Ageing and survival after different doses of heat shock: the results of analysis of data from stress experiments with the nematode worm Caenorhabditis elegans

Hormesis determines lifespan

This paper addresses how long lifespan can be extended via multiple interventions, such as dietary supplements [e.g., curcumin, resveratrol, sulforaphane, complex phytochemical mixtures (e.g., Moringa, Rhodiola)], pharmaceutical agents (e.g., metformin), caloric restriction, intermittent fasting, exercise and other activities. This evaluation was framed within the context of hormesis, a biphasic dose response with specific quantitative features describing the limits of biological/phenotypic plasticity for integrative biological endpoints (e.g., cell proliferation, memory, fecundity, growth, tissue repair, stem cell population expansion/differentiation, longevity). Evaluation of several hundred lifespan extending agents using yeast, nematode (Caenorhabditis elegans), multiple insect and other invertebrate and vertebrate models (e.g., fish, rodents), revealed they responded in a manner [average (mean/median) and maximum lifespans] consistent with the quantitative features [i.e., 30-60% greater at maximum (Hormesis Rule)] of the hormetic dose response. These lifespan extension features were independent of biological model, inducing agent, endpoints measured and mechanism. These findings indicate that hormesis describes the capacity to extend life via numerous agents and activities and that the magnitude of lifespan extension is modest, in the percentage, not fold, range. These findings have important implications for human aging, genetic diseases/environmental stresses and lifespan extension, as well as public health practices and long-term societal resource planning.

Biological resilience and aging: Activation of stress response pathways contributes to lifespan extension

While aging was traditionally viewed as a stochastic process of damage accumulation, it is now clear that aging is strongly influenced by genetics. The identification and characterization of long-lived genetic mutants in model organisms has provided insights into the genetic pathways and molecular mechanisms involved in extending longevity. Long-lived genetic mutants exhibit activation of multiple stress response pathways leading to enhanced resistance to exogenous stressors. As a result, lifespan exhibits a significant, positive correlation with resistance to stress. Disruption of stress response pathways inhibits lifespan extension in multiple long-lived mutants representing different pathways of lifespan extension and can also reduce the lifespan of wild-type animals. Combined, this suggests that activation of stress response pathways is a key mechanism by which long-lived mutants achieve their extended longevity and that many of these pathways are also required for normal lifespan. These results highlight an important role for stress response pathways in determining the lifespan of an organism.

Resistance to Stress Can Be Experimentally Dissociated From Longevity

On the basis of multiple experiments demonstrating that high resistance to stress is associated with long lifespan, it has been proposed that stress resistance is a key determinant of longevity. However, the extent to which high resistance to stress is necessary or sufficient for long life is currently unclear. In this work, we use a genetic approach to disrupt different stress response pathways and examine the resulting effect on the longevity of the long-lived insulin-like growth factor 1 (IGF1) receptor mutant daf-2. Although mutation of the heat shock factor gene hsf-1, deletion of sod genes, deletion of the p38 MAPK kinase gene pmk-1, or deletion of the transcription factor gene egl-27 all resulted in decreased resistance to at least one form of stress and decreased lifespan, the magnitude of change in stress resistance did not correspond to the magnitude of change in lifespan. In addition, we found that deletion of the glycerol-3-phosphate dehydrogenase genes gpdh-1 and gpdh-2 or deletion of the DAF-16 cofactor gene nhl-1 also results in decreased resistance to at least one form of stress but increases lifespan. Overall, our results suggest that while increased stress resistance is associated with longevity, stress resistance, and lifespan can be experimentally dissociated.

Aging causes decreased resistance to multiple stresses and a failure to activate specific stress response pathways

In this work, we examine the relationship between stress resistance and aging. We find that resistance to multiple types of stress peaks during early adulthood and then declines with age. To dissect the underlying mechanisms, we use C. elegans transcriptional reporter strains that measure the activation of different stress responses including: the heat shock response, mitochondrial unfolded protein response, endoplasmic reticulum unfolded protein response, hypoxia response, SKN-1-mediated oxidative stress response, and the DAF-16-mediated stress response. We find that the decline in stress resistance with age is at least partially due to a decreased ability to activate protective mechanisms in response to stress. In contrast, we find that any baseline increase in stress caused by the advancing age is too mild to detectably upregulate any of the stress response pathways. Further exploration of how worms respond to stress with increasing age revealed that the ability to mount a hormetic response to heat stress is also lost with increasing age. Overall, this work demonstrates that resistance to all types of stress declines with age. Based on our data, we speculate that the decrease in stress resistance with advancing age results from a genetically-programmed inactivation of stress response pathways, not accumulation of damage.

