Subfield history: caloric restriction, slowing aging, and extending life

drr-2 encodes an eIF4H that acts downstream of TOR in diet-restriction-induced longevity of C. elegans

Dietary restriction (DR) results in a robust increase in lifespan while maintaining the physiology of much younger animals in a wide range of species. Here, we examine the role of drr-2, a DR-responsive gene recently identified, in determining the longevity of Caenorhabditis elegans. Inhibition of drr-2 has been shown to increase longevity. However, the molecular mechanisms by which drr-2 influences longevity remain unknown. We report here that drr-2 encodes an ortholog of human eukaryotic translation initiation factor 4H (eIF4H), whose function is to mediate the initiation step of mRNA translation. The molecular function of DRR-2 is validated by the association of DRR-2 with polysomes and by the decreased rate of protein synthesis observed in drr-2 knockdown animals. Previous studies have also suggested that DR might trigger a regulated reduction in drr-2 expression to initiate its longevity response. By examining the effect of increasing drr-2 expression on DR animals, we find that drr-2 is essential for a large portion of the longevity response to DR. The nutrient-sensing target of rapamycin (TOR) pathway has been shown to mediate the longevity effects of DR in C. elegans. Results from our genetic analyses suggest that eIF4H/DRR-2 functions downstream of TOR, but in parallel to the S6K/PHA-4 pathway to mediate the lifespan effects of DR. Together, our findings reveal an important role for eIF4H/drr-2 in the TOR-mediated longevity responses to DR.

With TOR, less is more: a key role for the conserved nutrient-sensing TOR pathway in aging

Target of rapamycin (TOR) is an evolutionarily conserved nutrient-sensing protein kinase that regulates growth and metabolism in all eukaryotic cells. Studies in flies, worms, yeast, and mice support the notion that the TOR signaling network modulates aging. TOR is also emerging as a robust mediator of the protective effects of various forms of dietary restriction (DR), which can extend life span and slow the onset of certain age-related diseases across species. Here we discuss how modulating TOR signaling slows aging through downstream processes including mRNA translation, autophagy, endoplasmic reticulum (ER) stress signaling, stress responses, and metabolism. Identifying the mechanisms by which the TOR signaling network works as a pacemaker of aging is a major challenge and may help identify potential drug targets for age-related diseases, thereby facilitating healthful life span extension in humans.

Calorie restriction: what recent results suggest for the future of ageing research

BACKGROUND

Calorie Restriction (CR) research has expanded rapidly over the past few decades and CR remains the most highly reproducible, environmental intervention to improve health and extend lifespan in animal studies. Although many model organisms have consistently demonstrated positive responses to CR, it remains to be shown whether CR will extend lifespan in humans. Additionally, the current environment of excess caloric consumption and high incidence of overweight/obesity illustrate the improbable nature of the long-term adoption of a CR lifestyle by a significant proportion of the human population. Thus, the search for substances that can reproduce the beneficial physiologic responses of CR without a requisite calorie intake reduction, termed CR mimetics (CRMs), has gained momentum.

MATERIAL AND METHODS

Recent articles describing health and lifespan results of CR in nonhuman primates and short-term human studies are discussed. Additional consideration is given to the rapidly expanding search for CRMs.

RESULTS

The first results from a long-term, randomized, controlled CR study in nonhuman primates showing statistically significant benefits on longevity have now been reported. Additionally, positive results from short-term, randomized, controlled CR studies in humans are suggestive of potential health and longevity gains, while test of proposed CRMs (including rapamycin, resveratrol, 2-deoxyglucose and metformin) have shown both positive and mixed results in rodents.

CONCLUSION

Whether current positive results will translate into longevity gains for humans remains an open question. However, the apparent health benefits that have been observed with CR suggest that regardless of longevity gains, the promotion of healthy ageing and disease prevention may be attainable.

Genetic variation in the murine lifespan response to dietary restriction: from life extension to life shortening

Chronic dietary restriction (DR) is considered among the most robust life-extending interventions, but several reports indicate that DR does not always extend and may even shorten lifespan in some genotypes. An unbiased genetic screen of the lifespan response to DR has been lacking. Here, we measured the effect of one commonly used level of DR (40% reduction in food intake) on mean lifespan of virgin males and females in 41 recombinant inbred strains of mice. Mean strain-specific lifespan varied two to threefold under ad libitum (AL) feeding and 6- to 10-fold under DR, in males and females respectively. Notably, DR shortened lifespan in more strains than those in which it lengthened life. Food intake and female fertility varied markedly among strains under AL feeding, but neither predicted DR survival: therefore, strains in which DR shortened lifespan did not have low food intake or poor reproductive potential. Finally, strain-specific lifespans under DR and AL feeding were not correlated, indicating that the genetic determinants of lifespan under these two conditions differ. These results demonstrate that the lifespan response to a single level of DR exhibits wide variation amenable to genetic analysis. They also show that DR can shorten lifespan in inbred mice. Although strains with shortened lifespan under 40% DR may not respond negatively under less stringent DR, the results raise the possibility that life extension by DR may not be universal.

