Mice as a mammalian model for research on the genetics of aging

Mice are an ideal mammalian model for studying the genetics of aging: considerable resources are available, the generation time is short, and the environment can be easily controlled, an important consideration when performing mapping studies to identify genes that influence lifespan and age-related diseases. In this review we highlight some salient contributions of the mouse in aging research: lifespan intervention studies in the Interventions Testing Program of the National Institute on Aging; identification of the genetic underpinnings of the effects of calorie restriction on lifespan; the Aging Phenome Project at the Jackson Laboratory, which has submitted multiple large, freely available phenotyping datasets to the Mouse Phenome Database; insights from spontaneous and engineered mouse mutants; and complex traits analyses identifying quantitative trait loci that affect lifespan. We also show that genomewide association peaks for lifespan in humans and lifespan quantitative loci for mice map to homologous locations in the genome. Thus, the vast bioinformatic and genetic resources of the mouse can be used to screen candidate genes identified in both mouse and human mapping studies, followed by functional testing, often not possible in humans, to determine their influence on aging.     

Long-Term Bezafibrate Treatment Improves Skin and Spleen Phenotypes of the mtDNA Mutator Mouse

Pharmacological agents, such as bezafibrate, that activate peroxisome proliferator-activated receptors (PPARs) and PPAR γ coactivator-1α (PGC-1α) pathways have been shown to improve mitochondrial function and energy metabolism. The mitochondrial DNA (mtDNA) mutator mouse is a mouse model of aging that harbors a proofreading-deficient mtDNA polymerase γ. These mice develop many features of premature aging including hair loss, anemia, osteoporosis, sarcopenia and decreased lifespan. They also have increased mtDNA mutations and marked mitochondrial dysfunction. We found that mutator mice treated with bezafibrate for 8-months had delayed hair loss and improved skin and spleen aging-like phenotypes. Although we observed an increase in markers of fatty acid oxidation in these tissues, we did not detect a generalized increase in mitochondrial markers. On the other hand, there were no improvements in muscle function or lifespan of the mutator mouse, which we attributed to the rodent-specific hepatomegaly associated with fibrate treatment. These results showed that despite its secondary effects in rodent's liver, bezafibrate was able to improve some of the aging phenotypes in the mutator mouse. Because the associated hepatomegaly is not observed in primates, long-term bezafibrate treatment in humans could have beneficial effects on tissues undergoing chronic bioenergetic-related degeneration.     

Social stress shortens lifespan in mice

Stress and low socioeconomic status in humans confer increased vulnerability to morbidity and mortality. However, this association is not mechanistically understood nor has its causation been explored in animal models thus far. Recently, cellular senescence has been suggested as a potential mechanism linking lifelong stress to age‐related diseases and shorter life expectancy in humans. Here, we established a causal role for lifelong social stress on shortening lifespan and increasing the risk of cardiovascular disease in mice. Specifically, we developed a lifelong chronic psychosocial stress model in which male mouse aggressive behavior is used to study the impact of negative social confrontations on healthspan and lifespan. C57BL/6J mice identified through unbiased cluster analysis for receiving high while exhibiting low aggression, or identified as subordinate based on an ethologic criterion, had lower median and maximal lifespan, and developed earlier onset of several organ pathologies in the presence of a cellular senescence signature. Critically, subordinate mice developed spontaneous early‐stage atherosclerotic lesions of the aortic sinuses characterized by significant immune cells infiltration and sporadic rupture and calcification, none of which was found in dominant subjects. In conclusion, we present here the first rodent model to study and mechanistically dissect the impact of chronic stress on lifespan and disease of aging. These data highlight a conserved role for social stress and low social status on shortening lifespan and increasing the risk of cardiovascular disease in mammals and identify a potential mechanistic link for this complex phenomenon.     

An engineering approach to extending lifespan in C. elegans

We have taken an engineering approach to extending the lifespan of Caenorhabditis elegans. Aging stands out as a complex trait, because events that occur in old animals are not under strong natural selection. As a result, lifespan can be lengthened rationally using bioengineering to modulate gene expression or to add exogenous components. Here, we engineered longer lifespan by expressing genes from zebrafish encoding molecular functions not normally present in worms. Additionally, we extended lifespan by increasing the activity of four endogenous worm aging pathways. Next, we used a modular approach to extend lifespan by combining components. Finally, we used cell- and worm-based assays to analyze changes in cell physiology and as a rapid means to evaluate whether multi-component transgenic lines were likely to have extended longevity. Using engineering to add novel functions and to tune endogenous functions provides a new framework for lifespan extension that goes beyond the constraints of the worm genome.     

