Physical activity as a factor in the action of dietary restriction on aging: effects in Fischer 344 rats

Bring Back the Rat!

As 2020 is "The Year of the Rat" in the Chinese astrological calendar, it seems an appropriate time to consider whether we should bring back the laboratory rat to front-and-center in research on the basic biology of mammalian aging. Beginning in the 1970s, aging research with rats became common, peaking in 1992 but then declined dramatically by 2018 as the mouse became preeminent. The purpose of this review is to highlight some of the historical contributions as well as current advantages of the rat as a mammalian model of human aging, because we suspect at least a generation of researchers is no longer aware of this history or these advantages. Herein, we compare and contrast the mouse and rat in the context of several biological domains relevant to their use as appropriate models of aging: phylogeny/domestication, longevity interventions, pathology/physiology, and behavior/cognition. It is not the goal of this review to give a complete characterization of the differences between mice and rats, but to provide important examples of why using rats as well as mice is important to advance our understanding of the biology of aging.

APOE2 is associated with longevity independent of Alzheimer's disease

Although the ε2 allele of apolipoprotein E (APOE2) benefits longevity, its mechanism is not understood. The protective effects of the APOE2 on Alzheimer's disease (AD) risk, particularly through their effects on amyloid or tau accumulation, may confound APOE2 effects on longevity. Herein, we showed that the association between APOE2 and longer lifespan persisted irrespective of AD status, including its neuropathology, by analyzing clinical datasets as well as animal models. Notably, APOE2 was associated with preserved activity during aging, which also associated with lifespan. In animal models, distinct apoE isoform levels, where APOE2 has the highest, were correlated with activity levels, while some forms of cholesterol and triglycerides were associated with apoE and activity levels. These results indicate that APOE2 can contribute to longevity independent of AD. Preserved activity would be an early-observable feature of APOE2-mediated longevity, where higher levels of apoE2 and its-associated lipid metabolism might be involved.

Benefits of Caloric Restriction in Longevity and Chemical-Induced Tumorigenesis Are Transmitted Independent of NQO1

Caloric restriction (CR) is the most potent nonpharmacological intervention known to both protect against carcinogenesis and delay aging in laboratory animals. There is a growing number of anticarcinogens and CR mimetics that activate NAD(P)H:quinone oxidoreductase 1 (NQO1). We have previously shown that NQO1, an antioxidant enzyme that acts as an energy sensor through modulation of intracellular redox and metabolic state, is upregulated by CR. Here, we used NQO1-knockout (KO) mice to investigate the role of NQO1 in both the aging process and tumor susceptibility, specifically in the context of CR. We found that NQO1 is not essential for the beneficial effects of CR on glucose homeostasis, physical performance, metabolic flexibility, life-span extension, and (unlike our previously observation with Nrf2) chemical-induced tumorigenesis.

Continuous overexpression of thioredoxin 1 enhances cancer development and does not extend maximum lifespan in male C57BL/6 mice

We examined the effects of continuous overexpression of thioredoxin (Trx) 1 on aging in Trx1 transgenic mice [Tg(TXN)^+/0]. This study was conducted to test whether increased thioredoxin expression over the lifespan in mice would alter aging and age-related pathology because our previous study demonstrated that Tg(act-TXN)^+/0 mice had no significant maximum life extension, possibly due to the use of actin as a promoter, which may have resulted in loss of Trx1 overexpression during aging. To test this hypothesis, we generated new Trx1 transgenic mice using a fragment of the human genome containing the TXN gene with an endogenous promoter to ensure continuous overexpression of Trx1 throughout the lifespan. Universal overexpression of Trx1 was observed, and Trx1 overexpression was maintained during aging (up to 22–24 months old) in the Tg(TXN)^+/0 mice. The levels of Trx1 are significantly higher (approximately 4 to 31 fold) in all of the tissues examined in the Tg(TXN)^+/0 mice compared to the wild-type (WT) littermates. The overexpression of Trx1 did not cause any changes in the levels of Trx2, glutaredoxin, glutathione, or other major antioxidant enzymes. The survival study demonstrated that male Tg(TXN)^+/0 mice slightly extended the earlier part of the lifespan compared to WT littermates, but no significant life extension was observed over the lifespan. The cross-sectional pathological analysis (22–25 months old) showed that Tg(TXN)^+/0 mice had a significantly higher severity of lymphoma and more tumor burden than WT mice, which was associated with the suppression of the apoptosis signal-regulating kinase 1 (ASK1) pathway. Our findings suggest that the increased levels of Trx1 over the lifespan in Tg(TXN)^+/0 mice showed some beneficial effects (slight extension of lifespan) in the earlier part of life but had no significant effects on median or maximum lifespans, and increased Trx1 levels enhanced tumor development in old mice.

The effects of graded levels of calorie restriction: V. Impact of short term calorie and protein restriction on physical activity in the C57BL/6 mouse

Calorie restriction (CR) delays the onset of age-related disease and extends lifespan in a number of species. When faced with reduced energy supply animals need to lower energy demands, which may be achieved in part by reducing physical activity (PA). We monitored changes in PA using implanted transmitters in male C57BL/6 mice in response to graded levels of CR (10 to 40%) or matched levels of graded protein restriction (PR) for 3 months. Mice were fed at lights out and ad libitum controls were limited to dark-phase feeding (12AL) or 24hr/day. Total daily PA declined in a non-linear manner over the first 30 days of CR or PR, remaining stable thereafter. Total daily PA was not related to the level of CR or PR. Total daily PA over the last 20 days of restriction was related to circulating leptin, insulin, tumor necrosis factor-α (TNF-α) and insulin-like growth factor (IGF)-1 levels, measured after 3 months. Mice under restriction showed a high level of activity in the 2hrs before feeding (food anticipatory activity: FAA). FAA followed a complex pattern, peaking around day 20, falling on ∼day 37 then increasing again. FAA was also positively related to the level of restriction and inversely to leptin, insulin, TNF-α and IGF-1. Non-FAA, in contrast, declined over the period of restriction, generally more so in mice under greater restriction, thereby offsetting to some extent the increase in FAA. Mice under PR displayed no changes in PA over time or in comparison to 12AL, and showed no increase in FAA.

