The experimental manipulation of ageing by diet

Aging, longevity, and the role of environmental stressors: a focus on wildfire smoke and air quality

Aging is a complex biological process involving multiple interacting mechanisms and is being increasingly linked to environmental exposures such as wildfire smoke. In this review, we detail the hallmarks of aging, emphasizing the role of telomere attrition, cellular senescence, epigenetic alterations, proteostasis, genomic instability, and mitochondrial dysfunction, while also exploring integrative hallmarks - altered intercellular communication and stem cell exhaustion. Within each hallmark of aging, our review explores how environmental disasters like wildfires, and their resultant inhaled toxicants, interact with these aging mechanisms. The intersection between aging and environmental exposures, especially high-concentration insults from wildfires, remains under-studied. Preliminary evidence, from our group and others, suggests that inhaled wildfire smoke can accelerate markers of neurological aging and reduce learning capabilities. This is likely mediated by the augmentation of circulatory factors that compromise vascular and blood-brain barrier integrity, induce chronic neuroinflammation, and promote age-associated proteinopathy-related outcomes. Moreover, wildfire smoke may induce a reduced metabolic, senescent cellular phenotype. Future interventions could potentially leverage combined anti-inflammatory and NAD + boosting compounds to counter these effects. This review underscores the critical need to study the intricate interplay between environmental factors and the biological mechanisms of aging to pave the way for effective interventions.

High protein diet-induced metabolic changes are transcriptionally regulated via KLF15-dependent and independent pathways

High protein diet (HPD) is an affordable and positive approach in prevention and treatment of many diseases. It is believed that transcriptional regulation is responsible for adaptation after HPD feeding and Kruppel-like factor 15 (KLF15), a zinc finger transcription factor that has been proved to perform transcriptional regulation over amino acid, lipid and glucose metabolism, is known to be involved at least in part in this HPD response. To gain more insight into molecular mechanisms by which HPD controls expressions of genes involved in amino acid metabolism in the liver, we performed RNA-seq analysis of mice fed HPD for a short period (3 days). Compared to a low protein diet, HPD feeding significantly increased hepatic expressions of enzymes involved in the breakdown of all the 20 amino acids. Moreover, using KLF15 knockout mice and in vivo Ad-luc analytical system, we were able to identify Cth (cystathionine gamma-lyase) as a new target gene of KLF15 transcription as well as Ast (aspartate aminotransferase) as an example of KLF15-independent gene despite its remarkable responsiveness to HPD. These findings provide us with a clue to elucidate the entire transcriptional regulatory mechanisms of amino acid metabolic pathways.

Every-other day fasting prevents memory impairment induced by high fat-diet: Role of oxidative stress

Imbalance of diet consumption results in memory and learning deterioration. High-fat diet (HFD) causes neuronal damage and eventually cognitive impairment, which can be related to increasing oxidative stress in the brain. Using the every other day fasting (EODF) paradigm, as a method of dietary restriction is thought to provide protection of learning and memory in several experimental studies. In the current work, the preventive effect of EODF paradigm on memory impairment-induced by HFD was investigated. Adult male Wistar rats were fed with HFD using the EODF paradigm for six weeks. At the end of these six weeks, and while the previous treatment were continued, rats were examined for learning and memory (both the short-term and the long-term memory) using the radial arm water maze (RAWM). Oxidative stress in the brain, namely in the hippocampus was also assessed. Chronic administration of HFD induced impairment in both, short- and long- term memory that was prevented using EODF paradigm. Furthermore, EODF prevented HFD-induced decrease in the activities of the antioxidant enzymes, SOD and catalase along with reduction of glutathione (GSH) level and the ratio of reduced glutathione/oxidized glutathione (GSH/GSSG ratio). The EODF also inhibited rise in oxidized glutathione (GSSG) and thiobarbituric acid reactive substances (TBARS) seen with HFD. In conclusion, EODF ameliorated oxidative stress and memory impairment induced by chronic HFD. This probably, can be explained by the ability of EODF to normalize mechanisms involved in oxidative stress in the hippocampus.

Context-Dependent Roles for SIRT2 and SIRT3 in Tumor Development Upon Calorie Restriction or High Fat Diet

Calorie restriction (CR) is considered one of the most robust ways to extend life span and reduce the risk of age-related diseases, including cancer, as shown in many different organisms, whereas opposite effects have been associated with high fat diets (HFDs). Despite the proven contribution of sirtuins in mediating the effects of CR in longevity, the involvement of these nutrient sensors, specifically, in the diet-induced effects on tumorigenesis has yet to be elucidated. Previous studies focusing on SIRT1, do not support a critical role for this sirtuin family member in CR-mediated cancer prevention. However, the contribution of other family members which exhibit strong deacetylase activity is unexplored. To fill this gap, we aimed at investigating the role of SIRT2 and SIRT3 in mediating the anti and pro-tumorigenic effect of CR and HFD, respectively. Our results provide strong evidence supporting distinct, context-dependent roles played by these two family members. SIRT2 is indispensable for the protective effect of CR against tumorigenesis. On the contrary, SIRT3 exhibited oncogenic properties in the context of HFD-induced tumorigenesis, suggesting that SIRT3 inhibition may mitigate the cancer-promoting effects of HFD. Given the different functions regulated by SIRT2 and SIRT3, unraveling downstream targets/pathways involved may provide opportunities to develop new strategies for cancer prevention.