Lifespan of a Ceratitis fruit fly increases with higher altitude

Variation in lifespan may be linked to geographic factors. While latitudinal variation in lifespan has been studied for a number of species, altitude variation has received much less attention, particularly in insects. We measured the lifespan of different populations of the Natal fruit fly Ceratitis rosa along an altitudinal cline. For the different populations we first measured the residual longevity of wild flies by captive cohort approach and compared F(1) generation from the same populations. We showed an increase in lifespan with higher altitude for a part of our data. For the field collected flies (F0) the average remaining lifespan increased monotonically with altitude for males but not for females. For the F(1) generation, longevity of both males and females of the highest-altitude population was longer than for the two other lower-altitude populations. This relationship between altitude and lifespan may be explained by the effects of temperature on reproduction. Reproductive schedules in insects are linked to temperature: lower temperature, characteristic of high-altitude sites, generally slows down reproduction. Because of a strong trade-off between reproduction and longevity, we therefore observed a longer lifespan for the high- altitude populations. Other hypotheses such as different predation rates in the different sites are also discussed.     

Wild-derived inbred mouse strains have short telomeres

Telomere length and telomerase activity directly affect the replicative capacity of primary human cells. Some have suggested that telomere length influences organismal lifespan. We compared telomere length distributions in a number of inbred and outbred established mouse strains with those of strains recently derived from wild mice. Telomere length was considerably shorter in wild-derived strains than in the established strains. We found no correlation of telomere length with lifespan, even among closely related inbred mouse strains. Thus, while telomere length plays a role in cellular lifespan in cultured human cells, it is not a major factor in determining organismal lifespan.     

Role of insulin-like signalling in Drosophila lifespan

Regulation of lifespan by the insulin/insulin-like growth factor-like signalling (IIS) pathway has been conserved during evolution from the nematode worm to the mouse. In the insect Drosophila, regulation of lifespan by the IIS pathway was established by data showing that many mutations in single genes encoding IIS components result in an increase in lifespan. Recently, however, the focus has shifted from studying the effects of single gene mutations with ubiquitous effects to finding interventions that alter IIS in specific tissues and at specific stages in the life history of the fruitfly, in order to elucidate the signalling pathways at work and the mechanisms by which alterations in the IIS pathway can extend lifespan.     

Neuroendocrine and immune characteristics of aging in avian species

Avian species show a remarkable diversity in lifespan. The differing lifespan patterns are found across a number of birds, in spite of higher body temperature and apparent increased metabolic rate. These characteristics make study of age-related changes of great interest, especially for understanding the biology of aging associated with surprisingly long lifespan in some birds. Our studies have focused on a short-lived avian model, the Japanese quail in order to describe reproductive aging and the neuroendocrine characteristics leading to reproductive senescence. Biomarkers of aging used in mammalian species include telomere length, oxidative damage, and selected metabolic indicators. These markers provide confirming evidence that the long-lived birds appear to age more slowly. A corollary area of interest is that of immune function and aging. Immune responses have been studied in selected wild birds and there has been a range of studies that have considered the effects of stress in wild and domestic species. Our laboratory studies have specifically tested response to immune challenge relative to aging in the quail model and these studies indicate that there is an age-related change in the qualitative aspects of the response. However, there are also intriguing differences in the ability of the aging quail to respond that differ from mammalian data. Finally, another approach to understanding aging is to attempt to develop or test strategies that may extend lifespan and presumably health. One area of great interest has been to consider the effect of calorie restriction, which is a treatment shown to extend lifespan in a variety of species. This approach is routinely used in domestic poultry as a means for extending reproductive function and enhancing health. Our data indicate that moderate calorie restriction has beneficial effects, and that physiological and endocrine responses reflect these benefits.     

Partial reprogramming as a therapeutic approach for heart disease: A state-of-the-art review

Heart disease such as myocardial infarction is the first cause of mortality in all countries. Today, cardiac cell-based therapy using de novo produced cardiac cells is considered as a novel approach for cardiac regenerative medicine. Recently, an alchemy-like approach, known as direct reprogramming or direct conversion, has been developed to directly convert somatic cells to cardiac cells in vitro and in vivo. This cellular alchemy is a short-cut and safe strategy for generating autologous cardiac cells, and it can be accomplished through activating cardiogenesis- or pluripotency-related factors in noncardiac cells. Importantly, pluripotency factors-based direct cardiac conversion, known as partial reprogramming, is shorter and more efficient for cardiomyocyte generation in vitro. Today, this strategy is achievable for direct conversion of mouse and human somatic cells to cardiac lineage cells (cardiomyocytes and cardiac progenitor cells), using transgene free, chemical-based approaches. Although, heart-specific partial reprogramming seems to be challenging for in vivo conversion of cardiac fibroblasts to cardiac cells, but whole organism-based in vivo partial reprogramming ameliorates cellular and physiological hallmarks of aging and prolongs lifespan in mouse. Notably, cardiac cells produced using partial reprogramming strategy can be a useful platform for disease modeling, drug screening and cardiac cell-based therapy, once the safety issues are overcome. Herein, we discuss about all progresses in de novo production of cardiac cells using partial reprogramming-based direct conversion, as well as give an overview about the potential applications of this strategy in vivo and in vitro.     