Sirtuins, a promising target in slowing down the ageing process

Ageing is a plastic process and can be successfully modulated by some biomedical approaches or pharmaceutics. In this manner it is possible to delay or even prevent some age-related pathologies. There are some defined interventions, which give promising results in animal models or even in human studies, resulting in lifespan elongation or healthspan improvement. One of the most promising targets for anti-ageing approaches are proteins belonging to the sirtuin family. Sirtuins were originally discovered as transcription repressors in yeast, however, nowadays they are known to occur in bacteria and eukaryotes (including mammals). In humans the family consists of seven members (SIRT1-7) that possess either mono-ADP ribosyltransferase or deacetylase activity. It is believed that sirtuins play key role during cell response to a variety of stresses, such as oxidative or genotoxic stress and are crucial for cell metabolism. Although some data put in question direct involvement of sirtuins in extending human lifespan, it was documented that proper lifestyle including physical activity and diet can influence healthspan via increasing the level of sirtuins. The search for an activator of sirtuins is one of the most extensive and robust topic of research. Some hopes are put on natural compounds, including curcumin. In this review we summarize the involvement and usefulness of sirtuins in anti-ageing interventions and discuss the potential role of curcumin in sirtuins regulation.

Glucocorticoid antagonism limits adiposity rebound and glucose intolerance in young male rats following the cessation of daily exercise and caloric restriction

Severe caloric restriction (CR), in a setting of regular physical exercise, may be a stress that sets the stage for adiposity rebound and insulin resistance when the food restriction and exercise stop. In this study, we examined the effect of mifepristone, a glucocorticoid (GC) receptor antagonist, on limiting adipose tissue mass gain and preserving whole body insulin sensitivity following the cessation of daily running and CR. We calorically restricted male Sprague-Dawley rats and provided access to voluntary running wheels for 3 wk followed by locking of the wheels and reintroduction to ad libitum feeding with or without mifepristone (80 mg·kg(-1)·day(-1)) for 1 wk. Cessation of daily running and CR increased HOMA-IR and visceral adipose mass as well as glucose and insulin area under the curve during an oral glucose tolerance test vs. pre-wheel lock exercised rats and sedentary rats (all P < 0.05). Insulin sensitivity and glucose tolerance were preserved and adipose tissue mass gain was attenuated by daily mifepristone treatment during the post-wheel lock period. These findings suggest that following regular exercise and CR there are GC-induced mechanisms that promote adipose tissue mass gain and impaired metabolic control in healthy organisms and that this phenomenon can be inhibited by the GC receptor antagonist mifepristone.

The effects of graded levels of calorie restriction: VI. Impact of short-term graded calorie restriction on transcriptomic responses of the hypothalamic hunger and circadian signaling pathways

Food intake and circadian rhythms are regulated by hypothalamic neuropeptides and circulating hormones, which could mediate the anti-ageing effect of calorie restriction (CR). We tested whether these two signaling pathways mediate CR by quantifying hypothalamic transcripts of male C57BL/6 mice exposed to graded levels of CR (10 % to 40 %) for 3 months. We found that the graded CR manipulation resulted in upregulation of core circadian rhythm genes, which correlated negatively with circulating levels of leptin, insulin-like growth factor 1 (IGF-1), insulin, and tumor necrosis factor alpha (TNF-α). In addition, key components in the hunger signaling pathway were expressed in a manner reflecting elevated hunger at greater levels of restriction, and which also correlated negatively with circulating levels of insulin, TNF-α, leptin and IGF-1. Lastly, phenotypes, such as food anticipatory activity and body temperature, were associated with expression levels of both hunger genes and core clock genes. Our results suggest modulation of the hunger and circadian signaling pathways in response to altered levels of circulating hormones, that are themselves downstream of morphological changes resulting from CR treatment, may be important elements in the response to CR, driving some of the key phenotypic outcomes.

Significant life extension by ten percent dietary restriction

Although it is well documented that dietary restriction (DR) increases the life span of rodents and other animals, this increase is observed at relatively high levels of DR, in which rodents are typically fed 40% less than that consumed by rodents fed ad libitum. It is generally assumed that lower levels of DR will have a lesser impact on life span; however, there are very little published data on the effect of low levels of DR on life span. In this study, we show that 10% DR increased life span to almost the same extent as 40% DR. While both 10% and 40% DR resulted in similar changes in non-neoplastic lesions, 10% DR had no significant effect on the incidence of neoplasia (except for pituitary adenoma), and 40% DR resulted in a significant reduction (40%) in neoplasia. These data clearly demonstrate that the life span of F344 rats does not increase linearly with the level of DR; rather, even a low level of DR can substantially affect life span. This rodent study has important translational implications because it suggests that a modest reduction in calories might have significant health benefits for humans.

Deficiency in mTORC1-controlled C/EBPβ-mRNA translation improves metabolic health in mice

The mammalian target of rapamycin complex 1 (mTORC1) is a central regulator of physiological adaptations in response to changes in nutrient supply. Major downstream targets of mTORC1 signalling are the mRNA translation regulators p70 ribosomal protein S6 kinase 1 (S6K1p70) and the 4E-binding proteins (4E-BPs). However, little is known about vertebrate mRNAs that are specifically controlled by mTORC1 signalling and are engaged in regulating mTORC1-associated physiology. Here, we show that translation of the CCAAT/enhancer binding protein beta (C/EBPβ) mRNA into the C/EBPβ-LIP isoform is suppressed in response to mTORC1 inhibition either through pharmacological treatment or through calorie restriction. Our data indicate that the function of 4E-BPs is required for suppression of LIP. Intriguingly, mice lacking the cis-regulatory upstream open reading frame (uORF) in the C/EBPβ-mRNA, which is required for mTORC1-stimulated translation into C/EBPβ-LIP, display an improved metabolic phenotype with features also found under calorie restriction. Thus, our data suggest that translational adjustment of C/EBPβ-isoform expression is one of the key processes that direct metabolic adaptation in response to changes in mTORC1 activity.