Methodological considerations for heat shock of the nematode Caenorhabditis elegans

Stress response pathways share commonalities across many species, including humans, making heat shock experiments valuable tools for many biologists. The study of stress response in Caenorhabditis elegans has provided great insight into many complex pathways and diseases. Nevertheless, the heat shock/heat stress field does not have consensus as to the timing, temperature, or duration of the exposure and protocols differ extensively between laboratories. The lack of cohesiveness makes it difficult to compare results between groups or to know where to start when preparing your own protocol. We present a discussion of some of the major hurdles to reproducibility in heat shock experiments as well as detailed protocols for heat shock and hormesis experiments.

Transcript expression patterns illuminate the mechanistic background of hormesis in caenorhabditis elegans maupas

The animal model Caenorhabditis elegans was employed to study polyphenol- and humic substances-induced hormetic changes in lifespan. A detailed insight into the underlying mechanism of hormesis was uncovered by applying whole genome DNA microarray experimentation over a range of quercetin (Q), tannic acid (TA), and humic substances (HuminFeed(®), HF) concentrations. The transcriptional response to all exposures followed a non-linear mode which highlighted differential signaling and metabolic pathways. While low Q concentrations regulated processes improving the health of the nematodes, higher concentrations extended lifespan and modulated substantially the global transcriptional response. Over-represented transcripts were notably part of the biotransformation process: enhanced catabolism of toxic intermediates possibly contributes to the lifespan extension. The regulation of transcription, Dauer entry, and nucleosome suggests the presence of distinct exposure dependent differences in transcription and signaling pathways. TA- and HF-mediated transcript expression patterns were overall similar to each other, but changed across the concentration range indicating that their transcriptional dynamics are complex and cannot be attributed to a simple adaptive response. In contrast, Q-mediated hormesis was well aligned to fit the definition of an adaptive response. Simple molecules are more likely to induce an adaptive response than more complex molecules.

Genetic variation for stress-response hormesis in C. elegans lifespan

Increased lifespan can be associated with greater resistance to many different stressors, most notably thermal stress. Such hormetic effects have also been found in C. elegans where short-term exposure to heat lengthens the lifespan. Genetic investigations have been carried out using mutation perturbations in a single genotype, the wild type Bristol N2. Yet, induced mutations do not yield insight regarding the natural genetic variation of thermal tolerance and lifespan. We investigated the genetic variation of heat-shock recovery, i.e. hormetic effects on lifespan and associated quantitative trait loci (QTL) in C. elegans. Heat-shock resulted in an 18% lifespan increase in wild type CB4856 whereas N2 did not show a lifespan elongation. Using recombinant inbred lines (RILs) derived from a cross between wild types N2 and CB4856 we found natural variation in stress-response hormesis in lifespan. Approx. 28% of the RILs displayed a hormesis effect in lifespan. We did not find any hormesis effects for total offspring. Across the RILs there was no relation between lifespan and offspring. The ability to recover from heat-shock mapped to a significant QTL on chromosome II which overlapped with a QTL for offspring under heat-shock conditions. The QTL was confirmed by introgressing relatively small CB4856 regions into chromosome II of N2. Our observations show that there is natural variation in hormetic effects on C. elegans lifespan for heat-shock and that this variation is genetically determined.