Lifespan extension in genetically modified mice

Major advances in aging research have been made by studying the effect of genetic modifications on the lifespan of organisms, such as yeast, invertebrates (worms and flies) and mice. Data from yeast and invertebrates have been the most plentiful because of the ease in which genetic manipulations can be made and the rapidity by which lifespan experiments can be performed. With the ultimate focus on advancing human health, testing genetic interventions in mammals is crucial, and the mouse has proven to be the mammal most amenable to this task. Lifespan studies in mice are resource intensive, requiring up to 4 years to complete. Therefore, it is critical that a set of scientifically-based criteria be followed to assure reliable results and establish statistically significant findings so other laboratories can replicate and build on the data. Only then will it be possible to confidently determine that the genetic modification extends lifespan and alters aging.     

Mammalian models of extended healthy lifespan

Over the last two centuries, there has been a significant increase in average lifespan expectancy in the developed world. One unambiguous clinical implication of getting older is the risk of experiencing age-related diseases including various cancers, dementia, type-2 diabetes, cataracts and osteoporosis. Historically, the ageing process and its consequences were thought to be intractable. However, over the last two decades or so, a wealth of empirical data has been generated which demonstrates that longevity in model organisms can be extended through the manipulation of individual genes. In particular, many pathological conditions associated with the ageing process in model organisms, and importantly conserved from nematodes to humans, are attenuated in long-lived genetic mutants. For example, several long-lived genetic mouse models show attenuation in age-related cognitive decline, adiposity, cancer and glucose intolerance. Therefore, these long-lived mice enjoy a longer period without suffering the various sequelae of ageing. The greatest challenge in the biology of ageing is to now identify the mechanisms underlying increased healthy lifespan in these model organisms. Given that the elderly are making up an increasingly greater proportion of society, this focused approach in model organisms should help identify tractable interventions that can ultimately be translated to humans.     

Pharmacological maintenance of protein homeostasis could postpone age-related disease

Over the last 10 years, various screens of small molecules have been conducted to find long sought interventions in aging. Most of these studies were performed in invertebrates but the demonstration of pharmacological lifespan extension in the mouse has created considerable excitement. Since aging is a common risk factor for several chronic diseases, there is a reasonable expectation that some compounds capable of extending lifespan will be useful for preventing a range of age‐related diseases. One of the potential targets is protein aggregation which is associated with several age‐related diseases. Genetic studies have long indicated that protein homeostasis is a critical component of longevity but recently a series of chemicals have been identified in the nematode Caenorhabditis elegans that lead to the maintenance of the homeostatic network and extend lifespan. Herein we review these interventions in C. elegans and consider the potential of improving health by enhancing protein homeostasis.     

Recent advances in vertebrate aging research 2009

Among the notable trends seen in this year’s highlights in mammalian aging research is an awakening of interest in the assessment of age‐related measures of mouse health in addition to the traditional focus on longevity. One finding of note is that overexpression of telomerase extended life and improved several indices of health in mice that had previously been genetically rendered cancer resistant. In another study, resveratrol supplementation led to amelioration of several degenerative conditions without affecting mouse lifespan. A primate dietary restriction (DR) study found that restriction led to major improvements in glucoregulatory status along with provocative but less striking effects on survival. Visceral fat removal in rats improved their survival, although not as dramatically as DR. An unexpected result showing the power of genetic background effects was that DR shortened the lifespan of long‐lived mice bearing Prop1^df, whereas a previous report in a different background had found DR to extend the lifespan of Prop1^df mice. Treatment with the mammalian target of rapamycin (mTOR) inhibitor, rapamycin, enhanced the survival of even elderly mice and improved their vaccine response. Genetic inhibition of a TOR target made female, but not male, mice live longer. This year saw the mTOR network firmly established as a major modulator of mammalian lifespan.     

Biological features of the new A2G--adr mouse mutant with abnormal muscle function

A new mouse mutant (A2G-adr) with abnormal muscle function is described and has been compared with the 129 Re dystrophic mouse. The mutation, which is due to an autosomal gene defect, results in myotonic-like spasms, progressive muscle weakness and a reduced lifespan. Affected animals were consistently lighter than normal littermates; comparison of organ weights and organ-to-bodyweight ratios indicated a slower growth rate in the mutants.     

