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

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

Fitness cost of extended lifespan in Caenorhabditis elegans

An insulin/IGF-I-like signalling pathway determines the rate of aging of the adult nematode, Caenorhabditis elegans. Mutations in genes encoding this pathway can result in a doubling of lifespan. While such mutations may appear to have little effect on development or fertility, evolutionary theory predicts that large increases in lifespan will not be optimal for fitness. We demonstrate by laboratory natural selection that partial loss of function of the insulin receptor-like protein DAF-2 results in dramatically reduced fitness even under laboratory conditions. Despite long-lived mutants appearing healthy, they exhibit a heavy fitness cost consistent with an evolutionary theory of aging.     

Genes acting in longevity-related pathways in the endoparasitoid,Pteromalus puparum

Among insects, lifespans vary over a broad range, from the short-lived mayflies to the 17-year periodical cicadas. Generally, lifespans are determined by a phase in life, the reproductive lifespan, which varies among species. Numerous pathways, such as the insulin/insulin-like growth factor signaling pathway, the target of rapamycin pathway and the mitogen-activated protein kinase/extracellular signal-regulated kinases pathways, influence aging and lifespan. Components of these pathways were identified as lifespan-related genes, including genes mediating growth, metabolism, development, resistance, and other processes. Many age-related genes have been discovered in fruit flies, honeybees, and ants among other insect species. Studies of insect aging and longevity can help understand insect biology and develop new pest management technologies. In this paper, we interrogated the new Pteromalus puparum genome, from which we predicted 133 putative lifespan-related genes based on their homology with known lifespan-related genes of Drosophila melanogaster. These genes function in five signaling pathways and three physiological processes. The conserved domain structures of these genes were predicted and their expression patterns were analyzed. Amino acid sequence alignments and domain structure analysis indicate that most components remain conserved across at least six insect orders. The data in this paper will facilitate future work on parasitoid lifespans, which may have economic value in biocontrol programs.     

Genetics and epigenetics of aging and longevity

Evolutionary theories of aging predict the existence of certain genes that provide selective advantage early in life with adverse effect on lifespan later in life (antagonistic pleiotropy theory) or longevity insurance genes (disposable soma theory). Indeed, the study of human and animal genetics is gradually identifying new genes that increase lifespan when overexpressed or mutated: gerontogenes. Furthermore, genetic and epigenetic mechanisms are being identified that have a positive effect on longevity. The gerontogenes are classified as lifespan regulators, mediators, effectors, housekeeping genes, genes involved in mitochondrial function, and genes regulating cellular senescence and apoptosis. In this review we demonstrate that the majority of the genes as well as genetic and epigenetic mechanisms that are involved in regulation of longevity are highly interconnected and related to stress response.     

Movement decline across lifespan of Caenorhabditis elegans mutants in the insulin/insulin‐like signaling pathway

Research in aging biology has identified several pathways that are molecularly conserved across species that extend lifespan when mutated. The insulin/insulin‐like signaling (IIS) pathway is one of the most widely studied of these. It has been assumed that extending lifespan also extends healthspan (the period of life with minimal functional loss). However, data supporting this assumption conflict and recent evidence suggest that life extension may, in and of itself, extend the frail period. In this study, we use Caenorhabditis elegans to further probe the link between lifespan and healthspan. Using movement decline as a measure of health, we assessed healthspan across the entire lifespan in nine IIS pathway mutants. In one series of experiments, we studied healthspan in mass cultures, and in another series, we studied individuals longitudinally. We found that long‐lived mutants display prolonged mid‐life movement and do not prolong the frailty period. Lastly, we observed that early‐adulthood movement was not predictive of late‐life movement or survival, within identical phenotypes. Overall, these observations show that extending lifespan does not prolong the period of frailty. Both genotype and a stochastic component modulate aging, and movement late in life is more variable than early‐life movement.     