Hormesis does not make sense except in the light of TOR-driven aging

Weak stresses (including weak oxidative stress, cytostatic agents, heat shock, hypoxia, calorie restriction) may extend lifespan. Known as hormesis, this is the most controversial notion in gerontology. For one, it is believed that aging is caused by accumulation of molecular damage. If so, hormetic stresses (by causing damage) must shorten lifespan. To solve the paradox, it was suggested that, by activating repair, hormetic stresses eventually decrease damage. Similarly, Baron Munchausen escaped from a swamp by pulling himself up by his own hair. Instead, I discuss that aging is not caused by accumulation of molecular damage. Although molecular damage accumulates, organisms do not live long enough to age from this accumulation. Instead, aging is driven by overactivated signal-transduction pathways including the TOR (Target of Rapamycin) pathway. A diverse group of hormetic conditions can be divided into two groups. "Hormesis A" inhibits the TOR pathway. "Hormesis B" increases aging-tolerance, defined as the ability to survive catastrophic complications of aging. Hormesis A includes calorie restriction, resveratrol, rapamycin, p53-inducing agents and, in part, physical exercise, heat shock and hypoxia. Hormesis B includes ischemic preconditioning and, in part, physical exercise, heat shock, hypoxia and medical interventions.

Quantitative trait loci for longevity in heat-stressed Drosophila melanogaster

Longevity is a typical quantitative trait which is influenced by multiple genes. Here we explore the genetic variation in longevity of Drosophila melanogaster in both mildly heat-stressed and control flies. Quantitative trait loci (QTL) analysis for longevity was performed in a single-sex environment at 25°C with and without a mild heat-stress pre-treatment, using a previously reported set of recombinant inbred lines (RIL). QTL regions for longevity in heat-stressed flies overlapped with QTL for longevity in control flies. All longevity QTL co-localized with QTL for longevity identified in previous studies using very different sets of RIL in mixed sex environments, though the genome is nearly saturated with QTL for longevity when considering all previous studies. Heat stress decreased the number of significant QTL for longevity if compared to the control environment. Our mild heat-stress pre-treatment had a beneficial effect (hormesis) more often in shorter-lived than in longer-lived RIL.

Extension of lifespan in C. elegans by naphthoquinones that act through stress hormesis mechanisms

Hormesis occurs when a low level stress elicits adaptive beneficial responses that protect against subsequent exposure to severe stress. Recent findings suggest that mild oxidative and thermal stress can extend lifespan by hormetic mechanisms. Here we show that the botanical pesticide plumbagin, while toxic to C. elegans nematodes at high doses, extends lifespan at low doses. Because plumbagin is a naphthoquinone that can generate free radicals in vivo, we investigated whether it extends lifespan by activating an adaptive cellular stress response pathway. The C. elegans cap'n'collar (CNC) transcription factor, SKN-1, mediates protective responses to oxidative stress. Genetic analysis showed that skn-1 activity is required for lifespan extension by low-dose plumbagin in C. elegans. Further screening of a series of plumbagin analogs identified three additional naphthoquinones that could induce SKN-1 targets in C. elegans. Naphthazarin showed skn-1dependent lifespan extension, over an extended dose range compared to plumbagin, while the other naphthoquinones, oxoline and menadione, had differing effects on C. elegans survival and failed to activate ARE reporter expression in cultured mammalian cells. Our findings reveal the potential for low doses of naturally occurring naphthoquinones to extend lifespan by engaging a specific adaptive cellular stress response pathway.

Environmental challenges improve resource utilization for asexual reproduction and maintenance in hydra

Variation in life history can reflect genetic differences, and may be caused by environmental effects on phenotypes. Understanding how these two sources of life history variation interact to express an optimal allocation of resources in a changing environment is central to life history theory. This study addresses variation in the allocation of resources to asexual reproduction and to maintenance of Hydra magnipapillata in relation to differences in temperature and food availability. Hydra is a non-senescent, persistent species with primarily clonal reproduction. We recorded changes in budding rate and mean survival under starvation, which indicate changes in the allocation of resources to asexual reproduction and maintenance. In constant conditions we observed a clear trade-off between asexual reproduction and maintenance, where budding increased linearly with food intake while starvation survival stayed rather constant. In contrast, an environment with fluctuations in temperature or food availability promotes maintenance and increases the survival chances of hydra under starvation. Surprisingly, asexual reproduction also tends to be positively affected by fluctuating environmental conditions, which suggests that in this case there is no clear trade-off between asexual reproduction and maintenance in hydra. Environmental stresses have a beneficial impact on the fitness-related phenotypical traits of the basal metazoan hydra. The results indicate that, if the stress occurs in hormetic doses, variable stressful and fluctuating environments can be salutary for hydra. A closer examination of this dynamic can therefore enable us to develop a deeper understanding of the evolution of aging and longevity.