Lifelong wheel running exercise and mild caloric restriction attenuate nuclear EndoG in the aging plantaris muscle

Apoptosis plays an important role in atrophy and sarcopenia in skeletal muscle. Recent evidence suggests that insufficient heat shock proteins (HSPs) may contribute to apoptosis and muscle wasting. In addition, long-term caloric restriction (CR) and lifelong wheel running exercise (WR) with CR provide significant protection against caspase-dependent apoptosis and sarcopenia. Caspase-independent mediators (endonuclease G: EndoG; apoptosis-inducing factor: AIF) of apoptosis are also linked to muscles wasting with disuse and aging. However, the efficacy of CR and WR with CR to attenuate caspase-independent apoptosis and preserve HSPs in aging skeletal muscle are unknown. Therefore, we tested the hypothesis that CR and WR with CR would ameliorate age-induced elevation of EndoG and AIF while protecting HSP27 and HSP70 levels in the plantaris. Male Fischer-344 rats were divided into 4 groups at 11weeks: ad libitum feeding until 6months (YAL); fed ad libitum until 24months old (OAL); 8%CR to 24months (OCR); WR+8%CR to 24months (OExCR). Nuclear EndoG levels were significantly higher in OAL (+153%) than in YAL, while CR (-38%) and WR with CR (-46%) significantly attenuated age-induced increment in nuclear EndoG. HSP27 (-63%) protein content and phosphorylation at Ser82 (-49%) were significantly lower in OAL than in YAL, while HSP27 protein content was significantly higher in OCR (+136%) and OExCR (+155%) and p-HSP27 (+254%) was significantly higher in OExCR compared with OAL, respectively. In contrast, AIF and HSP70 were unaltered by CR or WR with CR in aging muscle. These data indicate that CR and WR with CR attenuate age-associated upregulation of EndoG translocation in the nucleus, potentially involved with HSP27 signaling.

Enhanced Locomotor Activity Is Required to Exert Dietary Restriction-Dependent Increase of Stress Resistance in Drosophila

Dietary restriction (DR) is known to be one of the most effective interventions to increase stress resistance, yet the mechanisms remain elusive. One of the most obvious DR-induced changes in phenotype is an increase in locomotor activity. Although it is conceptually perceivable that nutritional scarcity should prompt enhanced foraging behavior to garner additional dietary resources, the significance of enhanced movement activity has not been associated with the DR-dependent increase of stress resistance. In this study, we confirmed that flies raised on DR exhibited enhanced locomotive activity and increased stress resistance. Excision of fly wings minimized the DR-induced increase in locomotive activity, which resulted in attenuation of the DR-dependent increase of stress resistance. The possibility that wing clipping counteracts the DR by coercing flies to have more intake was ruled out since it did not induce any weight gain. Rather it was found that elimination of reactive oxygen species (ROS) that is enhanced by DR-induced upregulation of expression of antioxidant genes was significantly reduced by wing clipping. Collectively, our data suggests that DR increased stress resistance by increasing the locomotor activity, which upregulated expression of protective genes including, but not limited to, ROS scavenger system.

Do sirtuins promote mammalian longevity? A critical review on its relevance to the longevity effect induced by calorie restriction

Sirtuins (SIRTs), a family of nicotinamide adenine dinucleotide (NAD)-dependent deacetylases, are emerging as key molecules that regulate aging and age-related diseases including cancers, metabolic disorders, and neurodegenerative diseases. Seven isoforms of SIRT (SIRT1-7) have been identified in mammals. SIRT1 and 6, mainly localized in the nucleus, regulate transcription of genes and DNA repair. SIRT3 in the mitochondria regulates mitochondrial bioenergetics. Initial studies in yeasts, nematodes, and flies indicated a strong connection of SIRT with the life-prolonging effects of calorie restriction (CR), a robust experimental intervention for longevity in a range of organisms. However, subsequent studies reported controversial findings regarding SIRT roles in the effect of CR. This review describes the functional roles of mammalian SIRTs and discusses their relevance to mechanisms underlying the longevity effect of CR.

Studies of aging in ames dwarf mice: Effects of caloric restriction

Ames dwarf mice, which are small and deficient in growth homone (GH), prolactin (PRL), and thyroid stimulating hormone (TSH) live much longer (1-1.25 years) than their normal siblings. It was of interest to examine the response of these animals to caloric restriction (CR) because of the possibility that dwarf mice are voluntarily caloric restricted. We are testing the hypothesis that this possible natural caloric restriction will negate any benefits of an imposed CR on lifespan. Male and female Ames dwarf mice and their normal counterparts have been fed ad libitum (AL) or a 30% CR diet for 25-29 months. Animals were monitored daily and weighed weekly. At 12-15 months of age, CR mice weighed significantly less than their AL fed counterparts (normal females: -42%, normal males: -23%, dwarf females: -18.8%, and dwarf males: -22.2%). Only in dwarf females has this significant difference disappeared with age. At one year of age, a comparison of daily food consumption revealed that female dwarf mice consume significantly more food per gram body weight than normal females and a similar tendency is evident for males. Although they received 30% less food, CR mice ate the same amount as AL mice per gram body weight. On measures of total locomotor activity, CR mice were significantly more active than their AL-fed counterparts. On an inhibitory avoidance learning task, 18-21 month old dwarf mice exhibited significantly better retention than their age-and diet-matched normal counterparts. Histopathological analysis in aging dwarf versus normal mice suggested that the incidence of tumors does not differ between the two groups but tumors appear to develop later in dwarf than in normal mice. After 2.25 years on the study 27% of AL normals, 52% of CR normals, 74% of AL dwarfs, and 87% of CR dwarfs are still alive. We conclude that Ames dwarfs are not CR mimetics although they share many characteristics. It remains to be determined whether CR will delay aging and cause a further life extension in Ames dwarf mice.