Aneuploidy shortens replicative lifespan in Saccharomyces cerevisiae

Aneuploidy and aging are correlated; however, a causal link between these two phenomena has remained elusive. Here, we show that yeast disomic for a single native yeast chromosome generally have a decreased replicative lifespan. In addition, the extent of this lifespan deficit correlates with the size of the extra chromosome. We identified a mutation in BUL1 that rescues both the lifespan deficit and a protein trafficking defect in yeast disomic for chromosome 5. Bul1 is an E4 ubiquitin ligase adaptor involved in a protein quality control pathway that targets membrane proteins for endocytosis and destruction in the lysosomal vacuole, thereby maintaining protein homeostasis. Concurrent suppression of the aging and trafficking phenotypes suggests that disrupted membrane protein homeostasis in aneuploid yeast may contribute to their accelerated aging. The data reported here demonstrate that aneuploidy can impair protein homeostasis, shorten lifespan, and may contribute to age‐associated phenotypes.     

Gene Pathways That Delay Caenorhabditis elegans Reproductive Senescence

Reproductive senescence is a hallmark of aging. The molecular mechanisms regulating reproductive senescence and its association with the aging of somatic cells remain poorly understood. From a full genome RNA interference (RNAi) screen, we identified 32 Caenorhabditis elegans gene inactivations that delay reproductive senescence and extend reproductive lifespan. We found that many of these gene inactivations interact with insulin/IGF-1 and/or TGF-β endocrine signaling pathways to regulate reproductive senescence, except nhx-2 and sgk-1 that modulate sodium reabsorption. Of these 32 gene inactivations, we also found that 19 increase reproductive lifespan through their effects on oocyte activities, 8 of them coordinate oocyte and sperm functions to extend reproductive lifespan, and 5 of them can induce sperm humoral response to promote reproductive longevity. Furthermore, we examined the effects of these reproductive aging regulators on somatic aging. We found that 5 of these gene inactivations prolong organismal lifespan, and 20 of them increase healthy life expectancy of an organism without altering total life span. These studies provide a systemic view on the genetic regulation of reproductive senescence and its intersection with organism longevity. The majority of these newly identified genes are conserved, and may provide new insights into age-associated reproductive senescence during human aging.     

Lifespan regulation by evolutionarily conserved genes essential for viability

Evolutionarily conserved mechanisms that control aging are predicted to have prereproductive functions in order to be subject to natural selection. Genes that are essential for growth and development are highly conserved in evolution, but their role in longevity has not previously been assessed. We screened 2,700 genes essential for Caenorhabditis elegans development and identified 64 genes that extend lifespan when inactivated postdevelopmentally. These candidate lifespan regulators are highly conserved from yeast to humans. Classification of the candidate lifespan regulators into functional groups identified the expected insulin and metabolic pathways but also revealed enrichment for translation, RNA, and chromatin factors. Many of these essential gene inactivations extend lifespan as much as the strongest known regulators of aging. Early gene inactivations of these essential genes caused growth arrest at larval stages, and some of these arrested animals live much longer than wild-type adults. daf-16 is required for the enhanced survival of arrested larvae, suggesting that the increased longevity is a physiological response to the essential gene inactivation. These results suggest that insulin-signaling pathways play a role in regulation of aging at any stage in life.     

Tetz’s theory and law of longevity

Here, we present new theory and law of longevity intended to evaluate fundamental factors that control lifespan. This theory is based on the fact that genes affecting host organism longevity are represented by subpopulations: genes of host eukaryotic cells, commensal microbiota, and non-living genetic elements. Based on Tetz’s theory of longevity, we propose that lifespan and aging are defined by the accumulation of alterations over all genes of macroorganism and microbiome and the non-living genetic elements associated with them. Tetz’s law of longevity states that longevity is limited by the accumulation of alterations to the limiting value that is not compatible with life. Based on theory and law, we also propose a novel model to calculate several parameters, including the rate of aging and the remaining lifespan of individuals. We suggest that this theory and model have explanatory and predictive potential to eukaryotic organisms, allowing the influence of diseases, medication, and medical procedures to be re-examined in relation to longevity. Such estimates also provide a framework to evaluate new fundamental aspects that control aging and lifespan.     

