A putative gene causes variability in lifespan among genotypically identical mice

Body weight and high-fat diet are associated with epigenetic aging in female members of the BXD murine family

DNA methylation (DNAm) is shaped by genetic and environmental factors and modulated by aging. Here, we examine interrelations between epigenetic aging, body weight (BW), and life span in 12 isogenic strains from the BXD family of mice that exhibit over twofold variation in longevity. Genome-wide DNAm was assayed in 70 liver specimens from predominantly female cases, 6-25 months old, that were maintained on normal chow or high-fat diet (HFD). We defined subsets of CpG regions associated with age, BW at young adulthood, and strain-by-diet-dependent life span. These age-associated differentially methylated CpG regions (age-DMRs) featured distinct genomic characteristics, with DNAm gains over time occurring in sites such as promoters and exons that have high CpG density and low average methylation. CpG regions associated with BW were enriched in introns, tended to have lower methylation in mice with higher BW, and were inversely correlated with gene expression (i.e., higher mRNA levels in mice with higher BW). CpG regions associated with life span were linked to genes involved in life span modulation, including the telomerase reverse transcriptase gene, Tert, which had both lower methylation and higher expression in long-lived strains. An epigenetic clock defined from age-DMRs revealed accelerated aging in mice belonging to strains with shorter life spans. Both higher BW and the HFD were associated with accelerated epigenetic aging. Our results highlight the age-accelerating effect of heavier BW. Furthermore, we demonstrate that the measure of epigenetic aging derived from age-DMRs can predict genotype and diet-induced differences in life span among female BXD members.

Genetic cartography of longevity in humans and mice: Current landscape and horizons

Aging is a complex and highly variable process. Heritability of longevity among humans and other species is low, and this finding has given rise to the idea that it may be futile to search for DNA variants that modulate aging. We argue that the problem in mapping longevity genes is mainly one of low power and the genetic and environmental complexity of aging. In this review we highlight progress made in mapping genes and molecular networks associated with longevity, paying special attention to work in mice and humans. We summarize 40 years of linkage studies using murine cohorts and 15 years of studies in human populations that have exploited candidate gene and genome-wide association methods. A small but growing number of gene variants contribute to known longevity mechanisms, but a much larger set have unknown functions. We outline these and other challenges and suggest some possible solutions, including more intense collaboration between research communities that use model organisms and human cohorts. Once hundreds of gene variants have been linked to differences in longevity in mammals, it will become feasible to systematically explore gene-by-environmental interactions, dissect mechanisms with more assurance, and evaluate the roles of epistasis and epigenetics in aging. A deeper understanding of complex networks-genetic, cellular, physiological, and social-should position us well to improve healthspan.

Epigenetic age-predictor for mice based on three CpG sites

Epigenetic clocks for mice were generated based on deep-sequencing analysis of the methylome. Here, we demonstrate that site-specific analysis of DNA methylation levels by pyrosequencing at only three CG dinucleotides (CpGs) in the genes Prima1, Hsf4, and Kcns1 facilitates precise estimation of chronological age in murine blood samples, too. DBA/2 mice revealed accelerated epigenetic aging as compared to C57BL6 mice, which is in line with their shorter life-expectancy. The three-CpG-predictor provides a simple and cost-effective biomarker to determine biological age in large intervention studies with mice.

Epigenetic marks are chemical modifications found throughout the genome – the DNA within cells. By influencing the activity of nearby genes, the marks govern developmental processes and help cells to adapt to changes in their surroundings. Some epigenetic marks can be gained or lost with age. A lot of aging research focuses on one type of mark, called “DNA methylation”. By measuring the presence or absence of specific methyl groups, scientists can estimate biological age – which may differ from calendar age.

Recent studies have developed computer models called epigenetic aging clocks to predict the biological age of mouse cells. These clocks use epigenetic data collected from the entire genomes of mice, and are useful for understanding how the aging process is affected by genetic parameters, diet, or other environmental factors. Yet, the genome sequencing methods used to construct most existing epigenetic clocks are expensive, labor-intensive, and cannot be easily applied to large groups of mice.

Han et al. have developed a new way to predict biological aging in mice that needs methylation information from just three particular sections of the genome. Even though this approach is much faster and less expensive than other epigenetic approaches to measuring aging, it has a similar level of accuracy to existing models. Han et al. use the new method to show that cells from different strains of laboratory mice age at different rates. Furthermore, in a strain that has a shorter life expectancy, aging seems to be accelerated.