Short-term dietary restriction and fasting precondition against ischemia reperfusion injury in mice

Dietary restriction (DR) extends lifespan and increases resistance to multiple forms of stress, including ischemia reperfusion injury to the brain and heart in rodents. While maximal effects on lifespan require long-term restriction, the kinetics of onset of benefits against acute stress is not known. Here, we show that 2-4 weeks of 30% DR improved survival and kidney function following renal ischemia reperfusion injury in mice. Brief periods of water-only fasting were similarly effective at protecting against ischemic damage. Significant protection occurred within 1 day, persisted for several days beyond the fasting period and extended to another organ, the liver. Protection by both short-term DR and fasting correlated with improved insulin sensitivity, increased expression of markers of antioxidant defense and reduced expression of markers of inflammation and insulin/insulin-like growth factor-1 signaling. Unbiased transcriptional profiling of kidneys from mice subject to short-term DR or fasting revealed a significant enrichment of signature genes of long-term DR. These data demonstrate that brief periods of reduced food intake, including short-term daily restriction and fasting, can increase resistance to ischemia reperfusion injury in rodents and suggest a rapid onset of benefits of DR in mammals.

Short photoperiod initiated during adulthood sustains reproductive function in older female siberian hamsters more effectively than short photoperiod initiated before puberty

Reproductive aging in female mammals is characterized by a progressive decline in fertility and fecundity. Many women delay their first full-term pregnancy until an age at which their reproductive potential has already declined. No treatment is presently available to delay the aging process. In a limited number of rodent species, caloric restriction sustained reproductive function in older females, and in most investigations, sexual maturation was delayed because caloric restriction was initiated at weaning. We have previously reported similar outcomes in female Siberian hamsters that were reared in short photoperiod (SP), which profoundly inhibits reproductive physiology. When compared to hamsters held in long photoperiod (LP), females reared in SP matured much later and had greater reproductive success at 9 mo of age. Herein, we determined if delayed onset of sexual maturation was necessary for SP to decelerate reproductive aging. We initiated a 6-mo period of SP before or after sexual maturation and measured the reproductive success of females at 12 mo of age. Maintenance of hamsters in SP beginning after puberty was associated with significantly greater litter success (77%) compared to imposition of SP before puberty (35%); the difference in weaning success was even greater (73% and 12%, respectively). Regardless of which SP regime was used, litter success of females exposed to SP was substantially greater than that of 12-mo-old females held continuously in LP (6%). The efficacy of SP in decelerating female reproductive aging is manifest at several life stages and is greater when treatment is initiated after rather than before puberty.

Generation and bioenergetic analysis of cybrids containing mitochondrial DNA from mouse skeletal muscle during aging

Mitochondrial respiratory chain defects have been associated with various diseases and normal aging, particularly in tissues with high energy demands including skeletal muscle. Muscle-specific mitochondrial DNA (mtDNA) mutations have also been reported to accumulate with aging. Our understanding of the molecular processes mediating altered mitochondrial gene expression to dysfunction associated with mtDNA mutations in muscle would be greatly enhanced by our ability to transfer muscle mtDNA to established cell lines. Here, we report the successful generation of mouse cybrids carrying skeletal muscle mtDNA. Using this novel approach, we performed bioenergetic analysis of cells bearing mtDNA derived from young and old mouse skeletal muscles. A significant decrease in oxidative phosphorylation coupling and regulation capacity has been observed with cybrids carrying mtDNA from skeletal muscle of old mice. Our results also revealed decrease growth capacity and cell viability associated with the mtDNA derived from muscle of old mice. These findings indicate that a decline in mitochondrial function associated with compromised mtDNA quality during aging leads to a decrease in both the capacity and regulation of oxidative phosphorylation.

Divergent regulation of adipose tissue metabolism by calorie restriction and inhibition of growth hormone signaling

Calorie restriction (CR) and a reduced growth hormone (GH) signal affect insulin sensitivity and lifespan in mammals in a similar manner. We investigated the effects of CR and moderate inhibition of GH on glucose-stimulated activation of insulin signaling and the expression of genes related to fat metabolism in white adipose tissue (WAT) in rats. We used 10-month-old male, wild-type (W) Wistar rats, fed ad libitum (AL) or a 30% CR diet from 6weeks of age, and transgenic (Tg) rats with moderately suppressed GH signaling. Rats were killed 15min after an intraperitoneal injection of glucose or saline. In control W-AL rats, the levels of serum insulin, phosphorylated (p) insulin receptor (pY-IR), p-Akt, and the expression of glucose transporter (Glut) 4 in the membrane fraction were greater in the glucose-injected group than in the saline-injected group, indicating significant activation of insulin signaling in response to glucose loading. In the W-CR and Tg-AL rats, the serum insulin and pY-IR levels were lower than those in the W-AL rats. The Akt-Glut pathway was up-regulated even after saline-injection. Expression levels of adipogenic and lipogenic genes including PPARgamma, adiponectin, and its receptors, were higher in the W-CR rats than in the W-AL and Tg-AL rats. The present findings indicate adipose tissue metabolic profiles specific to CR.

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.

