer lived than at least some recently wild-caught strains. Typically, long-lived mutants are identified in relatively short-lived genetic backgrounds, and their effects are rarely tested in genetic backgrounds other than the one in which they were isolated or derived. However, the mutant's high-longevity phenotype may be dependent on interactions with alleles that are common in short-lived laboratory strains. Here we set out to determine whether one particular mutant could extend lifespan in long-lived genetic backgrounds in the fruit fly, Drosophila melanogaster. We measured longevity and resistance to thermal stress in flies that were transgenically altered to overexpress human superoxide dismutase (SOD) in the motorneurones in each of 10 genotypes. Each genotype carried the genetic background from a different naturally long-lived wild-caught Drosophila strain. While SOD increased lifespan on average, the effect was genotype- and sex-specific. Our results indicate that naturally segregating genes interact epistatically with the aging gene superoxide dismutase to modify its ability to extend longevity. This study points to the need to identify mutants that increase longevity not only in the lab strain of origin but also in naturally long-lived genetic backgrounds.

Conditional tradeoffs between aging and organismal performance of Indy long-lived mutant flies

Alterations that extend the life span of animals and yeast typically involve decreases in metabolic rate, growth, physical activity, and/or early-life fecundity. This negative correlation between life span and the ability to assimilate and process energy, to move, grow, and reproduce, raises questions about the potential utility of life span extension. Tradeoffs between early-life fitness and longevity are central to theories of the evolution of aging, which suggests there is necessarily a price to be paid for reducing the rate of aging. It is not yet clear whether life span can be extended without undesirable effects on metabolism and fecundity. Here, we report that the long-lived Indy mutation in Drosophila causes a decrease in the slope of the mortality curve consistent with a slowing in the rate of aging without a concomitant reduction in resting metabolic rate, flight velocity, or age-specific fecundity under normal rearing conditions. However, Indy mutants on a decreased-calorie diet have reduced fecundity, suggesting that a tradeoff between longevity and this aspect of performance is conditional, i.e., the tradeoff can occur in a stressful environment while being absent in a more favorable environment. These results provide evidence that there do exist mechanisms, albeit conditional, that can extend life span without significant reduction in fecundity, metabolic rate, or locomotion.

Breakdown in correlations during laboratory evolution. I. Comparative analyses of Drosophila populations

We provide evidence from comparisons of populations of Drosophila that evolutionary correlations between longevity and stress resistance break down over the course of laboratory evolution. Using 15 distinct evolutionary regimes, we created 75 populations that were differentiated for early fecundity, longevity, starvation resistance, desiccation resistance, and developmental time. In earlier experiments, selection for postponed aging produced increases in stress resistance, whereas selection for increased stress resistance produced increases in longevity. Direct estimates of correlations also indicated an antagonistic relationship between early fecundity on one hand and longevity or stress resistance on the other. Laboratory evolution of extreme values of stress resistance, however, led to a breakdown in these evolutionary relationships. There was no evidence that these significant changes in correlation resulted from genotype-by-environment interactions or inbreeding. These findings suggest that correlations between functional characters are not necessarily durable features of a species, and that short-term evolutionary responses cannot be extrapolated reliably to longer-term evolutionary patterns.

Synaptic Drosophila UNC-13 is regulated by antagonistic G-protein pathways via a proteasome-dependent degradation mechanism

UNC-13 is a highly conserved plasma membrane-associated synaptic protein implicated in the regulation of neurotransmitter release through the direct modulation of the SNARE exocytosis complex. Previously, we characterized the Drosophila homologue (DUNC-13) and showed it to be essential for neurotransmitter release immediately upstream of vesicular fusion ("priming") at the neuromuscular junction (NMJ). Here, we show that the abundance of DUNC-13 in NMJ synaptic boutons is regulated downstream of GalphaS and Galphaq pathways, which have inhibitory and facilitatory roles, respectively. Both cAMP modulation and PKA function are required for DUNC-13 synaptic up-regulation, suggesting that the cAMP pathway enhances synaptic efficacy via DUNC-13. Similarly, PLC function and DAG modulation also regulate the synaptic levels of DUNC-13, through a mechanism that appears independent of PKC. Our results suggest that proteasome-mediated protein degradation is the primary mechanism regulating DUNC-13 levels at the synapse. Both PLC- and PKA-mediated pathways appear to regulate synaptic levels of DUNC-13 through controlling the rate of proteasome-dependent DUNC-13 degradation. We conclude that the functional abundance of DUNC-13 at the synapse, a key determinant of synaptic vesicle priming and neurotransmitter release probability, is primarily regulated by the rate of protein degradation, rather than translocation or transport, convergently controlled via both cAMP and DAG signal transduction pathways.