Identification of mutations that delay somatic or reproductive aging of Caenorhabditis elegans

Aging is an important feature of animal biology characterized by progressive, degenerative changes in somatic and reproductive tissues. The rate of age-related degeneration is genetically controlled, since genes that influence lifespan have been identified. However, little is known about genes that affect reproductive aging or aging of specific somatic tissues. To identify genes that are important for controlling these degenerative changes, we used chemical mutagenesis to perform forward genetic screens in Caenorhabditis elegans. By conducting a screen focused on somatic aging, we identified mutant hermaphrodites that displayed extended periods of pharyngeal pumping, body movement, or survival. One of these mutations is a novel allele of the age-1 gene. age-1 encodes a phosphatidylinositol-3-kinase (PI3K) that functions in the insulin/insulin-like growth factor-1 (IGF-1) signaling pathway. age-1(am88) creates a missense change in the conserved PIK domain and causes dramatic extensions of the pharyngeal pumping and body movement spans, as well as a twofold extension of the lifespan. By conducting screens focused on reproductive aging in mated hermaphrodites, we identified mutants that displayed increased progeny production late in life. To characterize these mutations, we developed quantitative measurements of age-related morphological changes in the gonad. The am117 mutation delayed age-related declines in progeny production and morphological changes in the gonad. These studies provide new insights into the genetic regulation of age-related degenerative changes in somatic and reproductive tissues.     

Sexual dimorphism in the nutritional requirement for adult lifespan in Drosophila melanogaster

The nutritional requirements of Drosophila have mostly been studied for development and reproduction, but the minimal requirements for adult male and female flies for lifespan have not been established. Following development on a complete diet, we find substantial sex difference in the basic nutritional requirement of adult flies for full length of life. Relative to females, males require less of each nutrient, and for some nutrients that are essential for development, adult males have no requirement at all for lifespan. The most extreme (and surprising) sex differences were that chronic cholesterol and vitamin deficiencies had no effect on the lifespan of adult males, but they greatly decreased lifespan in females. Female oogenesis rather than chromosomal karyotype and mating status is the key cause of this gender difference in life‐sustaining nutritional requirements. These data are important to the way we understand the mechanisms by which diet modifies lifespan.     

Metabolic rate affects adult life span independently of developmental rate in parasitoid wasps

Developmental time and body size correlate with lifespan in a wide range of taxa, although not in insect parasitoids. When the rate of development is independent of adult metabolic rate, adult lifespan is free to adapt to the adult environment. We suggest that interspecific variation in intrinsic adult metabolic rates, differences in allocation of lipids to longevity, and reproduction and differences in the ability to use carbohydrates as an adult should all result in variation of adult lifespan, independent of development time. To test these ideas, we measured metabolic rate, lipid content and egg load at eclosion, developmental time, and lifespan of females with and without carbohydrate food in five species of Asobara, which represent parasitoids of Drosophila. No relationship between development time and adult longevity was found. As predicted, metabolic rates varied between species and appeared to trade off with adult longevity. We found no clear link between initial egg load and the longevity of a species, suggesting that lipid allocation may be less important in determining adult lifespan. The results obtained indicate that differences in metabolic rate have an important effect on adult lifespan, without affecting developmental rate in parasitoids. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103 , 45–56.     

New genes tied to endocrine, metabolic, and dietary regulation of lifespan from a Caenorhabditis elegans genomic RNAi screen

Most of our knowledge about the regulation of aging comes from mutants originally isolated for other phenotypes. To ask whether our current view of aging has been affected by selection bias, and to deepen our understanding of known longevity pathways, we screened a genomic Caenorhabditis elegans RNAi library for clones that extend lifespan. We identified 23 new longevity genes affecting signal transduction, the stress response, gene expression, and metabolism and assigned these genes to specific longevity pathways. Our most important findings are (i) that dietary restriction extends C. elegans' lifespan by down-regulating expression of key genes, including a gene required for methylation of many macromolecules, (ii) that integrin signaling is likely to play a general, evolutionarily conserved role in lifespan regulation, and (iii) that specific lipophilic hormones may influence lifespan in a DAF-16/FOXO-dependent fashion. Surprisingly, of the new genes that have conserved sequence domains, only one could not be associated with a known longevity pathway. Thus, our current view of the genetics of aging has probably not been distorted substantially by selection bias.     