HIF-1 modulates dietary restriction-mediated lifespan extension via IRE-1 in Caenorhabditis elegans

Dietary restriction (DR) extends lifespan in various species and also slows the onset of age-related diseases. Previous studies from flies and yeast have demonstrated that the target of rapamycin (TOR) pathway is essential for longevity phenotypes resulting from DR. TOR is a conserved protein kinase that regulates growth and metabolism in response to nutrients and growth factors. While some of the downstream targets of TOR have been implicated in regulating lifespan, it is still unclear whether additional targets of this pathway also modulate lifespan. It has been shown that the hypoxia inducible factor-1 (HIF-1) is one of the targets of the TOR pathway in mammalian cells. HIF-1 is a transcription factor complex that plays key roles in oxygen homeostasis, tumor formation, glucose metabolism, cell survival, and inflammatory response. Here, we describe a novel role for HIF-1 in modulating lifespan extension by DR in Caenorhabditis elegans. We find that HIF-1 deficiency results in extended lifespan, which overlaps with that by inhibition of the RSKS-1/S6 kinase, a key component of the TOR pathway. Using a modified DR method based on variation of bacterial food concentrations on solid agar plates, we find that HIF-1 modulates longevity in a nutrient-dependent manner. The hif-1 loss-of-function mutant extends lifespan under rich nutrient conditions but fails to show lifespan extension under DR. Conversely, a mutation in egl-9, which increases HIF-1 activity, diminishes the lifespan extension under DR. This deficiency is rescued by tissue-specific expression of egl-9 in specific neurons and muscles. Increased lifespan by hif-1 or DR is dependent on the endoplasmic reticulum (ER) stress regulator inositol-requiring protein-1 (IRE-1) and is associated with lower levels of ER stress. Therefore, our results demonstrate a tissue-specific role for HIF-1 in the lifespan extension by DR involving the IRE-1 ER stress pathway.

Dietary restriction (DR) is one of the most robust environmental manipulations that extend lifespan in various species. DR has also been shown to slow the onset of a number of age-related diseases. Studies in model organisms like C. elegans can be used to uncover biological mechanisms that determine the beneficial effects of DR. Previous studies suggest that the nutrient-sensing target of rapamycin (TOR) pathway is required for DR-mediated lifespan extension. However, the downstream mechanisms by which TOR modulates lifespan remain unclear. In mammalian cells, TOR and the downstream S6 kinase (S6K) activate expression of the hypoxia-inducible factor-1 (HIF-1), which is frequently up-regulated in various tumors. Using C. elegans as a model system, we characterized novel functions of HIF-1 in aging. We find that inhibition of HIF-1 extends lifespan under rich nutrient conditions, whereas enhanced levels of HIF-1 only allow partial lifespan extension by DR. We also demonstrated that increased lifespan by hif-1 or DR depends on the endoplasmic reticulum (ER) stress regulator inositol-requiring protein-1 (IRE-1) and is associated with lower levels of ER stress, which is caused by overloading of misfolded/unfolded proteins to ER. Thus, our results support the idea that HIF-1–mediated changes in protein homeostasis play a key role in the lifespan extension by DR.

Elevation of oxidative-damage biomarkers during aging in F2 hybrid mice: protection by chronic oral intake of resveratrol

Resveratrol (RSV), a naturally occurring phytoalexin that can be found in red wine, berries, and peanuts, has been shown to extend both mean and maximum life span in model organisms. RSV has also been reported to shift the physiology of middle-aged mice on a high-calorie diet toward that of mice on a standard diet. These beneficial effects of RSV have been suggested to resemble caloric restriction. Our study in F2 four-way cross-hybrid mice was the first to evaluate the effects of aging and long-term RSV treatment (14.09+/-3.4 mg/L in drinking water for 6 or 12 months) on biomarkers of oxidative damage to DNA, 8-hydroxy-2'-deoxyguanosine (8OHdG); lipid, 8-iso-prostaglandin(2 alpha) (8-iso-PGF(2 alpha)); and protein, protein carbonyl content (PCC). There was a significant age-dependent accumulation of oxidative damage to DNA, lipid, and protein as well as a clear increase in urine 8-iso-PGF(2 alpha) levels in the majority of mouse tissues. Rates of age-dependent increases in damage biomarkers varied between tissues. Chronic RSV treatment elevated total RSV plasma levels and reduced the observed age-dependent accumulation of (1) 8OHdG in liver and heart, (2) 8-iso-PGF(2 alpha) in heart and urine, and (3) PCC in liver and kidney. However, a 12-month RSV intake resulted in significant elevation of 8-iso-PGF(2 alpha) and PCC in kidney. Our studies demonstrate that RSV treatment consistently attenuated oxidative damage in tissues where age-related oxidative damage accumulation was prominent, but also suggested that chronic RSV treatment may induce nephrotoxicity.

Optimal window of caloric restriction onset limits its beneficial impact on T-cell senescence in primates

We have recently shown in non-human primates that caloric restriction (CR) initiated during adulthood can delay T-cell aging and preserve naïve CD8 and CD4 T cells into advanced age. An important question is whether CR can be initiated at any time in life, and whether age at the time of onset would modulate the beneficial effects of CR. In the current study, we evaluated the impact of CR started before puberty or during advanced age on T-cell senescence and compared it to the effects of CR started in early adulthood. Our data demonstrate that the beneficial effects of adult-onset CR on T-cell aging were lost by both early and late CR onset. In fact, some of our results suggest that inappropriate initiation of CR may be harmful to the maintenance of T-cell function. This suggests that there may be an optimal window during adulthood where CR can delay immune senescence and improve correlates of immunity in primates.