Studies on the regulatory mechanism of isocitrate dehydrogenase 2 using acetylation mimics

Mitochondrial isocitrate dehydrogenase 2 (IDH2) converts NADP+ to NADPH and promotes regeneration of reduced glutathione (GSH) by supplying NADPH to glutathione reductase or thioredoxin reductase. We have previously shown that under calorie restriction, mitochondrial deacetylase Sirt3 deacetylates and activates IDH2, thereby regulating the mitochondrial glutathione antioxidant defense system in mice. To investigate the regulatory mechanism of mIDH2 (mouse mitochondrial IDH2), we used lysine-to-glutamine (KQ) mutants to mimic acetylated lysines and screened 15 KQ mutants. Among these mutants, the activities of the K256Q and K413Q proteins were less than 50% of the wild-type value. We then solved the crystal structures of the wild-type mIDH2 and the K256Q mutant proteins, revealing conformational changes in the substrate-binding pocket. Structural data suggested that positively charged Lys256 was important in stabilizing the pocket because it repelled a lysine cluster on the other side. Glutamine (or acetylated lysine) was neutral and thus caused the pocket size to decrease, which might be the main reason for the lower activity of the K256Q mutant. Together, our data provide the first structure of an acetylation mimic of mIDH2 and new insights into the regulatory mechanism of acetylation of mIDH2.

Modeling evolutionary games in populations with demographic structure

Classic life history models are often based on optimization algorithms, focusing on the adaptation of survival and reproduction to the environment, while neglecting frequency dependent interactions in the population. Evolutionary game theory, on the other hand, studies frequency dependent strategy interactions, but usually omits life history and the demographic structure of the population. Here we show how an integration of both aspects can substantially alter the underlying evolutionary dynamics. We study the replicator dynamics of strategy interactions in life stage structured populations. Individuals have two basic strategic behaviours, interacting in pairwise games. A player may condition behaviour on the life stage of its own, or that of the opponent, or the matching of life stages between both players. A strategy is thus defined as the set of rules that determines a player׳s life stage dependent behaviours. We show that the diversity of life stage structures and life stage dependent strategies can promote each other, and the stable frequency of basic strategic behaviours can deviate from game equilibrium in populations with life stage structures.

Caloric restriction and the aging process: a critique

The main objective of this review is to provide an appraisal of the current status of the relationship between energy intake and the life span of animals. The concept that a reduction in food intake, or caloric restriction (CR), retards the aging process, delays the age-associated decline in physiological fitness, and extends the life span of organisms of diverse phylogenetic groups is one of the leading paradigms in gerontology. However, emerging evidence disputes some of the primary tenets of this conception. One disparity is that the CR-related increase in longevity is not universal and may not even be shared among different strains of the same species. A further misgiving is that the control animals, fed ad libitum (AL), become overweight and prone to early onset of diseases and death, and thus may not be the ideal control animals for studies concerned with comparisons of longevity. Reexamination of body weight and longevity data from a study involving over 60,000 mice and rats, conducted by a National Institute on Aging-sponsored project, suggests that CR-related increase in life span of specific genotypes is directly related to the gain in body weight under the AL feeding regimen. Additionally, CR in mammals and "dietary restriction" in organisms such as Drosophila are dissimilar phenomena, albeit they are often presented to be the very same. The latter involves a reduction in yeast rather than caloric intake, which is inconsistent with the notion of a common, conserved mechanism of CR action in different species. Although specific mechanisms by which CR affects longevity are not well understood, existing evidence supports the view that CR increases the life span of those particular genotypes that develop energy imbalance owing to AL feeding. In such groups, CR lowers body temperature, rate of metabolism, and oxidant production and retards the age-related pro-oxidizing shift in the redox state.

Calorie restriction does not increase short-term or long-term protein synthesis

Increased protein synthesis is proposed as a mechanism of life-span extension during caloric restriction (CR). We hypothesized that CR does not increase protein synthesis in all tissues and protein fractions and that any increased protein synthesis with CR would be due to an increased anabolic effect of feeding. We used short- (4 hours) and long-term (6 weeks) methods to measure in vivo protein synthesis in lifelong ad libitum (AL) and CR mice. We did not detect an acute effect of feeding on protein synthesis while liver mitochondrial protein synthesis was lower in CR mice versus AL mice. Mammalian target of rapamycin (mTOR) signaling was repressed in liver and heart from CR mice indicative of energetic stress and suppression of growth. Our main findings were that CR did not increase rates of mixed protein synthesis over the long term or in response to acute feeding, and protein synthesis was maintained despite decreased mTOR signaling.

A general life history theory for effects of caloric restriction on health maintenance

Background

Caloric restriction (CR) has been shown to keep organisms in a relatively youthful and healthy state compared to ad libitum fed counterparts, as well as to extend the lifespan of a diverse set of organisms. Several attempts have been made to understand the underlying mechanisms from the viewpoint of energy tradeoffs in organisms' life histories. However, most models are based on assumptions which are difficult to justify, or are endowed with free-adjusting parameters whose biological relevancy is unclear.

Results

In this paper, we derive a general quantitative, predictive model based on physiological data for endotherms. We test the hypothesis that an animal's state of health is correlated with biological mechanisms responsible for the maintenance of that animal's functional integrities. Such mechanisms require energy. By suppressing animals' caloric energy supply and biomass synthesis, CR alters animals' energy allocation strategies and channels additional energy to those maintenance mechanisms, therefore enhancing their performance. Our model corroborates the observation that CR's effects on health maintenance are positively correlated with the degree and duration of CR. Furthermore, our model shows that CR's effects on health maintenance are negatively correlated to the temperature drop observed in endothermic animals, and is positively correlated to animals' body masses. These predictions can be tested by further experimental research.

