Metabolic rate is not reduced by dietary-restriction or by lowered insulin/IGF-1 signalling and is not correlated with individual lifespan in Drosophila melanogaster

The heat dissipation limit theory and evolution of life histories in endotherms--time to dispose of the disposable soma theory?

A major factor influencing life-history strategies of endotherms is body size. Larger endotherms live longer, develop more slowly, breed later and less frequently, and have fewer offspring per attempt at breeding. The classical evolutionary explanation for this pattern is that smaller animals experience greater extrinsic mortality, which favors early reproduction at high intensity. This leads to a short lifespan and early senescence by three suggested mechanisms. First, detrimental late-acting mutations cannot be removed because of the low force of selection upon older animals (mutation accumulation). Second, genes that promote early reproduction will be favored in small animals, even if they have later detrimental effects (antagonistic pleiotropy). Third, small animals may be forced to reduce their investment in longevity assurance mechanisms (LAMs) in favor of investment in reproduction (the disposable soma theory, DST). The DST hinges on three premises: that LAMs exist, that such LAMs are energetically expensive and that the supply of energy is limited. By contrast, the heat dissipation limit (HDL) theory provides a different conceptual perspective on the evolution of life histories in relation to body size. We suggest that rather than being limited, energy supplies in the environment are often unlimited, particularly when animals are breeding, and that animals are instead constrained by their maximum capacity to dissipate body heat, generated as a by-product of their metabolism. Because heat loss is fundamentally a surface-based phenomenon, the low surface-to-volume ratio of larger animals generates significant problems for dissipating the body heat associated with reproductive effort, which then limits their current reproductive investment. We suggest that this is the primary reason why fecundity declines as animal size increases. Because large animals are constrained by their capacity for heat dissipation, they have low reproductive rates. Consequently, only those large animals living in habitats with low extrinsic mortality could survive leading to the familiar patterns of life-history trade-offs and their links to extrinsic mortality rates. The HDL theory provides a novel mechanism underpinning the evolution of life history and ageing in endotherms, and makes a number of testable predictions that directly contrast with the predictions arising from the DST.

Metabolism and longevity: is there a role for membrane fatty acids?

More than 100 years ago, Max Rubner combined the fact that both metabolic rate and longevity of mammals varies with body size to calculate that "life energy potential" (lifetime energy turnover per kilogram) was relatively constant. This calculation linked longevity to aerobic metabolism which in turn led to the "rate-of-living" and ultimately the "oxidative stress" theories of aging. However, the link between metabolic rate and longevity is imperfect. Although unknown in Rubner's time, one aspect of body composition of mammals also varies with body size, namely the fatty acid composition of membranes. Fatty acids vary dramatically in their susceptibility to peroxidation and the products of lipid peroxidation are very powerful reactive molecules that damage other cellular molecules. The "membrane pacemaker" modification of the "oxidative stress" theory of aging proposes that fatty acid composition of membranes, via its influence on peroxidation of lipids, is an important determinant of lifespan (and a link between metabolism and longevity). The relationship between membrane fatty acid composition and longevity is discussed for (1) mammals of different body size, (2) birds of different body size, (3) mammals and birds that are exceptionally long-living for their size, (4) strains of mice that vary in longevity, (5) calorie-restriction extension of longevity in rodents, (6) differences in longevity between queen and worker honeybees, and (7) variation in longevity among humans. Most of these comparisons support an important role for membrane fatty acid composition in the determination of longevity. It is apparent that membrane composition is regulated for each species. Provided the diet is not deficient in polyunsaturated fat, it has minimal influence on a species' membrane fatty acid composition and likely also on it's maximum longevity. The exceptional longevity of Homo sapiens combined with the limited knowledge of the fatty acid composition of human tissues support the potential importance of mitochondrial membranes in determination of longevity.

Evidence for only two independent pathways for decreasing senescence in Caenorhabditis elegans

Cold temperature, dietary restriction, reduced insulin/insulin-like growth factor signaling, and mutations in mitochondrial genes have all been shown to extend the lifespan of Caenorhabditis elegans (Kenyon et al., Nature 366:461-464, 1993; Klass, Mech Ageing Dev 6:413-429, 1977; Lakowski and Hekimi, Science 272:1010-1013, 1996). Additionally, all of them extend the lifespan of mice (Bluher et al., Science 299:572-574, 2003; Conti et al., Science 314:825-828, 2006; Holzenberger et al., Nature 421:182-187, 2003; Liu et al., Genes Dev 19:2424-2434, 2005; Weindruch and Walford, Science 215:1415-1418, 1982). The mechanism by which these treatments extend lifespan is currently unknown, but our study uses an epistatic approach to show that these four manipulations are mainly additive in terms of lifespan. Classical interpretation of this data suggests that these manipulations are independent of each other. However, using a Gompertz mortality rate analysis, the maximum mortality rate doubling time can be achieved through the use of only dietary restriction and cold temperature, suggesting that the mechanisms by which cold temperature and caloric restriction extend lifespan are the only independent mechanisms.

Mitochondrial energy metabolism and ageing

Ageing can be defined as "a progressive, generalized impairment of function, resulting in an increased vulnerability to environmental challenge and a growing risk of disease and death". Ageing is likely a multifactorial process caused by accumulated damage to a variety of cellular components. During the last 20 years, gerontological studies have revealed different molecular pathways involved in the ageing process and pointed out mitochondria as one of the key regulators of longevity. Increasing age in mammals correlates with increased levels of mitochondrial DNA (mtDNA) mutations and a deteriorating respiratory chain function. Experimental evidence in the mouse has linked increased levels of somatic mtDNA mutations to a variety of ageing phenotypes, such as osteoporosis, hair loss, graying of the hair, weight reduction and decreased fertility. A mosaic respiratory chain deficiency in a subset of cells in various tissues, such as heart, skeletal muscle, colonic crypts and neurons, is typically found in aged humans. It has been known for a long time that respiratory chain-deficient cells are more prone to undergo apoptosis and an increased cell loss is therefore likely of importance in the age-associated mitochondrial dysfunction. In this review, we would like to point out the link between the mitochondrial energy balance and ageing, as well as a possible connection between the mitochondrial metabolism and molecular pathways important for the lifespan extension.