The new approach developed by Han et al. will make it easier to study how aging in mice is affected by different interventions. Further studies will also be needed to better understand how epigenetic marks relate to biological aging.

Epigenetic perturbations in aging stem cells

Stem cells maintain homeostasis in all regenerating tissues during the lifespan of an organism. Thus, age-related functional decline of such tissues is likely to be at least partially explained by molecular events occurring in the stem cell compartment. Some of these events involve epigenetic changes, which may dictate how an aging genome can lead to differential gene expression programs. Recent technological advances have made it now possible to assess the genome-wide distribution of an ever-increasing number of epigenetic marks. As a result, the hypothesis that there may be a causal role for an altered epigenome contributing to the functional decline of cells, tissues, and organs in aging organisms can now be explored. In this paper, we review recent developments in the field of epigenetic regulation of stem cells, and how this may contribute to aging.

Genetic Regulation of Female Sexual Maturation and Longevity Through Circulating IGF1

We previously reported that insulin-like growth factor 1 (IGF1) was involved in coregulating female sexual maturation and longevity. To understand the underlying genetic mechanisms, based on the strain survey assays of development and aging traits, we crossed two mouse strains, KK/HIJ and PL/J, and produced 307 female F2 mice. We observed the age of vaginal patency (AVP) and the life span of these females. We also measured circulating IGF1 level at 7, 16, 24, 52, and 76 weeks. IGF1 level at 7 weeks significantly correlated with AVP. IGF1 levels at ages of 52 and 76 weeks negatively correlated with longevity (p ≤ .05). A gene mapping study found 22, 4 ,and 3 quantitative trait loci for IGF1, AVP, and life span, respectively. Importantly, the colocalization of IGF1, AVP, and life span quantitative trait loci in the distal region of chromosome 2 suggests this locus carries gene(s) that could regulate IGF1, AVP, and life span. In this region, proprotein convertase subtilisin/kexin type 2 has been found to be associated with female sexual maturation in a human genome-wide association study. We verified the roles of proprotein convertase subtilisin/kexin type 2 in regulating IGF1 and AVP by showing that depletion of proprotein convertase subtilisin/kexin type 2 significantly reduced IGF1 and delayed AVP in mice, suggesting that it also might be involved in the regulation of aging.

The mouse as a model organism in aging research: usefulness, pitfalls and possibilities

The mouse has become the favorite mammalian model. Among the many reasons for this privileged position of mice is their genetic proximity to humans, the possibilities of genetically manipulating their genomes and the availability of many tools, mutants and inbred strains. Also in the field of aging, mice have become very robust and reliable research tools. Since laboratory mice have a life expectancy of only a few years, genetic approaches and other strategies for intervening in aging can be tested by examining their effects on life span and aging parameters during the relatively short period of, for example, a PhD project. Moreover, experiments on mice with an extended life span as well as on mice demonstrating signs of (segmental) premature aging, together with genetic mapping strategies, have provided novel insights into the fundamental processes that drive aging. Finally, the results of studies on caloric restriction and pharmacological anti-aging treatments in mice have a high degree of relevance to humans. In this paper, we review a number of recent genetic mapping studies that have yielded novel insights into the aging process. We discuss the value of the mouse as a model for testing interventions in aging, such as caloric restriction, and we critically discuss mouse strains with an extended or a shortened life span as models of aging.

Concise review: hematopoietic stem cell aging, life span, and transplantation

Self-renewal and multilineage differentiation of stem cells are keys to the lifelong homeostatic maintenance of tissues and organs. Hematopoietic aging, characterized by immunosenescence, proinflammation, and anemia, is attributed to age-associated changes in the number and function of hematopoietic stem cells (HSCs) and their microenvironmental niche. Genetic variants and factors regulating stem cell aging are correlatively or causatively associated with overall organismal aging and longevity. Translational use of HSCs for transplantation and gene therapy demands effective methods for stem cell expansion. Targeting the molecular pathways involved in HSC self-renewal, proliferation, and homing has led to enhanced expansion and engraftment of stem cells upon transplantation. HSC transplantation is less effective in elderly people, even though this is the demographic with the greatest need for this form of treatment. Thus, understanding the biological changes in the aging of stem cells as well as local and systematic environments will improve the efficacy of aged stem cells for regenerative medicine and ultimately facilitate improved health and life spans.