Conclusion

Our model reveals how animals will alter their energy budget when food availability is low, and offers better understanding of the tradeoffs between growth and somatic maintenance; therefore shedding new light on aging research from an energetic viewpoint.

Caloric restriction-induced life extension of rats and mice: a critique of proposed mechanisms

In 1935, Clive McCay and colleagues reported that decreasing the food intake of rats extends their life. This finding has been confirmed many times using rat and mouse models. The responsible dietary factor in rats is the reduced intake of energy; thus, this phenomenon is frequently referred to as caloric restriction. Although many hypotheses have been proposed during the past 74 years regarding the underlying mechanism, it is still not known. It is proposed that this lack of progress relates to the fact that most of these hypotheses have been based on a single underlying mechanism and that this is too narrow a focus. Rather, a broad framework is needed. Hormesis has been suggested as providing such a framework. Although it is likely that hormesis is involved in the actions of caloric restriction, it also is probably too narrowly focused. Based on currently available data, a provisional broad framework is presented depicting the complex of mechanisms that likely underlie the life-extending and other anti-aging actions of caloric restriction.

Chronic calorie restriction increases susceptibility of laboratory mice (Mus musculus) to a primary intestinal parasite infection

Long-term calorie restriction (CR) has numerous benefits; however, effects of CR on susceptibility to intact pathogens are not well understood. Because CR enhances immune function of laboratory mice (Mus musculus), it was hypothesized that mice subjected to CR would be less susceptible to experimental infections of the intestinal parasite Heligmosomoides bakeri. Furthermore, because H. bakeri must combat a greater host immune response by CR mice compared to fully fed mice, it also was also hypothesized that (i) worms living in CR hosts would have lower reproduction than worms from ad libitum-fed mice, and (ii) CR mice would have a more female-biased sex ratio as male worms may be more vulnerable to host immune response than female worms. Mice were subjected to CR for 6.7 months and were then infected with H. bakeri for one additional month. As expected, CR mice had equal or enhanced immune response (eosinophils and immunoglobin G1 production) to H. bakeri infection compared to ad libitum-fed mice, and CR mice harbored a more female-biased sex ratio than ad libitum-fed mice. Contrary to predictions, CR mice had more worms than ad libitum-fed mice and the worms from CR mice produced more eggs than worms from ad libitum-fed mice. These data indicate that, despite the evidence that long-term CR enhances traditional measures of immune function, CR may actually increase susceptibility to intact parasite infection. Furthermore, changes in worm reproduction and differential survival of male vs. female worms may influence host-parasite transmission dynamics during long-term host CR.

Optimization of dietary restriction protocols in Drosophila

Dietary restriction (DR) extends life span in many organisms, through unknown mechanisms that may or may not be evolutionarily conserved. Because different laboratories use different diets and techniques for implementing DR, the outcomes may not be strictly comparable. This complicates intra- and interspecific comparisons of the mechanisms of DR and is therefore central to the use of model organisms to research this topic. Drosophila melanogaster is an important model for the study of DR, but the nutritional content of its diet is typically poorly defined. We have compared fly diets composed of different yeasts for their effect on life span and fecundity. We found that only one diet was appropriate for DR experiments, indicating that much of the published work on fly "DR" may have included adverse effects of food composition. We propose procedures to ensure that diets are suitable for the study of DR in Drosophila.

Adult-limited dietary restriction slows gompertzian aging in Caenorhabditis elegans

Dietary restriction (DR) delays the onset of age-related deterioration and extends the life span in a variety of model organisms. In many species, age changes in mortality obey the Gompertz equation, which describes an exponential increase with age in age-specific mortality rate. Recently, this model has been used in fruitflies and rodents to investigate the mechanism by which DR reduces adult mortality. We report that food restriction imposed by axenic culture reduces the exponential increase of age-specific mortality of Caenorhabditis elegans. Furthermore, the life span appears largely independent of nutritional status during development, as shown by shifting worms to different food concentrations shortly before adulthood. When DR was exerted after reproduction, a smaller reduction in Gompertzian aging was seen. Thus, the demographic changes exerted by DR in C. elegans resemble those seen in rats, yet are different to those seen in Drosophila and mice.

Role of hormesis in life extension by caloric restriction

Caloric restriction (CR) markedly extends the life of rats, mice and several other species, and it also modulates age-associated physiological deterioration and delays the occurrence and/or slows progression of age-associated diseases. The level of CR that retards the aging processes is a low-intensity stressor, which enhances the ability of rats and mice of all ages to cope with intense stressors. CR thus exhibits a hormetic action in these species, and therefore it is hypothesized that hormesis plays a role in the life-extending and anti-aging actions of CR. Both the findings in support of this hypothesis and those opposing it are critically considered. However, it is likely that hormesis is not the only process contributing to CR-induced life extension. It is proposed that two general processes are involved in CR-induced life extension. One is the reduced endogenous generation of damaging agents, such as reactive oxygen species. The second is hormesis, which enhances processes that protect against the action of damaging agents and also promotes processes that repair the damage once it occurs.