Genetic loci modulating fitness and life span in Caenorhabditis elegans: categorical trait interval mapping in CL2a x Bergerac-BO recombinant-inbred worms

Quantitative trait loci (QTL) can implicate an unbiased sampling of genes underlying a complex, polygenic phenotype. QTL affecting longevity in Caenorhabditis elegans were mapped using a CL2a x Bergerac-BO recombinant-inbred population. Genotypes were compared at 30 transposon-specific markers for two paired sample sets totaling 171 young controls and 172 longevity-selected worms (the last-surviving 1%) from a synchronously aged population. A third sample set, totaling 161 worms from an independent culture, was analyzed for confirmation of loci. At least six highly significant QTL affecting life span were detected both by single-marker (chi(2)) analysis and by two interval-mapping procedures--one intended for nonparametric traits and another developed specifically for mapping of categorical traits. These life-span QTL were located on chromosomes I (near the hP4 locus), III (near stP127), IV (near stP44), V (a cluster of three peaks, near stP192, stP23, and stP6), and X (two distinct peaks, near stP129 and stP2). Epistatic effects on longevity were also analyzed by Fisher's exact test, which indicated a significant life-span interaction between markers on chromosomes V (stP128) and III (stP127). Several further interactions were significant in the initial unselected population; two of these, between distal loci on chromosome V, were completely eliminated in the long-lived subset. Allelic longevity effects for two QTL, on chromosomes IV and V, were confirmed in backcrossed congenic lines and were highly significant in two very different environments-growth on solid agar medium and in liquid suspension culture.

Modelling human diseases in Drosophila and Caenorhabditis

Drosophila (fruitfly) and Caenorhabditis (nematode worm) are useful model organisms for understanding many molecular and cellular mechanisms of human disease. Work on them is powered by versatile gene discovery methods, output of their genome projects, and a biology that has much in common with that of humans. They contain homologues of most human disease genes. Many aspects of human disease, and of defences against disease, are also found in flies and worms. These include cancer, ageing, neurodegeneration, infectious disease, innate immunity, and dysfunction of neurotransmitter and endocrine systems.

Lifelong voluntary exercise in the mouse prevents age-related alterations in gene expression in the heart

We present the first quantitative gene expression analysis of cardiac aging under conditions of sedentary and active lifestyles using high-density oligonucleotide arrays representing 11,904 cDNAs and expressed sequence tags (ESTs). With these data, we test the hypothesis that exercise attenuates the gene expression changes that normally occur in the aging heart. Male mice (Mus domesticus) were sampled from the 16th generation of selective breeding for high voluntary exercise. For the selective breeding protocol, breeders were chosen based on the maximum number of wheel revolutions run on days 5 and 6 of a test at 8 wk of age. For the colony sampled herein, mice were housed individually over their entire lifetimes (from weaning) either with or without access to running wheels. The hearts of these two treatment groups (active and sedentary) were assayed at middle age (20 mo) and old age (33 mo). Genes significantly affected by age in the hearts of the sedentary population by at least a 50% expression change (n = 137) were distributed across several major categories, including inflammatory response, stress response, signal transduction, and energy metabolism. Genes significantly affected by age in the active population were fewer (n = 62). Of the 42 changes in gene expression that were common to both treatment groups, 32 (72%) displayed smaller fold changes as a result of exercise. Thus exercise offset many age-related gene expression changes observed in the hearts of the sedentary animals. These results suggest that adaptive physiological mechanisms that are induced by exercise can retard many effects of aging on heart muscle at the transcriptional level.

A search for doxycycline-dependent mutations that increase Drosophila melanogaster life span identifies the VhaSFD, Sugar baby, filamin, fwd and Cctl genes

BACKGROUND

A P-type transposable element called PdL has been engineered with a doxycycline-inducible promoter directed out through the 3' end of the element. Insertion of PdL near the 5' end of a gene often yields doxycycline-dependent overexpression of that gene and a mutant phenotype. This functional genomics strategy allows for efficient screening of large numbers of genes for overexpression phenotypes.

RESULTS

PdL was mobilized to around 10,000 new locations in the Drosophila melanogaster genome and used to search for genes that would extend life span when overexpressed. Six lines were identified in which there was a 5-17% increase in life span in the presence of doxyxcycline. The mutations were molecularly characterized and in each case a gene was found to be overexpressed using northern blots. Two genes did not have previously known phenotypes and are implicated in membrane transport: VhaSFD encodes a regulatory subunit of the vacuolar ATPase proton pump (H+-ATPase), whereas Sugar baby (Sug) is related to a maltose permease from Bacillus. Three PdL mutations identified previously characterized genes: filamin encodes the homolog of an actin-polymerizing protein that interacts with presenilins. four wheel drive (fwd) encodes a phosphatidylinositol-4-kinase (PI 4-kinase) and CTP:phosphocholine cytidylyltransferase-l (Cctl) encodes the rate-limiting enzyme in phosphatidylcholine synthesis. Finally, an apparently novel gene (Red herring, Rdh) was found in the first intron of the encore gene.

CONCLUSIONS

Screening for conditional mutations that increase Drosophila life span has identified genes implicated in membrane transport, phospholipid metabolism and signaling, and actin cytoskeleton organization.

Positive selection of Caenorhabditis elegans mutants with increased stress resistance and longevity

We developed selective conditions for long-lived mutants of the nematode Caenorhabditis elegans by subjecting the first larval stage (L1) to thermal stress at 30 degrees for 7 days. The surviving larvae developed to fertile adults after the temperature was shifted to 15 degrees. A total of one million F(2) progeny and a half million F(3) progeny of ethyl-methanesulfonate-mutagenized animals were treated in three separate experiments. Among the 81 putative mutants that recovered and matured to the reproductive adult, 63 retested as thermotolerant and 49 (80%) exhibited a >15% increase in mean life span. All the known classes of dauer formation (Daf) mutant that affect longevity were found, including six new alleles of daf-2, and a unique temperature-sensitive, dauer-constitutive allele of age-1. Alleles of dyf-2 and unc-13 were isolated, and mutants of unc-18, a gene that interacts with unc-13, were also found to be long lived. Thirteen additional mutations define at least four new genes.