Base excision repair, aging and health span

DNA damage and mutagenesis are suggested to contribute to aging through their ability to mediate cellular dysfunction. The base excision repair (BER) pathway ameliorates a large number of DNA lesions that arise spontaneously. Many of these lesions are reported to increase with age. Oxidized guanine, repaired largely via base excision repair, is particularly well studied and shown to increase with age. Spontaneous mutant frequencies also increase with age which suggests that mutagenesis may contribute to aging. It is widely accepted that genetic instability contributes to age-related occurrences of cancer and potentially other age-related pathologies. BER activity decreases with age in multiple tissues. The specific BER protein that appears to limit activity varies among tissues. DNA polymerase-beta is reduced in brain from aged mice and rats while AP endonuclease is reduced in spermatogenic cells obtained from old mice. The differences in proteins that appear to limit BER activity among tissues may represent true tissue-specific differences in activity or may be due to differences in techniques, environmental conditions or other unidentified differences among the experimental approaches. Much remains to be addressed concerning the potential role of BER in aging and age-related health span.

Awareness of hormesis will enhance future research in basic and applied neuroscience

Hormesis is defined operationally as responses of cells or organisms to an exogenous or intrinsic factor (chemical, temperature, psychological challenge, etc.) in which the factor induces stimulatory or beneficial effects at low doses and inhibitory or adverse effects at high doses. The compendium of articles by Calabrese entitled "Neuroscience and Hormesis" provides a broad range of examples of neurobiological processes and responses to environmental factors that exhibit biphasic dose responses, the signature of hormesis. Nerve cell networks are the "first responders" to environmental challenges--they perceive the challenge and orchestrate coordinated adaptive responses that typically involve autonomic, neuroendocrine, and behavioral changes. In addition to direct adaptive responses of neurons to environmental stressors, cells subjected to a stressor produce and release molecules such as growth factors, cytokines, and hormones that alert adjacent and even distant cells to impending danger. The discoveries that some molecules (e.g., carbon monoxide and nitric oxide) and elements (e.g., selenium and iron) that are toxic at high doses play fundamental roles in cellular signaling or metabolism suggest that during evolution, organisms (and their nervous systems) co-opted environmental toxins and used them to their advantage. Neurons also respond adaptively to everyday stressors, including physical exercise, cognitive challenges, and dietary energy restriction, each of which activates pathways linked to the production of neurotrophic factors and cellular stress resistance proteins. The development of interventions that activate hormetic signaling pathways in neurons is a promising new approach for the preventation and treatment of a range of neurological disorders.

Life span extension by calorie restriction depends on Rim15 and transcription factors downstream of Ras/PKA, Tor, and Sch9

Calorie restriction (CR), the only non-genetic intervention known to slow aging and extend life span in organisms ranging from yeast to mice, has been linked to the down-regulation of Tor, Akt, and Ras signaling. In this study, we demonstrate that the serine/threonine kinase Rim15 is required for yeast chronological life span extension caused by deficiencies in Ras2, Tor1, and Sch9, and by calorie restriction. Deletion of stress resistance transcription factors Gis1 and Msn2/4, which are positively regulated by Rim15, also caused a major although not complete reversion of the effect of calorie restriction on life span. The deletion of both RAS2 and the Akt and S6 kinase homolog SCH9 in combination with calorie restriction caused a remarkable 10-fold life span extension, which, surprisingly, was only partially reversed by the lack of Rim15. These results indicate that the Ras/cAMP/PKA/Rim15/Msn2/4 and the Tor/Sch9/Rim15/Gis1 pathways are major mediators of the calorie restriction-dependent stress resistance and life span extension, although additional mediators are involved. Notably, the anti-aging effect caused by the inactivation of both pathways is much more potent than that caused by CR.

Reduction in calorie intake is a well-established intervention that extends the life span of a variety of biological model organisms studied. Calorie restriction also delays and attenuates age-related changes in primates, although its longevity-promoting effect has not been demonstrated. Here, we utilized a single cell organism, baker's yeast, to examine the role of evolutionarily conserved genes in life span regulation and their involvement in calorie restriction. The yeast mutants lacking Ras2, Tor1, or Sch9 are long-lived. The anti-aging effect observed in these mutants depends on the protein Rim15 and several key regulators of gene expression that are essential in inducing cellular protection under stress. The beneficial effects of calorie restriction are much smaller in yeast that are missing these proteins, indicating their essential role in promoting longevity. Our study also showed that by combining the genetic manipulation and calorie restriction intervention, yeast can reach a life span ten times that of those grown under standard conditions. This extreme longevity requires Rim15 and also depends on other yet-to-be identified mechanisms. Our findings provided new leads that may help to elucidate the mechanisms underlying the anti-aging effect of calorie restriction in mammals.

Expression of dominant-negative Dmp53 in the adult fly brain inhibits insulin signaling

In Drosophila melanogaster, p53 (Dmp53) is an important mediator of longevity. Expression of dominant-negative (DN) forms of Dmp53 in adult neurons, but not in muscle or fat body cells, extends lifespan. The lifespan of calorie-restricted flies is not further extended by simultaneously expressing DN-Dmp53 in the nervous system, indicating that a decrease in Dmp53 activity may be a part of the CR lifespan-extending pathway in flies. In this report, we show that selective expression of DN-Dmp53 in only the 14 insulin-producing cells (IPCs) in the brain extends lifespan to the same extent as expression in all neurons and this lifespan extension is not additive with CR. DN-Dmp53-dependent lifespan extension is accompanied by reduction of Drosophila insulin-like peptide 2 (dILP2) mRNA levels and reduced insulin signaling (IIS) in the fat body, which suggests that Dmp53 may affect lifespan by modulating insulin signaling in the fly.