Gene expression profiling implicates OXPHOS complexes in lifespan extension of flies over-expressing a small mitochondrial chaperone, Hsp22

Aging is a complex process accompanied by a decreased capacity to tolerate and respond to various stresses. Heat shock proteins as part of cell defense mechanisms are up-regulated following stress. In Drosophila, the mitochondrial Hsp22 is preferentially up-regulated in aged flies. Its over-expression results in an extension of lifespan and an increased resistance to stress. Hsp22 has chaperone-like activity in vitro, but the mechanism(s) by which it increases lifespan in flies are unknown. Genome-wide analysis was performed on long-lived Hsp22+ and control flies to unveil transcriptional changes brought by Hsp22. Transcriptomes obtained at 45days, 90% and 50% survival were then compared between them to focus more on genes up- or down-regulated in presence of higher levels of hsp22 mRNA. Hsp22+ flies display an up-regulation of genes mainly related to mitochondrial energy production and protein biosynthesis, two functions normally down-regulated during aging. Interestingly, among the 26 genes up-regulated in Hsp22+ flies, 7 genes encode for mitochondrial proteins, 5 of which being involved in OXPHOS complexes. Other genes that could influence aging such as CG5002, dGCC185 and GstS1 also displayed a regulation linked to Hsp22 expression. The up-regulation of genes of the OXPHOS system in Hsp22+ flies suggest that mitochondrial homeostasis is at the center of Hsp22 beneficial effects on lifespan.

Metabolism as means for hypoxia adaptation: metabolic profiling and flux balance analysis

Background

Cellular hypoxia is a component of many diseases, but mechanisms of global hypoxic adaptation and resistance are not completely understood. Previously, a population of Drosophila flies was experimentally selected over several generations to survive a chronically hypoxic environment. NMR-based metabolomics, combined with flux-balance simulations of genome-scale metabolic networks, can generate specific hypotheses for global reaction fluxes within the cell. We applied these techniques to compare metabolic activity during acute hypoxia in muscle tissue of adapted versus "naïve" control flies.

Results

Metabolic profiles were gathered for adapted and control flies after exposure to acute hypoxia using ¹H NMR spectroscopy. Principal Component Analysis suggested that the adapted flies are tuned to survive a specific oxygen level. Adapted flies better tolerate acute hypoxic stress, and we explored the mechanisms of this tolerance using a flux-balance model of central metabolism. In the model, adapted flies produced more ATP per glucose and created fewer protons than control flies, had lower pyruvate carboxylase flux, and had greater usage of Complex I over Complex II.

Conclusion

We suggest a network-level hypothesis of metabolic regulation in hypoxia-adapted flies, in which lower baseline rates of biosynthesis in adapted flies draws less anaplerotic flux, resulting in lower rates of glycolysis, less acidosis, and more efficient use of substrate during acute hypoxic stress. In addition we suggest new specific hypothesis, which were found to be consistent with existing data.

Increased locomotor activity and metabolism of Aedes aegypti infected with a life-shortening strain of Wolbachia pipientis

A virulent strain of the obligate intracellular bacterium Wolbachia pipientis that shortens insect lifespan has recently been transinfected into the primary mosquito vector of dengue virus, Aedes aegypti L. The microbe's ability to shorten lifespan and spread through host populations under the action of cytoplasmic incompatibility means it has the potential to be used as a biocontrol agent to reduce dengue virus transmission. Wolbachia is present in many host tissues and may have local effects on diverse biological processes. In other insects, Wolbachia infections have been shown to alter locomotor activity and response time to food cues. In mosquitoes, locomotor performance relates to the location of mates, human hosts, resting sites and oviposition sites. We have therefore examined the effect of the virulent, life-shortening Wolbachia strain wMelPop on the locomotion of Ae. aegypti as they age and as the pathogenicity of the infection increases. In parallel experiments we also examined CO(2) production as a proxy for metabolic rate, to investigate a potential mechanistic explanation for any changes in locomotion. Contrary to expectation, we found that the infection increased activity and metabolic rate and that these effects were relatively consistent over the insect's lifespan. The results do not fit a standard model of bacterial pathogenesis in insects, and instead may reveal additional physiological changes induced by infection, such as either increased hunger or defects in the nervous system.

Multiple measures of functionality exhibit progressive decline in a parallel, stochastic fashion in Drosophila Sod2 null mutants

Oxidative damage has been proposed as an important factor in the progression of pathological and non-pathological age-related functional declines. Here, we examine functional deterioration in short-lived flies mutant for the mitochondrial antioxidant Manganese Superoxide Dismutase (Sod2). We find that the decline of several functional measures of aging occurs, in an accelerated fashion, in Sod2 mutants. Olfactory behavior, locomotor ability and cardiac performance were all seen to decline rapidly in Sod2 mutants. On average, functional declines in Sod2 mutants occur in a pattern similar to that seen in late-life Drosophila with a normal complement of Sod2. In longitudinal experiments, however, we find that functional failures occur in every possible sequence in Sod2 mutants. Significantly, failure of these functional measures is not irreversible, as spontaneous functional recovery was sometimes observed. These findings support a model where ROS-related damage strikes at multiple organ systems in parallel, rather than a "chain of dominos" model, in which primary organ failure contributes to the deterioration of further organ systems.