Metabolic aging and predicted longevity: results of a cross-sectional study in post-menopausal women

BACKGROUND AND AIMS

The extent to which general characteristics of metabolic aging contribute to differences in life span among individuals remains uncertain. The objective of this study was to examine the association of age-related physiological and metabolic variables with predicted longevity in postmenopausal women.

METHODS

Subjects were 33 healthy women aged 55-65 years. Total and resting energy expenditure, body temperature, immune function as assessed by a delayed-type hypersensitivity skin test (DTH), lipid profile, and reported dietary intake were measured.

RESULTS

There were no significant associations between longevity, energy expenditure, body temperature, lipid profile, or dietary intake. However, there was a significant association of predicted longevity with DTH (partial r=0.44, p=0.023).

CONCLUSIONS

These results suggest that immune function may predict familial differences in longevity, while energy expenditure, body temperature, lipid profile, and dietary intake are unrelated. Although the small sample size may have limited the ability to detect metabolic effects on longevity in this study, the general approach may be broadly applicable to examinations of metabolic aging in humans.

Dissociation between functional senescence and oxidative stress resistance in Drosophila

Many studies strongly suggest a causal link between oxidative stress and determination of life span. The relationship between oxidative stress and age-related functional declines, however, is less clear. Additionally, the full spectrum of functional declines associated with aging has not been systematically evaluated in the fruit fly, Drosophila melanogaster, one of the leading models for aging research. Toward a more comprehensive assessment of functional senescence in Drosophila, we evaluated a series of behaviors in control flies of increasing ages. Our studies reveal a novel age-dependent functional decline in the olfactory system and confirm previous reports of age-related locomotor defects in flies. Behavioral responses to electric shock and light are maintained in aged flies. Thus, some sensory systems senesce during the first several weeks of life while others do not. Interestingly, the age-dependent functional declines in olfactory and locomotor systems are indistinguishable in control flies and methuselah, a mutant with enhanced resistance to oxidative stress and increased life span. Our results indicate that enhanced resistance to oxidative stress and extension of life span do not necessarily confer protection from age-related functional declines.

Longevity determination genes in Drosophila melanogaster

Identification of longevity mutants is crucial for genetic approach to dissect the molecular mechanism of aging and longevity determination. In Drosophila melanogaster, several mutations have been shown to extend the longevity: methuselah encoding a putative G-protein coupled receptor, Indy encoding a sodium dicarboxylate cotransporter, chico encoding insulin receptor substrate, and InR encoding the insulin-like receptor. Extended longevity phenotypes were also observed in transgenic flies overexpressing antioxidant enzymes, Cu/Zn superoxide dismutase and Catalase, Cu/Zn SOD only, or a molecular chaperone, hsp70. Pleiotropism of mutations is a limitation associated with conventional mutagenesis for efficient detection of longevity determination genes. Using a conditional misexpression system, we identified Drosophila POSH (DPOSH), a scaffold protein containing RING finger and four SH3 domains, whose ubiquitous overexpression in adult stage extends the longevity. Neural-specific overexpression of DPOSH is sufficient to extend the longevity, whereas overexpression in non-neural tissues during development induces apoptosis through activation of JNK/SAPK pathway.

Age-dependent agonist-specific dysregulation of membrane-mediated signal transduction: emergence of the gate theory of aging

Although a general mechanism for the limited responsiveness of senescent cells has yet to be established, reduced responsiveness may in part be ascribed to deficits in the apparatus required for cell surface receptor-mediated signal transduction. Age-related changes of receptor-mediated signal transduction occur at many levels, and are known to include quantitative and qualitative changes in growth factor receptors, G-protein coupled receptors, and many other downstream signaling molecules. Here, we emphasize the prime role of the cellular surface in the perception and transmission of external stimuli in response to the aging process. As major means of cellular signal transduction, the receptor tyrosine kinase (RTK) system and the G protein-coupled receptor (GPCR) system of senescent cells were investigated. We observed that the RTK system was severely damaged, while the GPCR system was only partially inactivated by aging. These results suggest that the agonist-dependent dysregulation of and imbalance of signal transduction pathways might be responsible for the functional deterioration of senescent cells, and indicate a possibility of the functional recovery of senescent cells through agonist-specific signal system activation. Moreover, those data evoke the emerging concept that the senescent phenotype may be modulated by the membrance-associated signal system, implying the gate theory of aging.

Different age-specific demographic profiles are generated in the same normal-lived Drosophila strain by different longevity stimuli

We review the empirical data obtained with our normal-lived Ra control strain of Drosophila and show that this one genome is capable of invoking at least three different responses to external stimuli that induce the animal to express one of three different extended longevity phenotypes, each of which arises from one of three different antagonistic molecular mechanisms of stress resistance. The phenotypes are distinguished by different age-specific mortality patterns. Depending on the selected mechanism, the genome may respond by expressing a delayed onset of senescence (type 1), an increased early survival (type 2), or an increased late survival (type 3) phenotype, suggesting their different demographic effects. We suggest that the different demographic effects stem in part from the differential ability of each selection regime to reallocate the organism's energy from reproduction to somatic maintenance. These data document the complexity of the aging process and argue for a relationship between molecular mechanisms and longevity phenotypes.