Food, fertility and longevity

Some animals live in environments in which the food supply fluctuates. When it is scarce these animals do not breed, but invest resources into survival until food is again available, and they can reproduce. Under these circumstances the lifespan can be increased, just as it is after calorie restriction. Other animals have a fairly constant food supply, and it is predicted that these would not have an extended life span if subjected to calorie restriction. Hibernation is a natural form of calorie restriction, and in some cases may lengthen lifespan.

Caloric restriction and aging: controversial issues

It has long been held that food restriction extends the life span of rodents and other species by decreasing caloric intake and slowing the rate of aging. Recent findings challenge these concepts. This review assesses these controversial issues. The conclusion is that caloric restriction underlies the life extension of rats, but not of Drosophila. Mortality characteristics show that food restriction slows the rate of aging of rats and, in some studies, of mice. However, in other mouse studies and in Drosophila, mortality characteristics have been interpreted as indicating that it delays the start but does not slow the rate of aging; the author believes that this interpretation is faulty. These differences in mortality responses to food restriction provide a potentially powerful tool for uncovering basic mechanisms underlying its life-prolonging action. A hypothesis is presented for use in the search for these mechanisms.

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

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

Food restriction, evolution and ageing

The food restriction model for life extension is nearing "three-score and 10" years of age and remains in good shape, preserving much of the mystique of its youth. Although originally described for laboratory rodents, recent work shows that food restriction also appears to slow ageing processes in a range of other animal species, raising the question of whether this response represents some generalised evolutionary adaptation, perhaps a strategy to cope with periods of famine. If the food restriction response does have an adaptive basis, this would suggest that specific gene regulatory processes have evolved to shape the organism's physiological response to food restriction. It will then be important to investigate how these are organised and whether the same or different factors are at play in the various species in which food restriction extends life span.

Overview of caloric restriction and ageing

It has been known for some 70 years that restricting the food intake of laboratory rats extends their mean and maximum life span. In addition, such life extension has been observed over the years in many other species, including mice, hamsters, dogs, fish, invertebrate animals, and yeast. Since this life-extending action appears to be due to a restricted intake of energy, this dietary manipulation is referred to as caloric restriction (CR). CR extends life by slowing and/or delaying the ageing processes. The underlying biological mechanism responsible for the life extension is still not known, although many hypotheses have been proposed. The Growth Retardation Hypothesis, the first proposed, has been tested and found wanting. Although there is strong evidence against the Reduction of Body Fat Hypothesis, efforts have recently been made to resurrect it. While the Reduction of Metabolic Rate Hypothesis is not supported by experimental findings, it nevertheless still has advocates. Currently, the most popular concept is the Oxidative Damage Attenuation Hypothesis; the results of several studies provide support for this hypothesis, while those of other studies do not. The Altered Glucose-Insulin System Hypothesis and the Alteration of the Growth Hormone-IGF-1 Axis Hypothesis have been gaining favor, and data have emerged that link these two hypotheses as one. Thus, it may now be more appropriate to refer to them as the Attenuation of Insulin-Like Signaling Hypothesis. Finally, the Hormesis Hypothesis may provide an overarching concept that embraces several of the other hypotheses as merely specific examples of hormetic processes. For example, the Oxidative Damage Attenuation Hypothesis probably addresses only one of likely many damaging processes that underlie aging. It is proposed that low-intensity stressors, such as CR, activate ancient hormetic defense mechanisms in organisms ranging from yeast to mammals, defending them against a variety of adversities and, when long-term, retarding senescent processes.

Cytochrome P450 expression in rat gastric epithelium with intestinal metaplasia induced by high dietary NaCl levels

Drug metabolizing enzymes like cytochrome P450 (CYP) play an important role in determining the susceptibility of organs or tissue to the toxic effects of drugs or other xenobiotics. There is some evidence indicating that individual isoforms of CYPs are over-expressed in different types of malignant tumors including that of oesophagus, pancreas, breast, lung, colon and stomach. Nevertheless, it is not clear if this change in expression is previous or after the appearance of malignancy. This is important in order to clarify the possible role of xenobiotics in the development of gastric cancer. On the other hand, it has been reported that a high salt ingestion leads to histological changes in rat stomach mucosa including enhanced cell proliferation, lipid peroxidation and intestinal metaplasia. The aim of this study is to explore the expression and activity of CYP families involved in the metabolism of carcinogens in normal rat stomach mucosa and intestinal metaplasia induced by high NaCl ingestion. Male Wistar rats were exposed to diets containing different NaCl concentrations (0.6% control group, 6%, 12%, 18% and 24%) for 12 weeks and histological changes as well as CYP modulation were monitored in gastric mucosa. Chronic gastritis, regenerative hyperplasia and focal metaplasia were noted in animals receiving the 12%, 18% and 24% NaCl diets. In the same groups, induction of CYP1A1 and CYP3A2 was produced, mainly in areas of metaplasia. The expression of xenobiotic metabolizing enzymes in the gastric mucosa might contribute to chemical activation in the stomach, metabolizing both exogenous and endogenous compounds implicated in the development of gastric cancer.