Fat maintenance is a predictor of the murine lifespan response to dietary restriction

Dietary restriction (DR), one of the most robust life-extending manipulations, is usually associated with reduced adiposity. This reduction is hypothesized to be important in the life-extending effect of DR, because excess adiposity is associated with metabolic and age-related disease. Previously, we described remarkable variation in the lifespan response of 41 recombinant inbred strains of mice to DR, ranging from life extension to life shortening. Here, we used this variation to determine the relationship of lifespan modulation under DR to fat loss. Across strains, DR life extension correlated inversely with fat reduction, measured at midlife (males, r= -0.41, P<0.05, n=38 strains; females, r= -0.63, P<0.001, n=33 strains) and later ages. Thus, strains with the least reduction in fat were more likely to show life extension, and those with the greatest reduction were more likely to have shortened lifespan. We identified two significant quantitative trait loci (QTLs) affecting fat mass under DR in males but none for lifespan, precluding the confirmation of these loci as coordinate modulators of adiposity and longevity. Our data also provide evidence for a QTL previously shown to affect fuel efficiency under DR. In summary, the data do not support an important role for fat reduction in life extension by DR. They suggest instead that factors associated with maintaining adiposity are important for survival and life extension under DR.

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.

Quantitative trait loci (QTL) analysis of longevity in C57BL/6J by DBA/2J (BXD) recombinant inbred mice

BACKGROUND AND AIMS

Genes associated with longevity have been identified using both single gene and genome-wide approaches in a variety of species. The aim of this study was to identify quantitative trait loci (QTLs) that influence longevity in male and female mice from twenty-three C57BL/6J by DBA/2J (BXD) recombinant inbred (RI) strains.

METHODS

Approximately 12 animals of each sex for each RI strain were maintained under standard conditions until natural death or moribundity criteria were met.

RESULTS

A number of life span-relevant loci previously reported on chromosomes (Chrs) 7, 8, 10 and 11 were confirmed. In addition, 5 previously unreported QTLs for mouse life span on Chrs 1, 2, 6, 11, and X were identified as significant and 3 QTLs on Chrs 5, 8, and 16 were suggestive.

CONCLUSIONS

Several QTLs were coincident in males and females although the modest correlation between male and female median lifespans and the identification of sex specific QTLs provide evidence that the genetic architecture underlying longevity in the sexes may differ substantially. The identification of multiple QTLs for longevity will provide valuable resources for both reductionist and integrationist research into mechanisms of life span determination.

Theories of biological aging: genes, proteins, and free radicals

Traditional categorization of theories of aging into programmed and stochastic ones is outdated and obsolete. Biological aging is considered to occur mainly during the period of survival beyond the natural or essential lifespan (ELS) in Darwinian terms. Organisms survive to achieve ELS by virtue of genetically determined longevity assuring maintenance and repair systems (MRS). Aging at the molecular level is characterized by the progressive accumulation of molecular damage caused by environmental and metabolically generated free radicals, by spontaneous errors in biochemical reactions, and by nutritional components. Damages in the MRS and other pathways lead to age-related failure of MRS, molecular heterogeneity, cellular dysfunctioning, reduced stress tolerance, diseases and ultimate death. A unified theory of biological aging in terms of failure of homeodynamics comprising of MRS, and involving genes, milieu and chance, is acquiring a definitive shape and wider acceptance. Such a theory also establishes the basis for testing and developing effective means of intervention, prevention and modulation of aging.

Understanding and modulating ageing

Ageing is characterized by a progressive accumulation of molecular damage in nucleic acids, proteins and lipids. The inefficiency and failure of maintenance, repair and turnover pathways is the main cause of age-related accumulation of damage. Research in molecular gerontology is aimed at understanding the genetic and epigenetic regulation of survival and maintenance mechanisms at the levels of transcription, post-transcriptional processing, post-translational modifications, and interactions among various gene products. Concurrently, several approaches are being tried and tested to modulate ageing in a wide variety of organisms. The ultimate aim of such studies is to improve the quality of human life in old age and prolong the health-span. Various gerontomodulatory approaches include gene therapy, hormonal supplementation, nutritional modulation and intervention by free radical scavengers and other molecules. A recent approach is that of applying hormesis in ageing research and therapy, which is based on the principle of stimulation of maintenance and repair pathways by repeated exposure to mild stress. A combination of molecular, physiological and psychological modulatory approaches can realize "healthy ageing" as an achievable goal in the not-so-distant future.