Intramyocellular fatty-acid metabolism plays a critical role in mediating responses to dietary restriction in Drosophila melanogaster

Changes in fat content have been associated with dietary restriction (DR), but whether they play a causal role in mediating various responses to DR remains unknown. We demonstrate that upon DR, Drosophila melanogaster shift their metabolism toward increasing fatty-acid synthesis and breakdown, which is required for various responses to DR. Inhibition of fatty-acid synthesis or oxidation genes specifically in the muscle tissue inhibited life-span extension upon DR. Furthermore, DR enhances spontaneous activity of flies, which was found to be dependent on the enhanced fatty-acid metabolism. This increase in activity was found to be at least partially required for the life-span extension upon DR. Overexpression of adipokinetic hormone (dAKH), the functional ortholog of glucagon, enhances fat metabolism, spontaneous activity, and life span. Together, these results suggest that enhanced fat metabolism in the muscle and physical activity play a key role in the protective effects of DR.

The influence of acute, late-life calorie restriction on whole body energy metabolism in p66Shc(-/-) mice

It has been proposed that Shc proteins may influence aging by regulating insulin signaling and energy metabolism. Evidence suggests that deletion of p66Shc could partially attenuate weight gain on a high fat diet by increasing energy expenditure. However, the impact of p66Shc on the metabolic response to calorie restriction (CR) has not been determined. Thus, we used indirect respiration calorimetry to determine the impact of CR on energy expenditure (EE) and substrate utilization (RQ) in 18mo p66Shc(-/-) and wild-type (WT) mice. Calorimetry measurements were completed at baseline and following 3d of 40% CR and 2 mo of 26% CR. There was no difference (P>0.10) in EE and RQ between gentoypes, regardless of how EE data was normalized. Both p66Shc(-/-) and WT mice showed decreases (P<0.001) in EE normalized for body weight at 2 mo of CR. However, the response to 3d of CR was different between genotypes with only the p66Shc(-/-) showing a decrease (P<0.001) in 24 h EE expressed per mouse or normalized for body weight. The results indicate that p66Shc does not significantly influence EE in 18 mo mice at baseline or 2 mo of CR, although it may play a role in the EE response to very acute CR.

Distinct effects of calorie restriction and resveratrol on diet-induced obesity and Fatty liver formation

The potential of resveratrol to mimic beneficial effects of calorie restriction (CR) was investigated. We compared the effects of both CR (70% of ad libitum energy intake) or resveratrol (2 g/kg or 4 g/kg food) on high-fat diet-induced obesity and fatty liver formation in C57Bl/6J mice, and we examined their effects on calorimetry, metabolic performance, and the expressions of inflammatory genes and SIRT proteins. We found that resveratrol with 4 g/kg dose partially prevented hepatic steatosis and hepatocyte ballooning and induced skeletal muscle SIRT1 and SIRT4 expression while other examined parameter were unaffected by resveratrol. In contrast, CR provided superior protection against diet-induced obesity and fatty liver formation as compared to resveratrol, and the effects were associated with increased physical activity and ameliorated adipose tissue inflammation. CR increased expressions of SIRT3 in metabolically important tissues, suggesting that the beneficial effects of CR are mediated, at least in part, via SIRT3-dependent pathways.

Tail tendon break time: a biomarker of aging?

Research has attempted to identify biomarkers of aging that are predictive of longevity and specific age-related changes during animal life span. Tail tendon break time (TTBT), one presumed biomarker, measures collagen cross-linking, known to increase with age. Significant differences in the rate of increase of TTBT with age have been reported between mouse strains and animal species. We measured both TTBT and longevity in C57BL/6J, DBA/2J, and 23 recombinant inbred (RI) strains (B×D RIs), with TTBT measured at 200, 500, and 800 days of age. Longevity demonstrated considerable variability among these strains (116-951 days). TTBT, also highly variable, increased significantly with age in both sexes and all genotypes. Neither TTBT nor its rate of change correlated significantly with life span. There were suggestive trends for rate of TTBT change to correlate with male longevity and strain longevity to correlate with female TTBT. We conclude that for the range of genetic variation found among these mouse genotypes, TTBT cannot be considered a robust biomarker of longevity.

Changes of behavioral parameters during long-term food restriction in middle-aged Wistar rats

Food restriction (FR) has a beneficial effect on aging process and exerts a significant effect on the responses of rodents to standard behavioral tasks. The aim of this study was to assess the cumulative influence of FR on the behavioral and biochemical parameters in Wistar rats. Six-month-old rats were subjected to restrictive feeding (50% of the daily food intake, every-other-day feeding regimen) for one month or for six months until ages of 7 and 12months, respectively. We examined the habituation of exploratory movement, amphetamine (AMPH)-induced motor activity, as well as changes in serum corticosterone (CORT) and glucose levels. The results obtained from FR animals were compared with ad libitum (AL)-fed age-matched control rats. Habituation of motor activity was only affected by six months of restrictive feeding. The sensitization of the motor response to AMPH that was observed in animals exposed to FR for one month was not observed in animals that were exposed to the same feeding regimen for six months. Serum CORT was increased and serum glucose was decreased in both FR groups. These results clearly show that despite the similarity of the biochemical changes that were induced by one and six months of FR, the nature of the changes in motor activities in these two groups of animals during habituation and after AMPH treatment was different. Our findings indicate that long-term FR has complex behavioral consequences that need to be carefully evaluated with respect to animal age, duration of FR and severity of the diet.

A rapidly occurring compensatory decrease in physical activity counteracts diet-induced weight loss in female monkeys

To study changes in energy balance occurring during the initial phases of dieting, 18 adult ovariectomized female monkeys were placed on a low-fat diet, and available calories were reduced by 30% compared with baseline consumption for 1 mo. Surprisingly, there was not significant weight loss; however, daily activity level (measured by accelerometry) decreased soon after diet initiation and reached statistical significance by the 4th wk of dieting (18 +/- 5.6% decrease, P = 0.02). During a 2nd mo of dieting, available calories were reduced by 60% compared with baseline consumption, leading to 6.4 +/- 1.7% weight loss and further suppression of activity. Metabolic rate decreased by 68 +/- 12 kcal/day, with decreased activity accounting for 41 +/- 9 kcal/day, and the metabolic activity of the weight lost accounting for 21 +/- 5 kcal/day. A second group of three monkeys was trained to run on a treadmill for 1 h/day, 5 days/wk, at 80% maximal capacity, leading to increased calorie expenditure of 69.6 +/- 10.7 kcal/day (equivalent to 49 kcal/day for 7 days). We conclude that a diet-induced decrease in physical activity is the primary mechanism the body uses to defend against diet-induced weight loss, and undertaking a level of exercise that is recommended to counteract weight gain and promote weight loss is able to prevent the compensatory decrease in physical activity-associated energy expenditure that slows diet-induced weight loss.