Counting the Calories: The Role of Specific Nutrients in Extension of Life Span by Food Restriction

Reduction of food intake without malnourishment extends life span in many different organisms. The majority of work in this field has been performed in rodents where it has been shown that both restricting access to the entire diet and restricting individual dietary components can cause life-span extension. Thus, for insights into the mode of action of this intervention, it is of great interest to investigate the aspects of diet that are critical for life span extension. Further studies on the mechanisms of how food components modify life span are well suited to the model organism Drosophila melanogaster because of its short life span and ease of handling and containment. Therefore, we summarize practical aspects of implementing dietary restriction in this organism, as well as highlight the major advances already made. Delineation of the nutritional components that are critical for life-span extension will help to reveal the mechanisms by which it operates.

Melatonin is an efficient antioxidant

We have compared the peroxyl radical scavenger ability of melatonin with that of vitamin E, ascorbic acid (As.A.), reduced glutathione (GSH) and mannitol. All the antioxidants, except mannitol, prevented the lysis of human erythrocytes exposed to an azo-initiator of peroxyl radicals (2,2'-azo-bis(2-amidinopropane)dihydrochloride) at 37 degrees C. The percentage of this inhibition of erythrocyte lysis varied with the concentration of antioxidants, but the efficiency was melatonin > vitamin E > As.A. > GSH. Based on the assumption that each molecule of vitamin E scavenges two peroxyl radicals, the scavenging capacity of melatonin was four peroxyl radicals/molecule.

Dietary restriction, mortality trajectories, risk and damage

Restriction of food intake extends lifespan in evolutionarily diverse organisms, including mammals. Dietary restriction (DR) also delays the appearance of ageing-related damage and pathology and keeps organisms in a youthful state for longer. DR has hence been suggested to lower the rate of ageing. Analysis of mortality rates can be used to test this idea. During ageing, mortality rates in general increase, approximately exponentially. Lifespan can be extended either by a reduction in the rate of increase in mortality rate with age or a lowering of the initial rate of mortality. A reduction in the slope of a mortality trajectory has generally been taken to indicate that the rate of ageing has been lowered. Data on the effects of temperature on mortality in Drosophila are in accordance with this idea. Lowered temperature extends lifespan solely by lowering the slope of the mortality trajectory and flies with a hotter thermal history have permanently elevated death rates. In contrast, lowering of the initial rate of mortality has been taken to leave the rate of ageing unaffected. In Drosophila and in mice, but not in rats, DR extends lifespan by lowering the initial mortality rate. In Drosophila, the effect of DR is acute, and mortality rate switches rapidly between DR and control values with the corresponding changes in nutritional regime. DR in Drosophila therefore has no impact upon the rate of ageing. Possible mechanisms by which DR can both delay damage and pathology and yet act acutely to determine mortality rates are discussed. In rodents, some phenotypes associated with DR, including microarray profiles, show rapid switching with changed nutritional regime, pointing to potentially acute effects of DR in mammals.

The effect of aging and caloric restriction on mitochondrial protein density and oxygen consumption

It has been proposed that part of the anti-aging mechanism of caloric restriction (CR) involves changes in mitochondrial function. To investigate this hypothesis, mitochondria from various tissues of male Brown Norway rats (fully fed and CR) were isolated and respiration rates determined. In mitochondria from liver, heart, brain and kidney, there were no significant effects of CR on state 4 mitochondrial respiration rate. Further experiments using liver mitochondria under a variety of incubation conditions confirmed that CR does not alter mitochondrial respiration rate in this tissue. However, the respiration rate of mitochondria from brown adipose tissue (BAT) of CR animals was approximately three-fold higher compared to mitochondria from fully fed controls. Mitochondrial protein density was significantly higher in liver tissue of CR animals; it was significantly lower in heart and unchanged in BAT. It is concluded that whilst CR results in tissue-specific changes in mitochondrial respiration rate, these effects do not explain the CR-induced changes in free radical production reported previously for these organelles.

Mechanisms of ageing: public or private?

Ageing--the decline in survival and fecundity with advancing age is caused by damage to macromolecules and tissues. Ageing is not a programmed process, in the sense that no genes are known to have evolved specifically to cause damage and ageing. Mechanisms of ageing might therefore not be expected to be as highly conserved between distantly related organisms as are mechanisms of development and metabolism. However, evidence is mounting that modulators of the rate of ageing are conserved over large evolutionary distances. As we discuss in this review, this conservation might stem from mechanisms that match reproductive rate to nutrient supply.

Dietary restriction alters cell proliferation in rats: an immunohistochemical study by labeling proliferating cell nuclear antigen

Dietary restriction (DR) delays the onset of aging and lowers the incidence of both spontaneous and chemically induced cancers. The inhibition of cell proliferation has been suggested as a possible mechanism for this effect. We examined the effect of DR on cell proliferation in duodenum, forestomach, glandular stomach, and liver tissues of male Fischer 344 rats receiving 60% of the control feed intake for 24 months starting at 16 weeks of age. Rats were sacrificed, when 28 months old. Tissues were collected, histologically prepared, and stained immunohistochemically for proliferating cell nuclear antigen (PCNA). The PCNA-stained nuclei are detected in different phases of the cell cycle. A minimum sample of 2000 cells was counted in liver. The percentage of labeled S-phase cells per total cells counted was used as the labeling index for liver. The number of labeled S-phase epithelial cells per 1.1 mm of basement membrane or muscularis mucosa was used as the labeling index for duodenum, forestomach, and glandular stomach. Cell proliferation in glandular stomach and liver tissues was inhibited in rats DR for 24 months; however, cell proliferation in duodenum and forestomach mucosal tissues was unexpectedly enhanced by DR. These results indicated that while DR inhibits cell proliferation in tissues of rats, it is tissue-dependent. The decreased rate of cell division by DR in the designated tissues could be implicated in lowering the conversion of endogenous DNA damage or lesions to mutation and cancer.