A test of evolutionary theories of aging

Senescence is a nearly universal feature of multicellular organisms, and understanding why it occurs is a long-standing problem in biology. The two leading theories posit that aging is due to (i) pleiotropic genes with beneficial early-life effects but deleterious late-life effects ("antagonistic pleiotropy") or (ii) mutations with purely deleterious late-life effects ("mutation accumulation"). Previous attempts to distinguish these theories have been inconclusive because of a lack of unambiguous, contrasting predictions. We conducted experiments with Drosophila based on recent population-genetic models that yield contrasting predictions. Genetic variation and inbreeding effects increased dramatically with age, as predicted by the mutation theory. This increase occurs because genes with deleterious effects with a late age of onset are unopposed by natural selection. Our findings provide the strongest support yet for the mutation theory.

Matrix-mediated cellular rejuvenation

Biomaterial surface morphology and chemistry influence cell responses mediated via signaling cascades that regulate a wide range of metabolic processes. These responses may range from changes in surface adhesion and remodeling of the extracellular matrix to activation of cytokine, cytoskeletal and other biochemical pathways regulating or modulating cellular morphology and function. The present study has focused on collagen Type I, a key extracellular matrix protein, and its potential impact on the process of cellular aging. This study was undertaken for several reasons. First, several investigators reported that growth of cells on a collagen matrix markedly enhanced the resistance of cells to stresses. Second, a large body of accumulated data strongly indicated a relationship between the potential to respond to stresses and cellular aging with the former strongly influencing the rate of the latter. Finally, it has been recently demonstrated that in aged cells one of the key aging-related processes previously considered irreversible, attenuation of the expression of a major stress response protein, Hsp70, can be reversed. This fact together with a probable regulatory role of the stress response potential in cellular aging suggested a possibility that the cellular aging process as a whole can be altered. Indeed, in the present study, growth on a denatured collagen matrices reversed in aged cells not only the attenuation of Hsp70 expression but also other aging-related processes, such as beta-galactosidase expression, increase in protein oxidation and changes in cell morphology. Moreover, it appeared to reduce the rate of aging in young cells. Understanding the nature of collagen matrix-mediated cellular rejuvenation might suggest approaches for interfering with organismic aging. Some immediate applications include cell rejuvenation for purposes of cloning and reduction of the rate of aging during expansion of stem cells for purposes of tissue engineering.

Aging: the fitness-stress continuum and genetic variability

Assuming the stress theory of aging, longevity depends upon primary selection for stress resistance and metabolic efficiency. Predominantly based upon experimental studies in the insect Drosophila melanogaster, high genetic variability for fitness, especially mortality, occurs under extreme stress. Isofemale strains derived from the progeny of recently collected single inseminated Drosophila females from the wild should provide useful biological material for extrapolating to quantitative genetic studies in man. Furthermore, environments from the benign (hormetic) to the extreme can be incorporated. Survival to old age may depend upon genes for metabolic efficiency that respond to the environmental challenges of living as limits to adaptation are approached. Under this scenario the survival of longevity mutants in man to ages analogous to the extreme life spans found in some experimental organisms under benign or protected laboratory conditions is unlikely. More future emphasis is needed on genetic variation of longevity in natural populations of experimental organisms under an array of realistically stressful environments to act as an evolutionary model for longevity in our own species.

The complex genetic architecture of Drosophila life span

Continuous phenotypic variation in life span results from segregating genetic variation at multiple loci, the environmental sensitivity of expression of these loci, and the history of environmental variation experienced by the organism throughout its life. We have mapped quantitative trait loci (QTL) that produce variation in the life span of mated Drosophila melanogaster using a panel of recombinant inbred lines (RIL) that were backcrossed to the parental strains from which they were derived. Five QTL were identified that influence mated life span, three were male-specific, one was female-specific, and one affected life span in both sexes. The additive allelic effects and dominance of QTL were highly sex-specific. One pair of QTL also exhibited significant epistatic effects on life span. We summarize all of the QTL mapping data for Drosophila life span, and outline future prospects for disentangling the genetic and environmental influences on this trait.

Presynaptic regulation of neurotransmission in Drosophila by the g protein-coupled receptor methuselah

Regulation of synaptic strength is essential for neuronal information processing, but the molecular mechanisms that control changes in neuroexocytosis are only partially known. Here we show that the putative G protein-coupled receptor Methuselah (Mth) is required in the presynaptic motor neuron to acutely upregulate neurotransmitter exocytosis at larval Drosophila NMJs. Mutations in the mth gene reduce evoked neurotransmitter release by approximately 50%, and decrease synaptic area and the density of docked and clustered vesicles. Pre- but not postsynaptic expression of normal Mth restored normal release in mth mutants. Conditional expression of Mth restored normal release and normal vesicle docking and clustering but not the reduced size of synaptic sites, suggesting that Mth acutely adjusts vesicle trafficking to synaptic sites.

Mechanisms of aging: an appraisal of the oxidative stress hypothesis

The main purpose of this article is to provide a critical overview of the currently available evidence bearing on the validity of the oxidative stress hypothesis of aging, which postulates that senescence-associated attenuations in physiological functions are caused by molecular oxidative damage. Several lines of correlative evidence support the predictions of the hypothesis, e.g., macromolecular oxidative damage increases with age and tends to be associated with life expectancy of organisms. Nevertheless, a direct link between oxidative stress and aging has not as yet been established. Single gene mutations have been reported to extend the life spans of lower organisms, such as nematodes and insects; however, such prolongations of chronological clock time survival are usually associated with decreases in the rate of metabolism and reproductive output without affecting the metabolic potential, i.e., the total amount of energy consumed during life. Studies on genetic manipulations of the aging process have often been conducted on relatively short-lived strains that are physiologically weak, whereby life-span extensions can not be unambiguously assigned to a slowing effect on the rate of aging. It is concluded that although there is considerable evidence implicating oxidative stress in the aging process, additional evidence is needed to clearly define the nature of the involvement.