Pathways linking the early environment to long-term health and lifespan

The intrauterine environment is a major contributor to normal physiological growth and development of an individual. Disturbances at this critical time can affect the long-term health of the offspring. Low birth weight individuals have strong correlations with increased susceptibility to type 2 diabetes and cardiovascular disease in later-life. These observations led to the Thrifty Phenotype Hypothesis which suggested that these associations arose because of the response of a growing fetus to a suboptimal environment such as poor nutrition. Animal models have shown that environmentally induced intrauterine growth restriction increases the risk of a variety of diseases later in life. These detrimental features are also observed in high birth weight offspring from mothers who were obese or consumed a high fat diet during gestation. Recent advances in our understanding of the mechanisms underlying this phenomenon have elucidated several potential candidates for the long-term effects of the early environment on the function and metabolism of a cell. These include: (1) Epigenetic alterations (e.g. DNA methylation and histone modifications), which regulate specific gene expression and can be influenced by the environment, both during gestation and early postnatal life and (2) Oxidative stress that changes the balance between reactive oxygen species generation (e.g. through mitochondrial dysfunction) and antioxidant defense capacity. This has permanent effects on cellular ageing such as regulation of telomere length. Further understanding of these processes will help in the development of therapeutic strategies to increase healthspan and reduced the burden of age-associated diseases.

Stress to the rescue: is hormesis a 'cure' for aging?

Despite the fact that the phenomenon of hormesis has been known for many years it is still very much an area of controversy just how useful hormetic treatments are in preventing age-related human diseases and increasing life expectancy. Since there are no data in humans demonstrating hormesis as an effective anti-ageing strategy we turn to a simple model organism for insight. In this review we explore what can be predicted about the usefulness of hormetic treatments in humans based upon studies conducted in the soil nematode Caenorhabditis elegans.

Multiple mild heat-shocks decrease the Gompertz component of mortality in Caenorhabditis elegans

Exposure to mild heat-stress (heat-shock) can significantly increase the life expectancy of the nematode Caenorhabditis elegans. A single heat-shock early in life extends longevity by 20% or more and affects life-long mortality by decreasing initial mortality only; the rate of increase in subsequent mortality (Gompertz component) is unchanged. Repeated mild heat-shocks throughout life have a larger effect on life span than does a single heat-shock early in life. Here, we ask how multiple heat-shocks affect the mortality trajectory in nematodes and find increases of life expectancy of close to 50% and of maximum longevity as well. We examined mortality using large numbers of animals and found that multiple heat-shocks not only decrease initial mortality, but also slow the Gompertz rate of increase in mortality. Thus, multiple heat-shocks have anti-aging hormetic effects and represent an effective approach for modulating aging.

Catechin induced longevity in C. elegans: from key regulator genes to disposable soma

The flavanol catechin is a ubiquitous metabolite within the plant kingdom. Several health benefits have previously been reported, however, to date, most attention has been devoted to gallated forms of catechin. This study utilized the nematode Caenorhabditis elegans to assess potential life expanding effects of non-gallated catechin. Longevity was observed at three different catechin concentrations, an effect that was neither linked to a specific temperature nor to the viability of the feeding bacteria. Taken all tests into account, hormesis, calorie restriction, as well as the presence of simple antioxidative or antibacterial effects could be excluded. Likewise, the insulin/IGF-1 like signaling pathway and the calmodulin kinase II pathway were not considered to play a major mechanic role. Moreover, stress resistance was enhanced without a marked alteration in reproductive behavior. In addition, lifespan tests with various stress and lifespan relevant mutant strains revealed that the life span extending phenotype was absent in mev-1, daf-2, akt-2 and nhr-8. Finally, catechin elicited a significant reduction in body length, a finding that is in line with the "Disposable Soma Theory". It is proposed that catechin modulates an energy-intensive stress response and repair system that results in reduced body length and an enhanced lifespan.