The search for evolutionary developmental origins of aging in zebrafish: a novel intersection of developmental and senescence biology in the zebrafish model system

Senescence may be considered the antithesis of early development, but yet there may be factors and mechanisms in common between these two phenomena during the process of aging. We investigated whether any relationship exists between the regulatory mechanisms that function in early development and in senescence using the zebrafish (Danio rerio), a small freshwater fish and a useful model animal for genetic studies. We conducted experiments to isolate zebrafish mutants expressing an apparent senescence phenotype during embryogenesis (embryonic senescence). Some of the genes we thereby identified had already been associated with cellular senescence and chronological aging in other organisms, but many had not yet been linked to these processes. Complete loss-of-function of developmentally essential genes induce embryonic (or larval) lethality, whereas it seems like their partial loss-of-function (i.e., decrease-of-function by heterozygote or hypomorphic mutations) still remains sufficient to go through the early developmental process because of its adaptive plasticity or rather heterozygote advantage. However, in some cases, such partial loss-of-function of genes compromise normal homeostasis due to haploinsufficiency later in adult life having many environmental stress challenges. By contrast, any heterozygote-advantageous genes might gain a certain benefit(s) (much more fitness) by such partial loss-of-function later in life. Physiological senescence may evolutionarily arise from both genetic and epigenetic drifts as well as from losing adaptive developmental plasticity in face of stress signals from the external environment that interacts with functions of multiple genes rather than effects of only a single gene mutation or defect. Previously uncharacterized developmental genes may thus mediate the aging process and play a pivotal role in senescence. Moreover, unexpected senescence-related genes might also be involved in the early developmental process and regulation. We wish to ascertain whether we can identify such genes promptly in a comprehensive manner. The ease of manipulation using the zebrafish system allows us to conduct an exhaustive exploration of novel genes and small molecular compounds that can be linked to the senescence phenotype and thereby facilitates searching for the evolutionary and developmental origins of aging in vertebrates.     

The identification of zebrafish mutants showing alterations in senescence-associated biomarkers

There is an interesting overlap of function in a wide range of organisms between genes that modulate the stress responses and those that regulate aging phenotypes and, in some cases, lifespan. We have therefore screened mutagenized zebrafish embryos for the altered expression of a stress biomarker, senescence-associated beta-galactosidase (SA-beta-gal) in our current study. We validated the use of embryonic SA-beta-gal production as a screening tool by analyzing a collection of retrovirus-insertional mutants. From a pool of 306 such mutants, we identified 11 candidates that showed higher embryonic SA-beta-gal activity, two of which were selected for further study. One of these mutants is null for a homologue of Drosophila spinster, a gene known to regulate lifespan in flies, whereas the other harbors a mutation in a homologue of the human telomeric repeat binding factor 2 (terf2) gene, which plays roles in telomere protection and telomere-length regulation. Although the homozygous spinster and terf2 mutants are embryonic lethal, heterozygous adult fish are viable and show an accelerated appearance of aging symptoms including lipofuscin accumulation, which is another biomarker, and shorter lifespan. We next used the same SA-beta-gal assay to screen chemically mutagenized zebrafish, each of which was heterozygous for lesions in multiple genes, under the sensitizing conditions of oxidative stress. We obtained eight additional mutants from this screen that, when bred to homozygosity, showed enhanced SA-beta-gal activity even in the absence of stress, and further displayed embryonic neural and muscular degenerative phenotypes. Adult fish that are heterozygous for these mutations also showed the premature expression of aging biomarkers and the accelerated onset of aging phenotypes. Our current strategy of mutant screening for a senescence-associated biomarker in zebrafish embryos may thus prove to be a useful new tool for the genetic dissection of vertebrate stress response and senescence mechanisms.     