Age-related decline in cellular response to oxidative stress: links to growth factor signaling pathways with common defects

Accumulation of oxidative damage is believed to be a major contributor to the decline in physiologic function that characterizes mammalian aging, and recent studies suggest that how well you respond to acute oxidative stress is an important factor in determining longevity. Oxidant injury elicits a wide spectrum of responses ranging from proliferation to cell death. The particular outcome observed largely reflects the severity of the stress encountered and the relative degree of activation of various signal transduction pathways aimed at enhancing survival or inducing cell death. Herein we examine the relationship between pathways important in supporting cell survival in response to oxidant injury and those involved in regulating proliferation. We review evidence indicating that [Curr. Opin. Cell Biol. 10 (1998) 248] common pathways are indeed involved in regulating these responses, and [Physiol. Rev. 82 (2002) 47] alterations in shared signaling events likely account for the age-related decline in the ability of cells to respond to both proliferative signals and oxidant stimuli.

A Mutation in the ATP2 Gene Abrogates the Age Asymmetry Between Mother and Daughter Cells of the Yeast Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae reproduces by asymmetric cell division, or budding. In each cell division, the daughter cell is usually smaller and younger than the mother cell, as defined by the number of divisions it can potentially complete before it dies. Although individual yeast cells have a limited life span, this age asymmetry between mother and daughter ensures that the yeast strain remains immortal. To understand the mechanisms underlying age asymmetry, we have isolated temperature-sensitive mutants that have limited growth capacity. One of these clonal-senescence mutants was in ATP2, the gene encoding the β-subunit of mitochondrial F1, F0-ATPase. A point mutation in this gene caused a valine-to-isoleucine substitution at the ninetieth amino acid of the mature polypeptide. This mutation did not affect the growth rate on a nonfermentable carbon source. Life-span determinations following temperature shift-down showed that the clonal-senescence phenotype results from a loss of age asymmetry at 36°, such that daughters are born old. It was characterized by a loss of mitochondrial membrane potential followed by the lack of proper segregation of active mitochondria to daughter cells. This was associated with a change in mitochondrial morphology and distribution in the mother cell and ultimately resulted in the generation of cells totally lacking mitochondria. The results indicate that segregation of active mitochondria to daughter cells is important for maintenance of age asymmetry and raise the possibility that mitochondrial dysfunction may be a normal cause of aging. The finding that dysfunctional mitochondria accumulated in yeasts as they aged and the propensity for old mother cells to produce daughters depleted of active mitochondria lend support to this notion. We propose, more generally, that age asymmetry depends on partition of active and undamaged cellular components to the progeny and that this “filter” breaks down with age.

Lifespan in captive baboons is heritable

The effects of aging are evident in multiple organ systems, tissues, cell types, and molecules; all complex phenotypes affected by multiple shared and unique environmental factors and genes, which makes identifying the role of genetics in human aging difficult. Researchers have used yeast, nematodes, fruit flies, and mice to search for genes that influence the aging process. Given the phylogenetic distance and anatomic and physiologic dissimilarities of these organisms from humans, directly extrapolating these results to our species is problematic. However, nonhuman primates have a high degree of genetic, anatomic and physiologic similarity with humans and, thus, they may assist in the detection, characterization, and identification of genetic and environmental influences on human aging. Our goal is to demonstrate that effects of genes on variation in lifespan, a surrogate measure of aging, can be detected in a nonhuman primate species. Using variance component analysis, heritability of age at death was estimated to be 0.23+/-0.08 (P=0.0003) in 674 baboons from the Southwest Foundation for Biomedical Research (SFBR). This research demonstrates that lifespan is under partial genetic control. Given these findings, we believe that the baboon has potential as a model of human aging.

The biology of senescence: potential for prevention of disease

A recent series of advances in the understanding of mechanisms responsible for senescence have opened up potential avenues for delaying its onset and that of associated chronic diseases. Because the onset of senescence, like other biological processes, appears to be subject to regulation, advantage is being taken of pathways involved in this regulation to develop therapeutic interventions. These pathways include: (1) development of nutritional interventions based on the finding that caloric restriction extends maximum life span; (2) drugs to influence the metabolic pathways that link effects of caloric restriction to the changes in gene regulation that occur with aging; (3) drugs to prevent formation of advanced glycation end products resulting from reaction of reducing sugars with macromolecules; (4) agents to slow damaging effects of reactive oxygen species; and (5) methods to overcome effects of telomere shortening. Interventions to correct age-related, tissue specific changes in expression of transcription factors that enable cells to acquire specialized function are already in use (e.g., thiazolidinediones). In addition, because the aging process can be reset by factors present in oocytes, as shown by the cloning of healthy animals from senescent cells, methods to rejuvenate cells for transplantation or even intact tissues in individuals are within the realm of possibility. The hope in developing these interventions is to push back the onset of the chronic diseases associated with senescence and to prolong the period of adult vigor.