C. elegans STI-1, the homolog of Sti1/Hop, is involved in aging and stress response

Environmental and physiological stresses such as heat shock, oxidative stress, heavy metals, and pathogenic conditions induce cellular stress response. This response is often mediated by heat shock proteins that function as molecular chaperones. A stress-inducible cochaperone, Sti1/Hop (Hsp organizer protein), functions as an adaptor protein that simultaneously binds with Hsp70 and Hsp90 to transfer client proteins from Hsp70 to Hsp90. However, the biological role of STI-1 in vivo is poorly understood in metazoans. Here, we report the characterization of the Caenorhabditis elegans homolog of Sti1/Hop, which is approximately 56% identical with human STI-1. C. elegans STI-1 (CeSTI-1) is expressed in the pharynx, intestine, nervous system, and muscle from larvae to adults. Analysis of proteins immunoprecipitated with anti-STI-1 antibody by mass spectrometry revealed that CeSTI-1 can bind with both Hsp70 and Hsp90 homologs like its mammalian counterpart. sti-1 expression is elevated by heat stress, and an sti-1(jh125) null mutant shows decreased fertility under heat stress conditions. These mutants also show abnormally high lethality in extreme heat and may be functioning with DAF-16 in thermotolerance. In addition, sti-1(jh125) mutants have a shortened life span. Our results confirm that CeSTI-1 is a cochaperone protein that may maintain homeostatic functions during episodes of stress and can regulate longevity in nematodes.

Repeated temperature fluctuation extends the life span of Caenorhabditis elegans in a daf-16-dependent fashion

Thermocyclers were utilized to regularly shift nematodes between 12 degrees C and 25 degrees C throughout their life spans. When wild-type worms (N2) were "thermocycled" between 12 degrees C and 25 degrees C at 10-min intervals they lived almost as long as those that were incubated constantly at 12 degrees C. Shifting at 1-min or 1-h intervals lessened this effect. Similar results were observed for the long-lived mutants daf-2, eat-2 and clk-1, each of which prolongs life span through different mechanisms. In contrast, the life span of a daf-16 mutant was not prolonged by thermocycling worms, indicating that the effect is mediated by an insulin-like signaling pathway. To elucidate the molecular basis for the life span extension, two transgenic strains were employed in which heat shock proteins (HSPs) drove expression of the green fluorescent protein (GFP) gene. As expected, both HSPs were expressed at significantly higher levels in animals grown at 25 degrees C. Moreover, HSP expression in the thermocycled worms approximated that of animals grown at 25 degrees C more so than animals grown at 12 degrees C. This suggests that incubation at the higher temperatures for short time intervals induced stress-responsive gene expression that led to significant life span extension.

Hormesis in aging

Hormesis in aging is represented by mild stress-induced stimulation of protective mechanisms in cells and organisms resulting in biologically beneficial effects. Single or multiple exposure to low doses of otherwise harmful agents, such as irradiation, food limitation, heat stress, hypergravity, reactive oxygen species and other free radicals have a variety of anti-aging and longevity-extending hormetic effects. Detailed molecular mechanisms that bring about the hormetic effects are being increasingly understood, and comprise a cascade of stress response and other pathways of maintenance and repair. Although the extent of immediate hormetic effects after exposure to a particular stress may only be moderate, the chain of events following initial hormesis leads to biologically amplified effects that are much larger, synergistic and pleiotropic. A consequence of hormetic amplification is an increase in the homeodynamic space of a living system in terms of increased defence capacity and reduced load of damaged macromolecules. Hormetic strengthening of the homeodynamic space provides wider margins for metabolic fluctuation, stress tolerance, adaptation and survival. Hormesis thus counter-balances the progressive shrinkage of the homeodynamic space, which is the ultimate cause of aging, diseases and death. Healthy aging may be achieved by hormesis through mild and periodic, but not severe or chronic, physical and mental challenges, and by the use of nutritional hormesis incorporating mild stress-inducing molecules called hormetins. The established scientific foundations of hormesis are ready to pave the way for new and effective approaches in aging research and intervention.