Influence of calorie restriction on measures of age-related cognitive decline: role of increased physical activity

Controversy exists as to whether lifelong 40% calorie restriction (CR) enhances, has no effect on, or disrupts cognitive function during aging. Here, we report the effects of CR versus ad-lib feeding on cognitive function in male Brown Norway x Fisher344 rats across a range of ages (8-38 months), using two tasks that are differentially sensitive to age-related cognitive decline: object recognition and Morris water maze (MWM). All ages performed equally in object recognition, whereas, as a group, CR rats were impaired. In contrast, there was an age-related impairment in the MWM that was attenuated by CR as measured by time in proximity with and latency to reach the platform. Distance to the platform, a more sensitive measure, was not affected by CR. Finally, CR resulted in an overall increase in physical activity, one of several behavioral confounders to consider in the interpretation of cognitive outcomes in both tasks.

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.

Lifelong exercise and mild (8%) caloric restriction attenuate age-induced alterations in plantaris muscle morphology, oxidative stress and IGF-1 in the Fischer-344 rat

Muscle atrophy is a highly prevalent condition among older adults, and results from reduced muscle mass and fiber cross-sectional area. Resistive exercise training and moderate (30-40%) caloric restriction may reduce the rate of sarcopenia in animal models. We tested the hypothesis that lifelong, voluntary exercise combined with mild (8%) caloric restriction would attenuate the reduction of muscle fiber cross-sectional area in the rat plantaris. Fischer-344 rats were divided into: young adults (6 mo) fed ad libitum (YAL); 24 mo old fed ad libitum (OAL); 24 mo old on 8% caloric restriction (OCR); lifelong wheel running with 8% CR (OExCR). Plantaris fiber cross-sectional area was significantly lower in OAL than YAL (-27%), but protected in OCR and OExCR, while mass/body mass ratio was preserved in OExCR only. Furthermore, 8% CR and lifelong wheel running attenuated the age-induced increases in extramyocyte space and connective tissue. Citrate synthase activity decreased with age, but was not significantly protected in OCR and OExCR. Total hydroperoxides were higher in OAL than YAL, but were not elevated in OExCR, with out a change in MnSOD. IGF-1 levels were lower in OAL (-57%) than YAL, but partially protected in the OExCR group (+51%).

Effects of life-long caloric restriction and voluntary exercise on age-related changes in levels of catecholamine biosynthetic enzymes and angiotensin II receptors in the rat adrenal medulla and hypothalamus

We examined if life-long mild caloric restriction (CR) alone or with voluntary exercise prevents the age-related changes in catecholamine biosynthetic enzyme levels in the adrenal medulla and hypothalamus. Ten-week-old Fisher-344 rats were assigned to: sedentary; sedentary+8% CR; or 8% CR+wheel running. Rats were euthanized at 6 or 24 months of age. Tyrosine hydroxylase (TH) mRNA expression was 4.4-fold higher in the adrenal medullae and 60% lower in the hypothalamus of old sedentary rats compared to young (p<0.01). Life-long CR reduced the age-related increase in adrenomedullary TH by 50% (p<0.05), and completely reversed the changes in hypothalamic TH. Voluntary exercise, however, had no additional effect over CR. Since angiotensin II is involved in the regulation of catecholamine biosynthesis, we examined the expressions of angiotensin II receptor subtypes in the adrenal medulla. AT(1) protein levels were 2.8-fold higher in the old animals compared to young (p<0.01), and while AT(1) levels were unaffected by CR alone, CR+wheel running decreased AT(1) levels by 50% (p<0.01). AT(2) levels did not change with age, however CR+wheel running increased its level by 42% (p<0.05). These data indicate that a small decrease in daily food intake can avert age-related changes in catecholamine biosynthetic enzyme levels in the adrenal medulla and hypothalamus, possibly through affecting angiotensin II signaling.

Conjugated linoleic acid and chromium lower body weight and visceral fat mass in high-fat-diet-fed mice

More than half of the U.S. population has a body mass index of 25 kg/m2 or more, which classifies them as overweight or obese. Obesity is often associated with comorbidities such as diabetes, cardiovascular diseases, and cancer. CLA and chromium have emerged as major dietary supplements that reduce body weight and fat mass, and increase basal metabolic rate in animal models. However, studies show that CLA induces insulin resistance in mice and in humans, whereas Cr improves insulin sensitivity. Hence, we designed the present study to examine the combined effect of CLA and Cr on body composition and insulin sensitivity in a Balb/c mice (n = 10/group) model of high-fat-diet-induced obesity. CLA alone lowered body weight, total body fat mass, and visceral fat mass, the last of which decreased further with the combination of CLA and Cr. This effect was accompanied by decreased serum leptin levels in CLA-fed and CLA + Cr-fed mice, and by higher energy expenditure (EE) and oxygen consumption (OC) in CLA + Cr-fed mice. Serum levels of glucose, insulin, the pro-inflammatory cytokines, tumor necrosis factor-alpha (TNF-alpha), and interleukin-6 (IL-6), as well as insulin resistance index (IRI), decreased with CLA, whereas CLA and Cr in combination had significant effects on insulin and IL-6 concentrations and IRI. In summary, CLA + Cr decreased body weight and fat mass in high-fat-diet-fed mice, which may be associated with decreased leptin levels and higher EE and OC.