Protein turnover plays a key role in aging

Although the molecular mechanism of aging is unknown, a progressive increase with age in the concentration of damaged macromolecules, especially proteins, is likely to play a central role in senescent decline. In this paper, we discuss evidence that the progressive decrease in protein synthesis and turnover can be the primary cause of the increase in the concentration of damaged proteins with age. Conversely, protein damage itself is likely to be the cause of the decrease in protein turnover. This could establish a positive feedback loop where the increase in protein damage decreases the protein turnover rate, leading to a further increase in the concentration of damaged proteins. The establishment of such a feedback loop should result in an exponential increase in the amount of protein damage-a protein damage catastrophe-that could be the basis of the general deterioration observed in senescent organisms.

Caloric restriction and lifespan: a role for protein turnover?

Oxidative damage to cellular macromolecules has been postulated to be a major contributor to the ageing of diverse organisms. Oxidative damage can be limited by maintaining high anti-oxidant defenses and by clearing/repairing damage efficiently. Protein turnover is one of the main routes by which functional proteins are maintained and damaged proteins are removed. Protein turnover rates decline with age, which might contribute to the accumulation of damaged proteins in ageing cells. Interestingly, protein turnover rates are maintained at high levels in caloric restricted animals. Whether changes in protein turnover are a cause or a consequence of ageing is not clear, and this question has not been a focal point of modern ageing research. Here we survey work on protein turnover and ageing and suggest that powerful genetic models such as the nematode Caenorhabditis elegans are well suited for a thorough investigation of this long-standing question.

Complex adaptive systems, aging and longevity

Mortality and reproduction are intimately entwined in the study of aging and longevity. I apply the modern theory of complex adaptive systems (nonlinear, stochastic, dynamic methods) to questions of aging and longevity. I begin by highlighting major questions that must be answered in order to obtain a deeper understanding of aging. These are: (i) What should (in an evolutionary sense) mortality trajectories look like? (ii) Why does caloric restriction slow aging? (iii) Why does reproduction cause delayed mortality? (iv) Why does compensatory growth cause delayed mortality? I show how dynamic state variable models based on stochastic dynamic programming (Clark & Mangel, 2000) can be used to embed genetic theories of senescence (either mutation accumulation or antagonistic pleiotropy) in the somatic environment, as George Williams called for in 1957, and how they make the disposable soma theory of aging operational. Such models will allow unification of genetic and phenotypic theories of aging.

Ageing: definitions, mechanisms and the magnitude of the problem

All multi-cellular organisms undergo change with time. Conception heralds the onset of growth and development, leading to reproductive competence and propagation of the species. With time, organisms age, leading to death as a final end-point. Whilst our knowledge and definitions of growth and reproduction are firmly established, the concept of ageing remains less well understood. One of the reasons for the lack of a singular definition of ageing is that it can be considered in many different ways, according to social, behavioural, physiological, morphological, cellular and molecular changes. Research has led to a number of theories being proposed that may explain the ageing process. In this chapter, we will review some of these theories and address some of the following fundamental questions: What is ageing? How can ageing be measured? When does ageing begin? When is an organism defined as old?

Nutrition, oxidative damage, telomere shortening, and cellular senescence: individual or connected agents of aging?

There is substantial and long-standing literature linking the level of general nutrition to longevity. Reducing nutrition below the amount needed to sustain maximum growth increases longevity in a wide range of organisms. Oxidative damage has been shown to be a major feature of the aging process. Telomere shortening is now well established as a key process regulating cell senescence in vitro. There is some evidence that the same process may be important for aging in vivo. Very recently it has been found that oxidative damage accelerates telomere shortening. It is therefore possible for us to propose as an outline hypothesis that the level of nutrition determines oxidative damage which in turn determines telomere shortening and cell senescence and that this pathway is important in determining aging and longevity in vivo. We also propose that telomeres in addition to their well-recognized role in "counting" cell divisions are also, through their GGG sequence, important monitors of oxidative damage over the life span of a cell. This may explain the evolutionary conservations of this triplet in the repeat telomere sequence unit.

Calorie restriction and aging: a life-history analysis

The disposable soma theory suggests that aging occurs because natural selection favors a strategy in which fewer resources are invested in somatic maintenance than are necessary for indefinite survival. However, laboratory rodents on calorie-restricted diets have extended life spans and retarded aging. One hypothesis is that this is an adaptive response involving a shift of resources during short periods of famine away from reproduction and toward increased somatic maintenance. The potential benefit is that the animal gains an increased chance of survival with a reduced intrinsic rate of senescence, thereby permitting reproductive value to be preserved for when the famine is over. We describe a mathematical life-history model of dynamic resource allocation that tests this idea. Senescence is modeled as a change in state that depends on the resources allocated to maintenance. Individuals are assumed to allocate the available resources to maximize the total number of descendants. The model shows that the evolutionary hypothesis is plausible and identifies two factors, both likely to exist, that favor this conclusion. These factors are that survival of juveniles is reduced during periods of famine and that the organism needs to pay an energetic "overhead" before any litter of offspring can be produced. If neither of these conditions holds, there is no evolutionary advantage to be gained from switching extra resources to maintenance. The model provides a basis to evaluate whether the life-extending effects of calorie-restriction might apply in other species, including humans.