An explicit test of the phospholipid saturation hypothesis of acquired cold tolerance in Caenorhabditis elegans

Protection of poikilothermic animals from seasonal cold is widely regarded as being causally linked to changes in the unsaturation of membrane phospholipids, yet in animals this proposition remains formally untested. We have now achieved this by the genetic manipulation of lipid biosynthesis of Caenorhabditis elegans independent of temperature. Worms transferred from 25 degrees C to 10 degrees C develop over several days a much-increased tolerance of lethal cold (0 degrees C) and also an increased phospholipid unsaturation, as in higher animal models. Of the three C. elegans Delta9-desaturases, transcript levels of fat-7 only were up-regulated by cold transfer. RNAi suppression of fat-7 caused the induction of fat-5 desaturase, so to control desaturase expression we combined RNAi of fat-7 with a fat-5 knockout. These fat-5/fat-7 manipulated worms displayed the expected negative linear relationship between lipid saturation and cold tolerance at 0 degrees C, an outcome confirmed by dietary rescue. However, this change in lipid saturation explains just 16% of the observed difference between cold tolerance of animals held at 25 degrees C and 10 degrees C. Thus, although the manipulated lipid saturation affects the tolerable thermal window, and altered Delta9-desaturase expression accounts for cold-induced lipid adjustments, the effect is relatively small and none of the lipid manipulations were sufficient to convert worms between fully cold-sensitive and fully cold-tolerant states. Critically, transfer of 10 degrees C-acclimated worms back to 25 degrees C led to them restoring the usual cold-sensitive phenotype within 24 h despite retaining a lipid profile characteristic of 10 degrees C worms. Other nonlipid mechanisms of acquired cold protection clearly dominate inducible cold tolerance.

Public and private mechanisms of life extension in Caenorhabditis elegans

Model organisms have been widely used to study the ageing phenomenon in order to learn about human ageing. Although the phylogenetic diversity between vertebrates and some of the most commonly used model systems could hardly be greater, several mechanisms of life extension are public (common characteristic in divergent species) and likely share a common ancestry. Dietary restriction, reduced IGF-signaling and, seemingly, reduced ROS-induced damage are the best known mechanisms for extending longevity in a variety of organisms. In this review, we summarize the knowledge of ageing in the nematode Caenorhabditis elegans and compare the mechanisms of life extension with knowledge from other model organisms.

Hormesis and aging in Caenorhabditis elegans

Hormesis has emerged as an important manipulation for the study of aging. Although hormesis is manifested in manifold combinations of stress and model organism, the mechanisms of hormesis are only partly understood. The increased stress resistance and extended survival caused by hormesis can be manipulated to further our understanding of the roles of intrinsic and induced stress resistance in aging. Genes of the dauer/insulin/insulin-like signaling (IIS) pathway have well-established roles in aging in Caenorhabditis elegans. Here, we discuss the role of some of those genes in the induced stress resistance and induced life extension attributable to hormesis. Mutations in three genes (daf-16, daf-18, and daf-12) block hormetically induced life extension. However, of these three, only daf-18 appears to be required for a full induction of thermotolerance induced by hormesis, illustrating possible separation of the genetic requirements for stress resistance and life extension. Mutations in three other genes of this pathway (daf-3, daf-5, and age-1) do not block induced life extension or induced thermotolerance; daf-5 mutants may be unusually sensitive to hormetic conditions.

Lifespan extension of Caenorhabditis elegans following repeated mild hormetic heat treatments

Mild hormetic heat treatments early in life can significantly increase the lifespan of the nematode C. elegans. We have examined the effects of heat treatments at different ages and show that treatments early in life cause the largest increases in lifespan. We also find that repeated mild heat treatments throughout life have a larger effect on lifespan compared to a single mild heat treatment early in life. We hypothesize that the magnitude of the hormetic effect is related to the levels of heat shock protein expression. Following heat treatment young worms show a dramatic increase in the levels of the small heat shock protein HSP-16 whereas old worms are a 100-fold less responsive. The levels of the heat shock proteins HSP-4 and HSP-16 correlate well with the effects on lifespan by the hormetic treatments.