Aging and survival: the genetics of life span extension by dietary restriction

Reducing food intake to induce undernutrition but not malnutrition extends the life spans of multiple species, ranging from single-celled organisms to mammals. This increase in longevity by dietary restriction (DR) is coupled to profound beneficial effects on age-related pathology. Historically, much of the work on DR has been undertaken using rodent models, and 70 years of research has revealed much about the physiological changes DR induces. However, little is known about the genetic pathways that regulate the DR response and whether or not they are conserved between species. Elucidating these pathways may facilitate the design of targeted pharmaceutical treatments for a range of age-related pathologies. Here, we discuss how recent work in nonmammalian model organisms has revealed new insight into the genetics of DR and how the discovery of DR-specific transcription factors will advance our understanding of this phenomenon.

Energetics and longevity in birds

The links between energy expenditure and ageing are different at different levels of enquiry. When studies have examined the relationships between different species within a given class the association is generally negative--animals with greater metabolism per gram of tissue live shorter lives. Within species, or between classes (e.g. between birds and mammals) the association is the opposite--animals with higher metabolic rates live longer. We have previously shown in mammals that the negative association between lifespan and metabolic rate is in fact an artefact of using resting rather than daily energy expenditure, and of failing to adequately take into account the confounding effects of body size and the lack of phylogenetic independence of species data. When these factors are accounted for, across species of mammals, the ones with higher metabolism also have the largest lifetime expenditures of energy-consistent with the inter-class and intra-specific data. A previous analysis in birds did not yield the same pattern, but this may have been due to a lack of sufficient power in the analysis. Here we present an analysis of a much enlarged data set (>300 species) for metabolic and longevity traits in birds. These data show very similar patterns to those in mammals. Larger individuals have longer lives and lower per-gram resting and daily energy expenditures, hence there is a strong negative relationship between longevity and mass-specific metabolism. This relationship disappears when the confounding effects of body mass and phylogeny are accounted for. Across species of birds, lifetime expenditure of energy per gram of tissue based on both daily and resting energy expenditure is positively related to metabolic intensity, mirroring these statistical relationships in mammals and synergizing with the positive associations of metabolism with lifespan within species and between vertebrate classes.

Metabolic consequences of pregnancy-associated plasma protein-A deficiency in mice: exploring possible relationship to the longevity phenotype

Mice born with the deletion of the gene for pregnancy-associated plasma protein-A (PAPP-A), a model of reduced local IGF activity, live approximately 30% longer than their wild-type (WT) littermates. In this study, we investigated metabolic consequences of PAPP-A gene deletion and possible relationship to lifespan extension. Specifically, we determined whether 18-month-old PAPP-A knockout (KO) mice when compared with their WT littermates have reduced energy expenditure and/or altered glucose-insulin sensitivity. Food intake, and total energy expenditure and resting energy expenditure as measured by calorimetry were not different between PAPP-A KO and WT mice when subjected to the analysis of covariance with body weight as the covariate. However, there was an increase in spontaneous physical activity in PAPP-A KO mice. Both WT and PAPP-A KO mice exhibited mild insulin resistance with age, as assessed by fasting glucose/insulin ratios. Oral glucose tolerance and insulin sensitivity were not significantly different between the two groups of mice, although there appeared to be a decrease in the average size of the pancreatic islets in PAPP-A KO mice. Thus, neither reduced 'rate of living' nor altered glucose-insulin homeostasis can be considered key determinants of the enhanced longevity of PAPP-A KO mice. These findings are discussed in the context of those from other long-lived mouse models.

The oxidative stress theory of aging: embattled or invincible? Insights from non-traditional model organisms

Reactive oxygen species (ROS), inevitable byproducts of aerobic metabolism, are known to cause oxidative damage to cells and molecules. This, in turn, is widely accepted as a pivotal determinant of both lifespan and health span. While studies in a wide range of species support the role of ROS in many age-related diseases, its role in aging per se is questioned. Comparative data from a wide range of endotherms offer equivocal support for this theory, with many exceptions and inconclusive findings as to whether or not oxidative stress is either a correlate or a determinant of maximum species lifespan. Available data do not support the premise that metabolic rate and in vivo ROS production are determinants of lifespan, or that superior antioxidant defense contributes to species longevity. Rather, published studies often show either a negative associate or lack of correlation with species longevity. Furthermore, many long-living species such as birds, bats and mole-rats exhibit high levels of oxidative damage even at young ages. Similarly genetic manipulations altering expression of key antioxidants do not necessarily show an impact on lifespan, even though oxidative damage levels may be affected. While it is possible that these multiple exceptions to straightforward predictions of the free radical theory of aging all reflect species-specific, "private" mechanisms of aging, the preponderance of contrary data nevertheless present a challenge to this august theory. Therefore, contrary to accepted dogma, the role of oxidative stress as a determinant of longevity is still open to question.

Diet and aging

Interventions that extend life span by moderately reduced nutrient intake are often referred to as dietary or calorie restriction. Its efficacy in many species has led to the conclusion that a single, evolutionarily conserved, molecular mechanism operates in all cases to extend life. Here we discuss examples of diet/genotype interactions that show a more complex mechanistic view is required and that mild dietary modifications can dramatically change the interpretation of model organism aging studies.