Systemic delivery of a glucosylceramide synthase inhibitor reduces CNS substrates and increases lifespan in a mouse model of type 2 Gaucher disease

Neuropathic Gaucher disease (nGD), also known as type 2 or type 3 Gaucher disease, is caused by a deficiency of the enzyme glucocerebrosidase (GC). This deficiency impairs the degradation of glucosylceramide (GluCer) and glucosylsphingosine (GluSph), leading to their accumulation in the brains of patients and mouse models of the disease. These accumulated substrates have been thought to cause the severe neuropathology and early death observed in patients with nGD and mouse models. Substrate accumulation is evident at birth in both nGD mouse models and humans affected with the most severe type of the disease. Current treatment of non-nGD relies on the intravenous delivery of recombinant human glucocerebrosidase to replace the missing enzyme or the administration of glucosylceramide synthase inhibitors to attenuate GluCer production. However, the currently approved drugs that use these mechanisms do not cross the blood brain barrier, and thus are not expected to provide a benefit for the neurological complications in nGD patients. Here we report the successful reduction of substrate accumulation and CNS pathology together with a significant increase in lifespan after systemic administration of a novel glucosylceramide synthase inhibitor to a mouse model of nGD. To our knowledge this is the first compound shown to cross the blood brain barrier and reduce substrates in this animal model while significantly enhancing its lifespan. These results reinforce the concept that systemically administered glucosylceramide synthase inhibitors could hold enhanced therapeutic promise for patients afflicted with neuropathic lysosomal storage diseases.     

Rapamycin increases lifespan and inhibits spontaneous tumorigenesis in inbred female mice

The nutrient-sensing TO R (target of rapamycin) pathway is involved in cellular and organismal aging. Rapamycin, an inhibitor of TO R, extends lifespan in yeast, fruit flies and genetically heterogeneous mice. Here, we demonstrate that lifelong administration of rapamycin extends lifespan in female 129/Sv mice characterized by normal mean lifespan of 2 y. Importantly, rapamycin was administrated intermittently (2 weeks per month) starting from the age of 2 mo. Rapamycin inhibited age-related weight gain, decreased aging rate, increased lifespan (especially in the last survivors) and delayed spontaneous cancer. 22.9% of rapamycin-treated mice survived the age of death of the last mouse in control group. Thus we demonstrated for the first time in normal inbred mice that lifespan can be extended by rapamycin. This opens an avenue to develop optimal doses and schedules of rapamycin as an anti-aging modality.     

Down-regulating sphingolipid synthesis increases yeast lifespan

Knowledge of the mechanisms for regulating lifespan is advancing rapidly, but lifespan is a complex phenotype and new features are likely to be identified. Here we reveal a novel approach for regulating lifespan. Using a genetic or a pharmacological strategy to lower the rate of sphingolipid synthesis, we show that Saccharomyces cerevisiae cells live longer. The longer lifespan is due in part to a reduction in Sch9 protein kinase activity and a consequent reduction in chromosomal mutations and rearrangements and increased stress resistance. Longer lifespan also arises in ways that are independent of Sch9 or caloric restriction, and we speculate on ways that sphingolipids might mediate these aspects of increased lifespan. Sch9 and its mammalian homolog S6 kinase work downstream of the target of rapamycin, TOR1, protein kinase, and play evolutionarily conserved roles in regulating lifespan. Our data establish Sch9 as a focal point for regulating lifespan by integrating nutrient signals from TOR1 with growth and stress signals from sphingolipids. Sphingolipids are found in all eukaryotes and our results suggest that pharmacological down-regulation of one or more sphingolipids may provide a means to reduce age-related diseases and increase lifespan in other eukaryotes.     

Longevity is associated with relative brain size in birds

Brain size of vertebrates has long been recognized to evolve in close association with basic life‐history traits, including lifespan. According to the cognitive buffer hypothesis, large brains facilitate the construction of behavioral responses against novel socioecological challenges through general cognitive processes, which should reduce mortality and increase lifespan. While the occurrence of brain size–lifespan correlation has been well documented in mammals, much less evidence exists for a robust link between brain size and longevity in birds. The aim of this study was to use phylogenetically controlled comparative approach to test for the relationship between brain size and longevity among 384 avian species from 23 orders. We used maximum lifespan and maximum reproductive lifespan as the measures of longevity and accounted for a set of possible confounding effects, such as allometry, sampling effort, geographic patterns, and life‐history components (clutch size, incubation length, and mode of development). We found that both measures of longevity positively correlated with relative (residual) brain size. We also showed that major diversification of brain size preceded diversification of longevity in avian evolution. In contrast to previous findings, the effect of brain size on longevity was consistent across lineages with different development patterns, although the relatively low strength of this correlation could likely be attributed to the ubiquity of allomaternal care associated with the altricial mode of development. Our study indicates that the positive relationship between brain size and longevity in birds may be more general than previously thought.