Hormetic modulation of aging and longevity by mild heat stress

Aging is characterized by a stochastic accumulation of molecular damage, progressive failure of maintenance and repair, and consequent onset of age-related diseases. Applying hormesis in aging research and therapy is based on the principle of stimulation of maintenance and repair pathways by repeated exposure to mild stress. In a series of experimental studies we have shown that repetitive mild heat stress has anti-aging hormetic effects on growth and various other cellular and biochemical characteristics of human skin fibroblasts undergoing aging in vitro. These effects include the maintenance of stress protein profiles, reduction in the accumulation of oxidatively and glycoxidatively damaged proteins, stimulation of the proteasomal activities for the degradation of abnormal proteins, improved cellular resistance to ethanol, hydrogen peroxide and ultraviolet-B rays, and enhanced levels of various antioxidant enzymes. Anti-aging hormetic effects of mild heat shock appear to be facilitated by reducing protein damage and protein aggregation by activating internal antioxidant, repair and degradation 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.

Aging intervention, prevention, and therapy through hormesis

The phenomenon of hormesis is represented by mild stress-induced stimulation of maintenance and repair pathways resulting in beneficial effects for the cells and organisms. Anti-aging and life-prolonging effects of a wide variety of the so-called stressors, such as pro-oxidants, aldehydes, calorie restriction, irradiation, heat shock, and hypergravity, have been reported. Molecular mechanisms of hormesis due to different stresses are yet to be elucidated, but there are indications that relatively small individual hormetic effects become biologically amplified resulting in the collective significant improvement of cellular and organismic functions and survival. Accepting that some important issues with respect to establishing the optimal hormetic conditions still need to be resolved by future research, hormesis appears to be a promising and effective approach for modulating aging, for preventing or delaying the onset of age-related diseases, and for improving quality of life in old age.

Stressors and antistressors: how do they influence life span in HER-2/neu transgenic mice?

The purpose of this study is to investigate possible influences of different stressors (saline injections, light deprivation and constant light regimen) and geroprotectors (Epitalon and melatonin) on survivals of female HER-2/neu transgenic mice. We propose a semi-parametric model of heterogeneous mortality (frailty model) for the analysis of the experimental data. In this model, we assume that treatment influences parameters of both frailty distribution and baseline hazard. The unique design of the experiments makes it possible to compare the effects on survival produced by different treatments in terms of changes in population heterogeneity and underlying hazard. Parameters of the model help to describe the possible influences of various stressors, geroprotectors, and their dosage on the life span of laboratory animals. The proposed model helps to advance our understanding of the effects--such as debilitation, longevity hormesis and incomplete hormesis--which occur in the population as a result of different treatments.

Individual aging and mortality rate: how are they related?

Many researchers working in the area of aging and longevity base their conclusions on the behavior of empirical age trajectories of mortality rates. In such analyses, changes in the slope of the logarithm of the mortality curve are often associated with changes in the rate of individual aging. We show that such interpretation may be incorrect: the changes in the slope of this curve do not necessarily correspond to the changes in the rate of individual aging. We use three models of mortality and aging to illustrate this statement. The first one is based on the idea of frailty. We show that changes in frailty distribution alone may be responsible for changes in the slope. The second model exploits the idea of saving lives. It evaluates changes in mortality rate after elimination of lethal stressful events. The third model uses the idea of Strehler and Mildvan (1960). It shows that changes in the rate of individual aging may take place without changes in the slope of the logarithm of the mortality curve.

Longevity and heat stress regulation in Caenorhabditis elegans

Aging is the most complex phenotype for a multicellular organism. This process is now being under severe investigation. Here I will review the different processes known to affect longevity in the nematode Caenorhabditis elegans and their relationship with thermotolerance. All the longevity mutants that have been tested so far show an increase in stress resistance. In particular, long-lived mutants affected in the IGF/insulin pathway and those affected in the germ-line formation are both thermotolerant and long-lived. The mechanisms that activate the stress resistance are now been understood including the DAF-16 fork head transcription factor transport to the nucleus and the activation of genes involved in the defense to stress. The high correlation between stress resistance and longevity suggests that the same molecular activities that defend the cell from stress can defend the cell from the damage caused by aging.