Hydrogen peroxide: friend of the faux blonde,foe of the cell

Abstract

The oxidative theory of aging states (loosely) that the cumulative effects of oxidant damage may determine both the onset of senescence and time of death. A small avalanche of papers is now appearing, most of which generally support the theory. A recent one published in Nature is particularly notable. Using the small (～900 cell) metazoan Caenorhabditis elegans, Taub and colleagues¹ have found that three mutations associated with increased adult lifespan cause enhanced expression of an unusual cytosolic catalase (CTL-1). Furthermore, deficiency of CTL-1 caused by a nonsense mutation shortens the adult life span of these animals. Interestingly, these progeric mutants also show an abnormal accumulation of lipofuscin (or ceroid depending on the reader's bent) toward the end of life.     

Cost-free lifespan extension via optimization of gene expression in adulthood aligns with the developmental theory of ageing

Ageing evolves because the force of selection on traits declines with age but the proximate causes of ageing are incompletely understood. The ‘disposable soma’ theory of ageing (DST) upholds that competitive resource allocation between reproduction and somatic maintenance underpins the evolution of ageing and lifespan. In contrast, the developmental theory of ageing (DTA) suggests that organismal senescence is caused by suboptimal gene expression in adulthood. While the DST predicts the trade-off between reproduction and lifespan, the DTA predicts that age-specific optimization of gene expression can increase lifespan without reproduction costs. Here we investigated the consequences for lifespan, reproduction, egg size and individual fitness of early-life, adulthood and post-reproductive onset of RNAi knockdown of five ‘longevity’ genes involved in key biological processes in Caenorhabditis elegans. Downregulation of these genes in adulthood and/or during post-reproductive period increases lifespan, while we found limited evidence for a link between impaired reproduction and extended lifespan. Our findings demonstrate that suboptimal gene expression in adulthood often contributes to reduced lifespan directly rather than through competitive resource allocation between reproduction and somatic maintenance. Therefore, age-specific optimization of gene expression in evolutionarily conserved signalling pathways that regulate organismal life histories can increase lifespan without fitness costs.     

Molecular genetics of aging in the fly: Is this the end of the beginning?

How we age and what we can do about it have been uppermost in human thought since antiquity. The many false starts have frustrated experimentalists and theoretical arguments pronouncing the inevitability of the process have created a nihilistic climate among scientists and the public. The identification of single gene alterations that substantially extend life span in nematodes and flies however, have begun to reinvigorate the field. Drosophila's long history of contributions to aging research, rich storehouse of genetic information, and powerful molecular techniques make it an excellent system for studying the molecular mechanisms underlying the process of aging. In recent years, Drosophila has been used to test current theories on aging and explore new directions of potential importance to the biology of aging. One such example is the surprising finding that, as opposed to the commonly held assumption that adult life is a period of random passive decline in which all things are thought to fall apart, the molecular life of the adult fly appears to be a state of dynamic well-regulated change. In the fly, the level of expression of many different genes changes in an invariant, often age-dependent, manner. These as well as other molecular genetic studies and demographic analyses using the fly have begun to challenge widely held ideas about aging providing evidence that aging may be a much more dynamic and malleable process than anticipated. With the enormous success that Drosophila molecular genetics has demonstrated in helping understand complex biological phenomena such as development there is much optimism that similar approaches can be adapted to assist in understanding the process of aging.     

The role of mitochondrial DNA in the development of type 2 diabetes caused by fetal malnutrition

Epidemiological studies have revealed strong and reproducible links between indices of poor fetal growth and susceptibility to the development of glucose intolerance and insulin resistance syndrome in adult life. To explain these associations, the thrifty phenotype hypothesis has been proposed. Mitochondrial DNA abnormalities have been known to cause insulin deficiency, insulin resistance and diabetes mellitus. In this review, we propose that mitochondrial dysfunction is a link between malnutrition during early life and disease in adult life. The potential mechanism for mitochondrial dysfunction will be focused on availability of the taurine and nucleotides, and imprinting on the genes.