Down-regulation of AMP-activated protein kinase by calorie restriction in rat liver

AMP-activated protein kinase (AMPK) may act as a key enzyme for metabolic adaptation to calorie restriction (CR) or reduced growth hormone (GH)-insulin-like growth factor (IGF)-1 signaling, an experimental intervention for lifespan extension in animals. We investigated the protein levels of AMPKalpha and a downstream enzyme, acetyl-CoA carboxylase (ACC), by immunoblotting of liver and quadriceps femoris muscle (QFM) extracts from 6-month-old wild-type (W) and GH-suppressed transgenic (Tg) Wistar rats fed ad libitum (AL) or 30% CR diets from 6weeks of age. A modified alternate-day feeding regimen for CR yielded a fed-fasted cycle in CR rats, and therefore the effects of overnight fasting in W-AL rats were also evaluated. CR decreased threonine-172-phosphorylated AMPKalpha (p-AMPKalpha; an activated form) levels in the liver, whereas the CR-fed-fasted cycle or overnight fasting did not significantly affect the p-AMPKalpha level. In the QFM, the p-AMPKalpha level was slightly elevated in the CR-fasted phase, but greatly increased in the AL-fasted phase. Suppression of GH did not affect the p-AMPKalpha level. The phosphorylated-ACC levels did not alter in parallel with the p-AMPKalpha level, particularly in the liver. The present results suggest that CR down-regulates the AMPK activity in the liver on a long-term basis.

Reduced TOR signaling extends chronological life span via increased respiration and upregulation of mitochondrial gene expression

The relationships between mitochondrial respiration, reactive oxygen species (ROS), and life span are complex and remain controversial. Inhibition of the target of rapamycin (TOR) signaling pathway extends life span in several model organisms. We show here that deletion of the TOR1 gene extends chronological life span in Saccharomyces cerevisiae, primarily by increasing mitochondrial respiration via enhanced translation of mtDNA-encoded oxidative phosphorylation complex subunits. Unlike previously reported pathways regulating chronological life span, we demonstrate that deletion of TOR1 delays aging independently of the antioxidant gene SOD2. Furthermore, wild-type and tor1 null strains differ in life span only when respiration competent and grown in normoxia in the presence of glucose. We propose that inhibition of TOR signaling causes derepression of respiration during growth in glucose and that the subsequent increase in mitochondrial oxygen consumption limits intracellular oxygen and ROS-mediated damage during glycolytic growth, leading to lower cellular ROS and extension of chronological life span.

Health span extension by later-life caloric or dietary restriction: a view based on rodent studies

In spite of the potential benefit of lifelong food restriction to retard aging and extend life span, it is unrealistic in human. The restriction late in life may be more practical. There are, however, only limited studies on the effect of late onset caloric or dietary restriction. We and other investigators have shown that the late life restriction rejuvenates some parameters that decline with age in rats and mice. Although such studies may provide a basis for human application of late-life caloric or dietary restriction, the prolongation of maximum life span would not be expected in view of the current status of the long-lived population in which maximum life span potential appears to have already been achieved. The late life caloric restriction, however, could extend the health span if the extent were appropriate.

Effects of caloric restriction are species-specific

This article addresses two questions: (1) 'can caloric restriction (CR) extend the life spans of all species of experimental animals', and (2) 'is CR likely to slow the human aging process and/or extend the human life span?' The answer to the first question is clearly 'no', because CR decreases the life span of the housefly, Musca domestica, and fails to extend the life span of at least one mouse strain. The answer to the second question is unknown, because human CR has not yet been shown either to increase or curtail the human life span. However, recent efforts to develop insect models of CR have been unsuccessful and/or relatively uninformative, so any insights regarding the relationship between CR and human aging are more likely to arise from studies of established, mammalian models of CR.

Targeting mammalian target of rapamycin (mTOR) for health and diseases

The macrolide rapamycin is used clinically to treat graft rejection and restenosis. Mammalian target of rapamycin (mTOR) is a central controller of cellular and organism growth that integrates nutrient and hormonal signals, and regulates diverse cellular processes. New studies have linked mTOR to several human diseases including cancer, diabetes, obesity, cardiovascular diseases and neurological disorders. Recent data have also revealed that mTOR is involved in the regulation of lifespan and in age-related diseases. These findings demonstrate the importance of growth control in the pathology of major diseases and overall human health, and underscore the therapeutic potential of the mTOR pathway.