The Impact of Environment in Comparison with Moderate Physical Exercise and Dietary Restriction on BDNF in the Cerebral Parietotemporal Cortex of Aged Sprague-Dawley Rats

BACKGROUND AND AIMS

Moderate physical exercise, dietary restriction as well as enriched environment in separate studies have been reported to delay some of the adverse effects of aging on brain function, parallel to an increase in brain-derived neurotrophic factor (BDNF). In order to elucidate these influences in a comparative setting, we examined the tissue concentrations of BDNF in the cerebral parietotemporal cortex of old Sprague-Dawley rats.

METHODS

Male Sprague-Dawley rats were divided randomly into six groups, living from 5 months (baseline group BL) up to 23 months of age as follows: voluntary running in wheels (RW), food restricted by feeding to pair weight with RW animals (PW), forced running on treadmills (TM), and sedentary controls with ad libitum access to food, either housed individually (S1) or in groups of 4 animals (S4). BDNF concentrations were determined by a commercially available ELISA.

RESULTS

We found higher BNDF concentrations in the 5 months old animals than in the 23 months old animals of group S1. The old sedentary group S4 showed significantly higher BNDF concentrations in comparison with the old individually caged groups RW, TM, PW and S1. Their BNDF concentrations were even higher than those of the young baseline group.

CONCLUSIONS

Our data suggest that housing and social interactions have more influence on BDNF concentrations in the cerebral parietotemporal cortex of aging Sprague-Dawley rats than physical exercise and food restriction.

Impact of moderate physical exercise--in comparison with dietary restrictions--on age-associated decline in cell-mediated immunity of Sprague-Dawley rats

BACKGROUND AND AIMS

Moderate physical exercise and dietary restriction have both been demonstrated to delay some of the adverse effects of aging. In order to elucidate similarities or dissimilarities in their mode of action on the aging immune system in a comparative setting, we examined significant parameters of cell-mediated immunity in Sprague- Dawley rats.

METHODS

Male Sprague-Dawley rats, housed individually, were divided into four groups, living from 5 months (baseline group BL) up to 15, 19 and 23 months of age as follows: voluntary running in wheels (RW), food restricted by feeding to pair weight with RW animals (PW), forced running on treadmills (TM), and sedentary controls with ad libitum access to food (S1). White blood cell counts, capacity for lymphocyte proliferation in response to Concanavalin A, and interleukin-2 (IL-2) plasma concentrations were determined.

RESULTS

White blood cell counts and the cell numbers of lymphocytes, neutrophil and eosinophil granulocytes were significantly lower in the older RW and PW groups. We observed influences of forced exercise on lymphocyte proliferation: blastogenic reactivity was higher in TM animals compared with RW and PW animals at 23 months of age. Exclusively for RW animals, we found lower plasma concentrations of IL-2 at 23 months.

CONCLUSIONS

Our data support the idea that moderate physical exercise modulates age-associated decline in the cell-mediated immunity of old Sprague-Dawley rats significantly more than corresponding dietary restrictions.

Early‐onset calorie restriction conserves fiber number in aging rat skeletal muscle

The purpose of this work was to determine the effect of early-onset calorie restriction on sarcopenia in the aging rat. Ad libitum (AL) fed animals were examined at 5, 18, 21, and 36 months of age. Calorie-restricted (CR) rats, 40% restricted since 4 months of age, were examined at 21 and 36 months of age. By 36 months, vastus lateralis, rectus femoris and soleus muscles, from AL-fed rats, had significant muscle mass and fiber loss, and reduced muscle cross-sectional area. Mean fiber diameter decreased with age in the vastus lateralis and rectus femoris but not the soleus of AL-fed rats. The number of Type I fibers significantly increased in the vastus lateralis with age. Calorie restriction did not prevent muscle mass loss with age; however, it significantly reduced muscle mass loss between 21 and 36 months of age compared with age-matched AL cohorts. Calorie restriction prevented fiber loss with age, and this conservation of fiber number reduced muscle mass loss with age.

Voluntary exercise and mild food restriction effectively retard the collagen biomarker of aging

BACKGROUND AND AIMS

Obesity and lack of physical exercise characterize increasing numbers of people in Western societies, resulting in increasing morbidity and mortality. Regular physical exercise, on the other hand, has been shown to be beneficial. Beneficial effects have also been shown in laboratory rodent studies--lifelong physical exercise increases the mean life span but not the maximum life span, while food restriction increases both life spans. We studied male Sprague-Dawley rats in order to further analyze these beneficial effects.

METHODS

We compared voluntary running in wheels (RW) with food restriction to pair weight with RW (PW), and mild forced running in treadmill (TM)--all animals housed individually. Control groups were sedentary ad libitum-fed animals housed individually (S1) and four to a cage (S4). The collagen biomarker of aging was calculated as a composite parameter from thermal stability data for 5 (baseline), 15, 19 and 23 months of age.

RESULTS

In RW and PW, the increase in the composite parameter was retarded to the same extent. The retardation in TM was less efficient. S4 had the highest value for the composite parameter, together with the highest body weight and fat content. In groupwise comparisons, the composite parameter tended to predict survival in the groups up to the age of 23 months.

CONCLUSIONS

Voluntary running in wheels is more beneficial than mild-forced exercise on the treadmill. The composite parameter is a useful indicator of different interventions on aging rats.

Voluntary and forced exercise influence the survival and body composition of ageing male rats differently

The importance of maintaining physical fitness by engaging in exercise in a life-long perspective as well as the avoidance of obesity has been emphasised in recent years by epidemiological studies on human populations as well as studies on laboratory rodents. In laboratory studies, voluntary running in wheels and forced training in a treadmill have been used with beneficial results. Restriction of the food intake of sedentary laboratory rodents can be regarded either as life prolongation or prevention of life shortening by obesity. We compared the effects of these interventions on male Sprague-Dawley rats from the age of 5 to 23 months in the following groups: (1) RW=voluntary running in wheels; (2) PW=fed to pair weight with RW animals; (3) TM=forced training in a treadmill; and (4) S1=sedentary with ad libitum access to food. Each group consisted of 32 animals, all housed individually in cages. Two RW animals died, five died in each of the PW and S1 groups and 10 in the TM group (p<0.05). The S1 and TM groups gained most weight, the TM less after the age of 21 months (p<0.05). The body weights of the RW group was lower than those of the S1 and TM groups all the time (p<0.001) and the difference increased all the time. Body composition was analysed by bioelectrical impedance analysis. There were no differences in fat free mass (FFM) neither between RW and PW at any time, nor between S1 and TM. FFM was lower for RW and PW compared to S1 and TM. TM gained FFM until the age of 17 months, while S1 gained FFM all the time. S1 gained fat all the time, but the gain for TM levelled off. It stayed constant for RW until 13 months and decreased afterwards. We conclude that voluntary running in wheels enhances survival and keeps body fat lower than in PW animals up to the age of 17 months. Body composition and survival data suggest that voluntary running is more optimal than forced. Care must, however, be taken in analyses, since RW is a heterogenous group because there is a large variation between the animals with respect to how much they run.