Validation of manometric microrespirometers for measuring oxygen consumption in small arthropods

Scientists have used numerous techniques to measure organismal metabolic rate, including assays of oxygen (O2) consumption and carbon dioxide (CO2) production. Relatively few studies have directly compared estimates of metabolic rate on the same groups of animals as determined by different assay methods. This study directly compared measures of the metabolic rate of three lines of Drosophila simulans as determined either from direct measures of CO2 production using infrared gas analysis (IRGA), or from estimates of O2 consumption based on manometeric techniques. Determinations of metabolic rate of the same cohorts of flies using these two methods produced results that often differed widely. Typically metabolic rate as determined by the manometric method was significantly greater than that determined by CO2 output. These differences are difficult to explain by simple biotic or abiotic factor(s). Because of the idiosyncratic nature of these differences it is not possible to use a simple factor to convert from metabolic rate measurements done using manometric techniques to those expected from direct measures of CO2 output or O2 consumption. Although manometric devices are simple to construct and use, measurements of metabolic rate made with this method can vary significantly from measurements made by directly assaying CO2 production or O2 consumption.

Mitochondrial dysfunction as a cause of ageing

Mitochondrial dysfunction is heavily implicated in the ageing process. Increasing age in mammals correlates with accumulation of somatic mitochondrial DNA (mtDNA) mutations and decline in respiratory chain function. The age-associated respiratory chain deficiency is typically unevenly distributed and affects only a subset of cells in various human tissues, such as heart, skeletal muscle, colonic crypts and neurons. Studies of mtDNA mutator mice has shown that increased levels of somatic mtDNA mutations directly can cause a variety of ageing phenotypes, such as osteoporosis, hair loss, greying of the hair, weight reduction and decreased fertility. Respiratory-chain-deficient cells are apoptosis prone and increased cell loss is therefore likely an important consequence of age-associated mitochondrial dysfunction. There is a tendency to automatically link mitochondrial dysfunction to increased generation of reactive oxygen species (ROS), however, the experimental support for this concept is rather weak. In fact, respiratory-chain-deficient mice with tissue-specific mtDNA depletion or massive increase of point mutations in mtDNA typically have minor or no increase of oxidative stress. Mitochondrial dysfunction is clearly involved in the human ageing process, but its relative importance for mammalian ageing remains to be established.

Mitochondrial gene expression and increased oxidative metabolism: role in increased lifespan of fat-specific insulin receptor knock-out mice

Caloric restriction, leanness and decreased activity of insulin/insulin-like growth factor 1 (IGF-1) receptor signaling are associated with increased longevity in a wide range of organisms from Caenorhabditis elegans to humans. Fat-specific insulin receptor knock-out (FIRKO) mice represent an interesting dichotomy, with leanness and increased lifespan, despite normal or increased food intake. To determine the mechanisms by which a lack of insulin signaling in adipose tissue might exert this effect, we performed physiological and gene expression studies in FIRKO and control mice as they aged. At the whole body level, FIRKO mice demonstrated an increase in basal metabolic rate and respiratory exchange ratio. Analysis of gene expression in white adipose tissue (WAT) of FIRKO mice from 6 to 36 months of age revealed persistently high expression of the nuclear-encoded mitochondrial genes involved in glycolysis, tricarboxylic acid cycle, beta-oxidation and oxidative phosphorylation as compared to expression of the same genes in WAT from controls that showed a tendency to decline in expression with age. These changes in gene expression were correlated with increased cytochrome c and cytochrome c oxidase subunit IV at the protein level, increased citrate synthase activity, increased expression of peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) and PGC-1beta, and an increase in mitochondrial DNA in WAT of FIRKO mice. Together, these data suggest that maintenance of mitochondrial activity and metabolic rates in adipose tissue may be important contributors to the increased lifespan of the FIRKO mouse.

Chemical Complexity and the Genetics of Aging

We examine how aging is impacted by various chemical challenges that organisms face and by the molecular mechanisms that have evolved to regulate lifespan in response to them. For example, environmental information, which is detected and processed through sensory systems, can modulate lifespan by providing information about the presence and quality of food as well as presence and density of conspecifics and predators. In addition, the diverse forms of molecular damage that result from constant exposure to damaging chemicals that are generated from the environment and from metabolism pose an informatic and energetic challenge for detoxification systems, which are important in ensuring longevity. Finally, systems of innate immunity are vital for recognizing and combating pathogens but are also seen as of increasing importance in causing the aging process. Integrating ideas of molecular mechanism with context derived from evolutionary considerations will lead to exciting new insights into the evolution of aging.

Working harder to stay alive: metabolic rate increases with age in Drosophila simulans but does not correlate with life span

The hypothesis that metabolic rate is inversely correlated with life span has long been debated. Another area of controversy has been the relationship between metabolic rate and aging. In most molecular studies key aspects of cellular metabolism have been shown to decline with age. Less attention has been focused on metabolic rate as an organism ages. We studied the survival of three Drosophila simulans fly lines and measured whole organism metabolic rate, mitochondrial DNA copy number and walking speed. Metabolic rate as assayed by CO(2) production did not correlate with median lifespan but increased by 0.43-1.14%/d. In contrast, mitochondrial DNA copy number decreased by 0.56-1.06%/d. Physical activity, as assayed by mean walking speed, did not change with age but was positively correlated with mitochondrial DNA copy number. One explanation for these data is that metabolic rate was increased, in the face of a reduced mitochondrial DNA copy number and capacity for oxidative metabolism, to maintain a constant bioenergetic demand (physical activity). Alternatively, metabolic rate may increase to provide energy for the repair of cellular damage or due to a shift in metabolic substrate use over time.