Nitric oxide and mitochondrial biogenesis

The characteristic structural organization of mitochondria is the product of synthesis of macromolecules within the mitochondria together with the import of proteins and lipids synthesized outside the organelle. Synthetic and import processes are required for mitochondrial proliferation and might also facilitate the growth of pre-existing mitochondria. Recent evidence indicates that these events are regulated in a complex way by several agonists and environmental conditions, through activation of specific signaling pathways and transcription factors. A newly discovered role of this organelle in retrograde intracellular signaling back to the nucleus has also emerged. This is likely to have far-reaching implications in development, aging, disease and environmental adaptation. Generation of nitric oxide (NO) appears to be an important player in these processes, possibly acting as a unifying molecular switch to trigger the whole mitochondrial biogenesis process. High levels of NO acutely inhibit cell respiration by binding to cytochrome c oxidase. Conversely, chronic, smaller increases in NO levels stimulate mitochondrial biogenesis in diverse cell types. NO-induced mitochondrial biogenesis seems to be linked to proliferation and differentiation of normal and tumor cells, as well as in aging.

Why life oscillates--from a topographical towards a functional chronobiology

Chronobiology has identified a multitude of rhythms within our body as well as within each living cell. Some of these rhythms, such as the circadian and circannual, interact with our environment, while others run on their own, but are often coupled to the circadian or to other body rhythms. Recent evidence shows that these rhythms might be more important for our health than expected: Disturbance of the circadian rhythms by jet lag or shift work not only evokes autonomic disturbances but also increases the incidence of cancer, as shown in this issue of Cancer Causes and Control. The occurrence of rhythms in the organism obviously bears several advantages: (1) It increases organismic stability by calibrating the system's characteristics: Regulation curves in time and space are crucial for controlling physiological long-term stability. To determine its properties continuously the system varies its parameters slightly over several time scales at different frequencies-akin to what our body does, e.g. in heart-rate variability. (2) Tuning and synchronization of rhythms saves energy: It was Huygens who observed that clocks on a wall tend to synchronize their beats. It turned out later that synchronisation is a very common phenomenon observed in bodies' rhythms and can be found, for example, when we relax or sleep. At such times energy consumption is minimal, our body working most efficiently. (3) Temporal compartmentalization allows polar events to occur in the same space unit: there are polarities in the universe of our body, which cannot happen simultaneously. Systole and diastole, inspiration and expiration, work and relaxation, wakefulness and sleep, reductive and oxidative states cannot be performed efficiently at the same time and place. Temporal compartmentalization is probably the most efficient way to mediate between these polarities. Chronobiology and chronomedicine are opening a new and very exciting understanding of our bodies' regulation. The biological time and its oscillations gain more attention and importance as these interrelations are understood.

No decline in skeletal muscle oxidative capacity with aging in long-term calorically restricted rats: effects are independent of mitochondrial DNA integrity

We investigated if calorie restriction (CR) preserved skeletal muscle oxidative capacity with aging after accounting for life span extension by CR, and determined if mitochondrial content, mitochondrial DNA integrity, and peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) were involved. Ad libitum-fed (AL) and CR animals representing young adult, late middle age, and senescence were studied. Whereas citrate synthase and complex IV activities were lower in plantaris and gastrocnemius muscle of young adult CR animals, in contrast to the 15%-40% decline in senescent AL animals, there was no decline with aging in CR animals. There was no decline in citrate synthase protein in gastrocnemius with aging in either group, suggesting that CR preserves oxidative capacity with aging by protecting mitochondrial function rather than content. This protection was independent of mitochondrial DNA damage between groups. However, there was a slower decline in PGC-1alpha gene expression with aging in CR versus AL animals, suggesting a better maintenance of mitochondrial biogenesis with aging in CR animals.

Why life oscillates--biological rhythms and health

A multitude of biological rhythms have been identified in the whole organism as well as within each living cell. Some of these rhythms reflect adaptations to our environment, while others run on their own. Recent evidence shows that these rhythms and their interaction might be more important not only for recreation but also for our health. Disturbance of the circadian rhythms by jet lag or shift work not only disturbs our metabolic balance but also increases the incidence of cancer. Rhythms in the organism obviously stabilize systemic functions: They increase organismic stability by calibrating the system's characteristics. Regulation curves in time and space are crucial for controlling physiological long-term stability. To be continuously aware of its properties an autopetic system may vary its parameters slightly over several time scales at different frequencies--akin to what our body does, e.g. in heart-rate variability.

Gene-transfer technology: a preventive neurotherapy to curb obesity, ameliorate metabolic syndrome and extend life expectancy

Leptin insufficiency at crucial target sites in the hypothalamic circuitries that integrate energy intake and expenditure underlies abnormal rates of fat accumulation. The payload of this "fat burden" is metabolic syndrome, a cluster of life-threatening metabolic afflictions, and a shorter lifespan. Currently available therapies employed to combat obesity have disadvantages such as poor compliance for lifestyle modification or transient effectiveness and undesirable side-effects of pharmacological interventions. Recent studies suggest that neurotherapy comprising a single central administration of recombinant adeno-associated virus vector encoding the leptin gene severely depletes fat and ameliorates the major symptoms of metabolic syndrome for extended periods in rodents. These persistent benefits avert the deleterious impact of the "fat burden" and extend life expectancy. Thus, the novel approach of central gene-transfer technology has distinct advantages over current therapies and has the potential to correct or slow the progression of inherited or acquired hypothalamic diseases.