A genetic and genomic approach to identify longevity genes in mice

Natural variation in rate of aging or longevity in mice and humans shows a very complex inheritance pattern. Few targeted genetic screens have identified longevity genes in mammals, partly as a result from the fact that the genetics of longevity can only be studied reliably in cohorts of mice. In this paper we propose that a combined genetic and genomic analysis of large families of fully genotyped recombinant inbred mice may provide a crucial tool to the aging research community. As a proof of principle we describe preliminary studies in which variation in gene expression patterns in hematopoietic stem cells and brain were genetically linked to longevity.

Replicative senescence and the art of counting

The idea that aging is largely the result of (endogenous) stress appears to be at odds with the concept of biological 'clocks', which seem to programme and terminate cellular aging processes. Here, data are reviewed that show that telomeres, the major clock identified in human cells so far, do in fact measure stress and damage accumulation much more than simple mitotic time. Telomere shortening is significantly stress-dependent due to a telomere-specific damage repair deficiency. This identifies telomere-driven human cell replicative senescence as a stress response with high potential importance for human aging.

Deterministic regulation of hematopoietic stem cell self-renewal and differentiation

Most current theories assume that self-renewal and differentiation of hematolymphoid stem cells (HSCs) is randomly regulated by intrinsic and environmental influences. A direct corollary of these tenets is that self-renewal will continuously generate functionally heterogeneous daughter HSCs. Decisions about self-renewal versus commitment are made by individual, single HSCs and, thus, require examination on the clonal level. We followed the behavior of individual, clonally derived HSCs through long-term, serial repopulation experiments. These studies showed that daughter HSCs derived from individual clones were remarkably similar to each other in the extent and kinetics of repopulation. Moreover, daughter HSCs within a clone showed equivalent contributions to the myeloid or lymphoid lineages. Lineage contribution could be followed because of the discovery of a new subset of HSCs that gave rise stably to skewed ratios of myeloid and lymphoid cells. Overall, the data argue that self-renewal does not contribute to the heterogeneity of the adult HSC compartment. Rather, all HSCs in a clone follow a predetermined fate, consistent with the generation-age hypothesis. By extension, this suggests that the self-renewal and differentiation behavior of HSCs in adult bone marrow is more predetermined than previously thought.

The genetics of aging-- implications for pharmacogenomics

Lifespan experiments of lower organisms and mammals along with recent studies of centenarians are making inroads into delineating genetic factors that determine the ability to achieve exceptional longevity. These models may be helpful for the discovery of both longevity-enabling genes as well as genes associated with increased propensity to develop specific diseases. Both academic and commercial laboratories are putting substantial resources into discovering such genes in order to better understand the genetic and environmental underpinnings of how some people age more slowly than others and markedly delay or even escape age-associated diseases.

The genetics of aging

Once thought to be an extremely complex conundrum of weak genetic and environmental effects, exceptional longevity is beginning to yield genetic findings. Numerous lower organism and mammalian models demonstrate genetic mutations that increase life-span markedly. These variations, some of them evolutionarily conserved, inform us about biochemical pathways that significantly impact upon longevity. Centenarian studies have also proven useful as they are a cohort that, relative to younger age groups, lacks genotypes linked to age-related lethal diseases and premature mortality. Pedigree studies have demonstrated a significant familial component to the ability to survive to extreme old age and a recent study demonstrates a locus on chromosome 4 linked to exceptional longevity indicating the likely existence of at least one longevity enabling gene in humans. Thus, a number of laboratories are making substantial and exciting strides in the understanding of the genetics of aging and longevity which should lead to the discovery of genes and ultimately drugs that slow down the aging process and facilitate people's ability to delay and perhaps escape age-associated diseases.