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

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

Trends in oxidative aging theories

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

Dramatic age-related changes in nuclear and genome copy number in the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans has become one of the most widely used model systems for the study of aging, yet very little is known about how C. elegans age. The development of the worm, from egg to young adult has been completely mapped at the cellular level, but such detailed studies have not been extended throughout the adult lifespan. Numerous single gene mutations, drug treatments and environmental manipulations have been found to extend worm lifespan. To interpret the mechanism of action of such aging interventions, studies to characterize normal worm aging, similar to those used to study worm development are necessary. We have used 4',6'-diamidino-2-phenylindole hydrochloride staining and quantitative polymerase chain reaction to investigate the integrity of nuclei and quantify the nuclear genome copy number of C. elegans with age. We report both systematic loss of nuclei or nuclear DNA, as well as dramatic age-related changes in nuclear genome copy number. These changes are delayed or attenuated in long-lived daf-2 mutants. We propose that these changes are important pathobiological characteristics of aging nematodes.

Counting calories in Drosophila diet restriction

The extension of life span by diet restriction in Drosophila has been argued to occur without limiting calories. Here we directly measure the calories assimilated by flies when maintained on full- and restricted-diets. We find that caloric intake is reduced on all diets that extend life span. Flies on low-yeast diet are long-lived and consume about half the calories of flies on high-yeast diets, regardless of the energetic content of the diet itself. Since caloric intake correlates with yeast concentration and thus with the intake of every metabolite in this dietary component, it is premature to conclude for Drosophila that calories do not explain extension of life span.

Testing the rate-of-living/oxidative damage theory of aging in the nematode model Caenorhabditis elegans

The integration of the rate-of-living and oxidative damage theory of aging predicts that lifespan extension is linked to low energy metabolism, low ROS production rates, low molecular damage and a slow aging rate. In the long-lived Caenorhabditis elegans Ins/IGF-1 mutant daf-2(e1370), low carbonylation levels and postponed morphological decline comply with the latter two of these predictions. However, metabolic rates in daf-2(e1370) refute the rate-of-living theory. The apparent contradiction between increased ROS generation and long lifespan in daf-2(e1370) is reconciled by an enhanced stress defense, acknowledging oxidative damage as a probable cause of aging.

N-3 polyunsaturated fatty acids impair lifespan but have no role for metabolism

Although generally considered as beneficial components of dietary fats, polyunsaturated fatty acids (PUFA) have been suspected to compromise maximum lifespan (MLSP) in mammals. Specifically, high amounts of phospholipid PUFAs are thought to impair lifespan due to an increase in the susceptibility of membranes to lipid peroxidation and its damaging effect on cellular molecules. Also, there is evidence from in vitro studies suggesting that highly unsaturated PUFAs elevate basal metabolic rate (BMR). Previous comparative studies in this context were based on small sample sizes, however, and, except for one study, failed to address possible confounding influences of body weight and taxonomic relations between species. Therefore, we determined phospholipid membrane composition in skeletal muscle from 42 mammalian species to test for a relation with published data on MLSP, and with literature data on BMR (30 species). Using statistical models that adjust for the effects of body weight and phylogeny, we found that among mammals, MLSP indeed decreases as the ratio of n-3 to n-6 PUFAs increases. In contrast to previous studies, we found, however, no relation between MLSP and either membrane unsaturation (i.e. PUFA content or number of double bonds) or to the very long-chain, highly unsaturated docosahexaenoic acid (DHA). Similarly, our data set gave no evidence for any notable relation between muscle phospholipid fatty acid composition and BMR, or MLSP and BMR in mammals. These results contradict the 'membrane pacemaker theory of aging', that is, the concept of a direct link between high amounts of membrane PUFAs, elevated BMR, and thus, impaired longevity.

Life-long vitamin C supplementation in combination with cold exposure does not affect oxidative damage or lifespan in mice, but decreases expression of antioxidant protection genes

Oxidative stress is suggested to be central to the ageing process, with endogenous antioxidant defence and repair mechanisms in place to minimize damage. Theoretically, supplementation with exogenous antioxidants might support the endogenous antioxidant system, thereby reducing oxidative damage, ageing-related functional decline and prolonging life- and health-span. Yet supplementation trials with antioxidants in animal models have had minimal success. Human epidemiological data are similarly unimpressive, leading some to question whether vitamin C, for example, might have pro-oxidant properties in vivo. We supplemented cold exposed (7+/-2 degrees C) female C57BL/6 mice over their lifespan with vitamin C (ascorbyl-2-polyphosphate), widely advocated and self administered to reduce oxidative stress, retard ageing and increase healthy lifespan. No effect on mean or maximum lifespan following vitamin C treatment or any significant impact on body mass, or on parameters of energy metabolism was observed. Moreover, no differences in hepatocyte and lymphocyte DNA oxidative damage or hepatic lipid peroxidation was seen between supplemented and control mice. Using a DNA macroarray specific for oxidative stress-related genes, we found that after 18 months of supplementation, mice exhibited a significantly reduced expression of several genes in the liver linked to free-radical scavenging, including Mn-superoxide dismutase. We confirmed these effects by Northern blotting and found additional down-regulation of glutathione peroxidase (not present on macroarray) in the vitamin C treated group. We suggest that high dietary doses of vitamin C are ineffective at prolonging lifespan in mice because any positive benefits derived as an antioxidant are offset by compensatory reductions in endogenous protection mechanisms, leading to no net reduction in accumulated oxidative damage.