Identification of genes involved in Drosophila melanogaster geotaxis, a complex behavioral trait

Identifying the genes involved in polygenic traits has been difficult. In the 1950s and 1960s, laboratory selection experiments for extreme geotaxic behavior in fruit flies established for the first time that a complex behavioral trait has a genetic basis. But the specific genes responsible for the behavior have never been identified using this classical model. To identify the individual genes involved in geotaxic response, we used cDNA microarrays to identify candidate genes and assessed fly lines mutant in these genes for behavioral confirmation. We have thus determined the identities of several genes that contribute to the complex, polygenic behavior of geotaxis.

Extended leaf longevity in the ore4-1 mutant of Arabidopsis with a reduced expression of a plastid ribosomal protein gene

The longevity of plant leaf organs is genetically determined. However, the molecular mechanisms underlying the control of longevity are still largely unknown. Here, we describe a T-DNA-insertional mutation of Arabidopsis thaliana that confers extended leaf longevity. The mutation, termed ore4-1, delays a broad spectrum of age-dependent leaf senescence, but has little effect on leaf senescence artificially induced by darkness, abscisic acid (ABA), methyl jasmonate (MeJA), or ethylene. The T-DNA was inserted within the promoter region of the plastid ribosomal small subunit protein 17 (PRPS17) gene, and this insertion dramatically reduced PRPS17 mRNA expression. In the ore4-1 mutant, the leaf growth rate is decreased, while the maturation timing is similar to that of wild-type. In addition, the activity of the photosystem I (PSI) is significantly reduced in the ore4-1 mutant, as compared to wild-type. Thus, the ore4-1 mutation results in a deficiency in various chloroplast functions, including photosynthesis, which may decrease leaf growth. Our results suggest a possible link between reduced metabolism and extended longevity of the leaf organs in the ore4-1 mutation.

Organization of the stress system and its dysregulation in melancholic and atypical depression: high vs low CRH/NE states

Stress precipitates depression and alters its natural history. Major depression and the stress response share similar phenomena, mediators and circuitries. Thus, many of the features of major depression potentially reflect dysregulations of the stress response. The stress response itself consists of alterations in levels of anxiety, a loss of cognitive and affective flexibility, activation of the hypothalamic-pituitary-adrenal (HPA) axis and autonomic nervous system, and inhibition of vegetative processes that are likely to impede survival during a life-threatening situation (eg sleep, sexual activity, and endocrine programs for growth and reproduction). Because depression is a heterogeneous illness, we studied two diagnostic subtypes, melancholic and atypical depression. In melancholia, the stress response seems hyperactive, and patients are anxious, dread the future, lose responsiveness to the environment, have insomnia, lose their appetite, and a diurnal variation with depression at its worst in the morning. They also have an activated CRH system and may have diminished activities of the growth hormone and reproductive axes. Patients with atypical depression present with a syndrome that seems the antithesis of melancholia. They are lethargic, fatigued, hyperphagic, hypersomnic, reactive to the environment, and show diurnal variation of depression that is at its best in the morning. In contrast to melancholia, we have advanced several lines of evidence of a down-regulated hypothalamic-pituitary adrenal axis and CRH deficiency in atypical depression, and our data show us that these are of central origin. Given the diversity of effects exerted by CRH and cortisol, the differences in melancholic and atypical depression suggest that studies of depression should examine each subtype separately. In the present paper, we shall first review the mediators and circuitries of the stress system to lay the groundwork for placing in context physiologic and structural alterations in depression that may occur as part of stress system dysfunction.

The genetics of aging-- implications for pharmacogenomics

Lifespan experiments of lower organisms and mammals along with recent studies of centenarians are making inroads into delineating genetic factors that determine the ability to achieve exceptional longevity. These models may be helpful for the discovery of both longevity-enabling genes as well as genes associated with increased propensity to develop specific diseases. Both academic and commercial laboratories are putting substantial resources into discovering such genes in order to better understand the genetic and environmental underpinnings of how some people age more slowly than others and markedly delay or even escape age-associated diseases.

G protein-coupled receptors: dominant players in cell-cell communication

The G protein-coupled receptors (GPCRs) are the most numerous and the most diverse type of receptors (1-5% of the complete invertebrate and vertebrate genomes). They transduce messages as different as odorants, nucleotides, nucleosides, peptides, lipids, and proteins. There are at least eight families of GPCRs that show no sequence similarities and that use different domains to bind ligands and activate a similar set of G proteins. Homo- and heterodimerization of GPCRs seem to be the rule, and in some cases an absolute requirement, for activation. There are about 100 orphan GPCRs in the human genome which will be used to find new message molecules. Mutations of GPCRs are responsible for a wide range of genetic diseases. The importance of GPCRs in physiological processes is illustrated by the fact that they are the target of the majority of therapeutical drugs and drugs of abuse.

The genetics of aging

Once thought to be an extremely complex conundrum of weak genetic and environmental effects, exceptional longevity is beginning to yield genetic findings. Numerous lower organism and mammalian models demonstrate genetic mutations that increase life-span markedly. These variations, some of them evolutionarily conserved, inform us about biochemical pathways that significantly impact upon longevity. Centenarian studies have also proven useful as they are a cohort that, relative to younger age groups, lacks genotypes linked to age-related lethal diseases and premature mortality. Pedigree studies have demonstrated a significant familial component to the ability to survive to extreme old age and a recent study demonstrates a locus on chromosome 4 linked to exceptional longevity indicating the likely existence of at least one longevity enabling gene in humans. Thus, a number of laboratories are making substantial and exciting strides in the understanding of the genetics of aging and longevity which should lead to the discovery of genes and ultimately drugs that slow down the aging process and facilitate people's ability to delay and perhaps escape age-associated diseases.