Food restriction differentially affects pituitary hormone mRNAs throughout the adult life span of male F344 rats

Because neuroendocrine mechanisms may contribute to the antiaging effects of food restriction (FR), we measured the effect of FR on mRNAs encoding anterior pituitary (AP) tropic hormones. Slot blots or RNase protection assays were done on AP RNA from 3-, 6-, 12-, 18- and 24-mo-old male F344 rats consuming food ad libitum (AL) or food restricted (FR; to 60% of AL food intake) from 6 wk. Both AL and FR rats gained body weight during the study (P < 0.05), but FR rats weighed approximately 40% less (P < 0.0001). Messenger RNA levels were expressed in two ways, i.e., per total AP and per microgram total AP RNA. Proopiomelanocortin (POMC) mRNA/microg RNA was higher (P < 0.0005) in FR than in AL rats at all ages. Thyroid-stimulating hormone (TSH) beta mRNA declined with age (P < 0.05) in AL but not FR rats and was reduced by FR up to 12 mo (P < 0.01). Growth hormone (GH) mRNA/microg RNA declined with age (P < 0.05) in AL but not FR rats, and total GH mRNA in the AP was reduced by FR at early ages (P < 0.05). FR reduced prolactin (PRL) mRNA and its age-related increase (P < 0.0005). Levels of luteinizing hormone (LH) beta and follicle-stimulating hormone (FSH) beta mRNAs did not differ between AL and FR rats until 12 mo, but thereafter rose in FR (LH beta mRNA; P < 0.01, FSH beta mRNA; P < 0.05). Many of these changes in gene expression corroborate previously reported hormonal changes in FR rodents and mutant mice with extended life spans, and thus provide further support for the hypothesis that an altered hormonal milieu contributes to the antiaging effects of food restriction.

Restriction of energy intake, energy expenditure, and aging

Energy restriction (ER), without malnutrition, increases maximum life span and retards the development of a broad array of pathophysiological changes in laboratory rodents. The mechanism responsible for the retardation of aging by ER is, however, unknown. One proposed explanation is a reduction in energy expenditure (EE). Reduced EE may increase life span by decreasing the number of oxygen molecules interacting with mitochondria, thereby lowering reactive oxygen species (ROS) production. As a step toward testing this hypothesis, it is important to determine the effect of ER on EE. Several whole-body, organ, and cellular studies have measured the influence of ER on EE. In general, whole-body studies have reported an acute decrease in mass-adjusted EE that disappears with long-term ER. Organ-specific studies have shown that decreases in EE of liver and gastrointestinal tract are primarily responsible for initial reductions in EE with ER. These data, however, do not determine whether cellular EE is altered with ER. Three major processes contributing to resting EE at the cellular level are mitochondrial proton leak, Na(+)-K(+)-ATPase activity, and protein turnover. Studies suggest that proton leak and Na(+)-K(+)-ATPase activity are decreased with ER, whereas protein turnover is either unchanged or slightly increased with ER. Thus, two of the three major processes contributing to resting EE at the cellular level may be decreased with ER. Although additional cellular measurements are needed, the current results suggest that a lowering of EE could be a mechanism for the action of ER.

Analysis of the effects of growth hormone, voluntary exercise, and food restriction on diaphyseal bone in female F344 rats

The aim of this study is to examine the effects of growth hormone, exercise, and weight loss due to food restriction on tibial diaphyseal bone and on tibial muscle mass. Thirteen-month-old female F344 rats were divided into six groups: group 1, baseline controls (B); group 2, age-matched controls (C); group 3, GH treated (GH); group 4, voluntary wheel running exercise (EX); group 5, GH + EX; and group 6, food restricted (FR). The dose of GH was 2.5 mg recombinant human (rh) GH/kg body weight/day, 5 days per week, given in two divided doses of 1.25 mg at 9-10 A.M. and 4-5 P.M. Food-restricted rats were fed 60% of the mean food intake of the age-matched controls. All animals except the baseline controls were killed after 4.5 months. The baseline controls were killed at the beginning of the study. Growth hormone increased the body weight and tibial muscle mass of the rats markedly, while EX caused only a slight decrease in body weight and partially inhibited the increase caused by GH in the GH + EX group. Food restriction greatly decreased body weight below that of age-matched controls, but neither FR nor EX had a significant effect on the mass of the muscles around the tibia. Growth hormone and EX independently increased tibial diaphyseal cortical bone area (p < 0.0001, p < 0.0001), cortical thickness (p < 0.0001, p < 0.0001), cortical bone mineral content (p < 0.0001, p < 0.0001), periosteal perimeter (p < 0.0001, p < 0.0001), and bone strength-strain index (SSI) (p < 0.0001, p < 0.0001). The effects of GH were more marked and resulted in a greater increase in the weight of the mid tibial diaphysis (p < 0.0001). The combination of GH and EX produced additive effects on many of the tibial diaphyseal parameters, including bone SSI. GH + EX, but not GH or EX alone, caused a significant increase in endocortical perimeter (p < 0.0001). In the FR rats, cortical bone area and cortical mineral content increased above the baseline level (p < 0.001, p < 0.0001) but were below the levels for age-matched controls (p < 0.0001, p < 0.0001). In addition, marrow area, endocortical perimeter, and endocortical bone formation rate increased significantly in the FR rats (p < 0.01, p < 0.0001, p < 0.0001). Three-point bending test of right tibial diaphysis resulted in maximum force (Fmax) values that reflected the group differences in indices of tibial diaphyseal bone mass, except that GH + EX did not produce additive effect on Fmax. The latter showed good correlation with left tibial diaphyseal SSI (r = 0.857, p < 0.0001), and both indices of bone strength correlated well with tibial muscle mass (r = 0.771, Fmax; r = 0.700, SSI; p < 0.0001). GH increased serum IGF-I (p < 0.0001), and the increase was partially reduced by EX. Serum osteocalcin was increased by GH with or without EX (p < 0.01, p < 0.01), and FR or EX alone did not alter serum IGF-I and osteocalcin levels. The bone anabolic effects of GH with or without EX may relate, in part, to increased load on bone from tibial muscles and body weight, which were increased by the hormone. The osteogenic effect of EX with or without GH may relate, in part, to increased frequency of muscle load on bone as EX decreased body weight (p < 0.05), but had no significant effect on tibial muscle mass. The enhanced loss of endocortical bone by FR may relate, in part, to decreased load on bone due to low body weight (p < 0.0001), as FR did not cause a significant decrease in tibial muscle mass (p = 0.357). The roles of humoral and local factors in the bone changes observed remain to be established.