Calorie Restriction in Mice: Effects on Body Composition, Daily Activity, Metabolic Rate, Mitochondrial Reactive Oxygen Species Production, and Membrane Fatty Acid Composition

Different levels of calorie restriction (CR) (125, 85, 50, or 40 kcal/wk for 1, 3, and 6 months) were examined in mice by using the paradigm of Weindruch and colleagues. Lean and total body mass increased on 125 and 85 kcal/wk, but there was negligible growth on low-energy intake. There was no CR-induced reduction in either daily activity or mass-specific metabolic rate. There was no CR-effect on in vitro reactive oxygen species production by liver or muscle mitochondria at 3 months, but after 6 months the effect was significantly reduced in liver mitochondria from 40 kcal/wk mice compared to 125 kcal/wk mice. Changes in the fatty acid composition of phospholipids from liver, kidneys, heart, brain, and skeletal muscle were observed following 1 month of CR.

Microarray analysis of variation in individual aging C. elegans: approaches and challenges

Aging is generally defined and studied as a population phenomenon. However, there is great interest, especially when discussing human aging, in the identification of factors that influence the life span of an individual organism. The nematode Caenorhabditis elegans provides an excellent model system for the study of aging at the level of the individual, since young nematodes are essentially clonal yet experience a large range of individual life spans. We are conducting gene expression profiling of individual nematodes, with the aim of discovering genes that vary stochastically in expression between individuals of the same age. Such genes are candidates to modulate the ultimate life span achieved by each individual. We here present statistical analysis of gene expression profiles of individual nematodes from two different microarray platforms, examining the issue of technical vs. biological variance as it pertains to uncovering genes of interest in this paradigm of individual aging.

The naked mole-rat: a new long-living model for human aging research

Tremendous variation in maximum life span among species overshadows modest increases in longevity resulting from experimental manipulation. Few aging studies focus on long-lived mammals even though these species may expose mechanisms involved in resisting aging. Naked mole-rats (NMRs approximately 35 grams) are the longest-living (>28.3 years) rodents known. This review describes their biology and potential use in aging research. Lifestyle features concur with most evolutionary theories with the exception of the disposable soma theory. Indeed, maximum life span is similar in breeders and nonbreeders, and these highly fecund animals reproduce until they die. Shared characteristics with calorie-restricted, methionine-restricted, and dwarf mice models of extended longevity include reduced body temperature; reduced thyroid, and blood glucose concentrations; and low glycated hemoglobin; in addition to reduced incidence of cancer. Young naked mole-rats surprisingly have high levels of accrued oxidative damage. With their similar longevity quotient to humans, these rodents may provide a novel opportunity to examine mechanisms modulating aging.

Signaling and function of insulin-like peptides in insects

Insulin-like peptides (ILPs) exist in insects and are encoded by multigene families that are expressed in the brain and other tissues. Upon secretion, these peptides likely serve as hormones, neurotransmitters, and growth factors, but to date, few direct functions have been demonstrated. In Drosophila melanogaster, molecular genetic studies have revealed elements of a conserved insulin signaling pathway, and as in other animal models, it appears to play a key role in metabolism, growth, reproduction, and aging. This review offers (a) an integrated summary of the efforts to characterize the distribution of ILPs in insects and to define this pathway and its functions in Drosophila and (b) a few considerations for future studies of ILP endocrinology in insects.

Flies and their golden apples: the effect of dietary restriction on Drosophila aging and age-dependent gene expression

Reduced nutrient availability (dietary restriction) extends lifespan in species as diverse as yeast, nematode worms, Daphnia, Drosophila, and mammals. Recent demographic experiments have shown that moderate nutrient manipulation in adult Drosophila affects current mortality rate in a completely reversible manner, which suggests that dietary restriction in Drosophila increases lifespan through a reduction of the current risk of death rather than a slowing of aging-related damage. When examined in the light of the new demographic data, age-dependent changes in gene expression in normal and diet-restricted flies can provide unique insight into the biological processes affected by aging and may help identify molecular pathways that regulate it.

Diet, metabolism and lifespan in Drosophila

Dietary restriction (DR) by dilution of the food medium can extend lifespan in Drosophila. DR results in a state that is characterized by reduced fecundity, increased starvation resistance and higher total lipid levels. In the past, each of these correlated phenotypes has been proposed to play a causal role in the lifespan-extending effects of food reduction. However, more recent data show that each phenotype can be uncoupled from the long-lived state to varying extents. In this mini-review, we summarize the principal findings of the effects of DR on Drosophila in order to address what these phenotypes can tell us about the physiological remodeling required for Drosophila to be long-lived. Current data indicate lifespan-extension by DR is likely to involve both enhancement of various defense and detoxification mechanisms and a complex range of metabolic alterations that make energy available for these processes.

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

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

Dietary restriction in Drosophila

The fruit fly Drosophila is a useful organism for the investigation of the mechanisms by which dietary restriction (DR) extends lifespan. Its relatively short generation time, well-characterised molecular biology, genetics and physiology and ease of handling for demographic analysis are all major strengths. Lifespan has been extended by DR applied to adult Drosophila, by restriction of the availability of live yeast or by co-ordinate dilution of the whole food medium. Lifespan increases to a maximum through DR with a progressive dilution of the food and then decreases through starvation as the food is diluted further. Daily and lifetime fecundities of females are reduced by food dilution throughout the DR and starvation range. Standard Drosophila food ingredients differ greatly between laboratories and fly stocks can differ in their responses to food dilution, and a full range of food concentrations should therefore be investigated when examining the response to DR. Flies do not alter the time that they spend feeding in response to DR. Both mean and maximum lifespan are extended by DR. The nutrients critical for the response to DR in Drosophila require definition. The extension of lifespan in response to DR is very much greater in females than in males. Two nutrient-sensing pathways, the insulin/IGF-like and TOR pathways, have been implicated in mediating this response of lifespan to DR in Drosophila, as have two protein deacetylases, dSir2 and Rpd3, although the precise nature of this interaction remain to be characterised. Although female fecundity is reduced by DR, the response of lifespan to DR appears normal in sterile females, possibly implying that reduced fecundity is not necessary for extension of lifespan by DR. There is no reduction in metabolic rate or in the rate of generation of superoxide and hydrogen peroxide from isolated mitochondria in response to DR. DR acts acutely and rapidly (within 48 h) to reduce the mortality of flies that are fully fed to the level found in animals exposed to DR throughout life. This rapid mortality rate recovery provides a powerful framework within which to further investigate the mechanisms by which DR extends lifespan.