The genetics of exceptional human longevity

There is a substantial distinction to be made between the genetics of aging and the genetics of exceptional longevity. Twin studies suggest that the average set of genetic variations facilitates the average human's ability to live well into their octogenarian years. Other studies indicate that taking full advantage of this average set results in spending the majority of those years in good health. However, many people counteract such genetic endowment with poor health habits, resulting in a substantially lower average life expectancy and relatively more time spent in poor health. To live beyond the octogenarian years, life-span experiments in lower organisms and mammals and population and molecular genetic studies of centenarian sibships suggest that genetic factors play an important role in exceptional longevity. These factors are likely to influence basic mechanisms of aging, which in turn broadly influence susceptibility to age-related illnesses. Lacking genetic variations that predispose to disease, and having variations that confer disease resistance (longevity enabling genes), are probably both important to such a remarkable survival advantage. Recent studies indicate the likelihood that such factors will be elucidated in the near future.

Mutant and genetically modified mice as models for studying the relationship between aging and carcinogenesis

Increased interest is emerging in using mouse models to assess the genetics of aging and age-related diseases, including cancer. However, only limited information is available regarding the relationship between aging and spontaneous tumor development in genetically modified mice. Analysis of various transgenic and knockout rodent models with either a shortened or an extended life span, provides a unique opportunity to evaluate interactions of genes involved in the aging process and carcinogenesis. There are only a few models which show life span extension. Ames dwarf mutant mice, p66(-/-) knockout mice, alpha MUPA and MGMT transgenic mice live longer than wild-type strains. The incidence of spontaneous tumors in these mutant mice was usually similar to those in controls, whereas the latent period of tumor development was increased. Practically all models of accelerated aging showed increased incidence and shorter latency of tumors. This phenomenon has been observed in animals which display a phenotype that more closely resembles natural aging, and in animals which manifest only some features of the normal aging process. These observations are in agreement with an earlier established positive correlation between tumor incidence and the rate of tumor incidence increase associated with aging and the aging rate in a population. Thus, genetically modified animals are a valuable tool in unravelling mechanisms underlying aging and cancer. Systemic evaluation of newly generated models should include onco-gerontological studies.

Applying hormesis in aging research and therapy

Biology of aging is well understood at a descriptive level. Based on these data, biogerontological research is now able to develop various possibilities for intervention. A promising approach for the identification of gerontogenes and gerontogenic processes is through the hormetic effects of mild stress on slowing down aging. Although there are several issues remaining to be resolved, specially with regard to the notion of mild stress, application of hormesis in aging research and therapy is a powerful new approach.

Heterogeneity and its biodemographic implications for longevity and mortality

In the visible world, heterogeneity typically refers to the differences that exist among individuals in a defined population. These differences can arise from a variety of sources--biological, behavioral and social. Ever since Darwin, scientists have argued over the biological significance of differences observed at the individual, morphological, physiological, genetic, molecular and structural levels. A general consensus has been reached. Heterogeneity is ubiquitous, it is important, and it increases as observations are made at finer levels of biological resolution. Debates over the significance of heterogeneity have emerged once again as biologists and demographers work together in order to create the emerging field of biodemography. For these scientists, the debates center around the relative impact that individual heterogeneity has on population level statistics. It is argued here that in a world where the mortality barriers to long life for individuals have been dramatically weakened, the population consequences of heterogeneity are already visible and will grow in importance as biomedical technologies continue to usher progressively more people into the post-reproductive period of the lifespan.

Genetic analysis of lifespan in hybrid progeny derived from the SAMP1 mouse strain with accelerated senescence

The SAMP1 mouse, a senescence-accelerated mouse prone (SAMP) strain, shows accelerated senescence coupled with a short lifespan as a genetic trait, and has been used in gerontological research. The accelerated senescence and short lifespan of SAMP strains is considered to be under the control of multiple genes. To identify the chromosomal regions encompassing the genes for the accelerated senescence and short lifespan, we performed whole genome scanning with polymorphic marker loci in a progeny from a cross between the SAMP1 strain and normal B10.BR strain. A genetically recessive effect of the amyloidogenic Apoa2(c) allele from SAMP1 on chromosome 1 to shorten the lifespan was demonstrated in the progeny, consistent with the previous report. The recessive effect was observed also at D1Mit67, D5Mit267, D6Mit384 and D19Mit33, suggesting the presence of genes for accelerated senescence in the SAMP strains around these loci. Other markers on chromosomes 8, 14, 16, and 17, however, exhibited a dominant or additive effect to shorten or prolong the lifespan, demonstrating a complex genetic control of the trait.