Aging is a deprivation syndrome driven by a germ-soma conflict

Evolution through natural selection can be described as driven by a perpetual conflict of individuals competing for limited resources. Recently, I postulated that the shortage of resources godfathered the evolutionary achievements of the differentiation-apoptosis programming [Rev. Neurosci. 12 (2001) 217]. Unicellular deprivation-induced differentiation into germ cell-like spores can be regarded as the archaic reproduction events which were fueled by the remains of the fratricided cells of the apoptotic fruiting body. Evidence has been accumulated suggesting that conserved through the ages as the evolutionary legacy of the germ-soma conflict, the somatic loss of immortality during the ontogenetic segregation of primordial germ cells recapitulates the archaic fate of the fruiting body. In this heritage, somatic death is a germ cell-triggered event and has been established as evolutionary-fixed default state following asymmetric reproduction in a world of finite resources. Aging, on the other hand, is the stress resistance-dependent phenotype of the somatic resilience that counteracts the germ cell-inflicted death pathway. Thus, aging is a survival response and, in contrast to current beliefs, is antagonistically linked to death that is not imposed by group selection but enforced upon the soma by the selfish genes of the "enemy within". Environmental conditions shape the trade-off solutions as compromise between the conflicting germ-soma interests. Mechanistically, the neuroendocrine system, particularly those components that control energy balance, reproduction and stress responses, orchestrate these events. The reproductive phase is a self-limited process that moulds onset and progress of senescence with germ cell-dependent factors, e.g. gonadal hormones. These degenerate the regulatory pacemakers of the pineal-hypothalamic-pituitary network and its peripheral, e.g. thymic, gonadal and adrenal targets thereby eroding the trophic milieu. The ensuing cellular metabolic stress engenders adaptive adjustments of the glucose-fatty acid cycle, responses that are adequate and thus fitness-boosting under fuel shortage (e.g. during caloric restriction) but become detrimental under fuel abundance. In a Janus-faced capacity, the cellular stress response apparatus expresses both tolerogenic and mutagenic features of the social and asocial deprivation responses [Rev. Neurosci. 12 (2001) 217]. Mediated by the derangement of the energy-Ca(2+)-redox homeostatic triangle, a mosaic of dedifferentiation/apoptosis and mutagenic responses actuates the gradual exhaustion of functional reserves and eventually results in a multitude of aging-related diseases. This scenario reconciles programmed and stochastic features of aging and resolves the major inconsistencies of current theories by linking ultimate and proximate causes of aging. Reproduction, differentiation, apoptosis, stress response and metabolism are merged into a coherent regulatory network that stages aging as a naturally selected, germ cell-triggered and reproductive phase-modulated deprivation response.

Regulation of lifespan by histone deacetylase

Aging is a universal biological phenomenon in eukaryotes, but why and how we age still remain mysterious. It would be of great biological interest and practical importance if we could uncover the molecular mechanism of aging, and find a way to delay the aging process while maintaining physical and mental strengths of youth. Histone deacetylases (HDACs) such as SIR2 and RPD3 are known to be involved in the extension of lifespan in yeast and Caenorhabditis elegans. An inhibitor of HDACs, phenylbutyrate, also can significantly increase the lifespan of Drosophila, without diminution of locomotor vigor, resistance to stress, or reproductive ability. Treatment for a limited period, either early or late in adult life, is also effective. Alteration in the pattern of gene expression, including induction or repression of numerous genes involved in longevity by changing the level and the pattern of histone acetylation may be an important factor in determining the longevity of animals.

Genes, culture, and aging flies--what the lab can and cannot tell us about natural genetic variation for senescence

Model organisms cultured in the lab provide a powerful way to explore basic biological processes. However, lab culture can select for high early fecundity and dramatically shorten the life-span of lab organisms. Studies that use these short-lived organisms to identify aging-related genes might identify genes that simply restore the organism's original life-span. These results might not be fully relevant to wild populations. Experiments that reduce selection for shorter life-span or seek genes in naturally long-lived cohorts should lead to a more accurate understanding of aging.

The role of parental age effects on the evolution of aging

Many studies have found that older parents have shorter-lived offspring. However, the evolutionary significance of these findings is poorly understood. We carried out large-scale demographic experiments to examine the direct effect of maternal age and paternal age on offspring aging in inbred and outbred strains of the fruit fly Drosophila melanogaster. We found that the age of mothers and, to a lesser extent, the age of fathers can have a large influence on both offspring longevity and the shape of the age-specific mortality trajectory. In two independent experiments we found that older mothers generally produced shorter-lived offspring, although the exact effect of maternal age on offspring longevity differed among strains. These results suggest that maternal age effects on progeny aging may influence the evolution of aging.

Ageing: the many-headed monster

Ageing involves numerous pathways that are not functionally coordinated. The genome-wide profiling of transcription during Drosophila ageing is proving to be a powerful new tool for identifying these pathways.

Stress resistance as a determinate of C. elegans lifespan

It is difficult to exaggerate the progress that has been made in biogerontology over the last 15 years. As with all scientific revolutions, a few experiments in a small number of laboratories have changed the way in which we think about and design experiments. As a result of these experiments, there is much evidence to suggest that a rudimentary understanding of some of the processes that cause aging will be available in the next decade. One particular area of progress is the molecular genetics of lifespan. Although one may draw some distinctions between chronological lifespan and normal aging, extended lifespan remains one of the best indicators that an intervention in an aging process has been made. The isolation of a long-lived variant of a laboratory invertebrate is now essentially a trivial project but the information obtained from this approach is proving invaluable. As with most other biological problems, the most important experimental developments are coming from studying simple organisms in a reductionist fashion.