Membrane alteration as a basis of aging and the protective effects of calorie restriction

As has been experimentally determined, oxidative modification to biological systems can be extensive, although the identification and stochiometric relation of the reactive species that cause these alterations have not been fully elucidated. In this review, arguments are presented to support the notion that the combined effects of membrane lipid peroxidation and its by-products, reactive aldehydes are likely responsible for membrane-associated functional declines during aging. As evidence for a systemic response to overall oxidative stress, the molecular inflammation hypothesis of aging is discussed by considering that the activation of inflammatory genes act as a bridge linking normal aging to pathological processes.

Toward a unified theory of caloric restriction and longevity regulation

The diet known as calorie restriction (CR) is the most reproducible way to extend the lifespan of mammals. Many of the early hypotheses to explain this effect were based on it being a passive alteration in metabolism. Yet, recent data from yeast, worms, flies, and mammals support the idea that CR is not simply a passive effect but an active, highly conserved stress response that evolved early in life's history to increase an organism's chance of surviving adversity. This perspective updates the evidence for and against the various hypotheses of CR, and concludes that many of them can be synthesized into a single, unifying hypothesis. This has important implications for how we might develop novel medicines that can harness these newly discovered innate mechanisms of disease resistance and survival.

Nuclear-mitochondrial epistasis and drosophila aging: introgression of Drosophila simulans mtDNA modifies longevity in D. melanogaster nuclear backgrounds

Under the mitochondrial theory of aging, physiological decline with age results from the accumulated cellular damage produced by reactive oxygen species generated during electron transport in the mitochondrion. A large body of literature has documented age-specific declines in mitochondrial function that are consistent with this theory, but relatively few studies have been able to distinguish cause from consequence in the association between mitochondrial function and aging. Since mitochondrial function is jointly encoded by mitochondrial (mtDNA) and nuclear genes, the mitochondrial genetics of aging should be controlled by variation in (1) mtDNA, (2) nuclear genes, or (3) nuclear-mtDNA interactions. The goal of this study was to assess the relative contributions of these factors in causing variation in Drosophila longevity. We compared strains of flies carrying mtDNAs with varying levels of divergence: two strains from Zimbabwe (<20 bp substitutions between mtDNAs), strains from Crete and the United States (approximately 20-40 bp substitutions between mtDNAs), and introgression strains of Drosophila melanogaster carrying mtDNA from Drosophila simulans in a D. melanogaster Oregon-R chromosomal background (>500 silent and 80 amino acid substitutions between these mtDNAs). Longevity was studied in reciprocal cross genotypes between pairs of these strains to test for cytoplasmic (mtDNA) factors affecting aging. The intrapopulation crosses between Zimbabwe strains show no difference in longevity between mtDNAs; the interpopulation crosses between Crete and the United States show subtle but significant differences in longevity; and the interspecific introgression lines showed very significant differences between mtDNAs. However, the genotypes carrying the D. simulans mtDNA were not consistently short-lived, as might be predicted from the disruption of nuclear-mitochondrial coadaptation. Rather, the interspecific mtDNA strains showed a wide range of variation that flanked the longevities seen between intraspecific mtDNAs, resulting in very significant nuclear x mtDNA epistatic interaction effects. These results suggest that even "defective" mtDNA haplotypes could extend longevity in different nuclear allelic backgrounds, which could account for the variable effects attributable to mtDNA haplogroups in human aging.

Functional senescence in Drosophila melanogaster

The fruit fly Drosophila melanogaster is one of the principal model organisms used for studying the biology of aging. Flies are well suited for such studies for a number of reasons. Flies develop to adulthood quickly, have a relatively short life span, and are inexpensive to house. Most of the fly genome has been sequenced, powerful genetic tools are available to manipulate it, and most fly genes have obvious homologues in mammals. While the majority of aging studies in flies have focused on regulation of life span, the fly is emerging as a powerful model system for investigating the biology that underlies age-related functional decline. Key to the use of flies in this way is the striking number of parallels between functional senescence in Drosophila and humans. Here, we review age-related functional declines in Drosophila, human correlates of these age-related declines, and common mechanisms that influence longevity and specific aspects of functional senescence in flies.