Calorie restriction and age-related oxidative stress

Calorie restriction (CR) in mammals has been recognized as the best characterized and most reproducible strategy for extending maximum survival, retarding physiological aging, and delaying the onset of age-related pathologic conditions in mammals. The overwhelming majority of studies using CR have used short-lived rodent species, although current work using rhesus and squirrel monkeys will determine whether this paradigm is also relevant to manipulating the rate of primate aging. The mechanism by which restricted calorie intake modifies the rate of aging and pathology has been the subject of much controversy, although an attenuation in the lifetime accumulation of oxidative damage appears to be a central feature. Although the majority of studies have focused on the ability of cells from calorie-restricted animals to scavenge free radicals to explain the slower accrual of oxidative damage with age, it is not established that CR has a consistent effect to upregulate the activity of these enzymes in all tissues. A major effect of calorie-restricted feeding now appears to be on the rate of production or leak of free radicals from the mitochondria. The details of the adaptation and the signaling pathway that induces this effect are currently unknown.

Differential display analysis can reveal patterns of gene expression in immortalised hepatoma cells which are similar to those observed in young adult but not old adult liver cells

We used the differential display technique to examine whether there were any patterns of gene expression which were characteristic of both young adult rat liver and of immortalised rat hepatoma cell lines, but not of old adult rat liver. No genes were detected which appeared to be clearly expressed in young liver and immortalised cell lines, but not in old liver. However, 14 genes were detected in old liver which were down-regulated in young liver and the hepatoma cell lines. This observation lends support to the idea that immortalisation of malignant cells may involve, at least in some aspects, a reversal of the ageing process in these cells and that the genes involved have a recessive action.

Effect of dietary restriction on hepatic and renal phosphoenolpyruvate carboxykinase induction in young and old Fischer 344 rats

Food restriction is known to ameliorate many of the adverse physiologic effects of age. In this study, we have examined the effect of food restriction on the induction of the gluconeogenic enzyme, phosphoenolpyruvate carboxykinase (PEPCK) in liver and kidney following a 12-h fasting period in young (6 month) and old (24 month) ad libitum-fed and food-restricted male Fischer 344 rats. In the liver, following the 12-h fast, the activity of PEPCK increased approximately 2-fold in the young ad libitum fed rats and 3-fold in the young restricted animals. However, PEPCK activity remained unchanged in response to the 12-h fast in the 24 month old ad libitum fed rats. In the old restricted rats, the induction of PEPCK mimicked that of the young rats (PEPCK activity increased 2-fold within the 12-h fasting period). Therefore, dietary restriction not only enhanced the induction response in the liver in young rats, but also restored the induction of hepatic PEPCK in the old animals. In the kidney, there was no effect of age or dietary restriction on the induction of PEPCK as the activity of renal PEPCK did not change in response to the 12-h fast in any of the four groups of rats.

Influence of caloric intake on aging and on the response to stressors

Reducing the food intake of rodents to well below that of ad libitum fed animals increases the life span. This action, which gerontologists often refer to as the antiaging action of dietary restriction (DR), is due to the slowing of the aging processes. DR also maintains most physiological processes in a youthful state and delays the occurrence and/or slows the progression of age-associated disease processes. This antiaging action of DR results from the reduced intake of calories. Reduction of the body fat content does not play a causal role in the antiaging action of DR, nor does reduction in the metabolic rate. Alterations in the characteristics of carbohydrate metabolism and of oxidative metabolism in response to DR have been found that are of such a nature that they could, at least in part, underlie the antiaging action. Several theories have recently been proposed in regard to the mechanisms responsible for the antiaging action of DR, but none has been tested by rigorously designed studies. Of these theories, the one that seems most promising is based on the fact that DR protects rats and mice of all ages against the damaging actions of acute stressors. This protective action against stressors may play a major role in the antiaging action of DR.

Differential display analysis of gene expression indicates that age-related changes are restricted to a small cohort of genes

It is clear that there is a genetic component associated with the ageing process. Although evolutionary theory has suggested that the activity of certain genes may facilitate ageing by favouring resource utilisation by the germ cells at the expense of somatic cells, there is reason to believe that the senescent phenotype, which is the endpoint of the ageing process, may be due to alterations in the levels of expression of other genes. To investigate this situation we have used the differential display technique to survey gene expression during ageing of the rat brain, heart and liver. By optimising this technique it is possible to identify up to 10000-14000 PCR products, which represent genes expressed in the tissue under study. Interestingly, only a relatively small cohort (approximately 2%) of these genes appear to show significant changes in their levels of expression during ageing. Characterisation of the latter has so far revealed certain genes, such as glial fibrillary acidic protein, which are associated with the senescent phenotype. It has also revealed that the level of fos, a component of the AP-1 transcription factor, decreases with age, which has implications for AP-1 regulated genes. The differential display technique has also revealed an increase in mitochondrial RNA during ageing of the heart, which may be due to a gene dosage effect caused by the presence of increased numbers of mitochondrial genomes in myocytes in old age. The differential display technique therefore appears to offer a powerful tool for identifying genes which contribute to the emergence of a senescent phenotype.