Adaptive responses to oxidative damage in three mutants of Caenorhabditis elegans (age-1, mev-1 and daf-16) that affect life span

Oxidative damage shortens the life span of the nematode Caenorhabditis elegans (C. elegans), even in an age-1 mutant that is characterized by a long life and oxygen resistance. We found that daily short-term exposure (3 h) to hyperoxia further extended the life span of age-1, a phenomenon known as an adaptive response. age-1 also showed resistance to paraquat and heat. Acute hyperoxic treatment did not extend the life spans of wild type, daf-16 or mev-1. daf-16 mutant had a slightly shorter life span compared to wild type and was sensitive to heat and paraquat. The daf-16 phenotype resembles that of mev-1 showing a short life and oxygen sensitivity. We measured mRNA levels of superoxide dismutase genes (sod-1 through 4), catalase genes (clt-1 and ctl-2), known to encode anti-oxidant enzymes, and found they were elevated in age-1 young adults. On the other hand, in daf-16 and mev-1, the expression of sod-1, sod-2 and sod-3 genes was lower rather than in wild type. Conversely, ctl-1 and ctl-2 genes expression was significantly elevated in daf-16 and mev-1. This suggests that DAF-16, a forkhead/winged-helix transcription factor, whose expression is suppressed by AGE-1, phosphoinositide 3-kinase (PI3-kinase), regulates anti-oxidant genes as well as energy metabolism under atmospheric conditions. However, the level of gene expression of SOD and catalase was not elevated by short-term exposure to 90% oxygen in wild type, mev-1, daf-16 and even age-1. This suggests that SOD and catalase do not play a role in the adaptive response against oxidative stress under hyperoxia, at least under these experimental conditions.

New age patterns of survival improvement in Sweden: do they characterize changes in individual aging?

The parameters of the Gompertz approximation to the mortality curve are negatively correlated. Strehler and Mildvan [Science 132 (1960) 14] predicted this property of the mortality curve using a mathematical model of mortality and aging and then confirmed it in empirical studies. Despite the fact that their theory was based on the cohort model of mortality the SM correlation was also revealed in the analysis of period mortality data. In fact, most applications of the SM model to human data use Gompertz's approximation to the period mortality rate. Many researchers studying SM correlation consider it a universal demographic law. Such correlation prescribes a certain regularity in mortality changes. All mortality curves must intersect at one point. Mortality decline must produce the rectangularization of survival curves. In this paper we investigated the changes in the patterns of mortality decline in Sweden between 1861 and 1999. We found a difference in patterns of SM correlation for cohort and period mortality data. We investigated trends in survival improvement and found that the tendency to rectangularization of the survival curve existed for only a limited period of time. Then it was gradually replaced by near parallel shift of the survival curve to the right. We found that the pattern of SM correlation was relatively stable only at certain phases of the survival history of male and female populations. We analyzed past and recent patterns of survival changes and discussed possible causes for instability of SM correlation both in cohort and in period mortality data.

Hormesis and debilitation effects in stress experiments using the nematode worm Caenorhabditis elegans: the model of balance between cell damage and HSP levels

In this article, we discuss mechanisms responsible for the effects of heat treatment on increasing subsequent survival in the nematode worm Caenorhabditis elegans. We assume that the balance between damage associated with exposure to thermal stress and the level of heat shock proteins produced plays a key role in forming the age-pattern of mortality and survival in stress experiments. We propose a stochastic model of stress, which describes the accumulation of damage in the cells of the worm as the worm ages. The model replicates the age trajectories of experimental survival curves in three experiments in which worms were heat-treated for 0, 1, 2, 4, 6, or 8h. We also discuss analytical results and directions of further research. The proposed method of stochastic modelling of survival data provides a new approach that can be used to model, analyse and extrapolate experimental results.

Relationship between increased longevity and stress resistance as assessed through gerontogene mutations in Caenorhabditis elegans

We review the status of the hypothesis that interventions that increase the resistance to stress offer the potential for effective life prolongation and increased health. The work focuses on research in the nematode worm Caenorhabditis elegans and describes both published and unpublished results consistent with this hypothesis. Correlation between stress resistance and longevity among many gerontogene mutants is provided.