Body size, energy metabolism and lifespan

Bigger animals live longer. The scaling exponent for the relationship between lifespan and body mass is between 0.15 and 0.3. Bigger animals also expend more energy, and the scaling exponent for the relationship of resting metabolic rate (RMR) to body mass lies somewhere between 0.66 and 0.8. Mass-specific RMR therefore scales with a corresponding exponent between -0.2 and -0.33. Because the exponents for mass-specific RMR are close to the exponents for lifespan, but have opposite signs, their product (the mass-specific expenditure of energy per lifespan) is independent of body mass (exponent between -0.08 and 0.08). This means that across species a gram of tissue on average expends about the same amount of energy before it dies regardless of whether that tissue is located in a shrew, a cow, an elephant or a whale. This fact led to the notion that ageing and lifespan are processes regulated by energy metabolism rates and that elevating metabolism will be associated with premature mortality--the rate of living theory. The free-radical theory of ageing provides a potential mechanism that links metabolism to ageing phenomena, since oxygen free radicals are formed as a by-product of oxidative phosphorylation. Despite this potential synergy in these theoretical approaches, the free-radical theory has grown in stature while the rate of living theory has fallen into disrepute. This is primarily because comparisons made across classes (for example, between birds and mammals) do not conform to the expectations, and even within classes there is substantial interspecific variability in the mass-specific expenditure of energy per lifespan. Using interspecific data to test the rate of living hypothesis is, however, confused by several major problems. For example, appeals that the resultant lifetime expenditure of energy per gram of tissue is 'too variable' depend on the biological significance rather than the statistical significance of the variation observed. Moreover, maximum lifespan is not a good marker of ageing and RMR is not a good measure of total energy metabolism. Analysis of residual lifespan against residual RMR reveals no significant relationship. However, this is still based on RMR. A novel comparison using daily energy expenditure (DEE), rather than BMR, suggests that lifetime expenditure of energy per gram of tissue is NOT independent of body mass, and that tissue in smaller animals expends more energy before expiring than tissue in larger animals. Some of the residual variation in this relationship in mammals is explained by ambient temperature. In addition there is a significant negative relationship between residual lifespan and residual daily energy expenditure in mammals. A potentially much better model to explore the links of body size, metabolism and ageing is to examine the intraspecific links. These studies have generated some data that support the original rate of living theory and other data that conflict. In particular several studies have shown that manipulating animals to expend more or less energy generate the expected effects on lifespan (particularly when the subjects are ectotherms). However, smaller individuals with higher rates of metabolism live longer than their slower, larger conspecifics. An addition to these confused observations has been the recent suggestion that under some circumstances we might expect mitochondria to produce fewer free radicals when metabolism is higher--particularly when they are uncoupled. These new ideas concerning the manner in which mitochondria generate free radicals as a function of metabolism shed some light on the complexity of observations linking body size, metabolism and lifespan.

On the importance of fatty acid composition of membranes for aging

The membrane pacemaker theory of aging is an extension of the oxidative stress theory of aging. It emphasises variation in the fatty acid composition of membranes as an important influence on lipid peroxidation and consequently on the rate of aging and determination of lifespan. The products of lipid peroxidation are reactive molecules and thus potent damagers of other cellular molecules. It is suggested that the feedback effects of these peroxidation products on the oxidative stress experienced by cells is an important part of the aging process. The large variation in the chemical susceptibility of individual fatty acids to peroxidation coupled with the known differences in membrane composition between species can explain the different lifespans of species, especially the difference between mammals and birds as well as the body-size-related variation in lifespan within mammals and birds. Lifespan extension by calorie-restriction can also be explained by changes in membrane fatty acid composition which result in membranes more resistant to peroxidation. It is suggested that lifespan extension by reduced insulin/IGF signalling may also be mediated by changes in membrane fatty acid composition.

Membranes and the setting of energy demand

In his classic 1961 book, The Fire of Life, Max Kleiber presented a critique of the theories advanced to explain the BMR-body size relationship. One of the theories he dismissed was that the chemical composition of animals varies with body size. Since this time, however, much has been learned about the make-up of BMR in different animals as well as the chemical composition of different-sized animals. Specifically, in recent years it has become obvious that mammal species and bird species do vary in chemical composition in a systematic manner associated with the body size of the species. Small mammal and bird species have cellular membranes that are predominantly polyunsaturated, and as mammal and bird species increase in size, their cellular membranes become progressively less polyunsaturated. Since Kleiber's time, it has also become obvious that a substantial amount of the energy turnover of BMR is associated with the activity of membrane processes,specifically the maintenance of trans-membrane gradients, such as the Na+ gradient across the plasmalemmal membrane and the H+gradient across the mitochondrial inner membrane. The variation in both membrane composition and membrane processes associated with body size variation in metabolic rate has been combined in the `membrane pacemaker'theory of metabolism. This theory proposes that: (1) membrane-associated activities are significant and dominant components of BMR; (2) when BMR varies among species, all the activities that constitute BMR vary in unison; (3)species with high mass-specific BMR have highly polyunsaturated membranes while those with low BMR have less polyunsaturation of their membranes; (4)highly polyunsaturated membranes have distinctive physical properties that cause the proteins in the membranes to have a high molecular activity, and this results in higher rates of metabolism of cells, tissues and,consequently, the whole animal. Evidence supporting this theory is both correlative and experimental. Manipulation of membrane composition changes the molecular activity of membrane proteins. These differences in membrane composition may also represent a link between metabolism and aging. They probably explain the lifespan-body size relationship in mammals and birds and also the mammal-bird difference in lifespan.

Calorie restriction--the SIR2 connection

A nutritious diet low in calories improves the health and extends the life span of rodents. Recent studies identified a gene, SIR2, which encodes an NAD-dependent deacetylase and may mediate the effects of calorie restriction. In this review, we discuss SIR2 genes and calorie restriction in the lower organisms yeast and Drosophila. We then describe the physiological changes in mammals during calorie restriction and how they may lead to the observed health benefits. We summarize the roles of mammalian Sirt1 in mediating these changes in tissues and endocrine systems and propose that Sirt1 regulates calorie restriction by sensing low calories and triggering physiological changes linked to health and longevity.

Mitochondria, Oxidants, and Aging

The free radical theory of aging postulates that the production of intracellular reactive oxygen species is the major determinant of life span. Numerous cell culture, invertebrate, and mammalian models exist that lend support to this half-century-old hypothesis. Here we review the evidence that both supports and conflicts with the free radical theory and examine the growing link between mitochondrial metabolism, oxidant formation, and the biology of aging.