Mice and flies and monkeys too: caloric restriction rejuvenates the aging immune system of non-human primates

Humanity has been obsessed with extending life span and reversing the aging process throughout recorded history and this quest most likely preceded the invention of the written word. The search for eternal youth has spurred holy wars and precipitated the discovery of the new world (the 'Fountain of youth'). It therefore comes as no surprise that an increasingly greater amount of research effort is dedicated to improve our understanding of the aging process and finding interventions to moderate its impact on health. Caloric restriction (CR) is the only intervention in biology that consistently extends maximal and median life span in a variety of short-lived species. Several theories to explain the mechanisms of action of CR have been put forth, including the possibility that CR acts by retarding immune senescence. The question remains, however, whether CR will have the same beneficial impact on human aging, and, if so, how long does CR need to last to produce beneficial effects. To address this question, several groups initiated long-term studies in Rhesus macaques (RM) in the 1980s. Here, we review published data describing the impact of CR on the aging immune system of mice and primates, and discuss our unpublished data that delineate similarities and differences in the effects of CR upon T cell aging and homeostasis between these two models.

Ageing and metabolism: drug discovery opportunities

There has recently been significant progress in our understanding of the mechanisms that regulate ageing, and it has been shown that changes in single genes can dramatically extend lifespan and increase resistance to many diseases. Furthermore, many of these genes belong to evolutionarily conserved pathways that also control energy metabolism. In this review, we describe the shared molecular machinery that regulates ageing and energy metabolism. Although drugs to slow ageing face severe regulatory hurdles, it is likely that an understanding of ageing pathways will help to identify novel drug targets to treat metabolic disorders and other age-related diseases.

The unfortunate influence of the weather on the rate of ageing: why human caloric restriction or its emulation may only extend life expectancy by 2-3 years

Much research interest, and recently even commercial interest, has been predicated on the assumption that reasonably closely-related species--humans and mice, for example--should, in principle, respond to ageing-retarding interventions with an increase in maximum lifespan roughly proportional to their control lifespan (that without the intervention). Here, it is argued that the best-studied life-extending manipulations of mice are examples of a category that is highly unlikely to follow this rule, and more likely to exhibit only a similar absolute increase in maximum lifespan from one species to the next, independent of the species' control lifespan. That category--reduction in dietary calories or in the organism's ability to metabolize or sense them--is widely recognized to extend lifespan as an evolutionary adaptation to transient starvation in the wild, a situation which alters the organism's optimal partitioning of resources between maintenance and reproduction. What has been generally overlooked is that the extent of the evolutionary pressure to maintain adaptability to a given duration of starvation varies with the frequency of that duration, something which is--certainly for terrestrial animals and less directly for others--determined principally by the weather. The pattern of starvation that the weather imposes is suggested here to be of a sort that will tend to cause all terrestrial animals, even those as far apart phylogenetically as nematodes and mice, to possess the ability to live a similar maximum absolute (rather than proportional) amount longer when food is short than when it is plentiful. This generalization is strikingly in line with available data, leading (given the increasing implausibility of further extending human mean but not maximum lifespan in the industrialized world) to the biomedically and commercially sobering conclusion that interventions which manipulate caloric intake or its sensing are unlikely ever to confer more than 2 or 3 years' increase in human mean or maximum lifespan at the most.

Aging, longevity, and diet: historical remarks on calorie intake reduction

BACKGROUND

The link between longevity and diet is of great interest to biological and gerontological research. The fact that relevant knowledge has generally been available for many centuries is often not remarked upon.

OBJECTIVE

This article examines three aspects of early modern Western medicine which thematize the following links between the elderly, longevity and caloric intake: (1) the question of a diet specifically tailored to old age as background to certain theories of aging; (2) the transfer of these dietetic concepts to younger patients in order to improve health and extend life, and (3) the promotion of dieting in order to avoid the consequences of plethora and to retard the aging process.

METHODS

A number of Latin texts from premodern medical and health literature will be examined and their contents will be analyzed for material relating to diet for the elderly and longevity in their historic contexts.

RESULTS

We will clearly indicate fundamental parallels as well as differences between historic and modern scientific thought. We will thereby show that although a modern understanding of hormones and molecular genetics was obviously lacking, basic knowledge of the influence of nutrition on old age was prevalent. In contrast, the early modern lay concept of longevity through calorie reduction was based on coincidental observation.

CONCLUSION

These premodern, but nonetheless rational ideas must be integrated into the socio-cultural setting and the question must be raised as to the link between contemporary research aims and social reality.

Long‐term caloric restriction abrogates the age‐related decline in skeletal muscle aerobic function

The purpose of this study was to determine the effect of long-term caloric restriction (CR) on the age-associated decline of skeletal muscle aerobic function. Skeletal muscle maximal aerobic performance (VO2max) was assessed in ad libitum (AL) and CR rats aged 8-10 months and 35 months using a pump-perfused hindlimb model to match oxygen delivery to muscle mass between groups. Whereas there was a 46% decline in muscle mass-specific VO2max between 8-10 mo (524+/-13 micromol x min(-1) x 100 g(-1); mean+/- SE) and 35 mo (281+/-54 micromol x min(-1) x 100 g(-1)) in AL rats, not only did CR rats begin at the same point in 8-10 mo old rats (490+/-42 micromol x min(-1) x 100 g(-1)), we found no decline in 35 mo old CR animals (484+/-49 micromol x min(-1) x 100 g(-1)). Interestingly, although most markers of oxidative capacity began at a lower point in young adult CR animals, CR rats exhibited a higher in situ activity of complex IV at VO2max. This activity allows the young adult CR animals to exhibit normal aerobic capacity despite the lower oxidative enzyme activities. In stark contrast to the 19-41% decline in activities of citrate synthase, complexes I-III, and complex IV in homogenates prepared from the plantaris muscle and mixed region of gastrocnemius muscle with aging in AL rats, no age-related decline was found in CR animals. Thus, our results showed that CR preserves aerobic function in aged skeletal muscles by facilitating a higher in situ function of complex IV and by preventing the age-related decline in mitochondrial oxidative capacity.