Additive effect of voluntary exercise and growth hormone treatment on bone strength assessed at four different skeletal sites in an aged rat model

The aim of the study was to assess the effect of growth hormone (GH), voluntary exercise (Ex), and the combination of GH and Ex on bone strength, mass, and dimensions in aged, intact female rats. In addition, the effect of food restriction (FR) was studied. Fourteen-month-old virgin F-344 rats were divided into 6 groups with 13 animals in each: (1) baseline (BSL); (2) control + solvent vehicle (CTRL); (3) GH 2.5 mg/kg/day (GH); (4) exercise, voluntary: 0.6-0.7 km/day (Ex); (5) GH treatment and voluntary exercise (GH + Ex); and (6) FR. Group 1 was killed at the beginning of the study and served as baseline. All the other groups were killed after 18 weeks' treatment. The effects of aging and treatment regimes were measured at four different skeletal sites: lumbar vertebrae, femoral cortical bone, femoral neck, and the distal femoral metaphysis. Aging in itself induced a decline in vertebral body strength and ash density. At the appendicular skeletal sites, bone mass and strength were unchanged or increased. Treatment with GH alone induced a significant increase in the biomechanical parameters at the vertebral body and the femoral diaphysis, but not at the femoral neck or the distal femoral metaphysis. Voluntary exercise on its own increased load values significantly over CTRL at the vertebral body site, but not at any of the appendicular skeletal sites. The combination of GH and voluntary exercise resulted in an additive effect at the vertebral site and at the femoral diaphysis, and a synergistic (potentiating) effect at the two femoral metaphyses. FR, on the other hand, had a negative effect on cortical bone area and strength at the femoral diaphysis, but no significant effect on the other sites tested. We conclude that GH treatment and voluntary exercise both have skeletal anabolic effects; however, these effects are exerted to differing degrees at different sites. Importantly, when dosed together, GH and Ex have either an additive or synergistic anabolic effect on all sites (axial and appendicular).

The life-shortening effect of reduced physical activity is abolished by a fat rich diet

In female mice on a control diet (3.6% fat) reduced physical activity leads to a reduction of the average life span. So the average age at death of an inactive group is 500 +/- 166 compared to 565 +/- 175 days in an active control group. If the animals are kept on a fat rich diet (12.4% fat) this effect of physical activity restriction is no longer observable and the average age at death is 570 +/- 142 days, within the range of the control animals. The increased fat intake seems to reduce the stress or to increase the resistance to stress in the activity restricted animals. So stress is a crucial determinant of life span.

When will the biology of aging become useful? Future landmarks in biomedical gerontology

Where should we look to find the causes of and cure for aging? This essay considers a number of scientific discoveries, not yet made, that might dramatically increase the proportion of biogerontologists doing useful work. I will consider, in turn, problems and prospects in the areas of comparative biology, mammalian and invertebrate genetics, biomarker research, caloric restriction, and clonal senescence and then conclude with a discussion of potential links between aging and late life disease.

Effects of dietary restriction and exercise on the age-related pathology of the rat

Intervention of the aging process is an effective, experimental means of uncovering the bases of aging. The most efficacious and commonly used intervention used to retard the aging processes is dietary restriction (DR). It increases mean and maximum life spans, delays the appearance, frequency, and severity of many age-related diseases, and more importantly, attenuates much of the physiological decline associated with age. Although the subject of intense research, the mechanism by which DR alters the aging processes is still unknown. Physical exercise is another effective intervention shown to affect aging phenomena, especially when applied in combination with DR. Mild exercise in concert with DR is beneficial, but vigorous exercise coupled with DR could be deleterious. With regard to pathology, exercise generally exerts a salutary influence on age-related diseases, both neoplastic and non-neoplastic, and this effect may contribute to the increase in median life span seen with exercised rats. Exercise coupled with 40% DR was found to suppress the incidence of fatal neoplastic disease compared to the sedentary DR group. Exercise with mild DR suppressed the incidence of multiple fatal disease and chronic nephropathy, and also delayed the occurrence of many age-related lesions compared to the ad libitum (AL) control group. However, these effects may have little bearing on the aging process per se, as maximum life span is only minimally affected. Although not as intensively studied as DR, results from studies that utilize exercise as a research probe, either alone or in combination with DR, have helped to assess the validity of proposed mechanisms for DR and aging itself. Neither the retardation of growth rate nor the increase in physical activity, observed with either exercise or DR, appear to contribute to the anti-aging action of DR. Moreover, results from lifelong exercise studies indicate that the effects of DR do not depend upon changes in energy availability or metabolic rate. The mechanisms involving effects on adiposity or immune function are also inadequate explanations for the action of DR on aging. Of the proposed mechanisms, only one, as postulated by the Oxidative Stress Hypothesis of Aging, tenably accounts for the known effects of DR and exercise on aging.