Protein oxidation and enzyme activity decline in old brown Norway rats are reduced by dietary restriction

The effect of aging and diet restriction (DR) on the activity of creatine kinase (CK), glutamine synthetase (GS) and protein carbonyl formation in the cerebellum, hippocampus and cortex of male and female brown Norway (BN) rats has been investigated. It was demonstrated that CK activity in three different regions of the rat brain declines with age by 30%. Age-related decrease of GS activity was only 10-13% and did not reach statistical significance. Consistent with previously published studies, age-related increase of protein carbonyl content in each brain area studied has been observed. Preventive effects of a caloric restricted diet on the age-associated protein oxidation and changes of the activity of CK and GS in the brain was observed for both aging male and female BN rats. DR delayed the accumulation of protein carbonyls. Age-related changes of CK activity in rat brain were abrogated by DR. The activity of GS in the brain of old rats subjected to the caloric restricted diet was higher than that in the brain of young animals fed ad libitum. The results are consistent with the notion that DR may relieve age-associated level of oxidative stress and lessen protein damage.

Age-related changes in gene expression in the rat brain revealed by differential display

We have used the polymerase chain reaction (PCR)-based technique of differential display to analyse changes in gene expression during ageing of the rat brain. In this approach we have compared three young adult (6 months) with three old adult (20 months) animals. RNA preparations from the homogenised brains were subjected to reverse transcriptase (RT)-PCR using 36 different combinations of primer pairs. Any PCR product which was consistently found to be more prominent in the three young brains compared to the three old brains, and vice versa, was scored as potentially representing a gene which was differentially expressed during the ageing of this tissue. Out of a possible 2000+ PCR products we identified 44 that might represent genes that exhibit differential expression during ageing of the rat brain. An initial screen of these fragments, by Southern-blotting the PCR products and hybridising them with cDNA probes derived from either young or old brain RNA preparations, indicated that 40% of them represented genes that were differentially expressed. This approach is likely to prove invaluable for identifying cohorts of genes that show differential expression during the ageing process.

Age-dependence of the lateral mobility of lipids in hepatocyte plasma membrane of male rats and the effect of life-long dietary restriction

The lateral diffusion constant of lipids (D(1)) in hepatocyte plasma membranes was measured in liver smears by means of the fluorescence recovery after photobleaching (FRAP) method, applying the label, N-4-nitrobenzo-2-oxa-1,3-diazolyl phosphatidylethanolamine (NBD-PE). Nineteen ad libitum fed, male Fischer-344 rats in four age groups (2.1-29.8 months of age) were studied. A highly significant negative linear age-correlation of D(1) (cc = 0.958) was found. D(1) values were 1.39 x 10(-9) cm2/s in the young rats, and only 6.77 x 10(-10) cm2/s in the oldest rats. Lipid lateral mobility is changing in parallel with that of proteins, having been measured previously also with the FRAP method by the authors. Fractional recovery values (FR%) of the lipids were lower than those of proteins even in the young ages, but also decreased linearly with age, therefore, the parameter, D, x FR decreased even steeper with age than D(1) itself. D(1) was also measured in a group of six male Fischer 344 rats having been kept on dietary restriction (DR) since their age of 1 month until 30 months of age (applying the every-other-day (EOD) feeding). DR caused an increase of D(1), compared with the age-matched ad libitum fed animals: the mean was 9.24 x 10(-10) cm(2)/s. FR% and D(I), x FR again increased considerably under DR. The results are interpreted in terms of the increased protein and lipid turnover under DR.

The ontogeny of resource allocation in giant transgenic rat growth hormone mice

Dry mass budgets were conducted on transgenic metallothionein-1 rat growth hormone mice and normal Mus musculus to assess ontogenic changes in growth, feeding, and resource allocation. Younger mice had higher rates and efficiencies of growth than older mice. Young transgenic mice and normal controls were relatively similar for most features but became progressively dissimilar with time. The rate of growth of transgenics was never faster than the most rapid growth observed in normal mice, but they grew larger by maintaining a higher growth rate to a later age. On a mass-specfic basis, transgenic animals consumed less food than normal ones. Reduced feeding was not simply a reflection of the allometric scaling of food intake with larger body size, as younger transgenic mice ate less food than normal ones of equivalent size, even on an absolute basis. Transgenic mice achieved increased growth via superior production efficiency and ontogenically by maintaining greater efficiency to a later age. Differences in feeding and efficiency were detectable even before the mice diverged much in size. A single relationship relating production efficiencies and growth rates for older mice was confirmed, but younger transgenic mice and normal controls displayed fundamentally different relationships between efficiencies and rates of growth. Insights into growth regulation, feeding, life-history trade-offs, and allometric theory are discussed.

Aging and oxidative stress: modulation by dietary restriction

Aging is an inevitable biological process that affects most living organisms. Despite the enormous consequences associated with the aging process, until recently, relatively little systematic effort has been expended on the scientific understanding of this important life process. Society, however, urged by an ever increasing older population, is challenging scientists from many disciplines to explore one of nature's most complex phenomena-biological aging. For the past two decades, research directed toward the basic understanding of biological aging mechanisms and possible aging interventions have given us new insights into the molecular bases and the biological events that contribute to age-related deterioration. To further investigate the aging processes, one probe uniquely suited to exploring the progression of aging in animal models is dietary restriction, currently the only antiaging intervention accepted by gerontologists and nutritionists. Recent research renders a better understanding of how reduced dietary intake extends the life span, supplying evidence that dietary restriction is a diverse and effective modulator of oxidative stress. It has been proposed that this antioxidative mechanism is the underlying anti-aging action of dietary restriction.