The diet restriction paradigm: a brief review of the effects of every-other-day feeding

It has been known since the early 1900s that restriction of dietary intake relative to the ad libitum (AL) level increases stress resistance, cancer resistance, and longevity in many species. Studies investigating these phenomena have used three paradigms for dietary restriction. In the first, the AL intake of a control group is measured, and an experimental group is fed less than that amount in a specified proportion, e.g., 40%. In the second, food is provided AL to both the control and experimental groups: however, the experimental group is subjected to periods of fasting. Recent studies using this paradigm provide food every other day (EOD). Both of these paradigms have been in use since the early 1900s. A third paradigm that combines them was developed in the early 1970s: one or more days of fasting separate the provision of a limited amount of food. It was assumed for many years that the physiological responses to these paradigms were due exclusively to a net decrease in energy intake. Recently, however, it was found that some species and strains of laboratory animals, when fed AL every other day, are capable of gorging so that their net weekly intake is not greatly decreased. Despite having only a small deficit in energy intake relative to control levels, however, these animals experience enhanced longevity and stress resistance is enhanced in comparison to AL controls as much in animals enduring daily restriction of diet. These observations warrant renewed interest in this paradigm and suggest that comparisons of the paradigms and their effects can be used to determine which factors are critical to the beneficial effects of caloric restriction.

Stochastic dietary restriction using a Markov-chain feeding protocol elicits complex, life history response in medflies

Lifespan in individually housed medflies (virgins of both sexes) and daily reproduction for females were studied following one of 12 dietary restriction (DR) treatments in which the availability of high-quality food (yeast-sugar mixture) for each fly was based on a Markov chain feeding scheme--a stochastic dietary regime which specifies that the future dietary state depends only on the present dietary state and not on the path by which the present state was achieved. The stochastic treatments consisted of a combination of one of four values of a 'discovery' parameter and one of three values of a 'persistence' parameter. The results supported the hypotheses that: (i) longevity is extended in most medfly cohorts subject to stochastic DR; and (ii) longevity is more affected by the patch discovery than the patch persistence parameter. One of the main conclusions of the study is that, in combination with the results of earlier dietary restriction studies on the medfly, the results reinforce the concept that the details of the dietary restriction protocols have a profound impact on the sign and magnitude of the longevity extension relative to ad libitum cohorts and that a deeper understanding of the effect of food restriction on longevity is not possible without an understanding of its effect on reproduction.

Ink4a/Arf expression is a biomarker of aging

The Ink4a/Arf locus encodes 2 tumor suppressor molecules, p16INK4a and Arf, which are principal mediators of cellular senescence. To study the links between senescence and aging in vivo, we examined Ink4a/Arf expression in rodent models of aging. We show that expression of p16INK4a and Arf markedly increases in almost all rodent tissues with advancing age, while there is little or no change in the expression of other related cell cycle inhibitors. The increase in expression is restricted to well-defined compartments within each organ studied and occurs in both epithelial and stromal cells of diverse lineages. The age-associated increase in expression of p16INK4a and Arf is attenuated in the kidney, ovary, and heart by caloric restriction, and this decrease correlates with diminished expression of an in vivo marker of senescence, as well as decreased pathology of those organs. Last, the age-related increase in Ink4a/Arf expression can be independently attributed to the expression of Ets-1, a known p16INK4a transcriptional activator, as well as unknown Ink4a/Arf coregulatory molecules. These data suggest that expression of the Ink4a/Arf tumor suppressor locus is a robust biomarker, and possible effector, of mammalian aging.

An accelerated assay for the identification of lifespan-extending interventions in Drosophila melanogaster

Recent advances in aging research have uncovered genes and genetic pathways that influence lifespan in such diverse organisms as yeast, nematodes, flies, and mice. The discovery of genes and drugs that affect lifespan has been delayed by the absence of a phenotype other than survivorship, which depends on the measurement of age at death of individuals in a population. The use of survivorship to identify genetic and pharmacological interventions that prolong life is time-consuming and requires a large number of homogeneous animals. Here, we report the development of an assay in Drosophila melanogaster using the expression of molecular biomarkers that accelerates the ability to evaluate potential lifespan-altering interventions. Coupling the expression of an age-dependent molecular biomarker to a lethal toxin reduces the time needed to perform lifespan studies by 80%. The assay recapitulates the effect of the three best known environmental life-span-extending interventions in the fly: ambient temperature, reproductive status, and calorie reduction. Single gene mutations known to extend lifespan in the fly such as Indy and rpd3 also extend lifespan in this assay. We used this assay as a screen to identify drugs that extend lifespan in flies. Lipoic acid and resveratrol were identified as being beneficial in our assay and shown to extend lifespan under normal laboratory conditions. We propose that this assay can be used to screen pharmacological as well as genetic interventions more rapidly for positive effects on lifespan.