Stem cells from birth to death: The history and the future

The concept that adult stem cells, despite their impressive proliferative potential, are immortal has been challenged by experimental studies of hematopoietic stem cells. In this review, we discuss the properties that characterize a stem cell, the growing list of tissues in which stem cells are found, how they can be identified and isolated, how stem cells may transdifferentiate, and the findings that illustrate how age affects the hematopoietic stem cell population. We propose that an aging stem cell population affects tissue and organ homeostasis, particularly in response to environmental stresses, and we hypothesize that through this mechanism the functional status of stem cells affects the longevity of the organism.

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.

Rejuvenating views of the ageing process

Commonly held assumptions about the ageing process have guided and directed research in this important area over many years. Recent evidence, however, from disciplines as diverse as molecular genetics, clinical epidemiology and demography, provide a direct challenge to the validity of many of these assumptions. So, the time is ripe to re-examine these assumptions about the ageing process and to rethink the scientific foundations of the field.

The genetics of exceptional human longevity

There is a substantial distinction to be made between the genetics of aging and the genetics of exceptional longevity. Twin studies suggest that the average set of genetic variations facilitates the average human's ability to live well into their octogenarian years. Other studies indicate that taking full advantage of this average set results in spending the majority of those years in good health. However, many people counteract such genetic endowment with poor health habits, resulting in a substantially lower average life expectancy and relatively more time spent in poor health. To live beyond the octogenarian years, life-span experiments in lower organisms and mammals and population and molecular genetic studies of centenarian sibships suggest that genetic factors play an important role in exceptional longevity. These factors are likely to influence basic mechanisms of aging, which in turn broadly influence susceptibility to age-related illnesses. Lacking genetic variations that predispose to disease, and having variations that confer disease resistance (longevity enabling genes), are probably both important to such a remarkable survival advantage. Recent studies indicate the likelihood that such factors will be elucidated in the near future.

Life extension in Drosophila by feeding a drug

We report that feeding Drosophila throughout adulthood with 4-phenylbutyrate (PBA) can significantly increase lifespan, without diminution of locomotor vigor, resistance to stress, or reproductive ability. Treatment for a limited period, either early or late in adult life, is also effective. Flies fed PBA show a global increase in histone acetylation as well as a dramatically altered pattern of gene expression, including induction or repression of numerous genes. The delay in aging may result from the altered physiological state.

The genetics of aging

The genetic analysis of life span has only begun in mammals, invertebrates, such as Caenorhabditis elegans and Drosophila, and yeast. Even at this primitive stage of the genetic analysis of aging, the physiological observations that rate of metabolism is intimately tied to life span is supported. In many examples from mice to worms to flies to yeast, genetic variants that affect life span also modify metabolism. Insulin signaling regulates life span coordinately with reproduction, metabolism, and free radical protective gene regulation in C. elegans. This may be related to the findings that caloric restriction also regulates mammalian aging, perhaps via the modulation of insulin-like signaling pathways. The nervous system has been implicated as a key tissue where insulin-like signaling and free radical protective pathways regulate life span in C. elegans and Drosophila. Genes that determine the life span could act in neuroendocrine cells in diverse animals. The involvement of insulin-like hormones suggests that the plasticity in life spans evident in animal phylogeny may be due to variation in the timing of release of hormones that control vitality and mortality as well as variation in the response to those hormones. Pedigree analysis of human aging may reveal variations in the orthologs of the insulin pathway genes and coupled pathways that regulate invertebrate aging. Thus, genetic approaches may identify a set of circuits that was established in ancestral metazoans to regulate their longevity.

Optimizing detection of QTLs retarding aging: choice of statistical model and animal requirements

Quantitative trait locus (QTL) analysis makes no assumptions about the identity of genes involved in regulating aging. Moreover, it may be used as the first step in identifying such genes and, thus QTL analysis may be instrumental in formulating new hypotheses about aging. Genetic experiments, however, require hundreds to thousands of animals and are very expensive in mammals. Statistical power to detect longevity genes could be improved by excluding accidental, unrelated to aging mortality. While many early deaths are probably accidental, excluding early mortality altogether eliminates the age-related component, too. We used computer simulations to assess the effect of excluding early age-related, mortality on the statistical power of several common tests, such as t-test, Mann-Whitney and chi(2). Surprisingly, even the age-related, Gompertz component of early mortality reduces the statistical power of the t- and Mann-Whitney tests. For example, in a backcross design, to detect a gene slowing down the rate of aging and increasing mouse life span by 10% (P=0.0001; power=0.8), a regular t-test will require 640 mice, all kept for the entire life span and genotyped. If life spans of only 25% of the longest-lived animals from each of the two groups, carrying a putative longevity allele and not carrying it, are compared, population size can be reduced by two-fold, to about 300, and genotyping by seven-fold, to 90. Confirming simulation results, the significance of the effect of caloric restriction on life span increased from P=3.4x10(-5) to 1.1x10(-7), when life spans of only 40% of the longest-lived mice from each of the two groups, ad libitum fed and calorie restricted, were compared. Finally, finding the optimal combination of statistical test, the number of phenotyped and the number of genotyped animals, which would minimize experimental costs was addressed.