Biogerontology: the next step

After a long period of collecting empirical data describing the changes in organisms, organs, tissues, cells, and macromolecules, biogerontological research is now able to develop various possibilities for intervention. Because aging is a stochastic and nondeterministic process characterized by a progressive failure of maintenance and repair, it is reasoned that gene involved in homeodynamic repair pathways are the most likely candidate gerontogenes. A promising approach for the identification of critical gerontogenic processes is through the hormesis-like positive effects of mild stress. Stimulation of various repair pathways by mild stress has significant effects on delaying the onset of various age-associated alterations in cells, tissues, and organisms.

Progenitor cell mobilization by granulocyte colony-stimulating factor controlled by loci on chromosomes 2 and 11

Granulocyte colony-stimulating factor (G-CSF) can effectively mobilize hematopoietic stem and progenitor cells from bone marrow into blood, thereby allowing peripheral blood stem cells (PBSCs) to be used for transplantation. The efficiency of PBSC mobilization response to G-CSF is a multigene trait. DBA/2 (high-responder) and C57BL/6 (low-responder) mice were used for a genetic analysis of G-CSF–induced progenitor release. Significant linkages were found on chromosome 2 by analyzing segregation distortion among the high responders of 500 backcross mice and on chromosome 11 by using the quantitative trait locus analysis of 26 strains of BXD recombinant inbred mice.

Theoretical Gompertzian implications on life span variability among genotypically identical animals

Aging is a highly polygenic trait (Kirkwood, T.B.L., Franceschi, C., 1992. Is aging as complex as it would appear? Annals of the New York Academy of Sciences 663, 412-417) who's underlying mechanisms are still unresolved. Animal models are an important help for understanding this process and a recent report drew attention to a putative gene which causes variability in the life span among genotypically identical mice (de Haan, G., Gelman, R., Watson, A., Yunis, E., van Zant, G., 1998. A putative gene causes variability in life span among genotypically identical mice. Nature Genetics 19, 114-116). De Haan et al. observed that the time between the death of the first and last member of a group of inbred mice (the death range) is controlled by a locus, which they mapped to chromosome 11. The authors conclude that well-known effects like modifiers, suppressors or epistatic genes might not be able to explain how this variability in genetically identical organisms is generated and that such a trait has broad implications for genetic studies of the aging process. Here we give a possible answer to the question of what mechanism could cause such a phenotype. We show that all genes which affect the Gompertz parameters are possible candidates.

Genetic analysis of hemopoietic cell cycling in mice suggests its involvement in organismal life span

Normal somatic cells undergo replicative senescence in vitro but the significance of this process in organismic aging remains controversial. We have shown previously that hemopoietic stem cells of common inbred strains of mice vary widely in cycling activity and that this parameter is inversely correlated with strain-dependent mean life span. To assess whether cell cycling and life span are causally related, we searched for quantitative trait loci (QTLs) that contributed to variation of these traits in BXH and BXD recombinant inbred mice. Two QTLs, mapping to exactly the same intervals on chromosomes 7 and 11, were identified that were associated with variation of both cell cycling and life span. The locus on chromosome 11 mapped to the cytokine cluster, a segment that shows synteny with human chromosome 5q, in which deletions are strongly associated with myelodysplastic syndrome. These data indicate that steady-state cell turn-over, here measured in hemopoietic progenitor cells, may have a significant effect on the mean life span of mammals.

Developing mouse models of aging: a consideration of strain differences in age-related behavioral and neural parameters

Increased interest is emerging for using mouse models to assess the genetics of brain aging and age-related neurodegenerative diseases. Despite this demand, relatively little information is available on aging in behavioral or neuromorphological parameters in various mouse strains that are being used to create transgenic and null mutant mice. We review several issues regarding selection of appropriate strains as follows: (1) Does the behavioral parameter exhibit a significant age by strain interaction? (2) Do the strains differ in lifespan? (3) Are there potential intervening variables, such as strain-specific performance strategies or disease, in the behavioral task being investigated that would confound the desired conclusions? (4) Does the behavioral difference have an underlying neural correlate? In this context we present a conceptual model pertaining to the selection of mouse strains and behavioral parameters for genetic analyses. We also review the importance of applying stereological techniques for determining age-related structural changes in the mouse brain as well as the potential value of a database that would catalog this information. Thus, our intention is to underscore the growing importance of mouse models of brain aging and the concomitant need for additional information about mouse aging in general.